METHODE DE CONTROLE D'AFFAISSEMENT AUTOMATIQUE DESTINEE A DES CONVERTISSEURS MICROGRIP FONDEE SUR L'ANALYSE DE LA STABILITE D'UN PETIT SIGNAL

AUTOMATIC DROOP CONTROL METHOD FOR MICROGRID INVERTERS BASED ON SMALL-SIGNAL STABILITY ANALYSIS

Abrégé français

La présente invention concerne un procédé de commande automatique du fléchissement d'un onduleur de microréseau basé sur l'analyse de la stabilité des petits signaux. Le procédé de commande, qui est basé sur la régulation active de la pente d'une courbe de fléchissement P-f et de la pente d'une courbe de fléchissement Q-V, comprend les étapes consistant à : introduire l'analyse de la stabilité des petits signaux; vérifier la faisabilité des pentes après la régulation active; et mettre en uvre une régulation sans écart des tensions et des fréquences sous réserve que la stabilité d'un système soit assurée. D'après la présente invention, pour une génération distribuée dans le cadre d'une commande de fléchissement dans un état d'îlotage, l'exigence de variations de charge est satisfaite : en régulant automatiquement les pentes des courbes de fléchissement; en modifiant une valeur d'une puissance active de référence correspondant à une fréquence nominale des courbes de fléchissement et une valeur d'une puissance réactive de référence correspondant à une tension nominale; et en mettant en uvre des régulations sans écart de la fréquence et de la tension. La plage des pentes des courbes de fléchissement admise pour un fonctionnement stable du système est obtenue par l'intermédiaire de l'analyse de la stabilité des petits signaux. Quand les pentes de fléchissement sont régulées automatiquement, la plage doit être respectée. Cela permet d'éviter l'instabilité du système provoquée par une opération consistant uniquement à mettre en uvre les régulations sans écart de la fréquence et de la tension.

Abrégé anglais

The invention discloses an automatic droop control method for microgrid
inverters
based on small-signal stability analysis. On the basis of actively adjusting
the slopes
of P-f and Q-V droop curves, the control method introduces small-signal
stability
analysis to verify the feasibility of the actively adjusted slopes, thereby
realizing
no-deviating adjustment of voltage and frequency on the premise of
guaranteeing
system stability. For distributed power sources under droop control in an
islanding
status, the automatic droop control method for microgrid inverters based on
small-signal stability analysis involves varying an active power reference
value
corresponding to a rated frequency and a reactive power reference value
corresponding to a rated voltage in the droop curves by automatically
adjusting the
slopes of the droop curves, thus meeting the requirements of load changes and
realizing no-deviating adjustment of frequency and voltage; besides, the slope
ranges
of the droop curves allowed by stable system operation are obtained through
the
small-signal stability analysis; the ranges should be satisfied during the
automatic
adjustment of the droop slopes so as to prevent system instability caused by
unilateral
realization of no-deviating adjustment of frequency and voltage.

Revendications

CLAIMS:
1. An automatic droop control method for microgrid inverters based on
small-
signal stability analysis, comprising the following steps:
(1) providing voltage and frequency supports by distributed power sources:
providing voltage and frequency supports by distributed power sources under
droop control to
an islanding microgrid, wherein the inverters are under power frequency, "P-
f", and reactive
power voltage, "Q-V", control, by adjusting a frequency of an output voltage
of the inverters
using active power and adjusting a magnitude of the output voltage of the
inverters using
reactive power; output characteristics of the inverters satisfy an active
power-frequency droop
characteristic curve and a reactive power-voltage droop characteristic curve;
(2) adjusting the droop curves: when a load change causes the frequency of the
voltage to deviate from a rated value, automatically adjusting slopes of the
droop curves by an
adjuster;
(3) analyzing and verifying: obtaining upper and lower limits of the slopes of
the droop curves by means of small-signal stability analysis, and verifying
whether the
adjusted slopes are within an allowed range via calculation by a comparator;
(4) executing operations: if the adjusted slopes are between the upper and
lower limits and fall into the allowed range, recovering a stable operating
point of the
inverters to a rated voltage by the adjuster; if the adjusted slopes exceed
one of the upper and
lower limits, setting the droop curves according to a limiting value closest
to the adjusted
slopes;
wherein in the step (2), the slope Kp of a P-f curve is derived from a curve
equation: Kp = (f0 - f)/P , and during working, Kp is changed to a coefficient
Kpi generated
in real time according to system operating parameters:
<IMG>
12

