Interconnection studies

The following analyses and procedures should be performed for all new or modified interconnection facilities (generation, transmission, and end-user) to the ATC system to properly assess their reliability impact on the interconnected systems. For some analyses, a formal study report may be appropriate. For other analyses, a simple statement of assumptions and rationale may be sufficient.

Types of Analysis

The analyses are to include steady state, short-circuit, and dynamic assessments that include the requirements in TPL-001-4.

Compliance with Applicable Planning Criteria

The analyses and procedures are to comply with all applicable NERC, Regional Entity, and individual system planning criteria of the affected parties.

Coordination with Affected Entities

The results of the analyses will be jointly evaluated and coordinated by the affected entities.

Essential Documentation

All analyses should include the evaluation assumptions, system performance, alternatives considered, and any jointly coordinated recommendations.

Flow Regulating Equipment

To ensure continued operating flexibility during unexpected system conditions, consideration of adjustments to flow regulation equipment (including, but not limited to, phase angle regulators and high voltage direct current equipment) to accommodate the interconnection of new facilities (generation, transmission, and load) and the approval of Transmission Service Request is prohibited, except under unique circumstances, as defined by ATC. Mitigating system limitations via the adjustment of flow regulating equipment is allowed on a limited and carefully considered basis. Adjustments to flow regulation equipment could lead to conflicting objectives and is intentionally limited to cases that improve system robustness. Limiting adjustment of flow regulation equipment applies for equipment that is not primarily providing voltage support.

Specific Study Methodologies

Generator Interconnection Studies
  1. Shall utilize AC solution methods to screen for overloaded elements. Linear DC analysis may only be used to determine Distribution Factors (PTDF and LODF) for MISO generator interconnection studies and the impact of multiple Generator Interconnection Requests on a transmission facility for cost allocation purposes.
  2. Steady-state analysis shall utilize the following generation dispatch:
    • Shoulder Load & Summer Peak Levels: ATC will define the study area. In the study area, study generators, higher-queue generators, and existing local generators will be dispatched at their expected output level as determined by ATC regardless of the fuel type, or merit order dispatch. In general, engineering judgement and historical operating data should be utilized to determine a credible dispatch. Outside the study area, generators will be dispatched following the guidelines in the MISO Business Practice Manual for Generator Interconnections.
    • Additional/Alternative Seasonal Load Levels: If deemed necessary to adequately assess system reliability in the study area, other seasonal models may be required. Generating facilities should be dispatched at expected output levels, regardless of fuel type, in accordance with historical data and ATC Control Area merit order or ATC-wide merit order, depending upon what type of case is selected. In general, lighter load conditions should dispatch wind generation at 100 percent of their Pmax and winter peaking load conditions should dispatch wind generation at 20 percent of their Pmax.
  3. Dynamic stability studies shall dispatch generation in the study area to ensure expected more severe operating scenarios are assessed. Generally, this will involve dispatching all generation local to the study area regardless of fuel type, load level, or merit order. Engineering judgment and potentially sensitivity analysis should be utilized to determine a severe, yet credible dispatch.
  4. Existing generators in the study area with Interconnection Agreements allowing for higher seasonal output (e.g., combustion turbines with increased output capability at colder ambient temperatures) shall be modeled at that output level during dynamic stability studies. New Interconnection Requests with higher seasonal output levels will be analyzed at the higher output if the Interconnection Customer elects the additional capacity in the MISO Generator Interconnection Process.
  5. Power Factor Requirements for Interconnection Generating Units are as follows. ATC’s standard power factor range for synchronous and non-synchronous (e.g., wind turbines, solar) generation is 0.95 leading (when a Generating Facility is consuming reactive power from the Transmission System) to 0.90 lagging (when a Generating Facility is supplying reactive power to the Transmission System). These values have been approved by the FERC for use by ATC (cf. FERC Orders ER05-1475 and ER06-866). Static reactive power sources can only be used to make up for losses between the machine terminal and the POI for synchronous machines and losses between the terminal of the machines and the high side of the GSU for non-synchronous machines. All other reactive power to meet the power factor requirement must be provided by continuous and sustainable dynamic sources. Operation across the entire power factor range must be fully dynamic, variable, and capable of sustained indefinite operation. Static sources can be switched on or off in the range of seconds and provide reactive power in large discrete blocks. Cap Banks are considered static sources of reactive power. Dynamic sources can provide variable amounts of reactive power in a few milliseconds. Static Var Compensators (SVCs), Static Synchronous Compensators (STATCOMs), Flexible AC Transmission Systems (FACTs), inverters, and synchronous condensers are all considered dynamic sources of reactive power. The Generating Facility must be capable of maintaining ATC’s standard power factor range at all power output levels by providing continuous dynamic reactive power at the following locations:
    • The point of Interconnection (POI) for all synchronous generators
      • For synchronous machines, the interconnection studies will account for the net effect of all energy production devices and losses on the Customer’s side of the POI.
    • The high-side of the generator step up transformer (GSU) for all non-synchronous generators (FERC Order No. 827)
      • For non-synchronous machines, the interconnection studies will account for the net effect of all energy production devices and losses on the Customer’s side of the GSU.
      • Dynamic reactive power provided by non-synchronous generators muse meet the following requirement from FERC order 827 Item 35:
        • “Non-synchronous generators may meet the dynamic reactive power requirement by utilizing a combination of the inherent dynamic reactive power capability of the inverter, dynamic reactive power devices (e.g., Static VAR Compensators), and static reactive power devices (e.g., capacitors) to make up for losses.”
  6. The interconnecting generator must be capable of automatically and dynamically maintaining a POI voltage schedule that is specified by the Transmission Operator. Any generator interconnected within the ATC system is expected to maintain a voltage of 1.02 p.u. at its POI to facilitate transmission operations reliability under normal system conditions (system intact) and P1 and P2 Contingencies, unless another voltage level is communicated to the generator by the ATC Transmission Operator (cf. NERC Reliability Standard VAR-001).
Other ATC Interconnection Studies and Considerations

