Medium-voltage electrical system protection

09/25/2013


Low-voltage switchgear

Industry standard protection schemes for the transformer secondary include a circuit breaker equipped with long-time, short-time, instantaneous, and ground fault functions.

NEC Articles 215.10, 230-95, and 240.13 require ground-fault protection for solidly grounded wye systems of more than 150 V to ground circuits, which includes 277/480 V “wye” connected systems. The ground fault relay or sensor must be set to pick up ground faults that are 1200 amps or more and actuate the main switch or circuit breaker to disconnect all ungrounded conductors of the faulted circuit at a maximum of 1 s.

For hospitals, the substation feeding the distribution system is typically liquid-filled MV primary to 480/277 V secondary transformers connected to service switchboards with both main and feeder breakers. The switchboards shall be equipped with two-level ground-fault detection in accordance with NEC Article 517.17(B). Article 517.17(B) requires that both the main breaker and the first set of OPD downstream from the main have ground fault. Additionally, the ground fault protection shall be selectively coordinated per NEC Article 517.17(C).

For emergency and legally required standby feeders, NEC Articles 700.26 and 701.26 require that the ground fault device shall be alarm only.

For normal side circuits upstream from an automatic transfer switch (ATS), ground fault protection is required per NEC Article 230.95.

Suggested settings are:

  • Device 51 or function long-time pick-up (LTPU): Recommend 100% to 125% of the transformer FLA and set below the transformer and cable damage curves.
  • Long-time delay (LTD), STPU, and short-time delay (STD): Set to coordinate with downstream devices and below the transformer damage curve.
  • Device 50 or instantaneous: Set below cable damage curve and must be above the maximum fault current at the breaker total clear curve. 

MV distribution system protection

With protection for MV transformers addressed, the next step is to connect several transformers in a distribution system and to a utility system. In distribution design, the three objectives still apply: 

  1. Life safety
  2. Equipment protection
  3. Selectivity.

For example, if the NEC requirements for transformer overcurrent protection are considered without reference to applicable standards and code requirements, the system may address protection of transformers, while other elements of the distribution system (such as the feeders connecting the transformer(s) to the distribution system) may not be protected in accordance with the code.

Article 450 is specific and limited to requirements for the transformer. Ampacity of MV conductors feeding to and extending from the transformer, as well as necessary overcurrent protection of the conductors and equipment, are covered under the following:

  • NEC Article 240-100 and 240-101 applies to MV overcurrent protection over 600 V for feeder and branch circuit.
  • NEC 310.60(C) and Tables 310.77 through 310 list ampacity of MV conductors 2001 to 35000 V.
  • NEC Art 210.9(B) (1) requires the ampacity of the branch circuit conductors shall not be less than 125% of the design potential load.
  • NEC Article 493.30 lists the requirement of metal-enclosed switchgear.
  • NEC Section II (Article 300.31 through 300.50) covers MV wiring methods.
  • NEC Article 310.10 requires shield MV cable for distribution above 2000 V.
  • NEC Article 490.46 MV circuit breaker shall be capable of being lock-out or if installed in a draw-out mechanism, the mechanism shall be capable of being locked.
  • NEC Article 215.2(B) (1) through (3) outlines the size of the circuit grounding conductors.
  • NEC Article 490 covers equipment, over 600 V nominal.

Cold load pickup is defined as follows: Whenever a service has been interrupted to a distribution feeder for 20 minutes or more, it may be extremely difficult to re-energize the load without causing protective relays or fuses to operate. The reason for this is the flow of abnormally high inrush current resulting from the loss of load diversity. High inrush current is caused by:

  • Magnetizing inrush currents to transformers
  • Motor starting currents
  • Current to raise the temperature of lamps and heater elements.

Figure 6: This 2.47 kV to 480 V pad-mounted oil transformer require MV primary and LV secondary basic overcurrent and ground fault protection. Courtesy: JBA Consulting EngineersPer NEC Art 240.101, the continuous ampere rating of a fuse shall not exceed three times the ampacity of the conductors, and the continuous ampere rating of a breaker shall not exceed six times the ampacity of the conductor.

In industry practice, a feeder relay setting of 200% to 400% of full load is considered reasonable. However, unless precautions are taken, this setting may be too low to prevent relay misoperation on inrush following an outage. Increasing this setting may restrict feeder coverage or prevent a reasonable setting of upstream or source side fuses and protective relays. A satisfactory solution to this problem is the use of extremely inverse relay curves. Extremely inverse relay setting is superior in that substantially faster fault clearing time is achieved at the higher current levels.

The problem of setting ground relay sensitivity to include all faults, yet not trip for heavy-load currents or inrush, is not as difficult as it is for phase relays. If the 3-phase load is balanced, normal ground currents are near zero. Therefore, the ground relay should not be affected by load currents. For balanced distribution systems, the ground relay can be set to pick up as little as 25% of load current. If the 3-phase loads are unbalanced, then the ground relay should be set to pick up at about 50% of load current.

Figure 6: A 12.47 kV service switchgear has incoming utility connection sections, a visible means of disconnect, metering, and fused sections. Courtesy: JBA Consulting EngineersUnder a fault condition, the fault current can easily exceed the capacity of the cable tape shield or concentric neutral ground; hence, a separate ground wire is necessary. For example, Southwire Co. has published tape shields fault current capacity to be 1893 amps at 12.5% tape overlap, and 2045 amps at 25% tape overlap. Most solidly grounded MV distribution system short circuit currents can be well above 10,000 amps. Furthermore, NEC Art 215.2(B) requires a separate ground to handle short circuit currents. The ground conductor is required to be sized per Table 205.122.

For the coordination schemes presented in the examples, the breaker or fuse trip curves did not overlap. In practice, there may be overlapping non-selective protective schemes. In cases involving redundant protective devices, nonselective breaker operation is of little or no concern. Protective devices are redundant—no matter which device opens, the same outage occurs. To improve overall system protection and coordination, redundant devices are intentionally set to overlap (i.e., non-selectivity coordinate with one another).

For MV systems that are more complicated, a system protection engineer should be consulted.


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. 


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