Medium-voltage electrical system protection

Overcurrent protection is required for medium-voltage transformers, and connecting these transformers is common for medium-voltage distribution systems.

09/25/2013


Learning objectives

  1. Understand overcurrent protection requirements for medium-voltage distribution transformers.
  2. Understand overcurrent requirements for medium-voltage distribution.
  3. Learn about code and standard “minimums” that must be considered in the coordination of MV protective devices. 

Until recently, engineers didn’t work too frequently in the design of medium-voltage (MV) systems, mainly because anything over 600 V was primarily handled by the utilities. Exception included heavy electrical users such as government institutions, the mining industry, or industrial sites. However, in the past 15 years, there has been an explosion of MV electrical distribution systems used in large commercial complexes. Many of these complexes also have high-rise components with MV risers servicing unit substations at strategic locations on multiple levels. Another feature of large commercial complexes is the associated central plant function with MV chillers and unit substations.

The focus of this article is on overcurrent protection requirements for MV transformers, and connecting transformers to common MV distribution systems. MV designs are subjective and driven by the application. The intent is to illustrate code and standard “minimums” that must be considered in the coordination of MV protective devices. Sizing MV components such as motors, generators, transformers, wiring systems, the architecture of MV systems, or design of complicated protection schemes such as reclosers, zone interlocks, differential protection, etc., are all beyond the scope of this article.

Fundamental objectives

There are three fundamental objectives to overcurrent protection to include ground fault protection:

1. Safety: Personal safety requirements are met if protective devices are rated to carry and interrupt the maximum available load current as well as withstand the maximum available fault currents. Safety requirements ensure equipment is of sufficient rating to withstand the maximum available energy of the worst-case scenario.

2. Equipment protection: Protection requirements are met if overcurrent devices are set above load operation levels and below equipment damage curves. Feeder and transformer protection is defined by the applicable equipment standards. Motor and generator curves are machine specific and are normally provided in the vendor data submittal packages.

3. Selectivity: Selectively requirements are intended to limit system fault or overload response to a specific area or zone of impact and limit disruption of service to the same. Selectivity comprises two major categories:

a. Due to system operation limitations and equipment selection, selectivity is not always possible for non-emergency or optional standby systems.

b. NFPA 70: National Electrical Code (NEC) requires selectivity for:

i. Article 517.17(C): Hospital Ground-Fault Selectivity

ii. Article 700.27: Emergency Systems Coordination

iii. Article 701.27: Legally Required Standby Systems Coordination

Exception: NEC Article 240.4A and 695 allow conductors to be without overload protection where circuit interruption would create a hazard, such as fire pumps. Short-circuit protection is still required.

MV definition

MV is a term used by the electrical power distribution industry; however, various definitions exist.

IEEE 141 divides system voltages into “voltage classes.” Voltages 600 V and below are referred to as “low voltage,” voltages of 600 V to 69 kV are referred to as “medium voltage,” voltages of 69 kV to 230 kV are referred to as “high voltage,” and voltages 230 kV to 1,100 kV are referred to as “extra high voltage” with 1,100 kV also referred to as “ultra-high voltage.”

Per IEEE 141, the following voltage systems are considered MV systems:

 

Fuse manufacturer Littelfuse states in its literature that “The terms ‘medium voltage’ and ‘high voltage’ have been used interchangeably by many people to describe fuses operating above 600 V.” Technically speaking, “medium voltage” fuses are those intended for the voltage range of 2,400 to 38,000 Vac.

ANSI/IEEE Standard C37.20.2 - Standard for Metal-Clad Switchgear defines MV as 4.76 to 38 kV.

For this article, a good working definition of MV is 1 to 38 kVac as any voltage level above 38 kV is a transmission level voltage versus a distribution level voltage.

MV choice

The choice of service voltage is limited by the voltages the serving utility provides. In most cases, only one choice of electrical utility is available and typically there is limited choice of service voltage. As the power requirements increase, so too does the likelihood that the utility will require a higher service voltage. Typically, if the maximum demand approaches 30 MW, the utility typically may require an on-site substation. The norm, however, is that the utility will give several MV services that the engineer will need to integrate into an owner’s MV distribution system. 

In some cases, the utility may provide options for the service voltage. In this case an analysis of options must be conducted to determine the best option for the project. In general, higher service voltage results in more equipment expense. Maintenance and installation costs also increase with higher service voltages. However, for large-scale developments, equipment such as large motors may require a service voltage of 4160 V or higher. Typically, service reliability tends to increase as service voltages increase.

When connecting to an existing utility, the utility typically directs the interconnection requirements including protective device requirements. The utility will include required setting parameters and limitations based on the manufacturer for the protective devices.


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