Subsynchronous Oscillations (SSO)
and PowerWorld Applications at
ERCOT
Anuj Dixit and Priya Ramasubbu
ERCOT Planning and Grid Co-ordination
May 21, 2014
The Electric Reliability Council of
Texas (ERCOT)
• ERCOT ISO manages the flow of electric power to
23 million Texas customers - representing 85
percent of the state's electric load.
• Performs financial settlement for the competitive
wholesale bulk-power market
• Enables retail electric choice for Texas customers
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Presentation Outline
•
•
•
•
*CREZ Network and Resonance
What causes SSO?
Studying and Preventing SSO
How does this impact transmission
planning and operations?
• PowerWorld Applications
*Competitive Renewable Energy Zone
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Definition
Competitive Renewable Energy Zone – a
geographic area in the state of Texas initially
identified by AWS Truewind (as ERCOT’s
consultant) as an area suitable for multiple
100 MW wind development projects. Of the
original 25 zones the Public Utility
Commission (PUC) chose a small set based
on input from potential windfarm developers.
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CREZ Areas
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What and Where
 New CREZ 345kV lines built both inside
ERCOT load-serving areas and far into
areas where load is served by SPP.
 CREZ will not add any new connections
between ERCOT and SPP.
 CREZ areas are Panhandle A, Panhandle
B, Central, West, and McCamey.
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CREZ 345kV Lines
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Series Compensation
• The use of capacitors connected inline with a
transmission line.
• Benefits:
– Increases power flow by reducing line impedance.
– Relieve bottlenecks.
– Increases stability and reduces voltage variation.
8
Series Compensation in ERCOT
Series Compensation:
• CREZ
– Tule Canyon – Tesla
– Edith Clarke – Clear
Crossing
– Dermott – Clear Crossing
– West Shackleford – Sam
Switch/Navarro
– Big Hill – Kendall
– Clear Crossing – Willow
Creek (2016)
•
Rio Grande Valley
– Lon Hill – Rio Hondo
– Lon Hill – Edinburg
– Lobo – Edinburg (2016)
•
Introduction to Subsynchronous Oscillations
9/9/2013
HorseHollow GenTie
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How Resonance Can Occur in
Power System
• Complementary Storage Elements creates resonant
circuit (e.g. generator & series cap).
• Generator introduces negative damping.
• Low frequency power oscillations develop.
Generator
Load
Normal Energy
~
Generator
Normal Energy
Series Capacitor
Load
~
Oscillations
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Mohave SSO Incident (1970)
An example of SSO 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
interaction with nearby series capacitors.
• 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.
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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
•
•
•
•
CREZ Network and Resonance
What causes SSO?
Studying and Preventing SSO
How does this impact
transmission planning and
operations?
• PowerWorld Applications
Introduction to Subsynchronous Oscillations
9/9/2013
12
South Texas SSR Event (2009)
• A fault occurred on the Ajo to Nelson Sharpe line.
• 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.
• Undamped oscillations at 22 Hz.
• Voltages at generator doubled in ~150 ms.
• Damage to wind generators and series capacitors
occurred.
Introduction to Subsynchronous Oscillations
9/9/2013
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
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South Texas SSR Event (2009)
• Series capacitors installed on long 345 kV lines to allow
full loading.
• 1,000 MW of wind farms connected to Ajo.
345 kV series
compensated lines
Introduction to Subsynchronous Oscillations
9/9/2013
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Lessons Learned – South Texas 2009
• Wind farms are vulnerable to SSO due to nearby
series capacitors.
• Understanding and analysis techniques have
advanced considerably.
• Wind farms are now routinely analyzed for risk.
• AEP resolved this problem by installing
protection to automatically trip the wind farm.*
* This solution was site-specific. In other locations, protective tripping may not
Introduction to Subsynchronous Oscillations
be appropriate
as a first defense.
9/9/2013
15
Who’s At Risk? -- General Observations
• More Risk for SSO:
– Electrically closer to series capacitors.
– Long shaft / multi-mass generators (Coal, NG Steam,
Combined Cycle).
• Less Risk:
– Hydro, CTs, reciprocating engines.
– Solar inverters.
– HVDC ties.
• Because SSO is a concern for all generation
technologies, all generators are reviewed. ERCOT
notifies developers whose units might be at risk.
