U.S. patent application number 15/786996 was filed with the patent office on 2018-05-10 for power system and method of starting multiple power converters in grid forming mode.
The applicant listed for this patent is DYNAPOWER COMPANY LLC. Invention is credited to Apurva SOMANI.
Application Number | 20180131268 15/786996 |
Document ID | / |
Family ID | 60263025 |
Filed Date | 2018-05-10 |
United States Patent
Application |
20180131268 |
Kind Code |
A1 |
SOMANI; Apurva |
May 10, 2018 |
POWER SYSTEM AND METHOD OF STARTING MULTIPLE POWER CONVERTERS IN
GRID FORMING MODE
Abstract
A power system and method for performing a blackstart on a
microgrid. The power system includes at least a first power
converter and a second power converter. The first power converter
comprises a first controller having a plurality of startup
sequences for performing the blackstart. The second power converter
is electrically coupled to the first power converter at a point of
common coupling. During the blackstart, the first controller is
configured to select and perform one of the plurality of startup
sequences according to a point at which the second power converter
is within the second power converter's startup sequence during the
blackstart. The first controller selects the one of the plurality
of startup sequences according to a microgrid voltage at the point
of common coupling.
Inventors: |
SOMANI; Apurva; (South
Burlington, VT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DYNAPOWER COMPANY LLC |
South Burlington |
VT |
US |
|
|
Family ID: |
60263025 |
Appl. No.: |
15/786996 |
Filed: |
October 18, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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62410129 |
Oct 19, 2016 |
|
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|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G05B 15/02 20130101;
Y02E 40/70 20130101; H02J 13/0006 20130101; H02J 3/388 20200101;
H02J 3/38 20130101; H02M 1/36 20130101; H02J 3/381 20130101; Y04S
10/12 20130101; Y02P 80/14 20151101; H02J 2300/10 20200101 |
International
Class: |
H02M 1/36 20060101
H02M001/36; H02J 3/38 20060101 H02J003/38 |
Claims
1. A power system for performing a blackstart on a microgrid, the
power system comprising: a first power converter comprising a first
controller having a plurality of startup sequences for performing
the blackstart; a second power converter electrically coupled to
the first power converter at a point of common coupling, wherein,
during the blackstart, the first controller is configured to select
and perform one of the plurality of startup sequences according to
a point at which the second power converter is within the second
power converter's startup sequence during the blackstart, and the
first controller selects the one of the plurality of startup
sequences according to a microgrid voltage at the point of common
coupling.
2. The power system of claim 1, wherein the second power converter
comprises a second controller having the plurality of startup
sequences, the second controller being configured to control the
second power converter to perform one of the plurality of startup
sequences such that the first controller can synchronize with the
second controller during the blackstart according to the microgrid
voltage.
3. The power system of claim 1, wherein the first controller is
configured to select a first startup sequence when the microgrid
voltage is less than a first predetermined voltage threshold, and
in performing the first startup sequence the first controller is
configured to: close a first switch that is coupled between the
first power converter and the point of common coupling; start
gating of the first power converter; control a frequency of an
output voltage of the first power converter to be a first
predetermined frequency; ramp the output voltage level of the first
power converter from substantially zero to a first predetermined
voltage level over a first predetermined time period; hold the
output voltage level at the first predetermined voltage level and
hold the frequency of the output voltage at the first predetermined
frequency for a predetermined dwell period; ramp the output voltage
level from the first predetermined voltage level to a nominal
voltage level, and ramp the output voltage frequency from the first
predetermined frequency to a nominal output voltage frequency over
a second predetermined time period.
4. The power system of claim 1, wherein the first controller is
configured to select a second startup sequence when the microgrid
voltage is greater than a first predetermined voltage threshold and
less than a second predetermined voltage threshold, and in
performing the second sequence the first controller is configured
to: start phase lock loop synchronization to the microgrid voltage
and a microgrid frequency; implement a first wait time for waiting
unit the microgrid voltage has reached the first predetermined
voltage threshold; start gating of the first power converter to
output the first predetermined voltage threshold; ramp the output
voltage of the first power converter from the first predetermined
voltage threshold to a first predetermined voltage level over a
remaining time period, the remaining time period being a portion of
a first predetermined time period during which the second power
controller finishes ramping its output voltage from substantially
zero to the first predetermined voltage level; implement a second
wait time for waiting a portion of a predetermined dwell period,
the predetermined dwell period being a period during which the
second power converter is holding its output voltage and frequency;
close a first switch that is coupled between the first power
converter and the point of common coupling; implement a third wait
time for waiting a remaining portion of the predetermined dwell
period; ramp the output voltage level from the first predetermined
voltage level to a nominal voltage level, and ramp the output
voltage frequency to a nominal output voltage frequency over a
second predetermined time period.
5. The power system of claim 1, wherein the first controller is
configured to select a third startup sequence when the microgrid
voltage is greater than a third predetermined voltage threshold,
and in performing the third sequence the first controller is
configured to: implement a wait time for waiting unit the microgrid
voltage reaches a first predetermined portion of a nominal
microgrid voltage; start phase locked loop synchronization to the
existing microgrid voltage; determine whether the microgrid voltage
and frequency are within predetermined limits of the nominal
microgrid voltage and a nominal microgrid frequency; start gating
of the first power converter and set output voltage of the first
power converter to zero and frequency to nominal microgrid
frequency; ramp the output voltage of the first power converter
from zero to the microgrid voltage; close a first switch that is
coupled between the first power converter and the point of common
coupling.
6. The power system of claim 1, wherein the first controller is
configured to select a third startup sequence when the microgrid
voltage is greater than a third predetermined voltage threshold,
and in performing the third sequence the first controller is
configured to: catch an initial rising voltage and frequency of the
second power converter; synchronize the first power converter with
the existing rising voltage and frequency of the second power
converter; close a first switch that is coupled between the first
power converter and the point of common coupling; ramp the output
voltage and frequency of the first power converter from the initial
voltage and frequency over a remaining period of a final ramp of
the second power converter.
7. A power system for performing a blackstart on a microgrid, the
power system comprising: a first power converter electrically
coupled to the microgrid and comprising a first controller
configured to perform a plurality of startup sequences; a second
power converter electrically coupled to the microgrid, wherein,
during a blackstart, the first controller is configured to select
and perform one of the plurality of startup sequences according to
a microgrid voltage, the plurality of startup sequences including a
first start up sequence, a second start up sequence and a third
startup sequence, wherein, the first controller is configured to:
select the first startup sequence when the microgrid voltage is
less that a first predetermined threshold voltage; select the
second startup sequence when the microgrid voltage is greater than
the first predetermined threshold voltage but less than a second
predetermined threshold voltage; and select the third startup
sequence when the microgrid voltage is greater than the second
predetermined voltage.
8. The power system according to claim 7, wherein the second power
converter comprises a second controller that is also configured to
perform one of the plurality of startup sequences including the
first startup sequence, the second startup sequence and the third
startup sequence during the blackstart such that the first
controller can synchronize with the second controller according to
the microgrid voltage.
9. The power system according to claim 8, wherein: when the first
controller controls the first power converter to perform the first
startup sequence, the second controller has not begun one of the
first startup sequence, the second startup sequence and the third
startup sequence; when the first controller controls the first
power converter to perform the second startup sequence, the second
controller has begun the first startup sequence but has not
surpassed a predetermined point of the first startup sequence; and
when the first controller controls the first power converter to
perform the third startup sequence, the second controller has begun
the first startup sequence and has surpassed the predetermined
point.
10. The power system of claim 8, wherein, in performing the first
sequence the first controller is configured to: close a first
switch for coupling the first power converter to the microgrid;
start gating of the first power converter; control a frequency of
an output voltage of the first power converter to be a first
predetermined frequency; ramp the output voltage level of the first
power converter from substantially zero to a first predetermined
voltage level over a first predetermined time period; hold the
output voltage level at the first predetermined voltage level and
hold the frequency of the output voltage at the first predetermined
frequency for a predetermined dwell period; ramp the output voltage
level from the first predetermined voltage level to a nominal
voltage level, and ramp the output voltage frequency from the first
predetermined frequency to a nominal output voltage frequency over
a second predetermined time period.
