Author: Cao, Jinxin
Title: Transient analysis and protection design for power distribution line systems against lightning using a hybrid PEEC-MTL method
Advisors: Du, Yaping (BEEE)
Degree: Ph.D.
Year: 2022
Subject: Lightning protection
Electric power distribution
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xix, 196 pages : color illustrations
Language: English
Abstract: Continuous changes in climate and rising temperatures in recent years have led to an increase in thunderstorm severity in the world and caused a significant increase in lightning events in sub-tropic regions. The overhead distribution networks are subject to lightning frequently and are one of the industrial systems severely affected by lightning to this day. In the southern provinces of China, distribution system failures caused by lightning have been continuously reported in the medium-voltage (MV) overhead lines (OHLs). Note that the security and reliability of the distribution systems have been an increasing concern to the public. More effective lightning protection for the OH distribution lines needs to be considered and provided urgently.
This thesis proposes a new hybrid PEEC-MTL method for lightning transient simulation, which has a high computational performance, as well as great modeling flexibility for large-scale distribution line systems. Computer simulations are performed to address important issues in lightning protection in power systems, such as systematic parametric optimization and techno-economic investigation from a design point of view, which were not appropriately addressed in the literature. The concept of differentiated SA protection is introduced to study lightning protection from techno and economic aspects. A muliti-objective optimization procedure and other simplified procedures are developed to determine the optimal design. The work in this study provides valuable design recommendations and conclusions, as the reference for power companies and revision of related codes and standards.
In this study, a new Partial Element Equivalent Circuit (PEEC)- multi-conductor-transmission-line (MTL) hybrid numerical method is first proposed. This method retains both computational efficiency and accuracy when dealing with lightning surges in a system mixed with long wires and short wires, which is difficult to tackle with traditional computation methods. In this framework, overhead line wires are represented with the MTL model, poles, grounding electrodes, and others with the PEEC model. A model for mutual coupling among different parts including a lightning channel is proposed to integrate sub-systems. It is found in the study that the presence of a lightning channel has a significant impact on the surge response in a 500kV transmission line system. Ignoring the coupling from the lightning channel will underestimate the risk of lightning strikes. Also, the comparisons with various classic experiments have verified the simulation accuracy of the hybrid method.
With the proposed method, this thesis presents a systematic parametric investigation into lightning protection for MV overhead distribution lines, including a shield wire (SW), surge arresters (SAs), line insulation, and grounding configurations, from the design point of view. This thesis shows that the lightning channel positioned in front of an ungrounded pole could completely wipe out the effect of the SW. It is suggested to provide the wire grounding at every pole even if the grounding resistance at some poles is much higher than the design value. Minimizing the distance between the SW and outer-phase wires can effectively reduce lightning-induced voltages, rather than a protective angle. Besides, the OH distribution lines grounded with counterpoise wires under extremely low soil conductivity is investigated. It is found that the simple vertical grounding rod is not effective in lightning protection, while the performance is significantly improved with extended counterpoise wires. The counterpoise wire with a length of pole span performs best. However, if a higher lightning protection level is required, increasing the length of the counterpoise wire is not cost-effective and other schemes should be considered.
Furthermore, a comprehensive assessment of lightning protection schemes is introduced for the OH distribution lines from two aspects: lightning performance and economy. Traditional protection assessments tend to focus on the flashover rate of a line but ignore the failure rate of protection equipment itself, e.g., surge arresters. This leads to a situation in which the economy of a lightning protection scheme could not be addressed appropriately. This study proposes a techno-economic assessment method to consider the protective construction costs, the maintanence costs associated with failure rates and operational life. It is found that installing SAs on every pole results in the best protection performance, but could be extremely expensive due to the replacement of failed SAs. The combination of the SW and SAs provides much better performance under a wide range of soil conductivity at a moderate cost. The line insulation itself with a CFO of less than 200 kV is not sufficient for lightning protection. With other protection options, however, increasing the CFO can greatly improve the protection performance. Moreover, this study also investigates the lightning energy absorption in the dedicated SAs (SADT) provided for the transformers on 10kV OHLs. Several practical protection schemes are evaluated. It is found that the full SW scheme performs the best, but with the most expensive protection cost. The scheme of two or four extra SAs together with a partial SW can achieve a balanced performance of protection effect and economy for the OHLs. The SW for a section of the OHL alone is not enough to provide protection.
Finally, this thesis introduces differentiated protection for distribution systems and develops a novel procedure for the design of differentiated SA configurations, i.e., identifying appropriate positions for a limited number of SAs to achieve desired lightning performance. In addressing differentiated SA protection, a multi-objective optimization problem is formulated by minimizing both the flashover number and SA damage rate. An artificial neural network and an energy-distribution surrogate model are developed for efficiently evaluating these two parameters in the optimization. When addressing the flashover rate, a simplified probability-based approach is proposed. Compared with the traditional Monte Carlo method, the approach can achieve about a 10-fold increase in computational efficiency with reasonable accuracy. The energy absorption into SAs is statistically investigated, and empirical formulas are developed to quickly assess the damage rate of SAs. Apart from the optimization procedure, another selection strategy is also recommended for quick selection due to its simplicity and better performance, if the flashover number is mainly concerned.
Rights: All rights reserved
Access: open access

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