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    The secret of transformer operation

    Transformers can be divided into power transformers and special transformers (electric furnace transformers, rectifier transformers, power frequency test transformers, voltage regulators, mining transformers, audio transformers, intermediate frequency transformers, high frequency transformers, impact transformers, instrument transformers, and electronics Transformers, reactors, transformers, etc.). So how does the transformer operate, and is there any secret to the operation of the transformer? See below!


    1. The regulation of the transformer's shutdown and power transmission operation sequence and the classification of transformer working status

    1.1 Operation sequence of transformer power failure

    The operation sequence of main transformer shutdown and power transmission is: first stop the load side when power is cut off, then stop the power side; when power is transmitted, first send the power side, then the load side, the reason is:

    a. In the case of multiple power supplies, power outages in the above sequence can prevent reverse charging of the transformer. If the power supply side is stopped first, the protection may malfunction or refuse to operate in case of failure, extend the fault removal time and expand the power outage range.

    b. Power is sent step by step from the power supply side. In case of failure, it is convenient to check the power supply range.

    c. When the load-side bus voltage transformer is equipped with a low-frequency load shedding device and no current lock is installed, the power supply side will be stopped first after a power failure. The low-frequency load shedding device may malfunction due to the feedback of the large synchronous motor.

    1.2 Classification of transformer working status

    The working state of the transformer after operation is divided into four states, namely, running state, hot standby state, cold standby state and overhaul state.

    Operating status: the transformer's circuit breaker and isolating switch are all in the operating status of the closing position;

    Hot standby state: the transformer only opens the circuit breaker, and the isolating switch is still in the non-operational state of the closed position;

    Cold standby state: the non-operating working state where the circuit breaker and isolating switch of the transformer are in the open position;

    Maintenance status: All circuit breakers and isolating switches of the transformer have been disconnected, and the non-operational state of safety technical measures such as installation of ground wires, hanging of signs, and setting of temporary barriers.

    2. The operating voltage of the transformer

    2.1 The scope of operating voltage

    Generally, the operating voltage should not be higher than 105% of the rated voltage of the operating tap. For special use conditions, operation with a rated voltage not exceeding 110% is allowed.

    2.2 The relationship between current and voltage

    The relationship between current and voltage is as follows:

    When load current/rated current=K, (0≤K≤1), press


    The operating voltage U is limited.

    2.3 The influence and harm of over-voltage on the transformer

    The power supply voltage increases, the magnetic flux Фm increases, so that the excitation current Im increases. The excitation current is a reactive current, so the reactive power increases and the active power allowed by the transformer decreases.

    In addition, the voltage rises and the magnetic flux increases, so that the iron core saturates and generates overexcitation, which causes distortion of the voltage and magnetic flux waveform of the transformer (formation of peak waves), and increases the high-order harmonic components, thus increasing the additional loss of the motor and the circuit, resulting in a system The resonant overvoltage will destroy the insulation of electrical equipment, and at the same time, high-order harmonics will interfere with nearby communication lines.

    For the transformer itself, the voltage rise will cause overexcitation of the transformer. The overexcitation of the transformer will inevitably cause the transformer core to overheat, aging the core insulation, reducing the life of the transformer and even burning the transformer.

    2.4 Reasons for transformer overvoltage

    After the power system is decommissioned due to an accident, part of the system’s load rejection overvoltage, ferromagnetic resonance overvoltage, improper adjustment of the transformer tap switch gear, long line end with no-load transformer or other operations, the generator frequency is less than the rated value. Increase excitation current early, generator self-excitation, etc.

    2.5 Voltage adjustment of transformer

    The operating voltage adjustment of the transformer is realized through the tap switch. The transformer tap switch settings are divided into two types: non-excitation voltage regulation and on-load voltage regulation switch.

    The non-excitation voltage regulating switch can only be tapped and adjusted under the condition of power failure. When adjusting the single-phase switch, pay attention to the three-phase shifting gear to be consistent. After the shifting, the transformation ratio test must be done to confirm the three-phase gear. Only when the bits match.

