Power System Protection

Protection of power system

The electrical power system is the electrical network of conductors. We know that a transmission line means that high voltage system. The transmission line is divided into two, such as primary transmission and secondary transmission systems. Primary transmission carries large voltage (132KV, 230KV, 400KV, and many more). Whereas secondary transmission carries a lower voltage (33KV, 66KV) than the primary transmission. The severe faults occur in the high voltage transmission due to lightning stroke, and other causes. For this reason, to avoid the unexpected situation maintain continuity of power supply different kinds of high voltage electrical power system protection is used.

High voltage electrical power system protection

The protection of power system is a branch of electrical engineering that deals with the protection of electrical systems from faults by disconnecting faulty parts from the rest of the electrical network. The devices used to protect the electrical devices in the power system from faults are called protection devices. Power system protection is the disconnection of a faulty part of the electrical system directly from the live system so that the rest can operate satisfactorily without any serious damage due to the faulty part.

Power system design takes a lot of effort and money because lots of expensive components are used at the power station or substation. Of course, every investor wants to get the highest return on investment. The entire system may be at risk of serious damage and deterioration, resulting in endangering lives, property, and other equipment. Safety devices should be used to reduce the risk of damage due to these failures.

Protection zones of power system

Each specific protection project is known as a protection zone that protects a specific area. Basically, the safety zone is formed around each power tool. When an error occurs in a zone, only the circuit breaker trips that area. Therefore, only one faulty component disconnects without affecting other systems. Security zones of five sections are possible in a system, as mentioned below.

  • Protection of Transformers
  • Protection of Alternator
  • Protection of Bus bar
  • Transmission line Protection

Protection of high voltage transformers

The modern electric power system consists of very expensive and valuable components like a power transformer, alternator, transmission line, instrument transformer, and other switchgear components. But it is very important to protect such kinds of essential components. For this reason, some protective mechanisms are used. The power transformer is a very important component of the power system.

protection of power system The transformer consists of two windings, one is the primary winding and another is the secondary winding and it is a static device. Basically, the transformer is used to step up or step down the voltage level. The efficiency of the transformer is much higher than some other electrical devices. But sometimes a fault occurs in the transformer. If the fault occurs, then the efficiency becomes low, and the power supply is interrupted. There are some common faults that occur in the transformers, such as

  • Open circuit fault
  • Overheating
  • Short circuit fault

Transformer open circuit fault

The open circuit fault of a transformer is not a dangerous fault. Basically, the open-circuit fault occurs due to the undesired overheating of the transformer. Differential relay protection is not used due to the transformer open circuit fault. If an open circuit fault occurs, then the power transformer can be disconnected manually from the short circuit part to the healthy part. 

What is the reason for overheating?

Overheating of a transformer is a major issue because overheating can occur due to various reasons, such as overloading, short circuit, failure of insulation, and failure of the cooling system. The relay protection system is not provided for those reasons. Basically, banks of fans and thermal accessories are used to control those contingencies.

Short circuit fault of a Power Transformer

The short circuit fault of a power transformer is one kind of major fault of a transformer. A short circuit fault of a transformer occurs due to the deterioration of transformer winding insulation. This deterioration of transformer winding insulation occurs due to overheating, and mechanical injury. When an internal fault occurs in the transformer then, it is important to disconnect the transformer from the supply system because the internal fault is able to create an arc in the transformer. This arc can cause an oil fire. A relay protection system is widely used for the transformer internal fault. It is also called protection of power system.

What is the protection system of a Transformer?

The protection of the transformer against overloads is performed by dedicated protection, usually called a heat overload relay. This type of protection mimics the winding temperature of the transformer. The simulation is based on the measurement of current and the thermal time constant of the transformer. Dedicated types of transformers are protected against overheating due to potential overloads by dedicated relay monitoring thermal sensors embedded in the winding of the transformer. Internal faults are mainly inter-turn and phase-to-earth faults of a transformer. The fault of the short circuit occurs inside the dry-type transformers. There are different kinds of devices used for different kinds of faults, such as.


Buchholz relay: The Buchholz relay always provides protection against all kinds of faults like winding insulation failure, core heating, fall of oil level due to leaky joint, and others.

Earth fault relay: The earth fault relay is used to protect against transformer earth faults. But it is noted that a differential relay or differential protection system is used to protect against phase fault and earth fault.

Over-current relay: Over-current relay provides protection against transformer overloading and phase to phase fault. You need to use an over-current relay to provide protection against unexpected over-current.

What is the Buchholz relay?

The Buchholz relay is one kind of gas-actuated relay that is installed in the oil-immersed power transformer for all kinds of fault protection. When a fault occurs in the transformer, then the Buchholz relay gives an alarm and disconnects the transformer connection from the main supply for the severe internal fault. Buchholz relay is used in the transformer whereas the transformer rates more than 750 KVA. It contains more than 75% hydrogen gas.

What is the protection of an Alternator?

