U.S. patent application number 17/677890 was filed with the patent office on 2022-08-25 for loading calculation.
This patent application is currently assigned to S&C Electric Company. The applicant listed for this patent is S&C Electric Company. Invention is credited to Qing Guo, Boris Marendic, Michael Quinlan, Yoav Sharon.
Application Number | 20220268826 17/677890 |
Document ID | / |
Family ID | 1000006224405 |
Filed Date | 2022-08-25 |
United States Patent
Application |
20220268826 |
Kind Code |
A1 |
Guo; Qing ; et al. |
August 25, 2022 |
LOADING CALCULATION
Abstract
A system and method for determining loads throughout a power
distribution network having a plurality of switching devices
provided along a feeder. The method includes measuring the current
and/or voltage on one or both sides of the switching devices and
calculating a power flowing through each of the switching devices
and a load in each section using the current and/or voltage
measurements at predetermined sample times. The method further
includes storing a plurality of recorded current/voltage
measurements or calculated powers flowing through each device for
consecutive sample times. The method then determines a median load
from the measurements and power flows and calculates a load in each
of network sections.
Inventors: |
Guo; Qing; (Skokie, IL)
; Quinlan; Michael; (Chicago, IL) ; Sharon;
Yoav; (Evanston, IL) ; Marendic; Boris;
(Chicago, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
S&C Electric Company |
Chicago |
IL |
US |
|
|
Assignee: |
S&C Electric Company
Chicago
IL
|
Family ID: |
1000006224405 |
Appl. No.: |
17/677890 |
Filed: |
February 22, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63153657 |
Feb 25, 2021 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01R 19/2513 20130101;
G01R 31/086 20130101 |
International
Class: |
G01R 31/08 20060101
G01R031/08; G01R 19/25 20060101 G01R019/25 |
Claims
1. A method for calculating loads throughout a power distribution
network, the network including at least one power source, a primary
feeder, a plurality of switching devices positioned along the
primary feeder and a section between switching devices, wherein the
load on a section is the sum of the power flowing into the section
from all of the switching devices around the section, and wherein
the plurality of switching devices are in communication with each
other, the method comprising: determining a first downstream load
by a first switching device provided on the feeder downstream from
the power source using current and/or voltage measurements; sending
a total downstream load to a second switching device provided on
the feeder downstream from the first switching device; determining
a second downstream load by the second switching device using
current and/or voltage measurements; sending a total downstream
load to a next switching device provided on the feeder downstream
of the second switching device; continuing determining and sending
downstream loads to the switching devices provided along the feeder
downstream of the next switching device in this manner to a last
switching device on the feeder; and calculating the load in each
section along the feeder using the determined loads.
2. The method according to claim 1 further comprising knowing that
a fault has occurred in the network if calculating the load in each
section gives an incorrect load calculation in one of the
sections.
3. The method according to claim 1 wherein each switching device
includes a memory for an upstream side and a memory for a
downstream side of the device, the memories storing a plurality of
the determined loads taken at previous sample times.
4. The method according to claim 3 wherein an oldest determined
load is discarded when a new load is received.
5. The method according to claim 3 wherein each switching device
includes a median filter, and wherein determining the loads
includes determining the loads in a division as a median of a
recent determined load and past determined loads.
6. The method according to claim 5 wherein the median filters use
two-dimensional sample points for determining the loads for active
and reactive power.
7. The method according to claim 3 further comprising suspending
determination of the load in a switching device if any of the
switching devices reports a loss of voltage.
8. The method according to claim 7 wherein determining a load
includes using previously stored measurements before the loss of
voltage.
9. The method according to claim 7 wherein determining a load
includes using previously stored measurements if a short-term data
dissemination loss occurs in the network.
10. The method according to claim 3 wherein determining a load
includes computing and storing a ratio of load magnitudes of the
sections using previously stored loads if a long-term data
dissemination loss occurs in the network.
11. The method according to claim 3 further comprising clearing the
memories if a known change in a topology of the network occurs.
12. The method according to claim 1 wherein calculating the load in
each section along the feeder using the determined loads occurs in
a control device.
13. The method according to claim 12 wherein the control device is
one of the switching devices, more than one of the switching
devices or a sub-station.
14. The method according to claim 1 wherein the current and/or
voltage measurements are phasor values or magnitudes depending on
the measuring capability.
15. The method according to claim 1 wherein the switching devices
are reclosers, sectionalizers or circuit breakers.
16. A method for determining loads throughout a power distribution
network, the network including at least one power source, a primary
feeder, a plurality of switching devices positioned along the
primary feeder and a section between switching devices, wherein the
plurality of switching devices are in communication with each
other, the method comprising: measuring current and/or voltage on
one or both of an upstream side and a downstream side of each of
the switching devices; calculating a load in each of the switching
devices using current and/or voltage measurements at predetermined
sample times; storing a plurality of the calculated loads for
consecutive sample times; determining a median load from a
predetermined number of the loads stored; sending the median load
from the switching devices to a control device; and calculating a
load in each of the sections in the control device.
17. The method according to claim 16 further comprising knowing
that a fault has occurred in the network if calculating the load in
each section gives an incorrect load calculation in one of the
sections.
18. The method according to claim 16 wherein the control device is
one of the switching devices, more than one of the switching
devices or a sub-station.
19. A system for determining loads throughout a power distribution
network, the network including at least one power source, a primary
feeder, a plurality of switching devices positioned along the
primary feeder and a section between switching devices, wherein the
plurality of switching devices are in communication with each
other, the system comprising: means for measuring current and/or
voltage on one or both of an upstream side and a downstream side of
each of the switching devices; means for calculating a load in each
of the switching devices using current and/or voltage measurements
at predetermined sample times; means for storing a plurality of the
calculated loads for consecutive sample times; means for
determining a median load using from a predetermined number of the
loads stored; means for sending the median load from the switching
devices to a control device; and means for calculating a load in
each of the sections in the control device.
20. The system according to claim 19 wherein the control device is
one of the switching devices, more than one of the switching
devices or a sub-station.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority from the
U.S. Provisional Application No. 63/153,657, filed on Feb. 25,
2021, the disclosure of which is hereby expressly incorporated
herein by reference for all purposes.
BACKGROUND
Field
[0002] The present disclosure relates generally to a system and
method for determining loads throughout a power distribution
network and, more particularly, to a system and method for
determining loads throughout a power distribution network having a
plurality of switching devices provided along a feeder.
Discussion of the Related Art
[0003] An electrical power network, often referred to as an
electrical grid, typically includes power generation plants each
having power generators, such as gas turbines, nuclear reactors,
coal-fired generators, hydro-electric dams, etc. The power plants
provide power at medium voltages that are then stepped up by
transformers to a high voltage AC signal to be connected to high
voltage transmission lines that deliver electrical power to
substations typically located within a community, where the voltage
is stepped down to a medium voltage for distribution. The
substations provide the medium voltage power to three-phase feeders
including three single-phase feeder lines that carry current
120.degree. apart in phase. three-phase and single-phase lateral
lines are tapped off of the feeder that provide the medium voltage
to various distribution transformers, where the voltage is stepped
down to a low voltage and is provided to loads, such as homes,
businesses, etc.
[0004] Periodically, faults occur in the distribution network as a
result of various things, such as animals touching the lines,
lightning strikes, tree branches falling on the lines, vehicle
collisions with utility poles, etc. Faults may create a
short-circuit that increases the load on the network, which may
cause the current flow from the substation to significantly
increase, for example, many times above the normal current, along
the fault path. This amount of current causes the electrical lines
to significantly heat up and possibly melt, and also could cause
mechanical damage to various components in the substation and in
the network. Many times the fault will be a temporary or
intermittent fault as opposed to a permanent or bolted fault, where
the thing that caused the fault is removed a short time after the
fault occurs, for example, a lightning strike, and where the
distribution network will almost immediately begin operating
normally.
[0005] Fault interrupters, for example, reclosers that employ
vacuum interrupters, are provided on utility poles and in
underground circuits along a power line and have a switch to allow
or prevent power flow downstream of the recloser. These reclosers
detect the current and voltage on the line to monitor current flow
and look for problems with the network circuit, such as detecting a
fault. If fault current is detected the recloser is opened in
response thereto, and then after a short delay closed to determine
whether the fault is a temporary fault. If fault current flows when
the recloser is closed, it is immediately opened. If the fault
current is detected again or two more times during subsequent
opening and closing operations indicating a permanent fault, then
the recloser remains open, where the time between detection tests
may increase after each test. For a typical reclosing operation for
fault detection tests, about 3-6 cycles of fault current pass
through the recloser before it is opened.
