CONTENTS

WHAT IS POWER QUALITY?

WHY PQ IS IMPORTANT?

POWER QUALITY PROBLEMS

(A)Short duration voltage variation

(A.1)Voltage sag

(A.2)Voltage swell

(A.3)Interruption

(B)Voltage imbalance

(C)Waveform distortion (D)Long duration voltage variation

(D.1)Over voltage

(D.2)Under voltage

(D.3)Sustained interruption

(E)Voltage fluctuation

POWER QUALITY SOLUTIONS :

(1)Voltage Source Converter

(2) DVR Dynamic Voltage Restorer

(3) D-STATCOM

(4)Harmonic Filter

CONCLUSION

REFERENCES

ABSTRACT

Due to increasing complexity in the power system, voltage sags and swells are now becoming one of the most significant power quality problems. Voltage sag is a short reduction voltage from nominal voltage, occurs in a short time, voltage swell is an increase in the r ms voltage from its nominal voltage; they are bound to have a greater impact on the industrial customers. If the voltage sags exceed two to three cycles, then manufacturing systems making use of sensitive electronic equipments are likely to be affected leading to major problems. It ultimately leads to wastage of resources (both material and human) as well as financial losses. The increasing competition in the market and the declining profits has made it pertinent for the industries to realize the significance of high-power quality. This is possible only by ensuring that uninterrupted flow of power is maintained at proper voltage levels. Electric utilities are looking for solutions to ensure high quality power supply to their customers, a lot of solutions have been developed, but this project tends look at the solving the problems by using custom power devices such as Dynamic Voltage Restorer (DVR) and Distribution Static compensator (D-STATCOM). The Dynamic Voltage Restorer appears to be an especially good solution in the current scenario.

WHAT IS POWER QUALITY?

Power quality is simply the interaction of electrical power with electrical equipment. If electrical equipment operates correctly and reliably without being damaged or stressed, we would say that the electrical power is of good quality. On the other hand, if the electrical equipment malfunctions, is unreliable, or is damaged during normal usage, we would suspect that the power quality is poor. 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 inverters , with a dc bus having a reactive element such as a capacitor.

WHY POWER QUALITY IS IMPORTANT?

Both electric utilities and end users of electric power are becoming

increasingly concerned about the quality of electric power. The term

power quality has become one of the most prolific buzzwords in the

power industry since the late 1980s. It is an umbrella concept for a multitude

of individual types of power system disturbances. The issues

that fall under this umbrella are not necessarily new. What is new is

that engineers are now attempting to deal with these issues using a

system approach rather than handling them as individual problems.

There are four major reasons for the increased concern:

1. Newer-generation load equipment, with microprocessor-based controls

and power electronic devices, is more sensitive to power quality

variations than was equipment used in the past.

2. The increasing emphasis on overall power system efficiency has

resulted in continued growth in the application of devices such as

high-efficiency, adjustable-speed motor drives and shunt capacitors

for power factor correction to reduce losses. This is resulting in

increasing harmonic levels on power systems and has many people

concerned about the future impact on system capabilities.

3. End users have an increased awareness of power quality issues.

Utility customers are becoming better informed about such issues as

interruptions, sags, and switching transients and are challenging

the utilities to improve the quality of power delivered.

4. Many things are now interconnected in a network. Integrated

processes mean that the failure of any component has much more

important consequences.

POWER QUALITY PROBLEMS:

(A)Short duration voltage variation:

(A.1)Voltage Sag:

A voltage sag is a sudden reduction in the r. m .s voltage between 0.1 and 0.9 p. u. at a point in the electrical system, and lasting for 0.5 cycle to several seconds. Dips with durations of less than a cycle are regarded as transients. Figure 1 shows a waveform depicting a voltage sag.

Figure 1: Voltage dip (sag) waveform

A voltage dip may be caused by switching operations associated with a temporary disconnection of supply, the flow of heavy current associated with the starting of a large electric motors or the flow of fault currents or the transfer of load from one power source to another. These events may emanate from customers systems or from the public supply network. The main cause of momentary voltage dips is probably the lightning strike. Each of these cases may cause a sag with a special characteristics (magnitude and duration).The possible effects of voltage sags are:

Extinction of discharge lamps

Malfunctions of electrical low-voltage devices

Computer system crash

Tripping of contactor

(A.2) Voltage swell:

The increase of voltage magnitudes between 1.1 and 1.8pu is called Swell ,it sometimes accompany voltage sags. The most accepted duration of a swell is from 0.5 cycles to 1 minute. They appear on the switching off of a large load; energizing a capacitor bank ; or voltage increase of the un faulted phases during a single line-to- ground fault. Figure 2 shows a waveform of voltage swell

.

