GENERATOR COMPONENTS AND EXCITATION SYSTEM

Introduction

In a gas turbine set the rotating turbine drives the compressor and accessory equipment with available excess energy to produce shaft power which drives the generator. Approximately one half of the work produced is available to produce electricity; the other half is used to drive the compressor. The function of the generator is to convert mechanical energy of the turbine into electrical energy.

The generator is connected via a breaker and a generator transformer to the high voltage switchgear. During start-up of the gas turbine set, the generator is used as a motor to accelerate the gas turbine and the compressor. Therefore a static frequency converter is connected to the medium voltage switchgear. Its output is connected to the stator winding of the generator for feeding a rotating current of increasing frequency. For generation of a rotating magnetic field a static excitation system feeds the rotor of the generator of a direct current (DC). In order to couple the rotating magnetic field of the stator and the physically rotating rotor, the rotor is excited by DC. The thus resulting rotor field and stator field exert forces/torques upon each other coupling the “current rotation” in the stator to the rotation of the rotor.

THE ROTOR

The rotor is made from a single steel forging. The steel is vacuum-degassed to minimize the possibility of hydrogen-initiated cracking. Reheating and quenching also hardens the forging. Stress-relieving heat treatment is done following rough machining. Ultrasonic examination is performed at various stages of the rotor. The generator counter torque increases to 4 to 5 times the full-load torque when a short circuit occurs at the generator terminals. The rotor and turbine-end coupling must be able to withstand this peak torque.
ROTOR WINDING

Each winding turn is assembled separately in half-turns or in more pieces. The joints are at the centers of the end turns or at the corners. They are brazed together after assembling each turn,

to form a series-connected coil. The coils are made of high-conductivity copper with a small amount of silver to improve the creep properties. The gas exits through radially aligned slots.

Slot liners of molded glass fiber insulate the coils. These separators of glass fiber are used between each turn. They insulate against almost 10 V between adjacent turns.

The end rings and end discs are separated from the end windings by thick layers of insulation. Insulation blocks are placed in the spaces between the end windings to ensure the coils do not distort. The winding slots are cut in diametrically opposite pairs. They are equally pitched over two-thirds of the rotor periphery, leaving the pole faces without winding slots. This results in a difference between the stiffness in the two perpendicular axes. This difference leads to vibration at twice the speed. Equalizing slots are cut in the pole faces to prevent this problem from occurring. The slots are wider and shallower than the winding slots. They are filled with steel blocks to restore the magnetic properties. The blocks contain holes to allow the ventilating gas to flow.

The average winding temperature should not exceed 115°C. The hydrogen enters the rotor from both ends under the end windings and emerges radially from the wedges. Flexible leads made of thin copper strips are connected to the ends of the winding. These leads are placed in two shallow slots in the shaft. Wedges retain them. The leads are connected to radial copper studs, which are connected to D-shaped copper bars placed in the shaft bore. Hydrogen seals are provided on the radial studs. The D-leads are connected to the slip rings by radial connection bolts.
ROTOR END RINGS

The end rings are used to restrain the rotor end windings from flying out under centrifugal forces. These rings have traditionally been made from nonmagnetic austenitic steel, typically 18 percent Mn, 4 percent Cr. A ring is machined from a single forging. It is shrunk-fit at the end of the rotor body. The material of the end rings was proven to be susceptible to stress corrosion cracking. A protective finish is given to all the surfaces except the shrink-fit to ensure that hydrogen, water vapor, and so forth do not contact the metal.

The rings should be removed during long maintenance outage (every 8 to 10 years) and inspected for detailed surface cracking using a fluorescent dye. Ultrasonic scanning is not sufficient due to the coarse grain structure. A recent development has proven that austenitic steel containing 18 percent Mn and 18 percent Cr is immune to stress-corrosion cracking. New machines use this alloy. It is also used for replacement rings. This eliminates the need for periodic inspection. It is important to mention that a fracture of an end ring can result in serious damage to the machine and at least a few months’ outage. It is highly recommended to replace the traditional material with the new material.
Stator Casing

The stator core and core frame are mounted inside the casing. The casing must withstand the load and fault torques. It must also provide a pressure-tight enclosure for the hydrogen. Annular rings and axial members are mounted inside the casings to strengthen them and allow the hydrogen to flow. The end shields are made of thick circular steel plates. They are reinforced by ribs to withstand the casing pressure with minimal axial deflection. The stationary components of the shaft seal are housed in the end shields. The outboard bearing is also housed inside the end shields in some designs. The sealing of the end shield and casing joints must be leak free against hydrogen pressure.

BRUSHLESS EXCITATION SYSTEMS

Most modern gas turbines use brushless excitation systems. The rotating diodes are arranged as a three-phase bridge. The bridge arm consists of two diodes in series. If one of them fails (due to a short circuit), the second diode will continue to operate. Thus, the bridge continues to operate normally. If both diodes fail in the same arm, the fault is detected by a monitoring circuit, which trips the machine. Essential measurements, such as ground-fault indication, field current, and voltage, are taken by telemetry or instrument slip rings.
The Rotating Armature Main Exciter

Brushless machines require less maintenance than conventional ones. They also do not have sliding or rubbing electrical contacts that cause sparking and carbon dust. The main exciter is a three-phase rotating armature AC generator. The DC field is in the stator, and the AC winding is on the rotor. The exciter armature is made of low-loss steel laminations. The laminations are shrunk onto a shaft forged from annealed-carbon steel. Cooling air enters axial slots along the rotor body. The rotor conductors are made of braided strips in parallel. They are radially transposed to reduce eddy current losses. It is mounted on the outboard of the main AC exciter. The three-phase AC power is supplied from the main exciter to the silicon diode rectifier by axial conductors taken along the surface of the shaft. A steel retaining ring contains the components of the rectifier against centrifugal forces. The retaining ring is shrunk on the outside of the hub.

THE VOLTAGE REGULATOR
Background

Early voltage regulators used mechanical components. They had a large deadband, long response time, and required regulator maintenance. Modern AVRs use integrated circuits or digital microprocessor techniques.
System Description

The main function of the AVR is to maintain constant generator terminal voltage while the load conditions are changing. A dual-channel AVR with manual backup is normally used. The reliability of this design is high because the loss of one channel does not affect operational performance. The faulty channel can be repaired during operation.
The Regulator

The AVR is a closed-loop controller. It compares a signal proportional to the terminal voltage of the generator with a steady voltage reference. The difference (error) is used to control the exciter output.

When the load changes, the error increases. The channel A AVR applies a proportional-integral-derivative (PID) algorithm to the error and provides a corrective signal. This signal is amplified by the channel A converter. It is then sent to vary a field resistance. The excitation current will change, and the terminal voltage will change accordingly. It is critical to have a fast, stable response from the AVR. Special signal conditioning networks are introduced in the PID control to prevent instability. Accurate tuning (selection of PID coefficients) of the voltage response is achieved by having adjustable time constants. The AVR receives the generator terminal voltage signal through its own interposing voltage transformer. The voltage signal is rectified and filtered before comparing it with the reference voltage.

Neutral Grounding

The neutral ends of the three stator winding phases are connected together outside the casing. The star point is connected to ground through a neutral grounding device. It is designed to limit the fault current upon a ground fault in the stator winding. The neutral grounding device consists of a single-phase transformer. Its primary is connected between the generator star point and ground. Its secondary is connected to a resistor. This arrangement is chosen because the apparent impedance of the resistor appears on the primary side. This creates a very high impedance that limits the fault current to 15 A.

The End