Natural Science Research = Influence of Negative Sequence Current on Generators Jiang Wenlin Department of Electrical Engineering, Panzhihua University. Panzhihua 617000 capacity related factors, and starting from the characteristics of negative sequence current formation. The implementation requirements of generator negative sequence current protection are proposed.
China's power development has entered the development stage of large power grids and large units. The nationwide 500 transmission backbone network is forming a large-scale unit with a capacity of 200,600 and above. It has become the main unit of the power grid, and the safety of power grid operation is increasingly prominent. Because the machine is responsible for the main adjustment of the normal operation of the power grid and the accident, it is the heart of the power grid. Its operational reliability directly affects the operation of the power grid and the safety of the power supply. However, there are many accidents in the operation of the generator, and the nature of the accident is serious and the loss is huge. In the main accident of the generator, the negative sequence current burns the rotor repeatedly, and with the continuous expansion of the capacity of the power grid unit, the negative sequence current has become an important constraint factor for the safe operation of the generator, especially the large generator. In this paper, the different characteristics of the formation of negative sequence currents are analyzed. Based on this. The requirements of the generator rotor layer current protection are proposed. 1 Negative sequence current analysis of generator rotor damage characteristics The negative sequence current generates a reverse synchronous rotating magnetic field between the generator stator rotor air gap, and the rotor rotates in the forward direction, so the rotor The synchronous magnetic field has twice the synchronous rotational speed with respect to the negative sequence, and the magnetic field induces a frequency-doubled current in the damper winding excitation windings of various components of the rotor surface including the large-tooth small-slot wedge and the guard ring. Since the rotor of the turbo generator is a single forged body and has a strong damping effect, the frequency-induced current is small in the excitation winding; and because of the strong skin effect of the double-frequency current, the rotor surface Infiltration of only a few millimeters of circulation path has a relatively large average active resistance, so that when the generator is in asymmetric operation, the additional loss generated by the rotor surface is large, and the thermal effect is obvious.
The negative-sequence current flow path is concentrated near the lateral grooves of the large-toothed magnetic poles, and is distributed at both ends through the slot wedges, and the damping ring flows in the circumferential direction under the guard ring. Asymmetric faults around the rotor at home and abroad show that the rotor layer induces a frequency doubling current at the contact surface between the rotor end guard ring and the rotor body between the wedge walls, the wedge joint region damping ring and the damping strip, etc. The part with large shunt forms a local high temperature and burns the rotor. In more serious cases, there is melting in the above part or fatigue crack in the overheated area. If the temperature difference is too large, the main ring will cause the ring to be loose. 1 caused severe damage to the machine, 2 negative sequence current formation characteristics and countermeasures The normal operation of the power system is symmetrical, and now the phase of the phase power supply symmetry is symmetric. When the symmetrical operating state of the system or the hair dryer is palladium to destruction. There will be an unresolved phase voltage or current. The mountain component method can decompose it into a negative current of 12 and a zero-sequence current. The generator is often connected in a star shape, and the neutral point is not grounded or grounded through the impedance. The zero-sequence current is based on the characteristics of the operation of the power system. The influence of the negative sequence current is divided into the following two cases. 2.1 Long-term asymmetric operation 2. Long-term formation, asymmetry of the load, such as the widely used railway electrical machine, metallurgical single-phase arc A large-capacity single-phase load such as a furnace, thereby forming a large negative-sequence current, affects a generator that is closer to these loads.
2 Transmission network impedance is asymmetrical, such as high-voltage line, no power failure, phase-by-phase maintenance; low-voltage power grid, two-phase ground, power supply mode, etc.
2.1.2 Analysis of the ability of the generator to withstand negative sequence current for a long time. The increase of the single-machine capacity of the steam turbine generator depends on the size of the large motor. On the other hand, the cooling method is improved to improve the material utilization rate. Therefore, the electrode size does not increase proportionally with the capacity. Therefore, the heat load of the rotor surface of the large unit is correspondingly raised. In addition to the thermal load generated by the excitation current, in addition, the air gap magnetic density higher harmonics in the production of the heat of the production of 4 heat 6 redundant 1 pool is significantly improved, for example. Guo Guangli your steam turbine, magnetic dense generator, due to the high load of the stator line, the allowable value of the heat load generated by its negative sequence current should be reduced accordingly. Turbine generators manufactured by various factories in various countries have long-term tolerance of the negative-sequence current limit with the rated current as the base value, which is generally 1 for the hunger, 12 0.05, China's 60,000 large machine rules =0. Each flash large generator has a large difference due to differences in the internal motor manufacturing technology and power system. From the point of view of the operational needs of the power system, it is in a power system with a large asymmetric load such as a railway electric locomotive. These hair dryers with close electrical loads. Negative sequence currents that may be subjected to generational negative sequence capability, causing rotor damage accidents; may be taken as 1.=0.070.105 in other cases 1.=0.040.06. However, due to differences in generator stator rated line load, rotor materials and allowable The difference in temperature rise and the difference in the normal thermal load of the rotor make it possible for generators of the same capacity class of different manufacturers to have different values. Therefore, the actual capacity of the generator to withstand the negative sequence current for a long time needs to be verified by the negative sequence current test. 2.1.3 The current protection of the negative current constitutes the condition 1 action criterion, the generator should withstand the long-term negative sequence current capability requirements, should be in China The Rotating Electric Machine Standardization Committee recommends a long-term maximum allowable temperature of 130 as a negative sequence current test calibration standard; the test value is used as a basis for the setting of negative sequence current protection. 2 protection operation requirements When there is more than 12, the negative sequence current, the protection device should act reliably, emit sound and light signals for timely processing; when its duration reaches the specified value, and the negative sequence current has not been eliminated, it should act Remove the generator.
