Activities on power electronics in power systems at Chalmers

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1 Activities on power electronics in power systems at Chalmers Massimo Bongiorno Department of Energy and Environment Chalmers University of Technology

2 Chalmers University of Technology Two main campuses 17 Departments and 8 Areas of Advance Motto: Avancez

3 Division of Electric Power Engineering at Chalmers 45 (25 Ph.D Students) Work with energy flows, where at least on part is electricity Generation Transmission Consumption

4 Our interaction with society Chalmers University of Technology

5 Division of Electric Power Engineering at Chalmers One vision: contribute to decreased life cycle cost in sustainable energy production, distribution and usage Four main research areas E-mobility Power electronics in power systems Wind power Electrical machines and drives

6 The real breakthrough in the electric power systems AC transmission was first demonstrated at an exhibition in Frankfurt am Main kw transferred 175 km from Lauffen hydropower station to the exhibition area at V

7 The real breakthrough in the electric power systems The AC transmission system is based on several key inventions that have been implemented during a few decades only Transformer Practical handling of high voltage (Blathy, Déri, Zipernowsky, 1885) (Brown, 1891) Three-phase systems (Jonas Wenström, 1889) Synchronous machine Asynchronous 3-phase motor (Tesla, 1884) (Ferraris, Dobrowolsky, 1891)

8 How will the future power system look like? Today s electric power system Tomorrow s electric power system Traditional power plant Connecting ac system ac dc Bulk dc import/export How should the electric power system change in order to become fully sustainalbe, efficient and at the same time be available and reliable? Industrial plant Distribution feeder

9 Vision of the group Chalmers University of Technology Tomorrow s electric power systems must drastically change in order to become sustainable and efficient, meeting at the same time the changing societal challenges. Power electronic technologies in electrical energy generation, transportation and consumption will play a key role in enhancing the system feasibility, safety, availability, reliability and efficiency. Chalmers will be in the lead of this process by proposing, investigating and evaluating new solutions for reinforcing or supplant today s technologies and infrastructures.

10 The way forward How has the power systems evolved over the years? Connecting ac system ac dc Traditional power plant Bulk dc import/export Industrial plant Distribution feeder Today Tomorrow?

11 The way forward Is this too futuristic? Proposed dc-transmission between Trollhättan and Hamburg/Berlin (1909) Kangbao 1500MW Zhangbei 1500MW Yudaokou Fengning 1500MW Beijing 3000MW Current projects in China Tangshan

12 Power electronics will play a key role in the future power system

13 Example of future power grid Chalmers University of Technology

14 Two main research tracks Chalmers University of Technology Research on converter technology Aim is to conduct research on power converters, with main focus on converters for grid applications. Main topics of research are: Multilevel converter topologies for grid applications Converter control and modulation Emerging technologies (such as SiC) and their impact on the future converter topologies Research on converter application and system analysis Aim is to conduct research on the application of power converters in power systems and analysis of grids with high penetration of power electronics. Main research topics are: Control (outer control loops) of grid-connected power converters Investigation of new functionalities for power converters Stability studies Hybrid ac/dc grids

15 Our small team Massimo Bongiorno Btr. Professor Panagiotis Asimakopoulos PhD student Ehsan Behrouzian PhD student Mebtu Beza PostDoc Selam Chernet PhD student Nicolas Espinoza PhD student Joachim Härsjö PhD student Gustavo Pinares PhD student Georgios Stamatiou PhD student

16 Research in VSC technology

17 Specification priorities in VSC technology Every system that we deal with has to consider five basic design parameters Cost Efficiency Performance Reliability Size/weight OBS! Performance includes controllability, dynamic response, EMC and harmonic output

18 CHB Converters for STATCOM Applications To VSC s ac terminals Control computer phase a phase b phase c H-bridges Electronic ac supply Filter Y/D selector Star-connected Cascaded H-Bridge (CHB) Converter

19 CHB Converters for STATCOM Applications To VSC s ac terminals phase a phase b phase c Advantages : Modular design Reduced switching losses Allows transformerless operation Reduced harmonic pollution Disadvantages: Increased number of components Problematic capacitor balancing Star-connected CHB converter Reduced controllability under unbalanced conditions

20 Modular Multilevel Converters for grid applications Two configurations available in the market phase a phase b phase c phase a phase b phase c Star-connected MMC Delta-connected MMC

