Full Edited Project

8/2/2019 Full Edited Project

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Dynamic voltage restorer using AC Chopper for Distribution Systems

SBCE, KHAMMAM 1 DEPT.OF EEE

CHAPTER 1


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INTRODUCTION

1.1 OBJECTIVE OF THE PRESENT STUDY

Nowadays, modern industrial devices are mostly based on electronic

units such as programmable logic controllers and electronic drives. The electronic devices

are very sensitive to disturbances and become less tolerant to power quality problems

such as voltage sags, swells and harmonics. Voltage dips are considered to be one of the

most severe disturbances to the industrial equipments.

Voltage support at a load can be achieved by reactive power injection at the load pointof common coupling. The common method for this is to install mechanically switched

shunt capacitors in the primary terminal of the distribution transformer. The mechanical

switching may be on a schedule, via signals from a supervisory control and data

acquisition (SCADA) system, with some timing schedule, or with no switching at all. The

disadvantage is that, high speed transients cannot be compensated. Some sag is not

corrected within the limited time frame of mechanical switching devices. Transformer

taps may be used, but tap changing under load is costly.

Another power electronic solution to the voltage regulation is the use of a dynamic

voltage restorer (DVR). DVRs are a class of custom power devices for providing reliable

distribution power quality. They employ a series of voltage boost technology using solid

state switches for compensating voltage sags/swells. The DVR applications are mainly for

sensitive loads that may be drastically affected by fluctuations in system voltage.

1.2 LITERATURE SURVEY

C. Benachaiba and B. Ferdi [1] have proposed Power Quality

Improvement Using DVR. This paper describes the voltage sags and swells in the

medium and low voltage distribution network are considered to be the most frequent type

of power quality problems based on recent studies. Their impact on sensitive loads is

severe. The impact ranges from load disruptions to substantial economic losses up to

millions of rupees. Different solutions have been developed to protect sensitive loads

against such disturbances but the DVR is considered to be the most efficient and effective


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solution. Its appeal includes lower cost, smaller size and its dynamic response to the

disturbance.

This research described DVR principles and voltage restoration methods for

balanced and/or unbalanced voltage sags and swells in a distribution system. Simulation

results were presented to illustrate and understand the performances of DVR under

voltage sags/swells conditions. The DVR handled both balanced and unbalanced

situations without any difficulties and injected the appropriate voltage component to

correct rapidly any anomaly in the supply voltage to keep the load voltage balanced and

constant at the nominal value. The efficiency and the effectiveness in voltage sags/swells

compensation showed by the DVR makes it an interesting power quality device compared

to other custom power devices.

ROSLI OMAR, NASRUDIN ABD RAHIM etc. have presented Modeling and

Simulation for Voltage Sags/Swells Mitigation using Dynamic Voltage Restorer

(DVR). This paperanalyses the issue of voltage sags and swells and its severe impact on

non linear loads or sensitive loads. The dynamic voltage restorer (DVR) has become

popular as a cost effective solution for the protection of sensitive loads from voltage sags

and swells. The control of the compensation voltages in DVR based on dqo algorithm is

discussed. The proposed control scheme is simple to design. Simulation results carried

out by Matlab/Simulink verify the performance of the proposed method. The DVR

handles both balanced and unbalanced situations without any difficulties and injects the

appropriate voltage component to correct rapidly any anomaly in the supply voltage to

keep the load voltage balanced and constant at the nominal value. The main advantage of

this DVR is low cost and its control is simple. It can mitigate long duration voltage

sags/swells efficiently.

They also presented Design Requirements for a Dynamic Series Compensator

for Voltage Sags Mitigation in Low Voltage Distribution System. Power quality

issues have become an increasing concern due to an increase of sensitive loads in

distribution system.

Praveen.J, Bishnu P.Muni, etc.have presented Review of Dynamic Voltage

Restorer for Power Quality Improvement. Power qualify has always been important

for customers, but with increasing applications of electronic loads and controllers

sensitive to the power quality, the subject has attracted renewed interest in recent times.

Power quality encompasses several aspects: harmonics, over voltage, flicker, voltage sags

and swells, interruptions etc., lasting only a few cycles can cause significant damage for a


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manufacturing process and computer hardware installations. Voltage source converter

based Dynamic voltage restorer (DVR) can be used effectively for mitigation of voltage

sag and swells.

The present day industrial processes are automated with wide spread use of

embedded controllers and industrial computers. Further, commercial establishments and

software parks are also being affected by poor power quality as these establishments also

depend on computer and communication related products for their operation. Poor power

quality will alsoresult in loss of data and failure of sensitive equipment. The use of DVR

can help in reduction in financial losses associated with process shut down, failure of

expensive electronic equipment and poor quality of products. Excellent performance of

DVR is recorded at different industries round the globe. DVR is a cost effective solution

for improvement of power quality.

Short-lived sags may not cause much harm other than cause a slight flickering of

lights; temporary sag is bound to have a greater impact on the industrial customers. If the

sags exceed two to three cycles, then manufacturing systems making use of sensitive

electronic equipments are likely to be affected leading to major problems. It ultimately

leads to wastage of resources (both material and human) as well as financial losses.

The increasing competition in the market 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. Electric utilities are looking for solutions to ensure high quality power

supply to their customers. The Dynamic Voltage Restorer appears to be an especially

good solution in the current scenario.

The fundamental aspects of voltage sag production and their effects on power

quality as well as enhancing this power quality in distribution network, using FACTS

(Flexible AC Transmission System) Devices i.e. Dynamic Voltage Restorer (DVR). DVR

is a powerful custom power device for short duration voltage compensation, which is

connected in series with the load & hence it possesses some advantages.

Dynamic Voltage Restorer (DVR) and Static compensator (STATCOM) have

been recently used as active solution for voltage sag mitigation. It is a device that injects a

Dynamic controlled voltage in series to the bus voltage by means of a booster

transformer. DVR installed in front of a critical load will appropriately provide correction

to the load only.


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The majority of voltage sags are within 40% of the nominal voltage. Therefore, by

designing drives and other critical loads, capable of riding through sags, with magnitude

of up to 40%, interruption of processes can be reduced significantly. The DVR can

correct sags and swells resulting from faults in either the transmission or the distribution

system.

1.3 ORGANIZATION OF THE THESIS

The thesis is organized in to five chapters. The first chapter provides a

brief review of some papers about Dynamic Voltage Restorer.

The second chapter gives the brief discussion about power quality and power

quality problems like voltage sags, voltage swells, interruptions etc. Solutions for

power quality problems also discussed in this chapter.

