electrical power transmission

Understanding ISO and Utility Requirements for Solar PV Plants

When engineering SCADA systems for solar PV plants, one of the first questions that comes up is: Will the site have any interaction with a utility or ISO? If so, what requirements must the plant meet?

 

Let's take a look at the basics of ISO and utility requirements for solar PV plants.

 

First, what is the difference between an ISO and a utility?

Many people use the terms "ISO" and "utility" interchangeably, but they are quite a bit different once you look into them further.

 

Electrical utilities exist throughout the U.S. and the world. They are power supply companies that own and/or operate the facilities and infrastructure used for generation, transmission or distribution of electricity. Their primary job is to make sure that transmission is stable and customers have reliable access to electricity. Utilities can be publicly or privately owned.

 

Independent System Operators (ISOs) formed after the Federal Energy Regulatory Commission (FERC) deregulated the grid in the late 1990s. Unlike utilities, ISOs don't own any of the transmission or generation assets they manage.

 

ISOs coordinate, control and monitor the operation of the electrical power system, usually within a single U.S. state, but sometimes across multiple states. ISOs also act as a marketplace operator in wholesale power. They work on the bid side of things, and ensure that each generator is stable and able to handle their bids.

 

There are currently only a handful of ISOs across the U.S. Not all states and regions have them. As a result, the utilities in those areas don't engage in wholesale power markets. They must still conform to open access as mandated by FERC, but the power exchanges between utilities there are mostly facilitated through power purchase agreements (PPAs) and bilateral contracts.

 

A generation facility may work with both a utility and an ISO. For example, we have solar PV clients in Southern California that work with both the Southern California Edison (SCE) utility and California ISO (CAISO).

 

What is the common interaction between the SCADA provider and the utility and/or the ISO?

The operational requirements for a power plant depend on two contracts: the Power Purchase Agreement (PPA) and Interconnect Agreement (IA). The utility or ISO dictates the setpoints a generator must meet to connect to the grid. Meeting these setpoints is done through the programming of the plant controls, which is where the SCADA provider comes in.

 

At Nor-Cal, we review the IA and PPA during the design and engineering phase to ensure the plant will meet the control requirements. This often involves communication with the ISO and/or utility.

 

Generation facilities must also have what's called an RTU—Remote Telemetry Unit. A telemetry link allows a utility or ISO to receive data from the site's SCADA system, such as revenue meter data, production levels, energy readings, etc. The SCADA provider must communicate with the utility and/or ISO, as well as the site owners, to ensure they have all the data they need.

 

What are common practices for both ISO and utilities?

Both utilities and ISOs require telemetry testing to ensure the flow of data from the site to their systems.

 

Telemetry requirements for utilities vary by project. Some utilities have very minimal requirements and are hands-off when it comes to setting up the telemetry link. Other utilities are quite involved, which requires heavy coordination with the SCADA provider. Some utilities even interface with generation facilities via the same telemetry link as their regional ISO. This is true of SCE and CAISO, for example. Data transfer occurs between both of these systems at once.

 

ISOs need live site data from generation facilities in order to gauge and help stabilize the open energy market. In addition to generation data, they request data from weather stations, such as temperature and ATNS. They typically have special metering requirements as well.

 

ISO requirements can be quite rigorous and extensive. CAISO, for example, requires a telemetry device called a field Remote Intelligent Gateway (RIG) that allows them to collect data from the project site. A RIG is a data aggregator that collects data on a local basis, concentrates it, and sends it off to the ISO. At Nor-Cal, we offer a specialized service to help generation facilities meet these CAISO RIG requirements.

 

Another common practice for ISOs is the use of an Automatic Dispatch System (ADS). This functionality allows ISOs to communicate real time dispatch instructions to generators, and for generators to bid into the real time energy market. Participants can accept commands for more power at a higher bid price, or accept curtailment requests to support grid reliability. You can learn more about ADS systems in our CAISO ADS Markets article.

 

Does geographic region play a role in how stringent either utility or ISO requirements are?

The region absolutely plays a role in ISO requirements.

 

electrical substation

 

For instance, CAISO is responsible for maintaining the reliability of one of the largest electrical grids in the world. Its requirements are more stringent than ISOs for less heavily populated regions. CAISO and the Electric Reliability Council of Texas (ERCOT), for example, have somewhat similar requirements, though the communications, protocols and telemetry requirements differ. However, CAISO's interconnection procedures are far more rigorous.

 

With utilities, requirements vary on a utility-by-utility basis. In comparing utilities in one state versus another, there can be wildly different requirements.

 

Does plant size/capacity play a role in the ISO and/or utility involvement?

Definitely.

 

Most small DAS and DG sized sites don't require controls or telemetry. Once they reach a certain threshold, however, requirements "kick in" from the ISO and/or specific utility.

 

This threshold varies by ISO and utility. In Idaho, for instance, we're seeing new utility requirements come in even for smaller scale sites. These requirements necessitate the addition of hardware or equipment to ensure communication. Typically, the requirements for small scale sites aren't stringent.

 

Larger scale solar farms (typically 10 megawatts and above) will have utility and ISO requirements regardless of location.

 

How can Nor-Cal help with ISO and utility requirements?

With over 7GW worth of successfully commissioned solar PV projects across the country, our engineers and leadership are well versed in many different ISO and utility requirements.

 

Not only will we ensure that your SCADA solution will meet all the requirements, we can help walk you through the entire process. We're happy to answer any questions you have.

 

We also offer support specifically for CAISO NRI, RIG and metering compliance. Through our CAISO support services, we'll help prepare your solar facility for commercial operations.

 

Schedule a call with us today to learn more.

 

Seth White

Written by Seth White