Turbine Part 7

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Lecture 7

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Excitation System, Auxiliariesand Protections


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Generator Excitation

The purpose of the generator excitation system is:

To supply rotor field current to the main generator

To control generator output voltage

The excitation may be from a separate DC exciter or through self-excitation

The present trend is the use of separate DC exciter (static excitator) instead of self-excitation

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Following is the principle deployed for self-excitation:

Rotating shunt wound DC generator provides DC excitation

DC generator is coupled to the main generator and is driven by the main TG shaft

through a reduction gear, to reduce sparking at the commutator brushes

For startup, voltage is built up by residual magnetism and self-excitation

Main generator field current is controlled by field rheostat of the exciter

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The following is a description of a static excitation system of a typical NPP:

The excitation transformer is supplied from the Class IV supply

The voltage is reduced and supplied to the Silicon Controlled Rectifier (SCR)

SCR converts the AC voltage to DC

The output of the SCR is varied by the action of the Automatic Voltage Regulator

(AVR)

The DC voltage is then applied to the generator slip rings when the main field

breaker is closed

The generator output voltage varies as per the field voltage

The comparator compares this output voltage, sensed through PT, with the voltage

as set by the control room operator

Any change as sensed by the AVR is used to adjust the main field current to

eliminate the difference in the generator output voltage with

set value 4


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Simplified Version of Static

Excitater of a Typical NPP

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Generator Neutral Grounding

The purpose of the neutral grounding transformer is threefold:

To limit generator winding stresses to maximum design levels by limiting fault

currents

To raise neutral above the ground potential to provide sensitivity to the protective

relaying

To block capacitive voltage transients from any relaying operation

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Generator Auxiliaries

The generator auxiliaries consist of:

The generator stator water cooling system

The generator hydrogen seal oil system

The generator hydrogen cooling systems

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Generator Auxiliaries

Stator Water Cooling System (For Illustration from a Typical NPP)

2 x 100% pumps for circulation of water

Power: 415 V, Class IV, 22 kW

Flow: 60 m3/hr discharge pr 6.6 kg/cm2

Standby pump starts when either

i. Running pump trips or

ii. Discharge header pr < 5.5 kg/cm2

2 x 100% shell tube type HXsCooling NAPW

Water temperature at HX outlet is around 40C

Expansion tank is pressurized with nitrogen, at 0.1 to 0.15 kg/cm2

Low flow alarm at 38 m3/hr, TG trip at 25 m3/hr

Low or high conductivity alarm for operator action

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Simplified Diagram of a Typical

Stator Water Cooling System

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Seal Oil System

Hydrogen Seals

Required to prevent the escape of hydrogen from generator casing at the two ends

of the rotor shaft

Shafts seals are fitted at each end with compatible seal oil system

Oil flow also cools and lubricates the seal surfaces

Shafts seal of radial ring type uses seal ring that is babbitted on shaft journal side

Oil is used to seal the gap between the seal and shaft journal

The seal oil is supplied to the sealing gap from the seal body via radial holes and

groove

The seal rings are free and remain in floating condition

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Seal Oil System

Hydrogen Seals (contd)

The oil pressure in the annular gap is maintained slightly higher than the hydrogen

gas pressure to prevent hydrogen escape from the generator casing

The seal oil from the sealing gap drains out from both the side of seal ring

Seal body collects the drains from both the side in two separate drain chambers

and they are led by two separate drain lines to seal the oil system

The oil is supplied by a closed loop system called seal oil system

At least one seal oil pump must be fed by Class II because seal oil supply is

required at every instance, irrespective of shaft rotation

Hydrogen purity must be >96% to avoid an explosive hydrogen/air mixture

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Use of CO2for Purging

When changing the casing atmosphere from air to hydrogen or hydrogen to air, it

is necessary to have a CO2interface

Carbon dioxide is supplied for purging the casing whenever the generator is filled

or emptied of hydrogen

The use of this inert intermediary gas for scavenging prevents the formation of an

explosive mixture of hydrogen and air within the casing

Intermixing is prevented by the difference in the densities of the gases

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Gen Hydrogen Cooling System

It removes the heat accumulated by cooling hydrogen for maintaining the

generator rotor and stator iron within its temperature limits

The raw service water hydrogen coolers are within this circuit

Generator casing hydrogen pressure: 4.0 kg/cm2

2 x 100% refrigerant type dryer units

Hydrogen dryers location is the turbine building

Additional 10 m3capacity carbon dioxide tank provided to store CO2at lower

pressure

Provision to purge the hydrogen from the control room

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Turbogenerator Protections on

Electrical FaultsTo safeguard the generator and turbine, the following classes of trips are initiated on

detection of certain abnormal conditions

1. Class A trip

Tripping of generator breaker and simultaneous trip signal to turbine main trip

solenoid for tripping of turbine due to electrical fault in the generator/grid

system

2. Class B trip

Tripping of turbine and generator breaker trips on low forward power relay

due to the fault in mechanical parameters of TG and its auxiliary systems

3. Class C trip

The generator is isolated from the grid due to the grid low frequency, etc., andruns on station load

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Types of Protections

1. Differential protection (Differential-87)

This is a high speed protection against internal phase and phase to ground

faults of stator windings

2. Stator ground (64)

Following ground fault in the stator winding, an unbalanced voltage is sensed

by the overvoltage relay

Also, due to internal and phase to ground fault occurs excessive current flow

in the neutral circuit causing a voltage drop across the loading resistor in the

secondary of the neutral grounding transformer

This is monitored by another overvoltage relay

Nominal time delay of one second are included to prevent spurious trips

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3. Underfrequency protection

The generators are specified to be capable of continuous operation with

underfrequency up to say 5% or as specified by the manufacturer

A two stage underfrequency relay operates and trips all line breakers if

underfrequency limit is about to be crossed with appropriate delays (say 2

minutes or 0.1 second when very near the limit)

4. Loss of excitation protection (40)

This relay operates after a delay of 0.5 seconds, when reactive power flows

towards the generator beyond the underexcited limit

5. Phase unbalance protection (46)

The current in each phase is measured by CTs Based on these inputs, a relay is used to determine whether there is an

unbalance in the phase currents or phase/to ground faults

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6. Out-of-step (78)

Out-of-step protection is designed to prevent generator instability that could

lead to pole slipping and turbine overspeed

It provides some degree of loss of excitation protection as well as protection

against system related transients that could cause the generator to go out of

synchronism with the grid

The out-of-step protection monitors phase currents at the generator neutral

and phase voltages at the output. These are input relays, both of which must

be picked up to activate the trip.

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7. Overexcitation protection (59R, 64F)

The overexcitation protection is intended to protect the generator from

damage due to overfluxing

The protection is available during startups and shutdowns as well as normal

operation

The V/Hz ratio (i.e., generator terminal volts/frequency) is sensed as it is

proportional to the flux density in the iron circuits of the machines

Under overfluxed conditions, the core losses (and hence temperature rise)

becomes excessive, leading to damage and in the extreme case, structural

steel components of the generator

The main transformer and service transformers are also covered for theseprotections

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8. Low forward power relay (32A & 32B)

This relay trips the generator breaker when power flowing to the grid reduces

to 1% FP, e.g., due to reactor trip

9. Supplementary start

During startups and shutdowns, the supplementary start protection provides

two of the more basic protectionsstator ground and phase unbalanceat

frequencies other than 50 Hz, when all other protection may not be effective

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Turbine Part 7

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