communication infrastructures for distributed control .communication infrastructures for distributed
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Communication Infrastructures for DistributedControl of Power Distribution NetworksQiang Yang, Javier A. Barria, Member, IEEE, and Tim C. Green, Senior Member, IEEE
AbstractPower distribution networks with distributed gen-erators (DGs) can exhibit complex operational regimes whichmakes conventional management approaches no longer adequate.This paper looks into key communication infrastructure designaspects, and analyzes two representative evolution cases of ActiveNetwork Management (ANM) for distributed control. Relevantstandard initiatives, communication protocols and technologiesare introduced and underlying engineering challenges are high-lighted. By analyzing two representative case networks (meshedand radial topologies) at different voltage levels (33kV and11kV), this paper discusses the design considerations and presentsperformance results based on numerical simulations. This studyfocuses on the key role of the telecommunications provision whenupgrading and deploying distributed control solutions, as part offuture ANM systems.
Index TermsPower distribution networks, Active NetworkManagement, Distributed Generators, SCADA, communicationsystem, distributed control
Todays electric power networks were built with an inte-grated and vertical structure in mind. The power energy ismostly generated in centralized power plants and transportedover a long distance transmission network to a distributionnetwork before reaching the end users. In this context thedistribution network is regarded as a passive system used todeliver reliable unidirectional power flows to end users. Atpresent most Distribution Network Operators (DNOs) managetheir networks via single (backed up) control center relying onSupervisory Control and Data Acquisition (SCADA) systemswhich were designed for simple and centralized operations.
In recent years, medium voltage (MV) distribution networkshave been faced with a continuity of modifications as renew-able energy generation resources in the form of small-scaleDGs (e.g. wind turbines, Combined Heat and Power (CHP)and solar energy) are being connected to the utility grid.This trend has mainly been driven by advances in DistributedEnergy Resources (DER) technology and the desire to achievelow-carbon energy provision targets. Amongst other character-istics of DGs is that they can be deployed closer to the loads
Manuscript received September 1, 2010. Accepted for publication February1, 2011.
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Q. Yang is with the College of Electrical Engineering, Zhejiang University,Hangzhou, 310027 PRC (e-mail: firstname.lastname@example.org).
J. Barria is with the Department of Electrical and Electronic Engineering,Imperial College, London, SW7 2AZ, UK (tel:+44-(0)20-7594-6275; fax:+44-(0)20-7594-6274; email:email@example.com).
T. Green is with the Department of Electrical and Electronic Engineering,Imperial College, London, SW7 2AZ, UK (email:firstname.lastname@example.org).
and the surplus energy could be absorbed by the grid with theadded benefit that demand during peak times could be bettermanaged. The UK Government expects the renewable sourcesto provide 10% of the total national electricity supply by 2010,which implies about 14 GW of generation from current MVdistribution networks.
With DGs, the distribution grid is no longer a passivesystem, but an active system interconnecting generators (e.g.wind power) which allows coexistence of bi-directional powerflows. Moreover, the energy supplied by these generators isoften intermittent and hence more difficult to predict and con-trol. This brings about new operations and control challengesto cope with, e.g. voltage raise effect, increased fault level,protection degradation and altered transient stability . Weenvisaged that a great number of DGs will be integrated intofuture power grids across a large geographical span. Moreover,at present many power utilities are being faced with the realityof conventional centralized control systems limitations, as theycan greatly degrade due to the complexity of dealing withnetwork events that would require enormous amount of datato properly manage them. To meet this emerging challenge,Active Network Management (ANM) solutions need to beincorporated in the network when DGs are part of the system.
