40 electrical knowledge points, with 40 corresponding answers summarized

Thread Content

1. What is the difference between computer-based protection and conventional relay protection? The main difference lies in the fact that the original protection inputs are current and voltage signals, which are directly compared with each other on an analog basis, thereby allowing the analog values to be compared with the predetermined torque value in the device. Computers, on the other hand, can only perform numerical or logical operations. Therefore, it is first necessary to convert the instantaneous values of the input analog current and voltage into discrete digital values, before they can be sent to the computer’s central processing unit. There, calculations are performed using specified algorithms and programs, and the results of these calculations are continuously compared with the given values, so as to determine whether to trigger a trip. 2. How are the protection zones for each stage of the zero-sequence current protection defined? The I-section zero-sequence current setting allows the protection to bypass the maximum zero-sequence current resulting from a ground short circuit at the end of this line ； It is not possible to protect the entire length of the line, but it should be no less than 15%–20% of the total length of the line that needs to be protected ； The zero-sequence stage II protection generally covers the entire length of the line, extending into the stage I area of adjacent lines and working in coordination with them. Zero-sequence section III serves as a backup to sections I and II, and operates in conjunction with adjacent lines. Changhui Instruments yunrun.com.cn/ 3. What is post-acceleration in reclosing? When a fault occurs on the line, the protection operates according to the set values, the line switch is opened, and reclosing takes place immediately. In the case of a transient fault, the fault disappears after the line switch is disconnected, the reclosing is successful, and power supply to the line is restored ； In the case of a permanent fault, after reclosing, the protection time element is disabled, resulting in a trip time of 0 seconds; this is what causes an accelerated trip once the fault persists after reclosing, thereby cutting off the faulty circuit. 4. What should be done after mistakenly operating the isolating switch? ①When the isolating switch is pulled incorrectly, an arc is generated as soon as the blades separate from the static contacts; closing it immediately can extinguish the arc and prevent an accident. If the blades have already been fully separated, it is not allowed to close the switch that was pulled incorrectly ； ②When the isolating switch is pulled incorrectly, even if it is closed in the wrong manner or an arc is generated during closing, it must not be pulled open again, as operating the switch under load can cause a three-phase arc flash short circuit. 5. What is parallel R, L, C resonance? In a circuit in which resistors, inductors, and capacitors are connected in parallel, under the action of a sinusoidal power supply at a certain frequency, a special condition arises in which the voltage across the circuit and the total current are in phase, and the entire circuit behaves as a resistor. This condition is known as parallel resonance. 6. What are the advantages of using negative-sequence and zero-sequence incremental elements as the starting elements for distance protection? ①High sensitivity ； ②It can be seen as the starting element for an oscillating latching device ； ③It will not malfunction in the event of a break in the voltage secondary circuit ； ④The appearance of the symmetric component is independent of the phase of the fault; therefore, a single relay can be used as the starting element, making it relatively simple. 7. What conditions must be met by protective devices to be classified as Class I equipment? All protective devices of a category of equipment are in good technical condition, their performance fully meets the requirements for the safe operation of the system, and they satisfy the following main conditions: ① The protection panels, relays, components, auxiliary equipment, and secondary circuits are free of defects. ②The principle, wiring, and setting values of the device are correct, in compliance with relevant regulations, rules, and anti-accident measures. ③The drawing documents are complete and accurate. ④Operating conditions are good. 8. What are the inspection items and contents for the control switch? The inspection items for the control switch include: ① The casing is clean, free of oil stains, and intact. ②It should be installed firmly and remain stationary during operation. ③The sealing cover is well-sealed. ④All connections must be secure, without looseness or corrosion. ⑤It rotates smoothly, is in the correct position, and has good contact. ⑥Open the sealing cover and inspect it with a flashlight; the interior should be clean, the lubricant should not be dried out, and there should be no burn damage at the contact points. Test the contact piece with an insulated rod; the pressure should be adequate. 9. What aspects should be checked during the no-load commissioning of a transformer for its differential protection? For the differential protection of transformers, 5 no-load tests must be conducted on the transformer at its rated voltage during installation. Before connecting it under no-load conditions, the secondary wiring should be inspected to ensure it is correct. No-load switching tests should be conducted on the high-voltage side and the low-voltage side of the transformer. This is because the system impedance and the leakage reactance of the transformer can help limit the inrush current; the system impedance on the high-voltage side is lower, and generally the low-voltage windings of a transformer are located inside, resulting in lower leakage reactance. Therefore, a larger inrush current occurs when switching is performed on the high-voltage and low-voltage sides. In the test, the protective device should not operate even once; otherwise, the operating current of the relay should be increased. 