The University of Nottingham

Power Electronic Converters for HVDC Applications Prof Pat Wheeler Power Electronics, Machines and Control (PEMC) Group UNIVERSITY OF NOTTINGHAM, UK Email pat.wheeler@nottingham.ac.uk The presenter would like to acknowledge support from the Universidad Tecnica Federico Santa Maria and CONICYT through project MEC 80130065, "Estructuras de Avanzadas de Convertidores de Potencia para Conexion a Red".

Introduction Power Electronics for HVDC Power System Applications Applications in Renewable Energy Offshore wind power European DC Grid Power Converter topology options MMC Series Hybrid Alternate Arm Converter Parallel Hybrid

HVDC Applications

AC or DC Transmission Traditional overhead line with AC Overhead AC line with FACTS Cost of construction Components, space, transportation, Efficiency of operation Losses, downtime, Environmental impact Location, visual impact, resources, Cost Lines & Stations HVDC overhead line HVDC Applications Offshore wind farms DC Break Even Distance (illustrative) Remote generation Hydro, wind, wave, solar. Country interconnections Example: UK-France DC Convertor Stations AC Stations AC 800km Overhead Line 50km Submarine Cable Transmission Distance

AC or DC Transmission Traditional overhead line with AC Overhead AC line with FACTS Cost of construction Components, space, transportation, Efficiency of operation Losses, downtime, Environmental impact Location, visual impact, resources, Cost Lines & Stations HVDC overhead line DC Break Even Distance (illustrative) AC 800km Overhead Line 50km Submarine Cable Transmission Distance

Multiport HVDC Networks Offshore Wind Country A Offshore Wind Country C HVAC to HVDC Multi-port HVDC Transmission System HVAC to HVDC HVAC to HVDC HVAC to HVDC HVAC to HVDC T&D System Country A T&D System Country B T&D System Country C A Simplified Multiport HVDC system

Multiport HVDC Networks Multiport HVDC Test Facility at University of Nottingham M V3000 Full Power Wind Converters Low Voltage M V3000 Full Power Wind Converters Low Voltage M V3000 Full Power Wind Converters Low Voltage 3.3kV AC, 5kV DC, 5MW Circulating Power Research on energy management, protection, power electronic converters

HVDC Transmission UK investing 10GW of offshore wind capacity with HVDC links to shore STATCOM-less systems, grid control, ride through and protection rugged, cheap, proven, but traditionally large foot print Voltage Source IGBT Converters (VSC - HVDC Plus 500MW) full controllability, small foot print, black start VSC HVDC topologies for high efficiencies, fault performance F 160 140 120 Offshore grid voltage, kv 100 F F 80 Wind farm active (1) and reactive (2) powers, pu HF F F 1 0.5 0 1 2 HVDC DC-link current, ka 3 2 1 0 1 STATCOM active (1) and reactive (2) powers, pu Offshore Wind Farm 50-150km DC cable Land Station 1 0 2-1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Time, s

HVDC Converter Topologies

Conventional HVDC Transmission Tried and tested technology (50 year history) State of the art 132kV, 6400MW, 2000km Problems: Converter station footprint (filters approx 50%) Inability to work on dead networks - voltage needed for commutation of thyristors Limited scope for reactive power control Power reversal requires DC voltage reversal - Limits usage to certain cable types No scope for multi-terminal DC operation +V DC AC System Filter Upper Valve Group DC System Converter Transformer Lower Valve Group Ground Based on line commutated converter (LCC) thyristor technology

Voltage Source Converter (VSC) HVDC transmission Voltage source converter HVDC is growing rapidly ( 800kV 10GW) Many advantages over conventional HVDC eg: Black start can feed a dead network Active and reactive power control Much smaller filters/no compensation (station footprint much smaller) Constant voltage DC link multi-terminal applications No voltage reversal for power reversal Some key problems for VSC converters Efficiency (switching converter losses) Operation with faulted AC or DC side Scalability (modularity)

Amplitude Voltage Source Converter (VSC) HVDC transmission Two-level inverter based on series connected devices Simple concept DC supply (100+kV) One switch A Requires series devices 2-level operation needs relatively high PWM frequency 1 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8 High switching losses (total losses 1.7% per station) -1 0 0.005 0.01 0.015 0.02 Time /s 2-level PWM generates significant harmonics - filtering One phase of 3 shown

