1 Overview

The safe operation of the transformer insulation by the threat of accident, therefore, transformer manufacture, installation, maintenance and operation, the Isolation Transformer system security very seriously. This article focuses on analyzing the reasons for the accident caused the transformer insulation and the insulation accident prevention.

Insulation causes the accident 2

2? An insulation accident Overview

Transformer insulation system is a problem with insulation. Reasonable insulation coordination is transformer insulation withstand electric field strength (hereinafter referred to as "field) are greater than the role of field strength, and have a certain margin when the insulation with the damage, they cause accidents insulation 2 ? 2 role of field strength caused by the insulation out of control accidents

1) Long-term voltage

Long-term voltage control problem does not exist, but this does not mean that the role of field strength is not out of control. Because in a certain voltage, in case of electric field distortion, the role of field strength will change, causing the electric field distortion due to a suspension of metal conductor, the conductor on the cusp of the plot glitches and conductive dust collection, etc. For example: High pressure ball casing are installed or not tightened loose vibration in operation, to form a suspended conductor, producing enough to make the role of the oil gap breakdown field strength, caused by partial discharge and the acetylene decomposition of transformer oil.

2) the temporary over-voltage short-term increase in voltage or frequency resonance over-voltage referred to a temporary over-voltage. When the frequency increases the voltage exceeds the design value, the core may occur over-excitation. In the over-excitation of the situation, on the one hand the value of magnetizing current increases rapidly, the other exciting current harmonic component in the rapid increase. Over-excitation of multiple larger, more serious. The consequence of which is close to the core conductor of the coil overheating, causing inter-turn insulation breakdown. Foreign literature reported many times over such incidents.

3) Over-voltage

220kV voltage level than the operating over-voltage transformer to take effective protective measures, it has not been found in the operating voltage of the damage accident. 220kV and below the transformer over-voltage field effect has the possibility of out of control, enough to cause the accident. For example: when the load closing level occasionally coils short-circuit; low-pressure side of the cut compensation capacitor, low voltage wire to the fuel tank caused by the discharge; many different period closing, causing the breakdown and white high-pressure end of the casing.

4) lightning over-voltage

Transformer high side of the lightning protection more robust, tend to be more secure. However, some transformers, the low pressure side of the lower level of mine-resistant, resulting in lightning damage to the transformer when the accident occurred. Such as: 220kV transformers, low voltage 35kV side of the lightning damage accident occurred many times; 110kV three-winding transformer 35kV medium voltage side or low side has sent a health 10kV damaged by lightning.

2? 3 tolerance caused by decline in the insulation field accident

Tolerance field strength is the decline in the operation of the transformer due to the pollution leaving the insulation in a non-normal state. Caused by pollution are complex, common in the following three kinds:

1) wet insulation

Oil-paper insulation withstand normal field strength is high, in the normal operating voltage, the turn insulation breakdown is unlikely to occur the accident; but the reality is that the transformer winding insulation accident caused by the possibility of an accident turn insulation insulation accidents accounted for all 80% to 90%, the reason is oil-paper insulation has a great affinity for water, its dielectric strength after moisture will plummet.

Generally done at the factory the transformer insulation strength test, the insulation strength of the national standard. However, during transport or until the installation process, we can not guarantee the insulation is not damaged. So, before installing the transformer, generally required to lift the core inspection. Core shall be suspended before the sunny weather, relative humidity less than 65%, the body temperature is higher than the ambient temperature 10 ℃. Transformer oil insulation should meet the national standard in Table 1.

Table 1 Standard insulation of transformer oil breakdown voltage / kV micro-water content / ppm90 ℃ when the dielectric loss tanδ /%
≥ 55 ≤ 10 ≤ 0.5
As the core process of the body hanging insulation surface damp for get rid of moisture, heat must be carried out using a vacuum oil purifier oil recycling. Petroleum Heat oil in a heating cycle dehydration cylinder temperature control in 70 ~ 75 ℃, the tank temperature should be> 50 ℃, heat the oil circulation to 24h continuous time is appropriate, if below, to extend the cycle time in hot oil. In the cold of winter, in order to improve the efficiency of hot oil circulation and, if necessary, to take the body of the transformer to be necessary "insulation" measures to prevent the body temperature of the transformer due to lower ambient temperature is too low.

Wet insulation moisture accident can be divided into sudden accident and moisture gradient of the accident.

Sudden damp throwing incident is caused by water or smoke water accidents, the following situations: casing "the cap" Mifengbuyan, sucked into the water seepage along the outlet pressure to the coil, causing the central line The high voltage coil inter-turn short circuit; vacuum when the water hit the coil, causing high voltage coil burned; water cooler leaking, causing the coil burned; fuel tank cap on the tube or core ground pad Mifengbuyan locating pins into the water caused by the coil burned; ball-pressure water pipe, installation of water dumped into the coil, causing high voltage coil burned; transformer with oil transport in the damp, installation not found, put into operation the coil burned; not before oiling into the water will flush out the tubing and into the oil from the upper water mixed with oil poured into the device in the body, causing insulation breakdown; water storage cabinet, when the water will fill the oil into the coils, causing burning coil . Gradient of moisture is insulated overall wet accident, with the oil circulation, the local accumulation of water in the insulation to a certain extent, caused by the insulation accident. Such as: water within the oil gathers along Weiping plot, causing Weiping dendritic discharge; the largest in the electric field between the residues at the turn of the silica gel, silica gel after absorbing pass turn insulation, the turn insulation of local moisture, lead to inter-turn short circuit.

In short, the analysis of the normal operating voltage of the insulation accident, we must first consider whether there are wet insulation. Because the insulation under normal operating voltage large margin, in addition to water outside, there is little other damage to the insulation factors exist, able to withstand insulation strength of the normal operating voltage down to have lost its intensity.

2) Metal foreign body

The transformer body, such as residual metal conductors, the resulting partial discharge or insulation wear, or in the event of over-voltage under normal operating voltage insulation breakdown is likely to cause damage; but since the 1980s, such incidents occurred after the transformer is unlikely, because the manufacturers and on-site partial discharge tests are carried out for detection of metallic foreign body is very effective.

3) dust particles

Conductive particles, including dust particles, semi-conductive particles, guided magnetic particles, insulating fiber, and scraps of paper and so on.

Conductive, semi-conductive particles (copper end of the aluminum end of the carbon at the end, etc.) in the electric field along the power line will be arranged so that the electric field distortion, thereby triggering the discharge of an accident or charged oil flow phenomenon.

Guided magnetic particles (iron at the end) in the magnetic field along the magnetic field lines will be arranged, arranged after the multi-core ground prone to failure. This type of failure, the impact of current laws can be broken up; but restored to normal operation soon after the transformer, the ground may occur again. Again, the fibers in the oil drift, easy to absorb moisture, when the drift to the bare conductor between the electrodes, the formation of "easy breakdown point", to stimulate the breakdown voltage between the discharge wire. After this discharge, burning if the conductor is not serious, you can restore power, but the recovery after Power Transformer to observe whether the anomalies in the operation. 3 insulation accident prevention measures

A fundamental measure to prevent accidents insulation, from manufacturing, is in strict accordance with design requirements of the production process; from installation and maintenance, the installation and maintenance standards must be strictly enforced; from the run, the run in strictly ensure that the rated operating conditions next. In general, the transformer factory and transfer of test to prove through good insulation, but also in the running to be well maintained, the insulation is not damaged. Conversely, if the closed nature of the destruction of the transformer, the body of contaminated, even the best manufacturing quality transformers may also be damaged.