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Questions and Answers on Power Dispatch Technology--Short Answer Questions (Reposted) 1. What are power systems, power systems, and power grids? Answer: Usually, the unified whole of the production, transmission, distribution and use of electric energy and thermal energy composed of the power facilities and equipment of power generation enterprises and power generation, transmission, transformation, distribution, electrical equipment and corresponding auxiliary systems is called a power system. ； The unified whole of electric energy production, transmission, distribution and use consisting of power generation, transmission, transformation, distribution, electrical equipment and corresponding auxiliary systems is called a power system ； The unified whole that connects power generation and consumption, consisting of power transmission, transformation, distribution equipment and corresponding auxiliary systems, is called a power grid. 2. What are the characteristics of modern power grid? Answer: 1. The main grid is composed of a strong ultra-high voltage system. 2. The connections between various power grids are strong and the voltage levels are relatively simplified. 3. It has sufficient peak shaving, frequency modulation and voltage regulation capacity, can realize automatic power generation control, and has high power supply reliability. 4. Have corresponding safety and stability control systems, highly automated monitoring systems and highly modern communication systems. 5. Have a technical support system adapted to the operation of the electricity market, which is conducive to the rational use of energy. 3. What is the significance and function of regional power grid interconnection? Answer: 1. It can rationally utilize energy, strengthen environmental protection, and be conducive to the sustainable development of the power industry. 2. It can install large-capacity, high-efficiency thermal power units, hydropower units and nuclear power units, which will help reduce costs, save energy and speed up power construction. 3. Time differences and temperature differences can be used to stagger the peak power consumption, use the non-simultaneity of power consumption in various regions to adjust loads, and reduce spare capacity and installed capacity. 4. It can supply power to each other, exchange what is needed, and serve as backup for each other in various regions. It can reduce the backup capacity for accidents, enhance the ability to withstand accidents, and improve the security level of the power grid and the reliability of power supply. 5. It can withstand large impact loads and is beneficial to improving power quality. 6. Hydropower can be adjusted across river basins, and economic dispatch of water, thermal power and electricity can be carried out in a wider range to achieve greater economic benefits. 4. What are the principles of grid reactive power compensation? Answer: The principle of grid reactive power compensation is that grid reactive power compensation should basically be considered based on the principles of hierarchical partitioning and local balancing, and should be able to be adjusted with the load or voltage to ensure that the voltage at each pivot point of the system can meet the specified requirements both normally and after an accident, and to avoid the transmission of reactive power through long-distance lines or multi-stage transformers. 5. Briefly describe the difference between voltage characteristics and frequency characteristics of power systems? Answer: The frequency characteristics of the power system depend on the frequency characteristics of the load and the frequency characteristics of the generator (the characteristics of the load that change with the change of frequency are called the frequency characteristics of the load. The characteristic that the output of the generator set changes with the frequency is called the frequency characteristic of the generator), which is determined by the active load balance of the system and has little to do with the network structure (network impedance). Under non-oscillating conditions, the steady-state frequency of the same power system is the same. Therefore, the system frequency can be adjusted and controlled centrally. The voltage characteristics of the power system are different from the frequency characteristics of the power system. The voltage of each node in the power system is usually not exactly the same. It mainly depends on the balance of active and reactive power supply and demand in each area, and is also closely related to the network structure (network impedance). Therefore, the voltage cannot be adjusted centrally and uniformly across the entire network, and can only be adjusted and controlled by partition. 6. What are the system voltage monitoring points and central points? What's the difference? How is the voltage center point generally selected? Answer: The node that monitors the voltage value of the power system and assesses the voltage quality is called a voltage monitoring point. The important voltage support node in the power system is called the voltage pivot point. Therefore, the voltage center point must be the voltage monitoring point, but the voltage monitoring point is not necessarily the voltage center point. The selection principles for voltage pivot points are: 1) High-voltage busbars of regional water and thermal power plants (high-voltage busbars have multiple outlets) ； 2) Zoning selects 220kV substation busbars with larger busbar short-circuit capacity ； 3) Power plant busbars with large local loads. 7. Describe the impact of power system harmonics on the power grid? Answer: The main impacts of harmonics on the power grid are: The main harm of harmonics to rotating equipment and transformers is to cause additional losses and increased heat generation. In addition, harmonics can also cause rotating equipment and transformers to vibrate and emit noise. Long-term vibration can cause metal fatigue and mechanical damage. The main harm of harmonics to lines is to cause additional losses. Harmonics can cause the inductor and capacitor of the system to resonate, amplifying the harmonics. When harmonics cause system resonance, the harmonic voltage increases and the harmonic current increases, causing relay protection and safety automatic devices to malfunction, damaging system equipment (such as power capacitors, cables, motors, etc.), causing system accidents, and threatening the safe operation of the power system. Harmonics can interfere with communication equipment, increase the power loss of the power system (such as line loss), prevent reactive power compensation equipment from operating normally, etc., causing harm to the system and users. The main measures to limit the harmonics of the power grid are: increasing the pulsation number of the converter device ； Install AC filter and active power filter ； Strengthen harmonic management. 8. What is latent current? What impact does it have on reclosing? How to prevent it? Answer: When the faulty phase of the faulty line is removed from both sides, the capacitive coupling and inductive coupling existing between the non-faulty phase and the disconnected phase continue to provide current to the faulty phase, which is called latent current. Due to the existence of latent current, the arc extinguishing at the fault point is affected, which seriously hinders the arc channel dissociation during short circuit. Automatic reclosing can only be successful after the arc at the fault point is extinguished and the insulation strength is restored. When the latent current value is large, the arc extinguishing time of the fault point is longer, which will cause the reclosing failure. In order to reduce the latent current and improve the reclosing resynthetic power, on the one hand, measures can be taken to reduce the latent current: such as adding a small reactance to the neutral point of the high-voltage shunt reactor of the 500kV medium and long line, and placing fast single-phase grounding switches on both sides of the line for a short period of time. ； On the other hand, the measured arc extinction time can be used to set the reclosing time. 9. What is the theoretical line loss and management line loss of the power system? Answer: Theoretical line loss is an unavoidable loss in the process of transmitting and distributing electric energy. It is determined by the load condition of the power network and the parameters of the power supply equipment at that time. This part of the loss can be calculated through theoretical calculations. Management line losses are other losses and various unknown losses in the actual operation of the power network. For example, due to errors in the user's electric energy meter, the reading of the electric energy meter is too small. ； The amount of electricity lost is due to omissions and miscalculations in the readings of users' energy meters, leakage of electricity due to poor insulation of live equipment, as well as electricity consumption without electricity meters and electricity theft. 10. What is natural power? Answer: Operating transmission lines can both generate reactive power (due to distributed capacitance) and consume reactive power (due to series impedance). When a certain value of active power is transmitted in the line, the two reactive powers on the line can just balance each other. This value of active power is called the "natural power" or "wave impedance power" of the line. 11. How many types of neutral point grounding methods are there in power systems? What are large current and small current grounding systems? What are their classification standards? Answer: There are two main neutral point grounding methods in my country's power system, namely: 1. Direct neutral point grounding method (including the neutral point grounding method through a small resistance). 