Performance Analysis of Dynamic Voltage Restorer (DVR) using Sinusoidal and Space vector PWM Techniques

Performance Analysis of Dynamic Voltage Restorer (DVR) using Sinusoidal and Space vector PWM Techniques

B.Rajani,Phd.Research Scholar, Dr.P.Sangameswara Raju,

S.V.University.College of Engineerin Professor SV University, Dept.of Electrical Engg Tirupathi

Tirupathi ,A.P,INDIA

Abstract

This paper presents the researches on the Dynamic Voltage Restorer (DVR) application for power quality Improvement in electrical distribution network . Due to increasing complexity in the power system, voltage sags are now becoming one of the most significant power quality problem. voltag e sags is a short reduction voltage from nominal voltage, occurs in a short time . temporary Voltage sag is bound to have a greater impact on the industrial customers. If the voltage sags exceed two to three cycles, then manufacturing systems mak ing use of sensitive electronics equipments are lik ely to be effected leading to major problems .It ultimately leads to wastage of resources as well as financial losses. The increasing competition in the mark et and the declining profits has made it pertinent for the industries to realize the significance of high power quality. This is possible only by ensuring that uninterrupted flow of power is maintained at proper voltage levels .This can solve by using custom power devices such as DVR, Distribution static compensator (D-STATCOM) and un interruptible power supply (UPS). The Dynamic voltage Restorer appears to be an especially good solution in the current scenario. Two pulse width modulation based control techniques, viz. sinusoidal PWM and space vector PWM ,are presented for controlling the electronic valves in two level voltage source converter(VSC) used in the DVR system .The Simulation study of space vector PWM technique for DVR is compared with sinusoidal PWM. The performance of DVR as well as the adopted control algorithm is illustrated by simulation. The results obtained in PSCAD/EMTDC.

Keywords-Custom power , DVR, Power quality,Voltage sag, SPWM, SVPWM, PSCAD/ EMTDC.

1. Introduction

   Power quality proble ms to sensitive loads is one of the major concerns in electricity industry today

   .this is due to the advent of a large numbers of

   sophisticated electrical and e lectronics equipment, such as computers, programmable logic controllers, variable speed drives, and so forth. The use of these equipments very often requires power supplies with very high quality. voltage sag, which is a mo mentary decrease in rms voltage magnitude in the range of 0.1 to 0.9 per unit (p.u) [1], is considered as the most serious problem of power quality. It is often caused by faults in power systems or by starting of large induction motors. It occurs more frequently than any other power quality phenomenon does. Therefore , the loss resulted due to voltage sag problem for a customer at the load-end is huge .DVR and D-STATCOM are recently being used as the active solution for voltage sag mitigation. In this paper, DVR is proposed device to mit igate the voltage sag. Dynamic voltage restorer is a series compensator which is able to protect a sensitive load fro m the distortion in the supply side during fault or over loaded in power system. The basic principle of a series compensator is simple, by inserting a voltage of required magnitude and frequency ;the series compensator can restore the load side voltage to the desired amplitude and waveform even when the source voltage is unbalanced or distorted [2]

   .sinusoidal PWM and space vector PWM control

   techniques are used for controlling the DVR. space vector PWM can utilize the better dc voltage and generates the fewer harmonic in inverter output voltage . simulat ion results are compared for both the SPWM and SVPWM.

2. Mathematical model for voltage sag calculation

   Consider Figure 1 a norma l condition (no fault), current through load A and load B is equal

   (balance load).When theres fault on feeder 1, a high current (short circuit current) will flow to feeder 1. So, based on Kirchhoffs La w, currents flow to feeder 2 will be reduced. Consequently, voltage will also drop in feeder 2.This voltage drop will be defined as voltage sag. Assume

   Load A = ZLOAD_A ,Load B = ZLOAD_B

   Source reactance = XS , Feeder 1 Reactance = X1, Feeder 2 Reactance = X2 Cu rrent fro m supply

   the system voltage (Vth) drops, the DVR injects a series voltage VDVR through the injection transformer so that the desired load voltage magnitude VL can be ma intained.

