SPP's High Impact Large Load (HILL) Process Explained

By Keentel Engineering — Power System Studies | EMT & Dynamic Modeling | Interconnection Support

AI data centers, crypto mining facilities, hydrogen electrolyzers, and large industrial plants are connecting to the grid at a scale the power industry has never seen. A single AI campus can now demand more power than a mid-size city — and unlike a city, it can appear on the system in 18 months and change its consumption by hundreds of megawatts in seconds.

The Southwest Power Pool (SPP) responded with one of the most comprehensive large-load interconnection frameworks in North America: the High Impact Large Load (HILL) process, established through Revision Request 696 (effective January 15, 2026) and unanimously approved by FERC in January 2026. If you are developing a data center or any large load in the SPP footprint — Kansas, Oklahoma, Nebraska, the Dakotas, and beyond — this process now governs how your project gets studied, what your equipment must be capable of, and what models you must deliver before SPP will run a single simulation.

This guide walks through the entire process step by step, explains the engineering behind each requirement, and answers the technical questions we hear most often from developers, owner's engineers, and EPCs.

Part 1: What Is a HILL?

A HILL is a classification of load defined in the SPP Open Access Transmission Tariff (OATT). Your project is a HILL if it is a new commercial or industrial load — or an increase in load at a single site that meets either threshold:

• 
  10 MW or more connected at a voltage of 69 kV or below, or
• 50 MW or more connected above 69 kV.

Electric storage resources are excluded from the definition. Virtually every hyperscale or co-location data center, and most AI training campuses, will exceed these thresholds by an order of magnitude.

Why the special treatment?



Because large power-electronic loads behave nothing like the aggregate residential and commercial load the grid was planned around. A 245 MW data center is, electrically, a massive rectifier plant: UPS front ends, server power supplies, and variable-speed cooling drives. During a transmission fault, that entire load can disconnect in a few cycles — and either stay off or slam back on. Uncontrolled tripping of gigawatts of load during a voltage sag can turn a routine fault into a regional frequency and voltage event. SPP's HILL framework exists to make sure that doesn't happen.

Part 2: Choosing Your Pathway Attachment AQ, Attachment AX, or CHILL

Before any study begins, the Transmission Customer (typically the load-serving entity or network customer sponsoring your project) must choose a study pathway. As of RR720 (effective July 1, 2026), customers with sufficient Designated Resources have a choice.

Attachment AQ — the Delivery Point Assessment path.

This is the traditional route, used when the Transmission Customer already has enough Designated Resources (contracted generation) to serve the new load. The HILL is studied through a HILL Delivery Point Study (HDPS) alongside the host Transmission Owner's Load Connection Study.

Attachment AX — the Provisional Load Process.

This newer path addresses the reality that many large loads want service faster than new generation can clear the traditional interconnection queue. Under AX, the HDPS incorporates planned generation identified to serve the load, including a Generator Outlet Facility (GOF) analysis. The load receives provisional service tied to that generation; once the resource obtains firm service through the Aggregate Transmission Service Study, both move to a standard Network Integration Transmission Service Agreement (NITSA). Network upgrade costs are directly assigned to the customer during the provisional period, then migrate to Base Plan funding.

CHILL — Conditional HILL service.

For projects with the most aggressive energization timelines, SPP offers conditional access: quick study results and interconnection in exchange for accepting potential curtailment during periods of system stress.

Co-located generation: HILLGA and HILLGIA.

If your project pairs on-site or near-site generation with the load behind a common point of interconnection (POI) — increasingly the default for AI campuses — SPP's HILL Generation Assessment (HILLGA) process provides an interconnection agreement (HILLGIA) path, with requirements including a POI no more than two substations from the HILL and commercial operation within five years of the study agreement.

How Keentel helps:

Pathway selection is a commercial decision with deep technical consequences. Keentel Engineering supports developers and owner's engineers in evaluating AQ vs. AX vs. CHILL against Designated Resource positions, generation development timelines, and curtailment risk tolerance — before the application is filed, when the decision is still cheap to change.

Part 3: The Application Package What You Must Submit

There is no cluster window or filing deadline. HILL requests are processed on a rolling basis, which means the real "deadline" is self-imposed: the 90-day study clock does not start until your package is complete and validated. An incomplete submittal doesn't get rejected — it just sits, burning schedule.