2.2 short-term asymmetric operation 1 formation cause 1 power system asymmetry faults include + pay-as-called short-circuit and phase-breaking; 2 generator + pay-over faults include single-phase two-phase short circuit, branch short circuit, turn-to-turn short circuit; 3 single-phase coincidence Explain the action process; 4 outlet circuit breakers in the normal and fault conditions, failed to close all or open the action process. 5 When the high-voltage side of the main transformer is disconnected under normal and fault conditions, the action process of all the phases being closed or disconnected; 6 When the exit circuit breaker accidentally trips, the de-excitation switch fails to extinguish the arc in time.
2 generators with short, sequencing current capacity analysis. When analyzing the short-time listening current capability of the hair dryer, the action time is very short, and the rotor is regarded as an insulator. It is assumed that the frequency-doubled current generated by the negative sequence magnetic field flows only on the rotor body and the groove surface, and the losses are all used for the loss. The temperature of the household is rising. Neither the heat dissipation to the old medium nor the heat transfer along the rotor body to the center of the human axis defines the heating constant of the negative-counting rotor of the generator. The negative current of the negative-working current is 1 and the current is based on the generator. The product of the square and the duration, in seconds. 3. When the thermal effect color of 12 is obtained for 12 durations, the generator can operate safely. For domestic generators, specify indirect cooling turbine generators, take 30; indirect cooling turbine generators, take = 40, direct cooling generators are small, 30 and then, = 8.6 million = 7 national standard; 7064 86 stipulates that the stator line load of this type of generator is higher than that of indirect cooling.
3 protection features. 1 From the above analysis, it is known that in order to adapt to the ability of the generator to withstand short-term negative-sequence currents, the negative, flow, and operational criteria are analyzed by the value of the Lingchi force system and the limit of the industrial scale. The analysis of the worst case of short circuit is taken as an example. It is assumed that the two-phase short circuit of the machine is short-circuited. When the main protection and backup protection are respectively applied, the thermal effect of the negative sequence current 1 main protection action quickly jumps. breaker. The fastest rapid degaussing is caused by 2 5 for irreversible products; z, 4.0; the core magnetic field switch is at 14, the current is interrupted, and the inter-magnetic effect of the 3.51 demagnetization process is required. According to the positive sequence equivalent rule, by it, the printing juice heat effect 玢 is the rear positive ten 12 take the generator secondary transient 屯 resistance average position 1 = 0.2, the negative jin electric resistance takes the large position = 1.22, the turbine generator, such as 600,000 Jia, the national standard, 8706486 regulations, = 7, is bound to reduce the stator rated line load, which will inevitably lead to excessively large unit size, unable to meet the manufacturing cost requirements.
The conclusion is that the operation should be reduced from the operational aspect. The simplest and most effective method is to abandon the far-reserved performance of large generators while meeting the motor manufacturing requirements.
3 Conclusion 1 The rotor of the hair dryer is only based on the negative sequence overload protection setting. The long-term and short-term negative sequence current limit of the hair extension machine should be independently determined, and it cannot be adjusted with the generator backup protection; On, should be used as a backup protection μ for the generator. Yin. Electrical Engineering M. South China l Ligong 7 published f, w.
2 Wang Weiwei. Relay protection applies elbow. China Electric Power Press, 1998 3 Fan Xipu. The electrical part of the power plant is elbow. Buried workers 7 publishers. 17.
4 Wang Wei check Guilin Bi Daqiang + large generator transformer group relay protection discussion 1妃. China Electric Power. 2001., 134. 1.1 China Electric Power Press.
Hu Shide. , person 0; 1 skill and should. lead. The people of Beijing 1 went out of the company. 194.
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