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
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Product of the Year; Diagnose bearing failures; Asset performance management; Testing dust collector performance measures
Safety for 18 years, warehouse maintenance tips, Ethernet and the IIoT, GAMS 2016 recap
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
SCADA at the junction, Managing risk through maintenance, Moving at the speed of data
Safety at every angle, Big Data's impact on operations, bridging the skills gap
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role of plant safety and offers advice on best practices.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me