Introduction

As the grid evolves, there is an increasing need and motivation for existing renewable resources (e.g., solar PV) to add storage behind the same point of interconnection. With the coupling of storage to generators come many new challenges, including various challenges to coupling a storage resource on the direct current (DC) side of a renewable resource (i.e., behind the inverters). Now the hard part, how to do that within the rules and protocols of an ISO market, such as CAISO, and is it possible to separate the storage resource from the generation resource even if they are physically intertwined?

This article compares two different metering options for a solar+storage facility:

1. AC-coupled Storage with AC Low-Side Feeder Metering: AC low-side metering on the low-side of the step-up transformer placed on separated fuel type (e.g., one for PV and one for storage) feeders thus creating two resource ID’s; and
2. DC-coupled Storage with AC High-Side and Low-Side Metering: AC high-side and low-side AC metering and DC-side metering where the storage is coupled with the solar feeder. This allows for two resource ID’s and two fuel types even though the storage is on the DC-side.

In April 2017, the CAISO put into place metering enhancement rules within the CAISO Metering Business Practice Manual (BPM)
applicable within its Balancing Authority area (BAA). These rules set the stage for metering solar+storage facilities in a way that gives the resource owner and off-taker maximum flexibility.

The metering enhancement rules and Metering BPM give entities inside of the CAISO control area the option to use a Scheduling Coordinator Meter Entity (SCME) to submit settlement quality meter data (SQMD) into the CAISO settlement system. This change to the Metering BPM allows an entity to calculate or extract 5-minute settlement meter data without a CAISO-approved meter by collecting settlement quality data from an approved and accurate device either on the DC-side or AC (alternating current) side of the solar+storage facility.

The CAISO metering enhancement rules are designed to allow SQMD energy submissions into the ISO Market Result Interface Settlement (MRI-S) system directly without an ISO-polled meter. To do this, an entity will need to comply with the CAISO AC-side metering accuracy as directed by the CAISO Metering BPM. The same CAISO metering accuracy, if applied to other interconnection facility hardware (e.g., an inverter), can be used to collect data that can derive 5-minute settlement interval data. The SQMD 5-minute intervals will be submitted into MRI-S system by the selected Scheduling Coordinator.

AC-coupled Storage with AC Low-Side Feeder Metering
Concept: The AC low-side metering is the most common metering settlement concept when a solar+storage facility must have multiple CAISO resource identifiers (“resource ID’s”) for purposes of separately accounting for the solar and storage resources and allowing for two different fuel types (i.e., solar PV and non-generator resource), perhaps because there exist multiple power purchase agreements (PPAs), there is a need for the solar resource to be considered an Eligible Intermittent Resource (EIR), to account for renewable energy credits (RECs), or for the storage resource to provide ancillary services.

Interconnection Planning Model: AC-coupling storage to a solar PV facility uses a separate feeder for each storage layout without changing the interconnection engineering study and planning design of the solar facility. The CAISO production system will model each resource ID as its own fuel type (e.g., solar and storage separate and distinct). In addition, the two resource ID’s will be treated independently in CAISO’s EMS and market models. CAISO will not view the two resource ID’s as a co-located or hybrid resource.

Interconnection Planning Model Considerations: If the solar PV facility was not studied as a co-located fuel type (i.e., solar+storage), then adding storage will require the interconnection agreement to go through the material modification assessment (MMA) process.

Revenue Settlements: Storage and renewable resources electrically connected but on separate feeders on the AC low-side of the stepup transformer can be complex if the entity owner selects polled CAISO metering for the different resource ID fuel types. In order to not violate the CPUC’s Eligible Intermittent Resource (EIR) ruling, these two fuel types must be kept separate (i.e., each have their own resource IDs). To achieve this low-side AC metering on each feeder, or collection of busses under a feeder, a resource must have a CAISO-approved meter. Then each resource ID’s meter, or group of meters, are associated with a single fuel type resource ID. CAISO can aggregate the same fuel type resource meters (e.g., multiple solar PV meters associated with the same fuel type).

DC-coupled Storage with AC High-Side and Low-Side Metering:
Concept: AC high-side metering with a DC-coupled storage resource on the same DC collection bus as the renewable resource and distributed across one or multiple inverters.

Interconnection Planning Model: DC-coupling storage to a renewable resource uses the DC buses and feeders’ electrical connection infrastructure to couple the storage resource without changing the interconnection engineering, study, and planning design of the generator interconnection facility. There may be a need to study the short circuit capabilities when adding storage to a solar DC bus to ensure the inverter can handle the bidirectional power flow, it takes less time for this type of restudy than AC coupled storage.

Revenue Settlements: The storage and the solar resources must be settled separately using an SQMD plan for each resource ID fuel type
(e.g., storage and solar separate). The SQMD must be submitted by a SCME. The data in the SCME submission is derived from:

A. single high-side revenue quality metering,
B. low-side revenue quality metering, and
C. DC metering of the solar and storage output.

With this setup, the DC metering will interface with the low-side AC metering to logically separate the solar and storage production, thereby allowing for the solar resource to appear as its own resource and therefore qualify as an EIR.

Summary
As the saying goes, “the devil is in the details.” There are, of course, a number of complexities here but the important takeaway is to know, no matter the site’s physical configuration (i.e., DC or AC-coupled storage), it is possible to separate your solar resource from your storage resource in the CAISO market. It is just a matter of identifying and implementing the right metering solution within the CAISO rules. If you have any thoughts or questions on this topic, feel free to give us a call or send an email. We’d love to chat.

Interested in learning more
GridSME will be co-hosting a webinar on Operating Hybrid Resources in CAISO on July 15th at 10:30am PT. Join Customized Energy Solutions (CES), Buchalter, and us for an in-depth discussion on metering, telemetry, market participation, and ITC considerations for solar+storage resources operating in CAISO.