CHAPTER 1 OVERVIEW OF THE PROJECT

1.1 INTRODUCTION TO PROJECT

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Power quality is certainly a major concern in the present era; it become especially important

with the introduction of sophisticated devices, whose performance is very sensitive to the

quality of power supply. Modern industrial processes are based a large amount of electronic

devices such as programmable logic controllers and adjustable speed drives. The electronic

devices are very sensitive to disturbances and thus industrial loads become less tolerant to

power quality problems such as voltage dips, voltage swells, and harmonics. Voltage dips are

considered one of the most severe disturbances to the industrial equipment. A paper machine

can be affected by disturbances of 10% voltage drop lasting for 100ms. A voltage dip of 75%

(of the nominal voltage) with duration shorter than 100ms can result in material loss in the

range of thousands of US dollars for the semiconductors industry. Swells and over voltages

can cause over heating tripping or even destruction of industrial equipment such as motor

drives. Electronic equipments are very sensitive loads against harmonics because their control

depends on either the peak value or the zero crossing of the supplied voltage, which are all

influenced by the harmonic distortion. This project analyzes the key issues in the Power

Quality problems, specially keeping in mind the present trend towards more localized

generations (also termed as distributed and dispersed generation) and consequent

restructuring of power transmission and distribution networks. As one of the prominent

power quality problems, the origin, consequences and mitigation techniques of voltage sag

problem has been discussed in detail. The study describes the technique of correcting the

supply voltage sag in a distribution system by power electronics based device called

Distribution STATCOM (D-STATCOM). D-STATCOM injects a current into the system to

correct the voltage sag. The steady state performance of DSTATCOM

is studied for various levels of voltage sag levels.

2

CHAPTER 2HARMONICS IN POWER SYSTEM

2.1 INTRODUCTION TO HARMONICS

One of the biggest problems in power quality aspects is the harmonic content in the electrical

system. Generally, harmonics may be divided into two types: 1) voltage harmonics, and 2)

current harmonics. Current harmonics is usually generated by harmonics contained in voltage

3

supply and depends on the type of load such as resistive load, capacitive load, and inductive

load. Both harmonics can be generated by either the source or the load side. Harmonics

generated by load are caused by nonlinear operation of devices, including power converters,

arc-furnaces, gas discharge lighting devices, etc. Load harmonics can cause the overheating

of the magnetic cores of transformer and motors. On the other hand, source harmonics are

mainly generated by power supply with non-sinusoidal voltage waveform. Voltage and

current source harmonics imply power losses, Electromagnetic Interference (EMI) and

pulsating torque in AC motor drives. Any periodic waveform can be shown to be the

superposition of a fundamental and a set of harmonic components. By applying Fourier

transformation, these components can be extracted. The frequency of each harmonic

component is an integral multiple of its fundamental. There are several methods to indicate of

the quantity of harmonics contents. The most widely used measure in North America is the

total harmonics distortion (THD), which is defined in terms of the amplitudes of the

harmonics, Hn, at frequency nw0, 2 where w0 is frequency of the fundamental component

whose amplitude of H1 and n is integer. The THD is mathematically given by

2.2 ADVANTAGES OF MULTI LEVEL POWER INVERTER:

1. The multi level power inverter is a strong candidate topology for the future

naval ship propulsion systems considering advantages over traditional inverters

2. Lower switching losses

3. Higher voltage capability ,

4

4. Higher power quality ,and

5. They are suitable for medium to high power applications.

6. They are an ideal inter phase between a utility and renewable energy

sources such as photovoltaic or fuel cells.

7. Their efficiency is very high (>98%) because of the minimum switching

frequency

8. They can improve the power quality and dynamic stability for utility systems.

9. Switching stress and EMI are low.

10. Because of their modular and simple structure, they can be stacked up to an

almost unlimited number of levels.

2.3 AN INTRODUCTION TO POWER SYSTEM HARMONICS

The objective of the electric utility is to deliver sinusoidal voltage at fairly constant

magnitude throughout their system. This objective is complicated by the fact that there are

loads on the system that produce harmonic currents. These currents result in distorted

voltages and currents that can adversely impact the system performance in different ways. As

the number of harmonic producing loads has increased over the years, it has become

increasingly necessary to address their influence when making any

additions or changes to an Installation. To fully appreciate the impact of this phenomenon,

there are two important concepts to bear in mind with regard to power system harmonics. The

first is the nature

of harmonic-current producing loads (non-linear loads) and the second is the way in which

harmonic currents flow and how the resulting harmonic voltages develop.

2.4 Linear and non-linear loads

A linear element in a power system is a component in which the current is proportional to the

voltage. In general, this means that the current wave shape will be the same as the voltage

(See Figure2.1). Typical examples of linear loads include motors, heaters and incandescent

lamps.

