VOLTAGE SAG AND ITS MITIGATIONB.E. SEMINAR REPORT

Submitted to North Maharashtra University, Jalgaon in Partial Fulfillment of the Requirements for the Degree of BACHELOR OF ENGINEERING in

Electrical Engineering

By

MAYUR DILIP DHANDE

(Examination Number )

GuideProf. D. P. YAVALKAR

DEPARTMENT OF ELECTRICAL ENGINEERING

GOVERNMENT COLLEGE OF ENGINEERING, JALGAON 425002

2015-16

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GOVERNMENT COLLEGE OF ENGINEERING, JALGAON 425002

Department of Electrical Engineering

CERTIFICATE

This is to certify that the seminar entitled, “VOLTAGE SAG AND ITS MITIGATION”,

which is being submitted here with for the award of B.E. is the result of the work completed

by MAYUR DILIP DHANDE under my supervision and guidance within the four walls of

the institute and the same has not been submitted elsewhere for the award of any degree.

(Prof. D. P. Yavalkar)Guide

(Prof. G. K. Andurkar)Head of Electrical Department

(Prof. Dr. R. P. Borkar)Principal

Examiner

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DECLARATION

I hereby declare that the seminar entitled, “VOLTAGE SAG AND ITS MITIGATION”

was carried out and written by me under the guidance of Prof. D. P. Yavalkar Asst. Professor,

Department of Electrical Engineering, Govt. College of Engineering, Jalgaon. This work has

not been previously formed the basis for the award of any degree or diploma or certificate nor

has been submitted elsewhere for the award of any degree or diploma.

Place: MAYUR DILIP DHANDEDate:

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ACKNOWLEDGEMENT

It has indeed been a great privilege for me to have Prof. D. P. Yavalkar, as my mentor for

this seminar. Her awe-inspiring personality, superb guidance and constant encouragement are

the motive force behind this seminar work. I take this opportunity to express my utmost

gratitude to her. I am also indebted to her for her timely and valuable advice.

I am highly grateful to Prof. G.K. Andurkar, Head of Department of Electrical Engineering,

for providing necessary facilities and encouraging me during the course of work. We also

thankful to respected Principal Dr. R.P.Borkar for allowing me to work on this seminar.

I am thankful to all technical and non-teaching staff of the Department of Electrical

Engineering for their constant assistance and co-operation.

MAYUR DILIP DHANDE

(B.E. Electrical)

ABSTRACT

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Modern industrial processes are based on a large amount of electronic devices such as programmable logic controllers and adjustable speed drives. Unfortunately, electronic devices are sensitive to disturbances, and thus, industrial loads become less tolerant to power quality problems such as voltage sags, voltage swells, and harmonics. Voltage sags are an important power quality problem for which the dynamic voltage restorer (DVR) is known as an effective device to mitigate them. The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags and swells. A control method is used in this paper that is Phase Locked Loop. A phase locked loop is used to keep the load voltage synchronized continuously and track the source voltage. It is shown that the proposed method improves the performance of the DVR.

Keywords: voltage sag, power quality, Dynamic voltage restorer.

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CONTENTS

Chapter No. Title Page No.Certificate I

Declaration IiAcknowledgment iii

Abstract ivList of Figure viList of Tables vii

1 Introduction 11.1 Voltage sag 11.2 Multi-phase sags and single phase sags 11.3 Organization of Report 22 Literature Review 33 Voltage sag 4

3.1 Definition of voltage sag 43.2 Voltage sag 43.3 Standards associated with voltage sags 53.4 Sources of sags and short interruptions 73.5 General causes and effects of voltage sags 84 Voltage sag analysis 11

4.1 Characteristics of voltage sag 114.2 Effects of voltage dip on commercial installations 124.3 Measurement & characterization of voltage sags 134.4 Impact & cost of voltage sags 145 Voltage sag mitigation 15

5.1 Dynamic voltage restorer 155.2 D-STATCOM 185.3 Auto-transformer 215.4 Solid state transfer switch 246 Conclusion 27

References 28

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LIST OF FIGURES

Sr. No. Title Page no.

3.1 Voltage sag 4

3.2 Immunity curve for semiconductor

manufacturing equipment

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3.3 Revised CMEBA curve 7

3.4 Voltage sag due to cleared line-ground

fault

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3.5 Voltage sag due to motor starting 9

3.6 Voltage sag due to transformer energizing 10

4.1 Example of voltage sag 13

5.1 Basic structure of DVR 15

5.2 DVR without internal storage 16

5.3 Equivalent circuit diagram of DVR 17

5.4 Schematic diagram of D-STATCOM 18

5.5 Inductive mode of operation 19

5.6 Capacitive mode of operation 19

5.7 Block diagram of control circuit of D-STATCOM

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5.8 Circuit diagram of Auto-transformer 21

5.9 Voltage sag mitigation scheme using Auto-transformer

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5.10 Single phase circuit diagram during voltage sag 23

5.11 Block diagram of control circuit 23

5.12 Schematic representation of SSTS 24

5.13 Solid state transfer switch system 25

5.14 Thyristors on the alternate supply are turned ON on a sensing a disturbance on the preferred

source.

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LIST OF TABLES

Figure no. Title Page no.

