Srf Theory in Dvr

International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 6545(Print), ISSN 0976 6553(Online) Volume 4, Issue 5, September October (2013), IAEME

184

MITIGATION OF POWER QUALITY PROBLEMS IN THREE PHASE THREE WIRE DISTRIBUTION SYSTEMS USING DYNAMIC VOLTAGE

RESTORER (DVR)

B.Karthick1, S.Kalaivanan2 and N.Amarabalan3

1Depatment of Electrical and Electronics Engineering, Alpha College of Engineering & Technology, Puducherry (ACET), INDIA,

2Department of Electrical and Electronics Engineering, Alpha College of Engineering & Technology, Pudhucherry (ACET), INDIA,

3Department of Electrical and Electronics Engineering, Manakula Vinayagar Institute of Technology, Puducherry (MIT), INDIA,

ABSTRACT

The growth of power electronic technology is the field of electric power sector has caused a greater awareness on the power quality of distribution systems. This paper investigates the problems of voltage sag, swell and its severe impact on nonlinear loads nor sensitive loads. That problem can be mitigated with voltage injection method using custom power device Dynamic Voltage Restorer (DVR). The control strategy for extracting the compensation voltages in DVR is based on synchronous reference frame theory (SRF) along with PI controller. The control of the DVR is implemented through derived reference load terminal voltages. Simulation has been carried out to evaluate the performance of DVR based Synchronous reference frame theory (SRF) for the mitigation of voltage sag, swell with the help of Matlab/SIMULINK environment.

Keywords: Dynamic Voltage Restorer (DVR), Synchronous Reference Frame theory, Power Quality, Voltage Sag/Swell

I. INTRODUCTION

Power Quality has serious economic implications for customers, utilities and electrical equipment manufacturers. Modern industrial equipments are more sensitive to power quality problems such as voltage sag, swell, interruption, harmonic flickers and impulse transients. Failures due to such disturbances create impact on production cost. So now-a-days, high quality is became basic needs of highly automated industries. Power electronics and advanced control technologies have made it possible to mitigate the power quality problems and maintain the operation of sensitive loads. There are number of voltage sag/swell mitigating methods available but the use of custom

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ISSN 0976 6545(Print) ISSN 0976 6553(Online) Volume 4, Issue 5, September October (2013), pp. 184-195 IAEME: www.iaeme.com/ijeet.asp Journal Impact Factor (2013): 5.5028 (Calculated by GISI) www.jifactor.com

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power device is considered to be the most efficiency method. Dynamic Voltage Restorers (DVRs) [1]-[4] are a very effective series compensation device for mitigating voltage sag/swell. Dynamic Voltage Restorer (DVR) is a voltage source inverter (VSI) which is inserted in series between the supply and a critical load. The basic operating principle of the DVR is to inject appropriate voltage in series with the supply through an injection transformer to mitigate the power qulaity problems [5].

The voltage injection schemes and design of the self-supported DVR and the different control strategies for the controllers of the DVR have been discussed in [6]-[7]. E.g, adaline based fundamental extraction have been implemented in [8]. Instantaneous symmetrical component theory [9], space vector modulation, synchronous reference frame theory (SRFT) [10]-[11] based control techniques for a DVR are reported in this literature. In this paper, a new control algorithm is suggested based on SRF theory which includes P-I Controller for the generation of reference Vd and Vq. Reference load signal generation involves the conversion from three-phase to two-phase and vice versa. Moreover low pass filters are essential part of this algorithm which has slow dynamic response of the compensator.

The establishment of the paper are as follows. The basic introduction of the power qulaity problems and its remedies is discussed in the above section. The basic configuration of DVR and its components are enlighted in Section II. The operating principle and the voltage injection capabilities of the DVR is discussed in section III. The Control algorithm based on Synchronous Reference Frame Theory (SRF) is enumerated in Section IV. The Simulation results of the designed model are analyzed in Section V. Finally the Conclusion of the paper finds a place in Section VI.

