Submitted by: Name:Rajendra Kumar Choudhury Branch:Electrical Engg. Roll no:0601101160 7th sem

Introduction: In a generating station the generator and transformer are the most expensive equipments and hence it is desirable to employ a protective system to isolate the faulty equipment as quickly as possible to keep the healthy section in normal operation and to ensure uninterruptable power supply. The basic electrical quantities those are likely to change during abnormal fault conditions are current, voltage, phase angle and frequency . Protective relays utilizes one or more of these quantities to detect abnormal conditions in a power system.

Generator protection: Stator protection Differential Restricted earth fault Negative sequence current Loss of load

Cont.. Rotor earth fault Over speed Over voltage Loss of field Back –up over current

Differential protection: CTs are provided at each end of the generator winding which is to be protected.(as shown in fig.) When there is no fault the differential current (I1-I2) through the relay is zero. So the relay will not operate. When the fault occurs the balance is disturbed and differential current (I1-I2) flows through the operating coil of the relay causing relay operation and the trip circuit of the circuit breaker is closed.

Differential protection of generator:

Differential protection using balancing resistor:

Biased differential protection

Modified differential protection: Generally protection is made for 80 to 85% of the winding. If any fault occurs near the neutral point then the fault current is very small and relay does not operate. Modified differential protection scheme is used to over come this. Two phase elements (PC and PA) and balancing resistor(BR) is connected in star and the earth relay(ER) is connected between the star point and neutral pilot wire.

Modified differential protection

Restricted or balanced earth fault protection: In case of small size generators star point is not available because it is made inside the generator and grounded through some low resistance then percentage differential relay for ground fault is provided and is known as restricted earth fault protection. This scheme can be used only for ground faults but not for phase faults.

Restricted or Balanced earth fault protection

Stator protection: Stator faults include the following- Phase-to-earth faults Phase-to-phase faults Inter-turn faults From these phase faults and inter turn faults are less common ,these usually develop into an earth faults. This causes- Arcing to core Damage of conductor and insulation

Stator inter-turn fault protection: Inter-turn fault on the same phase of the stator winding cannot be detected by transverse differential protection as it does not disturb the balance between the currents in neutral and high voltage CTs. For protection against inter-turn faults the following protection schemes are used. (1)Cross differential protection. (2)Residual voltage protection.

Cross differential protection: Used in case of hydro-electric generator having double winding armature. As shown in figure relay Rc provides protection against phase to ground and phase to phase fault. The relay R1 provides protection against inter-turn faults.

Residual voltage protection: These method is used in case of alternators those don’t have parallel stator windings. During normal operation VRES=VRN+VBN+VYN=0. In case of fault VRES is not zero and this residual voltage operates the relay.

Rotor faults: Faults in the rotor circuit may be either earth faults or between the turns of the field winding . Field circuits are normally operated un-earthed. So a single earth fault will not affect its operation. But when a second fault arises then field winding is short circuited and produce unsymmetrical field system which leads to unbalanced forces on rotor and results in excess pressure and bearing and shaft distortion.

Rotor earth fault protection: The rotor earth fault protection is done by “dc injection method or ac injection method”. The dc or ac voltage is impressed between the field circuit and ground through a sensitive overvoltage relay and current limiting resistor or capacitor(in case of ac). But dc source is generally used as over-current relay in case of dc is more sensitive than ac. A single earth fault in rotor circuit will complete the path and the fault is sensed by the relay.

Rotor earth fault protection

Rotor temperature alarm It is provided in large generators. It indicates the level of temperature but not the actual hot spot temperature. The relay measures the temperature by measuring the resistance .(as shown in fig)

Loss of excitation protection: When the excitation of generator is lost it operate as a Induction generator. It derives excitation from the system and supply power at leading power factor. Which may cause- A fall in voltage & so loss of synchronism & system instability. Over heating of rotor due to induction current on it. A protection having MHO characteristic is used to detect loss of field.

Over voltage protection: Overvoltage protection is required in case of hydro-electric or gas turbine generators but not in case of turbo generators. Over voltage may be caused due to- Transient over voltage in the transmission line due to lightening. Defective operation of the voltage regulator. Sudden loss of load due to line tripping. The protection is provided with an over voltage relay. It is usually of induction pattern with an IDMT Characteristic

Overload protection: Overloading of the machine causes overheating in the stator winding. This can be prevented by using over-current relay with time delay adjustment. But overheating not only depends on over-current but also the failure of the cooling system in the generator. So temperature detector coils such as thermistors or thermocouples are used at various points in stator winding for indication of the temperature.

Reverse power protection: When prime-mover fails machine starts motoring and draws electrical power from the system and this is known as inverted operation . The generator can be protected from inverted operation by using single-element directional power relay(reverse power relay) which senses the direction of power flow.

Negative phase sequence protection: Unbalance may cause due to single phase fault or unbalanced loading and it gives rise to negative sequence current . This current in rotor causes rotor overheating and damage to the rotor. This can be protected by negative sequence current filter with over current relay.

Negative phase sequence protection:

Automatic field suppression and use of neutral circuit breaker: In case of a fault in the generator and though the circuit breaker is tripped ,the fault continues to fed as long as excitation will exist because emf is induced in the generator itself. Hence all protection system not only trip the generator circuit breaker but also trip the “automatic field discharge switch “.

Automatic field suppression and use of natural circuit breaker

External fault back up protection: Over-current and earth-fault protection is provided for back-up protection of large sized generators protected by differential protection. Induction type IDMT relay is used for this purpose.

External fault back-up protection

Conclusion: Protective relays are used to detect electrical faults and to alarm, disconnects or shutdown the faulted apparatus to provide personnel safety and equipment protection. A protective relay does not prevent the appearance of faults rather takes action only after a fault has occurred in the system.

References: Electrical power system by C.L.Wadhwa Electrical power by J.B.Gupta www.wikipedia.com