It is pointed out that the AC/DC integrated de-excitation protection can be used as the preferred solution for the de-magnetization protection of large hydro-generator rotors.
In the figure: Ufr-excitation power supply FMK excitation circuit breaking device LQ-generator rotor winding F-generator stator RV-zinc oxide varistor 2.1 series demagnetization mode In the excitation circuit, its representative product is DM2 auto-extinguish See the schematic diagram of the main circuit of the series demagnetization mode. Magnetic switch, which is simple in wiring and small in size, both interrupting and absorbing energy. In this de-excitation mode, the FMK magnetic field switch is connected in series on a small and medium-sized generator set with little energy of de-excitation. Main protection of the generator excitation system. When the main transformer connected to the generator and its unit fails, it mainly accomplishes two tasks. One is to quickly disconnect the magnetic field loop, that is, disconnect the generator rotor winding and the excitation power supply, so that the power supply no longer supplies energy to the rotor winding; The second is to quickly transfer and absorb the magnetic energy stored in the rotor, and limit the overvoltage at both ends of the rotor to protect the safety of the rotor.
The requirements for de-excitation protection are: safe, reliable and fast.
With the development of the power industry, the single-machine capacity of hydro-generators continues to increase, and the excitation power, rotor winding time constant and energy storage of the generators are constantly increasing, and thus the energy capacity of the de-excitation is greatly increased. In order to improve the stability of power system operation, the self-excited thyristor excitation system with fast response and dome value is widely used in the hydro-generator unit. This is an improvement, but it also protects the rotor from de-excitation. New and higher requirements have been proposed. Therefore, the demagnetization protection technology should also have corresponding development and progress to adapt to new changes. Seeking new and effective safe and reliable degaussing solutions is a serious issue before us.
2 Domestic main de-magnetic protection schemes and their characteristics There are many main loop wiring methods currently used in de-magnetic protection in China. However, in terms of its working principle, it is mainly the following two: certain protection. However, with the increasing capacity of the unit and the popularization and application of the thyristor static excitation system, the series demagnetization method has exposed many problems, such as insufficient magnetic energy capacity, difficulty in blowing small current, low arc voltage, and demagnetization time. long. In addition to the defects of the structure itself, the problem of refusal or misoperation often occurs, and the accident rate is high, which cannot meet the needs of the rapid development of the power industry.
Inverter demagnetization is also a type of series demagnetization. Large-scale hydro-generators mostly use self-excited system of the machine end. This kind of excitation system can use inverter demagnetization during normal shutdown, but it can not be used as the main means of accidental de-excitation protection of generators, because it is inside the generator. In the case of a short circuit, the voltage at the input end of the rectifying group is significantly reduced, thereby greatly slowing down the speed of demagnetization.
2.2 Parallel demagnetization mode Parallel demagnetization mode The main circuit schematic diagram is shown.
Parallel demagnetization is a new type of demagnetization developed in the early 1980s with the application of ZnO varistor. The characteristic of this method is to separate the two functions of current interruption and energy absorption: the magnetic circuit breaker only acts as a current interruption and energy transfer, and does not absorb energy by itself, the burden is greatly reduced, the probability of burning the switch is greatly reduced; the magnetic energy is eliminated. The task of voltage limiting is carried out by the non-linear resistance of zinc oxide, and the demagnetization speed is fast, close to the ideal constant voltage demagnetization. This is an ideal method of degaussing, and it is used more in China. However, this demagnetization method also puts forward higher requirements for the magnetic field circuit breaker, that is, the magnetic field circuit breaker is required to have both large current breaking capacity and high speed, and has the characteristics of high breaking voltage to meet the breaking current energy. need. Obviously, existing magnetic field switches such as DM2 are not competent.
