Metering is required by the utility to measure how much electricity is used by the customers, and it is referred to as revenue meter. These meters are usually installed at service entrances of properties. Since the addition of DG sources will introduce another energy source, it is required to accommodate for that addition by measuring the added electricity based on the facility and DG system size and interconnection policies at the location of installation. This can be accomplished using one of the following methods:
Using one meter that can operate in both directions (spins forwards and backwards) to measure the exported energy and subtract it from the imported energy. Some existing meters are capable of operating in both directions without any modifications while other old meters need to be upgraded by the utility company. Designers and customers should consult with their state rules for the eligibility of the net metering.
In this case and as the name entails, two-meters (unidirectional meters) are required to be present at the facility. This is usually common for larger PV systems. In this case and due to what is referred to as “net purchase and sale” in most places, excess energy produced at the customer location from any DG source can be purchased by the utility at a different rate from the customer rate when the customer buys the electricity. The rate is agreed upon when signing the contract.
Utility interconnection policies
One of the main barriers to the expansion and adoption of PV systems is the utility interconnection policies that are established by federal, state, and local governments. Local utility companies may enforce interconnection policies where other governmental policies are absent.
Public Utilities Regulatory Policy Act (PURPA)
A number of policies have been developed over the past 30 years that impacted the interconnection of privately owned power generation systems at the state and federal levels. In general, PURPA specifies the qualifying facility and agreements to meet certain technical and procedural requirements to be interconnected to the utility grid. PURPA practices are overseen by the Federal Electric Regulatory Commission (FERC), which is responsible for overseeing the electric utility industry in the US.
Interconnection agreement is a contract between a distributed power producer and electric utility under specific interconnection terms and conditions. Interconnection of PV systems must be approved by the local utility with cooperation of the local AHJ.
The interconnection process begins after submitting an interconnection plan to the local utility along with the system design application for a permit with the local AHJ.
After the permit is granted, PV installation can be completed and then inspected by the AHJ, and utilities in some cases, to approve the interconnection based on national codes and standards. Interconnection documents must include a conceptual system design showing the location of main parts of the systems with distances and the electrical one-line diagrams that show the main electrical calculations for PV system components and interconnection methods, protection devices, and disconnects used. Finally, the system can be interconnected, commissioned, and operated.
Interconnection agreement by utility in most cases mandates that any interconnected DG source including a PV system have liability insurance, system inspection and monitoring, system maintenance, and disconnects on the outside of the facility so the utility can have access to it at any time.