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Volume N2, June 2015

Innovation in the Power Systems

industry Engineers and specialists worldwide exchange

information and state-of-the-art world practicesto enhance knowledge related to power systemsin CIGREs latest publication.

In this Issue, Best of CIGRE 2014 Session Papers

CIGRE 21, rue dArtois, 75008 Paris - ISSN : 2426-1335

(Continued from CSE01)

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2/69C i g r e S c i e n c e & E n g i n e e r i n g2

with SIL (surge impedance loading), a subject whichis attracting increasing interest in recent years as de-signers and system planners are exploring the optionto increase the transfer capacity of transmission lines.

is is especially important for long distance inter-connections, where increased SIL of the lines can re-duce or even remove restrictions on power ow.

Last but denitely not least, this issue is hosting apaper on energy storage systems optimization, thisbeing an attractive solution to achieve a more envi-ronmentally-friendly energy mix while maintainingthe safety of electricity supply. What is importantabout this paper is, that it is the winning paper ofa student contest organised recently by the FrenchNational Committee on the theme of Smart Grids topromote Cigre among young academics.

Finally, let me thank you for your interest in CSEand encourage you to recommend this journal toyour colleagues but also consider it seriously for yournext paper.

Konstantin O. PapailiouChief Editor

[email protected]

Dear readers,You are holding in your hands the second ever issue

of the Cigre Science and Engineering Journal (CSE).e editorial team has done its best to put together

an attractive package of papers for you, which includethe remaining three best of the 2014 Paris session.Interestingly enough all these papers are related withthe increasing use and penetration of renewables inthe grid. ey cover permanent magnet direct drivecongurations for offshore wind farms, risks associa-ted to the communication malfunctions in medium voltage grids characterized by a high level penetra-tion of Distributed Energy Resources (DER) as wellas the topic of providing controllable reactive power

to the next-higher voltage level by controlling of saidresources.

e second group of papers in this issue is devotedto high quality work from some of Cigrs workinggroups, which in my view are better suited to be pu-blished in this journal than as Technical Brochures.

is is the case for the two papers on wind inducedmotion of bundled conductors (excluding galloping),namely Aeolian vibrations and sub-span oscillations,

prepared by WG B2.46, convened by Prof. GiorgioDiana, which continue the successful publication his-tory of Cigr on similar issues. Another paper deals

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Contents of this issue

Editorial

Effects of the modularity in PMSM synchronous machine behaviour

Reliable Controllable Reactive Power for the Extra High Voltage System By High VoltageDistributed Energy Resources

Security of communications in voltage control for grids connecting Distributed EnergyResources: impact analysis and anomalous behaviours

Energy Storage Systems Optimization

Wind Induced Motion On Bundled Conductors(Excluding ice galloping) - Part A Aeolian vibrations

Wind Induced Motion On Bundled Conductors(Excluding ice galloping) - Part B Subspan Oscillations

Evaluation of High Surge Impedance Loading (HSIL) solutionsfor increased natural capacity of 500 kV Overhead Lines

page

2

6

14

30

40

46

56

63

List of guresFigure 1: Section of th modelled PMSG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 2: Magnetic flux density comparison between no modular rotor (a) and modular rotor with 4 magnets/module (b),5 magnets/module (c) and 8 magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Figure 3: Comparison of magnetic flux density measured at rotor yoke between no modular rotor and modular rotorwith 4 magnets/module (a) and between no modular rotor and modular rotor with 4 magnets/module (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 4: Comparison of magnetic flux density measured at gap between no modular rotor and modular rotor with4 magnets/module (a) and between no modular rotor and modular rotor with 4 magnets/module (b). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Figure 5: Comparison between induced voltage obtained for non-modular and rotor modular with different number of magnets per module.. . 10Figure 6: Cogging torque in non-modular rotor (a) and comparison with modular rotor configurations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Figure 7: Comparison of voltage (a), current (b) and power (c) obtained for the no modular rotor machine and modularrotor with 16 magnets per module. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 1: Sketch of HV DER Q Provision for DSO-operated HV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 2: Sketch of HV DER Q Provision for TSO-Operated HV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Figure 3: Research Topics Related To CQ Provision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Figure 4: Schematic Representation of the EHV/HV System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Figure 5: DER PQ Capability Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Figure 6: Reactive Power Provision on Network Area Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Figure 7: Bandwidth of Controllable Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 8: Factors Determining Reactive Power Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19Figure 9: Expected Advantage from Optimizing EHV/HV Taps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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Figure 10: Overview of Simulated Active Power Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Figure 11: Base Case HV Wind Measured: Network Area P & Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 12: Base Case HV Wind Strong: Network Area P and Q . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 13: Network Area Q Bandwidth for Different Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 14: Loading of Transformer T2 for Different Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Figure 15: Voltage Level: DER Optimized, EHV/HV Taps Local . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 16: Voltage Level: DER + EHV/HV Taps Optimized . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 17: HV DER PCC Grid Strength and CQ Contribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23Figure 18: Specific Cost of HV Controllable Reactive Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 19: Cost Components Considered in Cost Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Figure 20: Specific Cost of Reactive Power from EHV Capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Figure 21: Decision Flowchart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Figure 1: Voltage Control use case Function Input/Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 2: Voltage Control use case SGAM mapping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31Figure 3: Voltage Control use case Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Figure 4: Benchmark grid - telecontrol topology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Figure 5: Benchmark grid - population distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Figure 6: Benchmark grid regional RES distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Figure 7: Benchmark grid RES distribution at substation level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34Figure 8: Voltage Control use case - Security Space . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 9: Voltage Control use case - SGIS Impact Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 10: Voltage Control use case - SGIS Likelihood Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 11: Voltage Control use case - SGIS Risk Calculus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 12: Voltage Control use case - SGIS Risk Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 13: Voltage Control use case - mapping of IEC 62351 series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Figure 14: Analysis steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Figure 1: Illustration of the storage system principle. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41Figure 2: Financial result achieved by different storage systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Figure 3: Flywheel simulation model. Modelling of saturation phenomena is not shown to enhance the figure clarity. Adapted from [21].. . . . . . 44Figure 4: Battery model used to simulate charge and discharge cycles. Based on [22]. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Figure 5: Evolution of according to time for flywheels (blue) and batteries (red). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Figure A.2: Predicted maximum Aeolian vibration antinode amplitude (0-peak) as a function of frequency when the same tensileload is applied on the subconductors and a constant low wind turbulence is considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Figure A.3: Predicted maximum strain (0-peak) on the bundle conductors at the suspension clampand at the spacer clamp as a function of frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51Figure A.4: Maximum strains at the suspension clamp with the Diana model : when the same tensile load is applied to the subconductorsof the bundle and a constant turbulence (It < 0.07) is considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Figure A.5: Maximum strains at suspension clamp with Claren-Cosmai model : when the same tensile load is applied to the subconductorsof the bundle and a constant turbulence (It = 0.05) is considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52Figure A.6: Maximum strains with Krispin model : when the same tensile load is applied to the subconductorsof the bundle and a constant, low, wind turbulence is considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Figure A.7 Cosmai hypothesis for variable wind turbulence index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53Figure A.8: Maximum strains at suspension clamp with the Claren-Cosmai model : tension differential neglected and variableturbulence (0.20 < It < 0.05) considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

Figure A.9 Maximum strains at suspension clamp with the Diana model : tension differential 1Dand constant turbulence (It < 0.07) considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54Figure A.10: Maximum strains at suspension clamp with Diana model : tension differential 10 Dand constant turbulence (It < 0.07) considered . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