10. What measures should be taken when working on the secondary wiring? When working on the secondary wiring, records must be kept ； During recovery. It should be canceled and recorded in the notebook. When there are many changes to the secondary wires, a tag should be attached to each wire end. When removing or laying secondary cables, tags 11 should still be attached to the beginning and end of the cables, as well as at the bends and junction points along their length. What are the anti-accident measures required for gas protection? ①The lower float of the gas relay should be of the baffle type, and its contacts should be changed to a vertical configuration in order to improve the reliability of operation in the presence of high levels of gas. ②To prevent the gas relay from short-circuiting due to water leakage, rainproof measures should be taken at its terminals and at the terminals inside the cable lead terminal box. ③The lead wires of the gas relay should be oil-resistant wires. ④The leads of the relay and the cables should be connected to the terminals inside the cable terminal box respectively. 12. What are the general principles for installing transformer protection? ①Gas protection against internal short circuits in the transformer iron casing and low oil level. ②Longitudinal differential protection or current quick-break protection for preventing phase-to-phase short circuits in the transformer coils and their leads, ground short circuits on the side of the coils connected to the high-current grounding system, as well as inter-turn short circuits. ③Overcurrent protection that prevents inter-phase short circuits outside the transformer and serves as a backup for gas protection and longitudinal differential protection (either overcurrent protection activated by composite voltage, or negative sequence current protection). ④Zero-sequence current protection against external ground short circuits in grids with high ground currents. ⑤Overload protection to prevent symmetrical overloads. 13. What is the function of the starting element in distance protection? ①During a short-circuit fault, activate the protection device promptly ； ②Activate the oscillation locking device, or serve as the measuring element for Section III ； ③Perform section switching ； ④Perform phase switching ； ⑤In transistor protection, if there is a failure in the DC logic section, the entire protection system is locked out. 14. What types of protection are generally installed on 10 kV transmission lines? ①Interphase short-circuit protection: Single-power supply circuits are generally equipped with two-stage overcurrent protection, namely instantaneous current protection and time-limited overcurrent protection. Dual-power supply lines are generally equipped with current velocity protection with or without direction indication, as well as overcurrent quick-break protection. ②Grounding protection: Generally, non-selective insulation monitoring protection, zero-sequence overvoltage protection, and power direction protection are installed. 15. What is the effect of negative feedback on the operating performance of an amplifier? ①Reduce the magnification factor, ② improve the stability of the magnification factor, ③ reduce waveform distortion, ④ widen the passband, ⑤ change the input and output resistances of the amplifier. 16. What are the causes of non-sinusoidal current? The generation of non-sinusoidal current can come from either the power supply or the load. Typically, the following reasons apply: ① Several different sinusoidal electromotive forces act simultaneously in the circuit, or alternating and direct current electromotive forces act together. ②The circuit has a non-sinusoidal periodic electromotive force. ③There are nonlinear components in the circuit. 17. In 6-35 kV power systems, surge arresters are connected to the voltage between the phase and ground. Why then should surge arresters be selected based on the rated line voltage? 6-35kV systems are systems with low ground short-circuit currents. Under normal conditions, the arrester is under the influence of the voltage relative to ground; however, in the event of a single-phase ground fault, the voltage of the non-faulty phases relative to ground rises to the line voltage. Such ground faults are allowed to exist for a certain period of time, during which the arrester should not operate. Therefore, the rated voltage of a lightning arrester must be the system’s rated line voltage rather than the rated phase voltage. 18. What conditions must be met by protective devices for them to be classified as Class III equipment? The protective devices of these three types of equipment are either inadequately equipped or have poor technical performance, thereby affecting the safe operation of the system. If the main protection device exhibits any of the following conditions, it is also classified as a Class III device: ① The protection does not meet the system requirements, and in the event of a fault it can cause system oscillation, lead to an accident, or severely damage the key equipment. ②The requirements for anti-accident measures are not met. ③The connecting pieces, handles, buttons, etc., designed for operation by personnel are marked. ④The drawings are incomplete and do not correspond to reality. ⑤The fault recorder is unable to record properly or is not in operation. 