Future Electricity Grids 2/2

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The European power system has changed considerably in the last 15 years. The liberalisation and unbundling of the electricity market has led to increased international power flows and reduced …

The European power system has changed considerably in the last 15 years. The liberalisation and unbundling of the electricity market has led to increased international power flows and reduced influence of the system operators. Meanwhile, renewable and other small-scale uncontrolled and variable energy sources are being installed in the system.

This session will focus on the use of power flow controlling devices as a means to control power flow in the system. Special attention will be paid to the coordination of power flow controllers and the possible negative effects on neighbouring systems.

* Coordination in transmission systems, especially with power flow controllers
* HVDC, VSC HVDC and the potential of supergrids.

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  • 1. Mini-Course on Future Electric Grids Part 2 of 2 Dirk Van Hertem — Dirk.VanHertem@ieee.org Electric power systems EKC2 , Controllable power systems Electrical engineering department Royal Institute of Technology, Sweden March 8, 2010 K.U.Leuven (Belgium) KTH, Stockholm (Sweden) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 1 / 47
  • 2. Introduction Course overview Who am I? Master in engineering from KHK Geel, Belgium Master of science in engineering from K.U.Leuven, Belgium PhD in engineering from K.U.Leuven, Belgium Currently Post-Doc researcher at the Royal Institute of Technology, Stockholm, Sweden Program manager controllable power systems group of the Swedish center of excellence for electric power systems (EKC2 ) Active member of both IEEE and Cigré Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 2 / 47
  • 3. Introduction Course overview Course overview and objectives Overview Part 1 New situation in the power system 1 Liberalization of the market 2 Increased penetration of smaller, variable energy sources 3 No single authority in Europe 4 Lacking investments in the transmission system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 3 / 47
  • 4. Introduction Course overview Course overview and objectives Overview Part 2 International coordination in the power system How this coordination is evolving (Coreso) Power flow controllers Coordination and power flow controllers The future “supergrid”. . . . . . and the road towards it Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 3 / 47
  • 5. Introduction Course overview What it is about and what not Not the grid of 2050 Main focus is Europe Not about smart grids (or not specifically) About transmission and not distribution Mainly from a grid operator point of view Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 4 / 47
  • 6. Introduction Course overview 1 Introduction Course overview 2 Coordination in the power system Situation sketch Information exchange between TSOs Steps towards increased coordination: Coreso example 3 Power flow controllers Introduction Controlling PFC in an international context Example: Losses in a grid Need for coordination How to coordinate? 4 Supergrids A supergrid? Technology requirements for the supergrid Controlling the supergrid Techno-Economic approach to a supergrid 5 Conclusions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 5 / 47
  • 7. Coordination in the power system 1 Introduction Course overview 2 Coordination in the power system Situation sketch Information exchange between TSOs Steps towards increased coordination: Coreso example 3 Power flow controllers Introduction Controlling PFC in an international context Example: Losses in a grid Need for coordination How to coordinate? 4 Supergrids A supergrid? Technology requirements for the supergrid Controlling the supergrid Techno-Economic approach to a supergrid 5 Conclusions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 6 / 47
  • 8. Coordination in the power system Situation sketch Power system control before liberalization Vertically integrated companies Generator company and grid operator are one company Power system operator controls the power system: Unit dispatch is done by system operators Topology changes: Line switching Reactive power: capacitor switching and VAr control of generators International/-zonal redispatch (at cost) All generation is centrally controlled Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 7 / 47
  • 9. Coordination in the power system Situation sketch Now: different involved parties Unbundling separated generator, transmission, distribution and suppliers Power exchanges were introduced Renewables were introduced Generation no longer directly controlled by transmission system operator Operator controls the transmission system: Unit dispatch can be requested by system operators at a cost Topology changes: Line switching Reactive power: capacitor switching, but VAr control of generators? International/-zonal redispatch (at cost) A significant increase of power flow controlling devices is noticed Less stable pattern due to market: high volatility Need for firm capacity for the market participants ⇒ Higher need for control with less “free” means Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 8 / 47
  • 10. Coordination in the power system Information exchange between TSOs Interconnected power system: information exchange The different zones are interconnected (synchronous zones) Operated independently International market operation Operation of the system effects the system cross-border Information is exchanged: Grid status (important outages) Day-ahead congestion forecasts Expected available capacities Any emergency with possible effects outside of the zone Not everything is exchanged Not all the generation data (aggregated) Grid data on a “need-to-know” basis Quite good working system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 9 / 47
