Chapter 9
Inverters: AC (From) DC
IN THIS CHAPTER
Getting the scoop on grid-direct inverters
Exploring the ins and outs of battery-based inverters
Selecting the right inverter for the system you’re designing
If PV modules (see Chapter 6) are the heart of any PV system, inverters are the brains. These devices take the direct current (DC) power produced by the modules (and stored in batteries) and turn that electricity into the alternating current (AC) electricity that people use in their homes and businesses. Not only that, but inverters are smart enough to realize when the utility is gone and when the batteries need some extra attention. Impressive, huh?
As I reveal in Chapter 2, you use one of two inverter types in any PV system:
- Utility-interactive inverters are classified into two subcategories:
- Grid-direct (for use in grid-direct PV systems)
- Battery-based (for use in utility-interactive, battery-based PV systems)
- Stand-alone inverters (for use in stand-alone, battery-based PV systems) are all battery-based.
In this chapter, I describe the operation and features of both grid-direct and battery-based inverters. I also help you discover what the considerations are when specifying the inverter for any PV system you design and install.
Getting the Goods on Grid-Direct Inverters
The majority of PV systems installed today feature inverters that connect directly to the PV array on one side and to the utility on the other side. These inverters don’t use any method of energy storage and are most often referred to as grid-direct inverters. They’re the most widely installed inverters due to their increased efficiencies and relatively simple installation, and they’re used only in grid-direct (battery-less) PV systems (I describe these in Chapter 2).
All grid-direct inverters are considered utility-interactive because they require the presence of the utility in order to operate; they work in parallel with the utility to supply power to common loads (including the lights in your home, your television, and anything else that uses electricity). They even have the ability to send power back into the utility’s grid.
In this section, I describe the basic operation of grid-direct inverters as well as their standard features, power output sizes, and technological differences.
Basic operation
Although the actual electronics inside a grid-direct inverter aren’t simple, the basic process of how they work is.
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First, DC power is delivered from the PV array to the inverter.
With very few exceptions, all types of grid-direct inverters work well with high-voltage PV arrays. The voltages typically fall between 150 V and 600 V, with 600 V being the absolute limit for residential and commercial grid-tied systems due to electrical code limitations. -
The inverter then takes this DC power and turns it to AC power through various methods.
The exact method for turning the DC to AC depends on the manufacturer and its choice of technology. I present the major technologies later in this chapter.
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Usually, the AC output from the inverter is then connected directly to a load center that’s also connected to the utility grid.
The load center is usually the main distribution panel (MDP; see Chapter 2).
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The AC power then either flows into loads that are connected to the load center, such as a refrigerator or lights, or goes back into the utility’s power lines and runs the meter backward.
This utility interconnection requires you to work with the client to let the utility know that the PV system will be connected to its utility grid and will have the potential of sending power back to the grid. (I present the different ways that system owners can connect to the utility and receive full credit for their PV systems in Chapter 2.)
Grid-direct inverters don’t employ any form of energy storage, so if the utility doesn’t provide them with a stable, consistent power source, they can’t run. In fact, grid-direct inverters are so aware of small changes in the utility’s power that they shut down on occasion to meet the necessary safety standards. This characteristic can be seen as a limitation, but it’s a deliberate feature of the inverters that allows them to operate safely with the utility grid (see the later “Safety features” section for more information).
Standard features
All grid-direct inverters have some basic standard traits, regardless of their size or technology. They all incorporate some basic safety features, use maximum power point tracking, and possess some type of user interface.
Safety features
One of the most important safety features in all grid-direct inverters (as well as utility-interactive, battery-based inverters, which I cover later in this chapter) is the ability to detect when the grid is suddenly disconnected. Grid-direct inverters have very sophisticated monitoring equipment that can detect the absence of the grid in fractions of a second and turn off the inverter automatically in response. The name given to this process is anti-islanding, and it’s a requirement for all grid-direct inverters connected to a utility grid.
