Problems that need to be understood when engaging in Power Supply

I. Glossary:

1. Three-phase alternating current (AC): An electric power system consisting of three AC circuits with the same frequency, equal potential amplitude, and Mutual Difference of 120 °.

2. One-time device: the device directly related to power generation and distribution is called one-time device. Including various high voltage circuit breakers, isolating switches, busbars, power cables, voltage transformers, current transformers, reactors, lightning arresters, arc suppression coils, parallel capacitors, and High Voltage Fuses.

3. Secondary equipment: Auxiliary Equipment that monitors, measures, controls, and protects a single device. Such as various relays, signal devices, measuring instruments, wave recording devices, telemetry, remote communication devices, various control cables, and small busbars.

4. High-voltage circuit breaker: Also known as high-voltage switch, it can not only cut off or close the no-load current and load current in the high-voltage circuit, but also when the system fails, through the action of the protective device, cut off the overload current and short-circuit current. It has a well-developed Arc Extinguishing structure and sufficient cut-off capability.

5. load switch: the construction of the load switch is similar to that of the classified isolation switch, but a simple arc extinguishing device is installed. It also has an obvious disconnection point, with a certain degree of cut-off capability, can be operated with load, but cannot directly disconnect the short-circuit current, if needed, it depends on the High-Voltage Fuses connected with it.

6. Air Circuit Breaker (automatic switch): It is a low-voltage switch that uses a manual (or electric) switch and uses a lock to keep the switch position. The trip mechanism acts on the switch and has an arc extinguishing device, it is widely used in AC and DC units below 500 V. When overload, short circuit, and voltage decrease or disappear in the circuit, the circuit can be automatically cut off.

7. Cable: cables consist of core wires (conductive parts), plus insulation layer and protective layer.

8. busbars: Electrical busbars are the access devices that collect and distribute electric energy. They determine the number of devices in the Power Distribution Device and indicate how to connect generators, transformers, and lines, and how to connect to the system to complete the transmission and distribution task.

9. Current Transformer: The instrument converter is an instrument that converts large current into small current.

10. Transformer: a static electrical device used to change the AC voltage of a certain value to another type or multiple types of AC voltage with the same frequency.

11. High-Voltage electroscope pen: it is a tool used to check whether high-voltage network power distribution equipment, overhead cables, and cables are charged.

12. Ground Wire: it is an important tool to ensure the staff when voltage occurs on the equipment and lines that have been powered off. According to ministerial provisions, the grounding wire must be made of bare copper cables larger than 25mm 2.

13. Notice Board: used to warn people not to be close to the equipment and charged part, instruct the staff to prepare a work place, and remind them to take safety measures, and notice board prohibiting the power-on of a certain device or line. It can be divided into warning, allow, prompt, and deny.

14. Cover bar: to prevent staff from accidentally touching the screen guard of charged equipment, there are two types of temporary and permanent blocks.

15. Insulation ROD: it is also called a linker rod, insulation rod, and Operation rod. Insulation rod consists of three parts: Work head, insulation rod and grip. It is used for closed or in-position high-voltage isolating switch, mounting and dismounting the portable grounding wire, as well as measurement and testing.

16. Xtep voltage: if there is a potential difference between the two points on the ground with a horizontal distance of 0.8 m, when the human body is in contact with the two points, the voltage will bear on the human body. This voltage is called Xtep voltage. The maximum cross-Step Voltage occurs between the grounding body and the grounding body at a horizontal distance of 0.8 m from the grounding body.

17. phase sequence: it refers to the order in which the instantaneous values of alternating current (AC) pass through zero values from negative values to positive values.

18. Power Grid: a power grid is a part of the power system. It is a unified network composed of various substations (sites) and transmission and distribution lines of different voltage levels.

19. Power System: The power system is part of the power system. It consists of generators and power distribution devices in power plants, boost-up and step-down substations, transmission and distribution lines, and electrical equipment of users.

20. Power System: power plants, substations, and the electrical equipment of users. The power system is connected by an electric network and a thermal network (or a hydraulic system.

