The IEC 60909 standard treats a Power Station Unit (PSU) in a different way as a separate generator and transformer. A combination of a synchronous generator and a machine transformer is considered as a single generating unit. In that case the generator and transformer are modelled jointly. Vision recognises this if the transformer is a "step-up" transformer. The calculation follows the method for an on-load tap-changer.
The IEC standard defines correction factors for the generator impedance and the transformer impedance. The definition of these correction factors depends on the short-circuit location:
•at the transformer high-voltage side
•at the transformer low-voltage side and the generator terminals
•at other locations inside the PSU
Vision detects the PSU through the step-up transformer with a generator connected to the low-voltage side. The step-up transformer can be specified using the "Step-up" check-box on the transformer tab "Connection".
The diagram from IEC-clause 4.2.1.3 can be used to further illustrate the method.
In this diagram transformer T is the step-up transformer.
In order to be able to recognise the combination of generator and transformer as part of a PSU, the transformer must be defined as a step-up transformer and the generator must be directly connected to the LV-node of the transformer.
IEC 60909 defines for the PSU:
•short-circuit at the step-up transformer's high-voltage side (Q)
•short-circuit at the step-up transformer's low-voltage side, also the generator node (F1)
•short-circuit at the step-up transformer's low-voltage side, inside the PSU, but not the generator node (F2 and F3)
Vision detects these situations as follows:
•If the short-circuit is located on the step-up transformer's high-voltage node, then the correction factors will be calculated for short-circuits on node Q.
•If the short-circuit is located on the step-up transformers's low-voltage node and the generator is connected to this node, then the correction factors will be calculated for short-circuits on node F1.
•If the short-circuit is located on a node with a rated voltage less than or equal to the voltage of the step-up low-voltage node of the transformer, then it will be concluded that the short-circuit is located inside the PSU and the correction factors will be calculated for short-circuits on nodes F2 or F3.
•If the short-circuit is located on a node with a rated voltage larger than the voltage of the step-up transformer's low-voltage node, then it will be concluded that the short-circuit is located outside the PSU and the correction factors will be calculated for short-circuits on node Q.
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