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International Journal of Latest Trends in Engineering and Technology (IJLTET)
Dynamics of Load Voltage Control using
DSTATCOM and DVR Compensators
K.Sravya
Department of Electrical and Electronics Engineering
AITS, Rajampet, Andhra Pradesh, India
K.Harinath Reddy
Department of Electrical and Electronics Engineering
AITS, Rajampet, Andhra Pradesh, India
C.Ganesh
Department of Electrical and Electronics Engineering
AITS, Rajampet, Andhra Pradesh, India
Abstract--Load voltage control is an indispensable tack in electrical power distributed systems in the presence of nonlinear probabilities of load. Traditionally series compensators and voltage-source converter-based shunt are used for load
voltage control. Deeper study into the load voltage control mechanisms in power distribution systems can ascertain facts
pertaining to performance bottlenecks. This knowhow can help in improving systems of power distribution. Towards this
end many researches came into existence in the recent past. Recently Gupta et al. studied the role and performance of
series compensators and VSC-Based shunt in power distribution systems for load voltage control. There are many powerquality problems like unbalancing, load voltage harmonic distortions, and voltage swells. They used DVR (Dynamic
Voltage Restorer) as series device and DSTATCOM (Distribution Static Compensator) as shunt device in order to
address quality problems. In this paper we built a custom simulator to analyze these problems further and also compare
the performance of compensators with strong and weak ac-supply besides studying the effect of parameters on control
performance of these compensators. We made extensive simulations to demonstrate the proof of concept. The empirical
results revealed significant differences between compensators of the two kinds.
Index Terms –Load voltage control, dynamic voltage restorer, distribution static compensator, power distribution
systems
I.
INTRODUCTION
Power distribution systems face many problems pertaining to voltage related power quality. The problems include
harmonic distortions, voltage dips, swells and sags due to voltage unbalancing in power distribution systems.
Therefore theses problems are to be addressed in case of loads which are voltage sensitive [1]. Of late there were
many researches on the usage of VSC for solving voltage problems in all grid-connected applications [2], [3], [4].
There have been new devices coming up in power distribution systems that cause non-linear load which causes
plethora of problems. Loads are associated with distribution systems with difference loads and levels of power based
on the applications. Due to this Short Circuit Current (SCC) levels at various lengths is experienced by power
distribution systems. The factors such as volt-ampere ratings, voltage, location of load and the size of distribution
system are to be considered. Feeder impedance also has its role to play in power distribution systems. Based on the
location on the feeder where load is connected, we can classify it into two types. When the long feeder has load
connected at the end with small short-circuit current value, it is known as weak ac-supply system [5]. The current
value of short-circuit and length of the feeders has impact on line impedance [6]. When load is connected close to
the feeder, it is known as strong ac-supplywhere line impedance is negligible. There are two VSC-based devices that
can be used to reduce voltage sags [7], [8]. The first device is known as DSTATCOM [9], [10], and [11]. Other
device is DVR which was widely explored in [12], [13], [14], and [15]. As studied in [16], [17] and [18] power
quality issues are addressed by these two devices in power distribution systems. The former device uses reactive
power compensation [19] while the latter uses load voltage control methods like closed loop and open loop [20].
As experiments proved in [21] the closed loop voltage control is best. Shun and series compensators are used in the
experiments in [22] as a common strategy. However, detailed study of these devices, their role and performance in
the power distribution systems is the need of the hour. In this paper we assume a three-phase system as explored in
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International Journal of Latest Trends in Engineering and Technology (IJLTET)
[25], [10], and [8] and [5] where phases are controlled independently. As per the phase basis, the results can be
easily analyzed and used for further investigations. In this paper we use two devices such as DVR and DSTATCOM
for analyzing voltage control in the presence of non-linear distribution of power. Figure 1 shows the structure of
compensator consisting of feeder, load and compensator.
Fig. 1 –Structure of compensator (excerpt from [23])
As can be seen in figure 1, there are three district parts in the compensator. They are known as feeder, load and
compensator. Closed loop frequency – response characteristics. Feeder impedance plays an important role in the
performance of the two devices. Performance comparison is made for weak and strong ac-systems for both DVR
and DSTATCOM. Three phase distribution systems are used for the experiments basedon VSC modulation. The
voltage source type is presented in figure 2 for nonlinear load.
