Visible from Equation 1), the adjustment coefficient! Indicates the relative change in generator voltage when the reactive current increases from zero to the rated value. The smaller the adjustment coefficient, the smaller the generator voltage change when the reactive current changes. So, the adjustment coefficient! Characterizes the ability of the excitation control system to maintain generator voltage. The characteristic curve is adjusted for three types of generators. It can be seen from the formula 1) that the characteristic curve having the positive adjustment coefficient of 6>0) is inclined downward, that is, the generator terminal voltage decreases as the reactive current increases. Has a negative adjustment coefficient! The characteristic curve of <0) is upturned, and the generator terminal voltage rises as the reactive current increases. Has no difference! The generator voltage of =0) is a constant value. Generator regulation characteristic curve! g generator terminal voltage; generator reactive current; /qs generator rated reactive current The above regulation characteristics are relative to a generator as an independent power source, however, in the power system, the synchronous generator is actually running in parallel If the power system is regarded as an infinite power source, when two or more generators are directly connected to the grid and run in parallel, changing the excitation current output of one of the generators does not significantly affect the terminal voltage, but only changes the power generation. The distribution of reactive power between machines. 2 multiple generators are directly connected in parallel, and the reactive power distribution between the generators is marked as the standard value of 2.1. The adjustment characteristics of 2.1 can not be stabilized in parallel operation. Two generators with no adjustment characteristics are not allowed on the common bus. Running in parallel. Because even if the voltage setting values ​​of the two generators are completely equal, the bus voltage remains unchanged, and the reactive power distribution between the two generators is arbitrary, and the system is in an unstable operating state. A generator with no difference regulation characteristics and a generator with positive adjustment characteristics can be stably operated in parallel, but the reactive load change of the system will all be reflected on the generator with no adjustment characteristics, so it is rarely use. If a generator operating in parallel on a common bus has a negative adjustment characteristic, according to the principle of negative adjustment, the other generator will lose its magnetism and the system will not operate stably. 2.2 Generator with stable differential characteristics Stable parallel operation assumes that two generators with positive-adjusting characteristics operate in parallel on the common bus, and their regulation characteristics are as shown. Since the terminal voltages of the two generators are the same, equal to the bus voltage Uei, the reactive currents of each generator are determined, which are IQ1 and 1"2. When the reactive load increases, the bus voltage drops, and the excitation The regulator will increase the excitation current to reach the new stable voltage U2. At this time, the reactive currents of the two generators are I"1 (and I"2' respectively. The two generators respectively take part of the increased reactive load. The size depends on the respective adjustment coefficients. It is also simplified by the formula 1). When the bus voltage changes from "G1 to G2", the equation 2) gives -1 from the equation 6) minus the equation 2), and the equation 4) shows that when the bus voltage fluctuates, the generator reactive current increases. It is proportional to the voltage deviation and inversely proportional to the adjustment factor, regardless of the voltage value. The negative sign indicates that in the case of positive adjustment, 5>0), when the bus voltage decreases, the generator reactive current will increase. Two generators with positive difference are operated in parallel. When the reactive load of the system fluctuates, the voltage deviation is the same. It can be seen from Equation 4) that if the standard value AI"- of each generator reactive current fluctuation amount is desired to be equal, it is required that each generator operating in parallel on the common bus has the same positive difference coefficient. 