REACTIVE POWER SHARING CONTROLLER AND CONTROL METHOD, AND
POWER GENERATING SYSTEM USING THE CONTROLLER
FIELD OF THE INVENTION
The present invention relates to a reactive power
sharing control in an isolated power grid having a
battery energy storage unit and renewable energy
generation systems installed.
10
BACKGROUND OF THE INVENTION
In these days, much renewable energy sources such
as solar or wind energy are introduced to power systems.
However due to their intermittent nature, integrating
15 renewable energy sources into power systems brings new
technical problems in their control because of frequency
and voltage fluctuations they cause.
Fluctuations in the active power generated by the
renewable energy sources can be smoothed by using battery
20 energy storage systems which consist of battery cells and
inverters for frequency conversion. When the inverter is
a self-commutated inverter, the inverter can control its
active power and reactive power injection independently
and freely.
^u^ The size or the capacity of an inverter is
determined by the maximum apparent power, which is
defined as the square root of the summation of square of
the active power and square of the reactive power. When
5 the inverter of the battery energy storage system is
commanded to output a large amount of reactive power
while delivering or drawing active power from the grid
for charging or discharging the battery, semiconductor
devices in the inverter, such as insulated-gate bipolar
10 transistors (IGBTs), can be overloaded and thereby
damaged.
To avoid such an overload, control schemes for
controlling the reactive power appropriately when voltage
fluctuations occur are necessary. One of the control
15 schemes is disclosed in the International publication
WO2012/070141, hereinafter referred to as "Patent
Literature 1". The Patent Literature 1, which is related
to an output control method and an output controller for
a wind turbine generation system and a battery energy
20 storage system connected to the wind turbine generation
system in parallel, describes a method for controlling an
optimal overall output of the wind turbine generation
system and the battery energy storage system to suppress
voltage fluctuation and frequency fluctuation of a power
_j^ grid , while preventing the overloading of the battery
energy storage system.
The Patent Literature 1 relates to a power system
connected to a power grid. However, a microgrid can be
5 operated either in a grid-connected mode or in an
isolated mode. When the renewable energy sources and the
battery energy storage system create an isolated power
system and supplies active and reactive power to a load
connected to the isolated power system, the isolated
10 power system has to always maintain the active and
reactive power balance that is: the summation of the
output active power from the renewable energy sources and
the output active power from the battery energy storage
system has to be equal to the active power required by
15 the load, and, the summation of the output reactive power
from the renewable energy sources and the output reactive
power from the battery energy storage system has to be
equal to the reactive power required by the load.
When the active power from the renewable energy
20 sources is not enough to supply the active power required
by the load, an amount of active power is automatically
withdrawn by the battery energy storage system to achieve
the active power balance. When the apparent power of the
inverter in the battery energy storage system gets larger
than its rated value, the inverter is overloaded and the
semiconductor devices can be damaged.
The control system shown in the Patent Literature 1
has a step where the state-of-charge (SOC) is calculated
5 by mean of a device in the battery energy storage system,
a step to send the SOC information to said master
controller, a step where said master-controller commands
to said slave controllers to regulate their reactive
power injection according to said second operation mode.
10 The control scheme shown in the Patent Literature 1 uses
a centralized control which requires reliable and fast
communication links between the power generation units
and a master controller. Especially, a step to detect the
SOC contains a certain length of time delay because the
15 calculation of SOC requires filtering to decrease the
effect caused by ripple current.
The second operation mode described above cannot
avoid a certain length of time delay and the time delay
can force the battery energy storage system to experience
20 overload even for short time.
To protect the inverter in the battery energy
storage system from the overload, some margin has to be
between its rated apparent power and the commands to the
inverter by taking care of the time delay explained above
This margin leads to a poor utilization factor of the
inverter.
SUMMARY OF THE INVENTION
5 In view of the previously mentioned issues, an
object of the present invention is to improve the
utilization factor of the inverter in a battery energy
storage system.
To solve the foregoing problems, the present
10 invention is configured as follows.
