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Author: Nikolaos Efkarpidis Co-authors: Carlos Gonzalez Thomas Wijnhoven

Coordinated voltage control scheme for Flemish LV distribution grids utilizing OLTC transformers and D-STATCOM’s. Author: Nikolaos Efkarpidis Co-authors: Carlos Gonzalez Thomas Wijnhoven Tom De Rybel Johan Driesen. Content. The problem The purpose of this work

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Author: Nikolaos Efkarpidis Co-authors: Carlos Gonzalez Thomas Wijnhoven

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  1. Coordinated voltage control scheme for Flemish LV distribution grids utilizing OLTC transformers and D-STATCOM’s Author: Nikolaos Efkarpidis Co-authors: Carlos Gonzalez Thomas Wijnhoven Tom De Rybel Johan Driesen

  2. Content • Theproblem • Thepurposeofthiswork • OLTCcontrolstrategies • Technicalimpactson LV grids • Inputs and assumptions • Evaluationof OLTC’s performance • Coordinated control scheme • Results • Conclusions-futurework Image Source: MR, “GRIDCON iTAP, The system solution for voltage regulated distribution transformers,” Tech. Rep., 2012

  3. TheProblem Higher loading of LV distribution networks • Increased electric power consumption • Distributed Energy Resources (DER) integration Technical impacts relating to: • Power Quality • Potential Equipment Overloads • Distribution System Efficiency

  4. The Problem The Solutions Traditional methods: • Installation of additional, parallel cables or replacement of the existing ones • Reduction of cable lengths increasing the number of substations Upcoming methods: • Active Network Management (ANM) strategies and technologies • Massive actual application at MV level • Evaluation of first implemented OLTC’s prototypes at LV level • Integration of Distributed Flexible AC Transmission Systems (D-FACTS)

  5. The Purpose of this Work • Evaluation of OLTCs in LV distribution grids with respect to: • Improvement of grid power quality • Supply voltage variations • Rapid voltage changes • Supply voltage unbalance • Supply voltage dips/swells • Power losses • Effects on the thermal limits of: • Distribution lines • Distribution transformers • Evaluation of a coordinative voltage control scheme with respect to: • Overvoltage limits • Voltage unbalance limits

  6. OLTC Control Strategies Conventional method • Line Drop Compensation (LDC) of the Active Voltage Regulator (AVR) • Local measurement of voltage on the secondary side of the transformer (Vbus1) • Estimation of load currents (I2,I3,I4) and line impedances (Z2,Z3,Z4) • Straightforward method • Low investment cost • Low complexity • Difficulties of current prediction • Low reliability

  7. OLTC Control Strategies Proposed method • Remote voltage measurement from all the Points of Common Coupling (PCC’s) • Application to each phase of the tap-changer individually • The range of the transformer ratio (0.8%-2.5%) and the number of steps depend on the design of the transformer and the OLTC. • The tap-changing time depends on the OLTC architecture • Umin,Umaxminimum and maximum of the PCC’s voltages • UL,UΗminimum and maximum allowable voltage • Ustep step voltage per tap-change • tap position of the tap-changer • ΔUmax=Umax–UΗ • ΔUmin=UL -Umin

  8. Technical Impacts on LV Grids Power quality issues • Customer voltage rise/drop EN-50160 requirements: • 10 minutes mean r.m.s voltage within the statutory limits 230 V +10%/-10% (253 V; 207 V) at least during 95% of the week • 10 minutes mean r.m.s voltage within the statutory limits 230 V +10%/-15% (253 V; 195,5 V) during 100% of the week Voltage drop due to: • Higher loading, large voltage drop over long distribution lines in radial networks. Voltage rise due to: • High DER penetration levels in radial networks, return current flowing through the neutral conductor in unbalanced four-wire LV networks.

  9. Technical Impacts on LV Grids Power quality issues • Voltage Unbalance Factor (VUF) where: V0, V1, V2 : zero, positive and negative sequence voltage components Va, Vb, Vc: phase-to-neutral voltages EN-50160 requirements: • VUF below 2% at least 95% of the week Voltage unbalance violations due to: • Disproportionate installation of single-phase generation units in LV distribution networks

  10. Technical Impacts on LV Grids Thermal constraints • Transformer thermal limits • IEEE Standard C57.91-1995-suggested limits for loading: • At low demand and high DER penetration levels the suggested limits might be exceeded • The OLTC mechanism can impose an asymmetrical power flow limit reducing the rated reverse power flow of the transformer • IEEE Standard C57.131-OLTC constraints:

  11. Technical Impacts on LV Grids Thermal constraints • Cable thermal limits In the literature: • Reported thermal ladder networks with equally-loaded conductors • Omission of interaction between the conductors • Same cross-section for all the conductors use of a 2-D finite element software • maximum permitted paper insulation temperature

