Interview Questions and Answers of Specialty Power System Electrical Engineering Set-4

Electrical Equipment

1. What is a power distribution unit? What devices does it include?

A: The distribution device is the switch electrical appliances. Current carrying conductor and protection and measurement of electrical equipment, according to certain requirements formed from the electrical construction, known as the distribution device. A device for receiving and distributing electrical energy. Power distribution devices, including switchgear (circuit breakers. Isolating switch. Load switch, etc.). Protection of electrical appliances (fuses. Relays. Surge arresters, etc.) Measurement of electrical (current transformer, voltage transformer, ammeter, voltmeter, etc.) and bus and Current carrying conductor. Power distribution devices according to their settings can be divided into indoor distribution equipment and outdoor power distribution devices, according to their voltage level is divided into high-voltage power distribution devices and low-voltage power distribution devices, according to its structure is divided into complete sets of power distribution equipment (switchgear) and assembly.

2. High-voltage circuit breaker. Load switch. Fuse role?What are their main structures and main types?

A: 1) high-voltage circuit breakers: high-voltage circuit breakers, also known as high-voltage switchgear in power plants and substations it is an important power distribution equipment. Its role has two aspects: 

① under normal circumstances, the separation of high-voltage circuit in the no-load current and load current. (In accordance with the requirements of power grid operation, starting for some of the power lines to stop. Transmission control role); 

② When the power system failure, it works with relay protection devices. Cooperated to quickly cut off the fault (short circuit) To reduce the power outage, to prevent the accident to expand, to ensure the safe operation of power systems. (I.e., protection)

The main structure of high-voltage switch can be divided into: 

① diversion part; 

② part of the arc device;

 ③ part of the insulation;

 ④ operating mechanism. 

The main types of high-voltage circuit breakers, according to arc quenching medium: oil switch (oil and less oil.) Compressed air breaker. Sulfur hexafluoride breaker. Vacuum breaker. At present the original high-pressure (110kV.220kV) and ultra-high voltage (330kV.500kV) power system plays a major role in the low oil breaker.Compressed air breaker are almost all replaced by SF6 switch; 10kV ~ 35kV power switch in the basic vacuum switch .

SF6 circuit breaker due to the development of SF6 gas it has excellent insulation properties and arc performance, making the switch off capacity greatly improved (a single fracture of the rated breaking current of 50kA ~ 63kA, 80kA), switch size and weight To reduce the maintenance and maintenance of the switch has become a great convenience and simple.

The development of vacuum switch and contact materials are closely related to the use of CuCr alloy contacts, making 12kV vacuum circuit breaker breaking current for a reach of 73kA or even 100kA. Vacuum switch interrupter is not overhauled, once the leak can only be replaced.

2) high-voltage load switch: high-voltage load switch is a small-capacity high-voltage circuit breakers. It has an arc extinguishing mechanism, but the interrupter capacity is small, it is mainly used to cut off or connected to the normal load current, can not be used to cut short-circuit current. In most cases, the load switch with the use of high-voltage fuses, load switch from the no-load or load current, by the fuse for short-circuit protection.Load switch for a large number of load switchgear. Ring power supply units and prefabricated substation. Load switch is mainly produced by gas-type load switch. Pressure gas load switch. Vacuum load switch. SF6 load switch. Vacuum and SF6 load switch in urban and rural areas in a large number of use.

3) High-voltage fuses:

High-voltage fuse is divided into current limiting and drop-type.

In the factory power supply system, the capacity is relatively small and relatively less important load, widely used fuses for power distribution lines and power transformers (including 35KV and below level voltage transformer) overload and short circuit protection. It is economical and can meet a certain degree of reliability, its structure is simple, easy maintenance and repair.

Fuse is dynamic. Static contact. Insulation support and indicators and other components. The action of the fuse has an inverse time characteristic, that is, the greater the short-circuit current through the melt, the shorter the time of the melt-blown current.

fuse usually made of Lead, tin, lead antimony (ti), aluminum alloy or zinc, etc. made of different specifications of round wire or flake,its specification is rated current. 

Interview Questions and Answers of Specialty Power System Electrical Engineering Set-2

8. Synchronous generators and power grid side by side What are the methods?

A: synchronous generator and grid parallel methods are self-synchronous method and quasi-synchronous method.Usually in order to reduce the impact of parallel current are generally used quasi-synchronous method, parallel to adjust the generator excitation current and speed, in the generator and the grid phase sequence of the same frequency equal to the voltage equal to the same phase and the same phase.