The third chapter introduces the Dynamic Voltage Restorer. This chapter also

describes the basic operation, structure and the existing control technique etcThis

chapter will give the reader a general idea about the Dynamic voltage restorer and its

functionality.

The fourth chapter presents an ac chopper based DVR, which can be used to

regulate the load side voltage and reduce the THD value. The control technique

designed and developed by the author to maintain the constant voltage. The designed

control technique was implemented and simulated using MATLAB. This chapter will

give a detailed description and reasoning about the construction of AC Chopper based

Dynamic voltage restorer.

In the fifth chapter the simulation results are outlined. The results are obtained by

using the simulink for different duty cycles from 10% to 99.99% for both with and

without filters and corresponding Fast Fourier transform analysis also presented.


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1.4 PROJECT OBJECTIVE

The objectives of this project are:

Protect the sensitive loads from disturbances of voltage fluctuations in the supply

system.

The control of DVR that injects a voltage in series with the distribution feeder in

order to regulate the load side voltage.

To simulate and analyze techniques using MATLAB/Simulink software.


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CHAPTER2


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POWER QUALITY

2.1 POWER QUALITY

Any power problems manifested in voltage, current or frequency deviations those

results in failure or miss operation of customer equipment.

In general maintaining a near sinusoidal power distribution bus voltages at rated

magnitude and frequency of energy supplied to the consumer must be uninterrupted. The

measure of power quality depends up on needs of equipment that is supplied.

2.2 POWER QUALITY PROBLEMS

2.2.1 Sources and effects of power quality problems:

Fig. 2.1 Single line diagram of power supply system

Power distribution systems, ideally, should provide their customers with an

uninterrupted flow of energy at smooth sinusoidal voltage at the contracted magnitude

level and frequency. However, in practice, power systems, especially the distribution

systems, have numerous nonlinear loads, which significantly affect the quality of power

supplies. As a result of the nonlinear loads, the purity of the waveform of supplies is lost.

This ends up producing many power quality problems. While power disturbances occur

on all electrical systems, the sensitivity of todayssophisticated electronic devices makes

them more susceptible to the quality of power supply. For some sensitive devices, a


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momentary disturbance can cause scrambled data, interrupted communications, a frozen

mouse, system crashes and equipment failure etc.

A power voltage spike can damage valuable components. Power Quality problems

encompass a wide range of disturbances such as voltage sags/swells, flicker, harmonics

distortion, impulse transient, and interruptions.

Voltage dip: A voltage dip is used to refer to short-term reduction in voltage of less

than half a second.

Voltage sag: Voltage sags can occur at any instant of time, with amplitudes ranging

from 1090% and a duration lasting for half a cycle to one minute.

Voltage swell: Voltage swell is defined as an increase in rms voltage or current at the

power frequency for durations from 0.5 cycles to 1 min.

Voltage 'spikes', 'impulses' or 'surges': These are terms used to describe abrupt,

very brief increases in voltage value.

Voltage transients: They are temporary, undesirable voltages that appear on the

power supply line. Transients are high over-voltage disturbances (up to 20KV) that last

for a very short time.

Harmonics: The fundamental frequency of the AC electric power distribution system

is 50 Hz. A harmonic frequency is any sinusoidal frequency, which is a multiple of the

fundamental frequency. Harmonic frequencies can be even or odd multiples of the

sinusoidal fundamental frequency.

Flickers: Visual irritation and introduction of many harmonic components in the

supply power and their associated ill effects. Voltage sags, Voltage swells, under

voltages, over voltages and interruptions for the voltage reduction standard of IEEE std of

1159-1995 as shown in the fig 3.3

Fig. 2.2 Voltage reduction standard of IEEE std of 1159-1995


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2.2.1.1 Causes of dips, sags and surges:

1. Rural location remote from power source

2. Unbalanced load on a three phase system

3. Switching of heavy loads

4. Long distance from a distribution transformer with interposed loads

5. Unreliable grid systems

6. Equipments not suitable for local supply

2.2.1.2 Causes of transients and spikes:

1. Lightening

2. Arc welding

3. Switching on heavy or reactive equipments such as motors, transformers, motor

drives

4. Electric grade switching

2.2.2 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.

2.2.2.1 IEEE Standard:

The 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.


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IEEE 493-1990 Recommended practice for the design of reliable industrial and

commercialpower 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:

Calculating voltage sag magnitude by calculating voltage drop at critical load with

knowledge of the network impedance, fault impedance and location of fault.

By studying protection equipment and fault clearing time it is possible to estimate the

duration of the voltage sag.

Based on reliable data for the neighborhood and knowledge of the system Parameters

an estimation of frequency of occurrence can be made.

IEEE 1100-1999, IEEE recommended practice for powering and grounding Electronic

equipment.

This standard presents different monitoring criteria for voltage sags and has a

chapter explaining the basics of voltage sags. It also explains the background and

application of the CBEMA (ITI) curves. It is in some parts very similar to Std. 1159 but

not as specific in defining different types of disturbances.

IEEE 1159-1995, IEEE recommended practice for monitoring electric power

quality. The purpose of this standard is to describe how to interpret and monitor

electromagnetic phenomena properly. It provides unique definitions for each type of

disturbance.

IEEE 1250-1995, IEEE guide for service to equipment sensitive to momentary

voltage disturbances.

This standard describes the effect of voltage sags on computers and sensitive

equipment using solid-state power conversion. The primary purpose is to help identify

potential problems. It also aims to suggest methods for voltage sag sensitive devices to

operate safely during disturbances. It tries to categorize the voltage-related problems that

can be fixed by the utility and those which have to be addressed by the user or equipment

designer.

The second goal is to help designers of equipment to better understand the

environment in which their devices will operate. The standard explains different causes of

sags, lists of examples of sensitive loads, and offers solutions to the problems.


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2.3 SOLUTIONS TO POWER QUALITY PROBLEMS

There are two approaches to the mitigation of power quality problems. Thesolution to the power quality can be done from customer side or from utility side First

approach is called load conditioning, which ensures that the equipment is less sensitive to

power disturbances, allowing the operation even under significant voltage distortion. The

other solution is to install line conditioning systems that suppress or counteracts the

power system disturbances.

Currently they are based on PWM converters and connect to low and medium

voltage distribution system in shunt or in series. Series active power filters must operate

in conjunction with shunt passive filters in order to compensate load current harmonics.