The term islanding refers to a situation in which the utility grid is out and alternate power sources from people’s homes are still connected to the grid and sending power back into those “dead” lines. Islanding presents a safety issue for utility workers who may be working on those lines; they may think the lines are safe to touch when in fact electricity is present from a different source. That’s why grid-direct inverters are required to recognize utility disturbances and stop producing power immediately. The inverters look at two parts of the utility power: the electrical frequency and the voltage. If either part goes out of the specified allowable range for the inverter’s operation, the anti-islanding feature activates, and the inverter turns off.
In the U.S. market, all grid-direct inverters require testing to the same standard to meet the anti-islanding requirement. The test is known as UL1741, and the testing procedure covers several safety issues for many of the electrical components used in PV systems. Many companies perform this test, but they all follow the same rules when doing so.
After the inverter turns off for anti-islanding, it monitors the grid to verify that the voltage and frequency values are within the specified ranges. When these two parameters are met for five continuous minutes, the inverter can then turn back on and resume producing power. This entire process is automatic within the inverter and doesn’t require any interaction from the user. (That’s good news for your client!)
Grid-direct inverters have to react to the frequency and voltage parameters they see from the grid. Therefore, it may be possible for the grid to experience a brownout situation where power isn’t entirely lost but the inverter still shuts down. Another situation that may cause a grid-direct inverter to shut down even though the grid is still up is when there are excessive voltage drops in the wires connecting the inverter to the utility.
Another important and standard safety feature in residential grid-direct inverters is ground fault protection (GFP). GFP is integrated into the inverters and is there as a fire-safety device. GFP is different from the ground fault circuit interrupter (GFCI) that most folks are familiar with. GFCI outlets, the ones with two buttons in the center, are used in homes and are required in locations such as bathrooms. The difference between GFP and GFCI is that GFCI outlets are installed to protect people from receiving a shock, whereas GFP devices are meant to prevent faults from starting fires. (I cover ground faults and GFP in depth in Chapter 10.)
Maximum power point tracking
All grid-direct inverters employ maximum power point tracking (MPPT) to produce as much power as possible from the PV array. MPPT allows the inverter to harvest the maximum amount of power and deliver it to the load center, just like some charge controllers (see Chapter 8) use it to deliver the maximum amount of power to a battery bank.
Note: Only one MPPT tracker in a grid-direct inverter looks at a system’s PV array. See the nearby “Multiplicity: Inverters with more than one MPPT” sidebar for when to use inverters with multiple MPPT inputs.
User interface
Because you can’t simply look at a PV array and know whether it’s working, inverter manufacturers have decided that some sort of display or interface is necessary to indicate how the array is functioning. Consequently, today’s inverters generally include options for the user to see what his entire PV system is doing. With very few exceptions, grid-direct inverters now come standard with a display built into the unit, a feature that allows your client to obtain critical information such as voltage, power output, and total energy production from the inverter itself.
In addition, inverter manufacturers offer a wide variety of ways to collect, store, and display the data gathered by the inverter. Many have methods for connecting the inverter to the Internet and allowing the user to visit a Web site and see real-time information about his system. Some even make it possible to connect the inverter directly to a computer, a feature that allows the system owner to collect and store data locally so he can review it periodically to identify potential issues if they occur.
Power output sizes
Even though all grid-direct inverters operate on the same principles, they come in a variety of power output sizes (which are very often referred to simply as the inverters’ size). Following are the three power output sizes you can expect to run into:
- Microinverters: These are inverters that connect to a single PV module rather than a string of modules. PV folk like these inverters because their ability to turn the individual modules’ power to AC reduces total system losses from factors such as shading. Microinverters are typically less than 250 W each, which matches well with the commonly used PV modules.
- String inverters: Although the true definition of a string inverter is an inverter that’s connected to a single series string (see Chapter 3 for details on series strings), the term has become synonymous with small (less than 15 kW) inverters that attach strings of PV modules for power outputs ranging from 1 kW to 15 kW.
- Central inverters: Associated with larger commercial projects, central inverters range in size from 15 kW to 1 MW. Central inverters operate a lot like the string inverters used in residential applications, just on a much larger scale (so they’re kind of like string inverters on steroids).
