In actual electrical work, many electricians are highly skilled and have strong practical abilities, but their knowledge of basic electrical knowledge is relatively weak. In some situations, this can sometimes be embarrassing for experienced electricians. Therefore, mastering the necessary basic knowledge is very important.
1. What are the uses of a current transformer?
Current transformers convert large currents into smaller currents according to a certain ratio, providing current for various instruments and relay protection, while isolating the secondary system from high voltage. They not only ensure the safety of personnel and equipment but also simplify and standardize the manufacturing of instruments and relays, improving economic efficiency.
2. What are the different wiring methods for current transformers?
The wiring methods for current transformers include two-phase V-connection and two-phase differential current connection using two current transformers; and three-phase Y-connection, three-phase Δ-connection and zero-sequence connection using three current transformers.
3. What are the different types of reactive power sources in a power system?
Reactive power sources in a power system include:
1. Synchronous generator; 2. Synchronous condenser; 3. Parallel compensation capacitor;
4. Series compensation capacitor; 5. Static compensator.
4. Why is it necessary to install a set of ZnO surge arresters between the power capacitor and its circuit breaker?
Installing ZnO surge arresters can prevent operational overvoltages that may occur during the opening and closing of power capacitors, ensuring the safe operation of electrical equipment.
5. Why must the secondary side of voltage transformers and current transformers be grounded?
Grounding the secondary side of voltage transformers and current transformers is a protective grounding process. This is because if the insulation of the primary, secondary, and current sides is damaged, high voltage from the primary side can travel to the secondary side, threatening the safety of personnel and equipment. Therefore, the secondary side must be grounded.
6. What are the functions of parallel reactors and series reactors?
Parallel reactors can compensate for the capacitive charging current of the line, limit the rise in system voltage and the generation of operational overvoltage, and ensure the reliable operation of the line.
Busbar series reactors can limit short-circuit current and maintain a high residual voltage on the busbar. Meanwhile, capacitor bank series reactors can limit high-order harmonics and reduce reactance.
7. What are the characteristics of a single busbar segmented wiring method?
Segmented connection of a single busbar can reduce the impact range of busbar faults and improve power supply reliability. When a section of the busbar is faulty, the sectional circuit breaker automatically trips with the cooperation of relay protection, disconnecting the fault and allowing the non-faulty busbars to maintain normal power supply. For important users, power can be drawn from different sections to ensure uninterrupted power supply.
8. What are the disadvantages of double busbar connection?
The double busbar system has the following disadvantages:
1. The wiring and operation are relatively complicated, and misoperation is prone to occur during switching operations.
2. The busbar disconnect switches are numerous, and the structure of the power distribution equipment is relatively complex, resulting in poor economic efficiency.
9. What is the compensation degree of the arc suppression coil? What is residual current?
The ratio of the difference between the inductor current and the capacitor current of the arc suppression coil to the capacitor current of the power grid is called the compensation degree. After the inductor current of the arc suppression coil compensates for the capacitor current, the remaining current flowing through the grounding point is called the residual current.
10. When a system with its neutral point grounded by an arc suppression coil is operating normally, does the arc suppression coil carry voltage?
During normal system operation, due to the imbalance of the three-phase capacitance to ground in the line, a certain voltage exists between the network neutral point and ground. The magnitude of this voltage is directly related to the capacitance imbalance. Under normal circumstances, the voltage generated at the neutral point should not exceed 1.5% of the rated phase voltage.
11. Why do batteries self-discharge?
The main reason for battery self-discharge is that impurities on the plates form small, localized cells, which then create a short circuit, causing self-discharge. Additionally, the uneven density of the electrolyte on the upper and lower surfaces of the battery leads to unequal electromotive forces on the plates, also contributing to self-discharge.
12. Why should batteries be charged and discharged regularly?
Regular charge-discharge, also known as verification charge-discharge, involves subjecting the plates of a float-charged battery to a significant charge-discharge reaction over a certain period of time. This process checks the battery's capacity, identifies aging batteries, and allows for timely maintenance to ensure normal operation. Regular charge-discharge should generally be performed at least once a year.
