Mitigation of Voltage Sag/Swell with CSI-IDVR - IEEE Xplore

2015 International Conference on Recent Developments in Control, Automation and Power Engineering (RDCAPE)

Mitigation of Voltage Sag/Swell with CSI-IDVR Ramchandra Nittala

Alivelu M. Parimi

K. Uma Rao

Research Scholar, Dept. of EEE BITS Pilani Hyderabad Campus Hyderabad, TS, India [email protected]

Assistant Professor, Dept. of EEE, BITS Pilani Hyderabad Campus Hyderabad,TS, India [email protected]

Professor, Dept. of EEE, R.V. College of Engineering Bangalore, Karnataka, India [email protected]

devices detect the level of voltage drop in the system and restore by injecting the required voltage into the system. The main disadvantages of these devices are requirement of large DC link to mitigate large voltage sags and also the devices are restricted to be connected to only one feeder.

Abstract— The distribution system contains various types of loads. These loads can be broadly classified as sensitive loads and non-sensitive loads. The sensitive loads are easily affected with power quality problems. Hence, power quality improvement is major concern in the distributed system. The power quality problems that are faced in the distribution side are voltage sag/swell, harmonics, flickering, distortions etc. One of the major power quality issue that is considered in this paper is voltage sag/swell. To handle these power quality issue Flexible AC Transmission Systems (FACTS) devices are considered. One of the FACTS device at the distribution side called Interline Dynamic Voltage Restorer (IDVR) is proposed to mitigate the voltage sag/swell. The building blocks of IDVR used for injecting the voltage can either be voltage source inverter (VSI) or current source inverter (CSI). In this paper, IDVR with two CSI as its building blocks is considered to mitigate voltage sag/swell. PV source is used at the DC link of IDVR to provide the necessary energy to mitigate sag/swell. The simulations are performed in MATLAB environment.

The solution to the drawbacks of DVR and UPQC is extended version of DVR called Interline Dynamic Voltage Restorer (IDVR). IDVR consists of two DVRs connected with a common DC link where each DVR is connected in series with different distribution lines [3-4]. When one DVR of IDVR mitigates voltage sag in one power line it consumes the energy from the DC link, while other DVR replenishes required energy at DC link through the second distribution power line. Therefore, IDVR is able to compensate even at longer voltage sags due the availability of amount of energy from DC link. Each DVR of the IDVR consists of current source inverter (CSI) as its building blocks, a proportional integral (PI) controller as its control system and a filter. The purpose of PI controller is to compare the actual or load voltage component with set or reference voltage component. The error between actual and reference voltage is given as input to PWM generator which generates gate pulses to inverter switches of CSI. The inverter injects voltage to the line through injection transformer. Due to the power electronic loads at the distribution side sometimes harmonics are also produced along with voltage sag. To eliminate the harmonics, an inductor is used as the filter which is connected along with building block of IDVR. The inductor value is designed based on the magnitude of highest order harmonics caused in the line.

Keywords— Interline Dynamic Voltage Restorer (IDVR), PV source, Super Conducting Magnetic Storage System (SMES), Current Source Inverter (CSI), Power Quality.

I. INTRODUCTION The numbers of loads on the distribution side are gradually becoming more sensitive. These sensitive loads are easily affected due to various disturbances in the system which is known as power quality issues. Therefore the power quality is a major concern at the distribution side to ensure the safety of sensitive loads. These sensitive loads have less tolerance towards these power quality issues. One of the major power quality concerns is voltage sag/swell. Voltage sag is defined as the reduction in RMS voltage from 10 to 90 percent of the nominal value for a period of half cycle to one minute. Similarly a voltage swell is raise in the voltage from 10 to 90 percent of the nominal value for the same duration. There are various traditional methods like capacitor banks, synchronous condenser etc., to mitigate the voltage sag/swell. The drawback of capacitor banks is limited range of protection and also the life span is less. Devices like synchronous condensers will produce lot of noise in the operation [1]. The alternative solution to these traditional devices is Flexible AC Transmission Systems (FACTS) devices. FACTS devices are static power electronic devices which are used to eliminate the power quality issues. The FACTS devices used at the distribution side are Dynamic Voltage Restorer (DVR) and Unified Power Quality Conditioner (UPQC). Both of the

