CURENT s Power Electronics Based Reconfigurable Grid Emulator. Leon M. Tolbert

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1 CURENT s Power Electronics Based Reconfigurable Grid Emulator Leon M. Tolbert Min H. Kao Professor October 18, 2017

2 Overview of CURENT NSF/DOE Research Center established in 2011 Grid Models - Large Scale Testbed Reconfigurable Grid Emulator Hardware Testbed

3 U.S. Wind and Solar Resources Population Wind Best wind and solar sources are far from load centers. Distance provides diversity of sources. Transmission networks must play a central role in integration. Solar 3

4 CURENT Vision A nation-wide transmission grid that is fully monitored and dynamically controlled for high efficiency, high reliability, low cost, better accommodation of renewable sources, full utilization of storage, and responsive load. A new generation of electric power and energy systems engineering leaders with a global perspective coming from diverse backgrounds. Monitoring and sensing Communication Control and Actuation Computation Multi-terminal HVDC 4

5 What is CURENT? Wide Area Control of Power Grid Power Grid Measurement &Monitoring HVDC Storage PMU0 PMU PMU PMU0 PMU FDR WAMS Communication Communication Solar Farm Actuation Wind Farm PSS Responsive Load FACTS Generator 5

6 Ultra-wide Area Wide Area Today s Operations Some Wide Area and Some Fast but not Both Traditional uncoordinated controls HVDC Minimal sensing Limited communication Distributed coordinated actuation with extensive measurements Balancing Authority Unit Commitment Economic Dispatch Region AGC RAS Schemes Substation SVC Fixed Comp. LTC Device AVR PSS Device Protection UFLS Day Hour Minute Second Cycle 6

7 CURENT Research Thrusts Engineered Systems Hardware Testbed Testbeds Large Scale Testbed Barriers System complexity Model validity Multi-scale Inter-operability Monitoring Modeling Control Actuation Enabling Technologies Situational Awareness & Visualization Estimation Communication& Cyber-security Control Design & Implementation System-level Actuation Functions Barriers Poor measurement design Cyber security Actuation & control limitation Fundamental Knowledge Wide-area Measurements Modeling Methodology Control Architecture Economics & Social Impact Actuator & Transmission Architecture Barriers Lack of wide-area control schemes Measurement latency Inflexible transmission systems 7

8 Overview of CURENT NSF/DOE Research Center established in 2011 headquartered at The University of Tennessee Grid Models - Large Scale Testbed Reconfigurable Grid Emulator Hardware Testbed

9 Engineered System: CURENT Concept Multiple functionalities: 1. Driving requirements for the four research thrusts. 2. Testing technologies developed from the four thrusts. 3. Demonstrating an engineered system that can Accommodate high penetration of renewables at reasonable costs Fully utilize grid capacity with new system security paradigm Systems will have elements of scaled physical transmission network and actuation, monitoring, control, and communication. 9

10 Engineered System Testbed Objectives Provide research platforms for testing thrust technologies, especially modeling and control thrusts. Study ways to increase the transmission capability, presently constrained due to network security considerations. Test different power electronics technologies and system architectures for improving power flow and reliability. LTB/H TB Develop scenarios to evaluate impact for high penetration of renewable energy sources, responsive loads, and energy storage on the future grid. Include real-time communication networks, real-time control, protection, cyber security, and actuation. Demonstrate CURENT-developed controls, wide-area responsive load, and wide-area renewable generation. 10

11 Engineered Systems CURENT Concept Multi-terminal HVDC High penetration of renewable energy sources Flexible DC and AC transmission Accommodate load and source variability, responsive load Improved situational awareness, ultra-wide-area control 11

12 CURENT Testbed Projects Large Scale Testbed (LTB): Virtual Grid Simulator with an Energy Management and Control System (Matlab based and Commercial-tool based) Regional and National Power Grid Models Hardware Testbed (HTB): Grid Emulator Development and Real-time Scenario Demonstration MISO PJM ERCOT 12

13 LTB Demonstration Plan Year 1~3 Year 4~6 Year 7~10 Generation I Regional grid models with > 20% penetration of renewables and HVDC connections Model development for primary and secondary frequency and voltage controls in regional grids Scaled down system models suitable for testing in RTDS and HTB Scenario development to include diverse system operating conditions Generation II Reduced North American system model with > 50% penetration of renewables and HVDC connections Extension of frequency and voltage control models to North American grid and for damping control and transient stability control Communication system modeling including cyber attacks Scenario development for North American grid Generation III Large model of North American system with >80% renewables and HVDC connections Fully integrated system model of real time communication, coordinated control, actuators, monitoring and load response Detailed scenarios for contingencies and cyber attacks sufficient to demonstrate resilience 13