The alternator is one of the main components of a power system. An alternator is a device that converts mechanical energy into electrical energy, and it is widely used in the power system to generate electricity. Most of the time, faults occur in the alternator, but electrical faults are always dangerous. For this reason, different kinds of protection systems are used against faults in the alternator. There are some faults that occur in the alternator, such as

Failure of the prime mover: When the input power of an alternator fails, then the alternator starts running as a synchronous motor (like constant speed) and creates some current from the supply system. This motoring condition is called alternator invert running. If this problem occurs, you need to isolate the machine from the control room because the failure of the prime mover is relatively riskless.

Failure of field: Failure of the field of the alternator is a very rare case. If the field fails, there is no immediate damage to the alternator. If this problem occurs, you need to disconnect the alternator from the system bus bar.

Overcurrent of alternator: Overcurrent of an alternator is a general issue this fault occurs due to the partial breakdown of alternator winding insulation and over-current can occur by overloading on the supply system of the alternator. There is no need to use an extra protection system for the overcurrent of an alternator because modern alternators are designed with very high internal impedance. If the overcurrent occurs then, you can disconnect the alternator manually from the supply system.

Overvoltage of an alternator: The overvoltage of an alternator occurs if the speed of the prime mover becomes very high due to the sudden loss of a load of an alternator. You can use an overvoltage relay to provide protection against the overvoltage of an alternator.

Over speed of an alternator: Sometimes over-speed occurs due to the loss of load, but modern alternators are made using mechanical centrifugal devices which are mounted on the driving shaft of the alternator so that when over speed occurs then the main valve of the prime mover can easily trip.

Stator winding fault of an alternator: The stator winding fault is a dangerous fault of an alternator. Basically, a stator winding fault occurs due to the insulation failure of the stator winding of an alternator. A relay protection system is absolutely necessary to provide protection against the stator winding fault of an alternator. An alternator’s stator winding fault can be classified into three types:

  • A fault within phase and ground
  • Phase to phase fault
  • Inter-turn fault

An automatic protection system or differential protection system is used against stator winding fault. The Merz-Price system or differential method protection system is employed because of reliability and sensitivity.

Protection of Bus bar

The bus bar is a crucial component of the power system. A bus bar is a copper road or thin aluminum tube where incoming and outgoing lines are connected. It cannot step up or step down the voltage level. It means that the bus bar cannot change the voltage level. The voltage level is changed by the electrical transformer.

Faults in the bus bars disrupt the entire power supply and disconnect all healthy feeders. Most of the faults are single phase in nature and these faults occur in temporary bus zones for various reasons, such as failure of support insulators, failure of circuit breakers, lightning stroke, etc. There are different kinds of faults that occur in the bus bar and different kinds of protection systems are used to protect the electrical bus bar, such as

Backup protection of bus bar

An easy way to protect the bus bar from this fault. Faults occurred in the bus bar due to the power supply system. Thus, backup protection is provided in the electrical supply system. The relay operating time will be 0.4 seconds. There are several disadvantages to bus-bar safety systems. These measures are mainly used to protect the transmission line.

However, as the security system is very economical, this security system is also used for bus-bar protection. Backup protection of the bus bar protective scheme is not used for small kinds of switchgear protection systems. Backup protection systems have many disadvantages, such as delayed operation, disconnection of more circuits for two or more transmission lines, and so on.

Differential over current Protection

The overcurrent protection scheme is based on the conventional current principle. This states that the current entry time in the current bus bar is equal to the current leaving repeatedly. The sum of incoming and outgoing junctions is equal to zero. If the sum of the current is not equal to zero, an error occurs in the system.

The differential protection scheme is used both for the protection of phase-to-phase faults and for ground faults. Current transformers are placed at both the incoming and outgoing ends of the bus bar. The secondary terminals of the current transformer are connected in parallel to each other. The combination of current transformer flows through the relay operating coil.

transformer-differential -protection

The current flowing through the relay coil indicates the short circuit current in the secondary of the current transformer. Thus, the relay sends signals to the circuit breakers to open the contacts. The disadvantage of this type of protection system scheme is that the faulty operation of the relay occurs when there is an external fault in the current transformer equipped with iron.

Transmission line Protection

The main purpose of the transmission protection relay is to identify the fault of the person. These relays are also used to identify the type of fault in the transmission line. Transmission line protection is very much relay-based. Several relay systems are used to protect the transmission lines. The use of some protective relay types in the transmission line is given below.

Protective relay: protective relay performs the related functions to detect faults and initiate appropriate control signals such as tripping signals.

Controlling relays: A relay controlling device manages a load of a load changer on a transformer.

Reconstitution and Integration Relay: This is a programmable relay whose function initiates a sequence of actions directed towards the automatic recovery of the circuit breaker.

Auxiliary Relay: This relay assists other relays by applying complementary functions.

Protective ties are usually very important for transmission line protection. We hope you have some basic ideas about electrical transmission line protection. In future articles, we plan to discuss other safety transmission line safety projects in the future.

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