Module (01) Introduction of Switchgears

7/29/2019 Module (01) Introduction of Switchgears

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Module

01Introduction of

Switchgears





1.1 GENERAL BACKGROUND

Every electric circuit needs a switching device and a protective device. The switching and

protective devices have been developed in various forms. For example, everyone is familiar with

low voltage switches and re‐wirable fuses. The switch is used for opening and closing the electric

circuit and the fuse is used for over‐current protection.

Switches, fuses, circuit‐breakers, isolators, relays, control panels, lightning arresters, current

transformers, and various associated equipments. Switchgear is an essential part of a power

system and also that of any electric circuit. Switchgears are also necessary at every switching point

in the power system, see Figure (1)).

Figure (1)

Location

of Switchgears

in a Typical

Power

System





Since, between the generation stations and final load points, there are several voltage levels and

fault levels hence, in various applications, the requirements of a switchgear vary according to:

Location Ratings Local Requirements.

1.2 SUB‐STATION EQUIPMENT

In every electrical sub‐station, there are generally various indoor and outdoor switchgear

equipments. Each of the equipments has certain functonal requirement (Table 1).

• A Circuit‐Breaker is "a switching and current ‐interrupting device". Basically, a circuit

breaker comprises a set of fixed and movable contacts. At high voltages these may hning a

circuit, which at domestic voltages is taken The contacts can be separated by means of an

operating mechanism. The separation of current carrying contacts produces an arc. The arc

is extinguished by a suitable medium such as dielectric oil, air, vacuum, SF6 gas. The circuit‐

breaker serves two basic purposes:

Switching during normal operating conditions, for the purpose of operation and maintenance.

Switching during abnormal conditions, such as short circuits, and interrupting the fault currents.

• Protective Relays are "automatic devices which can sense the fault and closes its contacts

when the actuating quantity/quantities reach(s) certain predetermined values to send

instructions to the associated circuit ‐breaker to open". Every part of the power system is

provided with a protective relaying system and an associated switching device.

Isolators are "disconnecting switches which can be used for disconnecting a circuit under

no‐current conditions". They are generally installed along with the circuit‐breaker. An

isolator can be opened after the circuit‐breaker.

• Earthing Switch is a switch which connects a conductor to earth so as to discharge the

charges on the conductor to earth. After opening the isolator, the earthing switch can be

closed to discharge the trapped electrical charges to ground. Earthing switches are

generally installed on the frames of the isolators.





Table (1) Various Sub‐station Equipment

S. No. Symbol Equipment Function

1 Circuit‐breaker switching during normal &

abnormal conditions, interrupt

the fault current

2 Isolator

(disconnecting

switch)

disconnecting a part of the

system from live parts, under

no‐load conditions

3 Earthing‐switch discharging the voltage on the

lines to earth, after

disconnecting them

4 Surge arrester diverting the high voltage

surges to earth and maintaining

continuity during normal

voltage

5

Current

transformer stepping

down

the

current

for

measurement, protection, and

control

6 Potential

transformer

stepping down the voltage for

the purpose of protection,

measurement, and control

• Current Transformer (CT) is a transformer whose current ratio is generally high (e.g.

500A/5A) and its volt‐ampere capacity is relatvely low (e.g. 50 VA) as compared with that

of power transformers. It is used for transforming the current to lower value for the

purpose of measurement, protection, and control

• Potential Transformer (PT), Voltage transformer (VT) is a transformer whose volt‐ampere

capacity is low (e.g. 100 VA) and its voltage rato is relatvely high (e.g. 132 kV/100 V). It is

used for transforming the voltage to lower value for the purpose of measurement,

protection,

and

control.

The

protective

relays

are

connected

in

the

secondary

circuits

of

CTs and PTs.





• Lightning Arrester (surge arrester) is equipment connected between the conductor and

ground to discharge the excessive voltages to earth and protect the sub‐station equipment

from over over‐voltages.

• Auto‐Reclosure is automatic closing of the circuit‐breaker after its opening. It is provided

to restore the service continuity after interrupting a transient fault.

• Contactor is a switching device capable of making, carrying, and breaking electric current

under normal and overload conditions.

• High Rupturing Capacity (HRC) fuse is used for over‐current protection in low voltage and

medium voltage circuits.

