Microgrids: reliable power while integrating renewable generation

Authors: Dr Maria Brucoli, and Kevin O’Halloran, Arup A microgrid is a cluster of loads and localised distributed generation and storage sources operating as a single controllable unit. Usually, a microgrid is a small part of the medium- or low-voltage distribution network, where the power and, sometimes, the heat demand are supplied by local energy sources (e.g. photovoltaic units, micro turbines and fuel cells) and storage devices (e.g. flywheels and batteries). A microgrid can operate in grid-connected mode by using the energy produced on site and importing/exporting any excess from/to the grid. However, in response to a grid power quality event or for economic reasons, it can be disconnected from the main grid and operate in islanded mode. Control is the key element that enables the microgrid to appear as a single, controlled unit to the distribution network operator, independent of how individual sources or loads behave within the microgrid network. MAIN COMPONENTS [caption id="attachment_15531" align="alignright" width="4882"] Figure 1: Typical microgrid network layout (click to enlarge)[/caption] A typical example of a microgrid with its key components is shown in Figure 1 below. Distributed generation (DG) and distributed storage (DS) units are directly connected to the distribution network, together with the local loads. Often, DG and DS units are referred as distributed energy resources (DERs). The term DG generally refers to a heterogeneous group of power sources. Sometimes, the term DG is associated with renewable energy sources (RES); however, not all DG technologies are environmentally friendly. DG technologies available on the market range from traditional power sources like diesel generators to new technologies like microturbines, fuel cells, photovoltaic systems and wind turbines. Typical DS units include batteries and flywheels. The microgrid is connected to the main grid at the point of common coupling (PCC) through a separation device, which in Figure 1 is referred to as a microgrid disconnecting device (MDD). Control is a key component in the microgrid. A microgrid central controller (MCC) is responsible for regulating power production and consumption within the microgrid and performing actions like grid synchronisation. Control and other functions like protection require monitoring and communication capabilities, which add to the costs and therefore need to be considered at an early stage of the design. The requirements of the interconnected loads shape the microgrid generation mix and operation. High-specification microgrids are designed to provide customised energy services to improve resilience and power quality. On the other hand, rural microgrids have a profound impact on society as they supply loads that improve education, health, safety and enable manufacturing and trade. Because of the close match and interaction between generation and consumption in a microgrid (i.e. a stiff grid capable of providing constant voltage and frequency is not always available as power is locally generated and consumed), loads cannot be considered as a passive element; on the contrary they can be defined as 'prosumers'. A ‘prosumer’ is a load that can consume power, but also participate (at different levels) in demand response (DR) by reducing its load consumption or shedding. DR enables the microgrid to manage peak loads, generation shortage/unavailability and, depending on the controls installed at load points, participate in the system stability. End users can also get a financial return by committing to curtail/shed power loads through the MCC. HOW DOES IT WORK? [caption id="attachment_15533" align="alignright" width="486"] Figure 2: Microgrid operating and transition modes[/caption] As a single controllable unit, a microgrid can be controlled in four different modes as shown in Figure 2.

Two operational modes (grid-connected and islanded);
Two transition modes (island forming and grid synchronisation).

When operating in grid connected mode, the DERs are controlled as active and reactive power sources so that they inject into the network a set, controllable amount of power while using the grid voltage and frequency as a reference. In islanded mode, the sources are controlled in order to maintain the voltage and frequency within acceptable limits. The transition between grid-connected and islanded mode can be planned or unplanned. A scheduled transition is an intentional event determined by factors like maintenance or economical convenience. Unscheduled transition is usually caused by an unexpected event like a major fault in the grid. During the island-forming transition, the microgrid control has to be designed to support the system frequency and voltage. Any transients produced by this transition should be sufficiently damped in order to allow the newly formed islanded microgrid to reach a stable operation. Alternatively, island forming can be on the basis of an interruption followed by separation from the grid and then ‘black-starting’ the microgrid. The transition between islanded and grid-connected mode (grid synchronisation) is controlled so that the microgrid, with all its generation, can be safely re-connected to the grid. Given their flexible architecture and operation and the variety of generation and storage sources connected to it, microgrids can be designed to suit the end-user power requirements. In addition, microgrids offer the opportunity to strike a balance in the provision of secure, affordable and sustainable power supply – the so-called ‘energy trilemma’. Microgrids bring a number of benefits (technical, monetised and non-tangible) to different players (owner, end-users and power utility), which can summarised as follows:

