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The protecting relays mentioned within the previous chapter may be productively utilized to notice the improper behavior of any circuit part and initiate corrective measures. As a matter of convenience, this chapter deals with the protection of alternators and transformers solely.

The most serious faults on alternators that need immediate attention area unit the mechanical device winding faults. the most important faults on transformers occur because of short-circuits within the transformers or their connections. the fundamental system used for defense against these faults is that the differential relay theme as a result of the differential nature of measurements makes this technique way more sensitive than different protection systems.

The generating units, particularly the larger ones, square measure comparatively few in range and better in individual value than most different instrumentation. Therefore, it’s fascinating and necessary to produce protection to hide the wide selection of faults which can occur within the trendy generating plant.
Some of the necessary faults which can occur on associate degree generator square measure :

  • Failure of prime-mover
  • Failure of the field
  • Overcurrent
  • Over speed
  • Overvoltage
  • Unbalanced loading
  • Stator winding faults
  • Failure of prime-mover. once the input to the prime-mover fails, the generator runs like an electric motor and attracts some current from the provisioning system. This driving condition is thought of as “inverted running”.
  1. Within the case of turbo-alternator sets, failure of steam offer might cause inverted running. If the steam offer is bitten by bit repaired, the generator can devour the load while not distressful the system.
  2. Within the case of hydro-generator sets, protection against inverted running is achieved by providing mechanical devices on the water-wheel. once the water flow drops to AN insufficient rate to take care of the electrical output, the generator is disconnected from the system.
  3. ICE driven alternators, once running inverted, draw a substantial quantity of power from the provisioning system and it’s a usual observe to supply protection against driving so as to avoid injury because of doable mechanical seizure. this is often achieved by applying reverse power relays to the alternators that isolate the latter throughout their driving action.
  • Failure of field. the probabilities of field failure of alternators area unit beyond any doubt terribly rare. although it will occur, no immediate harm is going to be caused by allowing the generator to run while not a field for a brief amount. it’s sufficient to trust the room attendant to disconnect the faulty generator manually from the system bus-bars. Therefore, it’s a universal apply to not give †automatic protection against this contingency.
  • Overcurrent. It happens in the main thanks to a partial breakdown of winding insulation or thanks to overload on the availability system. Overcurrent protection for alternators is taken into account unessential thanks to the subsequent reasons :
  1. The trendy tendency is to style alternators with terribly high values of internal electric resistance in order that they’re going to stand an entire short-circuit at their terminals for sufficient time while not serious warming. On the incidence of associate degree overload, the alternators are disconnected manually.
  2. The disadvantage of victimization overload protection for alternators is that such protection would possibly disconnect the alternators from the ability plant bus on account of some fleeting troubles outside the plant and, therefore, interfere with the continuity of electrical service.

  • Overspeed. The chief explanation for over speed is that the unforeseen loss of all or the main a part of the load on the generator. trendy alternators are typically supplied with mechanical centrifugal devices mounted on their driving shafts to trip the most valve of the prime-mover once a dangerous over speed happens.
  • Over-voltage. the sphere excitation system of contemporary alternators is therefore designed that overvoltage conditions at traditional running speeds cannot occur. However, overvoltage in associate generator happens once the speed of the prime-mover will increase thanks to the unforeseen loss of the generator load. within the case of steam-turbine driven alternators, the management governors are terribly sensitive to hurry variations.
  1. Trip the main circuit breaker to disconnect the faulty alternator from the system.
  2. Disconnect the alternator field circuit.
  • Unbalanced loading. Unbalanced loading implies that there area unit totally different part currents within the generator. Unbalanced loading arises from faults to earth or faults between phases on the circuit external to the generator. The unbalanced currents, if allowed to persist, might either severely burn the mechanical fixings of the rotor core or injury the sector winding.

The theme includes 3 line current transformers, one mounted in every part, having their secondaries connected in parallel. A relay is connected in parallel across the electrical device secondaries. underneath traditional operative conditions, equal currents flow through the various phases of the generator and their pure mathematics total is zero. Therefore, the total of the currents flowing within the secondaries is additionally zero and no current flows through the operative coil of the relay.

  • Stator winding faults. These faults occur principally thanks to the insulation failure of the mechanical device windings. the most styles of mechanical device winding faults, so as of importance, are :
  1. Fault between part and ground
  2. Fault between phases
  3. Inter-turn fault involving turns of an equivalent part winding

The mechanical device winding faults area unit the foremost dangerous and area unit seemingly to cause significant injury to dear machinery. Therefore, automatic protection is totally necessary to clear such faults within the fastest attainable time so as to reduce the extent of the injury. For the protection of alternators against such faults, the differential technique of protection (also is aware of as Merz-Price system) is most typically utilized thanks to its larger sensitivity and dependability.

Differential Protection of Alternators:

In this theme of protection, currents at the 2 ends of the protected section area unit compared. below traditional in operation conditions, these currents area unit equal, however, could become unequal on the prevalence of a fault within the protected section. The distinction of the currents below fault conditions is organized to submit to the in-operation coil of the relay. The relay then closes its contacts to isolate protected section from the system.