Figure 2. Voltage swell waveform

Swells can upset electric controls and electric motor drives, particularly common adjustable-speed drives, which can trip because of their built in protective circuitry. Swells can also put stress on delicate computer components and shorten their life span.

(A.3)Interruption:

An interruption occurs when the supply voltage or load current

decreases to less than 0.1 p u for a period of time not exceeding 1 min.

Interruptions can be the result of power system faults, equipment

failures, and control malfunctions. The interruptions are measured by

their duration since the voltage magnitude is always less than 10 percent

of nominal. The duration of an interruption due to a fault on the

utility system is determined by the operating time of utility protective

devices.

Instantaneous reclosing generally will limit the interruption

caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing

of the protective device may cause a momentary or temporary interruption.

The duration of an interruption due to equipment malfunctions

or loose connections can be irregular.

Some interruptions may be preceded by a voltage sag when these

interruptions are due to faults on the source system. The voltage sag

occurs between the time a fault initiates and the protective device operates.

(B)Voltage Imbalance:

Voltage imbalance (also called voltage unbalance) is sometimes defined

as the maximum deviation from the average of the three-phase voltages

or currents, divided by the average of the three-phase voltages or

currents, expressed in percent.

Imbalance is more rigorously defined in the standards6,8,11,12 using

symmetrical components. The ratio of either the negative- or zero sequence

component to the positive-sequence component can be used

to specify the percent unbalance. The most recent standards11 specify

that the negative-sequence method be used.

The primary source of voltage unbalances of less than 2 percent is

single-phase loads on a three-phase circuit. Voltage unbalance can also

be the result of blown fuses in one phase of a three-phase capacitor

bank. Severe voltage unbalance (greater than 5 percent) can result

from single-phasing conditions.

(C) Waveform Distortion:

Waveform distortion is defined as a steady-state deviation from an

ideal sine wave of power frequency principally characterized by the

spectral content of the deviation.

There are five primary types of waveform distortion:

DC offset

Harmonics

Interharmonics

Notching

Noise

DC offset: The presence of a dc voltage or current in an ac power system

is termed dc offset.

This can occur as the result of a geomagnetic disturbance

or asymmetry of electronic power converters. Incandescent light

bulb life extenders, for example, may consist of diodes that reduce the

rms voltage supplied to the light bulb by half-wave rectification.

Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause the electrolytic erosion of grounding electrodes and other connectors.

Harmonics: Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate (termed the fundamental frequency;

usually 50 or 60 Hz).

Periodically distorted waveforms can be decomposed

into a sum of the fundamental frequency and the harmonics.

Harmonic distortion originates in the nonlinear characteristics of

devices and loads on the power system.

Harmonic distortion levels are described by the complete harmonic

spectrum with magnitudes and phase angles of each individual harmonic

component. It is also common to use a single quantity, the total

harmonic distortion (THD), as a measure of the effective value of harmonic

distortion.

Interharmonics: Voltages or currents having frequency components

that are not integer multiples of the frequency at which the supply system

is designed to operate (e.g., 50 or 60 Hz) are called interharmonics.

They can appear as discrete frequencies or as a wideband spectrum.

Interharmonics can be found in networks of all voltage classes. The

main sources of interharmonic waveform distortion are static frequency

converters, cycloconverters, induction furnaces, and arcing devices.

Power line carrier signals can also be considered as interharmonics.

Since the first edition of this book, considerable work has been done

on this subject. There is now a better understanding of the origins and

effects of interharmonic distortion. It is generally the result of frequency

conversion and is often not constant; it varies with load. Such

interharmonic currents can excite quite severe resonances on the

power system as the varying interharmonic frequency becomes coincident

with natural frequencies of the system. They have been shown to

affect power-line-carrier signaling and induce visual flicker in fluorescent

and other arc lighting as well as in computer display devices.

Notching: Notching is a periodic voltage disturbance caused by the

normal operation of power electronic devices when current is commutated

from one phase to another.

