1. “Voltage and Reactive Power Compensation in
Wind Energy Systems through a STATCOM
using Resonant Controller”
Project C0014-2014-03 247099 Institutional Links
CONACYT-British Council
Modeling, analysis and digital/physical simulation of power systems
with integration of renewable energy sources; assessment of their
dynamic behavior and power quality impact
Meeting, April 06, 2016
2. Content
• Introduction
• STATCOM MODEL IN ABC REFERENCE FRAME
• ACTIVE AND REACTIVE POWER CONTROL IN THE ABC REFERENCE FRAME
• Balanced System Consideration
• Unbalanced System Consideration
• CASE STUDY: WT DISTURBANCES COMPENSATION WITH A STATCOM
• SIMULATION RESULTS
• EXPERIMENTAL RESONANT CONTROL IMPLEMENTATION USING OPAL-RT®
• CONCLUSIONS
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
2
4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
3. • Under voltage unbalance conditions, a decomposition process in positive
and negative sequence components for the three-phase power system
operation is required when this is manipulated by a PI controller. This
approach is associated with a considerable time delay.
• In different control structures is common use a Phase Lock Loop (PLL)
for the system elements synchronization with network frequency.
• For this reason, the control structure proposed in this presentation allows
the power electronic converters operation in the natural abc framework,
that is, without the need of a PLL and consequently avoiding a
considerable time delay in presence of disturbances in the system.
Introduction
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
4. WIND ENERGY SYSTEM
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 1: Wind energy system representation
5. STATCOM MODEL IN ABC REFERENCE FRAME
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 2: Modulating of the VSC
6. ACTIVE AND REACTIVE POWER CONTROL IN THE ABC REFERENCE FRAME
Balanced System Consideration
P =Va
ia
+Vb
ib
+Vc
ic
Q =
1
3
Va
-Vb( )ic
+ Vb
-Vc( )ia
+ Vc
-Va( )ib
é
ë
ù
û
The proposed control strategy
considers that the phase c depends of
the phases a and b, i.e.
Vc
= -Va
-Vb
Active (P) and reactive (Q) power control
law is inspired by the traditional three
phase concept, applied to balanced or
unbalanced power systems.
In order to regulate the reactive and active
power, the STATCOM output current
needs to be controlled. It is possible to
represent the system using only two
reference currents. In the current control
law, a resonant corrector is used.
ia
=
3Va
P - 3Q 2Vb
+Va( )( )
[6(Va
2
+Va
Vb
+Vb
2
)]
ib
=
3Vb
P - 3Q 2Va
+Vb( )( )
[6(Va
2
+Va
Vb
+Vb
2
)]
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
6
4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
7. ACTIVE AND REACTIVE POWER CONTROL IN THE ABC REFERENCE FRAME
Unbalanced System Consideration
In order to consider unbalanced conditions in the system, all sensors
need to be used. Then, without the consideration that the third phase
depends on the other two, the current reference are obtained as,
ia
=
Vc
+Va( )P - 3QVb( )+ Va
-Vb( )Vb
- Vc
-Va( )Vc
é
ë
ù
ûic
Va
Vc
-Va( )( )- Vb
Vb
-Vc( )( )
ib
=
Vc
+Vb( )P + 3QVa( )+ Vb
-Va( )Va
+ Vb
-Vc( )Vc
é
ë
ù
ûic
Va
Vc
-Va( )( )- Vb
Vb
-Vc( )( )
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
8. Voltage present at nodes A, B, C is analyzed. The circuit equations
are obtained using Kirchhoff’s Voltage Law (KVL), as follows:
AC grid voltage is obtained in
function of the current flowing
through the system
-Va
+VLa
+VRa
+
1
2
Vdc
Ma
= 0 Va
= La
dia
dt
æ
è
ç
ö
ø
÷+ia
Ra
+
1
2
Vdc
Ma
Va
s( )= sLa
Ia
s( )+ Ra
Ia
s( )+
1
2
Vdc
Ma
Ia
s( )=
Va
s( )-
1
2
Vdc
Ma
sLa
+ Ra
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 3: Equivalent block diagrams
in abc coordinates.
9. • Given the resonant controller characteristics, whose behavior is
mainly based on a capacitor, is only necessary to design a RL filter
for the STATCOM connection to the system.
