Source: Power Knowledge Forum
Summary:
1. The working principle of the brushless excitation system of the generator
2. The structure of the brushless excitation system of the generator
3. The drying system of the exciter
Content:
One. How the brushless excitation system works
Brushless excitation is divided into rotary diode type and rotary thyristor type according to the type of rotary rectifier, and the system schematic diagram is shown in Figure 1-1: at present, all the rotary diode type is actually used, and the technical problems such as transmitting and receiving trigger pulses and detection during rotation of rotary thyristor type are still in the research stage.
The brushless excitation system is generally composed of three parts: a permanent magnet machine, a main exciter, and a rotary rectifier, as shown in Figure 1-1
(a) (b)
Figure 1-1 Schematic diagram of the excitation system
(A. Rotating diode excitation system B. Rotating thyristor excitation system)
The magnetic pole of the permanent magnet machine, the armature of the exciter, and the rotating rectifier device are coaxially rotated with the generator. The high-frequency power supply generated by the permanent magnet armature is rectified through two sets of full-control rectifier bridges and becomes DC to supply the excitation winding of the main excitation machine, the intermediate frequency alternating current output of the main excitation armature is supplied to the rotary rectifier device, and the DC power output of the rotary rectifier is sent to the excitation winding of the generator rotor.
The excitation electromechanical armature winding is directly connected to the three-phase bridge full-wave rotary rectifier device, and the positive and negative poles of the rotary rectifier device are directly connected with the rotor of the main generator to supply the generator excitation. Therefore, the excitation system does not require brushes and slip ring devices, so that the brushless excitation system is constituted.
The rectifier device is a plate structure, which is easy to repair and replace; The fast fuse and capacitor are unit combination; The rectifier device circuit is a three-phase bridge full-wave rectification, and each route is connected in series with two parallel rectifiers and two parallel fuses, which can break the fault in time, and when 25% of the silicon rectifier tubes per phase are damaged, it can still meet the power generation requirements. The excitation voltage response time of this kind of excitation system is less than 0.1s, so it belongs to the brushless excitation system with high initial response.
The control angle of the thyristor rectifier that controls the automatic voltage regulation meets the requirements of regulating the excitation of the generator. Under normal circumstances, the magnitude of the generator excitation current is automatically adjusted by the automatic voltage regulator (AVR) according to the voltage deviation signal at the output end of the generator to maintain the voltage at the generator end at a given level.
The exciter cooling system is an important condition for its normal operation, the exciter is an air-cooled mode, there is no special fan design for ventilation, only the radial installation of the rotating diode, and the rotation produces wind pressure; There is a set of coolers in the exciter, and the cooling water is taken from the closed water system; In order to prevent the negative pressure generated by the rotation of the cylindrical armature of the exciter, the bearing leaks oil to the exciter, and a filter is provided on the top of the exciter to communicate with the atmosphere.
The brushless rotating diode excitation system has the characteristics of simple structure, easy maintenance and high reliability, but the brushless excitation mode has brought two new problems after canceling the slip ring and brush: first, it is impossible to directly measure the rotor current and rotor temperature, monitor the ground insulation of the rotor circuit, monitor the fuse on the rotating rectifier bridge, etc., and must use special measurement and monitoring methods; Second, the traditional demagnetization method of installing fast demagnetization switch and discharge resistance in the magnetic field circuit of the generator cannot be adopted, but can only be installed in the magnetic field circuit of the AC exciter, so the demagnetization time is relatively long, as shown in Figure 1-2, that is, the principle wiring diagram of the brushless excitation system.
Figure 1-2 Wiring diagram of the brushless excitation system
1-brushless main exciter armature; 2-permanent magnet armature; 3-permanent magnet machine pole; 4-rotary rectifier device; 5-main generator armature; 6-brushless main exciter magnetic field; 7-thyristor rectifier device; 8- Magnetic field of the rotor of the main generator
2. Structure and characteristics of brushless excitation system
The excitation system consists of a rectifier ring, a three-phase main exciter, a three-phase sub-exciter, a cooler, a metering and monitoring device. The structure is shown in Figure 1-3.
