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Series Compensation and SSR Concepts
Jonathan Rose
Planning Engineer
Resource Integration Department
Workshop on NPRR 562, Subsynchronous Oscillations
September 9, 2013
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?
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
Series Compensation in CREZ
Gray
Alibates
Windmill
50% Compensation
Ogallala
Oklaunion
WillowCrk
Long Draw
Navarro
SamSw
Killeen
4
Kendall
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)
Example of Benefit of Series Compensation
Scenario 1: Uncompensated Case
[2] pg 114 of Bergen – Vittal, “Power System Analysis”.
Example of Benefit of Series Compensation
Scenario 2: With Compensation
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 seriescompensated line
• Adverse effects on the generator units due to sub-synchronous
phenomena
1.2
Illustrative Voltage
Profile
1000MW
Voltage [pu]
1.15
1.1
1.05
1
0.95
Mid Line Caps
Sending End Caps
0.9
0
20
40
60
80
% Line Length from Receiving End
Line voltage profile for series capacitors at the end and middle of a line
100
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?
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.’
How does SSR happen?
Shunt Capacitors
~
Inductive
Capacitive
Generators,
Inductors, Load
Most power system elements are inductive.
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.
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.
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?
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.
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
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
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.
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?
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.
Gray
Alibates
Windmill
50% Compensation
Ogallala
Oklaunion
WillowCrk
Long Draw
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
Twin Oak
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.
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%
0%
N-3
98.4%
73.1%
62.6%
45.8%
9.6%
N-4
99.9%
98.4%
97.6%
96.0%
88.5%
N-5
99.997%
99.9%
99.9%
99.8%
99.5%
There is a 99.8% chance that a random N5 combination will not occur as a
simultaneous planned outage in 50 years.
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?
How Study for SSR?
• Frequency scans Network Impedance (N-5)
Impedance (ohms)
0.30
0.25
0.20
0.15
Resistance
0.10
Reactance
0.05
0.00
5
9
13
-0.05
17
21
25
29
33
37
41
45
49
53
57
Graph resistance &
reactance vs. frequency.
Look for dips &
crossovers. Less accurate
so designed to be
conservative.
Frequency (Hz)
• EMT1 Simulation
If frequency scan shows
possible exposure risk,
EMT simulation may be
able to dismiss the
exposure risk. EMT
simulations are more
accurate.
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.
1
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!
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?
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!
Mitigating SSR
Type
Entity
Several options to reduce or eliminate risk.
Cost
Notes
Involves avoiding certain outages or bypassing
ERCOT
$
• Outage coordination
Suggest 4-square chart:series
gen
vs TSP;
capacitor
when they occur. Very effective,
(&TSP)
low cost vs high cost. but only practical for mitigating rare conditions (e.g.
N-4 or higher). Bypassing manually performed by
• Special Protection Schemes*
operator via SCADA.
SPSs are effective but discouraged because they
Special Protection
TSP
$
•
Generator
PSS
tuning
are difficult to model in studies and may operate
Schemes
unexpectedly with unintended consequences. Any
proposals will undergo heavy scrutiny.
• Wind turbine controller adjustments
Upgraded control provides wide-band damping that
Wind Control System Generator
$
does not need tuning.
• Damping Filters
Upgrades
May only be effective in certain situations; would
Fossil Generator PSS Generator
$
needcapacitors
restudy when grid changes.
• Thyristor-controlled series
Tuning
Outage Coordination
SSR Filters
TSP /
Generator
$$$
Tuning may need adjustment as grid changes.
Tuning can be expensive.
Thyristor-Controlled
Series Caps (TCSC)
TSP
$$$
Theoretically very effective but unproven
technology.
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.
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.
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.
31
[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.
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?
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.
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.
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.
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…
Questions?