Other considerations

Project Constructability

ATC will consider the constructability of proposed additions, replacements or modifications to the transmission system as part of the company’s system planning process. In particular ATC will consider:

1. Whether addition, replacement or modification of a transmission line, transformer or other facility would result in violation of the System Performance Criteria, and
2. Whether addition, replacement or modification of a transmission line, transformer or other facility precludes the ability of ATC Operations to conduct maintenance activities on other transmission facilities.

Multiple Contingency Planning

There may be circumstances, where the risk to ATC and/or ATC customers of a multiple contingency event is sufficiently severe to warrant consideration for planning purposes. Examples of such an event would include:

1. The loss of a transmission facility during the period of maintenance or repair of another transmission facility
2. A multiple contingency arising from a common cause, such as a fire, flood, lightning etc., and/or a highly probable multiple contingency based on historical observance where studies indicate that there is potential for Adverse Reliability Impact
3. Failure of a transmission structure supporting multiple circuits
4. The loss of two transformers that are connected through a common breaker

ATC will consider the relative probability and consequences of certain selected multiple contingency scenarios to determine whether to apply a multiple contingency standard.

For more details regarding ATC’s multiple outage selection process, please refer to Multiple Outage Selection Process PR-0212-V03 and Detailed Multiple Outage Analysis Process PR-0213-V02.

Generator transient stability will be evaluated for at least one key contingency for two types of NERC Category D contingencies. These contingencies are three-phase faults on a transmission line with delayed clearing due to breaker failure (D2) and three-phase faults on a transmission transformer with delayed clearing due to breaker failure (D3).

Such multiple contingency scenarios may warrant consideration of operating guides or reinforcements. In these circumstances, ATC will document the potential event(s), the associated risks and potential mitigation measures, and will coordinate with affected customers, as appropriate.  (Applicable NERC Standard: TPL-003-1, TPL-004-1)

Terminal Equipment Limitations

Substation terminal equipment should not limit transmission facility ratings under NERC Category A or NERC Category B contingency conditions. This criterion would apply to new transmission facilities and should be reviewed when proposing modifications to existing facilities.

Maximization of Existing Rights-of-way

ATC will attempt to maximize use of existing rights-of-ways. Existing electric transmission, gas pipeline, railroad and highway corridors will be identified in all comparisons of alternatives and utilized where feasible. Environmental features of a right-of-way are also important to ATC operations. Environmental assessments are built into planning at a high level, and are continued into project assessments as projects move forward through to construction. In addition to avoiding and protecting environmentally sensitive areas, ATC is committed to working in partnership with regulators, environmental organizations and landowners to enhance areas of environmental significance.

Reduction of Transmission System Losses

ATC considers the benefit of reducing system losses along with other performance benefits and cost factors in evaluations of alternative transmission projects or plans. See Economic methodology.

Transmission system operating considerations in the planning process

Operating Guides

Operating guides are not preferred under normal conditions, but may be employed by ATC and/or entities with generation and/or distribution facilities interconnected with the ATC transmission system to avoid transmission facility loadings in excess of normal and emergency ratings provided such guides are practical for sustained periods, if they meet the following conditions:

1. Do not compromise personnel or public safety
2. Do not degrade system reliability
3. Do not result in a significant loss of equipment life or significant risk of damage to a transmission facility.
4. Do not unduly burden any entity financially.
5. Supervisory switching capability is available to accomplish these operating guides. Field switching will not be relied upon as a means to reduce facility loadings or to restore voltages to within acceptable levels.
6. ATC will strive to verify the efficacy of all operating guides that require on-site operations.

Operational Flexibility

System Planning – ATC will strive to plan the transmission system such that operating flexibility is maximized. ATC will accomplish this by considering as wide a variety of scenarios as practical, including maintenance scenarios, when evaluating alternative transmission projects or plans.

Special Protection Systems (SPSs)

Special protection systems (SPSs) are not preferred means of mitigating system limitations, but may be employed by ATC as temporary measures and are not normally considered a long term solution. Proposal of a new SPS may require ATC executive approval via the Asset Investment Management (AIM) process prior to becoming a formal alternative proposed by System Planning.

Radial transmission service

ATC will evaluate the risk of serving customer load from radial facilities. Such evaluations will consider the amount of load being served, the capability of the underlying distribution system and the amount of time that service is likely to be interrupted for the loss/failure of the radial facility.

Relaxation criteria

At times it may be appropriate to consider a relaxation of ATC-specific criteria, as long as NERC and Regional Entity (RE) standards are still satisfied. As system planners perform their work, they should evaluate when it may be appropriate to allow a relaxation of ATC-specific criteria. A decision to relax ATC-specific criteria should be made very carefully considering all of the issues involved (including – but not limited to – Electric Reliability Organization (ERO) and RE requirements and Federal Energy Regulatory Commission (FERC) directives related to transmission service requirements) and then only after performing a detailed assessment of the types and levels of risks involved in the decision. Planners are not permitted to relax ATC-specific criteria on their own. Instead, these situations should be identified and discussed with their manager for further evaluation. The final decision in this regard will be made by the Vice President of System Planning. If any decisions of this type are made, then these decisions will be documented and archived for future reference.

Steady state voltage stability margin identification

The steady state operating point will be identified by finding the nose of the P-V curve and applying the required 10% margin. If a P-V curve nose is not identifiable (no power flow solutions beyond the nose of the curve), then the last solved point prior to the nose will be used as the P-V curve nose. A pre-contingency margin of more than 10% will be identified, if needed, to avoid allowing a steady state operating point beyond the nose of the curve immediately following the worst case Category B or C contingency. ATC prefers the use of Powertech’s Voltage Security Assessment Tool (VSAT) to perform steady state voltage stability analysis.

Operations assumptions for three phase fault scenarios

The ATC Operations Department performs an operating horizon assessment taking into account operating horizon assumptions that may differ from the planning horizon assessment for certain three phase fault scenarios which are documented in certain ATC Transmission Operating Procedures (TOP).  The operating procedures reference any special circumstances in the planning studies and assessments and apply real time risk methodologies as outlined in the TOP procedures. (Note: There may be other potential ATC System Operations planning tasks that may interface with transmission planning tasks).

 

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