4.1 Classification of sag 11

5.1 Typical financial loss for voltage sags based on industry 14

5.2 Impact of voltage sag on industry 14

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1. INTRODUCTION

In the past, equipment used to control industrial process was mechanical in nature, which was rather tolerant of voltage disturbances. Nowadays, modern industrial equipment typically uses a large amount of electronic components, such as PLCs, adjustable speed drives and optical devices, which can be very sensitive to such voltage disturbances. Typical disturbances that cause problems for electronic equipment are voltage interruptions and sags, voltage fluctuations, capacitor switching transients, and harmonics [1].Voltage sags have been found especially troublesome because they are randomevents lasting only a few cycles. The process equipment may not keep continuing its normal operation during these sags for many cycles and will trip or shut down, although the supply voltage is totally recovered a few cycles after the sag occurrence. Therefore, from the point of view of industrial customers, voltage sags and momentary interruptions might produce the same effect to their processes. Besides, the occurrence of voltage sags at a certain point of adistribution system is much more frequent than the occurrence of momentary interruptions since the voltage sag can still be observed very far away from the fault location. The term “area of vulnerability” is usually employed to survey the voltage magnitude on a geographical map or one-line diagram due to the fault occurrence on a certain line [2].Many surveys regarding power quality problems related to voltage sags and momentary interruptions have been presented in literature. They are usually useful for determining the solution required for tackling the existing problem at a certain industrial facility, with special emphasis on the rating and ride-through capability of proposed mitigation equipment. These surveys agree usually in at least one point: faults on overhead transmission lines contribute tothe great majority of voltage sags verified in the distribution network and these sags are the most critical power quality problem to industrial customers. The subject of voltage sags and interruptions has even been the main topic covered by a recently published book.

1.1 VOLTAGE SAG

Voltage sag is a short-duration reduction in rms voltage caused by faults on the power system and the starting of large loads, such as motors. It is said that a voltage sag has taken place in an electrical network point when the voltage in one or more phases falls suddenly beneath an established limit (generally a 90% of the normal voltage), and recovers after a short period of time (usually between 10 ms and some seconds).

1.2 MULTI-PHASE SAGS AND SINGLE PHASE SAGSThey are three types of sags based on the number of phases are as follows :

SINGLE PHASE SAGS: The most common voltage sags, over 70%, are single phase events which are typically due to a phase to ground fault occurring somewhere on the system. This phase to ground fault appears as single phase voltage sag on other feeders from the same substation. Typical causes are lightning strikes, tree branches, animal contact etc. It is not uncommon to see single phase voltage sags up to 30% of nominal voltage or even lower in industrial plants.

PHASE TO PHASE SAGS: Two Phase, phase to phase sags may be caused by tree branches, adverse weather, animals or vehicle collision with utility poles. The two phase voltage sag will typically appear on other feeders from the same substation.

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THREE PHASE SAGS: Symmetrical three phase sags account for less than 20% of all sag events and are caused either by switching or tripping of a three phase circuit breaker, switch or recloser which will create three phase voltage sag on other lines fed from the same substation. Three phase sags will also be caused by starting large motors but this type of event typically causes voltage sags to approximately 80% of nominal voltage and is usually confined to an industrial plant or its immediate neighbour.

1.3 ORGANIZATION OF REPORT

Chapter 1 gives idea about voltage sag and its types. Chapter 2 focuses on various power quality problems associated with voltage sag and need for mitigation of voltage sag. Chapter 3 discuss about voltage sag, standards related with voltage sag and sources of sag. Chapter 4 deals with voltage sag analysis it includes characteristics of voltage sag, effects, measurement and characterisation and also cost impact of voltage sag. Chapter 5 includes various mitigation techniques which should be used for efficient and reliable mitigation of voltage sag. Chapter 6 includes advantages of using FACTS devices for mitigation of voltage sag. Chapter 7 includes the references.

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2. LITERATURE REVIEW

The quality of power delivered to the end user is very important as the performance of the consumer’s equipment is heavily dependent on it. But the power quality is affected by various factors like voltage and frequency variations, presence of harmonics, faults in the power network etc. Among them the voltage variations (sag) is one of the most frequently occurring problem. There are many methods to mitigate the voltage sag and among them the best way is to connect a FACT device at the point of interest. The well-known devices like DSTATCOM, DVR, and UPQC are used for this purpose. The world’s earliest DVR ‘installation was done at Duke Power Company’s 12.47kV substation in Anderson, SouthCarolina in 1996. After that immediately then ABB, Siemens and other companies have also focused and worked hard for several years to achieve the design patterns and finally developed their own patterns of the products to ensure the quality of voltage-sensitive load. Therefore, there is lot of research in this field. A survey on the structure and control strategies of the DVR is presented in. It discusses how a DVR can be controlled to mitigate voltage sag. It also presents the other advantages of connecting a DVR to the power network. The design of a DVR for voltage sag mitigation application is presented in. It also presents the response of the DVR when sag is created.The other FACT device that is used for voltage sag application is D-STATCOM. The basic structure of D-STATCOM is explained in. This paper discusses the working principle of the device. The different modes of operation of a D-STATCOM are clearly presented in. The control strategy to control the device is discussed in. A comparison between the DVR and D-STATCOM in mitigating voltage sag is given in. It states that the power injection required by D-STATCOM to mitigate a given voltage sag is more compared to that of DVR. But the D-STATCOM is capable of mitigating higher voltage sags without injecting active power. However, both these devices include switching losses. To overcome the drawback of these devices, a new technique to mitigate voltage sag is proposed in. It presents a PWM switched auto transformer to mitigate the voltage sag. As this topology uses only one power electronic switch, the switching losses are reduced greatly and the efficiency of the system is increased. This paper presents control strategy to control the IGBT switch such that the auto transformer is responded intact with the voltage imbalance. The proposed control strategy is validated with simulation results.

