Fault contribution from large photovoltaic systems in building power supply networks

• Large PV units in power system feeding building premises cause protection problems.
• PV unit of building where no fault has occurred contributes to the point of fault.
• Total fault current contribution from PV is determined using a number of factors.
• Relay settings should be reviewed after PV units are connected to the network.
• Relay setting should use delayed trips in instantaneous zones to maintain grading.

This paper presents a detailed analysis for determining the impact of adding large three phase photovoltaic (PV) systems in secondary (building) power distribution networks. The analysis highlights the protection relay coordination problems arising due to the increase in network fault levels caused due to the contribution from PV generators. A typical distribution network for power supply to large buildings with multiple apartments in a housing complex has been modeled and used as a test network. Simulation result shows that the magnitude of fault current contribution from PV system depends on a number of different factors, not only on the size of the PV system. The analysis emphasizes the requirement to review protection settings of similar installations prior to connection of PV generators to the network and suggests a method of protection coordination to minimize the requirement of reviewing the protection setting every time a new PV system is connected to the distribution network. It is found that fault current contribution from PV systems, depending on the size, can cause significant relay coordination problems in terms of discrimination and thereby reduce system reliability.