AIM: TO STUDY THE DIFFERENT SWITCHGEARS AND POWER SYSTEM PROTECTION IN POWER SYSTEM

DEFINITION OF SWITCHGEAR:

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnectors, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults downstream.DIFFERENT SWITCHGEARS EXPLAINED BELOW ARE:SWITCH :

A SWITCH is an electrical component that can break an electrical circuit interrupting the current or diverting it from one conductor to another.The most

familiar form of switch is a manually operated electromechanical device with one or more sets of electrical contacts.

Each set of contacts can be in one of two states: either 'closed' meaning the contacts are touching and electricity can flow between them, or 'open', meaning the contacts are separated and non-conducting.

AIR BREAK SWITCH:

POWER SYSTEM PROTECTION:

Power system protection is a branch of electrical power engineering that deals with the protection of electrical power systems from faults through the isolation of faulted parts from the rest of the healthy electrical network.

The objective of a protection scheme is to keep the power system stable by isolating only the components that are under fault, whilst leaving as much of the network as possible still in operation.

Protection systems usually comprise five components:

CURRENT AND VOLTAGE TRANSFORMERS (CTs & PTs) : to step down the high voltages and currents of the electrical power system to convenient levels for the relays to deal with;

RELAYS :to sense the fault and initiate a trip, or disconnection, order;

CIRCUIT BREAKER:to open/close the system based on relay and autorecloser commands;

BATTERIES :to provide power in case of power disconnection in the system.

COMMUNICATION CHANNELS : to allow analysis of current and voltage at remote terminals of a line and to allow remote tripping of equipment.

Failures may occur in each part, such as insulation failure, fallen or broken transmission lines, incorrect operation of circuit breakers, short circuits and open circuits. Protection devices are installed with the aims of protection of assets, and ensure continued supply of energy. The three classes of protective devices are:

1. Protective relays control the tripping of the circuit breakers surrounding the faulted part of the network.

2. Automatic operation, such as auto-reclosing or system restart.

3. Monitoring equipment which collects data on the system for post event analysis.

TYPES OF PROTECTION:

GENERATOR SET PROTECTION – In a power plant, the protective relays are intended to prevent damage to alternators or of the transformers in case of

abnormal conditions of operation, due to internal failures, as well as insulating failures.

HV TRANSMISSION PROTECTION– Protection on the transmission and distribution serves two functions: - Protection of plant -Protection of the public

At a basic level protection looks to disconnect equipment which experience an overload or a connection to earth.

OVER VOLTAGE PROTECTION – Overload protection requires a current transformer which simply measures the current in a circuit. If this current exceeds a pre-determined level, a circuit breaker or fuse should operate.

EARTH FAULT PROTECTION – Earth fault protection again requires current transformers and senses an imbalance in a three-phase circuit. Normally a three-phase circuit is in balance, so if a single (or multiple) phases are connected to earth ,an imbalance in current is detected. If this imbalance exceeds a pre-determined value a circuit breaker should operate.

DISTANCE PROTECTION– Distance protection detects both voltage and current. A fault on a circuit will generally create a sag in the voltage level. If this voltage falls below a pre-determined level and the current is above a certain level the circuit breaker should operate.

BACK-UP PROTECTION– At all times the objective of protection is to remove only the affected portion of plant . Sometimes this does not occur for various reasons which can include:

A) Mechanical failure of a circuit breaker to operate.

B) Incorrect protection setting and relay failures.

A failure of primary protection will usually result in the operation of back-up protection for recovery of fault.

DESIRABLE ATTRIBUTES OF POWER SYSTEM PROTECTION:

1.Dependability:

A relay is said to be dependable if it trips only when it is expected to trip.Dependability is the degree of certainty that the relay will operate correctly.Dependability can be improved by increasing the sensitivity of the relaying scheme.

2.Sensitivity:

It is the ability of the relay to pick up even on smallest possible faults.It is the ability of the relay to operate with low value of actuating quantity.

3.Security:

Security is a property used to characterize false trippings.A relay is said to be secure if it does not trip when it is not expected to trip.Security can be improved by improving selectivity of the relaying scheme.

4.Selectivity:

It is the ability to correctly locate and classify the fault and thus faulty section is isolated from the healthy one.A relay should be able to discriminate whether the fault is in its jurisdiction or not.and this jurisdiction of a relay is called zone of protection.Protection zones are classified into primary and back up zonesIt includes time grading principle according to the different zones of protection.

