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Power System Protection Tests

Written by Behnam Feizifar

Protection systems play a key role for the safe and reliable operation of today's electricity power systems. This is relevant for all power system grids in generation, transmission and distribution, industrial, railway, and marine applications. Properly working protection devices help to keep the safety of the system and to safeguard assets from damage while assisting to ensure security of supply. In order to guarantee the reliable operation of power systems, protection relays must be tested throughout their life-cycle, from their initial development through production and commissioning to periodical maintenance during operation.

Purposes of system protection

• Protect the public


• Improve system stability


• Minimize damage to equipment


• Protect against overloads


• Employ relay techs and engineers

Current Transformer (CT) is a device which transforms the current on the power system from large primary to safe secondary values. Thus, the secondary current will be proportional (as per the ratio) to the primary current.

Potential Transformer (PT) is a device which transforms the voltage on the power system from primary to safe secondary values. Thus the ratio is proportional to the primary value.

Protective relays monitor the current and/or voltage of the power system to detect problems with the power system. Currents and voltages to relays are supplied via CT's and PT's.

Which equipment should be protected?

• Generators


• Transformers, Reactors


• Lines


• Buses


• Capacitors

There are many different reasons why protection relays may suffer from malfunctions and these can vary depending on the various generations of relay that are in service today: Electromechanical relays may cause problems because of aging of mechanical components, such as coils or contacts, static relays are subject to drifting or can have failures of electronic circuits or components, and numerical relays may exhibit some undesirable effects because of software issues, e.g. following firmware updates. Hence, commissioning and routine testing should be performed thoroughly so that shortcomings in the reliability of the protection system are discovered before a power system fault occurs.

In designing, configuring and implementing protection systems, protection engineers main target to provide validated systems that operate with reliability, security, speed and selectivity. Testing is necessary to ensure that systems meet performance criteria, but the increasing complexity of interconnected equipment has made comprehensive testing more challenging. Testing of complex protection plans and coordinated wide-area protection poses still greater challenges. What is more, the need for increased power grid resilience and much smarter grids capable of handling separation of the bulk transmission system into separate microgrids, protection and control systems, requires tools capable of delivering realistic and high-fidelity test conditions for a broad range of system configurations.

Generator faults classification

• Internal faults


• Abnormal Service conditions


• External Faults (as back up protection)


• The hazards and problems considered


• Phase to phase faults in winding


• Phase-ground faults in winding


• Ground faults in the rotor

Generator abnormal conditions

• Loss or insufficient excitation


• Overload


• Overvoltage


• Under and Over frequency


• Unbalance current


• Inadvertent energization


• Out of step (loss of synchronism)


• Loss of prime mover (motoring)

Function number for generator protection

• Ground Fault (50/51-G/N, 27/59, 59N, 27-3N, 87N)


• Phase Fault (51, 51V, 87G)


• Backup Remote Fault Detection (51V, 21)


• Reverse Power (32)


• Loss of Field (40)


• Thermal (49)


• Fuse Loss (60)


• Over excitation and Over/Under voltage (24, 27/59)


• Inadvertent Energization (50IE, 67)


• Negative Sequence (46, 47)


• Off-Frequency Operation (81O/U)


• Sync Check (25) and Auto Synchronizing (25A)


• Out of Step (78)

Table 1. IEEE function number for generator protection

Transformer faults classification

• Winding failures due to short circuits (turn-turn faults, phase-phase faults, phase-ground, open winding)


• Core faults (core insulation failure, shorted laminations)


• Terminal failures (open leads, loose connections, short circuits)


• On-load tap changer failures (mechanical, electrical, short circuit, overheating)


• Abnormal operating conditions (overfluxing, overloading, overvoltage)


• External faults

Table 2. Protection functions for transformer

Transmission line protection

The high level factors influencing line protection include the criticality of the line (in terms of load transfer and system stability), fault clearing time requirements for system stability, line length, the system feeding the line, the configuration of the line (the number of terminals, the physical construction of the line, the presence of parallel lines), the line loading, the types of communications available, and failure modes of various protection equipment.

The more detailed factors for transmission line protection directly address dependability and security for a specific application. The protection system selected should provide redundancy to limit the impact of device failure, and backup protection to ensure dependability. Reclosing may be applied to keep the line in service for temporary faults, such as lightning strikes. The maximum load current level will impact the sensitivity of protection functions, and may require adjustment to protection functions settings during certain operating circumstances. Single-pole tripping applications impact the performance requirements of distance elements, differential elements, and communications schemes.

The physical construction of the transmission line is also a factor in protection system application. The type of conductor, the size of conductor, and spacing of conductors determines the impedance of the line, and the physical response to short circuit conditions, as well as line charging current. In addition, the number of line terminals determines load and fault current flow, which must be accounted for by the protection system. Parallel lines also impact relaying, as mutual coupling influences the ground current measured by protective relays. The presence of tapped transformers on a line, or reactive compensation devices such as series capacitor banks or shunt reactors, also influences the choice of protection system, and the actual protection device settings.

Table 3. Protection functions for transmission lines

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