For an Attachment AQ request, the submittal (sent to SPP's load studies group) must include:

1. A fully completed Delivery Point Assessment (DPA) Request Form (Addendum 1 to Attachment AQ).
2. A fully executed HDPS Agreement (template on SPP's OASIS studies page).
3. A ten-year load forecast with summer, winter, and light-load values for the delivery point, plus any associated changes at other delivery points.
4. A one-line diagram showing the anticipated load and changes to local delivery facilities.
5. Associated IDEV files (SPP's power-flow model change files).
6. The completed Additional HILL Characteristics Form (Business Practice 7850) — the detailed load-behavior questionnaire introduced by RR724.
7. Load modeling data — a CMLD and/or PERC1 model, with PERC1 preferred by SPP.
8. Any useful supplemental information — and SPP explicitly recommends including a PSCAD model at the time of submission (more on why below).

The Attachment AX list is similar but adds details of the planned generation — location, capacity, and the associated IDEV and DYR (dynamics) files. Under AX, the customer has 30 calendar days after submitting the request form to deliver the executed HDPS Agreement and study deposit.

The item that trips up most projects is #7.

Ten-year forecasts and one-lines are familiar territory. A validated PERC1 or CMLD parameter set that actually represents your UPS topology, cooling plant, and IT load ramp behavior is specialized power-systems work — and it is now a gating item for the study clock.

Part 4: The HDPS Step by Step

Once the package is complete:

Step 1 — Scoping call (within 10 days).

SPP, the Transmission Customer, and the host Transmission Owner meet to validate the request data, agree on study scope and assumptions, and set the study due date. Data validation at this call is what officially starts the 90-day clock.

Step 2 — The 90-day base study stage.

SPP performs the HDPS while the host TO simultaneously performs its Load Connection Study, and affected-system impacts are screened. The base stage comprises five analyses:

• Thermal overload study. Power-flow analysis under specified contingencies and operating conditions, comparing pre- and post-project loading on transmission equipment to identify overloads and required mitigations.
• Voltage stability study (steady state). Power-flow analysis across dispatch scenarios and contingencies to confirm the load does not cause voltage violations or non-convergent (voltage-collapse) conditions, identifying reactive support needs.
• Short circuit study. Fault-duty analysis per NERC TPL-001-5.1, comparing pre- and post-project fault levels to determine whether breaker or equipment mitigation is needed.
• RMS dynamic performance study. Transient stability analysis using SPP's network model and your RMS load model (this is where your CMLD/PERC1 gets exercised). SPP evaluates frequency response, voltage recovery, transient stability, load tripping and reconnection, ramping, energization, and fault ride-through against the SPP Disturbance Performance Requirements and NERC TPL-001-5.1.
• EMT screening — the SCRCCT gate. SPP screens the POI's system strength using Short Circuit Ratio (SCR), Weighted SCR (WSCR), Composite SCR (CSCR), and Critical Clearing Time (CCT). The thresholds: SCR/WSCR/CSCR ≥ 6.0 and CCT ≥ 0.15 seconds. Fall below either, and your project proceeds to the supplemental stage.

Step 3 — The supplemental study stage (if screening fails).

Here is the schedule trap: supplemental studies are performed outside the 90-day window, and they require a detailed EMT (PSCAD) model of your facility. If you didn't submit one with the application, you now build one while your project waits. This is precisely why SPP recommends — and we insist — that the PSCAD model be part of the original package.

The supplemental stage can include:

• EMT dynamic performance study — detailed electromagnetic-transient simulation of the facility against the transmission network.
• Sub-synchronous oscillation (SSO) screening and detailed studies — impedance frequency scans at the POI per CIGRE Technical Brochure 909, under contingencies up to N-5, checking for sub-synchronous resonance (SSR) and sub-synchronous control interactions (SSCI) between your converters and the network.
• Converter-driven stability studies — screening via SCR and weighted Multi-Infeed Interaction Factor (wMIIF), with detailed EMT analysis of control interactions between your facility, nearby converters, and third-party equipment.
• Emergency power control (EPC) study — verification that the facility's emergency power controls and ramp-rate capability perform as designed, tested against a Thévenin network equivalent.
• Active/reactive power capability and controller studies — confirmation of P/Q capability at the POI and step-response verification of the facility's power controllers.
• Power quality study — harmonic compliance, energization voltage-step limits, and operation under unbalanced conditions.
• Fault ride-through EMT study — direct demonstration of the facility riding through the required voltage/frequency envelopes.
• Network model verification and HILL model verification — SPP checks its own RMS-vs-EMT network consistency, and critically, checks that your RMS and EMT models agree with each other.

Step 4 — Study report and next steps.



SPP delivers the HDPS report within the 90 days (base stage). Results are valid for one year, and the Transmission Customer has one year from posting to notify SPP of intent to add the load to its NITSA (or Provisional Service Agreement under AX).

