1. Sub-Synchronous Problems and Prevention in ERCOT
John Adams
Principal Engineer
Resource Integration
RPG
September 16, 2011

2. Series Compensation
Series compensation is the addition of capacitor banks in series with a transmission line.
Series compensation can improve the stability and voltage controllability of the network, and it makes it possible to install fewer lines total.
Series compensation can also cause some generators to experience destructive sub-synchronous phenomena.
September 16, 2011
RPG

3. Series Compensated Lines in ERCOT
Existing (3):
Rio Hondo – Ajo 345kV
Edinburg – Lon Hill 345kV
Privately-owned Horse Hollow – Kendall 345kV
Future (13):
Two Silverton – Tesla 345kV
Two Edith Clarke – Clear Crossing 345kV
Two Dermott – Clear Crossing 345kV
Two Clear Crossing – Willow Creek 345kV
West Shackleford – Sam Switch 345kV and West Shackleford – Navarro 345kV
Two Big Hill – Kendall 345kV
Lobo – North Edinburg 345kV
All of these lines are 50% compensated.
September 16, 2011
RPG

4. Existing and Future Series-Compensated Lines in Valley
Lon Hill
Lobo
Lon Hill
Ajo
Ajo
Edinburg
Rio Hondo
Edinburg
Rio Hondo
September 16, 2011
RPG

5. Existing & Future Series Compensation to Kendall
Horse Hollow
Big Hill
Kendall
September 16, 2011
RPG

6. Future CREZ Series Compensation
Silverton
Tesla
Edith Clarke
Willow Creek
Clear
Crossing
D-FW
Dermott
West
Shackleford
Navarro
Sam Sw
September 16, 2011
RPG

7. Two Well-Known Sub-synchronous Problems
SSR = sub-synchronous resonance, an interaction between series capacitors and a long-shaft multi-mass generator. This can cause the generator shaft to be twisted, cracked, or even broken.
IGE = induction generator effect, the self-excitation of an induction machine due to the negative resistance of the machine being greater in magnitude than the positive resistance of the network at the system natural frequencies.
September 16, 2011
RPG

8. Early Sub-Synchronous Resonance Example: 1970 and 1971
In 1970 Mohave #2 experienced SSR due to being operated while connected radially to the rest of the system through a single 70% series-compensated 500kV line. The torsional oscillations caused the generator shaft to heat up so much that electrical insulation failed. The subsequent arcing burned a hole in the shaft.
The second failure a few months later occurred because the root cause analysis of the first failure was incorrect and the generator owner still didn’t know how to avoid It.
This information is part of the EPRI SSR Tutorial.
September 16, 2011
RPG

9. SSR Damage to Shaft
September 16, 2011
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10. Two Newer Sub-Synchronous Problems
SSTI = sub-synchronous torsional interactions, undesirable interactions between the mechanical mass system of a generator and power electronic devices such as HVDC terminals, static var compensators, or wind turbines.
SSCI = sub-synchronous control instability, undesirable interactions between series compensation and power electronic devices such as wind turbines, HVDC terminals, or static var compensators.
September 16, 2011
RPG

11. Sub-Synchronous Control Instability Example: ERCOT 2009
Series of Events
Ajo – Nelson Sharpe 345kV line was faulted single phase to ground and tripped out of service.
Two wind farms outside of Zorillo became radially connected to the Ajo – Rio Hondo series-compensated 345kV line.
The system experienced SSCI-induced overvoltages up to about 195% which caused the Ajo – Rio Hondo line to trip.
Both windfarms suffered numerous crowbar circuit failures.
Fault recorders indicated sub-synchronous currents at the series capacitors during the event.
September 16, 2011
RPG

12. Screening Study Section 1.6, part 1
“This interconnection point is vulnerable to sub-synchronous interactions (SSI) with nearby series capacitor banks and should be studied to ensure that these interactions are not a threat to the ERCOT Transmission Grid or to the proposed Resource. It is the interconnecting entity’s responsibility to select, purchase, and install turbines that are compatible with the series compensation and to provide ERCOT documentation that conclusively establishes that the project will not cause SSI problems.
September 16, 2011
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13. Screening Section 1.6, part 2
An interconnecting entity may meet this requirement by providing ERCOT with a letter from the turbine vendor stating the following:
What model and type of turbine the vendor is providing for this project.
That the vendor has studied sub-synchronous interactions for that turbine model in PSCAD or Matlab, or with equivalent engineering tools.
That the vendor’s turbines will not create sub-synchronous problems with series compensation on the ERCOT system.
Who performed the study, what system model was used in the study, and when the study was performed.
September 16, 2011
RPG

14. Who Is Responsible for What – ERCOT Staff
Educate the ERCOT market participants about the issue and establish clear rules.
Prevent future installation of equipment that will cause sub-synchronous problems for existing assets, unless appropriately mitigated.
Review studies and proposed mitigation efforts.
Obtain confidential data and non-disclosure agreements to allow their use in studies.
September 16, 2011
RPG

15. Who Is Responsible for What – Generator Owners
Owners of Existing Generators
Provide turbine design data to ERCOT when requested. The original turbine manufacturer may charge a fee to research this design data if it is not already at hand.
Review SSR studies involving your plant and make informed decisions whether or not to install additional protection, such as on-site filters or relaying.
September 16, 2011
RPG

16. Who Is Responsible for What – Generator Owners
Developers of Future Generators
Inform your turbine vendors of the location of the intended project and of all ERCOT series compensation, existing and known future installations.
Select a plant design that is compatible with the existing ERCOT grid and the known future series-compensated lines.
If requested: submit proper documentation that your proposed turbine and plant design have been studied and shown not to have sub-synchronous problems.
September 16, 2011
RPG

17. Who Is Responsible for What – Transmission Owners
If installing series compensation: detailed sub-synchronous analysis of possible interactions between the series compensation and existing vulnerable generators.
Review studies with ERCOT staff and the generator owner.
When needed: develop mitigation alternatives and if necessary incorporate the mitigation.
These are also required for HVDC development.
September 16, 2011
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18. Who Is Responsible for What – Turbine Manufacturers
Educate your design and sales staff about the issue and about ERCOT’s current and future use of series compensation.
If selling to a client for use in ERCOT, check that your turbine design will not experience sub-synchronous problems due to series compensation.
Recognize sub-synchronous currents and tripping the unit or plant is not acceptable as the primary response.
September 16, 2011
RPG

19. Who Is Responsible for What – Turbine Manufacturers
If selling to a client for use in ERCOT
“Riding through” a sub-synchronous event is not acceptable because the change of topology that initiated the event may be long-term rather than temporary.
ERCOT can provide a PSCAD worst-case testing module for your use.
Provide PSCAD turbine model to ERCOT on request. A black-boxed version of the detailed model will be sufficient if all user-defined constants and variables are left as inputs to the black-box section.
September 16, 2011
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20. Take Away
If you are developing a project in ERCOT, discuss this issue with your generator vendor and design your project to avoid sub-synchronous problems.
Tripping the generator off-line is not acceptable as the primary response to SSR or SSCI but is allowed as the backup level of protection.
If you are designing a facility which relies on inverters, you must consider both the fault current capability of the network at the interconnection point, and the total amount of nearby inverter-based equipment.
September 16, 2011
RPG