11. The power system of claim 8, wherein, in performing the second
sequence the first controller is configured to: start phase lock
loop synchronization to the microgrid voltage and a microgrid
frequency; implement a first wait time for waiting unit the
microgrid voltage has reached a predetermined portion of a first
predetermined voltage level; start gating of the first power
converter to output the predetermined portion; ramp the output
voltage of the first power converter from the predetermined portion
to a second predetermined voltage level for over a remaining time
period, the remaining time period being a portion of a first
predetermined time period during which the second power controller
finishes ramping its output voltage from substantially zero to the
second predetermined voltage level; implement a second wait time
for waiting a portion of a predetermined dwell period, the
predetermined dwell period being a period during which the second
power converter is holding its output voltage and frequency; close
a first switch that couples the first power converter to the
microgrid; implement a third wait time for waiting a remaining
portion of the predetermined dwell period; ramp the output voltage
level from the second predetermined voltage level to a nominal
voltage level, and ramp the output voltage frequency to a nominal
output voltage frequency over a second predetermined time
period.
12. The power system of claim 8, wherein, in performing the third
sequence the first controller is configured to: implement a wait
time for waiting unit the microgrid voltage reaches a first
predetermined portion of a nominal microgrid voltage; start phase
locked loop synchronization to the existing microgrid voltage;
determine whether the microgrid voltage and frequency are within
predetermined limits of the nominal microgrid voltage and a nominal
microgrid frequency; start gating of the first power converter and
set output voltage of the first power converter to zero and
frequency to nominal microgrid frequency; ramp the output voltage
of the first power converter from zero to the microgrid voltage;
close a first switch that couples the first power converter to the
microgrid.
13. The power system of claim 8, wherein. in performing the third
sequence the first controller is configured to: catch an initial
rising voltage and frequency of the second power converter;
synchronize the first power converter with the existing rising
voltage and frequency of the second power converter; close a first
switch that couples the first power converter to the microgrid;
ramp the output voltage and frequency of the first power converter
from the initial voltage and frequency over a remaining period of a
final ramp of the second power converter.
14. A method of performing a blackstart of a power converter
coupled to a microgrid having at least one other power converter,
the method comprising: sensing a microgrid voltage; selecting one
of a plurality of startup sequences according to the microgrid
voltage, the startup sequences including at least a first startup
sequence, a second startup sequence and a third startup sequence;
wherein selecting one of a plurality of startup sequences
comprises: selecting the first startup sequence when the microgrid
voltage is less that a first predetermined threshold voltage;
selecting the second startup sequence when the microgrid voltage is
greater than the first predetermined threshold voltage but less
than a second predetermined threshold voltage; and selecting the
third startup sequence when the microgrid voltage is greater than
the second predetermined threshold voltage; and controlling the
power converter to perform the selected startup sequence.
15. The method according to claim 14, wherein the microgrid voltage
being less that the first predetermined voltage indicates that the
other power converter has not begun its first sequence.
16. The method of claim 14 wherein when the first startup sequence
is selected, controlling the power converter to perform the first
startup sequence comprises: closing a first switch for coupling the
power converter to the microgrid; starting gating of the power
converter; controlling a frequency of an output voltage of the
power converter to be a first predetermined frequency; ramping the
output voltage level of the power converter from substantially zero
to a first predetermined voltage level over a first predetermined
time period; holding the output voltage level at the first
predetermined voltage level and holding the frequency of the output
voltage at the first predetermined frequency for a predetermined
dwell period; ramping the output voltage level from the first
predetermined voltage level to a nominal voltage level, and ramping
the output voltage frequency from the first predetermined frequency
to a nominal output voltage frequency over a second predetermined
time period.
17. The method of claim 14, wherein when the second startup
sequence is selected, controlling the power converter to perform
the second startup sequence comprises: starting phase lock loop
synchronization to the microgrid voltage and a microgrid frequency;
implementing a first wait time for waiting unit the microgrid
voltage has reached a predetermined portion of a first
predetermined voltage level; starting gating of the power converter
to output the predetermined portion; ramping the output voltage of
the power converter from the predetermined portion to a second
predetermined voltage level for over a remaining time period, the
remaining time period being a portion of a first predetermined time
period during which the other power controller finishes ramping its
output voltage from substantially zero to the second predetermined
voltage level; implementing a second wait time for waiting a
portion of a predetermined dwell period, the predetermined dwell
period being a period during which the second power converter is
holding its output voltage and frequency; closing a first switch
that couples the first power converter to the microgrid;
implementing a third wait time for waiting the remaining portion of
the predetermined dwell period; ramping the output voltage level
from the second predetermined voltage level to a nominal voltage
level, and ramping the output voltage frequency to a nominal output
voltage frequency over a second predetermined time period.
18. The method of claim 14, wherein when the third startup sequence
is selected, controlling the power converter to perform the third
startup sequence comprises: implementing a wait time for waiting
unit the microgrid voltage reaches a first predetermined portion of
a nominal microgrid voltage; starting phase locked loop
synchronization to the existing microgrid voltage; determining
whether the microgrid voltage and frequency are within
predetermined limits of the nominal microgrid voltage and a nominal
microgrid frequency; starting gating of the power converter and
setting output voltage of the power converter to zero and frequency
to nominal microgrid frequency; ramping the output voltage of the
first converter from zero to the microgrid voltage; closing a first
switch that couples the power converter to the microgrid.
19. The method of claim 14, wherein when the third startup sequence
is selected, controlling the power converter to perform the third
startup sequence comprises: catching an initial rising voltage and
frequency of the other power converter; synchronizing the power
converter with the existing rising voltage and frequency of the
other power converter; closing a first switch that is coupled
between the power converter and the microgrid; ramping the output
voltage and frequency of the power converter from the initial
voltage and frequency over a remaining period of a final ramp of
the other power converter.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to a power system
comprising a plurality of power converters; and more specifically,
to systems and methods for starting multiple power converters in
grid forming mode.
BACKGROUND OF THE INVENTION
[0002] A power system may include distributed power sources (e.g.,
distributed generators, battery banks, and/or renewable resources
such as solar panels or wind turbines to provide power supply to a
grid (e.g., a microgrid having local loads and/or a utility grid).
The power system may include a power converter, such as a power
inverter, for converting power between a power source and a grid.
Such power conversion may include AC/DC, DC/DC, AC/AC and
DC/AC.
[0003] A microgrid system can include a variety of interconnected
distributed energy resources (e.g., power generators and energy
storage units) and loads. The microgrid system may be coupled to
the main utility grid through switches such as circuit breakers,
semiconductor switches (such as thyristors and IGBTs) and/or
contactors. In the event that the microgrid system is connected to
the main utility grid, the main utility grid may supply power to
the local loads of the microgrid system. The main utility grid
itself may power the local loads, or the main utility grid may be
used in combination with the power sources of the microgrid to
power the local loads.
[0004] A controller comprising hardware and software systems may be
employed to control and manage the microgrid system. Furthermore,
the controller may be able to control the on and off state of the
switches so that the microgrid system can be connected to or
disconnected from the main grid accordingly. The grid connected
operation of the microgrid system is commonly referred to as "grid
tied" mode, whereas the grid disconnected operation is commonly
referred to as "islanded" or "stand alone" mode.
BRIEF SUMMARY OF THE INVENTION
[0005] Embodiments of the present invention include a power system
and method for performing a blackstart on a microgrid without the
necessity of communication between the power converters of the
power system.
[0006] In one aspect, a power system for performing a blackstart on
a microgrid includes a first power converter comprising a first
controller having a plurality of startup sequences for performing
the blackstart; and a second power converter electrically coupled
to the first power converter at a point of common coupling. During
the blackstart, the first controller is configured to select and
perform one of the plurality of startup sequences according to a
point at which the second power converter is within the second
power converter's startup sequence during the blackstart, and the
first controller selects the one of the plurality of startup
sequences according to a microgrid voltage at the point of common
coupling.
[0007] The second power converter may comprise a second controller
having the plurality of startup sequences, with the second
controller being configured to control the second power converter
to perform one of the plurality of startup sequences such that the
first controller can synchronize with the second controller during
the blackstart according to the microgrid voltage.
