How does Very High Fault Current impact EHV protection systems?

Bynitinnarvekar9

High fault current creates several problems in power system protection and equipment design.

1. CT’s require higher Knee Point Voltage
Depending on the substation’s specific X/R ratio, the fault current contains a large, decaying DC component. This unidirectional offset drives standard current transformer (CT) cores into deep saturation within a fraction of a cycle.
When fault current increases, CTs require higher knee point voltage and transient performance , thereby increasing CT cost.

2. Circuit breakers require higher interrupting capacity
When fault current increases, circuit breakers must interrupt much larger current safely.
Therefore higher interrupting capacity CBs are required which increases its size and cost.

3. Mechanical Shock (Electrodynamic Forces):
Current flowing through parallel conductors creates mechanical forces proportional to the square of the current .
A jump from 10kA to 40kA increases electrodynamic mechanical stress on transformer windings and busbar supports by 16 times. 

4. Transient overvoltage across transformer winding increases
High fault current produces high transient recovery voltage (TRV) across transformer windings and switchgear during fault interruption.
This can stress transformer and switchgear insulation and increase risk of insulation failure.
A Neutral Grounding Resistor (NGR) will be required to limit earth fault current and reduce transient overvoltage.
The use of NGR limits earth fault current, which can reduce relay sensitivity, complicate protection coordination, create thermal limitations, and increase touch and step potential during earth faults.

💡 Key Takeaway:
Severe fault currents transform standard protection scenarios into fast transient events.
High short-circuit current not only increases thermal stress, but can also:

  • create massive electrodynamic forces,
  • deform transformer windings and busbar supports,
  • increase CB duty and generate severe TRV stress across circuit breakers.
  • require CTs with higher knee point voltage and transient performance, increasing CT cost

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