Basic electrician knowledge 100 questions

1、 Yes 10kV change （ with ） What are the requirements for the grounding of the substation ？

transformer 、 Switchgear and transformers (PT、CP) It's a metal shell , Power Distribution Cabinet 、 Control protection panel 、 Metal frame 、 Lightning protection equipment 、 Cable head and metal barrier, etc . The grounding device has the following requirements ：

① The section of indoor angle steel foundation and support shall not be less than 25×4mm2 The flat steel of is connected as the grounding main line , Then lead out to the outdoors , Connect with outdoor grounding device ;

② The grounding body shall be away from the transformer （ with ） Three meters away from the wall of the substation , The length of grounding body is 2.5 rice , The distance between the two grounding bodies is in 5 Rice is the best choice ;

③ The form of grounding grid should be closed loop , If the grounding resistance cannot meet the requirements , External lead-in grounding body can be added ;

④ The grounding resistance of the whole grounding grid shall not be greater than 4 o .

2、 How to connect different sections , Cable cores of different metals ？

Connect different metals , For cables with different cross sections , The resistance at the connection point shall be small and stable . Cables with the same metal cross section are connected , The same metal material as the cable core conductor shall be selected , The special connecting pipe shall be processed according to the cross-section of the connected two pole core wire , Then connect by crimping . When cables of different metals need to be connected , For example, copper and aluminum are connected , Because the standard electrode positions of the two metals are quite different （ Copper is ＋0.345 v , Aluminum is -1.67 v ） There will be contact potential difference . When electrolytes are present , Aluminum as negative electrode will be formed , A primary battery with a positive electrode of copper , Cause electrochemical corrosion of aluminum , So as to increase the contact resistance , So when connecting two different metal cables , In addition to meeting the contact resistance requirements , Certain anti-corrosion measures shall also be taken . The general method is to brush a layer of tin on the inner wall of the copper pressure pipe before crimping .

3、 During the completion acceptance of explosion-proof electrical equipment , What items should be accepted in detail ？

During the completion acceptance of explosion-proof electrical equipment , We must pay attention to the following points ：

① Verification “ Explosion proof Certificate No ”;

② Explosion proof electrical equipment shall be of type 、 Group , Conform to the design ;

③ Explosion proof electrical equipment shall be free of cracks in the shell 、 Damage 、 The junction box shall be fastened , Fixing bolts and locking devices shall be complete ;

④ Explosion proof oil filled electrical equipment , The oil tank shall not leak oil , The oil level height meets the requirements ;

⑤ Redundant inlets for electrical equipment , Sealing shall be done as required ;

⑥ The installation of sealing device for electrical circuit shall comply with regulations ;

⑦ Wiring engineering of safety spark type electrical equipment , The line direction elevation shall comply with the design , The line shall be marked in sky blue ;

⑧ Grounding or neutral connection of electrical devices , It shall comply with the regulations , Anti static grounding , It shall meet the design requirements .

4、 What regulations should the installation of complete handcart cabinet comply with ？

The installation of complete handcart cabinet shall comply with the following provisions ：

① The handcart is flexible and light , No jamming and collision ;

② The center of dynamic and static contacts shall be consistent , Close contact , The hand is pushing into the working position , Meet product requirements ;

③ The switching contact of secondary circuit auxiliary switch shall act accurately , Reliable contact ;

④ The mechanical locking device should act accurately and reliably ;

⑤ The lighting in the cabinet is complete ;

⑥ The safety clapboard switch is flexible , Act accordingly with the entry and exit of the car cabinet ;

⑦ The position of control cable in the cabinet shall not prevent the access of handcart , And firmly fix ;

⑧ The grounding contact between the handcart and the cabinet , Should be in close contact , When the handcart is pushed into the cabinet , The grounding contact should be connected earlier than the main contact, and it is the opposite when pulled out .

5、 a distribution board （ ark ） Preparation before installation ？

① The installation of the switchboard should be finished indoors , The foundation meets the required strength , After cleaning, carry out ;

② The switchboard shall be inspected and accepted before installation , Check the model of the switchboard , Whether the electrical components in the panel meet the requirements , Whether there is mechanical damage ;

③ The foundation section steel shall cooperate with the civil engineering to install the embedded parts , The top surface of foundation section steel shall be higher than the ground level 10-20mm, The horizontal error of foundation section steel on the same horizontal plane in the same place shall not exceed 1/1000 The maximum horizontal error should not exceed 5mm, The foundation steel of trolley type distribution cabinet should be level with the ground inside .

6、 How to repair the general fault of asynchronous motor shaft ？

① Turning curve ： Take out the rotor and correct it according to the specific situation ;

② The iron core of the shaft is worn ： Then a ring groove shall be opened on the shaft at both ends of the iron core , Then put in two arc keys and weld them with the shaft ;

③ Journal wear ： Generally, it can be treated by falling flowers at the journal . If the wear is serious , A layer can also be deposited at the journal by electric welding , Then use the lathe to process to the required size ;

④ Shaft cracks ： If it is serious, replace it .

7、 In those cases , Before operation , Nuclear phase must be carried out ？

① All new installations 、 Refit the equipment and lines connected with the system ;

② Incoming line maintenance ;

③ Transformer maintenance 、 Fold the cable lead connector or adjust the tap switch ;

④ Redo the junction box for the system cable 、 Cable head 、 When moving cables and other operations that may change phases ;

⑤PT The secondary circuit of is connected with the synchronous circuit , When overhauling PT Or the change of secondary circuit shall be subject to false synchronization test .

8、PT Why is short circuit not allowed for secondary circuit during operation ？

PT In normal operation , Because the secondary load is the voltage of some instruments and relays, the coil impedance is large , It is basically equivalent to the no-load state of the transformer , The current passing through the transformer itself is very small , Its size depends on the size of the secondary load impedance , because PT Its impedance is small , The capacity is not big , When the transformer is short circuited for the second time , The secondary current is very large , The secondary fuse fusing affects the correct indication of the instrument and the normal operation of the protection , When the insurance capacity is improperly selected , When the fuse fails to fuse in case of secondary short circuit , be PT Easily burned .

