5-24 Misc Electrical Eqipments

8/10/2019 5-24 Misc Electrical Eqipments

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January 1979

Revised January 2001

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MISCELLANEOUS ELECTRICAL EQUIPMENT

Table of ContentsPage

1.0 SCOPE ............... ............... ............... ............... ............... ............... ................ ............... ............... ........... 3

1.1 Changes .......................................................................................................................................... 3

2.0 LOSS PREVENTION RECOMMENDATIONS .............. ................ ............... ............... ............... ............ 3

2.1 Batteries ........................................................................................................................................... 3

2.1.1 Electrical ................................................................................................................................ 3

2.1.2 Operation and Maintenance .................................................................................................. 3

2.1.3 Protection .............................................................................................................................. 3

2.2 Capacitors ........................................................................................................................................ 3

2.2.1 Electrical ................................................................................................................................ 3

2.2.2 Protection .............................................................................................................................. 4

2.3 Insulators ......................................................................................................................................... 42.3.1 Operation and Maintenance .................................................................................................. 4

2.4 Reactors .......................................................................................................................................... 4


2.4.2 Protection .............................................................................................................................. 4

2.4.3 Electrical ................................................................................................................................ 4

2.5 Disconnect Switches ....................................................................................................................... 4


2.6 Rectifiers and SCRs (Semiconductor-Controlled Rectifiers) ........................................................... 6


2.6.2 Electrical ................................................................................................................................ 6

2.7 Voltage Regulators .......................................................................................................................... 6


2.7.2 Protection .............................................................................................................................. 7

2.7.3 Electrical ................................................................................................................................ 72.8 Resistors and Rheostats ................................................................................................................. 7


2.9 Protective Relays and Instrument Transformers ............................................................................. 7


2.9.2 Electrical ................................................................................................................................ 7

3.0 SUPPORT FOR RECOMMENDATIONS ............... ............... ............... ............... ............... ............... ..... 8

3.1 Loss History ..................................................................................................................................... 8

4.0 REFERENCES ............. ................ ............... ............... ............... ............... ............... ............... ............... . 8

4.1 FM Global ........................................................................................................................................ 8

4.2 Other ................................................................................................................................................ 8

APPENDIX A GLOSSARY OF TERMS ....................................................................................................... 8

APPENDIX B DOCUMENT REVISION HISTORY ....................................................................................... 8

APPENDIX C SUPPLEMENTARY INFORMATION ..................................................................................... 8

C.1 Storage Batteries ............................................................................................................................ 8

C.2 Capacitors ....................................................................................................................................... 9

C.3 Insulators ....................................................................................................................................... 11

C.4 Reactors ........................................................................................................................................ 11

C.5 Disconnect Switches ..................................................................................................................... 12

C.6 Rectifiers And SCRs (Semiconductor Controlled Rectifiers) .................................................. 12

FM GlobalProperty Loss Prevention Data Sheets 5-24

2000 Factory Mutual I nsurance Company. All rights reserved. No part of this document may be reproduced,stored in a retrieval system, or transmitted, in whole or in part, in any form or by any means, electronic, mechanical,photocopying, recording, or otherwise, without written permission of Factory Mutual Insurance Company.


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C.7 Voltage Regulators ........................................................................................................................ 13

C.8 Resistors and Rheostats ............................................................................................................... 14

C.8.1 Resistors ............................................................................................................................. 14

C.8.2 Rheostats ............................................................................................................................ 14

C.9 Protective Relays And Instrument Transformers .......................................................................... 14

C.9.1 Protective Relays ................................................................................................................ 14C.9.2 Instrument Transformers ..................................................................................................... 15

List of FiguresFig. 1. Various acceptable disconnect switch arrangements. ........................................................................ 5

Fig. 2. Determination of current-limiting reactor size. .................................................................................. 12

Fig. 3. Rectifier input and output voltage wave forms. ................................................................................ 13

5-24 Miscellaneous Electrical EquipmentPage 2 FM Global Property Loss Prevention Data Sheets

2000 Factory Mutual Insurance Company. All rights reserved.


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1.0 SCOPE

This data sheet briefly describes the operation, function and protection of electrical equipment commonly

encountered in industrial power distribution systems for which no other FM Global standards are available.

The main distribution system components are described in the applicable FM Global Data Sheets.

1.1 Changes

January 2001. The recommendation for the smoke detection for electrical rooms was revised to provide

consistency within 5-series data sheets.

2.0 LOSS PREVENTION RECOMMENDATIONS

Provide ionization type smoke detection/alarms in electrical rooms to sound at a constantly attended loca-

tion regardless of any automatic sprinkler protection or heat detection that may exist. Response should include

notification of personnel capable of de-energizing the electrical equipment. The presence or absence of

smoke detectors does not change the need for sprinklers. Smoke detection spacing should be accordance

with Data Sheet 5-48, Automatic Fire Detectors.