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3.VOLTAGE SAG

3.1 DEFINITION OF VOLTAGE SAGS

The definition of voltage sags is often set based on two parameters, magnitude or depth and duration. However, these parameters are interpreted differently by various sources. Other important parameters that describe voltage sags are:i. the point-on-wave where the voltage sags occurs, andii.how the phase angle changes during the voltage sag. A phase angle jump during a fault is due to the change of the X/R-ratio. The phase angle jump is a problem especially for power electronics using phase or zero-crossing switching.

3.2 VOLTAGE SAG

A Voltage Sag as defined by IEEE Standard 1159 - 1995, IEEE Recommended Practice for Monitoring Electric Power Quality, is a decrease in RMS voltage at the power frequency for durations from 0.5 cycles to 1 minute, reported as the remaining voltage. The measurement of a Voltage Sag is stated as a percentage of the nominal voltage; it is a measurement of the remaining voltage and is stated as a sag to a percentage value. Thus a Voltage Sag to 60% is equivalent to 60% of nominal voltage, or 264 Volts for a nominal 440 Volt system. Voltage sag for a system is shown in fig.

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Fig.3.1 Voltage sag

3.3 STANDARDS ASSOCIATED WITH VOLTAGE SAGS

Standards associated with voltage sags are intended to be used as reference documents describing single components and systems in a power system. Both the manufacturers and the buyers use these standards to meet better power quality requirements. Manufactures develop products meeting the requirements of a standard, and buyers demand from the manufactures that the product comply with the standard.The most common standards dealing with power quality are the ones issued by IEEE, IEC, CBEMA, and SEMI. A brief description of each of the standards is provided in next subtopic.

IEEE StandardThe Technical Committees of the IEEE societies and the Standards Coordinating Committees of IEEE Standards Board develop IEEE standards. The IEEE standards associated with voltage sags are given below. IEEE 446-1995, “IEEE recommended practice for emergency and standby power systems for industrial and commercial applications range of sensibility loads” The standard discusses the effect of voltage sags on sensitive equipment, motor starting, etc. It shows principles and examples on how systems shall be designed to avoid voltage sags and other power quality problems when backup system operates.

IEEE 493-1990, “Recommended practice for the design of reliable industrial and commercial power systems” The standard proposes different techniques to predict voltage sag characteristics, magnitude, duration and frequency. There are mainly three areas of interest for voltage sags. The different areas can be summarized as follows [3]:

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i. Calculating voltage sag magnitude by calculating voltage drop at critical load with knowledge of the network impedance, fault impedance and location of fault.ii. By studying protection equipment and fault clearing time it is possible to estimate the duration of the voltage sag.iii. Based on reliable data for the neighbourhood and knowledge of the system parameters an estimation of frequency of occurrence can be made.

SEMI

The SEMI International Standards Program is a service offered by Semiconductor Equipment and Materials International (SEMI). Its purpose is to provide the semiconductor and flat panel display industries with standards and recommendations to improve productivity and business. SEMI standards are written documents in the form of specifications, guides, test methods, terminology, and practices. The standards are voluntary technical agreements between equipment manufacturer and end-user. The standards ensure compatibility and interoperability of goods and services. Considering voltage sags, two standards address the problem for the equipment.

SEMI F47-0200, “Specification for semiconductor processing equipment voltage sag immunity”The standard addresses specifications for semiconductor processing equipment voltage sag immunity. It only specifies voltage sags with duration from 50ms up to 1s. It is also limited to phase-to-phase and phase-to-neutral voltage incidents, and presents a voltage-duration graph, shown in Fig.

Fig.3.2 Immunity curve for semiconductor manufacturing equipment accordingto SEMI F47

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This standard defines a test methodology used to determine the susceptibility of semiconductor processing equipment and how to qualify it against the specifications. It further describes test apparatus, test set-up, test procedure to determine the susceptibility of semiconductor processing equipment, and finally how to report and interpret the results.

CBEMA (ITI) Curve

Information Technology Industry (ITI, formally known as the Computer & Business Equipment Manufacturers Association, CBEMA) is an organization with members in the IT industry. Within the organization, the Technical Committee 3 (TC3) has published the “ITI (CBEMA) curve application note”. The note describes an AC input voltage that typically can be tolerated by most information technology equipment. The note is not intended to be a design specification (although it is often used by many designers for that purpose), but a description of behaviour for most IT equipment. The curve assumes a nominal voltage of 120VAC RMS and 60Hz and is intended for single phase information technology equipment [IEEE 1100 – 1999].The voltage-time curve in Fig. describes the border of an area. Above the border the equipment shall work properly and below it shall shutdown in a controlled way.

Fig.3.3 Revised CBEMA curve, ITIC curve, 1996

3.4 SOURCES OF SAGS AND SHORT INTERRUPTIONS

Power systems have non-zero impedances, so every increase in current causes a corresponding reduction in voltage. Usually, these reductions are small enough that the voltage remains within normal tolerances. But when there is a large increase in current, or when the system impedance is high, the voltage can drop significantly. So conceptually, there are two sources of voltage sags:1) Large increases in current.2) Increases in system impedance.