A power generating system according to the present
invention comprises: a power generating device using
renewable energy; a first inverter connected to the power
generating device; a battery energy storage system; a
15 second inverter connected to the battery energy storage
system; and a controller, wherein: the first inverter and
the second inverter are connected at a point of common
coupling; and the controller operates to provide power to
load via the point of common coupling, the controller
20 changes a ratio of reactive power output from the first
inverter to reactive power output from the second
inverter.
According to the present invention, the utilization
factor of the inverter in a battery energy storage system
25 can be improved.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 illustrates an architecture of a standalone
power system including photovoltaic power
5 generation system, battery energy storage system and a
main controller for the embodiment 1 of the present
invention.
Figure 2 illustrates a control diagram of a main
controller for the embodiment 1 of the present invention.
10 Figure 3 illustrates a graph showing an example of
a voltage versus reactive power droop changing with an
output of another inverter for the embodiment 1 of the
present invention.
Figure 4 illustrates block diagrams of the voltage
15 versus reactive power droop control for the embodiment 1
of the present invention.
Figure 5 illustrates an architecture of a selfcommutated
inverter for the embodiment 1 of the present
invention.
20 Figure 6 illustrates block diagrams for calculation
of active and reactive power in the main controller for
the embodiment 1 of the present invention.
Figure 7 illustrates an another system
configuration with wind turbine generation system as the
renewable energy source for the embodiment 1 of the
present invention.
Figure 8 illustrates an another arrangement for the
controller of the renewable energy source for the
5 embodiment 1 of the present invention.
DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS
Before explaining an embodiment of the present
invention, the arts in the Patent Literature 1 are
10 explained. The Patent Literature 1 describes a control
method for a wind turbine generation system and a battery
energy storage system. The control method has two
operation modes selected on the basis of a command of
power required to the battery energy storage system.
15 The power system in the Patent Literature 1
comprises wind turbine generator controllers which
receive an active and reactive power command from a
master controller, a plurality of wind turbine generators
controlled by the wind turbine generator controllers, a
20 battery, an inverter and a battery energy storage system
controller which controls the inverter with receiving an
output command from the master-controller..
In a first mode of operation, the output commands
from the master controller to the wind turbine generator
25 controllers and the battery energy storage system
controller are lower than rated values of respective
apparent power.
In a second mode of operation, when the output
power command required to the battery energy storage
5 system controller by the master controller is greater
than a preset value which depends on the state-of-charge
level, the master-controller commands the slavecontrollers,
that are controllers of each wind turbine
generator and the battery energy storage system
10 controller, in the following way: the controller of the
battery energy storage system reduces the output command
of reactive power from the battery energy storage system,
while the controllers of the wind turbine generators
increase the new output command of reactive power from
15 the wind turbine generators by an amount of reduction
from the battery energy storage system.
The art in the Patent Literature 1 has problem as
discussed above.
An embodiment of the present invention, which can
20 solve the problem discussed above, will be described with
reference to the drawings. The controller of the
renewable energy source system of the embodiment commands
to increase the reactive power from the renewable energy
source system in the reactive power sharing between the
25 renewable energy source system and the battery energy
j ^ storage system supplying a load in an isolated mode of
operation when the active power of the battery energy
storage system is getting closer to the rated value. As a
result, the control system of the renewable energy source
5 system can avoid the overload and can improve the
utilization factor of the inverter in the battery energy
storage system. This effect is achieved by the controller,
which changes the ratio of the reactive power from the
power generating device to the reactive power from the
10 battery energy storage system depending on an active
power from the battery energy storage system, and is
explained in the embodiment below.
Embodiment 1
A stand-alone power system of the embodiment 1
15 according to the present invention is explained with
reference to FIG.l. The stand-alone power system includes
a renewable energy source system 1, a battery energy
storage system 2, a load 3, line impedances 5, 6, current
sensors 81, 83, voltage sensors 82, 84, and a controller
20 device 4.
