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 Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Leaders Under 40 program features outstanding young people who are making a difference in manufacturing. View the 2013 Leaders here.
The new control room: It's got all the bells and whistles - and alarms, too; Remote maintenance; Specifying VFDs
2014 forecast issue: To serve and to manufacture - Veterans will bring skill and discipline to the plant floor if we can find a way to get them there.
2013 Top Plant: Lincoln Electric Company, Cleveland, Ohio
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant 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.

Bring focus to PLC programming: 5 things to avoid in putting your system together; Managing the DCS upgrade; PLM upgrade: a step-by-step approach
Balancing the bagging triangle; PID tuning improves process efficiency; Standardizing control room HMIs
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

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.