9. What are the main data on the nameplate of asynchronous motor?

A: Rated power. Rated voltage. Rated current. Rated speed. Phase number. Model. Insulation class. Work. Allowable temperature rise. Power factor. Quality. Date of manufacture.

10. Asynchronous motor by structure how to classify?

A: divided into squirrel cage and winding type.

Power Systems

1. What is the power quality indicator?

A: Frequency. Voltage. Waveform. Symmetry

2. What are the indicators to characterize the economics of power system operation?

A: coal consumption (g/kw×h). Net loss rate

3. What are the voltage levels of our standard?

A: 0.22KV, 0.38KV, 0.66KV, 3kV, 6kV, 10kV, 35kV, 60kV, 110kV, 220kV, 330kV, 500kV, 750KV, 1000KV.

4. Which two types of power lines are divided according to the structure? The basic components of what?

A: The overhead line: wire. Lightning wire. Tower. Insulator

Cable line: conductor. Insulating layer

5. 220kv above the line often used split wire What is the purpose?

A: Improve the parameters of the line (to reduce line reactance) and to prevent the occurrence of corona phenomenon

6. What are the contents of the power flow calculation?

A: power loss calculation; current and power distribution calculation; voltage loss and node voltage calculation

7. What is the trend to calculate?

A: system design and design to select the knot line. Select the electrical equipment and wire cross-section; determine the system operation mode and develop maintenance plan; for relay protection. Automatic device design and setting calculation; Short circuit and stable calculation.

8. What are the losses of the transformer?

A: The changing losses (ie winding loss, with the load size and change); fixed loss (ie no-load loss or excitation loss)

9. What is the active power of the system? What is the use of spare capacity?

A: The active power supply is the generator; the standby capacity is used as: standby load. Accident backup. Overhaul spare. National economic reserve; Standby forms: cold standby (standby).

use)

10. What are the reactive power sources of the system?

Power supply - Static electricity compensator - Charging power for high - voltage lines

11. What are the system's voltage regulation measures?

A: The use of the generator voltage regulator; adjust the transformer variable voltage regulator; series capacitor voltage regulator; parallel compensation reactive power equipment regulator

12. What are the main uses of a capacitor in a system?

Improve the economy (increase power factor, reduce power loss); improve the stability of the system (series compensation, reduce the system reactance); improve the economy (increase power factor,

13. What are the short-circuit faults occurring in the power system? What is the highest probability of occurrence? Which equipment has the highest probability of a short-circuit fault?

Symmetrical short circuit: three-phase short circuit; the highest probability of occurrence of the short circuit: one phase short circuit; the highest probability of occurrence of short-circuit fault device : Power lines

Interview Questions and Answers of Specialty Power System Electrical Engineering Set-1

Interview Questions and Answers of Specialty Power System  Electrical Engineering Set-1

1. What is the role of the transformer in the power system?

A: The transformer is an important electrical equipment in the power system, play a role in the transfer of electricity, from the power plant to the user's entire hair. Transmission process, the transformer plays a role in raising and lowering the voltage.

2. When the secondary side of the transformer short-circuit what is the danger?

A: The transformer secondary side of a sudden short circuit, the winding will have a huge short-circuit current, the value of up to 20 to 30 times the rated current, such a large current damage to the transformer are:

(1) under the action of a huge short-circuit current, the winding will produce a large electromagnetic force, its value up to 400 ~ 900 times the rated electromagnetic force, so that the mechanical strength of the winding damage;

(2) a huge short-circuit current will make the winding heat, resulting in high temperature, may cause the winding burned.

3. What are the conditions of parallel operation of the transformer?

A: Two or more transformers running in parallel conditions: (1) high and low voltage side of the rated voltage are equal, the actual operating ratio should be equal; (2) to connect the same group; (3) short-circuit voltage per unit value (Percentages).

4. Why should the provisions of the allowable temperature of the transformer?

A: Because the higher the operating temperature of the transformer, the faster the insulation aging, not only affects the service life, but also because of insulation brittle and fragmentation, the loss of insulation winding protection. In addition, the higher the temperature the lower the dielectric strength of insulating materials, it is easy to be high voltage breakdown caused by failure. Therefore, the transformer can not exceed the permissible value during operation.