[0006] When a fault is detected, it is desirable that the first
fault interrupter upstream from the fault be opened as soon as
possible so that the fault is quickly removed from the network so
that the loads upstream of that fault interrupter are not
disconnected from the power source and service is not interrupted
to them. It is further desirable that if the first fault
interrupter upstream from the fault does not open for whatever
reason, then a next fault interrupter upstream from the fault is
opened, and so on. In order to accomplish this, it is necessary
that some type of communications or coordination protection scheme
be employed in the network so that the desired fault interrupter is
opened in response to the fault.
[0007] A sectionalizer is a self-contained, circuit-opening device
used in combination with source-side protective devices, such as
reclosers or circuit breakers, to automatically isolate faulted
sections of an electrical distribution network. A faulted circuit
indicator is a device that automatically detects and identifies
faults in an electrical distribution network, but does not have
switching capabilities to open a power line. The devices are
typically distributed between and among the reclosers to provide a
system for isolating smaller sections of the network in response to
a fault. Faulted circuit indicators and sectionalizers rely on
observing a sequence of fault currents and the presence and absence
of voltage either to indicate the presence of a fault or count the
number of reclosing attempts, and then perform circuit isolation
sectionalizing when the desired number of reclosing attempts has
been reached. Existing power distribution circuit sectionalizers
detect the passage of fault currents, including both the initial
fault event and subsequent recloser-initiated events, as part of
more elaborate fault isolation and restoration processes. These
processes may include counting discrete intervals of fault current
passage, or counting discrete intervals of voltage presence and
absence.
[0008] Modern power distribution networks of the type being
discussed herein usually operate as intelligent distribution
automation systems, i.e., provide intelligent control over
electrical power grid functions to the distribution level and
beyond, where the many of the devices talk to each other and
perform functions based on received information and data. These
systems require device control that needs to have knowledge of the
system topology, i.e., the architecture, for automation tasks,
where the main purpose of the knowledge dissemination is to enable
automatic power restoration in response to faults. For example, to
decide what extra network sections alternative power sources can
power, the automatic power restoration will need the relevant
network information, such as power being consumed in each
section.
[0009] In existing distribution automation systems, the system
topology needs to be manually pushed to the devices for distributed
control or a control center for centralized control, both at
deployment and every time the topology of the network changes. In
other words, the topology of the system needs to be manually loaded
into each device, and thus the topology stored in each device is
not automatically changed when the topology changes from devices
being added to or removed from the network or when switching events
occur to isolate faults. This not only requires a significant
amount of engineering work, but also complicates the logic when the
system topology changes due to automated operations. Further, if a
switching device automatically opens, such as in response to a
fault, other devices may not be made aware of this switching.
SUMMARY
[0010] The following discussion discloses and describes a system
and method for determining loads throughout a power distribution
network, where the network includes at least one power source, a
primary feeder, a plurality of switching devices positioned along
the primary feeder that are in communication with each other or
with a centralized control in a control center, and sections each
of which consists of power lines bounded by switching devices or
other current measuring devices. The method includes measuring
current and voltage or just current on one or both of an X side and
a Y side of each of the switching devices, and calculating power
flowing through each of the switching devices using current and
voltage or just current measurements at predetermined sample times.
The method further includes storing a plurality of the calculated
powers for consecutive sample times. The method then determines a
median power from a predetermined number of the stored powers,
sends the median power from the switching devices to a control
device or multiple control devices, and calculates a power in each
of the sections in the control device.
[0011] Additional features of the present disclosure will become
apparent from the following description and appended claims, taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a simplified schematic type diagram of an
electrical power distribution network;
[0013] FIG. 2 is a simplified schematic block diagram of a
switching device in the network shown in FIG. 1;
[0014] FIG. 3 is a simplified schematic type diagram of an
electrical power distribution network illustrating an extreme
linear case for a data dissemination process;
[0015] FIG. 4 is a simplified schematic type diagram of an
electrical power distribution network illustrating another extreme
case for a data dissemination process; and
[0016] FIG. 5 is a simplified schematic type diagram of an
electrical power distribution network for describing load
calculations.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0017] The following discussion of the embodiments of the
disclosure directed to a system and method for determining loads
throughout a power distribution network having a plurality of
switching devices provided along a feeder, where the method
includes calculating the loads in feeder sections, saves
consecutive calculated loads, and provides a median of the saved
loads is merely exemplary in nature, and is in no way intended to
limit the disclosure or its applications or uses.
[0018] This disclosure proposes providing a dynamic graph
representation for robust data dissemination between switching
devices in a power distribution network that can operate with
partial data resulting from communications problems. The dynamic
graph representation allows the switching devices that isolate
faults in the network to automatically obtain the network topology
with minimal manual input at device deployment, and enables an
automatic topology update in all of the devices when the topology
changes. The switching devices in the network are capable of
measuring voltage and/or current, and sending that data to other
devices or a control center by, for example, wireless
transmissions, cellular, fiber optics, etc. In the discussion
below, a team is defined as a section of powerlines bounded by
switching devices. In the discussion below, the device performing
the loading computation is sometimes referred to as the control,
where the control may reside in one or multiple switching devices,
a substation or control center. There will be data specific for
each switching device, such as open/closed status, and data
specific for each team, such as amount of load and whether there is
a fault within the team. A device list stores device data and a
team list stores team data of all of the devices and teams in a
certain region of the network. The boundary of this region is
specified by the distribution automation scheme.
[0019] FIG. 1 is a simplified schematic type diagram of an
electrical power distribution network 10 that employs distributed
control. The network 10 includes two AC sources 12 and 14, such as
electrical substations that step down high voltage power from a
high voltage power line (not shown) to medium voltage power. The
network 10 also includes switching devices 28, 30, 32 and 34 that
can be any suitable device for the purposes discussed herein, such
as reclosers, breakers, sectionalizers, etc. The devices 28, 30 and
34 are normally closed devices and the device 32 is a normally open
device. This configuration defines a feeder 18 between the source
12 and the devices 32 and 34 and a feeder 20 between the source 14
and the device 32. Lateral lines 22 are coupled to the feeder 18
and feed loads 24. A line section 38 of the feeder 18 is provided
between the source 12 and the device 28, a line section 40 of the
feeder 18 is provided between the devices 28 and 30, a line section
42 of the feeder 18 is provided between the devices 30, 32 and 34,
a line section 44 is connected to the source 14, and a line section
46 is connected to the device 34. The source 12 provides power to
the sections 38, 40 and 42 and the source 14 is prevented from
proving power to the sections 38, 40 and 42. A distributed
generation (DG) source 36, such as solar, wind, turbine, battery,
etc., is provided in the section 40.
[0020] The combination of the AC source 12, one side of the device
28 and the section 38 define a team, the combination of one side of
the devices 28 and 30 and the section 40 define a team, the
combination of one side of the devices 30, 32 and 34 and the
section 42 define a team, the combination of one side of the device
32, the source 14 and the section 44 define a team, and the
combination of one side of the device 34 and the section 46 define
a team, where some of the teams define a division 48 and where the
devices 32 and 34 are also tie devices between divisions. As will
be discussed in detail below, the topology, i.e., device
configuration, of the division 48 will be known by all of the
devices 28-34 by communications therebetween, where the topology of
the division 48 will be automatically revised and updated as
devices are removed and added to the network 10. A control center
50 may be in, for example, radio communication with the switching
devices 28-34 that provides switching control consistent with the
discussion herein.
[0021] FIG. 2 is a simplified schematic block diagram of a
switching device 60 intended to be a non-limiting general
representation of any one of the switching devices 28-34 in the
network 10, where the device 60 may include a switch 62,
voltage/current sensors 64, a control device 66 (sometimes referred
to herein as the control), a memory 68, having a size of N+1, and a
transceiver 70, where the device list and the team list may be
stored in the memory 68. Further, for some device designs discussed
below, the memory 68 is intended to represent a separate memory on
each of the upstream and downstream sides of the device 60. The
switching device 60 may also include a timer 72 and a median filter
74 for reasons that will become apparent from the discussion
below.
