Analytical approach to assess the loadability of unbalanced distribution grid with rooftop PV units

Photovoltaic (PV) power is a promising source of renewable energy, which is expected to provide a significant share of electrical energy in the near future. However, a stable voltage condition of the system must be always insured even with a high level of PV penetration. It should be noted that a large portion of PV systems are single-phase and they are being integrated into distribution network (DN). In addition, the DNs are unbalanced due to their loads and line characteristics. As a result, it is necessary to investigate the impact of power from both single and three phase PV units on the voltage stability of unbalanced DN. This paper has proposed an analytical methodology to investigate the static voltage stability of unbalanced DN with high PV penetration considering both single and three phase connections. For this purpose, the loadability of general unbalanced distribution system and how it is affected by PV power is analytically investigated for both single and three phase PV integration scenarios. In addition, the impact of reactive power capability of PV systems on the loadability has been assessed. Analytical outcomes are further studied and justified using results from IEEE 37-bus DN. Presented results conclude that both unbalanced system, and single and three phase PV penetrations must be taken into account to accurately assess the static voltage stability of practical DN.