Post-SGIA Interconnection Studies in ERCOT: From Agreement to Synchronization

Why the work after the SGIA decides your energization date

Signing the Standard Generation Interconnection Agreement (SGIA) does not mean you are cleared to energize. In ERCOT, a solar, wind or battery project still has to replace its preliminary models with as-built ones, pass ERCOT's model checks, clear a Quarterly Stability Assessment, and satisfy its Transmission Service Provider (TSP) before the first breaker closes.

This stretch is where schedules slip. The studies have to be built on final vendor data, they are reviewed by both ERCOT and the TSP, and each round of comments can cost a quarter.

This article covers what the post-SGIA phase involves, how it differs between ERCOT's TSPs, the studies you will need, and how Keentel Engineering supports EPCs, developers and owners through it. It closes with three anonymized case studies and a detailed FAQ.

Where post-SGIA sits in ERCOT's interconnection process

In ERCOT, "post-SGIA" covers everything from the signed interconnection agreement to Part 3 commercial operation. ERCOT's own process material puts a large project (10 MW or more) at 18 to 30 months from request to the Quarterly Stability Assessment (QSA), before construction and supply delays. The post-SGIA steps decide whether the back end of that schedule holds.

The sequence for a typical solar, wind or BESS project:

1. Generation Interconnection Request (GINR) studies. Screening Study, then the Full Interconnection Study (FIS), which includes the steady-state, short-circuit, stability, reactive and, where needed, sub-synchronous resonance (SSR) studies.
2. SGIA executed with the TSP. The execution date matters: projects with an SGIA on or after 1 August 2024 must meet ERCOT's preferred voltage ride-through requirements and IEEE 2800-2022 under NOGRR 245.
3. As-built model package. Final PSS®E, TSAT and PSCAD models built from the purchased equipment, with Model Quality Test (MQT) reports and unit model validation, submitted through ERCOT's RIOO system.
4. Quarterly Stability Assessment. ERCOT studies the project in a quarterly cycle. Models, PSCAD files and MQT reports must be in ahead of that cycle's deadline, or the project waits for the next quarter.
5. Resource registration and commissioning plan. Full registration data, telemetry and metering set-up, and a commissioning plan filed in RIOO.
6. Part 1: energization. The TSP confirms POI protection settings against the approved coordination study and back-feeds the plant.
7. Part 2: initial synchronization. Requires the ride-through capability package to be submitted first. Output above 20 MVA is released in hold-point steps.
8. Part 3: commercial operation. Field tests cover reactive capability, voltage regulation, primary frequency response and curtailment. The final model and verification package follows.

The studies and models in steps 3 to 8 are reviewed by two parties with different interests: ERCOT, which cares about system stability and model fidelity, and the TSP, which cares about its protection, power quality and switching procedures. Most delays come from one of the two sending comments back late in the cycle.

The Transmission Service Providers across ERCOT

ERCOT runs the grid and the market, but your plant connects to a Transmission Service Provider (TSP). The TSP signs the SGIA, builds or owns the interconnection facilities, and has its own technical requirements on protection, telemetry, power quality and switching. Keentel works with projects across all of ERCOT's TSPs, from the large investor-owned utilities to municipal systems and electric cooperatives.

The principal TSPs a renewable or storage project is likely to connect to:

The list is not exhaustive. Your TSP is named in your GINR and SGIA.

What changes from one TSP to the next

ERCOT's modeling and commissioning rules are the same for every project. What varies is the TSP layer on top:

• Facility interconnection requirements. Each TSP publishes its own requirements under NERC FAC-001, covering protection, metering, telemetry and design standards.
• Protection philosophy. Line differential versus distance schemes, direct transfer trip, breaker failure, relay platforms and the settings coordination process at the POI.
• Telemetry and communications. RTU type, ICCP points, communication paths and the data the TSP needs before back-feed.
• Power quality limits. How the TSP applies IEEE 519 harmonic limits and flicker criteria at the POI, and what background harmonic data it will share.
• Energization procedures. Switching orders, inrush limits, hold points and what must be studied before the first breaker close.
• Review cadence. How fast comments come back and which group reviews what.

On a project, we start by mapping the TSP's specific requirements against ERCOT's, so one set of models and studies satisfies both reviewers.

The post-SGIA study set, in depth

Every post-SGIA package answers one question for ERCOT and the TSP: does the plant as built behave the way the models say it will? The table below shows each piece of work, who reviews it and where it falls in the process.