5.Reliability:

It is defined as the probability of failure or ‘ not to fail in its action’.Reliability can be improved by providing. Back-up protection in the power system.

6.Speed

It is property which defines the life of equipments in the power system.The faster the equipments operate,the lesser the damage will occur to the equipments.

7.Discrimination:

It is the ability of power system to distinguish between the faulty part to the healthy part and also between the normal and abnormal conditions within the different zones of protection.

8.Stability:

This is the property which defines the operating stable conditions of the power system.

ZONES OF PROTECTION:

A relay’s zone of protection is a region which defines its jurisdiction.

It is essential that primary zones of protection should always overlap so that no portion of the system ever remain unprotected.

Overlapping of different zones are done so as to avoid ‘blind spots’ in any section.and different zones of protection are decided by the location of CTs.

FACTORS AFFECTING THE SELECTION OF SWITCHGEAR

For circuit-breakers , factors to be considered are :

condition of the secondary wiring, protection and control equipment;

interlocking and earthing arrangements in relation to current safety standards;

short-circuit ratings;

rating of the existing (fixed) equipment is adequate to ensure the replacement equipment can be used to its full rating.

CONTACTORS:

A contactor is a large relay,usually use to switch current to an electric motor or other high power load.

When a relay is used to switch large amount of electrical power through its contacts,it is designated as CONTACTORS.Contactors have generally multiple contacts and those contacts are normally open so that power to the load is shut –off when the coil is de-energised.

The lowest contact is an auxiliary contact which has a current rating much lower than the large motor power contacts.the auxiliary contact is often used in a relay logic circuit,or for some other part of the motor control scheme.one contactor may have several auxiliary contacts,either normally open or normally closed.

CIRCUIT BREAKER:

A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow. .

OPERATION:

A circuit breaker basically consists of two contacts:one fixed contact and

other is moving contact called electrodes.under normal operating

condition,these contacts are remain closed and will not open

automatically until and unless the system becomes faulty while contacts

can be operated through remote control or manually.

When fault occurs on any part of the system,the trip coils of the CB gets

energized and the moving contacts are pulled apart thus opens the ckt.

When the contacts of a CB are separated under fault condition,an arc is

struck between them.the current is thus able to continue until the

discharge ceases.This results into enormous production of heat energy

which can damage the contacts of the CB.

STANDARD CURRENT RATINGS OF CIRCUIT BREAKER :

International Standard IEC 60898-1 and European Standard EN 60898-1 define the rated current In of a circuit breaker for low voltage distribution applications as the current that the breaker is designed to carry continuously (at an ambient air temperature of 30 °C).

The commonly-available preferred values for the rated current are 6 A, 10 A, 13 A, 16 A, 20 A, 25 A, 32 A, 40 A, 50 A, 63 A, 80 A and 100 A.

The circuit breaker is labeled with the rated current in amperes, but without the unit symbol "A". Instead, the ampere figure is preceded by a letter "B", "C" or "D" that indicates the instantaneous tripping current, that is the minimum value of current that causes the circuit-breaker to trip without intentional time delay (i.e., in less than 100 ms), expressed in terms of In:

Type Instantaneous tripping currentB above 3 In up to and including 5 InC above 5 In up to and including 10 InD above 10 In up to and including 20 In

K above 8 In up to and including 12 In For the protection of loads that cause frequent short duration (approximately 400 ms to 2 s) current peaks in normal operation.above 2 In up to and including 3 In for periods in the order of tens of seconds.

Z For the protection of loads such as semiconductor devices or measuring circuits using current transformers;

BASIC TRIPPING CIRCUIT

FAULT CLEARING PROCESS

Relays are connected to CT secondary or PT secondary.During normal condition of operation ,voltage induced in CT secondary is small. Therefore current in relay operating coil is of insufficient magnitude to close relay contact.This scheme trips coil of CB denergise.therefore breaker contacts remain closed and it carry normal load current.

When fault occurs feeder circuit,CT transmits fault current to current coil of protective relay.similarly PT transmits voltage under fault conditions to potential coil of relay.

The relay operates and its contacts get closed which energises aux.relay coil.when aux.relay coil operates ,trip coil is energized and CB trips.The switch ‘a’ is an aux. mechanical switch.protective relay resets because of low flow of current through cc of relay.The protective and aux. relay will reset only when fault is cleared.