Analysis and Prevention of Insulation Accidents of Large Power Transformers

Introduction

1. The safe operation of the transformer is threatened by insulation accidents. Therefore, in the manufacture, installation, overhaul and operation of the transformer, the safety of the transformer insulation system is taken seriously. This article focuses on the causes of transformer insulation accidents and the prevention of insulation accidents.

Detailed introduction

2 Causes of insulation accidents

2.1 Overview of insulation accidents

The insulation system of the transformer is an insulation fit problem. Reasonable insulation coordination means that the withstand electric field strength of the transformer insulation (hereinafter referred to as "field strength" is greater than the field strength of the applied field, and has a certain margin. When the insulation coordination is damaged, it will cause an insulation accident.
2.2 Insulation accident caused by loss of control of field strength

1) Long-term working voltage

The problem of long-term working voltage out of control is non-existent, but this does not mean that the field strength is not lost. Because under a certain voltage, if the electric field is distorted, the field strength will change. The cause of the electric field distortion is the suspension of the metal conductor, the sharp burr on the conductor and the accumulation of conductive dust. For example, when the high pressure bushing is not tightened during installation or loosening during operation, a suspended conductor is formed, which generates a field strength sufficient to break the oil gap, causing partial discharge and decomposing the transformer oil out of acetylene.

2) Temporary overvoltage

The power frequency voltage rises briefly or the resonant overvoltage is collectively referred to as a temporary overvoltage. When the power frequency voltage rises above the design value, the overexcitation of the core may occur. In the case of overexcitation, on the one hand, the value of the exciting current rapidly increases, and on the other hand, the harmonic component in the exciting current rapidly increases. The larger the multiple of the overexcitation, the more serious it is. The consequence is that the conductor near the core coil is locally overheated, causing insulation breakdown between the turns. Foreign literature has repeatedly reported such accidents.

3) Operating overvoltage

Transformers with voltage levels exceeding 220 kV have taken effective protection against operating overvoltages, so no damage accidents under operating overvoltage have been found so far. The field strength of the operating overvoltage of 220kV and below transformers is likely to be out of control, which is enough to cause an accident. For example, when the no-load is closed, the coil turns or the interlayer short circuit occurs; when the low-voltage side compensation capacitor is cut, the low-voltage lead is caused to discharge the fuel tank; when the switch is closed for several times, the end of the high-voltage bushing is broken.

4) Lightning overvoltage

The lightning protection on the high voltage side of the transformer is relatively sound and generally safe. However, some transformers have low lightning resistance at the medium and low pressure sides, which may cause accidents caused by lightning strikes and damage to the transformer. Such as: 220kV transformer, low voltage 35kV side has suffered multiple lightning damage accidents; 110kV three-winding transformer medium voltage 35kV side or low voltage 10kV side also suffered from lightning damage accident.

2.3 Insulation accidents caused by the drop in field strength

The decrease in the withstand field strength means that the transformer is in an abnormal state due to contamination during operation. The causes of pollution are complex, and the following three types are common:

1) Insulation is damp

Normal oil-paper insulation withstands high field strength. Under normal operating voltage, 匝 insulation is unlikely to cause breakdown accidents; but the actual situation is that transformer winding insulation accidents are caused by 匝 insulation accidents, accounting for all insulation accidents. 80% to 90% of the reason is that the oil-paper insulation has a great affinity for water, and the insulation strength will decrease linearly after moisture.
The general transformer has been tested for insulation strength before leaving the factory, so the insulation strength has reached the national standard. However, there is no guarantee that the insulation will not be damaged during transportation or during installation. Therefore, before the transformer is installed, the core inspection is generally required. The weather before the hanging core must be sunny, the relative humidity is less than 65%, and the temperature of the body is 10 °C higher than the ambient temperature. The insulation of transformer oil should meet the national standards

Due to the moisture in the insulation surface of the body during the core lifting process, in order to remove moisture, a vacuum oil filter must be used for hot oil circulation. During the hot oil circulation, the temperature of the dehydration cylinder is controlled at 70-75 °C, the temperature of the tank should be >50 °C, and the continuous time of hot oil circulation is 24h. If the requirements are not met, the hot oil circulation time must be extended. In the cold winter, in order to improve the efficiency of the hot oil circulation, if necessary, the necessary "insulation" measures must be taken on the transformer body to prevent the temperature of the transformer body from being lowered due to the low ambient temperature.
Insulation and damp accidents can be divided into sudden damp accidents and gradual damp accidents.

Sudden damp accident refers to an accident caused by splashing water or sucking in water. There are several cases: the casing “General Cap” is not tightly sealed, and the sucked moisture seeps into the coil along the high-pressure outlet, causing the central incoming line. The high-voltage coil is short-circuited between turns; when the vacuum is applied, the water is hit onto the coil, causing the high-voltage coil to burn; the water cooler leaks, causing the coil to burn out; the iron core grounding sleeve or the positioning nail pad on the fuel tank top cover is not tightly sealed, entering The water causes the coil to burn out; the casing pressures the water inside the ball, and the water is poured onto the coil during installation, causing the high-voltage coil to burn; the transformer is dampened during oil transportation, and is not found during installation. The coil is burned after being put into operation; Rinse the water in the oil inlet pipe, and feed the oil from the upper part. The water is mixed in the oil and drenched onto the body, causing insulation breakdown. When the water is accumulated in the oil conservator, the water is flushed onto the coil when the oil is replenished, causing the coil to burn. .

Gradual damp accident refers to the insulation accident caused by the overall moisture of the insulation, and the accumulation of moisture in the insulation to a certain extent with the circulation of the oil. For example, moisture accumulates along the oil channel in the enclosure, causing a dendritic discharge in the perimeter; silica gel remains in the crucible at the maximum electric field, and the silica gel absorbs water and transmits it to the crucible insulation, causing the crucible insulation to be partially damp, causing a short circuit between turns.
In summary, when analyzing insulation accidents at normal operating voltages, first consider whether there is insulation and moisture. Because the insulation margin under normal working voltage is very large, there are few other factors that damage the insulation except for moisture, which can reduce the strength of the insulation to the strength that cannot withstand the normal working voltage.

2) Metal foreign bodies

If there is a residual metal conductor on the transformer body, due to partial discharge or wear and tear, the insulation breakdown may occur in the event of overvoltage or normal operating voltage. However, such accidents occurred in transformers since the 1980s. The possibility is not great, because the partial discharge test is carried out by both the manufacturer and the site, which is very effective for detecting metallic foreign bodies.

3) Dust particles

The dust particles include conductive fine particles, semiconductive fine particles, magnetic conductive fine particles, insulating fibers, paper dust, and the like.
Conductive and semi-conductive particles (copper powder, aluminum powder, carbon powder, etc.) are arranged along the power line under the action of an electric field, causing the electric field to be distorted, thereby causing a discharge accident or a phenomenon of oil flow charging.

The magnetically conductive particles (iron ends) are arranged along the magnetic lines of force under the action of a magnetic field, which may cause a multi-point ground fault of the iron core after being arranged. This type of fault can be shattered by the inrush current method; however, shortly after the transformer resumes operation, grounding may occur again. Furthermore, the fibers drift in the oil and easily absorb moisture. When drifting between the bare conductor electrodes, a "breakdown point" is formed, which excites the breakdown discharge between the low voltage leads. After this discharge, if the conductor is not burnt seriously, the power transmission can be resumed, but after the power is restored, it is necessary to observe whether the transformer is abnormal during operation.