What level of transformer (T/F) inrush current should be considered when nominating a generator to power a large T/F load
Most texts books advise that transformer inrush current can be as high as 10 times the transformer’s rated current (In) and furthermore will have a high harmonic content during this period. This advice is correct for when switching a transformer onto a mains supply where the source impedance of the supply is low. A much different inrush characteristic prevails when the transformer is being supplied from a generator where the source impedance is much higher.
When using a generating set, experience and available test data suggests that an unloaded transformer will typically ‘pull’ something in the order of 0.75 < 1.25 x the transformer’s name plate advised full rated kVA. (In x Vn x 1.732)
The variation about In of +/- 0.25pu is a factor based on whether the freshly applied voltage sine wave is in-line with, or opposing, the magnetic direction of the transformer’s magnetic cores ‘magnetic remanence alignment’ remaining from the when the transformer was last switched off.
The duration of this much lower inrush is over a longer time period; the same amount of magnetisation-work needing to be completed to allow the transformer to stabilise with the secondary voltage at the designed level. The transformer will likely be magnetised by a mains supply over a period of 10<15 cycles (<0.25s) the generator may take a period of 1<2 seconds to settle the transformer.
The inrush current content is swamped by kVAr as the current is driving the magnetics with Ampere-Turns. The copper losses within the transformer’s magnetising winding are present but constitute perhaps a peak momentary kW demand of only 25% of the transformer’s full rated kW. The harmonic distortion content is not usually an issue at these low magnetising levels. Both the above points do need to be considered as they may combine to trip a generators output breaker.
Once the transformer has settled the input to the transformer will be at a kVA level of typically 5<7% of the transformer’s full load rating.
In the case of sizing a generating set for a transformer related application then duly considering the above information will enable the expected peak inrush kVA can be identified. The next challenge is to ensure the level of TVDip% this step load imposes on the generating set is not too great and triggers the activation of an under-voltage trip or ‘disturb’ any already running loads.
Here the key consideration will be the ac generator’s excitation system and its related forcing capability. The acceptable sizing rule for one transformer powered by one generator is to select a ‘good’ excitation system (PMG or Auxiliary winding based – not shunt) and size the generator for the 1.25 x transformer kVA, and have this at the gen’s class ‘F’ rating. This sizing allows for the occasion when the transformer finds itself unexpectedly being magnetised with a small level of load still connected to the transformer’s secondary winding. With the transformer inrush being mostly kVAr, there is no real need to worry about engine load acceptance capability.
AMPS member companies can provide detailed performance of ac generators, their related excitation systems and performance capabilities; even provide a sizing service for troublesome applications.