    The on-load voltage regulating switch can be tapped and adjusted under electrified conditions, and the following requirements should be observed when adjusting: the voltage should be adjusted step by step, and the tap position, voltage and current changes should be monitored during voltage adjustment.

    Three-phase transformer split-phase installation switch, or single-phase transformer group on-load tap changer, should be three-phase synchronous operation.

    When on-load tap-changing transformers are operated in parallel, their tap-regulating operations should be performed step by step in turn.

    When the on-load tapping transformer and the non-excitation tapping transformer are operated in parallel, the tap voltages of the two transformers should be as close as possible.

    2.6 Transformer capacity during voltage adjustment

    When the voltage is adjusted, the capacity of the transformer is as follows:

    The tapping is changed within ±5% of the non-excitation voltage regulation, and the transformer capacity remains unchanged; when the load voltage regulation range is large, such as ±7.5% and ±10% tapping range, at the maximum negative tapping, it is -7.5 When tapping% and -10%, due to the limitation of the wire current, the transformer capacity should be reduced accordingly. If the manufacturer has no regulations, it is usually reduced by 2.5% and 5%.

    3. Monitoring and regulation of transformer operating temperature

    3.1 Ambient temperature

    The national standard GB1094.1-1996 "General Provisions of Part I of Power Transformers" stipulates as follows:

    The highest temperature +40℃;

    The highest annual average temperature +20℃;

    The lowest temperature is -25℃ (outdoor type) -5℃ (indoor type);

    The maximum temperature of the water cooler inlet is +25℃.

    3.2 Monitoring of 2 temperatures during transformer operation

    Including top oil temperature and winding temperature (if the winding thermometer is set) two temperatures.

    3.3 The top oil temperature regulation limit

    For self-cooled and air-cooled transformers, the temperature is 95°C. In order to prevent the transformer oil from aging too fast, it is usually controlled by reducing it by 10 degrees, that is, not exceeding 85°C. Each operating unit sets an 80°C alarm. For strong oil circulating transformers, it is 85°C, usually controlled by reducing 10°C, that is, not exceeding 75°C, and 70°C alarms are set for each operating unit.

    3.4 The specified limits of winding temperature:

    If the transformer is equipped with a winding thermometer, and the temperature displayed by the winding thermometer is the temperature of the hottest part of the transformer winding, the highest limit of the winding temperature is 95~100℃ (generally, the winding temperature is 10~15℃ higher than the temperature of the top oil layer. The temperature is controlled according to the limit value of 85℃, and the temperature of the winding is controlled according to the limit value of 95~100℃), and the alarm is usually set at 90~95℃.

    3.5 Regulations on the temperature rise limits of various parts of the transformer

    According to the national standard GB1094.2-1996 "Temperature Rise of Power Transformer Part Two":

    Temperature rise limit = maximum temperature-ambient temperature

    3.5.1 The temperature rise limits of various parts of the transformer are as follows:

    -The temperature rise of the top oil is limited to 55K (60K for full seal)

    -Transformers with strong oil circulation are specified as 40K

    -The temperature rise limit of the coil is 65K

    -The inner metal surface of the core and transformer is 80K

    -At the junction of the bushing terminal, the temperature rise in the air is not more than 55K, and the temperature rise in the oil is not more than 15K

    3.5.2 The temperature rise limits of the top oil and windings of the transformer are as follows:

    The transformer designed in accordance with the national standard GB1094, the normal life withstand temperature of Class A insulation is 98 ℃, the annual average temperature of the guaranteed normal life is 20 ℃, and the hottest point of the coil and the average temperature difference of the coil is 13K, so the coil temperature rise limit is 98 -20-13=65K. The maximum oil temperature for normal oil operation is 95℃, and the maximum temperature is 40℃, so the top oil temperature rise limit is 95-40=55K.

