Medium-voltage fused switchgear example

This example illustrates the results from an MV fused switchgear feeding a pad-mounted or dry-type transformer.

09/25/2013


Figure 1 illustrates medium-voltage (MV) fused switchgear feeding either a pad-mounted or a dry-type transformer. The NEC does not require the secondary overcurrent device to be close coupled. Where the primary and secondary overcurrent devices are located remotely, consideration must be given to protection of both feeder conductors between the transformer terminals and the terminals of the overcurrent devices.

Figure 1: The single line diagram and time-current curves illustrate a typical transformer protection scheme using primary MV fuses and low-voltage secondary breaker protection. Courtesy: JBA Consulting Engineers

Figure 1: The single line diagram and time-current curves illustrate a typical transformer protection scheme using primary MV fuses and low-voltage secondary breaker protection. Courtesy: JBA Consulting EngineersThe transformer circuit consists of:

  • Originating source: Customer-owned 12.47 kV metal fused switchgear
  • Primary fuse: 100E
  • Secondary circuit breaker: 2000 amp
  • Primary cable: #2-15 kV EPR MV 105 cables
  • Secondary cables: 9 sets of underground cables of 500 KCMIL per phase
  • Transformer rating: 1500 kVA, 12.47 kV D to 480 VU, Z=5.75% 

The following protective features are illustrated:

  • The 100E fuse curve is to the right of the transformer 12 times FLA inrush current point and to the left of the cable damage curve.
  • For a transformer of 6% impedance or less, the NEC requires both primary and secondary protection. The primary fuse must not be larger than 300% of primary FLA and the secondary 480 V overcurrent device must not be larger than 125% of secondary FLA.
    • Primary: The continuous rating of a 100E fuse is 130 amps; the FLA of a 1500 kVA transformer is 70 amps, which is 180%—well within the NEC requirements.
    • Secondary: A 2000 amp main breaker is 111% of the transformer FLA.
  • Both the primary and secondary cable damage curves are above the maximum fault current at 0.01 s.

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. 



No comments
Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
Pump technology design: Selecting pumps and pipes; Class A office buildings; NFPA 101-2015; ASHRAE 90.1 updates; Motors, drives, and HVAC efficiency
Renewable power systems: Designing a net zero energy building; NFPA 37; Data center design; Lighting systems
Salary survey: How much are you worth?; Dedicated outdoor air systems; Energy models and lighting
Understanding transfer switch operation; Coordinating protective devices; Analyzing NEC 2014 changes; Cooling data centers
Designing positive-energy buildings; Ensuring power quality; Complying with NFPA 110; Minimizing arc flash hazards
Implementing microgrids: Controlling campus power generation; Understanding cogeneration systems; Evaluating UPS system efficiency; Driving data center PUE, efficiency
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.