Model Overview

The Var-type loss-of-excitation relay is used with synchronous generators to detect loss of field excitation conditions. The 40Q relay operates on a level of the reactive power absorbed from the connected system, with a definite time setting. There are different scenarios which could lead to higher reactive power consumption of the synchronous generator, including voltage at a remote end of the power system being higher than that of the regulated machine terminal voltage. In case there is a loss of excitation (e.g. short-circuit or open-circuit of the field winding, for instance), the power system will  supply excitation current to the generator through the stator winding in the form of reactive current, which could lead to the acceleration of the machine, to loss of synchronism, and eventually unstable and damaging operation.

The 40Q relay monitors the three-phase voltages and currents at the stator windings of the synchronous machine. Reactive power is calculated by evaluating the positive-sequence of the input voltages and currents with a running window over one cycle of the fundamental frequency. When the reactive power exceeds the threshold and stays greater than the threshold for a duration longer than the time delay setting, the 40Q relay is asserted.

A fixed one-cycle start time is implemented in the 40Q relay in order to have the correct reactive power measurement. The 40Q relay will not operate during the first cycle of the simulation.

Mask and Parameters

General parameters

Inputs and Outputs

Inputs

Outputs

Available monitoring signals