21 CHB Converters for STATCOM Applications The problem of capacitor balancing under unbalanced conditions v a v b v c i a i b i c Uneven power distribution among the three phases! v 0 Star-connected MMC

22 CHB Converters for STATCOM Applications The problem of capacitor balancing under unbalanced conditions v a v b v c v a v b v c i a i b i c i a i b i c i 0 v 0 Star-connected MMC Delta-connected MMC Zero-sequence injection is needed to guarantee capacitor balancing

23 CHB Converters for STATCOM Applications v a v b v c i a i b i c v a v b v c i a i b i c Currents in phase Voltages in phase i 0 v 0 Currents in opposite phase Voltages in opposite phase

24 CHB Converters for STATCOM Applications What does this means? v a v b v c i a i b i c Voltages in phase i 0 Measured single-phase fault Voltages in opposite phase Measured double-phase fault

25 CHB Converters for STATCOM Applications What does this means? v a v b v c i a i b i c Currents in phase v 0 Currents in opposite phase VDE, E VDE-AR-N 4120: Technische Bedingungen für den Anschluss und Betrieb von Kundenanlagen an das Hochspannungsnetz.. We need more converter topologies/configurations!

26 Research in VSC integration and system analysis

27 Example: power oscillation damping using STATCOM P g Bus 1 Bus 2 Synchronous generator Stiff AC source E-STATCOM Uniform damping can be achieved by proper control of active and reactive power Mebtu Beza Power System Stability Enhancement Using Shunt-connected Power Electronic Devices with Active Power Injection Capability

28 transmitted power [pu] Energy [kj] Chalmers University of Technology Example: power oscillation damping using STATCOM P g Bus 1 Bus 2 Synchronous generator Stiff AC source E-STATCOM time [s] Classical (blue) vs proposed (red) control strategy Mebtu Beza Power System Stability Enhancement Using Shunt-connected Power Electronic Devices with Active Power Injection Capability

29 Risk for controller interaction in the ac grid High penetration of controllable objects can lead to unwanted phenomena in the grid Example: subsynchronous resonances in wind farms. First world incidend: Zorillo Gulf Wind,Texas 2009 Nelson Lon Hill Edinburg circuit breaker 93 MW 96 MW 345 kv series compensated lines Rio Hondo 33% series compensation 17% series compensation Ajo

30 Investigated system and modeling approach WT WT WT WT WT WT i g i L X T R L X L X c P R P f i R + i f u dc C _ v dc f WT WT WT v g infinite bus Power Electronic Converter v R E g Z (s) G (s) Y L Gear Box DFIG v s i s P out, Q out Transformer Di * D i Z G s Y L s Dv g

31 Analysis approach Analysis Closed-loop approach (eigen-value analysis) Open-loop approach (Nyquist) Di * D i Z G s Y L s Dv g Nyquist Stability Criterion u G s H s y jω T cl (s) = G s 1 + G s H s

32 Risk for controller interaction in the ac grid WT WT WT WT WT WT i g i L X T R L X L X c WT WT WT v g infinite bus Z (s) G (s) Y L Real (top) and imaginary part of open-loop transfer function for 55% series-compensated line as a function of closed-loop current controller bandwidth. Transmitted active power. Top: a cc =1 pu; bottom: a cc reduced to 0.5 pu

33 Converter interactions in VSC-based HVDC systems Direct-voltage controlled converter Power flow direction Active-power controlled converter P rated =1000MW AC Grid #1 P in AC filters Phase reactor VSC 1 + dc,1 - dc,2 dc-transmission link + - VSC 2 Phase reactor P out AC filters AC Grid #2 u dc, rated =±320kV dc-link length=300km P out [MW] P in [MW] dc1 [kv] time [s]

34 Converter interactions in VSC-based HVDC systems Interactions might occur between the sending station (direct-voltage controlled) and the remaining dc grid ac bus 1 +e 1 +e 2 ac bus 2 ac grid P g1 VSC phase reactor VSC 1 Direct-voltage control CL 12 -e 1 -e 2 CL 12 VSC 2 Power control P g2 VSC phase reactor ac grid

35 Converter interactions in VSC-based HVDC systems New approaches for stability analysis ac bus 1 +e 1 +e 2 ac bus 2 ac grid P g1 VSC phase reactor VSC 1 Direct-voltage control CL 12 -e 1 -e 2 CL 12 VSC 2 Power control P g2 VSC phase reactor ac grid dc-grid resonances G(j ) (pu) F(j ) (pu) Frequency (pu) appearance of resonance for large increase in power Re[1/F(j )] + Re[G(j )] (pu) Frequency (pu) Frequency (pu) : Stable system (Netdamping above zero) : Unstable system (Netdamping below zero at resonant frequencies)

