Discussion on the Design Factors of Large Full-speed Turbogenerator Rotor

introduction

my country's power industry has entered the stage of large units, high parameters and large power grids. As the main power supply mode, the capacity of steam turbine generators is increasing. However, large-capacity generators are expensive and complex in structure. Once a failure occurs, the maintenance period is long, causing significant economic losses to power generation companies. As the rotating part of the steam turbine generator, the reliability of the rotor will directly affect the safe operation of the generator.

1. Steam Turbine Generator Rotor Structure

The rotor is a key component of the generator, and its function is to transmit the load torque supplied by the steam turbine. When the excitation current is passed into the winding in the rotor axial slot, a strong rotating magnetic field will be generated as the rotor rotates. This magnetic field converts the mechanical energy of the steam turbine into electrical energy through the air gap and the stator winding, completing the generator's power generation task. The rotor also needs to withstand the torque under accident conditions and the huge centrifugal force generated by high-speed rotation. The main components include the shaft, excitation winding, guard ring, etc., as shown in Figure 1. The rated speed of the full-speed steam turbine generator rotor is 3000r/min or 3600r/min. It is a high-speed rotating and energized structural part. Its cross-section is shown in Figure 2. When designing, comprehensive performance such as electromagnetic insulation, ventilation, mechanical strength, vibration and torsional vibration must be considered comprehensively.

2 Rotor design elements

2.1 Electromagnetic design

As an important part of the generator magnetic circuit, the rotor must meet the electromagnetic design requirements of the motor during design. The shaft must have a high magnetization performance to reduce the magnetic potential voltage drop of the rotor teeth and yoke and reduce the excitation power. The number of turns, slot divisions, and slots of the rotor winding should be reasonably designed to obtain a better rotor magnetic potential waveform and reduce the additional loss caused by the high-order harmonics of the magnetic field on the stator surface. According to the electrical parameters of the electromagnetic design, the corresponding insulation level is selected. At present, the magnetic circuit analysis method and finite element simulation are mainly used for electromagnetic calculation.

2.2 Rotor cooling design

According to the rotor heat load obtained by electromagnetic calculation, a reasonable rotor cooling method is selected to ensure that the rotor coil temperature meets the insulation temperature rise limit requirements. There are two main ways to cool the rotor winding of a large steam turbine generator: ventilation cooling and water cooling. Ventilation cooling is currently the most commonly used cooling method for rotor coils. The coil structure is simple and easy to maintain. There are mainly three forms:

(1) Air gap air intake oblique flow ventilation, the stator core adopts axially partitioned multi-wind zone to match it. It is a self-ventilation cooling method. The gas circulation power mainly comes from the pumping pressure of the rotor itself. The main factor determining the gas flow in the rotor winding is the aerodynamic shape of the inlet and outlet air buckets at the rotor slot wedge. The advantages are that the gas has a high flow velocity in the oblique flow channel, the flow channel is long, the flow area is large, the heat dissipation coefficient is high, and the resistance of the ventilation system is low: the disadvantages are that the rotor surface and the air bucket structure are complex, and the slot wedge and coil processing is relatively cumbersome. This method is mostly used for large-capacity hydrogen-cooled generators. (2) Auxiliary slot radial ventilation is also a self-ventilation cooling method. Auxiliary slots are opened at the bottom of the rotor slot, and multiple radial air holes are opened along the axial direction of the winding. The cooling gas relies on the action of the fans at both ends of the rotor to flow through the auxiliary slots into the radial air duct of the coil and then into the air gap. The advantages are that the processing of the slot wedges and windings is relatively simple, the wind friction loss on the rotor surface is small, the cooling gas of the rotor comes directly from the fan, the initial temperature of the rotor winding temperature rise is low, the cooling effect is good, and the resistance of the ventilation system is low. The disadvantage is that the height of the auxiliary slot occupies radial space, and the rotor magnetic circuit is prone to local saturation. When the capacity increases to a certain limit, a larger rotor is required to match it to meet the rotor cooling requirements. (3) Axial ventilation is a ventilation cooling method that relies on an external high-pressure fan to provide power to maintain the flow of gas in the rotor winding. The gas enters the axial air duct of the rotor winding from the air inlet at the end of the rotor winding, and then flows out from the radial air outlet in the middle of the rotor. As the capacity of the motor increases, the rotor length becomes longer and longer, so the required fan air pressure is also higher. The advantage is that the rotor coil is simple to process: the wind friction loss on the rotor surface is small. The disadvantage is that the fan structure is more complicated and the power of the fan itself is larger: due to the long cooling air path, the coil temperature rise is more uneven. The rotor coil water cooling is to cool the deionized water through the hollow rotor coil. The advantage is that water has a strong specific heat capacity and heat dissipation capacity, and a high cooling capacity. The disadvantage is that the turbine generator rotor is a high-speed rotating body. To introduce static water into the rotor, cool the rotor winding and then discharge it reliably, a set of complex water channel structures is required, and the sealing of the rotor water channel must be ensured at the same time. Compared with gas cooling, water-cooled rotors have better cooling effects, but their structures are more complex, and there is a risk of water leakage accidents. The maintenance workload is relatively large, and only a few units use this method.

