ABB RELION REX640 Technical Manual page 936

Protection and control

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Protection functions

Figure 519: 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.

If a moderate system operates at 50 Hz, an underfrequency should be set for

different steps from 49.2 Hz to 47.5 Hz in steps of 0.3 – 0.4 Hz. The operating time

Set Restore delay time

1MRS759142 F

Technical Manual