Study of pole-top fire development in power distribution networks

摘要

Wooden utility poles are widely used in power distribution and transmission networks in many countries due to their low initial cost, high durability and electrical properties. However, the high number of pole failures in the networks due to wood deterioration and pole-top fires has become a major challenge for the network operators in the recent years. This thesis presents an original study of electrical characteristics of a wooden utility pole using finite element analysis and the electrical ladder network model. The influence of high voltage, leakage current and electric field in starting a pole-top fire is studied in order to find a solution to the issue. Firstly, leakage current and voltage distribution of a wooden pole are studied using a three dimensional resistance model based on the electrical ladder network for different scenarios that could occur in practice. The current flow, electric field and voltage distribution of a wooden utility pole are further investigated using finite element analysis. These results confirm that the electrical characteristics of the utility pole change significantly with the moisture content of the environment making the pole vulnerable to fire at high moisture contents. These analyses also show that high voltage and high electric field are present at the metal king bolt and there is a possibility of arcing development near this metal insertion. This thesis next studies the development of arcing in cellulose fibre. In particular, it focuses on arcing in microfibrils found in hardwood, caused by the creeping discharges under AC and DC voltages. The results show that the burning of cellulose materials is caused by sparks and arcing developed on the fibre surface. Higher moisture levels in the fibres reduce the time taken for the creeping discharges to develop into arcing. The results showed that the development of arcing is directly related to creeping discharges. This thesis next studies the creeping discharge and arcing development near wood / metal interfaces in a wooden utility pole. A small scale utility pole is tested in the high voltage laboratory under various configurations to investigate the development of arcing near the king bolt. The results show that arcing eventually develops into burning of the wooden surface near the king bolt, especially when the creepage distance of the insulators is reduced due to surface pollution. Finally, taking all these factors into consideration, a pole-top fire mitigation method using a nonconductive, high-strength king bolt made of fibreglass composite is proposed to overcome the pole-top fire issue. The feasibility of using fibreglass king bolts is studied using finite element analysis and laboratory experiments. The results show that the electric field at the cross-arm is reduced with the fibreglass king bolt and the proposed king bolt reduces the possibility of creeping discharge and arcing development at the cross-arm. The fibreglass king bolt is considered a low cost solution and can be retrofitted into existing distribution poles. This proposed solution has significant potential to eliminate the risk of pole-top fire by reducing the electric field of the bolt / cross-arm junction.