Transcript Findings of CREZ High Frequency Oscillations

Preliminary Study Results of the
High Frequency Oscillations in
the CREZ Reactive
Compensation Study
Shun-Hsien (Fred) Huang
ERCOT System Planning
ERCOT RPG Meeting
September 16, 2011
CREZ Reactive Power Compensation Study
• Study report is available at
http://www.ercot.com/calendar/2011/01/20110118-BOD
• One of the issues addressed in the study report,
“Fundamental frequency models: The main issue observed in this study was the
sensitivity of the models to low short-circuit ratios between the system strength and
the installed wind generation. Under these conditions high frequency oscillations
(sometimes in excess of 10 Hz) were observed. It was not clear if these
oscillations are a result of modeling issues or would actually exist in the
system. Additional work would be needed to confirm which is the case. If it is
found that the phenomenon is a modeling issue, then it is strongly recommended that
work be done to improve the models to prevent unwarranted conclusions from being
drawn based on study results using the model. (Note that in this study, it was
determined to address the issue by using “place holder” synchronous condensers to
increase the short-circuit ratios. If such an increase is actually needed, other
technologies may also be available to mitigate weak systems)”
• This follow up study is to address this particular issue.
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CREZ Reactive Compensation Study: High Wind Scenario
Without Synchronous
Condenser
Date
With Synchronous Condenser
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Meeting Title (optional)
High frequency oscillations - causes
• As a system weakens, the effective close-loop response of
voltage controller of the voltage support resources gets faster.
• At the weak grid (low short circuit ratio),
– Voltage is not stiff. (high dV/dQ, the same amount of Q support
results in larger voltage deviation)
– Undesired oscillatory response can be caused among wind
farms and other dynamic voltage devices (SVC/STATCOM/..)
when all of them try to regulate voltage at the same bus/area
with fast voltage controller.
– In the CREZ study case, the oscillation frequency is also based
on the combined effect of the voltage controller of wind
generators and SVCs.
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High frequency oscillations – potential solutions
• The oscillation can be mitigated through proper coordination of
voltage controller of the wind generation resource and SVCs
• A 5-bus test system is created to investigate the wind dynamic
model and SVC dynamic behaviors
– A SVC with simplified generic dynamic model.
– A wind plant with generic type 3 dynamic model.(Double Fed
Asynchronous Generator model)
– Potential solutions eliminates the oscillations.
• CREZ high wind case is also studied with the identified cause
and potential solutions.
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Test Case Study Scenarios
• Original case with high frequency oscillations. (large wind
generation + large SVC + weak grid)
• Potential solutions
– Coordination of SVC and wind generators.
• Tuning of SVC
• Tuning of wind generator
– Low-pass filter tuning of wind generator (not tunable in the
current PSS/e model)
– Proposed dynamic gain adjustment controller of wind generator
(not available in the current PSS/e model)
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Test Case Study Result
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CREZ High Wind Case Study Scenarios
• CREZ Maximum Export Case
• Potential solutions
– Coordination of SVC and wind generators.
• Tuning of SVC
• Tuning of distributed SVC
• Tuning of wind generator
– Low-pass filter tuning of wind generator (not tunable in the
current PSS/e model) (x)
– Dynamic gain controller of wind generator (not available in the
current PSS/e model) (x)
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CREZ High Wind Case Study Result
Original Case
SVC Tuning
Distributed SVC
Wind Generator
Tuning
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Solutions to resolve the high frequency oscillations
• Enhance the strength of the grid to obtain better voltage response:
(Synchronous machines, Transmission grid)
•
•
•
provide a larger margin (wider range) for voltage controller settings of
the existing/future wind farms/SVCs.
mechanism to install/operate/maintain synchronous condensers under
current market structure
longer lead time for transmission enhancement
• Voltage support coordination of the generation resources
•
•
•
A more robust voltage support controller may already be available (e.g.,
SVCs at Oncor, or the proposed dynamic gain adjustment in wind
machine). Need to obtain an adequate dynamic model for the study
purpose
Using several smaller SVCs at several substations instead of one big
SVC at one substation could improve the voltage response
Can be more challenge for voltage control coordination with future wind
farms/SVCs.
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Summary
• The cause and potential solutions are presented to resolve the
high frequency oscillations.
• Although the tuning of SVC or wind generator provides a
feasible solution to mitigate the high frequency oscillations, it
can also result in the need for additional dynamic support
device due to slower voltage recovery.
• A proposed controller function (dynamic gain adjustment)
demonstrates the potential to resolve the high frequency
oscillation without compromising the dynamic voltage support.
• All the findings and proposed solutions need to be discussed
with manufactures about the feasibility and obtain an adequate
model for study.
• This study results just provide one possible approach to
mitigate the high frequency oscillations.
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Thank you for your time.
Questions?
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