19. How to control the test environment conditions when testing relays? The requirements for experimental environmental conditions include temperature, relative humidity, and air pressure. These conditions not only affect the basic performance of the tested relays but also influence the operating condition of the testing instruments and equipment. The required conditions for the testing environment are as follows: ① Temperature: 15–35 degrees ② Humidity: 45–75% ③ Air pressure: 660–780 mmHg. 20. What principles should be considered when selecting testing instruments? ①Select the type of instrument based on the object being measured. First, determine whether the relay under test is DC or AC, and then select a DC meter or an AC meter accordingly. ②Select the internal resistance of the instrument based on the test circuit and the impedance of the relay coil being tested. ③Select the appropriate instrument based on the size being measured. ④Select the instrument based on the location of use and working conditions. 21. What are the main inspection items for a newly installed protection device after it is completed? The acceptance items are as follows: ① The measured parameters related to electrical equipment and circuits are complete and correct. ②All completion drawings of the protective devices are in line with the actual situation. ③The test value meets the requirements of the setting notice. ④The test items and results comply with the provisions of the testing regulations and relevant procedures. ⑤Verify the turns ratio and voltage-current characteristics of the current transformer, ensuring that its secondary load meets the error requirements. ⑥Check that the equipment in front of and behind the screen is neat and in good condition, that the circuit insulation is satisfactory, and that the markings are complete and correct. ⑦Acceptance tests are carried out using the rated load current and operating voltage to determine the polarity of the transformer, its turns ratio and the correctness of its circuitry, as well as the correctness of the components and wiring related to protection devices such as directional, differential, distance, and high-frequency protection devices. 22. How to test the zero-sequence voltage circuit of the zero-sequence directional protection in a large grounding current system during normal operation? To ensure the proper operation of the zero-sequence direction protection, a integrity check should be conducted on its zero-sequence voltage circuit. The method involves using a test busbar derived from the secondary winding of the voltage transformer connected in an open delta configuration to measure the voltage on the voltage buses YMN used for each set of zero-sequence direction protections; a voltage of 100V in these cases indicates normal operation. 23. In a radial power grid with low ground current, what is the difference in current between the faulty line and the non-faulty lines when a single-phase ground fault occurs? At the terminal of the faulty line, the zero-sequence capacitive current is equal to the sum of the zero-sequence capacitive currents of the other lines, and this current flows toward the busbar. The zero-sequence current measured at the sending end of a non-faulty line is the non-faulty relative ground capacitance current of that line, and it flows out toward the busbar. 24. In a system with high ground current, why is the operating time of inter-phase protection longer than that of zero-sequence protection? The time limit for protective actions is generally set according to a stepped principle. The operating time delay of interphase protection is determined by increasing the time delay at each stage from the user side towards the power source; whereas for zero-sequence protection, since most step-down transformers are connected in a Y configuration, no zero-sequence current exists on the high-voltage side when a ground short circuit occurs on the low-voltage side, so its operating time delay does not need to be synchronized with that of the low-voltage users connected to the transformer. Therefore, the operating time of zero-sequence protection is shorter than that of phase-to-phase protection. 25. What is power system oscillation? What are the common causes of oscillation? The phenomenon in which two systems or power plants operating in parallel lose synchronization is called oscillation. There are various reasons for oscillations; in most cases, they result from prolonged outage times that disrupt the dynamic stability of the system. In systems with weak connections, oscillations can also be caused by human error, generator demagnetization, fault-induced tripping, or the disconnection of certain circuits or equipment. 26. What requirements must a modulator meet? ①When the input DC signal Ui=0, the output signal U0=0. ②The amplitude of the output AC signal should be proportional to the magnitude of the DC signal. ③When the polarity of the DC signal Ui changes, the phase of the output AC signal also changes. 27. What are the principles for configuring line-to-line short-circuit protection in 27, 35 kV ungrounded neutral systems? The principle for configuring inter-phase short-circuit protection is as follows: ① When two-phase current protection is used, current transformers should be installed on the phases that share the same name (such as phase A and phase C). ②The protection device shall adopt a remote backup scheme. ③If a short circuit causes the voltage on the plant’s service busbars, the busbars at the connection points of the main power sources, or the busbars supplying important users to drop below 50%-60% of the rated voltage, the fault must be eliminated promptly. 28. What is the role of high-frequency protection in high-voltage power grids? High-frequency protection is used in long-distance high-voltage transmission lines; it enables the immediate isolation of any type of fault at any point on the protected line from both sides, thereby enhancing the stability of power system operation and the success rate of reclosing. 29. In a system with high ground current, why is the operating time of inter-phase protection longer than that of zero-sequence protection? The time limit for protective actions is generally set according to a stepped principle. The operating time delay of interphase protection is determined by increasing the time difference at each stage of protection from the user side towards the power source, whereas for zero-sequence protection, since most step-down transformers are connected in a Y,d11 configuration, no zero-sequence current exists on the high-voltage side when there is a ground short circuit on the low-voltage side; therefore, its operating time delay does not need to be coordinated with the low-voltage users of the transformer. Therefore, the operating time of zero-sequence protection is shorter than that of phase-to-phase protection. 