2.1 Batteries

2.1.1 Electrical

2.1.1.1 Arrange and protect storage batteries and battery areas electrically in accordance with Article 480

of the National Electrical Code.

2.1.2 Operation and Maintenance

2.1.2.1 Maintain them in accordance with Data Sheet 5-20, Electrical Testing.

2.1.2.2 Provide overcurrent protection by means of fuses or molded-case circuit breakers.

2.1.2.3 Take the following steps to limit the production of hydrogen and increase battery reliability:

1. Provide battery chargers with overcharge protection.

2. Provide suitable ventilation.

3. Maintain the battery room or area as close to 77F (25C) as possible.

2.1.2.4 Do not install batteries in applications requiring voltages in excess of the battery minimum useful volt-

age. In such applications, the battery will not carry its full load without its float or trickle charger.

2.1.3 Protection

2.1.3.1 Provide automatic sprinkler or water spray in the room or area for large installations utilizing com-

bustible battery containers. The choice of discharge density and area depends upon the type of container

material used the height to which they are installed. In enclosed areas of noncombustible construction where

an adequate extinguishing concentration can be maintained, Factory Mutual Research Approved auto-

matic CO2

or Halon 1301 system is an acceptable alternative. Where battery banks are not large enough

in themselves to warrant automatic protection, but present an exposure to important equipment, consider-

ation should be given to the installation of fire barriers or walls.

2.2 Capacitors

2.2.1 Electrical

2.2.1.1 Arrange and protect liquid-filled capacitors in accordance with Article 460 of the National Electric Code.

2.2.1.2 Provide liquid-filled capacitors other than those filled with Askarels with internal pressure and/or ther-

mally actuated interrupting devices set or adjusted to prevent case rupture in the event of an internal fault.

2.2.1.3 Protect capacitors with lightning arrestors when connected to exposed overhead lines.

Miscellaneous Electrical Equipment 5-24FM Global Property Loss Prevention Data Sheets Page 3



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2.2.2 Protection

2.2.2.1 Provide indoor installations, other than those located within a vault, containing 100 gal (379 dm3)

or more of flammable oil (approximately 1000 kVAR and up) with automatic sprinklers or water spray above

the capacitors and for 20 ft (6 m) in all directions. Sprinklers should be designed to meet a discharge den-

sity of 0.2 gpm/ft

2

(8 mm/min) over and 20 ft (6 m) beyond in all directions to a maximum operating area of3000 ft2 (279 m2).

2.3 Insulators


The following maintenance recommendations should normally be completed annually; however, the frequency

should be increased where the surrounding environment is unusually dusty, dirty, or where exposed to

mechanical damage.

2.3.1.1 Clean insulators periodically to eliminate accumulations of dust and dirt, which may result in a

flashover.

2.3.1.2 Inspect insulators closely for signs of cracking or chipping, and replace if necessary.

2.3.1.3 Perform periodic power factor testing to determine the insulating qualities of insulators. The maxi-mum allowable power factor varies greatly, depending upon the particular type of insulator, the operating

temperature and the surrounding environment. Refer to the manufacturers recommendations.

2.4 Reactors


2.4.1.1 Test and maintain reactor windings and insulating oils in accordance with Data Sheet 5-4,

Transformers.

Check tie rods and other connections for tightness at least annually to ensure that they retain their axial

and radial strength.

2.4.2 Protection

2.4.2.1 Provide automatic sprinklers or water spray for indoor, oil-insulated reactors, other than those located

within a vault, having a capacity of 100 gal (379 dm3) of oil or more. Sprinklers or water spray should be

designed for a discharge density of 0.2 gpm/ft2 (8 mm/min) over and 20 ft (6 m) beyond in all directions to

a maximum operating area of 3000 ft2 (279 m2).

2.4.3 Electrical

2.4.3.1 Outdoor, oil-filled reactors should be arranged and protected in accordance with recommendations

in Data Sheet 5-4, Transformers.

2.4.3.2 Arrange and protect reactors in accordance with Article 470 of the National Electrical Code.

2.4.3.3 Provide lightning protection in accordance with Data Sheet 5-11,Lightning and Surge Protection for

Electrical Systems.