Ii. multiple choice questions

1. When two resistors with the same rated voltage are connected in series to the circuit, the resistance with A large resistance value (A) is obtained ).

A. There is no significant difference between B and C.

2. The conversion switch of the multimeter is implementation ().

A. Switch of various measurement types and ranges B. Switch of multimeter current

C. Turn on the measurement switch of the tested object

3. Insulation rods should be (B) at ordinary times ).

A. Place it smoothly B. Keep them out of contact with the ground and walls to prevent moisture deformation C. Place it in the corner

4. the test period of insulated glove is (B ).

A. Once every year, once every B and six months, once every C and five months

5. the test period of insulation boots is (B ).

A. Once every year, once every B, once every six months, once every three months.

6. When the barrier needs to be removed or crossed during the duty period (c ).

A. A leader must be present B. a power outage must be performed first

C. A guardian must be present.

7. On-duty personnel inspect high-voltage equipment ().

A. Generally, two persons are responsible for B and the duty officer can do other work.

C. If any problem is found, you can handle it at any time.

8. After the switch operation ticket is executed, it must be (B ).

A. Save it to shift B. Save it for three months. C. Save it for a long time.

9. When receiving the switch operation command ().

A. The Guardian and operator shall be present and accepted by the guardian.

B. The operator is acceptable if the guardian is present.

C. It can be accepted by the substation Length

10. The positive phase paint of the DC bus is set to (c ).

A, blue B, white C, and blue

11. The grounding midline Paint shall be painted as ().

A, black B, purple C, white

12. The maximum allowable temperature for the connectors and clips of the substation (Station) equipment is ().

A, 85 deg c B, 90 deg C, 95 deg C

13. The maximum allowable temperature of the current transformer skin is (B ).

A, 60 deg c B, 75 deg C, 80 deg C

14. Power Cables shall not run at overload. in case of accidents, cables less than 10 k V can only run continuously (C.

A, 1 h overload 35% B, 1.5 h overload 20% C, 2 h overload 15%

15. The oil of the power transformer acts as (.

A. Insulation and arc extinguishing B. Insulation and rust-proof C. insulation and heat dissipation

16. The relay protection device is composed of (B.

A. Secondary Circuit components B and various Relays

C. including various relays and meter circuits

17. signal relays (C ).

A. the relay card or light indicator.

B. The lighting and audio circuit should be connected immediately.

C. The cards should be dropped on one side, and other signals should be connected when the contact is closed.

18. when the line is faulty, the faulty part can be quickly removed and (B ).

A. automatically relay the gate once. B. Send the signal.

C. Continue running in good condition

19. When the ground wire is installed, (B ).

A. First install phase B, first install the grounding end, and then install the two sides

C. First mount the Wire End

20. When wearing insulated gloves, put the outer cuff ().

A. Load the insulated glove B. Roll it up to C. Wrap it out of the glove

21. When a line is shut down and shut down for repair, and the line side is operated by a bypass, the indicator board of hanging (C) should be mounted on the switch operator.

A. Work Here B. Close prohibited C. Climb prohibited or high pressure dangerous

3. Fill in blank questions

1. Safety Work rules are as follows: the voltage of the device to the ground is higher than 250 V, the high voltage is lower than 250 V, the Safe voltage is less than 36 V, and the Safe Current is less than 10mA.

2. The staff on duty need to remove the barrier for work. The required security distance is 0.7 m for 10 k V, 1.0 m for 35 k V, and 110 m for 1.5 k V, 220 k V at 3.0 m.

3. When thunderstorms require inspection of outdoor high-voltage equipment, insulation boots should be worn, not close to lightning arresters, lightning rods and Grounding Devices.

4. When an electrical device is on fire, the power supply of the device should be cut off and then put out the fire.

5. Tools commonly used by the operator on duty include Wire Clamp, screwdrivers, electric knives, fl hands, sharp-mouth pliers, soldering iron and low-pressure test pens.

6. In the graphical symbol of the transformer, Y indicates three-phase coil star connection.

7. The signals in the Substation Control Room are generally divided into voltage signals, current signals, and resistance signals.