30. What are the basic requirements for an operational amplifier? ①The external impedance at the input terminal and the impedance value of the feedback circuit should be precise and stable ； ②The open-loop voltage gain should be large enough ； ③The open-loop input resistance ri must be large enough ； ④The open-loop output resistance should be low ； ⑤Zero-point drift and noise should be low. 31. What is the amplifier output resistance? At the amplifier output, the amplifier can be regarded as a signal source with a certain internal resistance, and this internal resistance is the output resistance. 32. What points should be noted when using the superposition principle to calculate linear circuits? By applying the superposition principle, it is possible to calculate the voltage and current in each branch under the effect of individual voltage sources and current sources, and then add these values together. Attention should be paid when using the superposition principle. ①This principle can only be used to calculate linear currents and voltages; it is not applicable to nonlinear circuits ； ②When performing superposition, pay attention to the direction of current and voltage, as well as the substitution number during superposition ； ③Neither the circuit connection method nor the values of the resistors in the circuit can be changed. When a current source is active, the voltage source is short-circuited; when a voltage source is active, the current source is open-circuited ； ④The superposition principle applies only to the superposition of voltages and currents; power cannot be calculated using this principle. 33. Why is overvoltage protection necessary for hydrogenerators? Due to the slow response of the turbine speed control system, unacceptable overvoltages can occur after an emergency load shedding, which is why it is required to install overvoltage protection. 34. What are low excitation of a generator and generator loss of magnetism? Low excitation refers to the excitation current of the generator being below the value corresponding to the static stability limit. Demagnetization refers to the loss of excitation current in a generator. 35. Why is overload and overcurrent protection for load voltage installed in generators? Why is a current transformer at the generator neutral point used for this protection? This is provided as differential protection for the generator or as backup protection for the next component, and it comes into action in the event of the following two faults: ① When there is an external short circuit and the protection device or relay of the faulty component fails to operate ； ②When a fault occurs within the range of the generator differential protection but the differential protection fails to operate. 36. What principles are generally followed in setting the negative-sequence voltage threshold for the composite voltage-started overcurrent protection of transformers? Why? When the system is operating normally, the three-phase voltages consist mainly of in-phase components, with very small out-of-phase components; therefore, the setting value for the out-of-phase voltage element is determined based on the unbalanced voltage output of the out-of-phase voltage filter under normal operating conditions, typically around 6–12 V (secondary voltage value). 37. Why are some large-capacity transformers and system interconnection transformers equipped with out-of-phase current protection and overcurrent protection for single-phase low-voltage starting as backup protections? Because this protection has the following advantages: ① It has high sensitivity in the event of an asymmetric short circuit ； ②When an asymmetric short circuit occurs behind the transformer, its sensitivity is independent of the transformer’s wiring configuration. 38. What is the role of intermediate relays in relay protection? ①The contacts of the measuring elements in protective devices are generally small in size and few in number; intermediate relays can be used to increase the capacity and quantity of contacts ； ②When tubular arresters are installed on the line, an intermediate relay can be used to introduce a delay in the activation of the protection device, thereby preventing premature activation of the fast-acting protection caused by the discharge of the arresters ； ③Meets the requirements of the protection logic circuit. 39. What are the differences in operating conditions between electromagnetic current relays and voltage relays? Voltage relays are generally connected to the secondary side of voltage transformers. Compared with current transformers, due to the higher voltage, these relays have more turns in their coils, thinner wires, and a higher impedance; moreover, the reactance of the coils increases, which results in a reduced current flow ； On the other hand, this reduces the magnetic reactance of the magnetic circuit; the decrease in current and the decrease in impedance compensate for each other, resulting in an unchanged electromagnetic torque during the operation of the relay, thereby eliminating its relay characteristics. 40. Why is it said that the protection zone of distance protection is basically unaffected by changes in system operation conditions? Since distance protection relies on the ratio of voltage to current at the start of the line as a criterion for operation, and since the short-circuit impedance varies only depending on the distance of the short-circuit point from the start of the line, the protection zone of such protection is essentially unaffected by changes in the system’s operating conditions.