IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 3, AUGUST 2008 1523

Actively Managing Wind Farm Power OutputRobert A. F. Currie, Student Member, IEEE, Graham W. Ault, Member, IEEE, Robert W. Fordyce,

David F. MacLeman, Mark Smith, and James R. McDonald, Senior Member, IEEE

Abstract—This letter presents the outcomes of a trial of an activenetwork management (ANM) scheme on part of the North-Scot-land electricity network. The ANM scheme has as its goal the fa-cilitation of increased connections of distributed generation (DG)from renewable resources on the Orkney Isles. The ANM scheme isthe first multigenerator application of ANM at distribution level inthe U.K. The ANM scheme is briefly discussed prior to the trial ofthe ANM scheme being introduced. The implications of the trial forfull ANM roll-out are discussed. The results of the trial of the ANMscheme are of direct relevance to those working in the areas ofANM, smart grids, and advanced distribution automation. The re-sults confirm that wind farms can be controlled within the thermalconstraints of a distribution network subject to certain operatingmargin considerations.

Index Terms—Active network management, distributed genera-tion, renewable energy.

I. INTRODUCTION

T HE Orkney Islands are the focus of significant activity inthe generation of electricity from renewable resources. A

collaborative project between the University of Strathclyde andScottish Hydro Electric Power Distribution plc (the network op-erator) resulted in the initial specification of the Orkney activenetwork management (ANM) scheme [1]. This formed the basisfor an application to the U.K. regulator to designate Orkneya Registered Power Zone (RPZ) [2]. This successful applica-tion paved the way for the detailed design and trial of the ANMscheme.

The archival value of this letter lies in the presentation of thefirst such ANM scheme to reach the closed-loop trial stage. Thetrial was undertaken in cooperation with an existing wind farmon Orkney, and the outcomes of the trial are of relevance to theplanning and design of future ANM schemes.

ANM addresses the concerns and technical challenges thatcan result from the connection and operation of distributed gen-eration (DG) units: power flow management, voltage control,and fault level management. ANM schemes are emerging thataddress one or more of these constraints, as described in [3].Previous work by the authors [4]–[6] provides a detailed intro-duction to ANM.

Manuscript received September 6, 2007; revised January 14, 2008. Paper no.PESL-00105-2007.

R. A. F. Currie, G. W. Ault, and J. R. McDonald are with the Advanced Elec-trical Systems Research Group, University of Strathclyde, Glasgow, G1 1XW,U.K.

R. W. Fordyce, D. F. MacLeman, and M. Smith are with Scottish Hydro Elec-tric Power Distribution plc, Perth, PH1 3AQ, U.K.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPWRS.2008.926722

II. ORKNEY ISLANDS DISTRIBUTION NETWORK

A. Existing DG Connections on Orkney

There are several DG units already connected to the Orkneydistribution network, as described in [1]. Further developmentof generation on Orkney is constrained by a lack of availableconnection capacity under existing arrangements.

B. Voltage Control on Orkney

Several reactive compensation devices have been installedin the Orkney network, including a dynamic Var compensatingdevice (DVAr). A more localized 33-kV radial voltage riseproblem has been addressed through the installation of addi-tional shunt reactors. The reactive compensation equipmentsolves short- and long-term voltage issues.

C. ANM Philosophy

The Orkney ANM scheme manages the output of new DGcapacity to match the available thermal capacity on the networkarising from diversity in both load demand and DG output. Theapproach to ANM is both corrective and preventative, designedto maintain current flow within safe limits but capable of takingcorrective action if limits are either reached or violated. Op-erating margins are employed to limit current flow below therated capacity of the critical section of the distribution network.A high level overview of the control solution can be found in[1] and [4]. The scheme takes a zonal approach to control ofthe Orkney “New Non-Firm Generation” (NNFG) units and thecontrol logic is capable of coping with a zonal nested controlproblem. NNFG units extend the permitted DG connection ca-pacity beyond normal firm limits.

III. TRIAL OF ORKNEY ANM SCHEME

A trial of the Orkney ANM scheme was completed inNovember 2006. The trial involved a programmable logiccontroller (PLC) that received measurements of current andpower being exported to mainland Scotland and a PLC at thesite of an existing Orkney wind farm which was the focus of thetrial. The PLCs communicated with each other through privatewire via messaged instructions. The wind farm PLC fed digitalsignals into the power network operator SCADA system, whichin turn issued instructions to the wind farm control system.

A. Output Regulation of Wind Farm During ANM Trial

Fig. 1 is a trace of wind farm output during one phase of theANM trial. The set-points that the ANM scheme issued to thewind farm are marked with arrows.