2.5 Disconnect Switches


2.5.1.1 Never operate disconnect switches under load unless specially designed for that type of operation.

2.5.1.2 Maintain disconnect switches in accordance with Data Sheet 5-19,Switchgear and Circuit Breakers.

2.5.1.3 Mark circuits in a switching area for ease of identification, and review them carefully before use.

2.5.1.4 Elevate or enclose switches to prevent unauthorized tampering.

2.5.1.5 Ensure that adequate clearances exist in relationship to other live conductors.




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2.5.1.6 Install disconnect switches between all circuit breakers and power bus. Provide them also between

transformer primaries and the public utility or other outside plant source. The disconnects should be inter-

locked with the main circuit breaker, whenever possible, so that they cannot be opened first.

2.5.1.7 Open generator disconnect switches after the generator is shut down, and keep them open until ready

to go back on line. This will prevent damage to the generator in the event that the circuit breaker is mistak-enly closed while the generator is off.

Fig. 1. Various acceptable disconnect switch arrangements.




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2.6 Rectifiers and SCRs (Semiconductor-Controlled Rectifiers)


2.6.1.1 Keep rectifiers, like all electrical equipment, clean, cool and dry. Check connections periodically for

tightness. Make inspections annually or more frequently depending upon the severity of the environments inwhich they are operated.

2.6.1.2 Maintain mercury-arc rectifiers in accordance with the following schedule (refer to manufacturers

specifications for details):

1. Each shift:

a) Check vacuum gauge readings and vacuum regulator operation.

b) Check operating temperature.

c) Check indicators for misfires.

d) Check for proper coolant level.

2. Weekly:

a) Check vacuum and water circulating pumps for excessive vibration.

b) Inspect electrolytic rods for excessive corrosion.

c) Check for proper operation of the water pressure relay.

3. Semiannually:

a) Check for the proper concentration of corrosion inhibitors.

b) Inspect mercury pump heater.

c) Change rotary pump oil and clean oil casing.

d) Repack the water circulating pump.

e) Check water regulator for proper operation.

f) Inspect thermostats for leaks, dirt in mechanisms and check for proper operation.

4. Whenever load is removed from the unit, check anode and water heaters for proper operation.

2.6.2 Electrical

2.6.2.1 Provide overload protection by means of circuit breakers or current-limiting fuses on the ac side of

the device.

2.6.2.2 Provide overvoltage protection by installing one of the following:

1. A combination resistor-capacitor suppressor on the incoming line.

2. Nonlinear resistive elements (such as selenium transient suppressors, Zener diodes or ceramic suppres-

sors) across the rectifier to be protected.

3. A special solid state circuit that shorts the line, absorbing the energy until the overvoltage ceases.

2.6.2.3 Provide sufficient heat sinking to prevent solid state junction temperatures from exceeding their maxi-

mum allowable temperature rise.

2.7 Voltage Regulators


2.7.1.1 Test and maintain voltage regulators and tap changers in accordance with Data Sheet 5-4,

Transformers.




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2.7.2 Protection

2.7.2.1 Provide automatic sprinklers or water spray for indoor, oil-immersed regulators, other than those

located within a vault, having a capacity of 100 gal (379 dm3) of oil or more. Sprinklers should be designed

to meet a discharge density of 0.2 gpm/ft2 (8 mm/min) over 3000 ft2 (279 m2).

2.7.3 Electrical

2.7.3.1 Provide lightning protection in accordance with Data Sheet 5-11,Lightning and Surge Protection for

Electrical Systems.

2.8 Resistors and Rheostats


2.8.1.1 Keep resistors and rheostats clean, cool and dry.

2.8.1.2 Check connections for tightness at least annually, and more frequently in areas exposed to severe

vibration.

2.9 Protective Relays and Instrument Transformers


2.9.1.1 Maintain relays annually as follows. Increase the frequency depending on the severity of the

surrounding environment.

1. Electromechanical relays

a) Clean relays of any dust, dirt, rust or moisture.

b) Check for loose hardware and connections as well as mechanical damage.

c) Check for smooth operation of plungers, armatures, cups or disks.

d) Check contacts for signs of pitting.

e) Check target operation.

f) Perform test of insulation resistance, pick-up current, time delays and control wiring, and compare to

previous records or manufacturers specifications.

2. Solid state relays

a) Check for loose hardware and electrical connections as well as mechanical damage.

b) Remove any dust, dirt, rust or moisture.

c) Perform tests of pick-up current, time delays and control wiring, and compare to previous records or

manufacturers specifications.

2.9.1.2 Maintain instrument transformers annually as follows. Again, the frequency of testing should be

increased if warranted by a severe surrounding environment.

1. Check for loose electrical connections.

2. Remove any dust, dirt and moisture.

3. Perform insulation resistance tests, turn ratio tests and polarity tests.

2.9.2 Electrical

2.9.2.1 Provide potential transformers with independent overload protection on each primary leg.




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3.0 SUPPORT FOR RECOMMENDATIONS

3.1 Loss History

The equipment described in this data sheet is responsible for losses totalling several million dollars every

year. The electrical failures alone have amounted to losses in excess of 10 million dollars over the last fiveyears.