For this reason, people have carried out the transformation of the magnetic field switch, although the efforts have not been effective. There are also a number of auxiliary measures around the magnetic field switch, such as the high-voltage fuse at the two ends of the switch, parallel manual zero-crossing circuit and other solutions. These auxiliary measures relieve the burden of the magnetic field switch to a certain extent, but they cannot be used independently, and some control links are added, and the reliability of the magnetic field switch cannot be separated. If the switch is rejected, the solution of these auxiliary measures will be powerless.
In the past ten years, some magnetic field circuit breakers have also been developed. For example, DM series magnetic circuit breakers have superior performance in terms of breaking speed, flow capacity, breaking voltage, safety and reliability, etc., and are used for hydroelectric power generation below 300MW. The rotor de-magnetic protection effect of the machine is very good, but for the 600MW or more (Three Gorges 780MW) hydro-generator unit, the existing DMX DC magnetic field circuit breaker has insufficient current-flow capacity and low breaking voltage (the existing DMX is the largest). Rated current 3500A, maximum breaking voltage 3 Turbine generator rotor de-excitation protection problems 3.1 Domestic generator self-excited thyristor static excitation system, the excitation transformer primary side and secondary side are mostly no circuit breaker, once The side is directly connected to the outlet end of the generator, and the secondary side is connected with the rectifier. During the de-excitation process, the huge energy of the generator is still fed back to the de-excitation system through rectification, which adds a heavy load to the breaking of the DC magnetic circuit breaker. Burden, this is one of the main reasons for the frequent occurrence of demagnetization accidents.
3.2 As mentioned earlier, the parallel demagnetization method has been widely used in China, and good results have been received, but the safety and reliability are still not very satisfactory, and accidents occur from time to time. The reason is mainly concentrated in the DC magnetic circuit breaker. The main problems in the flow interruption energy transfer are: 3.2.1 The existing DC magnetic field circuit breakers (switches) in addition to the DMX series magnetic field circuit breakers, most of which have smaller flow capacity, lower arc voltage and stable arc voltage. Poor sex, difficult to meet the needs of interrupted flow, energy transfer and demagnetization.
3.2.2 The special DC magnetic field circuit breaker has less market consumption and lower cost. The manufacturer is not willing to make a large investment, so it is limited in technical performance improvement and process quality improvement.
3.2.3 Mechanical DC magnetic field circuit breakers have inherent defects in principle. The breaking process is the process of arc formation, expansion, combustion, and extinction. If there is an arc, there will be ablation and damage; if there is mechanical action, there will be wear and tear, and mechanical jamming, refusal, and misoperation are inevitable, which cannot be fundamentally solved and eliminated.
3.3 In the design of the de-magnetic protection system, the parameter selection is unreasonable, and improper cooperation is also one of the causes of the occurrence of the de-excitation accident. 3.4 The de-excitation method is single and lacks backup.
In view of the existing problems of de-excitation protection, the author believes that an AC magnetic field circuit breaker should be installed on the AC side of the large-scale hydro-generator excitation system to develop a new type of electronic DC-type magnetic circuit breaker with high breaking voltage. The AC/DC integrated de-excitation protection technology is adopted to improve the safety, reliability and rapidity of the de-excitation protection of the large hydro-generator rotor.
4 AC and DC integrated de-magnetic protection scheme introduces the new AC-DC integrated de-excitation protection main circuit principle wiring diagram as shown, it is composed of AC magnetic field breaker AMK, electronic DC magnetic field breaker DDL and zinc oxide varistor energy absorbing component RV composition. The cut-off energy-shifting task can be completed by the AC circuit breaker or the electronic circuit breaker alone or together. The two kinds of circuit breakers can work independently and spare each other, which not only retains the characteristics of AC demagnetization, but also compensates for the lack of de-excitation of the AC circuit breaker alone. The two kinds of circuit breakers can complete the current interruption task independently or together, which can greatly improve the reliability of the magnetic protection action. The task of energy dissipation is accomplished by a zinc oxide varistor.
