ABB RELION 620 Series Technical Manual page 341

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1MAC504801-IB E

620 series ANSI

Technical Manual

stepwise in such a way that it does not lead the system back to the emergency condition.

This is done through an operator intervention or in case of remote location through an

automatic load restoration function. The load restoration function also detects the system

frequency and restores the load if the system frequency remains above the value of the set

restoration frequency for a predefined duration.

Start of restore delay counter

Restore delay counter suspended

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Operation of the load-shedding function

Power system protection by load-shedding

The decision on the amount of load that is required to be shed is taken through the

measurement of frequency and the rate of change of frequency (df/dt). At a single location,

many steps of load-shedding can be defined based on different criteria of the frequency

and df/dt. Typically, the load-shedding is performed in six or four steps with each

shedding increasing the portion of load from five to twenty-five percent of full load within

a few seconds. After every shedding, the system frequency is read back and further

shedding actions are taken only if necessary. In order to take the effect of any transient, a

sufficient time delay should be set.

Protection functions

RESTORE output activated

Restore delay counter continues

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