1. Can low-voltage AC and DC circuits share the same cable? Why?
Answer: No, because:
Sharing the same cable can reduce the insulation level of the DC system.
If the DC insulation fails, mixing AC and DC circuits can cause short circuits or malfunction of the relay protection system.
2. What size megohmmeter should be used to measure the insulation of secondary circuits, and what is the insulation standard?
Answer: Ideally, a 1000V megohmmeter should be used to measure the insulation resistance of secondary circuits. If a 1000V megohmmeter is unavailable, a 500V one can be used. The insulation standard is no less than 1 megohm for circuits in operation, no less than 20 megohm for new indoor installations, and no less than 10 megohm for outdoor installations.
3. What is the purpose of auxiliary contacts in oil circuit breakers?
Answer: The normally open and normally closed contacts of an oil circuit breaker change their position to connect the breaker’s trip and close circuits and alarm signal circuits. This enables the breaker to open or close circuits and correctly send alarm signals, initiate automatic devices, and lock protection circuits. When used in closing and tripping circuits, auxiliary contacts should be delayed.
4. What is the function of a current transformer?
Answer: A current transformer reduces large currents to smaller currents at a fixed ratio for use by instruments and protective relays while isolating the secondary system from high voltages. This ensures the safety of personnel and equipment and simplifies and standardizes the design and manufacture of instruments and relays, increasing cost efficiency.
5. What is the operating principle of overcurrent protection?
Answer: When a phase-to-phase short circuit occurs in the grid, the current suddenly increases while the voltage drops. Overcurrent protection works by setting the operating current of current relays based on the selectivity of the line. When the fault current reaches the set value of the current relay, the relay operates and, following the selective protection scheme, cuts off the faulted line.
6. What are the wiring configurations for current transformers?
Answer: Common wiring configurations for current transformers include:
Two-phase V-connection and two-phase differential connection using two current transformers
Three-phase Y-connection, three-phase Δ-connection, and zero-sequence connection using three current transformers.
7. What types of reactive power sources exist in power systems?
Synchronous generators
Synchronous condensers
Shunt capacitors
Series capacitors
Static VAR compensators.
8. How does the oil purifier of a transformer work?
Answer: During transformer operation, the temperature difference between the upper and lower layers of oil causes oil circulation within the oil purifier. Harmful substances in the oil, such as moisture, free carbon, and oxidation products, are absorbed by the silica gel in the oil purifier, purifying the oil and maintaining its electrical and chemical properties. This helps regenerate the transformer oil.
9. How does the cooling effect differ between directed and non-directed forced oil and air cooling systems in transformers?
Answer: In non-directed forced oil and air cooling systems, most of the oil flows through the gaps between the tank walls and windings, with only a small amount flowing through the core and windings, resulting in lower cooling efficiency. In contrast, directed forced cooling systems use oil flow guide plates to direct cooling oil effectively through the core and windings, enhancing the cooling effect and reducing winding temperature rise.
10. What is the purpose of thermometers, and what are the different measurement methods?
Answer: Thermometers are used to measure the top oil temperature inside the transformer tank and to monitor the operational status of the transformer. Depending on transformer capacity, temperature measurement methods include mercury thermometers, signal thermometers, and resistance thermometers.
11. What is the function of a buffer in high-voltage circuit breakers?
Answer: The opening buffer prevents damage to circuit breaker components caused by the large impact force when the spring releases energy. The closing buffer prevents excessive closing depth that could damage the bushing due to the impact force during closing.
12. What is meant by free tripping of a circuit breaker?
Answer: Free tripping refers to the ability of a circuit breaker to reliably open during the closing process if the protection system triggers the trip circuit. Circuit breakers with free-tripping capability can quickly open during short-circuit conditions, preventing the spread of the fault.
13. What are the arc-extinguishing characteristics of SF6 gas?
Answer: SF6 gas has the following excellent properties:
High conductivity in the arc column, low arc voltage, and low energy in the arc column.
Fast recovery of dielectric strength at current zero-crossing, approximately 100 times faster than air, giving it 100 times the arc-extinguishing capability of air.
High dielectric strength.
14. What are the characteristics of a vacuum circuit breaker?
Answer: Vacuum circuit breakers have short arcing times, small contact separation, and light contact wear during fault current interruption. This allows for lower operating energy and faster operation. Additionally, they are compact, lightweight, require little maintenance, and are fire- and explosion-resistant with low operating noise.
15. What does "synchronization" mean in a synchronous condenser?
Answer: Synchronization means that during operation, the rotating magnetic field of the stator and the rotor rotate in the same direction at the same speed.
16. Why is it not allowed to replace a voltage transformer primary fuse with an ordinary fuse?
Answer: For a 10kV voltage transformer, for example, the primary fuse has a rated current of 0.5A. Quartz-filled fuses offer superior arc-extinguishing performance and higher breaking capacity, which can also limit short-circuit currents. Ordinary fuses do not meet these requirements.
17. Why must the secondary side of voltage and current transformers be grounded?
Answer: Grounding the secondary side of voltage and current transformers is a protective measure. If the insulation between the primary and secondary sides fails, the high voltage from the primary side could reach the secondary side, endangering personnel and equipment. Grounding the secondary side prevents this risk.
18. What type of equipment does the nameplate FKL-10-2×750-6 represent?
Answer: FKL-10-2×750-6 is the nameplate of a split aluminum reactor with a rated reactance of 6%, a rated voltage of 10kV, and a rated current of 750A for each of its two branches.
19. What are the functions of shunt reactors and series reactors?
Answer: Line shunt reactors compensate for the capacitive charging current of the line, limit voltage rise, and reduce switching overvoltages, ensuring safe and reliable line operation. Busbar series reactors limit short-circuit current and maintain higher residual voltage on the bus. Series reactors in capacitor banks limit harmonic currents and reduce reactance.
20. What are the characteristics of single-bus sectionalized connections?
Answer: Single-bus sectionalized connections can limit the impact of bus faults. When a fault occurs in one bus section, the sectionalizing breaker automatically trips under relay protection, isolating the faulted section while keeping the non-faulted section in operation. For important users, power can be sourced from different sections to ensure continuous supply.
21. What are the drawbacks of double-busbar connections?
Answer: Double-busbar connections have the following drawbacks:
The wiring and operation are complex, and switching operations can lead to misoperation.
More busbar disconnectors are required, and the structure of the distribution device is more complex, reducing economic efficiency.
22. What is the function of a fault recorder?
Answer: A fault recorder in the power system automatically and accurately records electrical quantities before and after a fault. Analyzing these quantities helps diagnose accidents, verify correct relay operation, and improve the safe operation of the power system.
23. Why are expansion joints installed on rigid busbars?
Answer: Since materials expand and contract with temperature changes, busbars can lengthen or shorten as they heat up during operation. Expansion joints are installed to prevent excessive stress on busbars and support insulators, which could cause damage due to thermal expansion or contraction.
24. What is the compensation degree of an arc suppression coil, and what is residual current?
Answer: The compensation degree is the ratio of the difference between the arc suppression coil's inductive current and the network's capacitive current to the network's capacitive current. The remaining current flowing through the grounding point after the inductive current compensates the capacitive current is called residual current.
25. Does an arc suppression coil carry voltage during the normal operation of a system grounded via an arc suppression coil?
Answer: During normal operation, due to the imbalance in line-to-ground capacitances, there will be a voltage between the system's neutral point and ground. The magnitude of this voltage is directly related to the degree of capacitance imbalance and should not exceed 1.5% of the rated phase voltage.