  • 11. Coordination in the power system Information exchange between TSOs DACF: Day-ahead congestion forecasts Procedure Estimated zonal grid (cut at the borders) is provided Together with expected aggregated load/generation patterns The planned state of devices such as on-load tap changers and capacitors is provided Sum of generation, load and losses equals the planned exchange Exchange is set in the interconnections (X-nodes) Reactive power is set to a sensible amount Local load flow is run Data file is uploaded and merged Merged load flow is run and returned to TSO In case of congestion: TSOs negotiate appropriate actions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 10 / 47
  • 12. Coordination in the power system Information exchange between TSOs Still some problems Unexpected loop flows Uncertainty in the system remains high Black-outs or near black-outs due to lack off coordination and or communication August 2003: Italian black-out: Stopping pumped hydro (or reverse) might have helped Miscommunication was one of the main problems November 2006: UCTE near black-out Communication between operators failed Sequence of events that could have been avoided Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 11 / 47
  • 13. Coordination in the power system Information exchange between TSOs Limitations in cooperation Unforeseen events may occur Not everything is known With higher uncertainties and less control options, the system operator has limited tools available Some problems might be easily solved in another zone instead of costly local actions System-wide security assessments are not performed/updated during the day Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 12 / 47
  • 14. Coordination in the power system Steps towards increased coordination: Coreso example Steps towards increased coordination: Coreso example What is Coreso? The first Regional Technical Coordination Service Center (created Dec. 2008, in operation since Feb. 2009) Independent company, located in Brussels (www.coreso.eu) Shareholders are TSOs (founders Elia and RTE, and National grid), open to others Coreso does not operate the grid, but acts as a coordinated supervision for its members Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 13 / 47
  • 15. Coordination in the power system Steps towards increased coordination: Coreso example Steps towards increased coordination: Coreso example Service provider for TSOs Type of services: Pro-active assessment of the safety level of the network (day ahead and close to real time forecast) Proposing to the TSOs the implementation of optimized coordinated actions to master these risks Relaying significant information and coordinating the agreement on remedial actions Contributing to ex-post analysis and experience reviews of significant operating events for the appropriate area Providing D-2 capacity forecast Focus on: Supra national view on the network Cross-border impacts between TSOs Improved regional integration of renewable energy Area of interest: participating TSOs Security analysis extends to CWE (Benelux, France and Germany) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 13 / 47
  • 16. Power flow controllers 1 Introduction Course overview 2 Coordination in the power system Situation sketch Information exchange between TSOs Steps towards increased coordination: Coreso example 3 Power flow controllers Introduction Controlling PFC in an international context Example: Losses in a grid Need for coordination How to coordinate? 4 Supergrids A supergrid? Technology requirements for the supergrid Controlling the supergrid Techno-Economic approach to a supergrid 5 Conclusions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 14 / 47
  • 17. Power flow controllers Introduction What is power flow control Bending the laws of Kirchhoff In normal systems, power flows according to the laws of Kirchhoff Power flows in meshed networks depend on the relative impedance of the lines Using power flow controlling devices, these flows can be influenced Simplified: PFC work as a valve Overloaded lines can be relieved System can be adjusted to the situation: day-night, summer-winter, import-export, maintenance situations,. . . Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 15 / 47
  • 18. Power flow controllers Introduction Power flow control Power flow equations for a simple transmission line: |US | · |UR | Active power: PR = X · sin (δ) |US | · |UR | | UR | 2 Reactive power: QR = X · cos (δ) − X Receiving end power can be altered through voltage, impedance and angle Different technologies exist: mechanically switched,  ·I ·X thyristor based and fast switches UR Subset of FACTS (flexible AC transmission systems) IS X IR US δ US UR I Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
  • 19. Power flow controllers Introduction Power flow control Power flow equations for a simple transmission line: |US | · |UR | Active power: PR = X · sin (δ) |US | · |UR | | UR | 2 Reactive power: QR = X · cos (δ) − X Receiving end power can be altered through voltage, impedance and angle Different technologies exist: mechanically switched,  ·I ·X thyristor based and fast switches UR Subset of FACTS (flexible AC transmission systems) IS X IR US δ US UR I Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
