Need for protectionPower system must be kept in operation continuously without major breakdowns

This can be achieved in two ways:1.Implement a system adopting components, which should not fail : Not feasible2.Foresee any possible effects or failures that may cause long-term shutdown of a system.Restrict disturbance/failures/faults to limited area with continuity of power supply in rest of the areaThe special equipment adopted to detect such possible faults is referred to as ‘protective equipment or protective relay’ and the system that uses suchequipment is termed as ‘protection system’.

Need for protection

• protective apparatus is very necessary in the electrical systems, which are expected to generate, transmit and distribute power with least interruptions and restoration time.

• It can be well recognized that use of Protective equipment are very vital to minimize the effects of faults, which otherwise can kill the whole system.

Basic requirements of protectionA protection apparatus has three main functions/duties:•1. Safeguard the entire system to maintain continuity of supply•2. Minimize damage and repair costs where it senses fault•3. Ensure safety of personnel.

In order to carry out the above duties, protection must have the following qualities:• Selectivity: To detect and isolate the faulty item only.• Stability: To leave all healthy circuits intact to ensure continuity or supply.• Sensitivity: To detect even the smallest fault, current or system abnormalities and operate correctly at its setting before the fault causes irreparable damage.• Speed: To operate speedily when it is called upon to do so, thereby minimizing damage to the surroundings and ensuring safety to personnel.To meet all of the above requirements, protection must be reliable which means it must be:• Dependable: It must trip when called upon to do so.• Secure: It must not trip when it is not supposed to.

Basic components of protection1. Voltage transformers and current transformers: To monitor and give accuratefeedback about the healthiness of a system.2. Relays: To convert the signals from the monitoring devices, and giveinstructions to open a circuit under faulty conditions or to give alarms whenthe equipment being protected, is approaching towards possible destruction.3. Fuses: Self-destructing to save the downstream equipment being protected.4. Circuit breakers: These are used to make circuits carrying enormouscurrents, and also to break the circuit carrying the fault currents for a fewcycles based on feedback from the relays.5. DC batteries: These give uninterrupted power source to the relays andbreakers that is independent of the main power source being protected.

Power System Protection – Speed is Vital!!• Increased damage at fault location. Fault energy = current square × Rf × t, where t is time in seconds.The protective system should act fast to isolate faulty sections to prevent:• Danger to the operating personnel (flashes due to high fault energy sustainingfor a long time).• Danger of igniting combustible gas in hazardous areas, such as methane in coalmines which could cause horrendous disaster.• Increased probability of earth faults spreading to healthy phases.• Higher mechanical and thermal stressing of all items of plant carrying the fault current, particularly transformers whose windings suffer progressive and cumulative deterioration because of the enormous electromechanical forces caused by multi-phase faults proportional to the square of the fault current.

Sustained voltage dips resulting in motor (and generator) instability leading toextensive shutdown at the plant concerned and possibly other nearby plantsconnected to the system.

Various ground and phase faults

The protection system

Current transformer (C T)

Voltage (Potential) transformer ( V T /P T)

Trip circuit of a circuit breaker

Zones of protectionZones divided by equipment and available circuit breakersSix categories of zones are possible (1) generators and generator–transformerunits, (2) transformers, (3) buses, (4) lines (transmission, subtransmission,and distribution), (5) utilization equipment (motors, static loads, or other), and(6) capacitor or reactor banks (when separately protected).

Zones of protection

• Each of these six categories has protective relays, specifically designed for primary protection, that are based on the characteristics of the equipment bein g protected.

• The protection of each zone normally includes relays that can provide backup for the relays protecting the adjacent equipment.

• The protection in each zone should overlap that in the adjacent zone; otherwise, a primary protection void would occur between the protection zones . This overlap is accomplished by the location of the CTs —the key sources of power system information for the relays.

Primary and backup protection• Primary protection: First line of defense• Backup protection :Second line of defense• Backup protection should not have any

element in common with primary protection• Backup protection should preferably be

located at different place than primary protection

• Operating time of backup protection = operating time of primary protection + operating time of primary circuit breaker

Primary and backup protection

Primary and backup protectionRelay B with circuit breaker CBB provides primary protection to BC Relay A with circuit breaker CBA provides primary protection to AB and backup protection to BC

For fault on BC both primary relay RB and backup relay RA

start operating simultaneously.

In case the primary protection (provided by RB + CBB) operates successfully, the line B-C gets de-energized but the loads on buses A and B remain unaffected. Therefore, the back-up protection provided by RA + CBA resets without issuing a trip command.

Primary and backup protectionHowever, in case the primary protection fails to operate, the back-up which is already monitoring the fault, waits for the time in which the primary would have cleared the fault and then issues the trip command to its allied circuit breakers.