Understanding a PV system's power electronics requirements can help you specify the right equipment
Solar photovoltaic (PV) systems require installation of some type of power electronics to control the flow of energy within the system. For a discussion surrounding power electronics, the system types can be generalized as grid-direct and battery-based. Grid-direct systems refer to those that do not incorporate energy storage and use power electronics, typically referred to as inverters, to connect the PV array to the electric utility grid. Battery-based systems can be broken into two subcategories: stand-alone and electric utility backup. Stand-alone systems are commonly referred to as off-grid, where no electric utility connection is present. Backup refers to the presence of an electric utility connection and the ability of the inverter to send energy back onto the grid or accept energy to run loads as required. The classification utility-interactive is often used to describe these backup systems. Utility-interactive is a more general term, because it can apply to both grid-direct and backup systems that are working in parallel with the electric utility.
Grid-Direct Power Electronics
In the most common PV systems, the PV array is connected directly to an inverter that converts the DC energy produced by the PV array into AC energy that is directly connected to the electric utility. The inverters used for grid-direct systems come in a variety of physical sizes and a range of power output values. The inverters can be grouped into three general categories: micro, string, and central inverters (Photo 1).
Micro-inverters (Photo 2) are the smallest in terms of physical size and power output ratings. These inverters are typically connected to a single PV module and convert the module’s DC power directly into AC. Each micro-inverter is wired to the next via a special quick connect cable that parallels the AC output.
String and central inverters are connected to a number of modules wired in series to form a “string of modules.” These individual strings can vary from six to 20 modules, depending on module and inverter voltage values. To increase current and power input values to the inverter, the individual strings can be placed in parallel.
The difference between a string inverter (Photo 3) and a central inverter is not strictly defined, but the common industry terminology defines string inverters as an inverter that has less than 12kW of AC power output, accepts high-voltage DC input, and has a single-phase AC output for the electric utility interconnection. Conversely, central inverters are those that have a higher power output value and 3-phase AC output. Central inverters are primarily used in commercial applications. Some inverters on the market straddle both sides of these working definitions, but they are few in number.
Micro-inverters — Micro-inverter technology offers some advantages over the more traditional string inverter installation. One of the most recognizable advantages is that the conversion from DC to AC is happening at the module level, which results in a number of individual modules working independently from the modules surrounding them. Therefore, if a module is shaded from a tree or other object, it will not affect the performance of other modules as it would if the modules were all wired together in series, as in a string inverter configuration. This level of control also allows you to install computer-based monitoring that can display the power and energy values of individual modules — an effective tool for troubleshooting and energy production verification.
However, micro-inverters aren’t without disadvantages. One drawback is the location of the inverters. They are designed for mounting on the same rail system that supports the PV module. This typically places the inverters on a rooftop, which can be an extreme environment for power electronics to reside. Although micro-inverter manufacturers have constructed their products with this environment in mind, it’s still a consideration to keep in mind when designing a system. In addition, when an inverter fails, it requires the removal of a PV module to access it.
String inverters — The string inverter is considered the “traditional” inverter in the PV industry. At this time, there are numerous string inverter options from multiple inverter manufacturers. According to some current market research, these inverters account for a majority of residential installations, but with declining numbers due to the increase in micro-inverter installations. String inverters require a number of PV modules wired in series to reach the necessary start-up and operating voltages. Typical string inverters require at least 200VDC to 250VDC to operate and can accept up to 500VDC to 600VDC maximum. The exact values are determined by the manufacturer’s capacitors, transformers, and electronics. The AC output voltages are typically single-phase 208V, 240V, or 277V, allowing interconnection with most electric utility services.
The installation of string inverters requires that the system designer analyze the PV modules’ output characteristics and expected temperature variations on site to determine the necessary number of modules wired in series to match the inverter’s input requirements. Multiple online tools are available to assist designers in this calculation, including inverter manufacturer-specific tools. The result of these calculations generally results in multiple installation options, which requires the designer to make decisions based on the site-specific characteristics to optimally design the system. (These calculations and design decisions are the basis of a future article and will not be discussed here.)
Central inverters — For commercial applications, most installations use central inverters. These inverters vary in power output from ~12kW up to megawatt blocks. For commercial installations, another possibility is to connect multiple string inverters in parallel to achieve the same end result. This decision becomes a matter of what the designer and installer decide best optimizes the PV array.
The design and installation of central inverters is similar to string inverters. Central inverters have similar operating and maximum voltage ranges as string inverters, but there will be many more strings placed in parallel. This increases the overall power and current values. Some central inverters accept up to 1,000VDC for commercial and electric utility scale applications. At this time, they aren’t as common as the 600VDC inverters, but large-scale installations are moving toward the 1,000VDC systems.