    3.5.3 The temperature rise regulations for high altitude or areas where the ambient temperature exceeds the specified requirements:

    When operating in areas with an altitude higher than 1000m, the average temperature rise of the windings will reduce 1K for each 400m increase in the natural cooling transformer (AN), and 1K for each 250m increase in the air-cooled transformer (AF).

    When the ambient temperature exceeds the monthly average temperature of 30℃ or the annual average temperature of 20℃, the temperature rise limit of the top oil, winding, and iron core of the transformer should be reduced according to the value of the exceeding part.

    3.6 Regulations on the control settings of the thermostat

    a. The control setting of the top oil temperature (the user can adjust it by himself), if the user does not require it, it can be set as follows:

    At 80℃, temperature alarm (70℃ alarm for strong oil circulation)

    At 45°C (GB/T6451 stipulates 50°C), the air-cooled radiator fan stops (note: strong oil-air cooling transformers cannot be fully stopped).

    At 55°C (65°C specified in GB/T6451), the air-cooled radiator fan starts, and the auxiliary cooler of the forced oil air cooler is started.

    At 105°C, the transformer trips.

    b. The control setting of the winding thermometer (the user can adjust it by himself), if the user does not require it, it can be set as follows:

    The temperature alarms at 90~95℃.

    At 70°C, the fan of the air-cooled radiator is activated, and the auxiliary cooler of the forced oil air-cooler is activated.

    At 115°C, the transformer tripped.

    3.7 Relative thermal aging rate of transformer

    For transformers designed according to GB1094, the aging rate is related to the hot spot temperature of the winding. Under the rated load and normal ambient temperature, the common reference value of the hot spot temperature is 98°C. The transformer load guideline stipulates the relative aging rate at this temperature (98°C) Equal to 1. The relative thermal aging rate at a certain temperature is equal to the ratio of the thermal aging rate at that temperature to the thermal aging rate at 98°C, which is called the relative thermal aging rate at 98°C, namely:


    γ=heat aging rate at θ temperature/heat aging rate at 98℃

    Where θ——the temperature value during operation ℃

    γ——Relative thermal aging rate

    It can be seen from the formula that for every 6°C increase in temperature on the basis of 98°C, the aging rate doubles (the aging rate at 98°C is 1) and the lifespan is halved. This is the famous 6-degree rule, and its change rule is as follows (press 6 The degree of regular changes):


    4. Operating regulations under different load (load) conditions of the transformer

    4.1 Transformer load classification

    The transformer runs with load (load), and it is impossible to always carry the rated load invariably. Its load characteristics are divided into three states (types): normal periodic load, long-term emergency periodic load, and short-term emergency load.

    Normal periodic load: It is equivalent to the rated load used in the transformer design. Its periodic high and low load changes can be compensated for each other, that is, the ambient temperature is higher or exceeds the rated current during a certain period of time, but it can be used in other periods Compensated by the lower ambient temperature or lower than the rated current, the thermal aging can be equivalently compensated.

    Long-term emergency cyclical load: The transformer is required to operate for a long time at a high ambient temperature or exceeding the rated current. This load mode may last for several weeks or months, which will accelerate the aging of the transformer, but does not directly endanger the insulation strength. Safety.

    Short-term emergency load: The transformer is required to run at a large current exceeding the rated current in a short time. This load may cause the hot spot temperature of the winding to reach a dangerous level and temporarily reduce the insulation strength.

    4.2 Operating regulations for different loads of transformers

    a. Operating regulations for normal periodic loads:

    It can run periodically over rated current (when the aging factor is less than or equal to 1).

    When the transformer has serious defects, it is not easy to run beyond the rated current.

