ABB RELION REX640 Technical Manual page 860

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Protection functions

Set Restore delay time

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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.

The value of the setting has to be well below the lowest occurring normal frequency

and well above the lowest acceptable frequency of the system. The setting level, the

number of steps and the distance between two steps (in time or in frequency) depend

on the characteristics of the power system under consideration. The size of the largest

loss of generation compared to the size of the power system is a critical parameter. In

large systems, the load-shedding can be set at a high frequency level and the time delay

is normally not critical. In small systems, the frequency start level has to be set at a low

value and the time delay must be short.

1MRS759142 C

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