Study Methods for SSCI: State-of-the-art, and the way forward

Transcription

1 1 Study Methods for SSCI: State-of-the-art, and the way forward July 29, 2015 Andrew Isaacs Panel Presentation: Analysis of Subsynchronous Interactions in Systems with Renewable Generation Resources IEEE PES General Meeting, Denver CO

2 2 Presentation Outline: Introduction to SSCI Current analysis methods and their limitations Proposed study methodology Mitigation Conclusion

3 3 Introduction to SSCI: Distinction in Terms SSR : Sub-Synchronous Resonance Interaction between the mechanical/torsional masses in a generator (or wind turbine) and the electrical resonance from a series capacitor. TA : Torque Amplification: Increase in peak shaft torques leading to higher fatigue. SSTI : Sub-Synchronous Torsional Interaction Interactions between the mechanical/torsional masses in a generator (or wind turbine) and a power electronic device (such as an HVDC link, SVC, wind turbine etc ). SSCI : Sub-Synchronous Control Instability Interactions between a power electronic device (such as an HVDC link, SVC, wind turbine etc ) and a series compensated system.

4 4 Introduction to SSCI: Distinction in Terms Series Capacitor Power Electronics Gas Turbine Series Capacitor Power Electronics Gas Turbine --- SSCI SSR SSCI SSCI (Any Freq.) SSTI SSR SSTI ---

5 5 SSCI Analysis Methods 1. Screening Studies SSR/SSCI: Passive Harmonic Impedance Scans SSTI: Unit Interaction Factors 2. Perturbation Analysis SSR/SSTI: Used to determine generator electrical damping vs freq SSCI: Used to determine Effective Dynamic Impedance of a power electronic device 3. Advanced Screening Studies (research required) SSCI: Uses a combination of Harmonic Impedance Scans (linear portion of a system) and Perturbation Analysis (Effective Dynamic Impedance) 4. Eigenvalue Analysis SSR/SSTI/SSCI: Analytically determine small signal stability of system 5. Full Time Domain Analysis SSR/SSTI/SSCI: Uses fully detailed models of all devices to simulate entire system and determine stability

6 6 SSCI Screening Studies Harmonic Impedance Scans Determine net system impedance (as seen from behind the generator equivalent impedance) as a function of frequency Determines approximate frequency of electrical resonance Impedance dip an approximate indicator of the likelihood of SS interactions (large dips indicate closer to radial connections transition from positive to negative reactances) Limitations: How should nearby SVC/Statcom/HVDC/non-linear-devices be represented? What is the equivalent impedance of a wind turbine? Does SSCI depend on the magnitude of a disturbance/oscillation? If there are 2 or more wind farms nearby, how does a turbine non-linear controller affect the impedance as seen from the other farm? Screening Method used to determine high risk system conditions (for later study with more accurate methods).

7 7 Screening: Passive Harmonic Impedance Scans Harmonic Impedance from Generator Bus for SSCI Screening Analysis MC_0_SC - 1 series comp. line, 35 ohms MC_0_2xSC_1-2 series comp. line, 35 ohms each MC_0_2xSC_2 - as above, 2nd line not terminated MC_0_2xSC_3-2 series comp. lines, 2nd cap is 47 ohms MC_0_SC_4 - as above, 1st line out of service Ohms Frequency (Hz)

8 8 SSCI Dynamic Impedances Perturbation Analysis can be used to determine the Dynamic Effective Impedance of a non-linear device (wind farm): Perturb voltage with sub-synchronous components Measure sub-synchronous magnitude and phase of measured terminal current Impedance (Z) = V/I (performed with complex vectors at each frequency) Table of Z (R + jx) as a function of frequency Calculation for wind turbines (Dynamic Effective Impedances) can be added to linear system impedance (including the series capacitor). Limitations: Impedance may be non-linear with perturbation magnitude Non-linearities in the system and device models are not considered (SVCs? 2 wind farms? 3? 5?)