14 LTB - US Grid Model Development Objectives: o Develop several reduced-order models of the US interconnected power grid for use in demonstrating wide-area interconnections to transmit large amount of energy from renewable resources. o Evaluate disturbance scenarios for the design and verification of wide-area control methodologies. Goals o Developing a reduced model from a very large data set, such as the EI model o Improving and updating the existing reduced models - WECC model o Identify HVDC link opportunities Multi-Terminal DC system and overlay HVDC system in NPCC system Overlap HVDC grid for WECC system Interconnection of WECC, EI and ERCOT via B2B HVDC Implementation of HVDC Overlay with renewables 14

15 Overview of CURENT NSF/DOE Research Center established in 2011 Grid Modes - Large Scale Testbed Reconfigurable Grid Emulator Hardware Testbed

16 HTB Top-level Goals and Needs System-level outcome Interconnected (reduced bus model) EI/WECC/ERCOT with 80% renewable, featuring HVDC overlay and regional MTDC, fully-monitored transmission & some monitored loads, fully integrated closed-loop control on frequency, voltage, damping, and adaptive RES for improved transfer limit and reduced reserves HTB Modeling/building: reduced models of each of the 3 interconnections with variable RES levels up to 80% Control architecture: 3-layer traditional control with central control, regional control, and local control (also internal converter control). Protection architecture: Local level protection. Communication architecture: Ability to emulate power system communication Event capability: black start with renewables, restart, normal operation, fault, scheduled/unscheduled change of loads/sources/lines Operation: interactive, scenario setting, visualization Needs from: 1) actuation real-time capability/modes, Var sources, inertia source, HVDC transmission & flow control; 2) monitoring; 3) modeling/estimation, and 4) control 16

17 Hardware Grid Emulation System Testbed (HTB) Area 1 G km 110 km 25 km 10 km 10 km 25 km G3/WT III 3 Area 2 G2 2 L7 110 km C7 C9 66 km L9 4 G4 L km L13 14 G14 Area 3 Three-Area System VSC 4 DC cable 4 VSC 3 Multi-Terminal HVDC Wind Farm DC cable 2 DC cable 1 Wind Farm WT II VSC 2 DC cable 3 VSC 1 WT I Emulate various grid scenarios with interconnected clusters of scaled-down generators, loads, and energy storage. Demonstrate tools developed by research thrusts. 17

18 Hardware Testbed Architecture and Accomplishments Building Power Rectifier DC Bus Hardware Room Generator I Generator II Short Distance Transmission Line Emulator Long Distance Transmission Line Emulator Reworked hardware cabinets and software to make entire system more modular. Multiple simultaneous control functions and software hierarchy established. Cluster 1 CTs, PTs Load I Cluster 2 Cluster n Monitoring Output Inductors FDR, PMU Control Visualization and Control Room HVDC Cluster n+1 Cluster n+2 Cluster m CAN Bus Added new generation/load cabinet and transmission line cabinets so larger system can be emulated. Battery emulator for Lead-acid and Li-ion batteries and interconnecting two-stage converter. Virtual SG control of wind turbine converters. Transmission line fault emulation and power system protection function emulation. Analysis of PMU cyberattacks and multiple event detection. Realized different WECC scenarios with high penetration renewables and HVDC overlay: MBVSA, power system separation RAS, safety supervisor based emergency control. 7-18

19 HTB Hardware Expansion and Reconfiguration HTB Reconfiguration Installed a physical inductor cabinet, allowing quick hardware switching between the new 4-Area System and the previous 3-Area System Added input/output buses to all cabinets, simplifying inter-cabinet connections Front row Areas 1 and 4 Transmission line cabinet 1 Physical inductor cabinet PMU cabinet Middle row Areas 2 and 3 Transmission line cabinet 2 Back row 2 cabinets for Multiterminal HVDC system 1 wind farm cabinet 19

20 Hardware Testbed (HTB) Background Power circulates within a single area 20

21 Communication, Control, and Visualization(4 area) Visualization Room Layout Station 1 Station 2 Station 3 Station 4 Control Center functions Area control center: Control local area Independent from each other Dispatch transmission lines Implemented with AGC, local state estimation, voltage monitoring, etc. Central controller: Only for automatic scenario sequencing and demonstration Future system level testing Central Controller Computer 1 (Area 1) Computer 2 (Area 2) Computer 3 (RTDS) Computer 4 (Area 3) Computer 5 (HVDC) Computer 6 (Area 4) Visualization computer: Only for display of system information on the video wall 7-21