1. As-built dynamic models (PSS®E and TSAT)

The models used in the interconnection studies are usually built on early or generic data. Post-SGIA, they must be rebuilt around the equipment actually purchased: the inverter make, model and firmware, the power plant controller (PPC) settings, the main power transformer impedance and taps, and the collection system. ERCOT requires PSS®E power flow and dynamic files, plus TSAT versions, with ERCOT now moving its models to PSS®E version 36. A model that does not match field equipment is rejected, however well it has been tested.

2. PSCAD electromagnetic transient (EMT) models and studies

Positive-sequence tools cannot show fast control interactions. ERCOT requires a PSCAD model for inverter-based resources, built from the manufacturer's real control code and submitted on ERCOT's PSCAD template. Where the POI is weak (low short-circuit ratio) or close to other inverter-based plants, PSCAD studies show whether controls interact, oscillate or lose synchronism. Getting a usable, ERCOT-compliant PSCAD model from the inverter supplier is one of the most common schedule risks.

3. Model Quality Tests (MQT) and unit model validation

The MQT runs the same set of tests in PSS®E, TSAT and PSCAD and overlays the results. The tests include flat start, voltage and frequency steps, high and low voltage ride-through, phase angle jumps and weak-grid cases. The three models must agree with each other, and with hardware or factory test data. Hybrid plants that share a transformer or plant controller need a combined MQT.

4. Reactive power capability

ERCOT requires intermittent renewable and storage resources to provide reactive power across a 0.95 leading to 0.95 lagging power factor range at the POI at full output. The study accounts for collection-system and transformer losses and charging, and decides whether capacitor banks, reactors or extra inverter capacity are needed. The same capability is then demonstrated in the Part 3 field tests.

5. Voltage and frequency ride-through

Under NOGRR 245, projects with an SGIA on or after 1 August 2024 must meet ERCOT's preferred voltage ride-through requirements and IEEE 2800-2022. Older projects must meet the legacy requirements and get as close to IEEE 2800 as reasonably possible. Before Part 2, ERCOT needs a ride-through capability package: capability curves at the Resource level, inverter and protection limits, and evidence that rate-of-change-of-frequency and phase-angle-jump settings will not trip the plant inside the no-trip zone. NERC PRC-024 and PRC-029 add their own protection-setting and ride-through obligations.

6. Harmonics, flicker and power quality

The TSP will expect the plant to meet IEEE 519 harmonic limits at the POI. The study models the frequency response of the collection cables, main power transformer, inverters and any capacitor banks, looks for resonances, and checks distortion against the limits. Flicker and voltage-step studies cover fast output changes from passing clouds and capacitor or reactor switching. If filters are needed, it is far cheaper to find out before equipment is fixed.

7. Energization and switching studies

Before the first breaker close, the TSP needs to know what will happen when the main power transformer is energized. Inrush studies estimate the voltage dip on the transmission system and check that the inrush current will not trip protection. Transient overvoltage and ferroresonance screening covers breaker operations and the open-ended collection system. The results feed the energization plan and switching order.

8. Short circuit, protection and coordination at the POI

The plant's protection must coordinate with the TSP's relays at the POI and on the line. That means an updated short-circuit model with the as-built inverter fault contribution, coordination studies, and relay settings that the TSP reviews and approves. The TSP checks these settings before it authorizes Part 1 back-feed.

9. QSA and commissioning support

The QSA runs on a quarterly cycle, and the models, PSCAD files and MQT reports must be submitted ahead of each cycle's prerequisite deadline. Missing a deadline usually means waiting a full quarter. After the QSA, the work moves to registration data, the commissioning plan, the Part 1 to 3 checklists, and the field-test reports for reactive capability, voltage regulation, primary frequency response and curtailment.

10. Post-commissioning model verification

The obligation does not end at commercial operation. ERCOT requires a final model and verification package within 30 days of Part 3 approval, an updated verification report 12 to 24 months later, and a new package within 30 days of any change to settings that affect dynamic response.

What Keentel Engineering is doing for post-SGIA projects

Keentel supports EPCs, developers and owners through the post-SGIA phase on solar, wind, BESS and hybrid projects across ERCOT, with any TSP. Our aim is one consistent set of models and studies that clears ERCOT and the TSP on the first or second pass, on the QSA cycle you are targeting.

