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Understanding Protective Relays in Power Systems

Protective relays are critical components in power systems, providing essential protection for various elements such as generator sets, outgoing feeder and load networks, and incoming utility sources. These devices act as an investment "insurance," ensuring that equipment and systems are safeguarded against faults and abnormalities. The level or type of protection offered by these relays is dependent on the specific application, and they utilize current and voltage transformers to monitor the system. Here, we'll delve into some of the key protective functions provided by relays.


Key Protective Functions

27 - Undervoltage Function The undervoltage relay provides a trip signal when the sensed voltage decreases below the relay’s setting. It is used to detect low voltage conditions of a generator or utility and sometimes to check the availability of a voltage source. This function is typically combined with a 59 relay in the same case and is often caused by undersized or overloaded power sources. Undervoltage conditions can lead to significant operational challenges, such as decreased efficiency and potential damage to sensitive equipment. Implementing undervoltage relays ensures that power systems maintain optimal performance and reliability.

27 - Undervoltage Function

32R - Reverse Power Function The reverse power relay triggers a trip signal when the power flowing in the reverse direction exceeds the relay’s setting. This condition causes the generator to become a load or act as a motor. It also helps prevent the 'exporting' of generated power back into the utility grid, usually caused by a governor malfunction or an engine speed setting that is lower than the bus frequency. Reverse power scenarios can lead to costly damages and operational disruptions, making the reverse power function crucial for maintaining system stability and preventing unwanted power flow.

32R - Reverse Power Function

46 - Negative Phase Sequence Time Overcurrent Function This relay provides a trip signal when a level of negative phase sequence current exceeds the relay’s setting for a specified time. Negative phase sequence currents result from unbalanced loads on a three-phase generator, creating heat in the windings. The more significant the unbalance, the more severe the heating. Generators can typically operate with unbalanced loading in the 40% range without damage. Managing phase sequence currents effectively helps in prolonging the life of generators and preventing heat-induced failures.

46 - Negative Phase Sequence Time Overcurrent Function

47 - Undervoltage / Phase Sequence Function This relay combines undervoltage detection with phase sequence verification. It provides a trip signal when the sensed voltage decreases below the relay’s setting and ensures that the generator's phase rotation matches that of the bus. It is frequently used on mobile generators to verify correct phase rotation. Ensuring proper phase sequence is essential for the safe and efficient operation of mobile and stationary generators, preventing issues related to incorrect phase rotation.

47 - Undervoltage / Phase Sequence Function

50/51 - Instantaneous Overcurrent Function The 50/51 overcurrent relay is a protective relay that includes multiple set points. The instantaneous function (50) of the relay provides an instant trip signal when the level of current flow exceeds the setting of the relay’s 50 element. The time function (51) of the relay provides a trip signal when the level of current flow exceeds the setting of the relay’s 51 element for a given period of time. In low voltage applications this protection is usually provided by the generator circuit breaker.

50/51 - Instantaneous Overcurrent Function 

51G - Ground Overcurrent Function This relay detects overcurrent conditions specifically due to ground faults, providing targeted protection for equipment and personnel. Ground faults can be particularly hazardous, leading to electrical fires and severe equipment damage. Ground overcurrent relays help in quickly identifying and isolating these faults, enhancing system safety.


51G - Ground Overcurrent Function

59 - Overvoltage Function The overvoltage relay triggers a trip signal when the voltage exceeds the set threshold, protecting equipment from potential damage due to high voltage conditions. Overvoltage conditions can cause significant harm to electrical components, leading to premature failure and costly repairs. Implementing overvoltage protection ensures that voltage levels remain within safe operating limits.

59 - Overvoltage Function

81O/U - Over/UnderFrequency Function This relay monitors the system frequency and provides a trip signal if the frequency deviates from acceptable limits, protecting against both overfrequency and underfrequency conditions. Frequency variations can disrupt the stability and efficiency of power systems, making frequency protection relays essential for maintaining consistent performance and preventing system-wide issues.

81O/U - Over/UnderFrequency Function

86 - Lockout Relay Function The lockout relay is a critical safety device that remains in a tripped state until manually reset, ensuring that the fault condition is addressed before the system is re-energized. Lockout relays provide an additional layer of security, ensuring that faults are thoroughly investigated and resolved before the system resumes operation.

86 - Lockout Relay Function

87G - Generator Differential Overcurrent Function This relay provides protection by detecting differences in current between the generator’s input and output, indicating potential internal faults within the generator. Differential protection is highly sensitive and accurate, making it an essential function for safeguarding generators from internal failures and ensuring reliable operation.

87G - Generator Differential Overcurrent Function


Protective relays are indispensable in maintaining the safety and reliability of power systems. They provide various functions to detect and isolate faults, ensuring minimal damage to equipment and continuity of power supply. By understanding these key protective functions, engineers can better design and maintain robust power systems.

For comprehensive protection solutions and expert guidance on low and medium voltage switchgear, reach out to Enercon. Our team is dedicated to delivering reliable and efficient power system protection tailored to your needs. Visit us at to learn more and contact our experts today.


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