A85-Back Charging of Generator Transformers

Generator Transformers (GT) are normally energized from the LV side with the HV side breaker in the open position. The generator voltage and hence GT terminal voltage will go up gradually without generating inrush currents in the transformer. After building up the rated voltage, the transformer will be synchronized to the grid by closing the HV breaker. But sometimes charging from the HV side will be required and this procedure is generally termed as back charging of generator transformers.

1) Large power stations (thermal or nuclear) will take several years to complete. In case any problem is noticed in GT during the final commissioning stage from the generator side, the whole project schedule will be affected as spare GT may not be available. Hence Project Managers will insist on energizing the GT and confirm its healthiness as soon as transformer installation is over instead of waiting for the completion of Generator and Turbine installation and power station commissioning.

2) In a normal power station layout, the LV side of GT (11 – 27 kV) is directly connected to generator terminals without any breakers. High current Generator Circuit Breaker (GCB) is an expensive proposition and instead, a start-up transformer will be used to black start the station i.e. a dedicated step-down transformer to step down the grid voltage to GT LV voltage will be used to give supply to the station auxiliary transformers. LV side of the station auxiliary Transformer feed the station auxiliaries like boiler feed pump motor, large turbo-fan motor etc.) Once the boiler-turbine-generator is made operational, generator voltage will be built up energizing the GT from the LV side. Sometimes GCB is provided on the isolated busduct (IPB) between the transformer and generator. In that scenario, without the use of a separate station start-up transformer, black start is done by energizing the GT from the HV side with the GCB in the open position. GT LV will feed power to station auxiliaries and once the generator voltage is built up, synchronization to the grid will be done at the LV side of GT by closing the GCB.

In the above two scenarios, certain precautions are to be taken to avoid transformer failures during back charging due to abnormal high switching surges.

1. Back charging (energizing from the HV side with LV open) of generator transformers (GT) is more onerous than charging from the LV side due to the high switching surges transferred from the HV side to the LV side of the transformer. Unlike in normal transformers, the GT voltage ratio is high and this may result in high voltage surges causing flashover of LV transformer bushings. To take care of this, one trend is to use 36 kV bushings on the LV side, though the winding voltage is only 11-27 KV. Still, the transferred surge may exceed the Basic Insulation Level (BIL) of 36 kV bushings. During the back charging of the transformer, the generator protection cubicle (Surge Absorber and lightning arrester) shall be kept connected to the LV side of the transformer to protect it from the transferred surges.

2. When the GT back charging is done to confirm the healthiness of the transformer, it is better to put the transformer at the maximum tap with the entire tap winding in the circuit. This will slightly reduce the inrush current as the flux density will be less than at the rated tap. It will also result in increased 2nd harmonic content in inrush current reducing chances of maloperation of differential relay. This will avoid part winding resonance in the floating tap winding and consequent insulation failure in tap winding. This type of resonant overvoltage in tap winding can occur when the natural frequency of regulating winding is near the frequency components in the switching surge.

Case History: Once (Year 2005) a 600 MVA 21/420 kV GT bank was getting back charged. It had DETC taps at HV neutral for a voltage range of +- 10 %. Charging was done at the rated tap and hence tap winding (comprising 10 % of main winding) was floating at one end. Overvoltage developed at the floating end of winding (separate tap winding, outside of HV winding) and this caused a flash over across the tap winding. Immediately after charging, the transformer tripped out with gas in the Buchholz relay. Since the fault was at the neutral end of winding, the arc current was low (winding impedance limiting the current) and hence tank did not burst resulting in an oil fire. Took back transformer to the factory and repaired by replacing the tap winding.

3. Generator Circuit breakers are very special breakers capable of breaking huge fault currents. These breakers require a surge absorber and LA on the generator side for proper functioning. These are integral parts of GCB, In addition, the same protective parts are necessary on the transformer side to protect the transformer from transferred surges from the HV side. When a generator circuit breaker (GCB) is used, make sure that on the transformer side of the breaker, the surge absorber and LA are connected. On some models of GCB, LA and capacitor on the transformer side are optional items and hence shall be confirmed at the time of ordering for GCB.

Fig 3-Generator Circuit Breaker as mounted on Generator-Transformer bus duct (Hitachi Energy)

4. Some years back, I had to answer a question from the field. A 100 MVA 220 / 33 kV transformer is to be test charged from the LV side using a 10 MVA 132/33 kV Transformer. The rated LV current of 100 MVA is 10 times the LV rated current of 10 MVA transformer. Inrush current multiples of both will be almost the same -10 times rated current. Will the massive inrush current drawn by a 100 MVA transformer kill the 10 MVA unit? Did anyone try this?