[0022] The topology of the power distribution network 10 is
represented by specific fields in the device list and the team
list, where Tables 1 and 2 below are an example of the device list
and the team list, respectively, stored in each of the switching
devices 28-34 corresponding to the division 48. The local indices
ordering of the devices in the device list and the teams in the
team list are different in different devices. The device local
index identifies each device in the division 48 separately from the
device ID and the team local index identifies the teams in the
division 48. The local indices are only meaningful and can identify
a device or team in a specific devices memory 68. It is noted that
the teams do not need global indexing or IDs, where each team is
uniquely identified by the devices on the team. This makes a split
or a union of the teams due to the addition or removal of a device
easy to accomplish. The device specific data and the team specific
data in the lists can be anything relating to that device or team,
such a voltage measurements, current measurements, open/closed
status, etc. Besides the device specific data, each device in the
device list includes two fields, namely, a TeamOnX field that
specifies the local index in the team list of the team on each
device's X side, and a TeamOnY field that specifies the local index
in the team list of the team on each device's Y side, where the X
and Y sides of a device are determined by the physical orientation
of the device when it is installed. Knowing the orientation of the
devices is necessary because these devices are often capable of
measuring voltage on both sides of the device. Each team in the
team list includes a DevicesOnTeam field that specifies the local
indices in the device list (not device ID) of the devices on that
team.
TABLE-US-00001 TABLE 1 Local Index ID TeamOnX TeamOnY Device
Specific Data 1 28 A B -- 2 30 B C -- 3 32 C D -- 4 34 C E --
TABLE-US-00002 TABLE 2 Local Index DevicesOnTeam Team Specific Data
A 1 -- B 1, 2 -- C 2, 3, 4 -- D 3 -- E 4 --
[0023] The TeamOnX field and the TeamOnY field in the device list
point to the local indices of the entries in the team list, and the
DevicesOnTeam field in the team list points to the local indices of
the entries in the device list (not device IDs). This makes it
faster to access the information of the devices on a team, and the
information of teams on each side of a switching device by
eliminating the need for searches. Further, the device local
indices in the DevicesOnTeam field are sorted by the device ID. For
example, the DevicesOnTeam field for Team C is the switching
devices 30, 32 and 34. Because a team is uniquely identified by the
devices on the team, this makes the comparison between two teams
easier. Specifically, if two teams have the same number of devices,
and the devices have the same device IDs pairwise, then they are
the same team.
[0024] The topology of the division 48 can be recovered from either
the device list or the team list, and thus the two lists provide
mutual validation. In other words, if the topology recovered from
the device list and the topology recovered from the team list are
inconsistent, then there is an indication of data errors. Moreover,
the two lists also make the traversal of the graph more efficient.
For example, if a switching device needs to know which teams are
adjacent to Team A, it only needs to read which devices are on Team
A in its team list, and which teams are connected to these devices
in its device list. By contrast, for the same task, if only the
team list contains network topology information, after reading the
devices connected to Team A in the team list, the device will need
to search through all of the teams to find which team is connected
to those devices.
[0025] This graph representation makes it possible to automatically
discover the network topology by any of the switching devices.
Knowing the topology of the network 10 or the division 48 allows
power restoration in response to a fault, so that when there is a
fault and loads are dropped, knowledge of which loads have been
dropped and how big the loads are allows how the open and closed
status of the switching devices 28-34 is determined so that
alternate sources can be connected to some of the loads.
[0026] At deployment of a device, the engineer only needs to
manually specify the immediate neighbor devices on each side of the
new device and then the topology of the whole division 48 can be
provided in all of the devices 28-34 by transferring information
and data from device to device. The newly deployed device will
automatically build its own device list and team list with this
initial information. After sharing this information through
automated communications, the control may easily populate the
corresponding fields in its device list and team list and have the
topology information of the full division 48. For example, in the
network 10, if the control is in the switching device 28, then it
can start the device list and the team list with the information of
its immediate neighbor devices, as shown by Tables 3 and 4
below.
TABLE-US-00003 TABLE 3 Local Index ID TeamOnX TeamOnY . . . 1 28 A
B 2 30 B
TABLE-US-00004 TABLE 4 Local Index DevicesOnTeam . . . A 1 B 1,
2
[0027] When the switching device 28 receives a message that
contains information about the switching device 30 and the
immediate neighbor devices of the device 30, it can update its
device and team lists, as shown in Tables 5 and 6, respectively,
below. It is noted that the local indices in the message can be
different than the local indices in the control. Tables 7 and 8 are
the updated device and team lists, respectively, in the memory of
the device 28 after processing the message.
TABLE-US-00005 TABLE 5 Local Index ID TeamOnX TeamOnY . . . 1 30 A
B 2 28 A 3 32 B 4 34 B
TABLE-US-00006 TABLE 6 Local Index Local Team Indices . . . A 2, 1
B 1, 3, 4
TABLE-US-00007 TABLE 7 Local Index ID TeamOnX TeamOnY . . . 1 28 A
B 2 30 B C 3 32 C 4 34 C
TABLE-US-00008 TABLE 8 Local Team Indices . . . A 1 B 1, 2 C 2, 3,
4
[0028] After receiving the information of the device 32 and the
device 34 in subsequent messages, the complete device and team
lists will be built in the memory 68 of the device 28. One possible
reason that the device 28 may not have had the topology information
beyond the device 30 is that the system was just initialized.
Another possible reason is that the device 30 previously defined
the boundary of a division, and because of a change in its status,
the two divisions on its two sides are now combined as one
division.
[0029] For each switching device identified in a message, the
recipient device checks if it already has that device in its device
list. The same is done for each team. To avoid searches for these
checks, the recipient switching device maintains a mapping between
the local indices of the device list and the team list in the
incoming messages from a specific message source, for example, a
switching device, and the local indices of those lists in its own
memory. When the recipient device checks whether it already has a
device from the message in its own device list, it uses the map to
see if this device maps to the same device in its device list. If
yes, then the device updates the information in its device list
using the message and, if no, then the device updates the mapping
before updating its device list. The same procedure applies to the
team list. Note that none of the incoming messages needs to convey
a full view of the network. As long as the whole network is covered
by all incoming messages, the recipient will have up-to-date
information of the network. If a message is lost, a device may only
have outdated information for the part of the network covered by
that message. If the network is designed such that each device
calculates its local information, such as team loading, possibly by
gathering data from its immediate neighbor devices, and propagates
the result out to other devices, then the process requires no
central processing.
[0030] When a new switching device is added to the network 10, this
device only needs to be manually added to its immediate neighbor
device's settings as their new neighbor device, and the device
lists and team lists of these neighbor devices will update
themselves accordingly. As required before, the new device will
also only need its own immediate neighbor devices as the manual
input in its setting. The data dissemination will then propagate
the addition of the device automatically to other devices that need
the topology information. Similarly, when a device is removed from
the network 10, only its immediate neighbor devices will need to be
updated manually, and the updated topology information will
propagate automatically to other devices that need the topology
information.
[0031] The processes described above only adds or updates existing
elements (devices and teams) to the lists, and does not remove
elements from the lists. Elements do need to be removed from the
lists if a device is taken out of the system, or a division is
split into two divisions because a device becomes a boundary
device. To accomplish the removal of disconnected elements, a graph
traversal, such as breadth first search (BFS) or a depth first
search (DFS), well known to those skilled in the art, is initiated
starting from the device in which the control resides. This graph
traversal indicates all the elements that are still within the
division 48. Therefore, every element that hasn't been reached by
the graph traversal is removed.
[0032] As discussed above, in a power distribution network that
employs distributed control, knowledge acquired from each of the
switching device 28-34 needs to be disseminated through the
division 48 so that every device 28-34 in the division 48 has the
knowledge, where the division 48 is the basic unit for distribution
automation tasks, such as automatic power restoration. For example,
to decide what extra network sections each alternative source can
power, the automatic power restoration will need the relevant
network information, such as power being consumed in each of the
sections. Knowledge dissemination can also be used for other
purposes, such as supporting voltage regulation decisions.
[0033] This disclosure also proposes a knowledge or data
dissemination method that enables the switching devices 28-34 to
efficiently share their knowledge across the network 10. At the end
of each data dissemination period, each of the switching devices
28-34 will have up-to-date knowledge about all of the other devices
28-34 in the division 48. Compared to existing methods, this
knowledge dissemination method minimizes the number of messages
passed between the devices 28-34 as well as the size of the
messages. In particular, topological information is utilized to
minimize not just the number of messages, but also the number of
hops between the devices 28-34 each message needs to make.
[0034] The proposed knowledge dissemination method propagates data
from one device to another device in a distributed fashion. As a
manual input in the device settings, each device knows its
immediate neighbor devices on each side. An endpoint device is
defined as a device that does not have an immediate neighboring
device on one of its two sides or a device that defines the
boundary of the division that the distribution automation system
considers, such as open tie devices. For example, in the network
10, the devices 28, 32 and 34 are all endpoint devices, where the
boundaries of the division 48 are defined by open tie devices,
which are the normally open devices 32 and 34. The devices 28 and
30 are closed.
