Tag Archives: Aggregated Solar

How California plans to integrate distributed resources into its ISO market

By Herman K. Trabish, Utility Dive

A new era of grid operations is about to begin in California.

The state’s grid operator is preparing to offer aggregators of distributed energy resources (DERs) the opportunity to sell into its marketplace, the first in the nation to do so. DERs are the resources on the customer side or the distribution grid side of the electric system, such as rooftop solar, energy storage, plug-in electric vehicles, and demand response, and are typically below the 500 kW minimum size required to sell into the ISO system.

CAISO’s Final Plan

The “straw proposal,” an early draft of the ISO’s DERP initiative, was published last November to give stakeholders an opportunity to comment.  The final draft of the ISO’s plan answers many of the stakeholder concerns, with a focus on details of expanded metering and telemetry, the communications and counting methods, and the technologies the grid operator will need.

DER Aggregation

The ISO’s proposal provides a framework for the aggregation of DER to meet the ISO’s 0.5 MW minimum participation requirement and participate in ISO wholesale markets as an aggregated resource. The ISO proposes to classify a distributed energy resource provider or “DERP” as the owner/operator of one or more aggregations of individual distributed energy resources2 (DER) that participate in the ISO market as an aggregate resource rather than as individual resources.

Metering

In today’s California market, all of CAISO’s centralized generators have a resource identity (resource ID) and are required to have “revenue quality metering.” That can be via a direct interaction between the ISO and the resource ID, or it can be through a scheduling coordinator that mediates between the ISO and the resource ID. But for distributed resources, assigning a resource ID to each one is not feasible.

The proposal allows a scheduling coordinator to take administrative control of aggregated distributed energy accounts and meter them with any technology, including any online technology, that suits their purposes. The aggregator can be its own scheduling coordinator or can hire a third-party. A directly connected interface between the ISO and the aggregator is no longer required.

Locational dispersion and capacity of DERP aggregations

There are some 4,900 market pricing nodes (PNodes) on the ISO system. The system is also divided into load aggregation points (LAPs) that follow the territories of the state’s three investor-owned utilities. They are subdivided into sub-LAPs. With the issue of counting the DERPs clarified, the proposal takes up the question of how the ISO can keep track of the multiple sources and types and locations of DERs with which it will have to deal.

Under the new proposal, DERP aggregations may consist of one or more sub-resources at single or multiple locations. There can be multiple small resources across multiple PNodes, but they must be within one sub-LAP.  There is no minimum size limitation on the individual sub-resources in a DERP aggregation. This means that individual sub-resources may exceed the ISO’s minimum participation requirement of 0.5 MW. DERP aggregations across multiple PNodes may not exceed 20 MW, but for DERP aggregations limited to a single PNode, there is no MW size limitation.

Mixing DERs

For DERP aggregations limited to a single PNode, the sub-resources may be heterogeneous – that is, a mixture of sub-resource types is permitted, and there is no MW size limitation. It is not required that all of the sub-resources move in the same direction, only that the net movement of the aggregate of the sub-resources equate to the ISO dispatch instruction.

DERP aggregations across multiple PNodes may not exceed 20 MW. For DERP aggregations across multiple PNodes, all sub-resources within that sub-LAP must be homogenous and must move in the same direction as the ISO dispatch instruction. Homogenous aggregations are those in which all sub-resources are generation, energy storage acting together in charge or discharge only, or are load. For aggregations of energy storage, all sub-resources must be operating in the same mode (i.e., charging or discharging, but not a mix of the two) in response to an ISO dispatch.

The ISO performs network analyses to make certain the system is receiving what the market is selling into it. Sub-resources in an aggregation across multiple PNodes can cause distribution variability. But the PNode distribution variability must be minimized or “the congestion impacts estimated in the network analysis will be off.”  Until the ISO has enough operational experience to know whether the distribution variability would be a problem, it wants to limit DER aggregations “to those that move in the same direction as the ISO dispatch instruction.”

This is especially relevant to aggregated solar-plus-storage technologies that might be producing both load and generation, the final draft acknowledges. “The ISO recognizes that there is great interest in aggregating mixtures of rooftop solar, energy storage, plug-in electric vehicles, and demand response across multiple PNodes, without all the limitations required in this proposal. The ISO plans to examine such options in subsequent initiatives.”

Wait ‘Til Next Year

Several stakeholders suggested provisions be made for demand response (DR) in aggregations of distributed resources, but the ISO chose to limit its role, and does not include demand response participating as Proxy Demand Resource (PDR) or Reliability Demand Response Resource (RDRR) in the DERP proposal. In the proposal, the ISO clarifies that demand response participating as PDR or RDRR would continue to participate under its existing demand response framework and not under the DERP framework. The ISO says the existing PDR and RDRR framework already provides for market participation of aggregated demand response. This existing framework is designed to accommodate load reducing resources whose performance is assessed under a baseline methodology.

Stakeholders also suggested including in the DERP final proposal both the alternative baselines for PDR, and the alignment between distribution level interconnection and the ISO New Resource Implementation process. They are part of a separate energy storage initiative. These suggestions were declined. To facilitate bringing aggregated DERs into its marketplace, the ISO wants to include initially only those that can be directly metered under the specified terms.

The ISO will take formal comments on the final draft through June 24th. If approved by the Board in mid-July, the ISO will probably file by early autumn with the Federal Energy Regulatory Commission. That approval will require at least 60 days.

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