An accurate analysis of lightning overvoltages in mixed overhead-cable lines
Introduction
THE use of underground cables for transmitting energy has been constantly increasing worldwide over the last few decades. According to [1], this stems from technical changes and strong competition in the cable sector, which have reduced prices. Furthermore, increased urbanization and public concerns have increased the difficulty and time taken to obtain consents for overhead (OH) lines. The use of short underground (UG) sections has also been common, either due to complaints from residents or due to physical restrictions regarding OH line terminating structures arriving the substation, resulting in mixed overhead lines with short underground sections [2].
The study of lighting overvoltages for a mixed overhead- cable lines is an important issue [3]. A lightning strike on the shielding wire of the OH line can propagate into the sheath circuit since the transmission tower and cable sheath often share the same grounding system at the transition site. Also, a back flashover can occur across OH line insulators or even a lightning strike can directly hit the phase conductors due to a shielding failure. In these cases, the lightning surge can directly propagate into the cable core, inducing voltages at the cable sheath. In systems comprising short sections, multiple reflections and superpositions can occur in a short period of time and cause severe overvoltages [4]. The sheath overvoltage requires careful studies to avoid failures of sheath voltage limiters and sheath interrupts [3].
Typically, the study of transients in power transmission systems, including mixed overhead-cable lines, is carried out using EMT-type programs (for instance in [4,5]), that adopt some assumptions that may lead to inaccurate results, especially considering lightning overvoltages and short underground sections. The cable models normally adopt Pollaczek's formula of earth-return impedance and disregards ground displacement currents along with the earth-return admittance. The soil electrical parameters are also usually assumed constant and frequency independent. However, recent studies show that both aforementioned approximations can lead to inaccuracies in transients’ studies involving underground cables and frequency region of kHz up to MHz [6], [7], [8], [9], [10]. Furthermore, in most simulations, the sheath bonding and grounding is made through a simple lumped resistance, disregarding the frequency-dependent (FD) behavior of grounding system. It is to be noted that the level of the sheath overvoltage is highly dependent on the grounding performance at the transition site.
This paper assesses the influence of considering different modeling approaches in the computation of lightning overvoltages developed in short underground cable sections. The paper is organized as follows. The system under study, transmission line (TL) based parameters, FD soil model and grounding system are summarized in the Section II. In Section III, the wave propagation characteristics of overhead line and underground cable in frequency domain are investigated based on different approaches discussed in the Section II. Section IV performs the transient studies of a typical mixed overhead-cable line. Also, the influences of earth parameters, FD soil parameters and FD characteristics of grounding system on transient voltages of cable are made clear.
Section snippets
System under study and modeling guidelines
To focus on the fundamental aspects regarding the influence of several modeling approaches on the simulation of transient overvoltages developed in a mixed overhead-underground cable line, the single-phase equivalent circuit of 138 kV mixed line shown in Fig. 1 is considered. It consists of an overhead bare phase conductor (radius of 0.9155 cm and Rdc = 0.2076 Ω/km) positioned 12 m above ground, which, in the transition tower, is connected to an insulated cable that goes down vertically to the
Modal propagation constants for overhead line
Fig. 4 shows the calculated modal propagation constants of an overhead line shown in Fig. 1 by adopting the extended and classical TL approaches. The influences of CS model and AV FD soil model on the calculated modal propagation constants are also investigated in Fig. 4.
As shown in Fig. 4(a), it is clear that visible differences are observed for modal attenuation constants using classical and extended TL approaches at a high frequency region, i.e. MHz. The modal attenuation constants
Transient responses
In this section, the transient sheath voltages at the receiving end of the underground section are computed, considering different modeling approaches for the system components. The incoming surge in Fig. 1 is represented by an impulse voltage of 1.2/50 μs and amplitude of 650 kV, which corresponds to the typical Critical Flashover Voltage (CFO) of a 138 kV line. The aerial bare conductor was impedance matched to avoid unwanted reflections.
The time domain responses were obtained via the
Summary and conclusions
This paper presents an investigation of lightning overvoltages on a mixed overhead-cable line. From the presented analysis, the following main conclusions can be drawn.
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An abnormal phase velocity of earth-return mode of underground cable has been observed with increasing the frequency and for high-resistivity soils if their parameters are assumed constant. Thus, the FD soil parameters should be implemented in cable transient studies if the soil resistivity presents large values.
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Classical TL
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This paper was supported by the National Council for Scientific and Technological Development (CNPq) under grants 312763/2018-2.
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Paper submitted to the International Conference on Power Systems Transients (IPST2021) in Belo Horizonte, Brazil June 6–10, 2021.