[0008] The first controller may be configured to select a first
startup sequence when the microgrid voltage is less than a first
predetermined voltage threshold. In performing the first startup
sequence the first controller is configured to perform one or more
of the following: close a first switch that is coupled between the
first power converter and the point of common coupling; start
gating of the first power converter; control a frequency of an
output voltage of the first power converter to be a first
predetermined frequency; ramp the output voltage level of the first
power converter from substantially zero to a first predetermined
voltage level over a first predetermined time period; hold the
output voltage level at the first predetermined voltage level and
hold the frequency of the output voltage at the first predetermined
frequency for a predetermined dwell period; and ramp the output
voltage level from the first predetermined voltage level to a
nominal voltage level, and ramp the output voltage frequency from
the first predetermined frequency to a nominal output voltage
frequency over a second predetermined time period.
[0009] The first controller may also be configured to select a
second startup sequence when the microgrid voltage is greater than
a first predetermined voltage threshold and less than a second
predetermined voltage threshold. In performing the second sequence
the first controller is configured to perform one or more of the
following: start phase lock loop synchronization to the microgrid
voltage and a microgrid frequency; implement a first wait time for
waiting unit the microgrid voltage has reached the first
predetermined voltage threshold; start gating of the first power
converter to output the first predetermined voltage threshold; ramp
the output voltage of the first power converter from the first
predetermined voltage threshold to a first predetermined voltage
level over a remaining time period, the remaining time period being
a portion of a first predetermined time period during which the
second power controller finishes ramping its output voltage from
substantially zero to the first predetermined voltage level;
implement a second wait time for waiting a portion of a
predetermined dwell period, the predetermined dwell period being a
period during which the second power converter is holding its
output voltage and frequency; close a first switch that is coupled
between the first power converter and the point of common coupling;
implement a third wait time for waiting a remaining portion of the
predetermined dwell period; ramp the output voltage level from the
first predetermined voltage level to a nominal voltage level, and
ramp the output voltage frequency to a nominal output voltage
frequency over a second predetermined time period.
[0010] The first controller may also be configured to select a
third startup sequence when the microgrid voltage is greater than a
third predetermined threshold. In forming the third sequence, the
first controller is configured to perform one or more of the
following: implement a wait time for waiting unit the microgrid
voltage reaches a first predetermined portion of a nominal
microgrid voltage; start phase locked loop synchronization to the
existing microgrid voltage; determine whether the microgrid voltage
and frequency are within predetermined limits of the nominal
microgrid voltage and a nominal microgrid frequency; start gating
of the first power converter and set output voltage of the first
power converter to zero and frequency to nominal microgrid
frequency; ramp the output voltage of the first power converter
from zero to the microgrid voltage; close a first switch that is
coupled between the first power converter and the point of common
coupling.
[0011] The first controller may also be configured to select
another third startup sequence when the microgrid voltage is
greater than a third predetermined voltage threshold. In forming
this particular third sequence, the first controller is configured
to perform one or more of the following: catch an initial rising
voltage and frequency of the second power converter; synchronize
the first power converter with the existing rising voltage and
frequency of the second power converter; close a first switch that
is coupled between the first power converter and the point of
common coupling; ramp the output voltage and frequency of the first
power converter from the initial voltage and frequency over a
remaining period of a final ramp of the second power converter.
[0012] In another aspect, a power system for performing a
blackstart on a microgrid includes a first power converter
electrically coupled to the microgrid and comprising a first
controller configured to perform a plurality of startup sequences;
and a second power converter electrically coupled to the microgrid.
During a blackstart, the first controller is configured to select
and perform one of the plurality of startup sequences according to
a microgrid voltage, the plurality of startup sequences including a
first start up sequence, a second start up sequence and a third
startup sequence. The first controller may select the first startup
sequence when the microgrid voltage is less that a first
predetermined threshold voltage; select the second startup sequence
when the microgrid voltage is greater than the first predetermined
threshold voltage but less than a second predetermined threshold
voltage; and select the third startup sequence when the microgrid
voltage is greater than the second predetermined voltage.
[0013] The second power converter may comprise a second controller
that is also configured to perform one of the plurality of startup
sequences including the first startup sequence, the second startup
sequence and the third startup sequence during the blackstart such
that the first controller can synchronize with the second
controller according to the microgrid voltage.
[0014] In an aspect, in performing the startup sequences, when the
first controller controls the first power converter to perform the
first startup sequence, the second controller has not begun one of
the first startup sequence, the second startup sequence and the
third startup sequence; when the first controller controls the
first power converter to perform the second startup sequence, the
second controller has begun the first startup sequence but has not
surpassed a predetermined point of the first startup sequence; and
when the first controller controls the first power converter to
perform the third startup sequence, the second controller has begun
the first startup sequence and has surpassed the predetermined
point.
[0015] In performing the first sequence, the first controller may
be configured to perform one or more of the following: close a
first switch for coupling the first power converter to the
microgrid; start gating of the first power converter; control a
frequency of an output voltage of the first power converter to be a
first predetermined frequency; ramp the output voltage level of the
first power converter from substantially zero to a first
predetermined voltage level over a first predetermined time period;
hold the output voltage level at the first predetermined voltage
level and hold the frequency of the output voltage at the first
predetermined frequency for a predetermined dwell period; ramp the
output voltage level from the first predetermined voltage level to
a nominal voltage level, and ramp the output voltage frequency from
the first predetermined frequency to a nominal output voltage
frequency over a second predetermined time period.
[0016] In performing the second sequence, the first controller may
be configured to perform one or more of the following: start phase
lock loop synchronization to the microgrid voltage and a microgrid
frequency; implement a first wait time for waiting unit the
microgrid voltage has reached a predetermined portion of a first
predetermined voltage level; start gating of the first power
converter to output the predetermined portion; ramp the output
voltage of the first power converter from the predetermined portion
to a second predetermined voltage level for over a remaining time
period, the remaining time period being a portion of a first
predetermined time period during which the second power controller
finishes ramping its output voltage from substantially zero to the
second predetermined voltage level; implement a second wait time
for waiting a portion of a predetermined dwell period, the
predetermined dwell period being a period during which the second
power converter is holding its output voltage and frequency; close
a first switch that couples the first power converter to the
microgrid; implement a third wait time for waiting a remaining
portion of the predetermined dwell period; ramp the output voltage
level from the second predetermined voltage level to a nominal
voltage level, and ramp the output voltage frequency to a nominal
output voltage frequency over a second predetermined time
period.
[0017] In performing the third sequence, the first controller may
be configured to perform one or more of the following: implement a
wait time for waiting unit the microgrid voltage reaches a first
predetermined portion of a nominal microgrid voltage; start phase
locked loop synchronization to the existing microgrid voltage;
determine whether the microgrid voltage and frequency are within
predetermined limits of the nominal microgrid voltage and a nominal
microgrid frequency; start gating of the first power converter and
set output voltage of the first power converter to zero and
frequency to nominal microgrid frequency; ramp the output voltage
of the first power converter from zero to the microgrid voltage;
close a first switch that couples the first power converter to the
microgrid.
[0018] In performing the third sequence, the first controller may
instead be configured to perform one or more of the following:
catch an initial rising voltage and frequency of the second power
converter; synchronize the first power converter with the existing
rising voltage and frequency of the second power converter; close a
first switch that couples the first power converter to the
microgrid; ramp the output voltage and frequency of the first power
converter from the initial voltage and frequency over a remaining
period of a final ramp of the second power converter.
[0019] In another aspect, a method of performing a blackstart of a
power converter coupled to a microgrid having at least one other
power converter comprises: sensing a microgrid voltage; selecting
one of a plurality of startup sequences according to the microgrid
voltage, the startup sequences including at least a first startup
sequence, a second startup sequence and a third startup
sequence.
[0020] Selecting one of a plurality of startup sequences according
to the microgrid voltage may include selecting the first startup
sequence when the microgrid voltage is less that a first
predetermined threshold voltage; selecting the second startup
sequence when the microgrid voltage is greater than the first
predetermined threshold voltage but less than a second
predetermined threshold voltage; selecting the third startup
sequence when the microgrid voltage is greater than the second
predetermined threshold voltage; and controlling the power
converter to perform the selected startup sequence.
[0021] In an aspect, the microgrid voltage being less that the
first predetermined voltage indicates that the other power
converter has not begun its first sequence.