[0035] In the proposed dissemination method, the endpoint devices
28, 32 and 34 will periodically initiate data dissemination into
the division 48 as triggered by the timer 72, such as every five
seconds. When the timer 72 in each of the endpoint devices 28, 32
and 34 expires, the endpoint devices 28, 32 and 34 will reset their
timer 72 to expire in the same amount of time, and send a data
dissemination message including information and data it has stored
about itself, such as voltages, currents, loads on sections,
open/close status, etc., to their one-side neighbor devices. For
example, in the network 10, when the timer 72 expires, the device
28 will reset its timer and send a data dissemination message to
the device 30 containing its up-to-date information, the device 32
will reset its timer and send a data dissemination message to the
devices 30 and 34 containing its up-to-date information, and the
device 34 will reset its timer and send a data dissemination
message to the devices 30 and 32 containing its up-to-date
information. Note that it is beneficial if the timers are GPS
synchronized, but it is not a requirement as discussed below. It is
also noted that the communications of messages from one device to a
next device as described herein may reference a flow direction of
the messages.
[0036] As soon as a non-endpoint device receives messages from all
of its neighbor devices on its message receiving side, it will send
a message to all of its neighbor devices on its message sending
side along the flow direction, where the flow direction can be in
either direction. This message will contain the information about
the device itself, as well as the information it has about every
other device on its message receiving side. This includes the
information from its immediate neighbor devices on the message
receiving side and all of the devices beyond the immediate neighbor
devices. For example, in the network 10, after the device 30
receives the message from the device 28, it will store the
information and send a data dissemination message to the devices 32
and 34 containing the information about itself, as well as the
information about the device 28. Similarly, after the device 30
receives the messages from the devices 32 and 34, it will store the
information and send a data dissemination message to the device 28
containing the information about itself, as well as the information
about the devices 32 and 34. The non-endpoint devices know what
devices are on each of their sides in the division 48, and will not
to send its message until it receives the information from all of
the devices on a particular side. When a division boundary or
endpoint device receives messages from all of its neighbor devices
on one side, it will not send a message to all of its neighbor
devices on the other side because the neighbor devices on the other
side are in a different division. By this process, at the end of
each data dissemination period, each device will have the
information of all of the devices in the division 48.
[0037] FIG. 3 is a simplified schematic type diagram of an
electrical power distribution network 80 illustrating an extreme
linear case for the data dissemination process as discussed above.
The network 80 includes an AC power sources 84 and 86 and, in order
from the source 84 to the source 86, switching devices 90, 92, 94,
96 and 98, where the switching devices 90 and 98 are endpoint
devices and the switching device 98 is normally open. In this
configuration, a division 82 is provided between the source 84 and
the device 98 and includes a feeder 88. When its timer expires, the
switching device 90 sends its information to the switching device
92. When the switching device 92 receives the information from the
switching device 90 it sends its information and the information
about the switching device 90 to the switching device 94. When the
switching device 94 receives the information from the switching
device 92 it sends its information and the information about the
switching devices 90 and 92 to the switching device 96. When the
switching device 96 receives the information from the switching
device 94 it sends its information and the information about the
switching devices 90, 92 and 94 to the switching device 98.
Likewise, when its timer expires, the switching device 98 sends its
information to the switching device 96. When the switching device
96 receives the information from the switching device 98 it sends
its information and the information about the switching device 98
to the switching device 94. When the switching device 94 receives
the information from the switching device 96 it sends its
information and the information about the switching devices 96 and
98 to the switching device 92. When the switching device 92
receives the information from the switching device 94 it sends its
information and the information about the switching devices 94, 96
and 98 to the switching device 90.
[0038] FIG. 4 is a simplified schematic type diagram of an
electrical power distribution network 100 illustrating another
extreme case for the data dissemination process as discussed above.
The network 100 includes an AC power source 102 and a division 104.
The division 104 includes a feeder 106 coupled to the power source
102, switching devices 108, 110, 112, 114, 118, 122, 126, 130 and
132 coupled along the feeder 106, as shown, where the switching
devices 108, 114, 118, 126, 128 and 130 are endpoint devices of the
division 104. As above, along one flow direction, the switching
device 110 does not send its data dissemination message to the
right until it receives the messages from the switching devices 108
and 118, the switching device 122 does not send its data
dissemination message until it receivers the messages from the
switching devices 130 and 132, the switching device 112 does not
send its data dissemination message to the devices 110 and 112
until it receives the messages from the switching devices 110 and
122. Likewise, along another flow direction, the switching device
112 does not send its data dissemination message to the left until
it receives the messages from the switching devices 114 and 126,
the switching device 122 does not send its data dissemination
message to the switching devices 110 and 112 until it receives the
message from the switching devices 130 and 132, and the switching
device 110 does not send its data dissemination message to the left
until it receives the messages from the switching devices 112 and
122 that include information from all of the switching devices 112,
114, 122, 126, 130 and 132.
[0039] A self-tie is a series of two or more switching devices
where every device is both different from and a neighbor device of
the device preceding it, none of the devices in the series is an
endpoint device, and the same devices are listed both at the
beginning and the end of the series. If there is a self-tie in the
division, and no device in the self-tie is a division boundary
device, then one device in the self-tie will be designated as an
endpoint device by a predefined rule as explained below in
paragraph [0046]. This prevents the data dissemination from
entering an endless loop.
[0040] The discussion below is a comparison between the proposed
data dissemination method and two other known data dissemination
solutions, namely, a naive solution and a centralized solution. In
the naive solution, each switching device sends a message with its
own information to every other device. In the centralized solution,
one of the devices in a division is designated as a concatenator
device. Each switching device in the division will send a message
to the concatenator device with its own information. After
receiving messages from every device in the division, the
concatenator device will send a message with the information of the
whole division to its neighbor devices, which will then forward it
to their neighbor devices and so forth. It is noted that due to
geographical restrictions, when a switching device sends a message
to another device, the message may need to be routed through
multiple devices. To simplify the comparison, it is assumed that
each device can only send direct messages to its immediate neighbor
devices. Therefore, in order to send a message to another device
that is not a neighbor device, the message will be routed through a
neighbor device and the neighbor device's neighbor device until the
destination using the shortest path according to the network
topology. For example, in the network 10, when the device 28 needs
to send a message to the device 32, the device 28 will send the
message to the device 30, and then the device 30 will send it to
the device 32. To measure the message size, a unit of data is
defined as the data containing the voltage and current measures of
one device.
[0041] Examples can be produced that show that the data
dissemination method proposed in this disclosure requires a
relatively small number of messages compared to other methods, such
as naive and centralized methods, while the messages are still of a
reasonable size. It is also noted that the average units of data
each device sends (average number of messages x average message
size) is the minimum for the proposed approach compared to other
methods, such as naive and centralized methods. This has a direct
impact on the time it takes for each device to get the full picture
of the division 48. These advantages of the proposed method are
found in other types of network topologies as well, from linear
structures, such as the network 80, to full tree structures, such
as the network 100.
[0042] It is noted that for the centralized solution, the number of
messages depends on the choice of the concatenator device. Finding
a concatenator device that minimizes the number of messages in a
complex network is not trivial. Even if such a concatenator device
can be found, it may not result in fewer messages than the proposed
method, and it may still result in more data being passed around
than the proposed method. The main disadvantage of the centralized
solution is that if the concatenator device of a division
experiences any malfunction, no data will be passed. Furthermore,
if any communications link is down, then the devices downstream of
the communications link in relation to the concatenator device will
not receive any data about their neighbor devices who are also
downstream of this communications link.
[0043] For the proposed data dissemination method, it is beneficial
to maintain the information dissemination process relatively
synchronized between the devices. Specifically, the more
synchronized the process is the newer the information a device will
have about other devices. For a switching device that has multiple
neighbor devices on the receiving side and at least one neighbor
device on the sending side if it receives messages from all of the
neighbor devices on the receiving side around the same time, then
it can immediately send a message to the neighbor devices on the
sending side. By contrast, if it receives messages from its
receiving side neighbor devices one after another during a longer
time span, then it has to wait for a longer time to send a message
to its sending side neighbor devices, because it will only send the
message after it receives the message from the last receiving side
neighbor device. This results in the information that a device has
about the distant devices to be relatively old.
