1. 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

2. 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
Future?

3. 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

4. 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

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

6. 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]

7. 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!

8. 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

9. 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

10. 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

11. 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!

12. 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

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

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

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

16. 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.

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