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Figure B.1: Drag coefficient on windward rough cylinder as function of speed/Reynolds number from Wind tunnel experimental tests [8] . . . . 57Figure B.2: Rough cylinders energy with respect to x/D amplitude for the three considered frequencies: experimental numericalcomparison. Due to the model scale, f = 0.5 Hz corresponds to f = 1 Hz full scale is both the static and dynamic bundle rotation angle withrespect to the wind. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57Figure B.3: Test set-up showing towers, conductors and anemometers of the test line . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60Figure B.4: IREQ Measurements: Maximum peak to peak horizontal oscillations as function of mean wind speed. . . . . . . . . . . . . . . . . . . . . . . . . . 60Figure B.5: Benchmark results: Maximum peak to peak horizontal oscillations as a function of mean wind speed. . . . . . . . . . . . . . . . . . . . . . . . . . 6Diagram: Schematic representation of a Transmission Line in electric systems studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Figure 1: Racket Tower; the drawings distances and heights are in meters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Figure 2: Cross-Rope Tower (Chainette) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Figure 3: VX-Asymmetrical Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64Figure 4: VX-Symmetrical Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Figure 5: Cat Face Tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Figure 6: Monopole tower . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65Figure 7: Electric superficial gradients in 500kV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Figure 8: Graphic with the audible noise profiles (AN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Figure 9: Graphic with radio interference profiles (RI) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Figure 10: Graphic with the electric field profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Figure 11: Graphic with the magnetic field profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

List of tables

Table I: Input data for the analytical iterative procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Table II: Output data from the analytical iterative procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Table III: Rotor modularity options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Table IV:Torque period due to modules of the rotor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Table I: Voltage limits observed by OPF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2Table II: Simulated Scenarios and Strategie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21Table III: Abbreviations used in (2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table I: Voltage Control use case - Information Assets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32Table II: Voltage Control use case - Control Steps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33Table III: Security scenarios . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35Table IV: Voltage Control - security standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Table I: Analytical Methods and Assumptions Used in the Study (Numerical table entries refer to supporting references) . . . . . . . . . . . . . . . . . . . . 47Table II: Experimental tests characteristics and system configuration data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Table III: spacer-damper data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Table IV: wind power input and conductor self-damping for the analytical-analytical benchmark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50Table B.1:Subspan oscillation test case data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Table I: Minimum, Maximum and Average values of electric field on conductors surface. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Table II: Positive and Zero Sequence parameters of 500kV TLs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

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the generator mounted in a nacelle that sits at the top of alarge tubular tower. Therefore this ratio must be reduced asfar as possible, in order to achieve more compact and lighternacelle configurations.

Besides, in offshore applications the minimum maintenancerequirement it is especially important due to the difficultyof access to the turbines for long periods of time. Gearboxis one of the most critical components in wind turbines [1],and so, avoiding these components improves the reliabilityof the complete system. On the other hand, the increase ofwind turbine power rating becomes the direct drive optionmore attractive than the geared ones [2]. For these reasons,

direct drive configuration seems particularly suitable foroffshore wind turbines.

Direct drive electric generator are characterized by lowspeed and high torque and, therefore requires a high polenumber, which increases the generator dimension andweight if conventional synchronous or induction generatoris selected. Permanent magnet synchronous generator(PMSG) allow using small pole pitches due to the absenceof external excitation and rotor windings, and thereby theyachieve lower weight, improved thermal performance andhigher efficiency and energy yield [3]. Therefore, PMSGdirect drive configuration provides better performance,lower maintenance and it is lighter than other options asthe induction motor with a gearbox [1] - [4].

Different design configurations of PMSG suitable fordirect drive are proposed in literature, that can be groupedaccording to the directional flux paths in radial flux, axialflux and transversal flux PM machines. According to[4] radial flux PMSG with surface mounted magnets areeconomically better choice for large-scale wind turbinescompared to the axial-flux machine.

AbstractPermanent magnet direct drive configurations haveconsiderable advantages in offshore wind farms improvingreliability, longevity and lower maintenance by eliminatinggearboxes and external excitation systems. The size of themultipole permanent magnet machine, necessary for directdrive applications, exceeds manufacturing and transportallowed limits. Thus, new machine concepts based onmodular design are needed.

This paper presents the study of the influence of rotormodularity on the behaviour of a radial flux permanentmagnet synchronous generator with surface mountedmagnets and fractional slot windings. The electromagnetic

study is based on the finite element method and the solutionshas been obtained by using the software tool FLUX 2D.

The starting point of this study is a machine of 40 polepairs, 84 slots and an outer radius of 3.206 m. The studytakes into account the number of modules and the numberof permanent magnet in each rotor module.

1. IntroductionDevelopment of offshore wind power requires the designof high power wind turbines to allow better use of the windresource at such locations.

To select the wind turbine topology, it is necessary toconsider two key factors: the ratio torque/cost and the ratiotorque/mass. The torque/cost factor is essentially economicand the goal is to try to increase this rate to the extent possiblein order to obtain values of kW/ as low as possible. Thetorque/mass ratio is critical in applications where weightand size of the generator are design constraints. Typicallarge wind turbines use horizontal axis configuration with

Effects of the modularity in PMSMsynchronous machine behaviourStudy committee A1Working group A1.111

T. Arlabn a, M.P. Comechb b*, M.T. Villnb, M.Garca-Gracia b

a ACCIONA Windpower, Spainb Centre of Research for Energy Resources and Consumption e CIRCE, Universidad de Zaragoza, Spain

KEYWORDSDirect-drive wind turbine, Finite element analysis, Modular rotor, Permanent-magnet synchronous generator

*[email protected]

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Slot per pole per phase 0.35

Turns per coil 32

Rotor radius 3 m

Stator slot height 4.00%

Gap 0.300%

Maximum ratio L/D 250%

Rated Speed 10 r.p.m

Rotor material Steel M24

Stator material Steel M24

Conductor material Cu

PM material N30

Slot/tooth width ratio 1.5

Table I. Input data for the analytical iterative procedure.

Parameter Value Units

Rated Power 2400 kW

Rated Voltage 2.019 kVRated Current 710.974 A

Eciency 0.983 %

Frequency 6.667 Hz

PM length 29.25 mm

Magnet pole arc 144

Slot number 84

Stack length 1800 mm

Stator diameter 6412 mm

Rotor external diameter 6058.4 mm

Rotor internal diameter 5906 mm

Gap 9 mm

Magnet height 29.2 mm

Table II. Output data from the analytical iterative procedure.

3. Results of the FEM analysisThis section shows the finite element analysis carriedout in software FLUX 2D. Results obtained for the non-

modular machine described in the previous section aretaken as reference for comparison with modular rotormachine.

Figure 1 shows a section of the modular rotor machine

On the other hand, a high pole number with conventionalwinding structures involves a high slot number. Thefractional slot winding, which has a number of slots perpole and per phase less than one, has the advantage that itdoes not require many slots although the pole number ishigh, and as a result of this the iron and copper mass can bereduced, along with the material cost and the end windinglength [5].

Although required dimension in PMSG are smaller thanconventional generator, their size and mass, grow rapidlywith power capacity, and it is becoming a problem in termsof capital cost, logistics and assembly [6]. For example, [7]presents the design of a 10 MW direct drive PMSG with anexternal diameter of 10 m, that exceeds the limits imposedby transport and manufacture procedures. Hence, modulardesigns become suitable for high power direct drive PMSGconfigurations.

This paper presents the study of the influence of rotormodularity on electromagnetic behaviour of radial flux

surface mounted magnets PMSG with fractional slotwindings. Magnetic flux density distribution, induced voltage, cogging torque and generated power has beenanalysed. The electromagnetic study is based on the finiteelement method (FEM) and the solutions has been obtainedby using the software tool FLUX 2D.

Section 2 of this paper present the PMSG geometry ofthe non-modular PMSG taken as reference for this study.Section 3 shows the results obtained from FEM analysis forno load and load conditions and Section 4 summarizes themain conclusion obtained from this analysis.

2. Denition of analysed PMSGgeometryThe PMSG geometry has been developed following ananalytical iterative procedure from data shown in TableI. This analytical procedure results in the dimensions ofthe permanent magnet machine shown in Table II thatallow the modelling for finite element analysis of the non-modular PMSG taken as reference.