[0022] When the first startup sequence is selected, controlling the
power converter to perform the first startup sequence may include
one or more of the following: closing a first switch for coupling
the power converter to the microgrid; starting gating of the power
converter; controlling a frequency of an output voltage of the
power converter to be a first predetermined frequency; ramping the
output voltage level of the power converter from substantially zero
to a first predetermined voltage level over a first predetermined
time period; holding the output voltage level at the first
predetermined voltage level and holding the frequency of the output
voltage at the first predetermined frequency for a predetermined
dwell period; ramping the output voltage level from the first
predetermined voltage level to a nominal voltage level, and ramping
the output voltage frequency from the first predetermined frequency
to a nominal output voltage frequency over a second predetermined
time period.
[0023] When the second startup sequence is selected, controlling
the power converter to perform the second startup sequence may
include one or more of the following: starting phase lock loop
synchronization to the microgrid voltage and a microgrid frequency;
implementing a first wait time for waiting unit the microgrid
voltage has reached a predetermined portion of a first
predetermined voltage level; starting gating of the power converter
to output the predetermined portion; ramping the output voltage of
the power converter from the predetermined portion to a second
predetermined voltage level for over a remaining time period, the
remaining time period being a portion of a first predetermined time
period during which the other power controller finishes ramping its
output voltage from substantially zero to the second predetermined
voltage level; implementing a second wait time for waiting a
portion of a predetermined dwell period, the predetermined dwell
period being a period during which the second power converter is
holding its output voltage and frequency; closing a first switch
that couples the first power converter to the microgrid;
implementing a third wait time for waiting the remaining portion of
the predetermined dwell period; ramping the output voltage level
from the second predetermined voltage level to a nominal voltage
level, and ramping the output voltage frequency to a nominal output
voltage frequency over a second predetermined time period.
[0024] When the third startup sequence is selected, controlling the
power converter to perform the third startup sequence may include
one or more of the following: implementing a wait time for waiting
unit the microgrid voltage reaches a first predetermined portion of
a nominal microgrid voltage; starting phase locked loop
synchronization to the existing microgrid voltage; determining
whether the microgrid voltage and frequency are within
predetermined limits of the nominal microgrid voltage and a nominal
microgrid frequency; starting gating of the power converter and
setting output voltage of the power converter to zero and frequency
to nominal microgrid frequency; ramping the output voltage of the
first converter from zero to the microgrid voltage; closing a first
switch that couples the power converter to the microgrid.
[0025] When the third startup sequence is selected, controlling the
power converter to perform the third startup sequence may instead
include one or more of the following: catching an initial rising
voltage and frequency of the other power converter; synchronizing
the power converter with the existing rising voltage and frequency
of the other power converter; closing a first switch that is
coupled between the power converter and the microgrid; ramping the
output voltage and frequency of the power converter from the
initial voltage and frequency over a remaining period of a final
ramp of the other power converter.
BRIEF DESCRIPTION OF THE FIGURES (NON-LIMITING EMBODIMENTS OF THE
DISCLOSURE)
[0026] Other advantages of the present invention will be readily
appreciated, as the same becomes better understood by reference to
the following detailed description when considered in connection
with the accompanying drawings, wherein:
[0027] FIG. 1 shows an exemplary embodiment of a system for
starting multiple power converters in microgrid mode.
[0028] FIG. 2 shows an exemplary embodiment of a single power
converter's connection to a controller and its control system.
[0029] FIG. 3 is a flow chart illustrating a first start-up
sequence according to an embodiment of the present invention.
[0030] FIG. 4 is a flow chart illustrating a second start-up
sequence according to an embodiment of the present invention.
[0031] FIG. 5 is a flow chart illustrating a third start-up
sequence according to an embodiment of the present invention.
[0032] FIG. 6 is a flow chart illustrating a third start-up
sequence according to another embodiment of the present
invention.
DETAILED DESCRIPTION
[0033] Reference will now be made to the accompanying drawings,
which form a part hereof, and which show, by way of illustration,
specific exemplary embodiments. The principles described herein
may, however, be embodied in many different forms. The components
in the figures are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention. Moreover,
in the figures, like referenced numerals may be placed to designate
corresponding parts throughout the different views.
[0034] In the following description of the invention, certain
terminology is used for the purpose of reference only, and is not
intended to be limiting. For example, although the terms first,
second, etc. may be used herein to describe various elements, these
elements should not be limited by these terms. These terms are only
used to distinguish one element from another. As used in the
description of the invention and the appended claims, the singular
forms "a," "an," and "the" are intended to include the plural forms
as well, unless the context clearly indicates otherwise. It will
also be understood that the term "and/or" as used herein refers to
and encompasses any and all possible combinations of one or more of
the associated listed terms. It will be further understood that the
terms "comprises" and/or "comprising," when used in this
specification, specify the presence of stated features, integers,
steps, operations, elements, and/or components, but do not preclude
the presence or addition of one or more other features, integers,
steps operations, elements, components, and/or groups thereof.
[0035] Embodiments of the present invention includes systems and
methods for starting a plurality of power converters (e.g., a power
inverter) in grid forming mode (i.e., islanding mode) with a
powered down grid (i.e., a black grid). Power converters (e.g., a
bi-directional power inverter, DC/DC converter, AC/DC converter,
etc.) are used in microgrid applications to convert power between a
power source and a grid. The plurality of power converters may be
connected to a microgrid that includes the power converters and one
or more local loads. The microgrid may also include distributed
energy resources other than the plurality of converters. The
microgrid may, or may not, be capable of electrically connecting to
a utility grid.
[0036] When the microgrid is connected to the utility grid, the
microgrid may operate in a grid-tied mode in which the utility grid
is electrically connected to and supplies power to (or receives
power from) the microgrid and an islanding mode in which the
utility grid is disconnected from the microgrid. When the microgrid
is in an islanding mode, the power converters can be said to be in
a grid-forming mode. In the grid-forming mode, the power
converters--which are connected to power resources such as solar,
wing etc.,--assist in generating power to meet the electricity
needs of the one or more local loads of the microgrid. The power
converters interface with and control or "form" the microgrid. In
grid-forming mode, the power converters control both voltage
magnitude and frequency of the microgrid. Regardless of whether or
not the microgrid is connected to the utility grid, in a powered
down or black grid, no power is being supplied to the microgrid at
the point in time at which the power converters desire to supply
power to the loads (unlike, e.g., when the grid is transitioning
from grid-tied to islanding mode).
[0037] When starting multiple power converters with a black grid
(i.e., blackstart), certain issues arise. One issue is inrush
current of transformers and any motor loads that may be connected
to the microgrid. If a full voltage is instantaneously applied to a
microgrid that is at rest or is black, a large amount of inrush
current will be drawn by the microgrid from the source, which in
this case is the plurality of power converters (e.g., power
inverters). This in turn may end up tripping the power
converters.
[0038] Another issue involved with starting multiple power
converters with a black grid is the issue of synchronization. When
starting a plurality of power converters to power the microgrid,
the power converters should be controlled to be synchronized upon
startup, so that the power converters don't push power back and
forth between each other rather than supplying power to the local
loads. One method of synchronizing the power converters (e.g.,
power inverters) is to provide the power system with a master
controller that attempts to start all of the power converters at
the same time with some synchronization between the inverters.
However, this method has certain drawbacks. For example,
synchronization by way of a master controller requires additional
hardware, such as high speed digital lines (e.g., fiber or copper
channels), to synchronize the power converters on a millisecond
time basis.
[0039] Embodiments of the present invention provide a power system
in which the plurality of power converters can be synchronized
without the need for communication between the power converters or
communication between the power converters and a master controller.
In embodiments of the present invention, the power converters of a
power system implement a voltage and frequency ramp upon startup.
The same (or a similar) profile is programmed into (or received by)
the controller of each of the power converters. The profile
includes the parameters of the startup operation. The parameters
may include certain set frequencies, certain set voltages, ramp
times (i.e., a predetermined time during which the voltage or
frequency is ramped from one level to another), and hold times for
holding the voltage and/or the frequency in place for a
predetermined time. In an embodiment, these values are all kept the
same within each of power converters (i.e., the controller of each
of the inverters is programmed with (or receives) the same
parameters for performing the black start). Setting the parameters
in this way allows the individual power converters to "look at" (or
in other words, obtain) another power converters voltage to see if
the other power converter has started. In an embodiment, the power
converter may "look" at the other power converters voltage by, for
example, checking for voltage at its own terminals or checking the
voltage at the point of common coupling where the power converters
are electrically coupled to each other. The power converter can
discern whether the other power converter has started its
blackstart sequence based on the other power converter's voltage.