Regardless of their power output size, grid-direct inverters usually look pretty similar; Figure 9-1 shows you a typical one.
FIGURE 9-1: Your average grid-direct inverter.
The importance of transformers
Just like all cars perform the same basic function — transporting people from here to there — all grid-direct inverters have the same purpose: to take DC power from the PV array and turn it into AC power for the load center or the utility. But just like cars, a grid-direct inverter can have a number of different forms. The biggest difference between one inverter and the next is transformer related: Different manufacturers may use different transformers in their inverters, and some manufacturers may not use transformers at all. I present the main transformer-related differences among grid-direct inverters in the sections that follow.
Large transformers and low-frequency technology
Low-frequency grid-direct inverters are currently pretty common in PV systems. They take the high DC voltage from the PV array and, through a series of switches, turn that DC into AC with the help of a transformer that keeps the AC and DC isolated by inducing the switched DC voltage across the transformer and creating the AC current on the other side of it. Low-frequency inverters get their name because the frequency at which these switches are operating is relatively low in comparison to high-frequency inverters (described in the next section).
The advantages of low-frequency inverters include a very robust design that allows manufacturers to reduce the number of parts required to make AC from DC and keep overall costs low. However, because these inverters use a large transformer, power losses occur across that transformer. (Also, the transformer adds a lot of weight to the unit.)
Small transformers and high-frequency technology
High-frequency inverters use a switching technology that’s similar to that of their low-frequency counterparts. The difference is that high-frequency inverters do their switching much faster, which means they can use small, lightweight transformers rather than large, heavy ones.
Some manufacturers who work with this technology are able to use the high-frequency transformers to create multiple small inverters inside the same box. What’s the advantage of that, you ask? Well, if the PV array is producing low levels of power, one inverter can be off, letting the second inverter operate at a higher efficiency level. After the power level gets high enough, the second inverter turns on, preventing the first inverter from being overworked.
Transformer-less technology
Some inverters on the market don’t use any transformers at all. These inverters keep the DC and AC separated electronically and prevent any DC injection into the AC line by using firmware (small electronic programs) specialized for the inverter.
Transformer-less inverters are commonly referred to as ungrounded inverters because a connection to the grounding electrode system isn’t required due to the lack of a transformer. However, transformer-less inverters still need to include an equipment ground in order to reduce shock hazards. Turn to Chapter 17 for information on grounding requirements in PV systems.
The largest positive feature for transformer-less technology is the increase in overall inverter efficiency. Without a transformer present, the inverter has one less step to make and can turn the DC power from the array into AC power more efficiently. The major downside involves all the additional requirements you have to go through when installing a transformer-less inverter in order to be compliant with the NEC®. These extras can translate into higher costs during the design and installation process.
Investigating Battery-Based Inverters
When energy storage is a requirement for your client (calling for a utility-interactive, battery-based or stand-alone, battery-based PV system), a battery-based inverter is your go-to choice. Actually, battery-based inverters are better described as inverter/chargers because they have the ability (when needed) to accept an AC power source, such as the utility grid or a generator, and then turn the AC electricity into DC electricity for battery charging. Figure 9-2 illustrates your average battery-based inverter.
FIGURE 9-2: A typical battery-based inverter.
Utility-interactive, battery-based inverters operate almost identically to stand-alone, battery-based inverters, but a few major differences exist. In this section, you find need to know about the workings of both types of inverters and describe the features that come standard on any battery-based inverter.
Utility-interactive inverter operation
A utility-interactive, battery-based inverter works in different ways depending on whether the utility is up and running or down and out. I explain the differences in operation in the following sections.
When the utility is working
When the utility is up and running and the battery bank is full, power moves through a utility-interactive, battery-based inverter in the following route:
- The PV array starts producing DC power in the morning and sends it to the battery bank.
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If the battery bank is full, the charge controller “talks” to the inverter and sends the DC power toward it.
The battery bank will always be full unless there has been a power outage and the client has run loads using the battery bank.
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The inverter accepts the power from the PV array, changes it into AC power, and passes it though to the backup load center.
