Sub-Synchronous Resonance Protection & Mitigation

1. Sub-Synchronous ResonanceProtection & Mitigation John Adams Principal Engineer Planning Working Group - PLWG

2. What is Resonance • Resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. Frequencies at which the response is maximized are known as a system’s resonant frequencies. • At resonant frequencies, even small periodic drivers can produce large amplitude oscillations, because the system stores vibrational energy. • A common example of resonance is a playground swing, or pendulum, which has a natural frequency. • Resonance is what makes stringed instruments vibrate at a characteristic frequency for a given string length. • In resonant electrical systems, energy flows (oscillates) between the collapsing magnetic field of an inductor, and the charging of a capacitor.

3. What is Resonance in electrical circuits • In a “Tank” or LC circuit, a single pulse will induce an exchange energy between an inductors magnetic field, and a capacitors electrical field at a characteristic frequency of 1/√LC; exhibiting resonance.

4. Resonance in Series Compensated lines • When a series capacitor is added into a transmission line, it creates a resonant frequency. This is not a problem as long as energy is not injected at the resonant frequency. However, an interconnection which injects power at the resonant frequency can lead to damage.

5. Types of SSR • “Classic SSR” • Electric system resonates with generator mechanical system. • Note, even well-damped resonance can be harmful. May cause shaft fatigue. • SSTI: Sub-Synchronous Torsional Interaction • Interaction between a power electronic device (e.g. HVDC, SVC, wind turbine) and a generator mechanical system. • Possible where HVDC installed near conventional generator or series capacitor near a wind turbine. • SSCI: Sub-synchronous Control Instability • Interactions between power electronics (e.g. HVDC, SVC, wind turbine) and a series-compensated system. • Does not involve a mechanical component. Meeting Title (optional)

6. Protection Options Non-selective schemes should only be used for backup protection, not primary, due to risk of cascading events (large numbers of generator trips) * May not be available. ** Only protects series capacitor, not generator. Meeting Title (optional)

7. Mitigation Options Protection of generator should always be applied if any risk. Mitigation should be applied except in cases where SSR risk is low (i.e. number of contingencies). In these cases, protection should be sufficient. • At Generator • Passive Filters • Sub-synchronous excitation damping controller (SEDC) • Similar to a PSS • Wind Turbine Control System Upgrade • At Series Capacitor • Passive Filters • Thyristor Controls • Switching schemes • At ISO • Outage coordination • Deny certain outages or bypass series capacitor and curtail wind. Meeting Title (optional)

8. Identifying Risk: Protection and Prevention Risk Action • HIGH • N-0 / N-1 vulnerable • Prevention: Complete mitigation of risk. • Blocking filter or SEDC for conventional generator. • Control enhancement for wind turbine. • Partial or complete prevention. • Must ensure no cascading events. • Series capacitor switching schemes. • MEDIUM • NERC C,D contingency vulnerable Increasing Risk of SSR • LOW • Multiple failures / uncoordinated outages + contingency • Generator protection – always installed. • Series capacitor protection • ERCOT Outage Coordination Meeting Title (optional)

9. Coordinated & Overlapping Protection Coordination • At Generator • All generators, even low-risk, should have SSR protection • Trips generator • High risk generators should have preventive mitigation • Makes generator immune • At Series Capacitor • Sub-Synchronous current protection • Bypasses series capacitor. • Other options (for later discussion). • At ERCOT • Outage coordination. Secondary Tertiary Primary Meeting Title (optional)

10. SSR in Planning • ERCOT Screening Study • Perform SSR screening study. Identify whether a detailed study needed. • Full Interconnection Study • Perform detailed SSR study. Identify vulnerable contingencies. • High risk vulnerabilities require MITIGATION. • All vulnerabilities require prudent PROTECTION measures. • Low-risk vulnerabilities can be partially mitigated by OUTAGE COORDINATION. • Energization Check • Mitigation and protection plans reviewed and approved by ERCOT prior to energization. • Periodic Review • Some mitigation plans may be topology-dependent. These will require periodic review. Meeting Title (optional)

11. SSR in Operations Planning • ERCOT Outage Coordination • Maintain a list of vulnerable contingencies. • If asked to approve an outage which creates SSR vulnerability, will • negotiate modification to outage, • bypass series capacitor, or • curtail / outage vulnerable generators. Meeting Title (optional)

12. Concepts for planning guides • When a new generator is interconnected, screen for SSR/SSCI • examine if 5 contingencies can connect the new generator directly in series to the capacitor. • If 5 contingencies can series connect; ERCOT performs a level 1 screening study. • What screening studies are appropriate? • Level 1 screening– Grid side frequency scan - inject varying frequencies from generator terminals into grid model looking for resonant frequencies under contingency conditions up to 5 contingencies (single or double) . If no resonant frequencies are identified, no SSR/SSCI risk exists.

13. Level 1 Screening– Reactance looking into the Grid ERCOT examines the possibility of sub-synchronous interaction of a generator with the grid by modeling the interconnection, and replacing the generator with a frequency generator; then examining the response of the grid to various frequency injections.

14. Concepts for planning guides • What screening studies are appropriate - Continued? • Level 2 screening for torsional interaction– Similar to the control interaction studies, a model of the turbine-generator shaft may be modeled which is sufficiently detailed to include vibration of the turbine shaft. Alternatively, the torsional frequency modes of the shaft may be simply compared with the resonance modes of the grid. Resonance modes of shaft

15. Concepts for Planning Guides • If Level 1 and 2 screening are not sufficient to demonstrate the risk is negligible, Time domain studies may be required. • ERCOT may accept studies which demonstrate a facility type is invulnerable under any grid conditions and apply it to multiple locations • If studies indicate a proposal is vulnerable to sub-synchronous interaction with the grid, ERCOT may required the developer mitigate this vulnerability. • TSP and developer should be consulted • When a NERC B contingency can cause SSR it must be mitigated. • When a NERC C, or D contingency can cause SSR, it requires agreement of ERCOT staff and the interconnecting TSP to create a mitigation requirement.

16. Questions?