II. DYNAMIC VOLTAGE RESTORER

A. Basic Functioning of DVR Among the power quality problems (sags, swells, harmonics) voltage sags are the most

severe disturbances. In order to overcome these problems the concept of custom power devices is introduced recently. One of those devices is the Dynamic Voltage Restorer (DVR), which is the most efficient and effective modern custom power device used in power distribution networks. The function of the DVR will inject the missing voltage in order to regulate the load voltage from any disturbance due to immediate distort of source voltage. A dynamic voltage restorer (DVR) is a solid state inverter based on injection of voltage in series with a power distribution system. The DC side of DVR is connected to an energy source or an energy storage device, while its ac side is connected to the distribution feeder by a three-phase inter facing injection transformer. A single line diagram of a DVR connected power distribution system is shown in the Fig.1.Since DVR is a series connected device, the source current, is same as load current. DVR injected voltage in series with line such that the load voltage is maintained at sinusoidal nominal value. It is normally installed in a distribution system between the supply and the critical load feeder at the point of common coupling (PCC).

Figure1. Dynamic voltage restorer operation


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B. Components of DVR

The DVR can be divided into four components namely:

i) Voltage Source PWM Inverter PWM inverter using IGBT switches is used in the model. IGBT switches are commonly used

in series connected circuits. The insulated gate bipolar transistor or IGBT is a three-terminal power semiconductor device, noted for high efficiency and fast switching. Pulse-width modulation (PWM) is a very efficient way of providing intermediate amounts of electrical power between fully on and fully off. The voltage source converter is used to convert the DC to AC and then supply the voltage to distribution feeder through an injection transformer.

Figure2. Basic three phase inverter

ii) Injection Transformers The injection transformers connect the DVR to the distribution network via the high voltage

windings. They transform and couple the injected compensating voltages generated by the VSI to the incoming supply voltage. Basically injection transformers used in the model presented in this paper are three single phase transformers. The high voltage side of the injection transformer is connected in series to the distribution line, while the low voltage side is connected to the DVR power circuit. For a three-phase DVR, three single-phase or three-phase voltage injection transformers can be connected to the distribution line, and for single phase DVR one single-phase transformer is connected. The transformers not only reduce the voltage requirement of the inverters, but also provide isolation between the inverters.

iii) Energy Storage The energy storage unit supplies the required power for compensation of load voltage during voltage sag. A dc battery is used for this purpose.

iv) DC charging Circuit The dc charging circuit has two main tasks.

(i) The first task is to charge the energy source after a sag compensation event. (ii) The second task is to maintain dc link voltage at the nominal dc link voltage. Excess d.c link voltage rise will damage the d.c storage capacitor and switching device. Moreover the rise in d.c link voltage will nonlinearly increase switching loss and lower the DVR system efficiency. Thus aborting the reverse flow of energy is an important issue that needs to be restored. Many research studies in recent year focused on DVR energy optimization.


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III. OPERATING PRINCIPLE OF DVR

The schematic diagram of a self-supported DVR is shown in Fig.3 Three phase source voltages (Vsa, Vsb, and Vsc) are connected to the 3-phase critical load through series impedance (Za, Zb, Zc) and an injection transformer in each phase. The terminal voltages (Vta, Vtb, Vtc) have power quality problems and the DVR injects compensating voltages (VCa, VCb, VCc) through an injection transformer to get undistorted and balanced load voltages (VLa, VLb, VLc). The DVR is implemented using a three leg voltage source inverter with IGBTs along with a dc capacitor (Cdc). A ripple filter (Lr, Cr) is used to filter the switching ripple in the injected voltage. The considered load, sensitive to power quality problems is a three-phase balanced lagging power factor load. A self-supported DVR does not need any active power during steady state because the voltage injected is in quadrature with the feeder current.

Figure 3. Schematic diagram of self-supported DVR

The DVR operation for the compensation of sag, swell in supply voltages is shown in Fig.4. Before sag the load voltages and currents are represented as VL (presag) and Isa as shown in Fig.4(a). After the sag event, the terminal voltage (Vta) is gets lower in magnitude and lags the presag voltage by some angle. The DVR injects a compensating voltage (VCa) to maintain the load voltage (VL) at the rated magnitude. VCa has two components, VCad and VCaq. The voltage in-phase with the current (VCad) is required to regulate the dc bus voltage and also to meet the power loss in the VSI of DVR and an injection transformer. The voltage in quadrature with the current (VCaq) is required to regulate the load voltage (VL) at constant magnitude. During swell event, the injected voltage (VCa) is such that the load voltage lies on the locus of the circle as shown in Fig.4(b).

