Fachhochschule Frankfurt am Main University of Applied Sciences

Fachhochschule Frankfurt am Main University of Applied Sciences HVDC Systems Innovative Technologies and Systems Engineering for Advanced Power Systems Facts and Outlook in 2015 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 1

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems “Definition” of Advanced Power System Innovative systems engineering methods and tools to design complex systems Part A HVDC Classic – Technologies and Systems Engineering Part B VSC type HVDC – Technologies and Systems Engineering Part C HVDC Classic Controls Part D VSC Type HVDC Controls Part E Offshore Wind Farm Connection via HVDC Part F Transmission Grid Building with HVDC systems 3/3/2021 Widespread use of power semiconductor devices and systems in generation, transmission, distribution and storage Digital information technologies for control, supervision, protection and communication HVDC Systems - W. Kuehn - EE & IT - FH FFM 2

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Definition of Advanced Power System Convent. Bulk Power Generation Smart Transmission and Distribution Grid Advanced Power System Storage 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM Renew. Bulk Power Generation Distributed Generation from Renewable Energy Resources plus Prosumers 3

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Present transmission system Definition of Advanced Power System Plus increasingly used Future transmission system 3/3/2021 • AC lines (overhead, cable), AC power transformers, AC switchyards, AC switchgear (gas insulated, air insulated), AC compensation equipment (capacitors and reactors), Arrestors • power semiconductor compensating devices like TCR + FC, TSC, Statcom (FACTS devices) • High Voltage Alternating Current • High Voltage Direct Current • Flexible Alternating Current Transmission Devices HVDC Systems - W. Kuehn - EE & IT - FH FFM 5

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Definition of Advanced Power System Advanced transmission systems Areas of Application Classic HVDC uses thyristors Long distance transmission VSC type HVDC uses IGBTs Asynchronous interconnections Sea cable transmission • Bridging big water ways • Connecting offshore wind farms to onshore grid DC grids (future) 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 6

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Technologies Principle of Conversion 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 7

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Basic Properties of CSC and VSC CSC VSC P flow Both directions Q flow Only consumption difficult to control Consumption and supply easy to control Power Electronic Devices Thyristor – high voltage installations GTO/IGCT- medium voltage IGBT – high voltage installations Basic Circuits Two six pulse bridges in series on DC side, one star and one delta connected • • • Harmonic Currents, resp. voltages AC side constant current harmonics of order 11, 13, 25, … constant voltage harmonics order depends on switching frequency and method (PWM, carrier frequency method, unipolar, bipolar, other) Harmonic Currents, resp. voltages DC side constant harmonic voltages of order 12, 24, 36, … constant harmonic currents order depends on switching frequency and method 3/3/2021 Two level converter bridge Three level converter bridge Modular Multilevel Converter HVDC Systems - W. Kuehn - EE & IT - FH FFM 8

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Technological Advances HISTORY – HVDC installations From Mercury Arc Valves to Thyristor Valves • 1935 – 1945: Mercury Arc Valves - some experimental systems in SE, USA, CH, D • 1954: first commercial system 25 MW Gotland • 1970: first long distance transmission (1400 km) using Merc. Arc. Valves: Pacific Intertie DC - 1440 MW, +400 k. Vdc, 1. 8 k. Adc, three six pulse converters per pole with accordingly low order of harmonic currents (5, 7, 11, …) • 1975: thyristors, oil insulated and oil cooled in Cabora Bassa (Vdc = +500 k. V) • 1978: thyristors air insulated and water cooled - Nelson River Bipole 2 • 1985/1987: bipole 1/bipole 2 3150 MW, + 600 k. Vdc – Itaipu Brasil • PID after several upgrades: 3100 MW, +500 k. Vdc, 3. 1 k. Adc • 2011: China … • 2012: Madeira River HVDC / Brasil -Two bipoles x 3100 MW, + 600 k. Vdc, 2580 Adc, 2 x 2400 km DC line 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 9

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Technological Advances • • • Further Developments Metallic Oxid Arresters for valve protection Light triggered thyristors Quadruple or double valves / twin valves Frefabricated and factory tested valve modules Towers suspended under valve hall • Digital controls and protection, redundant and failsafe • Extensive use of simulators to develop controls and check real control cubicles before shipping • First use of B 2 B for power and continuous AC voltage control in 1985 • Continuous development towards higher DC voltage and Power: + 800 k. Vdc, 6000 MW • Through use of redundant equipment massive overload capability => advantage as compared to VSC 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 10