22 Hardware Test-bed Advantages Broad time scales in one system - microseconds for power electronics to miliseconds and seconds for power system event. Integrate real-time communication, protection, control, and power (and cyber security). Multiple power electronic converters (for wind and solar and energy storage) with separate controls. Capable of testing actual communication and measurements. A useful bridge from pure simulation to real power system application. 22

23 Modular Emulators in HTB 2017 Development Generator Emulator Load Emulator Synchronous generator Induction machine Constant impedance, constant current, and constant power load (ZIP) Emulators Wind Emulator Solar Emulator Transmission Line Emulator Wind turbine with permanent magnetic synchronous generator (PMSG) Wind turbine with doubly-fed induction generator (DFIG) Solar panel with two-stage PV inverter Back-to-back converter to emulate AC transmission lines Voltage Type Current Type Energy Storage Emulator RT Simulator Interface HVDC Emulator Combined Model Emulator Batteries (Li-Ion, Pb-Acid, and flow), supercapacitors, and flywheels Integrate RTDS and LTB with HTB Multi-terminal HVDC overlay Emulate combined model in single emulator Fault Emulator Emulate short circuit faults demonstrate system relay protections 23

24 Multiple Simultaneous Control Functions Central control level Scheduling Regional control level Measurements Pi, Qi, Pf, Qf, V, I, θij Topology Processor State Estimation Network Observability Check State Estimator V, θ Ctrl. Different Control Block Voltage Limit Monitoring and Control Transfer Active Power Limit Monitoring Margin Not Enough Reactive Power Support Frequency Difference Between Areas WADC WADC To Excitation Visualization Visualization Dispatch, Irradiance/Wind speed Variable irradiance level, wind speed, and load power consumption can be sent to the emulators. Renewable Energy Mode Selection Different operating modes can be selected: MPPT, inertia emulation, voltage regulation mode, etc. ACE Based AGC ω B 1 P 12 ACE K I s To Governor Energy Storage Renewable Energy Local control level Protection Under-voltage protection Under-frequency protection Over-frequency protection Over-voltage protection Overcurrent protection Generator Local Area Frequency Deviation PSS PSS Droop To Excitation ω 1/R To Governor 24

25 Current Controlled Emulator Power stage description o o o o o Inverter regulating v abc Three phase converter using IGBTs Three phase inductor as filter Three phase voltage controlled by paralleled inverter DC bus stiff and stable regulated by regenerative active regulator Testing structure of load emulator performance + Regenerative active rectifier regulating V dc - Load emulator regulating i abc grid Regenerative three phase current controlled emulator test schematic 25

26 Current Controlled Emulator (cont d) Controller design description o o o o Single closed loop current controller Proportional-integrator (PI) controller in dq domain Current references from emulated model, calculated with real-time data Converter dq0 frame frequency extracted from PLL i a v ab v bc v ca AUT i b i c v ab v bc v ca abc/dq PLL θ v d v q i d_ref Emulated model i q_ref i a i b i c abc/dq i d i q + - k p +k i /s + - k p +k i /s 3ωL V dc d a d b d c + abc/dq d d 3ωL V dc + d q + - Line voltage à Phase voltage v a v b v c θ abc/dq v q - 0 k p +k i /s + integral 1/s ω PI controller limit lowpass filter Control structure of current controlled emulator PLL structure 26

27 Current Controlled Emulator (cont d) Emulated model description o o o o Input with voltage and frequency Output with current references Mathematical model fed by real time data, output calculated within IGBT switching cycle Power system model dynamics are slow compared with power electronics controller v d v q v 0 ω External command Emulated model i d_ref i q_ref i 0_ref Emulated model input and output structure Example of induction motor dynamics and detailed current controller performance 27

28 Mitigation of Unstable Harmonic Resonances in HTB HTB: converter-based power system G V * clv1 1 + Z ov1 Connection Network G I * clc1 1 Unstable harmonic resonances G5: i G5 [10 A/div] G V * clv2 2 + Z ov2 Transmission line G Y oc1 I * clc Hz 605 Hz [time: 20 ms/div] FFT(i G5 )[Freq.: 250 Hz/div] Voltage-controlled converters Generators Renewables in voltagecontrol mode Non-ideal: non-passive output impedances Y oc2 Current-controlled converters Loads Renewables in currentcontrol mode HVDC Energy storage Non-ideal: non-passive output admittances Approach to Mitigation Adjusting converter controller parameters: Trade-off Increase the passivity of the converter impedances or admittances Reduce control bandwidth of converters (still sufficient for emulation of power system) Unstable harmonic resonances 7-28