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As a practical matter, most voltage sags are caused by increases in current. It is possible to think of the power system as a tree, with the customer sensitive load connected to one of the twigs. Any voltage sag on the trunk of the tree, or on a branch leading out to the customer twig, will cause voltage sag at its load. But a short-circuit out on a distant branch can cause the trunk voltage to diminish, so even faults in a distant part of the tree can cause a sag at customer load.The cause of most voltage sags is a short-circuit fault occurring either within the industrial facility under consideration or on the utility system. The magnitude of the voltage sag is mainly determined by the impedance between the faulted bus and the load, and by the methodof connection of the transformer windings. The voltage sag lasts only as long as it takes the protective device to clear the overcurrent condition (typically up to 10 cycles)[5], therefore the duration of the sag is determined by the fault-clearing time of that protectionsystem adopted. Moreover, if automatic reclosure is used by the utility, the voltage sag condition can occur repeatedly in the case of a permanent fault. Finally, depending on its magnitude and duration, the sag can cause an equipment trip, thus becoming a power qualityproblem[4].

The most common causes of facility-sourced voltage sags are:1) Starting a large load, such as a motor or resistive heater.2) Loose or defective wiring, such as insufficiently tightened box screws on power conductors.3) Faults or short circuits elsewhere in the facility (trees, animals, adverse weather such as wind or lightning).Voltage sags can also originate on the utility's electric power system. The most common types of utility-sourced voltage sags are:1) Faults on distant circuits, which cause a corresponding reduction in voltage on yourcircuit.2) Voltage regulator failures (far less common).

3.5 GENERAL CAUSES AND EFFECTS OF VOLTAGE SAGS

There are various causes of voltage sags in a power system. Voltage sags can caused by faults (more than 70% are weather related such as lightning) on the transmission or distribution system or by switching of loads with large amounts of initial starting or inrush current such as motors, transformers, and large dc power supply.

VOLTAGE SAGS DUE TO FAULTS

Voltage sags due to faults can be critical to the operation of a power plant, and hence, are of major concern. Depending on the nature of the fault such as symmetrical or unsymmetrical, the magnitudes of voltage sags can be equal in each phase or unequal respectively. For a fault in the transmission system, customers do not experience interruption, since transmission systems are looped or networked. Fig. shows voltage sag on all three phases due to a cleared line-ground fault.

Factors affecting the sag magnitude due to faults at a certain point in the system are:i. Dstance to the faultii. Fault impedanceiii. Type of fault

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iv. Pre-sag voltage levelv. System configurationa. System impedanceb. Transformer connections

The type of protective device used determines sag duration.

Figure 3.4Voltage sag due to a cleared line-ground fault

VOLTAGE SAGS DUE TO MOTOR STARTING

Since induction motors are balanced 3 phase loads, voltage sags due to their starting are symmetrical. Each phase draws approximately the same in-rush current. The magnitude of voltage sag depends on:i. Characteristics of the induction motorii. Strength of the system at the point where motor is connected.Figure 3.5 represents the shape of the voltage sag on the three phases (A, B, and C) due to voltage sags.

Figure 3.5Voltage sag due to motor starting

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VOLTAGE SAGS DUE TO TRANSFORMER ENERGIZING

The causes for voltage sags due to transformer energizing are:i. Normal system operation, which includes manual energizing of a transformer.ii. Reclosing actions

Figure 3.6 Voltage sag due to transformer energizing

The voltage sags are unsymmetrical in nature, often depicted as a sudden drop in system voltage followed by a slow recovery. The main reason for transformer energizing is the over-fluxing of the transformer core which leads to saturation. Sometimes, for long duration voltage sags, more transformers are driven into saturation. This is called Sympathetic Interaction. Fig. show the voltage sag due to transformer energizing.

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4. VOLTAGE SAG ANALYSIS

According to standard IEEE 1346-1998, Voltage Sag is defined as-“A decrease in rms voltage or current at the power frequency for durations of 0.5 cycle to1 min.Typical values are 0.1 to 0.9 pu.”

4.1 CHARACTERISTICS OF VOLTAGE SAG:

The voltage sag is characterized by its magnitude, duration and phase angle jump. Each of them is explained below in detail.

4.1.1 MAGNITUDE OF SAG: A sag magnitude is defined as the minimum voltage remaining during the event. The magnitude can be defined in a number of ways. The most common approach is to use the rms voltage. The other alternatives are to use fundamental rms voltage or peak voltage. Thus, sag is considered as the residual or remaining voltage during the event. In case of three-phase system where the dip in voltage is not same in all phases, the phase with lowest dip is used to characterize sag.The magnitude of voltage sag at a certain point depend on-

Type of fault Fault impedance System Configuration Distance of the fault from the point of consideration

4.1.2 DURATION OF SAG:

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Table-I Classification of Sag

The duration of sag is the time for which the voltage is below a threshold value. It is determined by the fault clearing time. In a three phase system all the three rms voltages should be considered to calculate the duration of the sag. A sag starts when one of the phase rms voltage is less than the threshold and continues until all the three phase voltages are recovered above the threshold value. Based on the duration of sag, the voltage sags are classified as shown in Table-I.

4.1.3 PHASE-ANGLE JUMP:

The short circuits in power system not only cause a dip in voltage, but also change the phase angle of the system. The change of phase angle is called as “Phase-Angle Jump”. It causes the shift in zero crossing of the instantaneous voltage. This phenomenon affects the power electronic converters which use phase angle information for their firing.