2. The neutral point is not directly grounded (including the neutral point being grounded through an arc suppression coil). In a system with a neutral point directly grounded (including a system with a neutral point grounded through a small resistance), when a single-phase ground fault occurs, the ground short-circuit current is very large. This system is called a large ground current system. In systems where the neutral point is not directly grounded (including systems where the neutral point is grounded through an arc suppression coil), when a single-phase ground fault occurs, the ground fault current is often much smaller than the load current because it does not directly form a short-circuit loop, so it is called a small ground current system. The classification standard in my country is: the system with X0/X1≤4~5 belongs to the large ground current system, the system with X0/X1>4~5 belongs to the small ground current system. Note: 12. In power system neutral point directly grounded and non-directly grounded systems, what are the characteristics when a single-phase ground fault occurs? Answer: The power system neutral point operation mode is mainly divided into two categories, namely direct grounding and non-direct grounding. The power supply reliability of directly grounded systems is relatively low. When a single-phase ground fault occurs in such a system, another ground point other than the neutral point appears, forming a short-circuit loop. The ground phase current is very large. In order to prevent damage to the equipment, the ground phase or even three phases must be quickly cut off. The power supply reliability of non-directly grounded systems is relatively high, but the requirements for insulation levels are also high. Because when a single-phase ground fault occurs in this system, a short-circuit loop is not directly formed, and the grounded phase current is not large, so it is not necessary to cut off the grounded phase immediately. However, at this time, the ground voltage of the non-grounded phase increases to 1.7 times the phase voltage. 13. In a small current grounding system, why is the neutral point grounded through an arc suppression coil? Answer: When a single-phase ground fault occurs in a small current grounding system, the grounding point will pass all the ground capacitance current of the voltage level grid corresponding to the ground fault line. If the capacitor current is quite large, intermittent arcing will occur at the grounding point, causing overvoltage and causing a large increase in the non-faulty phase-to-ground voltage. Under the action of arc grounding overvoltage, insulation damage may occur, resulting in two or more points of grounding short circuit, which may expand the accident. To this end, the measures taken by our country are: when a single-phase ground fault occurs in a small current grounding system power grid, if the ground capacitance current exceeds a certain value (10A for a 35kV power grid, 10A for a 10kV power grid, 30A for a 3-6kV power grid), an arc suppression coil is installed at the neutral point. The purpose is to use the inductive current of the arc suppression coil to compensate for the capacitive current during a ground fault, reduce the current at the ground fault point, improve the automatic arc extinguishing capability, and enable automatic arc extinguishing. Ensure continued power supply. 14. Under what circumstances is the single-phase ground fault current greater than the three-phase short-circuit fault current? Answer: When the zero-sequence comprehensive impedance of the fault point is less than the positive-sequence comprehensive impedance, the single-phase ground fault current will be greater than the three-phase short-circuit fault current. For example: In systems that use a large number of autotransformers, due to the large number of grounded neutral points, the zero-sequence comprehensive impedance of the system fault point is often smaller than the positive-sequence comprehensive impedance. At this time, the single-phase ground fault current is greater than the three-phase short-circuit fault current. 15. What are power system sequence parameters? What are the characteristics of zero-sequence parameters? Answer: In a symmetrical three-phase circuit, when currents of different phase sequences flow, the impedances encountered are different. However, the voltage and current of the same phase sequence still comply with Ohm's law. The ratio of the phase sequence voltage at both ends of any component to the corresponding phase sequence current flowing through the component is called the sequence parameter (impedance) of the component. The zero sequence parameter (impedance) is related to the network structure, especially the wiring method of the transformer and the neutral point grounding method. In general, the zero sequence parameters (impedance) and zero sequence network structure are different from the positive and negative sequence networks. 16. What is the relationship between zero sequence parameters and transformer wiring groups, neutral point grounding methods, overhead ground wires of transmission lines, and adjacent parallel lines? Answer: For transformers, the zero-sequence reactance is related to its structure (three single-phase transformer groups or three-column transformer), the connection of the windings (Δ or Y) and whether it is grounded or not. When one side of the three-phase transformer is connected in a delta or star shape with an ungrounded neutral point, the zero-sequence reactance of the transformer is always infinite from this side. Because no matter how the other side is connected, when a zero-sequence voltage is applied to this side, the zero-sequence current cannot be sent into the transformer. Therefore, only when the windings of the transformer are connected in star shape and the neutral point is grounded, the zero-sequence reactance of the transformer is limited (although sometimes it is still very large) when looking at the transformer from the star side. For transmission lines, the zero-sequence reactance is related to factors such as the number of loops of parallel lines, the presence or absence of overhead ground wires, and the conductive properties of the ground wires. The zero-sequence current is in the same phase in the three-phase line, and the mutual inductance is very large, so the zero-sequence reactance is larger than the positive sequence reactance, and the zero-sequence current will return through the ground and the overhead ground wire. The overhead ground wire acts as a shield for the three-phase conductors, reducing the zero-sequence flux linkage, even if the zero-sequence reactance is reduced. When zero-sequence currents in the same direction pass through two three-phase overhead transmission lines erected in parallel, not only the mutual inductance of any two phases of the first circuit and the third phase of the first circuit will produce a magnetizing effect, but also the mutual inductance of all three phases of the second circuit and the third phase of the first circuit will produce a magnetizing effect, and vice versa. This will further increase the zero-sequence impedance of this line. 17. What is the stable operation of the power system? How many categories of power system stability are there? Answer: When the power system is disturbed, it can automatically return to its original operating state, or transition to a new stable state with the help of control equipment, which is called stable operation of the power system. From a broad perspective, the stability of the power system can be divided into: 1. Stability issues of synchronous operation of generators (according to the different magnitudes of disturbances the power system endures, it can be divided into three categories: static stability, transient stability, and dynamic stability) ； 2. Voltage stability problems caused by insufficient reactive power in the power system ； 3. Frequency stability problems caused by insufficient active power in the power system. 18. Why can the transient stability be improved by using single-phase reclosing? Answer: After using single-phase reclosing, since the fault phase is removed during a fault instead of the three phases, the power transmitting end and the power receiving end do not completely lose contact between the fault phase and the reclosing period (the electrical distance is much smaller compared with the three-phase removal). This can reduce the acceleration area, increase the deceleration area, and improve transient stability. 19. Briefly describe the synchronous oscillation and asynchronous oscillation of synchronous generator? Answer: Synchronous oscillation: When the input or output power of the generator changes, the power angle δ will change accordingly. However, due to the inertia of the rotating part of the unit, δ cannot immediately reach a new steady-state value. It needs to oscillate around the new δ value several times before it can operate stably at the new δ value. This process is called synchronous oscillation, that is, the generator still maintains oscillation in a synchronous operating state. Asynchronous oscillation: The generator is greatly disturbed for some reason, and its power angle δ changes periodically between 0 and 360°, and the generator and the power grid lose synchronous operation. During asynchronous oscillation, the generator works in the generator state for a while and the motor state for a while. 