Meeting CAISO Rules for BESS Metering

Trimark’s Experts Weigh in on Separating Retail and Wholesale Meter Data for Battery Energy Storage Systems (BESS)

The rapid increase in the number of Battery Energy Storage System (BESS) participating in the California ISO (CAISO) market has resulted in changes to the Tariff designed to avoid duplicate data. Specifically, the rule requires separating wholesale and retail load in meter data. This concept is often called “BESS Idle.”

To introduce and explain this subject, we asked two Trimark experts to answer a few questions on the matter:

  • Casey James: Trimark Metering Manager, CAISO Inspector, and
  • Glen Perez: Trimark Consulting Services Director, formerly CAISO Manager of Market Services Quality and Control.

Q: What’s the simple explanation of the BESS Idle designation.

A:  When the BESS is in an “idle” state (e.g. Dispatch to Zero or Manual Mode) the CAISO revenue meter must be configured to record zeros in the load channel, thereby avoiding duplicate load charges in the wholesale settlement.

Q: What is the CAISO Tariff Rule driving this change?

A:  The CAISO Tariff Section states:

Storage resources participating in the CAISO markets may not charge their resources pursuant to a CAISO wholesale rate except to provide Energy or Ancillary Services to the CAISO Markets upon discharge.

To ensure that storage resources do not incur wholesale charges from the CAISO duplicative of any retail charges:

(a) CAISO Metered Entities’ revenue quality meters must be installed and programmed to exclude any retail Meter Data.

(b) Scheduling Coordinator Metered Entities’ SQMD Plans must describe how the Metered Entity’s metering arrangement or Validation, Estimation, and Editing procedure prevents commingling retail and CAISO Meter Data.”

Q: Do other ISO’s have a similar rule for BESS Idle?

A:  Some other authorities require a BESS Idle flag in the revenue meter program. As more BESS is commissioned for market participation, we expect the BESS Idle requirement to become a common metering practice.

Q: What does the term “BESS Idle” actually mean?

A:  “BESS Idle” is a shorthand term for a metering rule in the CAISO Tariff that aims to avoid double billing for wholesale and retail load. However, “BESS Idle” is an oversimplification of the issue. The language applies to the following situations:

  • BESS Dispatched at Zero: CAISO may dispatch a BESS at a target of zero which means the BESS is neither charging nor discharging – the classic “idle” state. Meanwhile the battery continues to draw a relatively small auxiliary (Aux) load to support internal functions. To avoid duplicating fees for this load, the CAISO meter must be programmed to record zeros for retail load during this period.
  • BESS in Manual Mode: A BESS may need to operate in manual mode during maintenance, cell replacement, or for other reasons. Manual mode could also occur when a utility needs the battery to absorb over-generation. In either case, the load should register as zero.

In this document, the term “BESS Idle” is used in its broadest context. The reader should recognize that the term applies to either of the operating states described above.

Q: What’s the difference between Station Service Power and BESS Aux Power?

Simplified drawing of a Sloth labeled "BESS" laying on a branch. Thought bubble says, "I'm Idle, Bro."

A:  Station Service Power (AKA “Station Aux”) is the load required to keep site equipment operational. Station Aux is frequently (though not always) supplied by a dedicated circuit that is separate from the charge/discharge circuit. This load is metered by a utility meter and billed at retail rates.

BESS Aux load is power used by the BESS to maintain its operations. The BESS Aux load is typically drawn from the charge/discharge circuit. When the BESS is charging or discharging, its Aux load is billed at its wholesale rate. When the BESS is “idle” its Aux load is billed at retail.

Q: How does the meter know when the BESS is “idle”?

A:  There are two basic ways to inform the meter that a metering interval must exclude load from settlement values:

  • Method 1 – CAISO Signal: The most accurate way to differentiate wholesale from retail meter data is to identify when CAISO sends a dispatch signal to the site. Upon authentication, the RIG or PLC passes a processed signal to the meter which determines if the command state should be “normal” or “idle.” There are only two CAISO-approved revenue meters that can accept an external signal in this manner and execute the required logic.
  • Method 2 – Calculative: A widely-used (though less accurate) method is to define an algorithm to determine conditions associated with non-charging loads. For example, if a BESS draws less than 500KW load for more than two minutes it is assumed that the meter is not ramping (or transient) and the load is insufficient for charging. Therefore, the BESS is idle. There is only one CAISO-approved revenue meter that can execute custom logic internally. This method is only available to SCMEs.

Q: How does the idle signal change what happens in the meter?

A: An external signal is sent to a specific register in the meter to indicate when the BESS is in idle state. The meter must be programmed to record, or not record, load data based on the signal. For example, when the signal indicates that CAISO is dispatching the BESS resource for charging, the meter will record the load in the wholesale channel. When the CAISO signal indicates that BESS is not charging or discharging, the meter will not record the load in the wholesale channel.

Q: Does the type of generation change anything related to BESS Idle?

A:  No. A battery can be affiliated with many types of generation. The BESS itself is classified as a Non-Generating Resource (NGR). New BESS resources are either standalone or built to supplement another resource, such as Photovoltaic (PV) or Wind. However, site design characteristics such as hybrid, co-located, or DC-coupling add complexity to the metering scheme which may affect the BESS Idle flag. Consult with a metering expert for guidance on your specific situation.

Trimark Metering Services

Trimark provides end-to-end metering services to help you meet Tariff rules – including the requirements described above.

  • End-to-End Solutions: We deliver complete solutions including metering schemes and networks, metering equipment, configuration, fabricated cabinets and panels, installing meters and connecting them to your CT/PTs, testing for accuracy and communications, supporting point-to-point testing, preparing forms, and inspecting and sealing the meter.
  • CAISO Revenue Metering: We’ll help you meet and maintain CAISO metering requirements – whether your resource is a CAISO Metering Entity (ISO ME) or Scheduling Coordinator Metering Entity (SC ME).
  • Complex Losses: Trimark designs solutions to ensure accurate settlements for complex environments such as PV and BESS clusters, multi-unit generation, multiple PPAs, and extended interconnection distances.
  • Dynamic Loss Compensation (DLC): When multiple sites share a generation tie line, losses change in real time. Trimark’s DLC solutions ensure accurate settlements.
  • DC Metering: Trimark supports the highest accuracy possible for DC-coupled PV+BESS resources.
  • Support: Trimark service agreements include automatic meter reading, meter data management, maintenance, repair, emergency services, SCME audits, and meter re-certification.

If you have further questions regarding configuring your meters to accommodate the BESS Idle requirement, Trimark is here to help.

Contact an Expert to Lean More >

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