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Example for CSC in long distance and in back-to-back Madeira River HVDC / Brasil 2 x 3100 MW, + 600 k. Vdc 2 x 2400 km DC line 2 x 400 MW Capacitor Commutated Converter (B 2 B) air core reactor to limit surge waves capacitors smoothing reactor only in dc leg because no lightning strike can hit the DC side 3/3/2021 air core reactor in DC leg smoothens DC current HVDC Systems - W. Kuehn - EE & IT - FH FFM 11

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems CSC HVDC – seems to remain competitive also for Back-to-Back systems Capacitor Commutated Converter • Permitting also Reactive Power Supply • and in connection with firing angle control continuous AC voltage control • No DC stress on transformer • Only small capacitors => tunable filters 3/3/2021 New topology “get rid” of the converter transformer => Lamar Grid Power Flow Controller Only grounding the neutral of AC filters on one converter side: avoids zero sequence harmonic currents HVDC Systems - W. Kuehn - EE & IT - FH FFM 12

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Technological Advances HISTORY – VSC for T&D 1994: GTO VSC 25 MVA railway frequency converter 1995: GTO VSC 100 MW frequency converter Bremer municipal utility • Parallel connection of one phase bridge on dc side, series connection on ac side via special transformer to obtain a 25 level converter => complex and expensive, • alternative: parallel 25 MVA units arranged in 75 MVA installations (actual solution in Germany) 1997: Introduction of IGBT VSC for Transmission and Distribution 2007: Estlink 1 rated power 350 MW, + 150 k. Vdc, max. DC current 1200 A 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 13

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems HVDC Classic or HVDC Light / PLUS? 3/3/2021 Decisive for selection of HVDC type • PRO Classic • Transmission distance • Power transfer level • Overload capacity • PRO VSC type • Operable at low short circuit power ratio • Black Start • Low or no Filtering requirements • Control features • Voltage stabilíty • Further criteria • Change of power flow direction in dc cable system • Availability/Reliability • Maintainability • Costs HVDC Systems - W. Kuehn - EE & IT - FH FFM 14

Classic HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems 12 pulse converters consist of two six pulse bridges connected in series on the DC side Part A HVDC Classic For each phase there is a so called quadruple converter valve consisting of four series connected valves 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Picture: Cepsi 2008 Siemens/Infineon Paper One valve comprises several valve modules One module comprises two valve sections LCC Line Commutated Converter HVDC Systems - W. Kuehn - EE & IT - FH FFM 15

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Thyristor Level • Parallel to a valve section is a grading capacitor Ck for even distribution of a steep wave shape • Snubber circuit in parallel to each thyristor for limiting overshoot at thyristor turn off and for AC voltage grading. The resistor RB damps oscillations due to the CB and circuit inductivity. • RDC is for DC voltage grading First power thyristor in the sixties: 33 mm diameter for 1000 A Idc peak blocking voltage Vrsm = 1600 V Today: 6 inch permits 4500 A Idc Vrsm = 8. 5 k. V (r reverse, s single, Picture: m maximum) Cepsi 2008 Siemens/Infineon Paper HVDC Systems - W. Kuehn - EE & IT - FH FFM 16

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Valve Section Smallest repeatable electrical unit of a valve Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Losses are produced in thyristors and snubber circuits • depend on DC current magnitude and firing angle • deionized cooling water flows through heat sinks and a heat exchanger (dry air cooler or wet surface cooler) HVDC Systems - W. Kuehn - EE & IT - FH FFM 17

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Two Valve Sections in one Valve Module Each valve consists of several valve modules Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Picture: Cepsi 2008 Siemens/Infineon Paper • Valve modules contain valve sections • Each section comprises several series connected thyristors, snubber circuits, controls and saturable valve reactors • 3 modules per valve HVDC Systems - W. Kuehn - EE & IT - FH FFM 18

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Valve Tower Part A Each twin tower contains four valves making up one quadruple HVDC Classic 1. HVDC Transmission Technologies Each valve is composed of three valve modules 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Example for valve tower: 3 valve twin converter towers (one per phase) I. e. , one quadruple valve consists of 12 modules, i. e. of 24 valve sections Picture: Cepsi 2008 Siemens/Infineon Paper HVDC Systems - W. Kuehn - EE & IT - FH FFM 19

