Several studies on voltage stability analysis of electric systems with high photovoltaic (PV) penetration have been conducted at a power transmission level, but very few have focused on small area networks of low-voltage. As a distribution system has its special characteristics-high R/X ratio, long tap switching delay, small PV units and so on, PV integration impacts also need to be investigated thoroughly at a distribution level. In this paper, the IEEE 13 bus system has been modified and extended to explore network stability impacts of variable PV generation and the results show that a voltage stability issue with PV integration does exist in distribution networks. Simulation comparisons demonstrate that distribution networks are traditionally designed for heavily loaded situations exclusive of PVs, but they can still operate under low PV penetration levels without cloud induced voltage stability problems. It is also demonstrated that voltage instability can effectively be solved by PV inverter reactive power support if this scheme is allowed by the standards in the near future.

Abstract-There are more large-scale PV plants being established in rural areas due to availability of low priced land. However, distribution grids in such areas traditionally have feeders with low X/R ratios, which makes the independent reactive power compensation method less effective on voltage regulation. Consequently, upstream Step Voltage Regulator (SVR) may suffer from excessive tap operations with PV induced fast voltage fluctuations. Although a battery energy storage system (BESS) can successfully smooth PV generation, frequent charge/discharge will substantially affect its cost effectiveness. In this paper, a real-time method is designed to coordinate PV inverters and BESS for voltage regulation. To keep up with fast fluctuations of PV power, this method will be executed in each 5s control cycle. In addition, charging/discharging power of BESS is adaptively retuned by an active adjustment method in order to avoid BESS premature energy exhaustion in a long run. Finally, through a voltage margin control scheme, the upstream SVR and downstream PV inverters and BESS are coordinated for voltage regulation without any communication. This research is validated via an RTDS-MatLab co-simulation platform, and it will provide valuable insights and applicable strategies to both utilities and PV owners for large-scale PV farm integration into rural networks.Index Terms--Coordinated voltage control, photovoltaic (PV), battery energy storage system (BESS), real-time control, state of charge (SOC) regulation.

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Abstract-The installed capacity of wind generation and photovoltaics (PV) in many countries is going to dominate generation fleets in a bid to meet growing renewable energy targets. Synchronous inertia has never been problematic as there was more available than needed, but it is being significantly reduced due to the increasing integration of non-synchronous renewable generation. When the low bidding priced generation of wind and PV becomes considerably large, conventional economic dispatch algorithms can result in less online synchronous inertia and put power system security at risk. However, the compromise of power system security due to synchronous inertia shortage is not well studied in literature. This paper develops a synchronous inertia constrained economic dispatch algorithm to satisfy the minimum required synchronous inertia of frequency control. Synchronous condensers and wind reserve are economically allocated to alleviate any shortage of synchronous inertia and frequency control ancillary services (FCAS). A Gaussian particle swarm optimization algorithm is introduced to simultaneously co-optimize the dispatch of synchronous generators and their FCAS, wind reserve and synchronous condensers.Index Terms-Synchronous inertia, economic dispatch, wind turbine, renewable energy, power system.  NOMENCLATURE   CtOverall power system operational cost in the economic dispatch cycle t in terms of net load   [2] are also crucial, the most basic and challenging problem lies in the electricity supply side.Economic dispatch algorithms proposed in literature have focused on the improvement of algorithm performance [3,4] and consideration of more operational constraints [5], such as spinning reserve and generator ramp rates. They did not dynamically evaluate the synchronous inertia adequacy of a dispatch result in terms of N-1 contingency [3][4][5]. This is due to the fact that synchronous inertia adequacy is not a problem for a conventional power system with limited non-synchronous generation.However, synchronous inertia is being materially reduced due to ongoing displacement of synchronous generators by asynchronous ones, such as wind turbine generators and photovoltaic (PV) panels. Electronic inverters used in wind turbines and PV panels lack the synchronous inertia that can help an RPS survive a major disturbance.If comprised of less online synchronous inertia, an RPS may routinely suffer a rapid rate of change of frequency (RoCoF) and a large frequency deviation following a disturbance. A great RoCoF of 6 Hz/s was recorded in the South Australian blackout on September 28, 2016 which was caused by the "lightness" of the South Australian power system [6]. The instantaneous penetration level of wind and PV generation was over 50% before the blackout in South Australia and only three thermal power plants were dispatched [6].This paper therefore proposes a synchronous inertia constrained economic dispatch to keep the minimum amount of synchronous inertia online. The proposed algorithm introduces a feedback loop...

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In a geographically small distribution area fast moving clouds may cover the whole area within a short period causing photovoltaic (PV) power to drop. When a feeder loses PV power support bus voltages will decrease. In an unbalanced network asymmetrical spacing and non-transposition of line configurations can result in different voltage drops for each phase. This may potentially cause some voltage problems after a decline in PV generation, such as an extremely low voltage magnitude of a certain phase and an unacceptable voltage imbalance level at a remote bus. This paper proposes a method of analyzing voltage variation sensitivity due to PV power fluctuations in an unbalanced network (unbalanced line configuration and phase loading levels). Based on this method a network reconfiguration solution is developed to solve the voltage problems. This solution utilizes unbalanced line characteristics and realizes the potential of the network, so no extra compensation devices are needed for network support.

Contribution of renewable energies in power systems is increasing due to continuous growth of wind and solar generators. Because of intermittency and uncertainty of these resources, conventional reliability evaluation methods are not applicable and different techniques have been developed to model these generators. However, most of these methods are time-consuming or may not be able to keep time dependency and correlations between renewable resources and load. Therefore, this paper intends to improve the existing methods and proposes a fast and simple approach. In this approach, wind power, PV generation and electricity demand are being modelled as time dependent clusters, which not only can capture their time dependent attributes, but also is able to keep the correlations between these data sets. To illustrate the effectiveness of this framework, the proposed methodology has been applied on two different case studies: IEEE RTS system and South Australia power network. The developed technique is validated by comparing results with sequential Monte Carlo technique.

In distribution power systems line transposition is not a common practice and phase loading levels are always changing. Therefore, perfect balance is never achieved at a distribution level. Non-transposed lines can cause unequal voltage drops between phases and this may be further exacerbated by uneven loads in each phase. Consequently, the voltage magnitude of a certain phase may suffer a severe decrease, breaching the lower voltage limit or unbalanced three-phase voltages at a remote bus may violate the percentage voltage imbalance accepted in the standards. This paper examines the mechanism of causing uneven voltage drops and investigates the impacts of unbalanced line configurations with different phase loading levels on network voltage imbalance. Methodologies are proposed to analyze voltage drops affected by line and load imbalance. Based on these methods, general guidelines are recommended to mitigate the voltage problems, which are useful for network analysis, planning and reconfiguration.