5

Figure2.1 –Voltageandcurrentwaveformsforlinear

On the other hand, the current wave shape on a non-linear load is not the same

16 as the voltage (See Figure2.2). Typical examples of non-linear loads include rectifiers

(power supplies, UPS units, discharge lighting), adjustable speed motor drives, ferromagnetic

devices, DC motor drives and arcing equipment

Figure 2.2 – Voltage and current waveforms for non-linear loads

The current drawn by non-linear loads is not sinusoidal but it is periodic, meaning that the

current wave looks the same from cycle to cycle. Periodic waveforms can be described

mathematically as a series of sinusoidal waveforms that have been summed together (See

Figure 2.3). The sinusoidal components are integer multiples of the fundamental where the

fundamental, in the United States, is 60 Hz. The only way to measure a voltage or current that

contains harmonics is to use a true-RMS reading meter. If an averaging meter is used, which

is the most common type, the error can be Significant.

6

Figure 2.3 Waveform with symmetrical harmonic component

Each term in the series is referred to as a harmonic of the fundamental. The third harmonic

would have a frequency of three times 60 Hz or 180 Hz. Symmetrical waves contain only odd

harmonics and un-symmetrical waves contain even and odd harmonics. A symmetrical wave

is one in which the positive portion of the wave is identical to the negative portion of the

wave. An un-symmetrical wave contains a DC component (or offset) or the load is such that

the positive portion of the wave is different than the negative portion. An example of un-

symmetrical wave would be a half wave rectifier. Most power system elements are

symmetrical. They produce only odd harmonics and have no DC offset. There are exceptions,

of course, and normally symmetrical devices may produce even harmonics due to component

mismatches or failures. Arc furnaces are another common source of even harmonics but they

are

notorious for producing both even and odd harmonics at different stages of the process.

2.5 Harmonic current flow

When a non-linear load draws current, that current passes through all of the impedance that is

between the load and the system source (See Figure2.4). As a result of the current flow,

harmonic voltages are produced by impedance in the system for each harmonic.

7

Figure 2.4 – Distorted-current induced voltage distortion

These voltages sum and when added to the nominal voltage produce voltage distortion. The

magnitude of the voltage distortion depends on the source impedance and the harmonic

voltages produced.

If the source impedance is low then the voltage distortion will be low. If a significant portion

of the load becomes non-linear (harmonic currents increase) and/or when a resonant

condition prevails (system impedance increases), the voltage can increase dramatically.

Power systems are able to absorb a considerable amount of current distortion without

problems and the distortion produced by a facility may be below levels recommended in

IEEE 519. However, the collective effect of many industrial

customers, taken together, may impact a distribution system. When problems arise, they are

usually associated with resonant conditions.

2.6 Harmonic currents can produce a number of problems, namely:

o Equipment heating

o Equipment malfunction

o Equipment failure

o Communications interference

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o Fuse and breaker mis-operation

o Process problems

o Conductor heating

CHAPTER 3 VOLTAGE DIPS

9

3.1 INTRODUCTION TO VOLATAGE DIPS

A voltage dip is a short-term reduction in, or complete loss of, RMS voltage. It is specified in

terms of duration and retained voltage, usually expressed as the percentage of nominal RMS

voltage remaining at the lowest point during the dip. A voltage dip means that the required

10

energy is not being delivered to the load and this can have serious consequences depending

on the type of load involved. Voltage sags – longerterm reductions in voltage – are usually

caused by a deliberate reduction of voltage by the supplier to reduce the load at times of

maximum demand or by an unusually weak supply in relation to the load.

Motor drives, including variable speed drives, are particularly susceptible because the load

still requires energy that is no longer available except from the inertia of the drive. In

processes where several drives are involved individual motor control units may sense the loss

of voltage and shut down the drive at a different voltage level from its peers and at a different

rate of deceleration resulting in complete loss of process control. Data processing and control

equipment is also very sensitive to voltage dips and can suffer from data loss and extended

downtime. The cost implications are very serious and are discussed in Section 2. There are

two main causes of voltage dips; starting of large loads either on the affected site or by a

consumer on the same circuit and faults on other branches of the network.

3.2 Dips caused by large loads

When heavy loads are started, such as large drives, the starting current can be many times the

normal running current. Since the supply and the cabling of the installation are dimensioned

for normal running current the high initial current causes a voltage drop in both the supply

network and the installation. The magnitude of the effect depends on how ‘strong’ the

network is, that is, how low the impedance is at the

22 point of common coupling (PCC) and on the impedance of the installation cabling. Dips

caused by starting currents are characterized by being less deep and much longer than those

caused by network faults –typically from one to several seconds or tens of seconds, rather

than less than one second. On-site problems, caused by too high resistance in the internal

cabling, are easily dealt with. Large loads should be wired directly back to the origin of the

appropriate voltage level – either the PCC or the secondary of the supply transformer. If the

problem is caused by the impedance of the PCC – i.e. the supply is too ‘weak’– then further

action is required. One solution, if applicable to the equipment in question, is to fit a soft

starter so that the starting current is limited to a lower value but is required for rather longer.