in which Pt-.DELTA.t represents the active power output by the inverters at
time (t-.DELTA.t);
when the active power of loads is invariant, a system frequency is f=fn; f0 is
a
no-load system frequency; when the active power of the loads increases at time
t, the active
power output by the inverters will increase to satisfy a system power balance;
however, Kpi is
calculated using the active power Pt-.DELTA.t of previous time .DELTA.t and
thus kept unchanged, and the
system frequency decreases; after a delay of time .DELTA.t, Kp starts to
decrease until P=Pn;
whereby an active power reference value Pn corresponding to a rated frequency
fn in the P-f
droop curve is increased by automatically decreasing the droop slope Kp, such
that the
frequency of the output voltage is still fn even when the inverters generate
more active power
P; such that the active power P output by the inverters cannot exceed a
maximum Pmax
allowed by normal operation of the inverters.
2. The automatic droop control method for microgrid inverters based on
small-
signal stability analysis of claim 1, wherein in the step (1), an equation of
the active power-
frequency droop characteristic curve, namely the P-f droop curve, is f = f0 ¨
KpP , in which P
represents the active power output by the inverters; f represents an actual
voltage frequency
output by the inverters; f0 represents a no-load system frequency; Kp
represents the droop
slope of the P-f curve; besides,fn is system rated frequency 50Hz and
corresponds to the
active power reference value Pn.
3. The automatic droop control method for microgrid inverters based on
small-
signal stability analysis of claim 1, wherein in the step (1), an equation of
the reactive power-
voltage droop characteristic curve, namely a Q-V droop curve, is V =V0 ¨ KqQ ,
in which Q
represents the reactive power output by the inverters; V represents the
magnitude of an actual
output voltage of the inverters; V0 represents a no-load system voltage; Kq
represents the droop
slope of the Q-V curve.
4. The automatic droop control method for microgrid inverters based on
small-
signal stability analysis of claim 1, wherein in the step (2), the droop slope
Kq of the Q-V
curve is derived from a curve equation: Kq = (V0 ¨V)/ Q; when the reactive
power of the
loads is invariant, the voltage is within a system allowed range [Vmin, Vmax];
when the reactive
13

power of the loads increases, the reactive power Q output by the inverters
will increase to
satisfy the system power balance; if the magnitude V of an outlet voltage
exceeds [Vmin, Vmax],
Kq is changed to a coefficient Kqi generated in real time according to the
system operating
parameters:
<IMG>
after a delay of time .DELTA.t, Kq starts to decrease until Q=Qn; whereby a
reactive
power reference value Qn corresponding to a rated voltage Vn in the Q-V droop
curve is
increased by automatically decreasing the droop slope Kq, such that the
magnitude of the
output voltage is still kept within the range [Vmin, Vmax] even when the
inverters output more
reactive power Q; nevertheless, the reactive power Q output by the inverters
cannot exceed a
maximum Qmax allowed by normal operation of the inverters.
5. The
automatic droop control method for microgrid inverters based on small-
signal stability analysis of claim 1, wherein in the step (3), the
automatically adjusted slopes
Kp, Kq of the droop curve satisfy the slope ranges [Kp min, Kp max], [Kq min,
Kq max] of the droop
curves obtained through the small-signal stability analysis; a microgrid
system composed of
the distributed power sources and the loads is described by n first-order
nonlinear ordinary
differential algebraic equations: , for an autonomous system: &y=f(x,u); small
disturbance is
applied to the system and the equation is linearized into:
&y = Ax + Bu, x(t0) = x0
y=Cx+Du
in which A, B, C, D are coefficient matrices; by an automatic control theory,
when the characteristic roots .lambda.=.sigma.+j.omega. of the matrix A have
negative real parts, the system has
damped oscillation and then recovers to be stable; when other variables of the
system are
14

