Each part of the transformer has different allowable temperature rises, and different operating conditions also require the safe operation of the transformer and the detection technology of the transformer.
Allowable temperature rise of the winding: The allowable temperature rise of the winding refers to the average temperature rise of the entire winding, measured by the resistance method. The allowable temperature rise is related to the insulation heat resistance grade. Oil-immersed transformers belong to Class A insulation. Since the traditional winding temperature rise measurement method is the resistance method, the measured temperature rise is the average temperature rise. The allowable average temperature rise of Class A insulation is 65K. The average temperature rise is related to the hottest point temperature of the winding. Suppose the difference in liter is 13K. When the annual average temperature is 20°C, the hottest point temperature of the Class A insulation winding is 20+65+13=98°C. At this time, the Class A insulation has a normal life. The allowable average temperature rise of various insulation types of dry-type transformers: A-level is 60K, E-level is 75K, B-level is 80K, F-level is 100K, H-level is 125K, and C-level is 150K. The winding temperature rise in winter is lower than the average temperature If the winding temperature rises higher than the average temperature rise in summer, the winding life will be sacrificed. If the operation exceeds the nameplate capacity, life will also be sacrificed. If the nameplate capacity is exceeded, the hottest point temperature of the oil-immersed transformer's Class A insulation winding cannot exceed 140°C. Even if the life is not sacrificed, it is not allowed to exceed 140°C, because when it exceeds 140°C, the oil will decompose into gas and affect the insulation. strength. Therefore, the hottest point temperature of Class A insulation of oil-immersed transformers cannot exceed 140°C, which is determined by the safe operation of the transformer.
Large-capacity transformers sometimes have several cooling methods, such as ONAN/ONAF. The rated capacity of the transformer generally refers to the allowable value under ONAF. When the fan loses power, the cooling will increase, so the capacity of the transformer must be reduced when operating in the ONAN cooling mode. , so that the average temperature rise of the winding does not exceed 65K.
In addition, in a two-winding or three-winding transformer, two or three windings should reach the same temperature rise at the same time. When one winding reaches an average temperature rise of 65K and the temperature rise of the other or two windings is lower than 65K, such a design is uneconomical. It is most economical for the temperature rise of the top oil surface of an oil-immersed transformer and the average temperature rise of several windings to reach the allowable temperature rise at the same time. That is, when the temperature rise of the top oil surface reaches 55K (60K when the oil is isolated from the air), the average temperature rise of the windings Reach 65K. In the design stage, the current density of each winding is reasonably selected, so that the temperature rise of each winding is close to 65K while the load loss does not exceed the standard value. At the same time, the top oil level reaches 55K. However, this is a problem for strong oil circulation. The temperature rise of the oil top layer of a strong oil-air-cooled transformer is generally 40K, and the temperature rise of the oil top layer of a strong oil-water-cooled transformer is generally 35K.
In fact, it is difficult for the temperature rise of the top oil surface and the average temperature rise of the winding to reach the limit allowable value at the same time. Therefore, the average temperature rise of the winding cannot generally be judged based on the temperature rise of the top oil surface. This is also the reason why large-capacity transformers are equipped with both oil surface temperature indicators and winding hot spot temperature indicators. For example, if a single oil surface temperature indicator is installed, it is sometimes difficult to judge the average temperature rise of the windings, especially for transformers with strong oil circulation cooling.
When analyzing the temperature rise of the transformer, we should also pay attention to the temperature of the cooling medium.
Generally, the cooling medium of air-cooled transformers is air, and the cooling medium of water-cooled transformers is water.
When a closed busbar is installed, although the medium of the low-voltage bushing in the closed busbar is air, the temperature is 80°C. Therefore, the allowable temperature of the low-voltage bushing is different when it is used in the open type and when it is used in the closed busbar. Generally, the rated current of bushings used in closed busbars should be reduced because the air temperature in the closed busbar is high.
It can be seen that the allowable temperature rise of the lead, bushing, on-load tap-changer or non-excited tap-changer depends on the temperature rise of the surrounding medium. The transformer has a certain ability to exceed the nameplate capacity, and its components should also have the same ability, having a certain lifespan without affecting safe operation.
The temperature rise of the top oil surface and the average temperature rise of the winding analyzed earlier refer to the sudden load dump under steady state. At this time, attention should also be paid to the time constant of temperature change. The time constant of oil is relatively large, which means that after the load changes, the oil surface temperature changes with the change of the load for a long time.
When the load increases, it cannot be considered that the oil surface temperature does not change, and the winding temperature does not change either. The time constant of the oil is large, and the oil surface temperature rises slowly. The winding time constant is small, and the winding temperature rises quickly. If there is a winding temperature indicator, this indicator should also have a good response time and a small time constant.
To control the start of the ONAF fan, the oil surface temperature indicator cannot be relied on. It can only be controlled by the winding temperature indicator, or controlled by the bushing current transformer.
In transformers, sometimes when the loss density caused by magnetic flux leakage is too large, local overheating will occur. Sometimes there will be local overheating on the box wall near the high-current lead wire, the box cover where the high-current bushing is led out, etc. The decomposition of oil into gas due to local overheating temperatures is not allowed and will cause a decrease in reliability. Therefore, measures should be taken to change the path of magnetic flux leakage, take magnetic isolation measures or use non-magnetic materials where magnetic flux leakage is concentrated.
When the transformer is in operation, there will inevitably be a short circuit. When the transformer is short-circuited, a short-circuit current will flow. At this time, the transformer will heat up rapidly. Due to the large short-circuit current, the transformer operates under adiabatic conditions without considering heat dissipation.
The allowable temperature of Class A insulated copper wire windings during short circuit is 250°C.
In order to keep this temperature not exceeded, the allowable current density under short-circuit current must be calculated in the design so that the copper conductor does not exceed 250°C during the allowed duration.
The allowable mechanical stress of copper conductors is related to temperature. If the operating temperature of copper conductors exceeds, the allowable stress will decrease. Therefore, when the elongation is 0.2%, the allowable stress should be the allowable value at 250°C.
When studying the allowable temperature rise of the transformer, there are several points that should be paid attention to:
a. The transition resistor in the on-load tap-changer of the on-load tap-changer should be such that the oil temperature rise of the resistor does not exceed 350K under continuous operation of the on-load tap-changer.
b. When doing the temperature rise test, do not allow external heat to flow back into the transformer. This is the case if the current density of the short-circuited lead is too high.
c. Sensors can be embedded in the windings and led out by optical fibers to measure the hot spot temperature of the windings. In this way, the transformer's operating capability beyond the nameplate can be measured.
d. For transformers operating at high altitudes, attention should be paid to the difficulty in heat dissipation at high altitudes, but at the same time the ambient temperature at high altitudes will drop, and the two can sometimes be compensated.
e. The core temperature of the dry-type transformer will affect the temperature rise of the windings near the core post.
f. There should be no residual air in the radiator that has not been released. There must be a vent plug on the radiator to deflate it before the temperature rise test.
g. It is better that the heat dissipation center of the radiator is higher than the heating center. h. There should be no dead oil area under the tank cover. i. The oil entering the oil tank from the radiator or cooler must be able to flow into the winding and cannot flow in a short circuit in the space outside the winding.
j. The unit used for temperature rise test must have sufficient capacity, and capacitor compensation can be used if necessary. Chromatographic analysis of gas in the oil before and after the temperature rise test is a detection method to detect whether there is overheating, but the temperature rise test time must be long enough. Liquid chromatography analysis can also be used to detect furfural content to determine whether there is low temperature overheating.