U.S. patent application number 10/713310 was filed with the patent office on 2008-10-02 for method, memory media and apparatus for detection of grid disconnect.
This patent application is currently assigned to General Electric Company. Invention is credited to Pengwei Du, Zhihong Ye.
Application Number | 20080238215 10/713310 |
Document ID | / |
Family ID | 36602175 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080238215 |
Kind Code |
A1 |
Ye; Zhihong ; et
al. |
October 2, 2008 |
METHOD, MEMORY MEDIA AND APPARATUS FOR DETECTION OF GRID
DISCONNECT
Abstract
A phase shift procedure for detecting a disconnect of a power
grid from a feeder that is connected to a load and a distributed
generator. The phase shift procedure compares a current phase shift
of the output voltage of the distributed generator with a
predetermined threshold and if greater, a command is issued for a
disconnect of the distributed generator from the feeder. To extend
the range of detection, the phase shift procedure is used when a
power mismatch between the distributed generator and the load
exceeds a threshold and either or both of an under/over frequency
procedure and an under/over voltage procedure is used when any
power mismatch does not exceed the threshold.
Inventors: |
Ye; Zhihong; (Schenectady,
NY) ; Du; Pengwei; (Troy, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
General Electric Company
|
Family ID: |
36602175 |
Appl. No.: |
10/713310 |
Filed: |
November 14, 2003 |
Current U.S.
Class: |
307/127 |
Current CPC
Class: |
G01R 31/68 20200101;
H02J 3/388 20200101; H02J 3/38 20130101 |
Class at
Publication: |
307/127 |
International
Class: |
H02B 1/24 20060101
H02B001/24 |
Goverment Interests
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The U.S. Government may have certain rights in this
invention pursuant to contract number NAD-1-30605-01 awarded by the
U.S. Department of Energy.
Claims
1-3. (canceled)
4. A method for preventing islanding in a power system that
includes a power grid having a feeder connected in circuit with a
distributed generator and at least one load, said method
comprising: determining a phase shift of a voltage based solely on
sequential frequency measurements at an output of said distributed
generator; comparing said phase shift to a threshold phase shift
that signifies a phase shift due to a disconnect of said grid from
said feeder; and if said phase shift is greater than said threshold
phase shift, issuing a command for a disconnect of said distributed
generator from said feeder, wherein said phase shift is determined
by: .theta. n = 2 .pi. ( 1 - f n - 1 f n ) , ##EQU00004## where
.theta..sub.n is said phase shift and f.sub.n and f.sub.n-1 are the
frequencies at a current zero-crossing and a previous zero-crossing
of said voltage, respectively.
5. A method for preventing islanding in a power system that
includes a power grid having a feeder connected in circuit with a
distributed generator and at least one load, said method
comprising: determining a phase shift of a voltage based solely on
sequential frequency measurements at an output of said distributed
generator; comparing said phase shift to a threshold phase shift
that signifies a phase shift due to a disconnect of said grid from
said feeder; and if said phase shift is greater than said threshold
phase shift, issuing a command for a disconnect of said distributed
generator from said feeder, wherein said phase shift is determined
by: .theta. total = .theta. n - 2 + .theta. n - 1 + .theta. n = 2
.pi. ( 3 - f n - 3 f n - 2 - f n - 2 f n - 1 - f n - 1 f n ) ,
##EQU00005## where .theta..sub.total is said phase shift,
.theta..sub.n, .theta..sub.n-1, and .theta..sub.n-2, are the
current, the first previous and the second previous phase shifts,
respectively, and f.sub.n, f.sub.n-1, f.sub.n-2, f.sub.n-3, are the
frequencies at a current, a first previous, a second previous and a
third previous zero-crossing of said voltage, respectively.
6-8. (canceled)
9. A controller for preventing islanding in a power system that
includes a power grid having a feeder connected in circuit with a
distributed generator and at least one load, said controller
comprising: a processor, a memory and an input/output unit, wherein
said memory includes a grid disconnect program that causes said
processor to perform the operations of; determining a phase shift
of a voltage based solely on sequential frequency measurements at
an output of said distributed generator; comparing said phase shift
to a threshold phase shift that signifies a phase shift due to a
disconnect of said grid from said feeder; and if said phase shift
is greater than said threshold phase shift, issuing a command for a
disconnect of said distributed generator from said feeder, wherein
said phase shift is determined by: .theta. n = 2 .pi. ( 1 - f n - 1
f n ) , ##EQU00006## where .theta..sub.n is said phase shift and
f.sub.n and f.sub.n-1 are the frequencies at a current
zero-crossing and a previous zero-crossing of said voltage,
respectively.
