Basler Electric BE1-24 Volts Per Hertz Overexcitation Relays

PURPOSE

BE1-24 Volts Per Hertz Overexcitation Relays are specifically designed to

protect transformers, generators and iron core reactors from the adverse

effects of excessive heating resulting from overexcitation.

Overexcitation exists whenever the per unit volts per hertz exceeds the

equipment design limitations.

OVEREXCITATION

Overexcitation is excessive flux density in the magnetic core of the protected

equipment. Once the magnetic core is at or near saturation, any increase in

the applied voltage will cause the excess flux to travel outside the core through

the air or the structural steel components. Structural steel components are not

laminated, and stray flux in these components cause large eddy current losses

and hysteresis losses. This generates excessive heat. Also, the excess flux

through the air requires a large current in the winding which creates excessive

heat.

Overexcitation of transformers, iron core reactors, and generators can cause

severe damage. This overheating causes the deterioration of adjacent insulation,

and if allowed to persist, will lead to equipment failure. Modern designs are

especially sensitive to overexcitation because they operate with high flux densities.

Overexcitation can result from an increase in voltage or a decrease in frequency.

VOLTS PER HERTZ PRINCIPLE

Protective relaying must determine the level of flux density from the secondaries

of voltage and/or current transformers. The following demonstrates the relationship

between the actual flux density, voltage, and frequency as monitored by the

protective relays.

RELAY CHARACTERISTICS

BE1-24 relay emulate heating and cooling characteristics of the protected equipment.

By accumulating time towards tripping whenever the timed trip volts/hertz pickup

setting is exceeded, the relays simulate heat build up within the protected equipment.

Once heated, metal does not cool instantaneously. To accommodate this characteristic,

the BE1-24 also has a linear reset characteristic which can be adjusted to follow the

cooling rate of the protected equipment. So, as heat builds up and dissipates within

the protected equipment due to overexcitation excursions, it is closely protected by

the BE1-24 tripping and reset characteristics.

The definite time alarm feature allows even more effective use of the equipment by

alerting operators that a potentially dangerous condition exists. Once alerted, the

operator can take corrective action to prevent the necessity of a relay trip.

Alternatively, the definite-time alarm output contact can be used to initiate automatic

corrective action.

The optional instantaneous trip provides high-speed tripping for the most severe

conditions.

APPLICATIONS

Volts/hertz overexcitation relays primarily protect directly connected generator unit

step-up transformers.

These transformers may be subjected to excessive volts/hertz during generator startup

or shutdown, power system islanding, or remote load rejection.

Typically, the generator field is applied when the machine is above 90% of rated speed.

If the field is applied early, or not removed soon enough, the terminal voltage may be

much higher than appropriate for the actual frequency. Many generator automatic

voltage regulators and excitation systems are equipped with a volts/hertz characteristic.

BE1-24 relays protect associated step-up transformers from overexcitation caused by

failure of the field application/removal scheme, or by failure of the automatic voltage

regulator during start up and shutdown.

An excessive load-to-generation ratio during power system islanding, or during local

loss of generation, can create an underfrequency condition which may result in the

overexcitation of the generator unit step-up transformer. The BE1-24 relay acts as

backup protection for underfrequency relaying, and for volts/hertz control functions

which may be incorporated in the generator voltage regulator.

The sudden removal of load from a generator results in an increase in terminal voltage

especially if a

transmission line remains connected as the only load. The automatic voltage regulator

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.