The power converter can then gauge, based on the magnitude/level of
the other power converter's voltage, at what point the other power
converter is within the other power converter's blackstart
sequence.
[0040] FIG. 1 is an exemplary embodiment of a system for performing
a blackstart on a plurality of power converters operating in grid
forming mode (i.e., islanding mode) with a powered down grid (i.e.,
a black grid). In the embodiment shown in FIG. 1, the power
converters 130 and 140 are bi-directional power inverters 130 and
140. However, it should be understood that the power converters 130
and 140 are not limited to power inverters and could be any
combination of DC/DC converters, AC/DC converters, etc.
Furthermore, FIG. 1 shows a first power converter and a second
power converter for convenience only, and it should be understood
that the power system 100 may include more than two power
converters.
[0041] Referring to FIG. 1, a power system 100 according to an
embodiment of the present invention may include power resources 110
and 120, power converters 130 and 140, external grid/AC source 150,
disconnect/islanding switch 160, load 170, AC bus 180, control
system 200, and sensors A and B.
[0042] In the embodiment illustrated in FIG. 1, the power resources
110 and 120 include a battery (or battery bank) 110 and a
photovoltaic cell 190. The power converters 130 and 140 are
bi-directional power inverters 130 and 140. The bi-directional
power converters convert between DC and AC. Each of the power
converters includes its own controller 230 or 240. The system may
also include an optional master controller 210 that may communicate
with the individual controllers 130 and 140 and receive readings
from the sensors A and B. Sensor A takes readings, such as voltage
magnitude, current magnitude, phase and/or frequency at the utility
grid 150 side of the switch 160. Sensor B takes readings, such as
voltage magnitude, current magnitude, phase and/or frequency at the
point of common coupling 180. Each of the utility grid, the first
and second power converters 130 and 140 and the load 170 are
electrically coupled at the point of common coupling 180.
[0043] If an external grid 150 is provided, the external grid 150
may be the main utility grid, a separate grid segment of the
microgrid, or even another AC or DC source connected to the
microgrid. Disconnect 160 may be an islanding switch for
electrically separating the microgrid from the external grid 150.
The disconnect 160 may be, for example, a static disconnect switch,
a motorized breaker, contactor, semiconductor AC switch, etc.
[0044] Load 170 represents the load that is actually consuming the
energy. Load 170 is represented in FIG. 1 on the AC side but may
also be a DC load.
[0045] The power converters are coupled together at a point of
common coupling (PCC) 180 to share the load 170. In the embodiment
illustrated in FIG. 1A, the PCC is an AC bus. AC bus 180 interfaces
with local load 170 on the microgrid.
[0046] In the embodiment illustrated in FIG. 1, the power
converters 130 and 140 are power inverters coupled to DC power
sources 110 and 120. However, it should be understood that the
invention is not limited to power inverters or DC sources. For
example, the power source 110 may be an AC source such as a wind
turbine, and the power converter 130 or 140 may include an AC/DC
converter coupled in series to an AC/DC power inverter between the
wind turbine and the AC bus 180. Moreover, the microgrid equipment
such as battery energy storage inverters, PV and wind systems,
diesel generators, etc. may be directly coupled to the bus 180 or
through isolation or autotransformers. Furthermore, some
distributed assets, such as a wind turbine, may be an AC source and
have an AC/AC converter where the input AC is from the turbine to
the converter and the output AC connection is to the grid. Power
sources 110 and 120 may be any DC source or combination of DC
sources and AC sources. Examples of such other sources that may be
used are generator(s), wind, PV (photovoltaic), fuel cell,
compressed air storage, etc. Power converters 130 and 140 may thus
be AC/DC, DC/DC, AC/AC or DC/AC.
[0047] Control system 200 may include a plurality of controllers
and sensors that communicate with each other for synchronization
and transition between grid-tied and microgrid modes. The control
system may include a plurality of individual power converter
controllers 230 and 240 each controlling one of the power
converters of the power system 100. The control system 200 may also
include an optional master controller 210 that is configured to
coordinate between individual inverter controllers 230 and 240. The
master controller 210 may be a separate site controller, may be one
of the individual controllers of one of the power converters, or
may be housed within one of the power inverters along with the
power converter's individual controller. The controller of one or
more of the individual power converters 230 or the master
controller 210 may be configured to monitor voltage magnitude,
current magnitude, phase and/or frequency at the utility grid 150
side of the switch 160 and the point of common coupling 180.
Commercially available transducers may be used at sensors A and B
to provide a signal to the control system for monitoring voltage
magnitude, current magnitude, phase and/or frequency.
[0048] FIG. 2 is a more detailed diagram of a single power
converter's 130 control system. It should be noted that FIG. 2 does
not show the connection of the second power converter 140, and is
provided only to further illustrate the coupling of the control
system of the power converter 130. In FIG. 2 controller 230 can
receive readings from sensors P, PF, V, I Hz, where P is power by
calculation, V is voltage magnitude measurement, I is a current
magnitude measurement, PF is a power factor calculation, and Hz is
a frequency measurement. The specific sensor layout of FIG. 1 is
exemplary only, and as would be appreciated by a person of ordinary
skill in the art, a different sensor orientation may be provided to
obtain the necessary readings for controller 140 to carry out the
present invention. The controller may receive power for its
operation from a converter (DC to DC) 261 coupled to power source
110 or a converter (AC to DC) 270 coupled to an AC source. The
optional master controller 210 is also illustrated in FIG. 2.
[0049] Referring again to FIG. 1, when operating in a grid-tied
mode, islanding switch 160 is closed and energy from energy sources
110 and 190 are coupled with the grid 150. Energy from energy
sources 110 and 190 may be used to provide power to the load 170 or
additional generation to the utility/grid 150 to support other
loads.
[0050] During a blackstart, the power converters 130 and 140 are
disconnected from the grid 150 and begin operation from a powered
down grid (i.e., black grid). In a blackstart condition, the power
converters 130 and 140 are required to start up in a synchronized
fashion to bring up a microgrid without any voltage source present
or operational. The power converter 130 and 140 may receive start
commands at different times due to differences in distance between
their local controllers and the master controller, lack of time
synchronous communication protocols, etc.
[0051] In embodiments of the present invention, the individual
power converters 130 begin a blackstart sequence without the
necessity of inter-unit communication. In embodiments of the
present invention, the sequence performed by a first individual
power converter 130 is dependent upon what point another second
individual power converter system is within its own sequence. The
first power converter may determine where the second power
converter is within the second power converters sequence based on
readings taken by sensor B. The readings may be taken directly by
the first power converter's own controller 230, or the values may
be received by the controller 230 from a master controller 210
which takes readings at sensor B.
[0052] In an embodiment, the startup sequence of an individual
controller 230 may be one of a plurality of sequences, and the
controller 230 determines which of the plurality of sequences to
perform based on the voltage level (i.e. magnitude) at its own
output terminal, which, in the embodiment shown in FIG. 1, is the
microgrid voltage level sensed by sensor B at the point of common
coupling 180.
[0053] FIG. 3 is a flow chart for illustrating the first sequence
according to an embodiment of the present invention. The first
sequence is performed by power converter 130 when there is
essentially zero voltage on the microgrid. In an embodiment, there
is essentially zero voltage on the grid when the output voltage
detected by sensor B is less than 1 percent (0.01 per unit). The
first sequence may include the following steps.
[0054] In step 310, an AC contactor is closed. The AC contactor is
different from the islanding switch 160. The AC contactor is
located between the point of common coupling 180 and the power
converter 130, and the AC contactor disconnects the power converter
130 from the point of common coupling 180.
[0055] In step 320, power converter gating is started. The power
converter 130 may include a plurality of switches for converting
power from DC to AC, DC to DC, AC to DC, etc. In step 320, these
switches begin receiving gating signals.