9、CT Why is open circuit not allowed in secondary operation ？

CT It is often used under high current conditions , At the same time as CT The current coil impedance of the instruments and relay devices connected in series with the secondary circuit is very small , Basically in short circuit state , therefore CT In normal operation , The secondary voltage is very low , If CT The secondary circuit is disconnected , be CT The core is seriously saturated with flux density as high as 1500 Above Gauss , Because the number of turns of the secondary coil is many times more than that of the primary coil , Therefore, a high voltage which is many times larger than the original voltage is induced at both ends of the secondary coil , This high voltage will cause great danger to all electrical equipment in the secondary circuit and the safety of workers , At the same time as CT The open circuit of the secondary coil will saturate the magnetic flux of the iron core, causing overheating and possible burnout , Moreover, the residual magnetism in the iron core will increase the transformer error , therefore CT Do not open the secondary circuit .

10、 What is current protection , How about the action principle ？

When the line is short circuited , One of the important features is the sharp increase of current in the line , When current flows through a predetermined .

11、 What is quick break protection , What are its characteristics ？

The starting current of overcurrent protection is set according to the principle of greater than the maximum load current , To ensure selectivity , It adopts the characteristics of step-by-step time limit , In this way, the action time limit of the protection device near the power end will be very long , This is not allowed in many cases . In order to overcome this shortcoming, the setting value is also increased , By limiting the range of motion , In this way, it is not necessary to increase the time limit to act instantaneously , Its action is considered by avoiding the short-circuit current under the maximum operation mode , Therefore, the full length of the line cannot be protected , It can only protect part of the line , The change of system operation mode affects the protection range of current quick break .

12、 What is grounding ？ What is zero connection ？ Why grounding and neutral connection ？

In the power system , Connect the neutral point of the equipment and electrical devices 、 A good electrical connection between the shell or support and the grounding device is called grounding .

Connecting the metal shell of electrical equipment and electrical devices to the system zero line is called zero connection .

The purpose of grounding and neutral connection , One is for the normal operation of electrical equipment , For example, working grounding ; Second, for personal and equipment safety , Such as protective grounding and neutral connection . Although in terms of the nature of grounding , And repeated grounding , Lightning protection grounding and electrostatic shielding grounding, etc , But its function is nothing more than the above two .

13、 Why are there black spots near the starting scale of some measuring instruments ？

Generally, it is on the dial of the indicating instrument , Are marked with the accurate grade scale of the instrument , Black spot near the starting end , It refers to the measuring range of the meter from this point to the full scale , Meet the standard grade of this table . Generally, the position of the black spot is based on the maximum value of the table 20% Standard method . for example , A bird that is full of this degree is 5 Ampere ammeter , Then the black dot is marked on 1A On . It can be seen from these , When selecting instruments , If the pointer indicates the part below the black dot during measurement , It shows that the measurement error is very large , Lower than the accuracy of the instrument , In this case, the instrument or transformer shall be replaced , Set the pointer to 20%-100%;

14、 When using a megger to measure the insulation resistance , What should be paid attention to ？

① When measuring the insulation resistance of the equipment , The power must be cut off first . For equipment with large capacitance （ Such as capacitor 、 transformer 、 Motor and cable line ） Discharge must be carried out first ;

② The megger should be placed in a horizontal position , Before wiring , First shake the megger to see if the pointer is “∞” It's about , then （L） and （E） Two terminals are short circuited , Slowly shake the megger to see if the pointer points to “ zero ” It's about , For semiconductor megohmmeter, short circuit calibration is not suitable ;

③ The lead wire of megger shall be multi strand flexible wire , And it shall have good insulation ;

④ Double circuit overhead lines and busbars that cannot be completely cut off , The induced voltage in the circuit under test exceeds 12 When you fall , Or overhead lines and electrical equipment connected with overhead lines when thunderstorms occur , Measurement is prohibited ;

⑤ Measuring capacitor , cable 、 Large capacity transformer and motor , There must be a certain charging time , The greater the capacitance , The longer the charging time . Generally, the reading after the megger rotates for one minute shall prevail ;

⑥ When telemetering insulation , The megger shall be kept at the rated speed . It's usually 120 turn / branch , When the capacitance of the measured object is large , To prevent the pointer from swinging , The speed can be appropriately increased （ Such as 130 turn / branch ）;

⑦ The surface of the tested object shall be wiped and cleaned , No dirt , To avoid missing the accuracy of measurement .

15、 When measuring insulation with a megger , Why is the telemetry time specified as １ minute ？

When measuring the insulation resistance with a megger , Generally, the reading after shaking for one minute shall prevail . Because after applying DC voltage to the insulator , The current flowing through an insulator （ Absorbing current ） It will gradually decrease with time . The DC resistivity of insulation is determined according to the steady-state conduction current , And insulators of different materials , The attenuation time of insulation absorption current is also different . But tests have shown that , The insulation absorption current of most materials has stabilized after one minute , Therefore, it is required to determine the insulation performance by the insulation resistance value after one minute of pressurization .

16、 How to choose a megger ？

Selection of megger , Mainly choose its voltage and measurement range , High voltage megger shall be used for high-voltage electrical equipment . Low voltage megger shall be used for low-voltage electrical equipment . The general selection principle is ：500 The electrical equipment below v shall be selected 500-1000 Volt megger ; Porcelain vase 、 a bus or bus bar 、 The knife brake should be 2500 Megger above volts .