Parameter Value Units

Rated Power (objective) 2400 kWRated voltage (objective) 1.32 kV

Number of phases 3

Pole pairs 40

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modelled in FLUX 2D indicating the different parts of thePMSG.

Figure 1 : Section of th modelled PMSG

To determine the number of rotor modules, it must be takeninto account that the designed machine has 80 poles andeach module must have the same number of magnets tomaintain machine symmetry. Hence, rotor can be divided

in different options as shown in Table III.Magnets/Module Modules

4 20

5 16

8 10

16 5

Table III . Rotor modularity options

3.1 Magnetic flux density distribution in no loadsimulation depending on the number of modules

First we will study the influence of the number of rotormodules has on the distribution of magnetic flux density.Figure 2 compares the magnetic flux density obtained fromfinite element simulation for each case. As it can be seen,non-modular rotor magnetic flux density distribution isuniform along rotor but rotor modularity affects magneticflux density distribution in both rotor and stator. In Figure2 higher magnetic flux density is represented in yellow. Inmodular design it must be ensured that saturation limits areno exceeded, if so, the initial design should be modified to

prevent this from happening.In order to facilitate comprehension of Figure 2, magneticflux density distributions measured at rotor yoke and at thegap are compared in Figure 3 and Figure 4 respectively.

Figure 2. Magnetic flux density comparison between no modular rotor (a)and modular rotor with 4 magnets/module (b), 5 magnets/module (c)

and 8 magnets/module (d).

Figure 3 compares the non-modular and the 4 magnets/module configuration. In Figure 3a) rotor yoke magneticflux densities are shown. As it was expected, magnetic fluxdensity decreases strongly where the gap between modulesis located, but also a diminution between the second andthe third magnet of each module appears. Moreover, itcan be seen that magnetic flux density increases betweenthe magnets at the end of the module and the adjacentones and decreases in the centre of the module. In fact, itcan be observed that magnets can be grouped in pairs, asit is observed in Figure 2b), in which the areas of highermagnetic flux density between magnets in the ends of theadjacent modules appears in yellow and the centre modulein red, indicating that lower values are reached in this area.Same behaviour can be observed in Figure 2d) for 8 magnets

per module configuration showing the same behaviour inpairs.

For modular rotor with five magnets per module, behaviourin pairs cannot occur, being an odd number. Figure 3 shows

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magnetic flux density comparison between non-modularand modular rotor design with 5 magnets per module. Asin the previous case an increase on magnetic flux densityin the magnets of the ends is observed while the magnetsin the central magnetic flux density remains unchangedrespect the non-modular configuration.

Magnetic flux density in the gap decreases in the area aroundthe rotor gap in both cases although there is no importantdiminution in the other positions, as it is shown in Figure 4.

3.2 Induced voltage depending on the number of modules

Besides the magnetic flux density behaviour, it is especiallyimportant to analyse how the rotor modularity affects to the

global behaviour of the machine by performing no load andload test.

Induced voltage depends on the gap magnetic flux densitybut also on the winding configuration. Designed PMSG has

a fractional-slot tooth concentrated winding.

Figure 5 compares the induced voltage obtained for thenon-modular and rotor modular machines with differentnumber of magnets per module. As it was expected, theinduced voltage decreases with the number of modules. 4magnets per module machine (20 modules) presents thedifference with 6 % respect the non-modular machine,and 16 magnets per module machine (5 modules) 1.4%. Regarding the selection of an even or odd number ofmagnets per module, there is no appreciable differencebetween the induced voltage of the machine with 4 and 5magnets per module.

3.3 Cogging torque depending on the number of modules

One of the relevant aspects to be analysed when PMmachines are being designed is the cogging torque,especially in low speed machines. Cogging torque is thetorque due to the interaction between PM of the rotor and

Figure 3. Comparison of magnetic ux density measured at rotor yoke between no modular rotor and modular rotorwith 4 magnets/module (a) and between no modular rotor and modular rotor with 4 magnets/module (b).

Figure 4. Comparison of magnetic ux density measured at gap between no modular rotor and modular rotor with4 magnets/module (a) and between no modular rotor and modular rotor with 4 magnets/module (b).

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the stator slots and it is analysed by means of no load tests.Cogging torque results in torque and speed ripple and it isa undesirable effect.

Cogging torque is position dependent and its periodicitydepends on the number of magnetic poles and the numberof stator teeth and period can be calculated as:

(1)

where LCM(nslots , n poles) is the least common multiple of the

number of slots and the number of poles of the machine. Inthis case, there are 84 slots and 80 poles and cogging torqueperiod obtained from (1) is 0.2143. Figure 6a) shows thecogging torque of the no modular machine for nine coggingtorque periods. In the no modular rotor machine thecogging torque peak value is 60 Nm.

In modular rotor machines, gaps between rotor modulesaffect cogging torque behaviour. Figure 6b) and Figure 6c)compares the cogging torque of the non-modular rotormachine with the obtained for modular rotor machines.

Figure 5. Comparison between induced voltage obtained for non-modular and rotor modular with different number of magnets per module.

Figure 6. Cogging torque in non-modular rotor (a) and comparison with modular rotor congurations.

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In these figures, it can be observed that a new componenthas been added to the cogging torque of the non-modularmachine and its amplitude and period depend on thenumber of modules of the rotor. To obtain the period ofthe component due to the rotor modules from, (1) must bemodified by adding the number of rotor modules:

(2)

Table IV shows the results for the different number ofmodules analysed, and as it can observed in Figure 6b)and c) these values of period agreed with those obtainedby FEM simulation. Furthermore, the amplitude of thecomponent due to the rotor modularity increases with thenumber of modules, as it can be observed in Figure 6. Thesecomponent periods must be taken into account to avoidtorsional and fatigue stress that can damage wind turbinestructure.

Modules LCM(nslots,nmodules)

Period(degrees)

20 420 0.857

16 336 1.071

10 420 0.857

5 420 0.857

Table IV. Torque period due to modules of the rotor.

3.4 Results of load simulation of the selected system

Figure 7 compares the results obtained on simulation of thenon-modular and modular machine with 16 magnets per

Figure 7. Comparison of voltage (a), current (b) and power (c) obtained for the no modular rotor machine and modular rotor with 16 magnets per module.

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pole when it is connected to a resistive load. Resistance value has been calculated in order to produce thegenerator rated power. As it can be seen there is hardlyany difference between voltage and current, 1.6 % inboth cases. Active power shown in Figure 7c) shows adifference between active powers obtained by both non-modular an modular machine of 75 kW, equivalent to3.4 %. Therefore, the five gaps introduced between themodules of the rotor produce additional losses of 3.4%in the rotor modular design.

4. ConclusionCurrent wind industry tendency is to develop increasingrated power turbines for offshore installation. PMSGdirect drive configuration presents several advantagesfor its application in offshore wind installation due totheir better reliability, longer life, improved performance,lower weight, and lower maintenance, especiallyimportant in offshore locations where the access to theturbines is limited during most of the year.

However, size of generator grows rapidly with powercapacity and it is becoming a problem in terms ofmanufacturing, logistic and assembly. Therefore,modular design appears as a solution for achieving evenhigher power avoiding these problems.

This paper analyse the influence of rotor modularityon the electromagnetic behaviour of an inner rotor,radial flux PMSG with surface mounted magnets andfractional slot windings. From FEM results obtainedit can be concluded that the behaviour of the modularrotor machine is close to the non-modular rotor oneand it is closer with lower number of modules, which isdetermined by the number of poles of the machine. Inthe analysed machine, the additional active power lossesdue to gaps between rotor modules are around 3.4 %.

A special mention should also be given to cogging torquein modular machine, which depends on the symmetry ofthe machine. Introducing gaps between rotor modulescauses no load torque components much higher (about100 times) than the cogging torque values in the non-modular rotor machine.