The renewable energy source system 1 outputs
electric power to feed the load 3. When the power
generated by the renewable energy source system 1 is
greater than the power required by the load 3, the excess
25 energy is stored in the battery energy storage system 2.
10
jp When the power generated by the renewable energy source
W
system 1 is lower than the power required by the load 3,
the battery energy storage system 2 discharges an amount
of power such that the total output power from the
5 renewable energy source system 1 and the battery energy
storage system 2 is equal to the power required by the
load 3.
In this embodiment, the line impedances 5 and 6 are
such that the resistances R_l and R_2 are negligible as
10 compared to the reactance X_l and X_2 respectively.
The active power P_LOAD required by the load 3 is
equal to the summation of the active power P_RES
delivered by the renewable energy source system 1 and the
active power P_BESS delivered by the battery energy
15 storage system 2, The active power P_RES is determined by
the available amount of the primary source and the
battery energy storage system 2 automatically delivers
the required active power P_BESS=P_LOAD-P_RES.
The battery energy storage system 2 includes a
20 battery unit or a battery bank 21, a self-commutated
inverter 22 connected to the battery bank and a
controller 23 which controls the self-commutated inverter
on the basis of voltage(V_PCC) at a point of common
coupling 7. The battery energy storage system 2 is
25 connected to the common point of coupling 7 through a
11
line impedance 6. The controller 23 of the battery energy
storage system 2 has an amplitude calculator to calculate
the voltage amplitude V_PCC_AMP of the voltage V_PCC at
the point of common coupling 7, a reactive power
5 controller to control reactive power provided by the
inverter 22 to an ordered level, and a voltage versus
reactive power droop controller. The voltage versus
reactive power droop controller calculates a reactive
power command. The controller 23 regulates the reactive
10 power delivered by the inverter 22 based on the reactive
power command with the reactive power controller to
maintain the voltage of the point of common coupling 7
within an acceptable range.
The voltage versus reactive power droop controller
15 calculates the reactive power command using MATH l(Eq.l).
MATH 1
V PCC_AMP = V*-k BESSxQ BESS (Eq.l)
In the Eq.l, V_PCC_AMP is the voltage amplitude at
the point of common coupling 7, V* is a reference voltage
20 for the point of common coupling 7, k_BESS is a constant
and Q_BESS is the reference value for reactive power to
be provided by the battery energy storage system 2 to the
load 3.
12
J-. FIG. 4a shows a block diagram of the voltage versus
reactive power droop control of the controller 23 in the
battery energy storage system 2.
A subtracter 101 evaluates the difference between
5 the said reference voltage V* and voltage amplitude
V_PCC_AMP calculated from the voltage V_PCC detected at
the point of common coupling 7 by a sensor 84 and outputs
the difference to a multiplier 102. The multiplier 102
calculates a reference value Q_BESS for reactive power
10 from the battery energy storage system 2 by
multiplication of the output of subtracter 101 and the
inverse of k_BESS in the Eq.l. The reference value
Q_BESS for reactive power outputted by the multiplier 102
is sent to the reactive power controller which is not
15 shown on FIG. 4a, and the reactive power from the
inverter 22 is controlled to the reference value Q_BESS
by the reactive power controller.
The renewable energy source system 1 of the
embodiment is a photovoltaic power generation system
20 which includes photovoltaic panels 11, a self-commutated
inverter 12 connected to the photovoltaic panels 11 and a
controller 13 controlling the inverter 12. The renewable
energy source system 1 is connected to the point of
common coupling 7 through a line impedance 5. The
25 controller 13 has at least two control units. The first
13
-^ control unit is a maximum power point tracker (MPPT)
which allows the photovoltaic power generation system to
output the maximum available power under a pre-determined
amount of the solar radiation. The second control unit is
5 a voltage versus reactive power droop control to maintain
the voltage of the point of common coupling 7 within an
acceptable range.
The MPPT is a well-known art.
The voltage versus reactive power droop controller
10 for the renewable energy source system 1 calculates a
reactive power command using MATH 2 (Eq.2).