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CSI are chosen as the building block of IDVR instead of VSI because CSI have some advantages over VSI like directly controlling the output current of the inverter and high reliability. The dc link current will be ripple free due to the presence of inductor at the DC link etc. In the research on IDVR with CSI, a superconducting magnetic storage system (SMES) is used in the DC link of IDVR to provide the required energy when the IDVR is mitigating the voltage sag or swell [5]. But this SMES coil should be large enough if the sags are occurring simultaneously in both the feeders at the same time and also when the depth of voltage sag is more. In addition to this, the economic cost will be more by the usage of the large SMES coil. An alternative solution to this is using the PV source with small size charged SMES coil. Whenever IDVR mitigates the voltage sag/swell in the system, the energy to mitigate the voltage sag/swell is taken from the PV source.

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Hence the SMES coil which is connected to the PV source has it energy maintained at the constant level even at the time of mitigation of voltage sag/swell. If in any voltage sag conditions the PV source is not able to give its maximum energy then the deficient energy which PV is unable to give can be utilized from the charged SMES coil. The simulations are performed using MATLAB software. The following sections discuss about the IDVR model, CSI and the simulation results. II. INTERLINE DYNAMIC VOLTAGE RESTORER The schematic diagram of IDVR is shown in Fig. 1. A two line IDVR is considered in this paper. The IDVR consists of two DVRs connected to two different feeders. PI controller along with CSI and filter is present inside the building blocks of IDVR. In Fig.1 Vs1 and Vs2 are source voltages while Vb1 and Vb2 are voltages of bus 1 (B1) and bus 2 (B2) at the distribution side where IDVR is connected. ZL1 and ZL2 are impedances of feeder 1 and feeder 2 respectively. Load 1 and load 2 are connected to each of the buses as shown with VL1,IL1 and VL2,IL2 as the load voltages and currents. Both these loads are assumed to be sensitive and they need protection from voltage sag and harmonics. Vinj1 and Vinj2 are voltages injected by DVR1 and DVR2 respectively during the voltage sag conditions. The building blocks of IDVR, CSI is discussed in the following section.

Fig. 2. Schematic diagram of CSI

A PV source is used at the DC link of IDVR to provide necessary energy while IDVR is mitigating the voltage sag. The next section covers total simulation results of IDVR system. IV. SIMULATION RESULTS The total system of IDVR with PV source connected to two feeders simulated in MATLAB is shown below in Fig. 3.

Fig. 3. Schematic diagram of total system simulated in MATLAB Fig. 1. Schematic layout of Interline Dynamic Voltage Restorer

In Fig. 3, feeder 1 and feeder 2 are at different voltage levels. The PV source is connected to SMES and the total set up gives the required energy at the DC link energy. Whenever IDVR mitigates the voltage sag/swell in the system, the energy to mitigate the voltage sag/swell is taken from the PV source. Hence the SMES coil which is connected to the PV source has it energy maintained at the constant level even at the time of mitigation of voltage sag/swell. If any voltage sag conditions the PV source in not able to give its maximum energy then the deficient energy which PV is unable to give can be utilized from the charged SMES coil. The parameters of the system shown in Fig. 3 is given in the below Table I. In general there are several conditions where voltage sag is produced. Here voltage sag/swell is created in four different conditions which is listed below

III. CURRENT SOURCE INVERTER CSI are the type of inverters fed with a current source with high impedance. The source at the AC side of the inverter may be a current source or inductor in series with a DC source [7]. As the input voltage is constant in VSI, in CSI the input current is constant but adjustable. The amplitude of the output current is independent of load. The waveform and magnitude of the output voltage depends on nature of load. Fig. 2 represents the three phase CSI. In Fig. 2, S1 to S6 represents the switches (MOSFET or IGBT) and D1 to D6 are the diodes connected in series to the switches to block the reversal of current. The advantages of CSI are its excellent current control capability, easy protection from short circuit or overcurrent, and output current is ripple free [8], though it is costlier than VSI.

