TPL-007 Transmission Planning Assessments: What’s Required and When, Complete Guide

Most Planning Coordinators and Transmission Planners don’t run into trouble with TPL-007 transmission planning because the physics is hard to understand. They run into trouble because the standard requires two different studies, on two different clocks, with two different distribution deadlines, and it’s easy to complete one half of the requirement while quietly missing the other.

TPL-007-4 Transmission System Planned Performance for Geomagnetic Disturbance Events exists because a solar storm can do something no thermal or stability contingency can: push quasi-DC current through your transformers for hours at a time. 

Get the applicability determination or the assessment cycle wrong, and you’re not looking at a paperwork gap. You’re looking at a High Violation Risk Factor finding tied to equipment that’s expensive and slow to replace.

This guide breaks down what TPL-007 transmission planning actually requires, who it applies to, and where entities most often lose the thread between the benchmark and supplemental assessments.

Why TPL-007 Exists

Geomagnetic disturbances (GMD) happen when a coronal mass ejection from the sun interacts with the earth’s magnetic field. That interaction drives geomagnetically-induced currents (GIC) slow, quasi-DC currents in the 0.0001–1.0 Hz range into long transmission lines and, from there, into transformer windings through the path of least resistance to ground.

Unlike a normal AC contingency, GIC doesn’t trip a relay and clear in cycles. It can drive a transformer into half-cycle saturation for hours, generating harmonics, consuming reactive power, and heating hot spots inside the transformer the kind of stress that led to the March 1989 Hydro-Québec blackout and contributed to FERC’s directive that NERC develop a planning standard specifically for this threat.

That directive produced two related standards: EOP-010 (operational procedures during an active GMD event) and TPL-007 (planning-stage vulnerability assessment). This article covers the planning side.

Need Engineering Support for Your GMD Vulnerability Assessment?

American Power Engineers delivers power system studies, GIC system modeling, substation design, NERC compliance support, and POI interconnection engineering across North America.

Explore Our Engineering Services

Who TPL-007 Transmission Planning Applies To

TPL-007-4 applies to four functional entities, each with defined responsibilities:

• Planning Coordinator (PC) — has a planning area that includes an applicable Facility
• Transmission Planner (TP) — has a planning area that includes an applicable Facility
• Transmission Owner (TO) — owns an applicable Facility
• Generator Owner (GO) — owns an applicable Facility

The applicability trigger is the Facility, not the entity type. TPL-007 applies to power transformers with a high-side, wye-grounded winding with a terminal voltage greater than 200 kV. Instrumentation transformers and station service transformers are explicitly excluded; they don’t carry enough GIC to matter.

This is where a lot of applicability determinations go wrong. Owners sometimes assume TPL-007 is a “high-voltage utility problem” and overlook it during interconnection studies for renewable projects.

But a GSU or collector-system transformer with a wye-grounded high side above 200 kV is in scope regardless of whether the facility is a legacy coal plant or a two-year-old solar farm which is exactly the kind of detail that should get flagged during early POI planning, not discovered during a NERC audit; see our SPP and MISO interconnection services for how GIC-relevant transformer configurations are handled during the interconnection process itself.

The Two Assessments: Benchmark vs. Supplemental

This is the single most misunderstood part of TPL-007 transmission planning. The standard requires two separate GMD Vulnerability Assessments, not one.

1. Benchmark GMD Vulnerability Assessment (Requirement R4)

• Uses the benchmark GMD event described in Attachment 1, a 1-in-100-year event based on a uniform geoelectric field applied across the system model.
• Evaluates steady-state voltage performance: whether the System remains stable, avoids voltage collapse, cascading, and uncontrolled islanding.
• Must include at least one System On-Peak Load year and one System Off-Peak Load year within the Near-Term Transmission Planning Horizon.
• Must be completed at least once every 60 calendar months.

2. Supplemental GMD Vulnerability Assessment (Requirement R8)

• Uses the supplemental GMD event based on the reference geomagnetic field waveform from the March 1989 event, but with a local enhancement factor applied to represent localized geomagnetic intensification that the uniform benchmark event doesn’t capture.
• Also evaluates steady-state performance, using the same on-peak/off-peak load conditions.
• Also required at least once every 60 calendar months.