[0044] If the devices have reliable synchronized clocks, it is
straightforward to synchronize the information dissemination
between the devices. In this disclosure, an alternative method of
roughly synchronizing the information dissemination through
messaging for the cases where reliable synchronized clocks are not
available is also proposed. To do this, one of the switching
devices 28-34 in the division 48 is designated as the synchronizer
device, where the synchronizer device periodically sends out sync
messages to its neighbor devices. When a device receives a sync
message from one side, it will propagate this message to all of the
neighbor devices on the other side. When an endpoint device
receives a sync message, it will reset its timer for data
dissemination. If an endpoint device is selected as the
synchronizer device, then it will reset its timer for data
dissemination as soon as it initiates the sync messages. The
synchronizer device sends the sync messages at a much lower
frequency than the information dissemination messages. For example,
the synchronizer device may initiate a sync message every five
minutes, and the endpoint devices may initiate regular information
dissemination messages every five seconds. Note that this method
does not require the synchronizer device to have a complete view of
the division, where each device in the division only needs to know
its own neighbor devices on each side. Also note that this method
can be used to initiate data dissemination. Since the sync messages
are propagated out from the synchronizer device, for different
possible locations of the synchronizer device in a division, the
total number of sync messages for the division 48 will be the
same.
[0045] If the synchronizer device of a division experiences any
malfunction, the information dissemination may not be synchronized.
However, unlike the concatenator device in the centralized
dissemination method, in this case, the information dissemination
will continue. The switching devices will still have updated
information of all of the other devices, albeit with longer delay.
To regain synchronism after a division loses its synchronizer
device, a non-synchronizer device that does not receive a sync
message for more than 1.5 times of the regular time interval for
sync messages, will start sending out sync messages to its neighbor
devices. While this may result in multiple devices thinking
themselves as the synchronizer device of the division, this can be
resolved as explained below.
[0046] In the case where more than one device thinks it is the
synchronizer device of the division, they will all initiate sync
messages. If a device receives a sync message initiated from a
different synchronizer device than the synchronizer device in its
memory, it will determine if the new synchronizer device is more
qualified as a synchronizer device than the old device based on
predefined rules, such as the one with the highest ID. If yes, it
will update its own memory of the new synchronizer device. If not,
it will ignore this sync message and not propagate it to other
devices. Because this qualification process happens in all devices
including those who think themselves to be the synchronizer device
of the division 48 at the end of this process, only the most
qualified device will remain the synchronizer device of the
division 48.
[0047] To deal with possible self-ties that do not include a
boundary device in the division, as mentioned earlier in paragraph
[0038], a device in the self-tie is designated as an endpoint
device. This can be done by exploiting the sync messages.
Specifically, if a device receives a sync message from one side and
later receives a sync message from the other side, both sync
messages are initiated by the same synchronizer device, and the
sync messages were originally sent from the same side of that
synchronizer device, then this device is designated as a virtual
endpoint device, and it does not propagate the second sync message.
This is necessary because if the sync messages were originally sent
from the different sides of that synchronizer device, it means that
the synchronizer device itself is in the self-tie, and it is an
endpoint device itself, so there is no need to create a virtual
endpoint device.
[0048] With distributed data dissemination methods as discussed
herein, the data messages are usually propagated from one device to
another device following certain flow paths, where the goal is for
each device to have the full and current information of all of the
other devices at the end of each dissemination process, and where a
flow path can either be a linear path or a more complex graph
structure with branching and joining lines. Any local interruption
to the flow of messages, either momentary or permanent, may block
the regular flow of the data dissemination. Specifically, any
momentary or intermittent interruption of the communications may
prevent a device from obtaining updated information about another
device for an extended period of time. And, if the communications
problem is permanent, then a device may never get any information
about another device.
[0049] With any communications problem, some degradation in
performance is inevitable. The purpose of the communications error
handling is to minimize the impact of communications problems so
that each device can still have some information about the other
devices. Although this information may be incomplete and relatively
outdated, it is better than not having any information. In
contrast, existing distributed data dissemination methods that rely
on or assume perfect communications will completely stop data
dissemination while communications problems exist, even if the
problems are intermittent or restricted to only a few devices.
[0050] This disclosure proposes two communications error handling
methods, where one method deals with short-term contingencies and
the other method deals with long-term contingencies. When the
communications problem appears temporary, the logic for short-term
contingencies will maintain the flow of data dissemination despite
the problem. When the communications problem appears permanent, the
logic for long-term contingencies will reroute the data
dissemination to circumvent the problem.
[0051] The following discussion illustrates the benefits of
applying the proposed communications error handling methods and not
applying them. For a one message loss error, the outcome without
the proposed error handling is that all of the feeder data is two
data dissemination periods old and the outcome with the proposed
error handling is that the feeder data is still one data
dissemination period old for adjacent sections and two data
dissemination periods old for other sections, where when a fault
occurs and the sections downstream of the fault need to be
restored, information for sections farther away upstream of the
fault is not needed. For intermittent message losses for several
dissemination periods, the outcome without error handling is the
newest feeder data would be the data collected before the first
message loss, and the outcome with error handling is the feeder
data is gradually older for devices farther away on the feeder. For
permanent communications loss of a device, where devices on both
sides of that device experiencing communications loss can still
communicate with each other, the outcome without error handling is
the newest feeder data would be the data collected before the
communications loss and fault restoration performed on old loading
data can lead to sub-optimal results, and the outcome with error
handling is the feeder data is up to date except for the loading
distribution between the sections on the two sides of the
problematic device and fault restoration will be optimal as the
device experiencing communications loss cannot be commanded to
operate anyway.
[0052] The network 80 shown in FIG. 3 can be used to describe the
proposed error handling scheme when communications losses occur. As
discussed, each endpoint switching device 90 and 98 periodically
sends messages to its immediate neighbor switching devices 92 and
96. The non-endpoint switching device 92, 94 and 96 wait until they
receive messages from all of their neighbor devices on one side,
and then send the aggregated information in a message to all of the
neighbor devices on the other side. For example, every t seconds,
i.e., the data dissemination interval, the device 90 sends a
message containing its own data to the device 92. When the device
92 receives the message from the device 90, the device 92 will send
a message to the device 94 with the data from the devices 90 and
92, and so on down the feeder 88. When the device 98 receives a
message from the device 96, the device 98 will just store other
devices' data from the message in its own memory. Similarly,
another message will flow down the feeder 88 in the opposite
direction from the device 98 to the device 90. After both message
flows are finished, all of the devices 90-98 will have the complete
and updated information of all of the other devices.
[0053] For the explanation of the proposed communications error
handling methods, only the data dissemination flow from the device
90 to the device 98 will be described. In this explanation, the
device 90 is the initiator device because it initiates the data
dissemination flow, the device 98 is the terminator device because
the dissemination flow stops at the device 98, and the other
devices 92-96 are intermediate devices. For this flow, a
from-neighbor device of a device is a neighbor device that this
device receives messages from, and a to-neighbor device is a
neighbor device that the device sends messages to. Note that these
definitions are specific to the particular message flow being
discussed, and are only introduced for the discussion below. The
devices 90-98 themselves do not make these designations as each
side of a device is treated exactly the same, and independently of
the other side of the device. In other words, when a device
disseminates aggregated device information to one side after
receiving all of the messages from the other side, it does so
regardless of how many messages it already received from the other
side. The only requirement for this scheme to work is that all of
the endpoint devices send messages to their neighbor devices every
data dissemination interval.
[0054] To handle short-term communications contingencies, the
intermediate devices 92-96 will use two timers. Whenever an
intermediate device sends a message to to-neighbor devices because
it received message(s) from from-neighbor devices, it will set its
timer to expire in the time defined as a backup interval, where it
is required that a data dissemination interval is less than the
backup interval, which is less than two times the data
dissemination interval. The timer will be reset each time the
device sends a message. With this condition, when communications
are working properly, the timers in the intermediate devices 92-96
will always be reset before expiring. However, when a message is
lost during transmission, the timer in an intermediate device may
expire. When the timer expires, this device will send a message to
the next device(s) along the flow direction even though it did not
receive a message from each of the preceding device(s) or
from-neighbor devices. It will then set its timer to expire in the
time of the data dissemination interval. For example, if the
message sent from the device 92 to the device 94 is lost, the timer
in the device 94 will expire, and it will send a message to the
device 96 although it did not receive a message from the device 92.
This message will contain the updated information of the device 94,
but the older information from the devices 90 and 92. In this case,
the device 94 is effectively a new initiator device for this round
of data dissemination for this direction of message flow. This
results in the device 98 having the updated information of the
devices 94 and 96, but the older information of the devices 90 and
92. Had the backup interval mechanism not been implemented, the
device 98 would not have updated information of any of the other
devices.