   Vth Rth jXth VDVR VL

   source = I,Current in feeder 1 = I1,Current in feeder

   DVR

   P +jQ

   VSC

   2 = I2,Fro m fig 1, by using KCL , L L

   Energy

   I = I1 + I2 (1)

   In normal condition (without fault in system)

   I V2

   X1 ZLOAD_ A

   V2

   X2 ZLOAD_ B

   (2)

   Fig 2: sche matic di agram of a DVR

   Consider the schematic diagra m shown in the above fig 2. By using KVL

   When a fault occurs (see Fig 1) in feeder

   1, because of short circuit, a high current will flow through feeder1 as well as source current I [3]. During this time , voltage in feeder 2 is decreased due to increasing of voltage drop across source reactance XS, this causes voltage sag.

   V2 X1

   Vth – Zth IL + VD VR = VL (5)

   =>VD VR + Vth = VL + Zth IL (6)

   Therefore, the series in jected voltage of the DVR can be written as

   VDVR = VL + Zth IL – Vth (7)

   Here,

   Vth = system supply voltage (Thevenin voltage) VL = load bus voltage

   Zth = system impedance (Thevenin impedance) IL = load current

   A P JQ *

   X I Fault

   IL =

   L L

   VL

   (8)

   p.u.

   V= 1 p.u

   S 1

   V=0

   I X2

   When VL is considered as a reference, equation (6) can be rewritten as

   VDVR =VL 0 0+ZthIL -Vth (9)

   Here , and are the angle of VDVR, Zth

   Fig .1: calcul ation for voltage sag

   and Vth respectively and is the load power factor angle with

   I V2 V2

   (3)

   =Tan-1(QL/PL). (10)

   The comple x power in jection by the DVR can be written as

   Here

   X1 X2

   ZLOAD_ B

   SDVR = VDVR I*L (11)

   V2=VS-IXS (4)

   VS = Source voltage (i.e ., V= 1 p.u)

   During the fault condition, voltage drop (IXS) increases and hence from equation (4), V2 decreases from its nominal va lue (i.e., V2 become as voltage sag).

3. Mathematical Model for voltage sag correction by Dynamic voltage restorer

   The schematic diagra m of a typical DVR is shown in Fig (2). The circuit left hand side of the DVR represents the Thevenin equivalent circuit of the system. The system impedance (Zth = Rth + jXth) depends on the fault leve l of the load bus. When

4. Test System Imple mented In PSCAD/EMTDC

Single line diagra m of the test system for DVR is shown in Figure-(4.1) and the test system emp loyed to carry out the various DVR simu lations presented in this section is shown in Figure-(4.2), which the same system is presented in [28]. Such system is co mposed by a 13 kV, 60 Hz generation system, feeding two transmission lines through a 3- winding transformer connected in Y/ / , 13/ 115/115 kV. Such transmission lines feed two distribution networks through two transformers connected in / Y, 115/ 11 kV

. Switching time duration at any sector:

The active and zero switching time for a particu lar sector are calculated by using the following formulae

T1 =

T2=

1. Ts .

   Vref

   Vref

   Vdc 3.Ts .

   Vdc

   . sin n

   3

   . sin n 1 3

   (12)

   (1 3)

   Fig 3: Singe line diagr am of the test system for DVR

   The DVR coupling transformer is connected in delta in the DVR side, with a leakage reactance of 10%. A unity transformer turns ratio was used, i.e., no booster capabilities e xist. Using

   T0 = Ts (T1 + T2) (14)

   Where, n=1 through 6, i.e sector 1 to 6 and

   0 600 .

   When T1+T2>Ts, the time T1 and T2 are simply rescaling as follo ws [9]

   the facilit ies available in PSCAD/ EMTDC, the DVR is simu lated to be in operation only for the duration of the fault, as it is expected to be the case in a practica l situation.

   Ta Ts T1

   Tb T1 T2 T2

   (15)

   1. DVR Control Strategy

      So that Ta+Tb=Ts and T0 = 0. According to this

      rescaling process, the converter can operate up to the modulation index 0.952.fro m the equations

      (12)-(15) duty cycles can be calculated by

      The main aim of the control system is to maintain the constant voltage magnitude at the point where a sensitive load is connected, under system

      disturbances [4].

      T1.or.Ta d Ts 1

      3.

      Vref

      Vdc

      . sin n

      3

      (16)

      2

      T2 .or.Tb d Ts

      3.

