Control of Power Converters in Low-Inertia Power Systems Florian D - PowerPoint PPT Presentation

Control of Power Converters in Low-Inertia Power Systems Florian D¨ orfler Automatic Control Laboratory, ETH Z¨ urich

Acknowledgements ! ! ! ! Marcello Colombino Jean Sebastien Brouillon Dominic Groß Irina Subotic Further: Gab-Su Seo, Brian Johnson, Mohit Sinha, & Sairaj Dhople 1

What do we see here ? Hz BEWAG UCTE *10 sec 2

Frequency of West Berlin re-connecting to Europe December 7, 1994 Hz BEWAG UCTE *10 sec before re-connection: islanded operation based on batteries & single boiler afterwards connected to European grid based on synchronous generation 3

The foundation of today’s power system Synchronous machines with rotational inertia M d dtω ≈ P generation − P demand Today’s grid operation heavily relies on 2 Mω 2 as safeguard against disturbances 1. kinetic energy 1 2. self-synchronization of machines through the grid 3. robust stabilization of frequency and voltage by generator controls We are replacing this solid foundation with ... 4

Tomorrow’s clean and sustainable power system synchronous machines renewables & power electronics + large rotational inertia – no rotational inertia + kinetic energy 1 2 Mω 2 as buffer – almost no energy storage + self-synchronize through grid – no inherent self-synchronization + robust control of voltage & freq. – fragile control of voltage & freq. – slow primary control + fast actuation & control what could possibly go wrong? 5

The concerns are not hypothetical issues broadly recognized by TSOs, device manufacturers, academia, agencies, etc. MIGRATE project: UPDATE REPORT ! M assive I nte GRAT ion of power E lectronic devices BLACK SYSTEM EVENT IN SOUTH AUSTRALIA ON ! !"#$%% 28 SEPTEMBER 2016 "&'()*%")+,-.)'%/),-)0% 1"2%/).3**)456(-34' % AN UPDATE TO THE PRELIMINARY OPERATING INCIDENT REPORT FOR THE NATIONAL ELECTRICITY MARKET. DATA ANALYSIS AS AT 5.00 PM TUESDAY 11 OCTOBER 2016. !"#$%& ! &$ ! &'" ! ()* ! +$,,-&&"" ! Impact of Low Rotational Inertia on Power System Stability and Operation lack of robust control: Andreas Ulbig, Theodor S. Borsche, Göran Andersson ETH Zurich, Power Systems Laboratory Physikstrasse 3, 8092 Zurich, Switzerland ulbig | borsche | andersson @ eeh.ee.ethz.ch “Nine of the 13 wind farms ERCOT CONCEPT PAPER PUBLIC online did not ride through the Future Ancillary Services in ERCOT Frequency Stability Evaluation six voltage disturbances Criteria for the Synchronous Zone ERCOT is recommending the transition to the following five AS products plus one additional AS that would be used during some transition period: of Continental Europe experienced during the event.” 1. Synchronous Inertial Response Service (SIR), 2. Fast Frequency Response Service (FFR), 3. Primary Frequency Response Service (PFR), – Requirements and impacting factors – 4. Up and Down Regulating Reserve Service (RR), and Renewable and Sustainable Energy Reviews 55 (2016) 999 – 1009 5. Contingency Reserve Service (CR). RG-CE System Protection & Dynamics Sub Group 6. Supplemental Reserve Service (SR) (during transition period) Contents lists available at ScienceDirect Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser However, as these sources are fully controllable, a regulation can be between the lines: added to the inverter to provide “synthetic inertia”. This can also be seen as a short term frequency support. On the other hand, these The relevance of inertia in power systems sources might be quite restricted with respect to the available capacity and possible activation time. The inverters have a very low Pieter Tielens n , Dirk Van Hertem overload capability compared to synchronous machines. conventional system would ELECTA, Department of Electrical Engineering (ESAT), University of Leuven (KU Leuven), Leuven, Belgium and EnergyVille, Genk, Belgium have been more resilient (?) !"#$"% &'()*)+,-.+'%-,#"$"/)%'-0)'(+"1',%',' %2*30+.*.4%'3.*1)*%)+' 6