Schematic arrangement. Identical current electrical device pairs CT1 and CT2 area unit placed on either facet of every part of the mechanical device windings. The secondaries of every set of current transformers area unit connected in star; the 2 neutral points and therefore the corresponding terminals of the two-star teams being connected along by suggests that of a four-core pilot cable.

In order, that burden on every current electrical device is that the same, the relays area unit connected across equipotential points of the 3 pilot wires and these equipotential points would naturally be situated at the center of the pilot wires. The relays area unit usually of magnetic force kind and area unit organized for fast action since fault ought to be cleared as quickly as potential.

Operation. below traditional in operation conditions, the present at each end of every winding are equal and therefore the currents within the secondaries of 2 CTs connected in any part also will be equal. Therefore, there’s balanced current within the pilot wires and no current flows through the in operation coils (R1, R2, and R3) of the relays. once AN earth-fault or phase-to-phase fault happens, this condition now not holds smart and therefore the differential current flowing through the relay circuit operates the relay to trip the electrical fuse.

The current within the affected part winding can flow through the core and frame of the machine to earth, the circuit being completed through the neutral attachment resistance. The currents within the secondaries of the 2 CTs in part R can become unequal and also the distinction of the 2 currents can flow through the corresponding relay coil (i.e. R1), returning via the neutral pilot.

The short-circuit current circulates via the neutral finish association through the 2 windings and thru the fault as shown by the dotted arrows. The currents within the secondaries of 2 CTs in every affected part can become unequal and also the differential current can flow through the operational coils of the relays (i.e. R2 and R3) connected in these phases.

Limitation. the 2 circuits for generator protection shown higher than have their own limitations. it’s a general follow to use neutral attachment resistance so as to limit the harmful effects of earth-fault currents. In such a state of affairs, it’s not possible to safeguard the entire of the stator coil windings of a star-connected generator throughout earth-faults.

Makers of protecting gear speak of “protecting eightieth of the winding” which suggests that faults within the two-hundredths of the wind close to the neutral purpose cannot cause tripping i.e. this portion is unprotected. it’s a usual follow to safeguard solely eighty-fifths of the wind as a result of the possibilities of Associate in Nursing earth fault occurring close to the neutral purpose area unit terribly rare because of the uniform insulation of the winding throughout.

Modified Differential Protection for Alternators:

If the neutral purpose of a star-connected generator is earthed through a high resistance, the protection won’t give comfortable sensitivity for earth-faults. it’s as a result of the high fastening resistance can limit the earth-fault currents to a coffee worth, necessitating relays with low current settings if Associate in Nursing adequate portion of the generator winding is to be protected.

Operation. beneath traditional operative conditions, currents at the 2 ends of every mechanical device winding are equal. Therefore, there’s a balanced current within the part pilot wires and no current flows through the operative coils of the relays. Consequently, the relays stay inoperative.


If Associate in Nursing earth-fault happens on any body part, the out-of-balance secondary current in CTs in this part can flow through the world relay ER and via pilot S1 or S2 to the neutral of this transformers. this may cause the operation of earth relay solely. If a fault happens between 2 phases, the out off-balance current can flow into round the 2 electrical device secondaries via any 2 of the coils PA, BR, computer (the combine being set by the 2 phases that area unit faulty) while not passing through the world relay ER. Therefore, solely the phase-fault relays can operate.

Balanced Earth-fault Protection:

In small-size alternators, the neutral ends of the three-phase windings are typically connected internally to one terminal. Therefore, it’s unacceptable to use Merz-Price current principle represented higher than as a result of there aren’t any facilities for accommodating the mandatory current transformers within the neutral affiliation of every section winding. underneath these circumstances, it’s thought of sufficient to produce protection against earth-faults solely by the employment of a balanced earth-fault protection theme.


Schematic arrangement. the schematic arrangement of balanced earth-fault protection for a 3-phase generator. It consists of 3 line current transformers, one mounted in every section, having their secondaries connected in parallel thereupon of one current electrical device within the conductor connection the starring purpose of the generator to earth. A relay is connected across the secondaries of the transformers. The protection against earth faults is proscribed to the region between the neutral and therefore the line current transformers.

Operation. underneath traditional operational conditions, the currents flowing within the generator leads and thence the currents flowing in secondaries of the road current transformers increase zero and no current flows through the relay. additionally, underneath these conditions, the present within the neutral wire is zero and therefore the secondary of the neutral current electrical device provides no current to the relay.

Stator Inter Stator Inter-turn Protection:


Merz-price circulating-current system protects against phase-to-ground and phase-to-phase faults. It doesn’t shield against turn-to-turn fault on an equivalent section winding of the mechanical device. it’s as a result of the present that this sort of fault produces flows during a native circuit between the turns concerned and doesn’t produce a distinction between the currents getting into and effort the winding at its 2 ends wherever current transformers area unit applied.

However, it’s typically thought-about reserve to supply protection for inter-turn faults as a result of they invariably grow to be earth-faults. In single flip generator (e.g. giant steam-turbine generators), there’s no necessity of protection against inter-turn faults.