ATC Experience with Cascading Outages Studies and

ATC Experience with Cascading Outages Studies and Preparing for

a NERC Audit

Charles Lawrence Planning Compliance Manager

American Transmission Company July 19, 2017

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American Transmission Company

• Owner and operator of ~ 9,482 miles of transmission line and 529 substations

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• Founded in 2001, as the first multi-state, transmission-only utility in the United States

American Transmission Company

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• In the area where compliance monitoring is performed by the North American Energy Reliability Corporation (NERC).

• Registered with NERC as a Transmission Owner (TO), Transmission Operator (TOP), Transmission Planner (TP)

• In the Midwest Reliability Organization (MRO)

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• Transmission Operations is focused on the avoidance of cascading that could lead to the loss of interconnected generation or distribution load due to operating horizon contingencies events

• Transmission Operations decides what mitigation measures need to be implemented and when to avoid unacceptable system performance deficiencies or loss of generation or up to 100 MW of firm load due to cascading either in real time or within the operating horizon.

Cascading Considerations Transmission Operations versus Transmission Planning

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• Transmission Planning is focused on the expected future impact of cascading on the potential loss of interconnected generation or distribution load due to operating horizon contingency events.

• Transmission Planning involves deciding when expected system performance deficiencies are significant enough to warrant the implementation of system modifications and when.

• Over-estimation of cascading impacts may lead to overspending, and earlier spending of money, than is appropriate on mitigation measures.

• Under-estimation of cascading impacts may lead to incurring inappropriate risk of potential loss firm load due to missed, inadequate, or late implementation of mitigation measures.

Cascading Considerations Transmission Operations versus Transmission Planning

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• ATC must either provide requested information related to applicable Transmission Planning and Cascading to MRO for compliance audits, spot checks, and self-certifications, or to be prepared to submit this type of information to MRO upon request.

• Therefore, ATC has developed and retains rationale, procedures, and evidence related to Transmission Planning Assessments and Cascading to help assure compliance with applicable NERC Reliability Standard requirements.

Reliability Standards related to Transmission Planning and Cascading

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• CIP-002 (Cyber Security) – Criteria 2.3 assess adverse BES reliability impact, which could include Cascading

• CIP-014 (Physical Security) – R1 risk assessments that consider Cascading

• FAC-002 (Interconnection Studies) – R1.3 steady state and dynamic studies, which could include Cascading


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• FAC-014 (System Operating Limits) – R4 establish planning horizon SOLs, which could include Cascading

• NUC-001 (Nuclear Plant Interface Coordination) – R9.2.3 perform types of planning analyses, which could include Cascading

• PRC-015 (Remedial Action Scheme Data and Documentation) R2 reviewed new or functionally modified RAS, which could include Cascading


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• TPL-001 (Transmission System Planning Performance Requirements) generally include Cascading

– R3.1 steady state analysis for planning events

– R3.2 steady state analysis for extreme events

– R4.1 stability analysis for planning events

– R4.2 stability analysis for extreme events

– R4.5 extreme event Cascading Analysis


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From the NERC Glossary of Terms

The uncontrolled successive loss of System Elements triggered by an incident in any location. Cascading results in widespread electric service interruption that cannot be restrained from sequentially spreading beyond an area predetermined by studies.

NERC Cascading

uncontrolled

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Uncontrolled – loss of System Elements as a consequence of resulting system conditions, rather than the operation of System Protection, other automatic controls, or system control operator actions, such as:

• Line circuits trip at specified overload levels

• Transformer circuits trip at specified overload levels

• Generation units trip at specified voltage levels or angular position levels

successive loss

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Subsequent loss (removal) of System Elements as a consequence of resulting system steady state or stability (dynamic) conditions, after all intended Protection System and other automatic control actions have occurred, in succession until there are no further subsequent uncontrolled removals of System Elements.

of System Elements

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From the NERC Glossary of Terms

System – A combination of generation, transmission, and distribution components

Element – Any electrical device with terminals that may be connected to other electrical devices, such as a generator, transformer, circuit breaker, bus section, or transmission line. An Element may be comprised of one or more components.

triggered by an incident in any location

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Incident Triggers – event contingencies

• Steady state contingencies

– Planning events (such as P1-P7 category contingencies)

– Extreme events (such as E1-E3 category contingencies)

• Stability (dynamic) incidents

– Planning events (such as P1-P7 category contingencies)

– Extreme events (such as E1-E2 category contingencies)

widespread electric service interruption

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• Electric service interruption is the loss of electric system load.

• For transmission planning purposes, it is the loss of firm forecasted load for a given year in the near term planning horizon (future years 1 through 5) and the long term planning horizon (future years 6 through 10).

• In the ATC area, widespread transmission planning load loss is the potential loss of 1000 MW or more of firm forecast load or the Total Load at Risk.

Total Load at Risk

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The Total Load at Risk is the sum of three types of potential load loss:

• Consequential Load Loss (NERC definition) All Load that is no longer served by the Transmission system as a result of Transmission Facilities being removed from service by a Protection System operation designed to isolate the fault.