The power generation and transmission landscape is rapidly evolving due to the growing use of new and emerging technologies. While the use of these technologies provides unique benefits, if not properly applied, their interactions with the system can outage or damage equipment or result in degraded system performance. To ensure that this does not occur, special studies are often required that have not traditionally been part of the planning process. This section is intended to provide an introduction to these studies, the phenomena they analyze, and when they might be required. It is not meant to be an exhaustive list of all possible new concerns or types of studies but rather a high level overview to illustrate the possible and probable areas of concern.

The implementation of new generation or transmission devices utilizing power electronic conversion equipment is one example of a situation which may require additional special studies. Alternatively, the siting of new traditional generation or transmission devices near existing devices with power electronic conversion equipment may also require additional studies.

The special studies are typically electro-magnetic transient (EMT) studies which ATC prefers to perform using the PSCAD/EMTDC software. As such, appropriate detailed PSCAD models will be required to be delivered for all projects which utilize power electronic converter based technologies. The use of a generic PSCAD model will typically not produce simulation results of acceptable accuracy. Models using binary/DLLs of the actual control and protection code from the converters and control and protection systems are usually required to accurately model complete device control details. Acceptability of specific models will be determined when they are delivered. ATC can be contacted to provide more detailed information on PSCAD model preferences and supported simulation features.

The specific types of studies of concern may include, but are not limited to, the following:

  1. Studies required in “weak grid” conditions
    1. Control interactions between multiple nearby power electronic or converter based devices (type 3 or 4 wind machines, solar PV, HVDC, STATCOM, SVC, etc.).
    2. Sub-synchronous studies (near converter based generation, HVDC, SVCs, STATCOMS, etc.) For example, Sub-Synchronous Torsional Interaction (SSTI), Sub-Synchronous Resonance (SSR), Sub-Synchronous Oscillations (SSO), Sub-Synchronous Control Interaction (SSCI), etc.
    3. Fault ride through performance verification (e.g. to support FERC order 828, PRC-024-2, etc.). This could be especially applicable to converter based generation which may have unusual responses during and after fault conditions.
    4. Other “weak grid” related studies for devices connecting to an area with low short circuit strength (voltage regulation, other control or protection system tuning, etc.).
  2. Control interaction studies for devices connecting to an area with low short circuit strength (voltage regulation, other control or protection system tuning, etc.)
  3. Power quality around generators or other devices utilizing power electronic converters; especially those with IGBTs. This includes harmonics, inter-harmonics, and other applicable power quality topics. This may apply to power electronic or converter based devices (type 3 or 4 wind machines, solar PV, HVDC, STATCOM, SVC, etc.)

Screening level studies and engineering judgement will be used, as appropriate, to determine the depth and breadth of detailed analysis required for the concerns and topics outlined above.

Next: Under-Frequency Load Shedding (UFLS)