PV Plant Interconnection Procedures

Author: Sandia National Laboratories^[1]

This article provides an overview of the Federal Energy Regulatory Commission (FERC) Standard Large Generator Interconnection Procedures (LGIP). It should be noted that the following examines the LGIP as of the issuance of FERC Order 2003-C.

In general, interconnection of new large-scale generating facilities takes place in accordance with the transmission provider’s pro forma Open Access Transmission Tariff (OATT) on file with the FERC^[2]. FERC Order 2003 sets forth the Standard LGIP for generators greater than 20 MW^[3] as well as the Standard Large Generator Interconnection Agreement (LGIA). These documents lay out the responsibilities of the both the transmission provider and the interconnection customer.

• 1 Application Process and Data Requirements
  • 1.1 Initiating an Interconnection Request
    • 1.1.1 Types of Interconnection Service
  • 1.2 Interconnection Study Stages and Their Scope
    • 1.2.1 Feasibility Study
    • 1.2.2 System Impact Study
    • 1.2.3 Facilities Study
  • 1.3 Queuing
    • 1.3.1 Serial and Clustering Approaches
    • 1.3.2 Queue Reform
• 2 Project Data and Modeling Requirements
  • 2.1 Load Flow Data
  • 2.2 Dynamics Data
• 3 References

Application Process and Data Requirements

When an interconnection request is made and deemed “perfected” or completed by the transmission provider, the request is entered into the interconnection queue. The transmission provider uses the queue to determine the sequence in which the interconnection request will be studied. After initiating an interconnection request, a three-step process is used to determine the interconnection costs and construction sequencing:

1. the Interconnection Feasibility Study (Feasibility Study);
2. the Interconnection System Impact Study (SIS); and
3. the Interconnection Facilities Study (Facilities Study).

The interconnection customer is obligated to pay the transmission provider a certain deposit for the performance of each study. After completion of the studies, the difference between the deposit and the actual cost incurred will be paid by or refunded to the interconnection customer. The interconnection customer is responsible for providing project data needed for the studies to proceed. The interconnection process concludes with the signing and execution of the LGIA, or a withdrawal of the application. During the interconnection study process, most proposed projects have not finalized aspects of their design, possibly including the project size. The interconnection procedures allows for modifications of the application to be made at certain junctures during the study process. Such modifications are evaluated by the transmission provider to determine if the change would impact the position on the interconnection queue.
The purpose of the interconnection studies is to determine the system upgrades necessary to interconnect the proposed project and the associated cost and construction schedule. It should be noted that securing delivery rights entails a separate application and study process not covered in this article. Additional upgrades may be required for delivery service. It should be noted that, in general, interconnection service does not convey delivery service.
The interconnection process can be technically complex when it involves multiple proposed projects in the same general location. Sometimes the studies must be conducted in coordination with other transmission providers that may be affected by the proposed interconnection. For these reasons, it can take months or even years for an interconnection request to go through entire interconnection process.

Initiating an Interconnection Request

To begin an interconnection request, the interconnection customers must complete the data request per Appendix A to the pro forma OATT and provide a refundable deposit to the transmission provider. The deposit will be applied towards the actual cost of the interconnection study. The data to be provided at the initiation of the interconnection request are the following:

• Type of interconnection service request (Network Resource Interconnection or Energy Resource Interconnection);
• Location of the proposed facility;
• Maximum MW electrical output during summer and winter seasons (Note: This is specific for conventional generators due to weather differences and cooling efficiencies between summer and winter. PV is different in that the max output is defined by the inverter MVA rating.);
• General description of the equipment configuration;
• Commercial operation date;
• Approximate location of the proposed Point of Interconnection (POI) (optional); and
• Generation facility data (Attachment A to Appendix 1 of the OATT)
  • Generator facility data
  • Data sheets for power flow and dynamic modeling.

Much of the generator facility data requested in Attachment A does not apply to inverter-based generators like PV plants. Instead relevant information regarding the inverters should be provided. In addition to (manufacturer, model name, number and version, other relevant information will be needed to conduct the studies.

Types of Interconnection Service

There are two types of interconnection service that can be requested at the onset of a large generator interconnection procedure:

• Energy Resource Interconnection Service or
• Network Resource Interconnection Service.

The choice of interconnection service may affect the assumptions under which the proposed project will be studied and required interconnection upgrades. Energy Resource Interconnection Service assumes that the proposed project will deliver its energy using the existing transmission system on an “as available” basis. Network Resource Interconnection Service roughly implies that the interconnection studies will determine network upgrades to allow for delivery of energy to the Transmission Provider’s network.

Interconnection Study Stages and Their Scope

The LGIP defines three study stages:

• the Interconnection Feasibility Study,
• the Interconnection System Impact Study, and
• the Interconnection Facilities Study.

Each of these studies comes with its own study agreement, study assumptions, and procedures.

