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Loughborough University Institutional Repository A novel architecture for power networks with distributed generation concept outline This item was submitted to Loughborough University’s Institutional Repository by the/an author. Citation: GARLICK, W.G., ZOLOTAS, A.C. and INFIELD, D.G., 2006. A novel architecture for power networks with distributed generation - concept outline. International Control Conference (ICC2006), Glasgow, Scotland, United Kingdom, 30th August - 1st September. Additional
  Loughborough UniversityInstitutional Repository A novel architecture for power networks with distributed generation -concept outline This item was submitted to Loughborough University’s Institutional Repositoryby the/an author. Citation:  GARLICK, W.G., ZOLOTAS, A.C. and INFIELD, D.G., 2006. Anovel architecture for power networks with distributed generation - concept out-line. International Control Conference (ICC2006), Glasgow, Scotland, UnitedKingdom, 30th August - 1st September. Additional Information: ã  This paper was presented at ICC2006: Conferences/Control2006 Metadata Record: Version:  Not specified Publisher:  ICC2006Please cite the published version.    This item was submitted to Loughborough’s Institutional Repository ( by the author and is made available under the following Creative Commons Licence conditions. For the full text of this licence, please go to:     A NOVEL ARCHITECTURE FOR POWER NETWORKS WITH DISTRIBUTED GENERATION - CONCEPT OUTLINE W. G. Garlick  1 , A. C. Zolotas 1 . D. G. Infield 1 . 1  Systems and Control Group, Loughborough University, Loughborough, LE11 3TU. Abstract: Providing a secure power network is already a complex task but as network complexity grows with each new distributed generation connection so the problem of assurance for the next source connection becomes more and more protracted. A fundamental change to the network architecture may be necessary and an architecture  based on cells containing both generation and loads has been proposed by some researchers. This paper proposes a novel power cell interface, replacing the conventional power transformer with an “active transformer” in order to provide a controllable, flexible and robust connection that will facilitate greater network management and business opportunities and new power flow control features. Copyright © 2006 GARLICK    Keywords: Active transformer, converter control, distributed generation, large power networks, renewable energy sources, system control.   1 INTRODUCTION Figure 1 shows the recently commissioned wind farm development at the Kentish Flats in the Thames estuary, one of the UK Department of Trade and Industry (DTI) and the Crown Estate first round sites. In December 2003, a second round of licences for off-shore wind farms was announced. Twelve developers were successful and 15 site leases offered, with a potential generation capacity ranging from 64 to 1200 MW, the largest being 1,200 MW at greater than 12 miles offshore. Existing power distribution networks have not been designed to accept extensive distributed generation and the size and distance offshore of the proposed generation  presents new technical challenges to their connection. The DTI and the Office of Gas and Electricity Markets (Ofgem) created and jointly chaired the Distributed Generation Co-ordination Group (DGCG). The Group was concerned with a wide range of issues related to the connection and operation of distributed electricity generation in Fig. 1. Kentish Flats wind farm during construction Great Britain, (Cooke, and Baker 2002; Collinson, 2003). The issues included the consideration and making of recommendations as to any research and development action that may be helpful to achieving Government targets for the generation of electricity  from renewable energy sources. A key objective of DGCG Workstream 3 (WS3), was to establish how to facilitate the connection of distributed generation to distribution networks, without driving reinforcement costs high and without impairing the quality and reliability of the supply to customers. The problems and the solutions that the group  proposed were categorized in terms of managing fault levels, voltage levels and network power flows. STATCOMs and Active Network Voltage Control solutions were identified as long term solutions requiring significant additional research and development. This paper describes a novel active network solution that addresses directly the problems identified by the DGCG. 2 BACKGROUND 2.1 Distributed Generation With the introduction of larger scale distributed generation, e.g. wind farms or larger biomass  projects, system stability and fault current capability needs to be assured before each new energy source can be connected. This is already a complex task but as the power network grows with each new connection so the problem of assuring stability and fault capability connection becomes more difficult. The Distribution Network Operators’ (DNOs) ability to increase the capacity of the current network is thus limited and a more flexible, active network will be needed to assure future electrical supplies and network development. The stable control of power flow and voltage have  been key issues since the early days of electrical  power generation and distribution (Steinmetz, 1920) and has been an ongoing concern as networks have grown to meet the increasing demand for electrical  power (Tesserson et al. , 1988). The integration of large wind farms, particularly those proposed for the more extensive off-shore sites, into existing power networks presents new management and control challenges for network engineers. The current approach is to examine each proposed connection and assess its compliance with the Grid Code (NGC (2004), Bolik, 2003) to determine the need for any special network support of control measures. These measures are generally costly and cannot be undertaken lightly. Control technologies, such as HVDC and Static Compensators (STATCOMs), are sometimes specified as control measures. These are based on  power electronic devices and may prove to be the only feasible and economic way of complying with current Grid Codes in the near term. Development of these control technologies is currently being undertaken in industry where the equipment is manufactured. HVDC can offer some significant benefits not normally available to network operators. They allow  power flow control and frequency decoupling and considerably increase the potential for meeting the Grid Code requirements. Further, the reduced operational power losses and cabling requirements for longer connections provide some cost advantage over an a.c. connection. Recently, small on-shore wind farms have been successfully connected using “HVDC Light”; e.g. 8MW scheme at Tjaereborg, Denmark, this scheme uses voltage source converters (VSC) with IGBT devices. Contemporary schemes are however unlikely to meet all future integration needs and a fundamental change to the network architecture and its control is  proposed to facilitate network development and robust integration of a high penetration of renewable energy generation. 2.2 Network Archi t ecture A power distribution system based on a single-phase high frequency resonant link was described by Sood and Lipo (1986) and in a subsequent paper, Sood and Lipo (1988), a pulse density modulated (PDM)  power converter utilizing zero voltage switching was  proposed. Little further research or applications have so far been found of this architecture until the  publication of Oates’ patents (2002, 2003) and, Dang (2006) at Nottingham University. However, research is currently being undertaken on new network architectures at UMIST (Strbac et al., 2004) and ETH Zurich (Geidl, 2004). Both schemes seek to divide power networks into small cells or micro-grids that incorporate electrical generation, storage and loads that are managed and controlled centrally by a dedicated controller. The UMIST scheme is based on the control of power flow and voltage whilst the ETH schemes controls energy. Both schemes use a conventional power transformer as the connection to the rest of the network. Whatever solutions are eventually used to connect distributed generation to power networks, their capital cost and reliability will be significant considerations. 2.3 Patents A patent by Oates (2002) describes an electrical substation, a “solid state substation”, based on high frequency d.c. link power converters that overcomes some of the limitations of conventional tap-changing  power transformers and permits a degree of control usually provided by a static VAR compensator (SVC). He also suggests the application of direct conversion and that “the input switching network may include a resonant circuit”. A high voltage
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