Figure 4. Phasor Diagram for (a) Voltage Sag (b) Voltage Swell


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IV. CONTROL ALGORITHM BASED SYNCHRONOUS REFERENCE FRAME THEORY (SRF)

The basic functions of a controller in a DVR are the detection of voltage sag/swell events in the system; computation of the correcting voltage, generation of trigger pulses to the PWM based DC-AC inverter, correction of any abnormalities in the series voltage injection and termination of the trigger pulses when the event has passed.

The compensation for voltage sags using a DVR can be performed by injecting/absorbing reactive power or real power. When the injected voltage is in quadrature with the current at the fundamental frequency, compensation is achieved by injecting reactive power and the DVR is self-supported with dc bus. But, if the injected voltage is in phase with the current, DVR injects real power and hence a battery is required at the dc side of VSI. The control technique adopted should consider the limitations such as the voltage injection capability (inverter and transformer rating) and optimization of the size of energy storage [12]

Fig.5 shows the control algorithm based on the comparison between reference load voltage and original load voltage. This is a closed loop system which requires DC link voltage of DVR and amplitude of load voltage to generate direct axis and quadrature axis voltages. When the load voltage drops or increases 10% of its reference load voltage in one or three phases of the system then the error signal generated by the DVR controller to create the PWM waveform for 6-pulse IGBT device.

Figure 5. Control Algorithm for DVR Controller

Fig.5 shows the control algorithm based on the comparison between reference load voltage and original load voltage. This is a closed loop system which requires DC link voltage of DVR and amplitude of load voltage to generate direct axis and quadrature axis voltages. When the load voltage drops or increases 10% of its reference load voltage in one or three phases of the system then the error signal generated by the DVR controller to create the PWM waveform for 6-pulse IGBT device.


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Figure 6. Control Block of DVR using SRF method of Control

Fig.6 shows the control block of the proposed DVR in which the synchronous reference frame (SRF) theory is used for the control of self-supported DVR. The voltages at PCC (Vt) are converted to the rotating reference frame using the abc-dqo conversion. The harmonics and the oscillatory components of voltages are eliminated using low pass filters (LPF). The components of voltages in d-axis and q-axis are,

)()()()(

acVdcVVacVdcVV

sqsqsq

sdsdsd

+=

+=

The compensating strategy for compensation of voltage quality problems considers that the load terminal voltage should be of rated magnitude and undistorted. The sag and swell in terminal voltages are compensated by controlling the DVR and the proposed algorithm inherently provides a self-supporting dc bus for the DVR. Three-phase reference supply voltages (VLa*,VLb*,VLc*) are derived using the sensed load voltages (VLa,VLb,VLc), terminal voltages (Vta, Vtb,Vtc) and dc bus voltage (Vdc) of the DVR as feedback signals.

The synchronous reference frame theory based method is used to obtain the direct axis (Vd) and quadrature axis (Vq) components of the load voltage. The load voltages in the three-phases are converted into the d-q-0 frame using the Parks transformation [6]. A three-phase PLL (phase locked loop) is used to synchronize these signals with the terminal voltages (Vta, Vtb, and Vtc). The d-q components are then passed through low pass filters to extract the dc components of Vd* and Vq*.

In order to maintain the DC bus voltage of the self-supported DVR, the error between the reference dc capacitor voltage (Vdc*) and the sensed dc bus voltage (Vdc) of DVR is given to a PI controller of which output (Vcad) is considered as the loss component of voltage and is added to the dc component of Vd` to generate Vd*. The reference d-axis load voltage is therefore as,


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losssdld VdcVV = )(*

Similarly, a second PI controller is used to regulate the amplitude of the load voltage (VL). The amplitude of load vol-tage (VL) at point of common coupling is calculated from the ac voltages (VLa, VLb, VLc) as,

)(32 222

LcLbLaL VVVV ++=

The amplitude of the load terminal voltage (VL) is employed over the reference amplitude (VL*) and the output of PI controller is considered as the reactive component of vol-tage (Vqr) for voltage regulation of load terminal voltage add-ed with the dc component of Vq` to generate Vq*.The refer-ence q-axis load voltage is therefore as,

qrsqLq VdcVV += )(*

The resultant voltages (Vd*, Vq*, Vo) are again converted into the reference supply currents using the reverse Parks transformation. Reference supply voltages (VLa*, VLb*, VLc*) and the sensed load voltages (VLa, VLb, VLc) are used in PWM current controller to generate gating pulses for the switches.

