0. JSIS March 4, 2015 Wide-Area Damping Monitoring using PMU Data
Joe H. Chow, Felipe Wilches-Bernal
Rensselaer Polytechnic Institute
X. Tony Jiang
Quanta Technology
Dmitry Kosterev
BPA

1. Damping assessment using relative phase information from PMU data
Power systems with long transmission lines for power transfer tend to have lightly damping interarea modes, e.g., Western US power system (WECC), Nordel power grid
The interarea mode damping is achieved by application of power system stabilizers (PSSs) on multiple generators.
Unfortunately, some of these PSSs may be poorly tuned because the interarea mode shapes have changed.
As a result, WECC wants to develop model identification techniques for generators, excitation systems, and PSSs, so that the damping contribution to the interarea modes can be properly assessed.
However, generator testing requires taking it off-line, which can be costly

2. Damping assessment using relative phase information from PMU data
Is it possible to assess the damping contribution of a particular generator (with its PSS) by measuring disturbance response?
This question is not addressed in control system theory for large-scale systems
We have recently developed a technique using linearized power system models to assess such damping contributions
The method looks at the phase differences between the generator rotor angle (or speed) and its terminal bus angle (or frequency)
The method can be applied to disturbance simulation or PMU data (requiring methods such as Prony, Eigensystem Realization Algorithm, or N4SID to extract the modal components)

3. Linearized Model Block Diagram of a Single-Machine infinite-Bus System
Heffron and Phillips
de Mello and Concordia

4. Synchronizing and Damping Torque Decomposition - SMIB

5. Interarea Mode Damping Result
For each generator, find the phase difference between the interarea mode content of the terminal bus voltage angle and the generator angle, for example, for generator 1:
If this difference is
Positive and large, then PSS contributes good damping
Negative, then this machine is contributing negative damping torque
Positive but small – need to look at the setting of the PSS
The damping for each generator is assessed using its own measurements
To apply the method in real time – after any disturbance resulting in sustained interarea mode oscillation, use the PMU data to check this phase difference for all critical generators. Note that the rotor angle can be measured with a regular digital recording device.

6. Alstom/Psymetrics Results
N. Al-Ashwal, D. Wilson, and M. Parashar, “Identifying sources of oscillations using wide area measurements,” 2014 Grid of the Future Symposium, CIGRE US National Committee, 2014.
Their method uses the rotor angle mode shapes to find the relative phase between different generators and use the information to determine damping contribution

7. DK WECC Simulation
Earlier Hongming Zhang asked Brian Thomas of GE to perform some WECC simulations for RPI. The scenarios selected did not show significant oscillations. Thus our method could not be used. Presented at the 10/2014 JSIS.
Then Dmitry provided a scenario with significant oscillations
2 cases for the scenario:
No PSS’s
A few PSS’s turned on
RPI was provided with the time response of a selected set of generators (rotor angle, rotor speed, generator terminal bus angle, generator terminal bus frequency).
RPI was not told which generators had the PSS turned on. The intent is to use the modal angle difference to find those generators with their PSS’s turned on.

8. Case 1: All PSS’s off

9. Case 1: A Few PSS’s On

10. Case 1: All PSS’s Off – Gen 1
blue: term. bus angle, green: rotor angle blue: term. bus frequency, green: rotor speed

11. DK Case Analysis
All generators in one coherent group, so they all have similar swings
The bus angles and rotor angles are plotted against an average angle. Modal information was difficult to extract from the angles (depending on the reference angle) – Brian has resolved this issue.
Instead, use an alternative approach of calculating the relative phase difference angle 𝑓 −angle(ω) associated with the interarea mode.

12. Angle diff change (deg)
DK Case Results
For each generator, compute the relative phase change between no PSS on to some PSS’s on.
Machines with large phase increases have effective PSS’s
Generator
Angle diff change (deg)
PSS 1
3.03 N 2
12.09 Y 3
11.21 4
1.79 5
1.11 6
5.00 7
5.24 8
-0.03 9
12.29 10
-6.80

13. Next Steps Partial results submitted to 2015 Power Tech Conference
Resolve some of the outstanding issues in modal information extraction
Testing of the method on a few generators in a real power system (like WECC)
PMU monitoring of generator terminal buses
Generator rotor speed and angle measurements with GPS signal and synchronized to PMU sampling
30 or 60 samples per second?

14. Acknowledgements
This work was supported primarily by the ERC Program of the National Science Foundation and DOE under NSF Award Number EEC
Other US government and industrial sponsors of CURENT research are also gratefully acknowledged.
Other industry/agency collaborators: NYPA, NYSERDA, Hitachi, Grid Protection Alliance, BPA, SCE, GCEP