Stator Protection of Generators: Top 7 Systems | Electrical Engineering

The following points highlight the top seven systems employed for stator protection of generators. The systems are: 1. Differential Protection for Generators 2. Modified Differential Protection for Generators 3. Biased Circulating Current Protection (Percentage Differential Relay Protection) 4. Self-Balance Protection System 5. Balanced Earth-Fault Protection 6. Stator Inter-Turn Protection 7. Stator Overheating Protection.

System # 1. Differential Protection for Generators:

The most common system employed for the protection of stator windings against earth faults and phase-to-phase faults makes use of circulating current principle. In this scheme of protection, currents at the two ends of the protected section are compared. Under normal operating conditions, these currents are equal but may differ on the occurrence of a fault in the protected section. The difference of the currents under faulty conditions is made to flow through the relay operating coil.

The relay then closes its contacts and makes the circuit breaker to trip and thus isolate the protected section from the system. Such a protection is called a Merz-Price circulating current system. Such a protective scheme is very effective for earth faults and faults between phases.

The schematic arrangement of differential protection scheme for a 3-phase generator is illustrated in Fig. 8.1. There are two sets of identical CTs, each set is mounted on either side of the stator phase windings. The secondaries of these current transformer sets are connected in star and their ends are connected through pilot wires. The relay coils are connected in star, the neutral point being connected to the current transformer common neutral and the outer ends one to each of the three pilot wires.

The relays are connected across equipotential points of the three pilot wires so that the burden on each CT is the same. The equipotential points of the pilot wires would naturally be located at the centre of these wires and so the relays are located midway as illustrated in Fig. 8.1.

However, it is always convenient to locate the relay coils adjacent to CTs near the main circuit breaker. This can be accomplished by inserting balancing resistances in series with the pilot wires to make equipotential points located near the main circuit breaker, as illustrated in Fig. 8.2. These resistances are usually adjustable in order to obtain the exact balance.

The relays employed in this protection scheme are generally of electromagnetic type and are arranged for instantaneous operation as faults are expected to be cleared as quickly as possible.

Under healthy conditions, the currents at both ends of each winding will be equal, emfs induced in secondaries of CTs will be equal and so no current will flow through the operating coils of relays. When an earth fault or phase-to-phase fault occurs, this condition no longer holds good and the differential current flowing through the relay operating coil makes the circuit breaker to trip.

Now assume that there is an earth fault on phase R due to breakdown of insulation to earth, as illustrated in Fig. 8.1. The currents in the secondaries of the two CTs in phase R will become unequal and the difference of the two currents will flow through the corresponding relay coil and the circuit breaker will get tripped.

If there is a short circuit between the two phases Y and B, as illustrated in Fig. 8.2, it will cause fault current to flow through two phases, as illustrated in the Fig. The currents in the secondaries of the two CT’s in each affected phase will become unequal and the differential current will flow through the operating coils of the relays connected in these phases. The relay then closes its contacts to trip the circuit breaker.

System # 2. Modified Differential Protection for Generators:

It is a general practice to use neutral resistance earthing in order to avoid the adverse effects of earth fault currents. In such a case it will not be possible to protect whole of the stator windings of a star-connected generator against earth faults with the protection schemes.

When an earth fault occurs near the neutral point, it will result in only a small fault current because of the small emf of short-circuited portion of the winding. This current, which is further reduced by the resistance of the neutral earthing, may not cause the relay to operate. The magnitude of the unprotected zone depends upon the value of resistance employed in neutral earthing and the relay setting.

Makers of protective gear speak of “protecting 80 or 85 per cent of the winding” which means that fault in the 20 or 15 per cent of the winding near the neutral point cannot cause tripping i.e., this portion of the winding remains unprotected. If the relays with very low settings are used for the protection of adequate portion of the generator stator winding, it will not be desirable for reliable stability on heavy through phase-faults.

In order to overcome above difficulty, the modified scheme shown in Fig. 8.3 has been developed. In this modified form of differential protection the setting of earth-faults is reduced without impairing stability. In this scheme two elements are arranged for phase fault protection and the third for earth fault protection only. The two phase elements (A and C) together with a balancing resistor (BR) are connected in star and the earth relay (ER) is connected between this star point and the neutral pilot wire.

The star-connected circuit is symmetrical as regards impedance and any symmetrical spill- current from the circulating current circuit, due to high through phase-fault conditions, will cancel at relay star point and will not flow through the earth fault relay. It is, therefore, possible with this scheme of protection to operate with a sensitive earth fault relay and still maintain a high degree of stability. This sensitive operation of earth fault relay enables low setting for the element which provides protection to a greater percentage of stator windings.

