enhancement of power quality using dstatcom

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CHAPTER-1

INTRODUCTIONINTRODUCTION

OVERVIEW:

The electric power system is considered to be composed of three

functional blocks - generation, transmission and distribution. For a reliable

power system, the generation unit must produce adequate power to meet

customers demand, transmission systems must transport bulk produce adequate

power to meet power over long distances without overloading system stability

and distribution systems must deliver electric power to each customers

premises from bulk power systems

Distribution system locates the end of power system and is connected to

the customer directly, so the power quality mainly depends on distribution

system. The reason behind this is that the electrical distribution network failures

account for about 90% of the average customer interruptions. In the earlier days,

the major focus for power system reliability was on generation and transmissiononly as these more capital cost is involved in these. In addition their

insufficiency can cause widespread catastrophic consequences for both society

and its environment.

But now a days distribution systems have begun to receive more

attention for reliability assessment. Initially for the improvement of power

quality or reliability of the system FACTS devices like static synchronous

compensator (STATCOM), static synchronous series compensator SSSC),

interline power flow controller (IPFC), and unified power flow controller

(UPFC) etc are introduced.

These FACTS devices are designed for the transmission system. But

now a days more attention is on the distribution system for the improvement of

power quality, these devices are modified and known as custom power devices.

The main custom power devices which are used in distribution system for

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power quality improvement are distribution static synchronous compensator

(DSTATCOM), dynamic voltage Restorer (DVR), active filter (AF), unified

power quality conditioner (UPQC) etc.

In this from the above custom power devices, D-Statcom and

D-statcom with LCL passive filter is used to reduce voltage sag and total

harmonic distortions.

2.1..LITERATURE SURVEY:

Power Quality in electric networks is one of today's most concerned

areas of electric power system. The power quality has serious economic

implications for consumers, utilities and electrical equipment manufacturers.

The impact of power quality problems is increasingly felt by customers -

industrial, commercial and even residential. Some of the main power quality

problems are sag, swell, transients, harmonic, and flickers etc

By custom power devices, we refer to power electronic static controllers

used for power quality improvement on distribution systems rated from 1 to 38

kV . This interest in the practice of power quality devices (PQDs) arises from

the need of growing power quality levels to meet the everyday growing

sensitivity of customer needs and expectations. One of those devices is the

Distribution Static Compensator (DSTATCOM), which is the most efficient and

effective modern custom power device used in power distribution networks. Its

application includes lower cost, smaller size, and its fast dynamic response to

the disturbance.

Several research papers and reports addressed the subject of improving

power quality in distribution system by the use of custom power devices. The

followings present a brief review of the work undertaken so far

N.G. Hingorani, [1] presents the concept of custom power is now becoming

familiar. The term describes the value-added power that electric utilities and

other service providers will offer their customers in the future. The enhanced

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level of reliability of this power, in terms of reduced interruptions and less

variation, will stem from an integrated solution to present problems, of which a

prominent feature will be the application of power electronic controllers to

utility distribution systems and/or at the supply end of many industrial and

commercial customers and industrial parks.

Yash Pal, A. Swarup, et al. [2] presents a comprehensive review of

compensating custom power devices mainly DSTATCOM (distribution static

compensator), DVR (dynamic voltage restorer) and UPQC (unified power

quality compensator). It is aimed at providing a broad viewpoint on the status of

compensating devices in electric power distribution system to researchers and

application engineers dealing with power quality problems. is highly required to

increase the reliability of the distribution system

2.2. SCOPE OF WORK :

From the literature review, it is observed that the work on the

investigation on power with compensating devices is very much diversified.

However it is observed that there is a scope to investigate the effectiveness of

compensating devices for different loads and with different depends on

distribution system. As the customers demand for the reliability of power

supply is increasing day by day, so the reliability of the distribution system has

to be increased. Electrical distribution network failures account for about 90%

of the average customer interruptions. So it is highly required to increase the

reliability of the distribution system.

The objective of the proposed work is to improve the power quality or

reliability in the distribution system with the use of custom power device.

Different conditions are considered to analyze the operation of D-Statcom for

the improvement the power quality in distribution system.

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CHAPTER-2

POWER QUALITY

3.1.INTRODUCTION

Power quality is The provision of voltages and system design so that

the user of electric power can utilize electric energy from the distribution

system successfully without interference or interruption. A broad definition of

power quality borders on system reliability, dielectric selection on equipment

and conductors, long-term outages, voltage unbalance in three-phase systems,power electronics and their interface with the electric power supply and many

other areas.

3.2 POWER QUALITY- A BIG ISSUE

Power quality in electric networks is one of today's most concerned



Modernization and automation of industry involves increasing use of

computers, microprocessors and power electronic systems such as adjustable

speed drives. Integration of non-conventional generation technologies such as

fuel cells, wind turbines and photo-voltaic with utility grids often requires

power electronic interfaces.

The power electronic systems also contribute to power quality problems

(generating harmonics). Under the deregulated environment, in which electric

utilities are expected to compete with each other, the customer satisfaction

becomes very important. The impact of power quality problems is increasingly

felt by customers - industrial, commercial and even residential.

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3.3 PROBLEMS ASSOCIATED WITH POWER QUALITY

3.3.1 MOMENTARY PHENOMENA

3.3.1.1 Transients:

Transients are unwanted decay with time and hence not a steady state

problem. A broad definition is that a transient is that part of the change in a

variable that disappears during transition from one steady state operating

situation to the other". Another synonymous term which can be used is surge.

