TROLIE Primary Concepts

Before reviewing examples, users may benefit from a basic understanding of the key constructs used by the TROLIE API.

UML concept model

Since TROLIE stands for Transmission Ratings and Operating Limits Information Exchange, we start there.

Operating Limit, or simply Limit

System Operating Limit is a well-defined industry term, but the salient point for TROLIE is that a Limit satisfies the most limiting of any provided reliability criteria. Limits for Transmission Facilities are determined by the Clearinghouse after considering all Ratings Proposals for the Power System Resources associated with that Transmission Facility during a particular Period of an Operational Window. TROLIE defines limits and ratings (described below) in 3 distinct time horizons, including near term “forecasts”, real-time and (in future releases) seasonal limits.

Forecast Limits

Forecast limits refer to the 240-hour-ahead forecasted AAR data set mandated by FERC order 881. While this is a forecast of limits as they would be in real-time, in practice these are often used for various processes involved in near-term planning of transmission services, including day-ahead markets and other look-ahead resource commitment processes, transmission scheduling and outage coordination.

Real-Time Limits

In addition to forecasts, TROLIE may also be used to exchange ratings within the current hour, either as an alternative or supplement to traditional telemetry protocols such as ICCP.

These are assumed to be “real-time” limits, based on measurements of ambient conditions as opposed to forecasts. These limits will be used by Transmission Providers in real-time grid operations processes, such as state estimator and real-time markets. The clearinghouse for real-time ratings may be run more frequently than the one for forecast ratings to adapt to real-world conditions.

Rating

For TROLIE’s purposes, a Rating is simply a proposed Limit for a Period of an Operational Window from a Ratings Provider for a particular Power System Resource.

Recourse Ratings

For background the reader is directed to sections 180 and 182 of the FERC Order 881 Final Rule as well as this paragraph from the pro forma Attachment M which states:

“The Transmission Provider must use Seasonal Line Ratings as a recourse rating in the event that an AAR otherwise required to be used under this Attachment is unavailable.”

Additionally, in section 180:

“Further, while this provision establishes the seasonal line rating as the default recourse rating, the transmission provider retains the ability […] to use a different recourse rating where the transmission provider reasonably determines such a rating is necessary to ensure the safety and reliability of the transmission system.”

Here we are concerned with three circumstances that require might require a recourse rating:

  1. A Ratings Provider cannot determine an AAR for a facility with a Ratings Obligation.
  2. The Clearinghouse Provider is not in possession of a rating for a Ratings Obligation.
  3. The Clearinghouse Provider determines that the rating in its possession is apparently inaccurate.

In no situation requiring a recourse rating does the TROLIE specification constrain or dictate a course of action. However, in order to have a working definition of “recourse rating” useful in documenting the specification, we examine anticipated uses of recourse ratings in the three circumstances identified in the preceding paragraph.

In the first case, under the assumption that the Clearinghouse Provider will receive the rating, it is anticipated that the Ratings Provider shall send an appropriate recourse rating, e.g., the effective seasonal rating, when the Ratings Provider cannot determine an AAR.

The second and third cases might occur when there is a communication outage or when the rating proposed by the Ratings Provider did not meet pre-coordinated validation criteria such as timeliness or lying within reasonability bounds. Regardless, in such circumstances the Clearinghouse Provider is obliged to use a recourse rating. Typically, the recourse rating in these circumstances is anticipated to be a rating provided by the Ratings Provider separately, such as the effective seasonal rating, a verbal override, or even a previously forecasted rating.

Power System Resource, or simply Resource

A term borrowed from CIM, in the context of TROLIE, a “Resource” is an object of the exchange whose identity–the resource-id–and other details have been pre-coordinated such that both parties can assign the ratings and/or limits associated with the resource-id to a particular element in their network model.

Clearinghouse Provider

Clearinghouse Provider is how this specification refers to the entity that hosts a TROLIE server for the purposes of collecting ratings from multiple Ratings Providers for a large footprint in order to determine and disseminate Limits Snapshots. Generally, this anticipated to be a Transmission Provider (FERC), such as an ISO.

A note on new terminology

“Clearinghouse Provider” along with “Ratings Provider” and “Ratings Obligation” are indeed terms introduced by TROLIE. Defining the specific roles in the exchange explicitly and using them ubiquitously is meant to reduce ambiguity and promote generality in the specification.

Ratings Provider

A Ratings Provider is a role defined by this specification to be responsible for a set of Ratings Obligations. Ratings Providers should typically implement clients to interact with TROLIE servers to fulfill these obligations. In some cases a Clearinghouse Provider might implement processes to obtain Ratings from a Ratings Provider who does not implement a TROLIE client, submitting Ratings on behalf of that Ratings Provider.

Ratings Obligation

A Ratings Obligation is a pre-coordinated expectation to provide compliant ratings for a given resource. Usually, this means providing forecast and real-time AARs for that resource. Ratings Obligations are used to authorize Ratings Proposals.

Transmission Facilities

We refer the reader to the definitions provided by FERC and/or their Transmission Provider’s tariff, but it suffices here to understand a Transmission Facility to be the power system resource that has its Limit determined by the Clearinghouse. Typically, the Transmission Facility is a transmission line or a transformer.

Segments

For the purposes of the TROLIE specification, a Segment is a logical construct: A Transmission Facility may be comprised of one or more Segments. A distinct Ratings Provider is assigned to each Segment. This modeling is pre-coordinated and is out of scope for TROLIE, so while a Transmission Facility is generally expected to have at least one Segment, the resource-id of a Ratings Proposal might nominate a Transmission Facility itself in some implementations.