The selection principle of the measuring range of the megger is ： The measurement range shall be adapted to the value of the measured insulation resistance to avoid large error in reading . For example, some megger readings do not start from zero , But from 1 Megohm or 2 Megohm begins . This kind of meter is not suitable for measuring the insulation resistance of low-voltage electrical equipment in humid environment . Because the insulation resistance of this equipment may be less than 1 Mega ohm , So that the meter can not get the reading , It is easy to mistake the insulation resistance as zero , And come to the wrong conclusion .

17、 What is fixed time limit ？ What is inverse time limit ？

In order to realize the selectivity of overcurrent protection , The protection action time of each section of the line shall be set according to the ladder principle , That is, the closer to the power end, the longer the time limit . The grade difference of each time limit is generally 0.5 second . The action time of the relay has nothing to do with the size of the short-circuit current . Using this action time limit method is called fixed time limit . The definite time overcurrent relay is electromagnetic , Equipped with time relay to obtain time limit characteristics , Its model is DL type .

Inverse time limit is to make the action time independent of the short-circuit current , When the action current is large , The action time is short , Otherwise, the action time is long , The relay made of this feature is called inverse time overcurrent relay . It is inductive , Models for GL type . The relationship between its action current and action time can be divided into two parts ： Part is fixed time , Part is inverse time limit . When the short-circuit current exceeds a certain multiple , The increase of current will no longer shorten the action time , At this time, it shows the definite time characteristic .

18、 hand over 、 Can the DC circuit share one cable ？

To put it simply 、 The DC circuit cannot share one cable , The main reason is ： hand over 、 The direct circuit is an independent system , Dangjiao 、 When DC shares one cable , hand over 、 DC interferes with each other , Reduce the insulation resistance to DC ; meanwhile , DC is an insulation system , AC is the grounding system , It is easy to cause short circuit between the two , old friend 、 DC cannot share one cable .

19、 There are several types of central signaling devices ？ What are their uses ？

The central signal device has two kinds of accident signal and warning signal . The purpose of the accident signal is to act as a circuit breaker .

20、 What is the effect of losing a card without returning ？

The signal that the card is dropped but not reset is generally reflected by the light plate and the alarm bell , Its characteristic is that in all control circuits , Any signal is not recovered , Can send out light signals , So as to remind the personnel on duty or the operator to find the fault according to the signal , There will be no omission or miscalculation .

21、 What is the effect of high or low DC bus voltage ？

When the DC bus voltage is too high , For electrical components with long-term live operation , Such as instrument relay , The indicator light is easy to be damaged due to overheating . When the voltage is too low , It is easy to make the protective device misoperate or refuse to operate . Generally, the allowable variation range of the specified voltage is ±10％.

22、 Transformer overcurrent protection action , How the attendant handles ？

① Go to the site to check if there is no obvious fault , Can send electricity ;

② It is determined that it is a human error , The protection causes the transformer switch to trip or contact the master control dispatcher to determine the system fault , Cause the overcurrent protection action , Then the transformer oil switch trips and can be put into operation immediately without inspection .

23、 Relay protection in case of abnormal fault , What responsibilities are allowed by the personnel on duty ？

a、 In case of any abnormality or accident inside the power station and the system ,（ If the current impulse voltage suddenly drops, the system oscillates 、 Overload , The cycle swings . Grounding and automatic tripping of switch ） The attendant must do the following work ：

① Check the condition that the signal drops and the card falls ;

② Check the sound 、 Light signal sending condition ;

③ Check that the switch trips automatically , The action of the automatic device ;

④ Monitor the current 、 Voltage cycle and active power change , Record the above information in the record book , Then reset the signal as required .

b、 Inform the shift supervisor of the results of the above inspection in time , System scheduling and workshop leaders .

c、 Under the following circumstances, the watchman can deal with it by himself ：

① Replace the bulb and fuse ;

② Select DC grounding .

24、 In three-phase four wire power supply system , Why not allow some equipment to be connected to neutral , The other part of the equipment adopts protective grounding ？

When the protective grounded electrical equipment is used to collide with the shell , Because the resistance of the earth is much greater than that of the neutral line , A short-circuit power supply is formed through the grounding electrode and grounding of the shell 、 It is often not enough to make the automatic switch and fuse operate , And grounding power supply , It also increases the neutral point potential of the power supply , Make all electrical equipment shells or cabinets connected to the neutral line have a voltage to the ground , It will cause more chances of electric shock .

25、 Try what the pencil does ？

The test pencil can measure whether the object is charged , There are several other uses ：

① It can measure whether any wire in the circuit is in phase or out of phase . The way is ： Standing on an object insulated from the earth , Hold a test pencil in each hand , Then test on the two wires to be tested , If the two test pens glow very bright , Then the two wires are out of phase , Otherwise, it is in phase .

② You can distinguish between AC and DC . During the test, if the two poles in the neon tube of the electric pen （ Both ends of the tube ） All glow , AC . If only one of the two poles emits light , It is direct current .

③ It can judge the positive and negative poles of direct current . Connected to DC circuit for test , The bright pole of the neon tube is the negative pole , The non illuminated pole is the positive pole .

④ Can judge whether the DC is grounded . In a DC system insulated from ground , You can stand on the ground and touch the positive or negative pole of the DC system with a test pencil , If the neon tube of the test pencil doesn't light , There is no grounding phenomenon . If it shines , It means that the grounding exists . It shines as brightly as at the tip of a pen , This means that the positive pole is grounded . If it shines on one end of the finger , The negative pole is grounded . However, those with grounding monitoring relays are not subject to this limitation .

26、 What is anti jump locking protection ？

So-called “ jumping ” When the circuit breaker is closed , Because the control switch is not reset or the control switch contact , The contact of the automatic device is stuck , Cause the tripping control circuit to still be connected and act to trip , In this way, the circuit breaker will reciprocate many times “ Jump and close ”, We call this phenomenon “ jumping ”. Anti jump locking protection is to use the mechanical locking of the operating mechanism itself or take other measures in the operating circuit （ Such as adding anti tripping relay ） To prevent jumping . Make the circuit breaker close to the fault line and trip , No longer switch on , Even if the operator still puts the control switch in the closing position , The circuit breaker will not happen “ jumping ”.