Since notching occurs continuously, it can be characterized through

the harmonic spectrum of the affected voltage. However, it is generally

treated as a special case. The frequency components associated with

notching can be quite high and may not be readily characterized with

measurement equipment normally used for harmonic analysis.

Noise: Noise is defined as unwanted electrical signals with broadband

spectral content lower than 200 kHz superimposed upon the power system

voltage or current in phase conductors, or found on neutral conductors

or signal lines.

Noise in power systems can be caused by power electronic devices,

control circuits, arcing equipment, loads with solid-state rectifiers, and

switching power supplies. Noise problems are often exacerbated by

improper grounding that fails to conduct noise away from the power

system. Basically, noise consists of any unwanted distortion of the

power signal that cannot be classified as harmonic distortion or transients.

(D)Long-Duration Voltage Variations:

Long-duration variations encompass root-mean-square (rms) deviations

at power frequencies for longer than 1 min. ANSI C84.1 specifies

the steady-state voltage tolerances expected on a power system. A volt- age variation is considered to be long duration when the ANSI limits

are exceeded for greater than 1 min.

Long-duration variations can be either overvoltages or undervoltages.

Overvoltages and undervoltages generally are not the result of

system faults, but are caused by load variations on the system and system

switching operations. Such variations are typically displayed as

plots of rms voltage versus time.

(D.1) Overvoltage:

An overvoltage is an increase in the rms ac voltage greater than 110

percent at the power frequency for a duration longer than 1 min.

Overvoltages are usually the result of load switching (e.g., switching

off a large load or energizing a capacitor bank). The overvoltages result

because either the system is too weak for the desired voltage regulation

or voltage controls are inadequate. Incorrect tap settings on transformers

can also result in system overvoltages.

(D.2) Undervoltage:

An undervoltage is a decrease in the rms ac voltage to less than 90 percent

at the power frequency for a duration longer than 1 min.

Undervoltages are the result of switching events that are the

opposite of the events that cause overvoltages. A load switching on

or a capacitor bank switching off can cause an undervoltage until

voltage regulation equipment on the system can bring the voltage

back to within tolerances. Overloaded circuits can result in undervoltages

also.

The term brownout is often used to describe sustained periods of

undervoltage initiated as a specific utility dispatch strategy to reduce

power demand. Because there is no formal definition for brownout and

it is not as clear as the term undervoltage when trying to characterize

a disturbance, the term brownout should be avoided.

(D.3) Sustained interruptions:

When the supply voltage has been zero for a period of time in excess of

1 min, the long-duration voltage variation is considered a sustained

interruption. Voltage interruptions longer than 1 min are often permanent

and require human intervention to repair the system for

restoration.

The term sustained interruption refers to specific power

system phenomena and, in general, has no relation to the usage of the

term outage. Utilities use outage or interruption to describe phenomena

of similar nature for reliability reporting purposes. However, this

causes confusion for end users who think of an outage as any interruption

of power that shuts down a process. This could be as little as

one-half of a cycle. Outage, as defined in IEEE Standard 100,8 does not

refer to a specific phenomenon, but rather to the state of a component

in a system that has failed to function as expected. Also, use of the

term interruption in the context of power quality monitoring has no

relation to reliability or other continuity of service statistics. Thus,

this term has been defined to be more specific regarding the absence

of voltage for long periods.

(E) Voltage Fluctuation:

Voltage fluctuations are systematic variations of the voltage envelope

or a series of random voltage changes, the magnitude of which does not

normally exceed the voltage ranges specified by ANSI C84.1 of 0.9 to

1.1 pu.

Loads that can exhibit continuous, rapid variations in the load current

magnitude can cause voltage variations that are often referred to

as flicker. The term flicker is derived from the impact of the voltage

fluctuation on lamps such that they are perceived by the human eye to

flicker. To be technically correct, voltage fluctuation is an electromagnetic

phenomenon while flicker is an undesirable result of the voltage fluctuation in some loads. However, the two terms are often linked

together in standards. Therefore, we will also use the common term

voltage flicker to describe such voltage fluctuations.

