Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 97-100___________________________________________________________________________________________________________Research on SVG Control Method Under Unbalanced Conditions
Zheng Zheng, Yousong Zhou*, Guopeng Zhang

School of Electrical Engineering and Automation, Henan Polytechnic University
Jiaozuo, 454000, China
E-mail: zhengzh@hpu.edu.cn,z2410767638@163.com,hpoyz@163.com
http://www.hpu.edu.cn/www/index.html

Abstract
In order to reduce the complexity of traditional methods, a PCI (Proportional Complex Integral) controller isadopted to control SVG under unbalance-grid condition, which has better performance than PI controller toeliminate steady-state error to compensate nonlinear loads. Based on the αβ frame, the proposed control scheme
considerably reduces total algorithm complexity. The current control loop is designed based on mathematicalmodeling under unbalanced conditions. Simulation and experimental results are provided to validate the correctnessof the proposed methods.
Keywords: SVG; complex filter; steady-state error; proportional complex integral control;

1. Introduction

2. Control Method

In recent years, the research of SVG (Static VarGenerator) is mainly focused on the fast and accurate
detection of reactive current component and thesimplification of reactive power control methods1,2.
Proportional-resonant control method3-4 can make theinput current achieve the zero steady state error.
However, the method has a high requirement for theaccuracy of the equipment parameters, which is difficult
to achieve the ideal state. Deadbeat control method andpredictive current control method are used in paper57.These control methods have some difficulties in thedesign of precise models, which can easily cause thesystem to appear unstable.
This paper presents PCI control method based on αβstationary reference frame. The control method not only
effectively eliminates the steady-state error of AC
current, but also improves the compensation
performance of the equipment.

The main circuit structure of the voltage-source SVG isshown in Fig.1.Phase voltage of three-phase power gridis expressed as ea, eb and ec; Three-phase load current isrepresented by ila, ilb and ilc ; The DC side voltage isexpressed as udc.

Fig.1. Main circuit topology of SVG*Corresponding author

Copyright © 2018, the Authors. Published by Atlantis Press.
This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).
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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 97-100___________________________________________________________________________________________________________2.1. Positive and negative component separation
method based on complex filter

variables, proportional complex integral control which
can be applied directly in three-phase coordinate system
is designed.

In this paper, a method of separating positive/negative
sequence components from the grid current by using thecomplex filter is proposed. The principle block diagram
is shown in Fig.2. In figure, ωc is the cut-off frequencyof the filter.

Fig.4.PCI control based on αβ stationary coordinate system
Fig.2.Separation of the positive/negative sequence current

The phase difference of each phase is 120°, which
can be given as
 j 3xa  xb  xc
(4) j 3xb  xc  xa j 3xc  xa  xb
The control diagram of three-phase orthogonalframe from (4) is shown in Fig.5.

From Fig.2, I  ( s ) and F ( s ) can be achieved as:
I ( s ) c ( s  jo )s  2c s  o
2F (s) 2I ( s )  F ( s ) I ( s )c (  o )(   )  4c 
2o

2

22(1)
(2)

Three-phase current transformed into αβ stationary
coordinate system can be written as:
1 1 I
1   a   I  I  I  2 2 2(3)  Ib      I 333   I  I  
 0 2  2   I c A specific block diagram is shown in Fig.3.

Fig.5.PCI control based on three-phase orthogonal frame

3. Analysis of simulation result
Fig.3. Separation of positive/negative sequence current under
three-phase unbalance

The relevant system parameters are given in Table 1.