Another solution is to negotiate with the supply company for a lower impedance connection –

but this may be expensive depending on the geography of the network in the area.

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Fig. 3.1 The cause of voltage dips

If the cause of the voltage reduction cannot be controlled, then other equipment will be

needed to compensate for it. Suitable equipment ranges from the traditional servo controlled

mechanical stabilizers to electronically controlled tap changers and dynamic voltage restorers

3.3 Dips network originating from faults

The supply network is very complex. The extent of a voltage dip at one site due to a fault in

another part of the network depends on the topology of the network and the relative source

impedances of the fault, load and generators at their common point of coupling. Figure 1

shows an example. A fault at position F3 results in a dip to 0 % at Load 3, a dip to 64 % at

Load 2 and to 98 % at Load 1. A fault at F1 will affect all users with a dip to 0 % at Load 1

and to 50 % for all other loads. Notice that a fault at Level 1 affects many more consumers

more severely than a fault at Level 3. Loads connected at Level 3 are likely to experience

many more dips than a load connected at Level 1 because there are more potential fault sites

– they are affected by Level 1 and level 2 faults. Loads at Level 2 and 1 are progressively less

sensitive to faults at Level 3. The‘closer’ the load is to the source, the fewer and the less

severe the dips will be. The

12

duration of the dip depends on the time taken for the protective circuits to detect and isolate

the fault and is usually of the order of a few hundred milliseconds. Since faults can be

transitory, for example when caused by a tree branch falling onto a line, the fault can be

cleared very soon after it has occurred. If the circuit were to be permanently disconnected by

the protection equipment then all consumers on the circuit would experience a blackout until

the line could be checked and reconnected. Auto reclosers can help to ease the situation, but

also cause an increase in the number of dips. An autorecloser attempts to reconnect the circuit

a short time (less than 1 second) after the protection equipment has operated. If the fault has

cleared, the autoreclose will succeed and power is restored. Loads on that circuit experience a

100 % dip between disconnection and autoreclose while other loads see a smaller, shorter dip

between the fault occurring and being isolated, as discussed above. If the fault has not cleared

when 24 the autorecloser reconnects, the protective equipment will operate again; the process

can be repeated according to the program set for the particular autorecloser. Each time the

autorecloser reconnects the faulty line another dip results, so that other consumers can

experience several dips in series. Utility performance in deregulated markets is partly - in

some countries, such as UK, solely - judged on the average ‘customer minutes lost’, taking

into account interruptions exceeding, typically, one minute. Minimizing this statistic has

resulted in the widespread application of autoreclosers and an increase in the probability of

dips. In other words, long term availability has been maximized but at the expense of quality.

3.4 Equipment sensitivity

Computers are now essential to all businesses, whether as workstations, network servers or as

process controllers. They are vital to all data processing transactions and many

communications functions, such as email and voice box systems. It was the introduction of

computer equipment that first highlighted the problem of voltage dips (in fact, most power

quality problems) and early installations were plagued with

seemingly random failures that resulted in considerable support effort being required. The

learning process resulted in the production of the Computer and Business Equipment

Manufacturers Association (CBEMA) curve (Figure 2). This curve has since been modified

and is now known as the Information Technology Industry Council (ITIC) curve (Figure 3)

and a version of it has been standardized by ANSI as IEEE 446 (Figure 4). Duration of an

event is plotted against voltage with respect to the nominal supply voltage and the curves

define the envelope within which equipment should continue to function without interruption

or data loss. As far as dips are concerned it is the lower limit line that is of interest. This line

13

represents the boundary between survivable and non-survivable dips. In an ideal world there

would be just one curve that represented real-world supply network performance and to

which all equipment would

comply. In fact, while quite a lot of equipment meets the requirement of one or other of the

standard curves, the performance of supply networks falls far short

Fig.3.2 CBEMA curve

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Fig.3.3 ITIC curve

3.4 ANSI curve

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3.5 Characteristics of equipment sensitivity

Electronic equipment power supplies, such as those used in personal computers (PC) and

programmable logic controllers (PLC) employ a reservoir capacitor to smooth out the peaks

of the full wave rectified waveform, so they should be inherently resilient to short duration

dips. The larger the capacitor, and the greater the difference between the stored capacitor

voltage and the minimum required for the internal voltage converters to operate, the better the

resilience will be. Designers will always try to reduce the size of the capacitor to a minimum

to reduce size, weight and cost while ensuring that the charge stored is just sufficient at

minimum voltage and maximum load. For good dip resilience a much larger capacitor is

required, at least twice as large to enable the equipment to ride through one cycle, and 100

times as large if a one-second 27 ride through was required. An alternative design strategy is

to keep the minimum input voltage as low as possible to maximize the hold up time of the

system.

