Subsynchronous Oscillations (SSO) and PowerWorld Applications at ERCOT Anuj Dixit and Priya Ramasubbu ERCOT Planning and Grid Co-ordination May 21, 2014

The Electric Reliability Council of Texas (ERCOT) ERCOT ISO manages the flow of electric power to 23 million Texas customers - representing 85 percent of the state's electric load. Performs financial settlement for the competitive wholesale bulk-power market Enables retail electric choice for Texas customers

Presentation Outline *CREZ Network and Resonance What causes SSO? Studying and Preventing SSO How does this impact transmission planning and operations? PowerWorld Applications *Competitive Renewable Energy Zone

Definition Competitive Renewable Energy Zone – a geographic area in the state of Texas initially identified by AWS Truewind (as ERCOT’s consultant) as an area suitable for multiple 100 MW wind development projects. Of the original 25 zones the Public Utility Commission (PUC) chose a small set based on input from potential windfarm developers.

CREZ Areas

What and Where New CREZ 345kV lines built both inside ERCOT load-serving areas and far into areas where load is served by SPP. CREZ will not add any new connections between ERCOT and SPP. CREZ areas are Panhandle A, Panhandle B, Central, West, and McCamey. About 2300 miles new ROW First est $5B, higher now.

CREZ 345kV Lines Sam = Venus - Tradinghouse Navarro = crossing point of Limestone - Watermill & Venus - Big Brown New subs in existing lines.

Series Compensation The use of capacitors connected inline with a transmission line. Benefits: Increases power flow by reducing line impedance. Relieve bottlenecks. Increases stability and reduces voltage variation.

Series Compensation in ERCOT CREZ Tule Canyon – Tesla Edith Clarke – Clear Crossing Dermott – Clear Crossing West Shackleford – Sam Switch/Navarro Big Hill – Kendall Clear Crossing – Willow Creek (2016) Rio Grande Valley Lon Hill – Rio Hondo Lon Hill – Edinburg Lobo – Edinburg (2016) HorseHollow GenTie

How Resonance Can Occur in Power System Complementary Storage Elements creates resonant circuit (e.g. generator & series cap). Generator introduces negative damping. Low frequency power oscillations develop. Generator Load ~ Normal Energy Load Generator Series Capacitor Normal Energy ~ Oscillations

Mohave SSO Incident (1970) An example of SSO Torsional Interaction Mohave generator: 1,580 MW coal-fired in NV. Gradually growing vibration that eventually fractured a shaft section. First investigations incorrectly determined cause. After 2nd failure in 1971 cause was identified as Subsynchronous Resonance interaction with nearby series capacitors. An electrical resonance at 30.5 Hz excited a mechanical resonance at 30.1 Hz. Problem was cured by reducing compensation percentage and installing a torsional relay. D. Baker, G. Boukarim, “Subsynchronous Resonance Studies and Mitigation Methods for Series Capacitor Applications,” IEEE 2005. D. Walker, D. Hodges, “Results of Subsynchronous Resonance Test At Mohave,” IEEE 1975.

CREZ Network and Resonance What causes SSO? Presentation Outline CREZ Network and Resonance What causes SSO? Studying and Preventing SSO How does this impact transmission planning and operations? PowerWorld Applications

South Texas SSR Event (2009) A fault occurred on the Ajo to Nelson Sharpe line. Fault cleared in 2.5 cycles by opening this line. The wind farms were then radially connected to the Ajo to Rio Hondo series compensated transmission line. Undamped oscillations at 22 Hz. Voltages at generator doubled in ~150 ms. Damage to wind generators and series capacitors occurred. Add additional slides of graphs of the event From AEP presentation by Paul Hassink, “Sub-synchronous Control Interaction,” Utility Wind Integration Group Spring Workshop April 15, 2011 Also: http://www.elforsk.se/Global/Vindforsk/Konferenser/HF_symposium_111206/Gotia_Power_V309_subsynchronus_resonence.pdf

South Texas SSR Event (2009) Series capacitors installed on long 345 kV lines to allow full loading. 1,000 MW of wind farms connected to Ajo. 345 kV series compensated lines Yellow color not good for projection screen. See comment jc1

Lessons Learned – South Texas 2009 Wind farms are vulnerable to SSO due to nearby series capacitors. Understanding and analysis techniques have advanced considerably. Wind farms are now routinely analyzed for risk. AEP resolved this problem by installing protection to automatically trip the wind farm.* * This solution was site-specific. In other locations, protective tripping may not be appropriate as a first defense.

