Siemens 6SR41 series - One Cycle Protection

Application and Operation
6.13 Input Side Monitoring and Protection
Product User Manual
Operating Instructions, Version AE 12/2009, A5E01454341C
137
Table 6- 4 List of Symbols Used in Figure "Block Diagram of Input-Side Monitoring"
Name Description
Er
ms
Average rms voltage (of all 3 phases)
E
d
Amplitude of voltage taking the transformer tap setting into account. This represents the actual
voltage being provided to the cells. If the tap setting is +5%, Ed will be 5% smaller than Erms, and
vice versa.
E
a,b,c
Zero sequence (DC offset) corrected input phase voltages
U Input frequency
U Angle of input-side flux
Ir
ms
Average rms current (in all 3 phases)
I
d
Real component of input current
I
q
Reactive component of input current
I
a,b,c
Single-phase components of input current
6.13.1 One Cycle Protection
NXG 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, 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
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 6-12 Implementation of One Cycle Protection