8. It is strictly prohibited to use the leather ruler, wire ruler, metal ruler and perform measurement work around the electrified equipment.

9. Dry Powder, 1211, and carbon dioxide fire extinguishers should be used when electric equipment is on fire. foam fire extinguishers should not be used to extinguish the fire.

10. Commonly Used DC power supplies in substations include battery, silicon rectification, and Capacitor storage.

11. The lighting of substations (sites) must be an independent power supply and cannot be mixed with common lighting circuits.

12. The operating terms of high voltage circuit breaker or isolating switch should be opened and closed.

13. Input and release operation terms of the relay protection device and automatic device.

14. The operation term for electrical installation and dismounting of the grounding wire is installation and removal.

15. Each operation ticket can only be filled in with one operation task. Each operation is marked with "√ ".

16. The executed operation ticket indicates "executed ". The words "Void" should be specified in the Void operation. These two ticket types must be saved for at least three months.

17. The output characteristics of a transistor are intercept, amplification, and saturation.

18. In the resistance, volume, and sense series circuit, only the resistance consumes power, while the inductance and Capacitor only undergo energy conversion.

19. The switch operation of the substation (Station) must be performed by two persons, who are the guardian of the equipment.

20. In case of any doubt during the switch operation, the ticket is not allowed to be changed without authorization. The ticket is to be reported to the on-duty dispatcher or the person in charge of duty, and the ticket will be processed after it is clarified.

21. In substation (Station) operations, accident handling without ticket operation, single operation for pulling and switching, unbinding the grounding knife gate, or removing only one set of ground wires of the whole plant.

22. Fill in the ticket, which should include the operation sequence of the Operation task, the sender, operator, guardian, and operation time.

23. Safe distance from the grounding point of the human body when the grounding fault of high-voltage equipment occurs: the distance between the grounding points should be greater than 4 m in the room and 8 m in the outside.

24. The primary current of the current transformer is determined by the load current of the primary loop. It does not change with the secondary loop impedance, which is the main difference from the working principle of the transformer.

25. The oil pillow is used to adjust the oil volume and prolong the service life of the oil. The volume of the oil pillow is generally of the total transformer volume.

26. When the transformer is faulty, the gas relay is connected to the signal circuit, and the underground is connected to the switch and trip circuit.

27. Transformer cooling methods include oil-immersed self-cooled type, Oil-Immersed Air-cooled type, strong oil-cooled type, and strong oil-water cooling type.

28, our factory main equipment rated current, 1000 k VA transformer: 57.7 A, J-752A: 52.5 A; J-201 A: 166.

29. The year, month, and major damaged equipment of the three rat damage accidents in our factory ,,,.

Iv. Q &

1. What is the purpose of relay protection?

A: ① When a fault occurs in the power grid that is sufficient to damage the equipment or endanger the safe operation of the power grid, the protected equipment will be quickly detached from the power grid; ② The abnormal operation of the power grid and the abnormal status of some equipment can issue alarm signals in a timely manner for rapid processing and recovery; ③ automation and automation of the power system are realized, and automatic control of industrial production.

2. What is the basic principle of the relay protection device?

Answer: when the power system fails, the basic features are sudden increase of current, sudden decrease of voltage, and change of the phase angle between current and voltage. Various relay protection devices seize these features, based on the changes in these physical quantities, different physical quantities, such as the internal and external faults, are protected by the difference between the normal and the fault, and the overcurrent protection is implemented when the reaction current increases, low-voltage protection with reduced Reactive voltage, over-current protection with changes in reactive current and phase angle, and distance protection with the ratio of Reactive voltage to current.

3. What are the requirements for relays?

Answer: ① the error of the action value should be small; ② the contact should be reliable; ③ the return time should be short; ④ the power consumption should be small.

4. What types of relays are commonly used?

Answer: depending on the physical quantity of the component, the relay can be divided into two types: Electric and non-electric. Non-electric relay include gas relay, speed relay, and temperature relay.

