Frequency Protection; Underfrequency Protection; Underfrequency Protection Implementation; Underfrequency Protection Logic - GE P642 Technical Manual

P64x
3

FREQUENCY PROTECTION

Power generation and utilisation needs to be well balanced in any industrial, distribution or transmission network.
These electrical networks are dynamic entities, with continually varying loads and supplies, which are continually
affecting the system frequency. Increased loading reduces the system frequency and generation needs to be
increased to maintain the frequency of the supply. Conversely decreased loading increases the system frequency
and generation needs to be reduced. Sudden fluctuations in load can cause rapid changes in frequency, which
need to be dealt with quickly.
Unless corrective measures are taken at the appropriate time, frequency decay can go beyond the point of no
return and cause widespread network collapse, which has dire consequences.
Normally, generators are rated for a particular band of frequency. Operation outside this band can cause
mechanical damage to the turbine blades. Protection against such contingencies is required when frequency does
not improve even after load shedding steps have been taken. This type of protection can be used for operator
alarms or turbine trips in case of severe frequency decay.
Clearly a range of methods is required to ensure system frequency stability. The frequency protection in this device
provides both underfrequency and overfrequency protection.
Frequency Protection is implemented in the FREQ PROTECTION column of the relevant settings group.
3.1

UNDERFREQUENCY PROTECTION

A reduced system frequency implies that the net load is in excess of the available generation. Such a condition can
arise, when an interconnected system splits, and the load left connected to one of the subsystems is in excess of
the capacity of the generators in that particular subsystem. Industrial plants that are dependent on utilities to
supply part of their loads will experience underfrequency conditions when the incoming lines are lost.
Many types of industrial loads have limited tolerances on the operating frequency and running speeds (e.g.
synchronous motors). Sustained underfrequency has implications on the stability of the system, whereby any
subsequent disturbance may damage equipment and even lead to blackouts. It is therefore essential to provide
protection for underfrequency conditions.
3.1.1

UNDERFREQUENCY PROTECTION IMPLEMENTATION

Simple underfrequency Protection is configured in the FREQ PROTECTION column of the relevant settings group.
The device provides 4 stages of underfrequency protection. The function uses the following settings (shown for
stage 1 only - other stages follow the same principles).
F<1 Status: enables or disables underfrequency protection for the relevant stage
●
F<1 Setting: defines the frequency pickup setting
●
F<1 Time Delay: sets the time delay
●
3.1.2

UNDERFREQUENCY PROTECTION LOGIC

Freq
Averaging
F<1 Setting
F<1 Status
Enabled
All Poles Dead
Freq Not Found
F<1 Timer Block

Figure 122: Underfrequency logic (single stage)

P64x-TM-EN-1.3
&
1
Chapter 12 - Frequency Protection Functions
DT
F<1 Start
F<1 Trip
V00861
259