Who’s At Risk? -- General Observations More Risk for SSO: Electrically closer to series capacitors. Long shaft / multi-mass generators (Coal, NG Steam, Combined Cycle). Less Risk: Hydro, CTs, reciprocating engines. Solar inverters. HVDC ties. Because SSO is a concern for all generation technologies, all generators are reviewed. ERCOT notifies developers whose units might be at risk. Answer for question #3

Presentation Outline CREZ Network and Resonance What causes SSO? Studying and Preventing SSO How does this impact transmission planning and operations? PowerWorld Applications Conclusion.

SSO Risk Assessment TOPOLOGY TEST TEN or less contingencies to radial? This simple test is performed by inspection: Does it take ten or fewer outages to make a generator radial to a series cap? This test will clear many projects, especially those far from the series capacitors and those connecting to the lower voltage network. This tests whether the network electrical characteristics are conducive to resonance. ERCOT performs this test as part of the transmission planning and GINR process. This tests the affected outage combinations to see whether they solve in a min load case. If the case doesn’t solve or has many overloads, the outage isn’t credible. If credible SSR risk exists, then a study should be performed. In lieu of a study, generator resources may obtain a letter from their manufacturer or they may also rework their proposed interconnection to reduce risk exposure. TOPOLOGY TEST TEN or less contingencies to radial? ERCOT SSO Screening Grid-side frequency scan shows risk? At 6 or fewer concurrent outages? POWER-FLOW TEST Outage combination solves in steady-state w/ few overloads? DETAILED STUDY REQUIRED Or rework the proposed interconnection. Or obtain letter from gen. manufacturer.

How Study for SSO? Frequency scans Time-Domain Simulation Graph resistance & reactance vs. frequency. Look for dips & crossovers. Less accurate so designed to be conservative. If frequency scan shows possible exposure risk, EMT1 simulation may be able to dismiss the exposure risk. EMT simulations are more accurate. 1 Electromagnetic Transient simulation: A time-domain analysis similar to a dynamic or “stability” analysis but capable of simulating off-nominal frequencies other than 60 Hz. Such simulations generally require more detailed models.

Detailed Study – Results & Action (As proposed in NPRR562) Detailed study will determine the system configurations that result in SSO, particularly the number of concurrent outages required. Procedural Mitigation: Outage Coordination procedures to avoid scheduling outages that would place a unit at increased risk. Structural Mitigation: Redesign, controller tuning, etc.   Potential Risk (# of Concurrent Transmission Outages) Required Action Responsible Entity ≤ 3 Protection and Structural Mitigation All affected ≤ 4 Protection 4, 5, or 6 Procedural Mitigation All affected and ERCOT

Protection vs. Mitigation? Involves forced tripping (removal of generator or series capacitor). Disruptive for a system that is already in a weakened state due to outages (“double blow”). Generally recommended as backup means of defense. Mitigation Involves reducing exposure to SSO risk. Generally allows the resource to continue operating, even when outages place the unit in stronger electrical coupling with a series capacitor. In many cases, may completely eliminate risk. E.g. Horse Hollow Energy Center installed mitigation which allowed the wind turbines to operate radially to the series-compensated transmission line owned by NextEra.

How to Solve SSO Problems? Mitigation Options: Control system upgrades (common for wind). Re-design of generating unit. Passive Filters. Some transmission level options available. Protection Options: Torsional relays (detect shaft twisting on conventional units). Subsynchronous Current relays.

Presentation Outline CREZ Network and Resonance What causes SSO? Preventing SSO How does this impact transmission planning and operations? PowerWorld Applications

Effect of Outages Outages are the best indicator for gauging risk for SSR. Five double-circuit outages make Big Brown radial to W.Shackelford – Navarro series compensated line.

Example Timeline of an SSR Event For a Generator that is “N-4 Risk” 5 seconds 10 days 8 days EMS Alarm EMS Alarm 1st Planned Outage 2nd Planned Outage 3rd Planned Outage Forced Outage Generator Damaged Take Action!!! Answer for question #4 SSR can happen very fast. Action should be taken well ahead of time.

Preventing Risk Example ERCOT Planning identifies all generators that might be at risk. ERCOT Planning orders detailed studies. Detailed studies indicate which outages place a generator at risk. EXAMPLE: For Wise County to be at risk, the following lines must be all out of service: WillowCreek – Hicks, WillowCrk – Parker, and WillowCrk - Jacksboro ERCOT Outage Coordination ensures that no two combinations of these outages go out at the same time. If unavoidable, takes mitigation action. Outage coordination mitigates risk.