  • 20. Power flow controllers Introduction Power flow control Power flow equations for a simple transmission line: |US | · |UR | Active power: PR = X · sin (δ) |US | · |UR | | UR | 2 Reactive power: QR = X · cos (δ) − X Voltage Receiving end power can be altered through voltage, UR impedance and angle Different technologies exist: mechanically switched, thyristor based and fast switches Subset of FACTS (flexible AC transmission systems) IS X IR US US UR Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
  • 21. Power flow controllers Introduction Power flow control Power flow equations for a simple transmission line: |US | · |UR | Active power: PR = X · sin (δ) |US | · |UR | | UR | 2 Reactive power: QR = X · cos (δ) − X Receiving end power can be altered through voltage, Impedance impedance and angle UR Different technologies exist: mechanically switched, thyristor based and fast switches Subset of FACTS (flexible AC transmission systems) IS X IR US I US UR Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
  • 22. Power flow controllers Introduction Power flow control Power flow equations for a simple transmission line: |US | · |UR | Active power: PR = X · sin (δ) |US | · |UR | | UR | 2 Reactive power: QR = X · cos (δ) − X Receiving end power can be altered through voltage, impedance and angle Different technologies exist: mechanically switched, UR Angle thyristor based and fast switches Subset of FACTS (flexible AC transmission systems) IS X IR US US UR Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
  • 23. Power flow controllers Introduction PFC devices: examples Phase shifting transformer US UR Mechanically switched device Basic principle of a transformer ∆U1 = 2 · k · UM23 k · UM23 How it works: Injects a part of UR 1 US1 UM3 UM2 UM1 UM1 the line voltage of opposing k · UM23 phases in series with the phase UM31 UM12 voltage to create an angle difference Different types: direct/indirect UM23 UM23 and symmetrical/asymmetrical UM3 UM2 Cheap, robust, efficient and slow Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 17 / 47
  • 24. Power flow controllers Introduction PFC devices: examples TSSC/TCSC: Thyristor switched/controlled TSSC ↔ TCSC series capacitor Compensate the natural series inductance of transmission lines Especially used for longer lines Possible to use for dynamic power system oscillation damping Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 17 / 47
  • 25. Power flow controllers Introduction HVDC: High Voltage Direct Current LCC HVDC Line commutated converter HVDC Exists for over 50 years High ratings, relative low losses Needs a strong AC grid to connect to 1 0 1 0 DC reactor 1 0 1 0 AC filter 1 0 1 0 1 0 1 0 1 1 0 0 1 0 1 0 Y /Y 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1111111111 0000000000 1 0 1 0 1 1 0 0 1 0 1 0 1 0 Y /∆ 1 0 1 0 Converter DC filter AC switchyard Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
  • 26. A three phase converter consisting of three 3-level phase units is illustrated in Figure 4.3. The single- As phase output voltage waveform, relative duration of the positive (and negative) output voltage with indicated in the figure, the assuming fundamental frequency switching,Power flow controllers is also shown in Figure Introduction respect to the duration of thedc terminals to is a function of or centre-tapped dc source. As seen, 4.3. The converter has three zero output connect to a split control parameter �, which defines the conduction interval of thevalves used as in the 2-level phase unit, and additional diodes are required to there are twice as many top upper, and the bottom lower valves. The magnitude of the fundamental connect to the dc supply centre-tap, which is the reference zero potential. However, with identical frequency component of the output voltage total dc supply the phase unitdoubled so that the parameter �. valve terminal-to-terminal voltage rating, the produced by voltage can be is a function of output HVDC: High Voltage Direct Current When � equals zero degreessame. maximum, while at � equals 90 degrees it is zero. Thus, one voltage per valve remains the it is advantage of the 3-level phase unit is that it has an internal capability to control the magnitude of the output voltage without changing the number of valve switchings per cycle. + The operating advantages of the 3-level phase unit can only be fully realised with some increase in circuit complexity, as well as more rigorous requirements for managing the proper operation of the Ud converter circuit. These requirements are related to executing the current transfers (commutation) between the four (physically large) valves, with well-constrained voltage overshoot, while maintaining the required di/dt and dv/dt for the semiconductors without excessive losses. +Ud UL1 Neutral (mid-) point UL2 An additional requirement is to accommodate the increased ac ripple current with a generally high UL3 triplen harmonic content flowing through the mid-point of the dc supply. This may necessitate the use -Ud � of a larger dc storage capacitor or the employment of other means to minimise the fluctuation of the mid-point voltage. However, once these problems are solved, the 3-level phase unit provides a useful VSC HVDC Ud building block to structure high power converters, particularly when rapid ac voltage control is needed. - The conduction periods for the inner and the outer valves is different, and therefore it is possible to use Voltage source converter two different designs of a VSC valve for the two positions. Figure 4.3: Three-phase 3-level NPC converter and associated ac voltage waveform for one phase By switching the valves more frequently, it is possible to eliminate more harmonics. A typical PWM Quite new switched waveform, using a carrier based control