Power Quality

INTRODUCTION

Power Quality is a technical term that has practical implications for your

business and your equipment. When power is generated, it has very predictable

characteristics. It energizes all electrical equipment equally and satisfactorily.

However, as the power travels through the wires and energizes the equipment, the

various pieces of equipment it energizes can change the quality of the power, making

it less suitable for the next application. These changes in power quality are especially

common in large industrial and commercial complexes and include increases and

decreases in voltage, momentary power outages, and noise on the electrical system.

At its most extreme, poor power quality can even cause equipment to malfunction.

Why Do You Need Power Quality Protection?

While power disturbances occur on all electrical systems, the sensitivity of

today’s electronics makes them more susceptible to them. For some sensitive devices,

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

frozen mouse, system crashes and equipment failure. A power voltage spike can

damage valuable components. Power quality issues can cause business problems such

as:

Lost productivity and idle people and equipment

Lost orders, good will, customers and profits

Lost transactions and orders not being processed

Revenue and accounting problem such as invoices not prepared, payment held up,

and early payment discount missed.

Customer and/or management dissatisfaction

Overtime required to make up for lost work time

According to Electric Light and Power magazine, 30 to 40 percent of all

business downtime is related to power quality problems. Businesses have a lot

invested in office and production equipment, and power quality protection is an

inexpensive insurance policy against incidents.

COMMON POWER DISTURBANCES

Common power quality disturbances include surges, spikes and sags in power

source voltage and harmonics (or noise) on the power line. Each of these occurrences

is discussed briefly below.

Surge – A rapid short-term increase in voltage Surges often are caused when

high power demand devices such as air conditioners turn off and the extra voltage is

dissipated through the power line. Since sensitive electronic devices require a constant

voltage, surges stress delicate components and cause premature failure.

Spike – An extremely high and nearly instantaneous increase in voltage with

a very short duration measured in microseconds. Spikes are often caused by lightning

or by events such as power coming back on after an outage. A spike can damage or

destroy sensitive electronic equipment. Turn the equipment off during a power

outage. Wait a few minutes after power is restored before turning it on, and then turn

on one device at a time.

Sag – A rapid short-term decrease in voltage. Sag typically is caused by

simultaneous high power demand of many electrical devices such as motors,

compressors and so on. The effect of sag is to tarve electronic equipment of power

causing unexpected crashes and lost of corrupted data. Sags also reduces the

efficiency and life span of equipments such as electric motor.

Noise – A disturbance in the smooth flow of electricity. Often technically

referred to as electro-magnetic interference (EMI) or radio frequency interference (RFI).

Harmonics are a special category of power line noise that causes distortions in electrical

voltage. Noise can be caused by motors and electronic devices in the immediate vicinity

or far Away. Noise can affect performance of some equipment and introduce glitches and

errors into software programs and data files.

Outage – Total loss of power for some period of time. Outages are caused by

excessive demands on the power system, lightning strikes and accidental damage to

power lines. In addition to shutting down all types of electrical equipment, outages cause

unexpected data loss.

What Causes Power Quality Problems?

Studies show that up to 80 percent of most small business’ power quality

problems are caused by disturbances created inside of a facility or business. (See the

chart below.) When large power users in a building, such as fans and air conditioning

equipment, cycle on and off, they can cause power dips and surges that affect other

equipment in the building. Lightning is another major source of disturbance,

accounting for more than 10 percent of power disturbances. That’s why it is important

to have power quality protection at the incoming utility meter and at each piece of

sensitive electronic equipment throughout your facility.

WHAT IS LINE CONDITIONER?

Also called a line conditioner, a device that sits between a computer and its

power supply, typically a wall outlet. The power conditioner provides protection

against surges in power just as a surge protector does, but a power conditioner also

maintains a continuous voltage fed to the computer during temporary voltage

reductions, such as a brownout. This is referred to as conditioning. Power

conditioners also can filter EMI emanating from a power source and can smooth the

rhythmic cycle of alternating current.

POWER CONDITIONING TECHNOLOGIES

Device and Principal functions General Description

Surge Protection devices

Divert or clamp surges.

Various types of surge protectors are

available to limit circuit voltages.

Devices vary by clamping voltage, and

energy handling ability. Typical devices

are “crowbar” types like air gaps, gas

discharge tubes, and non linear resistive

types like thyrite valves, avalanche

diodes, and metal oxide varistors. Also

available a reactive suppressors that are

able to clamp, or limit, surges regardless

of where on the power sine wave occurs.

These devices do not affect significantly

the energy consumption.

