PROTECTION AND EARTHING REQUIREMENTS OF LV AC AND DC DISTRIBUTION NETWORKS INTERFACED BY A SMART TRANSFORMER

Electrification of more transport and heat will place increasing demand on LV networks for which they have not been designed. This can potentially overload MV/LV transformers and LV cables, and may result in voltage drops outside statutory voltage limits. Deployment of solid state smart transformers (STs) at secondary substations has recently been considered as an alternative approach to address such challenges. However, their deployment will lead to a radical change in the operation of associated LV networks and, in particular, protection systems. Therefore this paper investigates in detail the impact of a smart transformer deployment at a secondary substation on the performance of conventional MV/LV protection, and provides recommendations on suitable earthing and protection for LVDC and hybrid AC/DC distribution networks interfaced by a smart transformer. INTRODUCTION The growth of low-carbon technologies (LCTs) such as distributed renewables, electrical storage, and electrical vehicles (EV) has increased the need for improving the infrastructure and operation of existing low voltage (LV) distribution and their associated substations [1][2]. Key operational parameters such as substation loading, fault levels, and LV voltage profiles are likely to be impacted. This is because LV distribution networks operate within relatively tight voltage limits, and electrification of more transport and heat will place increasing demand with the potential to overload MV/LV transformers and LV cables. In response to such challenges, SP Energy Networks’ LV Engine project has a key objective to design and trial the deployment of an electronic smart transformer (ST) within power distribution secondary substations [3]. In comparison to conventional transformers, STs have the potential to provide more effective voltage control, independent real and reactive power control, and bidirectional real power control. The ST deployment will be trialled within five different schemes, three of which will provide conventional 0.4kV AC supply with different network configurations, while the remaining two provide LV direct current (LVDC) and hybrid DC/AC supply. From protection perspective, the ST will provide reduced fault currents and potentially enable the use of equipment with lower ratings. However, its deployment at secondary substations will fundamentally change fault profiles on the associated LV networks. STs with suitable controls and configuration can have the ability to limit and block fault currents from the AC grid for the purpose of selfprotection against internal and downstream faults. Since the majority of existing LV networks are protected by simple overcurrent protection, reduced LV fault levels caused by the ST may impact fault detection, operating time, and selectivity of the overcurrent-based protection. Therefore, it is important to understand the impact of ST installation on existing MV/LV networks’ protection. This paper investigates through detailed discussion and simulation studies the impact of an ST deployment on the performance of conventional MV/LV protection. The paper provides recommendations on suitable earthing and protection for LVDC and hybrid AC/DC networks interfaced by an ST. IMPACT OF SMART TRANSFORMER ON LV PROTECTION PERFORMANCE Impact on fault level and protection coordination STs normally operate with reduced fault levels due to their power electronics limited short-circuit capabilities. Such constraints can potentially impact overcurrent protection operating time and coordination [4]. Another issue related to a reduced protection speed is the cause of severe voltage sags with increased duration. This can create a power quality issue on the customer side and stability issue on the ST side. Severe voltage sags can also be a problematic to any distributed generation connected to the network. Impact on LV earthing schemes When an ST is installed, the earthing requirements will be different in accordance to the ST outputs. For STs with only AC outputs, typical TN-S or TN-C-S earthing can be used, and the PE can be connected to the mid-point of the ST last stage inverter. But, for STs with LVDC outputs, the earthing will be different, and the following aspects should be considered. (1) Safety: on the customer side, TN-S system can only be used with certain voltage levels (<±380Vdc if electronic DC-RCDs are used) [5]. Above these voltages, it may require double isolation or floating IT earthing systems. (2) Corrosion: it is important to consider the avoidance of corrosion impact on any metal surfaces connected to the LVDC earth. (3) Common-mode noise and parasitic capacitors: when one ST provides AC 25th International Conference on Electricity Distribution Madrid, 3-6 June 2019