20. How to distinguish whether the oscillation occurring in the system is asynchronous oscillation or synchronous oscillation? answer: The obvious characteristics of asynchronous oscillation are that the system frequency cannot maintain the same frequency, and all electrical and mechanical quantities fluctuate significantly from the rated values. For example, the ammeters and power meters of generators, transformers and tie lines swing greatly periodically. ； The voltmeter swings greatly periodically, with the voltage at the center of the oscillation having the largest swing, and periodically falling close to zero. ； The transmission power of the link between the out-of-step power plants oscillates back and forth. ； The frequency of the sending system increases, while the frequency of the receiving system decreases and oscillates. When oscillating synchronously, the system frequency can remain the same, the fluctuation range of each electrical quantity is not large, and the oscillation attenuates within a limited time to enter a new balanced operating state. 21. What is the difference between system oscillation accident and short circuit accident? answer: The main differences between power system oscillation and short circuit are: 1. During oscillation, the voltage and current values at each point of the system swing back and forth, while during short circuit, the current and voltage values change suddenly. In addition, the current and voltage values change slowly during oscillation, but the current and voltage values suddenly change greatly during a short circuit. 2. During oscillation, the phase angle between the current and voltage at any point in the system changes with the change of the power angle. ； During a short circuit, the angle between current and voltage is basically unchanged. 3. The three phases of the system are symmetrical during oscillation. ； During a short circuit, the system may experience three-phase asymmetry. 22. What is the main cause of asynchronous oscillation in the power system? Answer: 1. The transmission power of the transmission line exceeds the limit value, causing static stability damage. ； 2. A short-circuit fault occurs in the power grid, cutting off large-capacity power generation, transmission or substation equipment, causing a large sudden change in load, etc., causing transient stability damage to the power system. ； 3. The ring system (or parallel double circuit line) suddenly opens the loop, causing the connection impedance of the two parts of the system to suddenly increase, causing stability damage and loss of synchronization. ； 4. Large-capacity units trip or lose excitation, which increases the load on the system tie line or severely drops the system voltage, causing the tie line stability limit to decrease and easily causing stability damage. ； 5. The asynchronous closing between power supplies failed to be brought into synchronization. 23. What is the general phenomenon when the system oscillates? Answer: 1. The voltmeter, ammeter and power meter of the generator, transformer, and line swing violently periodically, and the generator and transformer make a rhythmic roar. 2. The ammeter and power meter on the tie line connecting the out-of-synchronization generator or system will swing the most. The most intense voltage oscillation is at the system oscillation center, which drops to zero approximately once per cycle. As the distance from the oscillation center increases, the voltage fluctuation gradually decreases. If the impedance of the tie line is large and the capacitance of the power plants on both sides is also large, the voltage oscillation at both ends of the line will be small. 3. The power grid of the same period has been lost. Although there is electrical connection, there is still a frequency difference. The frequency of the sending end is high, and the frequency of the receiving end is low and slightly oscillating. 24. What is low-frequency oscillation? What is the main reason? Answer: The continuous oscillation phenomenon in the frequency range of 0.2 to 2.5 Hz that occurs between generators running in parallel under small interference is called low-frequency oscillation. The cause of low-frequency oscillation is the negative damping effect of the power system, which often occurs on weakly connected, long-distance, and heavy-load transmission lines. It is more likely to occur under the condition of using a fast, high-amplification excitation system. 25. What are the functions of ultra-high voltage power grid shunt reactors in improving the operating conditions of the power system? answer: 1. Reduce the capacitive effect on no-load or light-load lines to reduce power frequency transient overvoltage. 2. Improve voltage distribution on long-distance transmission lines. 3. Balance the reactive power in the line as much as possible when the load is light, prevent the unreasonable flow of reactive power, and also reduce the power loss on the line. 4. When large units are paralleled with the system, the power frequency steady-state voltage on the high-voltage bus is reduced to facilitate the paralleling of generators at the same time. 5. Prevent self-excited resonance that may occur with long lines in the generator. 6. When the neutral point of the reactor is grounded through a small reactance device, a small reactor can also be used to compensate the phase-to-phase and phase-to-ground capacitance of the line to accelerate the automatic extinguishing of the latent current and facilitate the use of single-phase rapid reclosing. 26. What is the role of adding a small reactance to the neutral point of a parallel high-voltage reactor in a 500kV power grid? Answer: Its function is to compensate the conductor-to-ground capacitance, make the relative-to-ground impedance tend to infinity, eliminate the longitudinal component of latent current, thereby improving the success rate of reclosing. The selection of the small reactance impedance size of the neutral point of the parallel high-voltage reactor should be calculated and analyzed to prevent ferromagnetic resonance. 27. What is subsynchronous oscillation of generator? What causes it? How to prevent it? answer: When the generator is connected to the system through a series capacitor compensated line, if the series compensation degree is high, the electrical resonance frequency of the network will easily resonate with the natural torsional vibration frequency of the large turbine generator shaft system, causing torsional vibration damage to the large shaft of the generator. This resonant frequency is usually lower than the synchronous (50 Hz) frequency and is called subsynchronous oscillation. For high-voltage direct current transmission lines (HVDC) and static var compensators (SVC), when their control parameters are improperly selected, subsynchronous oscillations may also be induced. Measures include: 1. By adding or modifying primary equipment; 2. Reduce the degree of series compensation; 3. Provide damping of the torsional vibration mode through secondary equipment (similar to the principle of PSS). 28. How many types of overvoltage are there in the power system? What are their causes and characteristics? Answer: Power system overvoltage is mainly divided into the following types: atmospheric overvoltage, power frequency overvoltage, operating overvoltage, and resonance overvoltage. The causes and characteristics are: Atmospheric overvoltage: caused by direct lightning strike. It is characterized by short duration and strong impact. It is directly related to the intensity of lightning strike activity and has nothing to do with the equipment voltage level. Therefore, the insulation level of systems below 220KV is often determined by preventing atmospheric overvoltage. Power frequency overvoltage: caused by the capacitance effect of long lines and sudden changes in the power grid's operation mode. It is characterized by a long duration and a low overvoltage multiple. It is generally not dangerous to equipment insulation, but it plays an important role in determining the insulation level in ultra-high voltage and long-distance power transmission. Operation overvoltage: caused by switching operations in the power grid, it is characterized by randomness, but the overvoltage multiple is high in the most unfavorable circumstances. Therefore, the insulation level of ultra-high voltage systems of 30KV and above is often determined by preventing operating overvoltage. Resonance overvoltage: caused by the system capacitance and inductance loop forming a resonant loop. It is characterized by high overvoltage multiples and long duration. 29. What is counterattack overvoltage? Answer: In power plants and substations, if lightning strikes the lightning rod, the lightning current will flow to the ground through the frame ground down conductor. Due to the existence of the frame inductance and ground resistance, a high ground potential will be generated on the frame. The high potential will cause a large potential difference to nearby electrical equipment or live wires. If the distance between the two is small, it will cause the lightning rod structure to discharge to other equipment or wires, causing counterattack flashover and causing an accident. 30. What is step voltage? answer: The current flowing into the ground through the grounding grid or grounding body will form a spatially distributed current field on the surface and deep underground, and generate a potential difference at different distances from the grounding body. This potential difference is called step voltage. The step voltage is directly proportional to the intensity of the ground current and inversely proportional to the square of the distance from the ground body. Therefore, in an area close to the grounding body, if a strong lightning current is encountered and the step voltage is high, it is easy to cause harm to people and livestock. 