Medium-voltage breaker switchgear example (oil-filled power transformer)

Below is an example of a MV breaker switchgear feeding a 12 MVA power transformer.


Figure 2: Single line and time-current curves illustrate a typical transformer protection scheme for transformer greater than 3 MVA using MV breakers and relay protection. Courtesy: JBA Consulting EngineersFigure 2 illustrates a medium-voltage (MV) breaker switchgear feeding a 12 MVA oil-filled power transformer. It includes the primary and secondary protection of the transformer circuit along with its associated feeder cables. The transformer circuit consists of:

  • Originating source: Customer-owned 22 kV metal enclosed switchgear vacuum breaker
  • Secondary circuit breaker: 4.16 kV vacuum breaker within enclosed breaker switchgear
  • Primary protection: Device 50 and 51 3-phase, 50/51GP ground fault protection
  • Secondary protection: Device 50 and 51 3-phase, 50/51GP ground fault protection
  • Primary cable: Two #500KCM underground 25 kV EPR MV 105 cables in conduit
  • Secondary cables: Two #500 KCMIL underground 5/8KV EPR 133% insulation cables in conduit
  • Transformer rating: 12/13.4 MVA, 12.47 kV D to 4.16 kV U, Z=9%

The following protective features are illustrated:

  • Primary overcurrent curve is to the right of the transformer’s 12 times FLA inrush point and to the left of the cable damage curve.
  • For primary and secondary protection with a transformer impedance of 10% or less, the NEC requires that the primary breaker not be larger than 400% of primary FLA and the secondary 4.16 kV breaker must not be larger than 250% of secondary FLA.
    • Primary: The relay overcurrent is set at 800 amps, which is 125% continuous of the 13.2 MVA/636 FLA of the transformer—within NEC guidelines
    • Secondary: The relay overcurrent is set at 2000 amps, which is 110% continuous of the 13.2 MVA/1834 FLA of the transformer—within NEC guidelines
  • Both the primary and secondary cable damage curves are above the maximum fault current at 0.01 s.
  • The minimum clear time between the breakers is 0.312 s, which is greater than the minimum 0.2 s guidelines.
  • Primary and secondary ground fault protection is selectively coordinated. 

Leslie Fernandez is senior project engineer, electrical at JBA Consulting Engineers. He has more than 28 years of engineering and design and field experience that includes MV distribution systems for military, mining, tunneling, food manufacturing, power production facilities, high-rise facilities, and casino resort complexes. 

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