3.3.1.2 Long Duration Voltage Variations:

When rms (root mean square) deviations at power frequency last longer

than one minute, then we say they are long duration voltage variations. They

can be either over voltages which is greater than 1.1p.u or under voltages which

is less than 0.9p.u.

Over voltage is due to switching off a load or energizing a capacitor

bank. Also incorrect tap settings on transformers can result in over voltages.

Under voltage are the results of actions which are the reverse of events that

cause over voltages i.e. switching in a load or switching off a capacitor bank.

3.3.1.3 Sustained Interruptions:

If the supply voltage becomes zero for a period of time which is greater

than one minute, then we can say that it is a sustained interruption. Normally,

voltage interruption lasting for more than one minute is often unending and

requires human intervention to restore the supply. The term outage is also

used for long interruption. However it does not bring out the true impact of the

power interruption. Even an interruption of half a cycle can be disastrous for a

customer with a sensitive load.

3.3.1.4 SHORT DURATION VOLTAGE VARIATIONS:

The short duration voltage variations are generally caused by fault

conditions like single line to ground or double line to ground and starting of

large loads such as induction motors. The voltage variations can be temporary

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voltage dips i.e. sag or temporary voltage rise i.e. swells or a absolute loss of

voltage which is known as interruptions .

Voltage Sags:

Voltage sag is defined as the reduction of rms voltage to a value

between 0.1 and 0.9p.u and lasting for duration between 0.5 cycle to 1 minute.

Voltage sags are mostly caused by system faults and last for durations ranging

from 3 cycles to 30 cycles depending on the fault clearing time. It is to be noted

that under-voltages (lasting over a minute) can be handled by voltage

regulation equipment. Starting of large induction motors can result in voltage

dip as the motor draws a current up to 10 times the full load current during the

starting. Also, the power factor of the starting current is generally poor.

VOLTAGE SWELLS:

A voltage swell is defined as a raise in rms voltage which is between 1.1

and 1.8p.u for time duration between 0.5 cycles to 1 minute. A voltage swell is

characterized by its magnitude (rms) and duration. As with sag, swell is

associated with system faults. A SLG (single line to ground) fault can result in a

voltage swell in the healthy phases. Swell can also result from energizing a

large capacitor bank. On an ungrounded system, the line to ground voltages on

the ungrounded phases is 1.73p.u during a SLG fault. However in a grounded

system, there will be negligible voltage rise on the un faulted phases close to a

substation where the delta connected windings of the transformer provide low

impedance paths for the zero sequence current during the SLG fault.

INTERRUPTION:

If the supply voltage or load current decreases to less than 0.1 p.u for a

period of time not more than one minute is known as interruption. Interruption

can be caused either by system faults, equipment failures or control

malfunctions.

The interruptions are measured by their duration alone. The duration

due to a fault is determined by the operating time of the protective devices.

Duration of an interruption due to equipment malfunction can be irregular.

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Some interruptions may also be caused by voltage sag conditions when there are

faults on the source side.

3.3.2 STEADY STATE PHENOMENA

3.3.2.1 Waveform Distortion

This is defined as a steady-state deviation from an ideal sine wave of

power frequency.

There are five types of waveform distortion:

(a) DC offset

(b) Harmonics

(c) Inter harmonics

(d) Notching

(e) Noise

3.3.2.2 Voltage Imbalance:

Voltage imbalance can be defined using symmetrical components. The

ratio of the negative sequence or zero sequence component to the positive

sequence component is a measure of unbalance. The main cause of voltage

unbalance is single phase loads on a three phase circuit which resulting in load

imbalance. Severe imbalance can be caused by single-phasing conditions in the

system.

3.3.3 VOLTAGE FLUCTUATIONS AND FLICKER:

Voltage fluctuations are systematic variations of the voltage or a series

of random changes in the voltage magnitude which lies in the range of 0.9 to

1.1p.u. High power loads that draw fluctuating current, such as large motor

drives and arc furnaces, cause low frequency cyclic voltage variations that result

in flickering of light sources like incandescent and fluorescent lamps which can

cause significant physiological discomfort or irritation in human beings.

The voltage flicker can also affect stable operation of electrical and

electronic devices such as motors and CRT devices. The typical frequency

spectrum of voltage flicker lies in the range from 1 Hz to 30 Hz.

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3.3.4 POWER FREQUENCY VARIATIONS:

Power frequency variations are defined as the deviations of the system

frequency from its particular value of 50 or 60 Hz. The variations in the

frequency begin from the changes in the load and the response of the generators

to meet the load. Thus the load characteristics which dependence on the

frequency and the control characteristics of the generators change the shift in

the frequency.

In current interconnected power systems, frequency variations are

insignificant most of the time unless governor and load frequency controls are

disabled under a system of power shortages and a lack of grid discipline.

Profitable incentives or disincentives that ensure balance between existing

generation and load may help control over frequency variations under normal

operating conditions.

3.4 SOLUTION OF POWER QUALITY PROBLEMS:

For the improvement of power quality there are two approaches.

According to first approach the solution to the power quality problems can be

done from the utility side. The 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 the power system disturbances.

In this approach the compensating device is connect to low and medium

voltage distribution system in shunt or in series. 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 inverters, with a dc source 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:

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3.4.1 THYRISTOR BASED STATIC SWITCHES:

The static switch is a versatile device for switching a novel element into

the circuit when the voltage support is desired. It has a dynamic response time

of about one cycle. To correct rapidly for voltage spikes, sags or interruptions,

such 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.

3.4.2 ENERGY STORAGE SYSTEMS:

Storage systems can be used to protect sensitive production equipments

from shutdown which is caused by voltage sag or temporary interruptions.

