HVDC Transmission Systems Based on Modular Multilevel Converters Maryam Saeedifard Georgia Institute of Technology ([email protected]) PSERC Webinar February 3, 2015 Presentation Outline • Introduction to HVDC Transmission Systems • Converter Requirements for HVDC Transmission Systems • The Modular Multilevel Converter (MMC) - Features - Operational Challenges - Solutions • Future Work 2 Introduction: AC Corridor’s Power Flow Control Boost or control ac voltage (V) Ref: ABB Reduce line reactance (X) Regulate phase angle (δ) 3 Introduction: DC Corridor’s Power Flow Control PP== VDC. IDC HVDC: High Voltage Direct Current Transmission 4 Introduction: AC vs DC Transmission AC Transmission × Loading a function of Z × Charging current a function of voltage level and cable capacitance × Distance limitation × 3 cables DC Transmission Power flow controlled No charging current effect or need for shunt compensation No distance limitation 2 cables 5 Introduction: AC vs DC Transmission • Due to reactive power charging, AC transfer capacity is dramatically reduced with distance • DC transfer capacity is almost independent of distance Ref: ABB 6 Introduction: Overhead Line Transmission Investment vs Cost Ref: ABB 7 Introduction: Types of HVDC Systems • Point-to-Point Systems - Overhead lines - Subsea or underground cables HVDC Converter Station HVDC Converter Station • Back-to-Back Systems - Interconnection of asynchronous AC grids 8 Introduction: Basics of HVDC Systems Ref: ABB 9 HVDC Technology: Converter Requirements Shortcomings: × × Ref: ABB Harmonic distortion Switching frequency and power losses 10 HVDC Technology: Converter Requirements + 𝑉𝑉𝑑𝑑𝑑𝑑 2 AC grid _ + 𝑉𝑉𝑑𝑑𝑑𝑑 2 _ Staircase voltage waveform ==> Reduced harmonic distortion and filtering size Low switching frequency ==> High efficiency 11 The MMC Small Number of Sub-Modules SubModule (SM) Large Number of Sub-Modules Features: Modular and scalable design Smooth and sinusoidal waveform Increased reliability and redundancy × Challenges: SM capacitor voltage balancing × Circulating currents 12 Equivalent Circuit of an MMC i dc + + SM1 SM2 v upa SM1 SM1 SM2 SM2 SM S1 SMn SMn SMn - Lo iupa VSM Lo Lo Lo Lo vdc Lo SM1 ilowa + SM2 SM1 SM1 SM2 SM2 v lowa − SMn SMn + − ia ib ic D1 C S2 D2 + vC − va vb vc SMn - 13 SM Capacitor Voltage Balancing i dc + + SM1 SM1 SM1 SM2 SM2 SM2 v upa SM S1 SMn SMn SMn - Lo iupa VSM Lo Lo Lo Lo vdc Lo SM1 ilowa + SM2 SM1 SM1 SM2 SM2 v lowa − SMn SMn + − ia ib ic D1 C S2 D2 + vC − va vb vc SMn - 14 SM Capacitor Voltage Balancing Example: Five-Level MMC 15 Circulating Current Control High circulating current: Rating value/size of components SM capacitor voltage ripple Power losses 16 Circulating Current Control • Circulating current – contains 2nd harmonic predominantly • Controllers to eliminate circulating current: • Proportional Resonant (PR) Controller • Predictive Circulating Current Controller 17 Circulating Current Control: PR Controller • Circulating current dynamics: diz ,abc Lo + Roiz ,abc = vz ,abc ≈ mz ,abcVdc dt • PR Controller: K i1s Ki 2 s K p1 + 2 + 2 2 s + ωn1 s + ωn22 • ωn1 and ωn2 are tuned to 2nd and 4th harmonic. 