When an under voltage situation occurs in an installation the reason can be: failure or wrong adjustment on the voltage regulator or loss of excitation current. All this is covered by the voltage relay which will trip when the voltage is out of limits.

However, the disadvantage when only using a voltage relay is that the voltage relay does not “know” which of the generators are causing the problem, when running two or more generators in parallel mode. All generators will be tripped and then a blackout occurs. Afterwards the user must examine every single generator to find the fault.

The advantage when using the T2100 Excitation Loss Relay is that only the faulty generator will be disconnected and a blackout is avoided depending on whether the remaining generators will be capable of handling the load.

How does the T2100 Excitation Loss Relay know which of the generators is the faulty one?
When running one generator in single mode the terminal voltage will be reduced when reducing the excitation. When running two generators in parallel mode the terminal voltage is equal for both generators and it is not possible to measure any deviation of the terminal voltage between them. If one of the generators during parallel operation has a low excitation, a high inductive current is running into the faulty generator from the well running generator.

This inductive current is measured and detected by the T2100 Excitation Loss Relay and the breaker of the faulty generator is tripped, thus avoiding overload on the other generators with a possible blackout of the system.

If only one voltage relay is used, the faulty generator will pull the voltage below the under voltage setting of the relay thus tripping both generators.