method with a frequency of 21 times fundamental frequency,waveform shown in the 4.4. For the purpose of voltage, assuming fundamental frequency been The ac is given in Figure figure is the phase-to-neutral this illustration, the dc capacitor has Fast switching (PWM) Figure: Scheme of a 3-level 3-phase VSC assumed to have anvalves. The neutral voltageno dc voltage ripple). switching of the infinite capacitance (i.e., is the voltage at the midpoint of the dc capacitor. As illustrated in Figure 4.3, the output voltage of the 3-level phase unit can be positive, negative, or zero. Positive output is produced by gating on both upper valves in the phase unit, while negative output is produced by gating on both lower valves. Zero output is produced when the upper and lower middle Highly dynamic 1 valves, connecting the centre tap of the dc supply via the two diodes to the output, are gated on. At zero output, positive current is conducted by the upper-middle controllable device and the upper centre- Makes its own rotating field Line-to-neutral voltage (pu) tap diode, and negative current by the lower-middle controllable and the lower centre-tap diode. 0 Relative high losses 4-4 Only two manufactures (ABB and 1 0 90 180 270 360 Siemens) Degree Figure 4.4 Single-phase ac voltage output for 3-level NPC converter with PWM switching at 21 times (→ Source: Cigré Tech. Rep. 269) fundamental frequency Figure: Voltage waveform of a 3-level 3-phase 4.2.4 Multi-Level Neutral Point Clamped Converter VSC with single phase output voltage In order to further reduce the harmonic content of the ac output voltage, the basic 3-level phase unit can be(fswitch a= 21 × 2n+1 phase unit (n=1,2,3,…) configuration. 2n dc supplies, provided by extended to multi-level, fn ) 2n dc storage capacitors (which are common to all three-phase units of a complete three-phase converter),Van Hertem (Electric Power Systems, KTH) Dirk are connected in series, providing 2n+1 discrete voltage levels. Mini-course onvalves are Four times n Future Electric Grids (2/2) 8/03/2010 18 / 47
  • 27. Power flow controllers Introduction HVDC: High Voltage Direct Current HVDC is a special power flow controller Allows full, independent active power flow control VSC HVDC also provides independent reactive power flow control The ultimate power flow controller, yet not a true power flow controller A B HVDC as a single link between two independent networks, no possibility for active power flow control (flow is equal to the imbalance in the zones) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
  • 28. Power flow controllers Introduction HVDC: High Voltage Direct Current HVDC is a special power flow controller Allows full, independent active power flow control VSC HVDC also provides independent reactive power flow control The ultimate power flow controller, yet not a true power flow controller HVDC as part of the meshed AC power system, HVDC can be operated as a PFC, with a flow independent on the rest of the system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
  • 29. Power flow controllers Introduction HVDC: High Voltage Direct Current HVDC is a special power flow controller Allows full, independent active power flow control VSC HVDC also provides independent reactive power flow control The ultimate power flow controller, yet not a true power flow controller A B Two meshed networks are connected through multiple HVDC. HVDC can be used as PFC when there is coordination Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
  • 30. Power flow controllers Introduction Power flow controlling devices: classification AC Network controller Conventional FACTS Devices (Switched) (Fast, static) R, L, C Thyristor Voltage Source Transformer Valves Convertor (IGBT) Switched Shunt Static VAr Controller Static Synchronous Shunt devices Compensation: (SVC) Compensator L and C Thyristor Controlled (STATCOM) Reactors (TCR),. . . Switched Series Thyristor Controlled Static Synchronous Series devices Compensation: and Thyristor Switched Series Compensator L and C Series Compensator (SSSC) (TCSC and TSSC). Phase Shifting Thyristor Controlled Unified Power Flow Combined Transformer(PST) Phase Angle Controller (UPFC) Series & Shunt Regulators (TCPST) VSC HVDC LCC HVDC Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 19 / 47
  • 31. Power flow controllers Introduction Existing/planned power flow controllers in the Benelux Norned u E UK-Fr Eu u Diele Meeden ' 1 HVDC interconnector UK-FR Gronau 2 Meeden PSTs (2×) BritNed u Eu 3 Gronau PST Zandvliet 4 Monceau PST u Eu Van Eyck 5 Norned HVDC c u$ W $ uy ˆ 6 Van Eyck PSTs Monceau ˆ 7 Zandvliet PST u   ©   8 Diele 9 BritNed (2011?) (source: UCTE) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 20 / 47
  • 32. Power flow controllers Introduction Existing/planned power flow controllers in the Benelux Cobra and/or Norned 2 Norned u E UK-Fr 1 HVDC interconnector UK-FR Eu u Diele Meeden ' 2 Meeden PSTs (2×) 3 Gronau PST Gronau 4 Monceau PST BritNedu Eu 5 Norned HVDC Zandvliet 6 Van Eyck PSTs u NEMO u Eu Van Eyck u Zandvliet PST 7 c u$ W $ uy ˆ 8 Diele u u Monceau ˆ 9 BritNed (2011?) BE-DE u ©    10 NEMO (2013?) 11 Belgium Germany (?) 12 Cobra and/or Norned 2 (?) Most are less than 10 years old (source: UCTE) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 20 / 47
  • 33. Power flow controllers Controlling PFC in an international context Control of PFC Locally controlled The investment is normally done by a TSOs Therefore control is done by the TSO to fulfill his own objectives Payed for by the local market participants, so “revenues” should be returned to the local market as well Optimal use of the transmission system Minimum losses Maximum security Maximum transmission capacity Effects are not local Devices are mostly placed on the border The effects of active power flow control can reach far into neighboring systems Some control actions are intended to influence “external” powers Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 21 / 47