These are generally DC storage systems such as UPS, batteries,

superconducting magnet energy storage (SMES), storage capacitors or even fly

wheels driving DC generators are used. The output of these devices can be

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

performing electronic switch like GTO or IGBT etc. Sufficient energy is fed to

the system to compensate for the energy that would be lost by the fault

conditions like voltage sag or interruption.

However 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. Flexible AC Transmission Systems (FACTS) for transmission

systems, the term custom power pertains to the use of power electronics

controllers in a distribution system, particularly, to deal with a variety of power

quality problems. Just as FACTS improves the power transfer capabilities and

stability limits, custom power makes sure customers get pre-specified quality

and reliability of supply.

There are many types of Custom Power devices like Active Power

Filters (APF), Battery Energy Storage Systems (BESS), Distribution static

synchronous compensators (DSTATCOM), Dynamic Voltage Restorer (DVR),

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

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

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Solid State Fault Current Limiter (SSFCL), and unified power quality

conditioner (UPQC).

CHAPTER-4CHAPTER-4

FACTS

4.1 INTRODUCTION:

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4.2 NEED OF CUSTOM POWER DEVICES:

Power quality is one of major concerns in the present era. Distribution

system locates the end of power system and is connected to the customer

directly, so the reliability of power supply mainly depends on distribution

system. It has become important, especially, with the introduction of

sophisticated devices, whose performance is very sensitive to the quality of

power supply.

Power quality problem is an occurrence manifested as a nonstandard

voltage, current or frequency that results in a failure of end use equipments. The

electrical distribution network failures account for about 90% of the average

customer interruptions. As the customers demand for the reliability of power

supply is increasing day by day, so the reliability of the distribution system has

to be increased. One of the major problems dealt here is the power sag.

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 system, 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 todays

sophisticated electronic devices makes them more disposed to the quality of

power supply.

For some sensitive devices, a temporary disturbance can cause

scrambled data, interrupted communications, a frozen mouse, system crashes

and equipment failure etc. A power voltage spike can damage valuable

components.

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To solve this problem, custom power devices are used. One of those

devices is the Distribution Static Compensator (DSTATCOM), which is the

most efficient and effective modern custom power device used in power

distribution networks. Its appeal includes lower cost, smaller size, and its fast

dynamic response to the disturbance.

4.3 CONFIGURATIONS:

The compensating type custom power devices can be classified on the

basis of different topologies and the number of phases. For power quality

improvement the voltage source inverter (VSI) bridge structure is generally

used for the development of custom power devices, while the use of current

source inverter (CSI) is less reported. The topology can be shunt

(DSTATCOM), series (DVR), or a combination of both (UPQC).

CHAPTER-5

D-STATCOM

5.1 INTRODUCTION:

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Among the power quality problems like sag, swell, harmonic etc,

voltage sag is the most severe disturbances in the distribution system. To

overcome these problems the concept of custom power devices is introduced

lately. One of those devices is the DSATCOM, which is the most efficient and

effective modern custom power device used in power distribution networks.

DSTATCOM is a recently proposed shunt connected solid state device

that injects voltage into the system in order to regulate the load side voltage. It

is generally installed in a distribution system between the supply and the critical

load feeder at the point of common coupling (PCC).Other than voltage sags and

swells compensation, DSTATCOM is used to reduce the Total harmonic

distortions.

5.2 PRINCIPLE OF D-STATCOM:

FIG: DSTATCOM

A DSATCOM is a solid state power electronics switching device

consisting of either GTO or IGBT, a capacitor bank as an energy storage device.

It is linked in shunt between a distribution system and a load that shown in

Figure.

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However, when voltage sag occurs in the distribution system, the

DSTATCOM control system calculates and synthesizes the voltage required to

preserve output voltage to the load by injecting a controlled voltage with a

certain magnitude and phase angle into the distribution system to the load. Here

a LCL passive filter is used in order to reduce the total harmonic distortions and

PWM , PI controller are being used.

5.3 BASIC ARRANGEMENT OF DSTATCOM:

Voltage Source Converter

Controller

Energy Storage Device

LCL Passive Filter

FIG: Schematic diagram of a D-STATCOM

EQUATIONS OF D-STATCOM:

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Where,

5.3.1 VOLTAGE SOURCE CONVERTER:

A voltage-source converter is a power electronic device that connected

in shunt or parallel to the system. It can generate a sinusoidal voltage with any

required magnitude, frequency and phase angle. The VSC used to either

completely replace the voltage or to inject the missing voltage. The missing

voltage is the difference between the nominal voltage and the actual.

It also converts the DC voltage across storage devices into a set of three

phase AC output voltages. It could be a 3 phase - 3 wire VSC or 3 phase - 4

wire VSC. Either a conventional two level converter or a three level converter is

used. For DSTATCOM application, the VSC is used to momentarily replace the

supply voltage or to generate the part of the supply voltage which is absent. The

VSC here is a two level i.e two phase.

5.3.2 ENERGY STORAGE DEVICE:

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The function of storage devices is to supply the required energy to the

VSC via a dc link for the generation of injected voltages. DC source is

connected in parallel with the DC capacitor. It carries the input ripple current of

the converter and it is the main reactive energy storage element. This DC

capacitor could be charged by a battery source or could be recharged by the

converter itself.

FIG: ENERGY STORAGE DEVICE

5.3.3 LCL PASSIVE FILTER:

LCL Passive filter is more effective on reducing harmonic distortion.

