Siemens SINAMICS PERFECT HARMONY GH180 Function Manual page 208

Operating the Control
7.9 Input Side Monitoring and Protection
as a result of bypass, the transformer's secondary winding currents are increased when the same
power is applied to the motor.
Since the actual power of the transformer secondary windings cannot be measured directly, an
approximation has to be made. The algorithm calls for the live calculation of the per-phase
instantaneous output power.
The per-phase power is divided by the number of active cells in the phase, to obtain a cell power
load on the associated secondary windings. This is then compared to the rated secondary
winding rating to determine if an overload exists. Additional scale factors for water-cooled (W/
C) and air-cooled (A/C) transformers are used for harmonic loading, but are different, to allow for
different architectures of the transformers.
The parameter "Harmonic Load Factor" (2024) specifies the amount of harmonic loading which
is applied to the transformer secondary winding as a function of winding power
This load factor cannot be determined by the manufacturer since they build and test with
sinusoidal waveforms, and is therefore specified by the engineering specs when ordered.
Nominally, it will be 1.12 for water-cooled transformers, and 1.2 for air-cooled transformers.
The "Xfrm Secondary Alarm" is set after one minute of the transformer secondary protection
rollback being active. This same alarm text and flag used when the rollback is disabled to indicate
the transformer secondary windings are overloaded. There is no fault associated with
transformer secondary winding protection when rollback is enabled.
Because the drive output power is proportional to the product of speed and torque, it becomes
impossible to overload the transformer below approximately 50% rated speed. This applies to all
load types. The rollback function will revert to rated torque below this speed – no reduced torque
limit. The torque current will be limited to 5% below the fault level as the smaller value during
rollback for the same reason.
Rollback Removal
"Power Rollback Enable" (7114) may be set to disable to prevent the transformer secondary
protection algorithm from affecting the torque delivered to the motor
If rollback is disabled, the transformer secondary windings will continue to be protected via an
alarm and fault. This is accomplished by adding two comparators to the output of the power
error integrator. One is used to set an alarm, and the other causes a trip. These levels are set as
0.85 and 0.70 PU Max cell power respectively. These were based on requirements needed to
protect the transformer from running continuously in this marginal power situation.
Rollback Algorithm Operation
Enter "Rated Secondary Power" (2022) according to the transformer nameplate or from the
Siemens engineering group, and then enter the appropriate "Harmonic Load Factor" (2024) for
the type of transformer (1.12 for water cooled or 1.2 for air cooled) unless another value is
available. These values are used along with the total of installed cells to calculate the
Transformer Cell Power Rating.
"Full Load Current" (1050) and "Cell Voltage" (2550) are also used to transition this power rating
to the same as used on the output of the drive, and so must be properly entered.
The instantaneous output power per phase is calculated, and scaled for cell rated output power.
This provides the cell power rating. The maximum value is subtracted from the rated value, and
if negative, is used to drive an integrator toward the rated value. The output is used as one of
many torque limits of which the lowest is applied to the speed regulator output.
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NXGPro+ Control Manual
Operating Manual, A5E50491925A