1. Integrating Wind on Weak Grids – CREZ Panhandle Region Sharyland Utilities
July 16, 2014

2. Background
In parallel with the ERCOT PREZ Study, Sharyland has been evaluating the transmission system performance in the Panhandle region, specifically:
Including generation resources meeting Section 6.9 of the ERCOT Planning Guides
Completing steady state voltage security, transient voltage stability and system strength assessments for the Panhandle region with updated generation models
Including the trigger levels identified in the ERCOT PREZ study report
Evaluating the assumptions of the 2400 MW Panhandle export limit and the corresponding 2,160 MW used in the ERCOT economic study

3. Sensitivity analysis around 200 MW @ Ogallala Evaluated
Background
ALIBATES
WINDMILL
TULE CANYON
COTTONWOOD
GRAY
0 MW
500MW
750MW
400 MW
559MW
355 MW
AJ Swope
OGALLALA
299 MW
RAILHEAD
WHITE RIVER
Sensitivity analysis around 200 Ogallala Evaluated
Panhandle WGRs meeting Section 6.9 of ERCOT Planning Guide (May 2014)

4. Investigation Objectives
Update steady state and dynamic analysis based on the latest set of generation resources meeting Section 6.9 requirements of the ERCOT Planning Guide
Evaluate ability of the as-is CREZ Panhandle transmission system to accommodate the units meeting the ERCOT requirements
Perform futuristic steady state voltage security, transient voltage stability and system strength assessments for the following scenarios
2nd circuit on the Alibates – Windmill – Ogallala – Tule Canyon loop
Large re-furbished SCs (320/250 MVAR)
Two (2) 50 MVAR SCs
Two (2) 100 MVAR SCs
Any other combinations derived from the Composite Short Circuit Ratio (CSCR) assessment
Key issues addressed by the analysis to date
Is the current system adequate to accommodate the resources meeting ERCOT Planning Guide Section 6.9 requirements (“FC Units”) from steady state and dynamic standpoint

5. Investigation Objectives
Key issues addressed by the analysis to date
Cost effective system additions required to accommodate FC Units + Signed IAs (with 2014 and 2015 COD – 4545 MW)
Evaluate effectiveness of 2nd loop only
Evaluate effectiveness of large re-furbished SCs only
Evaluate a combination of large re-furbished SCs and 2nd loop
Evaluate minimum amount of SC required to accommodate 4545 MW of generation in the Panhandle (in addition to the 2nd loop)
System Strength Assessment
Composite Short Circuit Ratio (CSCR) analysis for FC Units
Comparison with ERCOT’s WSCR approach
CSCR analysis for FC Units + Signed IAs (with 2014 and 2015 COD)
Additional SC requirements to ensure a minimum CSCR threshold to prevent control instability due to weak system conditions
Discussion on the “minimum CSCR/WSCR threshold” requirement
Wind turbine vendor survey – distinguish between POI SCR and CSCR/WSCR
Evaluate other options available in the industry for integration of wind generation to weak grids

6. Short Circuit Ratio Assessment
Study Approach
Composite SCR analysis performed to evaluate the following:
Assess the CSCR levels associated with Panhandle generation limited to FC units
Evaluate the Panhandle generation levels with FC units + Signed IAs (with COD of 2014 and 2015)
Identify adequate CSCR threshold level (for control stability) and associated SC requirements
Composite SCR definition
Model the gen tie line and the 34.5/345kV station transformer(s) associated with each WGR for the specific scenario
Gang the 34.5kV buses across all WGRs for the specific scenario (with a zero impedance connection)
Note that the fault contribution from the WGRs themselves is not included
Utilize the short circuit MVA on the 34.5kV bus in conjunction with the WGR capacity (MW) to determine CSCR
Perform the CSCR analysis across contingency conditions associated with loosing interfaces between Panhandle region and remaining ERCOT system

7. Short Circuit Ratio Assessment
Composite SCR Analysis – Illustrative Example

8. Short Circuit Ratio Assessment
Composite SCR Analysis – Panhandle Region – FC Units, 3263MW
(assuming Xs=Xd” = 0.2 per unit based on data from CREZ Reactive Power Study)
Composite SCR Analysis – Panhandle Region – FC Units + Signed IAs (2014/15), 4545MW