The line-filter between the converter and the grid can be reduced by using an

LCL-filter instead of an L-filter. The main drawback with this is that the LCL-

filter will introduce a resonance frequency into the system. Harmonic

components in the output voltage can lead to resonance oscillations and

instability problems unless they are properly handled.

One way of reducing the resonance current is by adding a passive

damping circuit to the filter. This damping circuit can be purely resistive,

causing relatively high losses, or a more complex solutions consisting of a

combination of capacitors and inductors.

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A typical LCL-filter is shown in Figure where V1 is the grid side

voltage, Vc is the voltage across the filter capacitor and V2 is the converter

output voltage.

FIG: LCL PASSIVE FILTER

EQUATIONS:

To design an efficient LCL Passive filters make sure that,

5.3.4 CONTROLLER :

In this project we use a Proportional plus integral controller.

Proportional- integral controller (PI Controller) is a feedback controller which

drives the system to be controlled with a weighted sum of the error signal

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(difference between the output and desired set point) and the integral of that

value.

FIG: PI CONTROLLER

In this case, PI controller will process the error signal to zero. The load

r.m.s voltage is brought back to the reference voltage by comparing the

reference voltage with the r.m.s voltages that had been measured at the load

point. It also is used to control the flow of reactive power from the DC capacitorstorage circuit.

PWM generator is the device that generates the Sinusoidal PWM

waveform or signal. To operate PWM generator, the angle is summed with the

phase angle of the balance supply voltages equally at 120 degrees. Therefore, it

can produce the desired synchronizing signal that required.

PWM generator also received the error signal angle from PI controller.

The modulated signal is compared against a triangle signal in order to generate

the switching signals for VSC valves.

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CHAPTER-6

POWER QUALITY PROBLEMS IN DSTATCOM

The major power quality problems here are voltage sag and harmonic

distortion.

6.1 VOLTAGE SAG

Voltage sags and momentary power interruptions are probably the

most important PQ problem affecting industrial and large commercial

customers. These events are usually associated with a fault at some location in

the supplying power system. Interruptions occur when the fault is on thecircuit supplying the customer. But voltage sags occur even if the faults

happen to be far away from the customer's site.

Voltage sags lasting only 4-5 cycles can cause a wide range of sensitive

customer equipment to drop out. To industrial customers, voltage sag and a

momentary interruption are equivalent if both shut their process down. A

typical example of voltage sag is shown in fig . The susceptibility of utilization

equipment to voltage sag is dependent upon duration and magnitude of voltage

sags and can be defined.

FIG : VOLTAGE SAG

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6.1.1 Characteristics of Voltage Sags:

Voltage sags which can cause equipment impacts are caused by faults on

the power system.Motor starting also results in voltage sags but the magnitudes

are usually not severe enough to cause equipment mis operation

How a fault results in voltage sag at a customer facility?

The one line diagram given below in fig. 3 can be used to explain this

phenomenon.

Consider a customer on the feeder controlled by breaker 1. In the case

of a fault on this feeder, the customer will experience voltage sag during the

fault and an interruption when the breaker opens to clear the fault. For

temporary fault, enclosure may be successful.

Anyway, sensitive equipment will almost surely trip during this

interruption. Another kind of likely event would be a fault on one of the

feeders from the substation or a fault somewhere on the transmission

system, In either of these cases, the customer will experience a voltage sag

during the actual period of fault. As soon as breakers open to clear the fault,

normal voltage will be restarted at the customer's end. Fig is a plot of rms

voltage versus time and the waveform characteristics at the customer's location

for one of these fault conditions.

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This waveform is typical of the customer voltage during a fault on a

parallel feeder circuit that is cleared quickly by the substation breaker. The total

duration of fault is 150m sec. The voltage during a fault on a parallel feeder

will depend on the distance from the substation to fault point. A fault close to

substation will result in much more significant sag than a fault near the end of

feeder. Fig 5 shows the voltage sag magnitude at the plant bus as a function of

fault location for an example system.

FIG: VOLTAGE SAG CHARACTERISTIC DURING FAULT

A single line to ground fault condition results in a much less severe

voltage sag than 3-phase fault Condition due to a delta--star transformer

connection at the plant. Transmission related voltage sags are normally much

more consistent than those related to distribution. Because of large amounts of

energy associated with transmission faults, they are cleared as soon as possible.

This normally corresponds to 3-6 cycles, which is the total time for fault

detection and breaker operation Normally customers do not experience an

interruption for transmission fault. Transmission systems are looped or

networked, as distinct from radial distribution systems. If a fault occurs

as shown on the 115KV system, the protective relaying will sense the fault

and breakers A and B will open to clear the fault.

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FIG: MAGNITUDE OF VOLTAGE SAG

While the fault is on the transmission system, the entire power system,

including the distribution system will experience Voltage sag. Fig shown the

magnitude of measured voltage sags at an industrial plant supplied from a 115

kV system .

FIG: VOLTAGE SAG OF MAGNITUDE AND DURATION

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Most of the voltages were 10-30% below nominal voltage, and no

momentary interrupts were measured at the plant during the monitoring period

(about a year). Fig given a three-dimensional plot illustrating the number of

sags experienced as a function of both the voltage sag magnitude and the

duration.

This is a convenient way to completely characterize the actual or

expected voltage sag conditions at a site. Evaluating the impact of voltage sags

at a customer plant involves estimating the member of voltage sags that can be

expected as a function of the voltage sag magnitude and then comparing

this with equipment sensitivity.

The estimate of voltage sag performance are developed by performing

short-circuit simulations to determine the plant voltage as a function of fault

location throughout the power system. Total circuit miles of line exposure that

can affect the plant (area of vulnerability) are determined for a particular sag

level.