determined, A is a function of the droop coefficients Kp, Kg:
.lambda.=f(Kp,Kq); the real part .sigma. of.lambda. is
set below zero, and then the ranges [Kp min, Kp max] [Kq min, Kq max] of the
slopes Kp, Kq of the
droop curves are obtained.

Description

CA 02950809 2016-11-30
J
AUTOMATIC DROOP CONTROL METHOD FOR MICROGRID
INVERTERS BASED ON SMALL-SIGNAL STABILITY ANALYSIS
Field of the Invention
The invention relates to the field of microgrid control, and in particular to
an
automatic droop control method for microgrid inverters based on small-signal
stability
analysis.
Background of the Invention
With the advent of energy crisis and the development requirements of energy
saving
and emission reduction, microgrids using lots of renewable energy are
developing
rapidly. Distributed power sources in microgrids are connected to the power
grid by
means of power electronic devices. When the microgrids are decoupled from the
large
grid to be in an islanding status in case of failures of the large grid, the
distributed
power sources within the microgrid systems are required to provide voltage and
frequency supports to the microgrid systems; this objective is achieved
extensively by
simulating synchronous power source characteristic droop control, such as P-f
and
Q-V droop control, P-V and Q-f droop control and the like. However, with
regard to
the determined droop curves, operating points vary correspondingly when loads
change; thus, no-deviating adjustment of voltage and frequency cannot be
realized.
The existing method involves actively adjusting the slopes of the droop curves
when
the load changes cause changes in voltage and frequency, i.e., changing the
value of the
rated power PN corresponding to the rated frequency 50Hz in the P-f droop
curve and
the no-load voltage value in the Q-V droop curve, such that the voltage and
the
frequency of the system are recovered to the rated operating points to realize
no-deviating adjustment. However, in this method, the limitations of
constraint
conditions of stable system operation to the droop slopes are not taken into
account;
therefore, the steady-state operating points after automatic adjustment may
cause
instability in system operation.
It is disclosed in invention patent (application No. 201210107053.4) an island
power
1

CA 02950809 2016-11-30
,
'
grid control and optimization method based on coordinate-rotated virtual
impedance,
which intends to design, for complex impedance characteristics in actual
microgrids,
the coordinate-rotated virtual impedance by using coordinate-rotated
orthogonal
transformation to improve the impedance characteristics of the microgrids.
However,
this patent mainly aims at computational analysis on the steady-state
operation of the
microgrids and at optimization of the operation without considering the
transient
adjustment process of real-time fluctuation of new energy and loads of the
microgrid
systems within short time, and thus cannot realize no-deviating adjustment of
voltage
and frequency in the islanding microgrid systems.
Summary of the Invention
An objective of the present invention is to solve the above problem and
provides an
automatic droop control method for a microgrid inverter based on small-signal
stability analysis. On the basis of actively adjusting the slopes of P-f and Q-
V droop
curves, the control method introduces small-signal stability analysis to
verify the
feasibility of the actively adjusted slopes, thereby realizing no-deviating
adjustment of
voltage and frequency on the premise of guaranteeing system stability.
In order to achieve the above objective, the present invention employs the
following
technical solutions:
An automatic droop control method for microgrid inverters based on small-
signal
stability analysis includes the following steps:
(1) providing voltage and frequency supports by distributed power sources:
providing
voltage and frequency supports by distributed power sources under droop
control to an
islanding microgrid, wherein the inverters are under P-f and Q-V control,
i.e., adjusting
a frequency of an output voltage of the inverters using active power and
adjusting a
magnitude of the output voltage of the inverters using reactive power; output
characteristics of the inverters satisfy an active power-frequency droop
characteristic
curve and a reactive power-voltage droop characteristic curve;
(2) adjusting the droop curves: when a load change causes the frequency of the
voltage to deviate from a rated value, automatically adjusting slopes of the
droop
2

