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Environmentally Friendly Compact Air-Insulated High-Voltage Substations energies Article Environmentally Friendly Compact Air-Insulated High-Voltage Substations Maurizio Albano 1,* , A. Manu Haddad 1, Huw Griffiths 2 and Paul Coventry 3 1 School of Engineering, Cardiff University, Cardiff CF24 3AA, UK; [email protected] 2 Electrical Engineering Department, Khalifa University of Science and Technology, Abu Dhabi 2533, UAE; huw.griffi[email protected] 3 National Grid, Warwick CV34 6DA, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-292-087-0672 Received: 23 August 2018; Accepted: 16 September 2018; Published: 19 September 2018 Abstract: This paper investigates the possible options for achieving a substantial reduction in a substation footprint using air-insulated switchgear as a more environmentally-friendly alternative to gas-insulated substations that use SF6 gas. Adopting a new approach to surge arrester location and numbers, International Electrotechnical Commission (IEC) minimum clearances can be successfully selected instead of the maximum clearances as currently adopted by many utilities, as is the case in the UK. In addition, innovative alternative compact busbar arrangements using vertical and delta configurations have been proposed by the authors. A further opportunity for compaction is offered by the application of compact and integrated technology offered from several manufacturers. The full overvoltage control within the entire substation under any surge condition is a key aspect of the feasibility of this type of substation. This work demonstrates that the new design option can be an attractive alternative for future substation configuration with minimum footprint. Keywords: surge arresters; compaction; EMTP; insulation; lightning; switching; substations; surges 1. Introduction Increasing the integration of renewable sources into the grid requires the reinforcement of power networks, and the construction of new lines and substations [1]. Restriction on land availability could suggest a wider application of gas-insulated substations (GIS) in future. However, despite the very efficient GIS space reduction, strong environmental pressures make this option less attractive due to the greenhouse effect of the insulating gas SF6 [2–5]. A sufficiently compact air-insulated substation would offer a cost-effective solution in many cases where restrictions in available land are encountered. For example, compaction could be successfully adopted for the construction of extension bays within existing substation boundaries. In many cases of substation replacement, the safety and operational issues associated with bay-by-bay replacement impose the selection of a GIS solution. However, with a compact design, a replacement substation could be constructed off-line on land of a reduced footprint that is adjacent to the existing substation, without affecting its availability during the construction period. The substation footprint is not only affected by the minimum clearances, but also by the overall design of the substation, which determines the required ground surface and volume that is occupied by the plant. Typical substation layout arrangements are: single bus, double breaker, multi-section double bus, and double bus, in addition to mesh configurations. The layout arrangement is usually selected to obtain the desired level of security of supply. The number of buses, circuit breakers, and disconnectors are a function of the selected layout, and consequently, the substation footprint. Furthermore, a significant area is required for the installation of instrument transformers. Conventional Energies 2018, 11, 2492; doi:10.3390/en11092492 www.mdpi.com/journal/energies Energies 2018, 11, 2492 2 of 14 high-voltage current and voltage transformers require separate equipment and clearances between them. Recent technologies offer similar or better performance using integrated equipment. This paper describes the combination of a number of existing and new compact solutions, IEC 60,071 minimum clearances [6,7], innovative busbar arrangements, and non-conventional equipment, which allow for the achievement of a substantial reduction of the substation footprint using air-insulated switchgears. All these solutions satisfy the safety requirements as described in current UK standards related to the substations [8,9]. An important evaluation for the adoption of minimum clearances is the performance of the compact substation under lightning and switching surge conditions. When significant reductions of air clearances between phases and the shortening of switch bay lengths are achieved, increased surge reflections and phase-to-phase couplings are expected. The effect of the installed High-Voltage (HV) equipment within the substation needs also to be taken