  • 34. Power flow controllers Controlling PFC in an international context Multiple zones, multiple PFC Load Load Load Load A A A A 50 % 50 % 20 % 80 % -10 % 110 % 50 % 50 % α β α α B D B D B D B D Gen Gen Gen Gen C C C C (A) (B) (C) (D) Example of possible problems with power flow control in multiple zones A: Generation in the south, load in the north, equal flow distribution B: Zone B invest in a power flow controller: power flow is shifted C: Overcompensation by B (following schedules, optimizing for zone B) D: D also invests in a power flow controller: two investments, no advantage Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 22 / 47
  • 35. Power flow controllers Example: Losses in a grid System losses with power flow control Higher losses in one line = higher system losses 0.1 pu R and 0.1 pu X in parallel 2 2 2 Ploss = R1 · I1 + R2 · I2 = R1 · I1 ⇒ shift power to the line with X R = 0.1 pu I1 I2 111111 000000 111111 000000 111111 000000 111111 000000 111111 000000 X = 0.1 pu A PFC can lower losses by pushing the current towards lines with lower resistance In case of a constant X /R ratio, the use of a PFC increases the overall losses in the system But also lowering local losses (while having higher system losses) Example IEEE39-bus system as test grid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 23 / 47
  • 36. Power flow controllers Example: Losses in a grid Example: Three zone system, two PFC Generators are circles, load busses are square Green lines are PFC Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 24 / 47
  • 37. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST 20 Contour plot of the losses in the 3 zones Phase shifter 2 (degree) 10 0 −10 −20 −25 −20 −15 −10 −5 0 5 10 15 20 25 Phase shifter 1 (degree) Losses in the 3 zones dependent on the settings of the two PSTs. Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 38. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST 20 d Contour plot of the losses in the 3 zones Phase shifter 2 (degree) 10 ‚   d d Zone 1, Zone 2 and Zone 0 ‚ d ©   3: 3 optima −10 −20 −25 −20 −15 −10 −5 0 5 10 15 20 25 Phase shifter 1 (degree) Losses in the 3 zones dependent on the settings of the two PSTs. Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 39. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST 20 Contour plot of the losses in the 3 zones Phase shifter 2 (degree) 10 Zone 1, Zone 2 and Zone 3: 3 optima 0 PST 1 is controlled by zone 2 −10 −20 −25 −20 −15 −10 −5 0 5 10 15 20 25 Phase shifter 1 (degree) Losses in the 3 zones dependent on the settings of the two PSTs. Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 40. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST 20 Contour plot of the losses in the 3 zones Phase shifter 2 (degree) 10 Zone 1, Zone 2 and Zone 3: 3 optima 0 PST 1 is controlled by zone 2 −10 PST 2 is controlled by zone 1 or 3 (interconnector) (example: 1) −20 −25 −20 −15 −10 −5 0 5 10 15 20 25 Phase shifter 1 (degree) Losses in the 3 zones dependent on the settings of the two PSTs. Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 41. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST 20 Contour plot of the losses in the 3 zones Phase shifter 2 (degree) 10 Zone 1, Zone 2 and Zone 3: 3 optima 0 PST 1 is controlled by zone 2 −10 PST 2 is controlled by zone 1 or 3 (interconnector) (example: 1) −20 Initial control zone is “bad” for zone 2 −25 −20 −15 −10 −5 0 5 10 15 20 25 Phase shifter 1 (degree) Losses in the 3 zones dependent on the settings of the two PSTs. Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 42. Power flow controllers Example: Losses in a grid Losses within multiple zones, two PST Suboptimal optimization 3 zones, 3 optimal phase shifter settings Phase shifters are not mutually controlled or coordinated Good for one can be bad for another Nash-equilibrium? Best solution for the system is not achieved Angle (PST1, PST2) Losses (MW) (−13◦ ,0◦ ) (−5◦ ,9◦ ) (0◦ ,2◦ ) (−5◦ ,6◦ ) Zone 1 11.4 13.2 12.3 12.4 Zone 2 11.6 8.72 9.8 8.91 Zone 3 12.0 9.18 9.17 9.23 Total 35.0 31.1 31.3 30.6 Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
  • 43. Power flow controllers Need for coordination Need for coordination. . . Different objectives Minimize local losses, not foreign Maximize export capacity to “B”, not import from “C” Objectives can be excluding What is good for zone “A”, is not necessary good for “B” And vice-versa Global objective is generally not reached when there are multiple objectives TSOs are no competitors, but each has his own objective Rather unwillingly obstructing other TSOs or grid users PFC control has financial repercussions Communication is key Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 26 / 47