Shunt active power filters operate as a controllable current source and series active power

filters operate as a controllable voltage source. Both schemes are implemented preferably

with voltage source PWM inverters, with a dc bus having a reactive element such as a

capacitor. However, with the restructuring of power sector and with shifting trend

towards distributed and dispersed generation, the line conditioning systems or utility side

solutions will play a major role in improving the inherent supply quality; some of the

effective and economic measures can be identified as following:

2.3.1 Lightning and Surge Arresters:

Arresters are designed for lightning protection of transformers, but are not

sufficiently voltage limiting for protecting sensitive electronic control circuits from

voltage surges.

2.3.2 Thyristor Based Static Switches:

The static switch is a versatile device for switching a new element into the circuit

when the voltage support is needed. It has a dynamic response time of about one cycle.

To correctquickly for voltage spikes, sags or interruptions, the static switch can used to

switch one or more of devices such as capacitor, filter, alternate power line, energy

storage systems etc. The staticswitch can be used in the alternate power line applications.


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2.3.3 Energy Storage Systems:

Storage systems can be used to protect sensitive production equipments from

shutdowns caused by voltage sags or momentary interruptions. These are usually DC

storage systems such as UPS, batteries, superconducting magnet energy storage (SMES),

storage capacitors or even fly wheels driving DC generators .The output of these devices

can be supplied to the system through an inverter on a momentary basis by a fast acting

electronic switch. Enough energy is fed to the system to compensate for the energy that

would be lost by the voltage sag or interruption.

Though there are many different methods to mitigate voltage sags and swells, but

the use of a custom Power device is considered to be the most efficient method. For

example, Flexible AC Transmission Systems (FACTS) for transmission systems, the term

custom power pertains to the use of power electronics controllers in a distribution system,

specially, to deal with various power quality problems. Just as FACTS improves the

power transfer capabilities and stability margins, custom power makes sure customers get

pre-specified quality and reliability of supply.

This pre-specified quality may contain a combination of specifications of the

following: low phase unbalance, no power interruptions, low flicker at the load voltage,

low harmonic distortion in load voltage, magnitude and duration of over voltage and

under voltages within specified limits, acceptance of fluctuations, and poor factor loads

without significant effect on the terminal voltage There are many types of Custom Power

devices. Some of these devices include: Active Power Filters (APF), Battery Energy

Storage Systems (BESS), Distribution STATIC synchronous Compensators

(DSTATCOM), Distribution Series Capacitors (DSC), Dynamic Voltage Restorer (DVR),

Surge Arresters (SA), Super conducting Magnetic Energy Systems (SMES), Static

Electronic Tap Changers (SETC), Solid-State Transfer Switches (SSTS), Solid State Fault

Current Limiter (SSFCL), Static VAR Compensator (SVC), Thyristor Switched

Capacitors (TSC), and Uninterruptible Power Supplies (UPS).


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CHAPTER 3


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DYNAMIC VOLTAGE RESTORER (DVR)3.1 INTRODUCTION

The technological advancements have proven a path to the modern

industries to extract and develop the innovative technologies within the limits of their

industries for the fulfillment of industrial goals. And their ultimate objective is to

optimize the production while minimizing the production cost and their by achieving

maximized profits while ensuring continuous production through the period.

As such stable supply of un-interruptible power has to be guaranteed during the

production process. The reason for demanding high quality power is basically the modern

manufacturing and process equipment, which operates at high efficiency, requires high

quality defect free power supply for the successful operation of their machines. More

precisely most of those machine components are designed to be very sensitive for the

power supply variations. Adjustable speed drives, automation devices, power electronic

components and examples for such equipments.

Failure to provide the required quality power output may sometimes cause

complete shutdown of the industries which will make a major financial loss to the

industry concerned. Thus the industries always demand for high quality power from the

supplier or the utility but the blame due to degraded quality cannot be solely put on the

hands of the utility itself. It has been found out most of the conditions that can disrupt the

process are generated within the industry itself. For examples, most of the non-linear

loads within the industries cause transients which can affect the reliability of the power

supply. Following shows some abnormal electrical conditions cased both in the utility end

and customer end that can disrupt a process.1. Voltage sags2. Voltage swells3. Phase outages4. Transients due to lightning loads, capacitor switching, non-linear loads etc.5. Harmonics

As a result of above abnormalities the industries may undergo burned-out motors,

lost data on volatile memories, erroneous motion of robotics, unnecessary downtime,


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increased maintenance costs and burning core materials especially in plastic industries,

paper mills and semiconductor plants.

As the power quality problems originated from utility and customer side, the

solutions come from both and are named as utility based solutions and customer based

solutions respectively. The best examples of both two types of solutions are FACTS

devices (Flexible AC Transmission Systems) and custom power devices. FACTS devices

are those controlled by utility, where as the custom power devices are operated,

maintained and controlled by customer itself and installed at the customer premises. Both

the custom power devices and FACTS devices are based on the solid state power

electronic components. As the new technologies emerged, the manufacturing cost and

reliability of those solid state devices are improved; hence the protection devices which

incorporate such solid state devices can be purchased at a reasonable price with better

performance than the other electrical or pneumatic devices available in the market.

Uninterruptible power supplies (UPS), Dynamic voltage restorer (DVR), and Active

power filters (APF) are examples of commonly used custom power devices. Among

those APF is used to mitigate harmonic problems occurring due to non-linear loading

conditions, where as UPS and DVR are used to compensate Voltage sags and sells

conditions. A new control technique to maintain the constant the load voltage for a single

phase DVR was developed and simulated using MATLAB software.

Voltage Sag (Fig. 3.1) is a momentary decrease in the root mean square voltage

between 0.1 to 0.9 per unit, with a duration ranging from half cycle up to 1 min .In other

word it is defined as a sudden reduction of supply voltage down 90% to10% of nominal

and followed by a recovery after short period of time. A normal duration of sag according

to standards is, 10 ms to 1 minute. It is considered as the most serious problem of power

quality. It is caused by fault in power system or by starting of large induction motor.

Fig. 3.1 Sag or dip


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It can interrupts or malfunction any electronic or electrical equipment which is sensitive

to load. Therefore huge losses result, due to voltage sag problem at customer load end.

Voltage swell (Fig. 3.2) is an increase between 1.1 and 1.8pu in rms

voltage (or) current at the power frequency for durations of 0.5 cycles to 2 secs. Voltage

swells are caused by system fault conditions, switching off large loads or energizing a

large capacitor bank.

Fig. 3.2 Swell

DVR (Fig. 3.3) is designed to mitigate voltage sags and voltage swells on lines

feeding sensitive equipment. A viable alternative to uninterruptible power systems

(UPS's) and other utilization voltage solutions to the voltage sag problem, the DVR is

specifically designed for large loads (2 MVA and up) served at distribution voltage. A

DVR is expected to be a lower cost alternative to UPS for applications at distribution

voltage. A DVR typically requires less than one-third the nominal power rating of the

UPS. Also, the DVR can be used to mitigate troublesome harmonic voltages on the

distribution system. The DVR is available in 2 MVA increment sizes up to 10 MVA.


