Inverter Replacement Considerations for Photovoltaic Energy Systems

Have you ever considered replacing the inverters on your photovoltaic site? Have you investigated the options and requirements for successful inverter replacements? NAES Renewables has extensive experience in identifying and sourcing replacement inverters for sites of all ages. In this discussion, Project Engineering Manager Frances Plourde provides an overview of the issues facing inverter replacement evaluations and provides insight on the many considerations needed during this process.

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Transcript

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Welcome to Power Bank, sponsored by NACE Renewables, where big ideas, bold conversations, and unstoppable energy collide. We're your hosts, Ann mc Broome and Francis ppl, and we're diving headfirst into everything you need to know, want to hear, and can't stop talking about in renewable energy. Let's get into it. Hello, and welcome to another episode of the Power Bank podcast sponsored by NACE Renewables. My name is Francis ppl and I'm a project engineering manager at NACE Renewables. And today I'm going to be discussing some considerations for inverter replacements performed on photovoltaic energy sites. This is a follow-up episode to our previous show where we discussed module replacement considerations. We delved deeper into some of the issues regarding module replacement, mostly regarding the lack of uniformity and consistency within module design characteristics. Today we're gonna be diving deeper into what it looks like to replace inverters on photovoltaic sites. Some of the issues are similar to that of module replacements, and then there's some issues that are unique to inverter functionality that make inverter replacements more complicated to pursue. As a little bit of background into what inverters are and why they're so important to photovoltaic energy systems, as well as a lot of other renewable energy systems. Photovoltaic energy sources are what we call direct current or DC energy sources. This means that the power that the current that is produced by these systems is only moving in one direction instead of the alternating current, AC systems which is what we use to power our electric grid. And almost every single appliance that you use. The majority of renewable energy sources that we use, including, photovoltaic power, wind, et cetera, these systems are all direct current, which means when they generate energy, that energy needs to be converted into alternating current in order to be transported within our electric grid, but then also to be utilized within our homes and businesses. So inverters are an essential system component that allows for that direct current to be converted into an alternating current power flow. They do this by rapidly switching the direction of the direct current input in order to set it to the desired frequency of the alternating current, in the United States, that is 60 hertz, so 60 times per second, the direction of the current is shifting. Inverters are absolutely necessary. To enable renewable energy sources, such as PV modules to be utilized to generate power that we can use within our grid. And then all of the appliances that we use, that run off of that grid. As with the photovoltaic modules themselves and with any other system component inverters will occasionally need to be repaired or replaced in order to ensure continuous. Safe and efficient function. Researchers at the Byrne University of Applied Sciences found that roughly 34% of all inverters will fail within the first 15 years of operation. So that's over a third will not make it to 15 years of operation. The reasons for this can include weather, damage, or damage due to things like fires or anything that can cause physical or structural damage to a photovoltaic system. Inverters are less susceptible to damage from weather conditions such as hail than PV modules are, but obviously hail, fire, flooding tornadoes, any other natural disaster that can cause. Damage to a photovoltaic system will inevitably lead to damage to an inverter. Another common reason for inverter failure is that of repeated faults leading to repeated tripping of the inverter. So if you have issues caused by repeated ground faults or other issues with the modules, with the existing electrical system that can back feed into an inverter and potentially lead to faults or thermal events within the inverters themselves, that can then require significant repair or replacement of that inverter. Another common reason why inverters may need to be replaced is because they've hit the age at which it becomes difficult or impossible to source replacement parts or skilled replacement labor in order to fix'em. Once you reach 10 to 15 or more years after inverter installation, it can be almost impossible to source replacement parts needed for those inverters to keep them functioning. And it also becomes very difficult to find technicians or inverter manufacturers that can provide technical support or repair services for those inverters, once they've hit that age, most inverters being installed presently have warranty coverage for five to 10 years after installation. So after that point, inverter manufacturers won't usually provide additional support. Or repair on those inverters, unless you're willing to pay additional fees. Once inverters have hit the age where it becomes difficult and expensive to keep them running continue to reform regular repair work on them, replacing those inverters becomes an attractive prospect. Now that we've covered some of the reasons for why inverter replacement may be necessary or desirable, I'd like to talk about some of the technical challenges involved with inverter replacements. As complicated and challenging as PV module replacements can be, inverter replacements can be far more complex because of their interconnection with so many other system components, both within the existing PD system as well as to the electrical grid at large. One of the first things to consider is to determine whether the existing inverters that are presently used in the site are