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Out-of-Step Relaying for Small Generators in Distributed Generation

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  • Create Date April 30, 2020
  • Last Updated August 4, 2021

Out-of-Step Relaying for Small Generators in Distributed Generation

This paper will examine out-of-step relaying for small generators protection. It will consider the value, purpose, and applicability and provide guidelines for setting of this type of relay, when employed. The recent popularity of distributed generation (DG) has caused an increasing number of small generators (rated between 100 kVA – 12.5 MVA) to be connected to the power system at the distribution level (480 V - 12.47 kV). Some utilities require that out-of-step relays be installed by the generation owner at the point of common coupling (PCC).

In an effort to accommodate the connection of DG on their systems, some utilities have adopted simplified interconnection requirements for these generators (often limited to 5 MVA or less). These requirements are implemented to protect the system by automatically disconnecting the generator in the event of a system fault or unusual generator operating conditions that may cause voltage or current disturbances on the system. Safeguards are also required to disconnect the generator in the event the utility de-energizes the distribution line being fed by the generator. In addition, protective relaying is also installed to protect the generator from damage due to system-originated disturbances and faults.

Included in these protection requirements is the need to automatically disconnect the generator from the power system if the generator loses synchronism, or goes out-of-step, with the system. Some utilities have interpreted this requirement by including an out-of-step relay (ANSI / IEEE Device No. 78) in the interconnection protection package.

This requirement when applied can present a challenge for the protection engineer trying to set this relay correctly. The traditional use of the No. 78 relay was to block inadvertent generator tripping. It is undesirable to allow a large generator to be tripped for a system disturbance that would not cause the generator to fall out-of-step or to trip and damage equipment not rated to trip in an out-of-step condition. An out-of-step relay can be employed to block the action of other relays to allow a large generator to ride-through disturbances that do not cause the generator to fall out-of-step, but which might cause other relaying for small generators to trip the generator. This relay is also used to block the tripping of breakers when conditions due to an out-of-step condition are too severe to allow the breaker to open until the machine swing has progressed to the point where safe interruption is possible.

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Application of Out-of-Step Relaying for Small Generators in Distributed Generation