Upgrading the short-circuit withstand capability of distribution networks to increase DG hosting capacity

(RES), creates significant challenges for Distribution Network Operators (DNOs), requiring targeted network interventions. While these mainly focus on mitigating DG’s impact on voltage profiles, another key limitation to DG hosting capacity is the short-circuit current withstand capability of network equipment, due to the additional fault current from DG sources.

This paper presents a pilot study aimed at upgrading the short-circuit current withstand capability of a HV/MV substation and its associated distribution lines. The study has two primary objectives:

1. To calculate the actual short-circuit current level of the examined HV/MV substation and assess its existing hosting capacity for photovoltaic (PV) units.

2. To develop a methodology for identifying the reinforcements required to upgrade the network’s short-circuit current withstand capability. The methodology is designed to ensure equipment resilience to short-circuit thermal stress, while avoiding overly conservative recommendations that would unnecessarily increase implementation costs. For the first task, a detailed model of the HV/MV substation and its supplied distribution lines was developed using dedicated power system simulation software. Short-circuit simulations were performed at the main MV buses of the substation, under various pre-fault operating conditions, to calculate the existing short-circuit current level and PV hosting capacity. The results show that the short-circuit current at the substation exceeds the present limit of 7.2 kA

(at 20 kV). Moreover, the study estimates the anticipated increase in PV hosting capacity after a potential upgrade of this limit to 10 kA.

As for the second task, the short-circuit current withstand capability of the HV/MV substation equipment was verified through site survey. On the other hand, evaluating distribution line equipment is more complex. This is because a site survey would be impractical in this case, while it is important not to make overly conservative, and thus costly, network reinforcement suggestions, which are based only on equipment nameplate ratings.

To address this challenge, a simulation-based methodology was developed to identify the necessary distribution line reinforcements, taking into account protection system operation. A generic, prototype simulation model of a distribution line was created, which accepts key input parameters for each actual line, such as external short-circuit level, length, conductor or cable type, pre-fault load and protection device settings. Using an automated short-circuit simulation process, the model determines the minimum distance from the line departure at which each type of equipment can safely withstand the worst-case short-circuit current. The study results show that the required network reinforcements are technically reasonable and practical.

The main contributions of this work are summarized as follows:

• • The study demonstrates that a detailed simulation analysis is essential for accurately estimating the short-circuit current level and associated DG hosting capacity of HV/MV substations, utilizing realistic pre-fault system data.

A comprehensive methodology is proposed to evaluate the short-circuit current withstand capability of distribution line equipment, providing safe yet practical recommendations for network reinforcements.