      Vref

      Vdc

      . sin n 1 3

      d0 1

      d1 d2

      (17)

      (18)

      Fig 4: sinusoi dal PWM based contr ol system for DVR

      Fig 5: space vec tor PWM-base d contr ol system for DVR

      These duty cycles are symmetrica lly distributed corresponding to each sector .By this symmetrica l distribution, only one phase is changing to on or off during the changing of adjacent state vector and hence swithing losses will be reduced

   2. Modeling of space vector PWM in PSCAD/EMTDC

      This section describes the modeling of space vector PWM in PSCAD/ EMTDC. To develop the model of SVPWM in PSCAD/ EMTDC, four user defined sub-systems ((a)-(d)) are c reated as shown in figure -(6.a) to figure-(6.d).

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      Fig 6: Test syste m i mple mente d i n PSCAD/ EMTDC.

      Fig 6a: Calcul ati on of sag magni tude and generation of reference signals .

      Fig 6 b: calcul ati on of reference vector magnitude and sector.

      Fig 6c: c alculation of turn-on ti me of two adjacent vectors and zer o vectors .

      Fig 6 d: calcul ati on of duty c ycles and generation of pulses to VSC.

      Sub-system (a): In this sub-system, at first angle delta, which is required to track the error to zero, and sag magnitude are determined for obtaining the reference signals.

      Sub-system (b): In this sub-system, the refe rence voltage (vector) magnitude (Vref) and phase ang le (Theta) are determined fro m the re ference three- phase voltage. To find out the sector, the range of calculated value of angle (Theta) must be between 0 and 2 .

      Sub-system (c): This sub-system is designed to calculate the turn-on time of t wo adjacent active vectors and the zero state vectors. Equation (15) is also imple mented in the simulat ion as shown in fig (6c).

      Sub-system (d): This sub-system is designed to calculate duty cycles and generate modulating signals. In this system carrier signals also generated and compared with modulating signals and generated pulses to DVR.

   3. Simulation results of DVR

      Fig.6 shows the test system,which is imp le mented in PSCAD/ EMTDC,used to carry out the various

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      DVR simulat ions presented in this section, which the same system is presented in[ 4]. Such system is composed by a 13 kV, 60 Hz generation system, represented by a thevenin equivalent ,feeding two transmission lines through a 3-winding transformer connected in Y/ / , 13/ 115/ 115 kV. Such transmission lines feed two distribution networks through two transforme rs connected in / Y, 115/11 kV.The DVR coupling transformer is connected in delta in the DVR side, with a leakage reactance of 10%. A unity transformer turns ratio was used, i.e., no booster capabilit ies exist. Here DVR is simu lated to be in operation only for the duration of the fault, as it is expected to be the case in a practical situation. In this section, the simu lation results of sinusoidal PWM and space vector PWM for a DVR are presented. Per unit voltage, per unit error, phase voltage and line – line voltage outputs of sinusoidal PWM and space vector PWM for different types of faults are shown in below figures. The simulat ion results are compared with Sinusoidal PWM. Simulat ion results are presented to demonstrate the validity of space vector modulation technique.

      Fig 7:Line to line voltage at sensiti ve load for three-phase short circuit without DVR

      Fig 8: Line to line vol tage at sensitive l oad for three-phase short circuit with DVR for SPWM

      Fig 9:Line to line voltage at sensiti ve load for three-phase short circuit with DVR for SVPWM

      Fig 10: phase voltages at sensitive l oad for three- phase short circuit without DVR

      Fig 11: phase voltages at sensitive l oad for three- phase short circuit with DVR for SPWM

      Fig 12: phase voltages at sensitive l oad for three- phase short circuit with DVR for SVPWM

      Fig 13: r ms per unit val ue at sensitive l oad for three-phase short circuit without DVR

      Fig 14: r ms per unit value at sensiti ve load for three-phase short circuit with DVR for SPWM

      Fig 15: r ms per unit value at sensiti ve load for three-phase short circuit with DVR for SVPWM

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      Fig 16:Line to line voltage at sensitive l oad for voltage s well wi thout DVR

      Fig 17: Line to line voltage at sensiti ve load for voltage s well with DVR for SPWM

      Fig 18: Line to line voltage at sensiti ve load for voltage s well with DVR for SVPWM

