Generator Protection

A generator could be part of:

• water turbine based hydropower station
• gas turbine based power station
• steam turbine based thermal power station
• nuclear power station

Generator Protection

• In case of a fault on a turbo-alternator, it is not enough to open the main circuit breaker connecting it to the power grid. For example, when a turbo-alternator driven by a steam turbine is tripped, the following must be done:
• Steam supply to the turbine is stopped or bypassed.
• Firing of the boiler is stopped.
• Coal mills are stopped.
• Coal supply to the coal mills is stopped.
• Field circuit of the alternator is interrupted.
• Field coils are connected across a resistor to dissipate the stored energy.
• Alternator is kept running at a slow speed (few rpm) with the help of barring gears till it cools down uniformly to avoid unequal expansions.

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Generator Protection

• Putting back the alternator on line is rather a slow process because all the parameters (temperatures and pressures) have to be progressively built up to avoid thermal shock resulting in uneven expansions which might cause unacceptable vibrations.
• Therefore, unscheduled outage of a thermal power station is avoided as far as possible. We have to keep in mind that a modern large turbo-alternator is a huge mass rotating at a very high speed (3000 rpm) in a very small air-gap.

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Generator Protection

• Thus, apart from the large electrical energies involved, there is tremendous amount of mechanical energy in the form of moment of inertia of the rotating mass and resultant forces on the shaft, the rotor body and the stator structure.
• Thus, any slight increase in temperature or uneven heating of the rotor may cause eccentricity, which gets accentuated because of the high speed of rotation and small airgap.
• The entire system has, therefore, to be run in a narrow range of various parameters like temperatures and pressures, displacements, flows, voltages, currents, power factor, and so forth. The operation outside the specified parameter range may cause a substantial decrease in the life of the equipment.

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Electrical Circuit of the Generator

Electrical Circuit of the Generator

Generator is never solidly grounded:-

If it were solidly grounded, the single line-to-ground fault current would be dangerously high. Apart from the high value of fault current, the resulting asymmetry in the rotating magnetic field inside the generator would cause unacceptably large vibrations and result in mechanical damage to the rotor.

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In order to limit the short-circuit current, the neutral of the generator is grounded through a resistance. In order to get a practicable value of the grounding resistor, it is connected through a step-down transformer, known as grounding transformer.

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Electrical Circuit of the Generator

Electrical Circuit of the Generator

The power plant has a sizeable auxiliary electrical load of its own, of the order of 10% of the power rating of the generator, which is supplied through the unit auxiliary transformer (UAT). It is to be noted that these auxiliaries require power even before the generator can be started, run up to speed and synchronized with the grid. Hence, there is the switching facility to energize the UAT directly from the grid.

Electrical Circuit of the Generator

The rotor of the generator houses the field winding. A separate dc generator, which is mounted on the turbo-alternator shaft, feeds the field. The dc system is kept floating with respect to the ac ground, i.e. neither the +ve nor the –ve terminal is grounded.

Electrical Faults and abnormal operating conditions

Stator Faults

• Stator Faults The three-phase armature winding on the stator can develop phase as well as ground faults. Another possibility is inter-turn faults between turns of the same phase.

Stator Phase and Ground Faults

Longitudinal percentage differential protection.

Stator Phase and Ground Faults

This gives rise to a larger spill current, during normal load and external faults, in case of the transformer differential protection than in case of the generator differential protection. Normally, the percentage bias setting of 5–10% is adequate for longitudinal differential protection of the generator stator winding whereas a setting of 20–40% may be required in case of power transformers.

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Transverse Differential Protection

Rotor Faults

Abnormal Operating Condition

• Unbalanced Loading

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Abnormal Operating Condition

• Unbalanced Loading

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Over-speeding

Loss of Excitation�

There are several possible causes due to which field excitation may be lost, namely:

• Loss of field to main exciter
• Accidental tripping of the field breaker
• Short circuit in the field winding
• Poor brush contact in the exciter
• Field circuit breaker latch failure
• Loss of ac supply to excitation system

Loss of Excitation�

Protection against Loss of Excitation Using Offset Mho Relay

Loss of Prime Mover

Directional relay for protection against loss of prime mover

Induction motor protection

Various Faults and Abnormal Operating Conditions

Starting Current

Electrical Faults�

• Fault on Motor Terminals
• Phase Faults Inside the Motor

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Electrical Faults�

Electrical Faults�

Ground Faults Inside the Motor�

Inter-turn Faults�

Abnormal Operating Conditions from Supply Side

Unbalanced Supply Voltage

Abnormal Operating Conditions from Supply Side

Unbalanced Supply Voltage

Abnormal Operating Conditions from Supply Side

Unbalanced Supply Voltage

Single Phasing

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Reduction in Supply Voltage

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Reversal of Phases

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Abnormal Operating Conditions from Mechanical Side

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Failure of Bearing and Rotor Jam

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Overload

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Resistance temperature detector relays (RTDs)

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Thermal replica relays

Data Required for Designing Motor Protection Schemes

• HP rating
• Supply voltage
• Full-load current
• Permissible continuous allowable temperature rise
• Locked rotor current
• Permissible maximum time with locked rotor
• Accelerating time
• Starting voltage