Application of infrared diagnosis technology in condition maintenance

Abstract: Through the application examples of infrared diagnostic technology in substation equipment, it is confirmed that infrared diagnostic technology is an effective detection tool for early detection of hidden dangers and defects in running equipment and timely implementation of equipment status maintenance.

Keywords: infrared diagnostic technology; substation equipment; condition-based maintenance

At present, the condition inspection of power equipment is actively carried out in the power system. To carry out condition inspection, equipment diagnosis must be carried out first. Diagnosing and discovering defects of electrical equipment under energized conditions provides a scientific basis for the condition assessment and condition inspection of substation equipment. Infrared diagnostic technology has been widely used in power systems. It is one of the effective and simple means to carry out condition inspection and find hidden defects by conducting energized tests on equipment. For heating defects such as wire clamp joints, disconnector interfaces, and internal insulation damage of primary and secondary equipment, infrared temperature measurement and imaging analysis can be used to timely handle defective equipment and prevent accidents caused by defect escalation.

1 Statistical analysis of hidden defects of substation equipment discovered by infrared diagnostic technology

In order to study the application of infrared diagnosis technology in condition maintenance of substation equipment, a statistical analysis was conducted on the substation equipment defects found by infrared diagnosis technology in Hangzhou Fuyang Power Supply Bureau from 2008 to 2009, as shown in Table 1 and Figure 1.

From the defect statistical analysis, the following preliminary conclusions can be drawn:

Infrared diagnosis found that there were many defects in the disconnector contacts, wiring connectors and other equipment. The main defect was that the contact resistance was too large, causing current-induced heating, which accounted for more than 70%.

Infrared diagnosis can also detect hidden defects inside primary equipment such as current transformers, reactors, capacitors, etc., accounting for 18%.

Infrared diagnosis has a low probability of finding defects in secondary circuits and low-voltage equipment, accounting for only 5%. Secondary equipment such as microcomputer protection, measurement and control devices, and transceivers generally have self-test functions, and when an abnormality occurs, it can mostly be discovered through device measurement, alarms, and other signaling.

2 Application examples of infrared diagnostic technology in equipment condition maintenance

2.1 Application in disconnector contacts and equipment connection points

Poor contact defects of disconnector contacts are common, and infrared diagnosis is relatively simple. The thermal image of the C-phase contact of the 220 kV positive mother disconnector of the #2 main transformer of a substation shows that the highest temperature is 110.7 ℃, and the measured temperature of the same position of the other two phases of the line is 40 ℃. After power outage inspection, it was found that the surface of the moving contact was oxidized and the operating lever was not installed in place. After adjustment and polishing, it resumed operation and the re-measured temperature was normal.

2.2 Application of the connection between through-wall insulators and conductors From the temperature measurement thermogram of the connection between the 35 kV through-wall insulator and the conductor of the #2 main transformer of a certain substation, it can be seen that the temperature of phase B is 75 ℃, phase C is 72 ℃, and phase A is 40 ℃. At that time, the current on the 10 kV side of the #2 main transformer was 290 A. After power outage inspection, it was found that the B and C phase joints of the through-wall insulators were weathered and rusted, which increased the contact resistance. After the bushing was replaced and put into operation, the temperature was re-measured and normal.

2.3 Application in case of internal defects in transformers

The load of voltage transformer TV is extremely low, mainly due to voltage-induced heating defects. Only when there is an insulation problem, abnormal heating will occur. TV failures are mainly caused by moisture, turn-to-turn breakdown, insulation degradation, etc. Usually only the overall temperature rise occurs, but there are also single-point shell connection causing point temperature to be too high, oil leakage causing three-phase temperature unevenness, etc. The infrared diagnostic thermal imaging can intuitively reflect the hidden dangers of failure.

Analysis of the infrared temperature measurement chart showed that there was an obvious hot spot inside the voltage transformer. The measured temperature of the hot spot was 45.9°C, the normal TV temperature was 26°C, the ambient temperature was 23°C, and the temperature rise of the hot spot was 22.9°C. After power outage and disassembly, it was found that the damping resistor screw inside the TV electromagnetic unit was in contact with the box wall. After adjusting the damping resistor, the unit was put into operation and the temperature was normal after retesting.