Many types of reactive power are generally divided:

① According to the principle of action, it can be divided into: electromagnetic, induction, rectification, and transistor; ②. The properties of reactive power include current relays and Voltage Relays; ③ According to the function can be divided into: Inter-electrical relays, time relays, signal relays, etc.

5. What are the inspection items of induction current relays?

A: induction current relays are anti-time over-current relays, which include induction components and speed-breaking components. Their common models are GL-10 and GL-20 series, the inspection items are as follows:

① External inspection; ② internal and mechanical inspection; ③ insulation inspection; ④. Start current inspection; ⑤. Action and return value inspection; ⑥. Fast Moving element inspection; 7. Inspection of operation time characteristics; testing of Operation Reliability of contacts.

6. How can I use the grounding table correctly?

A: Before the measurement, insert the two probe needles into the ground to connect the pole E, and the potential Probe needle P and the Current Probe needle C are in a straight line and 20 meters apart, P is inserted between E and C, and then E, P, and C are connected to the corresponding terminal of the instrument with special wires respectively.

During measurement, the instrument is first placed in the horizontal position to check whether the traffic gauge pointer is in the center line. Otherwise, the pointer can be adjusted to the center line by using the zero-bit regulator, then place the "signal rate scale" of the meter in the maximum multiple, slowly turn the shaker of the generator, and rotate the "measurement scale disk" to balance the flow meter pointer. When the pointer is close to the center line, speed up the speed of the generator's crank to reach more than 120 rpm, and then adjust the "measurement scale disk" so that the pointer is on the central line, multiply the readings of the measurement scale disk by the multiples of the rate scale, that is, the measured resistance value.

7. What external checks should the relay perform?

A: The relay should perform the following external checks during acceptance or regular inspection:

①. The relay housing should be intact and covered with the base; ②. Each element shall not be damaged or damaged, and shall be securely packed and neatly packed; ③ conductive parts of the screw and connecting wire parts should not be oxidized open welding and poor contact phenomenon, screws and wiring columns should be gasket and spring pad; ④, non-conductive parts such as spring, limit lever, etc., must be fixed with screws and use durable paint and seal.

8. What is the insulation absorption ratio of transformer?

Answer: During the maintenance of transformer, the insulation absorption ratio of transformer should be determined, it is equal to the ratio of the insulation resistance value measured in 60 seconds to the insulation resistance value measured in 15 seconds. That is, the ratio of R 60/B 15 to the absorption ratio can be used to determine whether the insulation is wet, contaminated, or has local defects, the procedures are as follows ~ 30 ℃, 35 ~ 60kV winding no less than 1.2, 110 ~ The 330kV winding must not be less than 1.3.

9. What are the inspection items of DX-11 type signal relays?

Answer: ①. External check; ②. Internal and mechanical tests; ③. Insulation Test; ④. DC resistance measurement; ⑤. Action value test; ⑥. Contact operation reliability test.

10, DX-11 type signal relays return value of action how to test?

Answer: The test wiring is the same as that of the intermediate relay. For the current type relay, its operation value should be 70 ~ of the rated value ~ 90%. for Voltage-type signal relays, the operation value should be 50 ~ of the rated value ~ 70%.

The return value shall not be less than 5% of the rated value. If the return value does not meet the requirements, you can adjust the spring tension or the distance between the armature and the core.

11. What is the selectivity of the relay protection device?

Answer: the selectivity of the protection device is determined by the protection scheme and Tuning Calculation. When the system fails, the relay protection device can quickly and accurately remove the faulty device, minimize the hazards caused by faults and the range of power outages, so as to ensure that non-faulty equipment continues to operate normally. The protection device can meet the above requirements, which is called selective.

12. What are the advantages of the rapid action of the relay protection device?

A: ① rapid action, that is, rapid fault removal, can reduce the working time of the user to reduce the voltage and accelerate the process of restoring normal operation; ②. Rapid fault removal, it can reduce the damage degree of electrical equipment affected by the fault; ③ it can be quickly removed to prevent the expansion of the fault.

