Under the action of heat, insulation materials will gradually decompose and age, until they lose their mechanical strength and insulation performance. Even in the event of system failure or normal operation, the insulation of transformers will be damaged, resulting in short circuits between the conductors of the damaged insulation parts and transformer failure.
Insulation materials all have a certain level of heat resistance, and the heat resistance levels of different insulation are shown in below table.
Thermal resistance level of insulation
| Thermal resistance level of insulation | Y | A | E | B | F | H | C |
| long-term operating temperature/℃ | 90 | 105 | 120 | 130 | 155 | 180 | ＞180 |
The insulation of oil immersed transformers belongs to Class A insulation,the long-term average operating temperature of its winding should not exceed 105 ℃.
The long-term operating temperature of transformers is specified in the standard GB1094.1-1996 "Power Transformers Part 1 General Principles". The normal operating conditions for oil immersed transformers are as follows:
Altitude: not exceeding 1000m;
Ambient temperature Maximum ambient temperature +40℃;
The average temperature of the hottest month +30 ℃;
Maximum annual average temperature +20℃;
Minimum temperature -25 ℃ (applicable to outdoor transformers);
Minimum temperature -5 ℃ (applicable to indoor transformers);
The maximum temperature of the cooling water at the inlet of the water cooler +25 ℃.
The standard GB1094.2-1996 "Power Transformers Part 2: Temperature Rise" specifies the temperature rise of oil immersed transformers
Top oil temperature rise
Transformer oil not in direct contact with atmosphere 60K ;
Transformer with direct contact between oil and atmosphere 55K;
The average temperature rise of the winding (measured by resistance method) 65K.
For electrical connection wires outside the iron core and winding, or structural components in the oil tank, no temperature rise limit is specified, but temperature rise is still required. It should not be too high, usually not exceeding 80K, to avoid thermal damage to adjacent components or excessive oil aging.
Standard GB/T 1094.7-2008 "Power Transformers - Part 7- Load Guide for Oil Immersed Power Transformers" for winding average temperature rise of the group (measured by resistance method) and the hot spot temperature rise of the winding are assumed to be 65K and 78K, respectively, which is the ratio of the hot spot temperature rise of the winding average temperature of the winding increases by 13K. The lifespan of transformer insulation is determined by the hot spot temperature of the winding, and the winding is subjected to this temperature rise condition.
The working environment temperature of the transformer complies with the provisions of GB 1094.1-1996 "Power Transformers Part 1 General Principles"
If it is fixed, the transformer has a specified working life.
If the ambient temperature is+40 ℃ and the average temperature rise of the winding is 65K, then the average temperature of the winding is (40+65) ℃=105 ℃,
This value corresponds to the long-term operating temperature of Class A insulation. At this time, the hot spot temperature of the winding is (40+65+13) ℃=118 ℃,it will be larger than 105 ℃.
The transformer allows the winding to operate at a hot spot temperature of 118 ℃, which varies from the ambient temperature within a year and every day,the relationship between the expected lifespan of oil immersed insulation in transformers and temperature is that when the temperature changes by 6 ℃, the insulation lifespan doubles. That is, when the temperature rises by 6 ℃, the insulation lifespan decreases to half of its original value; On the contrary, if the temperature decreases by 6 ℃, the insulation life will be doubled. Considering that the ambient temperature of the transformer varies, the standard specifies a maximum annual average temperature of 20 ℃, corresponding to an average winding temperature of (20+65) ℃=85 ℃, and a winding hotspot temperature of 98 ℃. When the ambient temperature is greater than 20 ℃, the hot spot temperature of the winding is greater than 98 ℃, and the insulation life decreases; When the ambient temperature is less than 20 ℃, the hot spot temperature of the winding is less than 98 ℃, and the insulation life is improved. Therefore, under normal temperature changes, transformers that meet the temperature conditions in GB1094.1-1996 "Power Transformers Part 1 General Principles" and GB1094.2-1996 "Power Transformers Part 2 Temperature Rise" have a specified thermal life.
