1.10.16

Sizing the Taps for Transformer Test Sets Load Loss and Heat Run Tests

Load loss and Heat Run Tests are performed by short-circuiting the low voltage windings of a transformer and energizing the transformer by applying voltage to the high voltage windings. The test is done this way because this means that the test set current rating can be lower but with a higher voltage than would be necessary if the test was done the other way around. When these tests are done, the full rated current of the LV windings is circulated and these current levels can be very high. To short-circuit the LV windings a buss bar is used to carry the very high current with minimal heat losses.

A Heat Run test requires the current to be increased even higher than rated value to simulate the heat generated in the magnetic core since the transformer is not at rated voltage during a heat run or load loss test. This means the test set must have a voltage tap rating higher than the actual current from the nameplate.

The test sets for LL and Heat Runs can also do the basic excitation tests An Example: 30 MVA 115kV to 13.8 kV with 9% Impedance HV winding is 115kV at 150 amps LV winding is 13.8 kV at 1255 amps The impedance voltage drop for the HV winding is 9% of 115 kV 10.35 kV Generally the copper losses in a power transformer are <2% of rated power For 30 MVA transformers that means we need 600kW of test power plus kVA to overcome the impedance losses. This means that we need a tap at 10.35 kV to cover the impedance voltage drop at rated current and 150 A to circulate rated current. This means the power for the test is 2.7 MVA. Of that we would supply at least 600kW but more likely a 1 MVA power regulator from the grid and 1.7 MVAr of capacitors. Since heat runs would be done too, we need to increase the rating by 1.1 to 1.5 to increase the current during the test which also then requires the tap voltage to be higher too. To figure out the taps of the test set, the real power and the reactive capacitive power we need to know all the HV winding voltages (we really don't care about the LV windings because they are always shorted out) the power rating for every one manufactured at that voltage and the highest impedance for EACH power and voltage rating.

Induced Voltage Tests Induced voltage tests are done at elevated frequency to increase the voltage that the transformer operates at. Mostly these tests are done below 200 Hz to eliminate the effects of the core losses but not too high to add capacitive loading to the circuit. During the induced test the transformer load appears capacitive and this capacitance load can be cancelled by inductors in the output of the test set just as we have capacitors in the output of the test set for load loss testing at power frequency. A general rule of thumb can be use to size the source power for induced tests. A value of 1-2 % of the nameplate is sufficient to energize to 150 to 200 % of rated voltage. The difficulty is to determine the ratio of real power required to reactive compensation. Example 30 MVA, 3 phase HV transformer 115 class 13.8 kV and 9% impedance 2% of nameplate is 600 kVA. For this example a 300 kW variable frequency source plus 300 kVA of compensation reactor can be used. The problem then is to decide on the voltage taps for the step up transformer. In this case the voltage ratio is not linear with frequency so the input voltage to create 115kV x 2 is not two times the 13.8 kV input rated LV winding it is usually less. General Comments The AC source for load losses can be supplied by a variable transformer from the line or from a VF source. If a VF source is used it may be larger than is necessary for the induced testing but the cost of a separate voltage regulator normally still makes a single VF source less expensive overall. Sizing the taps of the step-up transformer and the capacitor banks requires complete range information of the transformers being produced Reactors for Induced testing and transformer tap voltages are hard to size based on normal nameplate data. Prior test data is the best way to know the proper sizing otherwise it is an educated guess. The default setup is to calculate the taps for the step-up transformer based on load loss requirements and hope those taps will cover induced testing too. Of course additional capacitors or inductors can always be added to the system to increase the capacity. With our VF sources the real power of the test set can always be increased in blocks too.