A. Using sudden start of induction motor at feeder 1. B. With a LG fault at feeder 1. C. A multiple sag and swell at feeder 1.

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D. A simultaneous voltage sags in both feeders.

500 400 300

PARAMETERS OF THE TWO LINE IDVR SYSTEM

Parameter

Feeder 1

Load Voltage at Feeder 1 (in V)

TABLE I.

Feeder 2

Supply voltage (Vs)

415V

230V

Load Voltage ( Vl)

415V

230V

Load Resistance (Rl)

40

40

Load inductance (Ll)

95.5mH

95.5mH

Transformer resistance (Rt)

3

1

Transformer inductance (Lt)

10mH

6mH

Inductance at the DC link PV source at the DC link


0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

B. Voltage sag/swell with A LG fault at feeder 1 There are faults like LG (line to ground), LLG (double line to ground) and LLL ( triple line) which occur at the distribution end. Among these faults the frequently occurring fault is LG fault. The presence of LG fault makes the voltage zero in the particular phase, but this fault creates voltage sag or voltage swell in other two phases. A LG fault assumed near the load end of R phase in Fig. 3 and load voltage in this condition is shown Fig. 6.

300 200 100 0

Fig. 6 is load voltage without IDVR. As observed the LG fault in the R phase completely collapses the voltage in that phase and in turn it creates a voltage swell in the Y and B phase respectively. This sudden increase in the voltage may cause tripping of the circuit breakers in Y and B phases and if circuit breakers are not responding, then this leads damage of load. If the IDVR is placed at the feeder 1 the voltage sag/swell is compensated and the result is shown in Fig. 7.

−100 −200 −300 −400 0.25

0.1

Hence the load voltage is maintained at the rated value even when the induction motor is consuming reactive power. The required energy to mitigate the sag is given by PV source connected at the DC link. Due to the presence of CSI any harmonic content can be clearly eliminated. It is observed that the presence of IDVR clearly mitigated the voltage sag even at the sudden start of induction motor. The next case explains about the occurrence of voltage sag with LG fault.

400

0.2 Time (in S)

0.05

Fig. 5. Load voltage with induction motor as load with IDVR

1H 1.8KW

0.15

−200

−500 0

500

0.1

0 −100

−400

A. Using sudden start of induction motor at feeder 1 A 5hp, 415V, 1500 RPM induction motor is connected to the feeder 1. The total simulation is for 0.4 seconds. In this interval the induction motor is switched ON at 0.3 seconds. The load voltage at feeder 1 when the induction motor is ON is shown below

0.05

100

−300

In all the four conditions the performance of IDVR is observed. The simulation results are presented in each case

−500 0

200

0.3

0.35

0.4

Fig. 4. Load voltage with induction motor as load without IDVR

Fig. 4 is the load voltage at feeder 1 without IDVR connected to the feeder 1. When induction motor is ON at 0.3S, it consumes large reactive power at initial startup which leads to voltage sag for a few cycles which can be observed in Fig. 4. After few cycles of voltage sag, the voltage is being regained to the normal value. Apart from the induction motor, the sag created may cause any damage to the sensitive loads connected to that feeder. To mitigate this sag and protect the sensitive loads connected to the feeder 1, an IDVR is placed at the feeder 1 shown in Fig. 3.

800

Load Voltage at Feeder 1(in V)

600 400 200 0 −200 −400 −600

Fig. 5 represents the load voltage profile of feeder 1 with IDVR when the induction motor is ON. When the induction motor is ON at 0.3S, the PI controller in IDVR building block compares dropped voltage with reference voltage and allows PWM generator to generate the pulses to the CSI to inject the required voltage.