Critically re-visit system modeling/analysis/control Foundations and Challenges of Low-Inertia Systems (Invited Paper) David J. Hill ∗ and Gregor Verbiˇ Federico Milano Florian D¨ orfler and Gabriela Hug c University College Dublin, Ireland ETH Z¨ urich, Switzerland University of Sydney, Australia ∗ also University of Hong Kong email: federico.milano@ucd.ie emails: dorfler@ethz.ch, ghug@ethz.ch emails: dhill@eee.hku.hk, gregor.verbic@sydney.edu.au The later sections contain many suggestions for further New control methodologies, e.g. new controller to work, which can be summarized as follows: • mitigate the high rate of change of frequency in low New models are needed which balance the need to • inertia systems; include key features without burdening the model A power converter is a fully actuated, modular, and (whether for analytical or computational work) with • very fast control system, which are nearly antipodal uneven and excessive detail; characteristics to those of a synchronous machine. New stability theory which properly reflects the new • Thus, one should critically reflect the control of a devices and time-scales associated with CIG, new converter as a virtual synchronous machine; and loads and use of storage; The lack of inertia in a power system does not need to • Further computational work to achieve sensitivity • (and cannot) be fixed by simply “adding inertia back” guidelines including data-based approaches; in the systems. a key unresolved challenge: control of power converters in low-inertia grids → industry is willing to explore green-field approach (see MIGRATE project) [Milano, D¨ orfler, Hug, Hill, & Verbic, PSCC’ 18] 7

Cartoon summary of today’s approach Conceptually , inverters are oscillators that have to synchronize Hypothetically , they could sync by communication (not feasible) 8

Cartoon summary of today’s approach Colorful idea: inverters sync through physics & clever local control theory: sync of coupled oscillators & nonlinear decentralized control power systems/electronics experiments @NREL show superior performance 8

Outline Introduction: Low-Inertia Power Systems Problem Setup: Modeling and Specifications State of the Art: Comparison & Critical Evaluation Dispatchable Virtual Oscillator Control Experimental Validation Conclusions

Modeling: signal space in 3-phase AC circuits three-phase AC balanced (nearly true) synchronous (desired) � x a ( t ) � � x a ( t + T ) � � � � � sin( δ ( t )) sin( δ 0 + ω 0 t ) sin( δ ( t ) − 2 π sin( δ 0 + ω 0 t − 2 π x b ( t ) = x b ( t + T ) = A ( t ) 3 ) = A 3 ) x c ( t ) x c ( t + T ) sin( δ ( t ) + 2 π sin( δ 0 + ω 0 t + 2 π 3 ) 3 ) periodic with 0 average so that const. freq & amp � T 1 0 x i ( t ) dt = 0 x a ( t ) + x b ( t ) + x c ( t )=0 ⇒ const. in rot. frame T 1 1 1 x abc x abc 0 0 x abc 0 − 1 − 1 − 1 - π -2 π 0 2 π π - π -2 π 0 π 2 π - π -2 π 0 π 2 π δ δ δ assumption : balanced ⇒ 2d-coordinates x ( t ) = [ x α ( t ) x β ( t )] or x ( t ) = A ( t )e i δ ( t ) from currents/voltages to powers : active p = v ⊤ i and reactive q = v T R ( π 2 ) i � �� � 90 ◦ rotation 9

Modeling: the network interconnecting lines via Π -models & ODEs ◮ quasi-steady state algebraic model ∼ diffusive (synchronizing) coupling       . . . ... ... i 1 v 1 . . . . . .       . � n . = − y k 1 I 2 · · · · · · − y kn I 2 . j =1 y kj I 2 .       . .       . . . ... ... . . . i n v n . . . � �� � � �� � � �� � nodal injections nodal potentials Laplacian ⊗ I 2 with y kj = 1 / complex impedance ◮ salient feature: local measurement reveal global information � i k = j y kj ( v k − v j ) ���� � �� � local variable global information 10

Modeling: the power converter i dc i o i L R 1 network v dc v 2 v dc u G C DC port modulation control (3-phase) LC output filter AC port to power grid ◮ passive DC port port ( i dc , v dc ) for energy balance control → details neglected today: assume v dc to be stiffly regulated ◮ modulation ≡ lossless signal transformer (averaged) → controlled switching voltage 1 2 v dc u with u ∈ [ − 1 , 1] ◮ LC filter to smoothen harmonics with R, G modeling filter/switching losses well actuated, modular, & fast control system ≈ controllable voltage source 11

Control objectives in the stationary frame 1. synchronous frequency: � 0 � d − ω 0 dt v k = v k ∀ k ∈ V := { 1 , . . . , N } ω 0 0 ∼ stabilization at harmonic oscillation with synchronous frequency ω 0 2. voltage amplitude: � v k � = v ⋆ ∀ k ∈ V (for ease of presentation) ∼ stabilization of voltage amplitude � v k � 3. prescribed power flow: v ⊤ k i o,k = p ⋆ v ⊤ i o,k = q ⋆ k R ( π k , 2 ) ∀ k ∈ V k � �� � 90 ◦ rotation ∼ steady-state active & reactive power injections { p ⋆ k , q ⋆ k } 12

Main control challenges ω 0 v 2 v 2 ω 0 v 1 θ ⋆ 12 0 v 1 v ⋆ 0 � nonlinear objectives ( v ⋆ k , p ⋆ k , q ⋆ k ) & stabilization of a limit cycle � decentralized control: only local measurements ( v k , i o,k ) available � time-scale separation between slow sources & fast network may not hold + fully controllable voltage sources & stable linear network dynamics 13