Preventing Risk (Continued) Because Outage Coordination is monitoring, SSR is unlikely to become a Real Time issue. However, if it does… One of the new SSR Alarms in the Control Room sounds. Operators follow written mitigation procedures. Example Mitigation Procedure Call TSP. Ask if tripped line can be put back in service. If unable to re-energize, ask TSP to study bypassing series capacitor. TSP studies, sees minor adjustments required (e.g. move switched shunt or enforce a partial wind curtailment). TSP bypasses the series capacitor. SSR risk is averted. When possible, operators look for opportunity to place capacitors back in service and cancel the wind curtailment.

Role of ERCOT Planning in SSR ERCOT analyzed risk exposure of all existing and proposed power plants. All exposed plants are undergoing study. Several thermal and wind plants are already moving towards resolution. Until resolution, series capacitors will not be energized.* * If situation involves a new generation resource and existing series capacitors, then the new resource would not be allowed to energize.

How does this impact operators? List of units prone to SSR may have to be closely monitored under stressful system conditions Keeping an eye out for extreme contingencies near the series capacitors This will be done with the help of new SCADA displays and alerts

Bypassing: The “Emergency Kill Switch” Series Capacitor Current Interrupt Close to Bypass Bypassing removes the series capacitor from service but leaves the line still in operation. With capacitor removed, no SSR risk.

Bypassing: What effect will it have? Line impedance will double, thus less power will flow on line (and more on neighboring lines). Bus voltages may change by ~3%. A new voltage stability limit may appear. Because of this, it is recommended to run an STNET and possibly VSAT study before bypassing. Some generation adjustments may be necessary (e.g. wind curtailment). Answer for question #5

Example Display – SSR Alarm Source: ERCOT Advanced Network Applications

Example Display – SSR Alarm cont. Source: ERCOT Advanced Network Applications

Example Display – SSR Details Source: ERCOT Advanced Network Applications

Example Display – SSR Actions Source: ERCOT Advanced Network Applications

Presentation Outline CREZ Network and Resonance What causes SSO? Preventing SSO How does this impact transmission planning and operations? PowerWorld Applications

ERCOT System Planning ERCOT planning performs coordinated planning studies with input from NERC registered Transmission Planners (TPs), Transmission Owners (TOs) and other stakeholders to address region-wide reliability and economic transmission Performs transmission studies/assessment (both near-term and long-term planning horizon) to meet ERCOT and NERC standards (TPL-001-004, PRC-023, FAC-013) Assesses resource availability of existing units and future generators Load forecasting for the near-term and long-term Planning Horizon

Regional Transmission Plan (RTP) - Objective Regional Transmission Plan is developed annually by ERCOT in coordination with the Regional Planning Group (RPG) and the Transmission Service Providers (TSPs) Annual assessment to identify transmission needs of ERCOT system over the next six years Projects identified to meet the ERCOT/NERC reliability requirements (Reliability projects) and to reduce system congestion (Economic projects) that meet the ERCOT economic criteria

ERCOT RTP Process Case conditioning Initial start cases, contingency list and list of overloads ready N-1 SCOPF analysis G-1 screening followed by G-1+N-1 analysis X-1 screening followed by X-1+N-1 analysis Reliability analysis complete with all reliability issues resolved Economic case preparation Economic analysis Prepare the final RTP report

Use of PowerWorld in RTP

Case Building Assign ERCOT weather zones to buses and loads Update AGC flags for non-conforming loads Load scaling Control options for generators (Fast start, AGC, AVR, Enforce MW Limits) Geographic onelines

SCOPF / Contingency Analyses Import cost curves for generators Load throw over information Conditional modeling and global actions for SPSs SimAuto

PowerWorld Settings

Options and Settings in PowerWorld Solution Settings Common Options Enforce Generator MW Limits Island Based AGC Participation Factors LP / OPF Settings Constraint Options Maximum Violation Cost Control Options Fast Start Areas and Super Areas for OPF

Solution Settings

Options and Settings in PowerWorld (Contd.) Contingency Analysis Options Basics Full Power Flow Generator Participation Factors Advanced Limit Monitoring Do not report base case violations SCOPF Options Solve base case using optimal power flow

SCOPF Options

New Feature Recommendation N-1-1 capability with system adjustments