Electromagnetic Electrical Engineering Interview Questions 1

2. What is the Hall effect? What is the Rogowski coil?

The Hall effect is a phenomenon in which a lateral potential difference occurs across a current-carrying conductor in a magnetic field.

Rogowski coil is a kind of electromagnetic induction principle and the use of the full current law, measuring large inrush current (tens of kA to several hundred KA) or the rate of change of impact of the current.

3, What is Poynting vector?

The Poynting vector is the cross product of the electric field strength and the magnetic field strength, and the direction represents the direction of energy flow, and the size represents the unit time

The energy per unit vertical area. The unit is watts per square meter.

Poynting's theorem is the energy conservation theorem in the electromagnetic field, which means that the energy transfer is equal to the reduction of energy stored and the algebraic sum of the power supplies.

4, What is the the difference between free charge and bound charge, dielectric polarization and electrostatic induction and types?

Free charge is a free electron in the conductor (metal), ion (gas or liquid) and so on when the electric field force let the free movement of the charge. Bounded charge is defined as a charged particle in a dielectric that is bound by an atomic force, an intra molecular force, or an inter molecular bond,

They can move a little under the force of an electric field, but can not leave the range of molecules.

Polarization of the dielectric refers to the electric field in the dielectric molecules or atoms to form electric dipoles, the surface of the dielectric appears positive

And the phenomenon of negative binding charge. After polarization, the electric field created by the bound charge in the dielectric can generally attenuate the applied electric field.

Electrostatic induction of a conductor refers to the free-charge movement of the conductor in an applied electric field, accumulating on the surface of the conductor and establishing an electric field until The surface charge of the electric field and the external electric field in the conductor everywhere is the phenomenon. Due to electrostatic induction, electrostatic field In the conductor is internally charged, the charge can only be distributed in the conductor surface; charge inside the conductor is zero; its just near the conductor surface field strength and the surface perpendicular to the conductor is an equipotential body, the conductor surface is equipotential surface.

5, What is the difference between electrostatic shielding electromagnetic shielding?

Electrostatic shielding is the use of electrostatic equilibrium of electric field within the field strength of the conductor is made zero for shielding device.

Electromagnetic shielding is the use of good conductors in the vortex that can prevent high-frequency electromagnetic waves into this conductor that can be made while shielding the electric field and Magnetic field shielding device. It can suppress radiated interference and high-frequency conducted interference.

Magnetostatic shielding is the use of magnetic field in the high magnetic flux law of ferromagnetic materials that can make most of the magnetic field concentrated in the ferromagnetic circuit. A shielded device is provided which can shield the magnetic field.

The effect of static shielding is similar to that of electrostatic shielding, but the effect is not good for static shielding, since the conductivity of the metal conductor is of several dozen orders of magnitude than the air conductivity , and ferromagnetic material and air permeability difference of only a few orders of magnitude.

6, given the second type of boundary conditions, why the potential value of uncertainty?

Only partial derivatives do not have a base value

7, E at the interface between the two media is changing?

Tangent direction E unchanged; normal direction D unchanged.

8, In a circular coil vertical through a current into the wire, there is no induced coil emf

This depends on whether or not there is a change in the incoming current, since the principle of the induced electromotive force is that the induced magnetic field produces an induced electric field,

If there is a change in the current coil is induced electromotive force, if the current does not change then it is not.

9, In a balloon, the surface covered with a uniform charge, with the balloon constantly blowing large balloons inside and outside the electric field are how to change?

External constant, the internal field strength is zero

10, For the stability of the electromagnetic field conditions, physical significance When the frequency of the alternating current is low?

it is much smaller in the vicinity of the current than the wavelength of the electromagnetic wave at that frequency in the vacuum, Of the region, you can ignore the delayed effect of electromagnetic fields, such areas of the electromagnetic field is a quasi-stable or quasi-stable field).

Electrical Parts of Power Generation, Substation and Transmission Short Interview Questions Part 5

1. To measure power quality indicators are: voltage, frequency, waveform.

2. Power system consists of generators, substation substations, transmission lines, users.

3. Substation according to the status and role in the system is divided into hub substation, intermediate substation, regional substation, terminal substation.

4. Neutral point of the power system is often used in the grounding method , for the arc suppression coil grounded, directly grounded and grounded by low resistance.

5. In the 10kV system, the U-phase complete ground fault occurs. Then three-phase ground voltage: Uud = 0 kV, Uvd = 10 kV, Uwd = 10 kV; Neutral to ground voltage Und = 10 / kV.

6. In the 35kV system, U-phase completely ground fault occurs. Then the three-phase ground voltage: Uud = 0 kV, Uvd = 35 kV, Uwd = 35 kV; Neutral to ground voltage Und = 35 / kV.

7. In the 220kV system, U-phase full ground fault occurs. Then the three-phase ground voltage: Uud = 0 kV, Uvd = 220 / kV, Uwd = 220 / 3kV; Neutral to ground voltage Und = 0 kV.

8. In the electric arc, the disappearance of charged particles is mainly due to the two physical phenomena of recombination and diffusion. Diffusion is a phenomenon in which the charged particles in the arc column escape into the surrounding medium outside the arc column.