4.1 AC circuit breaker demagnetization technology AC circuit breaker demagnetization main circuit schematic diagram as shown, its basic principle is: a three-phase AC circuit breaker is set between the low voltage side of the excitation transformer and the rectifier. When demagnetization is required, the trigger pulse of the thyristor is first cut off, and then the AC breaker is tripped. At this time, the generator rotor is equivalent to a constant current source, which forms a closed loop by the two thyristors that are turned on during the pulse cutting and the secondary winding of the excitation transformer. In this circuit, there are both a constant current source formed by the rotor winding and an AC constant voltage source outputted from the secondary side of the excitation transformer. When the AC breaker is opened, the arc voltage generated by the fracture and the AC output voltage of the excitation transformer are superimposed to act on the closed main circuit. When the condition l/K+ is satisfied, the rotor current is completely converted into the ZnO energy absorbing element, and then the switch arc is extinguished, and the current interruption task is completed. When the peak value of the excitation transformer anode voltage is greater than the ZnO demagnetization residual voltage, even if the AC circuit breaker is not tripped, the rotor current can be transferred to the ZnO demagnetization resistor, and the rotor energy storage is absorbed by the ZnO varistor. It can be seen that by using the anode voltage of the excitation transformer, the breaking arc voltage of the AC circuit breaker can be greatly reduced.
See the test waveforms.
AC magnetic field circuit breaker main circuit schematic diagram of course, it is also feasible to use AC circuit breaker with DC breaking capacity as DC magnetic circuit breaker. In order to obtain a high enough breaking arc voltage, the main touch of the AC circuit breaker can be used. The heads are connected in series.
The fast-breaking excitation system is connected to the AC power source, which is beneficial to the rapid energy dissipation of the zinc oxide varistor.
The circuit breaker works in the AC circuit. During the breaking process, with the help of the reverse half-wave anode voltage, the breaking arc voltage of the AC circuit breaker is greatly reduced. Therefore, the arc ablation and the loss of the arc contact are relatively small and the safety is good.
The AC circuit breaker has a large amount of use, low cost, many manufacturers, and great investment and development, so the technology is mature, the product quality is high, the operating mechanism is flexible, and the reliability is high.
Compact, modular, small size and low price.
The pulse must be cut to increase the control link; when the thyristor breakdown short circuit occurs, the bridge arm is straight through, the AC circuit breaker does not work, only the freewheeling current extends the demagnetization time; when the generator internal, the outlet, and the excitation change occur In the event of a short circuit fault, the AC voltage is very low and the AC circuit breaker may not be able to shut down normally.
4.2 Electronic DC magnetic field circuit breaker demagnetization technology to fundamentally solve the inherent defects of mechanical magnetic circuit breaker, the author believes that should update the concept, change ideas, be brave to innovate, use power and power electronic components to develop a new magnetic field circuit breaker to replace the mechanical breaker.
The principle diagram of the main circuit of the electronic DC magnetic field circuit breaker is shown in the figure.
The electronic circuit breaker (the dotted line in the figure) consists of high-power main thyristors KP1 and KP2 (can increase or decrease the number of parallels and increase the number of series), auxiliary thyristor KP3, commutating reactor L, storage capacitor C The control trigger circuit CF and the capacitor charging power source Vc are composed. The opening of the circuit breaker is controlled by the G1 and G2 pulses of the control trigger circuit CF. The circuit breaker is controlled to be turned off by the G3 pulse of the control trigger circuit CF. At this time, the KP3 is turned on, and the capacitor C in which the electric energy is stored in advance is passed through the commutating reactor U, the main thyristor tubes KP1 and KP2, and the auxiliary thyristor. The tube KP3 is discharged, and a reverse current is applied to the main thyristor tube, forcing its current to cross zero to turn off, and the circuit breaker is broken.