CA 02950809 2016-11-30
=
curves by an adjuster;
(3) analyzing and verifying: obtaining upper and lower limits of the slopes of
the
droop curves by means of small-signal stability analysis, and verifying
whether the
adjusted slopes are within an allowed range via calculation by a comparator;
(4) executing operations: if the adjusted slopes are between the upper and
lower limits
and fall into the allowed range, recovering a stable operating point of the
inverters to a
rated voltage by the adjuster; if the adjusted slopes exceed one of the upper
and lower
limits, setting the droop curves according to a limiting value closest to the
adjusted
slopes.
In the step (1), an equation of the active power-frequency droop
characteristic curve,
namely the P-f droop curve, is f = fo¨ KpP , in which P represents the active
power
output by the inverters; f represents actual voltage frequency output by the
inverters;
1.0 represents a no-load system frequency; Kp represents the droop slope of
the P-f curve.
Besides, fn is system rated frequency 50Hz and corresponds to an active power
reference value P.
In the step (1), an equation of the reactive power-voltage droop
characteristic curve,
namely a Q-V droop curve, is V = Vo ¨ KgQ , in which Q represents the reactive
power output by the inverters; V represents the magnitude of an actual output
voltage
of the inverters; 170 represents a no-load system voltage; Kg represents the
droop slope
of the Q-V curve.
In the step (2), the slope Kp of the P-f curve is derived from a curve
equation:
Kp = (A - f)I P, and during working, Kp is changed to a coefficient Kp,
generated in
real time according to system operating parameters:
K
p
t-At 9
in which Pt_At represents the active power output by the inverters at time (t-
At).
When the active power of loads is invariant, a system frequency is f=fn; when
the
active power of the loads increases at time t, the active power output by the
inverters
will increase to satisfy a system power balance; however, Kp, is calculated
using the
3

CA 02950809 2016-11-30
=
active power Pt-At of previous time At and thus kept unchanged, and the system
frequency decreases; after a delay of time At, Kp starts to decrease until PP.
In other
words, the active power reference value Pn corresponding to the rated
frequency f0 in
the P-f droop curve is increased by automatically decreasing the droop slope
icp, such
that the frequency of the output voltage is still fn even when the inverters
generate
more active power P. Nevertheless, the active power P output by the inverters
cannot
exceed a maximum Pmax allowed by normal operation of the inverters.
In the step (2), the droop slope Kg of the Q-V curve is derived from a curve
equation:
K = (V, ¨V)1 Q . When the reactive power of the loads is invariant, the
voltage is
within a system allowed range [Vmin, Vrnax]; when the reactive power of the
loads
increases, the reactive power Q output by the inverters will increase to
satisfy the
system power balance; if the magnitude V of an outlet voltage exceeds [ Vmm,
Vmad, Kg
is changed to a coefficient Kcp generated in real time according to the system
operating
parameters:
v V
11
lµq1 = ¨ Qt-At
After a delay of time At, Kg starts to decrease until Q=Qn. In other words, a
reactive
power reference value Qõ corresponding to a rated voltage G in the Q-V droop
curve
is increased by automatically decreasing the droop slope Kg, such that the
magnitude
of the output voltage is still kept within the range [V,,,n, Vmax] even when
the inverters
output more reactive power Q. Nevertheless, the reactive power Q output by the
inverters cannot exceed a maximum Q. allowed by normal operation of the
inverters.
In the step (3), the automatically adjusted slopes Kp, Kg of the droop curve
satisfy the
slope ranges [Kp miiõ lc max], [Kg niln, Kg mad of the droop curves obtained
through the
small-signal stability analysis. A microgrid system composed of the
distributed power
sources and the loads is described by n first-order nonlinear ordinary
differential
algebraic equations: , for an autonomous system: &y=f(x,u). Small disturbance
is
applied to the system and the equation is linearized into:
4

