Distributed Energy Resources: Connection Modeling and Reliability Considerations

The North American Electric Reliability Corp. (NERC) report, Distributed Energy Resources: Connection Modeling and Reliability Considerations, helps entities, regulators, and policymakers better understand the differences between distributed energy resources (DERs) and conventional generation and how DERs affect the bulk power system.

As the DER integration on the distribution system increases, it can alter the power flow at the transmission-distribution interface. While effect of DER integration on the distribution network has been studied and analyzed extensively, the same is not true when it comes to the bulk power system. The impact of DERs on the bulk power system is a complex issue. NERC examines the potential reliability risks for increased levels of DER penetration and discusses mitigation approaches.

Key Takeaways

Risks and Mitigation Approaches

At lower penetration level, the impact of DER may not be significant on the bulk power system or transmission-distribution interface. However, as the DER penetration increases, issues related to transmission line loading, grid voltage, and system frequency during normal and disturbed operations may be a concern. Suitable care must be taken to ensure that the impact is addressed appropriately in planning and operating assessments. State regulators need to ensure that a collaborative effort is made with the utilities, DER experts, and ISO personnel to identify the risks to the bulk power system, model them appropriately, and take suitable measures to mitigate them. The report discusses various issues faced by different stakeholders and provides several approaches targeted to mitigate the risks.

Recommendations

The primary focus of the report is on modeling power system accurately enough for operation and planning for normal operations and disturbances to ensure bulk power system reliability. The foremost recommendation provided by the DER task force to the regulators is to develop a set of guidelines to assist in modeling and assessment such that the impact of DER can be accurately analyzed.

Modeling of DER includes data requirements to assess bulk power system reliability issues and modeling approaches. This can be categorized into steady-state and dynamic studies. The report delineates multiple modeling approaches for each study. Visibility and control are key issues at higher penetration as the grid operator is unable to control or communicate with the DER. In general, load and DER should be explicitly modeled, which yields valuable insights into how DERs can impact operating limits and margins in the interconnected systems. From the state interconnection rules' perspective, it is important to specify functional settings (trip and ride through requirements) because short circuit studies are significantly dependent on them.

Conventional Synchronous Generators and Non-Synchronous Distributed Energy Resources

The report also discusses the difference between a conventional synchronous generator and a non-synchronous DER. When the analysis was being conducted, the available standard was IEEE Std 1547-2003 where DERs were considered as passive devices that cannot participate in frequency and voltage regulation or communication. However, with the revised standard released, it is important for state regulators to consider the benefits of advanced inverter functionalities in the state interconnection rules depending on the integration levels.

The use case of California PUC is presented where the Smart Inverter Working Group identified the risks of high DER penetration and mitigation strategies, thereby driving the use of these functionalities. Under the grid modernization laboratory consortium, the effect of DER integration on transmission system was studied for California. The results confirmed the recommendations presented in the NERC report regarding DER modeling, data sharing, and advanced functionalities.

Addressing Shortcomings

Much of the previous work of the NERC Integration of Variable Generation Task Force discusses the shortcomings of the IEEE Std 1547-2003. The revised standard has identified and addressed them adequately. Additionally, California has expended considerable efforts into modeling the DER to study the impact on the bulk power system.