Historical fault performance (fault per year per 100 miles) can, then

be used to estimate the number of sags per year that can be expected below the

magnitude. A chart such as the one in fig 8. Can be drawn in splitting the

expected number of voltage sags by magnitude. This information can be used

directly by the customers to determine the need for power conditioning

equipment at sensitive loads in the plant.

6.1.2Voltage-Sag Analysis- Methodology

The methodology is outlined is (proposed) of IEEE Gold book (IEEE

standard 493, Recommended practice for the design of reliable industrial and

commercial power system) The methodology basically consists of the following

four steps:

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6.1.2.1Voltage Sag Calculation

Sliding faults which include line-line, line to ground, line to line- to

ground and three phase are applied to all the lines in the load flow. Each line is

divided into equal sections and each section is faulted as shown in fig 9.

6.1.2.2 Sag Occurrence Calculation:

Based upon the utilities reliability data (the number of times each line

section will experience a fault) and the results of load flow and voltage sag

calculations, the number of voltage sags at the customer site due to remote

faults can be calculated.

Depending upon the equipment connection, the voltage sag occurrence

rate may be calculated in terms of either phase or line voltages dependent upon

the load connection. For some facilities, both line and phase voltages may be

required. The data thus obtained from load flow, Voltage sag calculation,

and voltage sag occurrence calculation can be sorted and tabulated by sag

magnitude, fault type, location of fault and nominal system voltage at the fault

location

6.2 Study of Results of Sag- Analysis:

The results can be tabulated and displayed in many different ways to

recognize difficult aspects. Area of vulnerability can be plotted on a

geographical map or one - line diagram (fig above). These plots can be used to

target transmission and distribution lines for enhancements in reliability.

Further bar charts, and pie-charts showing the total number of voltage sags

with reference to voltage level at fault point, area/zone of fault, or the fault

type can be developed to help utilities focus on their system improvements

(figs.) To examining the existing system, system modifications aimed at

mitigating or reducing voltage sags can also be identified, thus enabling cost

benefits analysis. Possible such system structural changes that can be identified

include.

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Reconnection of a customer from one voltage level to another,

Installation of Ferro-resonant transformers or time delayed under voltage, drop

out relay to facilitate easy ride - through the sag Application of static transfer

switch and energy storage system., Application of fast acting synchronous

condensers, Neighborhood generation capacity addition , Increase service

voltage addition through transformer tap changing, By enhancement of system

reliability.

6.3 Solutions to Voltage Sag Problems:

Efforts by utilities and customers can reduce the number and severity of

sags.

A. Utility solutions:

Utilities can take two main steps to reduce the detrimental effects of

sags

(1) Prevent fault

(2) Improve fault clearing methods

Fault prevention methods include activities like tree trimming, adding

line arrests, washing insulators and installing animal guards. Improved fault

clearing practices include activities like adding line recloses, eliminating fast

tripping, adding loop schemes and modifying feeder design. These may reduce

the number and /or duration of momentary interruptions and voltage sags but

faults cannot be eliminated completely.

B. Customer solutions:

Power conditioning is the general concept behind these methods. Fig 12

is a schematic f the general approach used.

Power conditioning helps to

1. Isolate equipment from high frequency noise and transients.

2. Provide voltage sag ride through capability

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6.4 TOTAL HARMONIC DISTORTION:

Harmonics

Introduction:

The typical definition for a harmonic is a sinusoidal component of a

periodic wave or\quantity having a frequency that is an integral multiple of the

fundamental frequency. Some references refer to clean or pure power as

those without any harmonics. But such clean waveforms typically only exist in a

laboratory. Harmonics have been around for a long time and will continue to do

so. In fact, musicians have been aware of such since the invention of the first

string or wood wind instrument. Harmonics (called overtones in music) are

responsible for what makes a trumpet sound like a trumpet, and a clarinet like a

clarinet.

Electrical generators try to produce electric power where

voltageone frequency associated with it, the fundamental frequency. In the

North America, this frequency is 60 Hz, or cycles per second. In European

countries and other parts of the world, this frequency is usually 50 Hz. Aircraft

often uses 400 Hz as the fundamental frequency. At 60 Hz, this means that sixty

times a second, the voltage waveform increases to a maximum positive value,

then decreases to zero, further decreasing to a maximum negative value, and

then back to zero. The rate at which these changes occur is the trigometric

function called a sine wave, as shown in figure . This function occurs in many

natural phenomena, such as the speed of a pendulum as it swings back

and forth, or the way a string on a voilin vibrates when plucked. Fig . Sine wave

The frequency of the harmonics is different, depending on the fundamentalfrequency. For example, the 2nd harmonic on a 60 Hz system is 2*60 or 120

Hz. At 50Hz, the second harmonic is 2* 50 or 100Hz.300Hz is the 5th harmonic

in a 60 Hz system, or the 6th harmonic in a 50 Hz system. Figure shows how a

signal with two harmonics would appear on an oscilloscope-type display, which

some power quality analyzers provide.

In order to be able to analyze complex signals that have many different

frequencies present, a number of mathematical methods were developed. One of

the more popular is called the Fourier Transform. However, duplicating the

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mathematical steps required in a microprocessor or computer-based

instrument is quite difficult. So more compatible processes, called the FFT for

Fast Fourier transform, or DFT for Discrete Fourier Transform, are used. These

methods only work properly if the signal is composed of only the fundamental

and harmonic frequencies in a certain frequency range.

The frequency values must not change during the measurement period.