Noise filters(EMI/RFI) Series inductors with parallel capacitor

good for low energy high frequency

noise.

Voltage regulators

Provide a relatively constant steady state

output voltage level for a range of input

voltage.

A variety of voltage regulation

techniques are used. Common techniques

include Ferro-resonant transformers,

electronic tap switching transformers,

and saturable reactor regulator.

SURGE PROTECTORS

OVERVOLTAGE:

There are several instances when the elements of a power system are subjected

to over voltages i.e. voltages greater than the normal value. These over voltages on

the power system may be caused due to many reasons such as lightning, the opening

of a circuit breaker, the grounding of a conductor etc. Most of the over voltages are

not of large magnitude but may still be important because of their effect on the

performance of circuit interrupting equipment and protective devices. An appreciable

number of these over voltages are of sufficient magnitude to cause insulation

breakdown of the equipment in the power system.

VOLTAGE SURGE:

A sudden rise in voltage for a very short duration on the power system is

known as a voltage surge or transient voltage. Transients or surges are of temporary

nature and exist for a very short duration but they cause over voltages on the power

system. They originate from switching and from other causes but by far the most

important transients are those caused b lighting striking a transmission line. In most of

the cases, such surges ma cause the line insulators to flash over and ma also damage

the nearby transformers, generators or other equipment connected to the line if the

equipment is not suitably protected.

CAUSES OF OVERVOLTAGES:

1. INTERNAL CAUSES:

Switching surges

Insulation failure

Arcing ground

Resonance

2. EXTERNAL CAUSES i.e. lightning.

Internal causes do not produce surges of large magnitude. Surges due to internal causes

hardly increase the system voltage to twice the normal value. However, surges due to

lightning are very severe and may increase the system voltage to several times the normal

value. If the equipment in the power system is not protected against lightning surges,

these surges may cause considerable damage.

LIGHTNING:

An electric discharge between cloud and earth, between clouds or between the

charges centres of the same cloud is known as lightning.

EFFECTS OF LIGHTNING:

1. The traveling waves produced due to lightning surges will shatter the

insulators and may even wreck poles.

2. If the traveling waves produced due to lightning hit the windings of a

transformer or generator, it may cause considerable damage. The inductance

of the windings opposes any sudden passage of electric charge through it.

Therefore, the electric charges “piles up” against the transformer. This induces

such an excessive pressure between the windings that insulation may

breakdown, resulting in the production of arc. While the normal voltage

between the turns is never enough to start an arc, once the insulation has

broken down and an arc has been started by a momentary over voltage, the

line voltage is usually sufficient to maintain

3. If the arc is initiated in any part of the power system by the lightning stroke,

this arc will set up very disturbing oscillations in the line. This may damage

other equipment connected to the line.

PROTECTION AGAINST LIGHTNING:

Transients or surges on the power system may originate from switching and

from other causes but the most important and dangerous surges are those caused

by lightning. The lightning surges may cause serious damage to the expensive

equipment in the power system either by direct strokes on the equipment or by

strokes on the transmission lines that reach the equipment as traveling waves. It is

necessary to provide protection against both kinds of surges. The most commonly

used devices for protection against lightning surges are:

LIGHTNING ARRESTORS:

The earthing screen and ground wires can well protect the electrical system

against direct lightning strokes but they fail to provide protection against traveling

wave which may each the terminal apparatus. The lightning arresters or surge

diverters provide protection against such surges.

A lightning arrester or a surge diverter is a protective deice which conducts the

high voltage surges on the power system to the ground. It consists of a spark gap in

series with a non linear resistor. One end of the diverter is connected to the terminal

of the equipment to be protected and the other end is effectively grounded. The length

of the gap is so set that normal line voltage is not enough to cause an arc across the

gap but a dangerously high voltage will break down the air insulation and form an arc.

The property of the non linear resistance is that its resistance decreases as the voltage

increases and vice versa.

ACTION OF LIGHTNING ARRESTERS:

1. Under normal operation, the lightning arrester is off the line i.e. it conducts.

2. On the occurrence of over voltage, the air insulation across the gap breaks down

and an arc is formed, providing a low resistance path for the surge to the ground.

In this way, the excess charge on the line due to the surge is harmlessly conduced

through the arrester to the ground instead of being sent back over the line.

3. It is worthwhile to mention the function of non linear resistor in the operation of

arrester. As the gap sparks over due to over voltage, the arc would be a short

circuit on the power system and may cause power follow current in the arrester.

