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Transformers are static devices wholly encircled and usually, oil immersed. Therefore, the probabilities of faults occurring on them are terribly rare. However, the implications of even a rare fault could also be terribly serious unless the electrical device is quickly disconnected from the system. This necessitates providing adequate automatic protection for transformers against doable faults.

Small distribution transformers are typically connected to the availability system through series fuses rather than circuit breakers. Consequently, no automatic protecting relay instrumentation is needed. However, the likelihood of faults on power transformers is doubtless a lot of and therefore automatic protection is totally necessary.

Common transformer faults. As compared with generators, in which many abnormal conditions may arise, power transformers may suffer only from :

  1. Open circuits
  2. Overheating
  3. Winding short-circuits e.g. earth-faults, phase-to-phase faults, and inter-turn faults.


An electric circuit in one section of a 3-phase electrical device could cause undesirable heating. I follow relay protection isn’t provided against open circuits as a result of this condition is comparatively harmless. On the prevalence of such a fault, the electrical device is disconnected manually from the system.

Overheating of the electrical device is typically caused by sustained overloads or short-circuits and really sometimes by the failure of the cooling system. The relay protection is additionally not provided against this contingency associated thermal accessories are usually wont to sound an alarm or management the banks of fans.

Protection Systems for Transformers:

For the protection of generators, Merz-Price circulating-current system is certainly the foremost satisfactory. tho’ this can be mostly true of electrical device protection, there are cases wherever current system offers no specific advantage over different systems or infeasible on account of the difficult conditions obligatory by the large choice of voltages, currents, and attachment conditions invariably related to power transformers. underneath such circumstances, various protecting systems are used that in several cases are as effective because of the circulating-current system.

  • Buchholz devices
  • Earth-fault relays
  • Overcurrent relays
  • Differential system

The complete protection of the electrical device typically needs a mixture of those systems. selection of a selected combination of systems might rely on many factors like

  1. Size of the electrical device
  2. Form of cooling
  3. Location of the electrical device within the network
  4. Nature of load equipped and
  5. The importance of service that electrical device is needed.

Within the following sections, the higher than systems of protection are going to be mentioned intimately.

Buchholz devices:

Buchholz relay could be a gas-actuated relay put in in oil immersed transformers for defense against all types of faults. Named once its discoverer, Buchholz, it’s accustomed giving associate degree alarm just in case of inchoate (i.e. slow-developing) faults within the electrical device and to disconnect the electrical device from the availability within the event of severe internal faults.


Construction. the constructional details of a Buchholz relay. It takes the shape of a rounded vessel placed within the connecting pipe between the most tank and therefore the conservator. The device has 2 parts. The higher component consists of a mercury sort switch hooked up to a float. The lower component contains a mercury switch mounted on a hinged sort flap settled within the direct path of the flow of oil from the electrical device to the conservator. The higher component closes associate degree alarm circuit throughout inchoate faults whereas the lower component is organized to trip the fuse just in case of severe internal faults.

Operation. The operation of the Buchholz relay is as follows :

  1. Within the case of inchoate faults among the electrical device, the warmth thanks to fault causes the decomposition of some electrical device oil within the main tank. The merchandise of decomposition contains quite seventieth of atomic number 1 gas.
  2. If a heavy fault happens within the electrical device, a massive quantity of gas is generated within the main tank. The oil within the main tank rushes towards the conservator via the Buchholz relay and in doing, therefore, tilts the flap to shut the contacts of the mercury switch. This completes the trip circuit to open the fuse dominant the electrical device.


  1. it’s the best sort of electrical device protection.
  2. It detects the inchoate faults at a stage a lot of previous is feasible with different varieties of protection.


  1. It will solely be used with oil immersed transformers equipped with conservator tanks.
  2. The device will sight solely faults below the oil level within the electrical device. Therefore, separate protection is required for connecting cables.