In terms of TROLIE, ratings providers are obligated to provide rating data, in the form of Proposals, against segments. On jointly owned lines or tie lines for example, each stakeholder in the line (the Transmission Facility) may be responsible for submitting Ratings Proposals against a different Segment in the model allocated to that stakeholder.

Ratings Proposals

Proposals are forecasted or real-time ratings values submitted to TROLIE against a particular Segment. They are referred to as “Proposals”, as they are inputs to the limit “clearing” process internal to TROLIE server implementations that will integrate them into a final in-use rating set. In-use limit Snapshots are a distinct data set from Proposals. Proposals may be queried as well as submitted, so that the rating provider’s original input data is always kept separately from the in-use ratings. The values in rating proposals are also known as locally limiting ratings or LLRs.

Seasonal Ratings

Seasonal Line Ratings, or simply Seasonal Ratings in TROLIE, are based on the formal definition in the pro forma attachment M of FERC order 881, which states:

(3) “Seasonal Line Rating” means a Transmission Line Rating that:

(a) Applies to a specified season, where seasons are defined by the Transmission Provider to include not fewer than four seasons in each year, and to reasonably reflect portions of the year where expected high temperatures are relatively consistent.

(b) Reflects an up-to-date forecast of ambient air temperature across the relevant season over which the rating applies.

(c) Is calculated annually, if not more frequently, for each season in the future for which Transmission Service can be requested.

Seasonal ratings are used as Recourse Ratings in a TROLIE context, as described above. They also may be used for other purposes that go beyond the AAR timeline, such as planning and other long-term studies.

Seasonal Ratings are defined against Seasons.

Seasonal Overrides

A seasonal override is submitted by a Ratings Provider in order to instruct the Clearinghouse Provider to use an alternate rating instead of the seasonal rating when the seasonal rating would have otherwise been used as the limit. Typically, a seasonal override is needed for facilities that do not provide AARs and are being derated.

Temporary AAR Exception

For facilities that are not exempt from providing AARs under Order 881, there still exists the possibility of temporary operational conditions where the facility rating cannot be ambient-adjusted for due to reliability risk, for example. Under such conditions a temporary AAR exception is submitted by the Ratings Provider to the Clearinghouse Provider, instructing the latter to use the provided static rating in lieu of an AAR for a specified period of time.

Limits Snapshots

As implied above, Snapshots are generated in TROLIE server implementations based on proposals and other inputs to generate in-use ratings for each Transmission Facility. TROLIE allows for ratings providers to fetch the latest snapshot, aka the latest “version” of the ratings data. Depending on the context, limit snapshots may represent either RLRs or GLRs.

Forecast Window

See Forecast Windows

Monitoring Sets

Monitoring Sets are named sets of power system objects that may be used to filter ratings and limits returned by queries against these APIs. How Monitoring Sets are defined is beyond the scope of the TROLIE specification, and it is assumed that the sender and receiver have predefined the appropriate Monitoring Sets.

A typical implementation might define a Monitoring Set for each Ratings Provider, containing all of the power system objects of interest to that Rating Provider, such as their owned and/or operated facilities as well as any additional objects whose limits they might monitor. It is generally assumed that the Ratings Provider’s Monitoring Set would include all of the transmission facilities or other power system objects for which they have a Ratings Obligation as well as their so-called “tier 1” monitored elements.

Another typical Monitoring Set would be that which nominates the complete footprint for the Transmission Provider. A natural choice for the monitoring-set identifier is the NERC id of the entity that defines the monitoring-set, if applicable.

Seasons

According to pro forma attachment M of FERC 881, a season has the following definition:

… where seasons are defined by the Transmission Provider to include not fewer than four seasons in each year, and to reasonably reflect portions of the year where expected high temperatures are relatively consistent.

In practical terms, Transmission Providers have a wide variety of Seasons that they operate to.
Even if seasons across two Transmission Providers have the same name, such as “summer”, the start and end dates may differ. TROLIE is designed to be agnostic to these definitions as much as possible, so that the data may be exchanged in a more accurate and precise manner. While seasonal “aliases” such as “winter” may sometimes be provided as hints, seasonal rating schedules are ultimately defined in terms of start and end dates. Seasonal ratings in TROLIE are represented in terms of start and end dates, and TROLIE servers may decide how to enforce adherence to specific named seasons that they use.

Locally Limiting Rating

Locally Limiting Ratings (LLRs) refer to the output of individual ratings calculations, pre-clearinghouse. LLRs are what is represented in rating proposals in TROLIE. Nominally, LLRs are used as RLRs, and ultimately as GLRs. This is ultimately subject to the output of the clearinghouse.

Regionally Limiting Rating

Regionally Limiting Ratings (RLRs) refer to the output of rating clearinghouses for a particular Transmission Provider. For most facilities, these are the ratings the system is operated to. These ratings are the best they can be based on knowledge within the Transmission Provider’s region. RLRs for most facilities are naturally Globally Limiting Ratings (GLRs). However, for interties between transmission providers, known colloquially as “tie lines”, RLRs are incomplete. They need to be reconciled with the appropriate neighbor clearinghouse to be known as GLRs.

Globally Limiting Rating

Globally Limiting Ratings (GLRs) refer to the resulting ratings that the power grid is operated to after all ratings have been coordinated. For most facilities, these should simply be RLRs passed through. However, this is not the case for inter-ties across Transmission Providers, colloquially known as tie lines. RLRs on tie-lines must be reconciled across the different regions in order to become global. The word “global” implies that the rating now incorporates knowledge across regions.