System # 3. Biased Circulating Current Protection (Percentage Differential Relay Protection):

This system, also called the Merz-Price protection system, is the most common type of protection used for stator windings against phase-to-phase or phase-to-ground faults. It is the standard practice of manufacturers to recommend differential protection for generators rated 1 MVA or higher, and most of such generators are protected by differential relays. Above 10 MVA, it is almost universally the practice to use differential relays. Generally, percentage differential relaying is used, protecting about 80% of the stator winding.

When differential relaying is used for protection, the CTs at both ends of a generator winding must be of equal accuracy; otherwise if the error is excessive it will causes a mal-operation of the relay. To safeguard against such an operation, biased circulating current protection is used. Such a protection provides a biasing feature which automatically increases the relay setting in proportion to the load or through fault current, i.e., the relay is set to operate not at a definite current, but at certain percentage of the through current.

By suitably proportioning the ratio of the restraining coil turns to the operating coil turns, any amount of biasing can be achieved and compensation for unwanted operations due to spurious spill currents can be provided.

The advantages of such a scheme are enumerated below:

1. It does not require CTs with air gaps or special balancing features.

2. It permits a low fault setting to be used and this ensures maximum protection of the windings.

3. It ensures complete stability under the most severe through fault conditions.

The protection of a three-phase star-connected generator by means of percentage differential relay is illustrated in Fig. 8.4. CTs connected in star are provided on both the out-going sides and the machine winding connection to earth. The restraining coils are energised by the secondary connections of CTs in each phase, and the operating coils are connected to the central tapping on the restraining coils and the neutral pilot wire as illustrated in the Fig. 8.4.

For the greatest sensitivity of differential relaying the primary current rating of the CTs must be equal to the rated full-load current of the generator. In practice it is about 25% higher.

Under normal operating conditions the secondary outputs of the line CTs are equal, at any instant, to the outputs of the CTs at the neutral end. Thus there is balanced circulating current in the phase pilot wires and the relay restraining windings and so no current flows through the operating coils or in the common return pilot wire. If there is a fault due to a short circuit in the protected zone of the winding it produces a difference between the currents in the primary windings of CTs on both sides of the generator winding of the same phase.

This results in a difference between the secondary currents of the two CTs. Thus, under fault conditions, a current flows through the operating coils of those phase elements corresponding to primary phases on which the fault has occurred. If this current attains the preset magnitude, the relay operates and causes the circuit breaker to trip. The tripping can be arranged for a certain setting, and the restraint adjusted to a certain percentage of the winding to be protected. Generally 80% of the winding is protected.

System # 4. Self-Balance Protection System:

It necessarily consists of two cables connected to the two ends of each phase and the cables are passed through the circular aperture of the ring type CTs, as shown in Fig. 8.6. Under normal operating conditions the current flowing through two cables in the central aperture of the CT will be in opposite directions and so there will be no magnetization of the CT. In case there is an earth fault on any phase, the fault current passes only once through the circular ring of the CT of that particular phase.

This will set up magnetic flux and so induce an emf in the relay circuit which will trip the circuit breaker. This system is also effective for phase-to-phase faults but not useful for protection against fault between the turns of the same phase for ordinary generator winding. In case the winding of each phase is designed in two parallel paths such a protection will be useful.

This is very sensitive type of earth fault protection scheme but it has got some drawbacks, as enumerated below:

1. Under heavy short circuits large electromagnetic forces are developed in the CTs ring.

2. A different layout of the cable leads is required in this scheme.

3. For protection of the lead cable from the circuit breaker to stator terminal the ring CTs are to be placed near the circuit breaker which is again quite cumbersome. However, this difficulty can be overcome by providing a metallic sheath around the cable (but insulated from it) and earthing it in a manner illustrated in Fig. 8.7. In case the lead punctures anywhere, there will be flow of current through the sheet, as illustrated in the Fig. and relay will be energized resulting in tripping of the circuit breaker. In Fig. 8.7 connections for only one phase are shown.

System # 5. Balanced Earth-Fault Protection:

In small size generators, the neutral ends of the three phase windings are sometimes connected internally to a single terminal. Under such conditions, it is not possible to use the circulating current protection because neutral end is not accessible and protection is therefore, provided against earth faults only by using balanced earth-fault protection scheme. Such scheme does not provide protection against phase-to-phase faults until and unless they develop into earth faults, as most of them do.

Schematic arrangement of a balanced earth-fault protection for a 3-phase generator is illustrated in Fig. 8.8. In this scheme three line CTs, one mounted in each phase, have their secondaries connected in parallel with that of a single CT mounted on the conductor joining the star point of the generator to earth. A relay is connected across the secondaries of the CTs as illustrated in the Fig.