single phase or three phase inverters. This is particularly important when you're looking at older systems or systems where the inverters used were relatively small. You don't tend to see single phase inverters used in larger scale pB. Utilizations anymore. Single phase inverters are inverters that only output the AC current in one voltage phase, whereas three phase inverters use all three voltage phases. This becomes especially important to consider if you're replacing inverters that are smaller. Usually single phase inverters are under 10 kilowatts. But it's something to pay attention to, especially if you're looking at older systems that were designed with lower power outputs, or if you're looking at more residential or smaller commercial systems. Another aspect of inverter replacements to consider is whether the inverters that you're replacing are string inverters or central inverters. String inverters tend to be smaller and are more evenly distributed throughout the system, whereas central inverters, as the name suggests, are more centrally located and have multiple strings and sub arrays of a PV site directed into them. Central inverters tend to be more common in older systems. They used to be the standard for inverter installations, especially in larger utility scale sites. So sites approaching one megawatt or larger. So instead of having multiple smaller inverters, you would have a handful of very large central inverters that could cover a megawatt. Each of PV module inputs, central inverters have fallen out of fashion within the industry, mostly because of their size, their increased cost, and the lack of modularity within them. The reason why central inverters have fallen out of fashion within the industry is because of their increased equipment costs, their increased installation cost, and their lack of modularity. So these are huge pieces of cabinetry that require concrete pads to be laid down before installation, they require a significant amount of. Conduit runs, and as the name suggests, they're very centralized. That means that you have large portions of a photovoltaic site being directed into the same inverter, which means that if you have part of that array down, that impacts the rest of the inverter production as well. Many of the older utility scale systems that you may run into will use central inverters. But we're starting to see more and more developers and designers move over to string inverters. So string inverters historically were used in smaller commercial applications, but now we're starting to see them being used more and more in larger utility scale operations as well. And this is because string inverters accept fewer string and module inputs than central inverters do, they are significantly cheaper to install. The unit cost is significantly cheaper. They're easier to install, they only require, one or two technicians as opposed to an entire team To install a central inverter, you don't need to put down an expensive concrete slab like you do for central inverters, and they're highly modular. So that means that instead of having two or three central inverters on a site, you may have 20 or 30 string inverters. So the overall inverter capacity is the same, but because it's being directed to more equipment than for a central inverter, It's easier to address site issues that are going to specific inverters rather than having almost an entire system being pumped into one central inverter. Many of the reasons why we're starting to see central inverters, especially require replacement is because their lack of modularity means that certain areas of an array can be down for long periods at a time if that inverter is not returned to functionality quickly. And so many site owners and developers are wanting to move to a highly modular string inverter concept. So that's something to determine as well, if your existing inverter that you're looking to replace is string versus central inverters. Part of that discussion as well, considers inverters with internal transformers and what we call transformerless inverters. Again, this is an area of the industry that has rapidly shifted within the last 10 to 15 years. But most inverters, both string and central inverters that were manufactured 10 to 15 years ago had an internal transformer. So another way to think about this is that inverter had its own internal grounding. The reason why inverters have now shifted to mostly transformerless inverters is because they're significantly cheaper to manufacture and they're significantly cheaper to install. Because if you can imagine an inverter with an internal transformer, that's gonna be pretty heavy. That internal transformer is going to. Add a significant amount of weight, both mechanically, but then also when it comes time to install it. There was a change to the national electrical code that allowed for inverters to not need internal transformers. So this is what we call ungrounded or floating inverters. And so ever since then, most string inverters do not have that internal grounding and that internal transformer. The reason why this is becomes tricky when it comes to inverter replacements is that if you take an inverter that had a internal transformer, either a string or a central inverter, and then you replace it with a transformerless inverter, you open the site up for the risk of developing what we call potential induced degradation, or PID. This is a phenomenon that impacts PDV modules in systems where the inverters are not themselves grounded. It is degradation that presents itself due to potential differences between the modules themselves, specifically the module frames, and then the inverter that those modules are connected to. This can lead to leakage currents that can lead to overall performance degradation as high as 30% from the modules. Different PV modules are susceptible to PID at different rates. There are ways of testing PID susceptibility in PV modules, and certain PV modules are known to be more susceptible to PID than others. So if that's something that you're concerned about, if you're considering an inverter replacement from a grounded to an ungrounded inverter, we highly recommend that