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Fig. 3.3 Schematic diagram DVR system

A Dynamic Voltage Restorer (DVR) is a custom power device that is used to

compensate voltage sag and voltage swells. The DVR generally consists of voltage source

inverter (VSI), injection transformers, passive filters and energy storage (battery). The

efficiency of the DVR depends on the efficiency of the control technique involved in

switching the inverters. The inverters are switched using Space Vector Pulse Width

Modulation pulses (SVPWM) to maximize the usage of DC link voltage. During normal

operation supply voltage is provided by the network to the load at rated value.

3.2 NEED FOR DVR

Dynamic Voltage Restorer (DVR) was introduced commercially only in 1994.The primary function of DVR is to minimize the voltage sags on lines that cater to

sensitive equipment. It controls voltage applied to the load by injecting a voltage of

compensating amplitude, frequency and phase angle to the distribution line. The voltage

turns to the desired magnitude in case of any disturbances. The device functions as a filter

between the transmission line and the facility, thus enabling the facility to continuously

receive clean power. The DVR is primarily responsible for restoring the quality of voltage

delivered to the end user when the voltage from the source is not appropriate to be used

for sensitive loads. Usage of DVR enables consumers to isolate and protect themselves


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from transients and disturbances caused by sags and swells on the transmission lines or

distribution network.

3.3 BENEFITS OF DVR

DVRs offer a wide range of benefits to industrial and commercial end users,

some of them being:

A large part of the industrial machinery makes use of sophisticated electronics thatare quite sensitive to power disturbances. DVR plays a key role in ensuring the

smooth functioning of such equipments.

Power disturbances can lead to irregularities or in a worst-case scenario, stoppageof production processes. Whenever any kind of aberration in power is detected,

DVRs reduce the potential shutdown time for equipment within facilities that

ultimately saves a lot of time and money.

DVRs can also be used to tackle the problem of harmonics caused by non-linearload machinery in manufacturing facilities. If not corrected in time, the harmonic

voltages can spill over to the office power and cut into the productivity.

The insulation wear on transformers, motors and drivers caused by powerirregularities can also be reduced by DVR.

3.3.1 The Major Industries that are likely to benefit the most from DVRs are:

Utilities (transmission and distribution companies) Process industries (semiconductor plants, paper mills, plastic manufacturers) Automotive manufacturers Chemical plants & Steel plants Electronics (consumer electronic and computer manufacturers) Mining industry


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3.4 MERITS OF DVR OVER DEVICES LIKE D-STATCOM

1. Cost is less2. Small in size3. High energy capability4. To maintain load voltage is constant5. Effective custom power device to mitigate Sag and Swell6. It provides best effective solution for its size and capabilities7. Dynamic response towards disturbance8. DVR can mitigate for long period of Sag and Swell effectively

3.5 DEMERITS OF D-STATCOM

1. Complicated in structure and control2. High in cost3. Reduced Security4. Possibility of improper compensation5. Large in size

3.6 MODES OF DVR

The DVR has two modes of operation which are:

-Standby mode and

-Boost mode.

Standby mode: In this mode (VDVR=0), the booster transformers low voltage winding

is shorted through the converter. No switching of semiconductors occurs in this mode of

operation. Therefore, only the comparatively low conduction losses of the semiconductors

in this current loop contribute to the losses. The DVR will be most of the time in this

mode.

Boost mode: In this mode (VDVR>0), the DVR is injecting a compensation voltage

through the booster transformer due to a detection of a supply voltage disturbance.


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3.6.1 Equivalent Circuit:

Fig. 3.4 Equivalent Circuit of DVR

Figure shows the equivalent circuit of the DVR, when the source voltage is drop or

increase, the DVR injects a series voltage V injected through the injection transformer so

that the desired load voltage magnitude VLcan be maintained. The series injected voltage

of the DVR can be written as

V injected =VLV Supply

Where,

VL is the desired load voltage magnitudeV Supply is the source voltage during sags/swells condition

The load current IL is given by,

IL = ((PL j QL)/ VL)

3.7 VOLTAGE INJECTION METHODS OF DVR

Voltage injection or compensation methods by means of a DVR depend upon the

limiting factors such as; DVR power ratings, various conditions of load, and different

types of voltage sags. Some loads are sensitive towards phase angel jump and some are

sensitive towards change in magnitude and others are tolerant to these. Therefore the

control strategies depend upon the type of load characteristics.

There are four different methods of DVR voltage injection which are


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i. Pre-sag compensation method

ii. In-phase compensation method

iii. In-phase advanced compensation method

iv. Voltage tolerance method with minimum energy injection

3.7.1 Pre-sag/dip compensation method:

The pre-sag method tracks the supply voltage continuously and if it detects any

disturbances in supply voltage it will inject the difference voltage between the sag or

voltage at PCC and pre-fault condition, so that the load voltage can be restored back to

the pre-fault condition. Compensation of voltage sags in the both phase angle and

amplitude sensitive loads would be achieved by pre-sag compensation method. In this

method the injected active power cannot be controlled and it is determined by external

conditions such as the type of faults and load conditions

VDVR = V pr faultV sag

Fig. 3.5 Pre-sag compensation method

3.7.2 In-phase compensation method:

This is the most straight forward method. In this method the injected voltage is in

phase with the supply side voltage irrespective of the load current and pre-fault voltage.


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The phase angles of the pre-sag and load voltage are different but the most important

criteria for power quality that is the constant magnitude of load voltage are satisfied.

Fig. 3.6 In-phase compensation method

|VL|=|V pre-fault|

One of the advantages of this method is that the amplitude of DVR injection

voltage is minimum for certain voltage sag in comparison with other strategies. Practical

application of this method is in non-sensitive loads to phase angle jump.

3.7.3 In-phase advanced compensation method:

In this method the real power spent by the DVR is decreased by minimizing the

power angle between the sag voltage and load current. In case of pre-sag and in-phase

compensation method the active power is injected into the system during disturbances.

The active power supply is limited stored energy in the DC links and this part is one of

the most expensive parts of DVR. The minimization of injected energy is achieved by

making the active power component zero by having the injection voltage phasor

perpendicular to the load current phasor.

In this method the values of load current and voltage are fixed in the system so we

can change only the phase of the sag voltage. IPAC method uses only reactive power and

unfortunately, not al1 the sags can be mitigated without real power, as a consequence, this

method is only suitable for a limited range of sags.