How we run a post-SGIA engagement

1. Gap review in the first two weeks. We review your SGIA, the GINR study reports, the current models and the TSP's interconnection requirements. You get a list of what is missing, who owns it, and which QSA cycle is realistic.
2. Vendor data lock-down. We issue a data request to the inverter, PPC and transformer suppliers and follow it up until we have final data and an ERCOT-compliant PSCAD model.
3. Model build and MQT. We build or update the PSS®E, TSAT and PSCAD models and run the Model Quality Tests, with a short report ready for ERCOT.
4. TSP-facing studies. We run the reactive capability, harmonics, energization, short-circuit and protection coordination studies to the TSP's own criteria.
5. Submittals and comment resolution. We prepare the RIOO submittals and respond to ERCOT and TSP comments quickly, so the project holds its QSA slot.
6. Commissioning and verification. We support the commissioning plan, the Part 1 to 3 checklists and field-test reviews, then deliver the post-commissioning model verification report.

What you receive

• As-built PSS®E (v35 and v36 where required), TSAT and PSCAD model packages on ERCOT's templates
• Model Quality Test and unit model validation reports
• Reactive capability study, with sizing of any capacitor banks or reactors
• Ride-through capability package for Part 2, including NOGRR 245 and IEEE 2800-2022 evidence
• Harmonics, flicker and power quality study against IEEE 519 and the TSP's limits
• Energization study and input to the switching plan
• Short-circuit, protection coordination and relay settings review at the POI
• A comment-response log that tracks every ERCOT and TSP question to close-out
• Post-commissioning model verification report

Why EPCs bring us in

• One team, both reviewers. We build the models and the TSP-facing studies together, so they don't contradict each other.
• Scale of experience. Keentel's power system studies cover 4 kV to 765 kV, with interconnection, protection, NERC compliance and owner's engineer services under the same roof.
• Fits around your engineer of record. We take the specialist studies, and your EOR keeps the design. We agree the boundary at kick-off.
• Built for schedule. Every engagement is planned backwards from your QSA deadline and target synchronization date.

Three anonymized case studies

Client names, project names, locations and exact figures are withheld. The details are generalized to protect confidentiality.

Case study 1: Utility-scale solar, Gulf Coast, held its QSA window after an inverter change

Situation. An EPC took over a utility-scale solar project after the SGIA was signed. During procurement, the owner switched to a different inverter model from the one used in the interconnection studies. The existing PSS®E and PSCAD models no longer matched the plant, and the next QSA prerequisite deadline was about ten weeks away.

What we did. We ran a gap review in the first week and showed which studies had to be redone. We worked directly with the inverter supplier to get an ERCOT-compliant PSCAD model, rebuilt the PSS®E and TSAT models around the new equipment, and reran the MQT. The reactive capability study showed the plant was short of the 0.95 lagging requirement at full output, so we sized a capacitor bank before the substation design was frozen.

Outcome. The model package went in before the deadline and the project stayed in its planned QSA cycle. The capacitor bank was added during design, not as a field change.

Case study 2: Solar plus BESS hybrid, Central Texas, harmonic resonance found before energization

Situation. A developer's solar and storage hybrid shared one main power transformer. The TSP asked for a harmonics study before it would approve energization, and gave background harmonic data for the POI substation.

What we did. We built a frequency-domain model of the collection system, transformer, inverters and the plant's capacitor bank. The scan showed a parallel resonance close to a characteristic harmonic, which could push distortion at the POI above IEEE 519 limits when the capacitor bank was in service. We tested options and recommended changing the capacitor bank into a tuned filter, then rechecked the reactive capability with the change. We also produced the combined MQT ERCOT requires for hybrids sharing a transformer and plant controller.

Outcome. The TSP accepted the study and the filter design. The issue was fixed on paper before equipment was ordered, rather than after a power quality complaint.

Case study 3: Wind repower in West Texas, weak-grid PSCAD study and ride-through package

Situation. An owner was repowering a wind plant at a remote West Texas POI with a low short-circuit ratio and several other inverter-based plants nearby. The amended SGIA brought the project under ERCOT's preferred ride-through requirements and IEEE 2800-2022.

What we did. We ran PSCAD studies of the plant and its neighbours under weak-grid and fault conditions. These showed a control-interaction oscillation under one outage, which was resolved by retuning plant controller gains with the turbine supplier. We then assembled the ride-through capability package for Part 2, including Resource-level capability curves, protection limits, and evidence that frequency-change and phase-jump settings would not trip the plant inside the no-trip zone.

Outcome. ERCOT accepted the ride-through package, and the tuned settings were carried into the final model and post-commissioning verification report.