This paper deals with the use of the insulating properties of wood in transmission line practice. The electrical properties ?impulse strength and arc quenching?and the possible damaging effects of surge currents and power frequency leakage currents are considered. Prediction of the lightning performance of shielded and unshielded lines is discussed, and it is shown that the impulse strength and arc quenching properties of wood influence performance, the latter property being the dominant one for unshielded lines. A design criterion using this arc quenching property has been developed from experimental data on arc characteristics.

Line Design and Electrical Properties of Wood

Insulation degradation leading to leakage current flow through high voltage insulators could result in catastrophic failures such as pole fires and damages to nearby equipment. This paper examines the leakage current distribution originating from the high voltage conductor to various locations through the wooden pole of various age profiles. The specimens that are investigated in this paper range from a new sample to an aged pole sample of more than 20 years in service. The main results show that the aging effect on the wooden pole has great effect on the wood resistance. The decaying of the wood reduces the wood resistance and thus increases the leakage current. Thermal images showing heating of the king bolt due to leakage currents are also presented.

Aging effect on leakage current flow in wooden poles

A wooden pole is the most popular choice as the physical support structure for an electrical distribution network. A recent increase in the failures of wooden poles that lead to pole fires warrant further investigation into the performance of wooden poles and pole design. This paper examines the leakage current distributions on the radial, heartwood and sapwood section of the wood pole and the effect of the metal insertion in wooden structures using an electrical ladder network model. This paper presents the findings from two wooden pole models: a basic wooden pole and a complete wooden pole with cross-arm and supporting bars attached. The results show that the bulk of the leakage current flows through the internal section under wet weather conditions and the metal insertion along the radial of the wood increase the magnitude of the leakage current. The model takes into consideration the pole dimension, rain parameter, moisture content, air resistance, and preservative effect (chromated copper arsenate) on the wooden pole.

Study of Leakage Current Distribution in Wooden Pole Using Ladder Network Model

The electrical performance of two water-borne wood pole preservatives (ACA and CCA) has been compared to that, of pentachlorophenol preservatives. Laboratory tests have indicated that for water-borne preservative treated poles, fire inception currents are two to three times greater than for comparable penta- chlorophenol treated poles. This is likely due to different pole fire evolution mechanisms. Wood pole and hazard models are developed and augmented with fleld tests to demonstrate that, from the point of view of pole contact near ground level, a comparable degree of safety exists with both water-borne and oil-borne preservative treatments under normal operating conditions. With fire inception currents flowing, poles treated with water-borne preservatives could result in painful shocks.

The Influence of Wood Pole Preservatives on Wood Fire and Electrical Safety

ONE OF THE major unsolved problems in the insulation of electric power lines is that of obtaining trouble-free operation under conditions of severe atmospheric contamination. The presence of conducting film on the surface of outdoor insulators subjected to high voltage leads to a number of serious results. This may be summarized briefly as follows:

Insulator Surface Contamination

Wood pole or structure fires of leakage-current origin have caused serious wood-member damage and service interruption. These fires often occur at the end of prolonged dry periods. Operating experience indicates surface tree carbonization on crossarms and pocket burninig at the crossarm-pole junction to be the two general types of burn damage. Pocket burning on some properties has been responsible for the larger part of the cases that required wood-member replacement. Laboratory tests conducted on approximately 200 specimens indicate that a coincident occurrence of dry wood and selective wetting of pole and crossarm surfaces by a light rain or fog, wind directed, which leaves a dry wood area in series with the leakage path, can result in a fire of a pocket type. This dry area, being high in resistance in comparision with adjacent series wetted-wood surfaces, results in a voltage concentration across the dry zone. If the dry zone is sufficiently short for a given circuit voltage, electrical breakdown occurs across the dry zone. This electrical breakdown being located favorably in dry wood and encouraged by a breeze may result in a pocket burn. Shunting devices which by-pass leakage currents around dry high resistance areas have proven successful in laboratory tests in eliminating pocket fires within the by passed zone.

Paul M. Ross

Papers

Burning of Wood Structures by Leakage Currents