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Fig. 3.7 In-phase advanced compensation method

3.7.4 Voltage tolerance method with minimum energy injection:

A small drop in voltage and small jump in phase angle can be tolerated by the load

itself. If the voltage magnitude lies between 90%-110% of nominal voltage and 5%-10%

of nominal state that will not disturb the operation characteristics of loads. Both

magnitude and phase are the control parameter for this method which can be achieved by

small energy injection.

Fig. 3.8 Voltage tolerance method with minimum energy injection

3.8 DIFFERENCES BETWEEN VOLTAGE REGULATORS AND DVRS

Both are used to mitigate the effects of voltage dips. Dips are characterized by the

depth - the retained voltage - and the duration. Short and deep dips are best served by a

DVR while long and shallow dips are the province of the voltage regulator.


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A voltage regulator has no energy store. It has a transformer secondary winding in

series with the supply. When the input voltage moves outside the tolerance band the

primary of that transformer is driven to boost, or in anti-phase to reduce, the voltage

appropriately. Because the load voltage is kept constant, the power to the load is constant

so, when the input voltage falls, the input current increases. The current capability of the

supply and the device itself limits the working range to about +/-30 % of nominal voltage.

A DVR has an energy store, so requires no additional input power (in the short

term) to boost the voltage during a dip. A DVR can correct a dip to 0 % retained voltage.

But the DVR has a limited energy store and so is suitable for short-term effects only - it

cannot correct for long term under voltage, for example. Also, the store has to be

recharged between events so it is not suitable multiple dips are expected frequently.

Typically, DVRs use super capacitors, large secondary batteries or high-speed flywheels

as energy stores. Unsurprisingly, DVRs are more expensive than voltage regulators.


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CHAPTER4


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DVR USING AC CHOPPERS

4.1 INTRODUCTION

There are so many DVR schemes developed in order to reduce

the power quality problems like voltage sags and voltage swells is presented in the

previous chapter. But no one has discussed about reduction of total harmonic distortion

(THD) by using DVR. In this project work an AC chopper based DVR is developed in

order to regulate the load voltage and reduce the THD value. DVR is a power electronic

controller that can protect sensitive loads from disturbances of voltage fluctuations in

supply system. It is observed that DVR can regulate the voltage at the load by injectingthe voltage in to the system. . It is normally installed in a distribution system between

supply and critical load feeder. For the control operation on-off control of ac voltage

controller is designed with different duty cycles. The output of the chopper voltage will

inject to the distribution feeder through the injecting transformer in order to maintain the

constant voltage. The actual implementation of the DVR using chopper raises additional

issues of harmonics. The filter is designed to eliminate these harmonics.

4.2 STRUCTURE OF THE AC CHOPPER BASED DVR

A power electronic converter based series compensator that can protect

critical loads from all supply side voltage disturbances other than outages is called a

dynamic voltage restorer (DVR). The restorer is capable of generating or absorbing

independently controllable real and reactive power at its AC output terminal. This device

employs AC chopper or AC voltage controller structure. It injects output voltages in

series with the distribution feeder at the given load location. In 1996, Westinghouse

Electric Corporation installed worlds first dynamic voltage restorer in 12.47kV

substations in Anderson, USA. This was installed to provide voltage correction to an

automated rug manufacturing plant.

This work extends the concept of dynamic voltage restorer to maintain the load voltage

near the rated value. It can also perform the primary functions of the restorer, i.e., to

protect the load from temporary sag and swell.

This device is a AC chopper based DVR. The output of the chopper voltage will inject to


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the distribution feeder through the injecting transformer in order to maintain the constant

voltage.

1

2

1 2

Voltage injectingTransformer

Shunt transformer

AC Chopper

Line Impedance

Load

Voltage Bus Load Bus

Filter

Fig. 4.1 Block diagram of the AC chopper based DVR

For the control operation of DVR designs the on-off control of ac voltage

controller is used with different duty cycles).

A DVR is a recently proposed series connected solid state device that injects the

voltage in to the system in order to regulate the load side voltage. It is normally installed

in a distribution system between supply and critical load feeder location.

The voltage at load terminal (VL) is equal to sum of supply voltage (V Supply) and

the output voltage of the chopper (V injected).

VL= V Supply+

V injected

Where,

VL=Load Voltage

V Supply = Supply Voltage

V injected = Injecting Voltage


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4.3 CONFIGURATION OF DVR

The general configuration of a DVR consists of:

1. Shunt transformer,2. AC chopper,3. Injecting/Boosting transformer and4. Filter

4.3.1 Shunt Transformer:

The transformer block model shown consists of two coupled windings wound on

the same core.

Fig.4.2 Shunt transformer

The model takes into account the winding resistances (R1 R2) and the leakage

inductances (L1 L2), as well as the magnetizing characteristics of the core, which is

modeled by a linear (Rm Lm) branch.

The shunt transformer is a step down transformer. The supply voltage of 6350V,

which is step-down to 1270V by using this transformer and this voltage is given to the of

the ac chopper. The turns ratio of the shunt transformer is 1:5.


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4.3.2 AC Chopper (AC voltage controller):

AC voltage controllers (ac line voltage controllers) are employed to vary the RMS

value of the alternating voltage applied to a load circuit by introducing thyristors between

the load and a constant voltage ac source. The RMS value of alternating voltage applied

to a load circuit is controlled by controlling the triggering angle of the Thyristors in the ac

voltage controller circuits. In brief, an ac voltage controller is a type of thyristor power

converter which is used to convert a fixed voltage, fixed frequency ac input supply to

obtain a variable voltage ac output. The RMS value of the ac output voltage and the ac

power flow to the load is controlled by varying (adjusting) the trigger angle

Fig. 4.3 AC Chopper

There are two different types of thyristor control used in practice to control the ac power

flow

On-Off control

Phase control

4.3.2.1 On-Off control:

In On-Off control technique Thyristors are used as switches to connect the load

circuit to the ac supply (source) for a few cycles of the input ac supply and then to

disconnect it for few input cycles. The Thyristors thus act as a high speed contactor (or

high speed ac switch).

4.3.2.2 Phase control:

In phase control the Thyristors are used as switches to connect the load circuit to

the input ac supply, for a part of every input cycle. That is the ac supply voltage is


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chopped using Thyristors during a part of each input cycle. The thyristor switch is turned

on for a part of every half cycle, so that input supply voltage appears across the load and

then turned off during the remaining part of input half cycle to disconnect the ac supply

from the load. By controlling the phase angle or the trigger angle (delay angle), the

output RMS voltage across the load can be controlled. The trigger delay angle is

defined as the phase angle (the value of wt) at which the thyristor turns on and the load

current begins to flow. Thyristor ac voltage controllers use ac line commutation or ac

phase commutation.

