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20 MVA Generating Station – Transient Recovery Voltage Study

Home » 20 MVA Generating Station – Transient Recovery Voltage Study

Key Takeaways

  • Why TRV (Transient Recovery Voltage) can challenge hydro station breakers during faults and switching.

  • How simulation with EMTP-RV compared two upgrade scenarios: general-purpose vs. generator-duty breakers.

  • Why direct installation of surge arresters on generator terminals provides the best insulation protection.

  • The critical role of surge capacitors and arresters in meeting TRV compliance.

  • Why breaker procurement specs must demand proven TRV interruption capability from suppliers.

⏱ Estimated reading time: 5–6 minutes.

 

  1. Background

The switchgear of a 20 MVA hydroelectric generating station in British Columbia underwent an upgrade that included replacement of the existing station service and generator circuit breakers (CBs). In this context, WiseGrid’s experts performed a Transient Recovery Voltage (TRV) study to evaluate the required TRV capability of the future station service and generator medium-voltage circuit breakers.

This study was conducted in accordance with:

  • IEEE C37.011-2019 – Application Guide for TRV for AC High-Voltage Circuit Breakers
  • IEEE C37.04-2018 – Rating Structure for AC High-Voltage Circuit Breakers
  • IEC/IEEE 62271-37-013-2021 – AC Generator Circuit-Breakers

The primary objective was to determine the required TRV envelope and delayed zero-crossing time for the new breakers under various system fault and switching conditions.

  1. Study Methodology

The system including generators, circuit breakers, transformers, cables, and the equivalent utility network was modeled in EMTP-RV. Two main upgrade scenarios were analyzed:

  1. Scenario A – Retain existing general-purpose breakers
    • Breakers: 15 kV, 40 kA
    • Mitigation: Install 15 kV (12.7 kV MCOV) surge arresters and 0.15 μF surge capacitors on the transformer side of the main breakers.
    • Outcome: TRV issues resolved for all breakers (generator, station service, and main).
  2. Scenario B – Replace with generator-type breakers
    • Breakers: 15 kV, generator-duty type
    • Mitigation: Install 15 kV (12.7 kV MCOV) surge arresters on the transformer side of the main breakers.
    • Outcome: TRV issues resolved for all breakers.
  1. Technical Recommendations
  • Breaker Procurement

The technical specifications for the generator and main breakers must require suppliers to demonstrate that their proposed breakers can successfully interrupt under the modeled TRV and delayed zero-crossing conditions (per IEC/IEEE 62271-37-013 and IEEE TRV guidance).

  • Surge Protection

Based on insulation coordination results, all generators should be equipped with 15 kV (12.7 kV MCOV) surge arresters installed directly on the generator terminals to maximize surge suppression performance.

    • This location is superior to installing arresters on the generator side of the breaker switchgear, as the latter results in degraded protection.
  • Temporary Overvoltage (TOV) Considerations

The expected system TOV during ground faults and load rejection events must remain within the TOV capability of the proposed surge arresters.

  • Standards Compliance

The proposed surge suppression arrangement, while significantly improving TRV performance, does not provide full compliance with IEEE C62.21. The arrester voltage rating cannot be lower than 15 kV.

  1. Simulation Cases

TRV simulations were conducted for the following operating conditions:

  1. Combined source fault (generator + system)
  2. System source fault
  3. Generator source fault
  4. Out-of-phase fault
  5. Load switching
  6. System source fault
  7. Generator source fault
  8. Out-of-phase fault
  9. Load switching
  10. Combined source fault (generator + system)
  11. System source fault
  12. Generator source fault

Some waveforms of the simulations:

 

 

TRV waveform for case 1 (combined source fault at the terminal of the breaker)

 

Zoomed TRV waveform for case 1 (combined source fault at the terminal of the breaker)

 

 

Breaker delayed current zero during unloaded operation

Scenario 2 – 1.2 μs / 50 μs 60.2 kV peak impulse

 

 

  1. Conclusions
  • Without mitigation, the TRV requirements exceed the capability of the existing general-purpose breakers.
  • Both surge arresters and, in some cases, surge capacitors are required to ensure TRV compliance.
  • Direct installation of surge arresters on generator terminals is strongly preferred to maximize insulation protection.
  • The breaker specification process must include demonstrated TRV interruption capability from the supplier.