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.
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Figure 7-16
Implementation of One Cycle Protection
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
tr
, as
given below:
I
Reactive,Max
= 1.10 * (I
qMax
+ K
tr
* I
Real
2
)
The following figure shows a plot of the Maximum Reactive Current versus Real Current with a
transformer constant of 0.5.
Operating the Control
7.9 Input Side Monitoring and Protection
Control Manual
204
Operating Manual, A5E50491925A
Содержание SINAMICS PERFECT HARMONY GH180
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