Technical Analysis of Electric Brake of Hydrogenerator Units for Electric Power Automation Equipment Xu Qingshan\Le Xiuzhen, Chen Jun2, Zhang Xin3 (1. College of Electrical Engineering, Hohai University, Nanjing 210098, China; 2. Department of Electrical Engineering, Southeast University, Nanjing, Jiangsu, China 210096; 3. Jiangsu Power Grid Dispatching and Communication Center, Nanjing 210024, China) It has large braking torque, fast stopping speed, clean and pollution-free, but with the improvement of the current thrust bearing bush, mechanical braking is becoming more and more possible. In the new situation, some problems exist in the electric brake technology of the hydro-generator unit, such as the cost of installing the electric brake device, the difficulty of the site, and the system exposed on the actual operation of the unit with the electric brake installed. The dynamic current is too large, and the input time interval is too short, and the improvement measures of combining the electric brake with the excitation system, and the electric brake and the mechanical brake hybrid brake are proposed. 312:A Article ID: 1006― EleetroniePublishfebu The author suggests that if the mechanical braking performance of the unit is witnessed, the electric braking technology has gradually replaced the mechanical brake and has been widely used in large and medium-sized hydropower units. However, with the improvement and improvement of the mechanical manufacturing technology, the thrust bearing material of the unit has been greatly improved, and the shortage of mechanical braking is gradually improved; on the contrary, the unit with the installed electric brake device is exposed in practice. All kinds of defects, the funds that appear to be added to the electric brake equipment, the difficulty in the layout of the screen cabinet, etc., make the application of electric brakes affected. Therefore, in the new situation, how to recognize the role of electric braking and how to take countermeasures against the current deficiencies, it is necessary to conduct further research. 1 Re-recognition of electric braking 1.1 Basic principle and theory of electric braking When the hydro-generator and power system are disengaged, the unit enters the shutdown process. Since the rotating parts of the unit have a large inertia, the unit cannot be stopped in a short time. However, the unit bearings do not allow the unit to be in low-speed operation for a long time. This is because the oil film formation of the unit thrust bearing bush is related to the speed of the unit. The rotation of the unit at low speed will cause the oil film to break and then dry friction, and burn the bearing. . The working principle of electric braking is based on the armature reaction of the synchronous motor. When the unit is disconnected from the grid, the generator rotor is de-excited, the stator is short-circuited by three phases, and the excitation current is applied to the generator, so that it produces a direction opposite to the direction of the unit's moment of inertia, and has a strong braking electromagnetic moment. The excitation current is supplied by the rectified external power supply of the plant power system. Before the electric brake is put into operation, the following conditions must be met: the generator must be disconnected from the grid. The magnetic field must be demagnetized and the voltage at the generator terminal drops to the residual voltage. The vane is closed and the unit no longer has a motive force. The generator has no internal electrical faults. 1.2 Advantages of electric braking Traditional mechanical brakes generally use frictional braking and direct contact with brake rings to generate frictional resistance and act as a brake, but brake rings and dampers are susceptible to deformation due to mechanical fatigue; The generated dust is mixed into the circulating air of the generator, and adheres to the end of the stator coil and the surface of the iron core ditch with the oil mist, which affects the insulation and heat dissipation of the generator and seriously affects the normal operation of the generator. Electric braking is proposed to overcome the shortcomings of mechanical braking. Its biggest advantage is its large braking torque. According to the theory of synchronous motor, the braking torque Me can be expressed as 11/n, that is, the braking torque is large when the random group speed decreases, and reaches the peak when the speed is zero, which is the biggest advantage different from the mechanical braking torque. In addition to the large braking torque, the electric brake is also relatively clean, and easy to overhaul and maintain, meeting the requirements of system peaking and frequent operation of the unit. 1.3 The trend of braking mode 1.3.1 Synchronous generator braking device with separate excitation equipment The independent electric braking device shown is simple and versatile, suitable for all hydroelectric generating units, especially for the operating unit. The function is more suitable. However, this model has a large investment, and it is necessary to set up a separate cabinet, which adds difficulties in layout. In recent years, the improvement and improvement of the thrust bearing bushes has made this shortcoming of the electric brake better. The damper is flexible and has no card issuance phenomenon. The number of opening and stopping is not much, and the thrust bearing has been used or is ready to be changed into a plastic tile. A set of independent devices. 