GENERATION

SaskPower Plant Designed for "Black Start" Capability

The question is: Can a small generator furnish full magnetizing current to a large transformer during a black start, without being tripped?

Black Start

A new power plant built by SaskPower was designated as 'a black start capable'. The most recent blackout occurred about 5 years ago when a large part of the provincial power system collapsed due to a severe winter storm. During such situations, the power plant relies on its own emergency power sources to initiate power restoration. The new plant included a 500kW diesel to enable power system restoration following a blackout. The unit is capable of running the station service load and starting up one gas turbine generator.

Typically, the emergency lighting fed from the plant batteries comes on immediately. This is followed by an automatic start of the diesel engine within 15 seconds from the onset of a power outage. The delay is necessary in order to ascertain validity of the outage.

Having restored power to the station services, one gas turbine generator is started up and connected to the main transformer. This critical phase, if successful, is followed by adding load to the transformer and the transmission system to complete the power restoration.

Plant Configuration

The new plant as shown in Fig.1 comprises six relatively small gas turbine generators connected to the low voltage windings of a large step-up transformer. Each unit is connected through its own breaker. During a black start the transformer is energized from its low voltage side instead of high voltage side as is the case during normal power plant operation and synchronization. Black start is generally not a big concern for power plants. However, in this case, since the MVA ratio of generator to transformer (26.9/102MVA) is relatively low, there was a concern that the generator may not be able to provide the full magnetizing current to the transformer in order to establish the nominal voltage at its terminals. Consequently, the generator may overexcite at the instant of breaker closing and trip by its voltage restraint overcurrent relay.

Transformer Magnetizing Current

Pauwels, the transformer manufacturer was contacted to provide details on the expected magnetizing current, for the condition of 100 per cent and 20 per cent voltage applied to the Y winding while the windings X and Z were held open. The transformer is of the core form type. The expected magnetizing currents (Peak, RMS and Average) were calculated based on the known formulae (1,2) and are shown on Graphs 1 and 2. While the steady state magnetizing current is small, the peak inrush current is large even at 20 per cent voltage and can cause problems with the generator protection.

The flux required by a transformer is equal to the normal steady state flux plus a DC transient component. The initial transient current depends upon the magnitude of the supply voltage at the instant the transformer is energized, the residual flux in the core, and the impedance of the supply circuit. If the unit is energized as the voltage passes through zero, the transient is at its maximum. The current is further magnified by the residual flux, which is estimated to be in the order of 70 to 80 per cent of the nominal flux.

In the worst case scenario, a value of flux approaching 300 per cent of nominal can be attained, thus causing appreciable saturation of the magnetic core and a massive inrush magnetizing current in the order of 8 to 10 times full load current. The transient component decays rapidly during the first few cycles and more slowly thereafter. The damping is caused by the R/L factor, which is initially high due to the core saturation (L low) and decreases as the core becomes less saturated. Finally, the inrush current is limited by the source impedance, the generator impedance in this case, according to the equation;

Ii = Iio / (1+Iio*Xs), where;

Iio is the inrush current neglecting the supply source impedance.
Xs is the effective source supply impedance in per unit on the transformer kVA base.

Results

In order to limit the inrush current, it was decided to close the field breaker with a 20 per cent voltage. The voltage was gradually increased to 100 per cent over a period of 120 seconds. The value of the remnant flux in the core was not known. Furthermore, there were no means available of knowing and/or controlling the timing of the field breaker closure with respect to the magnitude of the supply voltage on its sine curve.

Graph 3 captured by a PML, model 7600 meter, shows the results of the test. The meter samples 128 samples/cycle and calculates various RMS values every 1/2 cycle.

The black start test and the procedures implemented were successful. The inrush current, dictated mainly by the transformer remnant flux, was not excessive and was lmited by the generator source impedance. The generator GTG 4 successfully energized the main step-up transformer to its full voltage without tripping and proved that the new plant in present configuration is 'black start capable'.

References:

[1] Blume L.F. et al, Transformer Engineering, J.Wiley & Sons, Inc., New York 1959
[2] Electrical T & D, Ref. Book by Westinghouse Electric Corp., Pittsburg, PA, 1964

Zark Bedalov is Principal Electrical Engineer with SNC-Lavalin, Vancouver, Naval Tauh is T & D Engineer with Saskpower, Saskatoon. Waldemar Ziomek is Electrical Engineering Manager with Pauwels Transformers, Winnipeg. ET