Failure of these rules to be maintained can result in mis-information. For

example, if a voltage waveform is comprised of 60 Hz and 200 Hz signals, the

FFT cannot directly see the 200Hz. It only knows 60, 120, 180, 240,..., which

are often called bins. The result would be that the energy of the 200 Hz signal

would appear partially in the 180Hz bin, and partially in the 240Hz bin.

An FFT-based processer could show a voltage value of 115V at 60 Hz,

18 V at the 3rdharmonic, and 12 V at the 4th harmonic, when it really should

havebeen30Vat200Hz.

These in between frequencies are called inter harmonics. There is

also a special category of inter harmonics, which are frequency values less than

the fundamental frequency value, called sub-harmonics.

For example, the process of melting metal in an electric arc furnace can

result large currents that are comprised of the

fundamental , inter harmonic, and sub harmonic frequencies being drawn from

the electric power grid. These levels can be quite high during the melt-down

phase, and usually effect the voltage waveform.

Effects of harmonics:

The presence of harmonics does not mean that the factory or office

cannot run properly .Like other power quality phenomena, it depends on the

stiffness of the power distribution system and the susceptibility of the

equipment. As shown below, there are a number of different types of equipment

that can have mis-operations or failures due to high harmonic voltage

and/or current levels. In addition, one factory may be the source of high

harmonics but able to run properly. This harmonic pollution is often carried

back onto the electric utility distribution system, and may effect facilities on the

same system which are more susceptible.

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Some typical types of equipment susceptible to harmonic pollution

include: - Excessive neutral current, resulting in overheated neutrals. The odd

triple n harmonics in three phase wave circuits are actually additive in the

neutral. This is because the harmonic number multiplied by the 120 degree

phase shift between phases is a integer multiple of 360 degrees. This puts the

harmonics from each of the three phase legs in-phase with each other in the

neutral, as shown in figure

Harmonic problems are almost always introduced by the consumers

equipment and installation practices. Harmonic distortion is caused by the high

use of non-linear load equipment such as computer power supplies, electronic

ballasts, compact fluorescent lamps and variable speed drives etc, which create

high current flow with harmonic frequency components.

The limiting rating for most electrical circuit elements is determined by

the amount of heat that can be dissipated to avoid overheating of bus bars,

circuit breakers, neutral conductors, transformer windings or generatoralternators.

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TOTAL HARMONIC DISTORTION:

THD is defined as the RMS value of the waveform remaining when the

fundamental is removed. A perfect sine wave is 100%, the fundamental is the

system frequency of 50 or 60Hz.

Harmonic distortion is caused by the introduction of waveforms at

frequencies in multiplies of the fundamental ie: 3rd harmonic is 3x the

fundamental frequency / 150Hz. Total harmonic distortion is a measurement of

the sum value of the waveform that is distorted.

FIG: HARMONIC DISTORTION

Power Measurement:

Despite the use of good quality test meter instrumentation, high current

flow can often remain undetected or under estimated by as much 40%. Thissevere underestimation causes overly high running temperatures of equipment

and nuisance tripping. This is simply because the average reading test meters

commonly used by maintenance technicians, is not designed to accurately

measure distorted currents, and can only provide indication of the condition of

the supply at the time of checking.

Power quality conditions change continuously, and only instruments

offering true RMS measurement of distorted waveforms and neutral currents

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can provide the correct measurements to accurately determine the ratings of

cables, bus bars and circuit breakers.

Neutral Currents

High harmonic environments can produce unexpected and dangerous

neutral currents. In a balanced system, the fundamental currents will cancel out,

but, triple- Ns will add, so harmonic currents at the 3rd, 9th, 15th etc. will flow

in the neutral. Traditional 3 phase system meters are only able to calculate the

vector of line to neutral current measurements, which may not register the true

reading. Integral 1530, 1560 and 1580 offer a 3 phase 4 wire versions with a

neutral 4th CT allowing true neutral current measurement and protection in high

harmonic environments.

Harmonic Profiles

There is much discussion over the practical harmonic range of a

measurement instrument; however study of the harmonic profiles of typically

installed equipment can guide the system designer to the practical solution. A

typical harmonic profile graph will show a logarithmic decay as the harmonic

frequency increases. It is necessary to establish the upper level at which the

harmonic content is negligible.

For Example:

A laptop switch mode power supply causes approximately 25% of 3rd

harmonic, 19% of 5th harmonic, 10% of 7th harmonic and 5% of 9th harmonic

etc. Therefore it can be seen that almost all the harmonic content in an IT

dominated load will be below the 15th harmonic. In a 3 phase load

incorporating 6 pulse bridge technology as is common in many variable speed

drives, UPS systems and DC converters, similar profiles will be observed but

extending to the 25th and 27th harmonic. It can therefore be deduced that in the

majority of industrial and commercial applications an instrument measuring up

to the 31st harmonic is ideal.

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6.5 MINIMIZATION OF HARMONICS:

Care should be undertaken to make sure that the corrective action taken

to minimize the harmonic problems dont actually make the system worse. This

can be the result of resonance between harmonic filters, PF correcting

capacitors and the system impedance. Isolating harmonic pollution devices on

separate circuits with or without the use of harmonic filters are typical ways of

mitigating the effects of such. Loads can be relocated to try to balance the

system better. according to the latest NEC-1996 requirements covering such.

Whereas the neutral may have been undersized in the past, it may now be

necessary to run a second neutral wire that is the same size as the phase

conductors. This is particularly important with some modular office partition-

type walls, which can exhibit high impedance values. Use of higher pulse

converters, such as 24-pulse rectifiers, can eliminate lower harmonic values.

