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

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

09/25/2013


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. 



No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Additive manufacturing advancements; Machine vision enhances robotics; Fieldbus evolution; Process safety; Advice from System Integrators of the Year; Road to IANA
Salary and career survey: Benchmarks and advice; Designing controls; Remote data collection, historians; Control valve advances; Hannover Messe; Control Engineering International
System integration: Best practices and technologies to help; Virtualization virtues; Cyber security advice; Motor system efficiency, savings; Product exclusives; Road to Hannover
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
PLCs, robots, and the quest for a single controller; how OEE is key to automation solutions.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
click me