Oncor Electric Delivery Company LLC

TOGETHER WE DELIVER

Planners Perspective on Series Compensated Transmission Lines Kenneth A. Donohoo, PE Director, System Planning Distribution and Transmission kenneth.donohoo@oncor.com

1 10/25/2012

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 = XL(1-XC/XL) = XL(1-k), where k = percent of compensation

2 10/25/2012

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

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BENEFIT OF SERIES COMPENSATION Scenario 1: Uncompensated Case

[2] pg 114 of Bergen – Vittal, “Power System Analysis”.

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BENEFIT OF SERIES COMPENSATION Scenario 2: With Compensation

5 10/25/2012

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 series-compensated line Complicates System Protection requirements Adverse effects on the generator units due to sub-synchronous oscillation phenomena

6 10/25/2012

SUB-SYNCHRONOUS RESONANCE (SSR) •Resonance: The tendency of a system under excitation to oscillate at certain frequencies.

•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.

Good Resonance Bad Resonance

* Some texts prefer ‘subsynchronous oscillations’ as a general term instead of ‘subsynchronous resonance.’

7 10/25/2012

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 2005. D. Walker, D. Hodges, “Results of Subsynchronous Resonance Test At Mohave,” IEEE 1975.

8 10/25/2012

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, 1990.

9 10/25/2012

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

10 10/25/2012

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: http://www.elforsk.se/Global/Vindforsk/Konferenser/HF_symposium_111206/Gotia_Power_V309_subsynchronus_resonence.pdf

11 10/25/2012

FAULT RECORDER, SOUTH TEXAS EVENT

Slide from AEP presentation by Paul Hassink, “Sub-synchronous Control Interaction,” Utility Wind Integration Group Spring Workshop April 15, 2011.

12 10/25/2012

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.

50% Compensation Windmill

Ogallala

Alibates

Gray

Long Draw

WillowCrk

Oklaunion

13 10/25/2012

EFFECT OF OUTAGES

Five double-circuit outages make Limestone radial to W.Shackelford – Navarro series compensated line.

Limestone

Navarro

Venus / Midlothian

Watermill

Twin Oak

Big Brown Wshackelford - Navarro

14 10/25/2012

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

15 10/25/2012

HOW STUDY FOR SSR?

•Frequency scans

•EMT1 Simulation

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.

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.

-0.05

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0.05

0.10

0.15

0.20

0.25

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5 9 13 17 21 25 29 33 37 41 45 49 53 57

Impe

danc

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Frequency (Hz)

ResistanceReactance

16 10/25/2012

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

17 10/25/2012

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!

18 10/25/2012

PROTECTION & MITIGATION

Location Responsibility

Option Notes

PROTECTION

Generation

Torsional Relays IEEE Recommended, most widely used technique for addressing risk, reliable protection method

SSR Blocking Filter Lack of adaptability/flexibility with changing system, may require multiple rebuilds as system changes

Supplemental exciter damping controls, (SEDC)

Generation excitation controls to mitigate SS

Transmission Automatic Bypass SC Operation

Fiber/communications in place, sufficient coordination to operate before generation Torsional Relays

MITIGATION Transmission

Single Circuit Segmentation Needs study; No change to current SC configuration, only how it is operated

SS Filters Single vendor, partial mitigation, lack of adaptability/flexibility with changing system, may require multiple rebuilds as system changes

Segmentation Both Circuits 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

19 10/25/2012

Mitigating SSR

Outage coordination

Special Protection Schemes*

Generator PSS tuning

Wind turbine controller adjustments

Damping Filters

Thyristor-controlled series capacitors

Suggest 4-square chart: gen vs TSP; low cost vs high cost.

Type Entity Cost Notes

Outage Coordination ERCOT (&TSP)

$ Involves avoiding certain outages or bypassing series capacitor when they occur. Very effective, but only practical for mitigating rare conditions (e.g. N-4 or higher). Bypassing manually performed by operator via SCADA.

Special Protection Schemes

TSP $ SPSs are effective but discouraged because they are difficult to model in studies and may operate unexpectedly with unintended consequences. Any proposals will undergo heavy scrutiny.

Wind Control System Upgrades

Generator $ Upgraded control provides wide-band damping that does not need tuning

Fossil Generator PSS Tuning

Generator $ May only be effective in certain situations; would need restudy when grid changes

SSR Filters TSP / Generator

$$$ Tuning may need adjustment as grid changes. Tuning can be expensive

Thyristor-Controlled Series Caps (TCSC)

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.

20 10/25/2012

Protecting Against SSR

Type Entity Notes Torsional Relays that trip Fossil Generator

Generator Fossil generators only. Selective and effective protection. Period of adjustment where occasional nuisance tripping possible.

Overvoltage / Overcurrent Relays that trip Generator

Generator Fast protection relays may protect against certain SSCI and IGE resonance. Low selectivity. Applicable to wind.

SSR Current Relays that bypass series capacitor

TSP Generally not fast enough to prevent damage to generators. Useful as backup protection. Selectivity?

SSR Current Relays that trip generator

Generator New technology. Not clear if fast enough to completely avoid damage. Applicable to both wind and fossil.

21 10/25/2012

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.

22 10/25/2012

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.

23 10/25/2012

Questions/Discussion