The AC output of central inverters is designed to have a balanced connection to 3-phase services. Most inverters come with either a 208VAC or 480VAC output, with some manufacturers offering 600VAC versions. Central inverters come in both 3- and 4-wire configurations, allowing for interconnection with both delta and wye electric utility services. Typically, central inverters are installed with data monitoring systems so the plant operator can identify any potential problem areas and aid in operations and maintenance procedures.
All PV systems that incorporate batteries in their design — either stand-alone or utility backup — require two different pieces of equipment that process the power generated and used within the system. The first piece is the charge controller. While the name lacks pizzazz, it is very descriptive for its function. The charge controller is wired in between the PV array and the battery bank to control the charging of the battery bank. Charge controllers are used on the DC side of the system, so they are not directly affected by the AC loads and inverter used in the system. Charge controllers are specified by the array voltage and current values as well as the battery bank voltage.
Currently, there are two commonly used technologies for charge controllers: pulse-width modulation (PWM) and maximum power point tracking (MPPT). PWM controllers are the less sophisticated and less expensive option of the two. PWM controllers are commonly used in conjunction with small PV arrays and battery banks. Although there are PWM controllers that can be used with large arrays, it is more common to see the MPPT controllers for larger PV arrays, such as ones sized to serve a whole house.
At the risk of oversimplifying it, the most notable difference between the two technologies is the way they process the power delivered from the array to the battery bank. MPPT controllers are able to maximize the power delivered by the array by bucking the array’s extra voltage to match the batteries’ voltage, while simultaneously boosting the current output from the array into the batteries. This enables the battery bank to receive the full power output of the array, allowing for more efficient charging. PWM controllers, on the other hand, match the voltage from the PV array to the battery bank voltage, regardless of the potential power production from the array. This can effectively leave power not used in the battery charging process “on the table.” Therefore, for larger systems, the additional costs for MPPT controllers can be justified.
The inverters used in battery-based systems are the other major electronic component employed. The inverters used in stand-alone and electric utility backup systems are very similar in appearance and function — the biggest difference is the ability of the backup versions to push current from the DC side of the system into the AC source (i.e., the electric utility system). This feature is disabled in stand-alone versions as the AC source is typically a standard generator.
These inverters are more appropriately called inverter/chargers, because they have the ability to invert the DC energy stored in the battery bank into AC for household usage as well as the ability to rectify AC to DC in order to charge the battery bank as required. Inverters are designed to connect to a battery bank at nominal 12VDC, 24VDC, or 48VDC and invert to 120VAC with AC power outputs ranging from hundreds of watts to the 6kW range. Multiple inverter output circuits can be connected in series to achieve the 240VAC necessary in most residential applications. This series connection also requires a communication cable installed in between the inverters to maintain the proper phase relationship. Some inverter models come with a standard 240VAC output, thus reducing the required components.
Summing it Up
The different types of power electronics detailed here should provide you with a good understanding of currently available products and their applications. Once you have identified your system type, you can then move forward to the process of specifying exact equipment. This will require examining the site-specific conditions and your level of comfort with the different products. When you move from one project to another, you will most likely have to adjust the components used to suit the different locations.
Mayfield is a principal with Renewable Energy Associates, Corvallis, Ore. He can be reached at: Ryan@renewableassociates.com.
Sidebar 1: DC Optimizers
Another module-level technology approach for PV installations is the DC optimizer. These power electronics work at the module level, similar to micro-inverters, but instead of inverting to AC at the module, the DC optimizers are paired with a specific string inverter that is optimized for operation and efficiency based in the output of the optimizers. This module-level control of the system offers advantages and disadvantages similar to micro-inverters. One safety feature many manufacturers incorporate is the ability to reduce and, in some cases, eliminate the DC voltage produced by the PV system in an emergency situation. DC optimizers also offer module level monitoring for the system owner.
These module-level power electronics are gaining popularity as the technology advances and their reliability is proven. In addition, some PV module manufacturers are accommodating different module level electronics, DC optimizers, and micro-inverters for direct integration into their modules. This allows contractors to choose the best electronics-based solution for the site and vary their installation practices from site to site while using the same base module.
Sidebar 2: AC Modules
Another product starting to infiltrate the industry is the AC module. These are PV modules that do not have any DC wiring associated with them — simply an AC output. There have been provisions for AC modules in the NEC for multiple cycles now, but true AC modules are just now becoming a viable option for installers. These modules are very similar to traditional DC modules connected to micro-inverters in the sense that the inverter is associated with a single module. In the case of the AC module, however, there aren’t two distinct products — the module and inverter are integrated into one seamless unit. One of the major advantages of these modules is the elimination of all DC wiring. This can increase the level of comfort for many electrical contractors and building departments, because this now more closely resembles other electrical installations being installed in buildings.