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Large induced currents in rotor leads to rotor end part over heating. 

Leading VAR taken from grid leads to severe voltage dips in grid, if grid is weak. 

Stability of machine lost. 

Stator overheating. 

Machine speed rises slightly. 

When a line is switched on high voltage will appear on the line due to its inductance and capacitance. This voltage is known as switching surges. 

a. Correct mV drops brush to be used. 

b. Brush tension adjustment. 

c. Air cleaning to reduce leakage current. 

d. Brush bedding before use. 

e. Field polarity change every 6 months.  

No only if generator breaker is off.  

To cater thermal expansion and contraction due to heating and to eliminate mechanical vibrations to the equipment. 

In TG motoring prevented due to the turbine limitation otherwise last stage blade will fails. 

In DG motoring prevented, because of unburned fuel catches fire in DG. 

This relay is suitable for protection of two or three winding power transformers, autotransformers or generators transformer units. 

The differential transformer protection measuring circuit is based on the well-known Merz-price circulating principle.

P * ∅ * Z * N / 60 * A

A = number of parallel paths. That is for lap winding it is equal to Z and for wave winding it equal to 2.  

For generator phase to phase faults and 3 phase faults. 

To prevent over fluxing of transformers or generator. 

 Emf = 4.44 f φ Z A

If f frequency is reduced due to less speed, 

φ = V / f Z A 

And flux will be more to saturate the core of transformers or generator. 

Non-magnetic stainless steel nut and bolts are used in IPBD to restrict magnetic effect at joints.

Filters, Vacuum pumps, Expansion tank, and magnetic filter. 

Surge capacitor and lightning arrestor. 

To detect rotor earth fault. 

No earth fault current limited to 5A, while differential setting is 10%

a. PT supply fail. 

b. Auto pulse fail. 

c. Supply of limiter fail. 

d. Supply of auto channel fail.

e. Regulated supply fail. 

f. High auto reference. 

Matt black paint, for efficient heat dissipation. 

a) Voltage drop due to resistance (R_a drop). This is negligible. 

b) Voltage drops due to leakage reactance (X_L). 

c) Voltage drops due to armature reaction. 

AVR, Protection & Metering.

Vibration increases. 

°C = (R-100) / 0.39 

The effect of armature flux on the main field flux is called Armature reaction, where armature flux weakens the main field flux. In Alternator, power factor contributes more importance in Armature reaction. 

a) In Unity, power factor field strength is average and effect is distortion. So voltage variation will not be too much. 

b) In lagging, power factor armature flux is directly opposite to the main field flux. That is armature flux is lagging 90ο by main field flux. So the result is demagnetizing the field. Due to less field flux, less voltage at the alternator terminals and excitation required is more. 

c) In leading, power factor armature flux is leading by 90ο to the main field flux. The result is additive and main field strength is more and excitation has to be reduced. Otherwise, end parts or overhang portion of the generator will heat.

Because loss of feedback to voltage corrector.  

For TG nil 

For DG 110% for 2 hours. 

There are three liquid detection devices provided for the same purpose.

The annular gap between stator and rotor of the generator are to be sealed to prevent hydrogen leak from the casing. 

Type of seal – ring type shaft seals 

Pressure of seal oil – 4 kg/cm²

Generator – GT overall differential protection. 

Burden of PT is less and burden of normal transformer is high. 

Partial discharges are electrical sparks, which occur in gas voids within the insulation when the voltage is high enough. The discharges are partial since there is some insulation remaining to prevent a complete breakdown. Partial discharge can erode the insulation and therefore contribute to insulation ageing.

This can be tested by an electromagnetic probe, which is a detector that is sensitive to the radio frequency signals produced by the partial discharges within the winding. With this probe test, it is possible to locate specific sites of deterioration within the winding when the winding is energized. 

A 100% stator earthfault protection is designed to detect earthfault occurring in the regions of machine winding close to the neutral end. This relay is a composite modular relay that gives 100% stator earthfault protection for machines, whose neutral are not directly earthed. It works on the principle involving monitoring of the neutral side and line-side components of the third harmonic voltages produced by AC generators in service. 

Rotor resistance = 98.1 mΩ

 Stator winding resistance’s 

R φ = 3.1 /3.1 Ω

 Y φ = 3.1 /3.1 Ω 

 B φ = 3.1 /3.1 Ω 

The cooling water must have an electrical conductivity less than 2.5 micro mho/cm. One portable polishing unit consisting of mixed bed is also provided in the system to remove impurities and maintain stator water conductivity at a less value. 

Magnetic filter is provided to catch the metal particles in stator water circuit, which are produced in the pipelines. This is mounted at the end of the circuit nearer to the inlet of the generator. 

Injecting inlet steam to the prime mover can vary active power. Reactive power can vary by the Generator main field voltage variation. An excitation change PF at which load is delivered. 

Active power is produced by source and used effectively. VAR is the power used for magnetization of core of transformers, motors, generators, overhead transmission lines (capacitive), household appliances etc.  

NGT – 1 phase, natural cooled, indoor dry type, 16.5 kV / 250V, 50 kVA. 

NGR – natural cooled, stainless steel grid type, 0.5Ω, 250V, 288A (continuous) and temperature rise allowed to 375 °C. 

This is to find out weakness in the stator winding. If the ambient conditions are right, and the insulation is weak, the leakage current will increase non-linearly. 

The record of voltage versus leakage current provides the condition of the winding for present and future use and may permit prediction of breakdown voltage whether it is within or slightly above the test voltage. 

HV test voltage = 1.5 * rated voltage for AC 

And DC voltage = (2E + 1 kV) 1.6 

Where E – rated voltage.

1.6 – AC/DC conversion factor.

Application of HV voltage also depends on the age factor or condition of the machine. 

Following are the findings of HV test. 

a) Capacitance charging current. 

b) Dielectric absorption current. 

c) Surface leakage current. 

d) Partial discharge current. 

e) Volumetric discharge current. 

In HV test starting leakage current should be more than switching off current in μ Amps. 

Surge protector – non-inflammable, synthetic liquid impregnated and hermetically sealed, 24KV, 0.25µ f (micro farad). 

PT – 16500/√3 /110/√3 volts. Fuse – 24kv, 3.15A. 

RSO test is performed to detect faults in rotor windings. The electrical faults in generator rotors fall into two main categories. The faults from the winding to the body and the faults between the parts of the winding (inter-turn faults). The existence of the faults will frequently display excessive mechanical vibration and cause serious concern.

The wave nature of radiation cannot explain the following : 

(i). The instantaneous ejection of photoelectrons 

(ii). The existence of threshold frequency for a metal surface. 

(iii). The fact that kinetic energy of the emitted photoelectrons is independent of the intensity of the incident light but depends upon the frequency of the incident light.

Correct Answer is: (b) 3Ze/πR³

Correct Answer is: (c) a = R

Correct Answer is: (d) none of these