I. CVETKOVIC, D. BOROYEVICH, R. BURGOS, C. LI, P. MATTAVELLI October 12, 2015 Paper Session 1D Synchronous Machine-Based Multi-Converter System With Online Interaction Monitoring Function I. CVETKOVIC, D. BOROYEVICH, R. BURGOS, C. LI, P. MATTAVELLI Presentation at: October 11-13, 2015 Chicago, IL

Motivation ES G DG DG DG ac transmission / distribution Consumption Generation L G ac transmission / distribution Electromechanically –based transmission / distribution substation ES DG dc transmission / distribution Enhanced transmission / distribution substation (power electronics-based) DG DG New grid-interface converter control strategies are needed for the high penetration of power electronics! Latest IEEE 1547 standard revision allows DG to regulate the voltage at the point of common coupling! DG DG ES DG ES DG ES DG ES DG 1890 1950 1990 2010 Future?

Synchronous Machine-based Control of Voltage Source Converters Concept from early 80s, grew interest after 2007 (200+ papers since then) Three main advantages for utilizing the synchronous machine – based control in grid-interface converter are: Im Re Forbidden Region Power-based synchronization Virtual Inertia Islanded mode Synchronous Generator Power Electronics Converter System Eigenvalues x Stealth Fighter Commercial Aircraft High Performance Low(er) Stability margins Low Performance High Stability Margins

Instability in Multi-Source Systems (Caused by the Phase-locked Loops) Microgrid subsystem example PV Energy Storage (ES) PLL 1 PLL 2 Grid ZS AC bus Load 1 Load 2 ES Operating point changes 1.5 Hz oscillations bus voltage ES Phase A Current PLL 1 frequency PLL 2 Phase-locked Loops interactions can become severe under weak grid conditions! PLL 1 PLL 2 PLL Signal Flow Graph PLL N *Dong Dong, 2013

Dynamically Dual Systems Synchronous machine: Dual Systems Power Electronics Converter: Virtual Synchronous Machine (VSM)

Dynamic Interactions in the WECC System Caused by Partial Loss of Generation ≈ 180 MW ≈ 370 MW Large transient causes overall system instability due to undamped power oscillations between Generators 1 and 3. WECC 9-bus system Active and Reactive Power at the bus No.4 during loss of generation transient Time [s] Power [MW, Mvar]

Decrease the virtual inertia 5 times Dynamic Interactions in the WECC System Caused by Partial Loss of Generation @ 5s ≈ 180 MW ≈ 370 MW System with VSM at the bus No.4 (with lower virtual inertia) is not unstable! System with exactly equaivalent VSM at the bus No.4 features same instability! WECC 9-bus system Active and Reactive Power at the bus No.4 during loss of generation transient Time [s] Power [MW, Mvar] Decrease the virtual inertia 5 times However, it has been shown show that lower virtual inertia is not always the best choice!

VSM decreases the initial virtual inertia 10 s after the transient Mitigating Instability in the WECC System Caused by the Partial Loss of Generation @ 5s ≈ 180 MW ≈ 370 MW VSM recognizes undamped power oscillations and adaptively readjust virtual inertia! WECC 9-bus system Active and Reactive Power at the bus No.4 during loss of generation transient Time [s] Power [MW, Mvar] VSM decreases the initial virtual inertia 10 s after the transient 10 s

Small-signal Stability Assessment Generalized Nyquist Criterion (GNC) Hybrid ac/dc microgrid system example To avoid instability the return ratio: Load 1 at the dc - interface AC bus Energy Storage must stay away from (–1,0) ! dc- interface ZS YL DC bus Load 2 Load 3 Load 4 Unstable System Stable System

Small-signal Stability Assessment Generalized Nyquist Criterion (GNC) Hybrid ac/dc microgrid system example To avoid instability the return ratio: Load 1 at the ac - interface ac- interface ZS YL AC bus eigenvalues Energy Storage must not encircle (–1,0) ! -3 -2 2 -1 -4 4 6 8 10 l1 ( j ) l2 ( j ) -4 -2 2 4 6 8 10 -1 l1 ( j ) l2 ( j ) DC bus Imaginary axis Load 2 Load 3 Load 4 Real axis Unstable System Stable System

Small-signal Stability Assessment Generalized Nyquist Criterion (GNC) Hybrid ac/dc microgrid system example The IMU is necessary to measure system impedances in order to: Evaluate system stability Assist in control design of new power electronics converters Check (transient) performance of the existing power electronics converters (e.g. voltage overshoot) Load 1 ac- interface ZS YL AC bus Example of the MV IMU (2.2 MVA) dc- interface ZS YL DC bus Load 2 Load 3 Load 4 At any desired interface point Impedance Measurement Unit (IMU) needed to assure stability!

Online (small-signal) Stability Monitoring Hybrid ac/dc microgrid system example In balanced & symmetrical three-phase systems: Line voltage is time-variant Line current is time-variant Power is time-invariant Load 1 AC bus Pi , Qi Energy Storage m Po DC bus Can (every) converter evaluate stability at its input and output, with no need for the IMU ? Load 2 Load 3 Load 4 Small-signal power contains information of the system eigenvalues ! No transformation to d-q coordinates Modulation index-to-power transfer function measured directly in abc

Implementation of Methodology on Large Synchronous Machines Simplified Representation of a Synchronous Generator Network Analyzer or similar Frequency Response Analyzer

Stability of Multi-Source Systems (Multi-machine system) Both converters emulate synchronous machines in this example

Stability of Multi-Source Systems Small-signal Active Power Small-signal Reactive Power System eigenvalues

and Future Work Concluding Remarks Presented is an alternative method for evaluation of small-signal system stability dual to GNC Stability evaluation can be done online; it can be implemented in any active source or load, and some passive where control terminal is available (e.g. synchronous machines) Practical Implementation of the technology is easy and inexpensive Get better understanding of the small-signal stability via both, active and reactive power measurements Explore opportunity to use small-signal power measurements not only for online stability monitoring, but also as an online health monitoring function – performance degradation, component failure, etc.

Thank you! Questions / Comments / Suggestions October 12, 2015 Paper Session 1D Thank you! Questions / Comments / Suggestions Presentation at: October 11-13, 2015 Chicago, IL