[0056] In step 330, the initial frequency reference is kept at a
lower value. In an embodiment, the initial frequency reference may
be 15 Hz for a 60 Hz system. The controller 230 controls the power
converter 130 to output a voltage having a frequency that is the
value of the frequency reference. Typically, there are rotational
loads on the microgrid. The rotational frequency of any motor
loads, or the rotational speed of any motor loads that are on the
microgrid, is directly proportional to the frequency of the
microgrid. Accordingly, by keeping the initial frequency at a low
value in step 330, the rotational loads are started in a soft
fashion, at a low speed. The speed can then be ramped up as the
frequency is ramped.
[0057] In step 340, the output voltage of the power converter 230
is ramped up from essentially zero to a low value over a
predetermined set time. In an embodiment, the magnitude of this low
value may be 15% of the rated voltage (i.e., the nominal voltage
magnitude) of the power converter, and the set time may be 1
second. Step 340 provides a reference for other power converters
(e.g., second power converter 140) for synchronization. For
example, first power converter 130 may receive a start command
prior to other power converters (e.g., second power converter 140)
due to, e.g., communication latencies or another reason. In step
230, at a fixed frequency, the output voltage of the first power
converter 130 is ramped from zero to a low voltage magnitude, which
gives a reference to the second power converter 140 that may
receive a start command shortly thereafter. Consider, for example,
the case in which only the first power converter 130 has received
the start command, and the first power converter 130 starts ramping
from zero to 15% of the rated voltage. If another second power
converter 140 receives the start command when the first power
converter 130 reaches 10% of the rated voltage, the second power
converter 140 can then look at the output grid voltage (i.e., the
voltage at the point of common coupling 180) using sensors and see
that the microgrid voltage is at ten percent. Thus, the controller
240 of the second power converter 140 can know the point at which
the first power converter 130 is within its startup sequence. The
controller 240 of the second power converter 140 has knowledge of
the startup profile of the first controller 13), because the
controller 240 has received or has programmed therein the same
profile. Thus, the second power converter 140 can know where the
first power converter 130 is as the first power converter 140 is
ramping its voltage to allow the second power converter 140 to
start, in a synchronized fashion, with the first power converter
130.
[0058] In step 350, the output voltage magnitude and frequency are
kept constant at 15% and 15 Hz for a set dwell period of time. In
other words, the power converter 130 is performing a hold during a
dwell period in step 350. Step 350 allows the second inverter (or
second or third or multiple other inverters), which sensed the
output voltage of the first inverter when the first inverter ramps
from 0-15%, enough time to synchronize with the existing low
voltage and low frequency.
[0059] In step 360, after the dwell period, the voltage and
frequency are ramped together to the rated/nominal values over a
predetermined set time. In an embodiment, the voltage is ramped
from 15% of the rated voltage to 100% of the rated voltage, the
frequency is ramped from 25%, 15 Hz to 100%, 60 Hz, and the
predetermined set time is 4 s. However, it should be understood
that the present invention is not limited to these particular
values, and other values may be used in their place.
[0060] Throughout the first sequence, droop control is enabled to
bring multiple inverters in synchronism.
[0061] The following control logic illustrates an embodiment in
which the power converter 130 is performing the first sequence. The
control logic is implemented by the controller 230 of the power
converter 130.
TABLE-US-00001 while (state=ready) if cmd = start, state = starting
endif if fault = true state = faulted endif if startconditionsmet =
false state = notready endif endwhile while(state=starting) if outV
< 0.01pu enable current_droop; disable power_droop; close KAC;
setfreq = 15; setV = 0.0; AC_PWM = true; rampV(setV, 0.15, 1000);
holdVF(0.15, 15, 1000); rampVF(0.15, 1, 15, 60, 4000) state =
runningUF enable power_droop; disable current_droop;
[0062] In the above control logic, initially, the controller 230
determines that power converter 130 is ready to begin operation.
`if outV<0.01 pu` refers to a determination as to whether the
controller 230 is to perform the first sequence (i.e. if the
controller 230 detects that the output voltage is substantially
zero, the first sequence is performed). `enable current_droop` and
`disable power_droop` disables the power droop and enables the
current_droop, so that the controller 230 performs a frequency
droop based on output active current and a voltage droop based on
output reactive current rather than performing droop based on
active and reactive power. The reason for switching from a power
droop to a current droop is that when ramping up, the output
voltage is fairly low, because at the start of the sequence the
output voltage is essentially zero. Thus, it is possible to supply
a large amount of current to the microgrid but at a low voltage,
which would mean that output power is low. To increase the
effectiveness of droop based sharing characteristics, current is
relied upon instead of power.
[0063] `setfreq=15` and `setV=0.0` are the predetermined values for
the initial frequency and output voltage magnitude of the power
converter 130 to 15 Hz and 0.0 V, and `AC_PWM=true` causes the
power converter 130 to start gating. `rampV(setV, 0.15, 1000)`
ramps the output voltage of the power converter 130 from setV to V1
over 1000 ms. In this case, the setV is 0.0 and V1 is 15% of the
rated output voltage of power converter 130. `holdVF(0.15, 15,
1000)` holds the voltage magnitude and frequency at 15 percent and
15 Hz, respectively, for 1000 ms. `rampVF(0.15, 1, 15, 60, 4000)`
ramps the output voltage and frequency from 15 percent to 100
percent and 15 Hz to 100 Hz, respectively, over 4000 ms.
`state=runningUF` means the startup sequence is finished and the
power converter 130 is in microgrid mode, powering local loads at
nominal voltage and frequency. `enable power_droop` and `disable
current_droop` disables the current_droop, which was performed
during the startup sequence, and enables the power droop.
[0064] A second sequence is performed by power converter 130 when
the power converter 130 detects a voltage present on the microgrid,
and the detected voltage is lower than a predetermined low voltage
level but is not essentially zero. This case will mean that one or
more of the other inverters (e.g., the second power converter 140)
have already started their start up sequence before the power
converter 130 received its start command. This case may exist when
the power converter 130 receives its start command shortly after
one or more of the other inverters, which have already started
their startup sequence using the first startup sequence described
above (e.g. the second power converter 140 receives the start
signal and begins the above-described first startup sequence prior
to the point at which the first power converter 130 receives the
start signal).
[0065] FIG. 4 is a flow chart for illustrating the second sequence
according to an embodiment of the present invention. The second
sequence may include the following steps.
[0066] In step 410, the power converter 130 senses a microgrid
voltage (i.e., the voltage at the point of common coupling), and
the power converter 130 (i.e., the controller 230 of power
converter 130) determines that the voltage is greater than a first
predetermined threshold voltage but less than a second
predetermined threshold voltage. In an embodiment, the first
predetermined threshold voltage may be 1% of the rated (i.e.
nominal) voltage of the power converter 230, and the second
predetermined threshold voltage may be 12% of the rated voltage of
the power converter 130. In this embodiment, the second
predetermined threshold voltage of 12% is 80% of the voltage
magnitude to which the power converter 130 is eventually ramped
during this second startup sequence. Of course, it should be
understood that values other than 12% and 15% may be selected.
Thereafter, internal phase locked loop synchronization to the
existing microgrid voltage, frequency and phase is started. The
internal phase locked loop synchronization is the internal phase
lock synchronization with the low voltage and low frequency present
on the grid.
[0067] In Step 420, a wait time is implemented for waiting until
the microgrid voltage reaches the second threshold voltage value
(i.e. 80% of the 15% of rated voltage to which power converter 130
will be ramped. At this point the power converter 130 will know the
point at which the second power converter 240 is in its startup
sequence. For example, the first inverter 130 will know that the
second inverter 140 is at 12%, because the first inverter 130
waited until the second inverter was at the second predetermined
threshold value. The overall time it takes to ramp from 0-15% of
rated voltage is also known, because the first power converter 130
has received (or has programmed therein) the same profile as the
second power converter 140. In this case, 1000 milliseconds (or 1
second) is programmed as the period of time for the ramp, and the
controller 230 can calculate how much of the 1000 milliseconds is
remaining after reaching an output voltage 12% of rated
voltage.
[0068] In step 430, the controller 230 controls the power converter
130 to begin gating. In this step, the controller 230 controls the
power converter 130 to have an output voltage that is 12% of the
rated voltage.