31. What are the main causes of power frequency overvoltage in power systems? Answer: 1. Capacitive effect of long no-load lines ； 2. Increase in non-fault phase voltage caused by asymmetric short circuit ； 3. Increase in power frequency voltage caused by load shedding. 32. What are the main measures to limit power frequency overvoltage in power systems? Answer: 1. Use parallel high-voltage reactors to compensate for the capacitive effect of no-load lines ； 2. Use static reactive power compensator SVC to compensate for the capacitance effect of no-load lines ； 3. Direct grounding of the neutral point of the transformer can reduce the increase in power frequency voltage caused by asymmetric ground faults. ； 4. The generator is equipped with an excitation regulator or voltage regulating device with good performance, so that when the generator suddenly sheds load, it can suppress the capacitive current's magnetizing armature reaction to the generator, thereby preventing the generation and development of overvoltage. 5. The generator is equipped with a responsive speed control system, which can effectively limit the power frequency overvoltage caused by the increase in generator speed when the load is suddenly shed. 33. What is operating overvoltage? What are the main ones? Answer: Operation overvoltage is an overvoltage caused by switch operation or fault tripping in the power grid. Mainly include: 1. Cut off overvoltage caused by no-load lines ； 2. Overvoltage caused when no-load line is closed ； 3. Cut off overvoltage caused by no-load transformer ； 4. Overvoltage caused by interstitial arc grounding ； 5. Unlock the overvoltage caused by the large loop. 34. What are the measures to limit operating overvoltage in the power grid? Answer: Measures to limit operating overvoltage in the power grid include: (1) Selecting a high-voltage switch with strong arc extinguishing capability ； (2) Improve the synchronicity of switching actions ； (3) Install a parallel resistor at the switch break ； (4) Use lightning arresters with good performance, such as zinc oxide arresters ； (5) Make the neutral point of the power grid directly grounded. 35. What is power system resonant overvoltage? How many types are there? answer: Some inductors and capacitors in the power system can form various oscillation circuits when the system is operating or malfunctioning. Under the action of a certain energy source, series resonance will occur, causing serious overvoltage in some components of the system. This phenomenon is called power system resonance overvoltage. Resonance overvoltage is divided into the following types: (1) Linear resonance overvoltage The resonant circuit is composed of an inductance component without an iron core (such as the inductance of a transmission line, the leakage inductance of a transformer) or an inductance component with an iron core whose excitation characteristics are close to linear (such as an arc suppression coil) and a capacitive element in the system. (2) The ferromagnetic resonance overvoltage resonant circuit is composed of inductive components with iron cores (such as no-load transformers, voltage transformers) and capacitive components of the system. Due to the saturation phenomenon of the iron core inductance element, the inductance parameters of the loop are nonlinear. This kind of loop containing nonlinear inductance elements will produce ferromagnetic resonance when certain resonance conditions are met. (3) Parametric resonance overvoltage consists of an inductive component whose inductance parameters change periodically (such as the synchronous reactance of a salient pole generator that changes periodically between Kd and Kq) and a system capacitive component (such as an unloaded line). When the parameters are matched, energy is continuously transferred to the resonant system through periodic changes in the inductance, causing parametric resonance overvoltage. 36. What are the functions of lightning wires and lightning rods? What is the function of lightning arrester? answer: The function of lightning protection wires and lightning rods is to prevent direct lightning strikes and reduce the probability of electrical equipment within their protection range (overhead transmission lines and substation equipment) being struck by direct lightning strikes. The function of the arrester is to cut the amplitude of the intruding flow wave and reduce the overvoltage amplitude of the protected equipment through the action of the parallel discharge gap or non-linear resistor. Lightning arresters can be used to protect against atmospheric overvoltage and operating overvoltage. 37. What are the dangers if the resistance of the grounding network does not meet the requirements? Answer: The grounding grid plays the role of working grounding and protective grounding. When the grounding resistance is too large: (1) When a grounding fault occurs, the neutral point voltage offset increases, which may cause the healthy phase and neutral point voltage to be too high, exceeding the level of insulation requirements and causing equipment damage. (2) When a lightning strike or lightning wave attacks, due to the large current, a high residual voltage will be generated, which will subject nearby equipment to the threat of counterattack, and reduce the lightning resistance level of the live conductors of the grounding grid's own protection equipment (overhead transmission lines and substation electrical equipment), which will not meet the design requirements and damage the equipment. 38. What are the main means of power grid peak regulation? Answer: (1) The pumped storage power plant changes the generator state to the motor state, and the peak shaving capacity is close to 200% ； (2) The hydropower unit reduces load and peaks or shuts down. The peaking is based on the minimum output (considering the vibration zone) close to 100%. ； (3) The load of the fuel (gas) unit is reduced, and the peak shaving capacity is above 50% ； (4) The peak shaving capabilities of coal-fired units are 50% (can be reduced to 60% if oil is added or a burner is installed), 100%, 100%, and 40% respectively for load reduction, start-stop peak shaving, less steam operation, and sliding parameter operation. ； (5) Load reduction and peak regulation of nuclear power units ； (6) Peak shaving and valley filling through load management on the user side. 39. What functional modules does the economic dispatch software include? Answer: (1) Load estimation (2) Unit optimization combination (3) Unit consumption characteristics and micro-increase consumption characteristics fitting and reorganization (4) Equal micro-increase dispatch (5) Line loss correction If it is a water and thermal power hybrid system, it is necessary to use the large system decomposition and coordination method or other algorithms to optimize the water electronic system and thermal electronic system respectively, and then coordinate the water and electricity equivalent coefficients between the water and thermal subsystems according to the total water consumption control in a day or the reservoir water level control conditions. 40. Briefly describe what basic information is required for economic dispatch of power systems? Answer: (1) Thermal characteristics of thermal power units need to be obtained through thermal tests to obtain the thermal characteristics of thermal power units under different load operating conditions, including boiler efficiency tests and turbine heat and steam consumption tests. ； (2) Consumption characteristics of hydropower units. This characteristic is the output-flow characteristics of the unit under different water heads. It should also be obtained through experiments or based on the manufacturer's design data. ； (3) Starting and stopping losses of thermal power units ； (4) Basic parameters for line loss calculation ； (5) Water-coal conversion equivalent coefficient. 41. What is a relay protection device? Answer: When the power components in the power system (such as generators, lines, etc.) or the power system itself fail or endanger its safe operation, it is necessary to promptly send a warning signal to the operating personnel on duty, or directly issue a trip command to the controlled switch to terminate the development of these events. An automated measure and equipment. The complete set of equipment that realizes this kind of automation measure is generally called a relay protection device. 42. What is the task of relay protection in the power system? Answer: The basic tasks of relay protection are mainly divided into two parts: 1. When a protected power system component fails, the relay protection device of the component should quickly and accurately issue a trip command to the switch closest to the faulty component, so that the faulty component can be disconnected from the power system in a timely manner to minimize damage to the power component itself, reduce the impact on the safe power supply of the power system, and meet certain specific requirements of the power system (such as maintaining the transient stability of the power system, etc.). 2. Respond to the abnormal working conditions of electrical equipment, and send signals according to the abnormal working conditions and equipment operation and maintenance conditions (such as whether there are regular personnel on duty), so that the personnel on duty can handle it, or the device can automatically adjust it, or remove the electrical equipment that may cause accidents if it continues to operate. Relay protection devices that respond to abnormal operating conditions are allowed to operate with a certain delay. 