9. Dynamic Assessment
CREZ Panhandle Dynamic Assessment – Summary Results

10. Dynamic Assessment
Scenario 2-a, Sustained Oscillations, Tule Canyon – Tesla DBL Outage

11. Dynamic Assessment
Scenario 2-j, Damped Oscillations, Tule Canyon – Tesla DBL Outage

12. Short Circuit Ratio Assessment
Comparative Analysis, Composite SCR Vs ERCOT Proposed Weighted SCR Approach

13. Key Investigation Needs – System Strength Issues
Discussion around the “technical basis” for identification of WSCR/CSCR threshold requirement for Panhandle
Important to differentiate between POI SCR requirements (turbine manufacturers typically specify this) and system level or super-node SCR requirements
Limited experience across industry (including manufacturers) in relating system level SCR requirements to control stability – this is a system level requirement
Sharyland developed survey for various wind turbine manufacturers
Request minimum POI/terminal SCR requirements to ensure control stability
Provide illustrative example of WSCR and/or CSCR approach
Request commentary on minimum WSCR/CSCR requirements - if vendors can comment on this
Survey spanned all major turbine vendors expected to comprise Panhandle WGRs
Sharyland passed survey by ERCOT prior to distribution to turbine vendors and intends to share results with ERCOT also
Other options being explored by Sharyland to provide “safety margin” on Panhandle weak grid issue
Weak grid model provided by certain turbine vendors – tuned controls specifically for weak grid conditions
Observed to have material impact on voltage recovery and alleviates control stability oscillations

14. Key Investigation Aspects for Future
Voltage (pu)
Dynamic VAR Issue or System Strength Issue?

15. Key Investigation Aspects for Future
Weak Grid Control assists in voltage recovery and damps oscillations but does not eliminate oscillations for 100% dispatch
CREZ Panhandle Voltage Response w/o and with Weak Grid Control

16. Key Discussion Points
Over-all CREZ Panhandle voltage stability and adequacy of various solutions presented by Sharyland not the focus of discussion at this workshop
Investigations performed by ERCOT & Sharyland have demonstrated potential control instability issues in the Panhandle due to low SCR and weak grid conditions
Synchronous Condensers have been proposed as one of the potential solutions for boosting system strength
Amount and nature of SCs required are a function of the desired system strength
Sharyland stressed on the need for a “technical basis” to arrive at a threshold for the CSCR/WSCR to be utilized for determining any additional equipment requirements
Sharyland also requested ERCOT to evaluate other options
Weak grid models for turbines expected to be utilized in the Panhandle
Wind turbine vendor survey – Sharyland conducted one and summary results are presented in this discussion
Any other technical analysis to further investigate the cause and potential mitigation for control instability oscillations stemming from weak grid conditions
Combination of weak grid models, SCs and other mitigation options expected to require close collaboration between WGRs and the operating TSP in the region

17. Wind Turbine Vendor Survey - Summary
Survey Request #1: Provide the minimum network SCR requirements at the POI that can be adequately handled by your wind turbine(s) when utilizing standard controls, without any control instability issues. If the turbine vendor has performed some tests to demonstrate the adequacy of the standard controls for the said SCR threshold, Sharyland requests documentation of the same.
Wind Turbine Vendor Response
Response #1 – 34.5kV station (so SCR limitations at POI will be higher and depend on the impedance between the 34.5kV and POI)
Response #2 – Current design based on an SCR of 5 or greater at the medium voltage bus (although could go lower if required and based on specific designs) Response #3 – Anywhere between 1.5 and 2.5 depending on the implementation of control modifications (or not)