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems HVDC Bipole Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Long Distance Transmission, e. g. + 500 k. Vdc/ 4. 5 k. Adc • Real Power Transfer in both directions • Decoupled Reactive Power Control at each Converter Station through switching of reactive power components • Bipole can compensate for outage of one pole through overload capability of the remaining pole HVDC Systems - W. Kuehn - EE & IT - FH FFM 20

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Main Components Converter Transformers • Provide the proper AC voltage level to the converter • Galvanic separation between ac and dc side • Limit short circuit current and harmonic current generation Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 AC Filter Circuits Smoothing Reactors • Cover reactive power demand of converters • Prevent harmonic currents from flowing to the grid • Yields smooth commutation voltage • Limit short circuit current at DC line fault • In combination with DC side filters limit harmonic currents flowing into the DC line HVDC Systems - W. Kuehn - EE & IT - FH FFM DC Side Harmonic Filters • Filter out harmonic currents superimposed to DC current 21

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 HVDC Schemes Back-to-Back (monopole) Back-to-Back with tapped AC line (monopole) Long Distance bipole Metallic return (MTRT) Long Distance with parallel converter poles and High Speed Switches (HSSW) for fast paralleling and deparalleling HVDC Systems - W. Kuehn - EE & IT - FH FFM 22

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Nelson River Bipole 1 and Bipole 2 Example for sustainability Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Source: Cigré 2012 HVDC Systems - W. Kuehn - EE & IT - FH FFM 23

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 HVDC Systems Engineering 3. HVDC Systems Engineering Basic Engineering using special computer programs EMTP Studies Single / Three Phase Diagram Equipment List and Layout Main Data and Control Concepts Reactive Power, AC Filters, Caps, Reactors Fault Studies overvoltages / overcurrents Special Breaker studies Controls RT Simulator Test Circuit Configuration and Stresses with Real Ccontrols Worst Case finding tests Develop commissioning test plan Insulation Coordination HVDC Systems - W. Kuehn - EE & IT - FH FFM 24

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Main Data Calculations Bipolar System: +500 k. Vdc, 2 k. Adc • => 2000 MW, 1400 km => 20 Ohm one way at ambient temp of 20 o C and nominal current • Nom. voltage AC grid 1: Vac 1 = 525 k. V (line voltage) • Nom. voltage AC grid 2: Vac 2 = 230 k. V (line voltage) • Conv. Transf. uk = 14 % • Nom. firing angle alpha = 15 deg • Nom. extinct. angle gamma = 18 deg Vdi 0 = Vd/(cos 18 o – uk/2) = 1. 135 x 500 k. V = 567. 5 k. V (valve reactors and losses neglected) • Vsec = Vdi 0 / 2. 7; • tranformer ratio Vac 1/Vsec 1 and Vac 2/Vsec 2 HVDC Systems - W. Kuehn - EE & IT - FH FFM 25

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Voltage Profile Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions Vd 1 = Vdi 01 (cos a – (dxn + dvn + drn)) x Id/Idn x Vl/Vln) Vd 2 = Vdi 02 (cos g – (dxn + dvn – drn)) x Id/Idn x Vl/Vln) or Vd 2 = Vdi 02 ( cos b + (dxn + dvn – drn)) x Id/Idn x Vl/Vln Pd = (Vd 1 – Vd 2) / Rline; Vd 2 = Vd 1 – Rline x Id 3. HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 26

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Characteristics HVDC Classic PQ-diagram of converter & Vdinv and Id versus Pd 1. HVDC Transmission Technologies Qd = Pd x tan j S 1 = SQRT(Pd²+Qd²) S = 1. 022 x S 1 or Vdi 0 x IL Part A 2. HVDC Schemes and their principle functions j = arccos (cos g – uk/2 Id/Idn Vln/Vl) 3. HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 27

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Principle Control Functions Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions Demo-code for rec/inv characteristics 3. HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 28

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems AC Filter Design Characteristic Harmonic currents In 0 = I 1 / n In = kred x In 0 kred = f(u, a) [1] fig. 5. 9 kred reduction factor Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Reactive power supply => Capacitor Size Harmonic Currents calculated over operating range Harmonic Resonance frequencies => Inductor Size Quality factor => Bandwidth Calculate Performance and Component Stress using max. Imped. search program HVDC Systems - W. Kuehn - EE & IT - FH FFM 29