27、 What causes the generator to generate shaft voltage ？ What harm does it do to the operation of the generator ？

The causes of shaft voltage are as follows ：

① Because the stator magnetic field of the generator is unbalanced , An induced potential is generated on the rotating shaft of the generator . The reason for the unbalanced magnetic field is generally that the local magnetic group of the stator core is large （ For example, the stator core is rusted ）, And set 、 Uneven air gap between rotors .

② Because the shaft seal of the steam turbine generator is not good , Due to high-speed steam leakage along the shaft or high-speed injection in the steam cylinder, the rotating shaft itself is statically charged . This kind of shaft voltage is sometimes very high , It can make people feel numb . But it has been grounded through carbon brush during operation , So it has actually been eliminated . The shaft voltage is generally not high , Usually no more than 2~3 v , In order to eliminate the shaft voltage passing through the bearing 、 The current circuit formed at the base and foundation , An insulating plate can be added under the bearing pedestal at the exciter side . Disconnect the circuit , But when the insulating pad is contaminated 、 Failure due to damage or aging , The shaft voltage is sufficient to break through the oil film between the shaft and the bearing and discharge occurs , in the course of time , It will gradually degrade the quality of lubricating and cooling oil , In serious cases, the rotating shaft and bearing bush will be burned , Cause shutdown accident .

28、 What stipulates that the temperature rise of transformer winding is 65℃？

The transformer will produce iron loss and copper loss during operation , These two losses will all be converted into heat , Heat the winding and iron core , Cause insulation aging , Shorten the service life of the transformer . The state stipulates that the temperature rise of transformer winding is 65℃ Is based on A Class insulation based .65℃+40℃=105℃ Is the limiting temperature of the transformer winding , In the oil immersed transformer, the A Grade insulation ,A The heat resistance of class a insulation is 105℃, Because the ambient temperature is generally lower than 40℃, Therefore, the temperature of transformer winding generally cannot reach the limit operating temperature , Even in a short time 105℃, Because of the short time , There is no direct danger to the insulation of the winding .

29、 What causes excessive no-load current of asynchronous motor ？

There are several reasons for excessive no-load current of asynchronous motor ：

① The power supply voltage is too high ： When the power supply voltage is too high , The motor core will produce magnetic saturation , Cause excessive no-load current ;

② The motor is improperly assembled or the gap is too large after repair ;

③ The number of turns of stator winding is insufficient or Ｙ Type a connection is wrongly connected △ V-shaped wiring ;

④ For some old motors , Due to corrosion or aging of silicon steel sheet , The no-load current is too large due to the weakening of magnetic field strength or the damage of insulation between sheets . For small motors , As long as the no-load current does not exceed the rated current 50% Can continue to use .

30、 The voltage and current of generator rotor are .

The generator rotor current is the excitation current . The voltage and current of the rotor are respectively taken from the DC bus and the bus shunt , Multi finger meter parameters , The excitation voltage and current are parameters supplied to the rotor winding to generate the magnetic field , It refers to the parameters used to generate the magnetic field .

Make a little progress every day ！

31、 What is the reason for the abnormal specific gravity of lead-acid battery electrolyte ？ How to deal with it ？

The phenomenon of abnormal specific gravity is ：

① The charging time is quite long , But the increase in specific gravity is little or no change ;

② The specific gravity decreases during floating charging ;

③ When fully charged , The specific gravity decreased greatly within three hours ;

④ The discharge current is normal, but the specific gravity of electrolyte decreases rapidly ;

⑤ Long time floating charge , The specific gravity of the upper and lower layers of electrolyte is inconsistent .

The main reasons for the abnormal specific gravity of electrolyte and the elimination methods are ：

① There may be impurities and turbidity in the electrolyte , It should be handled according to the situation , Replace the electrolyte if necessary ;

② Floating charge current is too small , The floating charge power supply should be increased , Further observation ;

③ Serious self discharge or leakage , The plates should be cleaned , Replace the clapboard , Strengthen insulation ;

④ The plates are seriously vulcanized , Relevant methods shall be adopted to deal with ;

⑤ Long term insufficient charging , As a result, the specific gravity is abnormal , After equalizing charging , Improve its operation ;

⑥ There is too much water or the sulfuric acid is not stirred evenly , Generally, the specific gravity shall be adjusted two hours before the end of charging ;

⑦ When the specific gravity of the upper and lower layers of electrolyte is inconsistent , Charge with a large current .

32、 How to deal with the instrument smoke ？

Instrument smoke is usually overload , Insulation decreases , The voltage is too high , Resistance deterioration , The current connector is loose, resulting in false connection and open circuit , When you find out , Short circuit the meter and the circuit quickly , The voltage circuit is disconnected , During operation, pay attention not to short circuit the voltage coil and open circuit the current circuit , Avoid protection maloperation and accidental contact .

33、 DC positive 、 What harm does negative pole grounding do to operation ？

DC positive pole grounding may cause protection malfunction , Because the general trip coil （ Such as outlet intermediate coil and trip coil ） All connected to negative power supply , If these circuits are grounded or poorly insulated again, it will cause protection malfunction , DC negative grounding and positive grounding are the same , Because the two-point grounding will short circuit the tripping or closing circuit , The relay contact may be burnt out .

34、 When the protection and instrument share one set of current transformer , How to short circuit when the meter circuit works ？ Pay attention to what ？

Protection and instrument share one set of current transformer , When the meter is working , The terminals of the meter must be short circuited , Be careful not to open circuit or short circuit the protection , At present, the secondary line of the current transformer is connected to the meter after it is connected to the protection , So the meter has a job , Short circuit the terminal of the meter itself , Does not affect protection .