CA 02950809 2016-11-30
g37 = Ax I- Hu. x(t0) = x.
y¨Cx+Du,
in which A, B, C, D are coefficient matrices. By an automatic control theory,
when the
characteristic roots 2=u+jco of the matrix A have negative real parts, the
system has
damped oscillation and then recovers to be stable. When other variables of the
system
are determined, 2 is a function of the droop coefficients Kp, Kg: 2=f(Kp,Kq).
The real
part a of 2 is set below zero, and then the ranges [lc mm, Kp mad, [Kg imm
konad of the
slopes K, Kg of the droop curves are obtained.
The present invention has the following advantages:
for the distributed power sources under droop control in the islanding status,
the
active power reference value corresponding to the rated frequency and the
reactive
power reference value corresponding to the rated voltage in the droop curves
are
varied by automatically adjusting the slopes of the droop curves, thus meeting
the
requirements of the load changes and realizing no-deviating adjustment of
frequency
and voltage; besides, the slope ranges of the droop curves allowed by stable
system
operation are obtained through the small-signal stability analysis; the ranges
should be
satisfied during the automatic adjustment of the droop slopes so as to prevent
system
instability caused by unilateral realization of no-deviating adjustment of
frequency
and voltage.
Brief Description of the Drawings
Fig. 1 is a structure diagram of a microgrid system;
Fig. 2 is a Thevenin's equivalent circuit of two inverters connected in
parallel;
Fig. 3 is a schematic diagram of a P-f droop curve;
Fig. 4 is a schematic diagram of a Q-V droop curve;
Fig. 5 is a schematic diagram of the P-f droop curve during Kp adjustment;
Fig. 6 is a schematic diagram of the Q-V droop curve during Kg adjustment;
Fig. 7 is a structure diagram of a microgrid example for small-signal
stability analysis;

CA 02950809 2016-11-30
Fig. 8 is a control block diagram of the present control method;
Fig. 9 are result figures based on PSCAD/EMTDC simulation software.
In the drawings, a represents active power generated by distributed power
sources in
the microgrid; b represents reactive power generated by the distributed power
sources;
c represents the voltage of a microgrid bus; d represents the frequency of the
microgrid system.
Detailed Description of the Embodiments
The present invention will be further described by combining the accompanying
drawings with embodiments.
An automatic droop control method for microgrid inverters based on small-
signal
stability analysis comprises the following steps:
Step (1): distributed power sources under droop control provide voltage and
frequency
supports to an islanding microgrid. Specific operations are as shown in Fig.
1. As a
general structure of the microgrid, the distributed power source DG1 is under
droop
control, while DG2 and DG3 are under PQ control. When the microgrid is in a
grid-connected operation mode, DG1 is in a grid-connected constant power
status
under droop control. When the microgrid is decoupled from a power distribution
network to be in an islanding status due to power distribution network
failures or
other reasons, DG1 provides the voltage and frequency supports to the
microgrid
system under droop control.
As shown in Fig. 2, the Thevenin's equivalent circuit of two inverters
connected in
parallel is illustrated. A relation between power and impedance transmitted on
the line
can be derived:
V,U0
_____ cos(-8,)--- ,--8 ,) ¨ cos 6),(1)
z, z,
v u U2
Q Z, Z, ¨ sin(0, ) sin 0,
(2)
The line impedance angle is very small and it thus can be considered
approximately
that sin 8 8 and cos g R-.; 1. If the output impedance of the inverters is
controlled to
6