This is the approach taken, by default, in equipment designed to work over a wide range of

voltage. The hold up time will be much greater with a 230 V supply than it will be with a 110

V supply. There is no technical problem in making a dip resistant power supply but it is not

done because it is not an issue that users raise with manufacturers and there are cost

implications. Nevertheless, the cost of making a PC or PLC resilient to 1 second dips is very

small compared to the cost of improving the network assets to prevent such a dip occurring.

Variable speed drives can be damaged by voltage dips and are usually fitted with under

voltage detectors that trip at 15 % to 30 % below nominal voltage. Variable speed drives with

enhanced ride through capability are the subject of a later section of this Guide.

Induction motors have inertia so they may help to support the load during a short dip,

regenerating energy as they slow down. This energy has to be replaced as the motor re-

accelerates and, if the speed has reduced to less than 95 %, it will draw nearly the full start-up

current. Since all the motors are ‘starting’ together, this may be the cause of further problems.

Relays and contactors are also sensitive to voltage dips and can often be the weakest link in

the system. It has been established that a device may drop out during a dip even when the

retained voltage is higher than the minimum steady state hold-in voltage. The resilience of a

contactor to dips depends not only on the retained voltage and duration but also on the point

on the waveform where the dip occurs, the effect being less at the peak. Sodium discharge

16

lamps have a much higher striking voltage when hot than cold, so that a hot lamp may not

restart after a dip.

The magnitude of dip that will cause a lamp to extinguish may be as little as 2 % at the end of

life or as high as 45 % when new. Most appliances and systems incorporate one or more of

the above elements, and so will exhibit problems when subject to dips. Figure 5, below,

suggests that it is cheaper and more reliable to design equipment to be resilient to dips, rather

than to try to make the whole process, whole plant, or the whole electricity distribution

system resilient. As shown here, the cost of solution increases rapidly as the point of cure is

moved from equipment through plant to infrastructure.

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CHAPTER 4SOLUTIONS TO POWER QUALITYPROBLEMS

4. SOLUTIONS TO POWER QUALITY PROBLEMS

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4.1 INTRODUCTION

There are two approaches to the mitigation of power quality problems. The solution 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.

A flexible and versatile solution to voltage quality problems is offered by active power filters.

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

operates as a controllable voltage source. Both schemes are implemented preferable with

voltage source PWM inverter s [5], with a dc bus having a reactive element such as a

capacitor. Active power filters can perform one or more of the functions required to

compensate power systems and improving power quality. Their performance also depends on

the power rating and the speed of response. 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.

4.1.1 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 correct

quickly 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

static switch can be used in the alternate power line applications. This scheme requires two

independent power lines from the utility or could be from utility and localized power

generation like those in case of distributed generating systems . Such a scheme can protect up

to about 85 % of interruptions and voltage sags.

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4.1.2 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 or the energy that would be lost by the voltage sag

or interruption. In case of utility supply backed by a localized generation this can be even

better accomplished.

4.1.3 Electronic tap changing transformer:

A voltage-regulating transformer with an electronic load tap changer can be used with a

single line from the utility. It can regulate the voltage drops up to 50% and requires a stiff

system (short circuit power to load ratio of 10:1 or better). It can have the provision of coarse

or smooth steps intended for occasional voltage variations.

4.1.4 Harmonic Filters

Filters are used in some instances to effectively reduce or eliminate certain harmonics. If

possible, it is always preferable to use a 12-pluse or higher transformer connection, rather

than a filter. Tuned harmonic filters should be used with caution and avoided when possible.

Usually, multiple filters are needed, each tuned to a separate harmonic. Each filter causes a

parallel resonance as well as a series resonance, and each filter slightly changes the

resonances of other filters.

4.1.5 Constant-Voltage Transformers:

For many power quality studies, it is possible to greatly improve the sag and momentary

interruption tolerance of a facility by protecting control circuits. Constant voltage transformer

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(CVTs) can be used [6] on control circuits to provide constant voltage with three cycle ride

through, or relays and ac contactors can be provided with electronic coil hold-in devices to

prevent disoperation from either low or interrupted voltage.

4.1.6 Digital-Electronic and Intelligent Controllers for Load-Frequency

Control:

Frequency of the supply power is one of the major determinants of power quality, which

affects the equipment performance very drastically. Even the major system components such

as Turbine life and interconnected-grid control are directly affected by power frequency.

Load frequency controller used specifically for governing power frequency under varying

loads must be fast enough to make adjustments against any deviation. In countries like India

and other countries of developing world, still use the controllers which are based either or

mechanical or electrical devices with inherent dead time and delays and at times also suffer

from ageing and associated effects. In future perspective, such cont rollers can be replaced by

their Digital –electronic counterparts.

