GRID‐FORMING CAPABILITIES of HVDC CONVERTERS Overview of Concepts, Motivations and Challenges

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GRID‐FORMING CAPABILITIES OF HVDC CONVERTERS Overview of concepts, motivations and challenges

Jon Are Suul, Salvatore D'Arco SINTEF Energy Research, Trondheim, Norway Outline

• Power system trends influencing the requirements for power converters control • Grid forming control of power converters • Principles of "grid‐following" vs "grid forming" control • Virtual Synchronous Machines (VSMs) as an example of grid forming control • "Grid‐forming" control vs virtual inertia • Grid forming control of HVDC converters • Motivations and limitations of grid forming functionality in HVDC converter terminals • Examples of results from Power‐Hardware‐in‐the‐Loop (P‐HiL) testing at SINTEF • Summary and open research questions

2 General development trend in power systems

• Increasing presence of power electronic converters • Renewable power generation • Wind turbines, photovoltaic generation etc. • HVDC transmission • Decommissioning of thermal power plants • Power converters do not inherently provide grid‐ forming capability or inertial response to support power system frequency regulation • Reduced equivalent inertia in the power system

3 Control of power converters in power systems

• "Grid‐following" converters • "Grid‐forming" converters • Synchronization to the measured grid voltage • Capability for voltage and frequency control • Typically by a Phase Locked Loop (PLL) • Inherently capable of islanded operation • Usually based on inner loop current control • Power‐balance‐based synchronization mechanism • Power control by active current component • Power control via voltage phase angle • Grid support functionality by auxiliary control loops • Outer loop control sharing of active and reactive power

Zload

Vs,ab

L2 Grid information Grid L 1 vs * Synchronization Ps I q C * cab, L1 1 g Frequency * ** Synchronizing p p&q P  P  Support s  s Power signal g Balance ic I Control Ps c, abc * * Control id v ,ref Qs g Reactive Pulse 16 * * Current q0 Power vˆ * Width Current V i v * s * q Control  ,ref Control i Voltage ** Voltage c gPWM Qs  Q Modulation Control V Support  s Reactive s Active  Control & Control Q Power Modulation s C p0 Power Control Icab, VDC DC Control 

CDC VDC vDC Virtual Synchronous Machines for grid‐forming control

• First publication on Virtual Synchronous Machine (VISMA)

concept by Beck and Hesse in 2006 Vg • Internal simulation of a Synchronous Machine (SM) Zg,tot v • Simulated machine model provides current references s q C Power L1 1 used for converter control p Calculation i • Can include high order synchronous machine model c Ic, abc * v Synchronous qo f • Main purpose: Emulate the main operational AVR Machine Current Simulation i* c Control gPWM iVSM characteristics of synchronous machines & p* v o VSM s Modulation • Grid forming functionality Governor

C • Inertial dynamics VISMA Concept DC vDC • The first proposals had higher detailing level than necessary Basis for Virtual Synchronous Machine (VSM) control

p o Inertia Model • Synchronization mechanism and power control * based on emulation of SM swing equation po  1      VSM 1 VSM b VSM  • Based on torque or power balance Ta s s pd • Linearized power balance is simpler for VSM applications:   k VSM  d  * g * dppkdVSMg   VSM o o dt Taa T T a p em Virtual Synchronous Machine swing equation • Ensures grid synchronization and inertial p* * r* VSM  p  1  1    response to grid frequency variations  k   VSM VSM b VSM    T p a s s  d • Typically combined with a simple power VSM   k VSM Frequency Droop d  ‐frequency droop ('governor') function PLL PLL Example of VSM‐based control for HVDC terminal

Phase PLL Locked Vg • Emulating impedance and inertial Loop p characteristics of SM o qo Z g vabc • Outer loops with equivalent Measurement o vo Simulated SM model abc functions as for SMs iul, Lf vˆo vo Processing • x Inner control loops for controlling iv

Iv the converter VSM

a a V Al La La Au • Including control of circulating * Voltage * cv qo vˆe Electrical Inner dq vvabc, Control *,abc abc b b * v g Al La La Au currents for Modular Multilevel vˆ ModelModel Control  ul, ul, ('AVR') * vc, abc Ac L L Ac Converter (MMC) based HVDC VSM l a a u

p Balancing o Modulation terminals p*  o PLL  VSM VSM * Frequency r* • Directly applicable to power‐  p Inertia vdc Control  Model VSM controlled terminals VSM ('Gover nor') • Can support multi‐infeed operation in 2

2VSM large offshore wind farms 2 * Circulating * i v dq • Conflicting with dc‐voltage control c Current c i 7 c Control abc Grid forming control in multi‐infeed HVDC‐connected offshore wind farms