you pursue that testing to ensure that your PD module production capacity won't be impacted by PID. As we discussed during our podcast on module replacement evaluations, the photovoltaic industry has increasingly tended towards focusing on producing equipment that is electrically larger than what came before it. This can be especially concerning for inverter replacements because oftentimes it can become very difficult to find inverters that are smaller and closer in size to what inverters manufactured 10 to 15 years ago were. For example, it can become very difficult to find an inverter with a capacity of 15 kilowatts. If the smallest inverter you can find nowadays is 25 or 30 kilowatts. Inverter size or inverter power capacity is a measure of how much AC power they're able to export into the electric grid. Inverters usually have a limit of DC power input that they're able to accept and that they can then turn into AC power. It's important to balance the DC to AC ratio for inverters. You don't usually want to exceed a 50% overload ratio, which means that you have 50% more DC power than AC power. This may cause issues with the inverter. You also wanna make sure you're providing enough DC input power to be able to export the AC power output that you want. It is also important to take a look at the modules themselves that are going to be connected to that replacement inverter. You wanna pay attention specifically to the replacement inverter's maximum PowerPoint or MPPT tracking capabilities. These are important to balance in order to make sure that the inverter can produce the maximum amount of power at all times. Most inverters will have a voltage and current limit that they have to remain between in order to both ensure safe and efficient, consistent function while also hitting their maximum PowerPoint tracking capabilities. So it's important to take a look at what the existing modules are connected to the original inverter, what string configurations they're in, and then figure out how many strings and how many modules the replacement inverter can accept in order to maximize inverter production. Additionally, especially in older systems where there was less uniformity in the voltage characteristics of the individual inverters, it can become very difficult to find inverters with the same voltage output as the existing inverters on a site. Before the industry became more standardized, there were multiple different output voltages for especially smaller string inverters. So you might have inverters that would output at 208 volts, 240 volts, 277 volts. As the industry has become more standardized, and especially when you're looking at larger inverters, this becomes less of an issue, but especially when you're looking to replace inverters that were. Installed 15 years ago or more. It can become incredibly difficult to find inverters that have the same output voltage as the existing inverters on site. This can become an important design characteristic when you're looking to incorporate replacement inverters into a system, because you want to ensure that the output voltage of those replacement inverters is going to be the same as what the output voltage was using the original inverters, there are some inverters that will allow you to adjust the output voltage, but this in many cases can lead to a decrease in the inverter's output power capacity. So it's usually best to focus on trying to find an inverter with the same output voltage as the existing inverter. As we've been discussing, there are many different considerations that need to be taken into account when you're identifying the proper replacement inverter for your particular application. Paramount to this is making sure that you're able to actually purchase the inverter that you want to use in your system. That it is commercially available and is available at a reasonable price. So this is where looking at the cost benefit analysis of specific design options is really important. Looking at what the additional cost would be of continuing to try to repair and maintain an existing inverter that is having issues versus what the cost would be to replace a new inverter. This is also where you should take into account the potential financial cost of any energy losses caused by inverter downtime. It's important to think about the financial impact of inverter downtime and inverter replacement costs, not only in terms of the actual cost of the energy itself, but also, if there are any clauses within the site's power purchase agreement or PPA that leave the site open to paying liquidated damages to the power purchaser in case of extended downtime or energy loss. So that's definitely something to consider as well. Weighing the cost of the lost electricity versus the cost of actually performing an inverter replacement. Sometimes that cost balance doesn't work out in favor of an inverter replacement, and it makes more financial sense to continue repairing and maintaining the existing inverters, but especially for larger sites that have been plagued with inverter issues for a long period of time, the cost of that power loss and any liquidated damages may outweigh the cost of replacing those inverters, so that's a very important consideration to take into account. It's also important to consider what availability there is for replacement inverters, especially if you're looking at older and smaller inverter types. The available inventory may not be there for you to perform a simple inverter swap, and so that's where you may need to go back to the drawing board. And figure out if you can replace several smaller string inverters with one larger new string inverter or one central inverter with several string inverters. So that's where the actual supply chain and available inventory is going to come into play in inverter replacement considerations, making sure that you can actually perform the replacement that you want with the inventory and the equipment available. There are several other important considerations that should be accounted for in inverter replacements that have less to do with the inverter function as a dC to AC conversion and more to do with the inverters configuration