Integration of distributed generation and storage with reduced impact on existing distribution network (e.g. voltage control, congestion management);
Local power production, including off-grid applications;
Reduction of greenhouse gas emissions;
Sustainable power production when using renewable energy sources;
Enhanced security of supply (e.g. natural or grid-based events);
Enhanced power quality;
Deferral of network investments;
Reduction of power losses associated with power transmission and distribution;
Economic benefits depending also on the regulatory framework;
Competitive electricity price in areas where electricity prices are high (e.g. remote areas relying on expensive diesel imports).

APPLICATIONS The shape and size of a microgrid can significantly vary because of the broad description associated with it. Microgrids can supply a single customer, a commercial park or a village as they operate in both grid-connected and off-grid mode. The table below summarises some of the key applications for microgrids.

Type	Description	Drivers
Remote & Off-Grid	Typically found in developing economies and geographical islandsCurrently the greatest number of operating microgrids	Absence of unreliable local access to gridHarvesting of local renewable energy sourcesCheaper electricity prices compared with diesel generators
Commercial & Industrial	Applies either to commercial parks, industrial/manufacturing facilities, office buildings, data centresSubstantial growth forecasted in the coming years	Security of supply (against natural and grid-based events)Custom-based power qualityReduced electricity costs (depending on location)Sustainability (green credentials)Improvement on distribution network operation
Community	Includes predominantly residential developments (e.g. land infill, planned communities, new towns)Application is gaining momentum following the recent storms and the increase in the residential load demandTo achieve widespread commercial acceptance needs standards in place and regulatory barriers removed	Security of supply (against natural and grid-based events)Reduced electricity costs (depending on location)Sustainability (green credentials)Integration of locally available renewable energy resourcesImprovement on distribution network operation
Military	Grid-connected military basesRemote/temporary military basesSecurity institutions	Independence from grid imports and fuel importsSecurity of supply (against natural and grid-based events)Integration of locally available renewable energy resources
Institutional & Campus	Major facilities including hospitals, prisons, university campuses, etcSingle owner microgridMarket segment prone to development of sophisticated microgrids	Security of supply (against natural and grid-based events)Custom-based power qualityReduced electricity costs (depending on location)Sustainability (green credentials)Improvement on distribution network operationEducational, state-of-the-art show piece

A real life microgrid in Cork PROJECT DESCRIPTION [caption id="attachment_15535" align="alignright" width="1006"]

Figure 3: Nimbus Building, Cork Institute of Technology[/caption] In 2012, Cork Institute of Technology (CIT), in partnership with United Technologies Research Centre Ireland Ltd (UTRC-I), installed a microgrid at the National Sustainable Building Energy Test-Bed (NSBET), which is located in CIT’s Nimbus Centre. NSBET is the first microgrid research test bed in Ireland. It is designed to support academic and industry research into smart grid technologies, demand-side management, microgrids and power electronics. The NSBET microgrid consists of multiple non-dispatchable and dispatchable distributed energy sources and energy storage integrated with an existing heating, ventilation and air-conditioning (HVAC) system and building management system (BMS). The main microgrid components include a 10 kW wind turbine, 50 kWe combined heat and power (CHP) unit with associated thermal storage, a 35kWh (85kW peak) lithium-ion battery storage system and a microgrid central controller. The wind turbine and the batteries are connected to the microgrid through a power electronics converter. [caption id="attachment_15536" align="alignright" width="1920"]