29 Synchronous Generator Emulator ~a Electrical Model Iabc ~b ~c Two-axis Model: Gate Signals Generator Model Vabc Uabc Machine Terminal Voltage Control Mechanical Model Excitation Model Droop AGC Governor Turbine P m P e 1 ω Ms D ω s ω g 1 δ s ω PSS U tref K A Tes 1 U t E fmin E fmax E f 29

30 Generator Emulator Synchronization T1 T2 T3 G1 X 1 I 1 I 2 Transition 1 I Load I 1 X 3 δ G2 V t X 2 I 2 V 2 Voltage Controller and PLL δ PLL δ r V tref Load I 2 Generator Electrical Model P e Generator Mechanical Model Frequency of G1 and G2 during synchronization Active power of G1 and G2 during synchronization Transition 2 Transition 3 Two generator emulators synchronized Current output of G1, G2, and load during synchronization G1 starts up with the load. Transition 1 G2 connect into the system open loop by using a PLL to lock the system frequency. Transition 2 Enable voltage closed loop control of G2 with PLL. Transition 3 Alternate frequency reference from PLL output to mechanical model output. Synchronization Process 30

31 Wind Turbine Emulator Gate Signals Full converter with permanent magnetic Wind Speed Control strategies Wind turbine model Vabc synchronous generator Generator Side Converter Grid Side Converter Output Filter Physical Models Wind power model and pitch model Electrical and mechanical models of PMSG Average model of the two converters Control strategies MPPT and reserved power control Droop and inertia emulation Reactive power control WT Current Grid Voltage Wind Speed 9m/s Variable wind speed experiment Wind Speed 12m/s Wind Speed 10m/s 1 p. u. Wind Speed 13m/s Detailed Waveforms 31 31

32 Virtual Synchronous Generator (VSG) Control of Type-4 Wind Turbine Objective: Let renewable energy sources behave like the synchronous generators in power system Comparison Carry out dispatch between command VSG MPPT under and variable traditional wind speed MPPT Generator Load Wind Machine Side Converter controls wind turbine speed MPPT Follow grid demands Grid Side Converter emulates the generator behavior Dispatch-able Variable emulated inertia Power (p.u.) Transition from VSG MPPT to VSG Normal Operation Turbine Speed (p.u.) 0 1 Storage SoC (p.u.) 0.5 Minute Level Energy Storage Provide energy buffer Storage Side Converter maintains DC Voltage Wind Speed (m/s) Time (s) VSG can track maximum power point, and provide inertial response 32

33 Solar Power Emulator Gate Signals Two-stage PV inverter Irradiance Control strategies Solar power model Vabc Physical Models PV panel model considering the irradiance and temperature Boost converter model Inverter model including the LCL filter Control strategies MPPT and reserved power control Droop and inertia emulation Reactive power control Low voltage ride through (LVRT) MPPT under irradiance (S: W/m 2 ) change i 1 : Generator current i 2 : PV current v g : Load voltage 33

34 ZIP Load Emulator Vabc Iabc + Vdc - ZIP load (constant impedance, constant current, and constant power) Load Model Gate Signals Iref Voltage Control Coefficients a + b + c = 1, d + e + f =1. Input voltage Coefficients and the output current amplitude in the ZIP load model change, which enables the load to switch from constant power to constant impedance/current during low voltage fault condition. I a & I b 34

35 Induction Motor Load Emulator Induction motor load IN: Converter voltages OUT: Current references PLL Torque Electrical model among V, I and flux Mechanical model Rotor speed Torque demand Phase A starting up current Mechanical torque 35

36 Induction Motor Emulator Startup Induction Motor Emulator i a i b i c v ab v bc v ca PLL i a i b i c d a d b d c abc/dq θ abc/dq abc/dq T em ω r Emulated model + - i d_ref + i q_ref - d d Controller d q Same input conditions & torque command i d i q ω r T em The comparison between simulation and experimental references results verifies the correct calculations of emulated model in DSP. 36