References[1] H. Fink, F. Devaux, B. Dolata, C. Perrier, New and innovative

Smart J. Ribrant and L. M. Bertling, Survey of Failures in WindPower Systems With Focus on Swedish Wind Power Plants During

1997-2005 IEEE Trans. on Energy Conversion, vol. 22, No. 1, March 2007.

[2] H. Polinder, F. F. A. van der Pijl, G. de Vilder, and P. J. Tavner,Comparison of Direct-Drive and Geared Generator Concepts forWind Turbines, IEEE Trans. on Energy Conversion, vol. 21, No. 3,September 2006.

[3] Li, H. and Chen, Z., Overview of different wind generator systemsand their comparisons, IET Renew. Power Gener., vol. 2, No. 2, pp.123138, 2008.

[4] K. Ahsanullah, R. Dutta, M.F. Rahman, Review of PM generatordesigns for direct-drive wind turbines, Australasian UniversitiesPower Engineering Conference, pp: 1 6, 2012.

[5] F. Libert, Design, Optimization and Comparison of Permanent Magnet Motors for a Low-Speed Direct-Driven Mixer, PhD,2004.

[6] R. Scott Semken, M. Polikarpova, P. Roytta, J. Alexandrova, J. Pyrhonen, J. Nerg, A. Mikkola, J. Backman, Direct-drive permanent magnet generators for high-power wind turbines:benefits and limiting factors, IET Renew. Power Gener, vol. 6, Iss.1, pp. 18, 2012.

[7] A. Damiano, I. Marongiu, A. Monni, M. Porru, Design of a 10 MW multi-phase PM synchronous generator for direct-drive windturbines, 39th Annual conference of the IEEE Industrial electronicssociety IECON 2013, pp. 5266 5270, 2013.

Biographies

Teresa Arlaban Gabeiras received the M.Sc. degree inIndustrial Engineer in 2007 and she has since then beenworking as an engineer in the ACCIONA WindpowerR&D Department. Currently, she is the Head of theResearch and Patents area. She has been involved in various wind research projects dealing mainly with gridintegration issues, power curve improvements, loadmitigation strategies and drive train analysis. She isthe co-author of more than 10 papers for internationalconferences and the co-author of more than 20 patentapplications related to control of wind turbines forgrid integration and load mitigation, transport andinstallation systems, generation concepts and offshoreapplications

Mara Paz Comech received the M.S. and Ph.D. degreesfrom the University of Zaragoza, Zaragoza, Spain, in 2003and 2008, respectively. She has been a Research Engineerin the electrical section of Centre of Research for EnergyResources and Consumption (CIRCE), Zaragoza, and is

currently teaching and researching in the Department ofElectrical Engineering, University of Zaragoza, and alsocollaborating with CIRCE. Her research interests includepower system modelling, FEM analysis, dynamic study,and wind energy conversion systems.

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Mara Teresa Villn received the M.S. degree inIndustrial Engineering from t he University of Zaragoza,Zaragoza, Spain, in 2003. She is a research engineer inthe electrical section of Centre of Research for EnergyResources and Consumption (CIRCE), Zaragoza. Herresearch interests include power system modelling, FEManalysis, and wind energy conversion systems.

Miguel Garca-Gracia was born in Saint-Brieuc, France,on April 23, 1963. He received the M.Sc., and Ph.D.

degrees from the University of Zaragoza, Zaragoza,Spain, in 1989, and 1996, respectively. He is currentlya Professor of Electrical Engineering and the AreaDirector of Electric Power System at Centre of ResearchEnergy and Resource Consumption (CIRCE), Zaragoza.His main research interests include power system, powersystem protection, electrical energy system, renewableenergy integration, lightning protections, and dielectrics.

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reactive power consumption from underlying networksand for the reactive power requirements of electricalequipment in his own system.

To fulfill the task, the TSO utilizes mechanicallyswitched compensating devices, synchronous generatorsconnected to the transmission system and sometimesFlexible AC Transmission Systems (FACTS) devices orreactive power capability of certain types of High VoltageDirect Current (HVDC) substations.

In certain regions of several countries e.g. UK, Ireland,Germany, Spain, Poland a significant locationalpenetration of wind and/or PV farms connected to thehigh voltage (HV) system is observed already to dateor expected for the short to medium term future [1]-[7]. Frequently, grid codes oblige Distributed EnergyResources (DER) to be able to provide CQ free of chargeto the HV system operator [8]-[12] . The term HVsystem operator is preferred here over a choice of eitherTSO or DSO (Distribution System Operator), since in anumber of countries, e.g. Switzerland, Germany, Poland,HV systems are operated by the DSO rather than by theTSO [13]. In many cases the CQ will be utilized for HVsystem purposes, e.g. to stabilize the voltage at a weakpoint of common coupling (PCC). In these cases DER CQwill only sporadically be available for other applications.These cases do not qualify for the investigations carriedout here. There is an increasing number of regionshowever, in which the abundance of CQ is to date or isexpected to become - such that providing reliable CQ asancillary service from HV to Extra High Voltage (EHV)may be considered. This is the point of departure for theinvestigations carried out here.

AbstractThe topic of providing controllable reactive power (CQ)to the next-higher voltage level based on controllingdistributed energy resources (DER) has recently receivedincreasing attention. From a theoretical perspective, theuse case of deferring or avoiding Extra High Voltage(EHV) investment in compensating equipment by theancillary service provision of controllable reactive powerfrom High Voltage (HV) to EHV would seem within

future reach in certain cases. Despite the comparativelyhigher losses related to HV DER controllable reactivepower provision, the use case is shown to be potentiallyeconomically attractive under certain circumstances.However, a considerable number of practical barriersexist that are presented in a systematic way based on adecision flowchart. Simulation results are based on aclose-to-real German EHV/HV system and measureddata. They highlight the benefit resulting from optimizeddispatch of both EHV/HV tap changer and HV DERproviding CQ.

1. Introduction1.1. Motivation

The paper at hand focuses on reactive power exchangeat the interface of high voltage and extra high voltagesystems. Typically, the Transmission System Operator(TSO) has among others - the task to ensure adequateavailability and dispatch of controllable reactive power(CQ). He will compensate both for the residual of vertical

Reliable Controllable Reactive Power for theExtra High Voltage System By High VoltageDistributed Energy Resources

Economic Attractiveness and Practical ImplicationsStudy committee A1

E. Kaempf1*, M. Braun (University of Kassel & Fraunhofer IWES, Germany)T. Stetz (Fraunhofer IWES, Germany)

H. Abele (TransnetBW GmbH, Germany)S. Stepanescu (Netze BW GmbH, Germany)

KEYWORDSAncillary Services, Compensating Equipment, Distributed Energy Resources, Reactive Power Provision

* [email protected]

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Beyond the deferral of investment in EHV compensatorsfurther use cases may be realized by HV CQ provision. Forinstance volt/var motivated redispatch of conventionalgeneration may be an issue from TSO perspective: Inthis case, due to locational shortage of CQ, conventionalgenerators currently not in service might need to betaken into service, and active power generation in someother part of the system reduced. Or an existing fossil-fuelled generator would need to change active poweroutput in order to be able to supply more CQ. In bothcases, the generators may need to be compensated by theTSO for the change made to their planned schedule.

The paper at hand channels the complexity of the discussedscenario into a decision flowchart that makes transparenthow the potential area of applicability is narrowed downby a large number of practical considerations. Differencesbetween the use cases are highlighted. The discussion isaccompanied and enhanced by simulation results froma real German EHV/HV system: The influence of EHV/

HV transformer control strategy on the resulting feasiblereactive power bandwidth under constraining operatingconditions is demonstrated.

1.2. Definitions and Scope

Generally, favorable and non-favorable connectingpoints of DER to the power system may be distinguished:A favorable connecting point allows to integrate the fullreactive power capability from connected resourcesbased on the existing EHV/HV transformer controlstrategy without causing unwanted voltage or loadingconditions. Existing utility-owned fossil-fuelled orhydro HV generators are frequently connected to suchfavorable points.