If the PV array is producing more power than the backup load center is consuming, the inverter takes the excess power, turns it into AC power, and sends it to the main distribution panel (MDP). -
The MDP then disburses power to the loads connected to it.
If the PV array is producing more power than the MDP and the backup load center combined, the inverter can push the AC power back into the grid, running the meter backward.
- As the PV array slows down and eventually stops producing power, the inverter stops sending power back toward the utility; the utility power then begins to flow into the MDP, just like it would if no PV system were present.
- The utility power continues through the inverter into the backup load center so that the loads always have power available and the batteries remain full.
The connection between the utility and a utility-interactive, battery-based inverter allows current to flow both ways (think of it as a two-way street). Stand-alone, battery-based inverters can’t do this; they only allow current to flow from the power source to the loads. Note: In both types of inverters, the AC output connection goes directly to a load center, so no power source is available to send power back toward the inverter (consequently, the AC output connection acts as a one-way street).
When the utility is down
No utility can stay live all the time, which is why utility-interactive, battery-based inverters are ready for the occasions when one goes out. They’re able to recognize the outage and automatically disconnect themselves from the utility connection, eliminating any possibility of the inverters islanding and sending power back to the utility. (I explain islanding in the earlier “Safety features” section.)
Because utility-interactive, battery-based inverters are connected to the utility, they have to conform to the same anti-islanding standards that grid-direct inverters do, which means they have to monitor the utility and disconnect themselves when the voltage and frequency levels are out of the specified parameters.
At the same time that it disconnects from the utility, a utility-interactive, battery-based inverter immediately begins drawing DC power from the battery bank and sends AC power to the backup load center. It then continues powering the loads from the battery bank until either the utility power returns or the batteries discharge and can’t support the loads.
If the power outage is extensive, the PV array can and will continue to charge the battery bank through the charge controller (I outline this process in Chapter 8), giving the battery bank an extended run time. When the utility power returns, the inverter reconnects to it, allowing the battery bank to recharge directly from the utility. This way the battery bank can be ready to supply power if another outage occurs. (Note: Most utility-interactive, battery-based inverters allow you to defeat this function if you’d rather have the PV array charge the batteries.)
Stand-alone inverter operation
During normal operation in a stand-alone system, the battery-based inverter accepts DC power from the battery bank, turns it into AC power, and delivers that AC power to the loads connected in the main distribution panel. Simple, right? As long as the capacity in the batteries remains high enough, the inverter is able to continue delivering power to the loads.
After a battery bank discharges to the level you’ve designed for, the inverter can either alert the system owner to manually start a generator or it can automatically start one in order to make sure the batteries don’t discharge more than he wants them to and cause damage (you can read all about the effects discharging has on a battery in Chapter 7). As soon as the generator connects to the inverter, the latter stops drawing DC power from the battery to make AC and starts passing the generator’s power through to the loads and charging the batteries with any remaining available power.
The charger built into a stand-alone inverter performs a multistage charging cycle similar to the one I describe in Chapter 8. Most systems are set up to allow the batteries a deep discharge before calling for help from the generator, so this cycle generally takes several hours to fully charge the battery bank. After the batteries are full again, the generator can be turned off (manually or automatically), and the inverter can return to its regular job of powering loads through the battery bank.
Standard features for all battery-based inverters
Battery-based inverters are the workhorses of the inverter world. They’re capable of handling a variety of environments and delivering high-quality, reliable power. In the sections that follow, I describe some standard features found in all battery-based inverters.
Most, but not all, of the battery-based inverters used in PV systems are actually inverter/chargers. On the other hand, some battery-based inverters are exactly that — inverters without the ability to charge a battery bank from an external AC source (such as the utility grid). With battery-based inverters, a number of features vary from manufacturer to manufacturer, so if you need a specific function, verify that the inverter can deliver what you want it to before you get too far into your design.
Considering safety
The safety features built into battery-based inverters tend to focus on the safety of the system. These inverters keep batteries from becoming too deeply discharged by alerting the system owner of a low-battery situation, starting a generator, or even shutting down; when they act as chargers, they make sure batteries are charged correctly by using multistage charging. And of course, when battery-based inverters are connected to the utility, they isolate themselves from the utility during power outages (just like grid-direct inverters do; see the earlier “Safety features” section for more information).
