Analysis and optimization of distribution networks with photovoltaic generation and battery energy storage system

Worldwide per capita consumption of electricity has been increasing and is poised to further rise in the foreseeable future. While efforts for reducing greenhouse gas emissions have gained momentum, the share of renewables in electricity generation is increasing day by day. As a result, power distribution networks are witnessing the rise of renewable energy source based Distributed Generation (DG) - at low to a medium voltage level as well - primarily due to the availability of power electronic based converters that provide an easy interface of DGs with the power grid. Penetration of Photovoltaic (PV) generation in Unbalanced Distribution Networks (UDNs) raises concerns for distribution system operator and also for PV plant owners. This research work is focused on grid side issues of integrating PVDGs in low voltage distribution networks and explores the use of grid-connected Battery Energy Storage System (BESS) to improve the daily operations of UDNs with and without PVDGs. Starting with a case-study based on an actual operating network in India with roof-top PV plants, this research work extends the analysis using IEEE standard test feeders and argues the high level of case-specificity in the influence of PVDGs on operations of UDNs. Further work explores the role of grid-connected BESS in improving the daily operations of UDNs with and without PVDGs. While IITGN-VGEC network is modeled and investigated for the impact of roof-top PVDGs, a utility feeder in Northwest USA is used for validation of novel multi-objective optimization approach for BESS applications. With a view to consider realistic scenarios in investigating the influence of PVDGs on daily operations of UDNs, an actual operating network – IITGN-VGEC network – was thoroughly modeled with an on-site survey of cable layout, roof-top PV systems and load compositions of major load centers. The modeled network was analyzed for overall voltage profile, overloading of cable sections, power factor at the source node and risk assessment of unintentional islanding. The cable overloading findings were found in agreement with few incidents of cable heating and subsequent outages, whereas risk assessment exercise of unintentional islanding hinted at the time of day and day of the year when unintentional islanding is possible to sustain for longer than few seconds. In an attempt to propose a switching sequence for manually switched capacitor bank in IITGN-VGEC network to maintain healthy power factor, Time-Series Distribution Power Flow (TSDPF) simulations were performed with annual metered measurements of real power demand – provided by Torrent Power Ltd, Gujarat, India – and on-site recorded measurements of real power injection from roof-top PV plants. TSDPF simulations were then used with IEEE standard test feeders to generalize the impact of PVDGs on daily operations of UDNs. Analyses of IEEE test feeders and IITGN-VGEC distribution network with TSDPF simulations led to a diverging set of results that imply high-level of case-specificity in the influence of PVDGs on UDN operations. The exhaustive analysis was performed using 4 distribution feeders, 19 feeder variants, and 132 case-scenarios to investigate the role of network-specific factors like (a) phase unbalancing, (b) geo-electric size of the feeder and (c) load density in influencing the daily operations of UDNs with increasing presence of PVDGs. Results signify the consideration of feeder-specific factors in planning exercise of grouping the ‘similar’ feeders for formulating the strategies to mitigate the unwanted influence of DGs on operations of distribution feeders. The demonstrated case-specificity also implies that optimization algorithms for improved operations with DGs will need to be based on an integrated approach that accounts for feeder-specific factors as well as cyclic variability of PVDGs. It is then imperative to formulate an integrated approach that optimizes the daily operations of UDNs considering the role of network-specific factors and cyclic variability of PVDGs. This is achieved by proposing a novel two-stage approach based on Fuzzy Multi-Objective Optimization (FMOO) platform and using a novel algorithm to characterize the daily load profiles from among the set of annual load profiles for a given network. Using BESS as a medium of energy storage, a two-stage FMOO approach optimizes the daily operations of UDNs for objectives including (a) peak shaving, (b) loss minimization, (c) reducing tap-changer operations, (d) improving feeder voltage profile and (e) minimizing discharge-recharge cycles for BESS. The FMOO approach is tested on IEEE 37 node feeder and also validated on a utility feeder in Northwest USA. The same approach is extended to include the variability of PVDGs while improving operations of UDNs with BESS. The whole work can thus be summarized in a sentence – role of network-specific factors in influencing the daily operations of UDNs with PVDGs imply a high level of case-specificity that needs to be accounted for by an integrated approach to mitigate the undesired influence of PVDGs and to improve network operations using BESS.by Kalpeshkumar Arvindbhai JoshiPh.D