37 Energy Storage Emulators Energy Storage Compressed Air Batteries and Ultra-capacitors Flywheels 37

38 BESS Emulator Overview Point of Emulation Technologies: Lithium Ion, Lead Acid, and Vanadium redox flow Models: battery and two-stage power electronics interface Control: command-based active and reactive power control Applications: frequency control and voltage support 38

39 Internal Battery Characteristics Li-Ion Cell Polarization Curve Li-Ion VRB Pb- Acid VRB Cell 39

40 Battery Energy Storage Emulator Inertia Emulation Control Test Load Loss Without IE With IE Point of Emulation Emulator Attributes Two modes of operation: Command and Inertia Emulation Choice between Lithium Ion, Lead Acid, Flow batteries Constant current constant voltage charging algorithm Independent active and reactive power control Operates on its own HTB inverter 7-40

41 Flywheel Energy Storage Emulator Flywheel working states triggered by frequency regulation demands: Acceleration: area frequency high/stable, absorbing power from grid Standby: reserve energy in spinning kinetic form Deceleration: area frequency sags/need power support, supplying load Acceleration Standby with load increase Deceleration Re-acceleration Flywheel current Generator 1 current Load 7 current G1 1 Flywheel Constant torque acceleration L7 7 8 C7 9 C9 L km 110 km 25 km 10 km 10 km 25 km Standby 11 4 G4 With flywheel 3 G3 Flywheel reaches ω min Without flywheel Acceleration mode index Deceleration mode index Constant power acceleration Deceleration 41

42 Combined Model Emulator Implementation u q u q u d u d i q i d q i d VSG Load e a e b e c n Standalone Load Simulation Standalone Load (a) Simulation VSG+LOAD I a,b,c u q u q u u d d i q i i q i d d Combined Load and (b) SG Simulation Combined Load and SG Simulation Standalone load and combined load and SG emulation results comparison (a) Standalone load emulation and (b) Combined load and SG emulation. Gate Signal Voltage Control L R i a e a ZIP Load Model I ZIP-ref V a,b,c-ref i b i c I a,b,c Generator Model e b e c n 42

43 Bus Short-Circuit Fault Emulation P EUT + - Fault Emulator Realize short-circuit faults by controlling the corresponding phase of emulator terminal voltages to zero Voltages of three-phase short circuit Line-to-line short circuit I a I b Z a Z b V a * V b * V a - PI PI - V b block Control block diagram of double-line-to-ground fault d a d b d c Currents of three-phase short circuit Single-line-to-ground short circuit with 1 Ω grounding resistance 43

44 Validation of the Developed SG Emulator under Three-Phase Fault Rotor Speed (p.u) Time (s) Rotor Speed Simulation Experiment 6 th -order synchronous generator model with saturation effect on d- axis Fault through shorted lines Phase A Current (A) Time (s) Phase B Current (A) Simulation Experiment Time (s) Phase C Current (A) Time (s) Phase A Current Phase B Current Phase C Current Amplitude and phase error are less than 5% when using second norm error for analyzing dynamic response over a frequency range of 0 to 200 Hz. 44

45 AC or DC Transmission Line Emulator G 208V V X V2 0 1 I P AC 1 P 2 Q 1 Q 2 L f i a i b i c Inverter V DC Rectifier L f 208V/480V Back-to-back structure with two terminals G 480V Transmission Line Emulator Attributes Vary the line length (impedance) for different scenarios Short circuit or open line faults Reclosing emulation Emulate multiple parallel lines Emulate FACTS applications such as CVSR G Line km 110 km 25 km 10 km 10 km 25 km 11 3 G3 Comparison between the emulator and simulation with line impedance change (line drop) G1 frequency Simulation in MATLAB/Simulink Emulator in HTB Emulator in HTB with DC offset control Area 1 L7 C7 C9 L9 Area G2 G4 45