CASE STUDY: WT DISTURBANCES COMPENSATION WITH A STATCOM
PB
=
3
2
æ
è
ç
ö
ø
÷ VB
*IB( )
IB
=
2
3
PB
VB
æ
è
çç
ö
ø
÷÷ =
2
3
3kVA
120V
æ
è
ç
ö
ø
÷ =16.6667A
ZB
=
VB
IB
=
120V
16.6667A
æ
è
ç
ö
ø
÷ = 7.2W
ZL
= 0.15( ) ZB( )= 0.15( ) 7.2W( )=1.08W
L =
ZL
w
=
ZL
2p f
=
1.08W
2p 60Hz( )
=
1.08W
377
= 2.865mH
The base power used is,
The base current IB that flowing through the PCC is,
The total base system impedance is obtained as,
The inductance is obtained using only the 15% of
the total base system impedance, i.e.
So the inductance is calculated. The
recommended resistance value lies is in the range
of 0.1 to 0.8.
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
9
4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
10. A random wind speed is
assumed; a simulation of 8
seconds is conducted, the
wind fluctuations speed
varies within a range of
7m/sec to 20m/sec.
Taking into account the last
values for L and R elements,
the gains for the controllers are
calculated by considering a
time response of 0.5ms for the
active power control PIact and
reactive power control PIreac.
PIactive PIreactive Resonant
Kp = 2.8 Kp = 26.85 n0= 2.522e4
Ki = 714.28 Ki = 435.6 n1= 49.74
n2= 0.4974
ωn= 2*π*(60Hz)
ˆR = 0.494W
ˆL = 2.578mH
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 4: Wind speed fluctuations
Table 1: Controllers parameters
11. SIMULATION RESULTS
Figure a shows that
despite the wind
fluctuations, the voltage
remains practically
constant. Besides,
different events take place
in the wind energy
system. However, the DC-
link voltage remains
constant due to the
efficiency of the proposed
control, as shown in
Figure a. The zoom of
DC-link voltage is shown
in Figure b.
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 5: Feedback capacitor voltage applying the
wind variation of Figure 4.
12. 4/6/16
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 6: PCC electric
variables without the
STATCOM interconnection in
wind energy system .
13. 4/6/16
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 7: PCC electric
variables with the STATCOM
interconnection in wind
energy system .
14. EXPERIMENTAL RESONANT CONTROL IMPLEMENTATION USING OPAL RT®
Figure 8: flow diagram
representing the wind
energy system.
Mechanical torque variations exemplify the behavior
of the synchronous generator in presence of wind
fluctuations at two different speeds.
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
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Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Figure 9: Feedback capacitor voltage.
15. 4/6/16
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
15
Figure 10: PCC electric
variables without the
STATCOM connection in
wind energy system;
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
16. 4/6/16
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
16
Figure 11: PCC electric
variables with the
STATCOM connection in
wind energy system.
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
17. Figure 12: Experimental
prototype implemented. 17
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The experimental set up
of the dynamic operation
of the wind energy
system was performed in
the real-time simulator
Opal-RT®.
18. • The use of resonant controller allowed to operate the STATCOM in
the abc reference frame (natural frequency) and also to remove
unbalanced voltage transients.
• It has been shown that the resonant controller structure can control the
wind energy system without requiring a PLL. The conducted case
study has allowed demonstrating the effective compensation by
STATCOM of wind speed fluctuations for the range of 7m/s to 20m/s,
thus contributing to the stability of the wind energy system.
• The correct operation of the resonant controller has been validated
through simulations with Matlab & Simulink and through laboratory
experimental implementation using a real-time simulator OPAL-RT.
Conclusions
“Voltage and Reactive Power Compensation in Wind Energy Systems
through a STATCOM using Resonant Controller”
18
4/6/16
Project C0014-2014-03 247099 Institutional Links CONACYT-British Council
Editor's Notes
The wind is the most promising and developed renewable energy source. The interaction between the AC grid and Wind Turbines (WT) brings the need of providing technical solutions to voltage regulation problems, reactive power compensation and stability problems caused by the wind nature fluctuations.
Power electronic converters, such as the Static Synchronous Compensator (STATCOM) are used for:
mitigate system oscillations.
maintains the voltage stability at the Point of Common Coupling (PCC).
satisfies the reactive power requirement of the WT.
keep load security
The proposed resonant control is now applied to analyze the dynamic operation of the test system of Figure 5. It contains the following elements: i) An infinite bus called “AC system”, considered as an ideal voltage source (magnitude, phase angle and frequency constant), ii) a 4kVA star-delta transformer to allow the connection of the infinite bus to the PCC; iii) STATCOM connection to the power network through a delta-star transformer of 3kVA and a filter RL; iv) a WT of 3kVA connected to the PCC through a 4kVA star-star. All transformers have a relation 1:1.