Thyristors in ac voltage controllers are line commutated (phase

commutated) since the input supply is ac. When the input ac voltage reverses and

becomes negative during the negative half cycle the current flowing through the

conducting thyristor decreases and falls to zero. Thus the ON thyristor naturally turns off,

when the device current falls to zero. Phase controls Thyristors which are relatively

inexpensive, converter grade thyristors which are slower than fast switching inverter

grade Thyristors are normally used. For applications up to 400Hz, if Triacs are available

to meet the voltage and current ratings of a particular application, Triacs are more

commonly used.

Due to ac line commutation or natural commutation, there is no need ofextra commutation circuitry or components and the circuits for ac voltage controllers are

very simple. Due to the nature of the output waveforms, the analysis, derivations of

expressions for performance parameters are not simple, especially for the phase

controlled ac voltage controllers with RL load. But however most of the practical loads

are of the RL type and hence RL load should be considered in the analysis and design of

ac voltage controller circuits.

4.3.2.3 Types of AC Voltage Controllers:

The ac voltage controllers are classified into two types based on the type of

input ac supply applied to the circuit.

Single Phase AC Controllers.

Three Phase AC Controllers.


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Single phase ac controllers operate with single phase ac supply voltage of 230V

RMS at 50Hz in our country. Three phase ac controllers operate with 3 phase ac supply of

400V RMS at 50Hz supply frequency. Each type of controller may be sub divided into

Uni-directional or half wave ac controller.

Bi-directional or full wave ac controller.

In brief different types of ac voltage controllers are

Single phase half wave ac voltage controller (uni-directional controller).

Single phase full wave ac voltage controller (bi-directional controller).

Three phase half wave ac voltage controller (uni-directional controller).

Three phase full wave ac voltage controller (bi-directional controller).

4.3.2.4 Applications of AC voltage controllers:

Lighting / Illumination control in ac power circuits.

Induction heating.

Industrial is heating & Domestic heating.

Transformer tap changing (on load transformer tap changing).

Speed control of induction motors (single phase and poly phase ac inductionmotor Control).

AC magnet controls.

4.3.2.5 Principle of On-Off control technique (Integral cycle control):

The basic principle of on-off control technique is explained with reference to a

single phase full wave ac voltage controller circuit shown below. The thyristor switchesT1and T2are turned on by applying appropriate gate trigger pulses to connect the input ac

supply to the load for n number of input cycles during the time interval ton.

The thyristor switches T1and T2are turned off by blocking the gate trigger pulses

for m number of input cycles during the time interval toff. The ac controller ON time ton

usually consists of an integral number of input cycles. Here on-off control technique for

ac voltage controller is used.


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R=RL= Load Resistance

Fig. 4.4 Single phase full wave AC Voltage Controller Circuit

Fig. 4.5 Waveforms for Single phase full wave AC Voltage Controller Circuit


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4.3.2.6 Principle of AC phase control:

The basic principle of ac phase control technique is explained with reference

to a single phase half wave ac voltage controller (unidirectional controller) circuit shown

in the below figure.

The half wave ac controller uses one thyristor and one diode connected in parallel

across each other in opposite direction that is anode of thyristor T1 is connected to the

cathode of diode D1and the cathode of T1 is connected to the anode ofD1. The output

voltage across the load resistor R and hence the ac power flow to the load is controlled

by varying the trigger angle . The trigger angle or the delay angle refers to the

value of wtor the instant at which the thyristor T1is triggered to turn it ON, by applying

a suitable gate trigger pulse between the gate and cathode lead.

The thyristor T1is forward biased during the positive half cycle of input ac supply.

It can be triggered and made to conduct by applying a suitable gate trigger pulse only

during the positive half cycle of input supply. When T1 is triggered it conducts and the

load current flows through the thyristor T1, the load and through the transformer

secondary winding.

By assuming T1as an ideal thyristor switch it can be considered as a closed switch

when it is ON during the period wt= to radians. The output voltage across the load

follows the input supply voltage when the thyristor T1is turned-on and when it conducts

from w t= to radians. When the input supply voltage decreases to zero at wt= , for a

resistive load the load current also falls to zero at wt= and hence the thyristorT1turns

off at wt=. Between the time period wt= to 2, when the supply voltagereverses and

becomes negative the diode D1 becomes forward biased and hence turns ON and

conducts. The load current flows in the opposite direction during wt = to 2 radians

when D1is ON and the output voltage follows the negative half cycle of inputsupply.


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Fig. 4.6 Half wave AC phase controller (Unidirectional Controller)

Fig. 4.7 Waveforms for Half wave AC phase controller (Unidirectional Controller)


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4.3.2.7 Disadvantages of Single phase Half wave AC voltage controllers :

The output load voltage has a DC component because the two halves of the outputvoltage waveform are not symmetrical with respect to 0 level. The input supply

current waveform also has a DC component (average value) which can result in

the problem of core saturation of the input supply transformer.

The half wave ac voltage controller using a single thyristor and a single diodeprovides control on the thyristor only in one half cycle of the input supply. Hence

ac power flow to the load can be controlled only in one half cycles.

Half wave ac voltage controller gives limited range of RMS output voltagecontrol. Because the RMS value of ac output voltage can be varied from a

maximum of 100% ofVsat a trigger angle =0 to a low of 70.7% ofVsat =

Radians.

These drawbacks of single phase half wave ac voltage controller can be overcomeby using a single phase full wave ac voltage controller.

4.3.2.8 Applications of RMS voltage controller:

Speed control of induction motor (polyphone ac induction motor). Heater control circuits (industrial heating). Welding power control. Induction heating. On load transformer tap changing. Lighting control in ac circuits. Ac magnet controls.

4.3.3 Voltage injection transformer:

The primary side of the injection transformer is connected in series with to the

distribution line, while the secondary side is connected to the DVR power circuit. For a

single phase DVR, one single phase injection transformer is connected to the distribution

line, and for the three phase DVR, three single phase or three phase voltage injection


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transformers are connected. For a single phase DVR, the single phase transformer is

connected as shown in fig. 4.8.

Supply side Load side

1

2

FromC

hopper

Fig.4.8 Voltage injection transformer

The basic function of the injection transformer is to increase the voltage supplied

by the filtered AC chopper output to the desired level while isolating the DVR circuit

from the distribution network. The transformer winding ratio is pre-determined according

to the voltage required in the secondary side of the transformer (Generally this is kept

equal to the supply voltage).