13. What are the characteristics of the electric flow rate cut protection?

A: The duration-free electric flow rate disconnection cannot protect the full length of the Line. It can only protect part of the line. Changes in the system operation mode will affect the protection range of the electric flow rate interruption. To ensure the selectivity of the action, the starting current must be adjusted according to the maximum running mode (that is, the current of the current line is the maximum running mode), but the protection range for other running modes is shortened, the minimum protection range required by the procedure should not be less than 15% of the total length of the Line.

In addition, the length of the protected line also affects the feature of the quick-stop protection. when the line is long, the protection range is large, and it is less affected by the system operation mode. Otherwise, if the line is short, it will be greatly affected, and the protection range will be even reduced to zero.

14, DS-110/120 type time relay action value and return value how to determine?

Answer, the maximum return voltage of the entry iron is the return voltage.

For DC time relays, the operating voltage shall not be greater than 65% of the rated voltage, and the return voltage shall not be less than 5% of the rated voltage. For AC time relays, the operating voltage shall not be greater than 85% of the rated voltage, if the operating voltage is too high, adjust the spring elasticity.

15. What are the requirements for the Circuit Breaker Control Circuit?

A: the control circuit of a circuit breaker varies according to the type of the circuit breaker, the type of the operating mechanism, and the different operation requirements. However, the wiring is basically similar, the control circuit of a General Circuit Breaker must meet the following requirements:

① Closed and trip coils are designed based on short-term current. After the task is completed, the loop current should be interrupted. ②, not only can manual remote control be implemented, it should also be able to automatically trip and brake during protection or automatic device action; ③ it should have a location signal reflecting the switch or trip of the circuit breaker; ④ there should be obvious signals of manual and automatic switch and switch; ⑤ There should be a "jump" locking device to prevent multiple switches of the circuit breaker; 6. You must be able to monitor the integrity of the power supply and the next operation circuit.

16. What is the electrical "ground?

A: When an electrical device is in operation, if a short-circuit grounding occurs, the short-circuit current will be connected to the earth and formed in a semi-sphere. The smaller the half sphere, the larger the dispersion resistance, the voltage drop of the ground short-circuit current increases. So in the close to the grounding body, the half sphere is small, the resistance is large, the current here is high, and the opposite is in the remote grounding body, because the half sphere is large, the resistance is small and its potential is low. The test proves that the spherical surface is already quite large and its resistance is zero when it leaves a single grounding earth or 20 m outside the ground pole. We call the point where the potential is equal to zero as electrical and "ground ".

17. What is the short-circuit voltage of a transformer?

A: The short-circuit voltage is a main parameter of the transformer. It is measured through a short-circuit test. The measurement method is to short-circuit the transformer side and pressurize the original side so that the current reaches the rated value, at this time, the voltage VD added by the original edge is called the short-circuit voltage. The short-circuit voltage is usually expressed by a hundred points, generally, the short-circuit voltage indicated by the transformer nameplate is the percentage of the short-circuit voltage VD and the rated voltage V e of the winding pressurized during the test, that is, VD % = VDe/Ve × 100%.

18. What should I pay attention to when measuring capacitors?

A: ① when measuring with A multimeter, select an appropriate gear position based on the capacitor and rated voltage. For example, capacitor commonly used in power equipment, generally the voltage is only a few volts to several thousand volts, if you use a multimeter R × 1ck, because the battery voltage in the table is 15 ~ 22.5 V, which is likely to cause capacitor breakdown, so the r × 1 K file should be used for measurement; ② For the capacitor just removed from the line, the capacitor must be discharged before measurement to prevent the residual electric chair in the capacitor from discharging to the meter and damage the meter. ③ for capacitors with high operating voltage and large capacity, sufficient discharge should be performed on the capacitor, and protective measures should be taken by the operator during discharge to prevent electric shock.

19. What is sine alternating current? Why is sine alternating current widely used?

A: The sine alternating current (SAC) refers to the variation of the current, voltage, and potential size and direction in a circuit according to the sine function pattern of time. The current that changes periodically with time is called an alternating current, communication for short.

