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...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
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 benefits; HMI and sensor tips; System integrator advice; Innovations from the industry
Robotic safety, collaboration, standards; DCS migration tips; IT/OT convergence; 2017 Control Engineering Salary and Career Survey
Integrated mobility; Artificial intelligence; Predictive motion control; Sensors and control system inputs; Asset Management; Cybersecurity
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This article collection contains several articles on how automation and controls are helping human-machine interface (HMI) hardware and software advance.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.

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

Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me