4.0 REFERENCES

4.1 FM Global

Data Sheet 5-2, Electrical Testing.

Data Sheet 5-4, Transformers.

Data Sheet 5-11, Lightning and Surge Protection for Electrical Systems.

Data Sheet 5-19, Switchgear and Circuit Breakers.

4.2 Other

National Electrical Code, Articles 460, 740, 480.

APPENDIX A GLOSSARY OF TERMS

This document does not have any defined terms.

APPENDIX B DOCUMENT REVISION HISTORY

January 2000. This revision of the document was reorganized to provide a consistent format.

APPENDIX C SUPPLEMENTARY INFORMATION

C.1 Storage Batteries

A battery is a device, consisting of a cell, or more than one cell electrically connected, which transforms chemi-

cal to electrical energy. A charging current reverses this reaction in a storage (secondary) battery.

Storage batteries are used, among many other things, for power generator transmission control, emer-gency power applications, telephone and alarm systems, radar and electrically powered industrial trucks.

Most common is the lead-acid battery. A lead-acid cell consists of a lead-dioxide coated positive plate and

sponge-lead coated negative plate immersed in an electrolyte consisting of sulphuric acid and water. The

resulting chemical reaction produces a potential difference across the terminals:

discharging

PbO2

+ Pb + 2H2

SO4

2PbSO4

+ 2H2

O

discharging

In ionic form:

Discharge

At negative plate: Pb

Pb++

+ 2ePb++ + SO

4 = PbSO

4

At positive plate: PbO2

+ 2H+ PbO + H2O - 2e

Charge

At negative plate: PbSO4

+ 2H++ 2ePb + H

2+ H

2SO

4

At positive plate: PbSO2

+ 2O4

= -2e Pb(SO4

)2

The equation read from left to right is the discharge reaction and from right to left, the charge reaction. The

process can be reversed by applying a floating or trickle charge and an occasional equalizing charge.




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A battery is formed by connecting several cells in series and placing them in a container constructed of glass,

hard rubber, asphaltic compositions or polypropylene plastic.

During charging, each cell produces hydrogen gas. Normally, this is vented to atmosphere through the vent

plugs. This gas is produced by a breakdown of water in the electrolyte by the excess charging current not

used by the active material. The rate at which the gas is produced increases toward the end of charging.Increased temperatures also increase the rate of hydrogen production.

In alkaline storage batteries such as the nickel cadmium battery, the electrolyte is composed of a caustic pot-

ash and water solution. The positive plate consists of nickel hydroxide and graphite and the negative plate,

cadmium and iron oxides. These batteries have a long life, require a minimum of maintenance and are usu-

ally smaller in size and weight than lead-acid batteries of similar capacity.

The discharge rate of a battery is dependent upon the total surface area of its plates, its internal resis-

tance and the connected load. For short duration, high discharge rate demands, such as engine starting, a

battery containing many plates is needed. For demands of longer duration but lower discharge rates, a bat-

tery constructed of fewer but larger plates should be used.

Batteries are rated in ampere-hours (Ah), the product of a discharge current and a nominal discharge period,

and cold cranking power. The ampere-hour rating is usually given during a 3 or 8 hour period at 25C (77F).

The actual sizing of battery cells to meet a particular need is best left to the manufacturer. Before the manu-facturer can determine the proper size, the total load to be served and the expected duty cycle of the system

must be determined.

For batteries to maintain their maximum capacity they must be kept charged. Rectifiers are used to con-

vert ac to dc to provide a float-charge or trickle-charge to the battery bank by applying a constant voltage

or constant current across the battery terminals. This charge counteracts the internal reactions of the bat-

tery, which tend to deplete it when not in use. After a period of discharge, an equalizing charge of higher

voltage is applied to the battery, restoring it to full capacity quickly. As an example, a typical lead-acid bat-

tery cell has a voltage of 2 V; the float charge is usually 2.15 V per cell and the equalizing charge 2.33 Vpc.

These values may vary slightly; therefore, the battery manufacturers recommendations should be followed.

Failures in storage batteries have resulted principally from internal short circuits, excessive loading and over-

charging. These failures may result in fires which spread across the tops of combustible battery cases,

possibly to other equipment and structures. The increased temperatures during the fire increase hydro-

gen production which adds to the fires intensity.

C.2 Capacitors

A capacitor is a device with the ability to store an electrical charge when a potential difference exists between

the terminals. When connected to an ac supply the alternating voltage results in the application of direct

potential in one direction during the first half cycle and the reverse direction in the second half cycle. As the

frequency of this cycle increases the effective current flow increases as can be seen from the following

formula:

l = 2fCE,

where f = Frequency (Hz)

C = Capacitance (farads)

E = Voltage

The corresponding reactive power is equal to 2fCE2.