Figure 4: Overview of Microgrid Central Controller Main Screen (click to enlarge)[/caption] Flexible system architecture ensures the microgrid can be expanded and reconfigured in the future. The control strategy also allows the different generating units to be integrated into the microgrid in a co-ordinated way and for the microgrid to operate in both grid-connected and islanded mode. In particular, the control is based on a hierarchical structure with different layers that start at the component level (e.g. voltage and current control loops), include islanding detection and synchronisation control and end with the microgrid energy management system (EMS). The EMS control is implemented through a SCADA (supervisory control and data acquisition) system designed and developed by UTRC-I, Neodyne and Arup. Arup’s role was key in the design and delivery of the NSBET microgrid. Since its inception, Arup supported CIT and UTRC’s ambitious vision for the project and provided technical, academic and yet practical advice. Given the challenging and novel nature of the project, Arup put together a diverse project team with a wide range of expertise including microgrid academic research, electrical and mechanical engineering, building control systems, project management, monitoring and commissioning. Arup’s specific design and project responsibilities included: microgrid system architecture; electrical network design; mechanical integration of the CHP and thermal storage into the existing building heating system; upgrading the BMS; specification of the microgrid central controller (MCC); procurement of specialist equipment; commissioning witnessing and construction monitoring. TESTING AND REAL-LIFE OPERATION [caption id="attachment_15537" align="alignright" width="941"]

Figure 5: Microgrid voltage during transition from grid-connected to islanded mode[/caption] The NSBET allows a wide range of research activities across several disciplines to be carried out. It has been in almost constant use since its installation and the facility has the ability to operate in ‘normal’ and ‘experimental’ modes with extensive control of each element allowed in ‘experimental’ mode. A wide range of activities can be carried out from monitoring and testing individual components (e.g. wind turbines) to microgrid network-wide system analysis. Figure 6 shows the metered wind turbine power output (as a function of wind speed), power flows within the microgrid and grid synchronisation. The capability of obtaining real test-data on the various installed generation and storage technologies, and the potential for collaboration between a wide range of interested parties, make the NSBET a hugely important and unique facility. The task of integrating these diverse components to function together correctly in a microgrid application is challenging. There must be a higher level of engagement from equipment manufacturers and suppliers than is usual in the construction of conventional building projects. The components used must be supplied ready for configuration into the microgrid, for example, suitably pre-configured power electronics. Project team • Clients: Cork Institute of Technology & UTRC Ireland • Lead consultant: Arup • Main contractor: Kiernan Electrical • SCADA: NeoDyne Further reading [1] Asmus. P. ‘Why microgrids are moving into the main stream.’ IEEE Electrification Magazine, Vol. 2, Issue 1, pp.12-19, March 2014. [2] Hatziargyriou, N; Asano, H; Iravani, R; and Marnay, C. ‘Microgrids.’ IEEE Power and Energy Magazine, vol. 5, no. 4, pp. 78–94, July/August 2007. [3] Lasseter B, et al. ‘Integration of distributed energy resources: The certs microgrid concept.’ Consortium for Electric Reliability Technology Solutions, April 2002. [4] IEEE Std. 1547.4-2011, ‘IEEE Guide for design, operation and integration of distributed resource island systems with electric power systems.’ IEEE Standards Association, July 2011. [5] Katiraei, F; Iravani, R; Hatziargyriou, N; and Dimeas, A. ‘Microgrids Management.’ IEEE PES Magazine, May/June 2008. Dr Maria Brucoli has more than 10 years’ experience working on microgrids. From 2004 to 2009, she was at Imperial College, London, first as a PhD student and then as a research associate. While at Imperial, her research was focused on the design of microgrid protection systems, modelling of inverter-interfaced distributed energy sources and developing suitable microgrid fault analysis techniques. Since joining Arup in 2009, she has been working on a number of microgrid projects including a research lab, the protection system for a rural microgrid, a number of off-grid systems and a research project on urban microgrids. Kevin O’Halloran is a mechanical engineer and energy consultant with six years’ experience in the commercial, science, industry and educational sectors in Ireland and New Zealand. O’Halloran is responsible for the delivery of co-ordinated building services design solutions from Arup’s Cork office. He has particular interest in sustainable building design and low-energy solutions. For more information about the work carried out in the field of microgrids, email john.burgess@arup.com and visit the Arup microgrid site.