The structure and working principle of steam turbine generator

What is a condensing steam turbine generator?

A steam turbine generator is a generator driven by a steam turbine. The superheated steam generated by the boiler enters the steam turbine and expands to do work, which makes the blades rotate to drive the generator to generate electricity.

A steam turbine generator is a kind of synchronous generator. It is an electrical device that uses a steam turbine as the prime mover to drive the rotor to rotate, and uses the principle of electromagnetic induction to convert mechanical energy into electrical energy. It is mainly used in thermal power plants or nuclear power plants.

Working principle of back pressure steam turbine

A steam turbine generator is a power generation device that uses a steam turbine as the prime mover to drive the rotor to rotate, and uses the principle of electromagnetic induction to convert mechanical energy into electrical energy. After the DC current is passed into the rotor winding of the generator, the rotor magnetic field is established, which is called the main magnetic field, and it rotates with the rotor of the turbo-generator.

The magnetic flux comes out from one magnetic pole of the rotor, passes through the air gap, the stator core, the air gap, and then enters the other adjacent magnetic pole of the rotor, thereby forming the main magnetic flux loop. Since the generator rotor rotates with the steam turbine, the magnetic pole of the generator rotates once, and the magnetic field lines of the main magnetic pole are cut in turn by the u, v, w three-phase windings (conductors) installed in the stator iron core. Three-phase alternating electromotive force with different phases is induced in the winding.

Assuming that the turbine generator rotor has a pair of magnetic poles (that is, one N pole and one S pole), when the turbine generator rotor and the turbine rotor rotate coaxially at high speed, such as when the turbine rotates at 3000 rpm, the generator rotor will Rotating at a constant speed of 50 cycles/second, the polarity of the magnetic pole also changes 50 times, then the induced electromotive force in the stator winding of the generator also changes 50 times, and at the same time, three-phase alternating electromotive force with different phases is induced in the three-phase winding of the stator. That is, a three-phase alternating electromotive force with a frequency of 50 Hz.

At this time, if the ends of the three-phase windings of the generator stator (that is, the neutral point) are connected to the ground, and the lead wire of the first end of the three-phase winding of the generator stator is connected to the electrical equipment, current will flow. This process It is the process of converting the mechanical energy input by the steam turbine rotor into electrical energy.

The structure of the condensing turbine

The steam turbine generators of thermal power plants or nuclear power plants are all of horizontal structure. As shown in Figure 1, the generators are matched with steam turbines and exciters to form a coaxial steam turbine generator set. The most basic components of a turbo-generator are stator, rotor, excitation system and cooling system.

Stator:

1. The stator core. The stator iron core is an important part that forms the magnetic circuit and fixes the stator winding. It is usually formed by laminating cold-rolled silicon steel sheets with a thickness of 0.5mm or 3.5mm and good magnetic conductivity. The stator iron core of the large-scale steam turbine generator is large in size, and the silicon steel sheets are punched into a fan shape, and then assembled into a circular shape with multiple pieces.

2. Stator winding. The stator windings are embedded in the stator slots of the inner circle of the stator iron core, and are arranged in three phases at an angle of 120° to each other, so as to ensure that the three-phase stator windings generate electromotive force with a phase difference of 120° from each other when the rotor rotates. Two sets of upper and lower insulated conductors (also known as wire rods) are placed in each slot, and each wire rod is divided into a straight part (placed in the iron core slot) and two terminal parts.

The straight part is the effective side of the conductor that cuts the magnetic field line and generates induced electromotive force, and the terminal part of the wire rod plays a connecting role, connecting the relevant wire rods according to a certain rule to form the three-phase winding of the stator of the generator. The stator bars of medium and small turbogenerators are all solid bars, while large turbogenerators mostly use internally cooled bars due to the need for heat dissipation, such as several solid bars and hollow bars that can pass water. composed in parallel.

3. Machine base and end cover. The function of the frame is to support and fix the stator core of the generator. The machine base is generally welded with steel plates, and must have sufficient strength and rigidity, and can meet the requirements of ventilation and heat dissipation. The function of the end cover is to cover the two ends of the generator body, and together with the frame, the stator core and the rotor, constitute a complete ventilation system inside the generator.

Rotor:

1. The rotor core. The generator rotor body is made of alloy steel with high strength and good magnetic conductivity. The grooves for the field windings are milled axially along the surface of the rotor body. The arrangement of the grooves is generally radial, and the part between the grooves is a tooth, commonly known as a small tooth. The unprocessed part is generally called the big tooth, and the big tooth is the pole body of the magnetic pole, which is the only way for the main magnetic flux.

2. Excitation winding. The excitation winding is a concentric winding composed of several coils, and the coils are wound with rectangular flat copper wires. After the excitation winding is placed in the slot, the straight part of the winding is compressed with slot wedges, the end is fixed radially with a retaining ring, and the axial fixing is made with a mica block and a center ring. The lead wire of the excitation winding is connected to the collector ring through the conductive rod, and then led out through the brush.

3. Guard ring and center ring. The speed of the turbo-generator is very high, and the end of the excitation winding is subjected to a large centrifugal force, so the guard ring and the center ring are used to fasten it. The guard ring tightly wraps the end of the excitation winding, so that the end of the winding does not have radial displacement and deformation; the center ring is used to support the guard ring and prevent the axial movement of the end.

4. Collector ring. The collector ring is divided into positive and negative collector rings, which are made of hard and wear-resistant alloy forged steel, and are installed on the outside of the excitation end of the generator rotor. The positive and negative collector rings are respectively connected to the two ends of the excitation winding through the lead wires, and are led to the generator excitation system by the brush device.

5. Fan. Fans are installed at both ends of the generator rotor to speed up the circulation of gas in the stator core and rotor parts, and improve the cooling effect.

Cooling system:

When the generator is running, various losses generated inside it will be converted into heat energy, causing the generator to heat up. Especially for large-scale steam turbine generators, because of their slender structure, the heat in the middle is not easy to dissipate, and the heat generation problem is even more serious. If the generator temperature is too high, it will directly affect the service life of the insulation, so cooling is a very important issue for large turbo generators.

Excitation system:

The main functions of the excitation system are:

1. When the generator is in normal operation, the excitation current is supplied and automatically adjusted according to the load of the main engine to maintain a certain terminal voltage and reactive power output.

2. When the generators run in parallel, make the reactive power distribution reasonable.

3. When a sudden short-circuit fault occurs in the system, the generator can be strongly excited to improve the stability of the system operation. After the short-circuit fault is removed, the voltage quickly returns to normal.

4. When the generator load suddenly decreases, it can be demagnetized forcibly to prevent the voltage from rising excessively.

5. When an internal fault occurs in the generator, such as a short circuit between turns or a two-point grounding fault on the rotor, the generator can be automatically demagnetized or demagnetized.