The most common applications of capacitors in electrical power systems are for power factor correction and

voltage regulation. These consist of two thin metal foils (plates) separated by a kraft paper or plastic sheet

dielectric. This is rolled into cylindrical form and left round or flattened to form an oval shape. The assem-

bly is then baked to remove all moisture from the paper, placed in a metal can and impregnated, under

vacuum, with a dielectric liquid to fill air spaces. The cans are then sealed and provided with leakproof ter-

minals. The high dielectric strength of the liquid gives capacitors a high breakdown point and long life (10-20

years) if properly maintained.

When a capacitor consists of the two flat, parallel plates, such as this, the value of its capacitance may be

calculated as follows:




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English

C = 2248 AK

t 1010

where C = Capacity in microfarads (f)

A = Area of dielectric (sq in.)K = Dielectric constant

t = Thickness of dielectric (in.)

Metric

C = 885.26 AK

t 108

where C = Capacity in microfarads (f)

A = Area of dielectric (m2)

K = Dielectric constant

t = Thickness of dielectric (m)

Shunt-connected capacitors improve the power factor of an electrical system by supplying some of the reac-

tive power normally supplied by the line. This reactive power is in the form of magnetizing energy for motorsand other inductive apparatus and does not contribute to work output. Since less reactive power (kVAR)

is drawn from the line, the apparent power (kW) approaches the total power used (kVA) and the power fac-

tor approaches unity.

kVA = (kW)2 + (kVAR)2

Power Factor (p.f.) = kW

kVA

The net result of improving the power factor of a plant is to make more of the systems capacity available

by removing the burden of supplying excessive magnetizing current, reduce line losses and improve volt-

age conditions, as well as save on electric power costs.

Many power companies now penalize industrial users for operating at too low a power factor because they

are billed on kW used rather than kVA. A low power factor means the utility company is supplying some powerfor which it not being paid.

Capacitors of similar construction can be connected in series to regulate line voltage. They reduce the volt-

age dips that occur when starting large motors or applying other large inductive loads.

Energy storage capacitors are usually oil-filled units of various sizes used to store large quantities of energy

for such things as welding, and X-ray or high intensity light sources. The amount of energy stored can be cal-

culated as follows:

W = 12 CE210-6

where W = Energy (joules)

C = Capacitance (f)

E = Potential across electrodes (volts)

Surge capacitors are also oil-filled. They are used to protect rotating machinery from lightning and other over-

voltages by making use of their time constants. The charging time of a capacitor depends upon what is known

as an RC time constant. This can be increased by increasing the capacitance or the resistance through which

it charges. The resultant reduction in the slope of the incoming wave front eliminates damage to motor

insulation.

Fluorescent light ballast capacitors are usually small (approximately 2-3 in. 2 in.1 in.) (51-76 mm 51 mm

25 mm), liquid-filled capacitors imbedded in the ballast asphalt encapsulation. Their function is to improve

the power factor of the fixture by offsetting the inductive effect of the ballast windings.

Motor start capacitors are sometimes used to provide the phase shift necessary in an auxiliary winding to

allow starting small, single phase motors. They are normally the dry dielectric type and are switched out of

the circuit when the motor begins to accelerate. In some cases an additional component known as a motor




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run capacitor is placed in parallel with the start capacitor. This is left in series with the auxiliary winding per-

manently, and offers the advantages of higher starting and breakdown torques, lower full load currents,

decreased core noise and increased efficiency. Motor run capacitors are usually liquid-dielectric-filled with

a maximum height of 9 in. (23 cm), containing just a few ounces of liquid.

Capacitors are also used in electronic equipment. These are chiefly small dry-dielectric capacitors con-structed of paper, mica, air, ceramic, glass or oxide dielectrics. They are used for power supply filtering, bypass

and coupling capacitors, to name a few.

Losses in capacitors of all types have been small, generally because of good electrical protection, but over-

voltages due to lightning or switching surges have caused liquid dielectric capacitor cans to burst, spreading

flaming oil to adjacent equipment. Contaminated oil or manufacturing defects can result in internal faults

which, if not properly protected from overloads, also may result in bursting cans and burning oil. Addition-

ally, several capacitors connected in a bank may be subject to multiple failures which could result in a chain

reaction.

C.3 Insulators

Insulators are nonconducting components used to isolate live electrical parts or conductors from other live

parts or ground.

