Main Turbine Generator & Exciter

Including:

Hydrogen, Stator Cooling Water, Generator Seal Oil, and Iso-Phase Bus & Duct

Systems

Enabling Objectives• STATE the function of

the Main Generator.

• LIST the three requirements for an electrical generator.

• STATE the purpose of the four major components of the Main Generator.

Enabling Objectives (cont..)

• LIST four main generator auxiliary systems and STATE the function of each.

• STATE why Hydrogen gas is used for generator cooling and STATE a special precaution associated with its use.

Function of Generator

• Converts the mechanical energy of the turbine into usable electrical energy

Picture of Generator

Generator

• Three phase, 4 pole AC generator

• Frame pressurized with hydrogen gas for cooling and low windage losses

• Stator windings cooled with circulating demineralized water

• Full load rating of 1504.8 MVA (1354 MW at .9 pf, 1203.84 MW at .8 pf)

Basic Generator Theory

• Requires three elements: conductor, magnetic field, and relative motion

• Power output is proportional to the number of conductors, strength of the magnetic field, and speed of rotation (motion or shaft torque)

Basic Generator Theory

Basic Generator Theory (cont.)

• Number of conductors in the generator is fixed

• Speed (or shaft torque when paralleled) is controlled by the attached steam turbine – provides for power control - MWatts

• Magnetic field strength is controlled by the rotor field current fed by the voltage regulator– provides for voltage control - MVars

Basic Generator Theory (cont.)

• MWatts - megawatts- is actual power (dollar$)

• MVAR - reactive power - represents the total current and voltage losses inherent with AC generation, transmission, and use.

Basic Generator Theory (cont.)

• When the generator is paralleled to the grid:

– power output (MW) is proportional to the torque (or push) of the turbine

– reactive power output (MVAR) is proportional to the rotor field current

Main Generator

Generator Details

• Enclosure Housing: Contains within its boundary the Rotor, Stator, and Hydrogen Cooler.

• The Enclosure Housing is pressurized with hydrogen gas which is used as the cooling medium for the rotor and stator windings.

Generator Details

• Stator : stationary winding through which electric current is induced by the rotating field windings

• Function - collect the induced current and provide a circuit path to the output of the generator (output bushings)

• Stator is cooled by Stator Cooling Water

Stator Bar

Generator Details (cont.)

• Rotor : directly couple to the shaft of the Main Turbine and rotates at 1800 rpm synchronous speed (or 60hz)

• Function - supply a regulated, rotating DC field to induce voltage, and resultant current flow in the generator stator windings

• Rotor is cooled by the hydrogen gas

Picture of Rotor

Generator Details

• Stator Core - securely holds stator windings

• Stator End-Turns - provides winding connections for a continuous circuit

• Hydrogen Blower - circulates cooling hydrogen

• Hydrogen Cooler - heat exchanger for hydrogen gas supplied by OLACW

Generator Details

• Bearings : supports shaft and bears load of the generator weight and torque

• Bushings : provides gas-tight electrical terminal for connection of output power to isolated-phase bus

Function of Exciter

• Controls generator output voltage by varying the current to the generator rotating field windings

Exciter

• Permanent Magnet Generator (PMG) supplies AC power to the Voltage Regulator

• Voltage Regulator supplies DC field current to the Exciter

• Exciter output is rectified and sent to the Rotor through leads in the shaft

Picture of Exciter

Exciter Diagram

Exciter Details

• Self powered from attached Permanent Magnet Generator (PMG)

• PMG consists of 24 permanent magnets located on the end of the exciter shaft. Produces : 120vac at 420hz – PMG also supplies power to the EH Governor

Cab. and Turbine Supervisory Panels

Exciter Details

• Power from PMG is rectified to DC, regulated by the voltage regulator, and applied to the exciter stationary field.

• Larger AC power is produced in the exciter rotating windings and is rectified to DC by diodes mounted on the shaft

• The DC is sent down the internal of the shaft to the attached rotor field

Exciter Details

• Exciter Housing cooled by internal air circulation which includes a heat exchanger cooled by CLACW

• Contains automatic rotor ground detection system, which provides for manual or automatic (once per 24hr) operation

• Exciter bearing ungrounded to prevent circulating currents in the shaft which will attack the bearing seating surfaces

Picture of Exciter

Exciter Details

• The output of the exciter is controlled by the Voltage Regulator– This controls the field current which controls

the output voltage of the generator

• While in parallel, the Voltage Regulator controls MVAR loading.