2.3 Mechanical Design

2.3.1 Mechanical strength of structural parts

The rotor has high mechanical properties to withstand the centrifugal force of its high-speed rotation. The stress level of the rotor teeth, guard rings, etc. needs to be calculated. The tooth root directly affects the selection of the electromagnetic scheme, so the electromagnetic calculation and stress calculation are usually performed alternately. The tooth root bears the centrifugal force of the tooth body, tooth head, copper wire in the slot, etc. The tooth root stress is inversely proportional to the tooth root size. In order to make full use of the rotor space and place more effective conductors, the slot cross-section size should be increased as much as possible. The tooth root size should be taken as small as possible under the condition that the stress meets the safety. The function of the guard ring is to protect the rotor end winding so that it will not fly away due to centrifugal force. The guard ring bears the centrifugal force of the rotor end coil and insulation block, etc. The centrifugal force itself and the heat-shrink stress of the shaft body work together. By calculating and selecting a reasonable heat-shrink tightening amount and the size and structure of the guard ring, it is ensured that the normal operation of the generator and the guard ring stress during the overspeed test meet the allowable requirements of material performance. At the same time, the separation speed of the mating surface between the guard ring and the shaft body is higher than the overspeed test speed.

2.3.2 Rotor vibration

Generator rotor vibration directly affects the safe operation of the generator. When the vibration exceeds the standard, it will cause bearing wear and oil leakage, affect the vibration of the steam turbine and even cause the unit to trip. The main causes of rotor vibration include rotor thermal imbalance, support stiffness difference, mass imbalance, uneven rotor body stiffness and other aspects. This article only discusses the vibration caused by the rotor itself.

The thermal imbalance of the rotor, i.e. the bending of the rotor after being heated, will lead to the change of the rotor balance state. The main reasons are the asymmetric cooling of the rotor and the obstruction of the expansion of the rotor coil. The asymmetric cooling problem can be solved by selecting a reasonable rotor cooling method, and the free expansion of the rotor coil can be guaranteed during the rotor structure design to avoid thermal stress concentration. The rotor mass imbalance refers to the deviation of the central inertia axis of the rotor from its rotation axis, and the centrifugal force produces forced vibration during the rotation of the rotor. The difference in bending stiffness between the lower wire slot and the large tooth of the full-speed rotor will produce vibration of twice the power frequency, which is not conducive to the safe operation of the unit and cannot be eliminated by adding counterweights. Usually, the crescent groove is processed in the large tooth interval to balance the stiffness difference with the lower wire slot. The tangential length of the crescent groove should take into account the distance between the adjacent lower wire slots to avoid excessive temperature caused by negative sequence current. Balance screw holes or balance grooves should also be set in the rotor body, coupling, fan seat ring, etc. to suppress the rotor vibration caused by mass imbalance.