2.2. PCI control method

Table 1.System Parameters

The traditional proportional complex integral control isbased on αβ stationary coordinate system as shown inFig.4. When the current is controlled in three-phase
coordinate system, the control computation is greatly
increased due to the coordinate transformation. In thispaper, according to the equivalent transformation ofParameter
Value
Grid Line Voltage 400V
Grid Frequency
50Hz
Grid Reactance
5mHParameter
DC side voltage
DC bus capacitor
The rated Capacity

Value700V
6μF
200kVarIn the system, the bandwidth is usually selected tobe 10 times higher than the fundamental frequency, andalso less than 1/5 of the switching frequency, so the98

Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 97-100___________________________________________________________________________________________________________range of bandwidth fb is 500 to 2000. The transfer
function of the closed loop of the system is obtained as
T ( s) I o ( s)I r (s)T (s) K (k p s  ki  jo k p )Ls  ( Kk p  R  jo L) s  Kki  jo ( Kk p  R)
2K k 2 (  o ) 2  ki2p( Lo  L  Kki )  ( Kk p  R) (  o )2

T ( s )  arctan(2k p (  o )ki)  arctan(22( Kk p  R )(  o )Lo  L  Kki
2(5)(6)) (7)Fig.7. (b) Positive and negative sequence component of load
current

When only kp is considered, the closed loop
amplitude frequency features can be achieved asT ( s) Kk p( L )  ( Kk p  R ) 22(8)Where fb=650Hz, ωb=4100rad/s, (8) give rises to
kp=0.1. Under the requirements of the system bandwidth,
the design make fb=690Hz, ωb=4330rad/s, (6) give rises
to kp=20. Under unbalanced conditions, the SVG control
strategy based on complex filter and PCI control can beshown in Fig. 6.

Fig. 8.Tracking error comparison of the output current
between PI control and PCI control

4. Experimental Verification
In this paper, RT-LAB semi-physical platform isverified to the proposed strategy as shown in Fig. 9. The
SVG control circuit is based on the core of DSP28335,
and the main part of the circuit is realized by the RTLAB platform. For the system, the phase voltage valueof input grid is 311V, the DC side voltage is 700V, andthe resistance of pure resistive load is 100 Ω.

Fig. 6. The Model of SVG

Fig.7. (a) The load current

According to above model, simulation is carried outin MATLAB/Simulink environment. Fig.7 shows theload current.Fig.8 shows tracking error comparison ofthe output current between PI control and PCI control.
When the system is controlled under PCI control after
0.5s, the SVG output current is close to reference
current, and the error is controlled to 0.1A. So PCIcontrol has a good effect on steady state error.

Fig.9. Schematic diagram of system structure

In order to avoid the disturbance caused by theequipment start-up, the equipment is cut off after thenormal running time of 1s, and the experimental

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Journal of Robotics, Networking and Artificial Life, Vol. 5, No. 2 (September 2018) 97-100___________________________________________________________________________________________________________

better compensation effect. From Fig.12, it can be seenthat unbalance degree is reduced from 0.28 to 0.06 forthe three phase current of the load, which achieve thestate's demand for unbalance degree.
The experimental results show that the SVG control
strategy based on complex filter and PCI control canrealize real-time compensation for reactive component
and negative component under unbalanced conditions.

waveforms of this system are shown in Fig. 10, Fig. 11
and Fig. 12.

5. Conclusion
Under unbalanced condition, the PCI controller isstudied using in SVG to compensate nonlinear loads.
Compared with traditional control method, the proposed
scheme can considerably reduce total algorithm
complexity. The current control loop is designed basedon the mathematical modeling under unbalanced input
conditions, and parameters of PCI controller is designed.
An experimental platform is built based on DSP 28335
and RT-Lab. Simulation and experimental results aregiven to demonstrate the effectiveness of the proposed
control scheme.

Fig. 10.Three-phase current on grid side

Acknowledgements
Fund projects: National Natural Science Foundation(U1504518).
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Fig.11. Voltage and current of A grid phase

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Fig.12. Three-phase load current unbalance

Fig.10 shows three-phase current state can bebalanced when the device is operating normally; while
the device is disconnected, the system state becomes
unbalanced. As shown in Fig.11, after compensating forreactive component and negative component, thevoltage of A phase is in accordance with the phase angleof this phasic current, which reveals that the system has100