1.3 FAULTS AND ABNORMAL CONDITIONS

During a fault, the fault impedance is low and accordingly, the fault currents are relatively high. At

the higher t Since the fault currents being excessive, they damage the faulty equipment and the

supply installation. During the faults, the power flow is diverted towards the fault; and the supply

to the neighboring zones is affected.

Faults can be classified as:

• Single Line to Ground Fault • Line to Line Fault • Double Line to Ground Fault • Simultaneous Fault • Three phase Fault • Open Circuit, etc. The other abnormal conditions which the system may be subjected to include:

• Voltage and Current Unbalance

• Under Frequency

• Over voltages

• Temperature Rise

• Reverse of Power

• Power Swing

• Instability, etc.





1.4 FAULT CLEARING PROCESS

The protective relays are connected in the secondary circuits of current and/or voltage

transformers. The relays sense the abnormal conditions and close the trip circuit of the associated

circuit‐breaker. The circuit‐breaker then opens its contacts. Arc is drawn between the contacts as

they separate. The arc is extinguished by suitable medium and technique. After final arc

extinction, a high voltage wave appears across the circuit‐breaker contacts tending to re‐establish

the arc. The transient voltage wave is called "Transient Recovery Voltage" (TRV).

1.5 FAULT CLEARING TIME:

It is the time elapsed between the instant of fault occurrence and the instant of final arc

extinction, in the circuit‐breaker. It is usually expressed in cycles. One cycle of 50 Hz system is

equal to 1/50 second. The fault clearing time is the sum of the relay time and the circuit‐breaker

time.

1.6 PROTECTIVE SCHEME

Protective scheme is a selected set of protective systems which protect one or two components of

the power system against abnormal conditions, e.g. generator protection scheme, transformer

protection scheme, etc.

The power system is covered by several protectve zones (Fig. 2). Each protectve zone covers one

or two components of the system. The neighboring protective zones overlap so that no part of the

system is left unprotected.

Figure 2





1.7 NEUTRAL GROUNDING (EARTHING) AND EQUIPMENT GROUNDING

The term "Grounding" or “Earthing" refers to the connection of a conductor to earth. The neutral

point of a generator or a transformer is deliberately connected to earth.

The Neutral Earthing has Several Advantages such as:

• It stabilizes the neutral point. • It is useful in discharging over‐voltages due to lightning to earth. • Simplifies design of earth fault protection. • Grounded systems require relatively lower insulation levels as compared with

ungrounded systems.

On the other hand, the "Equipment Grounding" refers to grounding of non‐current

carrying metal parts to earth. It is used for safety of personnel.

1.8 OVER‐VOLTAGE AND INSULATION CO‐ORDINATION

Over‐voltage surges in power systems are caused by various causes such as:

• Lightning, • Switching, • Resonance, etc.

The power system elements should withstand the over‐voltages without insulation failure. The

insulation level of a power system element refers to its values of power frequency and impulse

voltage withstand. The protective measures against over‐voltages due to lightning include:

• Use of overhead ground wires, • Low tower footing resistance, and • Ise of lightning arresters (surge arresters).

The surge arresters offer low resistance to over‐voltages and divert over‐voltages to earth.

1.9 ELECTROMECHANICAL RELAYS AND STATIC RELAYS

The electromechanical relays are based on the comparison between operating torque/ force and

restraining torque/force. The VA burden of such relays is high. The characteristics have limitations.

Each relay unit can perform only one protective function. Such relays are used for simple and less

costly protection purposes. For important and costly equipment and installation, static relays are

preferred. In static relays, sensing, comparison, and measurements are made by static (electronic)

circuits having no‐moving parts. Static relays have versatile characteristics, offer low burden, and

incorporate several protective/control/monitoring functions in one compact unit.





Recently (1980's), programmable statc relays incorporatng microprocessors have been

introduced. Microprocessor based relays have several superior features such as:

• Indication of operating values on demand and thereby no need for separate indicating instruments on panel.

• A single relay can perform 10 or more diff erent protectve functons, thereby reducing

number of separate relays and increasing reliability.

• Internal monitoring of own relay circuit. • Memory function e.g. a relay which has tripped on a fault can remember and flash on the display the magnitude of current and instant of time at the time of tripping.

• Better properties and extended range of application for generation, transmission, distribution, and industrial applications.