4.1.4 POINT-ON-WAVE:

To perfectly characterize sag, the point-on-wave where the sag starts and where it ends should be found with high precession. The point-on-wave is nothing but the phase angle at which the sag occurs. These values are generally expressed in radians or degrees.

4.2 EFFECTS OF VOLTAGE DIP ON COMMERCIAL INSTALLATIONS

CHILLER SYSTEM

It is a known fact that chillers plants and its related equipment are sensitive to voltage dips. Relays, control contactors and the electronic/computerized controls may drop off during dips, hence causing the trip. The impact to building occupiers is small; as these effects are likely to be transparent to them; or at most minor discomfort due to a slightly raised ambient temperature.

LIGHTING CIRCUITS

During a voltage dip, flickering of the lights can be observed. In general, problems only arise when High Intensity Discharge (HID) lamps are used. These types of lightings are known to be sensitive to voltage dips and temporarily extinguish after a voltage dip. Re-ignition time usually takes up to 10 minutes.

ESCALATORS

During a voltage dip, the control contactors and PLC of the escalator may drop off. AnotherCause could be the activation of the phase monitoring relay, which is meant to activate upon loss of mains. Governed by the Singapore Standard CP15:2004, escalators are designed to be

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brought to rest in a largely uniform deceleration in the event of loss of mains (thus not a safety concern). However public image may be affected.

GENERAL CIRCUITS

General circuits served by miniature circuit breakers (MCB) have also been reported to have tripped after a voltage dip. Immediately after a voltage dip, some MCBs may tripped due to large in-rush current drawn by motors, switch mode power supply, circuits containingControl transformer, etc. In the market there also exists Residual Current Device (RCD) that is equipped with under voltage detection. This type of RCD will trip automatically when the mains voltage falls below a specified level. This prevents voltage sensitive equipment from being operated at low voltages and avoids damage to such equipment.

4.3 MEASUREMENT & CHARACTERISATION OF VOLTAGE SAGS

Voltage sags are measured using specialised power quality monitoring instrumentation. The instrumentation must be configured with a sag threshold voltage. That is, a voltage level that will trigger a sag capture when the rms voltage falls below it. Voltage sags are characterised by reporting the duration for which the voltage variation persisted below the sag threshold combined with the maximum reduction in rms voltage, also known as depth. The depth is reported as the retained voltage. Figure 4.1 shows a graphical representation of a voltage sag including the sag threshold and the parameters (duration, retained voltage) used to report the sag. Note the use of a hysteresis value in Figure 4.1; this value is used to prevent voltage levels which are close to the sag threshold crossing the threshold multiple times and triggering multiple sags which are basically due to the same event. The theory of measurement, characterisation and reporting of voltage sags is considerably more complex than the basic overview given in this technical report.

Figure 4.1: Example of a Voltage Sag

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4.4 IMPACT & COST OF VOLTAGE SAGS

There is a strong argument that can be made to claim that voltage sags are the most costly of all power quality disturbances. While perhaps not as costly as interruptions, voltage sags are much more prevalent and in some cases may have the same impact as a supply interruption. Relatively shallow voltage sags can lead to the disruption of manufacturing processes due to equipment being unable to operate correctly at the reduced voltage levels. Industrial equipment such as variable speed drives and some control systems are particularly sensitive to voltage sags. In many manufacturing processes, loss of only a few vital pieces of equipment may lead to a full shut down of production leading to significant financial losses. For some processes which are thermally sensitive a significant loss of material as well as the time taken to clean up and restart the process must also be considered. There have been many studies which aim to quantify the cost of voltage sags. The results of these studies rangefrom relatively modest cost associated with voltage sags through to very high costs generally at high technology industrial plants (such as semi-conductor manufacturing). Table 1 below reproduced from [5] show the costs associated with voltage sags from a range of industries.

Industry Typical Financial Loss per Event ( )

Industry Financial loss per event (in €)Semiconductor Production 3 800 000

Financial Trading 6 000 000 per hour

Computer Centre 750 000

Telecommunications 30 000 per minute

Steel Works 350 000

Glass Industry 250 000

Table 1: Typical Financial Loss for Voltage Sags based on Industry [5]

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Table 2 reproduced from shows another summary of the impact of voltage sags on various industries from the US. The data presented agrees reasonably well with the data given in Table 1. It is stated in [6] that the cost to industry in the United States due to voltage disturbances is over $20 billion annually.Industry Loss per Voltage Sag ($US)

Industry Loss per voltage sagPaper Manufacturing 30 000

Chemical Industry 50 000Automobile Industry 75 000

Equipment Manufacturing 100 000Credit Card Processing 250 000Semiconductor Industry 2 500 000

Table 2: Impact of Voltage Sags on Industry [6]

5. VOLTAGE SAG MITIGATION

The voltage sag is a major problem that the power system network is facing nowadays. This is a severe problem and affects the functioning of the equipment. Therefore, this problem should be mitigated in order to maintain the efficiency of the power network. The use of custom power devices solves this problem. This chapter presents the basic structure and working principle of different devices like DVR, D-STATCOM, Auto Transformer used to mitigate the voltage sag.

5.1 DYNAMIC VOLTAGE RESTORER (DVR)

A Dynamic Voltage Restorer is a power electronic converter based gadget intended to ensure the discriminating burdens from all supply-side unsettling influences other than deficiencies [1]. It is connected in arrangement with the distribution feeder for the most part at the purpose of regular coupling.