3 generator-transformer group parallel operation of the generator's adjustment characteristics bookmark5 as shown when the reactive current is zero, the generator terminal voltage is Ugo; when the reactive current is rated value I"*, the generator terminal voltage is U, When the bus voltage is Ui, the reactive current of the curve 1 generator can be expressed by the formula 2). As shown, the typical generator transformer unit unit wiring method. Due to the inherent short-circuit impedance of the transformer, the parallel operating bus voltage on the high-voltage side of the main transformer is low. In order to compensate the voltage drop caused by the reactive current of the transformer, the generator excitation should adopt the negative adjustment adjustment characteristic, but the adjustment characteristic of the actual reaction on the high voltage side of the main transformer is positive adjustment. According to the negative balance characteristic of the generator, the generator will increase the excitation current as the reactive load of the system increases, maintaining the normal level of the system voltage at the high voltage side of the main transformer. In this way, the positive adjustment coefficient on the high voltage side is not too large, sensitive to the reactive load fluctuation of the system, and the voltage regulation characteristics are good, which is beneficial to the safe and stable operation of the system. If the negative tuning coefficient setting value of the generator excitation regulator is properly selected so as to be close to the voltage drop of the compensation reactive current on the transformer short-circuit impedance, the system voltage can be kept constant. Therefore, this negative adjustment link is actually a reactive current compensation device, which can maintain the voltage of the node in the system is basically constant, which is extremely beneficial for improving the system voltage quality and maintaining system stability. 4 Analysis of the adjustment characteristics of the two generators under the expanded unit wiring mode 4.1 Analysis The two units of the generator are operated in parallel under the expansion unit wiring mode, and the problem of stable and reactive power distribution is complicated. For the equivalent impedance map of a single machine to an infinite system, if the low voltage side of the main transformer is used as a common bus, for a parallel generator, the connected system is not an infinite system, and the internal resistance of the system is approximately equal to the short circuit impedance of the main transformer. XT. The reactive current sent by the two generators will produce a voltage drop across the short-circuit impedance of the main transformer. For the system voltage node on the high voltage side of the main transformer, it can be regarded as accessing the infinite system, that is, the system voltage of the high voltage side of the main transformer is basically unchanged. Under this premise, the author discusses the problem of widening the adjustment characteristics of two generators. First of all, in order to maintain stable operation, both generators must operate under positive adjustment characteristics. The system voltage on the high-voltage side of the main transformer is considered to be basically the same, but when one of the generators is excited, the main transformer low-voltage side voltage Will rise, at this time another generator feels the terminal voltage rises, will reduce the excitation to maintain the terminal voltage. As a result, while one generator sends out reactive power, the other unit absorbs reactive power, making it extremely difficult to obtain good reactive power regulation performance. Secondly, when one generator is connected to the grid on the low-voltage side of the main transformer and the other generator is juxtaposed, the system will still have reactive disturbance due to the fact that the characteristics of the two excitation systems are not completely equal, allowing the phase difference of the voltage to be synchronized. The two units are caused to generate reactive power disturbances at the time of parallel. If the positive adjustment coefficient is too small, it will cause one of the units to be reactively overloaded and the other unit to be demagnetized. Therefore, the positive adjustment coefficient needs to be set to be close to half of the main transformer short-circuit impedance X. This can solve the reliable parallel and stable operation between the two generators, but enlarge the unit to the system due to the large positive difference coefficient. The voltage regulation performance is poor), the response to the system reactive disturbance is weak, and the ability to maintain the constant voltage of the high voltage side of the main transformer is poor, which is very unfavorable to the system voltage quality and system stability. The following is an in-depth analysis of the situation that occurred during the start-up operation of Mianhuatan Hydropower Station. 4.2 The connection mode of the generator expansion unit of Mianhuatan Hydropower Station is shown as the main wiring diagram of the %2 main transformer unit of Mianhuatan Hydropower Station. The 3 and %4 machines are connected to the %2 main transformer low voltage through the 603 and 604 switches on the 15.75kV common bus. On the side, the 220kV system is connected to the high-voltage side switch 21B of the %2 main transformer. 