Transcript NPRR562 Outage Presentation2014 v5

Sub-Synchronous Oscillations*
ERCOT Comments on NPRR 562
Jonathan Rose
Resource Integration Department
April, 2014
* Sub-Synchronous Resonance is more precisely called Sub-Synchronous Oscillations.
Presentation Overview
• This presentation covers…
– Why ERCOT is proposing changes to NPRR562
– Better data has allowed a RELAXATION in the
requirements.
– The need for PROTECTION.
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Reason for Change
• ERCOT has received additional data
– Detailed outage records from Oncor and AEP show:
• Duration of outages is LESS than originally assumed.
• Double circuit outages are LESS frequent than originally
assumed.
• A modest relaxation in requirements is possible.
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Outage Statistics
• The data indicated a typical 345 kV line experienced an outage…
Type
Outage
Frequency
(Times / year)
Outage Duration
(Hours/Outage)
3.0
54
Concurrent Double 0.3
Circuit
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Single Circuit
• Only 15% of outages on double circuit lines involve both circuits
concurrently.
– Thus double circuit concurrent outages have a significantly lower
probability than single circuit outages.
Concurrent Double Circuit Outage – An overlapping outage such that there is a point in time when
both circuits of a double circuit line are outaged together.
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NPRR 562: Types of Action
Increasing Action
• Mitigation: Redesign of system to prevent SSO.
– Enables generator to operate normally even under elevated
transmission outages.
• Protection: Switching action to avoid impending damage.
– Result in generator tripping, transmission bypass, etc.
• Outage Coordination:
ERCOT avoids at-risk combinations
when scheduling outages.
• Detailed Study: Determines SSO risk.
• SSO Screening Study: Estimates SSO risk.
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NPRR 562: Risk and Action
Increasing Risk
Increasing Action
• Action commensurate with risk.
•
•
•
•
•
Mitigation
Protection
Outage Coordination
Detailed Study
Screening Study
• Risk gauged by likelihood of occurring
(number of simultaneous transmission outages).
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NPRR 562: Risk and Action
Increasing Risk
Increasing Action
1. First, a study is performed.
2. Then, action considered.
Mitigation
Protection
Outage Coordination
Detailed Study
Screening Study
• Generator with very high
risk situation may require
MITIGATION,
PROTECTION*, and
OUTAGE COORD.
• Generator with high risk
situation may require
PROTECTION and
OUTAGE COORD.
• Generator with moderate
risk may require only
OUTAGE COORD.
• A screening study which
estimates only a slight
risk might only require a
detailed study to confirm.
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*If mitigation is deemed fully effective, then protection would not be required.
Proposed Changes to NPRR
Original
Proposed
Action
N-0 to N-5 (10 ckts)
0 – 6 ckts
Detailed study.
N-4 to N-5 (10 ckts)
4 – 6 ckts
ERCOT outage coordination procedures.
-N-0 to N-3 (6 ckts)
4 ckts
Protection required.
0 – 3 ckts
Mitigation required.*
• Originally, double-circuits count as one simultaneous outage.
• NOW, circuits counted individually.
 Double-circuits count as two simultaneous outages.
For example, a gen 4 circuits away from SSO would require protection.
 Four and five double circuits no longer considered credible scenario.
• NOW, protection is mandatory in some cases.
•
Because ERCOT is proposing to substantially relax requirements, we would like a
layer of protection in the marginal (4 ckts) risk level.
* If mitigation not fully effective, then protection also required.
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Probabilities
The role of probabilities is to guide us when
we don’t have good intuition.
12 transmission outages to reach SSO  safe to ignore.
4 transmission outages to reach SSO 
2 transmission outages to reach SSO  action required!!!
Numerical Analysis
• Risk gauged by probabilities.
• Probabilities that a given set of n (single circuit) lines
have overlapping outages.
– In a year…
Probability of Simultaneous Outages
1
2
3
4
5
6
90%
10%
0.2%
0.005%
0.0001%
2e-6%
Notice these
probabilities are very
small. The probability of a
– Once in 50 years at any of 20* units…
particular set of 5 transmission lines
experiencing a concurrent outage is
very small.
Probability of Simultaneous Outages
1
2
3
4
5
6
100%
100%
90%
4.8%
0.1%
0.002%
The lower table should be used for ERCOT policy decisions, because policy impacts multiple units over
their lifetime.
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Risk versus Action
Probability of Simultaneous Outages
1
2
3
4
5
6
100%
100%
90%
4.8%
0.1%
0.002%
Unlikely  no action required beyond a
detailed study. ERCOT will perform outage
coordination just in case.
Possible  Protection required.
Likely to occur  Mitigation required.
Note overlapping layers of precaution.
Mitigation may be used in lieu of protection. If mitigation were applied and
shown to successfully eliminate risk, then protection would not be required.
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Single versus Double circuits
• Only 15% of outages on double circuit lines involve both circuits.
– A double circuit outage is far less likely than a single circuit outage.
Probability of Occurrence at Least once in 50 yrs*
Doubles
Singles
0
1
2
3
4
5
0
1
2
100.000% 100.000% 100.000%
100.000% 99.999% 21.956%
71.657%
2.584%
0.055%
0.302%
0.006%
0.000%
0.001%
0.000%
0.000%
0.000%
3
90.439%
0.519%
0.001%
0.000%
4
4.804%
0.011%
0.000%
5
0.101%
0.000%
6
0.002%
Probability of two double circuit outages occurring
simultaneously is in-between the probability of three singles
occurring and four singles occurring.
 May be possible to simply and just talk about “circuits.”
* At 20 vulnerable units.
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Presentation Summary
• This presentation covered…
– Why ERCOT is proposing changes to NPRR562
– Better data has allowed a RELAXATION in the
requirements.
– The need for PROTECTION.
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