The internal faults of power transformers are divided into two categories:
Overheating faults and discharge faults. According to the temperature, overheating faults can be divided into low temperature overheating, medium temperature overheating and high temperature overheating. Discharge faults can be divided into high energy discharge, low energy discharge and partial discharge according to the difference of energy density. Three types.
As for mechanical failures and internal water ingress and damp, they will eventually develop into electrical failures.
Overheating failure is due to accelerated insulation degradation caused by thermal stress.
If the thermal stress only causes the decomposition of the insulating oil outside the heat source, the special gases produced are mainly methane and ethylene, the sum of the two generally accounts for more than 80% of the total hydrocarbons, and as the temperature of the fault point increases, the proportion of ethylene Will increase, severe overheating will produce trace amounts of acetylene.
When overheating involves solid insulating materials, in addition to the above-mentioned substances, a large amount of carbon monoxide and carbon dioxide are also produced. If there is no CO and CO2, it may be a local overheating failure of bare metal.
Discharge faults are insulation degradation caused by high electrical stress.
High energy discharge fault, also known as arc discharge fault, this kind of fault produces a large amount of gas and produces violent gas. It is not easy to pre-diagnose it by measuring the dissolved gas in the oil. It is often based on the gas in the oil after the fault occurs. , Analysis of gas components, diagnosis of the nature and severity of transformer faults.
High-energy discharge fault gases are mainly acetylene and hydrogen, followed by ethylene and methane; if solid insulation is involved, the content of CO is also higher; low-energy discharge faults are generally spark discharge, and the fault gases are mainly ethylene and hydrogen.
Due to its small failure energy, total hydrocarbons are generally not high; partial discharge failure gas is characterized by the largest hydrogen content (accounting for more than 85% of the total hydrogen hydrocarbons), followed by methane, and the consequence of partial discharge is insulation aging. Its development can cause insulation damage and even accidents.
Diagnosis method of transformer internal fault
1. Measure the fault characteristic gas content (analysis data) and compare it with the attention value of the dissolved gas content in the oil. If the gas concentration reaches the attention value (the attention value of total hydrocarbon and hydrogen are both 150ppm, and the attention value of acetylene is 5ppm), attention should be paid to strengthen the tracking analysis to find out the cause.
2. Although the attention value has a certain reference in reflecting the probability of failure, it is difficult to be correct only based on the analysis result of the attention value due to the influence of related factors such as the gas content in the oil, the capacity of the transformer, the operation mode, and the operating life. Diagnosing the severity of a transformer fault must never be the only criterion for determining whether the equipment is faulty.
On this basis, the influence of gas production rate and other aspects should also be fully considered, and the transformers to be diagnosed and the characteristic gases checked should be emphasized and differentiated.
Only in this way can we further determine whether the transformer has a fault based on the analysis and make a preliminary estimate of the nature of the fault.
The gas production rate is directly related to the size of the fault energy, the fault location and the temperature of the fault point. The internal condition of the transformer can be further diagnosed by measuring the gas production rate of the faulty gas.
3. In order to clarify the real cause of gas generation and avoid misjudgments caused by non-fault reasons, when the transformer is diagnosed, it is necessary to fully understand the structure, manufacturing, installation and operation, maintenance and auxiliary equipment of the diagnosed transformer. In terms of the situation, a comprehensive analysis is carried out in combination with the chromatographic analysis data in order to correctly diagnose whether the transformer is faulty.