Unlike grid-direct inverters, ground fault protection (GFP) isn’t a standard feature of battery-based inverters, although it can be added to battery-based systems elsewhere. Other safety features, such as disconnects and overcurrent protection, are installed in external boxes located adjacent to battery-based inverters. See Chapter 10 for more details on these types of protection.
Interacting with users
So many variables require programming in battery-based inverters that some level of interface with the system user is necessary. Depending on the inverter’s manufacturer, this interface can take the form of a screen and touchpad built into the inverter or a hand-held controller wired directly to the inverter. Typically, the interface not only allows for inverter programming but also gives the user some basic system information (think battery voltage and current levels).
Sizes of battery-based inverters
Battery-based inverters come in a large range of power outputs (sizes). You can buy a small 100 W inverter that connects to the DC plugs in your car all the way up to 6 kW units. The most common types used in PV systems start at 1 kW of AC output and range up to 6 kW.
For battery-based systems that require more power than a single inverter can provide, multiple units can be stacked, or connected together in such a way that they can provide more power to the loads. Depending on the manufacturer, you can stack individual inverters together to provide up to 36 kW. Of course, having multiple inverters means the inverters need to talk with each other. This communication is typically handled by connecting the inverters together via a communications cable (similar to the cable you use to connect your computer to the Internet).
Low-frequency transformer technology
All battery-based inverters use inverter technology that’s similar to the low-frequency, transformer-based, grid-direct inverters. However, the battery-based inverters are limited to a 48 VDC nominal input.
Specifying Any Inverter
To specify an inverter is to decide which kind of inverter to use, either grid-direct or battery-based, depending on the system you’re designing. The following sections note the big-picture considerations you should have in mind as you work to pick the make and model of inverter that’s a good fit for your client’s needs. (When you’re ready to size the inverter you’ve chosen, head to Chapter 11 if you’re working with a grid-direct one or Chapter 12 if you’re working with a battery-based one.)
After designing and installing a few systems, most PV pros have one or two brands of inverters that they prefer, and they tend to stick with these brands. But just because you like a certain brand doesn’t mean you should stop checking out the market every now and again. Because the PV industry is growing rapidly, manufacturers are constantly releasing new and improved products that may serve future clients even better than your current favorite. You may wind up discovering your new favorite inverter simply by giving a new one a shot.
Grid-direct
Grid-direct inverters are designed to operate within all the AC voltages offered to residential and commercial buildings from the utilities in the United States. Consequently, you need to consider the operating range for the DC voltages from the PV array and the overall power output of the inverter when specifying an inverter for a grid-direct PV system. Of course, you also want to ensure that the inverter you choose works well for your client’s needs (as well as your own). The next sections are here to help you out.
Matching the inverter to the PV array
The DC voltage window (the range of allowable voltages for an inverter) and the AC power output value are your two big concerns when matching an inverter to a PV array. Following are some specifics about each one:
- DC voltage window: The manufacturer defines this window and whatever type of technology the inverter uses to turn DC power from the PV array to AC (this technology can be low frequency, high frequency, or transformer-less; I describe all three earlier in this chapter). The typical DC voltage window for grid-direct inverters used in residential systems is from 250 VDC (minimum) to 600 VDC (maximum). At first glance, this window seems extremely wide and relatively limitless, but as you discover in Chapter 11, this window narrows very quickly when you start evaluating real-world operating conditions.
- AC power output value: For grid-direct inverters, the AC power output value is directly tied to the DC input power value. The inverter’s power rating is therefore evaluated in relation to the PV array’s rated power values. As you find out in Chapter 6, PV modules vary in power output based on environmental conditions. PV systems also have inefficiencies all along the way, so grid-direct inverters are typically matched up with PV arrays that have larger power output ratings to make up for the energy losses. The exact amount of “extra” power from the array to the inverter is affected by a number of variables, but most grid-direct inverter manufacturers recommend between 15 percent to 25 percent more PV input power than the inverter’s power output rating. For example, if you have an inverter that’s rated to produce a maximum of 3,000 W (3 kW), the maximum recommended PV array (in terms of power output) would be between 3,450 W (3,000 W × 115%) and 3,750 W (3,000 W × 1.25%). Note: Generally, you already have a desired PV array size you want to install, so use this relationship to define the inverter size you need to match your chosen array.