The contribution at hand focuses on making availableCQ from resources previously not actively utilized formanaging the reactive power exchange, e.g. DER orindustry generators. While in the following, the termDER is used for simplicity, the results may be transferredto industry generators c. p.. Focus here is on provisionof controllable reactive power, i.e. on a situation wherereactive power output of HV DER is influenced onlineaccording to the varying requirements of the EHV system

operator with the aim of producing a defined exchangeof reactive power between EHV and HV.

In literature e.g. [14], [15], this topic of CQ provisionto the next-higher voltage level has so far usually been

The largest reactive compensating devices with thelowest losses will usually be connected to EHV. It istherefore of particular interest to compare the economiceffectiveness of reactive power provision from HV DERto that of EHV reactive power resources. At hand, theEHV connected capacitor a low-cost low-loss EHVresource of CQ - is chosen as benchmark for comparisonwith provision of CQ from HV DER units.

One might expect that under these conditions HVDER CQ cannot compete: Based on the simulation of aclose-to real German EHV/HV system it is shown thatindeed even loss-minimal dispatch of HV DER unitswill frequently increase losses in the HV system. Theincrease in network and DER losses expressed perMvarh delivered is in many cases significantly largerthan losses related to providing overexcited reactivepower from an EHV capacitor. In addition, dependingon the grid code, HV DER units may need to be paid forparts of their reactive power provision.

Yet, this fact is only the beginning of the analysescarried out here: If an EHV capacitor with expectedshort operating times is planned to be installed, this isequivalent to a very high specific cost of reactive power,i.e. a high cost per utilized Mvarh. The high cost isdue to the investment and capital costs related to theinvestment in an EHV capacitor. HV DER CQ will bethe more competitive, the more it is already paid off,i.e. reactive power can be provided at marginal cost.When it is not requested, it does not cause any costs.Should investment in DER e.g. retrofit be requiredexclusively to produce the desired HV CQ quantity andreliability, this competitive advantage is reduced.

The specific examples simulated focus on wind powerplants capable of providing reactive power even at zeroactive power output. The results are generally applicableto so far not utilized potentially controllable HV reactiveresources whose reactive power control capability isfinanced by other use cases. Further examples are e.g.suitable PV plants or industrial synchronous generators sofar frequently operated to maintain a fixed power factor.

For the vast majority of worldwide HV systems the

discussed scenario will seem rather hypothetic: Yet,with the observed fast expansion of installed DER - e.g.wind capacity in many countries it may be of interestto analyze the ICT, control infrastructure and grid coderequirements required for the discussed application.

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The research topic of CQ provision from HV to EHVaddresses a situation in which EHV network planning /operation is not (yet) taking into account the possibilitiesavailable from HV resources. This may occur e.g. for thefollowing reasons:

- HV is operated by the DSO, and therefore there is anorganizational boundary

- HV did previously not contain a significant amountof controllable resources that were technically able toprovide CQ

- EHV has so far had such an abundance of CQavailable that there was no point in making the effortof looking into HV resources.

The article at hand will thus only be of interest forsystems facing change in at least one of the abovecategories: At heart, a situation is addressed, where itshall be investigated whether enlarging the horizon ofanalyses e.g. in the above-listed ways - may produce

economic benefit.Hereafter, the effect of potential organizationalboundaries between HV and EHV system operator isnot further elaborated on, focus is on the principallyachievable potential that can be reached by coordinatedoperation of the EHV/HV system.

1.3. Literature Review

A comprehensive literature review on the topic maybe found in [17]. An overview of principally relevantresearch questions is given in Figure 3. Kaempf etal. [18] was the first source to provide a systematicoverview of use cases related to the topic. For any usecase analyzed it must be ensured that an economicallycompetitive, sufficient amount of CQ is provided atsufficient reliability. The requirements related to the termsufficient e.g. sufficient reliability, sufficient amount -will depend on each particular use case investigated.

First research results discussing feasible amounts andeconomic competitiveness are available [17]-[19]. In [20]it is pointed out that optimal power flow based dispatchof both DER and transformer tap changers yields relevant

reactive power bandwidths even under constrainedoperating conditions, thus providing a contributionto the discussion of required ICT infrastructure. Thequestion of long-term availability of HV reactive powerhas not been discussed so far. Neither has the topic of

treated in the context of integrating CQ from DERin situations where the interface of two voltage levelscoincided with the interface between TSO and DSO,Figure 1.

Figure 1: Sketch of HV DER Q Provision for DSO-operated HV

Figure 2: Sketch of HV DER Q Provision for TSO-Operated HV

If HV is operated by the TSO situation represented inFigure 2 - the reactive power provision process discussedhere will not be noticed as a separate process any more. Ifthe TSO possesses the ICT infrastructure described e.g.in [16], reactive power provision will be integrated intothe TSO closed-loop contingency constrained optimal volt-var control. HV network areas capable of providingCQ could in this case be considered as individual voltagecontrol zones. Alternatively, they could be integratedas sub-entities into an existing voltage control zone.Contingency constrained volt-var dispatch the tertiary voltage control will automatically ensure that the mostcost-effective controllable resource is activated whilemaintaining sufficient reserves. For a closer discussionof the concept of primary, secondary and tertiary voltagecontrol refer to [16]. Research interest in the article athand is in this case focusing on the question whetherit makes sense to integrate distributed controllable HVreactive power resources into EHV tertiary voltagecontrol. This again is determined by how much

dependable potential can be obtained from them, and atwhat cost. While costs of network losses are discussedhere, costs related to obtaining CQ from HV resourceswill depend on the grid codes and possibly on additionalcontracts negotiated bilaterally.

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The paper is organized as follows: Section 2 describesthe simulated EHV/HV system. Section 3 Methodologyderives among others - the choice of simulated DERpenetration and network infrastructure. Section 4Simulation Results presents achieved bandwidths ofCQ for different scenarios and control strategies. Thespecific cost of CQ provision based on control of HVDER is presented.

Section 5 starts by presenting the specific reactive powercosts related to the investment in an EHV capacitor.Subsequently, the practical requirements related tothe use case of deferring or replaying EHV capacitorinvestment by HV DER based Q provision are discussed.

2. Model Description andAssumptionsThe simulated system shall be systematically characterized

which allows to more easily estimate applicability of theconsiderations in the readers own target environment.

2.1. EHV / HV System

A real HV system and its related EHV subset aremodelled. The EHV subset consists of a detailed part,and the adjacent external network represented bynetwork equivalents. The subset of EHV modelled indetail consists of a total of 35 substations on 380 and 220kV levels. It is part of an interconnected transmissionsystem in which a minimum amount of transmissionconnected synchronous generation is always online [22].The connections to further modelled EHV nodes inFigure 4 merely summarize the modelled complex EHVsystem on a very schematic aggregation level, withoutdistinguishing between internal and external network.

The modelled EHV subset supplies two HV networkareas. One of these is the network area discussed here.This HV study network area is connected to EHV via sixtransformers, two of which operated in parallel.

The HV system supplies 73 medium voltage (MV)systems. The most direct EHV access for most of the ten

HV wind farms is via EHV/HV transformers T2 and T3a,T3b. As compared to the HV system status investigatedin [17] already, some lines considered to be shortlybefore realization of an expansion in [17] are assumedto be successfully upgraded here. The distance between

sufficient reliability been investigated systematically forHV DER CQ provision. Both are most relevant when usecases related to TSO network planning are investigated:Deferring EHV capacitor investment is the comparativelymost challenging use case in terms of the requirementsfor reliability, amount and long-term availability.

Figure 3: Research Topics Related To CQ Provision

In the context of a comprehensive overview of allaspects of grid integration of wind generation the topic

of providing CQ from subtransmission to transmissionlevel is raised in [21]. It is judged that due to the factthat wind generators are frequently connected at weakpoints of the subtransmission system, wind generatorsare not able to provide any substantial contribution tothe reactive power balance at the transmission level.The above statements were as may be seen in [21] motivated by the investigation into the impact of windgenerators on system stability in South Australia.

