Corona discharge detection using an ultraviolet imaging camera

July 15th, 2016, Published in Articles: EE Publishers, Articles: EngineerIT

 

By using a high sensitivity ultraviolet radiation receiver, corona and ultraviolet radiation can be recorded to evaluate equipment condition through data processing and analysis.

With the likely future increase in power demand and expansion of the grid, failures on high voltage equipment could increase the necessity for more preventative maintenance actions. Distribution equipment in high voltage yards will encounter problems like the reduction of insulation performance and structural defects resulting in partial discharge.

When corona and partial discharge phenomena occur, the discharge will emit large amounts of ultraviolet radiation which can be used as an indirect assessment of insulation condition of operating equipment and the detection of insulation defects. Presently various methods are used for discharge diagnosis, of which the optical method is the best with the highest sensitivity, resolution and suitability in all conditions.

By using a high sensitivity ultraviolet radiation receiver, corona and ultraviolet radiation can be recorded to evaluate equipment condition through data processing and analysis. A UVSee TD90 camera system was used for corona detection at a Chinese State Grid Corporation site. This article will elaborate on the use of the thermal imaging camera in the power industry based on the phenomena encountered.

Fig. 1: High voltage line – partial discharge. Fig. 2: Discharge caused by an insulator crack.

Fig. 1: High voltage line – partial discharge.
Fig. 2: Discharge caused by an insulator crack.

The principle of corona discharge detection using an ultraviolet imaging camera

When ionisation discharge occurs on high-voltage equipment, a corona, flashover or arc can be produced because of the difference in electric field strength or a high pressure difference of about 30 kV/cm. In the ionisation process, the electrons in the air continuously attract and release energy. When the electrons release energy, which can be called discharge, it will emit light and sound waves, ozone, ultraviolet, traces of nitric acid and more. UV imaging technology was developed to detect these ultraviolet signals produced during the discharge process. The technology further enables the detection of the location and intensity of the corona after data processing, imaging and overlay with visible light images. Amongst all emitted light frequencies, UV waves are proven to be the only efficient way for this type of testing. The UV wavelength range is usually 40 – 400 nm. Sunlight also contains ultraviolet, but due to the Earth’s ozone layer it absorbs a part of the long wave ultraviolet so the wavelength of solar ultraviolet reaching the earth’s surface is in fact more than 300 nm.

The main component of air is nitrogen. While the majority of the ultraviolet spectrum generated by ionisation of nitrogen in the region of wavelengths ranging from 280 to 400 nm, only a small part of the wavelength is less than 280 nm. Ultraviolet light waves less than 280 nm is in the solar blind area. If it can be detected, then it can only be from the Earth’s radiation. The principle of a camera such as the TD90 ultraviolet imaging camera is to use the solar blind interval by installing special filters, whereby the instrument can work between the wavelength 240 – 280 nm of ultraviolet during the day, so that even in daytime the corona can be observed. Some of the ultraviolet imaging cameras can only be used in a certain time of the day or in the evening due to the UV interference from the sun.

The corona is generally produced in sine wave crests or troughs, and the corona of high voltage equipment in the initial stage of discharge is not continuous, but fleeting. Ultraviolet imaging cameras enables the corona observation with two modes, according to the characteristics of the corona. One is a real-time monitoring pattern, observing the discharge of equipment in real-time, and displaying of a numerical value showing the proportional relation with the total amount of ultraviolet photons in certain areas, which will make it easy for quantitative as well as comparative analysis. The other one is an integrated mode, which displays and retains the amount of ultraviolet photons in a certain time and certain area (the region can be adjusted) on the screen, and is updated in real-time according to the first-in first-out and the dynamic average principle. In this mode, the shape and size of the discharge area of the device can be clearly seen if the instrument is properly adjusted.

The main factors affecting UV detection

The ultraviolet photon number observed by the UV camera is used as the intensity index of the corona of the target object. The ultraviolet photon count value is influenced by many factors. The detection distance, the humidity of the atmosphere, ambient temperature, pressure, altitude and the gain setting of the instrument all will have a direct impact on the measurement of the photon counter.

Detection distance

The detection distance has a significant effect on the results. When the detection distance is increased, the field of view will be reduced accordingly, with a corresponding reduction in sensitivity.

Humidity and contamination

The tests showed that the electric field intensity decreased with the increase in humidity. As the humidity increases, the surface conductivity of the insulator is enhanced, so it will be much easier for the discharge. During the tests it was found that the increased atmospheric humidity will absorb more ultraviolet light so that the detection of ultraviolet radiation is reduced. Humidity plays an important role and ways around this problem must be found to allow for analysis and recommendations. From the contamination tests, it is concluded that discharge activity of healthy insulators are greatly influenced by the surface humidity. When the relative humidity is more than 95%, the discharge times of the insulator are more than the low value of discharge times of the insulator strings, but in the dry condition (relative humidity less than 75%), the situation is the opposite.

Fig. 3: An aging insulator steel cap. Fig. 4: Discharge caused by a loose connection. Fig. 5: Tip discharge of a trap.

Fig. 3: An aging insulator steel cap.
Fig. 4: Discharge caused by a loose connection.

Effects of air pressure and temperature

When the air pressure is lowered or the temperature is increased, the air density is reduced which means the average distance between the air molecules is increased. The free path of electrons increases accordingly, so it will attract more energy in the electric field, making the air easy to be ionised, reducing the corona electric field intensity. Also, if the pressure rises or temperature decreases, the electric field intensity of the corona is increased.

Effects of altitude

When elevation increases, the pressure will decrease so the voltage which causes the initial corona drops. With the elevation increases of 2000 m, the initial corona voltage was reduced by 30%.

The influence of the gain of the detector

In corona spectra, the UV part accounts for a relatively low percentage and it encounters further losses through the optical transmission system, so the UV photon number eventually reaching the photosensitive device (CCD) is low, accounting for about 3% of the total amount received by the lens. In order to improve the sensitivity of the detector, the ultraviolet photons entering the optical system is under a gain process, so that the weak corona phenomenon can be observed. When in use, the gain of the camera is regulated to a reasonable value, so that the instrument can be sensitive to the corona, while the influence of the background interference can be reduced.

Case study: The Chinese State Grid Corporation

Fig. 5: Tip discharge of a trap.

Fig. 5: Tip discharge of a trap.

On 12 November 2012 the State Grid Corporation used the UV camera model UVSee TD90 for routine inspection of a 110 kV high voltage line and found very clear partial discharge as in Fig. 1.

The integrated mode was applied during the test with a gain of 62. It was found that the discharge image was very clear. In additional tests on the discharge area, the result was a discharge amplitude of 34 dB. According to the Charge Monitoring Guide of Hubei Electric Power Company the allowable amplitude should not exceed 20 dB and can be viewed as very serious. In this case the cable is contaminated with various elements which leads to a corona discharge. If the duration of power corona is relatively long it will cause the failure of cable insulation and ultimately lead to cable failure.

Additional applications can be seen in Figs. 2 to 5.

Conclusions

A UV imager can play an important role in the defect detection of high voltage equipment. Corona discharge can be detected by an ultraviolet imaging instrument which has the capability to pinpoint the actual location of the discharge through its high resolution measurement capability. Ultraviolet imaging is a relatively new technology, but it is easy to use, efficient, intuitive and the live monitoring makes detection and maintenance easier. There are many more applications in the power industry that can be exploited with this technology. Corona discharge detection is only one of them.

Contact Christo van der Walt, Engineering Dynamics, Tel 012 991-3168, christo@edprevent.com