V. SIMULATION RESULTS AND DISCUSSION

The performance of the DVR is demonstrated for different supply voltage disturbances such as sag and swells at terminal voltages. The DVR is modeled and simulated using the MATLAB and its Simulink and Sim Power System toolboxes. The MAT-LAB model of the DVR connected system is shown in fig.7. The Specification and parameters of the system were listed in the below Table I.

TABLE I PARAMETERS OF THE SYSTEM SPECIFICATIONS AND

SYSTEM PARAMETERS VALUES

AC Line Voltage 415 V, 50Hz Load 10 KVA, 0.80 pf Lag

PI Controller KP=5 Ki=120 DC Voltage 300 V

Harmonic Filter Lr = 2.0mH, Cr=10F, Rr = 4.8 PWM Switching Frequency 1080 Hz

Injection Transformer Turns Ratio 1:1 DC bus Capacitance of DVR 1000F


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Figure 7. MATLAB Model of DVR Connected System

A. Mitigation of balanced voltage sags In the simulation model, the voltage sags are generated by switching on impedance to the

ground at the Point of Common Coupling (PCC), upstream the DVR. This situation is equivalent to a remote short-circuit fault which results in voltage dips with a phase angle jump. The result for situation is equivalent to a remote short-circuit fault which results in voltage dips with a phase angle jump. The result for simulation of voltage is shown in Fig.8 (a-c). The 70% of Voltage sag is initiated for 5 cycles at the PCC is shown in Fig.8 (b). The injected and the load voltage is shown in Fig. 8 (b, c). A zoom on the load voltage at the sag starts shown in Fig.8 (d) while zoom at the sag end is shown in Fig.8 (e).


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Figure 8. a) Grid voltage; b) Injected voltage; c) Load voltage; d) Zoom at sag start in load voltage waveform and e) Zoom at sag end in load voltage waveform

B. Mitigation of unbalanced voltage sags An Unbalance sag is programmed to generate the grid voltage at the PCC as show in Fig.

9(a). One phase stays at 1 p.u. during the dip the other phases drop to 0.75 p.u. The injected voltage by the DVR and restored load voltage as shown in fig 9(b, c). When positive sequence of the grid voltage drops


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C. Mitigation of balanced and unbalanced voltage swells From the control point of view, the DVR should handle voltage swells in the same way it

handles voltage sags. In either case, the reference of the injected voltage the stepped and actual voltage has to track its reference. But it is a different perspective from the energy handle capability. In the case of voltage sags, the DVR delivers an active power to the load but in the case of voltage swells, the DVR may absorb the power from the grid. Since normally, the DVR is operated at the standby and the required energy is available in its energy storage, the power absorption may be dangerous unless the dissipative element is installed and the power dissipation is controlled.

The control of power dissipation in this context is referred to as the online over voltage protection. Therefore, the capability of the DVR to cope with voltage swells is strongly related to the online over voltage protection. However in some cases the voltage swells, may not causes an over voltage at the dc link. The voltage swell characteristics and the loading conditions are the main issues that determine the energy transfer of status from the grid to the DVR. Two cases of measured voltage swells are presented here. The 10% balanced voltage swell as programmed and measured at the PCC. The grid voltage as show in Fig. 10(a) where the swell of 10% has made for 5 fundamental voltage as shown in Fig 10(b-c). From Fig. 9(a-c) it is noted that the DVR as successfully kept at 1 p.u. load voltage.

Figure 10. a) Grid Voltage; b) Injected Voltage; Figure 11. a) Grid Voltage; b) Injected Voltage; c) Load Voltage c) Load Voltage


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Fig.(11) shows the case of unbalanced swell where the voltages of the supply programmed to have 10% voltage swells. The grid voltage is depicted as Fig 11a where the swell of 10% is made for 5 fundamental cycles. Injected voltage and the load voltage as shown in fig 10(b-c). From Fig 11(a-c) it is noted that the DVR has successfully kept the load voltage at 1 p.u.

Although, the swell is unbalanced the DVR can keep the load voltage balanced. During experiments, VSI was fed by a separate dc source and thus the dc voltage was fixed at 480V (0.8 p.u.) swells not influenced dc voltage.

VI. CONCLUSION

The modeling and simulation of a DVR using MATLAB/SIMULINK has been presented. From the Simulation analysis, the DVR based Synchronous Reference Frame theory (SRF) is able to detect different types of power quality problems without an error and injects the appropriate voltage component to correct immediately any abnormality in the terminal voltage to keep the load voltage balanced and constant at the nominal value. Simulation and experimental results show that, the proposed DVR successfully protects the most critical load against balanced and unbalanced voltage sags and swell. Moreover it has been found that DVR is capable of pro-viding a self-support to its dc bus by taking active power from the ac line.

