Respect for the Individual · Humility · Innovation · Excellence · Teamwork · Integrity Integrating Wind on Weak Grids – CREZ Panhandle Region Sharyland

Respect for the Individual · Humility · Innovation · Excellence · Teamwork · Integrity

Integrating Wind on Weak Grids – CREZ Panhandle RegionSharyland UtilitiesJuly 16, 2014

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BackgroundIn 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

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Background

Panhandle WGRs meeting Section 6.9 of ERCOT Planning Guide (May 2014)

ALIBATES

WINDMILL

TULE CANYON

COTTONWOOD

GRAY

0 MW

500MW

750MW

400 MW

559MW

355 MW AJ Swope

OGALLALA

299 MW

RAILHEAD

WHITE RIVER

400 MW

Sensitivity analysis around

200 MW @ Ogallala

Evaluated

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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

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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

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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

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Composite SCR Analysis – Illustrative Example

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Composite SCR Analysis – Panhandle Region – FC Units + Signed IAs (2014/15), 4545MW

Composite SCR Analysis – Panhandle Region – FC Units, 3263MW(assuming Xs=Xd” = 0.2 per unit based on data from CREZ Reactive Power Study)

Total Panhandle WGR Capacity (MW)

Contingency3 phase fault at "ganged

34.5kV bus" (A)

Short Circuit (MVA)

Composite Short Circuit Ratio (SCR)

Required Synch. Condenser (MVA)

for SCR = 1.2 No Contingency 81712 4882.76 1.07 114.15

Loss of double circuit Tule Canyon - Tesla 345kV line 66695 3985.41 0.88 293.62Loss of double circuit Tule Canyon - Cottonwood 345kV line 74194 4433.52 0.98 204.00

Loss of double circuit Tule Canyon - Ogallala 345kV line 79951 4777.53 1.05 135.20Loss of double circuit Tule Canyon - Alibates 345kV line 81035 4842.31 1.07 122.24

Loss of double circuit Gray - Tesla 345kV line 75327 4501.22 0.99 190.46

4544.6

Total Panhandle WGR Capacity

(MW)Contingency

3 phase fault at "ganged 34.5kV bus"

(A)

Short Circuit (MVA)

Composite Short Circuit Ratio (SCR)

Required Synch. Condenser (MVA)

for SCR = 1.2

No Contingency 73266 4378.06 1.342 NA

Loss of double circuit Tule Canyon - Tesla 345kV line 62665 3744.59 1.148 34.20

Loss of double circuit Tule Canyon - Cottonwood 345kV line 66478 3972.44 1.217 NA

Loss of single circuit Tule Canyon - Ogallala 345kV line 70724 4226.17 1.295 NA

Loss of double circuit Tule Canyon - Alibates 345kV line 69131 4130.97 1.266 NA

Loss of double circuit Gray - Tesla 345kV line 65119 3891.23 1.193 4.87Loss of single circuit Tule Canyon - Ogallala & Tule Canyon - Alibates

345kV lines68607 4099.66 1.26 NA

3263

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Dynamic Assessment

CREZ Panhandle Dynamic Assessment – Summary Results

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Base Case with Panhandle WGRs meeting Section 6.9 of Planning Guide

None Stable

2-a 2nd circuit on SU Loop Unstable

2-b Large SC (320/250) @ WM/OG Unstable

2-c Large SC (320/250) @ AB/WM Unstable

2-d Large SC (320/250) @AB/TC Unstable

2-e 2nd circuit on SU Loop & Large SC (320/250) @WM/OL Stable

2-f 2nd circuit on SU Loop and 50/50 SCs on WM/OL Unstable

2-g 2nd circuit on SU Loop and 100/100 SCs on WM/OL Sustained Oscillation

2-h 2nd circuit on SU Loop and 100/100 SCs on TC/AB marginally Stable

2-i 2nd circuit on SU Loop and 150/100 SCs on AB/TC Stable

2-j 2nd circuit on SU Loop and 100/150 SCs on AB/TC Stable

Scenario #

2015 Case with Panhandle WGRs meeting Section 6.9 & Ias with COD of 2015

Scenario Description Transmission Upgrades Key Observations

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Dynamic Assessment

Scenario 2-a, Sustained Oscillations, Tule Canyon – Tesla DBL Outage

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Dynamic Assessment

Scenario 2-j, Damped Oscillations, Tule Canyon – Tesla DBL Outage

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Comparative Analysis, Composite SCR Vs ERCOT Proposed Weighted SCR Approach

Total Panhandle

WGR Capacity

(MW)

ContingencyComposite

Short Circuit Ratio

Weighted Short Circuit

Ratio (ERCOT)

No Contingency 1.342 1.272Loss of double circuit Tule Canyon - Tesla 345kV line 1.148 1.072

Loss of double circuit Tule Canyon - Cottonwood 345kV line 1.217 1.017Loss of single circuit Tule Canyon - Ogallala 345kV line 1.295 1.172

Loss of double circuit Tule Canyon - Alibates 345kV line 1.266 1.089Loss of double circuit Gray - Tesla 345kV line 1.193 1.096

Loss of single circuit Tule Canyon - Ogallala & Tule Canyon - Alibates 345kV lines

1.26 1.09

3263

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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

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Key Investigation Aspects for Future

Dynamic VAR Issue or System Strength Issue?

Channel Plot

4813 - VOLT 560306 [G09-020-TAP 69.000] : fault_pwr_Siemens_new(VD4-108m)-wgc-enabledgfedcb4813 - VOLT 560306 [G09-020-TAP 69.000] : fault_pwr_Siemens-original-VD4modelgfedcb

Time (seconds)109876543210

1.1

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

Before MitigationAfter Mitigation

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Key Investigation Aspects for Future

2277 - VOLT 79501 [OGALLALA 345.00] : 3PH-4CY-NZRTH-LNDRAW-DCKT-Option#1gfedcb2282 - VOLT 79501 [OGALLALA 345.00] : 3PH-4CY-NZRTH-LNDRAW-DCKT-Option#1Agfedc2277 - VOLT 79501 [OGALLALA 345.00] : 3PH-4CY-NZRTH-LNDRAW-DCKT-Option#1Bgfedcb2277 - VOLT 79501 [OGALLALA 345.00] : 3PH-4CY-NZRTH-LNDRAW-DCKT-Option#1Cgfedc

Time (seconds)2017.51512.5107.552.50

1.1

1.05

1

0.95

0.9

0.85

0.8

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

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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

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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 – 1.7 @ 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)

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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

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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

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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

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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

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Questions ?

Documents

Respect for the Individual · Humility · Innovation · Excellence · Teamwork · Integrity Integrating Wind on Weak Grids – CREZ Panhandle Region Sharyland