Overfrequency Protection; Overfrequency Protection Implementation; Overfrequency Protection Logic; Application Notes - GE P24DM Technical Manual

P24xM
4

OVERFREQUENCY PROTECTION

An increased system frequency arises when the mechanical power input to a generator exceeds the electrical
power output. This could happen, for instance, when there is a sudden loss of load due to tripping of an outgoing
feeder from the plant to a load centre. Under such conditions, the governor would normally respond quickly to
obtain a balance between the mechanical input and electrical output, thereby restoring normal frequency.
Overfrequency protection is required as a backup to cater for cases where the reaction of the control equipment is
too slow.
4.1

OVERFREQUENCY PROTECTION IMPLEMENTATION

The following settings are relevant for overfrequency:
Stg (n) f+t Status: determines whether the stage is underfrequency, overfrequency, or disabled
●
Stg (n) f+t Freq: defines the frequency pickup setting
●
Stg (n) f+t Time: sets the time delay
●
4.2

OVERFREQUENCY PROTECTION LOGIC

Freq Averaging
Stg 1 f+t Freq
Stage 1
Enabled
Stg 1 f+t Status
Over
Adv Freq Inh
Freq Not Found
V<B Status
Enabled
UV Block
V00851

Figure 113: Overfrequency logic (single stage)

If the frequency is above the setting and not blocked, the DT timer is started and after this has timed out, the trip is
produced. If the frequency cannot be determined, the function is blocked.
4.3

APPLICATION NOTES

4.3.1

SETTING GUIDELINES

Following changes on the network caused by faults or other operational requirements, it is possible that various
subsystems will be formed within the power network. It is likely that these subsystems will suffer from a
generation/load imbalance. The "islands" where generation exceeds the existing load will be subject to
overfrequency conditions. Severe over frequency conditions may be unacceptable to many industrial loads, since
running speeds of motors will be affected. The overfrequency element can be suitably set to sense this
contingency.
An example of two-stage overfrequency protection is shown below using stages 5 and 6 of the f+t elements.
However, settings for a real system will depend on the maximum frequency that equipment can tolerate for a
given period of time.
P24xM-TM-EN-2.1
&
1
1
Chapter 11 - Frequency Protection Functions
DT
Note : This diagram does not show all stages . Other stages follow similar principles.
Stg1 f+t Sta
Stg1 f+t Trp
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