Figure 5 shows the wind energy system, with a power of 3kVA and a voltage of 120V.
Given the resonant controller characteristics, whose behavior is mainly based on a capacitor, is only necessary to design a RL filter, for the STATCOM connection to the system. The calculations the STATCOM parameters are described in [28], which derive from the active power present at the PCC.
Active (P) and reactive (Q) power control law is inspired by the traditional three phase concept, applied to balance or unbalanced power systems. It is represented by,
(5)
(6)
where Va, Vb, Vc and ia, ib, ic are the three-phase instantaneous sinusoidal voltages and currents at the PCC, respectively.
The proposed control strategy considers that the phase c depends of the phases a and b, that is,
(7)
In order to regulate the reactive and active power, the STATCOM output current needs to be controlled, hence (5) and (6) are expressed as:
(8)
(9)
Equations (8) and (9) are taken as current references to be applied in the current control of the STATCOM. It means, that ia becomes iaref in the control structure; the same applies for ib. Therefore, it is possible to represent the system using only two reference currents. In the current control law, a resonant corrector is used. This is due to the fact that reference currents are sinusoidal. The effectiveness of this corrector when sinusoidal references are used has been demonstrated [25-26].
The controller block diagram of the proposed control law is shown in Figure 3. It is important to notice that the structure of the control contains a feed-forward block with a derivative action on the references. The reference signals are sinusoidal and can be derived analytically. Also, there is an arrangement at the end of the diagram, which makes possible to assign from 2 control signals (M1 and M2) to 3 control signals, required for the converter arms.
Figure 3 Block diagram of power controller with sinusoidal references.
In Figure 3, filter elements and (resistance and inductance, respectively) are estimated values of the actual system parameters. As the control depends directly on the estimated values, a slight error on the controller parameters is assumed, in order to take into account the uncertainties due to estimation of parameters. The parameters used are given in the next section.
The resonant controller is given by equation (10).
(10)
The natural frequency ωn is adjusted to the input signal frequency in order to obtain an infinite open loop gain at this frequency [27]. The calculation of the parameters in the resonant controller is conducted using the pole placement approach. Is important to notice that in this configuration, unlike the traditional based dq0 transformation [21], [23], it is not necessary the use of PLL, since the control law is designed in the natural reference frame (three sinusoidal signals). The use of (8) and (9) which are already sinusoidal variables allows the above.
Two more controls are required in order to control the active and reactive power exchange. For these controls, the traditional PI (proportional + integral) controller is used. Figures 4a and 4b show the block diagram for the active and reactive power reference, respectively.
Figure 4 Control block diagrams. a) Active power; b) Reactive power.
The active (P) and reactive (Q) power inputs shown in Figure 4 are given by the PI output controls. The DC-link voltage control provides the active power and the RMS voltage control of the sinusoidal voltage at the PCC generates the reactive power.
In order to consider unbalanced conditions in the system, all sensors need to be used. This means that the considerations of equation (6), which allow to control the system with only 4 sensors (2 for currents and 2 for voltages) are not valid under unbalanced operation.
where Vdc is the DC bus voltage, Vabc is the AC system voltage, VLabc is the inductance voltage, VRabc is the resistance voltage, Mabc is the SPWM technique modulation index at each phase, respectively. the transfer function is described in the frequency domain by Laplace Transformation
For the resonant corrector gains, the pole placement approach is used, by considering a natural frequency ωn=60Hz. Table 1 shows the controller parameters used. It is important to notice that the values of and used in the STATCOM control (Figure 3) have a difference of 10% with respect to the R and L values in order to take uncertainty parameters
The first event is a voltage sag of 15% at the PCC, with a duration of 0.2s, i.e. from 0.5s to 0.7s. The next event occurs at 1s, when the STATCOM makes a reactive power exchange at the PCC. The third event is a voltage swell of 15% at the PCC, lasting 0.2s, i.e. from 1.5s to 1.7s in time. As a result, the proposed control law it is able to compensate and maintain the DC-link practically constant in each of the events that occurred in the wind energy system.
In this work a dynamic model of the STATCOM operating in power networks with integration of wind energy systems has been proposed.
The STATCOM has been able to generate or absorb reactive power at the PCC to keep the correct operation of the WT.
