PROTECTION OF ALTERNATOR               CHAPTER - 4

INDRODUCTION

Alternator is the most important and costly equipment in power system. The cost is high because the alternator unit is accompanied by prime mover, excitation system, voltage regulator, cooling system etc. The protection system provided should be therefore becomes elaborate and complex.

As soon as the fault develops on alternator, it should be immediately disconnected from he other generating plant to avoid the faulty machine from being fed by other sources.

ABNORMALITIES AND FAULTS

Following Table shows abnormal conditions (Faults) and protection schemes employed for the alternator.

SN	Abnormal Condition	Effect	Protection
1	Thermal overloading: i)    Continuous overloading ii)   Failure of cooling system	Overheating of stator winding and insulation failure.	Thermocouples of resistance thermometer impeded in stator slots and cooling system. Stator overloading protection with over current relays.
2	External fault fed by generator	Unbalanced loading stresses on winding and shaft, excessive heating for prolonged short circuit.	Negative phase sequence protection for large machines. Overload protection for small generators.
3	Stator faults: i)        Phase to phase ii)       Phase to earth iii)      Inter-turn	Winding burn-out, welding of core laminations down	Biased differential protection, sensitive earth fault protection, inter-turn fault protection.
4	Rotor earth faults	Single fault does not harm second fault causes unbalanced magnetic forces causing damage to shaft, bearings	Rotor earth fault protection
5	Loss of field: Tripping of field circuit breaker.	Generator runs as induction generator deriving excitation currents from bus-bar. Speed increases slightly.	‘Loss of field’ or ‘Field failure’ protection.
6	Motoring of generator. When input to prime mover stops, the generator draws power from bus-bars and runs a synchronous motor in the same direction.	Effect depends upon type of prime mover and the power drawn from he bus during motoring.	Reverse power protection by directional power relays direct the reversal of power.
7	Over-voltages	Insulation failure	Lightning arresters connected near generator terminals.
8	Over-fluxing of generator transformer and auxiliary transformer.	Heating of core relay.	Connected in voltage regulator circuit generator.
9	Under-frequency	Failure of blades of steam turbines.	Frequency Relays.

Something about diagrams: The differential protection scheme is applied for 3-phase alternator. For simplicity of understanding the star and delta connections are drawn in some different way .

                       

DIFFERENTIAL PROTECTION

This is the most common scheme used for alternator protection differential protection scheme for a star connected alternator.

Six identical CTs are used for this scheme. The secondary’s of CT area connected in star through pilot wires. The relay operating coils (R1, R2, R3) are connected across neutral pilot wire end. The relays are connected between equi-potential points respective line pilot wires of CT secondary side. The relays used are electromagnetic type. Operation under normal working conditions, the current entering altering the alternator winding is equal to current leaving the alternator winding.

There is induced current in secondary’s of CT these currents are also same in magnitude. These currents circulate in pilot wires but not through the relay coils.

When a fault develops, say earth fault or phase to phase fault, the current entering the alternator winding is not equal to current leaving the alternator winding. So CT secondary currents are no longer equal and difference current flows through operating coil. Thus, relay is energized, and trip signal operates the respective CB to isolate the faulty part.

                                   

Percentage Differential Protection of Alternator/Biased Differential Protection of Alternator

Fig. (a) and (b) shows percentage differential protection scheme for star and delta connected alternators.

                       

For star connected alternator the neutral is earthed through a resistance. The CTs are connected in star. One more coil called restraining coil is added in the circuit as shown. The restraining coil is connected centrally in the pilot wires. The respective operating coils are connected at centre of respective restraining coils. Under normal operating conditions the secondary currents of CTs are equal. Thus, there is balanced circulating current in the pilot wires and the restraining coil and current does not flow in the operating coil.

EARTH FAULT PROTECTION OF ALTERNATOR

When the fault current flows through earth return path, the fault is called earth fault. The earth faults are frequent. So earth fault protection is important.

Residual earth fault relay: When earth fault does not take place, the sum of three secondary currents of CT is zero. i.e. I_R +I_Y + I_B =0.

The sum of these currents is called residual current. When earth fault develops, the sum of three currents is not zero, the residual current flows through earth fault relay.

                                   

This protection is also called unrestricted earth fault protection. In another scheme, the relay is connected in neutral of the transformer.

                                   

The neutral acts as CT, primary. When fault occur, the fault current flows through this earth connection and secondary current increases causing earth fault relay operation. The amount of neutral current flowing does depend on type of earthing system provided.

This is also called unrestricted earth fault protection.

Earth Fault Protection with Core balance Current Transformer (CBCT)/zero Sequence CT.

A core balance CT is used in this scheme. The core of CBCT has large cross-sectional area so that it does not saturate when fault current produces high flux density in it.

The core embraces the three conductors R, Y, B. Under normal conditions, the components of fluxes produced by three currents are balanced and CT secondary current is negligible and relay does not operate.

When earth fault develops the balance is disturbed and sufficient current is induced in CT secondary, thus relay is operated.

                       

RESTRICTED EARTH FAULT PROTECTION FOR ALTERNATOR

                       

The CTs area connected in star as shown in Fig. The remaining connections are as per percentage differential protection scheme. In addition, one restricted earth fault relay is placed in the star connection of CT secondary’s as shown in Fig.

When earth fault occurs on the winding, the imbalance current flows through the operating coils and restricted earth fault relay. Thus, the restricted earth fault relay operates and trip signal is produced which operates the CB.