MATH 2
V_PCC_AMP = V*-k_RESxQ_RES (Eq.2)
In the Eq.2, V* is a reference voltage for the
15 point of common coupling 7 same as the V* in (Eq. 1), the
k_RES is a variable which depends on the active power
P_BESS outputted by the battery energy storage system 2,
and Q_RES is a reference value for the reactive power
delivered by the renewable energy source system 1 to the
20 load 3. The k_RES is calculated and provided to the
controller 13 by the controller device 4.
FIG. 4b shows a block diagram of the voltage versus
reactive power droop controller of the renewable energy
source system 1 implemented into controller 13.
14
^ A subtracter 103 evaluates the difference between
the said reference voltage V* and voltage amplitude
V_PCC_AiyiP calculated from the voltage V_PCC detected at
the point of common coupling 7 by the sensor 84 and
5 outputs the difference to a multiplier 104. The
multiplier 104 calculates a reference value Q_RES for
reactive power from the renewable energy source system 1
by multiplication of the output of subtracter 103 and the
inverse of k_RES in the Eq.2. The k_RES is calculated by
10 the controller device 4. The Q_RES outputted by the
multiplier 104 is sent to a reactive power controller of
renewable energy source system 1 which is not shown on
FIG. 4b.
FIG. 5 shows a main circuit configuration of the
15 self-commutated inverter 22. The principle of operation
of a self-commutated inverter is now described. A DC
energy source (or an energy storage device) is connected
at the terminals P and N and provides DC power to the
inverter. The provided DC power is then converted into AC
20 power by means of high frequency switching control of a
bridge of semi-conductor switching devices having a gate
terminal, in order to freely generate AC voltage or AC
current waveforms. The said gate of each herein semiconductor
switching device is controlled by a controller
25 23 by applying an appropriate gate signal for determining
15
^ the conduction periods of said semi-conductors. In FIG. 5,
the inverter 22 uses IGBTs 220m, 220n, 220o, 220p, 220q,
220r as semi-conductor switching devices and harmonics
filter 221, however other architectures based on
5 different types of semi-conductor switching devices, as
for instance IGCT, MOS-FET or GTO, can be used. The selfcommutated
inverters 12 have the main circuit
configuration same as that of the inverter 22 shown in
FIG. 5.
10 A control operation of the controller device 4 to
regulate the sharing of reactive power between the
renewable energy source system 1 and the battery energy
storage system 2 to deliver the reactive power required
by the load, is now explained.
15 FIG. 2 shows a control diagram of the controller 4
which controls the inverter 12 via the controller 13. The
controller 4 includes a power calculation unit 41 and a
voltage versus reactive power droop slope calculation
unit 42. The power calculation unit 41 calculates:
20 - the active power P_BESS outputted by the battery
energy storage system 2, with using the outputs of
current sensors 81 and voltage sensors 82;
- the reactive power Q_LOAD required by the load 3,
with using the outputs of current sensors 83 and voltage
25 sensors 84.
16
^p!^^^^
10
15
20
The unit 42 calculates the gain k_RES in the Eq.2
with using the P_BESS and Q_LOAD outputted by the power
calculation unit 41.
FIG. 6 shows two block diagrams implemented in the
power calculation unit 41 Line-to-line voltages V_BESS_ab
and V_BESS_bc detected by voltage sensor 82 detected by
the voltage sensors 82 are inputted into the power
calculation unit 41. The line-to-line voltages V_BESS_ab
and V_BESS_bc are transformed into phase voltages
V_BESS_a, V_BESS_b and V_BESS_c via control block llOp.
The phase-locked loop (PLL) 113p detects the grid voltage
phase 9_BESS and outputs two sinusoidal waveforms cos
e_BESS and sin e_BESS to blocks lllp and 112p.
The phase voltages V_BESS_a, V_BESS_b and V_BESS_c
outputted by the block llOp are transformed into d-q
coordinates via block lllp. The phase currents detected
by the current sensors 81 are transformed into d-q
coordinates via block 112p. The following MATH 3-6 (Eq.3-
6) explain the relationship between the phase voltages
and currents and the d-q coordinates.