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 AC Filter Reactor Protection Double contingency two consecutive ground faults Vcmax = c x 1. 41 x Vph c = 1. 4 Q = C x Vcmax 1. „HAND“ design Linearize Arrester Characteristic and use Thevenin‘s Equivalent Solve Diff. Equ. 2. EMTP simulation of equivalent circuit 3. Use EMTP setup of total system Additional trigger control and circuitry needed to induce flashovers HVDC Systems - W. Kuehn - EE & IT - FH FFM 30

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Arrestors Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 31

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Insulation Coordination Converter Valves Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions 3. HVDC Systems Engineering 3/3/2021 Forward Min Switch. Surge Protective Firing Vdio/2 Vd/2 Reverse 2. 55 Steep Front Withstand (1. 2 x S. F. P. L. ) 2. 2 SIL/BIL (1. 15 x S. S. P. L. /L. I. P. L. 2. 1 Steep front P. L. 1. 9 Switching Surge P. L. / Lightning Surge P. L. 1. 85 1. 65 Max. peak repetitive voltage 1. 12 1 Vdio/2 = Vd/2 / (cos gamma – uk/2) Vd/2 HVDC Systems - W. Kuehn - EE & IT - FH FFM 32

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part A HVDC Classic 1. HVDC Transmission Technologies 2. HVDC Schemes and their principle functions Determination of valve protection level and switching surge insulation level • Vd = 500 k. V => Vdi 0 = 500 k. V / (cos 18 o – 0. 07) = 567. 5 k. V (approx. calculation) • SSPL = (567. 5 k. V / 2) x 1. 9 = 475 k. V • or: • MCOV = (567. 5 k. V / 2. 7) x 1. 414 = 297. 2 k. V • SSPL = 297. 2 k. V x 1. 6 = 475. 5 k. V (includes arr. characteristic and uneven voltage distribution • SSIL = 1. 15 x SSPL = 1. 15 x 475. 5 k. V => 546. 8 k. V 3. HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 33

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part B VSC type HVDC 1. VSC Technologies 2. VSC-HVDC Configurations 3. VSC-HVDC Systems Engineering 3/3/2021 Vac VSC Technologies Fife years ago: Two-Level VSC Three-Level VSC Now all manufacturers: Modular Multilevel Converter (MMC) Þ Lower switching losses Þ No filtering necessary Switching elements: IGBT (Insulated gate bipolar transistor) with antiparallel diodes Id + • Constant DC Voltage • Change of power flow direction via DC Vdc current, i. e. no polarity reversal _ No DC stress on transformer if properly configured and grounded HVDC Systems - W. Kuehn - EE & IT - FH FFM 34

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part B VSC type HVDC VSC HVDC Configurations symmetric monopole converter ground on AC side MMC 1. VSC Technologies Source: Modern HVDC PLUS application of VSC in Modular Multilevel Converter Topology Kurt Friedrich, Siemens AG, Energy Sector, Power Transmission Solutions 2. VSC-HVDC Configurations 3. VSC-HVDC Systems Engineering 3/3/2021 alternative: asymmetric monopole converter ground on DC side (next slide) HVDC Systems - W. Kuehn - EE & IT - FH FFM 35

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Bipole with asymmetric Converters Part B VSC type HVDC 1. VSC Technologies 2. VSC-HVDC Configurations 3. VSC-HVDC Systems Engineering 3/3/2021 two asymmetric converters forming a bipole Þ DC stress on transformer! HVDC Systems - W. Kuehn - EE & IT - FH FFM 36

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems PQ-Diagram - Basic Calculation Pinv = 400 MW; Vt = 170 k. V; AC grid voltage 345 k. V transformer ratio 345 / 170 = 2. 029; (transformer not shown in single line diagram) Vd = 320 k. V delta = 100 Vc = 320 k. V/2 k. V x sqrt (3/2) = 196 k. V (at m = 1) Start X 2 = Vt x Vc x sin delta / P = 170 k. V x 196 k. V x sin 10 o / 400 MW = X 2 = 14. 46 Ohm Þ L 2 = 38 m. H at f = 60 Hz Maximum reactive power supply by converter Qcm = (Vt x Vc x cos delta – Vt² ) / X 2 = = 270 MVAR Apparent converter power (maximum) Scm = SQRT (Pinv² + Qcm²) = 482. 97 MVA E internal voltage AC grid; E is line voltage E = 345 k. V - j X 1 x (Pinv – j Qcm) / 345 k. V = 325. 92 k. V /-5. 340 End 3/3/2021 Grid reactance: assumed short circuit power 5000 MVA X 1 = 1. 1 x 345 k. V / 5000 MVA = 26. 19 Ohm HVDC Systems - W. Kuehn - EE & IT - FH FFM 37

