Protecting Transformer By Limiting Secondary Currents - Siemens SINAMICS PERFECT HARMONY GH180 Function Manual

Figure 7-17
Integral Timer
The integral timer gain can be calculated based on the desired response time (T
below:
I
gain
Where:
• Error is the maximum error (in per unit) that can be tolerated between I
reactive current I
• Slow_loop_sample_rate is the sample frequency of the slow loop, typically 450 to 900 Hz.
Note
Sampling Rate
If the sampling rate is below 4500, the slow loop is 1/5 of the sampling frequency (F
If the sampling rate is at 4500 or above, the slow loop is 1/10 of F
7.9.3

Protecting Transformer by Limiting Secondary Currents

Normally a GH180 drive is designed such that the transformer secondary windings cannot be
overloaded. However, in some instances drives have been provided with bypass, but not cell
redundancy. In this case bypass allows for continued drive operation, but the output power must
be reduced to limit the load on the transformer's secondary windings.
When a cell is bypassed, there are less cells available to supply the power to the motor. This
increases the power produced per cell. Also, the neutral point shift algorithm combined with the
motor's power factor shifts the distribution of power among the remaining cells. This means that
NXGPro+ Control Manual
Operating Manual, A5E50491925A
Maximum Reactive Current versus Real Current with Transformer Constant of 0.5
= T / (Error * Slow_loop_sample_rate)
trip
reactive
Operating the Control
7.9 Input Side Monitoring and Protection
Reactive,Max
.
samp
) as shown
trip
and actual
).
samp
205