    Under normal cyclical load, when running at over-rated current, the allowable excess load factor K2 and time shall be in accordance with GB/T15164 "Load Guide for Oil-immersed Power Transformers". Check the load diagram in Article 15 of Part Three ( Figure 9-Figure 12 graphs), there are 8 graphs in each graph, each graph is the relationship between the load factor K2 and time at different temperatures and the corresponding K1, namely -25, -20,- 10, 0, 10, 20, 30, 40 ℃ load diagram at eight temperatures.

    b. Long-term emergency cyclic load operation regulations:

    The operation of the transformer under this load mode will accelerate the aging of the transformer. Although it does not directly endanger the safety of the insulation, it will shorten the life of the transformer to varying degrees. This load mode should be minimized. The average relative aging rate at this time is greater than 1 (or even much greater than 1).

    Under the long-term emergency cyclical load, the load factor K2 and the time allowed to exceed the rated current are in accordance with the provisions of GB/T15164 "Load Guide for Oil-immersed Power Transformers". Check the load table (table) in Article 16 of Part Three. 7—Table 30). A total of 6 durations of daily life loss tables for 4 types of transformers are given.

    According to these daily life loss tables, the life loss of the transformer during the emergency load cycle can be obtained, and the load diagrams of K1 and K2 can also be obtained to calculate or find the allowable excess load factor K2 and time.

    c. Operating regulations for short-term emergency loads:

    The short-term emergency load operation makes the transformer operate at a large current exceeding the rated current in a short time. This load method may cause the hot spot temperature to reach a dangerous degree, and the relative aging rate is far greater than 1. Therefore, it is necessary to compress the load and reduce the time as much as possible. Generally, it is not allowed to exceed 0.5h. The short-term first aid load factor K2 within 0.5h can be included in the long-term first aid periodic load table or in the table in the operating regulations "DL/T572-95" Found in 3.

    4.3 When the transformer is running beyond the nameplate value (rated value), the regulation of the maximum current and temperature limit

    Under the premise that the transformer meets the operating regulations for different loads in Article 4.2, the current and temperature limits should also be paid attention to, that is, when the transformer is operated beyond the rated load specified on the nameplate, its current and temperature cannot exceed the maximum limit specified in the following table .


    5. Operating conditions of air-cooled radiator and strong oil cooler in transformer operation

    5.1 Operating conditions of air-cooled radiators

    Normally, when the top oil temperature reaches 55℃, the fan is switched on (GB/T6451 stipulates 65℃), and when the temperature drops to 45℃ (GB/T6451 stipulates 50℃), the fan stops; the current reaches 70% of the rated current (GB/T6451 stipulates 2/ 3) Turn on the fan when the current is lower than 50%, stop the fan. When the winding temperature reaches 70℃, the fan is put in, and the fan input and stop conditions can also be set by the user.

    After the oil-immersed air-cooled transformer stops the fan, if the temperature of the top oil layer does not exceed 65°C, it is allowed to run with the rated load.

    5.2 Operating conditions of strong oil-air cooler

    The coolers used in the strong oil-cooled transformer are divided into three types according to their roles in operation: working coolers, auxiliary coolers and backup coolers.

    It is generally stipulated during the normal operation of the transformer that when the load is below the rated load, the number of working coolers put in is determined by the following formula:


    Among them: N1-number of work stations

    β——load rate (actual load ratio rated load)

    N-total number of units

    N3-Number of spare units

    P0——Transformer no-load loss (kW)

    PK——Transformer load loss (kW)

    P——Total loss of transformer (kW)

    During the operation of the transformer, when the transformer load is low, the number of coolers required to be put in should be calculated according to the size of the load. The calculation method can also be carried out according to the following formula.


    Among them: Sn——The load (kVA) that the transformer can carry when n sets of coolers are turned on

    SH——The rated capacity of the transformer (kVA)

    Pk——Transformer load loss (kW)

    P'k——The allowable load loss when the transformer turns on n groups of coolers (kW)

    Where P'k=nP-P0

    n——The number of cooler groups opened

    P0——Transformer no-load loss (kW)

    P——The load of each group of coolers when the transformer is running (kW)


    N-the number of installed coolers

    When the rated load is more than 75%, or the top oil temperature reaches 55℃, or the winding temperature

    The primary winding of the step-down transformer is




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