Photovoltaic power plant system equipment

What is the main transformer?

What is the main transformer?

The main transformer usually boosts the AC current sent by the box-type transformer in the photovoltaic area to a voltage level of 110kV and above, and merges it into the public grid according to the grid-connected voltage level. Photovoltaic power plant equipment also requires the grid to maintain normal operation at night, and the main transformer is used to reduce the voltage to make the equipment in the plant operate normally. For small and medium capacity photovoltaic power plants connected to the grid at a voltage of 35kV, the main transformer is not a necessary system component.

A. Main transformer operation requirements

1. General inspection of main transformer trial operation
(1) When performing shock closing, the neutral point must be grounded.
(2) When the main squeezer is switched on for the first time, it can be switched on by a full voltage surge. If possible, the voltage should be boosted from zero; when the switch is switched on by shock, the main transformer is switched on from the high voltage side.
(3) The main transformer should be closed for five times of room load full voltage impulse, and there is no abnormal condition. The duration after the first power-up should not be less than 10min; the excitation current should not cause the protection device to malfunction.
(4) Several main transformers with on-load voltage regulation are generally not considered for parallel operation. If the load needs to be connected in parallel temporarily, the phase should be checked and the wiring group should be checked before the main transformers are paralleled. The tap changer is at the same position and the pressure difference is within the specified range.
(5) After electrification, check all welds and connecting surfaces of the body and accessories, and there should be no oil leakage.

2. Operational requirements for the commissioning and shutdown of the main transformer
(1) When the main transformer is put into operation, the power supply side with complete protection and small inrush current should be selected for charging. When the power fails, stop the low-voltage side first, and then stop the high-voltage side.
(2) When closing the no-load transformer, the neutral point of the operating side (high-voltage side) must be grounded.
(3) The commissioning and shutdown of the main transformer must be carried out with a circuit breaker.
(4) When the neutral grounding methods of the two main transformers in the large current grounding system are interchanged, the neutral grounding isolating switch of the transformer to be grounded should be closed first, and then the neutral grounding isolating switch of the original grounding transformer should be opened.
(5) Newly built or overhauled main transformers can only be operated in parallel with other transformers after phase verification is correct.
(6) When switching the main transformer, check that the input transformer has been loaded with a load before allowing the withdrawal of the transformer that needs to be shut down.

3. Putting into operation of the main transformer
(1) Parallel operation of main transformers should meet the following conditions:
1) The wiring group is the same.
2) The phase sequence and phase are the same.
3) Equal voltage ratio (allowable difference of 5%)
4) The short-circuit voltage is equal (10% difference is allowed).
(2) Before putting the main transformer into operation, the operating personnel should do the following inspections and make sure that the main transformer is in good condition and has live operating conditions.
1) The main transformer body has no defects, and the appearance is neat and tidy.
2) The oil level and oil color are normal, and there is no oil leakage or oil leakage.
3) The electrical test of the main transformer should be recorded and qualified.
4) The cooling device is normal, and the oil pipe passage valves should be opened.
5) The casing is clean, no cracks, the oil level and oil color are normal, and the lead wires are not loose.
6) All kinds of screws should be tightened, the main transformer shell should be reliably grounded, and the grounding resistance should be qualified.
7) There is no inflation or jam in the Buchholz relay.
8) The position indication of the on-load voltage regulating tap switch is correct, and there is no jamming of manual and electric voltage regulation.
9) The thermosyphon should not absorb moisture, it should be sky blue normally (white silica gel cannot be used), and the pipeline valve should be opened without clogging.
10) The pressure release device test should meet the requirements of the “Electrical Safety Work Regulations”.
11) The fixed value of the relay protection and the position of the connecting piece should meet the requirements. 