Amplitude Modular-Multilevel-Converter (M 2 LC) Concept more complex Switching of cells controls BOTH the DC side and AC side voltage 1 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8-1 0 0.005 0.01 0.015 0.02 Time /s DC SIDE B CELL E dc 2 phases of 3 shown A No bulk DC capacitor No need for series devices Multi-level operation low switching frequency + good harmonic performance Low switching losses (typical total losses 1% per station) Twice the number of devices required compared to 2-level approach Large number of capacitors of significant size Voltage on individual capacitors must be controlled

Modular-Multilevel-Converter (M 2 LC) I dc /3 + I AC /2 V dc /2 + V AC DC SIDE B A Multi-level AC voltage V AC and current I AC CELL V dc /2 - V AC E dc I dc /3 - I AC /2 2 phases of 3 shown

Hybrid Topologies Combination of: Multi-level wave shaping Series bridges Modest switching frequency Fractional rating low loss Series device wave steering Zero voltage switching Low loss Combination gives: High waveform fidelity Low loss Low device count Semiconductors Capacitors Scale demonstrator design 20MW, 20kV DC, 11kV AC 1200A, 3300kV IGBTs 1.5kV cell voltage

Parallel Hybrid Topology H-bridge- soft Switched (low loss) MMC type chain

Voltage [V] Parallel Hybrid Topology Topology has a very low component count compared to many alternatives Chain-Link converters perform wave shaping function Converters outside of main power path => low switching losses Mean Chain-link current typically <20% of DC current H-Bridge converters are zero voltage soft switched Device switching frequency = fundamental frequency Research Topics Theoretical work on energy control (unbalanced supply etc) Closed loop control and Energy management 150 100 50 0-50 -100-150 -0.02-0.01 0 0.01 0.02 Time [s] Line-Line voltages at the converter terminals

Parallel Hybrid Topology Chain-link cell capacitors have a tendency to charge or discharge Have to control the voltage of the individual converter chain-link capacitors Balancing can be achieved with a sorting algorithm based on the characteristics of the chain-link cells in a given chain-link position Change in chainlink cell voltage with the 'virtual position' of the cell in the chainlink for different power factors 0.6 0.4 1 0.2 V c / A 0-1 Chainlink cell Position 1 2 3 4 5 6 0-30 -60-90 -180-150-120 30 60 90 power factor angle 180 150 120 0-0.2-0.4-0.6 Capacitor Voltage Control one phase with set point = 50V

Series Hybrid Topology

Series Hybrid Topology - AAC Vcl1 S1 10kV 15 10 5 0-5 -10 S1 ON S2 ON Vs1 0 ZVS -15 0 5 10 15 20 S1 Vs2 10kV V OFF < 20kV Condition for zero energy exchange with chain-links V AC (peak) = 2E DC / Vcl2

Series Hybrid Topology - AAC ADVANTAGES Series IGBT switches commutate at near zero voltage Reduce switching losses Improves converter efficiency Series H-bridges can support the AC voltage when there is a DC side fault Actively control AC side current to zero No need to interrupt fault current with AC side breaker Actively control AC current to be reactive Gives option of STATCOM performance during DC side fault

Topology Comparisons 2-level converter Half-link M 2 LC Full-link M 2 MC Series hybrid Parallel hybrid Total Semiconductor count (pu) Total submodule DC capacitor rating (pu) Losses 1 2 4 2.5 1.5 0 1 1 ~0.5 <0.25 AC Harmonic performance Hard-switched IGBT valve needed? Ability to suppress DC faults?

Summary Power Converters will be an essential part of the future Electrical Energy Grid Renewable Energy Sources are not directly compatible with the grid Requirement for Power Conversion for all power source connections Challenges for Power Converter deployment in the Electricity Grid Cost [both purchase cost and cost of losses] Reliability/availability Current regulations and legacy equipment Many other topologies exist for AC/AC and AC/HVDC Newton-Picarte project between Universities of Nottingham/Talca/Concepcion will look at some alternatives Kick Off meeting was held in Talca a couple of weeks ago!

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