1. Series Compensation and SSR - Concepts
Jonathan Rose
Planning Engineer
Resource Integration Department
Workshop on NPRR 562, Subsynchronous Oscillations
September 9, 2013

2. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

3. Series 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

4. Series Compensation in CREZ
Gray
Alibates
Windmill
50% Compensation
Ogallala
Oklaunion
WillowCrk
Long Draw
Navarro
SamSw
Killeen
Kendall

5. Why use Series compensation?
Increase the power flow by reducing the line impedance
Improve the dynamic stability of the grid (by increasing the area under the power angle curve)
Reduce voltage variation
Relieve transmission bottlenecks
Increases capacity
Self-regulating (reactive power generated proportional to flow squared)

6. Example of Benefit of Series Compensation Scenario 1: Uncompensated Case
[2] pg 114 of Bergen – Vittal, “Power System Analysis”.

7. Example of Benefit of Series Compensation Scenario 2: With Compensation

8. Limitations of Series Compensation
The percent compensation limited to under 70%
Voltage profile – line voltage may reach 1.10 pu requiring extra insulation
Technically difficult to tie new generators into a series-compensated line
Adverse effects on the generator units due to sub-synchronous phenomena
Illustrative Voltage Profile

9. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

10. 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.’

11. How does SSR happen? ~ Inductive Capacitive
Shunt Capacitors ~
Inductive
Capacitive
Generators, Inductors, Load
Most power system elements are inductive.

12. Generators, Inductors, Load
How does SSR happen?
Shunt Capacitors
Series capacitors ~
Inductive
Capacitive
Generators, Inductors, Load
When inductive and capacitive equipment match at certain frequencies, they create complimentary energy tanks, which can give rise to resonance.

13. 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.

14. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

15. 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.

16. 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
Yellow color not good for projection screen. See comment jc1

17. 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:

18. 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.

19. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

20. 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

21. Wshackelford - Navarro
Effect of Outages
Five double-circuit outages make Limestone radial to W.Shackelford – Navarro series compensated line.
Even with all of these outages, case solves under min load conditions.
Venus /
Midlothian
Watermill
Navarro
Big Brown
Wshackelford - Navarro
Limestone
Jewett
Twin Oak

22. 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 is subject to SSR study.

23. Probability that Outages Don’t Overlap
Where did N-5 come from?
N-5 came from a calculation of outage probabilities in 2012 by Resource Integration.
Uses IEEE paper, “An IEEE Survey of U.S. and Canadian Overhead Transmission Outages at 230 kV and Above.” IEEE Transactions on Power, January 1994.
Probability that Outages Don’t Overlap
# outages
1 year
20 yrs
30 yrs
50 yrs
150 yrs
N-2
73.7%
0.2%
0.01% 0%
N-3
98.4%
73.1%
62.6%
45.8%
9.6%
N-4
99.9%
97.6%
96.0%
88.5%
N-5
99.997%
99.8%
99.5%
There is a 99.8% chance that a random N-5 combination will not occur as a simultaneous planned outage in 50 years.

24. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

25. How Study for SSR? Frequency scans EMT1 Simulation
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.

26. Who’s At Risk? -- General Observations
More Risk:
Electrically closer 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.
For new generators, ERCOT does not pre-approve certain technologies. A study or letter is required!

27. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

28. Protection vs. Mitigation?
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!

29. Special Protection Schemes* Generator PSS tuning
Mitigating SSR
Several options to reduce or eliminate risk.
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
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)
Theoretically very effective but unproven technology.
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.
Another interesting idea: Negative Sequence filter.
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.

30. 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
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
New technology. Not clear if fast enough to completely avoid damage. Applicable to both wind and fossil.

31. What was done elsewhere?
[1]
Excerpt from text [2]: “In contrast to Mohave, Navajo Project SSR is very complex due to the network. Significant technological advancement in the analysis of SSR [in the 3 yrs since Mohave incident], leading Navajo to announce it would proceed construction as planned.”
Mitigation for Navajo:
Pole amortisseur windings.
Arc gaps that reduce transient torque risk.
Static blocking filter.
Excitation damping control.
Torsional relay protection.
Note torsional relays pursued in most cases.
[1] D. Baker, G. Boukarim, “Subsynchronous Resonance Studies and Mitigation Methods for Series Capacitor Applications,” IEEE 2005.
[2] P. Anderson, R. Farmer, “Series Compensation of Power Systems.” PBLSH! 1996.

32. Presentation Outline What are series capacitors? What is SSR?
What causes SSR?
Simultaneous outages.
Understanding risk exposure.
Mitigating and Protecting against SSR risk.
What has ERCOT been doing?

33. What about the 2010 CREZ Reactive Study?
New Wind Generators
Reactive study tested the effectiveness various mitigations including bypass filter and TCSC but could not test the effectiveness of upgraded wind turbine control systems as details on these designs were not available at the time. Study did not issue a recommendation.
Existing Generators
The Reactive study included SSR studies of several existing generators that were thought might be at risk. Some of the studies confirmed risk and recommended additional analysis.
Additional analysis is now being performed. Some generators have been cleared as ‘no risk’ because better generator data became available and a more detailed analysis was performed.
ERCOT has since identified additional generators that require SSR studies. Many of these studies have already been completed; some are still ongoing.

34. Role of ERCOT Planning in SSR (Existing 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.

35. Role of ERCOT Planning in SSR (New 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.

36. Rare Events Do Happen
December 22, 1982: West Coast Blackout (from NERC)
This disturbance resulted in the loss of 12,350 MW of load and affected over 5 million people in the West. The outage began when high winds caused the failure of a 500-kV transmission tower. The tower fell into a parallel 500-kV line tower, and both lines were lost. The failure of these two lines mechanically cascaded and caused three additional towers to fail on each line. When the line conductors fell they contacted two 230-kV lines crossing under the 500-kV rights-of-way, collapsing the 230-kV lines. The loss of the 500-kV lines activated a remedial action scheme to control the separation of the interconnection into two pre-engineered islands and trip generation in the Pacific Northwest in order to minimize customer outages and speed restoration. However, delayed operation of the remedial action scheme components occurred for several reasons, and the interconnection separated into four islands…

37. Questions?