Since the characteristic of the resistor is to offer high resistance to high voltage, it

prevents the effect of a short circuit. After the surge is over, the resistor offers

high resistance to make the gap non-conducting.

TYPES OF LIGHTNING ARRESTER:

1. Rod gap arrester

2. Horn gap arrester

3. Multigap arrester

4. Expulsion type arrester

5. Valve type lightning arrester

NOISE &ITS ERADICATION

Recent years have witnessed tremendous advances in electronics. In the field

of personal computers, word processors and other computer related equipment, legal

restrictions regarding safety and noise generation have grown more strict with each

passing year. In most cases, electronic devices exported must now conform to the

noise regulations of the target country in order for them to be given market approval.

The following is an introductory description of the ways in which noise is

generated and the various noise regulations currently enforced throughout the world.

NOISE FILTER

NOISE GENERATION AND TRANSMISSION

The noise generated by electronic device consists of two kinds. Radiated

noise is transmitted directly into the air from an electronic device, taking the form of

an electric wave that interferes with other electronic devices. In contrast Conductive

noise interferes with other components and devices by being transmitted along power

lines and the wiring of electronic circuits. These two kinds of noise can be briefly

explained in the context of an electronic device by means of the following diagram

(Figure 1).

OPERATING PRINCIPLES OF NOISE FILTERS

A key counter measure taken against noise is the use of noise filters. The

operating principles of these devices are described below:

Viewed from the prescriptive of the circuit network, noise filter is a kind of

low range or low pass filter. It is designed to pass only frequency lower than cut off

frequency, while attenuating or blocking all ranges higher than cut off frequency.

Fig. 2

As shown in Figure 2, the filter operates according to a principle whereby

inductance connected directly in series with the line has virtually no affect on the

noise current at low frequencies, but at high frequencies it demonstrates a high

interruptive effect with respect to the noise current.

Also a capacitor connected in parallel with the line is used as side path to

return high frequency back to the power line. The result is that normal mode noise

passes through the capacitor and is then shunted back to the other line. In the case of

common mode noise, result is that noise passes through the midpoint of the two

capacitors to the ground.

The use of special materials such as amorphous alloys and toroidal cores

gives the Okaya noise filters excellent insertion loss characteristics and high voltage

pulse attenuation capability.

EVALUATION METHODS OF NOISE

FILTER CHARACTERISTICS

1. Static Characteristics

With a measuring impedance of 50 ohms, the amount of attenuation (insertion

loss) is determined by using a level meter to measure the voltage before and after

insertion of a noise filter into the test circuit. Using this method, both normal mode

and common mode attenuation can be measured.

…

Attenuation = 20log10 (V0 /V1) [dB]

V1: Level when test material is inserted

V2: Level when test material is not inserted

2. Dynamic characteristics:

In order to achieve measurement results as. near as possible to actual application

conditions, the following method is used: With a noise simulator as the noise

generating source, a rated current is allowed to flow through the test device and a

simulated power circuit network. The amount or normal mode and common mode

attenuation is measured.

3. Pulse Attenuation Characteristics:

The following method is used to measure the noise margin for the external noise

in an electronic device: a noise simulator is connected and the input/output voltages

are measured. The formula noted below is then used to calculate the amount of

attenuation in the form of the pulse absorption effect produced. In general, the noise

condition used to test malfunctions is a high voltage pulse of 50nsec.to 1μsec at 1kV

to 2kV in amplitude.

Normal mode Common mode

Application Precaution

The following points should be kept in mind with regard to the installation of

noise filters.

1. When mounting on the noise producing side, they should be mounted as close as

possible to the source of the noise with noise electrical or mechanical contact

between the input and output side of the filter

(Example) When the input/output lines are bundled together or arranged parallel with

each other, high frequency noise components induced on the input side, results in the

production of noise current on the output side.

Separation of input/output lines (good example)

Bundling or parallel arrangement of input/output lines (poor

example)

2. When the device is directly installed on the equipment exposed to interference, it

is important to mount the noise filter as close as possible to the machines power unit

or input wiring. If a power line is allowed to enter the case of the equipment without

passing through the noise filter, noise current can be radiated throughout the inside of

the equipment enclosure, affecting the internal electronics.

.3. Precautions should be taken to insure that the ground line for the noise filter has

lower impedance than that of the noise current. If this is not done, the noise

prevention effect will be lost. Also, ground lines should be as short as possible. The

use of long ground lines will result in substantial reduction of the noise prevention

effects (particularly in the high frequency ranges above several MHz.).

