Power System Protection

Electrical Power systems are essential for our daily lives. They provide electricity to homes, businesses, industries, and infrastructure. The demand for electric supply in our everyday life is very high, and we continuously need electric power at our premises, offices, and industries. However, power systems are also vulnerable to faults and disturbances, leading to damage and outages. Therefore, power system protection is essential to safeguard the system components, prevent or mitigate failures, and maintain the system’s stability and quality. Power system protection refers to techniques, equipment, means, and methods to disconnect power equipment such as generators, transmission lines, distributors, and other equipment under abnormal and faulty conditions. Power system protection is the set of devices, strategies, and protocols that detect, isolate, and clear faults and abnormal conditions in the power system. 

Desired Features of Power System

The traits of a successful, reliable system include selectivity, accuracy, and reliability, and power engineers always try to design protection systems that are fast, accurate, selective, and precise to avoid false trips, unnecessary interruptions, and cascading failures. The protection system should be designed and coordinated to cover the entire system, from generators to loads, and adapt to different operating conditions, such as fault location, fault type, system configuration, and load demand.

Basic Terms and Definitions Used in Power System Protection

Protective Relaying

Protective relaying refers to the scientific principles and operational practices that ensure continuous system service. It minimizes harm to property and individuals due to abnormal system behavior. The primary objective of relaying system is to safeguard the overall power system from the impact of a faulty component.


The reliability of a protective system is defined as its ability to function adequately when needed. This reliability can be further decomposed into two characteristics:

  1. Dependability: Refers to the feature that the system should and must operate in its assigned zone of protection in the presence of a fault
  2. Security: Refers to the feature that the system should abstain from unnecessary operations beyond its protective zone or in the non-existence of a fault.


It pertains to the system’s ability to identify abnormal conditions that surpass a predefined threshold value.


Protective System selectivity refers to the overall design strategy by which only the protective devices closest to a fault operate to isolate the faulty components.

Primary relaying system

Relays in the primary relaying system operate for prescribed abnormalities within that zone.

Backup relaying system

It involves relays positioned outside a primary protection zone but within an adjacent zone. These operate in response to prescribed abnormalities within the primary protection zone.

Types of Power System Relay Protection Methods

There are several types of power system protection. The most common types are overcurrent protection, distance protection, differential protection, and overvoltage protection. Each type has its principles and specific implementation techniques, but all aim to detect and isolate faults to minimize the impact on the rest of the system.

Overcurrent Protection

Overcurrent protection is the simplest and most widely used method for measuring the current flowing through a circuit. When the current exceeds a predefined threshold, the overcurrent relay trips the circuit breaker, disconnecting the faulty part from the rest of the system.

Overvoltage Protection

Overvoltage protection involves the relay that recognizes overvoltage in a circuit and triggers the corrective measures.

Distance protection

Distance relaying is a more advanced protection method based on measuring the impedance (resistance and reactance) between two points in the power system. The distance relay calculates the distance to the fault location based on the impedance, and if the distance is within a predefined range, the relay trips the circuit breaker. The operation of distance relays is governed by the ratio of the applied voltage to the current in the protected circuit. Distance protection is appropriate for faults involving transmission lines, where the fault location can be far from the source.

Differential protection

Differential protection is suitable for internal faults in components, where the current flowing in and out should be equal in normal conditions. Different relaying protection is a method that compares the current entering and leaving a component, such as a transformer, motor, or generator. If the phasor difference between the currents or two similar quantities exceeds a predefined threshold, the differential relay trips the circuit breaker, indicating a fault in the component.

Apart from the above methods of protection, there are other protection methods, such as frequency protection, voltage protection, and directional protection, that are used for specific applications or environments.

Protection of Power System Equipment

Electric power system comprises different components, including alternators, transformers, bus bars, transmission lines, and other equipment. Each of these is exposed to different types of faults, and it is essential to protect each from any potential spot or threat.

Protection of Alternators

Alternators in power plants are costly, and it is essential to implement protective measures to address a wide range of potential faults that may arise in the generating plant.

Several significant faults can occur in an alternator, including:

  1. Field failure
  2. Overvoltage
  3. Overcurrent
  4. Overspeed
  5. Prime-mover failure
  6. Unbalanced loading
  7. Stator winding faults

Each fault is mitigated using a specific technique and set of relays.

Protection of Transformers

Alternators and motors are rotating machines, while a transformer is a static device, and because of its static nature, the chance of faulty occurrence on the transformer is quite rare.

Transformers power system protection (PSP) involves four methods:

  1. Buchholz relays: These are employed to protect against incipient faults.
  2. Earth-fault relays: These are specifically designed to offer protection against earth faults
  3. Differential relays provide comprehensive protection against earth faults and phase faults. Differential relays can identify any imbalances that indicate a fault and take appropriate protective measures by comparing the current flowing into and out of a specific zone.
  4. Overcurrent relays: As the name implies, these are primarily used to protect against phase-to-phase faults and overloading overcurrent situations.

Busbar Protection

The busbars in power stations play a crucial role in connecting the incoming and outgoing circuits. In the event of a fault on a busbar, significant damage and disruption of the power supply can occur. A mechanism for quick-acting automatic protection to isolate the faulty busbar. Two widely employed schemes in this regard are:

  1. Differential protection: This scheme compares the currents entering and exiting the busbar. If an imbalance indicates a fault, the differential protection system activates and initiates the isolation of the faulty busbar.
  2. Fault bus protection: In this scheme, specific fault detectors are strategically placed along the busbar. These detectors monitor the condition of the busbar and promptly detect any faults. Once a fault is detected, the fault bus protection system intervenes to isolate the faulty busbar and prevent further damage or disruption.

Different Types of Faults in Power Systems

Given below is a list of a few types of faults that might occur in a power system:

  1. Line-to-Line Fault: It happens when two conductors of a three-phase power system come into direct contact.
  2. Line-to-Ground Fault: This fault occurs when one or more conductors come into contact with the ground or a grounded object.
  3. Double Line-to-Ground Fault: This fault involves two conductors contacting the ground or grounded objects simultaneously.
  4. Three-Phase Fault: This fault involves a short circuit between all three phases of a power system. It is considered the most severe fault and can cause significant damage and disruption.
  5. Open Circuit Fault: This fault occurs when a conductor or connection in a power system becomes open, causing a break in the circuit. It can result in a loss of power supply to the affected portion of the system.
  6. Short Circuit Fault: This fault occurs when a low-resistance path is formed between two points of differing potentials in a power system. It leads to a sudden increase in current flow, potentially causing damage to equipment and power interruptions.

Instrument Transformers

Instrument transformers are designed to transform high current and voltage levels into levels that instruments and protective devices can safely measure. Besides relaying systems, the Instrument transformers play a crucial role in ensuring the proper functioning of power systems. 

Protective relays and instrumentation devices such as voltmeters and ammeters use voltage and current to detect and isolate faults in the power system. Once a fault is detected, the protective relay will trip a breaker to isolate the fault and prevent damage to the system. The power system is rated high voltage, and high-voltage power lines are hazardous. Apart from safety hazards, the design of relays and instruments rated at very high voltage/current is uneconomical.

The instrument transformers provide a safe and accurate means of measuring voltage and current without exposing workers to potential hazards and without designing high-voltage relays and instruments.


In conclusion, power system protection is critical to power system operation and maintenance. The protection system ensures the safe and reliable operation of the power system, prevents damage and outages, and protects the equipment, the environment, and the public. Therefore, power system protection should be given the attention, resources, and investment it deserves to ensure the sustainability and resilience of the power system.

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