[0055] This short-term contingency handling logic is especially
useful for intermittent down communications links. Assume that the
communications link between the devices 92 and 94 and the
communications link between the devices 94 and 96 are not stable,
where the messages sent from the device 92 to the device 94 and the
messages sent from the device 94 to the device 96 get lost about
50% of the time. In this particular example, one of these two
communications links is always down in any round of the data
dissemination flow. Then, as illustrated below in Tables 9-13,
after several rounds of data dissemination, the device 98 will have
relatively up-to-date information about the other devices 90-96.
The number in each box in the Tables 9-13 indicates how current the
particular device stored data is relative to a certain round of
data dissemination. For example, the memory in the device 98
holding the information of the device 90 with number 2 means the
device 98 has the information of the device 90 from data
dissemination round 2. Table 9 is the stored data in each device
90-98 after data dissemination round 0, i.e., perfect
communication, Table 10 is the stored data in each device 90-98
after data dissemination round 1, i.e., the communications link
between the devices 92 and 94 is down, Table 11 is the stored data
in each device 90-98 after data dissemination round 2, i.e., the
communications link between the devices 94 and 96 is down, Table 12
is the stored data in each device 90-98 after data dissemination
round 3, i.e., the communications link between the devices 92 and
94 is down, and Table 13 is the stored data in each device 90-98
after data dissemination round 4, i.e., the communications link
between the devices 94 and 96 is down. If the short-term
contingency handling logic was not used in this example, then the
device 98 would never have any updated information about any of the
other devices.
TABLE-US-00009 TABLE 9 90's 92's 94's 96's 98's memory memory
memory memory memory 90's 0 0 0 0 0 information 92's 0 0 0 0
information 94's 0 0 0 information 96's 0 0 information 98's 0
information
TABLE-US-00010 TABLE 10 90's 92's 94's 96's 98's memory memory
memory memory memory 90's 1 1 0 0 0 information 92's 1 0 0 0
information 94's 1 1 1 information 96's 1 1 information 98's 1
information
TABLE-US-00011 TABLE 11 90's 92's 94's 96's 98's memory memory
memory memory memory 90's 2 2 2 0 0 information 92's 2 2 0 0
information 94's 2 1 1 information 96's 1 2 information 98's 2
information
TABLE-US-00012 TABLE 12 90's 92's 94's 96's 98's memory memory
memory memory memory 90's 3 3 2 2 2 information 92's 3 2 2 2
information 94's 3 3 3 information 96's 3 3 information 98's 3
information
TABLE-US-00013 TABLE 13 90's 92's 94's 96's 98's memory memory
memory memory memory 90's 4 4 4 2 2 information 92's 4 4 2 2
information 94's 4 3 3 information 96's 4 4 information 98's 4
information
[0056] For the long-term contingency handling, it is assumed that
the communications layer in each switching device 90-98 can
identify the states of communications links between that device and
the other devices and store them in a communications connectivity
table, i.e., whether each device can receive messages from and
transmit messages to the other devices. There are two flow
directions and three possible states for each communications link.
For a certain device, its communications layer identifies the other
devices it can transmit messages to and the other devices it can
receive messages from. The possible states for each direction of a
communications link are Yes ( ), No (x) and Unknown (?). The
long-term contingency handling logic only takes effect when the
communications layer decides that some of the communications links
are not in the Yes state. It is noted that if the device 90 can
receive messages from the device 92, then when the device 92 sends
a message to the device 90, the communications layer can find a
route for the message. This route could be one message directly
from the device 92 to the device 90, or multiple messages routed
from the device 92 to the device 90 through other devices using an
existing routing protocol provided by the communications layer. As
such, if the device 90 cannot receive messages from the device 92,
then there is no possible route for any message from the device 92
to the device 90.
[0057] The following are two examples of how such connectivity
tables can be used. In the first example, the device 92 cannot send
or receive messages, where the corresponding connectivity Tables
14-16 of the device 90, the device 92 and the device 94,
respectively, are shown below. From these tables, the device 90 and
the device 94 can communicate with each other despite the device 92
being down. This may be because the device 90 and the device 94 are
within each other's transmission range because there is a repeater
between them or because there are other nearby devices on other
feeders that can help route the messages between them.
TABLE-US-00014 TABLE 14 90 92 94 96 98 Transmit-to ? Receive-from
x
TABLE-US-00015 TABLE 15 92 90 94 96 98 Transmit-to ? ? ? ?
Receive-from x x x x
TABLE-US-00016 TABLE 16 94 90 92 96 98 Transmit-to ? Receive-from
x
[0058] In the second example, assume that one direction of a
communications link is down, for example, caused by vegetation,
resulting in the devices 90-98 breaking into two communications
groups, where a communications group is a subset of the devices
such that any two devices within this group can communicate with
each other with two-way communications. In this example, the
devices 90 and 92 in Group 1 can only send messages to, but not
receive messages from the devices 94-98 in Group 2. The
corresponding connectivity Tables 17-19 of the devices 90-94,
respectively, are shown below.
TABLE-US-00017 TABLE 17 90 92 94 96 98 Transmit-to ? ? ?
Receive-from x x x
TABLE-US-00018 TABLE 18 92 90 94 96 98 Transmit-to ? ? ?
Receive-from x x x
TABLE-US-00019 TABLE 19 94 90 92 96 98 Transmit-to x x
Receive-from
[0059] When the communications layer of a device detects that in
its communications connectivity table, the entry for a
from-neighbor device does not have Yes for both directions, it will
perform a graph search, such as a BFS or DFS, on the from-side of
the device to find the closest devices in the topology that have
Yes for both directions, and assign them as the new from-neighbor
devices. It is assumed that when the network is initially brought
up online, or when devices are installed or removed, there is some
time interval in which all of the devices communicate with each
other properly so they all have built an up-to-date topological
view of their surroundings. For example, in connectivity Table 16,
when the communications are perfect, the old from-neighbor device
of the device 94 is the device 92. Since in the connectivity Table
16 for the device 94, the device 92 no longer has Yes for both
directions, the device 94 will find the closest device on the
from-side in its topology with Yes for both directions for its new
from-neighbor device, where the new from-neighbor device is the
device 90.
[0060] In the connectivity Table 19 for the device 94, the device
92 is no longer a from-neighbor device, and there is no qualified
device as a from-neighbor device for the device 94. Without a
from-neighbor device, the device 94 becomes an initiator for the
flow. Note that for the device 94, even though the device 92 is not
a from-neighbor device anymore because the device 94 cannot
transmit messages to the device 92, it is still possible that the
device 94 can receive messages from the device 92. If a device
receives a message from a known device that is not a from-neighbor
device, it will process and integrate the data in the message, but
will not trigger the sending of a message to its to-neighbor
devices.
[0061] Similarly, when the communications layer of a device detects
that in its connectivity table, a to-neighbor device does not have
Yes for both directions, it will perform a graph search on its
to-side to find the closest devices in the topology that have Yes
for both directions, and assign them as the new to-neighbor
devices. In addition, it will also add the devices between itself
and the new to-neighbor devices for up to L levels in topology as
additional to-neighbor devices (L.gtoreq.1), where L can be a user
defined number such as 2. From those additional to-neighbor
devices, those who have a No for the transmit-to direction will be
excluded. For example, in the communications connectivity Table 14
for the device 90, the device 94 becomes a new to-neighbor device
and, for L=2, the device 92 is also a to-neighbor device since the
device 90 is uncertain about whether the device 92 can receive
messages from it. In the connectivity Table 18 for the device 92,
there is no device with two-way communications on the to-side and,
for L=2, the devices 94 and 96 will be added as the new to-neighbor
devices, which results in the split of the flow for data
dissemination. For Group 1, whenever the timer 72 for the
left-right direction of message flow in the device 90 expires, it
sends a message to the device 92, and the device 92 sends messages
to the devices 94 and 96. For Group 2, whenever the timer in the
device 94 expires, it sends a message to the device 96, and the
device 96 sends a message to the device 98. Note that the devices
94 and 96 will receive messages from the device 92, but they will
only process and integrate the data in the messages. The flow of
the data dissemination in Group 2 is still controlled by the timer
in the device 94. The end result is that the device 98 will have
the information of all of the other devices 90-96. But, for another
data dissemination flow from the device 98 to the device 90, the
unidirectional down communications link makes it impossible for the
device 90 to get updated information of the devices 94-98 in Group
2.