Part 5: The Fault Ride Through Requirements What Your Facility Must Actually Do

SPP's HILL Fault Ride-Through Requirements are where the framework moves from paperwork to physics. These are binding performance expectations, evaluated through modeling, commissioning tests, and operational monitoring. The highlights:

Voltage ride-through (measured at the POI).

The facility must operate continuously between 0.90 and 1.10 per-unit voltage. It must ride through 0.80–0.90 pu for at least 2.0 seconds, 0.50–0.80 pu for 0.5 seconds, voltages below 0.50 pu for 0.15 seconds, and overvoltage of 1.10–1.20 pu for 0.5 seconds. The envelope is aligned with the ITIC (formerly CBEMA) curve and ERCOT's large electronic load proposal.

Behavior during sags — not just survival.

During shallow sags (0.5–0.8 pu), the load must keep drawing power from the grid, with load reduction proportional to the voltage dip. For deep sags below 0.5 pu, the load may transfer to UPS or trip — but it must return to at least 90% of pre-disturbance consumption within one second of voltage recovering to 0.9 pu. In weak-grid conditions, that recovery time can be extended in consultation with SPP and the TO to avoid post-fault oscillations.

Constant current, not constant power.

Power-electronic loads must use constant-current control during disturbances. Constant-power control is prohibited — because a constant-power load increases its current draw as voltage falls, which is exactly the behavior that deepens sags and drives voltage collapse.

Reclosing ride-through.

Transmission faults are often cleared by breakers that automatically reclose — sometimes into a still-faulted line. Your facility must withstand up to six voltage fault-clearing attempts within a 90-second period before protection-driven disconnection is acceptable.

Transient overvoltage and frequency.

Instantaneous overvoltage withstand follows IEEE 2800-2022 Table 14 (cumulative durations over a one-minute window — e.g., 3 ms above 1.40 pu, 15 ms above 1.20 pu). Frequency ride-through follows a PRC-029-1-based profile: continuous operation from 58.8 to 61.2 Hz, and 299 seconds of ride-through out to 57.0 and 61.8 Hz.

The VSD exemption opportunity.

The requirements include a pragmatic carve-out: variable-speed drives (a large fraction of any data center's cooling plant) may be exempted from the constant-current requirement, the deep-sag minimum duration, and the transient overvoltage requirements — but only if the HDPS demonstrates the exemption will not adversely affect SPP grid reliability. That demonstration is a modeling exercise. Done well, it can meaningfully reduce your equipment specification and compliance cost.

Co-located generation has its own rules.

Inverter-based resources behind the same POI must meet IEEE 2800-2022 Clause 7 (with SPP-specified decision points, including reactive-current priority during faults); synchronous machines follow NERC PRC-024-4.

How Keentel helps:

Keentel performs facility-level FRT gap assessments — comparing your actual UPS transfer settings, VSD behavior, and protection philosophy against the SPP tables before models are built — and prepares the technical justification for VSD exemptions where the physics supports them. Finding a ride-through gap in Week 1 costs a design memo; finding it in the SPP study report costs a restudy.

Part 6: The Models CMLD, PERC1, and PSCAD

Three models sit at the heart of every HILL submittal, and SPP will formally verify that they agree with each other.

CMLD (Composite Load Model – Dynamic).

The WECC composite load model (CMPLDW) represents the facility as a mix of motor types, electronic load, and static load behind a feeder/transformer equivalent, with protection-based tripping and reconnection. For a data center, the parameterization must capture the electronic (UPS-served) fraction, the VSD-driven cooling fraction, any direct-connected motors, and the trip/reconnect thresholds that mirror the FRT envelope.

PERC1 — SPP's preferred model.

PERC1 is the newer performance-based electronic load model developed through industry load-modeling efforts specifically for power-electronic-dominant facilities. Rather than approximating a data center as a motor mix, PERC1 directly represents the behaviors SPP's FRT requirements are written around: proportional load reduction during sags, constant-current disturbance behavior, deep-sag transfer/trip logic, and the timed recovery ramp. SPP's submittal requirements list load modeling data as "CMLD and/or PERC1 (preferred)" — submitting a well-parameterized PERC1 signals to SPP's study engineers that your team knows the current framework.

PSCAD (EMT).

The electromagnetic-transient model represents the facility at the converter level: UPS rectifier front-ends with their ride-through, transfer, and recovery controls; VSD drive stages; transformers and the collector system; and facility protection logic. Because the supplemental studies exercise this model against Thévenin equivalents, impedance scans, unbalanced faults, energization events, and multi-reclose sequences, the model must be built to a much higher standard than "it runs": stable initialization, flat start, numerical robustness, and validated ride-through behavior.

Model verification is not optional.