5. What are the basic structure of the synchronous generator?

A: The stator includes: stator core, stator winding, frame, end cover, cooler and so on. The rotor includes a rotor core, a rotor winding, an inter-ring, a slip ring, a fan, and the like.

6. Synchronous generator "synchronization" What does this mean?

A: After the three-phase symmetrical load is applied to the generator, the three-phase symmetrical stator current produces a combined magnetic field which is in the same direction as the rotor and rotates at the same speed.

7. Synchronous generators according to the rotor magnetic pole shape how to classify? What are uses in the power plants?

A: The synchronous generator according to the rotor pole shape is divided into salient and hidden pole machine. Salient pole machine is usually more pole pairs, the rotor speed is low, generally used in hydropower plants.The hidden pole machine is usually a small number of pole, the rotor speed is high, usually in thermal power plants. Thermal power plants and nuclear power plants to use.

Electrical Power Systems Interview Knowledge Questions 6

Electrical Power Systems Interview Knowledge Questions 

power cable inspection which inspection items?

A: Power cable inspection inspection items are:

(1) Check the cable and terminal box for leakage of oil, insulating plastic is softened overflow.

(2) Insulator is clean and complete, whether there are cracks and flashover traces, wire connectors are intact and not heat.

(3) exposed cable sheath is complete, whether the solid support.

(4) the skin is good ground.

 main control room, relay protection indoor and 10 (6) kV power distribution indoor equipment normal patrol what projects? 

A: Indoor equipment, the normal inspection items are:

(1) no sound and Jiao smell.

(2) All instruments, signals, indicator windows should be consistent with the operating conditions, the instructions are correct.

(3) to protect the connection piece position is correct (should be consistent with the actual).

(4) system three-phase voltage balance (approximate), and in the specified range.

(5) power line, the main switch three-phase ammeter approximate balance.

 transformer oil shortages on the operation of what harm?

A: The transformer oil level is too low will make light gas action; serious lack of oil, the core and winding exposed to the air easily damp, and may cause insulation breakdown.

 forced oil circulation transformer stopped the oil pump why not allowed to continue to run?

A: The reason is that the transformer housing is flat, the cooling area is very small, and even can not transform the transformer no-load loss of heat generated by scattered out. Therefore, forcing the oil circulation transformer to completely stop the cooling system operation is dangerous.

take the operation of the gas in the transformer one should pay attention to what safety matters?

A: The security issues that should be noted are:

(1) to take gas gas must be carried out by two people, one person operation, one guardianship.

(2) climbing transformer should take a safe distance from the gas to prevent high-throw.

(3) to prevent accidental collision probe.

 What causes the transformer to issue unusual sound?

A: The following causes the transformer to make an unusual sound:

(1) Overload. (2) internal contact is bad, discharge spark.

(3) individual parts loose. (4) The system is grounded or shorted. (5) large motor starting to make a big change in load. 

 voltage transformer in operation, why not allow a short circuit twice?

A: The voltage transformer in normal operation, the secondary load impedance is very large, voltage transformer is a constant voltage source, the internal impedance is very small, small capacity, a winding wire is very small, when the transformer secondary short circuit, Current is great, if the second choice of improper fuse, the fuse can not be fused, the voltage transformer can easily be burned.

What happens when the capacitor should immediately exit the operation?

A. The capacitor should be removed from operation immediately upon any of the following conditions:

(1) casing flashover or serious discharge.
 (2) joint overheating or melting. 
(3) expansion of shell deformation.

(4) the internal discharge sound and discharge equipment abnormal sound.

 arc suppression coil switching tap the operation of what provision?

A: Operational requirements are:

(1) The tap-changer tap should be switched according to the position of the tap assigned by the dispatcher.

single busbar 10kV system single-phase grounding, the line-by-line power failure to find, grounding phenomenon is still not disappear What is the reason?

A: There are two reasons:

(1) Both lines are earthed at the same time.

(2) the bus station equipment grounding.

 fixed-time over-current protection is the setting principle?

A: The definite time overcurrent protection action current setting principle is: the action current must be greater than the load current, the maximum load current protection device does not act, when the next level line external short circuit, if the current relay has been in action, Subsequent protection After the fault current has been removed, the protection of this stage should be reliably returned.

 distribution lines why only installed over-current does not install quick-break protection?

A: 10kV distribution line power supply distance is short, the line short-circuit current and the end of the first little difference between the value, quick-break protection by bypassing the line short-circuit current setting, protection is too small; In both cases, it is not necessary to install a current quick-break protection.

 export intermediate relay contacts Why should series current coil?