The range of static relays is rapidly spreading.

1.10 SWITCHES

Generally, switches are used only where necessary for isolation purposes. Switches for Heating,

Ventilating, and Air‐Conditioning (HVAC) systems must be installed in conformance with NFPA‐70.

• Switch duty is defined by NEMA KS‐1, Enclosed Switches. General‐duty equipment is used

for nonessential applications and where equipment is subject to infrequent operation.

General duty equipment is intended for use on circuits of 240 V or less; therefore, heavy

duty equipment is required for higher voltages. Heavy‐duty equipment is used for

industrial application where reliability and continuity of service are prime factors and

where equipment is subject to frequent operation. It is intended for use on circuits of 600

V or less and where available fault current of more than 10,000 amperes are likely to be

encountered.

• Fusible switches combine isolation with protection of a particular component of the

circuit.

• Safety (disconnect) switches can be fused or non‐fused units operable up to 600 volts and

1,200 amperes of maximum contnuous current and are normally used for motor isolaton

or protection.

• Other switches such as heavy‐duty switches operable up to 600 volts and 1,200 amperes

of continuous current and load‐break pressure switches operable up to 600 volts and 5,000

amperes of continuous current are only used for application where circuit breakers are not

appropriate.





• Automatic transfer (and bypass/isolation) switches are designed to transfer power

sources under load in order to feed a system, typically an emergency system, on critical

loads. These devices are tested to meet basic short‐circuit testing requirements. Transfer

switches should always be evaluated on the basis of the maximum available short‐circuit

currents.

The automatic transfer switch must withstand:

• The magnetic stresses imposed by the instantaneous peak current available

at the point of application.

• The thermal stresses imposed by the available RMS short‐circuit current.

The short‐circuit current withstand rating of the transfer switch must be equal to or greater

than the available short‐circuit current at the point of application. When properly

coordinated with current‐limiting devices, automatic transfer switches can be used on

circuits having available short‐circuit currents greater than their unprotected withstand

short‐circuit current rating. Modern current‐limiting fuses, when properly sized, limit the

short‐circuit current to within the withstand rating of a transfer switch.

Transfer switches must withstand minimum short‐circuit currents at specified power

factors, as listed in U.L. Standard 1008, untl the over current protectve devices open.

Transfer switch manufacturers generally publish the withstand rating data for their

products. When the available short‐circuit current exceeds the withstand rating of the

transfer switch, current limitation is required. Properly sized modern current‐limiting fuses

ahead of the transfer switch limit the available short‐circuit current to within the withstand

rating of a transfer switch, thereby protecting the transfer switch. The transfer switch

manufacturer will mark the equipment with the fuse class and rating required to achieve

these higher short‐circuit ratings.

1.10.1 TYPES OF SWITCHES AND THEIR APPLICATION

There are a variety of switches used in the transmission and distribution of electric power.

The switches are designed for specific purposes and operational conditions. In general, the

switches are distinguished by the current handling capability; i.e., continuous, load break

or non‐load break, and fault current.

• A Disconnect Switch is one used for; closing, opening, or changing the connections in a

circuit or system, or for isolating purposes. It has no interrupting rating and is intended





to be operated only after the circuit has been de‐energized by some other means. A

series‐connected circuit breaker or circuit recloser should be open on all three phases

before a disconnect switch is opened or closed.

• An Interrupter Switch can use either air or oil as its interrupting medium. Load‐

interrupter switches are used to interrupt transformer‐magnetizing current, line‐

charging current, capacitor current to isolated banks, and load current within the limits

of their rating. They are normally used where the cost of a circuit breaker with fault‐

interrupting ability cannot be justified and where the use of air‐break switches is

hazardous because of the danger of inconvenient and uncontrolled arcs. Load‐

interrupter switches differ from circuit breakers and fault interrupter switches in that

they cannot interrupt overload or short‐circuit currents.

• A Grounding Switch is used to connect a circuit or a piece of apparatus to ground.