CA 02950809 2016-11-30
be inductive to keep the sum of the output impedance of the inverters and the
line
impedance still inductive, i.e., X0 R, ZzjX, then the following equations can
be
obtained:
P, =Vo,Uog, I X, (3)
Q, = (Vo,t ¨UO2)1 X, (4)
Further, the droop characteristics of frequency and voltage may be obtained as
follows:
f, = KpP, (5)
= Vo ¨ KqQ, (6)
In other words, the frequency of the voltage across the ports of the inverters
is
approximately in a linear relation to active power, and the magnitude of the
voltage is
approximately in a linear relation to reactive power. Thus, P-f and Q-V droop
characteristics are designed; the active power output by the inverters is
adjusted to
adjust the frequency, while the reactive power output by the inverters is
adjusted to
adjust the magnitude of the voltage.
As shown in Fig. 3, fo represents a no-load frequency; fn represents the
rated
frequency 50Hz of the system and corresponds to the reference active power Pn
in the
P-f droop curve; f represents the frequency of the actual output voltage of
the
inverters and corresponds to the actual active power P output by the
inverters; Kp
represents the droop slope of the P-f curve:
K =(fo¨ f)
P (7)
As shown in Fig. 4, Vo therein represents a no-load voltage; V,õõ and Vmax
represent
minimal and maximal voltages corresponding to the maximum reactive power
output
by the inverters and the maximum reactive power absorbed by the inverters,
respectively; V represents the magnitude of the actual output voltage of the
inverters
and corresponding to the actual reactive power Q output by the inverters; Kg
represents the droop slope of the Q-V curve:
7

CA 02950809 2016-11-30
K (V0 ¨V)
Q (8)
Step (2): When a change of loads occurs, the slopes of the droop curves are
adjusted
to realize no-deviating adjustment of frequency and voltage.
When the active load of the microgrid increases, the active power P output by
the
DG1 will increase to satisfy power balance.
As can be seen from Fig. 3, when the active power output by DG1 increases, the
frequency f of the output voltage will decrease. If the droop slopes are
adjusted
approximately, the operating point of DG1 can be translated back to the rated
frequency fn. Stated another way, as shown in Fig. 5, DG1 initially operates
at point A
in accordance with the droop curve 1; after the output active power increases,
the
frequency drops to f and DG1 operates at point B; the slope Kp of the droop
curve is
adjusted such that DG1 operates in accordance with the droop curve 2, and then
the
operating point turns to C and the output frequency is recovered to fn.
When the reactive load of the microgrid increases, the reactive power Q output
by
DG1 will increase to satisfy power balance.
As can be seen from Fig. 4, the magnitude of the output voltage of DG1 will
decrease
and is prone to exceed the system allowed limit range [Vmin, Vniax]. If the
droop slope
is adjusted approximately, as shown in Fig. 6, DG1 initially operates in
accordance
with the droop curve 1, and has the minimal voltage value Vminl
correspondingly
when generating the maximum reactive power and the maximum voltage value Vmaxi
correspondingly when absorbing the maximum reactive power. After the output
reactive power increases, the magnitude of the output voltage decreases to
exceed the
system allowed range; then, the droop slope K4 is adjusted to result in an
increase in
the corresponding minimal voltage when the maximum reactive power is
generated;
as a result, the magnitude of the output voltage falls back into the allowed
range.
Step (3): the above automatically adjusted slopes are verified using small-
signal
stability analysis.
Taking the microgrid shown in Fig. 7 as an example, the status equation of the
system
is established as follows:
8

