IEEE Guide for Protective Relay Applications to Power Transformers: Electrical Detection of Faults


Fire caused by unmitigated electrical fault 

Fuses are commonly used to provide fault detection for transformers with minimum nameplate ratings up to 5000 kVA, three-phase (Categories I and II). 

Transformers of 10 000 kVA and larger, three-phase, minimum nameplates (Categories III and IV) are generally protected by a combination of protective devices. 

Transformers that fall between these two ratings are protected by either fuses or relays. The choice of protection depends on the criticality of the load, the relative size of the transformer compared to the total system load, and potential safety concerns. 

System considerations, such as coordinating fuses with upstream relays or with transformer damage curves, may determine what protection is used. Some other considerations include types of faults, personal safety issues, speed of clearing, single phasing of load, and ferroresonance.

Fuse protection

Fuses have the merits of being economical and requiring little maintenance. Battery supply and a relay building are not needed. Fuses can reliably protect some power transformers against damage from primary and secondary external faults. They will provide limited protection for internal faults. Generally, more sensitive means for protection from internal faults is provided for transformers of 10 MVA and higher. Fuses have been used at higher transformer ratings, depending on the currently available fuse ampere ratings. Primary fuses for power transformers are not applied for overload protection, their main purpose being fault protection.

The selection of the fuse and proper ampere rating should be based on the following factors:

  • Fuse fault-interrupting capability and available system fault current. 
  • Maximum anticipated peak load current, daily peak loads, emergency peak loads, maximum permissible transformer load current, and the applicable transformer through-fault current duration curve. 
  • Fuse fault-interrupting capability and available system fault current.
  • Maximum anticipated peak load current, daily peak loads, emergency peak loads, maximum permissible transformer load current, and the applicable transformer through-fault current duration curve. 
  • Hot load pickup (inrush current on instantaneous reclosing of source-side circuit breaker) and cold load pickup (inrush current and undiversified load current after an extended outage).
  • Available primary system fault current and transformer impedance. 
  • Coordination with source-side protection equipment. 
  • Coordination with low-side protection equipment. 
  • Maximum allowable fault time on the low-side bus conductors. 
  • Transformer connections and grounding impedance as they affect the primary current for various types of secondary faults. 
  • Sensitivity for high-impedance faults. 
  • Transformer magnetizing inrush

Differential Protection

Current differential relaying is the most commonly used type of protection for transformers of approximately 10 MVA three-phase (self-cooled rating) and above. The term refers to the connection of CTs so that the net operating current to the relay is the difference between input and output currents to the zone of protection. Relays of three general classes are used with this current differential. 

They are: 

  • Time overcurrent relay, which may include an instantaneous trip unit having a high-current setting. 
  • Percentage differential with restraint actuated by the input and output currents. 
  • Percentage differential relay, with restraint actuated by one or more harmonics in addition to the restraint actuated by the input and output currents.

CT connections and ratios must be such that the net current in the relay operating coil or element for any type or location of the external fault is effectively zero unless relay current matching taps are available. Paralleling of two or more CTs for connection to a single restraint coil or element usually should be avoided for the most effective restraint action. 

Typical schematic connections for percentage differential protection of a ∆-Y transformer
Source: IEEE Guide for Protective Relay

If breaker CTs are used for input to the transformer differential, bypassing the breaker will affect the inputs for the differential relays. Assuming that the differential relay is removed from service and there are other relay schemes protecting the transformer, care must be taken to ensure that other breakers will trip in lieu of the bypassed breaker. If this alternative tripping is not available, the transformer breaker should not be bypassed.

Overcurrent Relay Protection

A fault external to a transformer can result in damage to the transformer. If the fault is not cleared promptly, the resulting overload on the transformer can cause severe overheating and failure. Overcurrent relays may be used to clear the transformer from the faulted bus or line before the transformer is damaged. On some small transformers, overcurrent relays may also protect for internal transformer faults, and on larger transformers, overcurrent relays may be used to provide relay backup for differential or pressure relays. Thermal relays may also be used to protect transformers from harmful overload. However, thermal relays often are used for alarm only. 

Overcurrent relay protection schemes for transformers: 

  • Phase time overcurrent
  • Phase instantaneous overcurrent
  • Tertiary-winding overcurrent.

Groud Fault Protection

Sensitive detection of ground faults can be obtained by differential relays or by overcurrent relays specifically applied for that purpose. Several schemes are practical, depending on transformer connections, availability of CTs, zero sequence current source, and system design and operating practices. 

Gound fault protection schemes for transformers: 

  • Faults in DELTA connected transformer windings.
  • Faults in WYE connected transformer windings. 
  • Case grounding
  • Impedance grounded system
  • Ground relays also used for sensitive ground fault protection.

Fault Detection for Special Purpose Transformers

Regulating Transformers
The exciting winding of a regulating transformer presents a special protection problem, since ordinary power transformer differentials are not sensitive enough to sense faults in this high-impedance winding. Regulating transformers can be either the most common in-phase type employing only voltage regulation, or the phase-shifting type that provides regulation of phase angle, or both. 

Sudden-pressure or fault-pressure relays will offer good protection for all three types. However, electrical protection may differ substantially between the in-phase type and the others. The tap changer mechanism compartment may be protected with gas or oil sudden-pressure relays. Pressure variations during normal tap changing arc interruption have not been found to cause false operation of the sudden-pressure relays. The use of vacuum interrupter switches in the tap changing mechanism eliminates any pressure variations in the tap changing compartment.

Combine Power and Regulating Transformers
A power transformer, such as a Y-∆ transformer, may also have regulating features, either in-phase, out-of-phase (such as quadrature), or both. Such transformers are called tap-changing-under-load, or load-tap-changing, transformers. 

The protection of a transformer of the in-phase variety has been previously covered in this guide. The electrical protection of the out-of-phase variety is even more difficult than the protection of the phase-shifting regulating transformer because the power transformer has no exciting winding since excitation is obtained from loaded windings. The comments on phase-shifting regulating transformers apply equally well to this type of transformer. In any case, the sudden-pressure or fault-pressure relay should be considered the first line of protection. 

Grounding Transformers
A grounding transformer can be either a zigzag (z-z) or a Y-∆-connected transformer. The electrical protection scheme is simple, consisting of overcurrent relays connected to ∆-connected CTs. Grounding transformers are seldom switched by themselves. However, when they are switched, they are subject to magnetizing inrush current, like any other type of transformer. Harmonic restrained overcurrent relays may be used to prevent inadvertent tripping upon energizing.

Protection of grounding transformers: zigzag



  • IEEE Guide for Protective Relay  Applications to Power Transformers
  • Sponsor: Power Systems Relay Committee of the IEEE Power Engineering Society.
  • Publisher: IEEE | Download 


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