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Presentation Outline
•
•
•
•
CREZ Network and Resonance
What causes SSO?
Studying and Preventing SSO
How does this impact transmission
planning and operations?
• PowerWorld Applications
• Conclusion.
17
SSO Risk Assessment
TOPOLOGY TEST
TEN or less
contingencies to radial?
This simple test is performed by inspection:
Does it take ten or fewer outages to make a
generator radial to a series cap? This test
will clear many projects, especially those far
from the series capacitors and those
connecting to the lower voltage network.
ERCOT SSO Screening
Grid-side frequency scan shows risk?
At 6 or fewer concurrent outages?
This tests whether the network electrical
characteristics are conducive to resonance.
ERCOT performs this test as part of the
transmission planning and GINR process.
POWER-FLOW TEST
Outage combination solves in
steady-state w/ few overloads?
This tests the affected outage combinations
to see whether they solve in a min load
case. If the case doesn’t solve or has many
overloads, the outage isn’t credible.
DETAILED STUDY REQUIRED
Or rework the proposed interconnection.
Or obtain letter from gen. manufacturer.
If credible SSR risk exists, then a study
should be performed. In lieu of a study,
generator resources may obtain a letter
from their manufacturer or they may also
rework their proposed interconnection to
reduce risk exposure.
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How Study for SSO?
• 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
-0.05
9
13
17
21
25
29
33
37
41
45
49
53
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Graph resistance &
reactance vs. frequency.
Look for dips &
crossovers. Less accurate
so designed to be
conservative.
Frequency (Hz)
• Time-Domain Simulation
If frequency scan shows
possible exposure risk,
EMT1 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.
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1
Detailed Study – Results & Action
(As proposed in NPRR562)
• Detailed study will determine the system configurations that result
in SSO, particularly the number of concurrent outages required.
Potential Risk
Required Action
Responsible Entity
(# of Concurrent
Transmission
Outages)
≤3
≤4
4, 5, or 6
Protection and
Structural Mitigation
Protection
Procedural Mitigation
All affected
All affected
All affected and ERCOT
• Procedural Mitigation: Outage Coordination procedures to avoid
scheduling outages that would place a unit at increased risk.
• Structural Mitigation: Redesign, controller tuning, etc.
Introduction to Subsynchronous Oscillations
9/9/2013
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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 SSO 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.
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How to Solve SSO Problems?
• Mitigation Options:
– Control system upgrades (common for wind).
– Re-design of generating unit.
– Passive Filters.
– Some transmission level options available.
• Protection Options:
– Torsional relays
(detect shaft twisting on conventional units).
– Subsynchronous Current relays.
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Presentation Outline
•
•
•
•
CREZ Network and Resonance
What causes SSO?
Preventing SSO
How does this impact transmission
planning and operations?
• PowerWorld Applications
23
Effect of Outages
• Outages are the best indicator for gauging risk for SSR.
• Five double-circuit outages make Big Brown radial to
W.Shackelford – Navarro series compensated line.
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Example Timeline of an SSR Event
For a Generator that is “N-4 Risk”
10 days
1st Planned
Outage
8 days
2nd Planned
Outage
3rd Planned
Outage
5 seconds
Forced
Outage
Generator
Damaged
Take Action!!!
• SSR can happen very fast.
• Action should be taken well ahead of time.
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Preventing Risk Example
ERCOT Planning identifies all generators that might be at risk.
ERCOT Planning orders detailed studies.
Detailed studies indicate which outages place a generator at risk.
EXAMPLE:
For Wise County to be at risk, the following lines must be all out of service:
WillowCreek – Hicks, WillowCrk – Parker, and WillowCrk - Jacksboro
ERCOT Outage Coordination ensures that no two combinations of these
outages go out at the same time. If unavoidable, takes mitigation action.
Outage coordination mitigates risk.
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Preventing Risk (Continued)
Because Outage Coordination is monitoring, SSR is unlikely to become a Real
Time issue. However, if it does…
One of the new SSR Alarms in the Control Room sounds.
Operators follow written mitigation procedures.
Example Mitigation Procedure
1. Call TSP. Ask if tripped line can be put back in service.
2. If unable to re-energize, ask TSP to study bypassing series capacitor.
3. TSP studies, sees minor adjustments required (e.g. move switched
shunt or enforce a partial wind curtailment).