The benchmark event tells you how the bulk system behaves under a broad, sustained storm. The supplemental event tells you how it behaves if the storm’s intensity spikes locally over your system. Entities that only run one of the two typically assume the assessments are redundant; they’re not. 

A system that clears the benchmark event cleanly can still fail the supplemental event if it has facilities concentrated in a way that a localized enhancement would stress.

Thermal Impact Assessments: Where TO and GO Obligations Kick In

Steady-state voltage performance is only half of the picture. GIC flow also heats transformers directly, which is where Transmission Owner and Generator Owner obligations under R5/R6 (benchmark-related thermal impact) and R9/R10 (supplemental-related thermal impact) come in.

The mechanics work like this:

1. The Planning Coordinator or Transmission Planner runs the GIC system model and provides each TO/GO the effective GIC flow information for their transformers (R5, R9).
2. Where that effective GIC exceeds the threshold defined in the standard, the TO or GO must perform a thermal impact assessment on that specific transformer (R6, R10), evaluating hot-spot heating and other thermal effects using OEM-specific transformer data, not generic assumptions.
3. Results feed back into whether a Corrective Action Plan is needed.

This step is where compliance packages most often stall internally; the PC/TP side of the study can be technically complete while the TO/GO side never receives, or never acts on, the transformer-specific GIC data needed to close the loop.

Corrective Action Plans and the 90-Day Clock

If either assessment identifies a System that doesn’t meet the performance requirements in Table 1, Requirement R7 requires a Corrective Action Plan (CAP) and FERC Order No. 830 specifically directed that CAPs be developed within one year of completing the GMD Vulnerability Assessment that identified the issue.

Separately, and just as easy to miss: the assessment distribution obligations require the responsible entity to distribute the completed benchmark or supplemental GMD Vulnerability Assessment to its Reliability Coordinator, adjacent Planning Coordinators, and adjacent Transmission Planners within 90 calendar days of completion, and to respond in writing to any comments received on that assessment within 90 calendar days of receipt.

Two separate 90-day clocks, one CAP deadline, all running off assessments that themselves run on a five-year cycle. Entities that track only the assessment due date and not the distribution and response clocks that follow it are the ones that show up with partial evidence packages at audit time.

Planning a Transmission, Substation, or Interconnection Project?

Our engineering team helps Planning Coordinators, Transmission Owners, and developers move from GMD vulnerability assessment through corrective action design and interconnection support.

View Engineering Services

Where TPL-007 Compliance Packages Fall Apart

A few patterns show up repeatedly across TPL-007 findings and near-misses:

• Incomplete GIC system models. The GIC system model needs accurate transformer winding configuration, grounding resistance, and substation grounding grid data for every applicable transformer data that’s often incomplete for older facilities or facilities acquired through M&A, where as-built records don’t match field conditions.
• Treating the benchmark event as sufficient on its own. As above, skipping or under-resourcing the supplemental assessment because the benchmark study “passed” is a common and incorrect shortcut.
• TO/GO transformer data gaps. Thermal impact assessments require OEM-specific hot-spot heating characteristics. For legacy transformers with incomplete manufacturer documentation, this data often has to be reconstructed rather than pulled from a datasheet.
• Multi-owner substation coordination. Where a Planning Coordinator’s planning area spans multiple Transmission Owners, the requirement to document individual and joint responsibilities for maintaining models and performing the assessment is frequently handled informally which becomes a problem the moment an auditor asks for the documented agreement itself, not just evidence that the study happened.
• Missing the distribution and response deadlines, described above, even when the underlying engineering is sound.

Building a Defensible TPL-007 Evidence Package

A GMD Vulnerability Assessment that holds up under audit generally includes:

1. Documented roles and responsibilities between the Planning Coordinator and each Transmission Planner in its area, in writing — not just implied through practice.
2. A current System model and GIC System model, with transformer grounding and winding data validated against as-built field records.
3. Written steady-state voltage performance criteria, referenced consistently across both assessments.
4. Benchmark and supplemental GMD Vulnerability Assessments, each covering an on-peak and off-peak load year, dated and version-controlled.
5. Effective GIC flow data delivered to every applicable TO/GO, with a documented process for triggering their thermal impact assessments.
6. Proof of the 90-day distribution and comment-response cycle for each completed assessment.
7. A Corrective Action Plan, where required, dated within one year of the triggering assessment.

