Planners Perspective on Series Compensated Transmission Lines

Transcription

1 TOGETHER WE DELIVER Planners Perspective on Series Compensated Transmission Lines Kenneth A. Donohoo, PE Director, System Planning Distribution and Transmission Oncor Electric Delivery Company LLC

2 SERIES CAPACITOR COMPENSATION The use of capacitors connected inline with transmission lines to cancel a portion of the transmission line impedance. The percentage of compensation refers to the percentage of transmission line impedance offset or cancelled. X = X L (1-X C /X L ) = X L (1-k), where k = percent of compensation 1

3 WHY USE SERIES CAPACITOR COMPENSATION? Increase the power flow by reducing the line impedance Increase utilization of existing facilities Relieve transmission bottlenecks Improve the dynamic stability of the grid Increases capacity/capability Reduce voltage variation 2

4 BENEFIT OF SERIES COMPENSATION Scenario 1: Uncompensated Case [2] pg 114 of Bergen Vittal, Power System Analysis. 3

5 BENEFIT OF SERIES COMPENSATION Scenario 2: With Compensation 4

6 LIMITATIONS OF SERIES COMPENSATION The percent compensation limited to less than 70% Voltage profile across line changes Voltage may reach 1.10 pu requiring extra insulation May require additional capability in facilities on each end of line Technically difficult to tie new generators into a seriescompensated line Complicates System Protection requirements Adverse effects on the generator units due to subsynchronous oscillation phenomena 5

7 SUB-SYNCHRONOUS RESONANCE (SSR) Resonance: The tendency of a system under excitation to oscillate at certain frequencies. Good Resonance Bad Resonance Subsynchronous Oscillations*: A phenomena where growing quantities of power are exchanged between equipment at frequencies lower than 60 Hz. Has the potential to break generator shafts, damage generator protection and series capacitor banks. Adding Series Compensation to a power system introduces SSR concerns. * Some texts prefer subsynchronous oscillations as a general term instead of subsynchronous resonance. 6

8 Mohave SSR Incident (1970) An example of SSR Torsional Interaction Mohave generator: 1,580 MW coal-fired in NV Gradually growing vibration that eventually fractured a shaft section First investigations incorrectly determined cause. After 2nd failure in 1971 cause was identified as Subsynchronous Resonance An electrical resonance at 30.5 Hz excited a mechanical resonance at 30.1 Hz Problem was cured by reducing compensation percentage and installing a torsional relay D. Baker, G. Boukarim, Subsynchronous Resonance Studies and Mitigation Methods for Series Capacitor Applications, IEEE D. Walker, D. Hodges, Results of Subsynchronous Resonance Test At Mohave, IEEE

9 SSR MANIFESTATIONS Different situations can cause SSR Torsional Interaction (TI) Electrical resonance frequency of system close to natural torsional resonance frequency of mechanical system. Can result in shaft failure. Usually takes several seconds. Transient Torque Electrical resonance frequency of system close to natural torsional resonance frequency of mechanical system, however adequate damping prevents growing oscillations. Can result in shaft fatigue. Induction Generator Effect (IGE) Purely electrical phenomenon; no mechanical component. Affects wind and fossil generators. Self excitation because synchronous motor circuit acts like an induction generator at subsynchronous frequencies. The effective slip can cause negative resistance, hence negative damping. Can result in rapidly growing currents or voltages. Subsynchronous Control Interaction (SSCI) Control system of power electronic device (e.g. HVDC, wind farms, or SVC) has an unintended resonant point close to the system electrical resonance. Result: Rapidly growing currents or voltages. P.M. Amderspm, B.L. Agrawal, Sybsynchronous Resonance in Power Systems. IEEE Press,

10 SOUTH TEXAS SSCI EVENT (2009) Series capacitors installed on long 345 kv lines to allow full loading. 1,000 MW of wind farms connected to Ajo. Many are Type III. 345 kv series compensated lines 9

11 SOUTH TEXAS SSCI EVENT (2009) A fault occurred on the Ajo to Nelson Sharpe line due to a downed static wire. 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. The Doubly-Fed Induction Generators (DFIG) controlled by a voltage source converter introduces negative damping. Undamped oscillations at 22 Hz. Voltages reacted approximately 2.0 pu in ~150 ms. The series capacitors bypassed approximately 1.5 seconds. Damage to wind generators and series capacitors occurred. From AEP presentation by Paul Hassink, Sub-synchronous Control Interaction, Utility Wind Integration Group Spring Workshop April 15, 2011 Also: 10

12 FAULT RECORDER, SOUTH TEXAS EVENT Slide from AEP presentation by Paul Hassink, Sub-synchronous Control Interaction, Utility Wind Integration Group Spring Workshop April 15,

13 EFFECT OF OUTAGES Can increase coupling between a series capacitor and generator. Two outages make the generator at Ogallala radial to the CTT series capacitors. Alibates Gray Windmill Ogallala 50% Compensation Oklaunion WillowCrk Long Draw 12

14 EFFECT OF OUTAGES Five double-circuit outages make Limestone radial to W.Shackelford Navarro series compensated line. Venus / Midlothian Watermill Navarro Wshackelford - Navarro Big Brown Limestone Twin Oak 13