The AC can change the voltage through a transformer. during long-distance transmission, the voltage is increased to reduce the loss of the line and the optimal economic effect is obtained. In use, you can change high voltage to low voltage by using a step-down transformer, which is conducive to safety and lowers your insulation requirements for the equipment. In addition, compared with DC motor, AC motor has the advantages of low cost and easy maintenance. Therefore, AC has been widely used.

20. Why can't I only measure the DC power but the AC power?

A: Because the magnetic field direction of the Permanent Magnet cannot be changed, only the DC current can be used to generate a stable deflection. For example, the AC current can be passed in the measurement mechanism of the electromechanical system, the generated rotation torque is also alternating, and the movable part is too late to rotate due to inertia. Therefore, this measuring mechanism cannot directly measure the AC. (The average value of alternating current is zero every week, so there is no deflection and the reading is zero ).

21. Why should the electrical measuring instruments, power meters and relay protection devices use different secondary Coils of CT as much as possible?

A: The measurement and protection levels of domestic high-voltage CT are separated to meet different requirements of Electrical Measurement and relay protection. Electrical Measurement has high requirements on CT accuracy, and the instrument should be affected by short-circuit current, so when the short-circuit current increases to a certain value, so that the measurement level Core saturation to limit the growth factor of the secondary current, protection level core should not be saturated in short circuit, the secondary current and the primary current proportional growth, to adapt to the protection sensitivity requirements.

22. Is the CT capacity labeled kVA (VA) labeled ohm (Ω? Their relationship?

A: The capacity of CT has a standard power voltammetry, that is, the number of power kVA consumed by the second rated current through the second rated load: W 2 = I 2 Z 2, sometimes the CT capacity is expressed by the Ohm value of the Secondary load, and its ohm value is the impedance value of the entire secondary Series loop of the CT. The CT capacity is proportional to the impedance. The impedance size of CT secondary circuit affects the accuracy of CT Series. Therefore, when the CT is running, the impedance value does not exceed the capacity voltammetry and Ohm value specified by the nameplate, to ensure its accuracy.

23. Is there any relationship between the speed of the shake test on the table and the tested resistance value? Why?

A: Generally, the speed of the shake test does not affect the measurement of insulation resistance. Because the reading value on the M/O table reflects the ratio of the generator voltage to the current, when the voltage changes, the current through the M/o Current Coil also changes proportionally, so the number of resistance values remains unchanged, however, if the rotor Number of the Table Generator is too slow, the voltage is too low at this time, it will also cause a large measurement error. Therefore, the table should be shaken according to the specified rotor number when using the table. It is generally set to 120 rpm, which can be changed by ± 20%, but cannot exceed ± 25% at most.

24. Which electrical equipment must be grounded or connected with zero protection?

Answer: ① base and housing of generators, transformer, motor high and low voltage electrical appliances and lighting appliances; ②. Secondary Coils of transformer; ③. Framework of power distribution board and Control Panel; 4. Transmission Device of electric equipment; 5. Metal architecture of power distribution device outside the house, concrete frame and metal fence; 6. Cable head and cable box housing, cable outer skin and pipe threading; 7. power line tower and switch equipment and capacitors mounted on Distribution Line poles.

25. What are the requirements for Grounding of cable lines?

Answer: ① When cables are laid underground, both ends of cables should be grounded; ② low-voltage cables except in particularly dangerous places (wet and corrosive gas conductive dust) grounding is not required in other environments; ③. High-voltage cables must be grounded under any circumstances; ④. metal casings and brackets cannot be grounded, if the outer cover of the cable is non-metallic materials such as plastic rubber hose and there is an insulating layer between the cable and the bracket, the bracket must be grounded; ⑤ Single-core Cables with a section below 16 mm² should be grounded at one end to eliminate eddy current.