10. A controller for preventing islanding in a power system that
includes a powder grid having a feeder connected in circuit with a
distributed generator and at least one load, said controller
comprising: a processor, a memory and an input/output unit, wherein
said memory includes a grid disconnect program that causes said
processor to perform the operations of; determining a phase shift
of a volt based solely on sequential frequency measurements at an
output of said distributed generator; comparing said phase shift to
a threshold phase shift that signifies a phase shift due to a
disconnect of said grid from said feeder; and if said phase shift
is greater than said threshold phase shift, issuing a command for a
disconnect of said distributed generator from said feeder, wherein
said phase shift is determined by: .theta. total = .theta. n - 2 +
.theta. n - 1 + .theta. n = 2 .pi. ( 3 - f n - 3 f n - 2 - f n - 2
f n - 1 - f n - 1 f n ) , ##EQU00007## where .theta..sub.total is
said phase shift, .theta..sub.n, .theta..sub.n-1, and
.theta..sub.n-2, are the current, the first previous and the second
previous phase shifts, respectively, and f.sub.n, f.sub.n-1,
f.sub.n-2, f.sub.n-3, are the frequencies at a current, a first
previous, a second previous and a third previous zero-crossing of
said voltage, respectively.
11-13. (canceled)
14. A memory media for a controller for preventing islanding in a
power system that includes a power grid having a feeder connected
in circuit with a distributed generator and at least one load, said
controller comprising a processor, a memory and an input/output
unit said memory media comprising, a grid disconnect detection
program that causes said processor to perform the operations of:
determining a phase shift of a voltage based solely on sequential
frequency measurements at an output of said distributed generator;
comparing said phase shift to a threshold phase shift that
signifies a phase shift due to a disconnect of said grid from said
feeder; and if said phase shift is greater than said threshold
phase shift, issuing a command for a disconnect of said distributed
generator from said feeder, wherein said phase shift is determined
by: .theta. n = 2 .pi. ( 1 - f n - 1 f n ) , ##EQU00008## where
.theta..sub.n is said phase shift and f.sub.n and f.sub.n-1 are the
frequencies at a current zero-crossing and a previous zero-crossing
of said voltage, respectively.
15. A memory media for a controller for preventing islanding in a
power system that includes a power grid having a feeder connected
in circuit with a distributed generator and at least one load, said
controller comprising a processor, a memory and an input/output
unit, said memory media comprising a grid disconnect detection
program that causes said processor to perform the operations of:
determining a phase shift of a voltage based solely on sequential
frequency measurements at an output of said distributed generator;
comparing said phase shift to a threshold phase shift that
signifies a phase shift due to a disconnect of said grid from said
feeder; and if said phase shift is greater than said threshold
phase shift, issuing a command for a disconnect of said distributed
generator from said feeder, wherein said phase shift is determined
by: .theta. total = .theta. n - 2 + .theta. n - 1 + .theta. n = 2
.pi. ( 3 - f n - 3 f n - 2 - f n - 2 f n - 1 - f n - 1 f n ) ,
##EQU00009## where .theta..sub.total is said phase shift,
.theta..sub.n, .theta..sub.n-1, and .theta..sub.n-2 are the
current, the first previous and the smcond previous phase shifts,
respectively, and f.sub.n, f.sub.n-1, f.sub.n-2, f.sub.n-3, are the
frequencies at a current, a first previous, a second previous and a
third previous zero-crossing of said voltage, respectively.
16. (canceled)
Description
BACKGROUND OF INVENTION
[0002] The present disclosure relates to a method and apparatus for
detection of a disconnection of the power grid. More particularly,
the present disclosure relates to islanding and the prevention
thereof.
[0003] Islanding of a grid-connected distributed generator (DG)
occurs when a section of the grid containing the DG is disconnected
from the main utility, but the DG continues to energize the grid
lines in the isolated section (termed as an island). For example,
consider a DG system connected to a feeder through a transformer. A
load (not owned by the DG owner) is also connected to the same
feeder through another transformer. If a grid disconnect device
(circuit breaker, recloser, fuse, or sectionalizer) opens, it is
possible for the DG to continue to supply current to the isolated
section of the grid. This is islanding, and the isolated section of
the grid being powered by the DG system is referred to as an
island.