A: The purpose of contact series current coil is to prevent the protection action after the contact jitter, vibration or a very short time can not switch on and off tripping. So to protect the middle of the outlet is usually used in contact with a series of current coil self-sustaining wiring. In the event of a fault, the contact is closed and the current coil in series is energized to produce a self-holding to ensure a reliable trip of the switch.

 transformer differential protection is based on what the principle of installation?

A: The transformer differential protection is installed according to the principle of circulating current. Two current transformers with the same model are installed on both sides of the transformer. In the normal and external fault, there is no current in the differential relay, and in the transformer internal phase short circuit occurs, the differential relay will have a great current flow.

transformer zero sequence protection under what circumstances put into operation?

A: Transformer zero sequence protection should be installed in the transformer neutral point directly to the ground side, to protect the side of the winding internal and lead-wire short-circuit, can also be used as the corresponding bus and line ground short-circuit protection, so when the Transformer neutral grounding knife into the joint, the zero sequence protection can be put into operation.

 What line of differential protection device installed cross-linked? What is the fault of the transverse differential protection response?

A: In the same impedance of the two parallel lines can be installed cross differential protection. Transverse differential protection protects against internal faults in parallel lines and does not respond to external faults in parallel lines.

synchronous reclosing under what circumstances do not move?

A: No action in the following cases:

(1) If the line has a permanent failure, with no reclosing the circuit breaker reclosing immediately after the break, the same reclosing will not move.

(2) no pressure reclosing refuses to move, the same period reclosing will not move.

(3) synchronous reclosing refuses to move.

 Under what circumstances will the reclosing of the circuit breaker exit operation?

A: The reclosing operation is exited in the following cases:

(1) the circuit breaker interrupter capacity is less than the bus short-circuit capacity, the reclosing out of operation.

(2) the number of circuit breaker failure trip exceeds the requirements, or though not more than the provisions, but the circuit breaker serious fuel injection, smoke, etc., should be agreed after the operation of the reclosing out.

(3) line with live operation, when the duty dispatcher command reclosing will be out of operation.

Electrical Power Systems Interview Knowledge Questions 4

 What is the acceleration after the reclosing?

A: When the protected line fails, the protection device selectively cut off the fault line, at the same time reclosing action, coincidence once, if coincident with a permanent fault, the protection device immediately without time limit, no action Disconnect the breaker again. This protection device is called reclosing after the acceleration, the general increase in an intermediate relay can be achieved.

What is automatic reclosing?

Answer: When the circuit breaker trips, can not manually operate and quickly make the circuit breaker automatically re-closing device called automatic reclosing.

ground distance protection What are the characteristics?

A: The grounding distance protection has the following characteristics:

(1) can protect a variety of ground fault, but only with a distance relay, wiring is simple. 

(2) allows a large grounding transition resistance. 

(3) protection action speed, good action characteristics. 

(4) by the system operation mode changes little effect. 

battery plate in the operation of what is the characteristics of curing?

A: The battery is running in the plate curing characteristics are:

(1) to take the bubble premature charging or a charge that began to take bubbles. (2) charging voltage is too high, the discharge voltage drops below normal. (3) positive plate brown with white spots.

 What causes the transformer oil shortage?

A: The transformer oil shortage is due to:

(1) long-term oil leakage transformer or a large number of oil spills. 

(2) repair the transformer, the oil does not put oil in time.

 (3) oil pillow capacity is small, can not meet the operational requirements. 

(4) the temperature is too low, oil pillow oil storage capacity.

battery in operation plate short circuit What are the characteristics?

A: There are three characteristics of the plate short-circuit:

(1) the voltage is low (sometimes zero) when charging or discharging. 

(2) the proportion of electrolyte during charging can not be increased.

 (3) Charging less bubbles and bubbles occur late.

 battery in operation in the plate bending What are the characteristics?

A: The plate bending characteristics have three points:

(1) plate bending. 

(2) plate cracking. 

(3) the cathode plate lead sponge swollen and into a moss-like tumor.

general circuit breaker operating handle on the "5-8", "6-7", "2-4" What is the contact? What do they turn on when operating the handle? What position is disconnected?

"6-8" is the closing contact, is in the "closing" position, after the "closing" position is disconnected; "6-7" is the trip contact, in the "tripping" position, "2-4" is the reclosing discharge contact, "Ready to trip" and "After the trip" position is turned on, "Ready to close" position is disconnected.

relay protection What is the role?