into account in order to estimate their influence on surge voltage magnitudes at different locations within the substation. All of these factors can lead to a different surge response of the substation. To quantify the surge voltages, an air insulated compact substation model is developed using the Electro Magnetic Transient Program (EMTP) in order to assess the impact of compaction within the substation. An evaluation of the potential footprint reduction is then performed, assuming a single line entrance bay as a case study for the comparison between conventional layout and compact configuration, which introduces the proposed compact solutions. The lightning surge performances of existing and compact substations were computed following a shielding failure on an incoming line. This is then followed by a switching surge investigation to evaluate the performances of the two substation designs under switching operations. Two switching operations were selected: a breaker closure after fault clearance, and the energization of an incoming line; the effect of high-speed re-closure and circuit-breaker pole scatter are also taken into account in the statistical studies. The benefits of an extended use of surge arresters within the substation are also explored. In this paper, how to reduce air clearances with overvoltage control (Section2), and then how to gain further compaction in adopting innovative alternative compact busbar arrangements (Section3) are discussed. A further step is the introduction of selected plant technologies (Section4). All of these compaction proposals have been introduced as example on the substation bay (Section5). 2. Reduced Air Clearances with Overvoltage Control The planning and the design of the super-grid system in England and Wales at 400 kV began in the 1960s [10]. The conductor clearances were established when overvoltage control was limited and the system was not subjected to environmental pressure. This led to the introduction of significant safety margins for minimum clearances in UK and international standards. Figure1 shows the evolution of minimum clearance distances for 400 kV and 275 kV systems in the UK since the 1960s. The original design was changed twice in order to avoid the effect of long wave-front switching surges, and to take into account additional electrical and civil tolerances. In the 1970s, the minimum phase-to-earth distance was set to 3 m, and the phase-to-phase clearance was set to 3.6 m. The green dotted line shows the targeted proposed compaction change from the current minimum clearances to the minimum proposed in IEC 60,071 values (Figure1). The last change set the phase-to-earth and phase-to-phase distances to the current values [9], as shown in Table1. Table 1. Minimum air clearances and insulation levels (LIWV and SIWV) according to IEC 60071. Existing Substation 1425/1050 kV Compact Substation 1050/850 KV Minimum Clearance [m] [m] phase-to-earth 2.80 2.10 phase-to-phase 3.60 2.60 Energies 2018, 11, 2492 3 of 14 Energies 2018, 11, x FOR PEER REVIEW 3 of 15 FigureFigure 1. 1. EvolutionEvolution of of UK UK HV HV minimum minimum clea clearancesrances and and the the proposed proposed one one [7]. [7]. IECIEC 60071 60071 [6,7] [6,7] suggests suggests three three levels levels of of switching switching impulse impulse withstand withstand voltage voltage (SIWV) (SIWV) for for 420 420 kV kV systems,systems, equal equal to to 850,950, and 1050 kV (peak value). The The associated lightning impulse withstand voltagevoltage (LIWV) (LIWV) is is between between 1050 1050 and and 1425 1425 kV (peak va value).lue). The The clearance clearance values values are are then then correlated byby the the selected selected LIWV LIWV and and SIWV SIWV values, values, as as described described in in Annex Annex A A of of IEC 60071-2 [7]. [7]. It It is is important important toto note note that that these these minimum minimum clearances clearances are are det determinedermined with with a a conservative conservative approach approach and and by by consideringconsidering common electrode configurations.configurations. WithinWithin the the standard standard specification, specification, it it is is clarified clarified that that it it is ispossible possible to to adopt adopt lower lower clearances clearances in in case case of ofmore more favorablefavorable gapgap configurationconfiguration or lower prospective prospective overvoltageovervoltage levels. levels. InIn fact, fact, the the dielectric dielectric strength strength of of the the air air insulati insulationon is is a a function of of many many factors, factors, such such as as the the geometricgeometric configuration configuration of of the the electrodes, electrodes, atmospheri atmosphericc conditions, conditions, and and the the shape shape of of the
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