Feasibility Study

The Feasibility Study Agreement is contained in Appendix 2 of the LGIP and must be signed and provided to the transmission provider along with a deposit for the performance of the study. After completion of the study, the difference between the deposit and the actual cost incurred will be paid by or refunded to the interconnection customer. The purpose of the Interconnection Feasibility Study is to identify any circuit breaker short circuit capability limits that may be exceeded as a result of the proposed interconnection, identify any thermal overload or voltage limit violations caused by the interconnection, and identify and estimate the cost of any facilities required to interconnect the proposed generation. Technically, the study consists of a power flow and a short circuit study. Before the studies begin, the interconnection customer must provide a designated POI and configuration to be studied as well as alternative POIs and configuration(s) as requested in Attachment A of LGIP’s Appendix 2.

System Impact Study

The SIS Agreement is contained in Appendix 3 of the LGIP and must be signed and returned to the transmission provider along with a deposit. The LGIP also requires that the interconnection customer demonstrate control of the proposed site before proceeding with the SIS. The purpose of the SIS is to evaluate the impact to the reliability of the transmission system after the interconnection of the proposed facility. The study consists of a short circuit analysis, stability analysis, and a power flow analysis. The interconnection customer must provide a designated POI and configuration to be studied along with alternative(s) POIs and configuration if these have changed from the Feasibility Study. Before the execution of the SIS Agreement, modifications to the interconnection request are permitted under certain instances without impacting the proposed project’s position on the queue. Per Section 4.4.1 of the LGIP, modifications allowed are:

• A decrease of up 60% of electrical output;
• Modifying technical parameters of the generating facility technology or the step-up transformer impedance; and
• Modifying the interconnection configuration.

Additional modifications may require an evaluation by the transmission provider to determine whether the modification is material. Any change to the POI constitutes a Material Modification, which as defined by the LGIPis a modification that has a material impact on the cost or timing of any interconnection request with a later queue priority date. A restudy may be required in some cases, for example, when there are significant changes in the interconnection request or in the projects that hold senior positions in the interconnection queue.

Facilities Study

The Facilities Study Agreement is contained in Appendix 4 of the LGIP and must be signed and returned to the transmission provider along with the greater of $100,000 or an estimate of the monthly cost of performing the Facilities Study. The purpose of the Facilities Study is to specify and estimate the cost of the equipment, engineering, and construction work needed to interconnect the proposed facility. The study will also identify electrical configurations of the transformer(s), switchgear, meters, and other station equipment. Finally, the study will also identify the nature and estimated cost of any transmission network upgrades needed as a result of the interconnection. Attachment A to Appendix 4 of the LGIP provides a customer schedule election for conducting the Facilities Study. The interconnection customer is provided with two options regarding the accuracy of the cost estimates to be provided in the Facilities Study. The options are:

1. ninety (90) calendar days with no more than a ± 20 percent cost estimate in the report, or
2. one hundred eighty (180) calendar days with no more than a ± 10 percent cost estimate.

Before the execution of the Facilities Study Agreement, modifications to the interconnection request are permitted under certain instances without affecting the queue position. Per Section 4.4.2 of the LGIP, modifications allowed at this stage are:

• An additional 15% decrease of electrical output; and
• Modifying technical parameters of the generating facility technology or the step-up transformer impedance.

Other modifications proposed before the initiation of the Facilities Study Agreement are subject to a material modification determination under the same rules set forth under the SIS Agreement.

Queuing

The interconnection queue is essentially a first-come, first-served process that determines the order in which a interconnection customer’s proposed facility will be studied. Upon receipt of a valid interconnection request, the transmission provider will assign the interconnection customer a queue position. The queue position is also important because it could determine the cost responsibility of the interconnection customer for the facilities needed to interconnect. Interconnection studies are conducted for each generator in the order in which they are filed and take into account all other interconnection requests and transmission service requests that hold senior positions in the queue. With the number of interconnection requests growing, especially for renewables, interconnection queues in regions with Regional Transmission Organizations (RTOs) and Independent System Operators (ISOs) have become extremely backlogged. In March 2008, FERC held a technical conference and subsequently issued an order directing transmission providers to provide the status of their queues along with proposals for queue process reforms.^[4] The FERC guidance was aimed at speeding the process of the interconnection studies by:

• Increasing staff to perform studies;
• Adopt more efficient modeling for studies; or
• Cluster interconnection requests in a single study.

The process of “clustering” multiple interconnection requests to evaluate the impacts to the transmission system in a single study is contained within the LGIP. Typically the transmission provider will have a “Queue Cluster Window” of no more than 180 days where all interconnection requests received within the window are studied together.
Many transmission providers are in the process of evaluating proposed reforms to the current FERC pro forma standards for queue process and management. Reform proposals are specific to the ISO or RTO and often include greater financial deposits or full demonstration of site control from interconnection customers to enter the queue (first-ready, first-served), interconnection studying techniques such as clustering, and tighter restrictions on suspension of interconnection status for projects.