A: The relay protection device can respond to electrical equipment failure and abnormal working conditions and automatically automatically and selectively act on the circuit breaker will be faulty equipment removed from the system to ensure trouble-free equipment to continue normal operation, the accident is limited to Minimum range, improve the reliability of system operation, the maximum guarantee to the user safety, continuous power supply.

What is the over-current protection delay characteristics?

A: The short-circuit current flowing through the protection device and the relationship between the operating time of the curve is called the protection device delay characteristics. Delay characteristics are divided into fixed time delay characteristics and inverse time delay characteristics. The operation time of the definite time delay is fixed, irrespective of the size of the short circuit current.Inverse time delay operation time and the size of short-circuit current, short-circuit current, short action time, short circuit current, action time. Short-circuit current and action time into a certain curve.

 "Electricity Act" What are the basic principles?

A: The basic principles of the Electricity Law are:

(1) The cause of electric power should be based on the needs of national economic and social development appropriate development ahead of the principle.

(2) The State encourages domestic and foreign economic organizations and individuals to invest in developing power sources according to law, to set up electric power production enterprises, Who benefits from the principle.

(3) The principle of the protection of power facilities and electric energy by the State.

(4) The principles of environmental protection and pollution prevention shall be protected according to law in power construction and electric power production.

(5) The state encourages and supports the principle of using renewable energy and clean energy for power generation.

(6) The power enterprises shall, according to law, implement the principle of self-management, self-financing and supervision.

(7) The principle of the state to help and support the development of power industry in ethnic minority areas, remote areas and poverty-stricken areas.

(8) the state encourages the use of advanced technology and management methods to develop the power industry principles.

Electrical Power Systems Interview Knowledge Questions 3

improve the power system static stability measures?

A: To improve the power system static and stable measures are: 

(1) to reduce the system components of the inductive reactance. 

(2) the use of automatic adjustment excitation device. 

(3) by reducing the load by frequency device.

(4) increase the power system of active and reactive power reserve capacity.

substation of the various energy meters should be equipped with what grade current transformer?

A: The active energy meter, should be equipped with an accurate rating of 1.0 or 2.0 of the current transformer; reactive energy meter should be equipped with 2.0 or 3.0 of the current transformer; transformers, transformers and stations with the station and the power meter used Other electric energy meters for the calculation of electricity shall be provided with current transformers of accuracy class 0.5 or 1.0.

What is compound rectifier? Commonly used multiple rectifier there are several?

A: Double-rectifier is connected to the voltage system by the power supply (voltage source) and the current system connected to the rectifier power supply (current source) with a series and parallel method of reasonable composition, in a system of various operating modes and Fault to ensure reliable, reasonable control of power supply. Commonly used compound rectifier has two kinds of single-phase and three-phase. Single-phase double-rectifier is divided into two parallel and serial wiring.

occurrence of sub-harmonic resonance over-voltage What are the risks?

A: The sub-resonant frequency of the system is quite harmful, in the sub-frequency resonance frequency and power-frequency voltage under the action of PT magnetic flux density quickly saturated, rapidly increasing excitation current, will PT overheating serious damage to the winding ( The same system all PT are threatened), or even cause a bus failure caused a large area blackout.

transformer oil level changes with what factors?

A: Transformer oil level under normal circumstances with the oil temperature changes, because the oil temperature changes directly affect the volume of transformer oil, so that oil within the standard oil level rise or fall. Factors affecting oil temperature changes are load changes, changes in ambient temperature, internal failure and cooling device operating conditions.

on-load tap-changer tap-changer failure is caused by what?

A: It is caused by the following reasons: (1) auxiliary contact in the transition resistance in the process of switching breakdown blown. (2) tap switch is not strict, the water caused by phase short circuit.

(3) As the contact roller stuck, so that the tap switch in the transitional position, resulting in interturn short circuit and burned. (4) tap the tank is short of oil. (5) voltage across the fault current encountered during the process.

 transformer on-load tap-changer action failure is what causes?

A: The main reason for the failure of the on-load regulator is:

(1) The operation power supply voltage disappears or is too low.

 (2) the motor winding burnout, starter motor pressure loss.

(3) interlock contact poor contact. 

(4) Rotating mechanism trip and pin off.

Transformer respirator inside the moisture absorber should pay attention to what?