[0004] Accordingly, there is a continuing desire for a method and
controller that prevent islanding.
BRIEF DESCRIPTION OF THE INVENTION
[0005] In a method embodiment of the invention islanding is
prevented in a power system that includes a power grid having a
feeder connected in circuit with a distributed generator and at
least one load. The method determines a phase shift of a voltage at
an output of the distributed generator. The phase shift is compared
to a threshold phase shift that signifies a phase shift due to a
disconnect of the grid from the feeder. If the phase shift is
greater than the threshold phase shift, a command is issued for a
disconnect of the distributed generator from the feeder.
[0006] In another method embodiment of the invention islanding is
prevented in a power system that includes a power grid having a
feeder connected in circuit with a distributed generator and at
least one load. The method determines a disconnect of the grid from
the feeder by a phase shift procedure when a power mismatch between
the distributed generator and the load exceeds a threshold. The
method further determines the disconnect by either or both of an
under/over frequency procedure and an under/over voltage procedure
when any power mismatch does not exceed the threshold.
[0007] In a controller embodiment of the invention islanding is
prevented in a power system that includes a power grid having a
feeder connected in circuit with a distributed generator and at
least one load. The controller comprises a processor, a memory and
an input/output unit. The memory includes a grid disconnect program
that causes the processor to determine a phase shift of a voltage
at an output of the distributed generator. The program also causes
the processor to compare the phase shift to a threshold phase shift
that signifies a phase shift due to a disconnect of the grid from
the feeder. If the phase shift is greater than the threshold phase
shift, the processor issues a command for a disconnect of the
distributed generator from the feeder.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is block diagram of an embodiment of a power grid
disconnect detection apparatus.
[0009] FIG. 2 is a waveform diagram of the output voltage of the
data generator of the FIG. 1 apparatus.
[0010] FIG. 3 is a block diagram of the controller of the FIG. 1
apparatus.
[0011] FIGS. 4 and 5 are flow diagrams of the grid disconnect
detection program of the controller of FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
[0012] Referring to FIG. 1, an electrical power grid 20 is
connected to a feeder 22 that is connected in a circuit with a load
24 and a DG 26. Grid 20 is connected with a public utility (not
shown). A disconnect device 28 is disposed in feeder 22 for the
purpose of disconnecting grid 20 from feeder 22 should abnormal
disturbances occur in grid 20. Another disconnect device 30 is
disposed in feeder 22 for the purpose of disconnecting DG 26 from
feeder 26. It will be apparent to those skilled in the art that
more than one load can be connected to feeder 22 and that DG 26 and
load 24 may be connected to feeder 22 via transformers (not
shown).
[0013] Disconnect devices 28 and 30 may be any suitable disconnect
devices, such as circuit breakers, reclosers, fuses, sectionalizers
and the like. DG 28 may be any distributed generator or a group of
distributed generators, such as a photovoltaic system, a fuel cell,
a microturbine, a small engine and the like.
[0014] If disconnect device 28 opens, it is possible for DG 26 to
continue to supply current to feeder 22, which would be isolated
from grid 20 due to the open disconnect device 28. This is
islanding and the isolated section of the grid (feeder 22 and
circuits connected thereto) is referred to as an island.
[0015] A voltage measuring device 36 monitors the voltage at the
output of DG 26. Voltage measuring device 36 provides samples of
the monitored voltage to a controller 40. Controller 40 processes
the voltage samples to detect a phase jump that occurs when grid 20
is disconnected by action of disconnect device 28.
[0016] Under normal conditions, the output voltage of DG 26 is
regulated by grid 20. When grid 20 is disconnected by the opening
of disconnect device 28, DG 26 becomes islanded. If there is a
power mismatch between DG 26 and load 24, the voltage vector will
shift its phase in order to balance the reactive power due to Ohm's
law. This phase shift normally occurs right after grid 20 is
disconnected.
[0017] In a first embodiment of the invention, detection of the
phase jump tries to capture the phase change of the voltage vector
in the first one or few cycles based on the voltage measurement and
then sends a trip signal to disconnect device 30, which opens. This
action disconnects DG 26 from feeder 22, thereby preventing
islanding.