]

4.2 USE OF CUSTOM POWER DEVICES TO IMPROVE POWER QUALITY

In order to overcome the problems such as the ones mentioned above, the concept of custom

power devices is introduced recently; custom power is a strategy, which is designed primarily

to meet the requirements of industrial and commercial customer. The concept of custom

power is to use power electronic or static controllers in the medium voltage distribution

system aiming to supply reliable and high quality power to sensitive users. Power electronic

valves are the basis of those custom power

devices such as the static transfer switch, active filters and converter-based devices.

Converter based power electronics devices can be divided in to two groups: shunt connected

and series-connected devices. The shunt connected devices is known as the DSTATCOM and

the series device is known as the Thyristor Controlled Swiched Capacitor(TCSC). It has also

been reported in literature that both the TCSC and DSTATCOM have been used to mitigate

the majority the power system disturbances such as voltage dips, sags, flicker unbalance and

harmonics,stability problems. For lower voltage sags, the load voltage magnitude can be

corrected by injecting only reactive power into the system. However, for higher voltage sags,

21

injection of active power, in addition to reactive power, is essential to correct the voltage

magnitude. Both TCSC and DSTATCOM are capable of generating or absorbing reactive

power but the active power injection of the device must be provided by an external energy

source or energy storage system. The response time of both TCSC and DSTATCOM is very

short and is limited by the power electronics devices. The expected response time is about 25

ms, and which is much less than some of the traditional methods of voltage correction such as

tap - changing transformers.

4.3 MODELING OF CUSTOM POWER DEVICE AND SIMULATION

RESULTS

As mentioned in the previous section that custom power devices could be the effective means

to overcome some of the major power quality problems by the way of injecting active and/or

reactive power(s) into the system.

This section deals with the modeling of DSTATCOM . Consequently some case studies will

be taken up for analysis and performance comparison of these devices. The modeling

approach adopted in the paper is graphical in nature, as opposed to mathematical models

embedded in code using a high-level computer language. The well-developed graphic

facilities available in an industry standard power system package, namely, MATLAB

(/Simulink) [12], is used to conduct all aspects of model implementation and to carry out

extensive simulation studies. The control scheme for these devices is shown in Fig.1. The

controller input is an error signal obtained from the reference voltage and the value rms of the

terminal voltage measured. Such error is processed by a PI controller and the output is the

angle δ, which is provided to the PWM signal generator. The PWM generator then generates

the pulse signals to the IGBT gates of voltage source converter

22

Fig 4.1 The PI Controller

CHAPTER 5DISTRIBUTION STATCOM

23

5.1 INTRODUCTION TO DSTATCOM

This chapter presents the operating principles of DSTATCOM. The DSTATCOM is basically

one of the custom power devices. It is nothing but a STATCOM but used at the Distribution

level. The key component of the DSTATCOM is a power VSC that is based on high power

electronics technologies.

The Distribution STATCOM is a versatile device for providing reactive compensation in ac

networks. The control of reactive power is achieved via the regulation of a controlled voltage

source behind the leakage impedance of a transformer, in much the same way as a

conventional synchronous compensator. However, unlike the conventional synchronous

compensator, which is essentially a synchronous

24

generator where the field current is used to adjust the regulated voltage, the DSTATCOM

uses an electronic voltage sourced converter (VSC), to achieve the same regulation task. The

fast control of the VSC permits the STATCOM to have a rapid rate of response.

The DSTATCOM is the solid – state based power converter version of the SVC. Operating as

a shunt – connected SVC, its capacitive or inductive output currents can be controlled

independently from its connected AC bus voltage. Because of the fastswitching characteristic

of power converters, the DSTATCOM provides much faster response as compare to SVC.

DSTATCOM is a shunt connected, reactive compensation equipment, which is capable of

generating and or absorbing reactive power whose

output can be varied so as to maintain control of specific parameters of the electric power

system. DSTATCOM provides operating characteristics similar to a rotating synchronous

compensator without mechanical inertia, due to the DSTATCOM employ solid state power

switching devices it provides rapid controllability of the three phase voltages, both in

magnitude and phase angle.

In addition, in the event of a rapid change in system voltage, the capacitor voltage does not

change instantaneously; therefore the DSTATCOM reacts for the desired responses. For

example, if the system voltage drops for any reason, there is a tendency for the DSTATCOM

inject capacitive power to support the dipped voltages.

5.2 Operating Principle of the DSTATCOM

Basically, the DSTATCOM system is comprised of three main parts: a VSC, a set of

coupling reactors and a controller. The basic principle of a DSTATCOM installed in a power

system is the generation of a controllable ac voltage source by a voltage source inverter (VSI)

connected to a dc capacitor (energy storage device). The ac voltage source in general, appears

behind a transformer leakage reactance. The active and reactive power transfer between the

power system and the DSTATCOM is caused by the voltage difference across this reactance.