43. Briefly describe the basic principles and construction methods of relay protection? Answer: Relay protection mainly uses the changes in electrical quantities (current, voltage, power, frequency, etc.) when components in the power system are short-circuited or abnormal to form the principle of relay protection action. There are also other physical quantities, such as the large amount of gas and the increase in oil flow velocity or the increase in oil pressure intensity that are accompanied by a fault in the transformer tank. In most cases, no matter which physical quantity is reflected, the relay protection device will include a measurement part (and a fixed value adjustment part), a logic part, and an execution part. 44. How to ensure the reliability of relay protection? Answer: Reliability is mainly guaranteed by relay protection devices with reasonable configuration, excellent quality and technical performance, and normal operation, maintenance and management. No electrical equipment (lines, busbars, transformers, etc.) is allowed to operate without relay protection. All operating equipment in power grids of 220kV and above must be protected by two sets of relay protection devices with AC and DC input and output circuits that are independent of each other and control different switches respectively. When any set of relay protection devices or any set of switches refuses to operate, another set of relay protection devices can operate another set of switches to eliminate the fault. In all cases, it is required that the DC power supplied to the two sets of relay protection devices and switches be supplied via different fuses. 45. In order to ensure the selectivity of power grid relay protection, what requirements should be met for the cooperation between the upper and lower level power grid relay protection? Answer: The setting between the relay protection of the upper and lower level power grids (including the same level and the upper and lower level power grids) should follow the principle of step-by-step cooperation to meet the selectivity requirements. That is, when the next level line or component fails, the relay protection setting value of the faulty line or component must cooperate with the relay protection setting value of the upper level line or component in terms of sensitivity and action time to ensure that the fault is selectively removed when the power grid fails. 46. Under what circumstances is it allowed to appropriately sacrifice the selectivity of relay protection? Answer: 1. For terminal lines connected to power supply transformers, whether one or more transformers are running in parallel (including multiple T-connected power supply transformers or power supply lines), the quick-acting section protection on the line side is allowed to avoid busbar fault settings on other sides of the transformer. When necessary, the line's fast-moving section protection can operate within a short time limit. 2. For series power supply lines, if the action time of the power side protection will be excessively prolonged according to the principle of step-by-step cooperation, some intermediate substations with smaller capacity can be treated as T-connected substations or non-cooperation points to reduce the number of levels of cooperation and shorten the action time. 3. The coordination of the internal protection of the double circuit can be considered based on the action of the main protection of the double circuit (such as cross-connected differential protection), or the conditions for the continuous action of the zero sequence current (or phase current quick break) protection on both sides when one circuit of the double circuit fails. ； When there are indeed difficulties, it is allowed that when one of the double circuits fails, the delay protection sections of the two circuits will not cooperate. 4. In the lines that constitute the operation of the ring network, it is allowed to set a predetermined deloading point or a deloading line. 47. In order to ensure sensitivity, how should the setting value of the last section of grounding protection be set? Answer: The last section of grounding protection (such as section IV of zero-sequence current protection) should be set according to the grounding fault that adapts to the following short-circuit point grounding resistance value: 220kV line, 100Ω ； 330kV line, 150Ω ； 500kV line, 300Ω. Corresponding to the above conditions, the operating current setting value of the last section of zero sequence current protection should not be greater than 300A. When a high-resistance ground fault occurs at the end of a line, the relay protection devices on both sides of the line are allowed to operate vertically to clear the fault. For 110kV lines, considering the operational sensitivity requirements in the case of possible high-resistance ground faults, the current setting value of the last section of zero-sequence current protection should generally not be greater than 300A. At this time, the zero-sequence current protection on both sides of the line is allowed to operate continuously to remove the fault. 48. Briefly describe the configuration principles of 220 kV line protection? Answer: For 220 kV lines, two sets of full-line quick-acting protection should be installed based on stability requirements or when there are difficulties in setting and coordination of backup protection. Ground short-circuit backup protection can be equipped with step-type or inverse-time zero-sequence current protection, or ground distance protection can be used supplemented by step-type or inverse-time zero-sequence current protection. Phase-to-phase short circuit backup protection generally should be equipped with staged distance protection. 49. Briefly describe the basic principles of line longitudinal protection? answer: Line longitudinal protection is a protection device that causes the switches on both sides to trip quickly at the same time when a line fault occurs. It is the main protection of the line. Its basic principle is: the specific relationship between the discriminant quantities on both sides of the line is used as the criterion, that is, both sides transmit the discriminant quantities to the opposite side through the channel, and then both sides determine the fault within the area or the fault outside the area according to the relationship between the discriminant quantities on the opposite side and the local side. Therefore, the discriminant quantity and channel are the main components of the longitudinal protection device. 50. What is "remote backup" of relay protection? What is "near fallback"? answer: ""Far backup" means that when a component fails and its protection device or switch refuses to operate, the protection device of the adjacent component on each power supply side acts to cut off the fault. ""Near backup" means: using a dual configuration to strengthen the protection of the component itself, so that when a fault occurs in the area, the possibility of the protection refusing to operate is reduced. At the same time, a switch failure protection is installed. When the switch refuses to trip, it is activated to cut off other switches on the same bus as the faulty switch, or to remotely switch the opposite switch. 51. Briefly describe the basic characteristics of directional high-frequency protection? answer: Directional high-frequency protection compares the fault directions seen at both ends of the line to comprehensively determine whether it is an internal fault in the line or an external fault. If the direction of the fault seen when an internal fault occurs on the protected line is the positive direction, then when there is an external fault on the protected line, one side will always see the reverse direction. Its characteristics are: 1) The forward discrimination starting component is required to have sufficient sensitivity for line end faults ； 2) Dual-frequency transceivers must be used. 52. Briefly describe the basic characteristics of phase difference high-frequency protection? Answer: Phase difference high-frequency protection is a high-frequency protection that compares the power frequency current phases on both sides of the protected line. When the fault current phases on both sides are the same, the protection is blocked, and when the current phases on both sides are opposite, the protection trips. Its characteristics are: 1) It can respond to various symmetrical and asymmetrical faults in all phases, and the device is relatively simple. ； 2) Does not reflect system oscillation. The protection can continue to operate under non-all-phase operating conditions and during single-phase reclosing. ； 3) Not affected by voltage circuit disconnection ； 4) The requirements for transceivers and channels are relatively high, and protection on both sides requires joint debugging during operation. ； 5) When the channel or transceiver is disabled, the entire protection will exit operation, so separate backup protection needs to be equipped. 53. Briefly describe the basic characteristics of high-frequency blocking distance protection? answer: High-frequency blocking distance protection uses a directional distance protection device installed on the line as the basic protection, adds corresponding sending and receiving equipment, and forms a longitudinal distance protection through the channel. Its characteristics are: 1. It can respond to various symmetrical and asymmetrical faults sensitively and quickly enough. ； 2. Still maintain the backup protection function ； 3. When the voltage secondary circuit is disconnected, the protection will malfunction, and disconnection blocking measures need to be taken to stop the protection from operating. 