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part B VSC type HVDC PQ-Diagrams Vc = m x Vd/2 x sqrt (3/2) fundamental rms m = 0 … 1 modulation index Pinv = Vc x Vt x sin (delta_conv) / X 2 Qinv = (Vc x Vt x cos (delta_conv) – Vt² ) / X 2 1. VSC Technologies 2. VSC-HVDC Configurations 3. VSC-HVDC Systems Engineering 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 38

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Control Part C HVDC Classic Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 39

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Pole/Bipole Control Part C HVDC Classic Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 40

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Stability Part C HVDC Classic Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 WEAK AC Grid: Short circuit power ratio less than 3. 5 SS. C. / Pdn = 2… 2. 5 corresponds to sum of differentials = 0 HVDC Systems - W. Kuehn - EE & IT - FH FFM 41

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Control Conventional PWM without dq-decomposition Part D VSC Type HVDC Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 42

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Control PWM with dq-decomposition Part D VSC Type HVDC Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 Idref = 0. 66 x [(Pref x Ud) + (Qref x Uq)] / (Ud² + Uq²) Iqref = 0. 66 x [(Pref x Uq) – (Qref x Ud)] / (Ud² + Uq²) Idref and Iqref can also be formed through PI-control of P and Q, where P = 1. 5 x [(Ud x Id) + (Uq x Iq)] and Q = 1. 5 x [(Uq x Id) – (Ud x Iq)] or both measured HVDC Systems - W. Kuehn - EE & IT - FH FFM 43

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Converter Control PWM with dq-decomposition Part D VSC Type HVDC Controls 1. Converter Control 2. Pole/Bipole Control 3. Stability 3/3/2021 Idref = 0. 66 x [(Pref x Ud) + (Qref x Uq)] / (Ud² + Uq²) Iqref = 0. 66 x [(Pref x Uq) – (Qref x Ud)] / (Ud² + Uq²) Idref and Iqref can also be formed through PI-control of P and Q, where P = 1. 5 x [(Ud x Id) + (Uq x Iq)] and Q = 1. 5 x [(Uq x Id) – (Ud x Iq)] or both measured HVDC Systems - W. Kuehn - EE & IT - FH FFM 44

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Pole Control Monopole Converter Station with metallic return Real Power Control via inverter Part D VSC Type HVDC Controls 1. Converter Control 2. Pole Control 3. Stability 3/3/2021 Alternatively: DC voltage control via inverter DC Voltage Control via rectifier Alternatively: Free running PLL at rectifier => Power transfer adapts to fluctuating WTG power without power order adaption, i. e. no communication needed; requires HVDC system with peak wind power transfer capability => proper rating of VSC type HVDC system or change to Classic HVDC with overload capability HVDC Systems - W. Kuehn - EE & IT - FH FFM 45

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part D VSC Type HVDC Controls Pole Control 400 MW / 320 k. V HVDC long distance transmission PSCAD simulation 1. Converter Control 2. Pole Control 3. Stability 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 46

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Stability Transmission Angle Stability Part D VSC Type HVDC Controls AC voltage control 1. Converter Control 2. Pole Control Limited AC voltage control 3. Stability 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 47

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part D VSC Type HVDC Controls 1. Converter Control 2. Pole Control 3. Stability 3/3/2021 Stability Inertia and Frequency How does it work? • VCO (voltage controlled oscillator) provides a control signal for PWM having the required grid frequency • VSC generates an output voltage of this frequency • Converters operating in parallel VCOs are synchronized by their PLL • Frequency stability relies on immediate load covering when a load step occurs • Converter has to deliver this power • For this it is equipped with virtual inertia • Accomplished simply by low gain in the PLL • Frequency regulation is done as for mechanical synchronous generators: DPref = - Df / R HVDC Systems - W. Kuehn - EE & IT - FH FFM 48

