Load Shedding And Restoration; Load Restoration Implementation; Holding Band - GE MiCOM P40 Agile Technical Manual

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8

LOAD SHEDDING AND RESTORATION

The goal of load shedding is to stabilise a falling system frequency. As the system stabilises and the generation
capability improves, the system frequency will recover to near normal levels and after some time delay it is
possible to consider the restoration of load onto the healthy system. However, load restoration needs to be
performed carefully and systematically so that system stability is not jeopardized again.
In the case of industrial plants with captive generation, load restoration should be linked to the available
generation since connecting additional load when the generation is still inadequate, will only result in declining
frequency and more load shedding. If the in-plant generation is insufficient to meet the load requirements, then
load restoration should be interlocked with recovery of the utility supply.
Whilst load shedding leads to an improvement in the system frequency, the disconnected loads need to be
reconnected after the system is stable again. Loads should only be restored if the frequency remains stable for
some period of time (minor frequency excursions can be ignored during this time period). The number of load
restoration steps is normally less than the load shedding steps to reduce repeated disturbances while restoring
load.
8.1

LOAD RESTORATION IMPLEMENTATION

The device uses the measurement of system frequency as the main criteria for load restoration. For each stage of
load restoration, it is necessary that the same stage of load shedding has occurred previously and that no
elements within that stage are configured for overfrequency or rising frequency conditions. If load shedding has
not previously occurred, the load restoration for that stage is inactive.
The device provides nine independent stages of Load Restoration. It is implemented in the FREQ PROTECTION
column of the relevant settings group. The following settings are relevant for Load Restoration:
Restore(n) Status: determines whether the stage is disabled or enabled
Restore(n) Freq: defines the frequency pickup setting
Restore(n) Time: Timer period for which the measured frequency must be higher than the stage restoration.
Holding Timer: Sets the holding timer value
8.2

HOLDING BAND

Load restoration for a given stage begins when the system frequency rises above the Restore(n) Freq setting for
that stage and the stage restoration timer Restore(n) Time is initiated. If the system frequency remains above the
frequency setting for the set time delay, load restoration of that stage will be triggered.
Unfortunately, frequency recovery profiles are highly non-linear and it would be reasonably common for the
system frequency to fall transiently below the restoration frequency threshold. If the restoration timer immediately
reset whenever a frequency dip occurred, it is likely that load restoration would never be successful. For this
reason, the protection has a "holding band". This holding band is a region defined by the restoration frequency and
the highest frequency setting used in the load shedding elements for that stage. The difference between these two
settings must always be greater than 0.02 Hz, otherwise a Wrong Setting alarm will be generated. Whenever the
system frequency dips into the holding band, operation of the stage restoration timer is suspended until the
frequency rises above the restoration frequency setting, at which point timing will continue. If the system
frequency dip is sufficiently large to cause any frequency element to start or trip in this stage, i.e. if the frequency
falls below the lower limit of the holding band, the restoration timer will immediately be reset. This is demonstrated
below.
P14xEd1-TM-EN-1
Chapter 11 - Frequency Protection Functions
251

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