MATH 3
Vd = Va--Vb--Vc]cos0 +
. " 2 - 2 -
— V b V c
2 ' 2 '
sin^ (£^.3)
MATH 4
17
c f
v.,-I
3
MATH 5
/ 1 1 V
V a—Vb—V c
~ 1 ' 1 '
(
sin^ + — V h V c
2 ~ 2 "
cos^
J
(EqA)
3
f. 1 1
/ a—lb—I c
r 2 - 2 -
(
cos^ + — / b / c
2 ' 2 '
%m.G (Eq.S)
MATH 6
' - !
/ 1 1
-\I a—lb—I c
I " 2 " 2 "
smO+ — / b / c
2 ' 2 ~
cos^ iEq.6)
The blocks 114p and 115p are multipliers and the
block 116p is an adder. The active power P_BESS is
obtained via the formula : P=V_d x I_d +V_q x I_q. using
blocks 114p, 115p, 116p and the d-q coordinates of
10 voltages and currents calculated by blocks lllp and 112p.
The line-to-line voltages V_PCC_ab and V_PCC_bc
detected by the voltage sensors 84 at the point of common
coupling 7 are transformed into phase voltages V_PCC_a,
V_PCC_b and V_PCC_c via block llOq. The phase-locked loop
15 (PLL) 113q detects the grid voltage phase e_PCC at the
point of common coupling 7 and outputs two sinusoidal
waveforms cos9_PCC and sin 9_PCC to control blocks lllq
and 112q. The phase voltages V_PCC_a, V_PCC_b and V_PCC_c
are transformed into d-q coordinates via block lllq and
20 the phase currents I_LOAD_a, I_LOAD_b and I_LOAD_c
detected by the current sensors 83 are transformed into
18
C d-q coordinates via block 112q based on the MATH 3-6
(Eq.3-6).
The blocks 114q and 115q are multipliers and the
block 116q is a subtracter. The reactive power Q_LOAD is
5 obtained via the formula : Q=V_d x I_q -V_q x I_d. using
blocks 114q, 115q, 116q and the d-q coordinates of
voltages and currents calculated by blocks lllq and 112q.
FIG. 3 illustrates the principle of operation of
the voltage versus reactive power droop slope calculation
10 unit 42 based on the output active power of the battery
energy storage system 2. The principle of operation of
the droop slope calculation unit 42, one of the features
of the embodiment, is now explained with reference to FIG,
3.
15 The slope 14 is fixed by the voltage versus
reactive power droop algorithm implemented in controller
23 with reference to (Eq. 1). If the battery energy
storage system 2 outputs a small amount of active power,
then it can output much reactive power,
20 If the battery energy storage system 2 outputs a
large amount of active power, the available reactive
power from the battery energy storage system 2 is low. In
order to avoid the overload of the inverter 22 in the
battery energy storage system 2, the voltage versus
25 reactive power droop slope calculation unit 42 changes
19
© the ratio Q BESS/Q RES based on the information from the
voltage and current sensors 81, 82, 83, 84, depending on
the amount of active power that the battery energystorage
system 2 delivers to the load 3.
5 The slope 43 shown in Fig. 3 is a slope for the
voltage versus reactive power droop control implemented
in the controller 13 for a value P_BESS=P_1 of active
power from the battery energy storage system 2, and the
slope 44 is another slope for the voltage versus reactive
10 power droop control implemented in the controller 13 for
a value P_BESS = P_2, wherein P_l is lower than P_2. For
a same amount of reactive power Q_LOAD required by the
load 3, in the steady state, the reactive power sharing
is such that Q_BESS_2/Q_RES_2 < Q_BESS_1/Q_RES_1. This is
15 because the voltage versus reactive power droop function
44 is based on k_RES, which is calculated from P_BESS and
Q_LOAD by calculation unit 42 in the controller device 4.
