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Generator Normal Operations

Before we begin to analyze the various malfunctions that can befall a generator and the connected auxiliary system, we must have an understanding of the parameters that define the normal operation. Our review of generator and generator auxiliary protection will be based on the sample system the Generator Step Up (GSU) transformer and auxiliary loads at 4.16 kV buses A and B through auxiliary transformers.

Protection principles & schemes of synchronous generators and induction motors
Protection principles & schemes of synchronous generators and induction motors (photo credit: Leif Bråtveit via Flickr)

When the unit is offline, the auxiliary load is supplied from the auxiliary transformer at Bus B. See Figure 1.

The GSU transformer connects to a moderate strength power system. With all lines in service, the power system and GSU transformer appear as a 14.8% impedance at the generator terminals. Line A is the strongest line connected to the 69 kV bus; its outage increases the impedance seen by the generator to 29%.

The system voltage also varies. During light load periods the voltage drop through system components such as lines and transformers is minimal. As load increases, the increased voltage drop caused by the flow of Watts and Vars through these components causes system voltage to fall. Because these power system components are highly inductive, the voltage drop caused by an amp of reactive current is greater than that caused by an amp of real current.

The system voltage regulation between light and peak load is amplified by the reactive characteristic of the long high-voltage (HV) transmission lines. These lines behave as capacitors when lightly loaded. The Vars they produce flow into the system, boosting voltage just as distribution capacitors do.

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