Loss Of Field Element(40). Miwii1000 - GE Multilin MIWII Instruction Manual

2.4 LOSS OF FIELD ELEMENT(40). MIWII1000

2 PRODUCT DESCRIPTION
2.4LOSS OF FIELD ELEMENT(40). MIWII1000
MIWII units provide a loss of field detection unit. When a generator operating at full load loses excitation, it starts to operate
as an induction generator, rotating at a higher speed than the synchronous speed. In this situation, the generator is still
providing active power to the system, but it starts consuming reactive power from the system to maintain its excitation.
Depending on the type of generator, an overheating situation is caused by the currents in the rotor steel, which can
seriously damage the equipment, although it is generally considered that this type of operation can be maintained during
several minutes before issuing a trip.
2
The generator field is adjusted so that the current injected by the generator to the system is slightly delayed with relation to
voltage, injecting both active and reactive power to the system. When a loss of excitation is produced, the generated
current becomes advanced to the voltage (first quadrant), where the generator produces active power but consumes
reactive power. Independently from the overheating effect that can be caused by this situation, generators need a minimum
excitation level to be steady at a load condition. If the system is not able to generate enough reactive power for the
excitation, this may cause a loss of synchronism in the generator.
Excitation systems for synchronous generators usually incorporate under-excitation limiters, that is, they control that the
excitation applied to a generator doesn't fall below a certain value, and they commonly use protection relays as backup.
MIWII units detect a loss of excitation situation by metering the apparent impedance. In a loss of field situation, apparent
impedance is capacitive, and is located at the third and fourth quadrants.
Commonly, two Mho circles are used, one for detecting partial excitation losses, and a smaller one for detecting total
losses.
The bigger circle is set as follows:
DIAMETER:Xd
OFFSET:X'd/2
The smaller circle is set as follows:
DIAMETER:0.7 * Xd
OFFSET:X'd/2
The loss of field unit incorporates the following settings:
Enabling unit 40 (ENABLE 40):
This setting determines whether the protection unit is operative and can issue pickup events (if the trip is also enabled, then
it will activate the trip contact) that can be configured to close outputs, light up LEDs or trigger the oscillography.
Enabling the Voltage Supervision (VOLT SUPERV 40):
This setting allows supervising the operation of unit 40 by a voltage level. The unit will not operate if this setting is set as
YES, and the voltage is lower than the value set in the following setting.
Voltage Supervision Level (VOLTAGE LEVEL 40):
This setting adjusts the voltage level that must be surpassed for allowing the operation of unit 40, if the previous setting V
SUP 40 is set as YES.
Circle 1 Trip Delay (TRIP ENABLE 40 1)
This setting allows the trip of Circle #1. In order for the unit to produce trip and close the trip contact, unit 40 must be
enabled, as well as this setting. If the unit is enabled but the trip is not, the unit is operative and will produce digital pickup
and virtual trip signals, that can be configured to close outputs, light up LEDs or trigger the oscillography; however, the unit
will not close the general trip contact (not programmable).
Circle 1 Diameter (DIAM CIRCLE 40 1)
This setting is used for fixing the diameter of circle #1, in secondary ohms, by passing the primary values to unit terminals,
taking into account the VT ratios and the installation current.
Circle 1 Offset (OFFS CIRCLE 40 1)
This setting is used for fixing the offset of circle #1, in secondary ohms, by passing the primary values to unit terminals,
taking into account the VT ratios and the installation current.
2-4 MIWII Directional Power Protection System GEK-113308