46 System model Transmission Line Fault Emulation Simulation Verification Transmission line L source1r L line /2 R line /2 L line /2 R line /2 source1 R source2 L V source2 V Phase A current on source 1 side source2 source1 100 Vsource1 V ab1 V bc1 V ca1 V ab2 V bc2 V ca2 L source1r source1 L-L to Phase L-L to Phase I a1 I a2 I b1 I b2 1/s I c2 I c1 V a1 V b1 V c1 V a2 V b2 V c2 V ab1 V bc1 V ca1 Transmission line emulator (V-IR/2)/(L/2) Transmission line emulation model (normal and fault condition) Ia1 I b1 I c1 Fault Normal (V x1 -V x2 )/2 Fault -1 I a2 I b2 I c2 I n V a(lr/2) n V b(lr/2) n V ab2 V bc2 V ca2 V c(lr/2) R source2 L source2 V source2 V a2(lr/2) V b2(lr/2) V c2(lr/2) V a1(lr/2) V b1(lr/2) V c1(lr/2) f V a1(lr/2) f V b1(lr/2) f V c1(lr/2) f V a2(lr/2) f V b2(lr/2) f V c2(lr/2) Mode Selection Fault Fault Normal Current (A) Current (A) Current (A) 0 Original Emulation Time(s) Phase B currenton on source 1 side Original Emulation Time(s) Phase C current on source 1 side Original Emulation Time(s) Fault Recover Fault Recover 7-46

47 Voltage Source Converter (VSC)-Based Multi-Terminal HVDC Overlay DC overlaying an AC system is suitable for transferring remote renewable energy to load centers Less converter numbers and potential cost benefit of multi-terminal configuration compared to building multiple point-to-point transmission lines Easy power flow reverse, smaller footprint by using voltage source converter (VSC) DC power flow controller and DC fault protection need to be addressed Summer Power Flow Winter Power Flow VSC 1 VSC 2 DC cable 3 VSC 1 VSC 2 DC cable 3 DC cable 2 DC cable 1 DC cable 2 DC cable 1 VSC 3 VSC

48 System Protection Inverter DSP s have been programmed to have their own protection based on system model parameters Parameters were chosen by WECC protection standards Automatic actions such as load shedding and generator trips can be taken Loads have under-voltage and under-frequency protection Generators have over-current, under-frequency, and overfrequency protection Load under-voltage Voltage Set-Point, p.u Tripping time, s Load Dropped, % Load under-frequency Frequenc y Set- Point, Hz Tripping time, Cycle Load Dropped, % 1 t( x) Generator Overcurrent Protection Curve t( x) x x Underfrequency Overfrequency Generator under and over frequency protection 20 Tripping time minutes seconds seconds seconds instantaneous 7-48

49 Vulnerability Assessment of Phasor Networks Vulnerability Exploitation SuperPDC Routers Storage Super PDC Relay Protection Office Routers Storage IT Center The communication protocol used by PMUs and PDCs: IEEE C Control Center Ethernet Router Log recorder Ethernet Router Lack of user and message authentication à Packet injection attack Attaker Communication Backbone Network Substation without PDC Routers Substation with PDC PDC Routers Ethernet Ethernet Routers PMUs FDR UGA Routers PMUs FDR UGA Security recommendations and best practices to be evaluated DNP3, TLS, WPA2 Phasor Communication Network

50 Multiple Event Analysis for HTB Background: Existing event detection algorithms can handle single event case by exploring their abnormality, e.g., frequency increment over a threshold indicates occurrence of load shedding. However, existing work fails to decompose and identify multiple events or cascading events which tangle the signals together. Objective: We propose a multiple-event analysis based on sparse coding to detect and recognize multiple, involved single events (i.e., generator trip, line trip, and load shedding) in an online fashion. Tasks: 1) Multiple events detection when signals from all buses are reliable. 2) Multiple events detection when there is 10% data hacked (missing signals). Results: We conducted the experiment based on a three-area HTB system under two testing scenarios. Table 1: Performance of testing scenario 1 Accuracy Bus1 Bus2 Bus3 Bus4 Bus5 Majoriy Voting S1C M2C Note: S1C denotes single event, M2C denotes double events. We can collect frequency signal from 5 buses. Majority Voting is applied to boost the performance. There are 10 S1C cases, and 5 M2C case. Table 2: Performance of testing scenario 2 (missing data) Accuracy -Bus1 -Bus2 -Bus3 -Bus2, 3 -Bus4, 5 -Bus1, 2, 5 S1C M2C Note: where - denotes the missing buses, e.g., -Bus1 indicates that the signal from bus 1 is missing, and -Bus1, 2, 5 denotes that buses 1, 2, and 5 are all missing. 3-50

51 Voltage Collapse Scenario G MW km 110 km 25 km 10 km 10 km 25 km G3 Load increase Collapsing point Collapsing point 2 L7 C7 C9 L9 4 G2 G4 Ramp the active power of the load on bus 9 (20% const. impedance, 20% const. current, 60% const. power) to 1.65 p.u. (1485 MW) Without Control The system enters a state of voltage instability (or voltage collapse) when the increase in load demand causes a progressive and uncontrollable decline in voltage. Stability margin goes zero Stability margin goes zero