Reconstructing items 2 and 6 after the fact particularly for entities with a mixed fleet of legacy transmission assets and newer renewable interconnections is where most internal planning teams bring in outside engineering support, especially when GIC modeling wasn’t built into the original interconnection studies.

How TPL-007 Connects to Broader NERC Planning Compliance

TPL-007 doesn’t sit in isolation from the rest of your planning obligations. It’s typically reviewed alongside the broader annual planning assessment and the O&P standards that govern the equipment data feeding into it. 

If you’re building out a full compliance program rather than addressing TPL-007 in isolation, our NERC audit readiness checklist covers how to structure evidence across standards ahead of an audit notification, and our complete NERC compliance overview walks through how TPL-007 fits into the wider O&P 693 framework for developers and owners.

FAQs

What is TPL-007 and why does it matter for transmission planning?

TPL-007 is the NERC reliability standard that requires Planning Coordinators, Transmission Planners, Transmission Owners, and Generator Owners to assess how their system performs during a geomagnetic disturbance (GMD) event. It exists because GIC flow during a solar storm can heat transformers and destabilize voltage in ways that normal thermal and stability planning studies don’t capture.

Who has to comply with TPL-007 transmission planning requirements?

Any Planning Coordinator, Transmission Planner, Transmission Owner, or Generator Owner with a Facility that includes a power transformer with a high-side, wye-grounded winding rated above 200 kV. Instrumentation and station service transformers are excluded.

What’s the difference between the benchmark and supplemental GMD Vulnerability Assessment?

The benchmark assessment (R4) evaluates a 1-in-100-year GMD event applied uniformly across the system. The supplemental assessment (R8) uses a waveform based on the March 1989 event with a local enhancement factor, capturing the risk of a geomagnetically intense event concentrated over part of the system. Both are required, one doesn’t substitute for the other.

How often must a TPL-007 GMD Vulnerability Assessment be performed?

At least once every 60 calendar months (five years) for both the benchmark and supplemental assessments, unless a triggering event or Regional Entity requirement shortens that cycle.

What happens if a GMD Vulnerability Assessment identifies a problem?

The responsible entity must develop a Corrective Action Plan (CAP) under Requirement R7. Per FERC Order No. 830, the CAP must be developed within one year of completing the assessment that identified the deficiency.

Does TPL-007 apply to solar, wind, and BESS interconnections?

Yes, if the facility includes a GSU, collector-system, or other transformer meeting the 200 kV wye-grounded high-side threshold. This is frequently overlooked during renewable interconnection planning, where GMD exposure isn’t always evaluated alongside the more commonly reviewed short-circuit and stability studies.

How is TPL-007 different from EOP-010?

TPL-007 is a planning standard that requires vulnerability assessments and corrective action plans developed ahead of time. EOP-010 is an operational standard, requiring operating procedures and plans used to respond in real time once a GMD event or warning is underway. They address the same physical threat at different stages: TPL-007 at the planning table, EOP-010 in the control room.

Why Work With American Power Engineers on TPL-007

American Power Engineers builds TPL-007 GMD Vulnerability Assessments backed by the same power system modeling depth used across our studies and interconnection work, not compliance documentation produced without the underlying GIC analysis behind it. Our support includes:

• Benchmark and supplemental GMD Vulnerability Assessments, including GIC system model development and steady-state analysis
• Reconstruction of transformer grounding and winding data for legacy facilities with incomplete records
• Thermal impact assessment support for Transmission Owners and Generator Owners
• Corrective Action Plan development aligned to the one-year FERC Order 830 deadline
• Audit-ready documentation packages covering the full requirement set, including the distribution and response evidence auditors ask for directly

Our team also supports the broader NERC O&P standard family — see our NERC O&P 693 Compliance Services for gap analysis, evidence build-out, and mock audit support across FAC, PRC, and TPL standards, and our Power System Studies team for the load flow, short circuit, and GIC modeling work that underpins a defensible TPL-007 assessment.

Contact American Power Engineers: Contact Form | Email: info@americanpowerengineers.com

---

This article is provided for general engineering and compliance education and does not constitute a compliance determination for any specific entity or Regional Entity. Applicability and evidence expectations should be confirmed against the current effective version of TPL-007 and your Regional Entity’s guidance.