Basler Electric BE1-24 Volts Per Hertz Overexcitation Relays

should act to bring the voltage down to a safe level before equipment damage occurs.

However, in the event of a regulator failure, the BE1-24 protects equipment from

damage.

Load tap changing (LTC) transformers and line voltage regulators may be subjected to

excessive volts/hertz during abnormal system frequency conditions.

This is the inevitable consequence of their constant voltage control function. Failure of

the LTC controller may result in a runaway condition producing dangerously high

voltages. The BE1-24 provides close protection for such equipment while allowing a

wide range of voltage control operations.

Figure 1-1 illustrates the advantage of applying the inverse square timing characteristic

rather than the two step protection scheme. This provides closer coordination and

prevents unnecessary tripping of the protected equipment.

In this example, two steps of definite time delay overexcitation protection are applied

to provide protection that approximates the transformer damage curve.

One step is set for 110 percent and 60 seconds. The second step is set for 118 percent

and 6 seconds. However, by using the inverse square characteristic, the damage curve

can be even more closely approximated. The resulting protection is more closely

coordinated with adequate margin. Note that the cross hatched area reveals the

overprotected area that results from use of a two step scheme.

When a transformer and generator are to be protected by a single volts-per-hertz

overexcitation relay, the overexcitation curves for both pieces of equipment must be 

considered. Figure 1-2 illustrates a situation where these two units have different

characteristics and yet can both be protected by the BE1-24. Utilizing the alarm function

(with its definite time delay), and in combination with the inverse square characteristic,

the transformer generator combination may be protected with a single BE1-24 Volts-Per

-Hertz Overexcitation Relay. If necessary, the instantaneous element can be set to a

percentage (105%) of the rated excitation to provide an early warning alarm.

This alarm can enable the operator controlling the excitation to recover and prevent

tripping.