−800 0

0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

Fig. 6. Load voltage at feeder 1 with LG fault without IDVR

35

0.35

0.4

500

500

400

400

300

300




200 100 0 −100 −200 −300

100 0 −100 −200 −300

−400 −500 0

200

−400 0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

−500 0

0.4

0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

Fig. 7. Load Voltage at feeder 1 with LG fault with IDVR

Fig. 9. Load voltage with multiple sags with IDVR

In Fig. 7 the load voltage is maintained in the normal manner during the fault condition with the presence of IDVR. The voltage is totally compensated in R phase with IDVR where the LG fault has occurred. In addition the voltage swells in Y and B phases are eliminated with IDVR. Hence this case also proves that the IDVR eliminates voltage swell in the distribution side. The previous two cases show the performance of IDVR with single sag condition while the next case represents the performance of IDVR with multiple sags.

Hence, the IDVR is performing well at the multiple voltage sag conditions. In all the three cases the sag conditions are applied to one feeder considering one feeder to be healthy and the mitigation process is observed in one feeder. In the next case the performance of IDVR is studied when both feeders get affected by voltage sag at the time. D. Simultaneous Voltage Sag in Both Feeders The two feeders of the system shown in Fig. 3 are being affected with voltage sag at the same time.

C. Multiple sags at feeder 1 The load voltage with multiple sags in the absence of IDVR is shown in Fig. 8

500 400


300

600


400

200

200 100 0 −100 −200 −300

0 −400 −500 0

−200

0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

0.3

0.35

0.4

−400

−600 0

Fig. 10. Load Voltage at feeder 1 without IDVR 0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

400

Fig. 8. Load voltage with multiple sags without IDVR Load Voltage at Feeder 2 (in V)

300

As shown in Fig. 8, the voltage sags are created between the intervals 0.15S to 0.2S and 0.25 to 0.3S respectively. The load voltage is distorted between the intervals of 0.2S to 0.25S and 0.3S to 0.35S are post sag conditions. The reason for the distortion is due to harmonics at the load side. This will be a major problem mainly to the sensitive loads. Due to this there may be loss of data and cause sensitive load to get damaged. To eliminate these problems, IDVR is connected to feeder and the performance of IDVR with these multiple sags is observed in Fig. 9.

200 100 0 −100 −200 −300 −400 0

0.05

0.1

0.15

0.2 Time (in S)

0.25

Fig. 11. Load Voltage at feeder 2 without IDVR

Fig. 10 and Fig. 11 represent the load voltages of feeder 1 and feeder 2 in the sag condition. The voltage is decreased from 415V to 380V in the first feeder and from 230V to 180V in second feeder respectively during the voltage sag. Due to the presence of harmonics the voltage is observed to be distorted in second feeder after the voltage sag.

In Fig. 9 the sags are completely mitigated by the IDVR connected to the feeder 1. The IDVR senses the decrease in voltage and injects the required voltage and maintains the required voltage. The PV source supplies the required energy to mitigate the sags. With presence of CSI and also with help of inductor which is used as the filter in IDVR, the total harmonics are also mitigated.

With the presence of IDVR connected to both the feeders, the voltage sag in both feeders is eliminated and the voltage is

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conditions and at each condition the performance of IDVR in mitigating the voltage sag is investigated. The total simulation is performed using MATLAB Simulink software. The results that are obtained in all the different cases indicate that the IDVR is mitigating the voltage sag/swell irrespective of the severity of voltage sag. In the presence of PV source at the DC link, the IDVR restores the DC link energy and thus mitigate the voltage sags which are occurring simultaneously in both feeders. The harmonics which are observed in some cases along with voltage sag are eliminated by the filter which is connected to the IDVR. Therefore a conclusion can be drawn that the IDVR along with PV source at its DC link is able to mitigate the voltage sag/swell in all the proposed conditions.

maintained at the normal value which can be observed in Fig. 12 and Fig. 13. 500 400


