Increase the Hosting Capacity of 4-wire Low Volage Supply Network for Embedded Solar Generators by Optimising Generator and Load Placement on the Three Supply Phases
نویسندگان
چکیده
Australia has topped the world in the penetration of residential roof-top solar generation systems. These residential solar generation systems are connected to the Low Voltage (LV) supply network which is not designed for two-way flow of electricity. Localized voltage rises due to excess solar generation feeding energy back into the supply network is commonly experienced. Supply utilities do not generally select the phases for the connection of solar generators. When load/generation is not balanced between the three LV supply phases, phase voltage unbalance arises as a result which aggravates the phase voltage condition due to neutral voltage shift. Balancing loads and solar generation on each phase is quite complex due to the stochastic nature of loads and solar generation. This paper proposes an allocation methodology based on minimizing voltage unbalance on the supply nodes, subject to the constraint that voltages on each supply node/bus are within the regulatory limits. The effectiveness of the approach is illustrated by computer modelling studies implemented in MATLAB and Open Distribution System Simulator (OpenDSS), using load and network data of a real LV network in Australia. INTRODUCTION Australia has topped the world in the penetration of residential roof-top solar generation systems. With a population of only 24 Million, there are almost 1.5 Million grid connected residential solar installations approaching 5,000MW of installed capacity by June 2016, and the number continues to grow. Similar to European LV network design, Australian electricity supply utilities run extensive 4-wire LV (230/400V) reticulations along the streetscape. The LV distribution network employs a Multiple Earthed Neutral (MEN) design where the neutral conductor is earthed at the distribution transformer and at points of connection into customer premises. Automatic voltage regulation is implemented upstream of the distribution transformer on the Medium Voltage (MV) network using on-load tap changing transformers, shunt capacitor banks and in-line voltage regulators (for long rural lines). Distribution transformers are equipped with off-load tap changing facility only. MV automatic voltage regulation is designed with peak load period in mind, results in steady-state supply voltage at the high side of the prescribed range most of the time [1]. This high LV supply voltage is not confined to Australian practice but has also been reported in other countries, e.g. UK [2]. The growth in residential roof-top solar generation systems has highlighted the undesirability of this voltage regulation approach, as it results in little headroom for the voltage rise caused by the connection of embedded solar generators. In addition, imbalance between loads and generations on the three phases results in voltage unbalance which affects both network and customer equipment [3], [4], and will exacerbate the steady-state voltage problem. Unless these challenges are overcome in a cost effective manner, curtailment of solar installations or expensive network augmentation would be required to accommodate the increasing penetration of solar installations. STANDARDS FOR VOLTAGE UNBALANCE A number of standards exist for defining voltage unbalance using either phase-to-neutral (Van, Vbn, Vcn) or phase-to-phase voltages (Vab, Vbc, Vca) [5], [6]: IEEE Standard 936 (1987): Phase voltage unbalance rate (PVUR) PVUR = Maximum {Van , Vbn , Vcn} − Minimum {Van , Vbn , Vcn} Mean {Van , Vbn , Vcn} x 100% (1) IEEE Standard 112 (1991): Modified phase voltage unbalance rate (PVURmod) PVURmod = Maximum deviation from Mean {Van , Vbn , Vcn} Mean {Van , Vbn , Vcn} x 100% (2) NEMA (National Electric Manufacturers Associations of the USA) Standard (1993): Line voltage unbalance rate (LVUR) LVUR = Maximum deviation from Mean {Vab , Vbc , Vca} Mean {Vab , Vbc , Vca} x 100% (3) IEEE Standard (1996) and IEC Standard 61000-4-27: Voltage unbalance true definition (VUTD) 24th International Conference on Electricity Distribution Glasgow, 12-15 June 2017
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