4. It is not an independent protection device. When the distance protection is deactivated or needs to be deactivated due to failure or abnormality, the protection must exit operation. 54. What is the main role of line longitudinal protection in the power grid? Answer: Since the line longitudinal protection can realize full-line rapid operation in the power grid, it can ensure the stability of the parallel operation of the power system, increase the transmission power, reduce the damage caused by faults, and improve the coordination performance between backup protections. 55. How many types of channels for line longitudinal protection can be divided into? Answer: 1. Power line carrier longitudinal protection (referred to as high-frequency protection). 2. Microwave longitudinal protection (referred to as microwave protection). 3. Optical fiber longitudinal protection (referred to as fiber optic protection). 4. Guide wire longitudinal protection (referred to as guide wire protection). 56. What are the main signals for line longitudinal protection? What are their functions? answer: The signals of line longitudinal protection are divided into three types: blocking signal, permission signal and tripping signal. Their functions are: 1. Blocking signal: It is a signal that prevents the protection from tripping. That is, no blocking signal is a necessary condition for protection to act on tripping. Only when the two conditions of local protection component action and no blocking signal are met at the same time, the protection will act on tripping. 2. Permission signal: It is a signal that allows protection to operate in tripping. That is, having a permission signal is a necessary condition for protection to operate in tripping. Only when the two conditions of local protection component action and permission signal are met at the same time, the protection will operate to trip. 3. Trip signal: It is a signal that directly causes tripping. At this time, it has nothing to do with whether the protection element operates. As long as the tripping signal is received, the protection will act on the tripping. Remote tripping protection uses the tripping signal. 57. Why does the phase difference high-frequency protection set up two starting components with different settings? Answer: The starting component starts the transmitter when a power system failure occurs to achieve phase comparison. In order to prevent the starting elements of the protection devices on both sides from operating at the same time due to external faults, starting the phase comparison element on one side first, and then opening the phase comparison element before the transmitter on the operating side has sent a signal will cause protection malfunction. Therefore, two starting elements with different settings must be set. The high-set value starting component starts the phase comparison component, and the low-setting value starts the transmitter. Since the low-set start-up element acts before the high-set start-up element, it can be ensured that when the high-set start-up element starts the phase comparison element during an external short circuit, the protection will definitely receive the blocking signal and no malfunction will occur. 58. Briefly describe the basic working principle of directional comparative high-frequency protection. Answer: The basic working principle of directional comparative high-frequency protection is to compare the fault directions measured on both sides of the line to comprehensively determine whether it is an internal or external fault of the protected line. If the fault direction measured when there is an internal fault on the protected line is the positive direction, then when there is an external fault on the protected line, there will always be a reverse direction measured on one side. Therefore, the discriminating element in directional comparison high-frequency protection is an element with its own directionality or a current element whose action value can distinguish forward and reverse direction faults. The so-called comparison of fault directions of lines is to compare the action behaviors of specific discriminating components on both sides. 59. What impact does the deactivation of line high-frequency protection have on the use of reclosing? Answer: When all line high-frequency protection is disabled, the use of line reclosing may be affected by the following two reasons: 1. The line does not operate with high-frequency protection, and the line fault needs to be removed by backup protection (delay period). That is, the fault cannot be removed quickly, causing the system stability limit to decrease. If reclosing is used to reclose a permanent fault, it will be even more detrimental to the stable operation of the system. 2. The setting of the line reclosing and reclosing time is coordinated with the line high-frequency protection. If the line high-frequency protection is disabled, the line backup delay period protection and the reclosing reclosing time will not match, and the reclosing may not be successful for instantaneous faults, adding an impact to the system. 60. When high-frequency protection is running, why do operators need to exchange signals every day to check the high-frequency channel? Answer: my country's power system often adopts a working mode in which there is no high-frequency current in the high-frequency channel under normal conditions. Since the high-frequency channel not only involves the equipment of the two plants and stations, but also is related to the operating conditions of the transmission line. The aging and failure of the processing equipment and transceiver components on the high-frequency channel will cause attenuation. Problems in any link on the high-frequency channel will affect the normal operation of high-frequency protection. When the system is running normally, there is no high-frequency current in the high-frequency channel, and it is not easy to find problems with the equipment on the high-frequency channel. Therefore, every day, the operator uses the start button to start the high-frequency transmitter to send high-frequency signals to the opposite side. The high-frequency channel is checked by detecting the corresponding current, voltage and corresponding indicator light on the transceiver to ensure that the high-frequency part of the protection device can work reliably in the event of a fault. 61. What is zero sequence protection? Why should a separate zero sequence protection be installed in a large current grounding system? Answer: After a ground fault occurs in a large short circuit current grounding system, zero sequence current, zero sequence voltage and zero sequence power will appear. The relay protection devices that use these electrical quantities to protect the ground short circuit are collectively called zero sequence protection. Although the overcurrent protection of three-phase star connection can also protect ground short circuit, its sensitivity is low and the protection time limit is long. This shortcoming can be overcome by using zero-sequence protection. This is because: ① When the system is operating normally and a phase-to-phase short circuit occurs, zero-sequence current and zero-sequence voltage will not occur, so the operating current of the zero-sequence protection can be set smaller, which is beneficial to improving its sensitivity. ； ②For Y/△ connection step-down transformers, ground faults after the △ side will not reflect zero-sequence current on the Y side, so the action time limit of the zero-sequence protection does not need to match the line protection after the transformer and takes a shorter action time limit. 62. Briefly describe the characteristics of directional zero-sequence current protection and its role in grounding protection? Answer: Directional zero-sequence current protection is a multi-stage current direction protection device that reflects the magnitude and direction of the zero-sequence current component when a ground fault occurs on the line. On the lines of power grids of different voltage levels in my country's high-current grounding system, directional zero-sequence current protection devices are installed as basic protection in accordance with the provisions of ministerial regulations. Power system accident statistics show that in large-current grounded power grids, line ground faults account for 80% to 90% of all line faults. The correct action rate of directional zero-sequence current protection is about 97%, which is one of the protections with the highest correct action rate among high-voltage line protections. Directional zero-sequence current protection has a series of advantages such as simple principle, reliable action, small equipment investment, convenient operation and maintenance, and high correct action rate. 63. What are the advantages of zero-sequence current protection? Answer: Answer: Zero-sequence current protection with and without directionality is a simple and effective grounding protection method. Its advantages are: 1. The structure and working principle are simple, and the correct action rate is higher than other complex protections. 2. The whole set of protection has few intermediate links, especially for nearby faults, which can achieve quick action and help reduce developmental faults. 3. Under the condition that the zero-sequence network of the power grid remains basically stable, the protection scope is relatively stable. 4. The absolute value of the protection response zero-sequence current is less affected by the fault transition resistance. 