IJIRAE:: Voltage Sag Mitigation in Real Time Using Booster Transformer

International Journal of Innovative Research in Advanced Engineering (IJIRAE) ISSN: 2349-2163 Issue 6, Volume 2 (June 2015) www.ijirae.com

________________________________________________________________________________________________________ 2014-15, IJIRAE- All Rights Reserved Page - 164

Voltage Sag Mitigation in Real Time Using Booster Transformer

Ms. Pallavi V. Pullawar*1, Dr. Sudhir R. Paraskar, Prof. Mr. Saurabh S. Jadhao 1, PG Student, Electrical Engg. Department

2 Professor & Head of Electrical Engg. Department 3 Assistant Professor of Electrical Engg. Department

1,2,3, Shri Sant Gajanan College of Engg.,Shegaon

Abstract This paper proposes a technique of power-quality mitigation for power system disturbances using Booster Transformer. Mitigation of voltage sag is carried out using dedicated experimental set up in laboratory with booster transformer. The result of booster transformer is presented in this work. The switching events like change of transformer taps, starting of induction motor are used to create the voltage sag. These events are controlled through FPGA based algorithm. Fuzzy logic controller is designed to achieve the firing angles which maintain voltage profile.

Keywords Booster Transformer, Voltage sag, Fuzzy logic, Thyristor, Firing angle, FPGA

I. INTRODUCTION

Voltage sags are regarded as one of the most harmful power quality (PQ) disturbances due to their costly impact on industrial processes. Mainly voltage sags in power systems are due to increased use of microelectronic control devices in modern industry [6]. Significant efforts have been taken to reduce the number of sags on the system and to mitigate the effects of sags in order to minimize the high associated costs of equipment misoperation [9]

This paper introduces the booster transformer for mitigation of voltage sag. The main objective is to mitigate the voltage sags in real time with the use of booster transformer. From booster transformer, the Comprehensive results are presented to assess the performance of each device to mitigate the voltage sag.

This paper is organized as follows: Section I consist of introduction, section II outline the background, section III describes mitigation methodology of Voltage Sag using Booster Transformer, section IV describes the experimental setup, section V shows the graphical results for various cases considered separately and section VI draws the conclusions.

II. BACKGROUND

A] Voltage Sag Classification

The voltage sag as defined by IEEE, Standard 1159-1995, and IEEE Recommended Practice for Monitoring Electric Power Quality is a decrease in root mean square (RMS) voltage at the power frequency for durations from 0.5cycles to 1 minute. The magnitude of voltage sag lies between the 90% to 10% of nominal voltage.

Momentary voltage sag is defined as the decrease in RMS voltage at a power frequency for duration from 0.5 cycles to 3 seconds. Temporary voltage sag is defined as the decrease in RMS voltage at a power frequency for duration from 3 seconds to 1 minute.

Fig. 1. Voltage Magnitude Events as used in IEEE Std. 1159-1995

B] Voltage Sag Characteristics 1. Magnitude The magnitude of the voltage sag can be determined in a number of ways. When the sag magnitude needs to be quantified in a number, one common practice is to characterize the sag through the remaining voltage during the sag then given as percentage of the nominal voltage. Thus, a 70% of sag in a 230 volt system means that, the voltage dropped to 161V. This method of sag characterizing is recommended in number of IEEE standards (493-1998, 1159-1995, and 1346-1998).



2. Duration Sag Duration is defined as the number of cycle during which the RMS voltage is below a given threshold value. Typical value of the threshold is around 90%. The start point of voltage sag is the instant at which the voltage falls below the 90% of nominal voltage and the end point of the voltage sag is the instant at which voltage rises above the 90% of the nominal voltage. The sag duration is the time between the start point and the end point. 3. Phase Angle Jump

The displacement in time of during-event voltage waveform is relative to the pre-event voltage waveform. A positive phase-angle shift indicates that, the phase angle of during-event voltage leads the pre-event voltage and in negative phase-angle shift the phase angle of during-event voltage lags the pre-event voltage.