POWER QUALITY SOLUTIONS:

(1) Voltage Source Converters (VSC):

A voltage-source converter is a power electronic device, which can generate a sinusoidal voltage with any required magnitude, frequency and phase angle. Voltage source converters are widely used in adjustable-speed drives, but can also be used to mitigate voltage dips. The VSC is 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. The converter is normally based on some kind of energy storage, which will supply the converter with a DC voltage. The solid-state electronics in the converter is then switched to get the desired output voltage. Normally the VSC is not only used for voltage dip mitigation, but also for other power quality issues, e.g. flicker and harmonics.

(2) Dynamic Voltage Restorer (DVR)

A DVR, Dynamic Voltage Restorer is a distribution voltage DC-to-AC solid- state switching converter that injects three single phase AC output voltages in series with the distribution feeder, and in synchronism with the voltages of the distribution system. By injecting voltages of controllable amplitude, phase angle, and frequency (harmonic) into the distribution feeder in instantaneous real time via a series- injection transformer, the DVR can restore the quality of voltage at its load side terminals when the quality of the source side terminal voltage is significantly out of specification for sensitive load equipment. It is designed to mitigate voltage sags and swells on lines feeding sensitive equipment.

A viable alternative to uninterruptible power systems (UPS) and other utilization solutions to the voltage sag problem, the DVR is specially designed for large loads of the order of 2 MVA to 10 MVA served at distribution voltage. A DVR typically requires less than one-third the nominal power rating of the UPS . DVR can also be used to mitigate troublesome harmonic voltages on the distribution network.

DVR comprises of three main parts:

1. Inverter

2. DC energy storage

3. Control system

The basic configuration of a DVR is shown in Figure 3 below;

Figure 3: Basic configuration of a DVR

(3)Distribution Static Compensator (D-STATCOM)

A D-STATCOM (Distribution Static Compensator), which is schematically depicted in Figure, consists of a two-level Voltage Source Converter (VSC), a dc energy storage device, a coupling transformer connected in shunt to the distribution network through a coupling transformer. The VSC converts the dc voltage across the storage device into a set of three-phase ac output voltages. These voltages are in phase and coupled with the ac system through the reactance of the coupling transformer. Suitable adjustment of the phase and magnitude of the D-STATCOM output voltages allows effective control of active and reactive power exchanges between the D-STATCOM and the ac system. Such configuration allows the device to absorb or generate controllable active and reactive power.

The VSC connected in shunt with the ac system provides a multifunctional topology which can be used for up to three quite distinct purposes:

1. Voltage regulation and compensation of reactive power;

2. Correction of power factor; and

3. Elimination of current harmonics.

Here, such device is employed to provide continuous voltage regulation using an indirectly controlled converter. The basic configuration of the D-STATCOM is shown in the diagram below, Figure 4;

Figure 4: Configuration of a D-STATCOM

(4) Harmonic Filters

Harmonic filters are used to reduce undesirable harmonics. They can be divided in two groups: passive filters and active filters.

Figure(5)

Passive filters (Fig. 5 left) consist in a low impedance path to the frequencies of the harmonics to be attenuated using passive components (inductors, capacitors and resistors). Several passive filters connected in parallel may be necessary to eliminate several harmonic components. If the system varies (change of harmonic components), passive filters may become ineffective and cause resonance.

Active filters (Fig.5 right) analyze the current consumed by the load and create a current that cancel the harmonic current generated by the loads. Active filters were expensive in the past, but they are now becoming cost effective compensating for unknown or changing harmonics.

CONCLUSION:

The availability of electric power with high quality is crucial for the running of the modern society. If some sectors are satisfied with the quality of the power provided by utilities, some others are more demanding.

To avoid the huge losses related to PQ problems, the most demanding consumers must take action to prevent the problems. Among the various measures, selection of less sensitive equipment can play an important role. When even the most robust equipment is affected, then other measures must be taken, such as installation of restoring technologies, distributed generation or an interface device to prevent PQ problems.

REFERENCES:

USING ACTIVE POWER FILTER TO IMPROVE POWER QUALITY BY LUIS A MORAN AND JUAN W. DIXON

POWER QUALITY ANALYSIS AND MITIGATION BY U.C.CHARLES

POWER QUALITY PROBLEMS AND NEW SOLUTION BY A. de Almeida, L. Moreira , J. Delgado

BOOKS:

ELECTRICAL POWER SYSTEMS QUALITY BY

Roger C. Dugan/Mark F. McGranaghan

Surya Santoso/H. Wayne Beaty