15 EFFECT OF OUTAGES Can increase coupling between a series capacitor and generator Must consider planned and forced outages SSR studies are labor-intensive and do not lend towards being studied on-demand Therefore possible outage combinations must be studied ahead-of-time by Planning Any generator that is up to FIVE outages away from being radial to a series capacitor may be subject to SSR study 14

16 HOW STUDY FOR SSR? Frequency scans Impedance (ohms) EMT 1 Simulation Frequency (Hz) Resistance Reactance Graph resistance & reactance vs. frequency. Look for dips & crossovers. Less accurate so designed to be conservative. If frequency scan shows possible exposure risk, EMT simulation may be able to dismiss the exposure risk. EMT simulations are more accurate. 1 Electromagnetic Transient simulation: A time-domain analysis similar to a dynamic or stability analysis but capable of simulating off-nominal frequencies other than 60 Hz. Such simulations generally require more detailed models. 15

17 WHO S AT RISK? More Risk: Electrically close to series capacitors Type III wind farms Long shaft / multi-mass generators (Coal, NG Steam, Combined Cycle) Less Risk: Type IV wind farms Type III wind farms with special damping controls Hydro, CTs, reciprocating engines Solar inverters HVDC ties 16

18 Protection vs. Mitigation? Protection Involves forced tripping (removal of generator or series capacitor). Disruptive for a system that is already in a weakened state due to outages ( double blow ). Generally recommended as backup means of defense. Mitigation Involves reducing exposure to SSR risk. Generally allows the resource to continue operating, even when outages place the unit in stronger electrical coupling with a series capacitor. In many cases, may completely eliminate risk. E.g. Horse Hollow Energy Center installed mitigation which allowed the wind turbines to operate radially to the series-compensated transmission line owned by NextEra. Protection or Mitigation? Recommend Both! 17

19 PROTECTION & MITIGATION Location Responsibility Option Notes Torsional Relays IEEE Recommended, most widely used technique for addressing risk, reliable protection method PROTECTION Generation SSR Blocking Filter Supplemental exciter damping controls, (SEDC) Lack of adaptability/flexibility with changing system, may require multiple rebuilds as system changes Generation excitation controls to mitigate SS Transmission Automatic Bypass SC Operation Fiber/communications in place, sufficient coordination to operate before generation Torsional Relays Single Circuit Segmentation Needs study; No change to current SC configuration, only how it is operated MITIGATION Transmission SS Filters Segmentation Both Circuits Single vendor, partial mitigation, lack of adaptability/flexibility with changing system, may require multiple rebuilds as system changes First phase toward ultimate future TCSC should system needs change; TCSC would integrate segmented platform, not replace Thyristor Controlled Series Compensation (TCSC) Maximum adaptability to system changes, similar to SVC technology CONTINGENCY PLANNING Clearances/ Outages ERCOT & TSP s Develop procedures to short/bypass SC 18

20 Mitigating SSR Type Entity Cost Notes Outage coordination Outage Coordination Special Protection Schemes* Generator PSS tuning Special Protection Schemes Wind turbine controller adjustments Damping Filters Wind Control System Upgrades ERCOT (&TSP) Suggest 4-square chart: $ Involves avoiding certain outages or bypassing series gen capacitor vs TSP; when low they occur. Very effective, cost vs high cost. but only practical for mitigating rare conditions (e.g. N-4 or higher). Bypassing manually performed by operator via SCADA. TSP $ SPSs are effective but discouraged because they Thyristor-controlled series capacitors Fossil Generator PSS Tuning SSR Filters TSP / Generator Thyristor-Controlled Series Caps (TCSC) are difficult to model in studies and may operate unexpectedly with unintended consequences. Any proposals will undergo heavy scrutiny. Generator $ Upgraded control provides wide-band damping that does not need tuning Generator $ May only be effective in certain situations; would need restudy when grid changes $$$ Tuning may need adjustment as grid changes. Tuning can be expensive TSP $$$ Theoretically very effective Also: Wind developers may select a different turbine model; new fossil plants may modify generator masses or install amortisseur windings; SVCs outfitted with special control schemes. 19

21 Protecting Against SSR Type Entity Notes Torsional Relays that trip Fossil Generator Overvoltage / Overcurrent Relays that trip Generator SSR Current Relays that bypass series capacitor SSR Current Relays that trip generator Generator Generator TSP Generator Fossil generators only. Selective and effective protection. Period of adjustment where occasional nuisance tripping possible. Fast protection relays may protect against certain SSCI and IGE resonance. Low selectivity. Applicable to wind. Generally not fast enough to prevent damage to generators. Useful as backup protection. Selectivity? New technology. Not clear if fast enough to completely avoid damage. Applicable to both wind and fossil. 20

22 Role of ERCOT in SSR (Existing Generation Resources) ERCOT analyzed risk exposure of all existing power plants. For exposed plants, Contacted TSP and generator. Coordinated study. Facilitate resolution. Several thermal and wind plants are already moving towards resolution. 21

23 Role of ERCOT in SSR (New Generation Resources) New resources analyzed for risk in GINR screening study Screening study report indicates whether exposed for SSR. If exposed, then developer must either: Run a detailed study. Typically not performed by TSP. Contract out. Obtain letter from generator manufacturer. Explains why not at risk for SSR and substantiated with technical reasoning or a study simulation. New resources not allowed to synchronize until SSR issues resolved. 22

24 Questions/Discussion 23