Medium-voltage electrical system protection


MV transformer protection

For discussion purposes, consider an appropriately sized transformer with a known rating. To be clear, a properly sized and rated transformer includes the following features:

  • Adequate capacity for the load to be served
  • Adequate temporary overload capacity (kVA size, or ratings)
  • Primary and secondary voltages properly rated for the electrical distribution system
  • Whether liquid filled or dry type transformers were correctly selected for the application.

The 2011 NEC requires that transformers be protected against overcurrent (NEC Article 450.3). Furthermore, NEC Article 450.3(A) specifically covers transformers over 600 V to include MV transformers.

Figure 4: This illustrates the applicable NEC articles that apply to MV service, feeders, and distribution equipment. Courtesy: JBA Consulting EngineersThree-phase MV transformers are required to be provided with both primary and secondary overcurrent protective devices (OPD) mainly because the primary and secondary conductors are not considered protected by the primary overcurrent protection. This is especially true for delta primary and wye secondary, where a secondary ground fault may not trip the primary protection. NEC Article 240.21 (C)(1) and NEC Article 450.3(A) validate this statement is true.

Although the primary windings are rated for MV, the designer must choose either fuses or circuit breakers to protect the transformer. As a general rule, 3000 kVA and smaller transformers installed as a stand-alone unit or as unit substations are usually protected by fuses. MV breaker protections are used for transformer sizes greater than 3000 kVA.

Unlike fuses and typical 600 V circuit breakers, MV circuit breakers rely on separate devices such as current transformers (CT), potential transformer (PT), and protective relays to provide the overcurrent protection. The majority of modern relays are multifunction type with the protection referred to by numbers that correlate with the functions they perform. These numbers are based on globally recognized IEEE standards as defined in IEEE Standard C37.2. A sample of a few of the protective function numbers that are used in this standard are shown in Table 1.

Table 1: These numbers are based on globally recognized IEEE standards as defined in IEEE Standard C37.2. Courtesy: JBA Consulting EngineersFigure 3: A single-line drawing illustrates the application of IEEE Standard C37.2 device numbers. Courtesy: JBA Consulting Engineers

Several factors influence the settings of transformer protection:

  • The overcurrent protection required for transformers is considered protection solely for the transformer. Such overcurrent protection does not necessarily protect the primary or secondary conductors, or the equipment connected on the secondary side of the transformer.
  • It is important to note that the overcurrent device on the primary side must be sized based on the transformer’s kVA rating and based on the secondary load to the transformer.
  • Before determining the size or rating of the overcurrent devices, observe that Notes 1 and 2 of NEC Table 450-3(A) permit the rating or setting of primary and/or secondary OPD to be increased to the next higher standard or setting when the calculated value does not correspond to a standard rating or setting.
  • When voltage is switched on to energize a transformer, the transformer core normally saturates, which results in a large inrush current. To accommodate this inrush current, overcurrent protection is typically selected with time-current withstand values of at least 12 times the transformer primary rated current for 0.1 s and 25 times for 0.01 s.
  • Engineers should ensure the protection scheme settings are below the transformer short-circuit damage curves as defined in ANSI C57.109 for oil-filled power transformers and ANSI C57.12.59 for dry-type power transformers.
  • Protective relay curves cannot be used in the same way as low-voltage circuit breaker curves or fuse curves. The protective relay curve only represents the action of a calibrated relay and does not account for the actions of the associated circuit breaker or the accuracy of the current transformers that connect the relay to the monitored circuit. The curve represents the ideal operation of the relay, and the manufacturing tolerances are not reflected in the curve. To coordinate an overcurrent relay with other protective devices, a minimum time margin must be incorporated between the curves. IEEE Standard 242 Table 15.1 recommended relay time margins are in Table 2.

Fuses and switchgear

E-rated power fuses are typically used in fused switches serving transformers. The purpose of the fuse is to allow for full use of the transformer and to protect the transformer and cables from faults. To accomplish this, the fuse curve should be to the right of the transformer inrush point and to the left of the cable damage curve. Typically, the fuse will cross the transformer damage curve in the long time region (overcurrent region). The secondary main device provides overcurrent protection for the circuit. “E” fuse ratings should always be greater than the transformer full load amps (FLA). The cable damage curve must be above the maximum fault current at 0.01 s.

For transformers 3 MVA and less, standard overcurrent protection schemes for MV switchgear breakers should include an instantaneous and overcurrent combination relay (device 50/51).

For transformers greater than 5 MVA, protection schemes become more complex. IEEE device numbers from IEEE C37.2 are used to outline the protection scheme. Transformer MV breakers may include the following protective device numbers:

In MV systems, current transformers (CTs) connect protective or metering devices. CTs interface the electronic device and the MV primary system. The MV primary system voltage and current levels are dangerously high and cannot be connected directly to a relay or meter. The CTs provide isolation from the cable’s high voltage and current levels and translate the primary current to a signal level that can be handled by delicate relays/meters. The rated secondary current is commonly 5 amp, though lower currents such as or 1 amp are not uncommon.

Protective relay’s CTs are expected to deliver about 5 amps or less under healthy load conditions.  The current will go to a high value when a fault occurs. Per ANSI C57.13, normal protective CT class secondary should withstand up to 20 times for a short period of times under fault conditions.  As a consequence, protective class CTs are accurate enough to drive a set of indication instruments, but will not be good enough for revenue class summation energy metering.

Other factors to consider:

  • CTs for protective relaying should be sized 150% to 200% of the full load amperage (FLA).
  • Unlike low-voltage breakers and fuses, MV circuit breakers do not have fixed trip. Settings do not correspond to those listed as standard in the NEC [NEC Article 240-6(a)].
  • Overcurrent, 51 device, should be set at 100% to 140% of FLA and set below the transformer cable ampacity.
  • Time dial should be set below the transformer damage curve and above the secondary main breaker device.
  • Instantaneous trip, 50 device, should be set below the transformer damage curves, below the cable damage curve at 0.1 set, and approximately 200% of inrush. In addition, the engineer must ensure that the setting does not exceed the maximum available fault current or the instantaneous trip will be rendered worthless.
  • For emergency and legally required standby feeders, NEC Articles 700.26 and 701.26 require the ground fault device shall be an alarm only. For MV systems, this can have significant negative consequences. The installation on neutral grounding resistor should be considered to limit ground fault currents to a safe level for MV generation systems.

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