The rating of the injection transformer is an important factor while deciding the

DVR performance, since it limits the maximum compensation ability of the DVR. Further

the leakage inductance of the transformer brings to a low value to reduce the voltage drop

across the transformer. In order to reduce the saturation of the injection transformer under

normal operating conditions it is designed to handle a flux, which is higher than the

normal maximum flux requirement. The winding configuration of the injection

transformer mainly depends upon the upstream distribution transformer.

For any type of load (i.e. R, RL and RC) the supply voltage is same because there

is no voltage drop due to source impedance except the drop in line impedance.

4.3.4 Filter:

Filters are used to convert the PWM inverted pulse waveform in to sinusoidal

waveform. This is achieved by removing the unnecessary higher order harmonic

components generated from the AC to Ac conversion in the AC Chopper, which will

distort the compensated output voltage. The filter can be placed at the secondary winding

of the injection transformer as shown in the figure. 4.9.


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Fig. 4.9 Filter placement

The implementation of DVR with AC chopper raises some additional issues of

harmonics. In order to eliminate these harmonics, filter is designed and the placement of

the filter is shown in fig. 4.8. In this project work, first we developed the series LC filter

with L=0.2026H and C=5mF by using the formulae

LC

f2

1 ,

Where, f=3 KHz and shown in the fig.4.10.

Fig. 4.10 series LC filter

By designing this series LC filter the Total Harmonic distortion (THD) value is

significantly more. Hence in order to decrease the THD value, one more LC filter is

added to the previous filter in cascade with the same values as shown in the fig. 4.11.


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Fig. 4.11 Cascaded LC filter

So, with the cascaded filter the obtained THD value is less as compared to the

single LC filter. But the THD value with cascaded LC filters also not tolerable. So we

must decrease THD value. For this we incorporate another filer (i.e. combination of high

pass and low pass filter) as shown in the fig. 4.12

.

Fig. 4.12 Modified Filters

In the above filter the values of L and C are chosen by trial and error method such that the

THD value becomes less.


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4.4 TOTAL HARMONIC DISTORTION

The total harmonic distortion, or THD, of a signal is a measurement of the

harmonic distortion present and is defined as the ratio of the sum of the powers of all

harmonic components to the power of the fundamental frequency. Lesser THD allows the

components in a loudspeaker, amplifier or microphone or other equipment to produce a

more accurate reproduction by reducing harmonics added by electronics and audio media.

A THD rating < 1% is considered to be in high-fidelity and inaudible to the human ear.

When the input is a pure sine wave, the measurement is most commonly the ratio

of the sum of the powers of all higher harmonic frequencies to the power at the first

harmonic, or fundamental, frequency:

1

2

1

432..........

P

P

P

PPPPTHD n

n

This can equivalently be written as

1

1

P

PPTHD

total

Measurements based on amplitudes (e.g. voltage or current) must be converted to

powers to make addition of harmonics distortion meaningful. For a voltage signal, for

example, the ratio of the squares of the RMS voltages is equivalent to the power ratio:

2

1

22

4

2

3

2

2.........

V

VVVVTHD

Where Vn is the RMS voltage ofnth harmonic and n=1 is the fundamental frequency.

THD is also commonly defined as an amplitude ratio rather than a power ratio,

resulting in a definition of THD which is the square root of that given above:

1

22

4

2

3

2

2.......

V

VVVVTHD

n

This latter definition is commonly used in audio distortion (percentage THD)

specifications. It is unfortunate that these two conflicting definitions of THD (one as a

power ratio and the other as an amplitude ratio) are both in common usage.
http://en.wikipedia.org/wiki/Signal_(electronics)http://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Fundamental_frequencyhttp://en.wikipedia.org/wiki/High-fidelityhttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Fundamental_frequencyhttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Root_mean_squarehttp://en.wikipedia.org/wiki/Fundamental_frequencyhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/High-fidelityhttp://en.wikipedia.org/wiki/Fundamental_frequencyhttp://en.wikipedia.org/wiki/Distortionhttp://en.wikipedia.org/wiki/Harmonichttp://en.wikipedia.org/wiki/Signal_(electronics)


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CHAPTER5


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SIMULINK/MATLAB AS A TOOL FOR SIMULATION

5.1 INTRODUCTION

Matlab simulation is used to develop the AC Chopper based Dynamic

voltage restorer after investigating the various literatures available for DVR. A systematic

procedure is described in this chapter for the simulation model development and the

results are presented further.

5.2 SIMULATION MODEL

Fig. 5.1 Main circuit diagram without filter


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The circuit model of an AC Chopper based DVR is shown in fig. 5.1. It is

developed by using Matlab/Simulink version 7.1. The brief discussion about the

components used in the simulation model is given in the appendix. It consists of three

main blocks. There are shunt transformer, Ac Chopper and injecting/ boosting

transformer. Shunt transformer is a step down transformer, which is used to step down the

voltage and this voltage is given to input of AC Chopper. The output of Chopper voltage

is injected in to the line through an injecting transformer. The sub circuit of AC Chopper

is given in fig. 5.2.

5.3 SUB CIRCUIT FOR AC CHOPPER

Fig. 5.2 On-Off control of ac chopper

An on-off control technique for Ac voltage controller with different duty cycles

(from 10 to 99.99%) is used. By varying the duty cycles from 10 to 99.99% the output

voltage is observed. To reduce the total harmonic distortion a filter is designed and

connected across the secondary winding of the injection transformer. The ac chopper

based DVR with filter is shown in fig. 5.3. The total harmonic distortion is decreased

with the increase in the duty cycle and vice-versa.


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5.4 AC CHOPPER BASED DVR CIRCUIT WITH FILTER

Fig. 5.3 Main circuit diagram with filter

The values of inductance (L) and capacitance (C) are chosen by trial and error

method i.e. L=0.2026H AND C=5mF such that the total harmonic distortion is less. By

varying the duty cycle from low to high the output voltage of the chopper will increase

and THD decreases.


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For any disturbances of supply voltage variations in the system,

injecting the voltage in to the distribution feeder through the injection transformer, the

constant voltage can be maintained. For any type of load (i.e. R, RL and RC) the load

voltage is same because there is no voltage drop due to source impedance.