The DSTATCOM is connected to the power networks at

a PCC, where the voltage-quality problem is a concern. All required voltages and currents are

measured and are fed into the controller to be compared with the commands. The controller

25

then performs feedback control and outputs a set of switching signals to drive the main

semiconductor switches (IGBT’s, which are used at the distribution level) of the power

converter accordingly. The basic diagram of the

DSTATCOM is illustrated in Fig 5.1.

Fig 5.1 Block Diagram of the voltage source converter based DSTATCOM

The ac voltage control is achieved by firing angle control. Ideally the output voltage of the

VSI is in phase with the bus (where the DSTATCOM is connected) voltage. In steady state,

the dc side capacitance is maintained at a fixed voltage and there is no real power exchange,

except for losses. The DSTATCOM differs from other reactive power generating devices

(such as shunt Capacitors, Static Var Compensators

etc.) in the sense that the ability for energy storage is not a rigid necessity but is only required

for system unbalance or harmonic absorption. There are two control objectives implemented

in the DSTATCOM. One is the ac voltage regulation of the power system at the bus where

the DSTATCOM is connected

and the other is dc voltage control across the capacitor inside the DSTATCOM. It is widely

known that shunt reactive power injection can be used to control the bus voltage. In

conventional control scheme, there are two voltage regulators designed for these purposes: ac

voltage regulator for bus voltage control and dc voltage regulator for capacitor voltage

control. In the simplest strategy, both the regulators are proportional integral (PI) type

controllers. Thus, the shunt current is split into d-axis and q-axis

26

components. The reference values for these currents are obtained by separate PI regulators

from dc voltage and ac-bus voltage errors, respectively. Then, subsequently, these reference

currents are regulated by another set of PI regulators whose outputs are the d-axis and q-axis

control voltages for the DSTATCOM.

5.3 Principle of Voltage Regulation

5.3.1 Voltage Regulation without Compensator:

Consider a simple circuit as shown in Fig 5.2. It consists of a source Voltage E, V is the

voltage at a PCC and a load drawing the current Il. Without a voltage compensator[8], the

PCC voltage drop caused by the load current Il, shown in fig as ΔV,

s l ΔV = E −V = Z * I ,

S =VI* ,so S* =V*I

The voltage change has a component ΔVr in phase with V and component ΔVx, which are

illustrated in Fig 5.2(a). It is clear that both magnitude and the phase of V, relative to the

supply voltage E, are functions of the magnitude and phase of the load current namely the

voltage drop depends on both the real and reactive power of the load. The component ΔV is

rewritten as

ΔV = I R + jI X

Fig 5.2 A Simple Circuit for demonstrating the voltage regulation principle.

27

Fig 5.2 (a) Phasor diagram for uncompensated

5.3.2 Voltage regulation with DSTATCOM:

Now consider a compensator connected to the system. It is as shown in Fig 5.2(b) shows

vector diagram with voltage compensation. By adding a compensator in parallel with the

load, it is possible to make E=V by controlling the current of the compensator.

Is =Ir + Il Where Ir is the compensating current.

Fig 5.2(b) Phasor diagram for voltage regulation with compensation

5.4 APPLICATIONS OF DSTATCOM

Typical STATCOM applications:

• Utilities with weak grid knots or

28

fluctuating reactive loads• Unbalanced loads• Arc furnaces• Wind farms• Wood chippers• Welding operations• Car crushers & shredders• Industrial mills• Mining shovels & hoists• Harbor cranes

CHAPTER 6MODELING OF DSTATCOM

29

6.1 DSTATCOM VOLTAGE REGULATION TECHNIQUE

The DSTATCOM improves the voltage sags and swell conditions and the ac output voltage at the

customer points is improved, thus improving the quality of power at the distribution side In this thesis

the voltage controller technique (also called as decouple technique) is used as the control technique

for DSTATCOM. The method is already discussed in the previous topic. This control strategy uses

the dq0 rotating reference frame because it offers higher accuracy than stationary frame-based

techniques. In this VABC are the three-phase terminal voltages, Iabc are the three-phase currents injected

by the DSTATCOM into the network, Vrms is the root-mean-square (rms) terminal voltage, Vdc is the

dc voltage measured in the capacitor, and the superscripts indicate reference values. Such a controller

employs a phase-locked loop (PLL) to synchronize the three phase voltages at the converter output

with the zero crossings of the fundamental component of the phase-A terminal voltage. The block

diagram of a proposed control technique is shown in Fig 6.4. Therefore, the PLL provides the angle φ

to the abc-to-dq0 (and dq0-to-abc) transformation. There are also four proportional-integral (PI)

regulators.