Voltage Regulator Diagram

Voltage Regulator Details

• Two modes of operation:– Manual - regulator output set by rheostat, non-

regulated or compensated. Sets base voltage output of generator

– Automatic - regulator monitors output and regulates to a desired setting. Sets the MVAR loading of the generator

Voltage Regulator Cabinet

Generator Protection

• Generator trips protect the Generator and Main Turbine in the event of a fault condition within the 25KV system

• Protection is provided by coordination of:– Voltage Regulator Trips / Protective Actions– Field Breaker Trips– Generator Output Breaker Trips

Generator Support Systems

• Generator Hydrogen Gas System

• Generator Stator Cooling Water System

• Generator Seal Oil System

• Isolated-Phase Bus and Duct Cooling System

• Closed Loop Auxiliary Cooling Water System

Hydrogen Gas System

Hydrogen Gas System

• Provides filtered, pressurized (75 psig) hydrogen cooling medium for generator internals to aid in heat removal

• Vital to obtaining full rated output of generator

• Also provides for safe purging of hydrogen gas with carbon dioxide

Hydrogen Gas System

• Besides cooling, the hydrogen gas system functions include– ability to purge to air using carbon dioxide– indication for pressure, temperature, moisture,

and purity– ability to dry and remove impurities– pressurizes Stator Cooling Water system tank

Main Generator

Hydrogen Gas System

• Disadvantages of using Hydrogen– Explosive over wide range of concentration

– Not detectable to senses

• Advantages of using Hydrogen– High thermal conductivity– High specific heat capacity– Low density - low windage losses– Readily available - inexpensive– Does not contribute to corrosion

Stator Cooling Water System

Stator Cooling Water System

• Uses filtered, demineralized water to remove heat from the stator coils.

• Allows for obtaining more power from less copper in the coils

• Also cools Main Lead Bushings, Neutral Bus and Bushings, and Current Transformers (CT’s)

Stator Cooling Water System

Stator Cooling Water System

• Besides providing cooling water to the stator bars, the system also provides:– Cooling to the generator bushings– Cooling to the neutral bus segments– Cooling to current transformers (CTs)

• Heat exchangers cooled by CLACW• Incorporates expansion tank which is maintained

with hydrogen pressure (2 psig) for corrosion control

Stator Cooling Water System

• Stator Cooling Water Skid (located beneath generator on 29’elev. east-side) contains:– 2 full-capacity motor driven pumps (1 run, 1 backup)– 2 heat exchangers– 2 demineralizer beds – 2 filter beds– 2 heater banks– Instrumentation, and alarms

• pressure, flow, core differential pressure• conductivity, oxygen concentration• tank level

Seal Oil System

Generator Seal Oil System

• Provides lubrication to the generator gland seals and acts as a separating medium between internal hydrogen and outside air.

• Two sub-systems– Air-Side Seal Oil – primary sealing oil against

internal hydrogen pressure– Hydrogen Side Seal Oil – secondary sealing oil,

increases efficiency of system to near 100%

Seal Oil System

• Provides for maintaining full rated hydrogen pressure at shaft penetrations to generator frame

• Each side has two seals; air-side and hydrogen-side

• Oil pressure is maintained about 12 psig above generator gas pressure – Differential between oil and gas pressure must be

maintained to prevent leakage

– Pressure at both seals about the same for gas-tight seal

Seal Oil System

• Air-side seal oil system shares oil with main lube oil system

• Hydrogen-side seal oil is a closed loop system with provisions for removing entrained hydrogen gas from the oil

• Each side has its own heat exchanger, supplied by CLACW

Seal Oil System

Seal Oil System

• Seal Oil Skid (located beneath generator on 29’elev. west-side) contains:– 3 seal oil pumps ; hydrogen-side, air-side, and air-

side backup– 2 oil coolers, 2 cuno-type filters – 3 tanks ; hydrogen-side receiver, hydrogen-side drain

regulator, air-side reservoir (includes heater)– 4 regulating valves, 3 pressure relief valves– Instrumentation ; pressures, differential pressures,

temperatures, tank levels, alarm

Isolated Phase Bus and Duct Cooling

Isolated-Phase Bus and Duct Cooling System

• Connects the output of Main Generator (25kV) to the primary (input) windings of the main transformers.

• Provides cooling of the enclosed circuit path bus conductors

Isolated Phase Bus and Duct Cooling

• Consists of– Isolated Phase Bus Conductors

• Tubular, all welded aluminum mounted on porcelain insulators

– Isolated Phase Bus Enclosure Duct• Continuous weather and dust tight aluminum housing suitable

for outside use

– Bus and Duct Expansion Joints• Allows for expansion and contraction of bus and duct due to

loading, temperature changes, and short circuit conditions

– Disconnect Links• Stranded flexible connectors that make connection between

the generator and the isolated phase bus

Isolated Phase Bus and Duct Cooling

• Also:– Isolated Phase Bus Cooling

• The busses between the main generator and the transformer are forced-air cooled

• Consists of two full-capacity fans and heat exchangers

– Surge Protection and Potential Transformer Cubicles

• Contains the components that provide signals to instrumentation and protective relaying

Main Generator Output Breaker

• Connects and disconnects generator output with the AC distribution system for protection of the generator

• 3 breakers poles – one per phase

• Integral part of bus ducting

• Operated by compressed air

Main Turbine Generator & Exciter

• Questions

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