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Oscillation Detection and Analysis
Applications in WECC
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November 19, 2012
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November 19, 2012
2
WECC Joint Synchronized Information Subcommittee
Oscillation Detection and Analysis Applications in WECC
November 19, 2012
The objective of this paper is to describe applications for oscillation detection and analysis to be
used by planning engineers, operational engineers and dispatchers.
1. Off-Line Applications for System Planning and Operating Engineers
Application 1A. Stand Alone Ringdown Analysis
System planners and operational engineers need to have an application to perform modal
analysis on the actual and simulated disturbance data.
Inputs and Signal Conditioning.
Disturbance data can include bus voltages, voltage angles, system frequencies, and active power
flows. A user shall be able to select a subset of signals for modal analysis. Several signals can be
combined together, such as adding power flows in several lines to calculate a path flow, or
calculate a difference between two signals. Derivative signals can be calculated by applying
linear filtering. Application needs to have an intelligence to patch data drop-outs.
Unlike PMU data, the simulated data is not equally sampled, and sometimes has two values at
the same time when switching events occur. The modal analysis application needs to properly resample the input data.
A user shall be able to select the time interval for modal analysis by visual inspection of signals.
The application needs to have data conditioning functions such as first-order de-trending,
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removing initial value, filtering and decimation that can be applied to all signals or individual
signals. The settings for signal conditioning can be saved in a configuration file so that they can
be used with another disturbance or a simulated case.
Analysis
Modal analysis application shall calculate oscillation frequencies, damping and mode shapes
(oscillation energy) for the selected signals. The calculations are to be recorded in a spreadsheet.
The application needs to identify both, inter-area and local oscillations.
Uses
System planners use disturbance events to validate power system models used in grid
simulations. A part of model validation criteria is to ensure that the system model reasonably
represents power oscillations, including oscillation frequencies, damping and mode shapes
(energy).
An operations engineer uses Modal Analysis Application to calculate frequencies, damping and
mode shapes for a disturbance event (e.g. generation outage) or a system test (e.g. Chief Joseph
brake insertion). An operations engineer can archive the calculation results as well as key system
conditions (such as power flows and phase angles). An operations engineer can baseline the
modal performance against that during previous similar events.
Comments
Prony analysis is often used for estimating oscillation damping for an oscillation ringdown.
Observed challenges with Prony analysis are summarized in Appendix A. It would be
worthwhile to improve robustness of Prony performance as well as to investigate other methods.
Application 1B. Automated modal analysis for multiple cases
System planners need to perform modal analysis on multiple simulation cases. During the
development of the oscillation damping procedures, system planners ran 1,000s of dynamic
simulations, including several contingencies applied to multiple base cases. There is a need to
perform modal analysis automatically on these simulation results to estimate frequencies and
oscillations of critical inter-area modes in the Western Interconnection and to tabulate these
results in a CSV/Excel file.
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Currently, BPA is using MATLAB-based application for automated Prony analysis of inter-area
oscillations developed by Dan Trudnowski. The application calculates frequency and damping of
selected inter-area oscillations for multiple cases and tabulates the results. BPA experience has
been positive in general for North-South oscillations. However, several issues are observed and
documented in Appendix A.
Application 1C. Oscillation Analysis Subroutine for System Studies
There are efforts under way to automatically calculate system operating limits. Under such
process, contingency analysis is performed on a given operating state, and should the
performance be found acceptable, the operating state is further stressed until the system
operating limit is reached. Oscillation damping analysis is one of the system performance
metrics, primarily the damping of inter-area power oscillations.
The following automated feedback scheme is envisioned. A master program calls a grid
simulator, e.g. GE PSLF or PowerWorld, to simulate a contingency and to export pre-selected
simulated quantities. A master program then calls an application to perform modal analysis on
the simulated quantities. The modal analysis application calculates oscillation frequencies and
damping, and then reports the results back to the master program.