1.4. Contribution and Outline

Based on the analysis of a German EHV/HV system thecontribution at hand takes up the concerns raised in [21]in the following ways:

- First, it is shown that the in southern Germanycurrently widely practiced local voltage controlof EHV/HV transformers will under certainconditions indeed lead to unsatisfactory minimalratios of installed CQ from DER to CQ that can bemade available to EHV.

- Second it is shown that this minimal ratio may besubstantially improved by introducing optimal-power flow based computation of setpoints for both

EHV/HV transformer tap positions and DER reactivepower output.- Third, an economic analysis compares specific costs

of EHV capacitors to the cost of reactive powerprovision obtained for the simulated network.

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0.9 at nominal power output. The broad green curve inFigure 5 indicates the assumption made here, while theblue lines delineate the grid code requirements relevantfor the simulated system [25].

The reliable generation of reactive power assumed heremay be obtained in the following ways:

(i) By contractually agreed retrofit of existing DERIt may be worthwhile investigating integration of thefrequently existing park capacitors connected to thepark collector bus into the reactive power managementduring zero active power output: This is possible, whenimplemented in the context of centralized coordinatedcontrol of EHV/HV tap changers and park capacitors, asshown in [17]. The advantages of combining capacitorsand DER reactive power control have already beenpointed out in [26]. Beyond that, numerous possibilitiesfor retrofit of existing plants with FACTS devices or with

wind generators containing STATCOM functionalityexist [27].

(ii) By deciding to change requirements for networkconnection in the future, or by foreseeing such capabilitywhen connecting new generators based on bilateralagreements.

(iii) By combining different types of resources, e.g.fossil fuelled industry generators and DER in a cost-minimal and reliable way.

Especially to introduce utilization of existing DERcollector bus capacitors may significantly increase thepossibility to provide low-cost overexcited reactivepower, as discussed in [17].

Figure 5: DER PQ Capability Assumptions

two EHV/HV substations, e.g. between T2 and T3 isusually less than 55 km. Wind farms are sized betweenPn = 10 MW and Pn = 58 MW. The distance betweenHV wind farms and the closest EHV coupling point isusually less than 45 km. HV-side measurements of 15minute averaged active and reactive power exchangewith MV systems were used. For EHV, the peak loadscenario was available and implemented. In this waythe typical planning condition of a German HV systemoperator is largely replicated, who usually has one or twocharacteristic network loading cases to represent EHV inhis simulations.

Figure 4: Schematic Representation of the EHV/HV System

2.2. Choice of simulated time period

Purpose here is to assess the potential of a partially highlywind-penetrated HV system to provide overexcited CQunder challenging conditions. Provision of overexcitedreactive power will usually be limited either by high voltages or by equipment loading. Therefore, in theinvestigated system, the high-feed-in low-load scenarioconstitutes a particularly challenging situation. Resultsare presented for a day featuring annual peak feed-backfrom MV systems combined with varying degrees of HVfeed-in from wind power plants.

2.3. CQ from HV DER

A significant variety of specifications regarding staticreactive power provision exists internationally [12],[23], [24]. In this case study capabilities offered by

manufacturers since several years but usually not yetinstalled in the field are assumed. The ability to provideoverexcited reactive power Q/Pn=0.484 from zero activepower generation to nominal power generation isassumed, Figure 5. This is equivalent to a power factor of

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3.2. Choice of DER penetration

The following investigation shall assess whether thegiven system is able to provide a significant amountof CQ even under constrained operating conditions.Ideally, the obtained results should be applicable for alonger time span. One possible approach to solve thisproblem consists of varying DER penetration, carryingout network expansion measures if necessary, andassessing the resulting CQ bandwidth for different DERpenetrations.

Here, a different approach is taken. Figure 8 illustratesthe influence factors on the resulting CQ bandwidth ofany given HV system.

Figure 8: Factors Determining Reactive Power Bandwidth

CQ provision is about making use of the remainingloading capacity, before a loading limit is hit, and makinguse of the remaining voltage bandwidth, before the CQ-provision-relevant voltage limit is reached. The morefrequently the system is close to some limit already inits base case, the harder it is to transport additionalCQ. Therefore, the moment when a network expansionmeasure is being planned, but has not yet been carriedout, is the most critical moment in the circle of increasedDER penetration and increased network expansion.Therefore in this study, a DER penetration is simulatedthat results in base case loading and voltage conditionsbeing close to permissible limits.

3.3. Assess Reactive Power Bandwidth Based onOperational Constraints: Contingency-ConstrainedOptimal Power Flow

If network expansion is oriented towards hosting DER

It is assumed here that in reality, when utilizing DERin the context of a volt-var Optimal Power Flow (OPF),DER would receive voltage setpoints, see e.g. [28].

3. Methodology3.1. Definition of Reactive Power Bandwidth

HV network areas are galvanically coupled regions thatmostly have two or more connection points with EHV.Reactive power exchange is here studied on a networkarea basis: The residual of reactive power exchange at allnetwork area EHV/HV connection points Figure 6 -is computed once for the base case depicted in cyanin Figure 7 - and once for the optimized case, depictedin brown in Figure 7. The difference between the twois the achievable bandwidth of controllable overexcitedreactive power, indicated by green arrows. In thiscontribution, only overexcited CQ is analyzed, therefore

the term overexcited is usually omitted when referringto the bandwidth in the following. The term reactiveflexibility is used as synonym to the term bandwidthhere. The consumer oriented counting system is appliedthroughout this contribution.

Figure 6: Reactive Power Provision on Network Area Level

Figure 7: Bandwidth of Controllable Reactive Power

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MV systems. Measured values in 15 minute resolutionat the HV terminals of HV/MV transformers wereavailable, and one feed-in measurement from a windfarm. The selected day features the peak annual feed-back from MV.

- A second scenario (Wind Strong) explores CQprovision under severely limited grid capacity: Themeasured values from MV systems are combinedwith wind feed-in such that the permissible n-1relevant loading limit of the transformer is reachedbetween 11:00 and 18:00. This scenario could occurfor a few hours per year, considering regionallypossible maximum coincidence of wind and PV[29]. DER are continually being added to the system.Network expansion measures to accommodateincreasing DER capacities tend to take several yearsfrom planning to implementation [30]. Thus, in thecontext of simulating dependable CQ provision fromHV to EHV, it is an interesting and relevant scenario

to investigate CQ provision in the event of capacitybottlenecks.

3.4. Formulation of the optimization problem

Cost minimal delivery of reactive flexibility is simulated.The minimum overexcited flexibility to be delivered(QFlexox,min) is the constraint, objective being costminimization:

Minimize:Cost (t) = Cost HVLineloss(t) + Cost EHVHVTrfLoss(t) (1)

subject to

QEHVHVetwArea (t) QFlexox,min

A loss-cost of 40 Euro/MWh was assumed, whichcorresponds to the average of the average annual EPEXspot market price Phelix Day Base of the years 2012and 2013 [31]. Cost of transformer tap-changing is notconsidered. For an analysis of its impact refer to [17].

Optimal power flow was implemented using the

heuristic optimization algorithm Mean VarianceMapping Optimization (MVMO) [32], whose superiorperformance for solving mixed-integer reactive powerproblems was shown e.g. in [33]. For further detailsregarding the chosen implementation of single swarm

at unity power factor, it is of particular importance to verify the feasible reactive power bandwidth based on acontingency constrained dispatch, Figure 9. In this wayit is ensured that

Figure 9: Expected Advantage from Optimizing EHV/HV Taps

the dispatch solution fulfills the requirement relevant foroperation of HV systems: N-1 security for loads mustbe observed at each time step. This approach allows to verify in how far the high reliability ancillary serviceprovision aimed at here may be practically feasible, andat what costs.

