The increasing interest in integrating intermittent renewable energy sources into microgrids presents major challenges from the viewpoints of reliable operation and control. In this paper, the major issues and challenges in microgrid control are discussed, and a review of state-of-the-art control strategies and trends is presented; a general overview of the main control principles (e.g., droop control, model predictive control, multi-agent systems) is also included. The paper classifies microgrid control strategies into three levels: primary, secondary, and tertiary, where primary and secondary levels are associated with the operation of the microgrid itself, and tertiary level pertains to the coordinated operation of the microgrid and the host grid. Each control level is discussed in detail in view of the relevant existing technical literature.

Trends in Microgrid Control

Many areas in power systems require solving one or more nonlinear optimization problems. While analytical methods might suffer from slow convergence and the curse of dimensionality, heuristics-based swarm intelligence can be an efficient alternative. Particle swarm optimization (PSO), part of the swarm intelligence family, is known to effectively solve large-scale nonlinear optimization problems. This paper presents a detailed overview of the basic concepts of PSO and its variants. Also, it provides a comprehensive survey on the power system applications that have benefited from the powerful nature of PSO as an optimization technique. For each application, technical details that are required for applying PSO, such as its type, particle formulation (solution representation), and the most efficient fitness functions are also discussed.

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Particle Swarm Optimization: Basic Concepts, Variants and Applications in Power Systems

Carbon capture and storage (CCS) is broadly recognised as having the potential to play a key role in meeting climate change targets, delivering low carbon heat and power, decarbonising industry and, more recently, its ability to facilitate the net removal of CO2 from the atmosphere. However, despite this broad consensus and its technical maturity, CCS has not yet been deployed on a scale commensurate with the ambitions articulated a decade ago. Thus, in this paper we review the current state-of-the-art of CO2 capture, transport, utilisation and storage from a multi-scale perspective, moving from the global to molecular scales. In light of the COP21 commitments to limit warming to less than 2 °C, we extend the remit of this study to include the key negative emissions technologies (NETs) of bioenergy with CCS (BECCS), and direct air capture (DAC). Cognisant of the non-technical barriers to deploying CCS, we reflect on recent experience from the UK's CCS commercialisation programme and consider the commercial and political barriers to the large-scale deployment of CCS. In all areas, we focus on identifying and clearly articulating the key research challenges that could usefully be addressed in the coming decade.

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Carbon capture and storage (CCS): the way forward

This paper provides an overview of grid code technical requirements regarding the connection of large wind farms to the electric power systems. The grid codes examined are generally compiled by transmission system operators (TSOs) of countries or regions with high wind penetration and therefore incorporate the accumulated experience after several years of system operation at significant wind penetration levels. The paper focuses on the most important technical requirements for wind farms, included in most grid codes, such as active and reactive power regulation, voltage and frequency operating limits and wind farm behaviour during grid disturbances. The paper also includes a review of modern wind turbine technologies, regarding their capability of satisfying the requirements set by the codes, demonstrating that recent developments in wind turbine technology provide wind farms with stability and regulation capabilities directly comparable to those of conventional generating plants.

A review of grid code technical requirements for wind farms

Plug-in electric vehicles (PEVs) present environmental and energy security advantages versus conventional gasoline vehicles. In the near future, the number of plug-in electric vehicles will likely grow significantly in the world. Despite the aforementioned advantages, the connection of PEV to the power grid poses a series of new challenges for electric utilities. This paper proposes a comprehensive approach for evaluating the impact of different levels of PEV penetration on distribution network investment and incremental energy losses. The proposed approach is based on the use of a large-scale distribution planning model which is used to analyze two real distribution areas. Obtained results show that depending on the charging strategies, investment costs can increase up to 15% of total actual distribution network investment costs, and energy losses can increase up to 40% in off-peak hours for a scenario with 60% of total vehicles being PEV.