III. MITIGATION METHODOLOGY

This paper intends to investigate mitigation technique that is suitable for the voltage sag. Voltage sag has been the focus of considerable research in recent years. It can cause expensive downtime. The mitigation techniques that will be studied is the booster transformer.

C. Booster Transformer The secondary of booster transformer is connected in series with the lines and booster transformer primary is supplied from

secondary of the regulating transformer fitted with on load tap changing gear. The output (secondary) winding of regulating transformer is so connected to the primary of booster transformer that the voltage injected in line, VB is in phase with supply voltage Vs., as shown in the Fig. 2. By changing the tapping on the regulating transformer, magnitude of VB can be changed and thus feeder voltage VF can be regulated. The advantage of above system are that the regulating equipment is independent of main transformer so that a failure in transformer will not through the later out of service for any length of time; and that it is much cheaper method when there is no main transformer at the point where regulation is desired. When the regulation is required at a point where a main transformer is to be placed, this system is costlier, requires more floor space, and increases the losses.

Fig. 2. Booster Transformer

D. Fuzzy Logic Control Schemes In fuzzy logic, basic control is determined by a set of linguistic rules which are determined by the system. The fuzzy logic

control is being proposed for controlling the voltage sag. Fuzzy logic controller is designed to achieve the firing angles for SVC such that it maintains voltage profile. In the decision-making process, there is rule base that linking between input and output signal. When the error voltage (Vpu Vbase pu) is small, firing angle is large.

IV. EXPERIMENTAL SETUP The online performance of various mitigation techniques for voltage sag is studied in the laboratories through tailor made

experimentation setup.

Fig. 3. Practical Experimental setup



Desired voltage signals are captured through data acquisition card and processed through mitigation. This section describes the hardware used during the experimentation.

Fig. 3 shows the experimental setup used for mitigation of voltage sag event in the laboratory. The main components of

the system are- 1. Single phase transformer - 2 KVA, 230/230 V, having taps at every 10 volts on primary as well as secondary side 2. A single phase induction motor of 2hp is used as source of voltage sag 3. A step down transformer of 230/6V is used as potential transformer to provide the signal of desired magnitude for the

measurement purpose. 4. Advantech data acquisition card (PCLD-8710) was used for collection of voltage signal samples. This system having 12

bit resolution and can provide the throughput rate up to 100 kS/s. Total 16 analog and digital channels are provided for the external interface This is PCI slot based data acquisition system can be fitted into PC directly.

5. FPGA based firing angle controller. The desired firing angle is input to the FPGA from the CPU and the desired firing angle pulse is developed from using the angle measurement. Same BASYS-2 board is used for this purpose.

6. FPGA based firing angle controller. The desired firing angle is input to the FPGA from the CPU and the desired firing angle pulse is developed from using the angle measurement. Same BASYS-2 board is used for this purpose.

7. FPGA based The Digilant BASYS2 board is used to host this controlling logic. 8. The miscellaneous devices like solid state relay, zero crossing detector and gain control circuit, digital isolators

V. RESULTS AND DISCUSSIONS

This section describes the result obtained from mitigation of voltage sag. Switching instants is defined as instant through which a voltage signal is passing. The voltage sag signal was captured at 198V tapping. The sag is created on the tailor made system using-

a) Starting of induction motor b) Tap changing

Mitigation of sag is carried out using the booster transformer. The booster transformer is shown in Fig. 4. The single

phase 120 VA, 230/12 volt transformer is used as a booster transformer.

Fig. 4. Booster transformer

Mitigation of voltage sag using Booster Transformer: The performance of booster transformer is checked in real time by creating the sag using induction motor start and

changing the tap position. Fig A of each fig shows the voltage signal (p.u.) captured during the starting of induction motor and by tap changing transformer (With/without compensation in case of induction motor and With/without compensation in case of tap changing of transformer). The sag flag denotes the detection of voltage sag in Fig B and Fig C shows the changes in the firing angle signal created by the fuzzy controller. As soon as the voltage sag detects, firing angle is changed as per sag and try to regulate voltage sag upto 1pu.