26. What regulations must be observed during high-pressure tests?

Answer: ① for High-pressure tests, enter the first ticket. Fill in a work ticket for both overhaul and test in an electrical connection section. However, prior to the test, the maintenance work should be permitted. In the same electrical connection part, after the work ticket for the high-voltage test is issued, the second Work Ticket is prohibited. For example, there is a sufficient safe distance between the pressure part and the maintenance part based on the test voltage, and the test can be performed on one side of the disconnection point when there is a short circuit, the other side can continue to work, but at this time there should be a "stop, high pressure danger!" (2) There shall be no less than two persons for the high-pressure test, and the test owner shall be experienced persons, before the test starts, the test owner should arrange the safety precautions in the test in detail for all the test personnel; ③ When the equipment joints need to be disconnected due to the test, the test personnel should be marked and inspected after the test; ④ The metal casing of the test device should be reliably grounded, and the high-voltage Lead should be shortened as much as possible. If necessary, the insulation should be firmly supported. The power switch of the test device should enable the double-knife brake that is obviously disconnected, to prevent false combination and switch, an insulating cover can be added to the blade. Two series power switches should be added in the low-voltage circuit of the test device, and an overload automatic brake device should be installed; ⑤ The test site should be equipped with barrier or barrier, hanging out "Stop, high pressure and danger!" Signs and sends guards. When the two ends of the device are not in the same location, another person is dispatched to guard the device. 6. Before pressurization, check carefully. Rate of the test wiring table, the regulator is in the Starting Status of the zero position and the instrument. It is correct to notify the relevant personnel to leave the device and obtain the permission of the person in charge before pressurization. The regulator should be monitored and Sung during pressurization, high-pressure test staff, in all pressurization, should focus on not chatting with others, at any time to guard against exceptions. The operator shall stand on the insulation pad; 7. When the wiring is changed or the test is completed, the test power supply should be disconnected first, and the high-voltage part of the boost equipment should be discharged and implemented through a short circuit; 1. If the device is tested with a large fused device without a ground wire, discharge the device before the test. During the high-voltage DC test, discharge the device to the ground several times for each ending section, and short-circuit grounding; when the test is completed, the test personnel should remove the self-installed short grounding line, and inspect and clean the tested equipment; and dig, special important electrical testing, detailed test measures should be taken and approved by the competent production leader (Chief Engineer.

27. Why is the transformer's original edge current determined by the secondary edge?

A: When the transformer carries load, when the secondary side current changes, the primary side current also changes accordingly. Why? According to the magnetic dynamic balance formula, the original and secondary edge currents of the transformer are reversed. The magnetic momentum generated by the secondary edge current plays a demagnetizing role in the original edge magnetic potential. That is, I 1 W 1 ≈-12 W 2. When the secondary edge current increases, the main magnetic flux in the Transformer Core must remain unchanged, and the original edge current must also increase accordingly to balance the production of the secondary edge current. This is what we see when the secondary current of the secondary side changes, the primary side current also corresponds to the principle, so the original edge current is determined by the secondary side.

28. Why does the stator current change with the rotor current when the load on the shaft of the three-phase asynchronous motor increases?

A: When the winding structure of an asynchronous motor is fixed, the size of the magnetic potential is determined by the stator current. Under normal circumstances, the current size depends on the load. When the power supply voltage increases and the load increases, the anti-torque of the motor shaft will increase, so the speed will decrease. According to the concept of "relative operation" in the basic working principle of the motor, the relative movement between the rotor conductor and the magnetic field (when the power supply voltage remains unchanged, the rotation speed will also remain unchanged) will increase, that is to say, the speed of the rotor conductor to cut the air gap magnetic field increases. Therefore, when the rotor is induced to be E2, the rotor current I2 and the rotor's magnetic potential F2 increase. It should be noted that the rotor's magnetic dynamic potential F2 does not play a degaussing effect on the main magnetic field of the stator. to offset the degaussing Effect of F2, the stator current I1 and the stator's magnetic potential F 1 will increase accordingly, therefore, the heavier the load on the motor shaft, the larger the Rotor Current I2 (of course, it cannot infinitely increase the load ). The stator current I1 also increases accordingly, so the stator current I1 changes with the Rotor Current I2.

29. Why is the measurement of small resistance accurate and high by using a double-Arm Bridge?

A: Because the two-Arm Bridge combines parasitic resistance into the error and causes the error to be equal to zero, the balance between the two-Arm Bridge is not affected by the parasitic resistance, this improves the measurement accuracy of the bridge.