  • 44. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Uncoordinated operation PFC coordination in a market environment Regional coordination Full system coordination New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 45. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Uncoordinated operation Solving local problem PFC coordination in a market environment (no coordination Regional coordination needed) Full system coordination New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 46. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Local objective Uncoordinated operation Do not take actions of PFC coordination in a market environment neighbor into account Regional coordination Coordinate only for Full system coordination safety New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 47. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Uncoordinated operation Optimize, knowing neighboring systems PFC coordination in a market environment Different objectives Regional coordination Nash-equilibrium Full system coordination New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 48. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system PFC control = money Include in the market mechanism? Local, single control objective PFC and flow based market coupling? Every party on its own Uncoordinated operation PFC coordination in a market environment Zone 1+2 Regional coordination Full system coordination New organization Single ISO approach Zone 2 Single TSO approach ? 6 Zone 1 Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 49. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective PFC influence is limited Every party on its own in distance Uncoordinated operation Possibilities to PFC coordination in a market environment implement in the current Regional coordination framework Full system coordination Coreso is taking first New organization steps Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 50. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Optimize social welfare Uncoordinated operation Additional organization: difficult PFC coordination in a market environment ISO: who will invest? Regional coordination Full system coordination TSO: national assets will have to merge New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 51. Power flow controllers How to coordinate? Possible control regimes of PFC for the European system Local, single control objective Every party on its own Optimize social welfare Uncoordinated operation Additional organization: difficult PFC coordination in a market environment ISO: who will invest? Regional coordination ⇒ most realistic first step Full system coordination TSO: national assets will have to merge New organization Single ISO approach Single TSO approach Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
  • 52. Power flow controllers How to coordinate? Regulatory framework Current framework PFCs are generally left out of the regulations UCTE operation handbook mentions PSTs as possible means of guaranteeing security No special required agreements exist to enforce PFC coordination Proposed changes For the TSOs/operators: ⇒ Increased communication Future European regulation PFCs and their effects should not be forgotten in forthcoming regulations Aim for more coordination through effective regulations Not only TSOs but also for regulators First step towards further integration, and insufficient on a long term Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 28 / 47
  • 53. Supergrids 1 Introduction Course overview 2 Coordination in the power system Situation sketch Information exchange between TSOs Steps towards increased coordination: Coreso example 3 Power flow controllers Introduction Controlling PFC in an international context Example: Losses in a grid Need for coordination How to coordinate? 4 Supergrids A supergrid? Technology requirements for the supergrid Controlling the supergrid Techno-Economic approach to a supergrid 5 Conclusions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 29 / 47
  • 54. Supergrids A supergrid? A supergrid? What is a supergrid? A popular definition: a supergrid is an overlay grid connecting different generation and load centers over larger distances It serves as a backbone Adds reliability and security of supply to the system A grid offers redundancy Sometimes also called “hypergrid” New? Recurring issue Electric transmission started from 1 generator to several local loads Grids became interconnected, at increasingly higher voltages The 400 kV grid became the supergrid of the 50’s Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
  • 55. Supergrids A supergrid? A supergrid? Early idea of a supergrid (after WW2) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
  • 56. Supergrids A supergrid? A supergrid? Early idea of a supergrid (after WW2) Implemented as a 400 kV AC grid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
  • 57. Supergrids A supergrid? Supergrid to connect remote renewable energy sources There is plenty of renewable energy available Solar from the Sahara, wind from the North Sea and hydro from Norway to balance (source: desertec) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 31 / 47
  • 58. Supergrids A supergrid? Supergrid to connect remote renewable energy sources There is plenty of renewable energy available Solar from the Sahara, wind from the North Sea and hydro from Norway to balance ±1 km between mills (1/km2 ) take 10 MW/mill (future) UCTE: 600 GW generation Capacity factor 1/3 Required surface to replace UCTE generation: 600 · 103 ×3 1×10 = 180 · 103 km2 square of 430 km × 430 km or 100 km wide, 1800 km long coastal track (Germany has about 2300 km coastline) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 31 / 47
  • 59. Supergrids A supergrid? Supergrids: current “proposals” Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 32 / 47
  • 60. Supergrids Technology requirements for the supergrid Technology for the supergrid Requirements High power transfer capabilities Long distances High transmission efficiency Cheap Offshore connections High reliability Compatible with the current infrastructure Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