A: The replacement of breathing apparatus inside the moisture absorption agent should be noted:

(1) should be heavy gas protection to change the signal. 

(2) Remove the ventilator should be blocked with the tube to prevent the suction air.

(3) put on a dry moisture absorption agent, the oil should be so that the oil did not breathe out the mouth to seal the respirator.

running in the transformer, according to the sound of its occurrence to determine the operation?

A: The transformer can run according to the sound to determine the operation. With one end of the stick on the transformer tank, the other end on the ear carefully listen to the sound, if it is continuous hum heavier than usual, it is necessary to check the voltage and oil temperature, if no abnormal, Loose core. When you hear a squeak, check the sleeve surface for flashover. When you hear crackling sound, it is the phenomenon of internal insulation breakdown.

What are the basic requirements for relay protection?

Answer: According to relay protection device in the power system in the task, relay protection device must meet the following four basic requirements: selectivity, rapidity, sensitivity and reliability.

Why does relay protection require device fast action?

A: Because the rapid action of the protection device can quickly remove the fault, to prevent the expansion of the accident, to prevent equipment from more serious Damage, but also can reduce the trouble-free low-voltage users in the work of the time and power outages, accelerate the resumption of normal operation process.

Electrical Power Systems Interview Knowledge Question 3

What are the main physical properties of SF6 gas?

A: SF6 gas is colorless, tasteless, non-toxic, non-flammable inert gas, with excellent insulation properties, and not aging deterioration, the proportion of about 5.1 times the air.

 transformer and factory transformer installed gas relay What are the requirements?

A: With the oil pillow 800kVA and above transformers, thermal power plant 400kVA and hydropower plants 180kVA and above plant transformers should be installed gas relay.

Why A-class insulation transformer winding temperature is set to 650C?

A: Transformer in operation to produce iron loss and copper loss, these two parts of the loss of all converted to heat, so that the core and winding heat, insulation aging, affecting the life of the transformer, so the international provisions of the transformer winding insulation more A-level Insulation, provides the winding temperature rise of 650C.

resistance limiting current on-load tap-changer which five main components? What are their uses?

A: The resistance limiting current on-load tap-changer composition and role as follows:

(1) Toggle switch: Used to switch the load current. (2) Selection switch: Used to switch the pre-selected tap.

(3) range switch: for commutation or coarse tap. (4) Actuator: is the power part of the tap, with interlocking, limit, counting and so on. (5) fast institutions: according to the procedures required to quickly switch.

side of the transformer tank installed thermosiphon filter What is the role?

A: The transformer oil in the operation will gradually dirty and oxidized, in order to extend the use of oil, so that the transformer in good conditions to run, you need to maintain good oil quality. A thermosyphon filter keeps the transformer oil in good working condition without severe aging. In this way, oil can be recycled for many years without special treatment.

 What is the unbalanced current transformer? What are the requirements?

A: Transformer unbalanced current refers to the three-phase transformer winding current difference between the terms.

Three-phase three-wire transformer, the phase load imbalance must not exceed 20%, in three-phase four-wire transformer, unbalanced current-induced neutral current must not exceed 25% of the rated current of low-voltage winding. If it does not meet the above requirements, should be adjusted load.

transformer new or overhauled after the transformer to determine why the cover and the oil pillow connecting pipe slope? What is the standard?

A: The gas relay side of the transformer has two slopes. One is along the gas relay direction of the transformer cover slope, should be 1% to 1.5%. Transformer cover slope requirements in the installation of the transformer from the bottom pad. The other is the transformer oil tank to the oil pillow connecting pipe slope, should be 2% to 4% (this slope is manufactured by the manufacturers good). The two slopes one is to prevent the storage of air in the transformer, the second is to facilitate the fault in the gas quickly and reliably into the gas relay to ensure correct operation of gas relays.

transformer which part of the normal operation of the winding is the hottest?

A: The temperature of the windings and the core are the upper part of the lower part of the lower. The general structure of the oil-immersed transformer windings, experience has proved that the temperature of the hottest height direction of 70% to 75%, horizontal from the winding diameter of one third of the calculation, the most hot spots of each transformer windings should be determined by the test.

 adjust the capacitor in the power system play a role?

A: The role of the capacitor is to transfer the system reactive power, improve power factor, reduce the loss in the network, adjust the network voltage and improve power quality have a better effect.

electrical wiring the main requirements are?

A: The main electrical wiring requirements are:

(1) has the reliability of power supply. (2) has the operational security and flexibility.

