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Hi,
You should normally follows the intructions given in the technical manual of the relay.
You will find enclosed some generic guidelines.
The following example represents a case at 100 times the CT rated current.
All currents are in per unit and the red curve is the primary current
At such high level of current, you will get the following issues:
- the CT secondary current (green) is highly saturated
- its magnitude is reduced compared to the primary current (RMS secondary current in brown)
- The A/D converter clips the measured values due to its limited measuring range (sample value in pink)
- The relays generally operates based on the fundamental component (blue) that appears to be a small fraction (15% in this example) of the primary current, however the measured value remains quite high (15 InCT)
- If the overcurrent is directional, the angle error is almost 90°
The thermal withtand of the CTs, wire and relays need to be checked as well.
Link copied. Please paste this link to share this article on your social media post.
Hi,
You should normally follows the intructions given in the technical manual of the relay.
You will find enclosed some generic guidelines.
The following example represents a case at 100 times the CT rated current.
All currents are in per unit and the red curve is the primary current
At such high level of current, you will get the following issues:
- the CT secondary current (green) is highly saturated
- its magnitude is reduced compared to the primary current (RMS secondary current in brown)
- The A/D converter clips the measured values due to its limited measuring range (sample value in pink)
- The relays generally operates based on the fundamental component (blue) that appears to be a small fraction (15% in this example) of the primary current, however the measured value remains quite high (15 InCT)
- If the overcurrent is directional, the angle error is almost 90°
The thermal withtand of the CTs, wire and relays need to be checked as well.
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