  • 61. Supergrids Technology requirements for the supergrid Technology for the supergrid Potential technologies Overhead lines AC connections OHL has high power ratings Allows long distances, but at high losses No offshore connections OHL are difficult to get permissions AC cables Limited length and rating Difficult system operation LCC HVDC (thyristor based) Current source inverter Parallel connecting of multiple terminals is troublesome Series connection gives reliability problems Cables are possible although limited capacity VSC HVDC (Fast switches) Voltage source converter: straightforward parallel connections Converter ratings are limited (but rising) Cables are possible although limited capacity Weak grids are possible Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
  • 62. Supergrids Technology requirements for the supergrid Technology for the supergrid Conclusion ⇒ No perfect solution. VSC HVDC for offshore supergrid AC OHL when possible? For Europe, VSC HVDC seems most appropriate AC system on shore is already quite strong Many load centers are located relatively close to the sea Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
  • 63. Supergrids Technology requirements for the supergrid Ratings } LCC HVDC OHL UDC [kV ] 6400 MW 800 { VSC HVDC OHL 2000 MW { 600 VSC/LCC HVDC Oil filled cable 2000 MW { “Super”grid needs to be bigger VSC/LCC HVDC than existing 400 kV AC 400 MI cable 2000 MW { systems VSC HVDC Existing AC: ≈ 2 GVA/circuit XLPE cable 1100 MW 200 ⇒ 5 GW? – 10 GW? New developments are needed, especially if cables are used IDC [kA] 0 0 1 2 3 4 Figure: Current possible ratings for HVDC systems (UDC refers here to the pole voltage, in a bipolar setup, P = 2 · UDC · IDC ). Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 34 / 47
  • 64. Supergrids Technology requirements for the supergrid Standards Similar to the AC system, standards are needed Standard voltages Once chosen, it is difficult to change What with the integration existing/upcoming lines? Different manufacturers must be able to connect to the same DC system (no vendor lock-in) The control systems of different manufacturers/owners must operate together and without detriment to the AC system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 35 / 47
  • 65. Supergrids Technology requirements for the supergrid How should the grid look like? DC Grid Option 1 Multi-terminal without redundancy AC Grid DC and AC system form each others redundancy Injections and thus DC flows are controlled Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
  • 66. Supergrids Technology requirements for the supergrid How should the grid look like? Option 2 DC Grid Grid of point-to-point DC lines Converter at both ends Some lines in the AC grid are replaced by DC lines AC Grid Full control AC connections and therefore AC protection devices Many expensive and lossy converters Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
  • 67. Supergrids Technology requirements for the supergrid How should the grid look like? Option 3 Meshed DC grid DC Grid Redundant lines Only converters at interface between AC and DC grid AC Grid Reduced losses DC flows can not be directly controlled Cigré workgroup B4-52 considers only this a real DC grid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
  • 68. Supergrids Technology requirements for the supergrid Connecting to the existing AC system The current AC system has not many infeed/withdrawal points for > 5 GW Reinforcements are needed in the existing AC system as well The complete grid build-up/orientation might change Originally from generation centers (near mines, mountains,. . . ) to load centers With supergrid: to from the nearest supergrid terminal (near the shore) to inland load centers Security N-1 connection: Serious disturbance in the system when a terminal is disconnected 1 or 2 connections per zone? What rating and how many connections to smaller synchronous zones: Ireland (7.8 GW installed capacity), Nordel (61 GW installed capacity),. . . Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 37 / 47
  • 69. Supergrids Technology requirements for the supergrid Protection Current VSC HVDC protection Interrupting DC currents is difficult AC protection is easy ⇒ Opening the AC system, disconnecting the complete DC circuit PS Figure: Protection system (PS) in existing VSC HVDC systems NOT USEFUL for supergrid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 70. Supergrids Technology requirements for the supergrid Protection Current VSC HVDC protection Interrupting DC currents is difficult AC protection is easy ⇒ Opening the AC system, disconnecting the complete DC circuit PS Figure: Protection system (PS) in existing VSC HVDC systems NOT USEFUL for supergrid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 71. Supergrids Technology requirements for the supergrid Protection Supergrid protection boundaries Fault causes rapidly changing currents in all lines Selectivity: Only the affected DC line must be switched IGBTs cannot withstand high overloads Fast enough (DC: no inductance XL to limit the current) Only in case of DC fault and not during load change or AC fault Consequences Fault location (branch) detection within a few milliseconds Too fast for communication between measurement devices Independent detection systems Opening at both sides of the faulted line No opening of other branches Backup in case this fails New superfast DC breakers must be developed Waiting longer results in more difficult switching and is lethal for the IGBTs Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 72. Supergrids Technology requirements for the supergrid Protection Example: 4 terminal MT HVDC system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 73. Supergrids Technology requirements for the supergrid Protection Fault occurs in the DC circuit (t = 0) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 74. Supergrids Technology requirements for the supergrid Protection Rapidly changing currents throughout the system di VDC = L · +R ·i dt VDC VDC R i (t ) = + I0 − · e− L · t R R Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 75. Supergrids Technology requirements for the supergrid Protection Protection system must indicate the faulted line PS Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 76. Supergrids Technology requirements for the supergrid Protection Opening of the faulted line (t < 5ms) Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