The protection against earth fault is limited to the region between the neutral and line CTs. As this scheme provides protection of only the stator winding against the earth fault in the stator and does not operate in case of external earth fault, so such a protection scheme is often called restricted earth fault protection scheme. This protection scheme is provided in large generators as an additional protection scheme.

Under normal working conditions, the currents flowing in the secondaries of the line CTs sum up to zero and current flowing in the secondary of neutral CT (CT₁) is also zero. Thus the relay remains de-energized. When an earth fault occurs within the protected zone (i.e., left of the line CTs), the fault current flows through the primary of neutral CT and the corresponding secondary current flows through the relay which trips the circuit breaker. In case the earth fault develops external to the protected zone (i.e. right of the line CTs), the sum of the currents at the terminals of the generator is exactly equal to the current in the neutral connection and as such no current flows through the relay operating coil.

However, such a scheme has a drawback. In case the earth fault occurs nearer the neutral terminal or when grounding of neutral is through a resistance or through a distributing transformer, the fault current may be so low that the secondary current of the CT becomes lower than the pick-up current of the relay. The relay thus remains inoperative and the fault continues to persist in the generator winding which is highly undesirable.

System # 6. Stator Inter-Turn Protection:

Merz-Price circulating-current protection system does not provide protection against turn-to-turn faults (short-circuits between the turns) on the same phase winding of the stator because the current produced by such a fault flows in a local circuit between the turns involved and does not create the difference between the currents entering and leaving the winding at its two ends where the CTs are mounted. However, it is usually considered unnecessary to provide such a protection since short circuits between the turns on the same phase invariably develop into earth fault.

The coils of modern large turbo-generators are usually single turn and hence they do not need turn-to-turn protection. However, inter-turn protection is provided for multi- turn generators such as hydroelectric generators. In case of large generators, stator windings are sometimes duplicated in order to carry heavy current. Advantage may be taken of this necessity to provide inter-turn protection. In this case the stator winding has two separate parallel paths, as illustrated in Fig. 8.9.

The primaries of CTs are inserted in these parallel paths and the secondaries are cross-connected, as shown in the figure. When there is no fault, currents flowing through the two parallel paths of the stator winding will be equal and therefore no current will flow through the relay operating coil. But during inter-turn fault in the phase winding, the currents flowing through the two parallel paths will be different and a current proportional to the difference of two currents will flow through the relay operating coil which will close the trip circuit and isolate the machine from the power system. Such a protection can be extremely sensitive.

Generators having single winding per phase or those generators whose parallel windings are not accessible can be protected by using zero sequence component of voltage caused by the reduction of the emf induced in the faulty phase. One such circuit is given in Fig. 8.10. In this arrangement a voltage transformer is connected between each phase terminal and the neutral of the winding.

The secondary leads are connected in an open delta. The zero sequence voltage (residual voltage of the generator terminals) appears across the tertiary winding of the voltage transformer which is connected to the operating winding of a three element directional relay. This winding in quadrature to this operating winding of the relay is energized by the secondary of the voltage transformer. During normal operation, the residual voltage is zero i.e.,

V_RES = V_RN + V_YN + V_BN = 0

This balance is disturbed during inter-turn fault on any of the single windings and the relay operating coil is energized by the residual voltage.

System # 7. Stator Overheating Protection:

Generally stator overheating is caused by sustained overloads or by cooling system failure. Overheating because of short-circuited laminations is very localized, and it is just a matter of chance whether it can be detected before serious damage is caused. It is not practicable to provide overload protection by backup stator fault overcurrent protection as backup overcurrent protection is usually set for sensing fault currents and should not trip for overloads. Electrical overcurrent relays cannot sense the winding temperature accurately because temperature rise depends on I²Rt and also on cooling. Electrical protection cannot detect failure of the cooling system.

The practice is to embed resistance temperature-detector coils or thermocouples in the slots below the stator coils of the generators of large capacity (above 1 MVA) for protection against overheating. Enough of such detectors are located at different places in the windings so that an indication can be obtained of the temperature conditions throughout the stator.

Several of the detectors that provide the highest temperature indication are selected for use with temperature indicator or recorder usually having alarm contacts, or the detector providing the highest indication may be arranged to operate a temperature relay to sound alarm. Supplementary temperature devices may be provided for monitoring the cooling system; such devices would give the earliest alarm in the event of failure of cooling system. But it is usually realized that the stator temperature detectors and alarm devices are sufficient