Guideline for the connection of small-scale inverter based distributed generation: an introduction and summary
Small-scale distributed generation (DG) in New Zealand, particularly photovoltaic (PV) generation, has been growing steadily over the past few years. In the last year alone to 31 March 2016, installed PV generation of all capacities has grown by a factor of about 1.6 to reach 37 MW. Approximately 90% (33 MW) of this installed PV capacity is made up of small-scale, single phase residential grid-tied systems with ratings below 10 kW. This corresponds, on average, to approximately 300-400 new PV systems being installed each month within low voltage (LV) distribution networks. Traditionally, the flow of power in electricity distribution networks has been largely unidirectional. However, distributed generation introduces reverse power flows into the LV network when the power produced by DG systems is greater than what can be consumed locally. This introduction of reverse power flows and the dynamic behavior of DG system inverters can negatively impact the electricity network, causing issues such as over-voltage, phase imbalance, overloading of conductors and transformers, and create unique safety challenges. As such, each DG connection application received by electricity distribution businesses (EDBs) presently needs to be carefully considered for its impact on the electricity network. The resourcing demand imposed by larger numbers of connection applications, and the difficulty of technical assessment including congestion evaluation, are likely to increase substantially as DG uptake intensifies. This has prompted the Electric Power Engineering Centre (EPECentre) via its GREEN Grid programme, with the assistance of the electricity industry based Network Analysis Group (NAG), to develop a small-scale inverter based DG connection guideline for New Zealand EDBs. This has been developed on behalf of the Electricity Engineers’ Association (EEA) specifically for the connection of inverter energy systems (IES) of 10 kW or less. This paper summarizes key aspects of this guideline. This includes a streamlined connection application evaluation process that enables EDBs to efficiently categorize DG applications into three groups. These groups vary from those with minimal or moderate network impact that can be auto-assessed, to those most likely to cause network congestion that require manual assessment. These categories are determined by looking at the DG hosting capacity specific to the LV network that the DG is connecting to. For two of these categories, mitigation measures for connection, are prescribed. It is also shown how DG hosting capacity can be used to simply evaluate LV network congestion in order to satisfy Electricity Industry Participation Code (EIPC) Part 6 requirements. Key technical requirements for all IES, appropriate for New Zealand conditions, are also summarized.