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The Var-type loss-of-excitation relay (40Q) operates when the reactive power that a synchronous generator consumes from the system exceeds the threshold. A tripping delay time corresponds to the threshold to prevent operation on transient conditions. The model works for 50 Hz or 60 Hz system.

Table of Contents

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




Enable and disable relay operation.



The nominal frequency of the measured signals. Both 50 and 60 Hz are supported in the model.

Q threshold


The threshold of the reactive power supplied from the connected system.

Time delay


Time delay associated with the threshold.

Sample Time


Sampling time for the relay

Inputs and Outputs




Three-phase voltages: Va, Vb, Vc


Three-phase currents: Ia, Ib, Ic


The reset signal for the relay. The relay will be reset when this signal becomes 1.



The output of 40Q relay. By default it is 0, if a loss-of-excitation scenario is detected, it becomes 1.

Available monitoring signals




The measured reactive power.

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