Application and Operation
6.13 Input Side Monitoring and Protection
Product User Manual
138 Operating Instructions, Version AE 12/2009, A5E01454341C
Transformer Model
The Transformer Model block in Figure "Implementation of One Cycle Protection" provides
the maximum value of the input reactive current for a given value of transformer constant,
Ktr, as given below:
I
Reactive,Max
= 1.10 * (0.05 + Ktr * I
Real
2
)
The following Figure shows a plot of the Max Reactive Current versus Real Current with a
transformer constant of 0.5.
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Figure 6-13 Plot of Max Reactive Current Versus Real Current with Transformer Constant of 0.5
Integral Timer
The integral timer gain can be calculated based on the desired response time (T
trip
) as
shown below:
I
gain
= T
trip
/ (Error * Slow_loop_sample_rate)
Where:
● Error is the maximum error (in per unit) that can be tolerated between I
Reactive,Max
and
actual reactive current I
reactive
● Slow_loop_sample_rate is the sample frequency of the slow loop (typically 450 – 600 Hz).
6.13.2 Excessive Drive Losses
NXG control utilizes input power and output power calculations to determine whether an
internal fault has occurred. Drive Power Loss is estimated as the difference between input
power and output power. This quantity is continuously checked with a pre-defined threshold
that is inverse time-based, i.e., if the threshold is exceeded by a large margin, then the trip
occurs a short time after the event, and vice-versa.
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