In Fig. 1, the wind farm is issued a set-point of 0.8 pu by theactive management scheme at around 120 s. Over the next 15

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1524 IEEE TRANSACTIONS ON POWER SYSTEMS, VOL. 23, NO. 3, AUGUST 2008

Fig. 1. Wind farm output during part of ANM trial.

TABLE IRESULTS OF ORKNEY ANM TRIAL

min, the wind farm output is reduced through set-points of 0.77pu and 0.73 pu at 300-s intervals. Set-point instructions are sentto the wind farm in response to current breaching acceptablelimits on the submarine cables to the mainland (the main thermalconstraint). The wind farm output remains below the set pointafter reducing from the initial higher value in each case over the15-min period. This holds true for the release of capacity to thewind farm towards the end of the trace when a 0.8 pu set-point isissued at around 1140 s and 0.84 pu set-point at around 1440 s.

Table I provides some key results from the ANM trial. It isimportant to note that the communications and control delayidentified in Table I does not distinguish between the commu-nications delay of the network utility SCADA system and theinherent delay within the wind farm control system.

The wind farm achieves the set point as issued each time bythe ANM scheme. There are different time delays for each setpoint. The ramp rate appears to be around 4%–5% of wind farmrated output per minute.

B. Main Outcomes of Orkney ANM Trial

1) Wind Farm Output Regulation Capability Confirmed: TheANM trial has shown that a wind farm can achieve a desiredset-point as issued by the ANM scheme. It has also been shownthat the MW output from the wind farm can remain consistentlybelow the set-point issued. The ability of the wind farm to main-tain power output at the designated set-point is dependent onwind conditions.

2) Control System Time Delays: Informal discussion witha wind farm developer suggests the response time can poten-tially be improved. Some grid connection requirements for windfarms place a more onerous requirement on the control of windfarm power output [7]. The time taken for the wind farm toachieve the set point must be taken into consideration in net-work control schemes that curtail DG power output based onlocal network constraints. It is clear, however, that preventiveand corrective actions will be required, based on monitoring ofthe wind farm response.

3) Ramp Rate Capability of NNFG: The results of thetrial show that the ramp rate of the wind farm is set around4%–5%/min. Setting the ramp rate higher will benefit theoperation of NNFG units in the full ANM scheme as smalleroperating margins will be employed and the curtailment ex-perienced by NNFG units will be reduced [5] for faster DGresponse rates.

IV. CONCLUSION

This letter has presented some key outcomes of a trial of anANM scheme on the Orkney distribution network. The resultsof the ANM trial are being fed into the design of the full ANMscheme, to be rolled out in the Orkney network in 2008. Thefurther implications of the trial for the full ANM scheme andthe wider DG community will be expanded upon by the authorsin future work. This closed loop field trial is an essential steptowards full deployment of ANM on Orkney and elsewhere inthe U.K. and beyond.

REFERENCES

[1] R. A. F. Currie, G. W. Ault, and D. Telford, Facilitate Generation Con-nections on Orkney by Automatic Distribution Network Management,DTI Project Final Rep., Contract: K/EL/00311/00/00, URN: 05/514,2005.

[2] Engineering Directorate, UK Energy Networks Association, Engi-neering Recommendation G85: Innovation in Electrical DistributionNetwork Systems; A Good Practice Guide, no. 1, 2005.

[3] R. A. F. Currie and G. W. Ault, Register of Active Management Pi-lots, Trials, Research, Development and Demonstration Activities, DTIProject Rep., Apr. 2006.

[4] R. A. F. Currie, G. W. Ault, and J. R. McDonald, “Methodology for thedetermination of the economic connection capacity for renewable gen-erator connections to distribution networks optimised by active powerflow management,” Proc. Inst. Elect. Eng., Gen., Transm., Distrib., vol.153, no. 4, pp. 456–462, Jul. 13, 2006.

[5] R. A. F. Currie, C. E. T. Foote, G. W. Ault, and J. R. McDonald, “Ac-tive power flow management utilising operating margins for the in-creased connection of distributed generation,” Proc. Inst. Elect. Eng.,Gen., Transm., Distrib., vol. 1, no. 1, pp. 197–202, Jan. 2007.

[6] G. W. Ault, R. A. F. Currie, and J. R. McDonald, “Active power flowmanagement solutions for maximising DG connection capacity,” inProc. IEEE Power Eng. Soc. General Meeting, Montreal, QC, Canada,2006, Invited Panel Paper.

[7] C. Jauch, P. Sorensen, and B. B. Jensen, “International review of gridconnection requirements for wind turbines,” presented at the NordicWind Power Conf., Mar. 1–2, 2004, Chalmers Univ. Technol., unpub-lished.