Supplying the right features for you and your clients
During the specification process, you also need to evaluate the feature set associated with the grid-direct inverters available for your system requirements. Some features may be very important to you as the installer; others will be there mainly for the user to enjoy over the years.
One of the features that’s of chief interest to you as the system installer is that many grid-direct inverters now come standard with disconnects that integrate to the inverter, allowing for fewer components during installation. This feature helps the installation process because the NEC® requires the installation of disconnects and specifies their location to the equipment they serve. By incorporating the disconnect into the inverter, you have less equipment to buy and install, which makes the system less expensive for your client and faster for you to install. (I cover safety components in Chapter 10).
From the client’s perspective, an important feature is the ability to access as much information about the system’s performance as he’s interested in. Some PV system owners go out to their inverters daily to check the energy-production values. Others, like me, know that if the information isn’t readily available (and in their faces), life will likely prevent them from checking the system on a daily basis. Find out what your client’s monitoring preference is and then seek out the user interface that makes the most sense for him based on those preferences, whether that’s a wireless display set up in a noticeable spot in his home or a data-transfer arrangement that puts the PV system’s data online or in a text message or e-mail.
Battery-based
Your first consideration when specifying a battery-based inverter should be this: What’s the AC power source — the utility grid or a generator? Answering “grid” means you need a utility-interactive, battery-based inverter; answering “generator” means you need a stand-alone, battery-based inverter. From there, the questions and considerations (including what special features are desired) are very similar to the kinds you’d have if you were specifying a grid-direct inverter. (Note that the considerations that follow are all unique to battery-based inverters.)
In some cases, you’ll want to have both the grid and a generator for multiple power sources. In this situation, you need to confirm that your chosen utility-interactive inverter can accept multiple power sources (not all of them can) before getting too far into the specification process.
After establishing the AC power source, you must determine the amount of power the inverter needs to supply continuously. As I show in Chapter 12, this means determining all the loads that the user plans to run simultaneously and comparing the amount of power needed to operate them all to the inverter’s continuous power output rating.
For stand-alone, battery-based systems, the inverter’s power output rating doesn’t need to account for the PV array’s power output because the battery bank is between the inverter and the PV array, acting as a buffer. When you’re installing a utility-interactive, battery-based system, however, you need to look at the relationship between the two. If the batteries are full and no loads exist, the inverter must have the ability to send all the PV power into the utility grid. In this situation, if the PV array can supply more power than the inverter can process into AC power and send into the grid, the additional power won’t be used, and the PV array won’t be operating at its maximum efficiency. Consequently, the client won’t send as much energy into the grid as he could have (which won’t make him too happy with you).
Another power-related item to consider is the surge rating. Any electrical load with a motor creates an electrical surge when started. The battery-based inverter you select for your client’s PV system must be able to handle this surge; if it can’t, the loads won’t start and could possibly cause the inverter to crash, stopping the power flow to all the loads. An inverter’s surge rating is listed on its spec sheet; this rating can show up as either watts or amps, so you may need to do some conversions to accurately compare the loads to the inverter (see Chapter 3 for help making the conversion).
Just as grid-direct inverters need a steady power source from the utility grid, battery-based inverters need a steady power supply from batteries and require a narrow supply voltage window from the batteries for operation. These inverters are designed to work in conjunction with a low-voltage battery bank — typically 12 V nominal, 24 V nominal, or 48 V nominal.
Finally, you need to make sure the inverter can deliver the correct voltages needed for the loads. Most deliver 120 VAC but can be configured for other voltages as necessary. For instance, some battery-based inverters come standard as 120/240 VAC for convenience, and a few manufacturers offer the ability to configure three inverters together for a 120/208 VAC system.