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part E – Offshore Wind Farm & HVDC Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 Requirements on Wind Farm and HVDC • Real and Reactive Power Control • Low Voltage Ride Through (LVRT) • Handle load rejection as well as gust winds • Rotor angle stability of parallel synchronous generators in onshore grid • Participation in Automatic Load-Frequency Control • Emulation of Inertia in VSI • Power System Stabilizer for offshore and onshore grid HVDC Systems - W. Kuehn - EE & IT - FH FFM 49

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier Turbine/Generator/Rectifier Various generator types available, the most important • Double Fed Induction Generator (DFIG) (type 3) • Full Converter Units (type 4) with • synchronous generators (SG) • asynchronous generators (AG) • permanent magnet generators (PM) Turbine speed control via turbine blade In this example a “VSC-Rec” supplies the AC generator voltage 2. Offshore Grid Forming 3. HVDC Connection 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 50

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Offshore Grid Forming Rec: AC voltage control and free running VCO*) Inv: Q control and Vd control Part E GR: AC voltage control free running VCO*) WR: Q control and Vd control Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 Grid Code Tenne. T TSO Gmb. H Oct. 5, 2010 *) different to currently applied method requires sufficient rating of converters HVDC Systems - W. Kuehn - EE & IT - FH FFM 51

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Transmitting fluctuating power Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 Problem: Rating of the VSC type HVDC system for peak power; Þ question of economy: Þ Higher investment cost versus higher energy supply in contrast: HVDC Classic has overload capability HVDC Systems - W. Kuehn - EE & IT - FH FFM 52

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems HVDC Transmission Participating in Load-Frequency Control ALFC controls have to be compatible with the existing AC System Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 53

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Stabilization and Operation along the maximum transferable power level of the Onshore Grid Part E Here: Power order ramps above MTP Level Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 Without energy storage With energy storage HVDC Systems - W. Kuehn - EE & IT - FH FFM 54

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Handle load rejection in AC onshore grid Here: AC line trip Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 Without energy storage With energy storage HVDC Systems - W. Kuehn - EE & IT - FH FFM 55

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Emulation of Inertia in Inverter Synchronous generator and VSC Inverter in parallel Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier low gain high gain => no inertia 2. Offshore Grid Forming medium gain 3. HVDC Transmission 3/3/2021 Rotor Angle of Synchronous Generator for different gains of PLL HVDC Systems - W. Kuehn - EE & IT - FH FFM 56

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems VSC Current Limitation Synchronous generator and VSC in parallel Part E Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 57

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems Part E Power System Stabilizer PSS Two VSC converters connected over 50 km 110 k. V ac line Offshore Wind Farm & HVDC 1. Turbine Generator Rectifier 2. Offshore Grid Forming 3. HVDC Transmission 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 58

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems HVDC Classic versus VSC type HVDC Multi-Terminal Configurations Part F HVDC Classic: 4 terminals with two bipolar DC lines at maximum DC grids • More terminals not realistic as long as there is no DC breaker available 1. HVDC Classic versus VSC type HVDC 2. • Currently HV High Speed AC switches used for fast paralleling and deparalleling but not for fault isolation • i. e. , fault on DC line will bring down the power of the entire DC Grid • Many design cases => High design demand on system engineers Offshore grids • Controls are complex and coupled 3. • Already with point-to-point DC transmission and AC/DC in parallel transient stability problem => multi-terminal DC increases the problem Onshore grids 3/3/2021 HVDC Systems - W. Kuehn - EE & IT - FH FFM 59

HVDC Systems: Innovative Technologies and Systems Engineering for Advanced Power Systems HVDC Classic versus VSC type HVDC Multi-Terminal Configurations Part F VSC type HVDC DC grids • More terminals possible than with HVDC Classic 1. HVDC Classic versus VSC type HVDC 2. Offshore grids • DC breaker needed to isolate fault with minimum impact on entire system – as of Nov. 2012 ABB’s press release a prototype is now available for a pilot project • Grounding problematic – once determined then valid for all added converters, i. e. symmetric and asymmetric cannot be mixed • Which control functions shall the converters exert? • Shall they be designed for peak power? 3. Onshore grids 3/3/2021 • Inertia emulation? • etc. HVDC Systems - W. Kuehn - EE & IT - FH FFM 60