If the base case investigated consists of a power systemalready operated close to the permissible limits, as isthe case here, integrating additional control variablesthat allow to redistribute power flows here: EHV/HV transformer tap changers is expected to increasethe feasible reactive power bandwidth. This will bedemonstrated in section 4.

While network planning may frequently be basedon analyzing selected loading conditions only, it ispreferred here to carry out time series simulations for the

assessment of highly reliable reactive power bandwidths.Two scenarios are investigated:

- The first scenario is based on the measuredcoincidence of HV wind feed-in and feed-back from

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protection in case of an outage of T2. Such curtailmentwould be a rare event in reality. Yet, to achieve high-reliability CQ provision, the feasible CQ bandwidthunder this kind of condition should be verified as well, atleast in systems facing considerable DER expansion. Theresidual of network area MV load (green) is assumed tobe the same for both the Wind Strong and the WindMeasured scenario.

Figure 10: Overview of Simulated Active Power Components

Figure 11 and Figure 12 highlight the resulting networkarea active and reactive power exchanges with EHV. Thebase case for assessing the reactive power bandwidthconsists of the reactive power exchange of the particularscenario: the blue line in Figure 11 represents thebase case reactive power exchange for scenario WindMeasured.

Figure 11: Base Case HV Wind Measured: Network Area P & Q

MVMO see [17]. Loading limits are assessed based onthe outcome of contingency screening: For lines, usuallyeither 70 % or 50 % loading limit under normal operatingconditions result. Throughout the scenarios analyzedhere, transformer T2 experiences highest loading:The limit for T2 is determined to be 61.4 % based oncontingency analyses.

Max. permissible voltage for 110 kV 1.100 p.u.Min. permissible voltage for 110 kV 0.982 p.u.

Max. permissible voltage for EHV side ofEHV/HV transformer

1.1030 p.u. (380 kV);1.1305 p.u. (220 kV)

Min. permissible voltage for EHV side ofEHV/HV transformer

1.0650 p.u. (380 kV);1.0363 p.u. (220 kV)

Table I: Voltage limits observed by OPF

Contingency analysis here focuses on the momentimmediately after a contingency, i.e. before DER voltagecontrol can adjust reactive power output and before EHV/HV taps change position. For this short moment here

maximum voltages of 1.105 p.u. have been permitted,otherwise values of Table I apply.

5. Simulation ResultsTable II provides an overview of the scenarios and controlstrategies whose results are discussed in the following.

Strategy Scenario

Base Case:EHV/HV

Taps VoltageControlled &DER Q = 0

EHV/HV TapsVoltage Con-

trolled (Local)

EHV/HV TapsControl Variable inOPF(Optimized),together with HV

DER Q

HV WindMeasured x x x

HV WindStrong x x x

Table II: Simulated Scenarios and Strategies

5.1. Base Scenarios: Wind Measured and Wind Strong.

Scenario Wind Measured allows to assess realisticfrequency distributions of specific loss costs of providingCQ. Scenario Wind Strong allows to analyze the networkarea reactive power bandwidth under challenging

normal operating conditions. Figure 10 features boththe measured (cyan plain) and the strong (cyan dotted)HV wind feed-in. The strong feed-in is slightly reducedby curtailment between 11:00 and 18:00. Curtailment isrequired to make sure that lines do not get tripped by

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normal operating conditions. This clearly highlights thenecessity to study reactive power ancillary service provisionto EHV based on network operation considerations, ratherthan purely on network planning assumptions.

Figure 13: Network Area Q Bandwidth for Different Strategies

Figure 14: Loading of Transformer T2 for Different Strategies

5.3. Analysis of Optimized EHV/HV Tap ControlEffects

The ability of integrated Tap + DER optimization to

Figure 12: Base Case HV Wind Strong: Network Area P and Q

5.2. Analysis of Achieved CQ Bandwidths

The assumed installed DER Q bandwidth amounts to 146Mvar overexcited. It is here aimed at providing 100 Mvar ofoverexcited CQ to EHV, which corresponds to the size oftypical smaller EHV capacitors, and is equivalent to 68 % of

installed CQ. Figure 13 displays the achieved reactive powerbandwidth for the different scenarios and control strategiesimplemented. The severe limitation of bandwidth forlocal voltage control of EHV/HV transformer taps duringdaytimes is clearly visible.

As expected from the analysis in Figure 9, the integrationof EHV/HV transformer tap positions as variables into theoptimization permits to achieve substantially improvedbandwidths throughout difficult operating conditions.From Figure 14 follows that in scenario Wind Strong.between approximately 10:30 and 18:00 the permissibleloading limit is reached. In fact, curtailment is required toobserve loading limits. Curtailment is assumed to be carriedout in steps of 10 % of installed DER capacity. This stepwiseactive power reduction see Figure 10 - results in slightlyadded freedom available for reactive power at certaintimes compare to the red dash-dotted line in Figure 13.If curtailment was carried out in steps of 5 %, less reactivepower could be made available, especially in the EHV/HVTap Local strategy.

It shall be pointed out here that the limitation in bandwidthobserved for the locally voltage controlled EHV/HV tapsin Figure 13 would not be observed if the n-1 contingency

constraint had not been imposed. By simulations it wasdetermined that if the post-contingency voltage limit waschosen to be higher, e.g. 1.2 p.u., or no n-1 constraint wasimposed, bandwidths would amount to 120 Mvar and more,while still observing the voltage bands of Table I under

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DER frequency distribution of reactive power during thesimulated 25 h is represented for each DER by a two-sided bean plot in Figure 17. Each bean plot consistsof two sides (red and blue) which allows to compareDER reactive power for the two control strategies. Theblue frequency distributions of reactive power valuesare clearly displaced towards negative, i.e. overexcited values, as compared to the red distributions that standfor locally voltage controlled EHV/HV taps. DER aresorted by the grid strength of their PCC, defined as theratio of short circuit power Sk over nominal active DERpower Pn, see [23]. Summarizing, the optimized controlof EHV/HV taps allowed to operate even those DERat least partially in overexcited mode that have fairlylow grid strength. Bean plots have been created usingpackage beanplot [34] from software R.

Figure 17: HV DER PCC Grid Strength and CQ Contribution

shift power flows shall now be analyzed in more detail.Figure 15 and Figure 16 highlight the difference innetwork voltage level resulting from the two controlstrategies. For locally voltage controlled EHV/HV taps voltages remain within a band between 1.03 and 1.08 p.u.The optimization of transformer taps allowed to makeuse of the full available specified voltage band until thelower boundary of 0.982. The upper voltage boundaryof 1.1 p.u. - Table I - is not reached, since correspondingsolutions do not observe the voltage boundaries in then-1 contingency calculations.

Figure 15: Voltage Level: DER Optimized, EHV/HV Taps Local

Figure 16: Voltage Level: DER + EHV/HV Taps Optimized

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5.5. Summary

Based on optimized dispatch of both EHV/HV transformersand HV DER reactive power output it was possible insimulations to reliably provide at least 57 % (83 Mvar) ofassumed installed DER reactive power capability on theday featuring 2013 annual peak feed-back towards HVfor this network area. Local voltage control of EHV/HVtransformer taps allowed to obtain bandwidths between -10and -55 Mvar, depending on the network loading condition.Thus, in the investigated network area, the widely practiced

local voltage control of EHV/HV taps does not qualify forproviding large quantities of CQ in a reliable way to EHV, ifn-1 contingency constraints are to be observed.

Cost of CQ provision varies substantially depending onnetwork operating conditions. Values between a saving of0.23 /Mvarh and a cost of 0.53 /Mvarh were observed.Due to the target of achieving a bandwidth of 100 Mvaroverexcited, frequently the loss-minimization modeof optimal power flow was not reached, and rather amaximization of reactive power bandwidth was carried out.The cost figures do not include the additional cost causedby CQ delivery inside the DER parks.

To implement DER based HV CQ provision as discussedhere, it would need to be ensured that DER are notdisconnected from the system for other reasons, e.g. dueto being part of a virtual power plant that contributes tothe reduction of overfrequency by occasionally completelydisconnecting DER from the grid.

