SEG Electronics GmbH Manual MRR1
10 DOK-TD-MRR1, Rev. B
4.2 Excitation current supervision
Failures in the excitation system of a synchronous generator may cause severe damage to the ro-
tor of the generator as well as undesirable operating conditions:
AVR failure; undercurrent
in parallel operation
Excessive reactive current flow to the generator
System stability endangered
AVR failure; undercurrent
in single operation
System under voltage if
System stability endangered
AVR failure; over current
in parallel operation
Excessive reactive current flow to the system
Thermal overload of the exciter winding
AVR failure; over current
in single operation
System overvoltage if in single operation
Thermal overload of the exciter winding
AVR failure; over current
Rotor damage due to overheating
Increased excitation current
overheating of the rotor system
Excessive excitation current
severe overheating of the rotor system
Table 4.1: Effects of different failures
Excitation current supervision:
The excitation current supervision comprises of three elements: under- and over current and ripple
component measurement. Those elements cover above mentioned failures of the generator
excitation system.
Undercurrent protection:
The undercurrent protection is a definite time element. The current setting (Ie<) should be chosen
at about 70% of the no load excitation current of the generator. The time delay tIe< should be set to
a value well above the AVR response time (e.g. 2 s), to allow transient de-excitation of the genera-
tor due to voltage regulator action.
If required the MRR1 may be blocked by connecting supply voltage to terminals D8/E8. This might
be necessary during start up of the generator.
Over current protection:
The over current protection is a definite time element. The current setting (Ie>) should be chosen at
about 120% of the nominal excitation current of the generator. The time delay tIe> should be set to
a value well above the AVR response time and required short circuit times (e.g. 3 s), to allow
transient over excitation of the generator due to voltage regulator action.
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