Figure 1-3 Brushless excitation system with rotating diode (system diagram)
1-three-phase sub-exciter; 2- Brushes and slip rings for grounding fault detection; 3-orthogonal axis measuring line 4-three-phase main exciter; 5-fuse response monitoring device; 6-diode rectifier device; 7-three-phase lead; 8-MULTI-CONTRACT JOINTS; 9-rotor winding; 10-stator winding; 11-automatic voltage regulator; 12-Fixed fuse response monitoring device.
The three-phase alternating current generated by the permanent magnet sub-exciter is rectified by the fully controlled rectifier bridge into DC, which is controlled by AVR to provide variable DC current to excite the main exciter. The three-phase alternating current induced by the rotor of the main exciter is rectified in the rotary rectifier bridge and enters the generator rotor winding through the DC lead of the rotor shaft.
Figure 1-4 Schematic diagram of the structure of the main exciter
1-coupling; 2-rectifier ring; 3-exciter rotor; 4 - fan
As can be seen from Figure 1-4 above, the rectifier ring and the exciter rotor are installed on the same shaft that is rigidly connected with the generator rotor, and are supported by a bearing located at its end, so that the generator rotor and the exciter rotor are supported by three bearings.
Through a multi-contact electrical contact system consisting of plug-in bolts and sockets, the mechanical coupling of the two shaft assemblies enables the simultaneous connection of DC leads located within the central shaft bore. This electrical contact system is also used to compensate for changes in lead length due to thermal expansion.
1. Rectifier ring (Figure 1-5, Figure 1-6)
In a three-phase bridge circuit, the main component of the rectifier ring is a silicon diode mounted on the rectifier ring. Figure 1-3 shows the internal structure of the diode. The necessary contact pressure of the diode is generated by a coil spring assembly and centrifugal force during rotation.
Figure 1-5 Combination of internal components of the rectifier ring
1-heat sink; 2-insulating material; 3-disc diode; 4-insulating bolt connection; 5-pressure-bearing parts with cooling holes; 6-MULTI-CONTACT FILM; 7-coil spring; 8-contact bridge
Figure 1-6 Combination of rectifier ring components
1-rectifier ring; 2-fuse; 3-diode; 4-coupling; 5-Multicontact connector
The parts shown in Figure 1-5 are additional parts that are installed in the rectifier ring. Each of the two diodes is mounted in a group of individual aluminium heat sinks, so that they can be connected in parallel. Connected to the individual heat sinks is a fuse, which disconnects one diode when it is not working.
In order to suppress transient voltage spikes due to rectification, six RC networks consisting of one capacitor and one damping resistor are installed in each rectifier loop. They are combined in a single resin-encapsulated device. The insulated and cold-shrunken rectifier ring acts as a DC bus on the positive and negative sides of the rectifier bridge. This arrangement allows for easy access to all components and the smallest electrical connections. The mechanical design of the two rectifier rings is the same, except that the forward direction of the diodes is different.
The DC current from the rectifier ring is fed through a radial bolt into the DC lead arranged in the center hole of the shaft. Three-phase alternating current is obtained by means of a copper wire mounted around the shaft between the rectifier ring and the three-phase main exciter. The wires are secured with strapping clamps and then fitted with snap-on lugs for internal connection of the diodes. Each heatsink bank is supplied with a three-phase wire with four diodes.
2. Three-phase main exciter
The main exciter adapts to the requirements of the rectifier load, and has a large reserve capacity, and the exciter does not produce harmful deformation or overheating when the generator outlet is short-circuited or asymmetrically short-circuited. The AC main exciter adopts 150Hz. The main exciter is a small three-phase cryptographic synchronous generator.
Figure 1-7 Schematic diagram of the structure of the three-phase main exciter
1-magnetic pole; 2-stator; 3-rotor; 4 - fan
The three-phase main exciter is a 6-pole rotating armature device, as shown in Figure 1-7. These 6 poles are mounted inside the stator frame with excitation and damping windings. The magnetic field windings are located on the poles of the laminated magnet. Busbars are installed on the pole shoes, the ends of which are connected to form damping windings. An orthogonal shaft is installed between the two poles to measure the induced current of the exciter. The rotor consists of multiple layers of laminations. The lamination is made by pressing through bolts on a compression ring. The three-phase winding is inserted into the groove of the laminated rotor, the winding conductor is crossed within the length of the core, and then the end turns of the rotor winding are fixed with fiberglass tape and connected on the side facing the rectifier wheel. The winding end is extended to a collector ring connected to the three-phase conductor of the rectifier ring, filled with synthetic resin and, after solidification, the entire rotor is hot-loaded onto the shaft. The bearings are located behind the fan and are forced to be lubricated by the turbine oil supply system.