35、 What are the common operating overvoltages ？

Switching overvoltage is easy to occur under the following conditions ：

① Disconnect the no-load transformer or reactor （ Including arc suppression coil , Transformer electric arc furnace group , Synchronous motor , Mercury rectifier, etc ）;

② Disconnect the capacitor bank or no-load long line ;

③ In the power network with ungrounded neutral , One phase to phase intermittent arc grounding .

36、 When the motor is running , The bearing temperature is too high , What should we look for ？

Improper grease , Too much or too little grease in the bearing housing , There are sundries in the grease ; The bearing goes through the inner ring or outer ring , Excessive motor vibration ; The bearing model is wrong ; Coupling misalignment .

37、 What is the main function of high voltage circuit breaker ？

The main function of high voltage circuit breaker is ：

① It can cut off or close the no-load current of the high-voltage line .

② It can cut off and close the load current of high-voltage lines .

③ It can cut off and close the fault current of high-voltage lines .

④ Cooperate with relay protection , It can quickly remove faults , Ensure the safe operation of the system .

38、 Work on high voltage equipment , What organizational measures are there to ensure safety ？

① Work ticket system ;② Work permit system ;③ Work guardianship system ;④ Work breaks 、 Transfer system ;⑤ End the system .

39、 How can the maintenance equipment be powered off ？

① Power failure of maintenance equipment , All aspects of the power supply must be completely disconnected ;

② It is forbidden to work on the equipment whose power is only disconnected by the switch ;

③ The disconnector must be opened , Make at least one obvious disconnection in each direction ;

④ The transformers and voltage transformers associated with the blackout equipment must be disconnected from both sides of the high voltage , Prevent reverse power transmission to the outage maintenance equipment .

40、 What tools and instruments are commonly used in substation maintenance ？

① Common electrical tools ( Wire pliers 、 bolt driver 、 Electrician knife 、 Movable handle 、 Needle nose pliers );

② Utility tools ( Set wrench 、 Pipe clamp 、 Flat nose pliers 、 Electric drill 、 Electric iron 、 Grinding wheel 、 Fitter tools and lifting tools );

③ Common measuring tools ( Steel ruler 、 Level ruler 、 Micrometer 、 Vernier caliper );

④ Meter ( A multimeter 、 Megger 、 Bridge );

⑤ Special tools for maintenance of substation equipment .

41、 What is circuit backup ？

It is represented by various components , device , Preparation of the current path composed of the device .

42、 What is a low voltage line ？

It is responsible for distributing electric energy overhead lines or cable lines , Voltage is 1KV The following power lines . Typical for 220V/380V Power lines .

43、 What is a low voltage electrical appliance ？

Low voltage apparatus is used for rated voltage AC 1000V Or DC 1500V And the following , In the circuit composed of power supply system and electric equipment, it plays a role of protection 、 control 、 Adjust the 、 Switching and switching electrical appliances .

44、 What is a low voltage package ？

The low-voltage complete set device is a complete set of equipment composed of low-voltage switchgear and control appliances .

45、 What are the two types of low-voltage package units ？

Electric control equipment ; Power distribution equipment （ Or power distribution device ） Two types of

46、 What is power distribution equipment ？

Power distribution equipment refers to all kinds of power plants 、 It acts as power in the low-voltage distribution system of substations and factories and mining enterprises 、 Complete equipment for power distribution and lighting .

47、 What are the working steps for power transformation and distribution outage ？

Disconnect the power supply 、 Electricity test 、 Install temporary grounding wire 、 Hang signs and install barriers .

48、 What does the low-voltage primary equipment mainly include ？

There are mainly low-voltage fuses ; Low voltage knife switch ; Low voltage knife fuse switch and load switch ; Low voltage circuit breaker .

49、 What are the main functions of the low-voltage fuse ？

It mainly realizes the short-circuit protection of low-voltage distribution system , Some can also realize overload protection .

50、 What are the main functions of the low voltage knife switch ？

No load operation , Used as disconnector .

51、 What are the main functions of low-voltage knife fuse switch and load switch ？

Low voltage knife fuse switch has dual functions of knife switch and fuse . The main function of the load switch can effectively turn on and off the load current , Short circuit protection .

52、 What are the main functions of low voltage circuit breakers ？

It can make and break the circuit with load , In short circuit 、 Overload and voltage loss automatic trip .

53、 What is electrical clearance ？

Between two adjacent conductors with potential difference in electrical appliances , The shortest distance through the air .

54、 What is the function of the circuit breaker arc extinguishing cover ？

① Guide the arc to blow out longitudinally , To prevent phase to phase short circuit ;

② Make the arc contact with the insulating wall of the arc extinguishing chamber , So as to cool down quickly , Increase de dissociation , Increase the voltage drop of arc column , Force the arc to extinguish .

55、 Why parallel compensation capacitors are used in industrial and mining enterprises ？

Parallel compensation capacitors are used to compensate the reactive power required by the inductive load of the electrical system , achieve ：

① Improve the voltage quality of power grid .

② Increase the power factor .

③ Reduce line loss .

④ Increase the output of transformer and line .

56、 What is electrical safety ? It includes those aspects ？

Electrical safety refers to the quality of electrical products , And installation 、 Use 、 No accident occurs during maintenance , Such as death by electric shock 、 Equipment damage 、 Electrical fire 、 Electrical explosion accident, etc . Electrical safety includes personal safety and equipment safety . Personal safety refers to the personal safety of electricians and other personnel participating in the work ; Equipment safety refers to electrical equipment and its auxiliary equipment 、 Safety of facilities .

57、 What are the common arc extinguishing methods ？

Common arc extinguishing methods are ： Quick pull arc extinguishing method 、 Cooling arc extinguishing method 、 Arc blowing method 、 Long arc cutting short arc extinguishing method 、 Narrow ditch or slit arc extinguishing method 、 Vacuum arc extinguishing method and sulfur hexafluoride arc extinguishing method .

