A new device for control of overvoltages during live-line work has been developed in Ukraine. The new portable protective gap (PPG) device provides improved limitation of overvoltages at the work site in comparison to traditional PPGs that have been used for decades. The new device consists of a rod air gap connected in series with a non-linear ZnO resistor. After extensive laboratory testing the device is now undergoing in-service validation tests.

The obvious advantage to using PPGs during live-line work is the reduction of overvoltage levels at the work site. In the case of excessive overvoltage, the discharge will be diverted to the PPG, which is typically installed on adjacent towers next to the work site. As a consequence, the minimum approach distance (MAD) for live workers is reduced. Decreasing the MAD simplifies live-line work procedures and improves work efficiency. Also, the use of PPGs expands the limits of live-line work by increasing the allowable number of defective insulators in a string that can be replaced during live work procedures.

At the same time, the use of conventional PPGs also introduces a few disadvantages such as the need to climb an adjacent tower at the work site to install the PPG and the remote possibility of introducing a short circuit on the line when the PPG sparks over. To address these shortcomings, a new PPG device was developed in Ukraine.

The new device consists of a typical rod air gap connected in series with a ZnO arrester. The line end of the unit is fastened to the energized conductor with the aid of a self-fastening terminal. The terminal attachment mechanism uses a system of levers to compress the conductor to assure good electrical contact. A metal rod attached to the terminal constitutes one electrode of the gap. The ground end of the unit is attached to the tower grounding system through a wire and portable base plate. A metal rod attached to the ground end of the unit and the wire constitutes the other electrode.

This new design is easily installed by two workers and eliminates the need to climb a tower during installation. The device is first raised into position by throwing a light insulating cord over the energized conductor with the aid of a small weight (i.e. piece of metal). It is generally easiest to begin installation at the low point of the sag of the conductor. The insulating cord and the PPG device are then moved closer to the transmission tower so that the device can be attached to the tower grounding system. The insulating cord is used to raise an insulating traction rope, which, in turn, lifts the PPG device into place. At this point, the self-fastening terminal should be located slightly above the conductor. With the aid of another insulating rope that is fixed to the bottom end of the unit, the terminal is rotated and hung on the conductor. As the traction rope is lowered, the terminal lever mechanism firmly grabs the wire.

The other electrode of the gap is positioned by rotating or raising it into place with the aid of the connected grounding cable and a control wire running through a metal tube that is fitted to the end of the arrester.

The device is removed by reversing the same sequence of steps. The traction rope is stretched, the terminal is released from the line, and the device is rotated and lowered to the ground.

Technical specifications for arresters used in this type of application are significantly less than other arresters used in high voltage networks. These arresters are designed for short term use only (a few hours per day), and there is no permanent current flow through the working elements of the unit. In addition, the arrester is not subjected to repeated operations. If the PPG device operates during live work, manual closing of the line is normally prohibited. Company personnel can control the characteristics of the arrester and confirm its integrity after each operation.

Laboratory tests performed at Kyiv Technical University and field tests on a 330-kV line have confirmed that it is possible to use a PPG in series with a surge arrester to offer additional work site protection during live-line work procedures. The application of these two devices in series:

-Allows live work on lines with reduced insulation (such as compact lines). -Allows workers to forego climbing a tower adjacent to the one they are working on to install a PPG. -Prevents a short circuit on the line when a PPG sparks over. -May eliminate the need to require blocking reclosure of the circuit breakers on the line being repaired. -Simplifies installation and removal of PPG devices.

As a final note, it is possible to simplify and shorten the installation time by using a sectionalized resistor that consists of resistive columns placed in separate tubes that are stacked at the work site.

Editor's Note: This article is adapted from the conference paper, "Device for Overvoltage Control in Live Line Working." The paper appeared in Proceedings of ESMO '98, 1998 IEEE 8th International Conference on Transmission & Distribution Construction, Operation & Live-Line Maintenance held in Orlando, Florida, U.S., on April 26-30, 1998.

V. Molchanov is first deputy director general and chief engineer for the ASELENERGO Ukrainian Research and Technology Association. He graduated in 1975 from the Kyiv Polytechnic Institute, Power Engineering Faculty with a specialty in electrical engineering. He worked in the Kyivenergo Power Co. He has worked in the Ministry of Energy of Ukraine.

V. Taloverya is center chief for the ASELENERGO Ukrainian Research and Technology Association. He graduated in 1960 from the Charkiv Polytechnic Institute. He obtained the Ph.D. degree in Belorussian Polytechnic Institute in 1989. He has worked in several areas of the power industry in Ukraine.

V. Brzezickiy is chief chair for the Kyiv National Technical University. In 1963, he received his qualification of engineer-electromechanic. He was a post-graduate student at the Institute of Electrodynamics Ukrainian Academy of Sciences. In 1981, he was invited to Kiev Polytechnical Institute as professor and chair, high voltage technique and electrophysics.

G. Gela is a senior research engineer for the EPRI Energy Delivery and Utilization Center-Lenox. He obtained the B.A.Sc., M.A.Sc. and Ph.D. degrees in 1973, 1975 and 1980 in electrical engineering from the University of Toronto. He has worked for Trench Electric and Ohio State University.

Calculating MAD Values An essential requirement for safe live-line working is the elimination of any possibility of a sparkover that could endanger workers. Since live-line work should only be performed in good weather, sparkovers caused by switching over- voltages need only be considered.

Normally, sparkovers at the work site are prevented by maintaining a minimum approach distance (MAD) between the worker and parts at different potential. The MAD is chosen based upon the highest switching overvoltages expected at the work site.

In Ukraine, the MAD between a person and parts at different potential is defined by: D = A + B x KOV x Sn where, A - distance that accounts for inadvertent movement of the worker (m) B - a factor, accepted as equal to 1.15 to 1.2 (with allowance for dispersion of the discharge voltage of air gaps) KOV - transient overvoltage factor (p.u.) Sn - Length of the air gap (m), the discharge voltage of which is equal to the highest voltage of the electrical installation

For electrical installations with grounded neutrals, the MAD is calculated by using A = 0.5 m and KOV = 2.1 to 3.0. The MAD values used in Ukraine are listed in Table 1.

In determining the proper MAD, it is also necessary to consider situations where the dielectric strength of the work site is lowered by factors such as: insulating devices and live working tools in the work zone; broken insulators; the presence of large conductive objects that may reduce the available air gaps; and the presence of electrically floating conductive objects.