Application 1D. Spectral Analysis Application
A user shall be able to perform spectral analysis on multiple signals. The analysis includes:
- a table with frequency peaks and mode shapes for each peak for multiple signals
- water-fall plots of signal spectrum over selected frequency range and time period of
interest
- spectrum of multiple signals over selected frequency range at a given time
- compass plots representing mode shapes
An operating engineer can use spectral analysis to identify and analyze oscillations.
Application 1E. Stand Alone Mode Meter Application
A system planner shall be able to run Mode Meter application manually for a selected data set.
The data set can be up to several hours. A system planner shall be able to modify Mode Meter
settings for manual runs. Mode Meter application needs to have data conditioning capabilities
similar to those for Ringdown Application.
A system planner will need to perform system model validation studies for system conditions
when Mode Meter indicates low damping. A system planner will use a validation base case
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representing system conditions at the time when low-damping condition was observed. Because
there is no practical way at this time to introduce load noise in large scale grid simulations, a
brake insertion can be used to initiate an oscillation in the system studies. Then, oscillation
frequencies, damping and mode shapes are calculated using ringdown application, and then
compared with oscillation frequencies, damping and mode shapes estimated by Mode Meter
using actual data.
A system planner can also use a small signal analysis program to calculate eigenvalues and mode
shapes for a validation base case, and to compare them with Mode Meter estimates. A system
planner can then perform contingency analysis using a small signal model to estimate damping in
post-contingency state.
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2. Real-Time Applications for Engineers
Application 2A. Oscillation Analysis
An Oscillation Detection application runs continuously by scanning bus voltages, frequencies,
active and reactive power for signs of sustained oscillations. An Oscillation Detection
application will create logs of the instances when sustained high energy oscillations are detected.
An operating engineer shall be able to examine an unusual oscillatory activity using off-line
applications described in section 1. An operating engineer shall be able to retrieve large data sets
and to perform oscillation analysis, including calculations of oscillation frequency and damping,
spectral and mode shape analysis. An operating engineer shall have tools to create reports on the
observed phenomenon.
Application 2B. Damping of Inter-Area Oscillations
Mode Meter is an application that estimates damping of electro-mechanical oscillations from
ambient data. Mode Meter application runs continuously with automatic settings and outputs
estimates of the oscillation frequency, damping, and critical mode shapes to a historian. A
system planner can retrieve the archived modal information together with the other power system
parameters (power flows, phase angles, etc) for baselining power system conditions:
- A planner can perform a correlation analysis between system configurations and modes of
oscillations, including their mode shapes
- A planner can perform a correlation analysis between damping and system stress
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3. Real-Time Applications for Dispatchers
Application 3A. Low Damping or Sustained Oscillation
The goal of the application is to provide an alarm to a dispatcher when a sustained oscillation or
low-damping condition exists in a power system.
A Mode Meter and Oscillation Detection applications run continuously by scanning bus
voltages, frequencies, active and reactive power for signs of sustained oscillations and lowdamping conditions. An alarm is issued when sustained oscillations or low-damping conditions
are detected. A post-processor is used to classify an oscillation as local or inter-area. Power
flows and phase angles are used to supplement the decision-making process. Signals with highest
oscillation activity are displayed to dispatchers.
Application 3B. Normal Operation
The goal of the application is to provide an alarm to a dispatcher that an un-dampened oscillation
can develop after the next contingency.
Mode Meter Application runs continuously to estimate damping of major inter-area power
oscillations from ambient data. BPA analysis indicates that pre-contingency damping alone is not
a good predictor of what post-contingency damping could be. Therefore, ambient damping
estimates need to be paired with other measurements, e.g. path flows, phase angles, key line and
generation statuses.
An alarm is issued when a set of pre-determined conditions is met, such as estimated damping is
below a threshold, and a combination of key flows and phase angles are above certain levels. A
damping display shows historic damping estimates, as well as key flows and phase angles.
Operating procedures are developed to reduce oscillation damping risks.
Dispatcher can ask a study engineer to perform a quick study to determine whether a postcontingency damping is acceptable.
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Appendix A: Observed Issues with Prony Analysis
Dealing with switching events. A large disturbance may involve multiple switching actions.