4. Whenever possible, the outer case of the noise filter should be mounted directly to

the outer case of the electronic equipment. When this is not possible, a short

grounding line should be used to link the outer case of the filter and the equipment.

VOLTAGE REGULATOR

Line voltage regulators do exactly what the name implies their purpose is to

maintain reasonably constant output voltage to the load in the face of variations in

power line voltage. There are many different ways of accomplishing this and among

the basic types there are innumerable variations, all with their own advantages and

limitation the following covers one or two popular or generic versions of five basic

types of voltage regulators.

(1). Most line regulators will correct utility voltage sags and swells of 15

percent to output voltages to the load ranging from ±3 to ±7 percent of nominal. Many

will bring extreme utility sags in the area of 20 to 30 percent, back to ±7 to ±10 percent

of the nominal output voltage. Some things to consider when picking a line voltage

regulator are cost, speed of response, output impedance, audible noise, efficiency at

part and full load, sensitivity to load power factor and unbalanced three-phase loads,

and harmonic distortion.

(2) A few types of line regulators can attenuate impulses to a modest degree,

but most perform the regulation function only and pass impulses right through. In fact,

some types may create additional impulse noise by internal switching.

(3) There are two types of motor-actuated regulators, the motor -driven brush

type and the induction regulator type. The motor-driven brush type moves across many

taps on an autotransformer, causing the series transformer to buck or boost the voltage

to the load. The same function is performed by an induction regulator where rotating

the regulator on direction or the other varies the magnetic coupling and raises or lowers

the output voltages. The following covers one or two popular or generic versions of five

basic types of voltage regulators.

(4). Most actuated voltage regulator are generally inexpensive and can handle

heavy loads, but they are slow in response and can only correct for gradual voltage

changes. In addition, their electromechanical parts require substantial maintenance and

output impedance is high. They are not often used with electronic equipment.

(5). The Ferro-resonant voltage transformer regulator is one of the more

popular regulator., especially in the lower size ranges of 0.5 to 2 KVA. The Ferroresonant

transformer core structure is so designed that the secondary operates in flux

saturation and secondary winding resonant with capacitor in tuned circuit. As a result of

this saturated operating mode changing the primary or line voltage may change the

current but will not vary the flux or the secondary induced voltage. Thus the unit

performs voltage regulation action. A unique characteristic of Ferro-resonant regulator

is its ability to reduce normal mode impulses. Since its regulating capability is based on

driving the secondary winding into saturation, transients and noise burst are clipped.

(a) The basic Ferro-resonant transformer puts out a high harmonic content square

wave, not suitable for supplying sensitive electronic loads. A properly selected

neutralizing winding cancels out most of the harmonic content of the output voltage and

yields a satisfactory low - distortion sine wave.

(b) The Ferro-resonant regulator has a response time of about 25 milliseconds or

1.5 cycles, good reliability, minimal maintenance requirements, reasonable cost, good

normal-mode impulse attenuation, and good regulation. Because of the tuned circuit on

the output, it is sensitive to frequency variations (1 percent frequency change causes 1.5

percent output voltage change), but this is not much of a problem with tight utility

network frequency control. More important are its high output impedance (again up to 30

percent of load impedance), sensitivity to both leading and lagging load power factors,

and low efficiency at partial loads. Efficiency is about 90 percent at rated load, but since

the unit wastes about 10 percent of rated load regardless of what load level it is operating

at, efficiency drops with load. This, in turn, results in heat dissipation and the generation

of audible noise.

momentary overloads such as might be caused by high motor starting current, and output

voltage collapses at somewhere about 150 percent of full load. Over sizing is not the

whole answer to this problem, since this cause reduced efficiency and increased heat

dissipation. In summary, the Ferro-resonant regulator is useful in small systems that do

not contain large motors.

(6) The last type of line voltage regulator to be looked at here is the electronic

tap-switching autotransformer. With this unit the proper triac (or back-to-back silicon

controlled Rectifier [SCR]) is energized depending on whether the power line voltage

must be increased or decreased. Voltage correction occurs in discrete steps rather than

continuously. Resolution depends on the closeness of the taps, and switching impulses

are minimized by changing taps when the line voltage is near a zero crossing.

(a) The electronic tap-switching transformer has many good features. Response

time is fast at about 0.5 cycles or 10 milliseconds, regulation is good, output impedance is

in the range of 5 percent of load impedance, efficiency is high a about 95 percent, and the

units are insensitive to load power factor and load unbalance.

(b) The disadvantages are a somewhat high cost and slightly poorer reliability

when compared to the more rugged units.

kVA and up.