They are normally constructed of glass, porcelain, or other compounds with similar characteristics. Porce-

lain insulators have a smooth, glazed surface to eliminate moisture absorption and minimize the accumulation

of dust. All insulators must have sufficient mechanical strength to withstand the maximum expected loads

from cables, wind and ice, mechanical abuse, lightning, high energy arcs and magnetic forces resulting from

the flow of short circuit currents. They must be able to prevent a flashover under various temperatures and

humidity levels.

Failures in insulators are usually due to external causes such as flashover from lightning or mechanical dam-

age from flying objects. The material in a properly constructed insulator does not deteriorate with age, but

improperly constructed pieces can eventually break down from thermal or mechanical stress and result in a

fault.

Insulator design is dependent upon the length of the leakage path needed for the particular voltage class,

the mechanical strength needed for expected loads, and the wet flashover characteristics desired. These fea-

tures result in the ripple-type shape used for most insulators.

C.4 Reactors

Reactors are devices that are used to add a form of impedance to a circuit. They have two principal uses

in power systems. The most common application is the series-connected, current-limiting reactor. This type

is used to limit the magnitude of short circuit fault currents in feeders, ties and generator leads by adding

a low resistance impedance in the form of an inductance in the power line. This inductance offers a high

impedance to large fault currents (see Figure 2).

Current-limiting reactors are often added to a power system to allow the use of lower interrupting capacity

switchgear, thereby cutting down on installation costs. Problems arise if a failed reactor is bypassed to restore

power. This leaves circuit breakers exposed to fault currents beyond their interrupting capacities, and could

result in a catastrophic failure should additional faults occur.

Shunt reactors are connected line-to-line or line-to-ground and are used on high voltage transmission lines

to prevent objectionable voltage rises that result from charging currents.

Reactors consist of insulated windings on glass reinforced polyester resin, concrete or epoxy resin core forms,

and can be the air-cooled, dry type or liquid dielectric insulated. Usually the air-cooled, dry type is used

indoors for low and medium voltage applications. Liquid immersed outdoor reactors, which consist basi-

cally of air-core reactors placed in a steel tank and filled with insulating oil, are used for high voltage

applications.

High fault currents subject reactors to large thermal and mechanical stresses. They should be adequately

braced against the maximum expected stresses. Other types of reactors are used for purposes such as

neutral grounding, to limit disturbances due to ground faults, and for motor starting, to reduce starting currents.




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C.5 Disconnect Switches

Disconnect switches are used to isolate the conductors of a circuit from the source after the current flow

has been interrupted by a circuit breaker or contactor. They are essentially manual safety devices that visu-

ally indicate an open circuit. Disconnect switches enable complete de-energization of such equipment as

circuit breakers, starters, contactors and transformers for inspection and repair.

Switches can be of the knife blade or horn gap types. Those intended for high voltage applications are not

intended to open load current unless specially designed. Even disconnect switches provided with horn gaps

should not be used to break load current unless completely analyzed and found to be of adequate capac-

ity. Disconnects used in high voltage applications are provided with a means of holding the switch closed

under the large magnetic forces resulting from short-circuit currents.

C.6 Rectifiers And SCRs (Semiconductor Controlled Rectifiers)

Rectifiers are devices that transfer electric energy from alternating current circuits to direct current circuits.

They are used in etching and plating, welding, battery charging, dc motors, excitation for synchronous motors

and generators, control systems, switching applications and uninterruptible power supplies.

Rectifiers can be of the solid-state or mercury-arc type. Newer rectifiers and SCRs are of solid-state con-

struction. They are formed by joining two pieces of material (usually silicon or germanium) with different

electron contents. The junction formed will conduct only when the applied voltage is of a certain polarity. Con-

duction ceases when the polarity reverses (see Figure 3).

Another type of rectifier is the mercury-arc rectifier. Basically, the mercury-arc rectifier consists of an evacu-

ated metal chamber containing an anode of iron, carbon or other material and a cathode consisting of a

Fig. 2. Determination of current-limiting reactor size.




13/16

pool of liquid mercury. A spark ignitor, also provided in the chamber, vaporizes a portion of the mercury, caus-

ing the cathode to emit electrons. When a positive potential is applied to the anode it attracts the electrons,

causing an arc to form between the anode and cathode. The rectifier will not conduct when the anode is nega-

tive with respect to the cathode.

Semiconductor Controlled Rectifiers, also known as thyristors, are a special type of solid state rectifier used

primarily in motor speed control systems, light dimmers, battery chargers and exciters. An additional gate

terminal is used to control the point at which the device begins to conduct. As the magnitude of the gate

pulse increases, the terminal voltage at which the SCR conducts decreases. After the device is triggered by

this gate pulse, it will continue to conduct independent of the gate until the current flow decreases enough

to stop conduction.