36 Converter interactions in VSC-based HVDC systems New approaches for stability analysis ac bus 1 +e 1 +e 2 ac bus 2 ac grid P g1 VSC phase reactor VSC 1 Direct-voltage control CL 12 -e 1 -e 2 CL 12 VSC 2 Power control P g2 VSC phase reactor ac grid Imaginary part (pu) concerned poles Real part (pu) Net-damping at N [pu] Damping factor of concerned poles Correlation between dominant poles and net-damping Imaginary part (pu) concerned poles Real part (pu) Net-damping at N [pu] Damping factor of concerned poles Top: increase of power flow; bottom: increase of dc-cable length

37 Converter interactions in VSC-based HVDC systems Ongoing investigation: DC-side input admittance of MMC converter in HVDC (direct-voltage controlled) P 0 Investigation on damping properties of MMC converter and comparison with two-level converter u + MC 1 MC 2 MC 1 MC 2 MC 1 MC 2 i dc d 2 - MC N MC N MC N Include impact of Number of submodules AC Current Control Circulating Current Control Direct-Voltage Control s f Z s (s) PCC s g s i s L R R L i u i l Time-varying direct voltage AC-grid strength PLL dynamics l + - MC 1 MC 2 MC N MC 1 MC 2 MC N MC 1 MC 2 MC N d 2 Y MMC

38 Risk for controller interaction in the dc grid phase grid VSC Based AC/ DC Terminal-1 External DSP controlled VSC Based AC/ DC Terminal-3 3 phase grid VSC Based AC/ VSC Based AC/ 3 phase grid DC Terminal-2 DC Terminal-4 Controller inside RTDS External DSP controlled Controller inside RTDS 3 phase grid Real part of F(j ) [pu] Frequency [pu] imaginary part of F(j ) [pu] Frequency [pu] Frequency response of one of the converters on the MTDC system System modeling real part of dc grid impendace [pu] Frequency [pu] Resonance peak at the dc node where the converter is connected dc voltage [pu] Id current [pu] Time [s] Voltage and current at VSC 1 for power increase. Instability develops at VSC 1

39 Electromagnetic Transient study of wind Farms connected by HVDC Offshore-WindParks in the Nordic See

40 The BorWin Offshore-WindFarm Chalmers University of Technology

41 However Chalmers University of Technology

42 Study approach Wind Farm ACC Bus HVDC System Wind turbine 1... Wind turbine N AC cable AC cable AC filters Phase reactor Offshore VSC DC cable Onshore VSC Phase reactor AC filters AC grid Frequency dependent impedance models should be developed for each component (Wind turbine converters and HVDC converters) Useful method if detail model of components is not easy to get. Moreover, stability analysis will be easier as the various subsystems can be studied separately. Control and system settings have significant impact on the results. System Nyquist plot in case of short (blue) and long (ref) long AC cable

43 Risk for controller interaction in the dc grid If an existing HVDC network shows an acceptable performance, will the addition of a new element (converter, cable) influence the performance of it?

44 Which is the most suitable dc-grid topology? DC Grid level DC Grid level AC Grid level AC Grid level Independent HVDC links Radial connection DC Grid level DC Grid level AC Grid level AC Grid level Ring connection Meshed connection The meshed connection seems to be the preferred solution both from industries and from the research community

45 Which is the most suitable dc-grid topology? DC Grid level AC Grid level Represents the dc replica of the existing ac-transmission system Meshed connection Interconnection of transmission lines in the ac grid gives: increased reliability in power supply effective use of power generation installations Reduced power peaking Allows utilization of uncontrolled energy sources (e.g. renewables)

46 Which is the most suitable dc-grid topology? DC Grid level AC Grid level Represents the dc replica of the existing ac-transmission system Meshed connection What do we need for a meshed dc grid? Good models and understanding of system dynamics FACTS kind controllers (such as power-flow controllers) Dc/dc transformers Dc breakers Are we sure that this is the way to go?

47 To sum up Tomorrow's electric power systems must drastically change in order to become sustainable and efficient Power electronics will play a key role in the future power system The technology is changing We must have a system approach to understand the benefits and possible pitfalls of high-penetration of power electronics in the future power system

48 Thank you very much for your attention!