5.1.1 BASIC STRUCTURE:The DVR is a series connected power electronic device used to inject voltage of required magnitude and frequency. The basic structure of a DVR is shown in Fig. 5.1. It contains the following components-

1) Voltage Source Inverter (VSI)2) DC storage unit3) Filter circuit4) Series Transformer

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Fig.3.1 Basic Structure of DVR

VOLTAGE SOURCE INVERTER (VSI): The VSI consists of solid state switches like IGBT’s or GTO’s used to convert the DC input to AC. It is used to inject the AC voltage to compensate the decrease in the supply voltage. The switches of the VSI are operated based on the pulse width modulation (PWM) technique to generate the voltage of required magnitude and frequency.

DC STORAGE UNIT: The storage unit may consist of batteries, capacitors, flywheel, or super magnetic energy storage (SMES). For DVR with internal storage capacity, energy is taken from the faulted grid supply during the sag. This configuration is shown in Fig. 5.2. Here a rectifier is used to convert the AC voltage from the grid to DC voltage required by the VSI.

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Fig.3.2 DVR without Internal Storage

FILTER CIRCUIT: An LC filter is connected at the output of the VSI to filter the harmonics that are present in the output voltage of VSI. It also reduces the dv/dt effect on the windings of the transformer [2].SERIES TRANSFORMER: A series transformer is used to connect the DVR with the distribution feeder. In case of three phase system, three single phase transformers are used to connect the DVR with the power network.

5.1.2 OPERATING PRINCIPLE:The main operation of the DVR is to inject voltage of required

magnitude and frequency when desired by the power system network. During the normal operation, the DVR will be in stand-by mode. During the disturbances in the system, the nominal or rated voltage is compared with the voltage variation and the DVR injects the difference voltage that is required by the load. The equivalent circuit of a DVR connected to the power network is shown in Fig. 5.3. Here Vsis the supply voltage, Vinjis the voltage injected by the DVR and VL is the load voltage.

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Fig .5.3 Equivalent Circuit Diagram of DVR

5.1.3 CONTROL STRATEGY:The principle contemplations for the control of a DVR are-

identification of the begin and completion of the hang, voltage reference era, transient and unfaltering state control of the infused voltage and security of the system [6].Any control technique implemented to control the DVR should fulfil all the above aspects. The basic idea behind the control strategy is to find the amount by which the supply voltage is dropped. For this the three phase supply voltage is compared with the reference voltage Vref. If there is voltage sag (or any other voltage imbalance) then an error occurs. This error voltage is then sent to the PWM generator, which generates the firing pulses to the switches of the VSI such that required voltage is generated. The whole control strategy can be implemented in 2-ϕrotating (d-q) coordinate system.

5.1.4 APPLICATIONS OF DVR:

There are many applications of DVR in addition to mitigate voltage sag. They are

1) DVR can be used to compensate the load voltage harmonics and improves the

power quality of the system.2) DVR can be used under system frequency variations to provide the

real powerrequired by the load. This is done by connecting a uncontrolled rectifier at the input of the VSI.

3) DVR can also protect the system against voltage swell or any other voltage

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Imbalances that occur in the power system.

5.2 D-STATCOMA Distribution Static Compensator is in short known as D-STATCOM.

It is a power electronic converter based device used to protect the distribution bus from voltage unbalances. It is connected in shunt to the distribution bus generally at the PCC.

5.2.1 BASIC STRUCTURE:D-STATCOM is a shunt connected device designed to regulate the

voltage either by generating or absorbing the reactive power. The schematic diagram of a D-STATCOM is as shown in Fig. 5.4. It contains-

DC Capacitor Voltage Source Inverter (VSI) Coupling Transformer Reactor

.

Fig.5.4 Schematic Diagram of D-STATCOM

As in the case of DVR, the VSI generates voltage by taking the input from the charged capacitor. It uses PWM switching technique for this purpose. This voltage is delivered to the system through the reactance of the coupling transformer. The voltage difference across the reactor is used to produce the active and reactive power exchange between the STATCOM and the transmission network. This exchange is done much more rapidly

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than a synchronous condenser and improves the performance of the system.

5.2.2 OPERATING PRINCIPLE:

A D-STATCOM is capable of compensating either bus voltage or line current. It canoperate in two modes based on the parameter which it regulates. They are-

Voltage Mode Operation: In this mode, it can make the bus voltage to which it

is connected a sinusoid. This can be achieved irrespective of the unbalance ordistortion in the supply voltage.

Current Mode Operation: In this mode of operation, the D-STATCOM forces

the source current to be a balanced sinusoid irrespective of the load current harmonics.The basic operating principle of a D-STATCOM in voltage sag mitigation is to regulate the bus voltage by generating or absorbing the reactive power. Therefore, the DSTATCOM operates either as an inductor or as a capacitor based on the magnitude of the bus voltage.

Inductive Operation: If the bus voltage magnitude (VB) is more than the rated

voltage then the D-STATCOM acts as an inductor absorbing the reactive power from the system. The circuit and phasor diagram are shown in Fig.5.5.

Fig.5.5 Inductive Mode of Operation

Capacitive Operation: If the bus voltage magnitude (VB) is less than the rated voltage then the D-STATCOM acts as a capacitor generating the reactive power to the system. The circuit and phasor diagram of this mode of operation are shown in Fig. 5.6.