Power rectifiers can generate a considerable amount of heat. Increases in the junction temperature due to

overloads can affect all their characteristics, since they do not have the thermal capacity inherent in large

motors and transformers that utilize heavy copper windings and iron or steel cores. It is, therefore, impor-

tant to provide a means of dissipating this heat to prevent damage to the rectifiers or changes in the system

characteristics.

The maximum allowable junction temperature rise, and the heat sinking necessary to limit this rise, are depen-dent upon the ambient temperature, the continuous current drawn, the conducting time and the thermal

conductivity, density, specific heat and cross-sectional area of the material used.

The selection of the best heat sinking also depends on such factors as size and weight limitations and eco-

nomics. Common types include natural convection cooling, forced convection cooling and water cooling.

Forced convection cooling is now the most widely used.

In natural convection cooling the rectifier is mounted on a finned metal heat sink, which is cooled by exchang-

ing heat with the surrounding air. The cooling can be increased by forcing air past the fins. Water cooling

is the most efficient method in use and it has size and cost advantages over the others.

Another problem encountered with solid state rectifiers is voltage transients as a result of lightning, or more

often, switching surges. The immediate failure or gradual deterioration of the device depends upon the sever-

ity of the overvoltage and the frequency of occurrence.

C.7 Voltage Regulators

A voltage regulator is a device that maintains the voltage level within certain limits. Voltage levels usually

change when the load on a power system changes considerably. Reduced or increased voltage operation

of most equipment results in inefficient operation as well as excessive heating of apparatus with subse-

quent increases in operating costs and decreases in equipment life.

Voltage level can be controlled by tap changing on power transformers, regulating autotransformers, induc-

tion voltage regulators or static voltage regulators. The operation of power transformers and autotransformers

is described in Data Sheet 5-4, Transformers.

An induction voltage regulator operates on the principle of a variable ratio autotransformer. The primary wind-

ing supplies the magnetization while the secondary or series winding provides the incremental voltage

Fig. 3. Rectifier input and output voltage wave forms.




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changes needed to adjust the line voltage. The voltage is controlled by changing the mutual inductance

between the two windings. The mutual inductance is changed by repositioning the windings mechanically.

The current in the primary winding counterbalances the magnetizing or de-magnetizing effect of the second-

ary winding.

Regulation is automated by supplying a voltage regulating relay from a potential transformer connected acrossthe line. This relay controls a small regulator motor that rotates the secondary winding in response to volt-

age variations to change the mutual inductance. Regulation of10% of line voltage can be made smoothly

without opening the circuit.

Induction voltage regulators are usually oil-immersed and self-cooled, but large units are cooled like large

transformers by the use of radiators and fans.

Newer types of voltage regulators are statically operated devices. They operate on the principle of chang-

ing magnetic field flux densities to control voltage levels. These are easier to maintain than tap changers or

induction voltage regulators because there are no moving parts, bearings or contacts.

C.8 Resistors and Rheostats

C.8.1 Resistors

A resistor is a device in a circuit that determines the rate at which current is converted to radiant energy.

They may be constructed of (1) a wire embedded in enamel, cement, sand or suspended in air; (2) a metal

ribbon; (3) a cast metal grid; or (4) carbon plates, disks or rods.

The principal uses of resistors are for field discharge across a motor or generator field winding to allow the

gradual decay of the magnetic field and reduce sparking when shutting down, for grounding of power sys-

tems to limit ground fault currents, and in electronic equipment to reduce or divide current and voltage.

C.8.2 Rheostats

A rheostat is a resistor that may be varied in value without opening the circuit. It consists of a resistance

element of formed metal wire or ribbon mounted on an insulating material.

Rheostats are used principally on motors and generators to regulate the field strength which in turn regu-

lates generator output voltage, motor power factor or speed; to adjust battery charging potential from a

constant potential source; to reduce current thereby dimming lights; and in electronic equipment to adjust volt-age, current, and sound levels, calibration, etc.

A saturable core inductor performs the same functions as a rheostat but operates on a different principle.

A dc source is used to vary the flux in the core of the reactor. As the core saturates, its impedance decreases.

With no dc, the core is unsaturated and the reactor impedance is high. This method provides smooth, step-

less control of alternating voltage using low levels of dc. It has the additional advantage of requiring no

moving parts.

C.9 Protective Relays And Instrument Transformers

C.9.1 Protective Relays

A relay is a device in the system that recognizes a fault and operates a set of contacts to sound an alarm

or initiate the operation of a sufficient number of circuit breakers or contactors to clear the fault with a mini-

mum of damage and disturbance to the system.

