A Method for Equalizing the Voltage Distribution Along the Surface of a Dielectric

A. A. Vorob'ev, V. S. Dmitrevskii

An analysis of the methods currently used for equalizing the voltage distribution along the surface of a dielectric [1] has shown that none of the known methods fully satisfies the requirements imposed.

To equalize the voltage distribution along the surface of a dielectric, it is proposed to install conducting rings or strips [2] on its inner surface. Such electrodes, located at a certain distance from one another, will increase the total conductivity of the section of the column and reduce the field strength in the section between the conducting rings.

The physical action of the proposed device may be explained as follows. When a positive charge appears on the high-voltage electrode, negative charges will be induced on the nearest section of the first voltage-equalizing ring. Positive charges will arise on the remote part of the ring; on the outer surface of the dielectric, surface charges of the corresponding signs will be induced, and, in the case of field nonuniformity and high voltage, surface equalizing currents will appear, equalizing the voltage distribution along the outer surface of the dielectric.

Figure 1 shows a possible arrangement of the device.

Fig. 1. Cross section of a flat specimen with internal equalizing strips.

Action of Internal Auxiliary Electrodes Under Alternating Voltage

The action of internal conducting rings on the distribution of voltage along the surface of a dielectric under alternating current was investiga-

was carried out on flat specimens made of getinax. The specimen thickness was 10 mm. The distance between the electrodes was 750 mm. The surrounding medium was air. Figure 2 shows the voltage distribution along the surface of a getinax plate with and without internal conducting strips. As can be seen from Fig. 2, the installation of internal conducting strips leads to a more uniform voltage distribution under alternating current.

Effect of internal conducting rings and strips on the voltage distribution along the surface of a dielectric under impulses

We have studied the effect of internal conducting rings or strips on the voltage distribution along the surface of a dielectric under impulses.

Figure 3 gives the change in the voltage distribution along a pressboard plate of thickness 1 mm as a function of the location of the internal auxiliary electrodes.

Fig. 2. Voltage distribution along the surface of a getinax plate under alternating current with (curve 1) and without (curve 2) internal equalizing strips.

Fig. 3. Voltage distribution along the surface of a pressboard plate under impulses:
\(*\) — without equalizing strips.
\(\odot\) — \(a_1=5\) mm, \(a_2=\infty\), \(a_3=90\) mm
\(\times\) — \(a_1=5\) mm, \(a_2=50\) mm, \(a_3=90\) mm
\(\triangle\) — \(a_1=10\) mm, \(a_2=50\) mm, \(a_3=90\) mm
\(\circ\), \(\oplus\) — \(a_1=20\) mm, \(a_2=50\) mm, \(a_3=90\) mm

From Fig. 3 it is seen that, as the distance \(a_1\) decreases, the voltage distribution over the surface of the pressboard plate becomes more uniform. Measurements of the voltage distribution were carried out under im-

pulses of negative polarity (waveform 0.15/32). The magnitude of the applied voltage was 71.5 kV. The length of the specimen was 750 mm. Fig. 4 shows the voltage distribution along the surface of the getinax plate for different distances between the internal auxiliary electrodes.

Fig. 4. Voltage distribution along the surface of a getinax plate under pulses

* — without grading strips
⊕ — \(a_1 = 5\) mm, \(a_2 = \infty\), \(a_3 = 90\) mm
○ — \(a_1 = 5\) mm, \(a_2 = 50\) mm, \(a_3 = 90\) mm
* — \(a_1 = 10\) mm, \(a_2 = 50\) mm, \(a_3 = 90\) mm
× — \(a_1 = 20\) mm, \(a_2 = 50\) mm, \(a_3 = 90\) mm
⊙ — \(a_1 = 3\) mm, \(a_2 = \infty\), \(a_3 = 200\) mm

the auxiliary electrodes. The length of the getinax plate was 750 mm. The experiments were carried out under pulses of negative polarity. The magnitude of the applied voltage was 71.5 kV.

Fig. 5. Dependence of the field strength at the high-voltage electrode of a getinax plate on \(a_1\)

The measurements carried out showed that, when the distance between the auxiliary electrodes located near the high-

of the high-voltage electrode, the voltage distribution over the surface of the getinax plate becomes more uniform. Fig. 5 shows the change in field strength at the high-voltage electrode as a function of the arrangement of the internal auxiliary electrodes. It is seen from Fig. 5 that, when the distance \(a_1\) is reduced, the field strength at the high-voltage electrode decreases and, at a distance equal to 3 mm (the width of the auxiliary electrode), amounts to 2 kV/cm, i.e., is almost equal to the field strength in a uniform field at the same applied voltage.

Experiments carried out on cylindrical specimens confirmed the data obtained on flat specimens. The installation of internal auxiliary electrodes—rings—at certain distances from one another led to an equalization of the voltage distribution along the surface of dielectrics under pulses.

Conclusions

The proposed method of equalizing the voltage distribution along the surface of a dielectric has the following advantages:

1. Equalization of the voltage distribution along the surface of a dielectric is achieved under pulses and alternating current without increasing the external dimensions of the structures.

2. It does not cause a reduction in the insulating distances between the axes of adjacent insulating columns.

3. The effectiveness of the method of internal conducting rings and strips is no lower than the effectiveness of the method of external auxiliary electrodes.

REFERENCES

1. Vorob'ev A. A., Dmitrevskii V. S. Methods and devices for equalizing the voltage distribution along the surface of a dielectric under pulses (present collection).

2. Vorob'ev A. A., Dmitrevskii V. S. A method for regulating the voltage distribution over the surface of insulation by installing conducting rings and strips along the internal surface at specified distances from one another. Application for an inventor’s certificate in the Ministry of the Electrotechnical Industry No. K-801.