(3) simple, easy to operate. (4) with the construction and operation of the economy. (5) Consideration should be given to the possibility of future expansion.

Electrical Power System Interview Knowledge Question 2

11. Try to analyze why the zero-sequence reactance of a single-loop transmission line is less than the equivalent zero-sequence reactance of a double-loop transmission line from the physical concept?

Parallel to the erection of double-circuit lines are through the zero sequence current, in a return line with a phase wire cross-linked flux not only with the other two-phase zero sequence current mutual inductance flux, there is another loop three-phase zero Sequence current generated mutual inductance flux, and are formed through the earth loop. Zero-sequence current in the same direction, so mutual magnetic flux from the magnetic effect. (0), then the zero-sequence impedance of the single-loop transmission line is Z (0), Z (0) is the impedance of each line, = Zs + 2zm, zero-sequence impedance of the double-loop transmission line 'Z (0) = Z (0) + Z (0).

13. Why does the non-damper winding of synchronous generator transient potential Eq in the short-circuit remain the same? What is the practical value of this point?

Damped synchronous generator because of its Eq and excitation winding flux linkage f is proportional to, and f in the running state of the sudden moment of conservation, so Eq in the running state of sudden changes in the moment is also conserved. For generators with undamped windings, this value can be calculated from the steady state at the moment before the mutation and directly applied to the calculation after the moment of the mutation.

14. What is the physical meaning of the Park transformation (ie, why do Park transforms)?

Park transformation from the physical point of view is actually a reference system transformation. By Park transformation, the observer's view is transferred from the stationary stator to the rotating rotor. Stator windings from the stator, the stator winding is three stator winding, dd winding, qq winding, 00 windings to replace, in which dd winding, qq windings were d axis, q axis axis, with the rotor synchronization Movement, 00 winding axis in the rotor rotation axis direction (with d axis, q axis perpendicular). Because there is no relative motion between the stator equivalent windings and the rotor windings, the mutual inductance between the equivalent windings of the stator, the mutual inductance between the equivalent windings, and the mutual inductance between the stator and rotor windings do not vary with the stator- Of the change, that does not change with the α, does not change with time. In this way, Park's flux linkage equation and the Park voltage equation are combined to obtain constant coefficient differential equation to solve.

15. With the damper winding of the synchronous generator-side three-phase short-circuit, short-circuit current in the stator winding cycle component can be divided into several parts? Which parts are attenuated?What time constant attenuation?

At this time, the periodic components in the stator windings can be divided into steady-state forced component and free component, the former is not attenuated, the latter is divided into sub-transient component and transient component of the two parts to zero attenuation, sub-transient component to vertical The time constant Td of the shaft-damper winding is attenuated and the transient component decays to zero with the time constant Td 'of the field winding.

16. Why is it necessary to implement reactive power in-place balance? 

 power transmission voltage loss is V = '' 'P QPR QX (R jX), in the high-voltage power line, the power loss is 2VV22

In most cases, the line reactance is larger than the resistance, and the reactance of the transformer is much larger than the resistance. Therefore, the long-distance transmission of reactive power increases the voltage drop and power loss of the grid during transmission. Therefore, from improving the voltage quality of reducing the loss of the network point of view, should try to achieve the reactive power of the local balance.

17. Describe why series capacitor compensation can improve power system stability.

If the line reactance is XLeq = XL-XC <XL, the total reactance X of the system decreases, and the power limit Pm = EV of the simple power system increases accordingly. The operation of the line Stability also increased accordingly with X

18. Short-circuiting of the power system, the closer to the short-circuit point, the more positive sequence voltage (low), negative sequence voltage (high), zero sequence voltage (high).

19. Non-damper winding The synchronous generator transient potential Eq 'is directly proportional to the flux linkage of the (excited) winding, and can be maintained at the instant of a sudden change in operating conditions.

20. Power system three-phase short circuit, the impact of current (that is, the shortest possible maximum instantaneous current value) as iim, RMS (short-circuit current maximum possible Of the effective value) as Iim, the existence of iim = 2im? why?

 Iim refers to the maximum possible instantaneous value of the short-circuit current, which occurs 0.01 seconds after the fault. Iim is short-circuit after the first cycle of short-circuit current instantaneous value of the root mean square value, that is famous value. Since the short-circuit current includes the period component and the non-cycle component, the instantaneous value of the non-periodic component