4.3 After the accident of 3 generator tripping caused by the negative adjustment characteristic, the single machine has 2 main test runs normally. On June 29, 2001, the 3 units of Mianhuatan Hydropower Station operated normally P'=150MW, Q'= 0), 4 machines are operated by the operation personnel from the start to the quasi-synchronization. In the short 400ms when the 4 machine exit 604 switch is closed, the 3 machine exit 603 switch trips, and the 3 generators disengage the accident stop. Afterwards, check the SOE, event record and protection action as follows: 4 machine terminal voltage instantaneously rises to U4=16.01kV, reactive power mutation reaches Q4=239Mvar emits reactive power), active P4=0; 3 machine active unchanged 33 =150MW) reactive power mutation to Q3=-249.5M>a absorption reactive power), although the low excitation limit of the 3 excitation system is action, but the reason is not timely, the unit is protected by loss of field and low voltage overcurrent protection. Discharge the shutdown. 4.4 Analysis of the cause of accidental trip caused by negative adjustment difference In the normal operation of the 3 units of Mianhuatan Hydropower Station, the reactive current increases when the 4 machines are connected to the grid. According to the negative adjustment characteristic, the 4 machine feels that the terminal voltage U" decreases. Increasing the excitation current tries to maintain the terminal voltage, which leads to further increase of the reactive power sent by the 4 generators; /4 increase leads to the increase of U"; at this time, the 3 machines feel that the terminal voltage is high, and the 3 machines try to maintain the terminal voltage. Reduce the excitation current and the reactive power/reduction of the 3 generators. In this way, the reactive power generated by the four machines is completely absorbed by the three machines, and a reactive current circulating current is formed between the three machines and the four machines, thereby losing stability, eventually resulting in demagnetization of the three-machine excitation system, and the loss-of-magnetism protection of the three generators. Low-voltage over-current protection action exit jumps. 5 Microcomputer type excitation regulator itself defects Currently, the microprocessor-based excitation regulator handles the analog quantity by isolating the stator three-phase secondary voltage and current and sending it to A/D sampling, usually in a cycle of 20ms. Each phase voltage and current are sampled at intervals of 32 points. When the voltage and current in one cycle are collected, the digital signal processor (DSP) calculates the effective value, active power and reactive power of the three-phase voltage and current through fast Fourier transform FFT. When the generator enters phase, the microcomputer-type low-excitation limiting unit calculates the maximum allowable phase-in reactive power corresponding to the P-/characteristic curve based on the actual active power/, if the actual reactive power is generated! b>! Bc, that is, the low excitation limit action value is reached, and the low excitation limit signal is delayed by 60ms or longer in a very short time, thereby starting the low excitation limit program and limiting the phase reactive power to the allowable value /bc. This is to avoid the oscillations caused by the control action being too frequent. The software of the microcomputer type excitation regulator adopts the PID control with dead zone. Therefore, when the normal generator is in phase operation, the adjustment is slowly adjusted to reduce the excitation, and the low excitation limit can be exerted. When the generator excitation system is demagnetized, the excitation current is reduced rapidly, and the low excitation limit unit is judged and limited. Due to the defect of the microcomputer excitation itself, the excitation current cannot be kept up when the generator excitation system is demagnetized. The reduced speed causes the low excitation to limit the reduction of the excitation current, which in turn leads to a deeper phase loss of the generator phase until the stable operation is lost. 6 Problems in the operation of the two-machine positive adjustment characteristic under the expansion unit wiring mode Table 1 Two-machine grid-connected online reactive power distribution test data change The two units in the expansion unit of the ff3 machine Mianhuatan Hydropower Station have used positive adjustment characteristics, although It can run stably under the condition that the reactive load of the system is not disturbed, but there are still problems in the actual operation. 6.1 When two units are juxtaposed, it is easy to form a reactive circulation flow during operation. The 3 and 4 machine excitation regulators of Mianhuatan Hydropower Station operate in the 'voltage closed loop mode', and the adjustment coefficient is set to +3.0%. Network online reactive power distribution test, at different active power load points P total, adjust 3 machine excitation current, respectively change the reactive power of 3, 4 machines, record 220kV booster station in different 3 or 4 machine reactive power points Voltage U!2kV, 2 main transformer high-voltage side reactive power/total, 3 machine active power P3 and reactive power/3, 4 machine active power P4 and reactive power/4, the data is shown in Table 1. The above test data analysis shows Two generators are operated in parallel, and