5. The protection setting value is not affected by the load current, and is basically not affected by short-circuit faults in other neutral points of the ungrounded power grid, so the sensitivity of the protection delay period allows for higher settings. 64. Why is the zero-sequence current protection set up with a sensitive section and an insensitive section? Answer: For lines using three-phase reclosing or comprehensive reclosing, in order to prevent the zero-sequence current protection from malfunctioning when oscillation occurs during the three-phase closing process or the non-full-phase operating state of the single-phase reclosing process, a four-stage protection consisting of two first sections is often used. The sensitive section is set according to the maximum zero sequence current that occurs when a single-phase or two-phase grounding short circuit occurs at the end of the protected line. Its operating current is small and the protection range is large, but it is blocked in the non-all-phase operation state after the single-phase fault is removed. At this time, if other phases fail again, you must wait for the reclosing to reclose and rely on the accelerated tripping after reclosing. Making the tripping time longer may cause adjacent lines in the system to trip due to unmatched protection. Therefore, a set of insensitive protection section is added. The insensitive section is set to avoid the maximum zero-sequence current that occurs when operating in non-full phases and causing oscillation. Its operating current is large and can avoid the zero-sequence current in the above-mentioned non-full-phase conditions. Both are instantaneous actions. 64. What are the advantages of grounding distance protection? Answer: The biggest advantage of grounding distance protection is: the protection range of the instantaneous period is fixed, and it is easier to obtain the second stage of grounding protection with shorter delay and sufficient sensitivity. Especially suitable for the first and second section protection of short lines. For short lines, a feasible grounding protection method is to use grounding distance protection for the first and second sections and supplement it with complete zero-sequence current protection. The two types of protection are adjusted separately and perform their respective responsibilities: the ground distance protection is used to obtain the instantaneous protection section of the line and the second section protection of the entire line with a shorter time limit and sufficient sensitivity. ； The main task of zero-sequence current protection is to protect high-resistance faults and ensure reliable selectivity with the zero-sequence current protection of adjacent lines. 65. What is the principle of step-by-step coordination of multi-stage zero-sequence current protection? What are the consequences of not complying with the principle of step-by-step cooperation? Answer: The principle of step-by-step cooperation of adjacent protections is that adjacent protections are required to cooperate with each other in terms of sensitivity and action time. No crossover points are allowed between the action characteristics of the upper and lower levels of protection, and a certain margin should be left. Practice has proved that the principle of step-by-step cooperation is an important principle to ensure selective action of power grid protection. Otherwise, over-level protection tripping will inevitably occur, causing serious consequences of the expansion of power grid accidents. 66. What is distance protection? What are the characteristics of distance protection? Answer: Distance protection is a protection device based on distance measuring elements. Its action and selectivity depend on the comparison results of local measurement parameters (impedance, reactance, direction) and the set parameters of the protected section. The impedance and reactance are proportional to the length of the transmission line, hence the name distance protection. Distance protection is mainly used for the protection of transmission lines, usually three-section or four-section type. The first and second sections are directional and serve as the main protection of the line. The first section protects 80% to 90% of the line. The second section protects the entire line and serves as backup protection for adjacent busbars. The third section has a direction or no direction, and some also have a fourth section without a direction, which serves as backup protection for the main line and adjacent lines. The complete set of distance protection includes fault start, fault distance measurement, corresponding time logic loop and AC voltage loop disconnection blocking. Some are also equipped with basic links such as oscillation blocking and continuous monitoring of the entire set of protection. Some ground distance protection is also equipped with a separate phase selection component. 67. What impact does the error of voltage transformer and current transformer have on distance protection? Answer: The error of voltage transformer and current transformer will affect the accuracy of distance measurement of impedance relay. Specifically, the angular error and transformation ratio error of the current transformer, the angular error and transformation ratio error of the voltage transformer, and the voltage drop on the secondary cable of the voltage transformer will cause phase errors and numerical errors in the voltage and current on the impedance relay terminals, thus affecting the accuracy of impedance measurement. 68. What are the locking devices for distance protection? What are the functions of each? Answer: There are two blocking devices for distance protection, AC voltage disconnection blocking and system oscillation blocking. AC voltage disconnection lockout: When the secondary circuit of the voltage transformer is disconnected, the protection may malfunction due to the voltage applied to the relay dropping as if it were a short-circuit fault, so a lockout device must be added. Oscillation blocking: When a system fault occurs and a negative sequence component occurs, the protection is opened (0.12-0.15 seconds) to allow action, and then the protection is released to prevent malfunction of the protection when the system oscillates. 69. When the power system oscillates, what impact does it have on the relay protection device? Answer: When the power system oscillates, it will have an impact on the current relay and impedance relay of the relay protection device. 1. Impact on current relays. When the oscillating current reaches the operating current of the relay, the relay operates ； When the oscillation current decreases to the return current of the relay, the relay returns. Therefore, the current quick-break protection will definitely malfunction. Generally, the oscillation period is short. When the time limit of the protection device is greater than 1.5 seconds, it is possible to avoid the oscillation without malfunction. 2. Effect on impedance relay. During periodic oscillation, the voltage at any point in the power grid and the current flowing through the line will change with the change in the phase angle between the electromotive forces of the power supplies on both sides. The oscillation current increases, the voltage drops, and the impedance relay may operate. ； The oscillating current decreases, the voltage increases, and the impedance relay returns. If the impedance relay contacts are closed for a long time, the protection device will malfunction. 70. What is automatic reclosing? Why is automatic reclosing used in power systems? Answer: The automatic reclosing device is an automatic device that automatically re-engages a switch that has been tripped for some reason as needed. Power system operation experience shows that the vast majority of faults in overhead lines are transient, and permanent faults are generally less than 10%. Therefore, after the short circuit fault is removed by the relay protection action, the arc will automatically extinguish, and in most cases the insulation at the short circuit can automatically recover. Therefore, automatic reclosing recloses the switch, which not only improves the safety and reliability of power supply and reduces power outage losses, but also improves the transient stability level of the power system, increases the transmission capacity of the line, and can also make up for or reduce losses caused by incorrect tripping of switches or relay protection devices. Therefore, overhead lines generally need to use automatic reclosing devices. 71. How to classify automatic reclosing? Answer: (1) According to the classification of reclosing action, it can be divided into mechanical type and electrical type. (2) According to the way reclosing acts on the circuit breaker, it can be divided into three types: three-phase, single-phase and comprehensive reclosing. (3) According to the number of actions, it can be divided into one-time type and two-time type (multiple type). (4) According to the usage conditions of reclosing, it can be divided into single-side power reclosing and double-side power reclosing. Bilateral power supply reclosing can be divided into verified non-voltage, verified synchronous reclosing and non-synchronous reclosing. 