9 9 SSCI Dynamic Impedances Dynamic Impedance as seen from the 345 kv Bus (Varying Perturbation Magnitudes) Ohms Frequency (Hz) R - 1 kv R - 2 kv R - 3 kv R - 4 kv R - 5 kv R - 6 kv R - 10 kv R - 20 kv R - 30 kv R - 40 kv R - 20 kv (Damped)

10 10 Eigenvalue Analysis If enough is known about the system state variables, stability can be accurately predicted and instabilities directly mitigated. New commercial tools available (eg. TGSSR) Limitations: - It is hard to get enough detail about proprietary turbine controls, and complex system device state variables

11 11 Time Domain Analysis Ultimate Simulation Model the entire system including multi-mass shaft models, HVDC/SVC/STATCOMs, wind farms etc. Apply a small signal disturbance and evaluate damping Apply faults and observe large signal disturbances (and watch for tripping/ride through) Limitations: Time consuming (varying loadflow conditions, contingencies, wind turbine combinations, two segment series capacitors ) Must be used in conjunction with screening studies (to focus on mostconcerning cases)

12 12 Study Process Step 1: Pre-screening Purpose: Eliminate generators not requiring analysis Identify susceptible generators by visual inspection (SLD s, etc). Generators may be lumped at common electrical nodes If there are many simultaneous contingencies required (eg. N-6) to make a generator radial (or the contingency is improbable), and the electrical distance is far, the generator may be omitted from the studies. A Problem condition + 1 technique can be applied

13 13 Study Process Step 2: Screening Purpose: Identify problem generators and network conditions 1. Create an EMT case as large as possible/practical 2. Select frequency scan locations based on pre-screening, and perform passive harmonic impedance scans. Automate if possible to capture all possible problem contingencies 3. If impedance dip is large for probable contingencies, mark contingency for detailed study.

14 14 Study Process Step 2: Screening (con t) Purpose: Identify problem generators and network conditions 4. Identify generators for detailed studies. The selection of the generator is dependent on the following factors: - Level of impedance dip at probable contingencies. - Type of the wind turbine (type 3 or 4) and whether a proven SSCI damping controller is available. - The shape of the damping curve of the wind turbine for SS frequencies (dynamic frequency scan) - Whether torsional frequencies are closer to the series resonance frequencies (for SSR concerns and conventional generators).

15 15 Study Process Step 3: Detailed SSCI Study Purpose: Definitively evaluate stability 1. Obtain detailed EMT models of problematic wind farms 2. All power electronic and dynamic devices should be modelled (HVDC, SVC, wind, PV, etc) 3. Study WPP alone to determine baseline for overall effect on system damping 4. Test turbine in problematic contingencies identified in Step 2. (again, use problem condition + 1 technique). Note damping effects of nearby elements (eg. GT s, loads, supply positive damping, SVCs may supply positive or negative damping, etc.) 5. Repeat problematic conditions with full system model (including other wind plants). Evaluate small signal stability and response to faults.

16 16 Study Process Step 3: Detailed SSR Study (For reference) Purpose: Definitively evaluate stability 1. Carry out Eigenvalue analysis and identify torsional frequencies (torsional data is required from the manufacturer). Obtain mechanical damping of the generator at torsional frequencies if available. 2. Carry out perturbation analysis (identifying electrical network contribution to damping at varying frequencies) for previously identified problematic contingencies with all possible series capacitor combinations. This analysis should be done with the detailed PSCAD model. 3. Check whether known torsional frequencies are close to the series resonance. If the torsional frequencies are inside any negatively damped regions, steps should be taken to prevent possible SSR issues. 4. As a final test, run the PSCAD cases with a full multi-mass model for the generator of concern included, and check for growing oscillations with torsional frequencies for any cases predicting negative damping. This will further confirm SSR issues.

17 17 SSCI Mitigation Also: Avoid radial conditions with SPS Combination of turbine types (some may add damping) SSCI protection relays Exotic methods NGH damping Blocking filters Stabilizers in nearby controllers FACTS (TCSC, SSSC, UPFC )