52 Voltage Collapse Scenario with STATCOM G MW km 110 km 25 km 10 km 10 km 25 km G3 Reactive power injected 2 L7 C7 C9 L9 4 G2 G4 Ramp the active power of the load on bus 9 (20% const. impedance, 20% const. current, 60% const. power) to 1.65 p.u. (1485 MW) With Control When the margin is close to a defined threshold, e.g. 15%, it will trigger the control action, such as reactive power compensation. Reactive power injected Stability margin goes under threshold

53 Digital Interface 1 2 Digital Interface Power Interface Power Interface RTDS Interface with HTB Interface Algorithms: Voltage Type Current Type V S V S Z s Z s Simulator Hardware I L V L I M =I L V M =V L +ε I s V M =V L Simulator I M =I s +ε V L V L Hardware Z L Z L Emulator 1 G1 G mh 1.2 mh Area mh LD7 Emulator 2 Emulator 3 Two RTDS interfaces with the HTB LD12 Area 1 and HTB2 in HTB mh Transmission Line RTDS 10 mh 6 mh mh HVDC1 HVDC mh LD9 Emulator 6 LD mh 0.7 mh Area mh Area 3 in RTDS Emulator 4 G3 G4 Emulator 5 G5 Combination of Voltage and Current Type Digital Interface Voltage Reference Digital Side Voltage Current Reference V abc PLL I abc abc/dq0 θ=ωt abc/dq0 V dq0 I dq0 Filter Filter Closed-Loop Control dq0/abc dq0/abc Closed-Loop Control 1 2 Filter Voltage Modulate Filter V dq0fb I dq0fb Δθ V abc/dq0 θ PWM θ abc/dq0 Voltage Delay Correction Feedback Voltage Power Converter Feedback Current RTDS Interface Attributes Expand HTB to more than 40 buses Unique system that has both control and power hardware interface Δθ I Current Delay Correction 53

54 3-Area CURENT HTB Structure Area 1 Area 2 G km 110 km 25 km 10 km 10 km 25 km G3/WT III 3 G2 2 L7 110 km C7 C9 66 km L9 4 G km L13 14 L12 Area 3 G14 Wind Farm WT II VSC 2 DC cable 2 VSC 4 Multi-terminal HVDC DC cable 4 DC cable 3 VSC 3 Three-area system including multi-terminal HVDC and wind farm DC cable 1 VSC 1 Wind Farm WT I Three-area system: Three areas interconnected by three long transmission lines (220 km, 220 km, and 132 km respectively) Area 3 connected to a multi-terminal HVDC (MTDC) system with DC lines (two 100 km, one 60 km, and one 70 km) 54

55 Scenario - High Renewable Penetration Inertia Emulation Scenario Replace G2 by an onshore wind farm. Together with offshore wind provided by HVDC, system renewable penetration can reach 80%. Event triggered by a HVDC converter failure. Solution Frequency and voltage support from onshore wind farm and the HVDC converters Curtailment and voltage mode control when necessary Integration of energy storage to further enable grid support controls Base case with generator Expected outcome System frequency and voltage within acceptable range MPPT MPPT with inertia emulation Voltage mode Voltage mode with storage HTB scenario test results Wind Turbine Active Power (p.u.) Area Frequency (Hz) Time (s) 55

56 Scenario Wide-area Damping Controller Scenario o o 50% renewable penetration One poorly damped inter-area oscillation mode between Area 1 and Area 2 Wide-area Damping Controller (WADC) o o o Based on the measurement-driven model, which is identified by using ringdown data and ambient data Controller input: frequency difference between Area 1 and Area 2 Controller output: added to exciter voltage reference of Generator 1 System disturbance o Load increase (Bus 9, 0.7 p.u. to 1.1 p.u.) f 1 - f 4 (p.u.) Damping Ratio (%) 2 x t(s) without control with control Frequency deviation Tie-line power, Bus 7-Bus 9 without control with control P Bus7-Bus9 (p.u.) t(s) without control with control time (s) Estimated damping ratio using Matrix Pencil 56