      Fig 19: phase voltages at sensitive l oad for voltage s well without DVR

      Fig 20: phase voltages at sensitive l oad for voltage s well with DVR for SPWM

      Fig 21: phase voltages at sensitive l oad for voltage s well wi th DVR for SVPWM

      Fig 22: r ms per unit val ues at sensitive l oad for voltage s well wi th out DVR

      Fig 23: r ms per unit val ues at sensitive l oad for voltage s well with DVR for SPWM

      Fig 24: r ms per unit val ues at sensitive l oad for voltage s well with DVR for SVPWM

   4. Conclusion

      The power quality problems such as voltage sags and swells are presented in this paper. An electromagnetic transient model of custom power equip ment, name ly DVR was presented and applied to the study of power quality. The highly developed graphic facilit ies available in PSCAD/ EMTDC were used to conduct all aspects of model imp le mentation and to carry out e xtensive simu lation studies.Two pulse width modulation – based control techniques, viz sinusoidal PWM and space vector PWM, have been imple mented for controlling the electronic valves in two level Vo ltage Source Converter (VSC) used in the DVR system. The control techniques were tested and the reliability and effectiveness of control schemes are shown in results. It is observed that space vector PWM utilized the better dc voltage and reduced the harmonic d istortion in line voltage as well as phase voltage when compared with sinusoidal PWM and it effective ly mit igated the voltage sags and swells. Co mpared with sinusoidal PWM, This space vector PWM technique can be applied to any Vo ltage Source Converter (VSC) based application as an aptitude for wide linear modulat ion range for output line-to-line voltages and phase voltages are the notable features of space vector modulation. The PWM control scheme controls the magnitude and the phase of the injected voltages, restoring the rms voltage, phase voltages and line voltage very effectively.

   5. References

1. IEEE recommended practice for evaluating electric power system compatibility with electronic process equipment ,IEEE standard 1346-1988.1988

2. A..Ghosh and G.Ledwich,Power quality Enhancement using Custom Power deviceskluwer Academic publishers 2002.

3. Shairul Wizmar Wahab and Alias M ohd Yusof, Voltage Sag nd M itigation Using Dynamic Voltage Restorer (DVR) System, electrika, vol.8, No.2, pp. 32-37, 2006.

4. O. Anaya-Lara, E.Acha, M odeling and analysis of custom power system by PSCAD/EMTDC, IEEE Trans., power delivery, vol.PWDR-17, pp266-272, 2002.

5. A. Hernandez, K.E. Chong, G. Gallegos, and

   E. Acha, The implementation of a solid state voltage source in PSCAD/EM TDC, IEEE power eng. Rev., p p. 61-62, dec. 1998.

6. I.T.Fernando, W.T. Kwasnicki, and A.M .Gole, M odeling of conventional and advanced static var compensators in electromagnetic transients simulation

   program[online].Available:http://www.ec.uma nitoba.ca / hvdc.

7. N.M ohan, T.M . Undeland, and W.P. Robbins,

   Power electronics: Converters, applications and design. New York: Wiley, 1995.

8. E.Acha, V.G. Agelidis, O. Anaya-Lara, and

   T.J. E.M iller, Electronic control in electrical power systems. London, U.K, Butter-Worth- Heinemann, 2001.

9. S.M .M uyeen, M ohammad Abdul M annan, M ohd.Hasan ali, Rion Takahashi, Toshiaki M urata and Junji Tamura, Simulation technique and Application of space vector PWM method in PSCAD/EM TDC, ICICT – 2007, 7-9 M arch 2007, Dhaka, Bangladesh.

Authors Biography

B.Rajani rece ived B.Tech degree in Electrical &Electronics Engineering fro m S.I.S.T.A.M college of Engineering, Srika kula m 2002 and M.E degree in Power Systems and Automation fro m Andhra university,Visakhapatnam in the

year 2008.she presently is working towa rds her Ph.D degree in S.V.Un iversity, Tirupathi. Her areas of interest are in power systems operation &control and power quality imp rovement.

Dr. P.Sang ameswar arar aju received Ph.D fro m

Sri Venkateswara Univerisity, Tirupathi, Andhra Pradesh. Presently he is working as professor in the department of Electrical & Electronics Engineering, S.V. University.

Tirupati, Andhra Pradesh .He has about 50 publications in National and International Journals and conferences to his credit.His areas of interest are in powe r system operation &control and stability.