The booster transformer is used for reduction in this experiment. It is observed that depth and duration of sag improves to 17% and 62.69 % respectively. The depth and duration recorded for various switching instants are given table 1.

Fig. 5[B], it is clear that, the duration of sag for without compensation technique is 72.59 cycles and reduces to 9.9 cycles when compensated through booster transformer. Fig. 5[C] shows that the changes in the firing angle created by the fuzzy controller.

Expanded view of Fig. 5 is shown in the Fig. 6, due to induction motor starting and Fig. 7,due to tap changing transformer . In this figure it is found that the duration of sag reduces considerably. The performance of this topology during various switching instants is given in the table 1.



Fig. 5. Online mitigation of voltage sag due to induction motor starting using booster transformer topology (INSTANCE 2)

Table 1: Performance of Booster Transformer Topology at random instant in case of Induction Motor starting and Tap Changing Transformer

VARIOUS

INSTANCE

DEPTH OF SAG (Lowest value of voltage after disturbance in pu) DURATION OF SAG (cycles)

WITHOUT COMPENSATION

WITH COMPENSATION

WITHOUT COMPENSATION

WITH COMPENSATION

Case of Induction Motor starting

INSTANCE 1 0.7023 0.7056 127.82 30

INSTANCE 2 0.7073 0.7056 72.59 9.9

INSTANCE 3 0.7192 0.7305 13.96 12

Case of Tap Changing Transformer

INSTANCE 1 0.844509278 0.910271795 102.1 1

INSTANCE 2 0.8704114299 0.928343299 66.7 0.8

INSTANCE 3 0.87796571 0.935829825 93.9 0.8

It can be observed that from table1, in both cases of voltage sag due to induction motor starting and tap changing

transformer, depth of voltage sag is lower in with compensation technique than in without compensation technique. This shows that by using booster transformer, sag is mitigated.

Fig. 6. Expanded view of voltage sag due to induction motor starting graph plotted with verses without compensation technique using booster transformer

topology (INSTANCE 2)


________________________________________________________________________________________________________ 2014-15, IJIRAE- All Rights Reserved Page -168

Fig. 7. Expanded view of voltage sag due to tap changing starting graph plotted with verses without compensation technique using booster transformer topology

(INSTANCE 2) The comparative results by considering first 20 samples for mitigation of sag created due to induction start and

transformer tap changing for any random instants are given in table 2.

Table 2: Tabular calculation of expanded view of different topologies in case of induction motor and tap changing transformer

MITIGATION METHODS

WITHOUT COMPENSATION (pu)

WITH COMPENSATION (pu)

SAG MITIGATION (pu)

SAG MITIGATION (%)

BOOSTER TRANSFORMER TOPOLOGY

Case of Induction Motor starting

0.79256 0.758015 0.74309

0.84786 0.781645 0.849255

0.0553 0.02363 0.106165

6.5223 3.0231 12.5009

Case of Tap Changing Transformer

0.844509278 0.870411429 0.877965714

0.910271795 0.928343299 0.935829825

0.065762517 0.05793187 0.057864111

7.224 6.240 6.183

From table 2, it can be seen that (by taking average of first 20 samples of sag in both with and without compensation technique), by using booster transformer sag is mitigated.

VI. CONCLUSION In this paper, online mitigation of voltage sag is carried out using Booster Transformer and results are obtained through

dedicated experimental test- bench in the laboratory. The paper contributes in the following aspects towards the objective, Mitigation of voltage sag using Booster Transformer topology: From the results obtained for the case of voltage sag created due starting of induction motor, twelve percent mitigation is possible, where as for the same event seven percent mitigation is possible with tap changing transformer topology.

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[3] Nagendrababu Vasa, sreekanth G, Narender Reddy Narra, Dr. Srujana A, Series Compensation Technique for Voltage Sag Mitigation, IOSR Journal of Engineering (IOSRJEN) ISSN: 2250-3021 Volume 2, Issue 8 (August 2012), PP 14-2

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IJIRAE:: Voltage Sag Mitigation in Real Time Using Booster Transformer

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