Series Compensation and SSR - Concepts Resource Integration Department

Objectives At the completion of this course you will be able to: Identify the purpose of series compensation Identify at least two places in ERCOT where series compensation is installed Identify scenarios of risk for subsynchronous resonance Identify the methods used to mitigate subsynchronous resonance

Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. How does this impact Operations? Example SCADA Alarms.

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. Answer for question #1

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 Answer for question #2 Clear Crossing – Willow Creek is currently on hold, if SSR issues are resolved then it could be in by December 2015

What are series capacitors? What is SSR? What causes SSR? Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. How does this impact Operations? Example SCADA Alarms.

Sub-Synchronous Resonance (SSR) Resonance: The tendency of a system under excitation to oscillate at certain frequencies. Subsynchronous Resonance: A phenomena where growing quantities of power are exchanged between equipment at frequencies lower than 60 Hz. Has the potential to break generator shafts and damage transmission equipment. Adding Series Compensation to a power system introduces SSR concerns. Good Resonance Bad Resonance

How does SSR happen? ~ Inductive Capacitive Shunt Capacitors ~ Inductive Capacitive Generators, Inductors, Load Most power system elements are inductive.

Generators, Inductors, Load How does SSR happen? Shunt Capacitors ~ Series capacitors Inductive Capacitive Generators, Inductors, Load When inductive and capacitive equipment match at certain frequencies, they create complimentary energy tanks, which can give rise to resonance.

Who’s At Risk? -- General Observations More Risk: Electrically closer to series capacitors. Long shaft / multi-mass generators (Coal, NG Steam, Combined Cycle). Most wind farms in ERCOT Less Risk: Hydro, CTs, reciprocating engines. Solar inverters. HVDC ties. Because SSR is a concern for all generation technologies, all generators are being reviewed. Answer for question #3

Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. Mitigating and Protecting. How does this impact Operations? Example SCADA Alarms.

South Texas SSR Event (2009) Series capacitors installed on long 345 kV lines to allow full loading. 600 MW of wind farms connected to Ajo. 345 kV series compensated lines Yellow color not good for projection screen. See comment jc1

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

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.

Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. How does this impact Operations? Example SCADA Alarms.

SSR: Poor Visibility SSR is not visible in STNET, RTCA, etc. SSR is not visible in dynamic tools such as TSAT. SSR only visible in specialized planning tools (PSCAD).

Preventing SSR How do you prevent SSR? Most prevention done in planning. Planning conducts or contracts SSR studies. Studies must incorporate wide variety of operating scenarios Scenarios identified as being at risk must be incorporated into an operating procedure matrix. Similar as how West-to-North stability limit was handled before online TSAT studies were done.

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: Willow Creek – Hicks, Willow Creek – Parker, and Willow Creek - 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.

Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. How does this impact Operations? Example SCADA Alarms.

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

Presentation Outline What are series capacitors? What is SSR? What causes SSR? Preventing SSR. How does this impact Operations? Example SCADA Alarms.

Example of SSR SCADA Alarm for Big Brown Big Brown unit is one of the existing units prone to SSR Detailed studies show will have destructive resonance under four simultaneous outages (N-4 condition). Thus, for example, design alarm at an N-3 situation, so alarm sounds when generator is one outage away from resonance and operators have time to act.

Example Display #1 – Line Monitoring Source: Lonestar SSR presentation at the NPRR 562 workshop

Example Display #1 - Alarms Source: Lonestar SSR presentation at the NPRR 562 workshop

Example Display #2 – SSR Alarms Source: ERCOT Advanced Network Applications/EMMS Development

Example Display #2 – SSR Details Source: ERCOT Advanced Network Applications/EMMS Development

Example Display #2 – SSR Actions Source: ERCOT Advanced Network Applications/EMMS Development

Questions?

What is the primary purpose of adding series compensation to a long transmission line? To allow the line to deliver more power by reducing the impedance and increasing the stability limit To reduce line sag by increasing the thermal rating of the conductor

Locate two existing or planned series compensated lines in ERCOT. Edith Clarke – Clear Crossing 345kV and Lon Hill – Rio Hondo 345kV South Texas Project – White Point 345kV and South Texas Project – W.A. Parish 345kV Dermott – Cottonwood 345kV and Odessa – North McCamey 345kV None of the above

Which of the following scenarios poses a higher risk for SSR? A 765 MW combined-cycle plant connected directly to a series compensated 345kV line A 25 MW solar photovoltaic farm connected to a 69kV line electrically distant from series compensation

Due to forced outages, a generator is one line outage away from a SSR condition. A mitigation plan exists to bypass the series compensation. The transmission operator should __________. Take action immediately Wait, there will be time to bypass the series capacitors if the last line trips out.

Under normal conditions, bypassing the capacitors on a 50% series compensated line should _________ the power flow on the line due to the line impedance doubling. Decrease Increase