Two Systems, One Solution: Transitioning from Residential Traditional String Inverters to DC Converter and Micro-Inverter Module Level Power Electronics (MLPE) solutions in the United States

Martin Herzfeld

Solar Module Installation

A growing practice within the United States, and more specifically California, is the installation of solar modules with a tilt with the azimuth to the East or a tilt with the azimuth to West module-level power electronics. As reported, there now are more than approximately 2 million solar energy system installations in the United States and approximately 1 million solar energy system installation in California alone. For the world, if you are in the northern hemisphere where you tilt the modules to the South or vice versa for the southern hemisphere where you tilt modules to this North. Both Solar Energy Systems (SES) and Energy Storage Systems (ESS) are now considered two systems. The largest growth in MLPE has been for small and medium size systems.

Solar Energy Systems & Energy Storage Systems:

As the number of Solar Energy Systems (SESs) continue to grow in California – there are additional grid services that become required with Rule 21 interconnection, which was established on September 8th, 2017. As a result, the interactive utility interconnection could become a hindrance, not only by revenue impacting events if the utility is unavailable, but also if the utility is unavailable for a substantial period of time. For example, there are systems which were installed which did not initially meet the deadline for UL 1741 SA and were blocked from interconnection or hindered until the new requirements were met for Rule 21 with typically with new equipment. In addition, rates schedules are changing requiring Time of Use (TOU) with higher peak rates in the late afternoon which is resulting in self consumption and load shifting. A system may be designed to install the array to the South West to peak in the afternoon, for instance 3 PM with the higher rates and benefit from the Net-Metering. Although the array is larger to the SW and the warranty and the support of the systems may be approximately 20 years, there is less energy produced, but at a higher value. On the other hand, an Energy Storage Systems (ESSs) could be to an SES resulting in an ac coupled systems to provide load shifting and self consumption. Although there is a benefit of load shifting, the greatest requirement, in order to counterbalance the process, would be to provide backup power in the event of a utility system outage. However, depending on the ESS technology, the I-Codes set both threshold capacities and maximum allowable capacities with additional requirements on the location of where ESS whether it will be allowed in a non-habitable space, garage, or outside the home is an ongoing issue for a residence and whether the system is less than 100 volts and if the live parts are not accessible. A critical loads panel board must be installed to meet the requirement of battery backup or load shifting. On the other hand, ESSs rated in the maximum MWh of approximately 10 years may not meet the same warranty and support requirement as the SES which includes the modules at approximately 20 years depending on the manufacturer. This, combined with Module Level Power Electronics (MLPE) assists in meeting the new electrical Code requirements both Arc Fault and Rapid Shutdown when an SES combined with ESS systems. There are limited MLPE vendors which work well with traditional inverter manufacturers. Now with the two systems combined, if the SES is to the South East, then based on the TOU rate, it may be advantageous to charge the batteries in the morning and discharge them in the evening. If the SES is at a South West azimuth on an Electric Vehicle (EV) Time of Use Rate, it may be advantages to increase the size of the array in the SES to the South West with the benefit of the warranty of the solar modules would be 20 years as compared to a 10 year or MWh limit for an ESS. Therefore, options on what the best method or technique would be to improve the return-on-investment whether it's maximizing the number of kilowatt hours per year or maximizing the returned based on time-of-use involves more intricate considerations.

Transitioning and Updating to an SES + ESS Solution with MLPE:


Many SES installations are beginning to age, and there is an advantage of solar energy systems using MLPE. The solar modules degrade at different rates and have and have the benefit of traditional inverter systems with higher voltages in a residence. Due to this, it may be time to update the modules on a rooftop or ground-mounted system with higher power modules. In other words, although there are approximately 10 years left on the warranty on the modules, some people are considering an upgrade or an update to the PV system. As a result, if a traditional rooftop systems is removed for a new roof, then transition could be to a dc converter solution. For example, I transitioned the ground-mounted solar energy system I use in practice from a traditional string inverter to a DC converter solution after 15 years of service from a defective inverter with new updated modules as well. The DC electrical alteration was completed using existing wire and the practical safeguarding would be increased in a DC ungrounded non-isolated system from a DC grounded system. I transition from a traditional string inverter solution to a micro-inverter solution, which involved a multi-conductor cable compared to using single conductor cable and complete system rewiring. Therefore, if the system did not have the minimum number of modules required by the DC converter system, a solution using micro-inverters which would require rewiring would have been easier to conduct. Many of the systems from the past did not have the clear advantages of systems today including diagnostic capabilities to identify mismatched modules. With diagnostic and IV curve tracers at the string level, even though they are time consuming to identify a defective module, by using the existing modules in a DC converter MLPE solution, it could identify defective modules in an older traditional inverter configuration and the system may be restored. I’ve observed from experience many third-party inspections with specific visits to the sites, when both Solar and Energy Systems (SES + ESS) are not monitored, and the systems are typically abandoned soon after. Ultimately, the benefits to a larger array size, instead of adding ESS independently, would be the ongoing maintenance and support of the system as well as the minimization of the systems that would end up abandoned had the array size not been increased. To summarize, traditional string inverters in comparison to the other types listed previously degrade at different rates, hence said modules becoming defective in a string.


Ongoing Installation Practices and Support


One of the most prevalent issues in the solar industry would be following the manufacturer's instructions when installing modules, inverters and MLPE equipment. A solar energy system should be installed with appropriate spacing between the modules, such as approximately 10 mm, using the correct size of mid-clamps and following the match module manufacturers instructions on where the attachment zones are, if using rails on mounting locations. It is also important after an installation that the correct field applied marking is applied, and all circumstances, such as weather and/or environment are accounted for. One of the challenges for installation, however, is the display for the monitoring of a system, especially when it comes to new DC converter systems, in which the inverters may not include a display. If there is no display, then the power and energy generated can not be quickly checked at a glance. For third-party inspections, this issue becomes challenging when companies may have transitioned to systems that now require support and maintenance. A way to counteract this occurrence is through module-level power electronics. Module-level power electronics provide that unique capability of diagnosing problems and issues specifically to the module. With these types of power electronics these issues all but go away and reduce the maintenance time in trying to identify and isolate issues.


Intricate Solutions Based on Requirements


For right now, specifically in Northern California, an ideal solution would be the Electric Vehicle
Time of Use (TOU) rate schedules with a large South West array using MLPE with DC converters without
ESS using the EV as an “ESS” with net-metering. Depending on your requirements, online software tools
can identify best options. Due to the ever evolving updates to systems, and the popularity of the solar
industry, solar and energy storage systems combined are more complicated than they used to be.

Martin Herzfeld is one of the few Interstate Renewable Energy Council (IREC) Certified Master Trainers ™ for Photovoltaics (PV) Installation Professional in California and an adjunct energy professor at a local college. A California licensed contractor since 2004, Martin holds specialty classifications in Solar (C46), Electrical (C10), Trenching (D56), Pole Installation & Maintenance (D31), Low-Voltage Systems – Instrumentation (C7) and is an Underwriters Laboratories (UL) Certified PV Systems Installer and an OSHA-Authorized Construction Trainer in worker safety. In addition, Martin is a Contract Solar (PV) Technical Inspector for 3rd Party Inspections since 2011.

 

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