Serial and Clustering Approaches

The serial approach to interconnection studies is the traditional style of the FERC LGIP. This is the first-come, first served approach in which interconnection requests are studied individually with the transmission system model based on the time the request was submitted and any modifications made from earlier interconnection requests.
The clustering approach is one that many transmission providers are moving towards. In a clustering approach, transmission providers have a window of opportunity during which interconnection requests are accepted (typically this happens twice a year). Upon the closure of the window, all valid interconnection requests are studied simultaneously. The benefit of doing this is that the interconnection studies are done for multiple projects at once. Network upgrade costs for a cluster are often shared among the interconnection projects in the cluster.

Queue Reform

As a consequence of the time sensitive governmental incentives described for renewable energy described above, there has been a charge to submit interconnection requests in order to secure interconnection rights for access to transmission. That charge has created a severe backlog of interconnection requests in many areas across the nation. The FERC has taken notice of the backlog issue and has directed RTOs and ISOs to report on the status of their efforts to improve the processing of their interconnection queues.
RTOs and ISOs have identified the two biggest problems with the original interconnection requirements as the ease of entry into the interconnection queues and the serial study approach. Many transmission providers are increasing the deposit amount to enter the queue as well as requiring the applicant to meet certain milestones such as power contracts, permit acquisitions, and transmission service security deposits in order to secure a position within the queue. Transmission providers are also opting to move to the clustering approach for interconnection studies and are tightening their requirements for allowing generating projects to suspend their interconnection requests.
Interconnection queues and study processes vary from region to region. Below are some of the reforms the various RTOs and ISOs have taken to improve the process:^[5]

• California ISO (CAISO), Midwest Independent Transmission System Operator (MISO), New York ISO (NYISO), and PJM have moved from a serial approach to a cluster or group study approach;
• MISO, Southwest Power Pool (SPP) and NYISO have moved from a first-come, first-served approach to a first-ready, first-served approach; and
• ISO-NE has increased the deposit levels throughout the interconnection process.

Project Data and Modeling Requirements

The interconnection study process entails a fair amount of simulation and analysis to determine the impact of the proposed project and identify mitigation alternatives. Traditionally, three types of analysis are conducted to study the performance of the bulk power system. They are:

1. short circuit,
2. steady-state power flow, and
3. dynamic transient stability.

In these studies, the proposed project is represented along with the rest of the transmission system and its respective generators, loads, transformers, and other electrical equipment. Models and other project data must be provided in a format that transmission providers can incorporate in their simulation platforms. While there are many power flow and dynamic simulation tools available, most transmission providers in the United States use Positive Sequence Load Flow (PSLF) and Power System Simulation for Engineering (PSSE) programs for power flow and dynamic simulations.
To date there has been a lot of work understanding the data requirements for wind generators on the system but little for PV system. Recently, the Renewable Energy Modeling Task Force (REMTF) of the Western Electricity Coordinating Council (WECC) expanded its scope to address the modeling issues of PV systems. A good resource for PV system modeling an REMTF’s document titled WECC Guide for Representation of Photovoltaic Systems in Large-Scale Load Flow Simulations^[6].

Load Flow Data

Large central PV systems have a complex internal configuration and it is not practical or necessary to represent each of the components in the context of interconnection studies. The WECC Guide recommends that large-scale PV systems be modeled as a single machine equivalent.
Quoting from the WECC Guide,
“In this model, the equivalent generator represents the total generating capacity of all the inverters, the equivalent pad-mounted transformer represents the aggregate effect of all step-up transformers, and the equivalent collector system branch represents the aggregate effect of the PV plant collector system. With the proper model parameters, this model should approximate PV plant load flow characteristics at the interconnection point, collector system real and reactive losses and voltage profile at the terminals of the “average” inverter in the PV plant.”
An important observation is that data requirements listed in the pro forma LGIP and LGIA were intended for conventional generators. A different set of data is needed for PV systems. Appendix A of the WECC Guide contains a sample PV plant data request that is more adequate for power flow representation of PV plants. It is recommended that this type of information be used to supplement the data request in the pro forma LGIP.

Dynamics Data

The power system is dynamic system and, as such, the full spectrum of possible behaviors cannot be predicted with a steady-state, static model. Dynamic issues within the power system, such as transient stability of rotating machines (generators and motors), are addressed using transient stability programs that examine the system from tens of milliseconds up to several seconds after an event. These programs require dynamic models of the synchronous machines, turbines and governors, loads, high-voltage direct current (HVDC) transmission lines, static var compensators (SVCs), inverters, and other fast-acting devices.
Access to dynamic models for PV (and wind) generators has been an issue for the industry.^[7] Manufacturer-specific, user-written, and often proprietary models are often used for interconnection studies because standard models do not yet exist or are not adequate. The effort required to work with such models is significant, and can impact study cost and time significantly. In addition, such models may not be adequate to meet North American Electric Reliability Corporation (NERC) modeling requirements for regional planning.^[8] REMTF is currently working to improve standardized dynamic models for PV (and wind) generation.

References