72. What are the starting methods of automatic reclosing? What are the characteristics of each? Answer: There are two starting modes for automatic reclosing: starting mode where the position of the circuit breaker control switch does not correspond to the position of the circuit breaker and protection starting mode. The advantages of the non-corresponding starting method: it is simple and reliable, can also compensate for or reduce the impact and loss caused by the circuit breaker accidentally touching or tripping, can improve the reliability of power supply and the stability of system operation, has good operating effects in power grids at all levels, and is the basic starting method for all reclosing. The disadvantage is that when the auxiliary contact of the circuit breaker is in poor contact, the corresponding starting method will fail. The protection startup mode is a supplement to the non-corresponding startup mode. At the same time, during the single-phase reclosing process, some protection needs to be blocked, the fault phase needs to be selected and fixed in the logic loop, etc., and a reclosing starting component activated by the protection is also needed. Its disadvantage: it cannot compensate for and reduce the impact and loss caused by the malfunction of the circuit breaker. 73. When reclosing occurs due to a permanent fault, what adverse effects will it have on the power system? Answer: 1. The power system will be impacted by the fault current again. ； 2. The working conditions of the switch become more serious because the switch has to cut off the fault current twice in a short period of time. 74. What are the principles for selecting reclosing methods for single-sided power transmission lines? Answer: 1. Under normal circumstances, three-phase one-time reclosing is used. 2. When the interrupting capacity of the switch allows, secondary reclosing can be used under the following circumstances: 1) Single-circuit lines without remote control from a substation without regular personnel on duty. ； 2) A single-circuit line that supplies important loads and has no backup power supply. 3. If the secondary reclosing method is used, it must be verified by stability calculation and reclosing is allowed. 75. What are the special requirements for reclosing of bilateral power transmission lines? Answer: In addition to meeting the basic requirements for automatic reclosing devices, the reclosing of bilateral power transmission lines should also meet the following requirements: (1) When a fault occurs on the line, the protection devices on both sides may trip with different time limits. Therefore, the reclosing on both sides of the line must ensure that the switches on both sides are tripped before reclosing (2). When a fault occurs on the line and a trip occurs, there is often a question of whether the power supplies on both sides are synchronized during reclosing and whether asynchronous closing is allowed. 76. Why can capacitive automatic reclosing be able to reclose only once? Answer: Capacitive automatic reclosing uses the instantaneous discharge and long-term charging of the capacitor to achieve one reclosing. If the switch is tripped by a protective action due to a permanent short circuit, the switch will trip for the second time after the automatic reclosing once. At this time, the tripping position relay will restart. However, since the time relay contacts are closed for a long time before the entire group of reclosing is reset, the capacitor is tapped by the coil of the intermediate relay and cannot continue to charge. The intermediate relay cannot be restarted. After the entire group is reset, the capacitor still needs 20 to 25 seconds to charge. This ensures that the reclosing can only send out a closing pulse. 77. What is acceleration before reclosing? What are its advantages and disadvantages? Answer: The pre-reclosing acceleration protection method is generally used in radial lines with several series connections, and the reclosing device is only installed on a section of the line close to the power supply. When a fault occurs on a line (including adjacent lines and subsequent lines), the protection close to the power supply side first acts non-selectively and instantaneously to trip, and then relies on reclosing to make up for this non-selective action. The disadvantage is that it takes a long time to remove the permanent fault, and the circuit breaker of the closing device operates more times. Once the circuit breaker or reclosing refuses to operate, the scope of the power outage will be expanded. The pre-reclosing acceleration protection method is mainly suitable for direct distribution lines below 35kV led from power plants or main substations. 78. What is post-reclosing acceleration? Why is post-acceleration not used when calibrating synchronized reclosing? Answer: When a line fault occurs, the protection selectively removes the fault, and the reclosing switch performs a reclosing operation to restore power supply. If the reclosing occurs due to a permanent fault, the protection device will act to open the circuit breaker without time limit and without selection. This method is called post-reclosing acceleration. Verification synchronous reclosing is when one side of the line is reclosed without voltage, and the other side is reclosed only when the frequency at both ends does not exceed a certain allowable value. If the line has a permanent fault and closes without pressure and then disconnects again, the calibrated synchronous reclosing will not reclose at this time. Therefore, it is meaningless to use the calibrated synchronous reclosing to accelerate after reinstallation. If it is a transient fault, after no-voltage reclosure, that is, the line has been reclosed successfully and the fault no longer exists, so there is no need to accelerate after installation. Synchronous reclosing does not use post-acceleration, which can avoid malfunction caused by closing surge current. 79. A line has two sets of microcomputer protection, and the line is in single-phase reclosing mode. How should the two sets of microcomputer protection and reclosing be used? Answer: A line has two sets of microcomputer protection. The handles of the two sets of microcomputer reclosing are both in the single position, and only one set of the closing outlet is connected. If the closing outlets of two sets of reclosing circuit breakers are connected, it may cause the circuit breaker to reclose twice in a short period of time. 80. What electrical quantities are usually recorded by microcomputer fault recorders? Answer: For voltage systems of 220 kV and above, microcomputer fault recorders generally need to record the voltage (UA, UB, UC, 3U0) and current (IA, IB, IC, 3I0) ； High-frequency protection high-frequency signal, protection action status and switch position and other switching signals. 2006-4-1 15:18:07 Power dispatch technology Q&A--Short answer question 81. What are the characteristics of transformer excitation inrush current? Answer: 1. It contains a large non-periodic component, which often causes the inrush to deviate to one side of the time axis. 2. Contains a large number of high-order harmonic components, mainly the second harmonic. 3. There is a discontinuity between the excitation inrush waveforms. 82. What are the current methods to prevent the influence of excitation inrush current in transformer differential protection? Answer: The current methods to prevent the influence of excitation inrush current mainly include: 1. Use differential relays with fast saturation cores. 2. To identify the difference between short-circuit current and excitation inrush current waveforms, the discontinuity angle is required to be 60° to 65°. 3. Using second harmonic braking, the braking ratio is 15% to 20%. 83. What is the reason for the unbalanced current in the steady state of transformer differential protection? Answer: 1. Due to the different models of current transformers on each side of the transformer, that is, the unbalanced current caused by the different saturation characteristics and excitation current of the current transformers on each side. It must meet the requirements of the 10% error curve of the current transformer. 2. Unbalanced current caused by the difference between the actual current transformer ratio and the calculated ratio. 3. Unbalanced current caused by changing the voltage regulating tap of the transformer. 84. How is the unbalanced current generated under transient conditions of transformer differential protection? Answer: 1. Since the non-periodic component of the short-circuit current is mainly the excitation current of the current transformer, its iron core is saturated and the error increases, causing unbalanced current. 2. The excitation inrush current when the transformer is closed without load has current only on one side of the transformer. 85. How is the neutral point gap grounding protection of the transformer constructed? answer: The neutral point gap grounding protection of the transformer adopts the parallel connection of zero-sequence current relay and zero-sequence voltage relay, with a time limit of 0.5S. When a ground fault occurs in the system and there is a zero-sequence current during discharge in the discharge gap, the zero-sequence current relay of the special current transformer located at the grounded end of the discharge gap is activated. ； If there is no discharge in the discharge gap, the zero sequence voltage relay is used to operate. When intermittent arc grounding occurs, the time element shared by the gap protection must not return halfway to ensure reliable action of the gap grounding protection. 86. What are the configuration principles and characteristics of high-impedance differential protection of transformers? answer: Transformer high impedance differential protection is usually configured on large transformers as another set of transformer main protection with different principles. Its differential CT uses the 500KV side, 220KV side of the transformer (both three-phase) and the neutral point