CA 02950809 2016-11-30
%
1
1DQ A mg XINV
iline ilkeDQ
iloadDQ = itoadDiz (9),
in which
_ -
A, + /3/Nv RN MiNvCiNv, BiNv RNMNET BiNvRNMload
Amg = B1NET RN MEW CINVc B2 NET CINV ro ANET + B1NET RN MNET B1NET RN
Mload
_B1LOAD RN MINV CINVe + B2 LOAD CINV co B1 LOAD RN
MNET Aload + B1LOADRNMload _
Amg is the characteristic matrix of the system.
The characteristic roots of Amg are solved with the real parts set to be
negative, and
then the stable ranges of the droop coefficients of the microgrid system are
as follows:
1.57x 10-5 < Kp < 1.90x 10-4
3.17x 104 < Kg < 4.79x 10-3
Step (4): determination and adjustment setting: if the slopes are within the
allowed
ranges, no-deviating adjustment of frequency and voltage can be realized; or
otherwise, the droop curves are set according to the allowed maximum or
minimal
slopes obtained through the small-signal stability analysis.
As shown in Fig. 8, a microsource is connected with a microgrid bus by means
of the
inverters, an LC filter and a line. The outlet voltage and current of the
filter are
measured to obtain the output active power, the output reactive power, and the
magnitude and frequency of the voltage. D represents a delay link with a delay
of a
time interval t. The solving boxes of Kp and Kg are automatic solving
processes for
realizing no-deviating adjustment of frequency and voltage of the islanding
microgrid;
then, the small-signal stability analysis is performed to limit the magnitude.
V
represents the magnitude of the outlet voltage of the LC filter. PI represents
a
proportional-integral controller to improve the dynamic response
characteristic of the
magnitude of the voltage. Vm and 6m represent a space vector magnitude of
three-phase output phase voltage synthesis and a phase angle reference value
required
for SPWM (Sinusoidal Pulse Width Modulation), respectively.
During working, the active power and reactive power output by the inverters
are
measured. After a delay of time At, the automatically adjustable droop
coefficients Kp,
9

CA 02950809 2016-11-30
=
=
Kg may be calculated. The system is modeled by the small-signal stability
analysis,
and then the slope ranges [Kp mm, Kp max] and [Kg nun, Kg max] of the droop
curves
capable of keeping the system stable may be calculated to limit the magnitudes
of the
previously calculated K, Kg. Reference voltage and frequency values for the
inverters
are calculated according to equations (5), (6) to further generate control
pulses. As
generation of the control pulses from the reference voltage and frequency is
the prior
art, it is not redundantly described herein.
PSCAD modeling is carried out on the microgrid as shown in Fig. 7, wherein DG1
is
an energy storage system to which the automatic droop control method for the
inverters with consideration to the small-signal stability analysis of the
present patent
is employed; DG2 is a photovoltaic power generation system to which constant
power
control may be employed within 12 seconds of simulation on the assumption of
invariant illumination; DG3 is a wind power generation system to which
constant
power control may be employed within 12 seconds of simulation on the
assumption of
invariant wind speed.
At the fourth second, 10kW step pulse load is applied, and three distributed
power
sources output the active power, the reactive power, a bus voltage and a
system
frequency, as shown in Fig. 9.
As can be seen from Fig. 9, the automatic droop control method for the
microgrid
inverters based on the small-signal stability analysis has good adjustability:
for the
distributed power sources under droop control in the islanding status, the
reference
active power value corresponding to the rated frequency and the reference
reactive
power value corresponding to the rated voltage in the droop curves are varied
by
automatically adjusting the slopes of the droop curves, thus meeting the
requirements
of the load changes and realizing no-deviating adjustment of frequency and
voltage;
besides, the slope ranges of the droop curves allowed by stable system
operation are
obtained through the small-signal stability analysis; the ranges should be
satisfied
during the automatic adjustment of the droop slopes so as to prevent system
instability
caused by unilateral realization of no-deviating adjustment of frequency and
voltage.

CA 02950809 2016-11-30
, t
t
Although the specific embodiments of the present invention are described above
in
conjunction with the accompanying drawings, they are not limit to the
protection
scope of the present invention. It will be understood by those skilled in the
art that
various modifications or variations that can be made by those skilled in the
art without
creative work on the basis of the technical solution of the present invention
still fall
into the protection scope of the prevent invention.
11

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