18 Circulating Current Control: PR Controller vc, p ,1,abc vc, p , 2, abc Software SM Capacitor Voltage Balancing vc, p ,n ,abc Measured upper-arm SM Capacitor Voltages Hardware θe iqe iabc i p ,abc Ac-side current controller e θe ωe Leqiˆd iqe,ref K Kp + i s e d i Kp + Ki s ide ,ref i p ,abc in,a b c iz ,abc ,ref 1 2 Vdc 2 iz ,abc u e d v 2 Vdc K i1 K i2 + s + ω n21 s 2 + ω n22 2 1 SM2 SMn 2 i p ,abc Measured arm in ,abc currents m0e = 0 v z ,abc 1 Vdc vc, n ,1, abc vc, n, 2, abc vc,n,n ,abc Measured lower-arm SM Capacitor Voltages mz ,abc Circulating current controller in ,abc PWM Generator-1 Upper-arm switching signals mde ωe Leq iˆqe K p1 + 2 mabc qd 0 → abc e d SM1 mqe eq abc → qd 0 in,a b c vqe uqe Phase-b Phase-a i p ,abc 1 Phase-c PWM Generator-2 1 2 SM1 SM2 Lower-arm switching signals SMn SM Capacitor Voltage Balancing Si1 SMi ~Si1 C vci VS i-th Sub-module 19 Circulating Current Control: PR Controller 20 Circulating Current Control: Predictive Current Controller From KVL: diupa Vdc di − vupa= l + Ria + L a + vsa , 2 dt dt Vdc di di − vlowa = l lowa − Ria − L a − vsa 2 dt dt Discrete model of the ac-side phase current: Discrete model of the ac-side phase current: ia (k + 1) = 1 vlowa (k + 1) − vupa (k + 1) L' − vsa (k + 1) + ia (k ) K' 2 Ts l L' = + L 2 K'= L' +R Ts Discrete model for circulating current and SM capacitor voltages: Ts iDiscrete k + = − vlowa (k + 1) − vupacapacitor ( 1) (k + 1) ) + iz (k ) for (Vdcmodel z 2l Vcij (k + 1) = Vcij (k ) + voltages: il (k ) Ts C 21 Predictive Circulating Current Control of MMC Prediction based on cost function minimization: i dc + + SM1 SM2 v upa SM1 SM1 SM2 SM2 SM S1 SMn SMn SMn - Lo iupa VSM Lo Lo Lo Lo vdc Lo SM1 ilowa + SM2 SM1 SM1 SM2 SM2 v lowa − SMn SMn + − ia ib ic V = J λ ∑ Vcij − dc + λz izj n i D1 C S2 D2 + vC − va vb vc SMn - 22 Closed-Loop Control of MMC-HVDC Transmission Line MMC-2 MMC-1 23 Predictive Control of MMC-HVDC 24 DC-Side Fault in MMC-HVDC Systems 25 SM Technologies: Normal Operation Full-Bridge SM Clamp-Double SM 26 SM Technologies: DC-side Short-Circuit Fault Operation Full-Bridge SM Clamp-Double SM 27 DC-side Short-Circuit Fault Operation of Full-Bridge MMC 28 DC-Side Fault in MMC-HVDC Systems: Full-Bridge MMC Case 29 Power Losses for Various SM Circuits Power Losses of Single SMtype MMCs Normalized with Respect to Half-Bridge MMC 30 Hybrid Design of MMC-HVDC Systems 31 DC-Side Fault in MMC-HVDC Systems: Hybrid MMC Case 32 Power Losses for Various Hybrid MMCs Power Losses of Hybrid MMCs Normalized with Respect to Half-Bridge MMC 33 Future Work • Control and protection of multi-terminal HVDC systems based on the MMC • Accurate and efficient modeling and simulation tools for MMCHVDC systems • Operation of the MMC-HVDC systems under fault conditions 34 Acknowledgement • This presentation contains data and graphs from ABB publications/presentations available on the public domain including: • Mats Larsson, Corporate Research, ABB Switzerland Ltd, “HVDC and HVDC Light: An alternative power transmission system”,Symposium on Control & Modeling of Alternative Energy Systems, April 2, 2009. • Gunnar Persson, Senior Project Manager,Power Systems - HVDC, ABB AB Sweden, “HVDC Converter Operations and Performance, Classic and VSC”, Dhaka, September 18, 2011. 35
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