58、 Low voltage switchgear and control equipment ( Abbreviated as complete equipment ) What does it mean ？

There are one or more low-voltage switchgear and associated controls , measurement , The signal , Protect , Adjustment and other equipment , The manufacturer is responsible for all internal electrical and mechanical connections , An assembly that is completely assembled with structural components .

59、 In the package , What are the meanings of main circuit and auxiliary circuit ？

The main circuit is all conductive circuits that transmit electrical energy ; All controls outside the main circuit of the auxiliary circuit 、 measurement 、 Conductive circuit in signal and regulation circuit

60、 What standards should the low-voltage distribution cabinet design comply with ？

IEC4391（ Low voltage switchgear and control equipment ）、GB7251（ Low voltage switchgear package ）、ZBK36001（ Low voltage withdrawable switchgear sets ）

61、 How to divide the protection range of each section of zero sequence current protection ？

Zero sequence current I The section avoids the setting of the maximum zero sequence current flowing through the protection through the grounding short circuit at the end of the line ; The whole length of the line cannot be protected , However, it shall not be less than... Of the total length of the protected line 15%-20%; zero sequence II The total length of the general protection line , And extend to... Of adjacent lines I Within a paragraph , And cooperate with it . zero sequence III Paragraph is I,II Backup segment of segment , And cooperate with adjacent lines .

62、 What is the difference between the computer protection and the original relay protection ？

The main difference is that the original protection input is current 、 Voltage signal , Directly compare and deal with analog quantities , Compare the analog quantity with the given resistance torque in the device . The computer can only perform numerical or logical operations . therefore , First, it is required to input analog current 、 The instantaneous value of the voltage is transformed into a bit discrete digital quantity , Then it can be sent to the central processing unit of the computer , Operate according to the specified algorithm and program , And compare the operation result with the given number at any time , Finally, judge whether to trip .

63、 What is the post acceleration of reclosing ？

When the line fails , The protection operates according to the setting value , The line switch is disconnected , Reclosing will act immediately . If it is a transient fault , After the line switch is disconnected , The fault disappears , Coincidence succeeded , Restore power supply to the line ; In case of permanent failure , After coincidence , The protection time element is exited , Turn it into 0 Second trip , This is the accelerated tripping when the fault does not disappear after the reclosing action , Trip to clear the fault point .

64、 How to deal with the wrong operation of the disconnector ？

① When pulling the disconnector by mistake , As soon as the knife switch leaves the static contact, an arc occurs , Close it immediately , You can extinguish the arc , Avoid accidents , If all the knife switches have been opened , Do not close the knife switch that was pulled by mistake ;

② When pulling the disconnector by mistake , Even if the combination is wrong , Even an arc occurs when closing , No more pulling , Because the loaded knife switch will cause three-phase arc short circuit .

65、 What do you mean R、L、C Parallel resonance ？

resistance 、 A circuit in which inductance and capacitance are connected in parallel , Under the action of sinusoidal power supply with a certain frequency , The voltage at the circuit end is in phase with the total current , The whole circuit is in a special state of resistance , This state is called parallel resonance .

66、 The starting element of distance protection adopts negative sequence 、 What are the advantages of zero sequence incremental components ？

① High sensitivity ;

② It can be seen that the starting element of the oscillation locking device ;

③ There will be no maloperation when the voltage secondary circuit is disconnected ;

④ The occurrence of symmetrical components is independent of the fault phase , Therefore, the starting element can adopt a single relay , So it's simpler .

67、 What conditions the protection device meets can be evaluated as class I equipment ？

All protective devices for class I equipment , Its technology is in good condition , The performance fully meets the requirements for safe operation of the system , And meet the following main conditions ：

① Protective screen 、 Relay 、 Components 、 Auxiliary equipment and secondary circuit are free of defects .

② The principle of the device 、 The wiring and setting value are correct , Comply with relevant regulations 、 Regulations and anti accident measures .

③ Drawings and data are complete , In line with reality .

④ Good operating conditions .

68、 What are the inspection items and contents of the control switch ？

The inspection contents of the control switch include ：

① The housing is clean and free of oil dirt , Intact .

② The installation should be firm , Inactive during operation .

③ The sealing cover is well sealed .

④ All wire ends shall be firmly connected , Not loose , No rust .

⑤ Flexible rotation , The position is right , Good contact .

⑥ Open the sealing cover , Check with a flashlight , The interior should be clean , The lubricating grease is not dry , There is no burning loss at the contact point . Test the contact piece with an insulating rod , The pressure should be good .

69、 What should the transformer differential protection check when the transformer is put into no-load operation ？

Differential protection of transformer , During the new installation, the transformer must be made under the rated voltage 5 No load test . Before no-load operation , Secondary wiring should be checked , And make sure it is correct . The no-load input test shall be conducted on the transformer

70、 When removing the secondary line , What measures should be taken ？

When removing the secondary line , Records must be kept ; When you recover . It should be written off in the log book . When the secondary line changes a lot , A tag shall be attached to each thread end . Remove or lay the secondary cable , Tags shall also be attached at the beginning and end of the cable and at the turns and crossing elements along the cable .

71、 What are the requirements of anti accident measures for gas protection ？

① Put the lower pontoon of the gas relay into the baffle type , The contact is changed to vertical , To improve the reliability of heavy gas action .

② To prevent short circuit of gas relay due to water leakage , Rain proof measures shall be taken at its ends and on the terminals in the cable lead terminal box .

③ The outgoing line of gas relay shall be oil proof wire .

④ The outgoing line and cable of the gas relay shall be respectively connected to the terminals in the cable lead terminal box .

72、 What are the general principles of transformer protection installation ？

① Gas protection against internal short circuit of transformer iron shell and oil level reduction .

② Protect transformer coil and outgoing line from phase to phase short circuit , The grounding at the coil leading out side of the grid side with large grounding current Longitudinal differential protection or current quick break protection for short circuit and coil turn to turn short circuit .