Most relays now in use are electromechanical types. There are two basic operating principles. The electro-

magnetic attraction type consists of either a magnetic plunger drawn into a coil or a hinged armature attracted

by an electromagnet. The more common type, the electromagnetic induction relay, is similar in operation to

an induction motor. Current flowing in a stator winding creates a torque which rotates a metal cup or disk.

This, in turn, operates a set of contacts against a restraining spring.

Relays can be made to operate instantaneously or with a time delay. Instantaneous relays have no inten-

tional delay, and operate in less than 12 to 6 cycles after initiation of a fault. Time delay relays are classified

into four categories. Definite time relays have a constant delay. Inverse, very inverse, and extremely inverse

relays increase in operating speed as the magnitude of the fault current increases. The extremely inverse

relay is the most sensitive but the inverse relay is the most common general application device.




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Protective relays are available which operate on various electrical quantities such as current, voltage, imped-

ance, frequency, power, etc. They can be further classified according to functions of these quantities such

as direction, differential, over and under, distance and balance, to name a few.

Non-directional relays operate on current magnitude only, regardless of direction of flow. They usually con-

sist of an electromagnetic attraction type instantaneous element and an electromagnetic induction type timedelay element. Pick-up current (current at which contacts begin to move) settings are varied by changing tap

settings. Time delays are adjusted by a time dial which acts to vary the tension on the restraining spring.

Directional relays operate when current flows in a predetermined direction only. This type allows selective trip-

ping when other sources of fault current such as local generation or other feeders are available. It uses a

small directional control element (senses power) that must operate before a standard instantaneous and/or

time delay element.

Differential relays operate on the principle that under normal operation the current flowing into a circuit must

equal the current flowing out. They measure two currents, such as those flowing on either side of a motor

winding, and compare them. They are set to operate on a predetermined difference. This difference can be

either a constant percentage difference or a percentage that increases as the fault current increases. The

latter type accounts for instrument transformer errors that result from large current flows.

Distance relays compare circuit voltage to current, the ratio of which is the impedance. The impedance is

used as a measure of distance. These are mainly used on long transmission lines, rather than in industrial

applications where power lines are usually too short to make them practical.

Over and under relays detect quantities such as current and voltage that exceed or drop below predeter-

mined levels.

Solid state protective relays are now available to perform most functions offered by electromechanical relays

but they overcome many of their limitations. They have the following advantages:

1. Faster switching allows improved tripping.

2. Wide adjustability.

3. Reduced size and space requirements.

4. Improved seismic capability due to lack of moving parts.

5. Temperature stability.

6. Greater sensitivity allows closer coordination.

7. Less susceptibility to dust and dirt due to lack of moving parts.

8. Easy testing.

9. Load characteristics avoid instrument transformer problems.

One of the biggest factors is that they are now becoming competitive in price with electromechanical relays.

Solid state relays replace moving contacts, which are subject to so many types of failures, with transistor

and semiconductor controlled rectifier switches. They are low energy devices and often self-powered. This

eliminates the need for a trip source such as batteries or a motor-generator set.

A solid state relay consists of three basic sections. The input section takes a signal from a current sensor

or instrument transformer, rectifies it and adjusts its level. A timing section sets the operating time delays, andthe trip circuit actuates the circuit breaker trip coil or other tripping device.

The particular type of relay protection needed for any system depends upon the type and importance of the

equipment and the degree of protection economically justifiable.

C.9.2 Instrument Transformers

Instrument transformers are key components of a system, because they determine the success of a con-

trol or protection scheme. They supply metering equipment or protective relays at reduced voltage or current

levels, isolated from the power system. This provides greater personnel safety as well as reducing the size

and cost of monitoring and control equipment.




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Potential transformers are connected between phases or from phase to ground. The ratio of the transformer

is usually chosen to provide a 115 volt secondary output. Those for use on systems of less than 15 kV are

usually dry type and those used on systems 15 kV and above are oil-immersed.

Current transformers are installed with their primary windings in series with the power line. Their secondary

windings are usually designed for a 5 ampere secondary current. As was the case with potential transformers,current transformers for service at voltages of less than 15 kV are usually dry type and those 15 kV and

above, oil-immersed.

There are four types of current transformer construction: Wound primarycurrent transformers consist of

multiturn primary and secondary windings insulated and permanently assembled on a core.Bar typeshave

primary and secondary windings insulated and permanently assembled. The primary winding is single turn,

consisting of a bar type conductor mounted through the center of the toroidal core. Window typecurrent trans-

formers consist of a secondary winding only, insulated and permanently assembled on a core. The primary

winding consists of the power cable or bus bar placed through the center of the toroidal core. Bushing types

are similar to the window types in that only a secondary winding is assembled on the core. This type is

assembled as part of a transformer or circuit breaker bushing.

FMELPC Oct 1978


5-24 Misc Electrical Eqipments

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