Grounding switches are normally subdivided into two separate groups: manually

operated and high‐speed. Where a manually operated grounding switch is installed, it

is normally connected to an air‐break or disconnect switch. It is used to effectively

ground a line after the air‐break or disconnect switch has isolated it. The manually

operated grounding switch is generally interlocked with its associated switch so that

the grounding switch cannot be closed until the disconnect switch is open. A high‐

speed grounding

switch

has

a stored

‐energy

mechanism

capable

of closing

the

switch

automatically, within a specified rated closing time. The switch is opened either

manually or by a power‐operated mechanism. High‐speed grounding switches are used

to provide protection to a differential relayed area in coordination with a remote circuit

breaker. Normally, the arrangements are such that the differential relay detects the

fault and initiates the closing of the high‐speed grounding switch and results in tripping

the remote circuit breaker to clear the fault.

1. Air Switches.

Air‐break switches are normally mounted on top of their supporting structure. They are

either manually operated by means of an operating handle or electrically operated by

means of a motor‐operated mechanism.

They are used to perform various switching assignments such as isolating transformers,

bypassing circuit breakers, and for line sectionalizing (where small amounts of magnetizing

or charging currents are to be interrupted).





2. Oil Switches

An oil switch has its main contacts submerged in oil. Oil acts as an insulator to help quench

the arc between the contacts. In addition, since the tank is airtight, the vaporized oil

caused by the arc develops pressure which assists in breaking the arc. If the voltage is not

very high, a three‐pole switch can be placed in a single tank. At higher voltages, three

separate tanks are used to make it impossible for a phase‐to‐phase fault to occur. Oil

switches will normally open only load current. A separate trip coil is necessary to interrupt

overload or fault currents. Oil switches are generally used in capacitor switching,

distribution sectionalizing, and transformer primary switching.

3. Vacuum Switches

Vacuum switches are interrupters which use vacuum chambers for contact separation.

They are generally used to interrupt load, capacitor, or transformer magnetizing currents.

Unlike oil switches, vacuum switches require virtually no maintenance. They can be used

for submersible or pad mount operation.

1.10.2 SWITCH ACCESSORIES

Switch accessories are devices that perform a secondary or minor duty as an adjunct or

refinement

to

the

primary

operation

of

a

switch

or

to

assist

in

the

operation

of

a

switch.

Some accessories that are commonly associated with switches are as follows:

• Operating Mechanisms. The operating mechanism of a switch is a power‐operated or

manually‐operated mechanism complete with an assembly of levers, mechanical linkages,

and interphase connecting rods by which the contacts of all poles are actuated

simultaneously.

• Hook Sticks. A hook stick is a hook provided with an insulating handle (usually specially

treated wood) for opening and closing hook‐stick‐operated switches. When not being used,

hook sticks should be stored in a dry location.

• Interlocks. An interlock is a device applied to two or more movable parts, preventing or

allowing a movement of one part only when one or more other parts are locked in a

predetermined position. An interlock system is a series of these devices applied to

equipment to allow operation of the equipment only in a prearranged sequence. Switches

used only for isolating purposes must be interlocked to prevent opening of the isolating





switch under load, or the switch must be provided with a highly visible sign warning against

opening the switch under load. Interlocks are classified into three main divisions:

mechanical interlocks, electrical interlocks, and key interlocks.

1.10.3 AUXILIARY SWITCHES

Auxiliary switches are low‐voltage switches that are attached to the operating mechanism

of gang‐operated switches. The open or closed position of auxiliary switches is governed by

the position of the main contacts. Auxiliary switches are used for electrical interlocking,

remote position indication, or control of electrically operated switches.

1.11 FUSES

The only high‐voltage fuses fitted in an HV switchboard are those which form a back up to a

contactor (air‐break or vacuum) and those on the HV side of a voltage transformer. All are of the

high rupturing capacity (HRC) type.

The contactor fuses are of the open type but are embodied in the contactor unit itself. This forms

adequate protection since it is necessary to isolate the unit, and in the case of the air‐break type

to withdraw it, in order to gain access to the fuses.

Where fuses, whether HV or LV, are used in series with a contactor, their purpose is to protect the

contactor itself against having to open on a fault current which is in excess of its rating. Fuses used

in this manner are termed 'back‐up fuses' and are selected with reference to the contactor's own

inverse‐time characteristic.

Voltage transformer HV fuses form part of the VT itself. The VT compartment can only be opened

after isolation, after which the fuses are accessible. Although the VT fuses have a very small

current rating, they still have to be able to break a full scale short‐circuit current if a fault should develop on the VT itself.

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Module (01) Introduction of Switchgears