IntelliGrid  and SmartGrids  are two research ini-tiatives that investigate the realization of future smart orintelligent energy networks. Another notable effort is theAdvanced Metering Infrastructure (AMI) (e.g. see ) whichis designed to measure, collect and analyze energy usagethrough interacting with smart meters via various commu-nication media. AMI allows distribution of information tocustomers, utilities and service providers enabling them toparticipate in or provide demand response solutions, oftenwith a requirement of significant communication infrastructurereinforcement. The AuRA-NMS (autonomous regional activenetwork management system) project  has also investigateda cost-effective ANM solution by implementing distributedand intelligent active network control to enhance energy secu-rity and quality of supply. The key idea behind this approachis to devolve the management authority from DNOs controlcenter to networked regional controllers deployed locally tocarry out management tasks in either autonomous or cooper-ative operational regimes. The AuRA-NMS identified manyrequirements on the underlying communication system whichcurrent SCADA systems can barely meet and that needs tobe well understood. This paper investigates communicationsinfrastructure design aspects, and analyzes two representativeevolution cases of Active Network Management (ANM) fordistributed control. We envisage that a plausible transition to
a fully automated ANM solution will be incremental due tothe nature of the current installed facilities, and hence, wetake the view that gradual upgrades of the communicationsinfrastructure will be the most likely roadmap. In this sensewe also constrain our analysis to established technologiesand standards so that ANM solutions would not depend oncustomized solutions and/or proprietary communications pro-tocols. To obtain the results reported in this paper we developa simulation environment that is flexible enough to assessvarious scenarios in which different technologies and protocolscould co-exist 1. The simulation environment includes all themajor features of legacy communication technologies so thatthe selected solutions could also be assessed in terms of itsbackward compatibility. In this paper we present:
1) A generic and flexible methodology for modeling andassessing different types of data traffic and its associ-ated communications system, to support distributed con-trol using ANM systems. Communications requirementsfrom distributed sensors, control commands and agentcommunications, can be easily specified.
2) Two case studies: the communication system designsare analyzed through two representative medium voltagenetworks (33kV and 11kV) based on aforementionedmodeling approach and assessment framework.
Even though, the proposed methodology is flexible enoughto analyze a variety of heterogeneous end-to-end communi-cations infrastructures performance, when different communi-cations technologies inter-operate, this paper presents only asubset of results due to space limitations.
The remainder of the paper is organized as follows: SectionII presents an overview of current communication provisionin DNOs. Section III explains the regional active networkmanagement approach and its key features, followed by adiscussion of related communication standards, protocols andtechnologies in Section IV. The major communication systemengineering challenges are discussed in Section V. Section VIpresents in details the approach of communication system andtraffic modeling. Section VII and VIII discusses the designconsiderations of two case networks and provides numericalresults obtained from simulation experiments, respectively.Finally, some concluding remarks are given in Section IX.
II. BACKGROUND: CURRENT DISTRIBUTION NETWORKMANAGEMENT
Current power distribution network operations rely mostlyon simple extension of SCADA systems which were mostlydesigned with a centralized architecture in mind interconnect-ing a master terminal (at control center) and a large numberof Remote Telemetry Units (RTUs) located at geographicallydispersed sites, as shown in Figure 1. The underlying feature ofthese systems is their heterogeneity in terms of communicationmedium (e.g. leased digital fibre, private wire, telephone lines,satellite and mobile radio) and channel capacity (from a fewhundred to a few thousand bits per second). These char-acteristics mean frequent communication media conversions
1The simulation environment source code can be obtained from the corre-sponding author.
with the undesirable impact on its end-to-end availability andaverage Bit Error Rate (BER). Furthermore, SCADA protocolsare mostly proprietary and designed specifically with errordetection and message retry mechanisms to guarantee datadelivery under most circumstances.
Fig. 1: Current control schemes: central control (a) and localcontrol (b)
RTUs act as relays to collect network operational states fromsensors (analogue and/or binary) and route control signals toactuating devices. Data acquisition is often carried out throughpolling in a non-continuous fashion, e.g. every 10-20 secondsat higher voltage sites (e.g. 33kV) and hours or even daysat lower voltage sites (e.g. 11kV). In addition, RTUs mayalso be able to send event driven data, e.g. field alarms. Atthe control ce