CHAPTER-7

D-STATCOM TEST MODELS

DSTATCOM is a device, which is used to

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Improve the Voltage Sag

Reduce the Total Harmonic Distortion

7.1 D-STATCOM TEST SYSTEM:

FIG: SINGLE LINE DIAGRAM OF THE TEST SYSTEM

The above figure comprises of a 230kV, 50Hz transmission system,

which is represented by a Thevenin equivalent, feeding into the primary side of

a 3-winding transformer connected in Y/Y/Y, 230/11/11 kV. A varying load is

connected to the 11 kV, secondary side of the transformer. A two-level D-

STATCOM is connected to the 11kV tertiary winding to provide instantaneous

voltage support at the load point.

A 750 F capacitor on the dc side provides the D-STATCOM energy

storage capabilities. Breaker 1 is used to control the period of operation of theD-STATCOM and breaker 2 is used to control the connection of load 1 to the

system.

7.2 METHODOLOGY:

STEP-1:

Start the program.

Design a distribution system using Matlab version R2009A.

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Create distortions by inserting different types of faults such as

Three phase to ground Fault(TPG)

Single Line to ground Fault (SLG)

Double Line to ground Fault (DLG)Line to Line (LL) into the distribution system.

STEP-2:

Run the simulation.

Vary the values of the fault resistances for different types of faults.

If voltage sag >0.9p.u , it means the condition satisfies go to STEP-3

Otherwise i.e., if voltage sag


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(breaker-2) to the load. Another winding is connected to the breaker (breaker-

1) to DSTATCOM with LCL Filter.

FIG: SIMULINK TEST MODELOF DSTATCOM WITH

LCL PASSIVE FILTER

7.4 RESULTS WITHOUT DSTATCOM:

The scope-2 gives the output wave forms of simulink model without

DSTATCOM. For different types of fault resistances we get different

wave forms.

6.4.1 OUTPUT WAVEFORMS:

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FIG : VOLTAGE SAG AT 0.2 *10^-6 for SLG FAULT

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FIG: VOLTAGE SAG AT 6.2*10^-7 FOR DLG FAULT

FIG: VOLTAGE SAG AT 4.99*10^-7 FOR TPG FAULT

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FIG: VOLTAGE SAG AT 8.1*10 -7 FOR LL FAULT

From the figure we can see the voltage sag i.e., the reduction in voltage.

For different values of fault resistances we get different types of waveforms.

Here we can see the waveforms for different types of faults. The wave forms are

not greater than 0.9 p.u which shows that there is a voltage reduction or voltage

sag.

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7.5 OUTPUT WAVEFORMS WITH DSTATCOM:

The scope 1 gives the output waveforms of simulink model with

DSTATCOM . The following are the figures of voltage sags

FIG: VOLTAGE SAG AT 1.3p.u FOR SLG FAULT

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FIG: VOLTAGE SAG AT 1.3p.u FOR DLG FAULT

FIG: VOLTAGE SAG AT 1.4p.u FOR TPG FAULT

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FIG: VOLTAGE SAG AT 1.41p.u FOR LL FAULT

From the figure we can see the voltage sag i.e., the reduction in voltage.

For different values of fault resistances we get different types of waveforms.

Here we can see the waveforms for different types of faults. The wave forms are

greater than 0.9 p.u which shows that there is a improvement in voltage sag.

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7.6 OUTPUT WAVE FORMS OF DSTATCOM WITH LCL PASSIVE

FILTER:

The power gui opens an FFT analysis which gives the output wave

forms

0 0 . 0 5 0 . 1 0 . 1 5 0 . 2 0 . 2 5 0 . 3

- 0 . 5

0

0 . 5

S e l e c t e d s i g n a l : 1 5 c y c l e s . F F T w i

T i m e ( s )

0 5 1 0 1 5 2 00

0 . 2

0 . 4

0 . 6

0 . 8

H a r m o n i c o r d e r

F u n d a m e n t a l ( 5 0 H z ) = 1 , T H D .

M

ag

(%

ofFundam

ental)

FIG: OUTPUT WAVEFORMS OF DSTATCOM WITH

LCL PASSIVE FILTER

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7.7 SIMULINK MODEL OF DSTATCOM WITHOUT LCL PASSIVE

FILTER:

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FIG: SIMULINK TEST MODEL FOR DSTATCOM WITHOUT

LCL PASSIVE FILTER

RESULTS:

7.7.1 OUTPUT WAVEFORMS OF DSTATCOM WITHOUT LCL PASSIVE

FILTER:

0 0 . 0 5 0 . 1 0 . 1 5 0 . 2 0 . 2 5 0 . 3

- 0 . 0 5

0

0 . 0 5

S e l e c t e d s i g n a l : 1 5 c y c l e s . F :

T i m e ( s )

0 5 1 0 1 5 2 00

1

2

3

4

5

6

H a r m o n i c o r d e r

F u n d a m e n t a l ( 5 0 H z ) = 8 . 8 2

M

ag

(%

ofFundam

ental)

FIG: OUTPUT WAVEFORM OF DSTATCOM WITHOUT

LCL PASSIVE FILTER:

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CHAPTER-8

CONCLUSION

Power quality in electric networks is one of today's most concerned



Most occurring power quality problems are voltage sag and total

harmonic distortion. These are due to faults such as increase in load while

starting a motor, transformer energizing etc., The impact of power qualityproblems is increasingly felt by customers - industrial, commercial and even

residential.

In this project we discuss about the improvement of voltage sag in

distribution system using DSTATCOM and reduction of total harmonic

distortion by using LCL passive filter with DSTATCOM in Distribution system.

