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.

Short-circuit study. Understanding the magnitude and duration of short-circuit currents within an electrical system not only is a basic life and safety issue, but also is essential to the power engineering studies described below. NEC section 110.9 requires that equipment intended to clear a short circuit carry an appropriate interrupting rating. Simply speaking, the nominal maximum current rating of an overcurrent protection device—which manufacturers and testing laboratories determine based on performance under standard test conditions—must be at least equal to the available fault current at the line terminals of the equipment.

In practice, calculating maximum available short-circuit current available in an electrical system is not as intuitive as you might expect, in part because OCPDs with the same nominal interrupting rating can have very different time-current characteristics. Figure 1, for example, shows that a 100 A–rated Bussmann Series LPJ Class J fuse will open a 200 A fault in roughly 300 seconds (5 minutes) and a 1,000 A fault in approximately 0.2 seconds. Another 100 A–rated fuse may have different time-current characteristics even though its nominal interrupt rating is the same.

Though the IEEE standards pertaining to ac circuit breakers date back to the 1990s, working groups have recently revised the standards relating to both high-voltage and low-voltage devices. In 2016, IEEE published a revised version of IEEE C37.010, which covers the application of OCPDs rated above 1,000 Vac. In 2015, the international standards association published the most recent edition of IEEE C.37.13, which relates to the application of low-voltage ac OCPDs. These IEEE standards describe two short-circuit calculation methods, both of which are based on Thévenin-equivalent circuit models for each bus node within a single-line diagram.

Large-scale PV power plants integrate dozens or even hundreds of inverters, each of which the IEEE standards define as a generator. While it is impractical to manually determine all of the short-circuit calculations at each electrical node in a utility-scale PV plant, power engineering software can perform these calculations quickly and accurately.

Minimum AIC ratings. While this analysis goes hand-in-hand with a short-circuit study, it emphasizes not the magnitude and duration of a fault but rather the ability of the OCPD to clear a fault without extensive damage to the equipment or electrical system. Manufacturers and testing laboratories certify and mark listed OCPDs with an available interrupting capacity, expressed in the units AIC or kAIC. For example, low-voltage ac panelboards and circuit breakers typically carry withstand ratings from 14 kAIC to as high as 65 kAIC, whereas fuses for some applications carry ratings as high as 200 kAIC.

Not surprisingly, equipment costs within the same voltage class increase for products with a higher withstand rating. All else being equal, the installing contractor will purchase the lowest-cost product that meets engineering design specifications, which means it is very important that electrical engineers specify minimum AIC ratings for OCPDs, panelboards and other electrical equipment. Fielding improperly rated equipment could lead to catastrophic failures in the event that instantaneous fault currents exceed equipment withstand ratings.

When reviewing interrupting capacity ratings, note that NEC Section 240.86 allows for both fully rated and series-rated protection systems. In a fully rated system, each OCPD carries an AIC rating greater than or equal to the available fault current. Evaluating compliance with this parameter is a very straightforward process, and the fully rated design resists the introduction of errors over time. In a series-rated system, the available short-circuit fault current may exceed individual component withstand ratings for tested combinations of equipment or combinations of equipment selected under engineering supervision. While this option may allow for lower up-front installation costs, a licensed professional engineer needs to evaluate, document and stamp every subsequent change to the electrical system.

Protection coordination. Each branch of a multibranch electrical system has its own overcurrent protection, whether a fuse, breaker, relay or other similar device. Should a fault occur somewhere in the system, the OCPD installed between the fault-current source(s) and closest to the fault should be the one that opens and clears the fault. The main purpose of a protection coordination study, therefore, is to review the settings and ratings of each OCPD to ensure the proper fault-clearance time sequence. Short-circuit study values are obviously a primary input for a protection coordination study.

The IEEE Buff Book (Standard 242-2001) is the primary sourcebook for protection and coordination principals. Depending on the project-specific equipment, a multitude of other IEEE standards may also apply to the protection coordination study calculations. Standards in the IEEE C37 series provide information pertinent to different types of breakers, relays, air break switches and other equipment. IEEE C57.12.59-2015 provides guidance on fault-current duration calculations for dry-type transformers, whereas IEEE C57.109-1993 provides similar guidance for liquid-immersed transformers.

Arc-flash hazards. NEC 110.16 provides a representative list of electrical equipment—switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures and motor control centers—that requires field- or factory-applied markings to warn qualified persons of potential arc-flash hazards. In general, electrical equipment on or in commercial, industrial or utility facilities that requires servicing, adjustment or maintenance should have an arc-flash hazard warning label so that workers know what level of PPE to wear when working on the equipment. These arc-flash hazard analysis and warning requirements extend to PV system dc collection equipment, including combiner boxes, recombiners and inverters.