By having the controller device 4 comprising a
calculation unit 42 to calculate a slope of the voltage
20 versus reactive power droop depending on the active power
from the battery energy storage system 2 and the reactive
power required by the load 3, the ratio of reactive
powers can be kept in proper range. And the absolute
value of the proportionality constant is decreasing
20
^p, function to the active power of the battery energy
storage system 2.
Here, Q_BESS_1 and Q_RES_1 are values of reactive
power output from the battery energy storage system 2 and
5 the renewable energy source system 1 respectively, for PI
of P_BESS. Q_BESS_2 and Q_RES_2 are values of reactive
power output from the battery energy storage system 2 and
the renewable energy source system 1 respectively, for P2
of P_BESS.
10 The calculation process to obtain a slope
k_RES(P_BESS) for the voltage versus reactive power droop
slope calculation unit 42 is now explained. In the
steady-state, the voltage of the point of common coupling
7 is represented by the following formula based on the
15 MATH 1 and MATH 2 (Eq.l and Eq.2).
V_PCC_AMP=V*-k_BESSxQ_BESS=V*-k_RESxQ_RES
T h e r e f o r e , in s t e a d y - s t a t e , MATH 7 and MATH 8 make
s e n s e .
MATH 7
k RF'^
20 Q BESS = - X Q LOAD (Eg. 7)
k_BESS + k_RES
MATH 8
Q RES = f^-B^^^ ^Q lOAD (Eg. 8)
k BESS + k RES
21
^^ In this embodiment, the reactive power required by
the load 3 to the battery energy storage system 2 is
smaller than the available reactive power from the
battery energy storage system 2, shown by MATH 9.
5 MATH 9
Q _ BESS _ available = ^{S _ BESS _ ratedf -{P _ BESSf (Eg. 9) .
In order to avoid the overload and to improve the
utilization factor of the inverter 22, a possible choice
for k;_RES which depends on the active power P_BESS
10 outputted by the battery energy storage system 2 is
represented by MATH 10.
MATH 10
kRES = k RES(P BESS) = k BESS. #-^^g,,:^g^)M^3^^ ,^^,„)
Q _ LOAD - ^{S _ BESS _ ratedf -(P_ BESSf
15 In the MATH 10 (Eq.lO), S_BESS_rated is the rated
apparent power of the battery energy storage system 2,
and k_RES is a decreasing function of the active power
P_BESS.
In the embodiment 1 explained above, a power
20 generating system comprises the photovoltaic generation
panel 11 as a power generating device using renewable
power, the self-commutated inverter 12 as a first
inverter connected to the photovoltaic generation panel
11, the battery bank 21 having batteries, the self22
commutated inverter 22 as a second inverter connected to
the battery bank 21, and the controller device 4, wherein
the self-commutated 12 and the self-commutated inverter
22 are connected at the point of common coupling 7 and
5 the controller device 4 controls power provided to the
load 3 via the point of common coupling 7. The controller
device 4 changes a ratio of reactive power outputted from
the self-commutated inverter 12 and the reactive power
output from the self-commutated inverter 22 on the basis
10 of the active power outputted by the self-commutated
inverter 22. This ratio is changed on the basis of the
voltage versus reactive power droop function. The
controller device 4 can prevent the inverter 22 for a
connection between the battery energy storage system 2
15 and a power grid from overloading by increasing the
reactive power from the renewable energy system 1 and
decreasing the reactive power from the battery energy
storage system 2, in case that the battery energy storage
system 2 outputs too much active power.
20 By avoiding overload of inverter the 22, control
margin can be less, and utilization rate can be higher
than the method of Patent Literature 1.
The power generating system in the embodiment 1
further comprises the voltage sensors 84 as a first
25 voltage sensor to detect voltage of the point of common
23
j-j coupling 7, the current sensors 83 as a first current
sensor to detect current of the point of common coupling
7, the voltage sensors 82 as a second voltage sensor to
detect voltage of the point of common coupling 7 side of
5 the self-commutated inverter 22; and the current sensors
81 as a second current sensor 81 to detect current of the
point of common coupling 7 side of the self-commutated
inverter 22. So, active power of the battery energy
storage system 2 and reactive power demanded from the
10 load 3 can be calculated based on the values from the
current sensors and voltage sensors. The calculations are
implemented by power calculation unit in the controller
device 4.
