OVER EXCITATION PROTECTION FOR POWER TRANSFORMER BASICS AND TUTORIALS

Overexcitation of a transformer can occur whenever the ratio of the per unit voltage to per unit frequency (V/Hz) at the secondary terminals of a transformer exceeds its rating of 1.05 per unit (PU) on transformer base at full load, 0.8 power factor, or 1.1 PU at no load.

For the generator, the limit is 1.05 PU (generator base). When an overexcitation condition occurs, saturation of the laminated steel cores of the generator and transformer can occur. Stray magnetic fields increase in magnitude, particularly at the ends of the cores.

Nonlaminated components at the ends of the cores, which were not designed to carry these higher levels of flux, begin to heat up because of the higher losses induced in them. This can cause severe localized overheating in the transformer and generator and eventual breakdown in the core assembly or winding insulation.

The permissible short-time overexcitation capability of a specific transformer or generator should be obtained from the manufacturer. Figure below shows V/Hz limiting curves provided by three different transformer manufacturers.

Overexcitation is of major concern on directly connected generator unit transformers. One of the primary causes of excessive V/Hz on generators and unit transformers is operation of the unit under regulator control at reduced frequencies during generator start-up and shutdown. Another cause of excessive V/Hz is inadvertent manual overexcitation during generator start-up and shutdown.

Overexcitation can also occur during complete load rejection that leaves transmission lines connected to a generating station. Under this condition the V/Hz may exceed 1.25 PU. With the excitation control in service, the overexcitation will generally be reduced to safe limits in a few seconds.

With the excitation control out of service, the overexcitation may be sustained and damage can occur to the generator and/or transformers. Failures in the excitation system or loss of signal voltage [i.e., blown voltage transformer (VT) fuse] to the excitation control can also cause overexcitation.


Occasionally a transformer remote from a generation station will be exposed to overflux conditions that may not be protected by an overvoltage relay or by the V/Hz protection associated with a generation station. A typical case would be a transformer on the end of a long line connected to a generating plant.

During a load rejection in which this transformer is connected to the generator, the transformer may have a significantly higher V/Hz than that at the generator facility as a result of the Ferranti effect. In this case, or similar cases, V/Hz protection should be applied to the remote transformer.


Overexcitation protection for the transformer is generally provided by the generator overexcitation protection, which uses the VTs connected to the generator terminals. So, the curves that define generator and transformer V/Hz limits must be coordinated to properly protect both pieces of equipment.

Generally, the transformer V/Hz curve is put on a generator voltage basis Therefore, a 13.2/115 kV transformer being used as a step-up transformer for a 13.8 kV generator will reach its continuous no-load V/Hz limit of 110% at a generator voltage of 105.2% of the generator rated voltage.

Generator manufacturers recommend an overexcitation protection system as part of the generator excitation system. These systems (V/Hz limiters) will limit the V/Hz to a safe value in the automatic control mode.

To provide protection when the unit is under manual control, the V/Hz limiter may send a relay alarm signal during an overexcitation condition and, if the condition persists, decrease the generator excitation or trip the generator and field breakers, or both. The generator manufacturer should be asked for recommendations for overexcitation protection.

It should be noted that, if the generator can be operated leading, the high-side voltage of the transformer may have a higher PU V/Hz than the generator V/Hz. This aspect may need to be considered in proper V/Hz protection of the transformer.

It is common practice to apply separate V/Hz protection in addition to the protection built into the excitation control system. Several forms of protection are available including definite time, preprogrammed inverse time curves, and user-programmable inverse time curves. A detailed discussion on various forms of V/Hz relays can be found in 4.5.4 of IEEE Std C37.102-1995 [B86].

When the transformer rated voltage is equal to the generator rated voltage, the same V/Hz relay that is protecting the generator may be set to protect the transformer. In some cases, however, the rated transformer voltage is lower than the rated generator voltage and protection may not be provided.

It may, therefore, be desirable to provide supplementary protection for the transformer. Since the V/Hz capabilities of transformers may differ appreciably, it is not possible to provide definitive protection recommendations that would cover all units.