[0062] In distribution automation power networks of the type being
discussed herein, it is important for the control devices to have
updated load information for each section in the network so as to
know what loads can be powered by what sources so that the sources
are not overloaded when load switching occurs in response to
faults, where a section's boundary is defined by switching devices
or other current measuring devices. The load of each section can
either be computed by actual current and voltage measurements of
the switching devices on the boundary or by actual current and
estimated voltage measurements of those devices. However, it is
difficult to always get accurate and locally synchronized
measurements for the load computation. The purpose for computing
updated loading information is to enable automatic restoration from
faults, or automatic reconfiguring of the electric network for
optimal load balancing. Specifically, to decide what extra network
sections each alternative source can power, the power being
consumed in each section must be known.
[0063] This disclosure also proposes a robust statistical method to
compute load power that smooths out unstable measurements, which
improves the existing methods of computing loads in many aspects.
In this disclosure, loads are considered at the section level. As
referred to above, each section is defined as the powerlines
bounded by the switching devices, or other devices capable of
measuring current magnitude. This proposal works the best for
devices that are also capable of measuring voltages, but this is
not required. In closed loop topologies where two or more power
sources are allowed to be combined to power the same loads, the
direction of the current flow with respect to voltage is assumed to
be measurable by the use of phasor voltage and current
measurements, remote synchro-phasor voltage measurements combined
with local synchro-phasor current measurements, or other means.
Using the method in this disclosure, if the measurements are in the
phasor domain, i.e., complex values, the resulting power will be
complex power. In radial topologies, if only current magnitude
measurements are available, the method can still be applied,
although the results will only be power magnitudes. The control is
assumed to have the network topology information because the load
of each section is computed from the devices around the
section.
[0064] This loading estimation scheme must be accompanied by a
communications scheme in which each device's latest voltage and
current measurements are periodically sent to the control, either
directly or indirectly depending on the data dissemination scheme.
The control then computes the updated load for each section.
Alternatively, where there are multiple controls distributed
throughout the network, the computation of the load can be
distributed such that a control at each section computes the load
for that section, and communicates the result to controls in other
sections. Measurements of different devices may arrive at the
control at different times. At the end of each data dissemination
period, the control should have received the latest measurements
from all of the devices. Such measurements can be the complex power
flowing through each device. If the phasors, i.e., complex voltages
and currents, are periodically sent to the control for other
purposes, then there is no need to send complex powers in addition
to the phasors because the complex powers can be computed from the
phasors.
[0065] The loads for the sections are computed from measured or
derived power measurements, i.e., using the measured or estimated
voltages and measured currents. The load of a section s for each
phase p, where p can be phase A, B or C, is the sum of the power
flowing into the section from all of the devices around the
section, i.e., net power flow, on the phase p. Assume that for a
device k bounding a section s for phase p, the measured current is
I.sub.k,p, the measured voltage, if available, is V.sub.k,p, and if
the device lacks a voltage sensor, the estimated voltage is
V.sub.e,k,p, where V.sub.k,p, V.sub.e,k,p and I.sub.k,p can be
phasor values or just magnitudes depending on the measuring
capability. Where applicable, ctp.sub.k,s stands for the current
transformer polarity, where ctp.sub.k,s=1 if current flowing
through the device k into section s is measured as positive
current, and ctp.sub.k,s=-1 otherwise. Table 20 below shows how the
power flowing through the device k into the section s on phase p,
denoted as P.sub.k,s,p, is computed.
TABLE-US-00020 TABLE 20 Device's measure capability Power
Calculation Measured voltage phasor V.sub.k, p and current P.sub.k,
s, p = ctp.sub.k, s .times. phasor I.sub.k, p V.sub.k, p .times.
I*.sub.k, p Estimated voltage synchrophasor V.sub.e, k, p P.sub.k,
s, p = ctp.sub.k, s .times. and current synchorophasor I.sub.k, p
V.sub.e, k, p .times. I*.sub.k, p Voltage magnitude V.sub.k, p and
current P.sub.k, s, p = ctp.sub.k, s .times. magnitude I.sub.k, p
with some measure of V.sub.k, p .times. I.sub.k, p .times.
directionality, for example, by measuring the dir(I.sub.k, p) time
difference between the voltage's zero crossing and the current's
zero crossing; specifically, dir(I.sub.k, p) = 1 if current angle
is close to voltage angle, or dir(I.sub.k, p) = -1, if current
angle is about 180.degree. off of voltage angle Voltage magnitude
V.sub.k, p and current P.sub.k, s, p = V.sub.k, p .times. I.sub.k,
p .times. magnitude I.sub.k, p, in a radial topology dir.sub.k, s
where dir.sub.k, s = -1 if segment s is between device k and the
source, as determined by topology, and dir.sub.k, s = 1 otherwise
Estimated voltage magnitude V.sub.e, k, p P.sub.k, s, p = V.sub.e,
k, p .times. I.sub.k, p .times. and current magnitude I.sub.k, p in
a radial dir.sub.k, s topology where dir.sub.k, s is defined as
above
[0066] If the device k is not equipped with a voltage sensor, the
voltage at the device k, as a phasor or just magnitude, must be
estimated from the measurement(s) of nearby device(s). For example,
the voltage of the nearest voltage-measuring device can be used, or
an average voltage can be computed from several nearby
voltage-measuring devices. In non-radial networks, if some
switching devices are not equipped with a voltage sensor, all
voltage and current measurements need to be phasors, and these
phasor measurements need to be synchronized (synchro-phasors). In
radial networks, nominal voltage can be used as well, where no
actual voltage sensing is required. If a step-down or step-up
transformer exists between the device k and the voltage-measuring
device, then the voltage needs to be adjusted based on the
transformer.
[0067] With the power of each switching device around the section
s, the load of this section on phase p is then
L.sub.s,p=.SIGMA.keKs P.sub.k,s,p, where K.sub.s is the set of all
of the devices around this section. If phasors are used in the
computation, then the load is a sum of complex values and is itself
a complex value.
[0068] If there is load in a section having a constant-voltage
source, i.e., substation, and the control has no access to the
measurements on the source side of the section, then the load
cannot be calculated with the above described method. For example,
in the section 38 of the network 10, when computing the available
additional power capacity the source 12 can provide, which may be
necessary to determine whether the source 12 can provide power to
additional sections for switching calculations during a fault event
so that the source 12 is not overloaded, a conservative estimate
can be used. More particularly, if the maximum power that the
source 12 can provide is P.sub.source, the maximum load allowed on
the section 38 is L.sub.max, and the load 24 on the section 38
computed using the method above from measurements from the
switching device 28 is L.sub.1, which should be negative since
power is flowing out of the section 38, then the available
additional capacity P.sub.addition that the source 12 can provide
is P.sub.addition=P.sub.source-L.sub.max+L.sub.1.
[0069] If there is a DG source, such as a solar, wind, turbine,
battery, etc., with constant power in a section, it will be treated
as a negative load, where it would not be known how much power the
source can provide or is providing. In this case, the load
calculation using the measurements from other devices around this
section will be the same as described above. For example, if the DG
source 36 in the section 40 is outputting more power than the
customers of the section 40 are consuming, i.e., the load 24, then
the load of the section 40 will be negative using the computation
described above. In other words, the load of the section 40 that is
computed using the computation described above is the sum of the
power output of the DG 36 and the power consumption of the load
24.
[0070] The control maintains a memory, for example, the memory 68,
for each device on each side to store the recent history of the
measurements. The memory 68 is used for the improvement of
robustness and the handling of unidentified transient behaviors due
to faults, switching, etc. The size of the memory 68 for each side
of each device is fixed. Whenever a new measurement is received, it
will be stored in the corresponding memory, and the oldest value of
that measurement in the memory 68 is erased.
[0071] To handle transient behaviors due to faults, switching, etc.
and smooth out unstable measurements, instead of computing the
section load from the latest measurements, the median filter 74
using N recent measurements is applied for the load computation.
Specifically, for the devices around a section, with the recent
history of the measurements in the memory 68, Load #1 is computed
from the latest measurements of each device, Load #2 is computed
from the second latest measurements of each device, . . . , and
Load #N is computed from the Nth latest measurements of each
device. Then, the median of Load #1, Load #2, . . . , Load #N is
taken, and this median value is used as the load for that
section.
[0072] The reason that a median filter is chosen over other
possible filters is that a median filter can handle up to (N-1)/2
outliers with N samples. In other words, if up to (N-1)/2 samples
are affected by arbitrarily large errors, the median estimate will
still be based on one of the correct remaining samples. In
contrast, by that definition, an average filter cannot properly
handle even a single outlier. Another feature of a median filter is
that for most of them, the filtered result matches one of the
sampled values.
