1. 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.

2. 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.

3. ERCOT has received additional data
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.

4. Outage Statistics
The data indicated a typical 345 kV line experienced an outage…
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.
Type
Outage Frequency (Times / year)
Outage Duration (Hours/Outage)
Single Circuit
3.0 54
Concurrent Double Circuit
0.3 56
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.

5. Mitigation: Redesign of system to prevent SSO.
NPRR 562: Types of 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.
Increasing Action

6. Action commensurate with risk.
NPRR 562: Risk and Action
Action commensurate with risk.
Risk gauged by likelihood of occurring (number of simultaneous transmission outages).
Mitigation
Protection
Outage Coordination
Detailed Study
Screening Study
Increasing Risk
Increasing Action

7. First, a study is performed. Then, action considered.
NPRR 562: Risk and Action
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.
First, a study is performed.
Then, action considered.
Mitigation
Protection
Outage Coordination
Increasing Risk
Increasing Action
Detailed Study
Screening Study
*If mitigation is deemed fully effective, then protection would not be required.

8. 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. --
4 ckts
Protection required.
N-0 to N-3 (6 ckts)
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.

9. 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!!!
The extreme upper and lower ends of the risk spectrum are obvious in terms of action required. It’s the middle of the spectrum where we need probabilities to guide us.

10. Risk gauged by probabilities.
Numerical Analysis
Risk gauged by probabilities.
Probabilities that a given set of n (single circuit) lines have overlapping outages.
In a year…
Once in 50 years at any of 20* units…
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 particular set of 5 transmission lines experiencing a concurrent outage is very small.
The lower table should be used for ERCOT policy decisions, because policy impacts multiple units. The reason why “20 units” will be explained.
Probability of Simultaneous Outages 1 2 3 4 5 6
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.

11. Probability of Simultaneous Outages
Risk versus Action
Probability of Simultaneous Outages 1 2 3 4 5 6
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.

12. 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*
Singles
Doubles
Notice that a dbl ckt outage is in same ballpark as two single ckt outages.
Two double circuit outages are in the same ballpark as 3 to 4 single circuit outages.
Therefore, we may be able to simply and just talk about circuits.
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.

13. This presentation covered…
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.