  12. Inputs and Assumptions Loads and generation units • 39 households, whereof 10 are single-phase • load profiles with step size of 10 min • Single-phase PVs of 5 kVA Investigated LV grid SPECIFICATION OF THE INVESTIGATED SCENARIOS

  13. Inputs and Assumptions Transformer and cable • Oil-type distribution transformer of 250 kVA capacity • Reactor type OLTC • Underground four-core sector-shaped conductor with cross-section 3x70 + 1x50 mm2

  14. Evaluation of OLTC’s performance Elimination of the violations of various constraints • Voltage statutory limits: • Under-voltage (85% Un, 90% Un)

  15. Evaluation of OLTC’s performance Elimination of the violations of various constraints • Voltage statutory limits: • Over-voltage (110% Un)

  16. Evaluation of OLTC’s performance Increase of the violations of voltage unbalance constraints: • Voltage Unbalance Factor (VUF) limit (2%) Reason: Independent OLTC control of every phase

  17. Evaluation of OLTC’s performance Decrease of the maximum values of the grid elements • Transformer thermal indicators: • Top oil-temperature (120 oC) • Hottest-spot conductor temperature (200 oC) • Short-time loading (300 %) • Cable thermal indicators: • Paper insulation temperature (80 oC)

  18. Evaluation of OLTC’s performance Effects of the DG penetration level on: • Annual losses • Paper insulation temperature

  19. Evaluation of OLTC’s performance OLTC parameters

  20. Evaluation of OLTC’s performance Conclusions • Partly improvement of the over-voltage and under-voltage indicators • Deterioration of voltage unbalances in the grid • Decrease of annual network losses and paper insulation temperature of the cables • Distinct drop of both the temperature indicators and the maximum short-time loading of the transformer Next step • Combination of the proposed voltage control algorithm with additional ANM technologies Efkarpidis N., González de Miguel C., Wijnhoven T., Van Dommelen D., De Rybel T., Driesen J. 2013. Technical Assessment of On-Load Tap-Changers in Flemish LV Distribution Grids. In Solar Integration Workshop. London, 21-22 October 2013 (London) , pp. 94-101

  21. Coordinatedcontrol scheme Possible solutions Traditional reactive power compensation devices: • Static Var Compensators (SVCs) • Switched capacitor banks • Other fixed impedance devices Distributed Flexible AC Transmission Systems (D-FACTS): • Distributed Static Synchronous Compensator (D-STATCOM) • Dynamic Voltage Restorer (DVR) • Distributed Power Flow Controller (DPFC) • Active Power Filter (APF) Two D-STATCOM’s of 70 kVA capacity

  22. Coordinated control scheme D-STATCOM controller for positive-sequence voltage regulation via active power management

  23. Coordinated control scheme D-STATCOM controller for negative-sequence voltage regulation via reactive power management where φ is the phase angle of the negative-sequence voltage V-

  24. Results Case 1- 50% DG / 103.78% LB • Drop of the over-voltage indicator under 1.1 pu • High loadings are imposed for short periods of time (~ 3min) • Slight influence of KVUF for zero values

  25. Results Case 1- 50% DG / 103.78% LB • Reduction of the under-voltage indicator (2) remaining above the limit (0.9 pu) • No influence of KVUF

  26. Results Case 1- 50% DG / 103.78% LB • VUF and maximum current are affected by both gains

  27. Results Case 2- 40% DG / 96% LB • Drop of the maximum voltage increasing either KVUF or KV • Slight current drops because of KVUF increase

  28. Results Case 2- 40% DG / 96% LB • Minimum voltage under the statutory limit (0.9 pu) => Disconnection of D-STATCOM device for positive-sequence control when not needed !!!

  29. Results Case 2- 40% DG / 96% LB • Increase of KVUF causes VUF drop => Disconnection of D-STATCOM device for positive-sequence control when not needed !!!

  30. Results Comparison of evaluated methods

  31. Results Evaluation in terms of the location in the grid

  32. Conclusions-future work Conclusions • Partly improvement of overvoltage indicators and deterioration of voltage unbalances with independent tap-changing control per phase • Full remediation of the violations of the voltage indicators via active power management • Considerable reduction of voltage unbalances via reactive power management Future work • Combination of the proposed voltage control algorithm with additional ANM technologies Efkarpidis N., Wijnhoven T., González de Miguel C., De Rybel T., Driesen J. 2014. Coordinated voltage control scheme for Flemish LV distribution grids utilizing OLTC transformers and D-STATCOM’s.In DPSP-IET Events. , Copenhagen 2014, pp. 1-6

  33. Thank you for your attention Questions ??

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