INTER-TURN FAULT PROTECTION OR STATOR

INTER-TURN FAULT PROTECTION

The normal differential protection scheme can protect the alternator against phase in ground or phase to phase faults. It cannot protect the alternator from inter-turn faults. This is because current entering and leaving, the alternator winding remains the same. Even though some turns of any phase are shorted. So CT secondary currents do not change and operating coil does not carry any current.

The inter-turn faults later on develop earth fault, so inter-turn fault must be detected at an early stage.

Fig. shows arrangement for inter-turn fault protection scheme. We have shown it for one phase only for simplicity of under standing. For three phases alternators there will be such three groups.

                                               

This scheme is used for multi-turn alternator, the winding of each phase is divided into two equal sections (S₁ and S₂) as shown in figure. Two identical CTs are connected in respective windings. The CT secondary’s are connected in phase opposition and a relay R is connected in CT secondary loop.

The functioning of this scheme is similar to differential protection principle.

Under normal operating conditions, the currents through S₁ and S₂ are same, so CT secondaries carry same current. And no current flows through relay R.

If the adjacent turns of S1 are shorted, currents shared by S1 and S2 differs hence there is difference in CT secondary currents. The difference current flows through the loop and relay now carries current and produces trip signal for CB.

OVERHEATING PROTECTION

Stator Overheating Protection: Overheating in stator may be caused by the failure of the cooling system, over loading or core fault like shorted laminations.

Modern alternators 9above 2MW) employ two methods to detect overheating.

In one method, the inlet and outlet temperatures of cooling medium are measured. Cooling medium may be gas (hydrogen) or liquid (water). The inlet and outlet temperatures of cooling medium are compared and if drastic difference is there, it means over heating is occurring. Thus, overheating can be detected.

In second method, temperature sensors are embedded in stator slots. The temperature sensors may be a RTD (Resistance Temperature Detector), thermistor, thermocouple.

                                               

Whetstone's bridge principle is used to detect overheating. P, Q and S are the fixed value resistances. The RTD is connected in one arm as shown. This RTD is kept in stator slot of alternator. When temperature is within limits, point a and b are he same potential and relay coil does not carry any current i.e. bridge is balanced. When overheating occurs, RTD resistance changes causing imbalance and some potential difference is created across a and b. The relay coil is thus energized and relay operates the alarm circuit.

Negative Sequence Protection of Alternator Against Unbalance Load

When stator currents are unbalanced, they produce negative sequence component. This component rotates at synchronous speed and in opposite direction to rotor. This cause double frequency currents to be induced in the rotor. So, rotor iron losses increase and heating of rotor occurs. The double frequency current also produces vibrations and heating of stator.

Figure shows a scheme for protection against unbalance loads.

Three CTs are connected in star and the secondary’s are connected to negative sequence filter, over current relay is connected to sequence filter.

The negative sequence filters consists of number of inductors and resistors. This negative sequence filter detects presence of negative sequence component due to unbalance and operates.

                                   

REVERSE POWER PROTECTION

The turbine drives the alternator. Alternator is connected to supply system through transformers, bus bars etc. And at a time many alternators are connected to the supply system. But when input to any one turbine is stopped, the alternator continues to rotate as a motor i.e. it takes electrical power from supply and supplies mechanical input to the turbine so it drives the turbine.

So previously the power flow was from turbine à alternator à supply system and no it is from supply system à motor (alternator acting as a motor) à turbine i.e. power flow reverses. When electrical power flows in reverse direction, it still maintains the balance in three phases. So reverse power flow can be detected by using a single relay in any one phases.

Directional relay can be used to detect reverse flow. Figure shows induction type directional relay to detect reverse power flow.

                                   

The shunt magnet coil and series magnetic coil are excited from any one alternator winding. When power flow direction is correct the disc rotates in normal direction. But when power flow reverses, the disc rotation is in opposite direction and opposite rotation causes trip circuit contacts to close. Thus, trip signal is produced.

SOLVED EXAMPLE

A 20 MVA 11kV, 3-phase star connected alternator is protected by Mertz-Price protection scheme. The star point is earthed through a resistance of 5Ω. If CTs have a ratio of 1000/5 and the relay is set to operate when there is an out of balance current of 1.5 Amp.

Calculate:

i)        The percentage of each phase of the stator winding which is unprotected.

ii)       The minimum value of earthing resistance to protect 90% of the winding.

Solution: i) Let X% of the winding is unprotected, Earthing resistance r_e = 5Ω

                                          11 x 10³

V_ph = Voltage per phase = ---------  = 6350.85 volts

                                              √3

The minimum fault current to operate the relay = 1000/5 x 1.5 = 300 Amp.

e.m.f. induced is X% of stator winding, = V_ph x (X/100) = 63.508 x Volt.

Earth fault, current that will flow because of voltage induced in X% winding

=  63508X     63.508

------------- = ------------ = 12.7X amps.

       r_e              5

This current should be equal to minimum fault current required to operate the relay.

      12.7X = 300       so X = 23.62%

ii) Let re = minimum earthing resistance required to protect 90% of the stator winding. The 10% winding will be unprotected.

                                                                        63.508 x 10

i.e.         X = 10%    300 = 63.508X/re    300 = ---------------- =  12.7X Amp,  re = 2.12Ω.

                                                                                r_e

EXERCISE

1.    State the abnormalities and faults in alternator with necessary protection.

2.    Draw a diagram of differential protection scheme for a star connected alternator and explain its working.

3.    Draw a diagram of percentage differential protection scheme for delta connected alternator and explain its working.

4.    Explain earth fault protection of alternator using CBCT.

5.    With a neat sketch explain overheating protection using Wheatstone bridge.

6.    Draw a protection scheme for double frequency current using negative sequence filters.

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