3. Three-phase sub-exciter
The sub-exciter adopts a permanent magnet intermediate frequency generator, which has good external characteristics, and its terminal voltage changes no more than 10% of the rated value from the generator to the forced excitation. Equipped with fault low voltage and overcurrent detection relays and voltage and current meters for alarms.
Figure 1-8 Schematic diagram of the structure of the three-phase sub-exciter
1-bearing; 2-stator; 3-permanent magnet rotor; 4 - stator
The three-phase sub-exciter is a 16-pole rotating magnetic field device. The frame of the exciter is equipped with laminated cores with three-phase windings. The rotor consists of a hub with a suspension pole, as shown in Figure 1-8. Each pole consists of 10 separate permanent magnets housed in a non-magnetic metal shell and bolted between the hub and the outer pole shoe. The rotor hub is hot-mounted at the free end of the shaft.
4. Cooling of the exciter
Figure 1-9 Schematic diagram of the exciter structure
1-three-phase sub-exciter; 2-fan; 3-three-phase main exciter; 4-rectifier ring
The exciter (Figures 1-9) is air-cooled. The cooling air is closed circulation and recooled in two chiller units mounted across the exciter. The entire exciter is housed in a casing through which cooling air circulates through.
Figure 1-10 Schematic diagram of exciter cooling
As shown in Figure 1-10, the rectifier ring draws cold air from both sides and discharges the hot air to a chamber located under the substrate. Another stream of cold air passes through the auxiliary exciter, then through the fan, and the casing of the main exciter receives this cold air. Cool air enters the main exciter from both ends and is conveyed to the transport pipe under the rotor body, where it is discharged to the lower chamber through the radial groove of the rotor core. The hot air is returned to the main housing through the cooler area.
3. Drying system of exciter
The exciter is also fitted with a dryer dehumidifier designed to prevent condensate from forming inside the exciter or on the crank unit when the turbine generator is shut down.
Figure 1-11 Dryer structure diagram
1-dry air outlet; 2-shut-off valve; 3-regenerative air inlet; 4-temperature regulating device; 5-dryer housing; 6-dry air inlet; 7-regenerative air outlet; 8-strainer
Figure 1-12 Diagram of the dryer working principle
1-regenerative air outlet; 2-dryer wheel; 3-heater; 4-ventilator; 5-filter, 6-air outlet; 7-shut-off valve
The dryer is used to remove moisture from the air inside the exciter casing. The dryer wheel is made of non-flammable material, and as can be seen from the figures (1-11, 1-12), the dryer wheel is fitted with a cylindrical piping system on its inlet side, the surface of which is filled with highly hygroscopic material. The tubular tubes are machined to the required dimensions so that they can achieve laminar flow with low pressure loss, even at high gas velocities.
When positive hot air passes through the dryer wheel, which rotates in the opposite direction to the incoming air, the moisture absorbed by the dryer wheel is removed in the regeneration section and discharged into the atmosphere. The material of the dryer wheel is regenerated and can reabsorb moisture
The process of absorbing moisture and regenerating the material of the dryer wheel can be carried out simultaneously with an independent air flow, which also ensures that the air is continuously dried. Installing a shut-off valve in the dry air outlet line prevents the power plant's contaminated air from being sucked in during exciter loading.
Dehumidification is performed when the dryer wheel rotates slowly (approximately 7 revolutions per hour). The honeycomb dryer wheel is made of magnesium silicide alloy containing crystalline lithium chloride, and the inside of the dryer wheel is divided into 4 parts, of which 1/4 part is used for the regeneration of the dry material and 3/4 part is used for water absorption.
For the absorption section, the air to be dehumidified passes through the moisture absorption section of the dryer wheel, and part of the moisture in the air is removed by the adsorption material, i.e., lithium chloride. In the regeneration section, the regeneration air in the dryer wheel removes the accumulated moisture from the dryer wheel. The dryer wheel rotates continuously to ensure the continuous removal of moisture from the air inside the exciter.