300 200 100 0 −100 −200 −300 −400 −500 0

0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

Fig. 12. Load voltage at feeder 1 with IDVR

REFERENCES V.K. Mehta, Principles of Power System. S.Chand Publications, 4th revised edition 2008 [2] A. Ghosh and G. Ledwich, “Compensation of distribution system voltage using DVR,” IEEE Trans. Power Del., vol. 17, no. 4, pp. 1030– 1036, Oct. 2002 [3] S.S. Choi, B. H. Li, and D. M. Vilathgamuwa, “Dynamic voltage restoration with minimum energy injection,” IEEE Trans. Power Syst., vol. 15, no. 1, pp. 51–57, Feb. 2000 [4] L.Gyugyi, K. K. Sen, and C. D. Schaude, “The interline power flow controller concept: A new approach to power flow management in transmission system,” IEEE Trans. Power Del., vol. 14, no. 3, pp. 1115– 1123, Jul. 1999. [5] J.Singaravelan, Abdul, K.Suresh, “Simulation of Interline Dynamic Voltage Restorer”, Internation Journal of Engineering Science and Technology (IJEST), Vol 3 No.8 Aug.2011 [6] Vilathgamuwa, D.M.; Wijekoon, H.M.; Choi, S.S.; , "A Novel Technique to Compensate Voltage Sags in Multiline Distribution System;The Interline Dynamic Voltage Restorer," Industrial Electronics, dddIEEE Transactions on , vol.53, no.5, pp.1603-1611, Oct. 2006 [7] M.S.Jamil Asghar, Power Electronics. PHP Publications, 3rd edition. [8] Kinhal, V.G.; Agarwal, P.; Gupta, H.O., "Performance Investigation of Neural-Network-Based Unified Power-Quality Conditioner," Power Delivery, IEEE Transactions on , vol.26, no.1, pp.431,437, Jan. 2011. [9] Ho, C.N.-m.; Chung, H.S.-H., "Implementation and Performance Evaluation of a Fast Dynamic Control Scheme for Capacitor-Supported Interline DVR," Power Electronics, IEEE Transactions on , vol.25, no.8, pp.1975,1988, Aug. 2010. [10] Sweeka Meshram; Omprakash Sahu, “Application of ANN in economic generation scheduling in IEEE 6 bus system” IJEST vol no.3, March 2011. [1]

250 200


150 100 50 0 −50 −100 −150 −200 −250 0

0.05

0.1

0.15

0.2 Time (in S)

0.25

0.3

0.35

0.4

Fig. 13. Load voltage at feeder 2 with IDVR

The PV source is able to give the required energy for IDVR to mitigate both sags simultaneously at the two feeders. The limitation to this case is the availability of PV source and charge in SMES coil. If the PV source and charged SMES coil are unable to give the required energy during the mitigation of voltage sag in both feeders, then the compensation process is not possible. Therefore it is observed, IDVR with PV source at its DC link has mitigated the voltage sag/swell in different scenarios. This IDVR topology also solves the problem of simultaneous sag because of the PV source presented at the DC link. Due to the presence of CSI as the building blocks of IDVR and additionally with the presence of filter, the harmonics which are observed in some cases along with voltage sag are also eliminated. The PV source is able to give the required energy to IDVR for mitigating the sags. Hence, the sensitive loads at the distribution end can be protected with IDVR irrespective of severity of the voltage sag/swell V. CONCLUSION In this paper the performance of IDVR with CSI at its building block along with a PV source at its DC link is proposed. The purpose of IDVR is to mitigate the voltage sag/swell which occurs at the load end. To provide necessary energy for IDVR to mitigate voltage sag/swell, a PV source is provided at the DC link of IDVR. The PV source is connected along with charged SMES coil, which acts as back up for PV source. The charged SMES coil provides necessary energy for IDVR to mitigate voltage sag if the PV source fails to operate. In this paper, the voltage sag/swell is created in four different

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