[0073] There are different types of median filters suitable for
loading computations with two-dimensional sample points. This is
the case when the power is represented as a complex value, one
dimension being the real part (active power) and the other
dimension being the imaginary part (reactive power). One example is
described below of such a median filter for complex power. For N
complex numbers c.sub.1, c.sub.2, . . . , c.sub.N, the median of
the real part M.sub.Re=median.sub.n=1, . . . ,N{Real(c.sub.n)} and
the median of the imaginary part M.sub.im=median.sub.n=1, . . .
,N{Imaginary(c.sub.n)} are computed. These parts are then combined
as the reference point M.sub.Ref=M.sub.Re+iM.sub.im. Note that
M.sub.Ref may not belong to the set of the sample points. Finally,
a complex number is chosen in the samples that is the closest to
the reference point M.sub.Ref as the median of the samples, i. e.,
M=argmin.sub.cn=c1,c2, . . . ,cN{|c.sub.n-M.sub.Ref|} Another
possible alternative median filter to the above one is the
geometric median. In general, geometric medians are computationally
more intensive.
[0074] The following is an example illustrating the issues with
computing loads in sections using only the latest measurements, but
not using the measurement stored in the memory 68 and the median
filter 74. FIG. 5 is a simplified illustration of an electrical
power distribution network 140 including an AC source 142, a feeder
144 coupled thereto, and switching devices 146, 148 and 150 coupled
to the feeder 144, where a source section 152 is defined between
the source 142 and the device 146, a load section 154 is defined
between the devices 146 and 148, a load section 156 is defined
between the devices 148 and 150 and a load section 158 is defined
downstream of the device 150, and where the section 154 has a load
of 160, the section 156 has a load of 162 and the section 158 has a
load of 164, where the load 160 is 50 kW, the load 162 is 20 Kw and
the load 164 is 40 kW.
[0075] At a first step, the device 146 takes load measurements,
calculates the power flowing through the device 146, which is 110
kW in this example because of the total load downstream of the
device 146, and sends the power calculation to the device 148. At a
second step, the device 148 takes current and voltage measurements,
calculates the power flowing through the device 148, which is 60 kW
in this example because of the total load downstream of the device
146, and sends the power calculations for both of the devices 146
and 148 to the device 150. At a third step, the device 150 takes
current and voltage measurements and calculates the power flowing
through the device 150, which is 40 kW in this example. With the
data collected by the device 150, the load in each of the sections
154, 156 and 158 can be calculated correctly at 50 kW, 20 kW and 40
kW.
[0076] If the device 148 receives the power of 110 kW flowing
through the device 146, but before the device 148 can take a
voltage and current measurement, a fault may occur in the section
158, which increases the power consumption in the section 158 by 50
kW. If this happens, in the second step when the device 148 takes
the current and voltage measurements, it will include the extra
consumption and its calculation of power flowing through itself,
i.e., the device 148 will be 110 kW. In the third step when the
device 150 takes the load measurements and tries to calculate the
load in each of the sections 154, 156 and 158, the load for the
section 154 will be incorrectly calculated as 0 kW because the
device 150 thinks he power flowing through the device 146 is 110
kW, and the power flowing through the device 148 is also 110 kW.
This miscalculation is caused by the fault. Using the measurement
memory 68 and the median filter 74 as discussed in this disclosure,
the outlier caused by the measurement mismatch in this example will
be filtered out, and the resulting section loading will be the
loading during normal conditions. That is what is desired for
applications such as automatic fault restoration.
[0077] An alternative approach to solve the issue in this example
is to reject any measurement that is associated with a fault. This
requires additional rules and filtering criteria for identifying a
fault, and it would be difficult to capture all of the possible
fault scenarios. By contrast, the proposed approach using the
measurement memory 68 and the median filter 74 does not need to
specify such rules and criteria, and can reject other transient
behaviors that may not be associated with faults.
[0078] In certain cases, such as when any one of the devices 146,
148 and 150 reports a loss of voltage, i.e., the voltage is below a
predefined threshold, the measurements in the memory 68 should not
be used for loading computations because the loads computed from
such measurements may be incorrect. For this reason, this
disclosure proposes that during a data dissemination period, if any
device 146-150 reports loss of voltage, the latest measurements of
the devices that have been sent to the control are discarded in the
memory. When the control receives the measurements from the
remaining devices during the same data dissemination period, those
measurements are not accepted. After the retraction, since the
memory 68 has the size of N+1, the median filter 74 for load
computation still has enough measurements to compute the load using
the median of the recent N measurements without disturbing
events.
[0079] In the example discussed above, assume that after the second
step, the feeder 144 loses power from the source 142. If memory
retraction is not used, then after receiving (N+1)/2 measurements
since the loss of voltage, the loads on all of the sections 154,
156 and 158 will be computed as zero. These are not the loads
desired for the computation of automatic restoration. Using the
mechanism in this proposal, when the source 142 is lost, at least
one of the devices 146-150 will report a loss of voltage.
Therefore, the latest measurements of this new period of data
dissemination are discarded. Assuming the remaining recent
measurements in the measurement memory are the same as referred to
above, the load calculation will still be correct and reflect the
load per section prior to the loss of voltage, which is desired.
These desired loading values will stay the same even if the loss of
voltage lasts for an extended time. This is because for every new
dissemination period, the new measurements will not be recorded by
the memory 68 due to the loss of voltage.
[0080] During changes in topology when the current may be coming
from a different source, combining power flow measurements taken
before and after the topology change will lead to erroneous load
calculating results. In addition, topological changes, which
typically follow a disturbance, may lead to a change in loading,
for example, because a DG dropped offline. Therefore, when the
control makes a change, or is aware of changes in topology that may
directly change the section loads, it is faster to obtain the
changed loads if the loading computation only uses the measurements
after the change. When such control-aware changes occur, the
control will clear the memory 68 by discarding the existing
measurements and saving the updated measurement in the memory 68.
At this point, the memory 68 will contain only one recent
measurement. Note that before the memory 68 is filled with at least
N measurements, the median filter 74 used for computing the loading
can be applied to the fewer measurements in the memory 68. With the
continuation of receiving updated measurements, the memory 68 will
eventually be filled with N+1 recent measurements again.
[0081] If the memory 68 is not cleared after the control-aware
changes, using the median filter 74, the new values would only be
reflected after receiving more than (N+1)/2 updated measurements.
That would take the time of (N+1)/2 periods of data dissemination.
Moreover, if the change caused a significant increase in the
section loads, the slow update would put the network at risk of
overloading the power sources. One example is a DG, which is
modeled as negative load. When a DG goes offline due to reasons
such as fault protection of the main feeder, the control
immediately knows this. The control then should assume that the
section with the DG has a load of zero. If merely the measurement
giving zero load was added to the memory 68, but the older
measurements in the memory 68 were not cleared, then using the
median filter 74, a fault restoration logic would think this
section is still outputting power because of the negative load, and
try to restore extra loads that should not be restored.
[0082] If at the end of a data dissemination period the control has
not received the latest measurements from some devices, the loading
computation using the median filter 74 proceeds unchanged.
Immediately following the data loss, this may result in the load
being computed from measurements taken at different data
dissemination periods. However, assuming the load changes slowly
relative to the data dissemination interval, this should result in
negligible error in the load computation. Furthermore, after a few
data dissemination periods with no communication contingency, the
measurements in the memories 68 will realign automatically so that
again loading will only be computed from measurements taken at the
same data dissemination period.
[0083] If a certain device lost communications, the control in
other devices or the control center 50 will not receive any updated
measurements from that device. The loss of communications of that
device should be detected by the control's communications layer or
other methods such as a timer in the control. As soon as the
control detects the loss of communications of the device, it
computes and stores the ratio of the load magnitudes of the section
on two sides of the device using the last updated loads of these
sections. Later, when the control receives the updated measurements
from devices other than the non-communicating device, it computes
the combined load of the sections on two sides of the
non-communicating device, and then it distributes the combined
loads to the sections using the saved ratios from the last actual
loads.
[0084] For example, in the network 10, if the device 30 lost
communications, and the last computed loads of the section 40 and
the section 42 are 40 kW and 20 kW, respectively, then the control
stores the ratio of the loads between the section 40 and the
section 42, which is 2:1. Then, with the topology information known
to the control, it uses the measurements of the devices 28, 32 and
34 to compute the combined load of the section 40 and the section
42. If the combined load is 66 kW, then it records 44 kW for the
load of the section 40 and 22 kW for the load of the section
42.
[0085] The foregoing discussion discloses and describes merely
exemplary embodiments of the present disclosure. One skilled in the
art will readily recognize from such discussion and from the
accompanying drawings and claims that various changes,
modifications and variations can be made therein without departing
from the spirit and scope of the disclosure as defined in the
following claims.
* * * * *