Fig. 2 - Voltage source type for non-linear load (excerpt from [23])
As can be seen in figure 2, the non-linear load considered in this paper is of bridge rectifier kind. The input
impedance is represented as L lac , R lac as explored in [2]. Output voltage is fed to resistive load R ldc which is
supported by a capacitor C ldc . For a given large ac inductance L lac and dc capacitor C ldc Thevinien equivalent voltage
source is represented by (L lac , R lac ) as explored in many researches such as [24], [18], and [25]. In this paper we
made experiments with the said devices and compared performance of series compensator and VSC-Based shunt in
load voltage control in distribution power systems. The remainder of the paper is structured as follows. Section II
provides details of the proposed system. Section III presents experimental results while section IV concludes the
paper.
II. VSC – BASED SHUNT AND SERIES COMPENSATORS
We used DVR (Dynamic Voltage Restorer) as series device and DSTATCOM (Distribution Static Compensator) as
shunt device in order to address quality problems.This section provides details about these devices and how they are
able to address the quality problems in power distribution systems. These are the two compensator devices that can
be used for load voltage control in power distribution systems. These devices work differently and their functionality
is explored here in terms of two models namely DSTATCOM model and DVR model. Both work in the
environment where nonlinear load is distributed as shown in figure 2.
DSTATCOM Model
The DSTATCOM compensator with single-phase equivalent circuit is presented in figure 3 where VSC is best used
to inject voltage under the closed loop settings. Dc link voltage is helped by a dc link capacitor. The voltage source
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is non-linear in nature. In the shunt path current is injected. The state space representation in case of DSTATCOM
compensator is derived as follows.
X=A x + b 1 V s + b 2 V + b 1 V d V l =cx
(1)
Fig. 3 –Equivalent circuit of DSTATCOM-compensated distribution system (excerpt from [23])
As can be seen in figure 3, the DSTATCOM compensator is capable of addressing power quality problems in power
distribution system. The next sub section explores the other compensator model named DVR model.
DVR Model
It is a direct control scheme. Its equivalent circuit is distribution system is as shown in figure 4 where DVR injects
controllable voltage.
Fig. 4 –Equivalent circuit of DVR-compensated power distribution system (excerpt from [23])
As seen in figure 4, the ac voltage is injected by DVR under closed loop settings. The dc link voltage is supported
by separate energy storage as explored in [26] or grid connected rectifier as discussed in [19]. The current flown
through VSC is termed as series current represented by i sc . In the case of DVR the state space representation is
derived
as
follows.
Load Voltage Control
Closed loop control settings have been used for DSTATCOM [18], [25], [10] and DVR [13] models earlier. The
mechanism used in this paper is a simple output voltage feedback control. In the experiments the load voltage
represented by V l is fed back to reference voltage V lref and both are compared. The error obtained is used to produce
switch action appropriately. The controller transformer function is obtained as follows.
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Where el is the error function. The derivative and proportional gains are represented as k1 and K2. Noise
disturbances are controlled by low pass filter by limiting amplification. The load voltage control process using both
DSTATCOM and DVR are presented in figure 5.
Fig. 5 –Block diagram for load voltage control using DVR and DSTATCOM
III. EXPERIMENTAL RESULTS
We made extensive simulations to find the performance differences between the compensators DSTATCOM and
DVR in compensating power distribution systems in order to address power quality problems such as unbalancing,
load voltage harmonic distortions, and voltage swells.The experimental results are as presented below.
Fig. 6 – Comparison of voltage control (DSTOTCOM non – stiff supply)
As seen in figure 6, load voltage and tracking error are presented under non-stiff supply system using DSTOTCOM
compensation model in power distribution systems.
Fig. 7–Load voltage control showing distorted source voltage and voltage raise condition (DSTATCOM non-stiff supply)
As seen in figure 7, load voltage and source voltage and nominal voltage are presented under non-stiff supply
system using DSTATCOM model.
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Fig. 9 – Load voltage control with load voltage and voltage dip conditions (DVR non-stiff supply)
As can be seen in figure 9, the load voltage, source voltage, and voltage dip conditions are presented besides
nominal voltage using DVR non – stiff supply system.
Fig. 10 – Load voltage control with voltage and voltage dip conditions (DVR stiff supply)
As can be seen in figure 10, the load voltage, source voltage, and voltage dip conditions are presented besides
nominal voltage using DVR stiff supply system.