SPP's HILL Model Verification study compares the responses of your RMS and EMT models to confirm consistency. If your CMLD/PERC1 and PSCAD models were developed by different parties at different times from different data, discrepancies surface during SPP's study — the most expensive possible moment. The models should be developed as a matched pair and benchmarked before submittal.

A note on PNNL reference models.

Pacific Northwest National Laboratory has published excellent reference models and parameterizations for data center loads, and many projects rightly start there. But reference models represent a generic facility. SPP's process — particularly the FRT validation and model verification steps — requires parameters traceable to your equipment: your UPS topology and transfer settings, your chiller and DLC plant, your block-load ramp rates, your protection settings. Refinement from PNNL baselines to site-specific, SPP-format models is exactly the specialized scope most project teams need help with.

How Keentel helps:

Keentel Engineering develops CMLD and PERC1 parameter sets and PSCAD facility models as a matched, mutually benchmarked pair — validated against the full SPP FRT envelope (voltage sag tables, IEEE 2800 TOV cumulative-duration logic, frequency profiles, six-reclose sequences, and one-second recovery) and packaged in SPP-submittal formats, including IDEV-compatible records and Business Practice 7850 documentation. We build every PSCAD model supplemental-stage-ready, so a failed SCRCCT screen costs your project analysis time, not model-development time.

Part 7: A Realistic Timeline

For a well-prepared project, the sequence looks like this:

The two levers a developer actually controls are at the top of the table: how fast the package gets complete, and whether the PSCAD model is in it. Everything after that runs on SPP's clock.

Part 8: The Seven Most Common Mistakes We See

1. Submitting without load models and assuming SPP will "work with you." The clock simply doesn't start.
2. Submitting CMLD only, when SPP prefers PERC1. At best a data-validation comment cycle; at worst, a model that can't represent the required disturbance behaviors.
3. Skipping the PSCAD model to save budget. If SCRCCT screening fails at a weak POI — common in load pockets — the project waits months while a model is built.
4. Developing RMS and EMT models independently. SPP's model verification study will find the inconsistencies for you.
5. Modeling a facility the equipment can't deliver. If the UPS transfer settings or VSD ride-through don't actually meet the FRT tables, a beautiful model just documents non-compliance.
6. Ignoring the VSD exemption pathway. Teams over-specify equipment to meet requirements the tariff would have exempted them from — if anyone had run the demonstration.
7. Treating the Additional HILL Characteristics Form as paperwork. The form's answers must match the models. Inconsistencies between the form, the forecast, and the model parameters are the fastest route to a deficiency notice.
   

Frequently Asked Questions

How Keentel Engineering Supports HILL Projects

Keentel Engineering provides end-to-end dynamic modeling and study support for large load interconnections in SPP and across North American markets:

• CMLD and PERC1 dynamic load model development — site-specific parameterization from your equipment data, benchmarked against reference models (including PNNL baselines), encoded to the SPP FRT envelope, and delivered in SPP submittal formats with Business Practice 7850 documentation.
• PSCAD EMT model development and validation — supplemental-stage-ready facility models covering UPS front-ends, VSD cooling plant, protection and reconnection logic; validated against the full SPP ride-through requirements including reclose sequences, TOV cumulative-duration logic, and recovery performance.
• RMS-vs-EMT model verification — matched-pair benchmarking that pre-satisfies SPP's HILL Model Verification study.
• FRT gap assessments and VSD exemption studies — equipment-versus-requirement gap analysis before model build, and HDPS-basis technical justification for VSD exemptions.
• Submittal package support — Additional HILL Characteristics Form, IDEV files, and coordination of the load forecast data set.
• Study-cycle coordination — scoping call support, SPP/TO comment response, model resubmittals, and on-call engineering through base and supplemental study stages.
• Pathway and strategy advisory — AQ vs. AX vs. CHILL evaluation, co-located generation (HILLGA/HILLGIA) coordination, and weak-grid mitigation planning.

Every engagement is performed under the responsible charge of a licensed Professional Engineer, with mobilization typically within two business days of notice to proceed.

Ready to start — or stuck in a comment cycle?

Contact Keentel Engineering at contact@keentelengineering.com or visit keentelengineering.com to schedule a technical consultation. Bring your one-line and your UPS datasheets; we'll bring the roadmap.

Keentel Engineering — Power System Studies | EMT & Dynamic Modeling | Interconnection Support

This article reflects the SPP HILL framework as of mid-2026, including RR696, RR720, and RR724, the HDPS Overview, and SPP's HILL Fault Ride-Through Requirements (V2.0). SPP's requirements continue to evolve through the stakeholder process; always confirm current requirements against SPP's OASIS studies page and governing documents before submittal.