Some of these actions, e.g. dropping generation or a series capacitor insertion, affect the
system damping, and therefore the modal analysis needs to be done on a time window after
such switching event. Figure A-1 shows an example of Prony application proving a bad fit
because of a large switching event in the event window.
Other minor switching, such as shunt capacitor switching, have minimal impact on the actual
damping, but distort electrical signals and the “perception” of damping. Because of these
distortions, it may be difficult to find a “clean” window large enough to do a good damping
estimate. It will be desirable for analytical methods to be capable of handling signal
distortions / discontinuities due to a minor switching.
13hs_146g04-12hs41y-12hs_ld15a-bc-alb_B.csv, Prony Fit 1
GCL_500.FREQ-PALO_VERDE_500.FREQ
10
8
6
4
2
0.323 Hz, -6.36 %D, 0.399
0.282 Hz, -1.35 %D, 2.04
0
-2
-4
-6
-8
Fort Rock series
-10
5
10
capacitor switching
PSLF
Prony
15
20
25
30
Time (sec.)
35
40
45
50
Figure A-1:Prony analysis affected by a series capacitor switching
Prony application needs to provide some type of indicator of the quality of the damping
estimate.
In automated mode, BPA runs Prony application over two windows several seconds apart
and compares the results for consistency. Very different damping results would trigger a
quality flag, flowed by manual examination of the results.
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Figure A-2 shows Prony fit done on the same event as shown in Figure A-1 but using a
window 5 seconds later. Damping estimates are better, but we see a low energy negatively
dampened mode, probably due to non-linearity in the signals.
13hs_146g04-12hs41y-12hs_ld15a-bc-alb_B.csv, Prony Fit 2
GCL_500.FREQ-PALO_VERDE_500.FREQ
10
8
6
4
2
0.306 Hz, 0.72 %D, 4.36
0.296 Hz, -1.97 %D, 0.107
0
-2
-4
-6
-8
PSLF
Prony
-10
15
20
25
30
35
Time (sec.)
40
45
50
Figure A-2:Prony analysis run 5 seconds later on the same event as shown in Figure A-1
Dealing with non-linearities. Prony methods assume linear system response. Events involving
large generation outages include non-linear response and as well as transition between
equilibriums as event progresses.
Set-up. Prony method seems to be an art, as the results are sensitive to the selected window,
signal conditioning, etc. Even for lightly dampened oscillations, Prony method can give
ambiguous results. Figure A-3 shows Prony fit to a simulated Chief Joseph brake test, identifying
two nearly identical modes with comparable energy but different damping ratio. Figure A-4
shows a Prony fit to simulated Palo Verde event. The fundamental oscillation is undampened.
Prony shows two modes at close frequencies, a higher energy slightly dampened mode and
negatively undampened low energy mode.
A better guidance is required on setting up Prony calculations and signal conditioning. Also,
post-processing intelligence is needed.
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It is also advised to look into methods alternative to Prony, such as Matrix-pencil, or SteiglitzMcBride.
13hs_146b02-12hs41y-12hs_ld15a-brk_B.csv, Prony Fit 2
GCL_500.FREQ-PALO_VERDE_500.FREQ
10
8
6
4
2
0.306 Hz, 0.92 %D, 3.27
0.305 Hz, 2.49 %D, 1.7
0
-2
-4
-6
-8
PSLF
Prony
-10
10
15
20
25
30
35
Time (sec.)
40
45
50
55
Figure A-3: Prony fit to a simulated Chief Joseph brake test
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13hs_146b02-12hs41y-12hs_ld15a-2pvng_C.csv, Prony Fit 2
GCL_500.FREQ-PALO_VERDE_500.FREQ
8
6
4
2
0
0.297 Hz, 0.63 %D, 3.11
0.283 Hz, -6.42 %D, 0.309
-2
-4
-6
-8
PSLF
Prony
-10
10
15
20
25
30
35
Time (sec.)
40
45
50
55
Figure A-4: Prony fit to a simulated Palo Verde outage
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