4. TSP bypasses the series capacitor. SSR risk is averted.
5. When possible, operators look for opportunity to place capacitors back
in service and cancel the wind curtailment.
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Role of ERCOT Planning in SSR
• ERCOT analyzed risk exposure of all existing
and proposed power plants.
• All exposed plants are undergoing study.
• Several thermal and wind plants are already
moving towards resolution.
• Until resolution, series capacitors will not be
energized.*
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* If situation involves a new generation resource and existing series capacitors, then the new resource would not be allowed to
energize.
How does this impact operators?
• List of units prone to SSR may have to be
closely monitored under stressful system
conditions
• Keeping an eye out for extreme contingencies
near the series capacitors
• This will be done with the help of new SCADA
displays and alerts
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Bypassing: The “Emergency Kill Switch”
Series
Capacitor
Current
Interrupt
Close to
Bypass
• Bypassing removes the series capacitor from
service but leaves the line still in operation.
• With capacitor removed, no SSR risk.
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Bypassing: What effect will it have?
• Line impedance will double, thus less power will
flow on line (and more on neighboring lines).
• Bus voltages may change by ~3%.
• A new voltage stability limit may appear.
• Because of this, it is recommended to run an
STNET and possibly VSAT study before bypassing.
– Some generation adjustments may be necessary
(e.g. wind curtailment).
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Example Display – SSR Alarm
Source: ERCOT Advanced Network Applications
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Example Display – SSR Alarm cont.
Source: ERCOT Advanced Network Applications
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Example Display – SSR Details
Source: ERCOT Advanced Network Applications
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Example Display – SSR Actions
Source: ERCOT Advanced Network Applications
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Presentation Outline
•
•
•
•
CREZ Network and Resonance
What causes SSO?
Preventing SSO
How does this impact transmission
planning and operations?
• PowerWorld Applications
36
ERCOT System Planning
• ERCOT planning performs coordinated planning studies with input
from NERC registered Transmission Planners (TPs), Transmission
Owners (TOs) and other stakeholders to address region-wide
reliability and economic transmission
• Performs transmission studies/assessment (both near-term and
long-term planning horizon) to meet ERCOT and NERC standards
(TPL-001-004, PRC-023, FAC-013)
• Assesses resource availability of existing units and future generators
• Load forecasting for the near-term and long-term Planning Horizon
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Regional Transmission Plan (RTP) Objective
• Regional Transmission Plan is developed
annually by ERCOT in coordination with the
Regional Planning Group (RPG) and the
Transmission Service Providers (TSPs)
• Annual assessment to identify transmission
needs of ERCOT system over the next six years
• Projects identified to meet the ERCOT/NERC
reliability requirements (Reliability projects) and
to reduce system congestion (Economic
projects) that meet the ERCOT economic criteria
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ERCOT RTP Process
Case conditioning
Reliability analysis
complete with all
reliability issues resolved
Economic case
preparation
Initial start cases,
contingency list and list
of overloads ready
X-1 screening followed by
X-1+N-1 analysis
Economic analysis
N-1 SCOPF analysis
G-1 screening followed
by G-1+N-1 analysis
Prepare the final RTP
report
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Use of PowerWorld in RTP
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Case Building
• Assign ERCOT weather zones to buses and
loads
• Update AGC flags for non-conforming loads
• Load scaling
• Control options for generators (Fast start,
AGC, AVR, Enforce MW Limits)
• Geographic onelines
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SCOPF / Contingency Analyses
• Import cost curves for generators
• Load throw over information
• Conditional modeling and global actions
for SPSs
• SimAuto
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PowerWorld Settings
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Options and Settings in PowerWorld
• Solution Settings
– Common Options
• Enforce Generator MW Limits
– Island Based AGC
• Participation Factors
• LP / OPF Settings
– Constraint Options
• Maximum Violation Cost
– Control Options
• Fast Start
• Areas and Super Areas for OPF
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Solution Settings
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Options and Settings in PowerWorld
(Contd.)
• Contingency Analysis Options
– Basics
• Full Power Flow
• Generator Participation Factors
– Advanced Limit Monitoring
• Do not report base case violations
• SCOPF Options
– Solve base case using optimal power flow
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SCOPF Options
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New Feature Recommendation
• N-1-1 capability with system adjustments
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