Atlast we have seen the output waveforms regarding them using

Matlab/simulink.

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ENHANCEMENT OF POWER QUALITY

DISTRIBUTION SYSTEM USING D-STATCOM

INDEX

Abstract

List of Figures:

List of Tables:

CHAPTER-1: MATLAB INTRODUCTION 1-30

CHAPTER-2: INTRODUCTION 31-33

2.1LITERATURE SURVEY

2.2SCOPE OF WORK

CHAPTER-3: POWER QUALITY 34-40

3.1 INTRODUCTION

3.2 POWER QUALITY-A BIG ISSUE

3.3 PROBLEMS ASSOCIATED WITH POWER QUALITY

3.3.1 MOMENTARY PHENAMINA

3.3.1.1 TRANSIENTS

3.3.1.2 LONG DURATION VOLTAGE VARIATIONS

3.3.1.3 SUSTAINED INTURRUPTION

3.3.1.4 SHORT DURATION VOLTAGE VARIATIONS

3.3.2 STEADY STATE PHENMINA

3.3.2.1 WAVE FORM DISTORTION

3.3.2.2 VOLTAGE IMBALANCE

3.3.3 VOLTAGE FLUCTUVATIONS AND FLICKERS

3.3.4 POWER FREQUANCY VARIATIONS

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3.4 SOLUTIONS OF POWER PROBLEMS

3.4.1 THYRISTER BASED STATIC SWITCHES

3.4.2 ENERGY STORAGE SYSTEM

CHAPTER-4: CUSTOM POWER DEVICES 41-47

4.1 INTRODUCTION

4.2 NEED OF CUSTOM POWER DEVICES

4.3 CONFIGURATIONS

4.3.1 CONVERTER BASED CLASIFICATION

4.3.2 TOPOLOGY BASED CLASIFICATION

4.4 BENEFITS WITH THE APPLICATION OF CUSTOM

POWER DEVICES

CHAPTER-5: D-STATCOM 48-53

5.1 INTRODUCTION

5.2 PRINCIPLE OF D-STATCOM

5.3 BASIC ARRANGEMENTS OF D-STATCOM

5.3.1 VOLTAGE SOURCE CONVERTER

5.3.2 ENERGY STORAGE DEVICE

5.3.3 LCL PASSIVE FILTER

5.3.4 CONTROLLER

CHAPTER-6: POWER PROBLEMS IN D-STATCOM 54-66

6.1 VOLTAGE SAG

6.1.1 CHARACTERSTICS OF VOLTAGE SAG

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6.1.2 VOLTAGE GENERATING ANALYSIS-

METHODOLOGY

6.1.2.1 VOLTAGE SAG CALCULATIONS

6.2. STEADY OF RESULT SAG CALCULATIONS

6.3 SOLUTION OF VOLTAGE SAG PROBLEMS

6.4 TOTAL HORMONIC DISTORTION

6.5 MINIMISATION OF HORMONICS

CHAPTER-7: D-STATCOM TEST MODELS 67-79

7.1 D-STATCOM TEST SYSTEM

7.2 METHODOLOGY

7.2.1 PROCEDURE

7.3 SIMULINK MODEL FOR THE TEST SYSTEM

7.4 RESULTS WITHOUT D-STATCOM

7.5 RESULTS WITH D-STATCOM

7.6 OUTPUT WAVE FORM OF D-STATCOM WITH LCL

PASSIVE FILTER

7.7 SIMULINK MODEL OF D-STATCOM WITH OUT LCL

PASSIVE FILTER

7.7.1 OUTPUT WAVE FORM OF D-STATCOM WITHOUT

LCL PASSIVE FILTER

CHAPTER-8: CONCLUSION 80

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LIST OF FIGURES:

FIGURE NUMBER NAME OF THE

FIGURE

PAGENO

5.2.2 45

5.2.3(a) 48

5.2.3(b) 49

50

LIST OF TABLES:

TABLE NUMBER NAME OF THE

TABLE

PAGE NO

2.2 Types of HT Motors 23

ABSTRACT

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This project presents the enhancement of voltage sags, harmonic

distortion using Distribution Static Compensator (D-STATCOM) with LCL

Passive Filter in distribution system. The model is based on the Voltage Source

Converter (VSC) principle. The D-STATCOM injects a current into the system

to mitigate the voltage sags. LCL Passive Filter was then added to D-

STATCOM to improve harmonic distortion and low power factor.

A new PWM-based control scheme has been implemented to control the

electronic valves in the D-STATCOM. The D-STATCOM has an additional

capability to sustain reactive current at low voltage, and can be developed as a

voltage and frequency support by replacing capacitors. Voltage sag is a short

time event during which a reduction in r.m.s voltage magnitude occurs. Voltage

sags are improved with insertion of D-STATCOM. When the value of fault

resistance is increased, the voltage sags will also increase for different types of

fault.

Suitable adjustment of the phase and magnitude of the D-STATCOM

output voltages allows effective control of active and reactive power exchanges

between D-STATCOM and AC system. The PI controller will process the

error signal to zero. The load r.m.s voltage is brought back to the reference

voltage by comparing the reference voltage with the r.m.s voltages that had

been measured at the load point. It also is used to control the flow of reactive

power from the DC capacitor storage circuit. The PWM generator can produce

the desired synchronizing signal that is required. PWM generator also receives

the error signal angle from PI controller.

The modulated signal is compared against a signal in order to generate

the switching signals for VSC valves. To enhance the performance of

distribution system, D-STATCOM was connected to the distribution system.

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enhancement of power quality using dstatcom

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