Another embodiment is described in Fig.7. Fig. 7
15 shows another example of a power generating system having
a renewable energy source system 1. Fig.7 is different
from Fig. 1 only in having a wind turbine generation
system 11 instead of the photovoltaic generation system.
Fig. 8 also shows another embodiment. The only difference
20 in Fig. 8 from Fig. 1 is that the controller device 4 is
a component of the renewable energy source system 1.
The present invention is not limited to the above
embodiment. The type of the renewable energy source
system is not limited to the photovoltaic generation
25 system. It is, for instance, possible to use the wind
24
jp turbine generation system as shown on FIG. 7. The w
position of the controller device 4 can be changed. It is,
for instance, possible to integrate the controller device
4 in the controller of the renewable energy source system
5 1 as shown on FIG. 8.
10
15
20
25

Claims
1. A power generating system comprising:
a power generating device (11) using renewable
energy;
5 a first inverter (12) connected to the power
generating device (11);
a battery (21);
a second inverter (22) connected to the battery
(21); and
10 a controller (4),
wherein:
the first inverter (12) and the second inverter
(22) are connected at a point of common coupling (7); and
the controller (4) operates to provide power to a
15 load (3) via the point of common coupling (7),
characterized in that:
the controller (4) changes a ratio of reactive
power from the first inverter (12) to reactive power from
the second inverter (22) based on active power from the
20 second inverter (22).
2. A power generating system according to claim 1
further comprising:
a first voltage sensor (84) to detect voltage at
25 the point of common coupling (7);
26 6 0',;,'6 \^^^^
'0555
€ a first current sensor (83) to detect current at
the point of common coupling (7);
a second voltage sensor (82) to detect output
voltage of the second inverter (22); and
5 a second current sensor (81) to detect output
current of the second inverter (22).
V^-
')^ ^i^
3. A power generating system according to claim 1 or 2
wherein:
10 the controller changes the ratio based on a voltage
versus reactive power droop.
4. A power generating system according to claim 3,
wherein the controller further comprises:
15 a calculation unit to calculate a slope of the
voltage versus reactive power droop based on an active
power from the battery and a reactive power required by
the load.
20 5. A controller for a power generating system having a
power generating device using renewable energy and a
battery
characterized in that:
. < ^ 0555 % \ % ,
n n* ii\^
the controller changes a ratio of reactive power
from the power generating device to a reactive power from
the battery based on an active power from the battery.
10
6. A controller according to claim 5, wherein:
the active power is calculated based on current and
voltage in the battery; and
the reactive power is calculated based on the
current and voltage from the battery and current and
voltage at the point of common coupling (7).
15
7. A controller according to claim 5 or 6 wherein:
a ratio of reactive power based on a voltage versus
reactive power droop.
8. A controller according to claim 7, further
comprising:
a calculation unit to calculate a slope of the
voltage versus reactive power droop based on an active
20 power from the battery and a reactive power required by a
load.
9. A method for controlling a power generating system
having:
25 a power generating device using renewable energy;
28 .,G, Sv^ -ft%^55^^1^
a first inverter connected to the power generating ^ V^''
device;
a battery; and
a second inverter connected to the battery:
5 wherein the first inverter and the second inverter are
connected at a point of common coupling and a power is
supplied through the point of common coupling,
characterized in that:
changing a ratio of reactive power output from the
10 first inverter (12) and reactive power output from the
second inverter (22) based on a voltage versus reactive
power droop.
10. A power generating system, substantially as herein
described with reference to accompanying drawings and
15 examples.
11. A controller for a power generating system,
substantially as herein described with reference to
accompanying drawings and examples.
12. A method for controlling a power generating system,
20 substantially as herein described with reference to
accompanying drawings and examples.