1. Risk Assessment and Probabilistic Transmission Planning Methods
Seattle City Light
Risk Assessment and Probabilistic Transmission Planning Methods
ColumbiaGrid Puget Sound Area Study Team
Kurt Conger, Consultant to Seattle City Light

2. Transmission Enables:
Efficient bulk power markets
Hedges against generation outages
Hedges against fuel price changes
Low-cost access to renewable energy
Operational flexibility
- Donald Morrow and Richard Brown, InfraSource

3. All Lines in Service Study Case
Planning base case assumption of All Lines In Service (ALIS) and N-1 performance requirements may not reflect operating realities
Operating state of the system may be N-2, N-3, or N-k for extended periods of time due to planned outages
The need for maintenance scheduling cannot be ignored in planning studies
Increasing frequency and duration of maintenance outages
Overlapping outages are common
Operating staff assume responsibility for operating a system that is seldom in ALIS state.
“Planners should model the system as operators would see it in real life.” (Stephen Lee)

4. Deterministic Approach: NERC Planning Standards
NERC TPL-001-0
ALIS Performance
NERC TPL-002-0
Single Contingency (N-1) Performance
“R Include the planned outage of any bulk electric equipment at those demand levels for which planned outages are performed.”*
Question: if we know that planned outages will be likely, but don’t know which elements, what do we study?
NERC TPL and TPL-004-0
* including maintenance

5. Risk Assessment Methods, Metrics and Criteria
PRA – Probabilistic Reliability Assessment (Stephen Lee, EPRI)
“Community Activity Room” (CAR) concept
Holistic approaches, including IRP
Probabilistic Transmission Planning (Wenyuan Li and Paul Choudhury, BCTC)
Monte Carlo simulation or state enumeration
Probabilistic Economic Analysis

6. Factors Affecting Risk
Load Growth
Age of Equipment
Ability to repair and maintain
Replacement is inevitable at some point in the future
Equipment Load Cycling
Frequency and Duration of Forced and Planned Outages
Length of Transmission Lines
Environmental (weather-related) Factors
Generating Resource Constraints and Retirements
Changes in gen patterns
e.g. emission restrictions, fish biops, RPS

7. Factors Affecting Future Planned Outages
Drivers:
Load Growth
Need to Restore Ratings – improve reliability performance
Changes to Protection Systems
Age of Transmission Facilities (repair or replace)
External organization (e.g. WSDOT) Requirements
Unplanned construction delays
Approaches:
Repositioning under-built circuits (no outage)
Increase Physical Clearance (variable outage duration)
Retensioning (long outage duration)
Reconductoring (long outage duration)
Tower Replacements (long outage duration)
Sometimes Double-Circuit towers!

8. Managing the Model Technical Challenges: Approaches:
Computational: N-1-1 combinations on 1,000 elements = 1,000,000 solved cases!
Combined Reliability and Congestion modeling (e.g. OPF redispatch)
Describing method, assumptions and results
Dynamic Stability?!
Approaches:
Process to “de-clutter” the planning model
Probabilistic methods to eliminate improbable system states
Use of representative system states and indices

9. Cost/Benefit Quantification
Total Cost Method
Total Cost =
Investment Cost +
Operation Cost +
Unreliability Cost*
* Unreliability Cost
Economic damage function based on GDP considering:
LOLP – Loss of Load Probability
LOLE – Loss of Load Expectation
EENS – Expected Energy Not Served

10. Planning for Scheduled Outages
How should a realistic and manageable set of outage scenarios be developed for a planning case?
Develop a Monte Carlo type approach
Study the drivers behind historical maintenance outage schedules and identify trends
Extend outage planning process milestones into future planning periods
Select an outage group based on known maintenance needs, age/mortality (Iowa curves), other.

11. Survival Rates – Wood Poles (Labrador, NF)

12. Mitigation Measures
Costs and risks of implementing short-term mitigation measures
Load shedding  economic damage cost
RAS arming  increased risk of tripping
Sectionalizing transmission  may affect distribution system reliability
Distribution load transfers  increased losses, decreased resilience and operational flexibility
Question:
At what point will operational mitigation measures no longer be effective or become intolerable without new infrastructure?