Power grid recruitment examination questions

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17. In the process of bus switching operation, if the standby bus is successfully charged, (a) ensure the equipotential of two groups of buses.

A, keep the state of disconnectors on both sides of bus circuit breaker unchanged; B, cut off the control of bus circuit breaker; C, cut off the operating power supply of disconnectors on both sides of the bus circuit breaker; D, quickly switch all branches to the standby bus.

18. When there is a metal three-phase short circuit on the line, protect the residual voltage on the bus at the installation position (c). A the highest b is the line voltage drop between the fault point and the protection installation. C is the same as the short-circuit point. D can't judge.

19. When the transformer has an external fault, a large penetrating short-circuit current flows through the transformer, and then the differential protection of the transformer (C).

A immediate action b delayed action c no action d short circuit time depends on 20. The alternating main magnetic flux in the core of current transformer is generated by (C). A voltage across the primary winding b current through the secondary winding c current through the primary winding d terminal voltage of the secondary winding 2 1. Split winding transformer is mainly used for (a). A, when the primary side is short-circuited, limit the short-circuit current b, improve the voltage distribution c of the winding when the shock wave invades, and improve the heat dissipation conditions of the winding.

The voltage waveform is improved and the third harmonic component 22 is reduced. The meaning of no-load DC motor is (C).

A there is no current in the armature winding of the motor, and the main power switch is not closed.

There is no load on the shaft of motor C, and motor D is not excited.

23. The phase relationship between input voltage and input current of asynchronous motor is (A). A input voltage is before input current, b input voltage is after input current, c input voltage and input current are in phase, and d input voltage and input current are in phase. Changing the connection relationship between A, B and C power lines and asynchronous motor can change the direction of motor, and the correct method is (C). Three lines A, B and C rotate counterclockwise, three lines B and C rotate clockwise, any two lines C, B and C are interchanged once, and any two lines D, B and C are interchanged twice. 25. The incorrect DC motor speed regulation method is (D). A reduce the excitation current b armature series resistance c reduce the power supply voltage d and change the number of poles.

26. When calculating the short-circuit current of high-voltage system, the circuit should be in the state of (B). A short circuit process state b normal operation state c switching process state d starting process state 27. When the DC motor works normally, (b) remains unchanged. The magnetic flux c in the main pole of the induced electromotive force b in the armature winding flows through the magnetic flux in the rotor core 28 of the commutator D. The possible characteristics of the AC synchronous generator relative to the load are (A). A arbitrary b resistance c inductance d capacitance

29. No-load closing of transformer may produce a great impact current, mainly because of (B).

The instantaneous voltage of network A is too high, and the magnetic circuit B has a certain remanence, and the main flux changes instantaneously. The main flux changes instantaneously. The magnetic circuit D has a certain remanence.

30. The main difference between transformer and asynchronous motor magnetic circuit structure is (B). The asynchronous motor is rotating while the transformer is stationary.

There is an air gap in the magnetic circuit of B asynchronous motor, and the magnetic circuit of transformer can close the air gap in the magnetic circuit of C transformer, and the magnetic circuit of asynchronous motor can close the mutual influence of the magnetic circuit of D transformer, but the magnetic circuits of asynchronous motor are independent of each other.

1. In high-frequency protection, the function of high-frequency arrester is (to stop high-frequency current).

2. The unbalanced current in longitudinal differential protection is essentially the difference between the two ends of the protected equipment (current transformer excitation current).

3. The sensitivity of the longitudinal differential protection is the highest when a phase-to-phase short circuit occurs at the same position inside the generator.

4. Fault removal time includes (protection action time) and (circuit breaker trip time). 5. In distance protection, amplitude comparison circuit and phase comparison circuit can be used to compare two voltages. The former can be divided into (balanced type) and (cyclic type), while the latter usually adopts (polarity matching phase comparison) and (annular rectification phase comparison) circuits.

6. For the longitudinal differential protection of transformer, when the power supply is unilateral, the braking coil should be connected to the (load) side, because there is no (braking) effect when the fault occurs in this area, which improves the protection (sensitivity).

7. The delay (not required) of circuit breaker failure protection matches the action time limit of other protections, because it starts timing after other protections (actions).

8. The basic requirements of power system operation refer to (reliability, economy and power quality). 9. The purpose of using split conductor is to (reduce corona loss and line reactance).

10. Capacitors are connected in parallel in the system, and their reactive power is proportional to the line voltage (square).

1 1. The relay protection device consists of (measuring part, logic part and executing part). 12. The requirements of power system for relay protection are (selectivity, rapidity, sensitivity and reliability).

13. The condition of AC arc extinguishing is that the arc gap recovery voltage is less than the arc gap breakdown voltage. 14. At the same time, the percentage of the ratio of power loss to power supply in power network is called (network

Loss rate).

15. Power quality refers to (voltage, waveform quality, frequency). 16. The main measure to adjust the terminal voltage of users is to change the transformer ratio.

17. In order to install less compensation equipment capacity, the transformer tap should be selected (all capacitors are put into use in the largest way).

18. When the transmission line is empty, the terminal voltage is higher than the terminal voltage.

19. The dynamic stable current of electrical equipment should not be less than the maximum (three-phase impulse short circuit) current passing through electrical equipment.

20. The stability of power system refers to (the ability of system synchronous generators to run in parallel).

What is the significance of developing UHV in China?

China is a big energy consuming country in the world, with the total coal consumption ranking first and electricity consumption ranking second in the world, but the primary energy distribution and productivity development level are very unbalanced. The economically exploitable hydropower resources in China are 4.065438 billion kilowatts, of which more than two thirds are distributed in Sichuan, Yunnan and Tibet. More than two thirds of the proven reserves of coal resources are distributed in Shanxi, Shaanxi and Mongolia provinces. The demand for energy and electricity is mainly concentrated in the economically developed areas in eastern and central China. This national situation, in which the energy distribution and productivity development level were once unbalanced, determines the need to optimize the allocation of energy resources throughout the country. From the economic comparison between coal transportation and power transmission, it is an inevitable choice to develop power transmission, especially UHV power transmission with remarkable economic benefits.