  • 77. Supergrids Controlling the supergrid Power balance and flows At any time, the power balance must be zero: ( i PAC →DC ) − Ploss = 0 Injections can be fully controlled (DC) but compensation for losses is needed Slack bus or distributed slack bus Power flows are according to the laws of Kirchhoff Redispatching of DC injections might be needed to change DC flows and avoid congestion The DC system flows are determined by the DC voltages at the converter side Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 39 / 47
  • 78. Supergrids Controlling the supergrid Interaction between AC and DC system DC system will have a profound influence on AC system flows Changing the power injections between nodes can have important consequences How the interaction will/should be is not trivial, especially with multiple zones and multiple synchronous zones A VSC HVDC terminal is highly dynamic Operation may not jeopardize AC system security (interactions between AC and DC controls) Operation of electrically close terminals may interfere Potential to increase stability and damping in the system Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 40 / 47
  • 79. Supergrids Controlling the supergrid Segmenting the AC system? In synchronous AC systems, events propagate throughout the system By subdividing current synchronous zones in different smaller zones, this can be limited Part of the synchronizing power would be lost as well Might be an option for currently loosely or non-synchronized systems (USA?) DC Grid AC Grid Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 41 / 47
  • 80. Supergrids Techno-Economic approach to a supergrid Potential benefits of a supergrid Income: 4 clear economic benefits 1 Access to remote energy sources 2 Higher penetration of renewable energy sources by improved balancing 3 Improved grid security 4 Reduced congestion in the system Costs: expensive installation HVDC terminals and cables are expensive There are other resources besides renewables (generation mix) Radial HVDC links to shore are possible as well AC system upgrades might be sufficient for many years Pay-back time Is it interesting from an economic point of view to install a supergrid? Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 42 / 47
  • 81. Supergrids Techno-Economic approach to a supergrid Regulations and ownership Many operational questions remain Who will own/invest in the supergrid? TSOs (ENTSO-E?) Governments/EU Generator companies Private investors The investor wants a return on investment! The owner determines how the grid will look like How many connections Which connection points How is the combined AC and DC power system operated? How will money be earned? Regulated market Merchant grid Connection charges for offshore generators Who will be the regulating authority? Multi-zonal regulations? Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 43 / 47
  • 82. Conclusions 1 Introduction Course overview 2 Coordination in the power system Situation sketch Information exchange between TSOs Steps towards increased coordination: Coreso example 3 Power flow controllers Introduction Controlling PFC in an international context Example: Losses in a grid Need for coordination How to coordinate? 4 Supergrids A supergrid? Technology requirements for the supergrid Controlling the supergrid Techno-Economic approach to a supergrid 5 Conclusions Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 44 / 47
  • 83. Conclusions Conclusions 1: coordination In the multi-zonal transmission system, coordination is not trivial Cooperation exists, but can be better Coreso is a new and promising initiative Power flow controlling devices are increasingly present in the grid PFCs influence losses, transmission capacity, security,. . . PFCs influence the operation of the local transmission system . . . also that of neighbors Make coordination even more important Different manners of coordination are possible Until now, no true coordination exists First step: communicate Second step: implemented in the regional initiatives framework/coreso Optimum would be full coordination, with a single European TSO? The current situation is not ideal nor a full implementation of the IEM Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 45 / 47
  • 84. Conclusions Conclusions 2: supergrid The DC supergrid is often seen as the ultimate solution to integrating renewable energy sources The potential is great But many challenges remain Technical: Ratings are currently insufficient Protection is an issue Offshore grid will not solve all problems Operation and control: The power balance must be controlled The new system must remain secure (N-1) The combined AC and DC system interact Economic: What is the rate of return? and who will pay? What about regulations? Who and how will the supergrid be controlled? ⇒ A supergrid? Yes, but not tomorrow. . . Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 46 / 47
  • 85. Conclusions Questions Dirk Van Hertem (Electric Power Systems, KTH) ? Mini-course on Future Electric Grids (2/2) 8/03/2010 47 / 47