within the system itself. Something important to consider is the space or the mounting method of the inverter that you're going to be using. Especially if you're moving from a central inverter to a string inverter, you're going to need to develop a new method of mounting and installing those string inverters into your system. So where are you going to put them? Especially in hot and dry environments, we know that heat and UV exposure severely damages equipment over time. And so in many cases it's desirable to construct some sort of shade structure or make sure to mount inverters in an area where they're shaded and not getting direct sunlight for the entirety of a day. So that's something to consider as well. If you are considering a, if you're considering installing a new central inverter into a system, you also need to incorporate the cost of putting in a new concrete pad, getting a crane on site to be able to remove an existing central inverter and install a new central inverter. So there are significant costs and considerations with the mounting and installation of new inverters. That should be considered. Another important concept is communications. Inverter Communication is a way of being able to see which inverters are producing the way that they should. And if there's specific issues with inverters or strings, it gives a really good idea as to the health of the site and how the site is performing. So replacement inverters will need to be able to work with the existing communication system on site. Do they have the right configurations? Can they be incorporated into whatever existing data collection and data monitoring system exists on site. So that's very important to consider as well. Another important thing to consider is the trenching or conduit needs for replacement inverters, especially as it has to do with the number of DC inputs that that inverter can take. Inverters will differ in the number of DC inputs they can take, depending on how they are set up. So some inverters will only be able to take in a single DC input. So all of the PV module strings will need to be combined before being input into an inverter, whereas some inverters allow for multiple DC inputs, and so you don't have to do that combination of string inputs prior to input within the inverter. This can lead to differences in needs for available trenching or conduit on site. So especially if you're needing to incorporate more input strings than what the existing inverter allowed for, you may need to consider adding additional trenching or conduit runs to allow for those additional connections. So that's definitely something to consider as well, that. Cost can add up quickly, and the cost of incorporating the new inverter into the site may end up outweighing the cost benefit of replacing that inverter. There are also several contractual and regulatory issues involved with replacing inverters that absolutely need to be considered. The first involves looking at the power purchase agreement for the site. The power purchase agreement, or the PPA is a contract between the site owner and the purchaser of that power, so that's usually gonna be a local utility. The power purchase agreement dictates the power export from the site. That can be expected by the utility in order to make sure that the site is producing consistently and effectively. Power purchase agreements often contain clauses that state that the site owner needs to alert the utility of any material changes to the power output capacity of the system, especially in cases where the inverter name plate ratings, the amount of power that the inverter is technically able to produce is changing. This may trigger those clauses within the PPA that require the utility to be notified and approve of any changes to the system. So that's definitely something to consider. In addition to utilities, regulatory bodies or other governing agencies may also have limitations or requirements for inverter replacement procedures. Depending on the size of your site and the state it's located in, there may also be governmental or regulatory requirements to pursue when you are exploring an inverter replacement option. Some states, including states that have independent system operators such as the California Independent System Operator or caso, or the Electric Reliability Council of Texas or Ercot may have requirements that need to be met during the inverter replacement process. It's common for utility scale sites to need to prove that their power reliability and their power flow will be consistent with existing standards and codes. Once inverter replacements are performed, in many cases they require PS CAD model or PSS model proof that the inverter configuration used in a replacement will allow for consistent or improved energy reliability at the system compared to what the system was before the replacement. So it's important to consider that that process is not trivial, and so especially if you're pursuing an inverter replacement on a larger scale site, there may be additional requirements to prove that that inverter replacement will lead to increased system efficiency and functionality over time. Whatever the reason for an inverter replacement study, whether it's due to wear and tear on an older inverter, or significant damage to an inverter that requires replacement NACE renewables. Experience team of engineers is up to the task of solving any inverter replacement challenge that you may have. If you have any questions about what we've discussed today or if you'd like to get in touch with us for an inverter replacement evaluation or for any other services, please reach out to us by sending us an email at Power Bank at na NAE s.com, and we'd love to hear from you. Thank you for joining us today on the Power Bank, and we'll see you again soon. Thank you so much for joining us on today's episode of The Power Bank, sponsored by NACE Renewables. If you have any questions regarding the topic of our discussion today, or you would like to reach out to us about special projects for photovoltaic sites, we would absolutely love to talk with you. You can send us an email at Power bank@nace.com. Thank you and keep powering on.