Fig. 11- Load voltage control against sag in source voltage (DVR stiff supply)
As seen in figure 11, the source voltage, load voltage, against sag in the source voltage is presented under DVR with
stiff supply system.
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Fig. 12–Load voltage for three phase distribution system (DSTATCOM model)
As seen in figure 12, for three phase distribution system using DSTATCOM compensator model in power
distribution systems.
Fig. 13 – Seven-level inverter output voltages
As seen in figure 13, for three phase distribution system seven-level inverter output voltages are presented. The
voltage is controlled in the presence of non-linear load against harmonic distortion and unbalance in the source.
Fig. 14–Three phase load voltage control against 25% sag (DVR model)
As can be seen in figure 14, three phase load voltage control against 25% sag is presented according to DVR
compensation model for power distribution systems.
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Fig. 15 – Three phase load voltage control against 15% swell (DVR model)
As can be seen in figure 15, three phase load voltage control against 15% swell is presented according to DVR
compensation model for power distribution systems.
IV. CONCLUSION
In this paper we studied the role and performance of series compensators with strong and weak ac-supply and VSCBased shunt in load voltage control in power distribution systems. The analysis is made in presence of non-linear
load spread across the load bus. For our experiments DVR (Dynamic Voltage Restorer) as series device and
DSTATCOM (Distribution Static Compensator) as shunt device are used to address quality problems in power
distribution systems. Especially we focused on the performance comparison of the devices in load voltage control in
the presence of non-linear distribution of voltage. Various system parameters are considered for the study to know
the effects of them on the performance of compensators. We built a custom simulator which demonstrates the proof
of concept. The empirical results through extensive simulations we could find the significant differences between
the compensators of both the kinds. The results will help in making well informed decisions in power distribution
systems with respect to the usage of the devices that have been studied in this paper.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
P. R. Sánchez, E. Acha, J. E. O. Calderon, V. Feliu, and A. G. Cerrada, “A versatile control scheme for a dynamic voltage restorer for
power-quality improvement,” IEEE Trans. Power Del., vol. 24, no. 1, pp. 277–284, Jan. 2009.
K. Selvajyothi and P. A. Janakiraman, “Reduction of voltage harmonics in single phase inverters using composite observers,” IEEETrans.
Power Del., vol. 25, no. 2, pp. 1045–1057, Apr. 2010.
P. Samuel, R. Gupta, and D. Chandra, “Grid interface of photovoltaicmicro turbine hybrid based power for voltage support and control
using VSI in rural applications,” presented at the IEEE Power Eng. Soc. Gen. Meeting, Calgary, AB, Canada, 2009.
Y. A. R. I. Mohamed and E. F. E. Saadany, “A control method of grid-connected PWM voltage source inverters to mitigate fast voltage
disturbances,” IEEE Trans. Power Syst., vol. 24, no. 1, pp. 489–491, Feb. 2009.
A. Ghosh and G. Ledwich, “Load compensating DSTATCOM in weak AC systems,” IEEE Trans. Power Del., vol. 18, no. 4, pp. 1302–
1309, Oct. 2003.
IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, IEEE Std. 519-1992, Apr. 12, 1993.
M. H. Haque, “Compensation of distribution system voltage sag by DVR and D-STATCOM,” in Proc. IEEE Porto Power Tech Conf., Sep.
2001, vol. 1,5, pp. 10–13.
A. Ghosh, “Performance study of two different compensating devices in a custom power park,” Proc. Inst. Elect. Eng., Gen. Transm.
Distrib., vol. 152, no. 4, pp. 521–528, Jul. 2005.
B. Singh, A. Adya, A. P. Mittal, J. R. P. Gupta, and B. N. Singh, “Application of DSTATCOM for mitigation of voltage sag for motor loads
in isolated distribution systems,” in Proc. IEEE Int. Symp. Ind. Electronics, Jul. 9–13, 2006, vol. 3, pp. 1806–1811.
M. K. Mishra, A. Ghosh, and A. Joshi, “Operation of a DSTATCOM in voltage control mode,” IEEE Trans. Power Del., vol. 18, no. 1, pp.
258–264, Jan. 2003.
G. Ledwich and A. Ghosh, “A flexible DSTATCOM operating in voltage or current control mode,” Proc. Inst. Elect. Eng., Gen.,Transm.
