New Micro-Parallel Inverters Aim to Reduce System Costs, Improve Performance

Traditionally, there have been two main options for solar installation designers and engineers to convert DC to AC power: string inverters or micro inverters. Both present significantly different advantages and disadvantages for installers, with installation expenses varying up to 20 percent depending on the inverter selection. Either solution had drawbacks: what one would gain in convenience, it would sacrifice in performance and maintenance and vice versa. But a new class of inverter is entering the market that combines the installation simplicity of a string inverter with the intelligence and energy harvest advantages of a micro inverter.

Analysis by the U.S. Department of Energy (DOE) showed installation labor to be largest expense category associated in PV applications.  With installation and maintenance costs outweighing the panels, framing, and other associated expenses by a factor of two to three, a new solution was needed to make solar energy installations more affordable for residential and small/mid-sized commercial applications. 

The micro-parallel inverter (MPI) is a new emerging class of inverter; aimed at helping to further the proliferation of solar energy into the residential and small commercial market segments (5 kW-200 kW) by reducing installation and maintenance costs. A micro-parallel inverter is designed to invert four panels in parallel and provide advanced data, communications and IT features. The MPI system can also be utilized for off-grid, grid-tied, and grid-tied with emergency backup applications. It has a wide enough input; voltage, current and frequency range to operate with generator or battery based grids for application flexibility that surpasses its two predecessors.

Incorporates the Latest Communication Technology

Designed around the concept of easing the ergonomic burden of installation, registration, and maintenance, the micro-parallel inverter tackles the difficult chore of reducing the cost of acquisition and ownership by simplifying the installation, cabling and activation processes.

MPIs benefit from newer technological innovations in controller and communication modules, allowing additional features previously not available in existing inverters.  The resulting innovations give the MPI improved intelligence, ubiquitous data display with remote control, and continuously adjusting energy harvest schemes controlling the MMPT for each individual panel. This control automatically throttles energy conversion based on temperature to maximize the energy production of each panel. Fault alert processing notifies maintenance when a specific part has failed, and isolates the location and identifies the specific failed part for expedited ordering and replacement. These features will reduce unneeded maintenance truck rolls to repair and replace.

MPI Installation Is Simplified

Wiring for residential or commercial 240-VAC single or 3-phase MPI systems is similar to standard building construction.  PV system installation with the MPI is configured as a modular, plug-n-play approach.  

Each micro parallel inverter controls multiple 300-W solar panel modules via individual channels.  The parallel connectivity eliminates the shading dropout or of a string inverter approach.  The individual inverter channel design -1 channel per panel (module), four channels per inverter; continuously optimizes the maximum Power Point Tracking (MPPT) for each individual panel (module). This overcomes the shortfall of the string inverter architecture, where efficiency is lost using a single MPPT for each string. 

In addition, the AC design of the micro parallel inverter eliminates the need for combiner boxes, DC optimizers and DC GFCIs required by string inverters.  MPI Panels are mounted in 4.8-kW clusters of four rows of four panels. The MPI’s can be mounted to a panel frame, the array racking, or directly to the roof.  Panel inter-connectivity uses quick connect, DC terminals leading from the MPI to each of the four solar panels. The MPI’s can handle a mix of panel technologies ranging from 100-300 W.

Each MPI is then connected to a 240-VAC, 20A, 4.8-kW trunk cable. The trunk cables lead to a single disconnect (visible from each MPI) which is wired directly to the site’s breaker panel. The trunk cabling supports 16 panels, with just one disconnect at the breaker, unlike the 16 separate disconnects incorporated into each micro inverter’s AC connector.

Efficiency and Maximum Harvest

Graceful degradation and enhanced fault knowledge is enabled by a unique channel design within the MPI that uses power sharing, and channel to channel health and status communications. This power sharing and communication provides the necessary information to discern from a dirty panel needing cleaning, a failed inverter channel requiring a service visit, or a failed panel needing replacement. With the modular architecture, each channel is designed to be serviceable in the field. Micro inverter failures lose this extra level of knowledge. Additionally, the MPI remains operational with as few as one of the four panels active, on any channel generating DC power. String inverter architecture can lose generation of the entire string due to the shading of a single panel.

Self-Mapping and Automated Control

Installation, operation, and trouble shooting is aided and controlled by the system’s Electricity Monitor and Control (EMC). This unit plugs into any AC receptacle near the breaker panel and connects to the site’s internet router or, when internet is not available, directly to a computer via an Ethernet cable. A built-in cell phone or satellite phone version of the EMC is also available for installations where internet connectivity is limited.  

The EMC communicates with the micro parallel inverters via PLCC. Since each MPI communicates to 4 panels, the EMC has the ability to monitor 4x the number of panels compared to a micro inverter architecture. The owner, installer, or user log into a private, self-launched EMC web site and the software walks them through startup, checkout, and commissioning. There is no need to contact the manufacturer during installation and activation. A Self-Mapping feature finds and maps each inverter location to its physical location in the array, the automated process directed by the EMC. Installers simply close breakers, and correct faults detected during startup. This feature improves the accuracy of the map and eliminates the time-consuming and costly practice of hunting the field for erroneously mapped panels.

The EMC monitors and controls the array, communicates with the user, and takes commands from the system owner. Maintenance alerts are e-mailed to the maintainer/owner identifying system component failures that have been isolated to the location, the affected micro parallel inverter, and individual panel.  These emailed alerts are limited to reporting only actual failures, minimizing unnecessary truck rolls. System performance is always readily available via the web interface for other analysis, monitoring or prognostic purposes.  

The EMC software architecture allows for remote upgrades for future sustainability against obsolescence. Remote control allows the system owner the ability to remotely deactivate the system when necessary. The cell/sat phone versions of the EMC provide additional connectivity for remote or hard to reach locations and installations. Production limits can be self-monitored to assure commercial generators do not exceeded their generation allotments.  System owners can remotely activate/de-activation generation at specific installations, and when integrated into a smart grid installation this feature can be controlled by the utility to better manage grid loading.  

Rugged and Durable

New technology provides micro parallel inverters with a thermal survival feature that improves reliability, availability and energy harvest. This feature is a unique patented power output throttling circuit thatprotects the MPI against heat-related failures. Other benefits enter due to its abilities to be activated remotely or programed to maximize energy harvest and prevent over generation when limits are imposed. This improved energy harvest increases the payoff associated with using oversized panels. Instead of clipping generation at a set wattage the throttling adjusts generation based on circuit temperature providing the maximum energy harvest for the system’s panels.   

With less cabling, a more simplistic plug and play installation approach, and better activation and monitoring abilities, the micro parallel inverter stands to bridge the gap between micro and string inverters, while reducing the associated installation costs by 75 percent. 

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Arnold Kravitz is the Commercial VP, for TRC. The founder and GM of the Sarnoff's (SRI) ISS and Imaging Business, he has been the VP of Advanced Technology, and the VP of Engineering at L-3, the the Chief Engineer of Small Satellite Business at BAE, department head for both The Microwave, Space and Mission Electronics business and The Optics Center of Excellence. Arnold has received numerous industry awards, and patents, and has contributed to the USDOE’s ANL Fusion Power program, several NOAA/ NASA Optical and Radar imaging satellites, and other concealed object detection, spectroscopy, surveillance, and cryptographic systems and products He received his BSEE from the University of Hartford, a MSEE from Rensselaer Polytechnic Institute ( RPI) and is a graduate of GE’s Edison Engineering Program, Lockheed's Advanced Leadership Development Program, BAE’s Intensive Leadership Development program, and Wharton's Executive Development Program.

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