Earth-Fault Leakage Protection:


An earth-fault sometimes involves a partial breakdown of winding insulation to earth. The ensuing escape current is significantly but the short-circuit current. The earth-fault could continue for a protracted time and cause goodish injury before it ultimately develops into a short-circuit and far from the system. underneath these circumstances, it’s profitable to use earth-fault relays so as to confirm the disconnection of earth-fault or leak within the early stage. associate degree earth-fault relay is actually associate degree overcurrent relay of low setting associate degreed operates as before long as an earth-fault or leak develops.

The 3 leads of the first winding of power electrical device area unit taken through the core of a current electrical device that carries one secondary. The operational coil of a relay is connected to the current secondary. underneath traditional conditions (i.e. no fault to earth), the resultant of the 3 section currents is zero and there’s no resultant flux within the core of current electrical device notwithstanding what proportion the load is out of balance. Consequently, no current flows through the relay and it remains down.

Combined Leakage and Overload Protection:


The core-balance protection represented higher than suffers from the disadvantage that it cannot offer protection against overloads. If a fault or discharge happens between phases, the core-balance relay won’t operate. it’s a usual observe to supply combined discharge and overload protection for transformers. the planet relay has a low current setting and operates underneath earth or discharge faults solely. The overload relays have a high current setting and are organized to work against faults between the phases.

Applying Circulating current System to Transformers:

Merz-Price current -the current principal is usually used for the protection of power transformers against earth and section faults. The system as applied to transformers is basically an equivalent as that for generators however with sure complicating options not encountered within the generator application. The complicating options and their remedial measures are briefed below :


  1. During a power electrical device, currents within the primary and secondary are to be compared. As these 2 currents are typically completely different, therefore, the employment of identical transformers (of same flip ratio) can offer differential current and operate the relay even underneath no load conditions.
  2. There’s typically a section distinction between the first and secondary currents of a 3-phase power electrical device. notwithstanding CTs of the correct turn-ratio are used, a differential current could flow through the relay underneath traditional conditions and cause relay operation.
  3. The correction for section distinction is established by applicable connections of CTs. The CTs on one facet of the facility electrical device is connected in such some way that the resultant currents fed into the pilot wires are displaced in the section from the individual section currents within the same direction as, associate degreed by an angle adequate to, the section shift between the power-transformers primary and secondary currents.
  4. The table below shows the sort of connections to use for CTs so as to complete the section distinction within the primary and secondary currents of power electrical device.
  5. Most transformers have suggests that for faucet dynamical that makes this drawback even harder. faucet dynamical can cause differential current to flow through the relay even underneath traditional operative conditions.
  6. Another complicating consider electrical device protection is that the magnetizing inpouring current. underneath traditional load conditions, the magnetizing current is incredibly little.

In order to beat them on top of the problem, differential relays area unit set to control at a comparatively high degree of unbalance. This methodology decreases the sensitivity of the relays. In apply, the advantage is taken of the actual fact that the initial in-rush currents contain the distinguished second-harmonic element.

Circulating-Current Scheme for Transformer Protection:

Merz-Price circulating-current theme for the protection of a 3-phase delta/delta power electrical device against the phase-to-ground and phase-to-phase faults. Note that CTs on the 2 sides of the electrical device square measure connected in star. This compensates for the section distinction between the facility electrical device primary and secondary.


During traditional in operation conditions, the secondaries of CTs carry identical currents. Therefore, the currents getting into and deed the pilot wires at each ends square measure constant and no current flows through the relays. If a ground or phase-to-phase fault happens, the currents within the secondaries of CTs can now not be constant and therefore the differential current flowing through the relay circuit can clear the breaker on either side of the electrical device. The protected zone is proscribed to the region between CTs on the high-voltage facet and therefore the CTs on the low-tension facet of the facility electrical device. it’s worthy to notice that this theme conjointly provides protection for short-circuits between activates constant section winding. once a short-circuit happens between the turns, the turn-ratio of the facility electrical device is altered and causes unbalance between current electrical device pairs.