Power Engineering Software for Large-Scale Solar Applications

As PV systems get larger, AHJs are more likely to require power engineering reports that are unfamiliar to some solar professionals but well known to traditional power engineers.

In her SolarPro article “The Solar Software Ecosystem” (May/June 2016), Pamela Cargill provides an overview of the diverse software platforms developed specifically for solar market applications. Here I take an in-depth look at the types of electrical design calculations and reports that utilities and other AHJs require in large-scale PV plant applications, and the power engineering software that supports these calculations. Note that a licensed professional engineer must supervise these types of calculations to ensure accuracy and compliance with applicable codes, standards and any local conditions or special circumstances.

Electrical Design Calculations

Utilities often require additional electrical engineering studies in large-scale PV power plant applications as opposed to distributed generation sites. Depending on the AHJ and the scale and location of the proposed interconnection, large-scale PV project developers may need to provide reports related to medium-voltage conductor ampacity calculations, short-circuit studies, minimum ampere interrupting capacity (AIC) ratings, protection coordination, arc-flash hazards and harmonic resonance. In many cases, once the engineering team has completed the electrical design, including the technical specifications for each component, a single software package can perform all the necessary calculations for each report.

It is important to recognize that the maximum available utility fault current is a critical piece of information in many power engineering analyses. This value is relevant not only to short-circuit studies, but also to the follow-on studies that build on their results. The challenge is that utilities are often reluctant to provide the maximum available fault current, whether because they are pressed for time, lack the knowledge or do not wish to expose themselves to liability. Some utilities will provide this information only for a fee and after a significant time delay.

If the utility has not provided a maximum available fault-current value, engineers and designers should assume that the available fault current is infinite. This conservative approach will provide the highest degree of protection and safety. However, it can also lead to unnecessarily stringent personal protective equipment (PPE) requirements for service technicians. All else being equal, it is preferable to use a maximum value that the utility company provides. If the PV system is large enough to require its own substation or generator step-up transformer, the utility fault information is necessary to program the final OCPD settings at the PV power plant to ensure that protective equipment at nearby substations operates properly.

It is also important to recognize that conditions on the utility grid and within a PV power plant are not static. Over the operational life of the plant, engineers should continue to conduct many of the studies that utilities require during the initial project build stages. Project stakeholders should perform power engineering studies related to the equipment at the point of interconnection at least every 5 years or any time there is a substantive change to the project. If an inverter fails and must be replaced with a different make or model, for example, it is important to reevaluate electrical design calculations, as conditions within the power plant have changed.

Conductor ampacity. Many PV design professionals, including licensed contractors and engineers, can perform conductor ampacity calculations in low-voltage systems (≤2,000 V) under most common design scenarios by referring to NEC Article 310. In large-scale PV applications, power engineering software calculations provide the most benefit within the medium-voltage collection system, which operates at 2001 V or higher. While Article 310 does provide some tables for conductor application and insulation rated 2,001 V–35,000 V, these design values are based upon a set ambient temperature or soil thermal resistivity conditions that may not exist at a specific project location. Power engineering software can calculate medium-voltage conductor ampacity more accurately based on project-specific conditions.

Most MV collection systems involve conductors directly buried in native soil or installed on overhead poles. In direct-buried systems, the design needs to account for soil thermal resistivity, cable construction (single core or triplex), electrical skin resistance, neutral type, insulation temperature rating, insulation thickness (100%, 133% or 173% insulation level) and other factors. While most overhead systems use uninsulated single conductor cables, the design needs to account for maximum droop; ground wire size, if present, and whether optical or metallic; and insulator sizing. Power engineering software can evaluate medium-voltage cable ampacities based on these considerations as well as other site-specific conditions.

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