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5.6 Capacitive Mode of Operation

5.2.3 CONTROL STRATEGY:

The main aim of the control strategy implemented to control a D-STATCOM used for voltage mitigation is to control the amount of reactive power exchanged between the STATCOM and the supply bus. When the PCC voltage is less than the reference (rated) value then the D-ATACOM generates reactive power and when PCC voltage is more than the reference (rated) value then the D-ATACOM absorbs reactive power. To achieve the desired characteristics, the firing pulses to PWM VSI are controlled. The actual bus voltage is compared with the reference value and the error is passed through a PI controller. The controller generates a signal which is given as an input to the PWM generator. The generator finally generates triggering pulses such that the voltage imbalance is corrected. The block diagram of the control circuit is shown in Fig. 5.7

Fig.5.7 Block Diagram of the Control Circuit of D-STATCOM

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5.2.4 APPLICATIONS OF D-STATCOM:The applications of the D-STATCOM are-

1. Stabilize the voltage of the power grid2. Reduce the harmonics3. Increase the transmission capacity4. Reactive power compensation5. Power Factor correction

5.3 AUTO-TRANSFORMERAn auto transformer is a single winding transformer where there is

no isolation between the primary and secondary windings. This device requires less conductor material in its construction and is of less size and weight when compared to the normal two winding transformer. This device can be used in mitigating the voltage sag when controlled properly. The principle of operation and the control technique are explained below.

5.3.1 BASIC STRUCTURE:The basic structure of an auto transformer is shown in Fig. 5.8 In

this circuit configuration the secondary voltage is more than the primary voltage and the transformer operates as a step-up transformer. This configuration is used in voltage sag mitigation.

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Fig.5.8 Circuit Diagram of an Auto-Transformer

From the circuit diagram,VP is the primary voltageVL is the load voltageIS is the source currentIL is the load currentThe turns ratio N1:N2 is taken as unity and the relation between primary andSecondary voltages and currents is given by the equation.

VL/VP= IS/IL= (N1+N2)/N1

5.3.2 OPERATING PRINCIPLE:The auto transformer is controlled by a PWM operated power

electronic switch. The single-phase diagram of a power system network with a PWM switched auto transformer used for voltage sag mitigation is shown in Fig. 5.9.

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Fig.5.9 Voltage Sag Mitigation Scheme Using Auto Transformer

The circuit contains the following components-1. An IGBT Switch: This switch is operated based on the pulses

generated by the PWM generator and controls the auto transformer operation.

2. Auto-Transformer: It is used to boost the voltage so that the load voltage remains constant irrespective of the variations in the supply voltage. It is controlled by the IGBT switch.

3. Ripple Filter: The output voltage given by the auto-transformer contains harmonics along with the fundamental component. Thus, these harmonics should be filtered out to maintain the THD for the given system voltage at the load should be within the IEEE standard norms. Therefore, a ripple filter is used at the output of the auto-transformer.

Bypass Switch: There is a bypass switch made of SCR’s connected in antiparallel. This switch is used to bypass the auto-transformer during the normal operation. During voltage sag condition, this switch remains off and autotransformer operates.The single-phase circuit diagram during voltage sag condition is shown in Fig. 5.10.Here the bypass switch is off and the auto-transformer works based on the IGBT switchoperation to generate required voltage on the load side [7].

5.3.3 CONTROL STRATEGY:The main aim of the control strategy is to control the pulses

generated to the IGBT switch such that the auto-transformer generates desired voltage to mitigate the voltage sag. The RMS value of the load voltage is compared with a reference value (Vref). Under normal operating

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conditions there is no error and no pulses are generated to the IGBT switch and auto-transformer do not work. When there is voltage sag then an error occurs and based on the error value PWM generator generates pulses to the IGBT switch. Accordingly, the auto-transformer operates and the load voltage is maintained constant. The block diagram of the control Strategy is shown in Fig. 5.11

Fig.5.10 Single-phase Circuit Diagram during Voltage Sag

Fig.5.11 Block Diagram of Control Circuit

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The voltage error is passed through a PI controller and it generates a phase angle δ. With this phase angle a control voltage is generated [7] using sine wave generator by using equation

Vcontrol = Fa Sin (ωt+Ϩ)Where ma is the modulation indexThe magnitude of the control voltage is dependent on the phase angle δ. The phase angle is proportional to the degree of disturbance [7]. Here the voltage which has been generated called control voltage is compared with the triangular voltage Vtrifor the cause to generate the pulses which can be fed to the IGBT switch. In this way the auto transformer is controlled to mitigate the voltage sag.

5.3.4 ADVANTAGES:

The PWM switched auto-transformer is advantageous over the other devices inMitigating the voltage sag. The advantages are as follows-

1. Less cost2. Less number of switches required3. Reduced gate driver circuit size4. No energy storage device

5.4 SOLID STATE TRANSFER SWITCH (SSTS)

The SSTS can be used very effectively to protect sensitive loads against voltage sags, swells and other electrical disturbance [8]. The SSTS ensures continuous high quality power supply to sensitive loads by transferring, within a time scale of milliseconds, the load from a faulted bus to a healthy one. The basic configuration of this device consists of two three phase solid state switches, one for main feeder and one for the backup feeder. These switches have an arrangement of back-to-back connected thyristors, as illustrated in Figure 5.12

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Figure 5.12Schematic representations of the SSTS as a custom power device.

