Planning Guide concepts for dealing with Sub Synchronous Resonance in ERCOT & other issues

1. Planning Guide concepts for dealing with Sub Synchronous Resonance in ERCOT & other issues ERCOT John Adams September 14, 2012

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 Sub Synchronous Resonance (SSR) • Torsional Interaction (TI)– Torsional interaction between the turbine-generator shaft and the electrical grid. The originally experienced SSR at the Mohave project in Arizona. What people are normally referring to when they discuss SSR. • Sub-Synchronous Control Interaction (SSCI) – The feedback based control systems of some generators, particularly type 3 wind turbines, can interact with the series compensation to produce positive feedback and current amplification at sub-synchronous frequencies. • Induction Generator Effect (IGE) – a purely electrical phenomenon due to the flow of sub synchronous currents in the armature of a synchronous generator creating the appearance of a negative resistance looking in; effectively leading to amplification.

6. 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.

7. 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.

8. Concepts for planning guides • What screening studies are appropriate - Continued? • Level 2 screening – If a SSR/SSCI risk exists within the 5 contingency criteria, a turbine side scan may be performed. In this technique, varying frequency’s are injected into a very detailed generator control model. (Typically for wind turbines)

9. 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

10. 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 • Agreement of ERCOT staff and TSP creates a mitigation requirement.

11. Other issues for Planning Guides • Developer to inform ERCOT when purchase of major equipment orders placed • ERCOT to cancel SGIAs which have not proceeded to construction within 3 years of the proposed operational date unless directed by the BoD to retain these requests. • In evaluation of reactive requirements, ERCOT to confirm design can meet the dynamic reactive requirements with a proposed tap setting for a range of POI voltage set points ranging from 1.0 pu to 1.05 pu without adjusting tap settings.