[0018] The relationship between the power mismatch and the phase
jump .theta. can be derived as follow:
arc tan ( .DELTA. Q / P 1 + .DELTA. P / P ) .ltoreq. .theta.
threshold ( 1 ) ##EQU00001##
where .DELTA.P and .DELTA.Q are power mismatch and
.theta..sub.threshold is the phase jump setting threshold.
[0019] If the phase shift .theta. is greater than the threshold
phase shift .theta..sub.threshold, the phase jump (i.e., grid 20
disconnect) is detected. If the phase shift .theta. is equal to or
less than the threshold phase shift .theta..sub.threshold, any
phase shift is assumed to be due to normal disturbances.
[0020] The phase shift .theta..sub.threshold is selected based on
the maximum possible disturbances of the system in which the device
is used. The threshold should be larger (with certain margin) than
the phase shift caused by the disturbances so as to avoid a falxe
trip during normal grid connected operation.
[0021] Equation (1) reveals that if the power mismatch .DELTA.P and
.DELTA.Q at the time of grid disconnect satisfy the condition
(.theta..ltoreq..theta..sub.threshold), then the islanding cannot
be detected because the actual phase jump is smaller the
threshold.
[0022] It can also be seen from Equation (1) that the effectiveness
of the phase jump method is not influenced by the quality factor Qf
of load 24. Under/over frequency protection becomes less effective
with higher quality factor load.
[0023] In a second embodiment of the invention, the phase shift
detection method is combined with under/over frequency and
under/over voltage protection. This combination reduces the
non-detection zone (defined in .DELTA.P and .DELTA.Q space,
.DELTA.P and .DELTA.Q being small enough within the zone for the
detection processes to respond) of under/over frequency protection.
Also, it can be seen that the phase jump method is not very
effective with high active power mismatch, but under/over voltage
protection is sensitive to high active power mismatch. Therefore,
combing the phase jump, under/over frequency, and under/over
voltage will result in a reduced anti-islanding non-detection zone.
In this embodiment, the phase shift procedure is used to determine
the grid disconnect if a power mismatch between DG 26 and load 24
exceeds a threshold and either or both of the under/over frequency
procedure and the under/over voltage procedure is used if any power
mismatch does not exceed the threshold. This embodiment may employ
any conventional under/over procedure for frequency and under/over
voltage procedure.
[0024] Voltage measuring device 36 continuously monitors the
voltage at the output of DG 26 and sends samples thereof to
controller 40. Based on the measured voltage, frequency can be
computed, either based on zero-crossing, phase lock loop, DQ
(direct quadric) phase lock loop, or other frequency tracking
procedures.
[0025] Referring to FIG. 2, an example of the zero-crossing
technique is depicted by a voltage characteristic waveform 42 after
grid 20 is disconnected. Before grid 20 disconnected, the frequency
can be measured based on the time period, defined as T.sub.n-1, of
the two consecutive waveform zero crossings. After grid 20 is
disconnected, there will be a phase shift due to DG 26 and load
power mismatch. Then the next time period, defined as T.sub.n, of
the two consecutive zero-crossings will be different from
T.sub.n-1. Here T.sub.n and T.sub.n-1 are the inverse of the two
consecutive frequency measurement: f.sub.n=(1/T.sub.n) and
fn-1=(1/T.sub.n-1). Based on the previously measured frequency
f.sub.n-1, the next zero crossing time can be calculated as if
there were no grid disconnection. The angle between the calculated
zero-crossing and the actually measured zero crossing is the phase
shift, .theta..sub.n, due to the grid disconnection and is given
by:
.theta. n = 2 .pi. ( 1 - f n - 1 f n ) ( 2 ) ##EQU00002##
[0026] To improve the performance, an alternative embodiment of the
invention, instead of using the phase change of one period with two
consecutive measurements, uses three periods. In this case,
although more storage is needed, the security and reliability are
significantly improved. The reason is, due to grid transients, some
phase change may occur momentarily even without grid disconnection.
Controller 40 may pick up those transients as an islanding event
and cause a false trip. Those transients are normally causing the
frequency up and down momentarily, rather than drifting away in one
direction. Therefore, by doing three periods, the short-term
frequency oscillation will not cause too much phase change. The
implementation of the three-period measurements can be expressed
below:
.theta. total = .theta. n - 2 + .theta. n - 1 + .theta. n = 2 .pi.