What is the main transformer?
main transformer

B. Main transformer operation and maintenance specifications

1. Inspection of the main transformer
(1) Normal patrol inspection items of the main transformer.
1) The sound of the main transformer should be normal.
2) The oil level and oil color of the oil pillow and casing are normal, and there is no oil leakage.
3) Whether the silica gel of the respirator has changed color.
4) The casing should be free of damage, cracks and discharge marks.
5) The lead joints of the casing are in good contact, and there is no heat generation.
6) The Buchholz relay is filled with oil and no gas.
7) The main transformer measuring meter should indicate correct and no abnormal phenomena.
8) The pressure regulating device is normal, the indication is correct, the secondary circuit is good, and the damp resistance is normal.
9) Whether the main transformer terminal box is tightly sealed, and whether it is dry and clean.

(2) Special inspection of the main transformer.
In one of the following situations, maintenance personnel should conduct special inspections of the main transformer to increase the number of inspections.
1) According to the specific conditions of the site (dust, pollution, heavy fog, icing, etc.), the number of inspections should be increased. In windy weather, check the leads and whether there is any hanging debris. In foggy weather, check the porcelain bushing for discharge. In snowy weather, check whether the joints are hot according to the melting of snow, and deal with the icicles in time.
2) After a thunderstorm, check the casing for discharge, and check the action of the whistle avoider and the protective gap.
3) In the event of a sudden climate change (cold or hot), additional inspections should be carried out on the oil level of the main transformer.
4) When the gas relay sends out a signal, perform body oil level and external inspection.
5) When overloaded, the number of inspections should be increased. Focus on monitoring changes in load, oil temperature and oil level, good contact of lead joints and no heat generation, and cooling system operation.
6) After a short-circuit failure, focus on checking whether the contacts of the relevant equipment are abnormal.

2. Maintenance of the main transformer
(1) During the first month of operation of the main transformer, take oil samples every week for pressure breakdown test. If the pressure value of the oil drops by 15% to 20% from the factory test value, the oil should be filtered at this time; If the withstand voltage of the oil is lower than 35kV2mm, the main transformer must be stopped immediately. In the process of oil filtering, if there are black carbides on the surface of the filter paper, the body inspection must be carried out. The inspection requirements are the same as the procedures and the acceptance inspection of the body. Find out the point of failure and eliminate it in time.
(2) For the main transformer in operation, take an oil sample for color and gas chromatographic analysis. Analyze the composition and content of the gas in the oil to determine whether the main transformer is faulty and the nature of the fault. If it is judged from the gas content in the oil that the safe operation of the main transformer will not be dangerous for the time being, the main transformer can continue to operate, but the monitoring must be strengthened at any time. If the fault is serious, measures must be taken and a handling decision must be made.
(3) There is an iron core grounding bushing on the top cover of the main transformer tank. The grounding bushing can be used to monitor the body insulation. Open the grounding wire of the grounding bushing for measurement. Take care to avoid instantaneous open circuit during measurement.
(4) Accidental oil draining, an accidental oil draining valve with a glass plate is installed on the lower fuel tank of the main transformer. This glass plate is usually used as a gate valve sealing cover. If an accident occurs and the oil must be discharged urgently, the main transformer oil in the oil tank can be quickly discharged by smashing the glass plate immediately.

3. Operation and maintenance of main transformer accessories
(1) Pressure relief valve.
1) After the pressure release valve in operation is activated, the mechanical and electrical signals of the release valve should be reset manually.
2) The pressure release valve has oil leakage, and measures should be taken to solve the problem. The main cause of oil leakage is generally high pressure in the tank due to some reason, which has exceeded the sealing pressure of the release valve, but has not yet reached the opening pressure, causing leakage. leak. Just eliminate the factors that increase the pressure.
3) Some of the three types of sealing rings in the valve have aged and failed, and the failed rubber rings should be replaced in time. Foreign matter on the sealing surface should be removed in time without adjustment.
4) Take advantage of the opportunity of each power outage maintenance of electrical equipment to perform the following maintenance on the pressure release valve: whether the opening action is sensitive, and if there is jamming, remove it; remove foreign objects in the valve; whether the sealing rubber ring is aging, deformed or damaged; Whether it is corroded, deformed or damaged; whether the signal switch action is flexible.
5) The rubber ring of the pressure release valve must be replaced every ten years from the date of leaving the factory to avoid oil leakage or even failure of the release valve due to the aging of the rubber ring.

