An overview of numerical and mathematical modelling-based distributed generation (DG) system optimisation techniques is presented in this review paper. The objective is to compare different aspects of these two broad classes of DG optimisation techniques, explore their applications, and identify potential research directions from reviewed studies. Introductory descriptions of general electrical power system and DG system are first provided, followed by reviews on renewable resource assessment, load demand analysis, model formulation, and optimisation techniques. In renewable resource assessment model review, uncertain solar and wind energy resources are emphasised whereas applications of forecasting models have been highlighted based on their prediction horizons, computational power requirement, and training data intensity. For DG optimisation framework, (solar, wind and tidal) power generator, energy storage and energy balance models are discussed; in optimisation technique section, both numerical and mathematical modelling optimisation methods are reviewed, analysed and criticised with recommendations for their improvements. In overall, this review provides preliminary guidelines, research gaps and recommendations for developing a better and more user-friendly DG energy planning optimisation tool.

Review of distributed generation (DG) system planning and optimisation techniques: Comparison of numerical and mathematical modelling methods

The rapid development of photovoltaic (PV) systems in electrical grids brings new challenges in the control and operation of power systems. A considerable share of already installed PV units is small-scale units, usually connected to low-voltage (LV) distribution systems that were not designed to handle a high share of PV power. This study provides an in-depth review of methods and strategies proposed to prevent overvoltage in LV grids with PV and discusses the effectiveness, advantages, and disadvantages of them in detail. On the basis of the mathematical framework presented in this study, the overvoltage caused by high PV penetration is described, solutions to facilitate higher PV penetration are classified, and their effectiveness, advantages, and disadvantages are illustrated. The investigated solutions include the grid reinforcement, electrical energy storage application, reactive power absorption by PV inverters, application of active medium-voltage to LV transformers, active power curtailment, and demand response. Coordination between voltage control units by localised, distributed, and centralised voltage control methods is compared using the voltage sensitivity analysis. On the basis of the analysis, a combination of overvoltage prevention methods and coordination between voltage control units can provide an efficient solution to increase the PV hosting capacity of LV grids.

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Methods and strategies for overvoltage prevention in low voltage distribution systems with PV

Photovoltaic embedded generation in low-voltage ac networks is quite popular; however, despite its benefits, there are some problems especially when photovoltaic (PV) penetration exceeds certain thresholds. Among others, voltage violation is of prime importance. Our review of the literature focused on PV penetration limits due to voltage violations in low-voltage (LV) networks. The review revealed that voltage violations can occur at a penetration level as low as 2.5% when a large distributed generator (DG) is installed at a single point. Alternatively, a LV network can host a large number of photovoltaic distributed generators (PVDGs), with a penetration level up to 110% if evenly distributed over shorter lengths. However, an LV network has no rules of thumb for safe penetration limits. Penetration-level calculations have been found that they used numerous approaches, which we have analyzed and discussed to adopt a more rational and unified approach. Our literature review revealed that, in LVs, a very high penetration level can be achieved as compared with medium-voltage (MV) networks. However, MV voltage-level control problems impose a limit for PV hosting in LV networks. There is a need to evolve strategies for robust voltage control at the MV level and to develop certain rules of thumb for PV penetration limits in LV networks independent of the MV level, to increase the PV hosting capacity.

PV Penetration Limits in Low Voltage Networks and Voltage Variations

A plug-and-play connection of spatially distributed microgrids with renewable and non-renewable energy sources is one of the proposed descriptions of the evolving smart grid. Moreover, there is a gradual shift on distributed generation (DG) over the years to focus more on renewable generation due to global incentivization, climatic issues (such as greenhouse gas emission), and energy market deregulation. This article places attention on photovoltaic (PV) system as an emerging power generation paradigm enabled by the evolving smart grid. One of the critical constraints in this game-changing technology is the high propensity for a mismatch between its power output and load profile characteristics, which could result into severe voltage violations, high losses and a larger reverse power during low load conditions. Consequently, the concept of dispatchability with load-following function for various PV deployment scenarios is crucial for the 21st century grid modernization. The dispatchable PV unit output is controllable when deployed with storage systems making it suitable for ancillary services and demand-side management application. Also, dispatchability implies the ability to remotely start and stop the operation of PV unit when needed to maintain grid reliability. This article presents various deployment scenarios of dispatchable PV units and their impacts on the evolving smart grid. Additionally, this paper reviews the evolution of control, monitoring and communication systems in the active distribution network, utility-interactive inverter technologies, dispatchable grid-connected hybrid system deployments, and communication network configuration for dispatchable distributed energy resource (DER) units integration into the electric power system (EPS).

Evolution of dispatchable photovoltaic system integration with the electric power network for smart grid applications: A review

Optimal oversizing of utility-owned renewable DG inverter for voltage rise prevention in MV distribution systems

This study deals with the overvoltage problems caused by the increased photovoltaic (PV) penetration in typical rural radial distribution systems, where small PV systems and household consumers are connected. For this study, the authors used an iterative algorithm to solve the power flow problem in radial grids and considered three methods for the correction of unacceptable overvoltages, which are based on different principles. The system under consideration is an almost symmetrical three phase, low-voltage radial network, consisting of nine PV and consumer connection points. The system was simulated for a whole year with a sample time of 15 min, taking into account the variations of the solar radiation and loads. The results verified that the control of reactive power on the line can successfully manage overvoltage issues. The additional demand of reactive power and consequent line losses are used to evaluate various correction methods.

Ioannis A. Drougakis

Papers

Photovoltaic systems in low-voltage networks and overvoltage correction with reactive power control