18. Wind Turbine Vendor Survey - Summary
Survey Request #2: Turbine vendors offer “weak grid” versions of their turbine controls (both in terms of model and actual variations in controls in the field) that are specifically designed for turbines that are expected to connect to weak grids. In relation to the above, please provide the following information:
Does the turbine vendor support a “weak grid” version of the turbine?
Response #1 – Site specific studies may allow vendor to go lower than standard
Response #2 – Vendor has developed control parameters for low SCR applications (as low as 1.5) and applied them as needed
Response #3 – No, currently there is no specific WTG version for weak grids marketed at this time
Are the modifications required to derive a “weak grid” version limited to firmware upgrades for control loop settings or does the vendor anticipate major changes in the turbine design to derive the weak grid version?
Response #1 – Modifications made to the plant were strictly software settings
Response #2 – Mainly software changes, but improved HW filtering may be needed
Response #3 – Limited to control changes

19. Wind Turbine Vendor Survey - Summary
Survey Request #2: Turbine vendors offer “weak grid” versions of their turbine controls (both in terms of model and actual variations in controls in the field) that are specifically designed for turbines that are expected to connect to weak grids. In relation to the above, please provide the following information:
Does the turbine vendor have a “Weak grid” model in PSS/e to be studied as part of the planning studies?
Response #1 – Vendor’s experience that weak grid instabilities cannot be simulated accurately in PSS/E. Although most of the setting modifications associated with weak grid operation can be implemented in the PSS/E model, the basic instability associated with weak grid interconnections (i.e., oscillatory instability) cannot be simulated reliably in a simplified positive sequence model like PSS/E.
Response #2 – Model is the same but some parameters will be tuned
Response #3 – Vendor doesn´t have a special PSSE model of weak grid connections
Can the turbine vendor confirm that this weak grid version has been deployed successfully in the field in some part of NA or across the world with similar weak grid issues?
Response #1 – N/A
Response #2 – Yes, with an SCR of 1.3 for several months now
Response #3 – Yes, with an SCR of 1.5 as reported in publications

20. Wind Turbine Vendor Survey - Summary
Survey Request #3: Provide the minimum CSCR requirements for a region such as the Panhandle based on the representative example provided in Appendix A
Response #1 – Use of a single simplified regional parameter of merit, like the CSCR or WSCR, can lead to incorrect conclusions, particularly if mixed technologies (e.g., full converter and DFIG) are used. Hence no recommendations
Response #2 – 1.7 standard. Will need site specific studies to determine how much lower we can go.
Response #3 – 5 (POI SCR) along with additional comment that dynamic simulations are needed to clarify the impact of other WPPs in the area and come with a final number. No comment on CSCR threshold
Survey Request #4: Provide the minimum WSCR requirements for a region such as the Panhandle based on the representative example provided in Appendix B
Response #1 – 5 (POI SCR) along with additional comment that dynamic simulations are needed to clarify the impact of other WPPs in the area and come with a final number. No comment on WSCR threshold
Response #2 – Information to answer request is not available. A special Panhandle area specific study will be needed to determine the answer to this question.
Response #3 – Use of a single simplified regional parameter of merit, like the CSCR or WSCR, can lead to incorrect conclusions, particularly if mixed technologies (e.g., full converter and DFIG) are used. Hence no recommendation

21. Additional Investigation by Sharyland
CSCR/WSCR “purely indicative metrics” of potential control instability when viewed in isolation
Additional uncertainty due to absence of any industry standard on “comfortable thresholds” for such metrics
Such metrics are also system dependent since both CSCR and WSCR are not based on a single POI location albeit on a combination of few stations (weak region of the grid)
Hence the “non-committal” response on CSCR/WSCR limits by turbine vendors
Need to further develop a “technical theory” that links WSCR/CSCR with necessary/sufficient conditions for damped and/or growing oscillations
Key ingredients for development of such a theory
Voltage controller gain estimation – As viewed from the grid
System strength estimation – As a function of volt-VAR variation
Under varying conditions of interest i.e. critical contingencies
Linking the above to system strength and SCR conditions
Arrive at necessary and sufficient condition for damped system oscillations
Possibly also at the same N&S condition for un-damped system oscillations
DNV GL/PWR Solutions, in collaboration with Sharyland, will present an overview of such a “technical approach

22. Questions ?