(2) Maintenance of gas relay.
1) The relay should be inspected once a year for visual inspection and the reliability of the signal circuit and the reliability of the trip circuit.
2) The operating relay should be opened once a year to check the internal structure and operation reliability.
3) A power frequency withstand voltage test shall be carried out every five years for the relays that have been in operation.

(3) The use of gas box.
When you need to collect gas, first remove the cover, and then gently rotate the screw plug or needle valve of the lower gas plug to gradually release the main transformer oil in the gas box, and close the plug when the oil level drops to the required amount of gas Or a needle valve, and collect gas from the upper gas plug in the same operation method. After the collection is completed, the gas box should be filled with main transformer oil to ensure the authenticity of the next sampling, and the disassembled parts should be reinstalled.

(4) Maintenance of cooler.
In the long-term operation, the surface of the air inlet is clogged by insects, dust, branches and leaves and other debris, which reduces the cooling capacity. Therefore, the cooling tube clusters should be cleaned during the regular inspection of the main transformer shutdown.

(5) Main transformer temperature controller.
The main transformer temperature controller is mainly composed of elastic elements, capillary tubes, temperature bulbs and micro switches. When the bulb is heated, the volume increase produced by the thermal expansion of the temperature-sensitive medium in the bulb is transferred to the elastic element through the capillary tube, causing the elastic element to produce a displacement. This displacement indicates the measured temperature and drives the micro The switch works to control the input or withdrawal of the cooling system.
1) The temperature controller must be inspected in accordance with the verification procedures for pressure thermometers. That is, the temperature bulb must be completely immersed in the constant temperature bath, and the meter head must be installed vertically.
2) When installing on site, it must be noted that the bulb should be fully inserted into the oil-filled casing, and the depth of the bulb into the oil-filled casing should not be less than 150mm.

(6) Maintenance of on-load tap changer.
Before the newly installed or overhauled on-load tap-changing main transformer is put into operation, the operating personnel and the construction personnel should conduct the following inspections:
1) Review the inspection, commissioning and handling reports of the construction unit.
2) Perform visual inspection (including oil level, seal, dustproof, rainproof, temperature control, mechanism transmission device, gas relay, explosion-proof device, electrical control circuit, etc.)
3) Manually operate a cycle (from 1→N and from N→1), the construction personnel will demonstrate, and explain to the duty personnel the number of revolutions of each gear switching operation, and check that the position relay, stroke indicator, and counter are correct; The mechanical interlocking, manual and electric interlocking of the extreme positions should be reliable.
4) Two cycles of electric operation. First operate a cycle on site, and then operate a cycle in the control room. Check that the terminals of each part of the electrical circuit are in good contact, the contactor is reliable in action, and the electrical lockout of each indicator and limit position should be correct and reliable.
5) The gas relay of the on-load switch has passed the inspection (flow rate is 1.4/s±10%), its heavy gas contact should be tripped, light gas contact should be connected to the signal, check the trip function of the gas relay, press the trip The test button should be able to cut off the power supply of the main transformer.
6) Check the aging of each joint position of the main transformer, and the DC resistance is qualified; check the insulation oil of the on-load switch should meet the standard.
The patrol inspection of the on-load tap changer should be carried out at the same time as the patrol inspection of the transformer. The items are as follows:
1) The voltage indication should be within the specified range.
2) The device indicator should be consistent with the position of the tap changer.
3) The switch has a tube oil level, the oil color and the moisture absorber should be normal.
4) There is no oil leakage in the switch box and gas relay.
5) Operators should check the sealing condition of the control box once a month. After heavy rain, check whether water has entered in time. The lubricating oil in the gearbox of the drive motor and the sector gear should be kept on the oil level line without leakage.
6) In the drive control box, the operator should clean it once a month (preferably using a vacuum cleaner), and focus on inspections and handle the important parts of the AC contactor and terminal block.
7) In spring and winter, the control box of the on-load voltage regulating device should be put into the tide resistance.

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