9. Isolating the role of the switch are: isolated power, switching operation, sub-small current.

10. High-voltage circuit breaker according to the use of the arc medium, can be divided into oil circuit breakers, air circuit breakers, sulfur hexafluoride circuit breakers, vacuum circuit breakers.

11.LW10B-252 / 3150-40 is sulfur hexafluoride circuit breaker, the rated voltage of 252kV, rated current of 3150A, rated breaking current of 40kA, dynamic and stable current of 100kA.

12. The main basic technical parameters of high-voltage circuit breakers are rated voltage, rated current, rated breaking current, thermal stability current, dynamic and stable current, closing current.

13. Pure SF6 gas is colorless, tasteless, non-toxic, non burning, non-combustion and non-metallic compound.

.

14. Gas breakdown voltage increases with the increase of gas pressure and then decreases.

15. Outdoor isolation switch according to the number of support insulators can be divided into single column, double column, three columns.

16. The current transformer primary winding is connected in series of circuit, secondary winding and measuring instruments and relays and other string The current transformer secondary side can not be open, the current transformer running close to the short-circuit state.

17. The primary winding of the voltage transformer is connected in parallel with the primary circuit. The secondary winding is connected in parallel with the measuring instrument and the relay. The secondary side of the voltage transformer can not be short-circuited. The running state of the voltage transformer is close to the open circuit.

18. Current transformer error includes current error and phase error, the main factors affecting the error are primary current, core quality and structure size, the secondary side of the load and its power factor.

19. Voltage transformer error including voltage error and phase error, the main factors affecting the error of a primary voltage, primary and secondary side of the winding impedance, the secondary side of the load and its power factor.

20. Electrical main wiring requires of basic requirements of reliability, economy, flexibility.

21. The operating mechanism of the circuit breaker can be divided into manual operating mechanism, electromagnetic operating mechanism, spring operating mechanism, pneumatic actuating mechanism and hydraulic operating mechanism according to different energy forms.

22. There are basic types of bus wiring are single bus, double bus connection, one and half breaker wiring.

23.Basic types of bus-type wiring are: unit wiring, bridge wiring, polygonal wiring.

24. In the power failure of the switching operation, you should first disconnect the circuit breaker, and then disconnect the isolator on both sides; in the transmission operation, you should close both sides of the isolation switch, and then disconnect the circuit breaker.

25. High-voltage power plant voltage levels are 3kV, 6kV, 10kV.

26. Station power supply has working power, backup power, starting power, emergency security power.

27. Factory power wiring generally consists of single bus connection.

28. Neutral point indirect grounding system Compared with the neutral point direct grounding system, its reliability is high, the line and the equipment insulation investment is big (for same voltage grade).

29. The role of the circuit breaker under normal circumstances is to connect and disconnect the circuit, the emergency case operate for automatic fault removal.

1. What are the basic requirements for the power system?

A: 

(1) to meet the demand for electricity; 

(2) safe and reliable power supply; 

(3) to ensure power quality; 

(4) to ensure the economic operation of the power system.

2. Which Arc suppression coil compensation is commonly used in the system, and why?

A: 

(1) full compensation; 

(2) under-compensation; 

(3) over compensation. 

(4) Over compensation; 

because when the ground current decreases, the over-compensation current is larger and does not become fully compensated. In addition, even if the future development of the power grid, the original arc suppression coil can also be used.

3. What is the role of arc suppression coil?

A: When the single-phase-to-ground fault occurs in the ungrounded neutral system, a large capacitive current flows through the fault point, which makes the grounding arc which is not easy to extinguish. To tackle with this often neutral point arc suppression coil is installed in system and electricity Sense current compensation capacitor so that the fault arc quickly extinguishes.

4. According to different high-voltage circuit breaker interrupter medium, which can be divided into several types?

A: oil circuit breakers, air circuit breakers, vacuum circuit breakers, SF6 circuit breakers.

5. What is the current transformer secondary winding operation, why it is not safe to let it open?

A: If CT secondary side is open, the secondary current is zero, the secondary side of the demagnetization magnetic potential is zero, while the primary side of the magnetic potential is unchanged, its secondary core appears with highly saturated voltages. On secondary side The winding will induce few thousand volts of electromotive force that will cause personal and equipment safety hazards, so the CT secondary side of the operation is not allowed to open.

8. A circuit breaker model LW10B-550 / 3150-50, try to say the model in the letters, numbers represent the meaning. 

A: That is the outdoor SF6 circuit breaker, design number 10B type, rated voltage of 550kV, rated current 3150A, rated breaking current 50kA.

9. What are the basic methods of extinguishing AC arc?

With the use of special metal materials for the arc contact; 

With the blowing arc;

With the use of multi-fracture arc quenching; 

With circuit breaker contact separation speed; 

With the installation of metal gate extinguishing device.

10. What are the role of the transformer?

A: (1) to the measuring instruments and relays and other secondary equipment power supply, and can make it standardized and miniaturized.

(2) to the use of low-voltage small-section control cable, to achieve long-range measurement and control.

(3) When in a short circuit, it protects the measuring instruments and relays of the current coil from large current damage.

11. What are the advantages of Zinc Oxide Arresters?

A: 

(1) no gap

(2 no free flow

(3) flow capacity 

(4) stable performance, anti-aging ability, good stain resistance.