the reactive power of one unit is completely reflected in the reactive power of the other unit, and a reactive current circulation is formed between the two units. Moreover, if only one unit is adjusted Reactive power is unable to adjust the voltage of the system. Only when the two machines increase the excitation at the same time or reduce the excitation at the same time, the system voltage and the reactive power of the system can be adjusted. The reason is that for the generator excitation regulator, only one unit is adjusted. Generator excitation When the magnetic current is output, only the setting value of the tuning unit increases, the output characteristic curve of the regulator moves up in parallel, and the reactive power regulation characteristic of the generator also moves up, and the adjustment characteristic remains unchanged, that is, unchanged. 6.2 Two machines in parallel, the instantaneous reactive power fluctuation is large. The authors set up the on-line reactive power dynamic impact test of the two machines in the 3rd and 4th excitation regulators of Mianhuatan Hydropower Station. The recorded data are shown in Table 2. Table 2 is not connected to the grid. Power dynamic impact test data The adjustment coefficient of the two machines is set. The two machines are juxtaposed before the two machines are juxtaposed (3 machine low excitation limit action). Before the two machines are running in parallel, the voltage of the two generators is not They are completely equal, and there is inevitably a transient shock. Japanese companies have developed spherical solar cells. Kyoto Semiconductor Corporation of Japan has announced the successful trial of spherical solar cells, and will begin mass production at the end of next year. The company has exhibited this new product at the *001 International New Technology Exhibition in Tokyo and has attracted the attention of visitors. This spherical micro solar module module is made of spherical balls with a diameter of only 1.5mm. Made of monocrystalline silicon. These pellets are arranged in a line of 16 vertical and 30 horizontal, and are hardened with a conductive adhesive silver paste and covered with a transparent glass. It has a photoelectric conversion efficiency of 16%, a current output power of 16%, and a current output power of 0.380 to 0.400 mV. The spherical single crystal silicon used in this solar cell is produced in a gravity-free state. And the reactive power disturbance is large, which is not conducive to the safe and stable operation of the unit. It is recommended to reduce the reactive disturbance caused by the differential pressure after closing by reducing the differential pressure allowed in the generator synchronizing device. 6.3 The system side adjustment coefficient is too large. The generator adopts the positive adjustment coefficient, plus the short-circuit impedance of the main transformer. The response coefficient of the reaction on the 220kV system side is too large, and it is not sensitive enough to the reactive disturbance response of the system. It is too soft, so each generator is not sensitive to the reactive load fluctuation response of the system, which is unfavorable for the safe and stable operation of the system. 6.4—The loss of magnetism of the generator causes another generator to overcurrent. In the case of loss of magnetism in one generator, the other generator will increase the excitation current in order to maintain the voltage on the busbar in parallel. Causes the generator to overcurrent. 7Constructive concept Through the above-mentioned principle of the adjustment characteristics, as well as the adjustment characteristics of the main unit wiring mode of the expansion unit and the test data of the Miantan Hydropower Station, the author believes that the excitation adjustment of the two generators for expanding the unit wiring mode The reactive power sent by the two generators should be monitored. After the monitoring signals are processed, the respective excitation regulators are controlled to realize the reactive current compensation and adjustment of the generator, so as to adjust the reactive power between the generators. Distribute, alleviate the instantaneous reactive power transient shock of the generator, improve the system voltage quality, and maintain the stability of the system. Specific improvements and options are detailed elsewhere. The granular silicon is placed in a heating and melting device for instantaneous high-temperature heating and melting, and then vertically falls in a gravity-free state. During the process of 1.51 drop of 14 m, the silicon solidifies into a single crystal silicon ball due to the surface tension. Spherical solar cells are the idea of ​​Texas Instruments. Compared with the existing planar solar cells, it has the advantages of large light receiving area, high utilization efficiency of solar energy, less use of silicon materials and waste materials in the production process, and low production cost. Therefore, it will contribute to the popularity of solar energy. 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