Assessment of the Impact of Plug-in Electric Vehicles on Distribution Networks

Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation

This paper describes a methodology for assessing the transmission network adequacy in systems with significant contribution of wind generation. The historical network design standards were conceptualised for conventional generation and applying those standards to wind generation, given its lower reliability performance, is shown to lead to transmission overinvestment when wind generation is located in an exporting area or to underinvestment and a significant increase in the risk of loss of supply when it is located in an importing area. The objective of this work is therefore to update the methodology for determining the adequacy of transmission capacity to reflect the characteristics of wind generation, while maintaining the overall philosophy of the existing standards. This work is conducted to inform the review of the Great Britain (GB) network design standards to incorporate wind generation. Case studies are presented in the context of a simplified GB transmission system model to demonstrate that wind power tends to drive less transmission capacity than conventional generation.

Reliability-driven transmission investment in systems with wind generation

The UK Government is working towards ambitious targets on renewable generation and CHP. Depending on the load factor assumed, this will require the installation of significant generation capacity, in the order of 10 GW by 2010. Significant proportion of this generation is expected to be connected to distribution networks (i.e. at voltages of 132 kV and below). However, under the present conditions the owners and operators of the distribution networks, the Distribution System Operators (DSOs) anticipate that they can integrate only a much more limited capacity of distributed generation (DG) without a major reinforcement. Recently, significant amount of work was carried out to analyse and demonstrate the benefits that can be derived from changing the operation philosophy of distribution networks from passive to active. The emphasis was on the design of control strategies of active distribution systems that would enhance the ability of the existing networks to accommodate additional distributed generation, and reduce the demand for reinforcements. Hence there is a need to understand the impact of various active management control strategies on the overall operation and investment performance of the entire distribution network. This kind of analysis will be essential for DSOs in order to take the most cost effective strategic decisions to optimally integrate DG in their networks. This paper presents a methodology for quantifying the value of various active distribution network management for various DG development scenarios, in the context of the UK distribution system. First the main network issues related to the connection of DG are discussed, and how they could be resolved by using active management. Indicative results quantifying the benefits of active management in the entire UK distribution network are then presented.

Benefits of active management of distribution network in the UK

In this study, the implementation of virtual power plant (VPP) as a means to coordinate the use of distributed resources for different control objectives by transmission system operator and distribution system operator is described. In order to illustrate the concept, a range of illustrative studies demonstrating the application of VPP concept on a real 11 kV system in Brixton will be presented, using data from the Low Carbon London project. The studies demonstrate the changes in the operating characteristics of the VPP area over a range of system operating conditions.

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Virtual power plant: managing synergies and conflicts between transmission system operator and distribution system operator control objectives

Within a general Microgrid framework, this paper presents a model to simulate closed loop energy and ancillary services markets operated by a Microgrid Central Controller (MGCC) and to investigate the performance of such a market mechanism in managing disturbances both within and outside the Microgrid cell. The proposed approach can be seen as an alternative to a more traditional control approach in which the central operator optimises the dispatch of all generators taking into account their bids and then instructing them to produce their outputs at the specified prices according to merit order and network constraints. In the alternative approach discussed here, this dispatch decision is decentralised to each controllable unit. The central operator only influences the decision taken by each unit via price signals. The main advantage of the approach is to preserve the independency of the local controllers and to decentralise the operation decision, making the system more able to cope with a large number of generating units. The performance of the proposed market mechanism is examined via different case studies that simulate the control actions of MGCC, micro-sources and controllable loads in managing small and large disturbances within and outside the Microgrid cell. Copyright © 2010 John Wiley & Sons, Ltd.

Danny Pudjianto

Papers

Strategic Development of North Sea Grid Infrastructure to Facilitate Least-Cost Decarbonisation

Reliability-driven transmission investment in systems with wind generation

Benefits of active management of distribution network in the UK

Virtual power plant: managing synergies and conflicts between transmission system operator and distribution system operator control objectives

Closed loop price signal based market operation within Microgrids