[0069] In step 440, the output voltage of the power converter 130
is ramped to 15% of rated voltage from 12% (i.e. 80% of 15%) of
rated voltage. In step 440, the output voltage of the power
converter 130 is ramped from 12% of rated voltage to 15% over the
remaining time calculated by the controller 230 (e.g., 800
milliseconds).
[0070] In Step 450, a wait time is implemented by the controller
230 of the first power converter 130. The wait time of the first
power converter 230 occurs at the same time as the dwell period of
the second power converter 140 (which, in this case, is performing
the first startup sequence) described in step 350 above. In an
embodiment, the profile programmed in controller 230 calls for the
wait time to be 500 milliseconds (i.e., half of the 1 second dwell
period of step 350). The 500 millisecond wait time ensures that the
voltage and frequency of the first power converter 130 will be
synchronized with the second power converter 140, because it allows
the first inverter to use the phase lock loop a sufficient amount
of time to synchronize with the voltage and frequency generated by
the second power converter 140.
[0071] In step 460, the AC contactor is closed so that the power
converter 130 is connected to the microgrid. Because the first
power converter and the second power converter are synchronized at
this point, the closure of the AC contactor in step 460 will be a
soft closure. The soft closure (connect) means that the AC voltage
on each side of the AC contactor is matched in amplitude, frequency
and phase. After the closure of the AC contactor, the controller
230 of the first power converter 130 waits for the remainder of the
dwell period (i.e. 500 milliseconds), at which the controller 230
of the first power converter knows that it should begin ramping
up.
[0072] In step 470, the output voltage and frequency of the power
converter 430 are ramped together to the rated values over a
predetermined time. In an embodiment, the output voltage and
frequency are ramped from 15%, 15 Hz to 100%, 60 Hz over 4 s. Step
470 occurs at the same time as step 360, and thus, during the
second power converter is ramping at the same time as the first
power converter in step 470.
[0073] Throughout the second sequence, droop control is enabled to
keep the plurality of inverters in synchronism.
[0074] The following control logic illustrates an embodiment in
which the power converter 130 is performing the second startup
sequence. This control logic may be implemented along with the
first control logic (and the later described third control logic),
so that the appropriate sequence is selected based on the microgrid
voltage (i.e., the sensed voltage at the point of common coupling).
The control logic is implemented by the controller 230 of the power
converter 130.
TABLE-US-00002 elseif (outV>0.01pu and outV<0.12pu) enable
current_droop; disable power_droop; start_sync;
waittill(outV=>0.12pu) setV = outV; setfreq = 15; AC_PWM = true;
trem = 1000*outV/0.15; rampV(setV, 0.15, trem); wait(500ms); close
KAC; wait(500ms); rampVF(0.15, 1, 15, 60, 4000) state = runningUF;
enable power_droop; disable current_droop;
[0075] In the above control logic, elseif (outV>0.01 pu and
outV<0.12 pu) checks whether there is a voltage on the microgrid
(i.e., a voltage at the point of common coupling) that is below a
predetermined low voltage level. When a sensed microgrid voltage is
between a first predetermined threshold voltage level (i.e. 1
percent of the rated voltage) and a second predetermined threshold
value (i.e., 12 percent of the rated voltage) the second startup
sequence is performed. enable current_droop and disable power_droop
disables the power droop and enables the current droop, so that the
controller 230 performs a frequency droop based on output active
current and a voltage droop based on output reactive current rather
than performing droop based on power. start_sync starts internal
phase locked loop synchronization to the existing microgrid voltage
and frequency. waittill(outV=>0.12 pu) implements a wait time
during which the power converter 130 holds until the microgrid
voltage has reached a predetermined portion of the second threshold
voltage value (i.e., 12% of rated voltage). setV=outV and
setfreq=15 set the output voltage of the power converter 130 to the
microgrid voltage and the frequency to 15 Hz, and AC_PWM=true
causes the power converter 130 to start gating. trem=1000*outV/0.15
calculates the amount of time remaining in the ramp of the second
power converter 140 (which is performing the first startup
sequence) when the second power converter 140 is ramping from 0 to
15% of the rated voltage (see step 340 above). rampV(setV, 0.15,
trem) ramps the output voltage of the power converter from the
microgrid voltage to 15% of rated voltage over the calculated
remaining time trem. wait(500 ms) implements a wait time, which
ensures that the first inverter can use the phase lock loop for a
sufficient amount of time to synchronize with the phase lock loop
of the second power converter 140. close KAC instructs the AC
contactor to close. wait(500 ms) waits the remainder of the dwell
period (i.e. 500 milliseconds). rampVF(0.15, 1, 15, 60, 4000) ramps
the output voltage and frequency of the power converter 130 from 15
percent to 100 percent and 15 Hz to 100 Hz, respectively, over 4000
ms. state=runningUF means the power converter 130 is in microgrid
mode. enable power_droop and disable current_droop disables the
current_droop, which was performed during the startup sequence, and
enables the power droop.
[0076] A third sequence is performed by power converter 130 when
one or more other inverters (e.g., the second power converter 140)
have already started their start up sequence and have moved
substantially along the startup sequence before the first inverter
receives a start command. This case may exist when the first
inverter receives its start command after the one or more other
inverter(s) which have already started their startup sequence under
sequence 1 or sequence 2. In one example in which the third
sequence is utilized, a second power converter 140 is performing
the first start up sequence, and the power converter 130 receives
its start signal after the second power converter 140 has entered
the dwell period and begun its hold (step 350). During the dwell
period, the output voltage and frequency of the second power
converter 140 are maintained at a constant level, and thus the
microgrid voltage is at a constant level. Therefore, the controller
230 of the first power converter 130 cannot realize where the
second power converter 130 is within the dwell period solely by
sensing the microgrid voltage, because the microgrid voltage is at
a constant level rather than being ramped. The following exemplary
third sequence will be referred to as a first exemplary third
sequence and may be used when another power converter has begun the
first sequence and either within its hold period or beyond its hold
period and performing its final ramp.
[0077] FIG. 5 is a flow chart for illustrating the third sequence
according to an embodiment of the present invention. The third
sequence may include the following steps.
[0078] In step 510, a wait time is implemented until the microgrid
voltage has reached a predetermined portion of the rated voltage.
In an embodiment, the predetermined portion may be 85% of the rated
voltage. At this point, the microgrid voltage is fairly close
nominal operation of 100% of rated voltage at 60 Hz.
[0079] In step 520, internal phase locked loop synchronization to
the existing microgrid voltage is started by the controller 230 of
the power converter 130.
[0080] In step 530, the voltage level and frequency of the
microgrid are checked to determine whether they are within
predetermined limits. In an embodiment, the predetermined limits
are 85% to 110% of the rated voltage and 60+/-5 Hz for the grid
frequency.
[0081] In step 540, synchronous gating is started to imitate the
microgrid voltage within the controller 230 of the first power
converter 130. In step 540 the output voltage of the power
converter 130 is initially set to OV and the frequency is set to
the nominal frequency 60 Hz.
[0082] The reason for setting the output voltage to zero and
ramping it is for the power converter 130 itself to limit its
internal transients. At this point, the AC contactor is still open,
so any energization that the power converter 130 is performing is
internal. The power converter 130 may, for example, be an inverter
that includes transformers and capacitors as output filters within
them. If a large voltage is applied to the power converter, there
may a large level of inrush current within the inverter. Thus, the
power converter 130 is preventing the supply of its own inrush
current by ramping the voltage from 0 to the microgrid voltage.
[0083] In step 550, the AC contactor between the first power
converter 130 and the point of common coupling 180 is closed so
that the power converter 130 is electrically coupled to the
microgrid. At this point, the first and second power converters 130
and 140 are operating in parallel.
[0084] In step 560, droop mode is enabled by the controller 130 of
the power converter 130 to facilitate power sharing.
[0085] The following control logic illustrates an embodiment in
which the power converter 130 is performing the first exemplary
third startup sequence. This control logic implemented along with
the first and second sequence control logic, so that the
appropriate sequence is selected based on the microgrid voltage
(i.e., the sensed voltage at the point of common coupling. The
control logic is implemented by the controller 230 of the power
converter 130).
