Integration studies for New Zealand Low-Voltage Distribution Networks with Inverter-based Energy Systems.

Abstract

Electrical distribution networks are undergoing drastic changes with inclusion of distributed energy resources, especially on Low Voltage grid. The uncontrolled market and reducing price of inverter based energy systems means increased penetration of renewable generation on varying topologies. However, current network planning strategies can successfully take distributed generation connected to Transmission and Medium voltage distribution networks into account, but fail at Low voltage due to unpredictable customer behaviour. Rapid uptake of low voltage distributed generation is part of Smart Grid domain and will form critical un-factored generation that can eventually disrupt normal operation of distribution networks. Subsequent detailed analysis of low voltage network must be carried out to evaluate protection and control strategies upstream to minimize adverse effects of high penetration of such micro-generation. The biggest impediment in protection and control of distribution network with distributed generation is lack of understanding the behaviour of inverter based systems under fault. As protection and voltage control strategies utilised at distribution are by and large rigid, problems arising from bi-directional power flow can be aggravated by strict ride through criteria, and undefined fault contribution of Inverter Energy Systems (IES). However, such issues can be mitigated by understanding fault behaviour of inverter based distributed generation, and leverage modern commercially available low voltage inverters with reactive and active power control. Several issues have been documented in countries with high penetration of distributed generation arising from lack of control and visibility on low voltage distribution network. Unfortunately, standards around interconnection of LV IES are not adequate to assuage problems arising from increased penetration. For example, fault current limitation does not form part of interconnection standards, hence fault contribution from IESs is by and large uncontrolled, leading to unpredictable protection problems. System stability suffers from adverse impacts of strict Low Voltage Ride Through (LVRT) criteria that does not leverage wide voltage tolerance of modern commercial inverters. In context of leeway available in interconnection standards, this research attempts to understand the protection and voltage control issues on different Low Voltage networks, and mitigate them using existing solutions. In this thesis, a test bench and respective methodology is developed from a thorough review of existing interconnection standards in order to quantify fault behaviour of commercially available LV inverters. This includes defining LVRT envelope and fault contribution of IES that can be incorporated into simulation platforms. The derived properties through physically based fault modelling is utilised for simulating protection and power quality problems. Various New Zealand networks are used as case studies, and problematic feeders based on network strengths are isolated as problematic feeders. This allows to quantify problematic penetration levels, while simultaneously explore safety and protection concerns across different zones of protection within Low Voltage network. Voltage violations with high penetration of IES are identified on benchmark models to understand role of reactive and active power in different network topologies. Subsequently, reactive and active power control as seen in commercially available inverters are assessed for different networks for mitigating voltage violations arising from high penetration of IES in low voltage distribution network.