The first one is responsible for controlling the terminal voltage through the

reactive power exchange with the ac network. This PI regulator provides the reactive current reference

Iq*, which is limited between +1pu capacitive and -1pu inductive. Another PI regulator is responsible

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for keeping the dc voltage constant through a small active power exchange with the ac network,

compensating the active power losses in the transformer and inverter. This PI regulator provides the

active current reference Id*. The other two PI regulators determine voltage reference Vd*, and Vq*,

which are sent to the PWM signal generator of the converter, after a dq0-to-abc transformation.

Finally, Vabc* are the three-phase voltages desired at the converter output.

6.2 IMPEMENTATION OF D-STATCOMThe test system employed to carry out the simulations concerning the DSTATCOM actuation for

voltage sag compensation is shown in Fig.6.1 Such system is composed by a 230 kV, 50 Hz

transmission system, represented by a Thevenin equivalent, feeding a distribution network through a

3-winding transformer connected in Y/Y/Y, 230/11/11 kV. To verify the working of a DSTATCOM,

a variable load is connected at bus 2. During the simulation, in the period from 500 to 900 ms, the

switch S1 is closed .The above test system is simulated under the environment of Matlab - Simulink

and power system block set (PSB) the model used for this purpose is shown in the fig.6.2.

Fig 6.1 block diagram DSTATCOM

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Fig6.2. MATLAB Simulation model of test system with DSTATCOM

A set of simulations was carried out for the test system shown in Fig.6.5. The

simulations relate to three main operating conditions.

1) In the simulation period 500–900 ms, the load is increased by closing switch S1. In this

case, the voltage drops by almost 27% with respect to the reference value.

2) At 900 ms, the switch S1 is opened and remains so throughout the rest of the simulation.

The load voltage is very close to the reference value, i.e., 1 pu.

3) In order to gain insight into the influence that capacitor size has on D STATCOM

performance, simulations were carried out with different size of capacitors. The total simulation

period is 1.4 s. The rms voltage at the load point for the case when the system operates with

no D-STATCOM. Similarly, a new set of simulations was carried out but now with the D-STATCOM

connected to the system. Where the very effective voltage regulation provided by the D-STATCOM

can be clearly appreciated.When the Switch S1 closes, the D-STATCOM supplies reactive power to

the system, shows the regulated rms voltage corresponding to a 750 F capacitor, where a rapid

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regulation response is obtained and transient overshoots are almost nonexistent. This contrasts with

cases where the capacitor is undersized. For instance the rms voltage for the case when a 75 F

capacitor is employed.

CHAPTER 7RESULTS AND ANALYSIS

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7.1 DSTATCOM RESULTS FOR TEST SYSTEM:-

7.1.1 Voltage response of the test system without DSTATCOM:-Fig7.1 Voltage response of the test system without DSTATCOM

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Fig.7.1.

shows the rms voltage at the load point for the case when the system

operates with no D-STATCOM. It has been seen observed that there is voltage dip when the load is

increased by closing the circuit breaker of fig.6.6.

71.2 Voltage response of the Test system with DSTATCOM (With 750ufcapacitor)

Fig 7.2(a) Voltage response of the test system with DSTATCOM

(With 750uf capacitor)

Fig. 7.2(a) shows voltage response of the test system with D-STATCOM (With

750uf capacitor). The result shows that, the D-STATCOM supplies reactive power to the system and

compensated the voltage dip to maintain constant voltage profile. It also shows that a rapid regulation

response is obtained and transient overshoots are almost nonexistent with a 750uf capacitor.

7.1.3 Voltage response of the test system with DSTATCOM (With 75uf

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capacitor)

Fig7.2(b) Voltage response of the test system with DSTATCOM

(With 75uf capacitor)

Fig. 7.2(b) shows voltage response of the test system with D-STATCOM (With 75uf

capacitor). The result shows that, the D-STATCOM supplies reactive power to the

system and compensated the voltage dip to maintain constant voltage profile. It also shows that a

voltage regulation response is obtained with little transient overshoots using a 75uf capacitor. From

the above diagrams it is proved that DSTATCOM reduces voltage dips and so that it improves the

power quality.

