The Effects of Corona On Insulators
ME Special Project
Lorraine Shave
Breakdown of insulators on power lines is a problem many utilities have. This can lead to power interruptions and early replacement of parts. The breakdown can be attributed to localized discharges called corona. Corona can be defined as a type of localized discharge that results from high, non-uniform electric fields. It causes deterioration of the insulator and sometimes causes complete breakdown. Proper design can reduce how often corona occurs, thus creating better protection and longer life of the insulator.

Corona can be detected in a number of ways. The obvious ways are by sight and sound. At high voltages, corona produces visible light, and audible noise. Corona can also be detected by using various types of measuring equipment. When observed, corona may first appear as a faint glow. As the current and potential are increased the glow spreads eventually producing streamers. The glow may have a paintbrush like appearance. The audible noise that corona causes can be described as crackling or hissing sounds. The noise is caused by explosion gas expansions (Teixeira, p7). The visible light and audible noise can be observed simultaneously.

The voltage at which corona begins in called the discharge inception voltage. Conversely, the voltage at which corona ceases is called discharge extinction. Finally, using the partial discharge detector, the actual charge, q, of the corona can be obtained.
 
 

Experiments:

Figures 1 and 2 show the actual test equipment and set up. There is a high voltage source, a partial discharge detector, a transformer with measuring capacitors attached, a high volt line attached to the test insulator, a night vision lens, a video camera, monitor and VCR. The last four items were used to record the actual corona discharges on the insulator.

Figure 1: Test equipment: Left: voltage source,
              Right: partial discharge detector
 
 
 Figure 2: Test setup: Left rear: transformer,
Left top: test insulator,
         Right: measuring capacitors
 

After energizing the insulator, the voltage was slowly increased until voltage inception was detected. Voltage inception was at 42 kV. The current recorded at this time was 22 milliamperes (mA), and the charge, obtained from the partial discharge detector, was approximately 1500 picocoulombs (pC). At this point the voltage was raised to about 45 kV (see figure 3), and then reduced until discharge extinction occurred. The voltage at which extinction occurred was 39.5 kV.

 

Figure 3: Corona discharge at 45 kV

Finite Element Analysis:

Different amounts of voltage were applied in the simulation software to see what the electric fields would be on the insulator. The electric field plots from the simulator software show where corona should begin. As stated before, corona exists in high, non-uniform electric fields. Corona can first be observed at the point which air breaks down. Under ideal conditions, the breakdown of air occurs at 30 kV/cm. The greater the value of the electric field the greater amount of corona can be observed. As can be observed in figure 4, the greatest electric field value exists on the very edge of the conductive cap, and is around the 30kV/cm range.

 
Figure 4: Electric field plot at 44 kV
 
 
Figure 5: Highest values of the electric field plot at 44 kV
 

Conclusion:

The high electric fields found on insulators can be attributed to the sharp edge between the conductive cap and the polymer used for the sheds. Proper design of the insulator can improve the insulator’s performance. Three important things that need to be taken into consideration are the shape of the electrodes (conductive caps/pins), the quality of the polymer, and the quality of the cement used in attaching the electrodes to the insulating polymer. If sharp edges are avoided when designing the electrodes, corona can be deterred. Likewise, the polymer and the cement used should be smooth and free of air bubbles.
 

 

Reference:

  1. Teixeira, Paul A., Thesis: Corona Discharge and Impulse Breakdown Characteristics of Covered-Conductor Distribution Systems. Potsdam, New York: Clarkson University, 1983