978-1-6654-4981-6/21/$31.00 ©2021 IEEE A Multi-infeed HVDC System: Assessment of Transient Overvoltages Roberto Benato Deapartment of Industrial Engineering University of Padova Padova, Italy roberto.benato@unipd.it Giorgio Maria Giannuzzi TERNA S.p.A. Rome, Italy giorgio.giannuzzi@terna.it Antonio Chiarelli Department of Industrial Engineering University of Padova Padova, Italy antonio.chiarelli@phd.unipd.it Cosimo Pisani TERNA S.p.A. Rome, Italy cosimo.pisani@terna.it Sebastian Dambone Sessa Department of Industrial ENgineering University of Padova Padova, Italy sebastian.dambonesessa@unipd.it Roberto Zaottini TERNA S.p.A. Rome, Italy roberto.zaottini@terna.it Abstract— The paper presents an electromagnetic-transient (EMT) analysis of a multi-infeed HVDC cable system in order to assess the transient overvoltages on the cables due to different fault conditions. It is well known that the HVDC-MMC configuration needs surge arresters to reduce the transient overvoltages on the DC cable system. This work highlights how the voltage perturbations on the HVDC-LCC link due to the faults on the HVDC-MMC link are not negligible and how a reduction of transient overvoltage is necessary for both the HVDC systems. The results of this study can be useful to improve the insulation coordination of the multi-infeed HVDC system and their reliable operation. Keywords— transient overvoltage; insulation coordination; HVDC transmission; MMC; VSC; LCC I. INTRODUCTION The use of High Voltage Direct Current transmission systems in ac grids is gaining more and more importance especially with the ever growing penetration of renewable energy sources in the electrical networks, which determines several issues for a reliable operation of the grids[1-2]. It is also worth noting that the fewer number of power cables required for the DC transmission lines compared with the AC ones and the negligible electromagnetic field emissions [2] make the HVDC systems fully compatible with the installation in railway/highway infrastructures [3-9]. The HVDC technologies can be summarized in two main typologies: LCC and VSC. In particular, VSC-based systems are becoming increasingly important, due to their flexibility. The key point of VSC technology concerns the use of the modular multilevel converter (MMC). Each leg of the MMC converter has a number of sub-modules (SMs) in cascade proportional to the number of needed voltage levels. The main benefits of this technology are the lower switching frequency, which allows a converter power losses reduction, and the possibility to avoid the filter devices if a suitable number of levels per leg is used [10]. Moreover, since the VSC technology allows the reverse of the power flow without reversing the polarity of the voltage, cross-linked- polyethylene-extruded (XLPE) cables can be used with this converter typology [11]. There are three basic submodule configurations (see fig.1), Half-Bridge (HB), Full-Bridge (FB), Hybrid-Bridge. Each configuration offers pros and cons, depending on the type of application [2]. A converter reactor is located in series with the SMs in order to limit short-circuit currents through the valves in a fault occurrence and to reduce the balancing currents between individual phases to very low values. Fig. 1. HVDC-MMC converter topologies One of the major challenges concerning the HVDC power cables is related to the transient overvoltages (TOVs). During the design phase of a HVDC project, the study of the insulation coordination is very important because it allows knowing the maximum overvoltage value which the equipment shall withstand. It has to be noted that an equipment over-sizing can result in design issues and too high cost, while an equipment under-sizing could cause a failure of the HVDC link. A widely used guide concerning the recommendations for carrying out tests on extruded transmission cables for HVDC applications is the Cigrè TB496 [12]. Nowadays, the usage of several HVDC transmission systems connected to the same ac grid is becoming increasingly common. The converters, which can be of different technologies, share the same ac bus or are connected to electrically close ac bus. This type of HVDC configuration is called “Multi-infeed HVDC system” (MIHVDC) [13-23]. Hybrid HVDC transmission systems exploit the advantages offered by the VSC converter of a flexible reactive power control, which can improve the voltage stability of the AC system and also can prevent commutation failures in the LCC system when the AC bus voltage drops and supplies power to a passive network. Hence, an EMT analysis is required in order to evaluate the worst overvoltage value, both in the case of a single HVDC-MMC link and a MIHVDC link. There are some meaningful contributions in literature in this filed. Saad et al. [24] and Iravani et al. [25] highlight the main aspects of the transient overvoltages, showing the worst TOV cases for a HVDC-MMC monopolar configuration and the crucial importance of surge arresters in order to limit the overvoltage. Goertz et al. [26] and Freye et al. [27] carry out an electromagnetic transients (EMT) study by varying the fault position and the fault impedance for monopolar and bipolar HVDC configurations. Half- Bridge Full- Bridge Hybrid- Bridge 2021 AEIT HVDC International Conference (AEIT HVDC) | 978-1-6654-4981-6/20/$31.00 ©2021 IEEE | DOI: 10.1109/AEITHVDC52364.2021.9474603 Authorized licensed use limited to: University of Johannesburg. Downloaded on March 18,2022 at 09:27:22 UTC from IEEE Xplore. Restrictions apply.