13. What is the unbalanced current of a transformer? What are the requirements?
The unbalanced current of a transformer refers to the current difference between the windings of a three-phase transformer. In a three-phase three-wire transformer, the load imbalance of each phase must not exceed 20%. In a three-phase four-wire transformer, the neutral current caused by the unbalanced current must not exceed 25% of the rated current of the low-voltage winding. If the above regulations are not met, the load should be adjusted.
14. Which part of the transformer winding is the hottest during normal operation?
The temperature of the windings and the core is higher at the top and lower at the bottom. For general oil-immersed transformer windings, experience has shown that the hottest point is 70%-75% of the height in the vertical direction, or one-third of the way from the inner diameter of the winding in the transverse direction. The hottest point of each transformer winding should be determined by testing.
15. What could cause a transformer to make an abnormal sound?
The main reasons are as follows: 1. Overload. 2. Poor internal contact, causing arcing. 3. Loose individual parts. 4. Grounding or short circuit in the system. 5. Large load changes due to the starting of a large motor.
Why are integrated reclosing devices installed on 16,220kV lines?
The 220kV line is a neutral-point directly grounded system. Because single-phase grounding faults are the most common type of fault in the system, circuit breakers are equipped with phase-by-phase operating mechanisms. When a single-phase grounding fault occurs, the protection system only trips the circuit breakers on both sides of the faulty line, leaving the unfaulty phases untripped. This prevents operational overvoltage and improves system stability. When a phase-to-phase fault occurs, the protection system trips the three-phase circuit breakers on both sides. Furthermore, when single-phase tripping and single-phase reclosing or three-phase tripping and three-phase reclosing are required, this can also be accomplished by a combined reclosing mechanism.
17. What are the different operating modes of integrated reclosing? How do they each work?
The integrated reclosing is achieved in three ways by the switching switch QK.
1. Integrated reclosing method. After a single-phase fault trips, the single-phase reclosing occurs, and the three phases trip on the permanent fault. After a phase-to-phase fault trips the three phases, the three phases reclose, and the three phases trip again on the permanent fault.
2. Three-phase reclosing method. For any type of fault, trip all three phases and reclose them (check for synchronism or no voltage). If the reclosing occurs on a permanent fault, trip all three phases again.
3. Single-phase reclosing mode. After a single-phase fault trips, the single-phase reclosing occurs. If the reclosing occurs on a permanent fault, all three phases trip. After a phase-to-phase fault trips all three phases, reclosing does not occur.
18. What is surface discharge?
In practical insulation structures, solid dielectrics are often surrounded by gaseous or liquid dielectrics. For example, line insulators are surrounded by air, and the solid insulation of oil-immersed transformers is surrounded by transformer oil. In this case, discharge often occurs along the interface between the two dielectrics; this type of discharge is called surface discharge.
19. What factors affect the surface discharge voltage?
The main factors affecting surface discharge voltage are: 1. the uniformity of the electric field; 2. the difference in dielectric constant of the dielectric surface; 3. whether it has been rained on; and 4. the degree of contamination.
20. What are the components of theoretical line loss in power grid energy loss?
1. Variable losses, whose magnitude varies with load changes, are proportional to the square of the load power or current passing through each component of the power grid. These include copper losses in overhead transmission and distribution lines and cable conductors at all voltage levels, transformer copper losses, and copper losses in equipment such as synchronous condensers, voltage regulators, reactors, wave traps, and arc suppression coils.
2. Fixed losses are independent of the current carrying the load power of the components, but are related to the voltage applied to the power grid components. They include iron losses of transmission and distribution transformers, synchronous condensers, voltage regulators, reactors, arc suppression coils, and other equipment; corona losses of 110kV and above overhead transmission lines; insulation dielectric losses of cable capacitors; leakage losses of insulators; iron losses of current and voltage transformers; and losses of voltage windings and other accessories of user electricity meters.
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