The electronic circuit breaker is completely different from the mechanical circuit breaker in terms of working principle, process structure and control mode. It has no arc and no mechanical transmission part, which eliminates the principle of arc ablation, mechanical jamming, and the possibility of refusing to move. The shutdown time is short, only a few hundred microseconds, the current is fast, and the breaking voltage is high, which is very beneficial to the need of commutation and energy transfer. The development of power electronics technology provides us with high-capacity, high-voltage, high-reliability switching components, and high-capacity electronic circuit breakers have good development prospects.
Characteristics of electronic circuit breaker demagnetization The electronic circuit breaker is de-excited without arc, no sound, no mechanical transmission, long life, high safety and reliability.
The flow capacity is large, the single pipe can reach 5000A; the breaking speed is fast, less than 1m; the breaking voltage is high, and the single pipe can reach 5000V; it is beneficial to the need of rapid demagnetization.
The control power is small, the degree of automation is high, and it can undertake frequent on-off operations, especially for small-duty, unattended power stations and peak-shaving units.
The perfect state monitoring and self-diagnosis system can easily realize the communication connection with the program control system or the computer monitoring system, which further improves its safety and reliability.
Since the electronic circuit breaker was put into operation at the Hongshan Power Plant of Baishan Power Plant in 1990, the de-excitation and over-voltage protection devices composed of 9 electronic circuit breakers have been put into operation at the Xiaoshan Power Station, the Lotus Power Station and the Fengman Power Plant. The rated current and the breaking voltage are continuously improved. The lotus 140 MW machine demagnetization simulation test has a breaking current of 3196 amps and a de-excitation energy of 2.54 megajoules. The electronic circuit breakers for Hongshi units have been safely operated for more than 10 years. The electronic circuit breakers on other units have been safely operated for 4 to 5 years. The success rate of the de-magnetic protection actions is 100%, and significant social and economic benefits have been obtained. Deeply affected by 4.3 zinc oxide varistor energy absorbing element (valve) zinc oxide varistor element has flat volt-ampere characteristics, minimal leakage current, large surge energy absorption and fast response, and is used as a de-energized energy-limiting component. , you can get close to the ideal demagnetization effect. In the past ten years, nearly 1,000 de-excitation devices assembled by us have performed well in various power plants at home and abroad. The first 300MW hydroelectric generating unit de-excitation device has been operating safely for more than ten years. After the decommissioning, the valve piece was tested by the microcomputer, the characteristics changed little, and the aging test was still passed smoothly. This shows that as long as the stable performance and high quality zinc oxide valve piece are selected, reasonable design, string combination and current sharing can be consistent, the life of the valve piece is very long, and the demagnetization and pressure limiting energy absorption are full. reliable. The microcomputer test and combination technology of the valve plate won the third prize of Science and Technology Progress Award of the Chinese Academy of Sciences. Zinc Oxide Nonlinear Resistance Demagnetization Technology won the second prize of Science and Technology Progress Award of the Chinese Academy of Sciences and the first prize of Anhui Science and Technology Class.
S Conclusion 5.1 AC magnetic field circuit breaker de-excitation, compared with mechanical DC magnetic field circuit breaker de-excitation, whether it is in its own internal quality, or in the environmental conditions of de-excitation work have obvious advantages, safety and reliability are rather embarrassing.
5.2 The electronic circuit breaker overcomes the inherent defects of the mechanical DC magnetic circuit breaker and has outstanding advantages. It can be used as an alternative to the latter and should be optimized and popularized.
5.3 AC and DC integrated de-magnetic protection scheme, the two kinds of circuit breakers have different working principles, different structural forms and different control modes, each working independently and cooperating to form a redundant de-magnetic protection system, which can de-magnetic protection Reliability is a level.
5.4 Large-scale hydro-generator rotor de-excitation protection adopts AC and DC comprehensive protection, which is safe, reliable and fast, and has the best performance-price ratio. It can be used as the preferred scheme for rotor degauss protection of large hydro-generators such as the Three Gorges.
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