IEEE 1584-2002 is the international standard that governs arc-flash hazard calculations. A companion standard, IEEE 1584.1-2013, details the scope and deliverables for an arc-flash hazard study. The standards provide guidance regarding the input data and calculation methods for determining the arc-flash hazard distance and incident energy in a variety of scenarios, including dc systems, ac systems under 1,000 V, and ac systems between 1,000 V and 20 kV.

Whereas general arc-flash hazard warning labels meet minimum code requirements for equipment rated less than 1,200 A, Section 110.16(B) requires more-detailed labels for equipment rated 1,200 A or greater. To assist the servicing electrician with PPE selection, these equipment labels should detail the nominal system voltage, available fault current, clearing times for OCPDs and date of label application. Note that an exception to NEC 110.16(B) allows the use of alternative labels that conform to NFPA 70E, Standard for Electrical Safety in the Workplace.

Harmonic resonance. The shattering of a wine glass when a singer hits a particular note is a well-known example of harmonic resonance, a phenomenon described by physics that affects many different types of systems. A system tends to oscillate or vibrate at a larger amplitude when exposed to certain frequencies. The fact that materials and systems have a resonant frequency is generally a nonissue, as evidenced by the fact that a wine glass does not routinely shatter at the slightest vocal provocation. However, harmonic resonance can have unexpected and destructive consequences. These can occur when a source applies input energy to a system at a specific resonant frequency, having a substantially stronger physical effect on the system than expected.

The fundamental frequency of the North American electrical power system is 60 Hz. Each multiple of this frequency is an ordinal harmonic, meaning that 120 Hz is the second harmonic, 180 Hz is the third harmonic and so on. Generators and power supplies have a tendency to produce harmonics when they operate. We refer to the sum total of these harmonics as total harmonic distortion (THD). To comply with UL 1741, the inverter listing and test standard, inverters must also meet the requirements in the IEEE 1547 and IEEE 519 standards, which cover distributed resource interconnection and harmonic control requirements, respectively. According to IEEE 519, the THD a given inverter in isolation produces is limited to 3% current waveform distortion over the first 60 harmonics. Meanwhile, IEEE 1547 states that the total demand distortion (TDD) at the point of interconnection cannot exceed 3% of the current waveform distortion. These requirements are related as TDD is a measure of THD that also accounts for the peak demand load current.

The ac collection system for a large-scale PV power plant has a natural resonant frequency. Generally speaking, the resonant point for most electrical systems is located somewhere between the fifth and seventh harmonic. While resonance is usually not a problem in PV power plants, issues do arise in some projects because the resonant condition is excited where the harmonic characteristics of the inverters and any harmonic contributions from the utility match up exactly with the resonant frequency of the collection system. Though this issue may occur in just a handful of projects, it can incur substantial mitigation costs due to the time required to design, implement and test the efficacy of mitigation measures.

Performing a harmonic resonance study is an insurance policy against possible commissioning costs in the future. It is the only way to determine whether a project will experience resonance issues. If so, engineers can design mitigation measures in advance, which will avoid difficulties and delays during the compressed timelines associated with many commissioning and performance-testing activities. After the engineering team has completed the previously discussed engineering studies, it only needs the inverter manufacturer’s IEEE 519 test results and the interconnecting utility’s harmonic content information to perform a harmonic resonance study. With proper planning, you can obtain this information from both parties at the same time that you gather the fault-current contribution information needed for the short-circuit study.

Software Platforms

In theory, it is possible to perform any of these electrical calculations by hand or using self-created spreadsheet-based tools. The IEEE standards detail the required input data, essential calculations and report deliverable. In practice, the sheer volume of the calculations required at each node of the electrical system strongly favors commercial software platforms. Performing manual calculations or developing complex spreadsheets is both a tedious and error-prone process, whereas using commercially available power engineering software ensures more consistent results. Moreover, software developers offer products that not only provide the necessary electrical engineering reports but also come with equipment specifications preprogrammed or available via download.

Electrical engineers have dozens of software packages to choose from, as the underlying design equations are readily available. However, some software platforms have existed for 30 years or more, meaning their original development period dates back to the days of DOS operating systems. On the one hand, that longevity may speak to a company’s ability to adapt to changing market needs. On the other, that legacy may limit advanced functionality compared to software developed more recently in Microsoft Windows or cloud-based environments.

Power engineering software tools typically operate from intelligent single-line diagrams. The software graphically links electrical devices—such as fuses, relays, breakers, load centers, motor control centers, var compensators, transformers, PV modules, inverters, variable frequency drives and many others—with conductors to create buses. Users can either manually enter device ratings and specifications or load these data from prepopulated databases. The software links these data with each line-diagram component. Depending on the platform, users can enter vendor-supplied data, select from default options or let the software calculate values automatically. Most products in this class offer minimal or no support for iOS environments.

Qualifying vendors. An overview of four representative power engineering software platforms follows. These descriptions do not represent specific recommendations or endorsements for your particular needs. Whereas some power engineering software developers specialize in meeting the needs of utility transmission and distribution planners, other vendors focus on developing tools for industrial plant designers. Some platforms are better for plant or network design, while others support real-time power engineering analyses. To identify the best software option, you must evaluate the software vendor’s target market and applications as well as your needs and use cases.