③ It can prevent the phase to phase short circuit outside the transformer and act as the backup overcurrent protection for gas protection and pilot differential protection （ Or over-current protection started by composite voltage 、 Or negative sequence current protection ）.

③ Zero sequence current protection against external grounding short circuit in power grid with large grounding current .

⑤ Overload protection against symmetrical overload .

73、 What is the function of the starting element of the distance protection ？

① In case of short circuit fault , Start the protection device quickly ;

② Start the oscillation locking device , Or also as the first III Measuring element of segment ;

③ Perform segment switching ;

④ Carry out phase switching ;

⑤ In transistor protection , If the DC logic section fails , Lock the whole set of protection .

74、10 What protection is generally installed on kV transmission lines ？

① Phase to phase short circuit protection ： Single power lines are generally equipped with two-stage overcurrent protection , Current quick break protection , Definite time overcurrent protection . Dual power lines are generally equipped with directional or non directional current speed protection and overcurrent quick break protection .

② Grounding protection ： Generally, non selective insulation monitoring protection is installed 、 Zero sequence overvoltage protection 、 Power direction protection .

75、 What is the effect of negative feedback on the performance of the amplifier ？

① Reduce magnification ,

② Improve the stability of magnification ,

③ Improve waveform distortion ,

④ Broaden the passband ,

⑤ Change the input and output resistances of the amplifier .

76、 What is the cause of non sinusoidal current ？

Generation of non sinusoidal current , It can be a power supply , It can also be a load . There are usually the following reasons ：

① There are several different sinusoidal electromotive forces acting simultaneously in the circuit , Or AC and DC electromotive force work together ,

② There is a non sinusoidal periodic electromotive force in the circuit .

③ There are nonlinear elements in the circuit .

77、6kV-35kV The lightning arrester in the power system is connected to the phase to ground voltage , Why should the lightning arrester be selected according to the rated line voltage ？

6kV-35kV The system is a small grounding short-circuit current system , Under normal circumstances , The lightning arrester is under the action of phase to ground voltage , However, in case of single-phase grounding fault , The voltage to ground of the non fault phase rises to the line voltage , This kind of ground fault is allowed to exist for a period of time , At this time, the lightning arrester shall not act . therefore , The rated voltage of the arrester must be the rated line voltage of the system rather than the rated phase voltage .

78、 What conditions can the protective device meet be rated as class III equipment ？

The protective devices of class III equipment may not be fully equipped , Or poor technical performance , Thus affecting the safe operation of the system . If , The main protective device has one of the following conditions , Also rated as class III equipment ：

① The protection does not meet the system requirements , It can cause system oscillation in case of fault , Collapse accident or serious damage to main equipment .

② Fail to meet the requirements of anti accident measures .

③ Connecting piece for operator operation 、 handle 、 Buttons, etc. are provided with signs .

④ Incomplete drawings , And it does not conform to the reality ,

⑤ The fault oscillograph can not record waves in good condition or is not put into operation .

79、 When testing the relay , How to master the test environmental conditions ？

The requirements for test environmental conditions include temperature 、 Relative humidity 、 And air pressure . These conditions not only affect the basic performance of the tested relay , And it also affects the working state of testing instruments and equipment . The requirements for test environmental conditions are as follows ：

① temperature ：15-35 degree

② humidity ：45-75%

③ pressure ：660-780mmHg

80、 When selecting test instruments , What principles to master ？

① Select the type of instrument according to the measured object . First, it depends on whether the relay under test is DC or AC , Select DC instrument or AC instrument .

② The internal resistance of the instrument shall be selected according to the test circuit and the coil impedance of the measured relay .

③ Select appropriate instruments according to the measured size .

④ Choose instruments according to the place of use and working conditions .

81、 After the newly installed protective device is completed , What are the main acceptance items ？

The acceptance items are as follows ：

① Relevant measured parameters of electrical equipment and lines are complete 、 correct .

② As built drawings of all protective devices conform to reality .

③ Check that the setting value meets the requirements of the setting notice .

④ The inspection items and results comply with the inspection regulations and relevant regulations .

⑤ Check the transformation ratio and volt ampere characteristics of current transformer , The secondary load meets the error requirements .

⑥ Check the front of the screen 、 The equipment behind the screen is neat , In good condition , The circuit insulation is good , The signs are complete and correct .

⑦ Conduct acceptance test with primary load current and working voltage , Judge the transformer polarity , Correctness of transformation ratio and its circuit , Determine the direction , Differential , distance , Correctness of relevant components and wiring of high-frequency and other protection devices .

82、 How to check the zero sequence voltage circuit of zero sequence directional protection of large grounding current system in normal operation ？

To ensure the correct action of zero sequence directional protection , The integrity of zero sequence voltage circuit of zero sequence directional protection shall be checked . The method is to use the test small busbar from the secondary winding of the open delta connection of the voltage transformer to the voltage small busbar for each set of zero sequence directional protection YMN The measured voltage is 100V, It's normal .

83、 When single-phase grounding fault occurs in radial power network of small grounding current system , What is the difference between the current of a faulty line and that of a non faulty line ？

The zero sequence capacitor current measured at the sending end of the fault line , Equal to the sum of zero sequence capacitive currents of other lines , And flow to busbar . The zero sequence current measured at the sending end of the non fault line is the non fault phase to ground capacitive current of the line , And out of the bus .

84、 In large grounding current system , Why is the time limit of interphase protection action longer than that of zero sequence protection ？

The action time limit of protection is generally set according to the ladder principle . Action time limit of interphase protection , It is composed of one time limit differential increment for each level of protection from the user to the power supply , The zero sequence protection is mainly due to the step-down transformer Y/ connection , When the low-voltage side is short circuited to ground , There is no zero sequence current at the high voltage side , Its action time limit does not need to cooperate with the low-voltage users of the transformer . Therefore, the action time limit of zero sequence protection is shorter than that of phase to phase protection .