VSM-controlled

8 VSM as example of grid forming control

• VSM‐based control can be considered a sub‐group of grid‐forming control • Main characteristic feature is the explicit emulation of the inertia and damping • Example: Equivalence between VSM‐based control and power‐frequency droop

p Active Power Droop Controller o Inertia Model 0

* po   1  1    VSM VSM b VSM p      0   N Ta s s mP pd  s

  p  f p k VSM m * d  s   P-reg * f

11  spp  *  11 0,el pu g pu Tk, fpmm p ad    f mmpp Inertia term Damping term 9 Salvatore D'Arco, Jon Are Suul, "Equivalence of Virtual Synchronous Machines and Frequency-Droops for Converter-Based MicroGrids," in IEEE Transactions on Smart Grid, Vol. 5, No. 1, January 2014, pp. 394-395 The role of inertia in power systems

• Limits the frequency transients in response to disturbances • Provides power response proportional to the frequency derivative d pJ r r dt of generation • Challenges with reduced inertia in grids

with remaining traditional power plants: Idealized response to a loss • Reduced minimum frequency (Nadir) in response to disturbances J. Fang, H. Li, Y. Tang, F. Blaabjerg, " On the Inertia of Future More‐ • Increased maximum Rate‐of‐Change‐of‐ Electronics Power Systems," IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 7, No. 4, pp.2130‐2146, December 2019 Frequency (RoCoF)

10 Grid‐forming control vs virtual inertia

• Grid forming control can include inherent inertia emulating features • For instance: VSM‐based control or large filtering time‐constant in power‐frequency droop control • Virtual or synthetic inertia can be implemented as auxiliary function in grid‐ following converters • Does not imply any grid‐forming capability

Synchronous Machine swing Equivalent response to equation enforced frequency variations

 d dg Jppr   pJ rdt mech em VI VI VI dt 11 Grid forming control of HVDC converters

• Main motivations • Challenges • High power rating from a single unit • Current limitations and energy availability for • Comparable size and influence on local grid as providing inertial response traditional large generation plants • Power and energy availability for compensating load • High controllability of Modular Multilevel variations Converters (MMC)‐based HVDC terminals • Conflict between grid forming control and dc • Typically customized design and control voltage control • Compatible with requirements for black‐start • Need for energy storage for providing grid forming capability functionality without other dispatchable sources

12 Examples of results at SINTEF

• Ongoing research project on provision of virtual inertia from HVDC transmission systems • Evaluation of implementations and power system impacts from different control strategies for providing virtual inertia • Power‐Hardware‐in‐the‐Loop (P‐HiL) testing conducted in the National Norwegian Smart Grid Laboratory at NTNU/SINTEF

13 Experimental setup for P‐HiL testing

• Two MMCs in point to point configuration • One unit controlling the dc link voltage and the other controlled with inertia support • Real‐time simulation of islanded power system with synchronous machine and variable resistive load • Utilized for Power‐ Hardware‐in‐the‐Loop (PHiL) experiments • Actuated by high‐ bandwidth 200 kW grid emulator 14 Operation in ideal grid with fixed frequency

• Comparison of multiple strategies for implementing VSM/grid‐forming functionality • Test with converter connected to a grid emulator generating a stiff voltage with fixed frequency and amplitude • Response to a step in the power reference for different VSM implementations • Large variations in dynamic response • Sensitive to control system implementation Salvatore D'Arco, Tuan T. Nguyen, Jon Are Suul, "Evaluation of Virtual Inertia Control • Sensitive to emulated inertia Strategies for MMC-based HVDC Terminals by P-HiL Experiments," in Proceedings of the IEEE 45th Annual Conference of the Industrial Electronics Society, IECON'2019, Lisbon, Portugal, 14- 17 October 2019, pp. 4811-4818 15 Operation in islanded grid with low inertia

All VSM at J = 2 6000 VCVSM J = 2 D CCVSM QSEM J = 2 D Response to CCVSM DEM J = 2 D All control 5000 DFDT J = 2 D load‐step in No inertia D strategies VCVSM J = 2 ND inertia and droop CCVSM QSEM J = 2 ND the islanded CCVSM DEM J = 2 ND behave 4000 DFDT J = 2 ND similarly No inertia ND system

3000 power [W] power 2000

only inertia 1000

16 -1000 0 2 4 6 8 10 12 14 time [s] Summary and open research questions

• Grid forming operation of HVDC terminals in modern low‐inertia power systems • Promising opportunity due to high power rating and high controllability • Wide range of control schemes proposed or under development • Different stability characteristics but equivalent response for inertia emulation in islanded power systems • Main limitations related to energy availability and dispatchability • Open research topics • Required share and functionality of grid‐forming converters in modern power systems • Need for virtual inertia for supporting remaining traditional generation units • Optimal location and transient response of converter units for providing virtual inertia • Converter control during fault conditions • Required changes to power system protection and operation with increasing share of converters 17 Thank you for your attention!

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