5.5 MODELING OF CIRCUIT

Components: Quantity:

1. Single phase supply 02

2. Line impedance 01

3. Linear transformer 02

4. Switch 01

5. Pulse generator 01

6. Ground 07

7. Voltage measurement 04

8. Controlled voltage source 01

9. Filter 01

10.Series R-load 01

11. Scope 04


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5.6 CIRCUIT RATINGS

Contents: Ratings:

1. Voltage 6350V

2. Frequency 50Hz

3. Line resistance 1ohm

4. Line reactance 10mohms

5. Shunt Transformer power: 30KVA, f=50Hz

Primary/Secondary Voltage: 6350/1270V

Magnetization Resistance: 2000

Magnetization Inductance: 500

6. High pass Filter Inductance 7UH

7. High pass Filter Capacitance 0.09046F

8. Low pass Filter Inductance 0.20264UH

9. Low pass Filter Capacitance 5mF

10. Injecting Transformer power: 30KVA, f=50Hz

Primary/Secondary voltage: 1270/1270V

Magnetization Resistance: 2000

Magnetization Inductance: 500

11. R-Load 635ohms


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CHAPTER 6


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SIMULATION RESULTS

6.1 OUTPUT WAVEFORMS of INJECTED VOLTAGE WITHOUT FILTER:

The voltage across the load for different duty cycles from 10% to 90% without

filter is shown in the fig. 5.4 and corresponding Fast Fourier Transform (FFT) analysis is

also shown in fig.5.5. In the below Fig. 5.4 the X-axis represents the time (sec) and Y-

axis represents the Injecting voltage (volts). Here it is observed that the THD values

decreases from 271.50% to 30.17%.

For 10%:

For 20%:


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For 30%:

For 40%:

For 50%:


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For 60%:

For 70%:

For 80%:


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For 90%:

Fig. 6.1 Output voltage waveforms for different duty cycles

6.2 FFT ANALYSIS of LOAD VOLTAGE WITHOUT FILTER:

FOR 10%:

FOR 20%:


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For 30%:

For 40%:

For 50%:


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For 60%:

For 70%:

For 80%:


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For 90%:

Fig. 6.2 FFT Analysis for different duty cycles without filter

6.3 THD WITHOUT DISTORTION:

After the FFT analysis without filter the results are presented in the

tabular form in table. 6.1. With the increase in the duty cycle from low to high the

injecting voltage is increased and THD is decreased.

Table.6.1 THD Values without filter

Duty

Cycle (%)

Injecting

voltage (V) THD (%)

10 271.14 127

20 254 188.13

30 381 146.85

40 508 117.75

50 635 96.6

60 762 78.64

70 889 62.94

80 1016 47.03

90 1143 30.17


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6.4 CHARACTERISTICS OFF TIME& ON TIME vs RMS VOLTAGE:

For the load voltage without filter the RMS value of the voltages with

respect to different on times and off times are recorded and same is plotted in figs.6.6 and

6.7. The RMS voltage is increased by increasing the on-time as shown in the Fig. 6.6.

And the effect of off time on RMS voltage is shown in Fig. 6.7

Fig. 6.3 On-time vs. Vrms voltage

Fig. 6.4 Off-time vs. Vrms voltage


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6.5 WAVEFORMS OF LOAD VOLTAGE WITHOUT FILTER:

The voltage across the load for different duty cycles from 10% to 99.99% with

filter is shown in the fig.5.8 and corresponding FFT analysis is also shown in fig.5.9. In

the below Fig. 5.8 the X-axis represents the time (sec) and Y-axis represents the Load

voltage (volts). Here it is observed that the THD values decreases from 30.17% to 0.01%

and the three waveforms indicates the supply voltage, injected voltage and load voltage

i.e.

For 10%:

For 20%:


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For 30%:

For 40%:

For 50%:


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For 60%:

For 70%:

For 80%:


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For 90%:

For 99.99%:

Fig. 6.5 Output voltage waveforms for different duty cycles with filter

6.6 FFT ANLYSIS of LOAD VOLTAGE WITH FILTER:

For 10%:


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For 20%:

For 30%:

For 40%:


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For 50%:

For 60%:

For 70%:


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For 80%:

For 90%:

Fig. 6.6 FFT Analysis for different duty cycles with filter

6.7 TOTAL HARMONIC DISTORTION WITH FILTER:

After analyzing the FFT analysis with filter the results are presented in the tabular

form in table. 6.2. With the increase in the duty cycle from low to high the injecting

voltage and load voltage is increased and THD is decreased.


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Table.6.2 THD Values with filter

Duty cycle

(%)

Injecting

voltage(V) THD (%)

Load voltage

(V) THD (%)

10 132.8 32.39 6469 0.66

20 265.6 30.63 6598 1.23

30 398.4 27.94 6727 1.65

40 531.1 24.53 6857 1.95

50 663.8 20.60 6986 1.89

60 796.5 16.34 7116 1.82

70 929.1 11.97 7246 1.53

80 1062 7.66 7376 1.1

90 1194.2 3.61 7506 0.57

After analysing without and with filter a look up table is prepared as shown in the

table. 6.3. For supply voltage variation from +25% t0 -25% around the rated voltage in

steps of 5%. The load voltage is controlled with in a 5% of the nominal load voltage. It

is also observed that the THD is in between 2.26 to 0.01%.


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Table.6.3. Look-up table

Duty

Cycle

(%)

Supply Voltage(V)

(6350V)

Load Voltage(V)

THD (%)Total voltage

at supply

Voltage

variation in

Percentage

Total

voltage at

load

Voltage at load

in Percentage

99.99 7937.5 +25 6642 +4.6 0.01

80 7620 +20 6581 +3.6 1.23

50 7302.5 +15 6648 +4.69 2.05

30 6985 +10 6588 +3.74 1.68

5 6667.5 +5 6528 +3.8 0.33

99.99 4762.5 -25 6052 -4.6 0.01

80 5080 -20 6108 -3.8 1.85

50 5397.5 -15 6036 -4.9 2.26

30 5715 -10 6094 -4.0 1.82

5 6032.5 -5 6087 -4.1 0.36


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CONCLUSION

By designing the AC Chopper based dynamic voltage restorer load voltage ismaintained very near to the rated value. The harmonic analysis has been done with the

different duty cycles. The results are presented for the same duty cycles. The Total

harmonic distortion (THD) decreases with the increase in duty cycle. For any type of load

up to 100Amps the voltage across the load is controlled to within a 5% of the nominal

load voltage. It is also observed that the THD is in between 2.26 to 0.01%. The author

feels that this method is used for the first time used in DVR schemes. The use of series

AC chopper for the voltage restoration is much simpler than the previously used schemes.

FUTURE SCOPE

This work can be extended to a 3-phase system.


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[4] MOZDER, A. JR. BOSE, B. K., Three-Phase AC Power Control Using PowerTransistors, IEEE Trans. Ind. Appl., IA-12 (1976), pp. 499505.

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