In addition to insulation defects, current transformers TA also have electrical conduction faults. In normal operation, the transformer itself has little power loss and the overall temperature rise is generally very low. Once a fault occurs, the temperature rise is very obvious. Therefore, the application of infrared diagnostic technology on TA has the best effect. In the application, it can detect internal connection faults, insulation defect faults, external connection faults, oil shortage faults, etc.

On July 11, 2008, the temperature of the B phase of the outdoor oil-filled TA of a 220 kV substation 35 kV line was 33.3 ℃, and there were no abnormal hot spots in the other two phases, with an average temperature of 27 ℃. After a power outage test, it was found that the dielectric loss of this phase exceeded the standard by 6.5%, which was significantly higher than in previous years, and the power was immediately shut down for replacement. It was determined that this was heating caused by an internal fault.

2.4 Application in Reactors

A 220 kV substation #1 capacitor reactor had a heating failure. One capacitor reactor had been burned out, and several 10 kV capacitor reactors had heated up to over 100 °C or the overall thermal temperature distribution was uneven. The cause was that the insulating paint on the reactor surface peeled off (or the hydrophobic performance decreased), causing the coil to be partially damp. After replacement, the temperature was re-measured and normal.

2.5 Application in cable joints

The three phases of the 35 kV line cable head of a substation had some abnormal conditions, but no abnormalities were found during power outage inspections and routine tests. After dismantling the equipment, it was confirmed that the welding of the outer shielding ring grounding lead was poor and the manufacturing process was poor. The heating of this part has caused part of the insulation layer to become charred. From the surface temperature gradient, the abnormal point is 10 ℃ higher than the normal temperature, but the internal disintegration situation shows that the temperature of the fault point may be higher. After the cable head was remade and put into operation, it was normal again.

The heating of cable heads is a very common problem. The breakdown of cable insulation due to overheating or poor workmanship is an important cause of cable head accidents. Therefore, the thermal imaging detection of cables is actually the detection of cable heads. The main defects of cable heads are poor external connectors of cable head outlets, poor crimping of internal connections such as cable head noses, and poor insulation of cable heads.

This type of fault generates heat that is conducted from the base of the cable head to the outside and is higher than the temperature of the cable body, but the total temperature rise will not be too high. This is usually the result of poor manufacturing process of the cable head or aging of the insulation.

3 Limitations of infrared diagnosis of power equipment faults

It should be admitted that no advanced technology can be perfect, and infrared diagnosis is no exception. At present, the main disadvantage of infrared diagnosis is that it is difficult to calibrate. Although the temperature measurement sensitivity of infrared diagnostic instruments is very high, the accuracy is greatly affected by the emissivity of the surface being tested and the environmental conditions; different testers have different detection methods, so it is difficult to obtain very accurate temperature measurement results.

It is still difficult to make an effective diagnosis for some internal defects that are not easy to observe, such as fully sealed electrical equipment, central switchgear, composite insulators, line insulators, etc., or abnormal heating failures affected by convection or other interfering heat sources, and failures inside large and complex high-voltage electrical equipment or thermal power equipment.

Since infrared thermal imaging has relatively high requirements for the measured environment, including ambient temperature, humidity, and wind speed, it is necessary to avoid sunlight, direct light, thunderstorms, fog, and snow as much as possible to prevent interference from other high-temperature radiation.

4 Conclusion

Infrared diagnosis is one of the effective means of current substation equipment condition inspection and maintenance, but mobile infrared temperature measurement also has certain limitations and needs to be gradually improved in the future development in combination with actual conditions. Infrared temperature measurement must be combined with other traditional methods, such as installing fixed infrared detection devices and pasting temperature indicating sheets, visually observing the changes in phase color paint and metal color, changes in odor, and other temperature measurement methods, plus other traditional oil chromatography and dielectric loss tests to comprehensively analyze and judge the operating status of the monitoring equipment, carry out predictive condition inspection and maintenance, and establish a complete infrared temperature measurement technology management system for electrical equipment, which plays an important role in carrying out condition inspection and maintenance of electrical equipment and improving the safety, stability and economic operation of the power grid.