Siemens SINAMICS PERFECT HARMONY GH180 Function Manual page 206

Operating the Control
7.9 Input Side Monitoring and Protection
The control utilizes input reactive current to determine whether a "hard" fault on the secondary
side of the transformer has occurred. For example, a short circuit in one of the secondary
windings will result in poor power factor on the high voltage side of the transformer. A model
of the transformer, based on the power factor at rated load (typically 0.95), is implemented in
the control processor. The drive input reactive current is continuously checked with the
predicted value from the model. An alarm/trip is generated if the actual reactive current exceeds
the prediction by more than 10%. This check is avoided during the first 0.25 seconds after
medium voltage power-up to avoid the inrush current from causing nuisance trips.
Implementation
The following figure shows the implementation of one cycle protection.
Figure 7-16
Transformer Model
The transformer model in Figure Implementation of One Cycle Protection provides the
maximum value of the input reactive current for a given value of transformer constant, K
given below:
I
Reactive,Max
The following figure shows a plot of the Maximum Reactive Current versus Real Current with a
transformer constant of 0.5.
204
Implementation of One Cycle Protection
= 1.10 * (I + K * I
qMax tr Real
)
2
tr
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Operating Manual, A5E50491925A
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