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37

LITERATURE SURVEY

38

LITERATURE SURVEY

The last decade has seen a marked increase on the deployment of end user equipment that is

highly sensitive to poor quality control electricity supply. Several large industrial users are

reported to have experienced large financial losses as a result of even minor lapses in the

quality of electricity supply . A great many efforts have been made to remedy the situation,

where the solutions based on the use of latest power electronic technology figure

prominently. Indeed custom power technology, the low voltage counterpart of the more

widely known flexible as transmission system (FACTS) technology, aimed at high voltage

power transmission applications, has emerged as a

credible solution to solve many of the problems relating to continuity of supply at the end

user level. The various power quality Problems at the Distribution level are voltage sag and

swells, fluctuations, harmonics, flickering etc . Recently, various power electronic technology

devices have been proposed especially to be applied to medium voltage networks, generally

named custom power. Custom power concept introduced by N.G.Hingorani has been

proposed to ensure high quality of power supply in distribution networks using power

electronics devices. Additionally, various custom power devices are based on the voltage

source converter technology introduced by N.G.Hingorani and L.Gyugyi . At present, wide

range of very flexible controllers, which capitalize on newly available power electronics

components, are emerging for custom power applications. Among these the Distribution

static Compensator (DSTATCOM) and dynamic voltage restorer (DVR), both of them based

on the VSC principle given by L.Gyugyi , and the SSTS are the controllers which have

received the most attention. The modeling and analysis of these custom power devices has

applied for the study of power quality by Olimpo Anaya-Lara and E Acha presenting

comprehensive results to assess the performance of each device as a potential custom power

application. The different control techniques of DSTATCOM are discussed in papers . Sung-

Min Woo, Dae- wook kang, Woo-Chol Lee, Dong-Seok Hyun, have demonstrated a new

control technique for 4 reducing effect of Voltage Sag and Swell with DSTATCOM . In this

Thesis the DSTATCOM is simulated with Voltage Regulation Technique . The interest in

distributed generation has considerably increased due to market deregulation, technological

advances, governmental incentives, and environment impact concerns . At present, most

distributed generation Installations employ induction and synchronous machines, which can

39

be used in thermal, hydro, and wind generation plants . Although such technologies are well

known, there is no consensus on what is the best choice under a wide technical perspective.

In the paper by Prof.Mrs. P.R.Khatri,

Prof.Mrs. V.S.Jape, Prof.Mrs. N.M.Lokhande, Prof.Mrs. B.S.Motling, they

discussed the main problems associated with DG and also how to interface the DG to the

utility systems.

M. I. Marei, E. F. El-Saadany and M. M. A. Salama, in their work dealt with the

Flexible Distributed Generation proposed a novel control scheme for the nonlinear link

connecting DG to the distribution network using a current controlled Voltage Source

Inverter (VSI).

40

CONCLUSION

Power quality measures can be applied both at the user end and also at the utility level.

The work identifies some important measures that can be applied at the utility level without much

system upset (or design changes).This project has presented models of custom power equipment,

namely D -STATCOM, and applied them to mitigate voltage dip which is very prominent as per

utilities are concerned. The highly developed graphic facilities available in MATLAB/ SIMULINK

were used to conduct all aspects of model implementation and to carry out extensive simulation

studies on test system. A new PWM-based control scheme has been implemented to control the

electronic valves in the two –level VSC used in the D-STATCOM . As opposed to fundamental

frequency switching schemes already available in the MATLAB/SIMULINK. This characteristic

makes it ideally suitable for low-voltage custom power applications. It was observed that in case of

DSTATCOM capacity for power compensation and voltage regulation depends mainly on the rating

of the dc storage device. It can be concluded from the TCSC results that the impedance can be varied

from capacitive mode to inductive mode so that the voltage can be raised during heavy inductive load

periods and the voltage can be reduced to within limits so that voltage is controlled. By using TCSC

no harmonics are introduced. Since the supply current is pure sinusoidal stability can also be

improved by varying the reactance.

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REFERENCES H. Hingorani “Introducing custom power” IEEE spectrum, vol.32 no.6

June 1995 p 41-48.

Ray Arnold “Solutions to Power Quality Problems” power engineering journal 2001 pages:

65-73.

John Stones and Alan Collinsion “Introduction to Power Quality” power engineering journal

2001 pages: 58 -64.

Gregory F. Reed, Masatoshi Takeda, "Improved power quality solutions usingadvanced solid-

state switching and static compensation technologies," PowerEngineering Society 1999

Winter Meeting, IEEE

G. Venkataramanan and B. Johnson, “A pulse width modulated power line conditioner for

sensitive load centers,” IEEE Trans. Power Delivery, vol. 12, pp. 844– 849, Apr. 1997.

N.G. Hingorani and L. Gyugyi, “Understanding FACTS: Concepts and Technology of Flexible

AC Transmission Systems”, 1st edition, The Institute of Electrical and Electronics Enginee

rs,2000.

F. Z. Peng, H. Akagi, and A. Nabae, “Compensation characteristics of the combined system of

shunt passive and series active filters,” IEEE Trans. Ind. Applicat., vol. 29, pp. 144–151,

Jan./Feb. 1993.

M.H.Haque “Compensation of distribution system voltage sag by DVR and DSTATCOM”

Power Tech Proceedings, 2001 IEEE Porto , Volume: 1 , 10-13 Sept. 2001 Pages:5 pp. vol.1

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