Distrib., vol. 149, no. 2, pp. 215–224, Mar. 2002.
Y. W. Li, D. M. Vilathgamuwa, F. Blaabjerg, and P. C. Loh, “A robust control scheme for medium-voltage-level DVR implementation,”
IEEETrans. Ind. Electron., vol. 54, no. 4, pp. 2249–2261, Oct. 2006.
D. M. Vilathgamuwa, H. M. Wijekoon, and S. S. Choi, “A novel technique to compensate voltage sags in multiline distribution system-The
interline dynamic voltage restorer,” IEEE Trans. Ind. Electron., vol. 53, no. 5, pp. 1603–1611, Oct. 2006.
H. Kim and S. K. Sul, “Compensation voltage control in dynamic voltage restorer by use of feed forward and state feedback scheme,” IEEE
Trans. Power Electron., vol. 20, no. 5, pp. 1169–1177, Sep. 2005.
J. Godsk, M. Newman, H. Nielsen, and F. Blaabjerg, “Control and testing of a dynamic voltage restorer (DVR) at medium voltage level,”
IEEE Trans. Power Electron., vol. 19, no. 3, pp. 806–813, Aug. 2002.
Vol. 3 Issue2 November 2013
67
ISSN: 2278-621X
International Journal of Latest Trends in Engineering and Technology (IJLTET)
[16] F. A. L. Jowder, “Design and analysis of dynamic voltage restorer for deep voltage sag and harmonic compensation,” Proc. Inst.
Eng.Technol. Gen. Transm. Distrib., vol. 3, no. 6, pp. 547–560, 2009.
[17] I. R. Filho, J. L. S. Neto, L. G. Rolim, and M. Aredes, “Design and implementation of a low cost series compensator for voltage sags,” in
Proc. IEEE Int. Symp. Ind. Electronics, Jul. 9–13, 2006, vol. 3, pp. 1353–1357.
[18] R. Gupta, A. Ghosh, and A. Joshi, “Switching characterization of cascaded multilevel inverter controlled systems,” IEEE Trans. Ind.
Electron., vol. 55, no. 3, pp. 1047–1058, Mar. 2008.
[19] P. S. Sensarma, K. R. Padiyar, and V. Ramnarayan, “Analysis and performance evaluation of a distribution STATCOM for compensating
voltage fluctuations,” IEEE Trans. Power Del., vol. 16, no. 2, pp. 259–264, Apr. 2001.
[20] J. G. Nielsen, F. Blaabjerg, and N. Mohan, “Control strategies for dynamic voltage restorer compensating voltage sags with phase jump,” in
Proc. Appl. Power Electr. Conf. Exp., Mar. 4–8, 2001, vol. 2, pp. 1267–1273.
[21] G. Joos, S. Chen, and L. Lopes, “Closed-loop state variable control of dynamic voltage restorers with fast compensation characteristics,” in
Proc. IEEE Ind. Appl. Conf., Oct. 3–7, 2004, vol. 4, pp. 2252–2258.
[22] M. A. E. Alali, Y. A. Chapuis, S. Saadate, and F. Braun, “Advanced common control method for shunt and series active compensators used
in power quality improvement,” Proc. Inst. Elect. Eng., Elect. PowerAppl., vol. 151, no. 6, pp. 658–665, Nov. 2006.
[23] Rajesh Gupta, Arindam Ghosh and Avinash Joshi, “Performance Comparison of VSC-Based Shunt and Series Compensators Used for Load
Voltage Control in Distribution Systems”, IEEE, p1-11.
[24] F. Z. Peng, “Application issues of active power filters,” IEEE Ind. Appl. Mag., vol. 4, no. 5, pp. 21–30, Sep./Oct. 1998.
[25] R. Gupta and A. Ghosh, “Frequency-domain characterization of sliding mode control of an inverter used in DSTATCOM application,”
IEEETrans. Circuits Syst. I, Reg. Papers, vol. 53, no. 3, pp. 662–676, Mar. 2006.
[26] J. G. Nielsen and F. Blaabjerg, “A detailed comparison of system topologies for dynamic voltage restorers,” IEEE Trans. Ind. Appl., vol.
41, no. 5, pp. 1272–1280, Sep./Oct. 2005.
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