CYME / 800.361.3627 / CYME.COM

Now part of Eaton, CYME International is based in Quebec, Canada, and has offered power engineering software solutions since 1986. Though the CYME power engineering software primarily supports transmission and distribution network modeling, users can adapt the platform for industrial plant design. In February 2017, the company launched an integration capacity analysis module to help utilities simplify distributed energy resource integration and growth planning. The platform also includes software modules for modeling dc or ac conductor ampacity, dc or ac short-circuit studies, protection coordination and harmonic analysis.

According to Eaton, power engineers can use CYME software to plan and design all types of electrical installations, from transmission down to the meshed secondary networks of distribution systems. In solar applications, users can load irradiance profiles and 8,760 hourly-load data to model the variable output of a particular PV power plant on the distribution and transmission networks. The software can also model the impacts of advanced grid-support features from inverters, such as volt-var compensation or other power factor adjustments, as well as real or total apparent power curtailment. Users can also model anti-islanding impacts based on IEEE 1547 trip settings or other values. This functionality is useful where a PV project is part of a larger microgrid application or where multiple inverters share a single point of interconnection.

EASYPOWER / 503.655.5059 / EASYPOWER.COM

Based near Portland, Oregon, EasyPower has provided power system software solutions since 1984. The EasyPower software suite automates many common code- or AHJ-mandated electrical engineering calculations and reports. Since the platform is Windows based, instead of relying on command prompts, it features a graphical interface that today’s workforce will find intuitive.

EasyPower offers a variety of preconfigured starter electrical design suites. Users can customize and add features to these core packages by downloading additional software modules. The arc-flash hazard analysis tool, for example, is an add-on module for EasyPower’s short-circuit analysis engine. EasyPower also offers harmonic analysis and OCPD coordination modules. Though users can download modules that automate certain design activities, such as conductor or OCPD sizing, these automated calculation capabilities do not eliminate the need for engineering oversight and review. The output reports can focus and expedite these activities.

The reporting functions in EasyPower also include arc-flash hazard label templates. After running ac and dc arc-flash incident energy calculations based on the OCPD ratings and the generator sources, users can generate equipment labels complete with optional QR codes that embed the input data. This feature allows field maintenance teams to perform or adjust the calculations shown on the labels and determine the correct PPE before servicing equipment.

ETAP / 800.477.3827 / ETAP.COM

Based in Irvine, California, and founded in 1986, ETAP produces one of the most popular industrial plant design platforms on the US market. The eponymous software package offers electrical power analysis and operation features for a broad range of applications. In addition to developing tools for utility transmission and distribution operators, ETAP offers modeling tools specifically for PV and other distributed generation applications. The company offers software modules for each of the electrical engineering reports discussed, as well as many others that facility design teams may find useful.

The database for ETAP’s intelligent, interactive one-line diagrams includes most common protective components and generators. While users typically have to enter information manually from PV module and inverter datasheets and test reports, the software saves these data in user-defined libraries for later retrieval. ETAP can run calculations for both the ac and the dc side of a PV project. Whereas the arc-flash hazard capabilities of some power engineering software are limited to 1,000 V, ETAP can analyze arc-flash hazards in ac systems up to 800 kV. Additionally, it calculates interrupt protection on both series and fully protected systems.

The company’s latest release, ETAP 18, includes a mobile app that allows users to collect, verify or edit information in the field. The app allows users to geotag photos, create or edit one-line diagrams, or view ETAP data on a mobile device. etapAPP works on tablets running Microsoft Windows or Apple iOS.

POWER ANALYTICS / 844.367.2572 / POWERANALYTICS.COM

Based in Raleigh, North Carolina, and founded 25 years ago, Power Analytics is now on its sixth-generation software release, Paladin DesignBase 6.0. The company targets its software to two audiences: transmission and distribution operators, and industrial generator plant operators that sell energy on a merchant basis to the utility market. Based on its target markets, Power Analytics differentiates its products by placing a heavy emphasis on economic decision-making capabilities. Users can model traditional revenue streams based not only on demand or time-of-use pricing signals but also on nontraditional revenue streams such as blockchain and cryptocurrency sources.

Paladin DesignBase allows users to evaluate all types of generators, from rotating machines with various fossil or renewable fuel sources to inverter-based PV power systems with energy storage. The company is working with San Diego Gas & Electric and the California Energy Commission to develop enhanced power models of the Borrego Springs microgrid project. These enhanced power modeling capabilities are valuable for distributed generation feasibility studies and for evaluating microgrid islanding capabilities. Based on customer need, Power Analytics has developed a consulting arm to provide professional services such as site surveys, energy audits, feasibility studies, project design support and real-time plant operations.

CONTACT

Bill Reaugh / Solar Rae Design / Sacramento, CA

Article Discussion

Related Articles