Each time a fault condition is detected in the main feeder, the control system swaps the firing signals to the thyristor in both switches, in example, Switch 1 in the main feeder is deactivated and Switch 2 in the backup feeder is activated. The control system measures the peak value of the voltage waveform at every half cycle and checks whether or not it is within a pre-specified range. If it is outside limits, an abnormal condition is detected and the firing signals of the thyristors are changed to transfer the load to the healthy feeder.

5.4.1 BASIC CONFIGURATION AND FUNCTION OF SSTSThe SSTS as shown in Figure 5.13 is a high speed, open transition switch which

enables the transfer of electrical loads from one ac power source to another within a few milliseconds.

Fig.5.13 Solid State Transfer Switch system

The open-transition property of the SSTS means that the switch break contact with one source before it makes contact with the other source. The advantage of this transfer scheme over the closed-transition mechanical switch is that the electrical sources are never cross-connected unintentionally. The cross connection of independent ac sources, with the alternate source switching on to a faulted system is discouraged by electric utilities. The solid state transfer switch consists of two three phase ac thyristors switches. The thyristor, operating in its two modes, forms the key component of the SSTS. In the ON-state mode, low impedance forward conduction of current takes place. In the OFF state mode, an open circuit with almost infinite impedance occurs in the thyristor. The basic ON-state and OFF-state properties of the thyristor are used to form an intelligent switch which can choose between two upstream power sources providing the better quality of supply available to the electrical load downstream. The basic configuration is based on anti-parallel thyristor group on preferred and alternate sides of the switch. A thyristor allows conduction only in forward direction. Figure 4.8 illustrate how the thyristors of transfer switch 1 can conduct either in the positive or the negative half cycle of the ac sinusoid and the supply path is indicated by the bold line.

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During normal operation, thyristors associated with the preferred source are in the ON-state normally closed (NC) position, while those associated with the alternate source are in the OFF-state normally open (NO) position. Current sensing circuits constantly monitor the states of the preferred and alternate sources and feed the information to the monitoring high speed controller. Upon detecting the loss of the preferred source or voltage that is not within the preset range, the controller blocks the firing impulse signals to the gate-driven thyristors of transfer switch 1 and instructs the thyristors of transfer switch 2 to turn ON with a fail-safeinterlocking mechanism.

Figure 5.14Thyristors on the alternate supply are turned ON on a sensing a disturbance on the preferred source.

The mechanical bypass equipment provides conventional transfer switch functionality when the SSTS is in a thermal overload condition or is out of service for testing or maintenance.

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6. CONCLUSIONSThe demand for electric power is increasing at an exponential rate and at the same time the quality of power delivered became the most prominent issue in the power sector. Thus, to maintain the quality of power the problems affecting the power quality should be treated efficiently. Among the different power quality problems, voltage sag is one of the major one affecting the performance of the end user appliances. In this report the methods to mitigate the voltage sag are presented. From this report, the following conclusions are made-

Among the different methods to mitigate the voltage sag, the use of FACT devices is the best method

The FACT devices like DVR, D-STATCOM are helpful in overcoming the

voltage unbalance problems in power system DVR is a series connected device and injects voltage to

compensate the voltageimbalance

D-STATCOM is a shunt connected device and injects current into the system

These devices are connected to the power network at the point of interest toprotect the critical loads

These devices also have other advantages like harmonic reduction, power factor

correction The amount of apparent power infusion required by D-STATCOM

is higher thanthat of DVR for a given voltage sag

DVR acts slowly but is good in reducing the harmonic content Both DVR and D-STATCOM require more number of power

electronic switchesand storage devices for their operation

To overcome this problem, PWM switched auto-transformer is used for

mitigating the voltage sag Here the number of switches required are less and hence the

switching losses arealso reduced

The size and cost of the device are less and hence PWM switched auto

transformer is an efficient and economical solution for voltage sag mitigation.

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7. REFERENCES

[1] R.C. Dugan, M.F. McGranaghan, and H.W. Beaty, “Electric Power Systems Quality,” McGraw-Hill, 1996.[2] “IEEE Recommended Practice for the Design of Reliable Industrial and Commercial Power Systems,” IEEE Std. 493-1997, December 1997.[3] IEEE Standards Board (1995), “IEEE Std. 1159-1995”, IEEE Recommended Practice for Monitoring Electric Power Quality”. IEEE Inc. NewYork[4] AmbraSannino. Mitigation of voltage sags and short interruptions through distribution system design. Dept. of Electrical Engineering University of Palermo, pp 1-6,2000.[5] David Chapman, Power Quality Application Guide - The Cost of Poor Power Quality, Copper Development Association, 2001.[6].Haque, M. H., "Compensation of distribution system voltage sag by DVR and DSTATCOM,"Power Tech Proceedings, 2001 IEEE Porto , vol.1, no., pp.5 pp. vol.1,, 2001.[7]Venkatesh, C.; Reddy, V.P.; Siva Sarma, D.V.S.S., "Mitigation of voltage sags/swells using PWM switched autotransformer," Harmonics and Quality ofPower, 2008. ICHQP 2008. 13th International Conference on , vol., no., pp.1,6, Sept. 28 2008-Oct. 1 2008.[8] K. Chan, A. Kara, and G. Kieboom, “Power quality improvement with solid state transfer switches,” in Proc. 8th ICHQP 1998, Athens, Greece, Oct. 1998, pp. 210-215

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