GE Multilin G650, Manual

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GEK-113285A G650 Generator Protection & Control System 5-99
5 SETTINGS 5.5 CONTROL ELEMENTS
5
5.5.11 LOSS OF EXCITATION (40)
This function is used to detect loss of excitation on synchronous machines. It includes two mho characteristics looking into
the machine, each with adjustable reach, center, and time delay.
Excitation can be lost due to inadvertent tripping of the field breaker, open or short circuit on the field winding, regulator
failure, or loss of the source to the field winding. Loss of excitation can be damaging to the machine and/or detrimental to
the operation of the system. When a synchronous generator loses excitation, it will tend to act as an induction generator: it
will run above normal speed, operate at reduced power and receive its excitation (VARS) from the system. The impedance
seen by a relay looking into a generator will depend on the machine characteristics, the load flow prior to the loss of
excitation, and the type of excitation failure.
• The
stage 1
characteristic is typically set to detect severe cases of excitation failure. This is achieved with a mho
element with a diameter equal to the base impedance of the machine and an offset equal to half the machine transient
reactance (X'd).
The stage 1 element should be time delayed to allow for blocking by the VT fuse failure element (60 ms).
• The
stage 2
characteristic is typically set to detect a loss of excitation for all load conditions. This is achieved with a
mho element with a diameter equal to the synchronous reactance of the machine and an offset equal to half the
machine transient reactance.
Some stable power swing conditions may momentarily enter the stage 2 characteristic For security of the element under
such conditions, it is recommended to delay stage 2 by a minimum of 0.5 seconds.
Figure 5–23: OPERATING CHARACTERISTIC
( )
2
1
2
'
1
Zb
Radious
d X Zb
Center
=
+
=
( )
2
2
2
'
2
Xd
Radious
d X Xd
Center
=
+
=