( 3 - f n - 3 f n - 2 - f n - 2 f n - 1 - f n - 1 f n ) ( 3 )
##EQU00003##
[0027] Referring to FIG. 3, controller 40 includes a processor 50,
an input/output (I/O) unit 52, a memory 54 and a bus 56. Bus 56
interconnects processor 50, I/O unit 52 and memory 54. I/O unit 52
includes any desired I/O device, such as a keyboard, display,
printer, communications device for communicating over a network,
ports for receiving signals (e.g., the voltage samples from voltage
measuring device 36) or for sending signals (e.g., a trip signal to
disconnect device 30) and the like. Memory 52 includes an operating
system 58 and a controller program 60. Controller program 60
includes a grid disconnect detection program 62. A memory media 64
(e.g., a disk) contains a copy of grid disconnect program 62 and
may also include a copy of operating system 58, controller program
60, or other software, which can be loaded into memory 54.
[0028] Operating system 58 controls processor 50 to execute
controller program 60 and grid disconnect detection program 62 for
detecting a disconnect of grid 20 from feeder 22. Grid disconnect
program 62 processes the voltage samples to determine a phase shift
.theta..sub.n between a current zero crossing and one or more
previous zero crossings. The phase shift .theta..sub.n is compared
to the phase shift threshold .theta..sub.threshold and if greater,
the phase shift .theta..sub.n indicates a grid disconnect. A
command is then issued that causes a trip signal to be sent to
disconnect device 30 to disconnect DG 26 from feeder 22.
[0029] Referring to FIG. 4, a first embodiment of grid disconnect
detection program 62 measures the output voltage of DG 26 at step
70. Step 70 receives the voltage samples and determines the
zero-crossing times. Step 72 determines the current frequency
f.sub.n, based on the current zero-crossing. Step 74 determines the
phase change .theta..sub.n based on the current frequency f.sub.n
and the previous frequency f.sub.n-1. For example, step 74 uses
Equation 2. Step 76 compares the current phase shift .theta..sub.n
to the threshold phase shift .theta..sub.threshold. If the current
phase shift .theta..sub.n is greater, step 78 issues a command to
generate a trip signal that trips disconnect device 30, thereby
disconnecting DG 26 from feeder 22. If the current phase shift
.theta..sub.n is equal to or less than the threshold phase shift
.theta..sub.threshold, step 80 stores the current frequency f, for
use during the next iteration. Steps 70 through 76 and 80 are
repeated continuously until step 76 determines that the current
phase shift .theta..sub.n is greater than the threshold phase shift
.theta..sub.threshold. Step 78 then issues the command for the trip
signal and resets program 52.
[0030] Referring to FIG. 5, another embodiment of grid disconnect
program 62 is the same as the embodiment shown in FIG. 4 except
that the current phase change .theta..sub.n is determined based on
the current frequency f.sub.n and the three previous frequencies
f.sub.n-1, f.sub.n-2, and f.sub.n-3. Steps in FIG. 5 that are
identical to those shown in FIG. 4 bear the same reference
numerals. Step 74 of FIG. 4 is replaced in FIG. 5 by step 84 which
makes the phase shift determination based on current frequency
f.sub.n and the three previous frequencies f.sub.n-1, f.sub.n-2,
and f.sub.n-3. For example, step 84 may use Equation (3).
[0031] Regardless of where the instructions for the program steps
are stored, when they are executed by the processor, they provide a
technical effect of facilitating the detection of grid disconnect
from circuits that include a DG.
[0032] It should also be noted that the terms "first", "second",
"third", "upper", "lower", and the like may be used herein to
modify various elements. These modifiers do not imply a spatial,
sequential, or hierarchical order to the modified elements unless
specifically stated.
[0033] While the present invention has been described with
reference to one or more exemplary embodiments, it will be
understood by those skilled in the art that various changes may be
made and equivalents may be substituted for elements thereof
without departing from the scope of the present invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the disclosure without
departing from the scope thereof. Therefore, it is intended that
the present invention not be limited to the particular
embodiment(s) disclosed as the best mode contemplated for carrying
out this invention, but that the invention will include all
embodiments falling within the scope of the appended claims.
* * * * *