Medium-voltage breaker switchgear example (800 amp service)

Below is an example on an MV breaker switchgear feeding a loop distribution system.

09/25/2013


Figure 5: Time-current curves illustrate a typical transformer MV distribution protection scheme with six loop-fed transformers. Courtesy: JBA Consulting EngineersFigure 5 illustrates a medium-voltage (MV) breaker switchgear feeding a loop distribution system with six-unit substation 1500 kVA transformers. It also includes two utility 800 amp service, main-tie-main MV metal clad switchgear, and 400 amp loop distribution feeders. The distribution circuit consists of:

  • Utility sources: Two 12.47 kV 800 amp service
  • Incoming mains: 1200 amp 15 kV 750 MVA vacuum breakers
  • Incoming mains CTs: 1200 amps (1.5 x 800 amps)
  • Loop circuit breakers: 1200 amp 15 kV 750 MVA vacuum breakers
  • Loop distribution wiring: U/G 3 to 500 KCM with #2 copper-ground wire
  • Loop circuit CTs: 600 amps (incoming main CTs: 600 amps (1.5 x 400 amps)
  • Main and feeder protective relays: SEL 351 with 50 and 51 3-Phase, 50/51 GP ground fault

The following protective features are illustrated:

  • Loop overcurrent curve is to the right of the combined six transformer inrush point; under emergency conditions, either breaker can feed all six transformers. Under normal conditions, each loop circuit breaker will serve half of the loop’s transformers.
  • The loop overcurrent curve is to the left of the cable damage curve.
  • NEC Art 210.9(B) (1) requires the loop ampacity of the conductors shall not be less than 125% of the design load. The design is for 400 amp distribution, which should be sufficient for six transformers loaded to 90% or 375 amps. Per Table 310.609(C)(75) 500 KCM MV 105 cable is rated for 525 amps, which equates to 133%—well within the NEC requirements.
  • The minimum clear time between the main breakers and loop breakers is 0.312 s, greater than the minimum 0.2 s guidelines.

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...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
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.
Learn how to create value with re-use; gain productivity with lean automation and connectivity, and optimize panel design and construction.
Go deep: Automation tackles offshore oil challenges; Ethernet advice; Wireless robotics; Product exclusives; Digital edition exclusives
Lost in the gray scale? How to get effective HMIs; Best practices: Integrate old and new wireless systems; Smart software, networks; Service provider certifications
Fixing PID: Part 2: Tweaking controller strategy; Machine safety networks; Salary survey and career advice; Smart I/O architecture; Product exclusives
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Look at the basics of industrial wireless technologies, wireless concepts, wireless standards, and wireless best practices with Daniel E. Capano of Diversified Technical Services Inc.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.

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

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.