• Subsequent Cascading load loss The load that is outaged as a result of the subsequent element-based outages during the remaining cascading tier analysis. This type of load loss is a result of either load tripping when load buses are below 0.8 p.u. or load islanding due to lines/transformers tripping.

• Expected Load Shed loss The sum of the load curtailment that may need to be performed by manual or automatic control to restore the ATC system to be within emergency limits and the additional load curtailment that may need to be performed by manual or automatic control to restore the ATC system to be within normal limits.

that cannot be restrained from sequentially spreading beyond an area

predetermined by studies

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• ATC includes Cascading analysis in its planning assessment studies to be compliant with applicable NERC Reliability Standard requirements.

• ATC performs both steady state and dynamic cascading analysis simulations to identify the possible extent and impact that cascading circumstances might have on ATC system performance for appropriate system condition models and applicable contingencies.

Cascading Analysis Process Summary

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• Steady state analysis

– Stage 1: simulate an extensive list of applicable contingencies to find potential system performance deficiencies and cascading analysis candidates

– Stage 2: find potential mitigation measures for non-cascading contingencies with potential system performance deficiencies

– Stage 3: simulate cascading (tiers) for candidate contingencies and find potential system performance deficiencies, as well as potential mitigation measures for contingencies that caused system performance deficiencies

Cascading Analysis Process Summary

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• Stability (dynamic) analysis

– Stage 1: simulate a list of contingencies based on past experience and engineering to find potential system or generating unit voltage or angular performance deficiencies.

– Stage 2: simulate the subsequent loss of any interconnected generation or load within the transient time of about 20 seconds due to voltage instability or loss of synchronism.

– Stage 3: find potential mitigation measures for contingencies with applicable system performance deficiencies

Cascading Trip Criteria

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The ATC cascading analysis methodology has cascading trip criteria for the following elements.

• Line circuit overload trip (overhead conductor sag) 345 kV, 230 kV, 161 kV, 138 kV, 115 kV, 69 kV

• Transformer circuit overload trip

• Underground/submarine cable overload trip

• Generator undervoltage level trip

Consideration of Voltage Sensitive Load Loss

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The ATC considers the potential effects of voltage sensitive load loss in its cascading analysis methodology.

Voltage sensitive loads, typically motors power electronic devices, trip at low enough steady state and transient voltage levels.

The tripping of voltage sensitive load tends to result in ‘self healing’ and reduces the progression of cascading (successive losses).

Cascading Load Loss Criterion

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ATC presently approximates the effect of steady state voltage sensitive load tripping in its cascading analysis methodology with a simple cascading load loss criterion.

When the voltage level at a load interconnection bus drops below 0.8 per unit of nominal system voltage, or the case will not solve for the next tier in the cascading simulation, then enough load is simulated to be tripped at the load interconnection buses with a voltage level below 0.8 per unit. Algorithms are used to identify the next most likely interconnected load to trip among load interconnection buses with a voltage level below 0.8 per unit.

This approach serves as a proxy for the minimum amount of self-tripping voltage sensitive load to restore all interconnected hat would self-trip for sufficiently low voltage at actual voltage sensitive load terminals.

Cascading Analysis Stop Criteria

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Steady state cascading analysis, which represent successive losses as simulation tiers, has four simulation stop criteria:

• No more tripping

• Number of BES lines tripped

• Total load at risk threshold

• Unmitigated voltage instability

Cascading Analysis Tools

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ATC’s cascading analysis is performed using V&R Energy Resources POM/OPM software and custom cascading analysis scripts.

The V&R cascading analysis scripts

o Are tailored to ATC’s cascading process

o Are easy to use with different input options

o Tabulate results in a spreadsheet format

o Come with a helpful User Guide

ATC Cascading Analysis Example

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ATC generally has the following cascading analysis information each year

• Steady state analysis – About 250,000 transmission planning contingencies

– About 800 candidate cascading contingencies

– An occasional few NERC Cascading contingencies

• Stability (dynamics) analysis – About several 100 transmission planning contingencies

– An occasional few NERC cascading contingencies

Further Study

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The accuracy of cascading analysis would be improved if the loss of voltage sensitive load was more accurately simulated.

The following prospective approach should be investigated because it would allow a more reasonable approximation of voltage sensitive load over a large range of actual voltage related load tripping. The proposed approach is to:

• Designate a specific percentage of expected voltage sensitive load loss for each interconnection load at each of four different voltage levels (0.9, 0.8, 0.7 and 0.6).

• Simulate the tripping of the voltage sensitive load at the interconnection load with the lowest bus voltage first and then the next lowest voltage until the voltages at interconnected load have no more voltage level percentages below their thresholds.

Further Study (continued)

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• Develop a nominal correlation between four existing dynamic load model categories of commercial, residential, inducstrial and rural and the percent of load that would trip a the 0.6, 0.7, 0.8, and 0.9 voltage levels.

• Use the four load type correlation to estimate the four different voltage level percentages at a given load interconnection when the percentage of the four load types are known or can be estimated for a given load interconnection.

Documents

ATC Experience with Cascading Outages Studies and