57 Active Power (p.u.) Active Power (p.u.) Scenario Measurement-Based Voltage Stability Assessment and Control for Load Area (3-Area) Collapse scenario: load increase on Bus 13 until system collapses. Stable scenario: with the same load increase, the remedial action (Q support from MTDC) is taken against the voltage instability triggered by the stability margin monitoring. New MBVSA algorithm: Rebuild a N+1 buses equivalent (For 3-area HTB, N = 2) using boundary buses measurements Stability Margin P 7to12 P 7to12Limit P 7to12 (after Q support) P 7to12Limit (after Q support) Stable Collapse Time (s) Calculate the transfer limits for each tie line Stable Monitor the stability margins. If margin < threshold, take remedial action. Equivalent circuit model 0.6 Stability Margin 0.5 P 9to13 P 9to13Limit 0.4 P 9to13 (after Q support) P 9to13Limit (after Q support) Time (s) 57

58 HTB 4-Area Structure S8 L8 Area 2 Area 3 G1 1 S9 9 3 WT 3 G2 550 km c WT 4 T L9 T3-4 L2 4 Four-Area System 580 km VSC 1 DC Line 2-3 DC Line km Multi-terminal HVDC VSC km DC Line km VSC km S11 11 Area 1 Four-area system including multi-terminal HVDC overlay, large wind farms, solar energy, and energy storage (battery-based) PV7 7 c L7 L11 T km L6 6 PV S10 PV5 L10 Area 4 58

59 Scenario Measurement-Based Voltage Stability Assessment and Control (4-area) Collapse scenario: load increase on Bus 11 until system collapses. Stable scenario: with the same load increase, the remedial action (reactive power support from HVDC at bus 6 and energy storage at bus 11) is taken against the voltage instability triggered by the stability margin monitoring. MBVSA algorithm: Rebuild a N+1 buses equivalent (For 4-area HTB, N = 2) using boundary buses measurements. Without reactive power support Calculate the transfer limits for each tie line. Monitor the stability margins. If margin < threshold, take remedial action. Equivalent circuit model With reactive power support 7-59

60 Scenario Power system separation remedial action scheme Collapse scenario: three-phase short circuit at the end of line 3-4, line 3-4 is tripped by the overcurrent protection, the system becomes unstable Stable scenario: when overcurrent protection is triggered it sends the signal to trip line 2-8, this action separates the system into two stable islands. Without the separation With the separation 7-60

61 Scenario Development Interactive voltage control scenario: To demonstrate the application of energy storage in voltage control. Users can increase load at one or multiple load buses (8,9,10 or 11), the energy storage injects reactive power to the corresponding bus when the active power reaches 90% of the limit, saving the system from voltage collapse. State predictor scenario: To demonstrate the application of state predictor in transient instability prevention using a semi-analytical approach. After a generator trip, the system will show real time and 10-second future predicted rotor angle of generators. All energy storage in the system injects active power to prevent transient instability when predictor detects instability. 61

62 HTB Future Work Continued HTB Construction and Expansion Provide more flexible control and visualization options easier reconfigurability Expand the HTB structure to the national grid with RTDS or LTB. Continue integration of multi-terminal HVDC into HTB with MMC structure. Functionality enhancement Add more capabilities to the communication architecture including the ability to test cybersecurity on HTB and robustness to failures in communication, sensor failures, and actuator failures. Enhance the HTB control function under the condition of the loss of monitoring or measurement. Enhance the HTB control function with load identification function, cross regional AGC control. Comprehensive integration of protection schemes for power electronics connected sources (wind, PV, etc.) Use testbed to develop microgrid controller to be demonstrated on EPB s system in Chattanooga 62

63 HTB Demonstration Plan Year 1~3 Generation I Hardware implementation of the power electronics based emulators, including large wind / solar / storage farm emulation. Integrate PMU/FNET data into HTB. Multiple load and scenario demonstrations (multi-terminal HVDC, hybrid AC/DC, multi-area oscillation and control, high renewable energy penetration). Year 4~6 Generation II Implementation of sensing, monitoring, actuation, and protection in real-time. Integrate with real-time simulation. Scenario demonstrations (multiple HVDC links between wide areas, major tie line and wind farm outage dynamic effects, coordinated power flow control over large distances, demonstrate system resilience to attacks, energy storage impact). Year 7~10 Generation III Coordinated high penetration renewable control demonstration. Automatic real time reconfiguration for selected outage scenarios. Ultra-wide-area coordinated real-time communication and control on a system hardened against coordinated cyber attack. 7-63

64 Acknowledgements This work was supported primarily by the ERC Program of the National Science Foundation and DOE under NSF Award Number EEC and the CURENT Industry Partnership Program. Other US government and industrial sponsors of CURENT research are also gratefully acknowledged. 64