85、 What is power system oscillation ？ What are the general causes of oscillation ？

The phenomenon that two systems or power plants running in parallel lose synchronization is called oscillation . There are many reasons for oscillation , Most of them are caused by the destruction of the dynamic stability of the system due to the long time of clearing the fault , In the system with weak connection, it may also be due to misoperation , Loss of excitation or fault tripping of generator 、 To cause oscillation by cutting off a line or equipment .

86、 What requirements should the modulator meet ？

① When input DC signal Ui=0 when , The output signal U0=0

② Amplitude of output AC signal , It shall be proportional to the size of the DC signal ,

③ When DC signal Ui When the polarity of changes , The phase of the output AC signal also changes .

87、35kV In an ungrounded neutral network , What is the configuration principle of line phase to phase short circuit protection ？

The configuration principle of phase to phase short circuit protection is ：

① When two-phase current protection is adopted , Current transformers shall be installed on two phases of the same name （ for example A,C phase ）.

② The protection device shall adopt remote backup mode .

③ If the line is short circuited, it will make the auxiliary bus of the power plant 、 The voltage of the tie point bus or important user bus of the main power supply is lower than the rated voltage 50%-60% The fault shall be removed quickly .

88、 In the high voltage power grid , What is the function of high frequency protection ？

High frequency protection is applied to long-distance high-voltage transmission lines , All kinds of faults at any point of the protected line can be removed from both sides instantaneously , Thus, the stability of power system operation and the success rate of reclosing can be improved .

89、 In large grounding current system , Why is the time limit of interphase protection action longer than that of zero sequence protection ？

The action time limit of protection is generally set according to the ladder principle . The action time limit of phase to phase protection is composed of a time limit difference increased by each level of protection from the user to the power supply , The zero sequence protection is mainly due to the step-down transformer Y,d11 connection , When the low-voltage side is short circuited to ground , There is no zero sequence current at the high voltage side , Its action time limit does not need to cooperate with the transformer low-voltage users . Therefore, the action time limit of zero sequence protection is shorter than that of phase to phase protection .

90、 What are the basic requirements for operational amplifiers ？

① The external impedance of the input terminal and the impedance of the feedback circuit should be accurate 、 Stable ;

② The open-loop voltage amplification factor shall be large enough ;

③ Open loop input resistance r i Big enough ,

④ Open loop output resistance should be small ;

⑤ Zero drift and noise should be small .

91、 What is the amplifier output resistance ？

At the amplifier output , The amplifier can be regarded as a signal source with certain internal resistance , This internal resistance is the output resistance .

92、 What should be paid attention to when using the superposition principle to calculate the linear circuit ？

Applying the superposition principle, the voltage and current of each branch under the independent action of each voltage source and current source can be calculated respectively , Then the superposition principle adds up , Attention should be paid to when applying the superposition principle

① This principle can only be used to calculate linear current and voltage , Not applicable to nonlinear circuits

② Pay attention to the direction of current and voltage when stacking , Take the algebraic sum when superimposing

③ The circuit connection mode and the size of each resistance in the circuit shall not be changed . When the current source acts , The voltage source is short circuited , When the voltage source acts , The current source is open .

④ The superposition principle is only applicable to the superposition of voltage and current , And power cannot be calculated by superposition principle .

93、 Why should the hydro generator be provided with overvoltage protection ？

Due to the slow regulation of the turbine governing system , After accident load shedding , It is prone to unacceptable overvoltage , Therefore, overvoltage protection is required .

94、 What is generator low excitation and generator loss of excitation ？

Low excitation means that the excitation current of the generator is lower than the excitation current corresponding to the static stability limit . Loss of excitation refers to the loss of excitation current of the generator .

95、 Why should the generator be equipped with overcurrent protection for load voltage operation ？ Why? This protection shall use the current transformer at the neutral point of the generator ？

This is set as the generator differential protection or the backup protection of the next element , It works when the following two faults occur ：

① When there is an external short circuit , When the protection device or relay of the faulty component refuses to act ;

② When there is a fault within the scope of generator differential protection and the differential protection refuses to act .

96、 The negative sequence voltage setting value of transformer composite voltage starting overcurrent protection is generally set according to what principle ？ Why? ？

When the system is running normally , The three-phase voltage is basically a positive sequence component , The negative sequence component is very small , Therefore, the setting value of negative sequence voltage element is set according to the output unbalanced voltage of negative sequence voltage filter during normal operation , Generally go to 6-12V（ Secondary voltage value ）

97、 Why is it said that the protected area of distance protection is basically not affected by the change of the system operator ？

Because the distance protection uses the ratio of the voltage and current at the beginning of the line as the criterion to form the protection , Because the short-circuit impedance changes only with the distance between the short-circuit point and the beginning of the line , Therefore, the protected area is basically not affected by the change of the operation mode of the system .

98、 Why do some large capacity transformers and system interconnection transformers use negative sequence current and single-phase low-voltage starting overcurrent protection as backup protection ？

Because this protection has the following advantages ：

① In case of asymmetric short circuit , Its sensitivity is high ;

② In case of asymmetric short circuit behind the transformer , Its sensitivity has nothing to do with the wiring mode of the transformer .

99、 What is the role of intermediate relay in relay protection ？

① The contact of the measuring element in the protection device is usually very small , The quantity is also small , The capacity and number of contacts can be increased through the intermediate relay ;

② When a pipe type lightning arrester is installed on the line , The delay of the action of the protection device can be obtained by using the intermediate relay , To prevent malfunction of quick action protection caused by lightning arrester discharge ;

③ Meet the needs of protection logic circuit .

100、 What is the difference between the working conditions of electromagnetic current relay and voltage relay ？

The voltage relay is generally connected to the secondary side of the voltage transformer , Compare with current transformer , Due to high voltage , Therefore, there are many turns of relay coil 、 The conductor is thin 、 High impedance , And the reactance of the coil increases , Even the current .