6. Economic attractiveness ofhighly dependable controllablereactive power from HV6.1. Specific reactive power cost of EHV capacitors

At hand it is assumed that the TSO alternative to sourcing

5.4. Cost of DER CQ-Provision for Simulated Period

The resulting PQ-capability curve for both the WindMeasured and the Wind Strong scenario is displayedin Figure 18. The Base Case was chosen as reference forreactive power provision, therefore, no cost is associated(black dots). The specific cost of CQ provision iscomputed to:

SpCQC Scen i =C Ctr,Scen i _C Base,Scen i

(Q NA,Ctr,Scen i _Q NA,Base,Scen i)t (2)

Term Unit Explanation

SpCQC /Mvarh Specic Cost of Controlled Reactive Power

Scen i -Scenario i:i=1: HV wind as measuredi=2: HV wind strong

Ctr j -Controlled Case j: j=1: DER Optimized., EHV/HV Tap VoltageControlled (Local) j=2: DER + EHV/HV Tap Optimized

Base -Base Case:DER Q = 0, EHV/HV Tap Voltage Con-trolled (Local)

C Cost = Loss Cost * Losses, see (1)

Q NA MvarResidual of Network Area Reactive PowerExchange with EHV, as measured at theEHV terminals of EHV/HV transformers

t h Time

Table III: Abbreviations used in (2)

Providing larger amounts of CQ under the simulateddifficult operating conditions comes at the cost of higherlosses, as may be observed by comparing the colors offilled (EHV/HV Tap local) and empty (EHV/HV TapOptimized) symbols of same shape (e.g. circles).

Figure 18: Specific Cost of HV Controllable Reactive Power

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capacitor incl. its EHV switching field, and operationalcosts of 2 % of investment costs per year [35]. Losscosts were assumed to 40 /MWh, see section 3.4.Not considered are possible long term price increasesfor operational costs and loss costs. From Figure 20follows that if the capacitor is switched on only duringrare contingency events, the specific costs per Mvarhdelivered may become very high.

Once an EHV capacitor has been purchased, it ischaracterized by extraordinarily low losses in the order of

0.35 kW/Mvar1

, i.e. 0.014 Eur/Mvarh under the assumedloss cost. Specific reactive power cost of EHV capacitorsis thus dominated by investment and capital cost.

With respect to HV DER CQ provision from the analysesof the previous sections the following may be concluded:The higher the amount of CQ available free of chargeto the DSO due to grid codes, and the higher therequirements of grid codes with respect to providing CQeven at zero active power output, the lower the relevantfixed costs for making available reliable CQ at times ofzero DER active power output. Especially if capacitorsconnected at the collector bus of many existing DER parksexist and are utilized, this results in a low-loss, highlyavailable source of CQ. Moreover, this resource is alreadypaid off, i.e. beyond the potentially required upgrade ofInformation and Communication Technology (ICT) andControl Infrastructure, no additional investment andcapital costs ensue. If modern PV inverters are used, ableto provide reactive power down to 3 % of active poweroutput without installing additional compensatingequipment, and EHV demand for reactive power isexpected to occur only during daytimes, then utility-scale PV plant based reactive power provision may alsobe a very attractive option [36].

1 Courtesy TransnetBW GmbH: Value applies to EHV mechanically switchedcapacitor with damping network of 250 Mvar operated at 420 kV. It includesmain and ancillary capacitors as well as the inductance and damping resistor.Losses related to harmonic currents are not considered.

overexcited reactive power from HV DER consists ofinstalling an EHV-connected capacitor. Principally theTSO may have further investment alternatives at hand,e.g. FACTS devices. These usually have the capability toprovide further ancillary services and therefore assessingthe specific cost per Mvarh provided while consideringinvestment and capital costs tends to be more complex.EHV capacitors are being installed in many countries.They are a typical source of voltage support for slowphenomena [21]. Here the aim is to compare reactivepower cost of EHV capacitors to that caused by CQ

provision from HV DER as illustrated above.Generally, the cost per Mvarh of reactive power deliveredis influenced by the cost components shown in Figure19. Here, costs from the perspective of the networkoperator(s) are computed, without distinguishingwhether they occur on EHV or on HV side. Generally, HVDER CQ requires ICT & OPF integration of substantiallymore components than CQ from EHV capacitors.Therefore in Figure 19, ICT & OPF integration costs ofthe EHV capacitor are not indicated separately. ICT andOPF integration costs of HV DER CQ may constitutea relevant cost component. They require more detailedanalysis to take into account potential synergies withother use cases and further ancillary services and aretherefore not considered here.

Maintenance cost of HV DER would be in theresponsibility of the HV DER owner. However, thehigher number and different nature of components tobe managed in the context of HV DER CQ will causeincreased administration costs for the network operator.DER CQ is assumed to be available free of charge andpotentially added losses within DER parks are notconsidered.

Based on these assumptions Figure 20 shows the specificcost of reactive power from EHV capacitors as a functionof capacitor utilization rate. The figure was compiledassuming a cost of 3.3 Mio. Euro for the 100 Mvar

Figure 19: Cost Components Considered in Cost Comparison Figure 20: Specific Cost of Reactive Power from EHV Capacitors

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power in the event of a contingency of a heavily loadedtransmission line.

Figure 21: Decision Flowchart

Step 3 is devoted to assessing the available CQ fromHV under the defined target conditions. Step 4 is ofparticular importance: In many cases it may be requiredto study HV CQ bandwidth under non-normal andcontingency operating conditions, in order to satisfy thereliability requirements from EHV. The combined resultsof steps 3 and 4 will determine whether the CQ that canbe provided at the required reliability level is sufficient.This check will rule out a large number of potentialapplications. During the check the CQ potential ofneighboring network areas might also be considered, if

tolerable from EHV perspective.Check 7 constitutes a further stepping stone for theconsidered use case: If retrofit of ICT and controlinfrastructure of DSO and DER is required, and possibly

Summarizing, in many cases the dominating costcomponent of providing CQ based on HV DER will bethe operational costs (losses, administration). Unlikean EHV capacitor characterized by long equipmentlifetime the DER based reactive power of HV networkareas may be flexibly adjusted to the EHV requirements.Therefore, the risk of investing into an EHV capacitorthat may not be needed any more a decade later due tochanged system operating conditions, could be avoidedby recurring to CQ from HV.

Thus, from the perspective prior to the investment intoan EHV capacitor, it seems attractive to contemplatethe HV alternative. The strong points of this alternativein case of expected low EHV capacitor utilizationrates having been listed, the next section is devoted todiscussing the practical impediments related to such anundertaking.

6.2. Requirements related to the use case of deferring

or replacing EHV capacitor investmentThe decision flowchart of Figure 21 summarizes theconsiderations: Point of departure for the investigationis the intention to install an EHV capacitor characterizedby low expected utilization rates. The first question tobe clarified is whether the HV system has potentiallyCQ resources that have not yet been considered in thedimensioning of the EHV capacitor. If this is the case,the conditions motivating the EHV capacitor installationshould be clearly defined, in order to derive the conditionsunder which HV CQ would be utilized.

A possible scenario might be to compensate forthe increase of PV-feed-in related reactive powerconsumption in LV and MV systems under conditions ofhigh solar irradiation and low load (target condition),see [37], [38]. This would limit relevant CQ deliveryto daylight times. Thus, modern PV plants capable ofproviding reactive power support down to very lowactive power feed-in would be the ideal candidate forcompensating this reactive power consumption fromMV and LV systems on HV level. This compensationmay in fact be complemented by similar action from MVPV plants.

The most challenging outcome of this step consists ofan EHV reactive power requirement that is uncorrelatedwith HV reactive power capability. Such situation mayarise when the capacitor is installed to provide reactive

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implemented based on the heuristic optimization MeanVariance Mapping Optimization. This allowed to analyzeHV

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