REFERENCE

[1] Power Quality Problems and New Solutions by A. de Almeida, L. Moreira. J. Delgado [2] Math H.J. Bollen, Understanding power quality problems: voltage sags and interruptions,

IEEE Press, New York, 2000 [3] Performance of DVR under different voltage sag and swell conditions by T. Devaraju, V. C.

Reddy and M. Vijaya Kumar [4] Voltage Quality Improvement Using DVR by Chellali BENACHAIBA, Bra-him FERDI [5] Adeline-Based Control of Capacitor Supported DVR for Distribution System Bhim Singh*,

P. Jayaprakash, and D. P. Kothari [6] Control of three level inverter based DVR by 1S.LEELA, S.DASH [7] Rating and Design Issues of DVR Injection Transformer by Sasitharan S., Mahesh K. Mishra,

Member, IEEE, B. Kalyan Kumar, and Jayashankar V.,member, IEEE [8] Particle Swarm Optimization based Dynamic Voltage Restorer [9] Power Quality Enhancement Using Custom Power Devices by A. Ghosh and G. Ledwich.

2002. Kluwer Academic Publishers. [10] FACTS controllers in power transmission and distribution by K. R. Padiyar. [11] Modeling and simulation for voltage sags/swells mitigation using DVR by Rosli omar,

Nasrudin Abd Rahim, Marizan Sulaiman. [12] A. Ziane-Khodja, M. Adli, S. Bacha ,Y. Zebboudj and A. Khireddine, Control of Voltage

Sensitive Load using a Dynamic Voltage Restorer Commanded in Current, International Journal of Electrical Engineering & Technology (IJEET), Volume 3, Issue 1, 2012, pp. 167 - 179, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

[13] Neha Kaushik, Power Quality, Its Problem and Power Quality Monitoring, International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 1, 2013, pp. 46 - 57, ISSN Print : 0976-6545, ISSN Online: 0976-6553.

[14] M. Vishnu Vardhan, N.M.G.Kumar and Dr. P. Sangameswararaju, Unified Power Quality Conditioner for Compensating Power Quality Problem: Adaptive Neuro-Fuzzy Interference System (ANFIS), International Journal of Electrical Engineering & Technology (IJEET), Volume 4, Issue 4, 2013, pp. 74 - 92, ISSN Print : 0976-6545, ISSN Online: 0976-6553.


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BIOGRAPHIES

B.KARTHICK was born in 8th September 1988. He received his B.Tech Degree in Electrical & Electronics Engineering in 2010 from Pondicherry University, INDIA and M.Tech in 2012 from Pondicherry Engineering College, Pondicherry University, INDIA with the specialization in Electrical Drives & Control (EDC). Currently he is working as an Assistant Professor, in Electrical & Electronics Engineering Department (EEE), in Alpha College of Engineering & Technology (ACET), Pondicherry University, INDIA. His area of interests includes Power System, Power Quality and Power Electronics.

S.KALAIVANAN This author born in Pondicherry, INDIA on 16th February, 1980. He received graduation in 2004 from Pondicherry Engineering College, Pondicherry University, INDIA and M.E from Mailam Engineering College, Anna University, Chennai, Tamil Nadu, INDIA with the specialization in Power Electronics and Drives (PED) in the year 2012. He is having more than 6 years in Industrial Experience. His area of interests includes Power System, Power Quality and Power Electronics. Currently he is working as an Assistant Professor, in Electrical & Electronics Engineering department (EEE), Alpha College of Engineering & Technology (ACET), Pondicherry, Pondicherry University, INDIA.

N.AMARABALAN was born in 17th April 1981. He received his B.Tech Degree in Electrical & Electronics Engineering in 2004 from Pondicherry Engineering College, Pondicherry University, INDIA and M.E from Mailam Engineering College, Anna University, Chennai, Tamil Nadu, INDIA with the specialization in Power Electronics and Drives (PED) in the year 2009. His area of interests includes Power System and Power Quality. Currently he is working as an Assistant Professor, in Electrical & Electronics Engineering department (EEE), Manakula Vinayagar Institute of Technology (MIT), Pondicherry, Pondicherry University, INDIA.

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Srf Theory in Dvr