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.


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.

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.


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.

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.

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.