Page 1 15 August 2008

Generator Protection
V.Rakesh, P.Sankara Santhoshini, M.K.Hemalatha, T.Sujeeth Kumar

ABSTRACT

Generator is the heart of the power system and it is costly Equipment. Many types of faults will
be occurring in the power system. So, there is necessity to protect the generator from those faults.

In generator faults may occurs due to short circuit, voltage balance, over voltage, field failure,
over flux, stator and rotor earth faults and other abnormal conditions.

If a short circuit persists on a generator for a longer period, it may cause a fire. It may spread in
the system and damage a part of it.

Our article constitutes the Need for Protection, Causes and Effect of various Faults on generator
and employed few protective techniques.

INTRODUCTION

1.1. NEED FOR PROTECTION:

If a short circuit persists on a generator for a longer period, it may cause a fire. It may spread in the system and
damage a part of it. The system voltage may reduce to a low level and individual generators in a power station or group
of generators in different power stations may lose synchronism. Thus, an unclear heavy short circuit may cause the total
failure of the system. So there is need for protection techniques to overcome the faults occur in the generator.

1.2. CAUSES OF FAULTS:

Faults are caused either by insulation failures or by conducting path failures. Over voltages due to lighting or
switching surges cause flashover on the surface of insulators resulting in short circuits.

1.3. FAULTS IN GENERATOR:

STATOR FAULTS

1) Phase-to-phase faults,
2) Phase-to-ground faults, and
3) Inter-turn faults.
The danger of these faults is that they may lead to damage the laminations due to heat generated. Hence it
needs particular re insulation and re building of the core, which is very costly and time consuming.

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ROTOR FAULTS:

There may be ground faults or short between the turns of the field winding, caused by the severe mechanical
and thermal stresses acting upon the winding insulation.

ABNORMAL CONDITIONS:

These conditions involve:

1) Unbalanced loading,
2) Overloading,
3) Over speed,
4) Over voltage,
5) Loss of excitation.

The unbalanced loading results in circulation of negative sequence currents in the stator winding which
gives rise to a rotating magnetic field. This field rotates at double the synchronous speed with respect to the rotor
and induces a voltage of double the frequency in the rotor conductor. If the degree of unbalance is large these
currents will overheat the rotor stamping and the field winding.

Overloading of the stator will overheat the stator winding which may damage the insulation depending
upon the degree of overloading. In case of hydraulic generators a sudden loss of load results in over speeding of the
generator because the water flow to the turbine cannot be stopped quickly because of mechanical and hydraulic
inertia.

Over voltages are caused by over speeding of the generator or due to faulty operation of the voltage
regulator.

The loss of excitation of a generator may result in loss of synchronism and slightly increased generator
speed since the power input to machine remains unchanged. The machine, therefore, behaves as an induction
generator and draws its exciting current from the system which is equal to its full load rated current. This leads to
overheating of the Stator winding and rotor body because of currents induced in the rotor body due to slip speed.

EFFECTS OF FAULTS:

The most dangerous type of fault is a short circuit as it may have the following effects on a generator, if it
remains uncleared.
1. Heavy short circuit current may cause damage to equipment due to over heading and high mechanical forces set
up due to heavy current.
2. There may be reduction in the supply voltage of the healthy feeders, resulting in the loss of industrial loads.
3. Short circuits may cause the unbalancing of supply voltages and currents, thereby heating rotating machines.
4. Arcs associated with short circuits may cause fire hazards. Such fires, resulting from arcing, may destroy the
faulty element of the system. There is also a possibility of the fire spreading to other parts of the system if the
fault is not isolated quickly.

VOLTAGE BALANCE PROTECTION

DESCRIPTION:
Three attracted armature units are employed as shown in fig. Unit-A is fed from a
resistance/capacitance network, which under healthy conditions with negligible harmonics has Zero output.

Unit-B has three changes over contacts and this unit drop off an operation of Unit-A.

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Unit-C is a DC auxiliary unit energized by normally closed contact of Unit-B. The Unit-C has 30-
40nsec time delay on pick up.

Fig 1.1 Voltage Balance Relay Circuit

OPERATION:
In normal condition the 3-phase supplies is balanced and coil A is not energized. Thus coil switch of A
is closed. This leads to current through coil B from rectifier. Thus energizes the coil B, this opens B switch and C is
not energized when supply is given. If a fault occurs the imbalance takes place and fault current flows through coil
A from rectifier. This leads to opening of coil A switch and coil B deenergises. During this coil B switch is closed
and DC positive is extended to the coil C and coil C is energized. This changes the contacts of trip and alarm
circuits. Thus operates the relay and circuit breaker.

OVER VOLTAGE PROTECTION

PRINCIPLE OF OPERATION:
The generator output voltage is given to the relay as input by using a P.T. If the generator output
voltage exceeds the setting value the relay will operate.

DESCRIPTION:
The rectifier output is applied to a series circuit comprising a resistor and a zenor diode. The voltage
across the resistor varies with the input and is fed via a level setting potentiometer as a signal to the measuring
circuit.

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Fig 1.2 over Voltage Protection Relay

RLY1 for Alarm

RLY2 for trip ckt

The level detector compares this signal with a preset voltage. If the present value is crossed, a
timing capacitor begins charging at a rate determined by the resistance of a time setting potentiometer. This charging
continues until the voltage exceeds a present level, at which time a drive is triggered. A positive feedback
arrangement ensures a rapid circuit to operate the hinged armature output element. This element operates contacts
for trip and external alarm purpose as well as an operating indicator.

When the single voltage is restored to normal a fast discharge circuit ensures very short rest and
overshoots times. The mechanical flag indicator must be hand reset by means of a level which protects through the
case.
1. External over voltage due to lightening
2. Internal over voltage due to switching operation.

OVER FLUX PROTECTION

DESCRIOTION:
In general this scheme is applied for protection of generator transformers. The flux density ‘B’ in the
transformer core is proportional to V/F (B; V/F). Power transformers are designed to with stand continuously for
supplying core is designed such that V/F causes higher core loss and core heating. The capability for V/F generator
transformer and UAT occur if full excitation is applied to generator before full synchronous speed is reached V/F relay is
provide in AVR of generator, this block and prevents incasing excitation current before full speed and frequency is
reached.

The magnetic flux density of a transformer core is a function of V/F. hence the relay senses the magnetic flux
condition. Over fluxing relay is provided with enough time lags.

This over flux relay is not required in case of substation since V/F relay is provided in.

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Fig 1.2 over Flux Protection Relay Schematic Diagram

OPERATION:
The fig. shows block diagram of over fluxing relay. In general negative DC supply is extended to VAA relay
through fuse. When over coil is energized. Thus contacts in relay are changed. This leads to AVR supply decreasing
automatically and manually to generator. At the same time opens the terminals of AVR raise manually and
automatically. If the fault is not isolated the relay 2 is activated for certain time delay. Supply is extended to generator
field circuit breaker trip coil and trips the faulty terminals. Thus the fault is isolated (During this fault the magnetic core
will be saturated).

CONCLUSION:
We conclude that, there is a need for generator protection techniques to overcome the faults and we included
few protection techniques in this article.

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