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I would like to understand how a voltage restrained overcurrent protection works. Is this different from a normal overcurrent protection, in which the relay will pickup when the secondary CT current exceeds the set level of the relay?

This option I understand is used in numerical relays for better fault clearance and to prevent false operation. I would like to know how voltage restrained protection can be used for overcurrent protection.

There are lot of vendors who makes voltage restraint relays for extended protection for generators, e.g. GE makes one. It has a Voltage restraint U-Magnet and coil to produce a restraint torque that opposes the operating coil to avoid false trips.

Typical Voltage-restrained relay has a voltage element that provides restraining torque proportional to voltage and thus actually shifts the relay pickup current. Hence the relay becomes more sensitive larger the voltage drop, but is relatively insensitive at normal voltage. The relay is set to ride through permissible power swings at nominal voltage. since the voltage and overcurrent units of the voltage-controlled relay are independently adjustable, it is preferrable to a voltage restrained type. Typically these type of relays are recommended for generator back up protection.

Nanda
Sr. Controls Engineer
nandakumar. gopal @ gmail. com

Responding to Sanu’s 17-Jan (01:02) query... two fundamental goals of protective relaying are: 1) maintain supply as long as possible; and 2) provide backup! The voltage controlled overcurrent relay provides both:

To meet goal #1, the voltage-restraint feature causes the relay to function as a pseudo-impedance relay, thereby meeting the first goal! That is, it discriminates between a close-in fault (lower voltage, quicker response) and a remote one (higher voltage, slower response!)

To meet goal #2, it provides backup protection for faults not cleared by the system’s primary relays and breakers!

Sanu, if you are interested in additional information or references, contact me on-, or off-list!

Regards, Phil Corso (cepsicon@aol.com)

This is in response to those providing off-list questions and comments:

First some history! The voltage-controlled overcurrent relay #51V (ANSI Identifying Function Number) was specifically designed to provide backup protection for generators. Sometimes a generator’s synchronous reactance is so large that it can cause a sustained fault-current to be less than full-load current. That means a current-only relay is ineffective for protection. Thus, the voltage-controlled relay was developed.

Essentially, as explained by Nanda in his 18-Jan (01:26) reply, a restraining torque produced by the relay’s voltage-input is inversely proportional to bus voltage, hence it negates current-input produced torque. This causes the relay to operate (pickup) faster for a close-in fault, but more slowly for a remote fault!

Regarding the relay’s backup function, here are two examples: a) provides faster pickup for phase-phase bus faults in systems that aren’t equipped with bus-differential relays, because of the consequent voltage-drop; and b) provides backup to a generator’s differential relays while the generator is off-line but its field energized! However, this relay is not used for protection in systems with neutral grounded (earthed) NGR or NGT systems! Furthermore, except for application in very large low-voltage (< 1,000-V) machines I would be very skeptical of anyone suggesting it for backup!

Regarding reference text on the subject of power systems, a good start is the IEEE colored-book series! And, in my opinion, the best text on protective relaying is J. Lewis Blackburn’s "Applied Protective Relaying!" Last century, when Prof Blackburn was my teacher, it was known as "Silent Sentinels!"

Regards,
Phil Corso, PE (cepsion@aol.com)