TABLE-US-00003 else waittill(outV=>0.85pu) start_sync;
waitill(outFreq<65 and outFreq>55) setfreq = outFreq; setV =
0; AC_PWM = true; rampV(setV, outV, 1000); close KAC; enable
power_droop; state = runningUF; endif
[0086] In the above control logic, else refers to the situation
other than when the microgrid voltage (i.e., a voltage at the point
of common coupling) is between the first predetermined threshold
voltage level and the second predetermined threshold voltage level.
In other words, this is the situation in which the sensed microgrid
voltage is greater than 12% of the rated voltage.
waittill(outV=>0.85 pu) controls the power converter 130 to wait
until the microgrid voltage has reached a predetermined portion of
the rated voltage (in this case, 85% of rated voltage). start_sync
starts internal phase locked loop synchronization to the existing
grid voltage. waitill(outFreq<65 and outFreq>55) controls the
power converter 130 to wait until the microgrid frequency is within
limits of the nominal frequency (in this case 60+/-5 Hz).
setfreq=outFreq sets the frequency of the power converter 130 to
the microgrid frequency. setV=0 sets the output voltage of the
power converter 130 to 0 V. AC_PWM=true starts synchronous gating
to imitate the microgrid voltage within the first power converter
130. rampV(setV, outV, 1000) ramps the output voltage from 0 to the
microgrid voltage over 1000 milliseconds. close KAC closes the AC
contactor between the first power converter 130 and the point of
common coupling 180. enable power_droop enables droop mode to
facilitate power sharing among the power converters 130, 140.
state=runningUF indicates that the state of the power converter 130
is microgrid mode.
[0087] In another second example of the third sequence, the first
power converter 130 and its controller 230 may sense the microgrid
voltage and frequency when another second power converter is
performing the first sequence and is beyond the hold period. Thus,
for example, the second power converter is ramping from 15% of
rated voltage and 15 Hz to 100% of rated voltage and 60 Hz.
[0088] FIG. 6 is a flow chart for illustrating the third sequence
according to another embodiment of the present invention. The third
sequence may include the following steps.
[0089] In step 610, the controller 230 of the first power converter
130 "catches" on to the existing rising voltage and frequency of
the microgrid. The microgrid voltage and frequency is rising as the
second power converter is performing its final ramp. Accordingly,
the controller 230 of the first power converter can determine where
the second power converter is at in its final ramp by sensing the
microgrid voltage and frequency. This step is different from step
610 in the first exemplary third sequence in that instead of
waiting for the microgrid voltage to reach the predetermined
portion of the rated voltage (e.g., 85% of rated voltage), the
controller 230 "catches" on to the existing rising voltage and
closes in.
[0090] In step 620, the controller 230 synchronizes the power
converter 130 with the microgrid voltage.
[0091] In step 630, synchronous gating is started to imitate the
microgrid voltage within the first power converter 130.
[0092] In step 640, the AC contactor between the first power
converter 130 and the point of common coupling 180 is closed so
that the power converter 130 is electrically coupled to the
microgrid. At this point, the first and second power converters 130
and 140 are operating in parallel.
[0093] In step 650, current droop is enabled and voltage droop is
disabled, so that the controller 230 performs a frequency droop
based on output active current and a voltage droop based on output
reactive current rather than performing droop based on power. Once
current droop is enabled, the controller 230 ramps the output
voltage and frequency of the power converter 130 from the initial
microgrid voltage and frequency sensed in step 610 over the
remaining period of the final ramp of the second power
converter.
[0094] The first and second power converters each have the same (or
a similar) profile programmed into (or received by) its controller.
Thus, the first controller 230 knows the slope of the final ramp
(step 360 above) performed by the second power converter during the
first sequence, the voltage at which the second power converter
begins and ends the final ramp (e.g., begins at 15% of rated
voltage and ends at 100% of rated voltage), and the amount of time
that the second power converter takes to perform the final ramp (4
s). The first controller also knows the output voltage of the
second power converter, because the microgrid voltage sensed by the
first controller is the output voltage of the second power
converter. Accordingly, the first controller 230 can use the output
voltage of the second power converter as well of its knowledge of
the profile of the final ramp to calculate the time remaining in
the final ramp (e.g., how much of the 4 s is left. The first
controller 230 than controls the power inverter 130 to perform its
ramp over this remaining time period, so that the first power
converter is performing its ramp at the same time that the second
power converter is performing its final ramp.
[0095] In step 660, once the voltage and frequency ramp is over,
current droop is disabled and power droop is enabled.
[0096] The following control logic illustrates an embodiment in
which the power converter 130 is performing the second exemplary
third startup sequence. This control logic implemented along with
the first and second sequence control logic, so that the
appropriate sequence is selected based on the microgrid voltage
(i.e., the sensed voltage at the point of common coupling). The
control logic is implemented by the controller 230 of the power
converter 130.
TABLE-US-00004 else start_sync; setfreq = outFreq; setV = outV;
AC_PWM = true; close KAC; enable current_droop; disable
power_droop; rampVF(Vout, 1, Fout, 60, 4000*(1-Vout)); disable
current_droop; enable power_droop; state = runningUF; endif
[0097] In the above control logic, else refers to the situation
other than when the microgrid voltage (i.e., a voltage at the point
of common coupling) is between the first predetermined threshold
voltage level and the second predetermined threshold voltage level.
In other words, this is the situation in which the sensed microgrid
voltage is greater than 12% of the rated voltage. In this
embodiment, if the sensed microgrid voltage and frequency are the
same as the microgrid voltage and frequency during the hold period
of the second power converter, the first controller and power
converter waits until the microgrid voltage starts its subsequent
ramp (i.e., the final ramp of the second power converter, during
which the microgrid voltage is ramped to nominal levels).
start_sync starts internal phase locked loop synchronization to the
existing grid voltage. setfreq=outFreq and setV=outV catches the
output voltage and frequency of the second power converter, which
is the same as the sensed microgrid voltage and frequency.
AC_PWM=true starts synchronous gating to imitate the microgrid
voltage within the first power converter 130. close KAC closes the
AC contactor between the first power converter 130 and the point of
common coupling 180. enable current_droop and disable power_droop
disables the power droop and enables the current droop, so that the
controller 230 performs a frequency droop based on output active
current and a voltage droop based on output reactive current rather
than performing droop based on power. rampVF(Vout, 1, Fout, 60,
4000*(1-Vout)) ramps the output voltage of the first power
converter 130 from the microgrid voltage and frequency to the
nominal voltage and frequency (i.e., 60 Hz, rated voltage) over the
remainder of the 4000 ms ramp time of the second power converter.
Here, Vout represents the percentage of the rated voltage of the
output voltage outV of the second power converter. enable
power_droop and disable current_droop disables the current_droop,
which was performed during the startup sequence, and enables the
power droop. state=runningUF means the power converter 130 is in
microgrid mode.
[0098] For the control logic of the first through third sequences
discussed above, current droop can be defined by the following
equations:
Fout=Fnon-Kpf*Vnom/Vout*(Vgrid*Id);
Vout=Vnom-Kqv*VnomNout*(Vgrid*Iq).
[0099] Power droop can be defined by the following equations:
Fout-Fnom-Kpf*(Vgrid*Id);
Vout=Vnom-Kqv*(Vgrid*Iq);
[0100] In the above equations, Vout is applied voltage; and Vnom is
nominal voltage. When current droop is enabled, the droop slopes
are scaled by Vnom/Vgrid, which is higher than one (Vout<Vnom)
during blackstart. When power droop is enabled, the droop slopes
are not scaled. Vout*Id is a measure of output active power. Vourlq
is a measure of output reactive power.
[0101] The above described embodiments are described as a microgrid
connected to a utility grid as the external grid. However, it
should be understood that the external grid is not limited to a
utility grid. For example, the microgrid could be further segmented
into multiple microgrids. Each of the microgrids would have an
energy source (renewable, generators, storage) and a load. The
microgrids could then re-connect and disconnect from/to each other
as needed.
[0102] It will be apparent to those skilled in the art that various
modifications and variations can be made to the disclosed power
system without departing from the scope of the disclosure. Other
embodiments of the present disclosure will be apparent to those
skilled in the art from consideration of the specification and
practice of the present disclosure. It is intended that the
specification and examples be considered as exemplary only, with a
true scope of the present disclosure being indicated by the
following claims and their equivalents.
* * * * *