1.3.3 Electrical Braking and Mechanical Braking The electric braking is only suitable for the normal shutdown of the unit or the unit shutdown of non-mechanical and electrical accidents. Therefore, the mechanical brake of the unit is still indispensable. The unit can adopt the hybrid braking mode of mechanical brake at low speed, because the mechanical brake is also used to minimize the wear at low speed (10% rated speed), and it will not cause pollution to the generator winding. Of course, in view of the fact that the electrical brake may fail, the mechanical brake must be able to reliably stop the brake. Generally, the mechanical brake should be put at 30%~40% of the rated speed. 2 Perfect measures for current electric braking 2.1 Optimizing the selection of electric braking parameters The braking current in the shutdown is an extremely important parameter, which is input by the electric braking torque. For units in the system with peaking and frequency modulation, due to the frequent opening and closing operations, it is still necessary to solve the problem of the electrical brake input time interval of the unit. It is necessary to ensure normal and quick start-stop, and to ensure that the unit is not subject to large wear during the shutdown process. The minimum interval can be determined in consultation with the dispatching department in the system as necessary. The brake is blocked during the interval. The specific implementation method is: adding a time relay in the circuit for starting the braking, and each time after starting once, it is necessary to go through a predetermined time interval before automatically returning to cancel the electric braking. 2.2 Dealing with the relationship between electrical braking and electrical protection 2.2.1 Setting the inherent protection of the electric braking system The electric braking system should be equipped with its own protection device. When the electric brake is put into operation, the rectifier transformer or the rectifier may be short-circuited, or the rectifier unit may be out of control and cause an accident. Therefore, overcurrent protection should be set on the high voltage side of the rectifier transformer, and overcurrent protection should be set on the stator three-phase short-circuit switch side to ensure that the electric brake can safely exit when it fails. 2.2.2 Ensure that the electric brake and other protection coordination rod joint melting phenomenon kflf issued Me to avoid a tired brake electric electric furnace Teng ingHouse. After the electric brake input, the generator stator current frequency decreases with the decrease of the rotational speed. For the generator set whose neutral point is grounded by the arc suppression coil, when the electric brake stop speed drops to 1/3 of the rated speed, the fundamental voltage caused by the third harmonic potential in the stator winding of the generator resonates, and the secondary voltage of the arc suppression coil The increase in value directly leads to misoperation of the zero-sequence overvoltage type stator grounding protection. The solution is to short-circuit the brake short-circuiting switch QFdk to ground () to eliminate the capacitive reactance variable in the fundamental voltage resonant circuit, thereby preventing the fundamental voltage resonance from occurring. This method is simple and feasible, safe and reliable. Many units in China have used electric braking methods to stop. The short circuit point of the electric brake is also generally in the differential protection zone. Therefore, the differential current formed during the braking process will result in a differential protection action. The solution is to activate the contactor to short-circuit the current transformer circuit or to lock the differential protection during electrical braking after the electrical brake input condition is met. It is also used in foreign countries to install a three-phase TA on the three-phase short-circuit switch QFdk, which is connected to the differential protection current loop. Therefore, the protection will not be disturbed due to current balance during electrical braking, so there is no need to withdraw the differential protection. Electrical braking also affects the generator's negative sequence current protection. The reason is that the general negative sequence current relays are designed according to the conventional 50Hz condition, and there is no low frequency compensation measure. In the low frequency region of the electric braking process, for the current conversion device containing the electromagnetic component in the relay, due to the influence of the saturation of the core and the change of the parameters of the LC component, the parameter balance is destroyed, and the unbalanced output is large, at a certain frequency. May be greater than the set value, causing the actuator to malfunction. There is a more detailed discussion on the in-depth study in this area. 3 Conclusion Electric brake is a recognized environmentally friendly braking method because of its good braking characteristics at low speeds and no pollution to the motor. Electric braking can effectively reduce downtime and effectively improve the operating conditions of the thrust bush. After the mechanical brake has a good improvement on the thrust bush of the unit, although it can be braked at a lower speed, the mechanical damage and dust hazard caused by the brake friction are alleviated to some extent, but the problem has not been fundamentally solved. It is only a reduction in the degree of harm, and at the same time inevitably leads to defects in long downtime. So nowadays, the fast and clean electric braking method should also be considered as the first choice. Especially for the mode of combining electric braking and excitation system, it has good economy and can effectively reduce the field equipment and its floor space. In addition to being considered as a brake backup, the mechanical brake can be used with electric brakes at low speeds in order to brake more effectively.
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