Shun-Hsien (Fred) HuangERCOT System Planning

ERCOT RPG MeetingSeptember 16, 2011

Preliminary Study Results of the High Frequency Oscillations in the CREZ Reactive Compensation Study

2

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.

3

CREZ Reactive Compensation Study: High Wind Scenario

Meeting Title (optional)Date

With Synchronous CondenserWithout Synchronous

Condenser

4

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.

5

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.

6

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)

7

Test Case Study Result

8

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)

9

CREZ High Wind Case Study Result

Original Case

SVC Tuning

Distributed SVC

Wind Generator Tuning

10

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.

11

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.

12

Thank you for your time.

Questions?