Three-phase string inverters have made giant leaps forward over the past several years. Prices have fallen rapidly relative to central inverters, and along with rapid shutdown and arc fault requirements, their use has become standard in larger projects. According to GTM Research's latest report, The Global PV Inverter and MLPE Landscape 2016, in the USA, three-phase string inverter use increased progressively over the last few years from 5 percent of all inverter shipments in 2012 to 11 percent in 2015. Over that time period, the threshold for which string inverters making clear financial sense has risen from about 500 kilowatts to around 5 megawatts today.
The appeal of PV string inverters from the prospective of a plant owner, there are four major areas string inverters excel. (1) The acquisition cost per MW is much less than central inverters. (2) If an inverter faults, the generation loss is significantly smaller than a central inverter. Because 33 string inverters are equal to one central inverter, losing one string inverter (vs. one central inverter) is less impactful to revenue. If you lose one string inverter, it’s not an emergency, but if you lose one central inverter, it’s all hands on deck to get it repaired. (3) String inverters produce common, off-the-shelf 480v AC, whereas central inverter output is more uncommon (315v AC). Often the transformer and other infrastructure is more readily available, and often a little less expensive. (4) Design for string inverters system is much less complicated
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For string inverter use to take off, the broader value proposition must be proven for large-scale projects. Price declines are a major driver; however, the upfront cost of the inverter is far from the only determining factor. Associated balance-of-systems, installation costs, system performance, and lifetime operations and maintenance needs are also taken into account.
Preliminary analysis and anecdotal submissions from leading EPCs suggests that for 20-megawatt and larger utility projects overall system pricing for string and central inverters are relatively close. Though the calculations are complicated and entail a number of assumptions, string inverters typically make up for some of the upfront premium of 2 cents to 4 cents per watt in their lifetime cost savings since they require less maintenance. Additionally, in the event of a failure, availability is minimally affected, and inverters can quickly be swapped out without the need of an electrician. However, the complexity of the calculations and the numerous project-specific factors such as slope, land shape, racking type, and interconnection requirements render rules of thumb for string versus central inverter use difficult to come by.
String Inverter Configuration
Disadvantages of PV string inverters – Over their lifetime, string inverters are more expensive. But it’s not a simple cost per watt calculation. There are additional costs that don’t usually factor into a purchasing manager’s site estimation equation, such as:
Complexity – From an instrumentation and implementation standpoint, string inverters are complex. They have a lot more data from a lot more devices, which means more points in the data acquisition system, which is often licensed by numbers of points (a hidden cost adder).
Instrumentation – String inverters produce more data points to monitor. As a rule of thumb, assume each analog data point in a SCADA system typically costs $10 after adding up all software, engineering, and implementation costs. Since it takes 33 string inverters to equal a single central inverter, and because each string inverter typically has 50 points, you’re actually looking at monitoring 1,650 data points ($16,500) vs. a central inverter’s 100 data points per inverter ($1,000).
Maintenance - String inverters don’t provide too many diagnostics. It’s expected that when these inverters fail, they’re swapped out instead of fixed. Instead of storing spare parts, you’re paying for several full-on inverters to sit in your inventory until they’re needed.
Communication - String inverter communications technology uses serial communications rather than Ethernet. This means data transferring speeds are lower, the cost of cable is higher, and the amount of field wiring increases. Even inverter manufacturers who have done away with serial communications have their drawbacks. A recently completed project using string inverters. The communications to the inverters are conducted over power conductors, not serial communications cables. This reduces the installation cost of serial cables…but also creates a single point of failure equivalent to a central inverter (negating one of the key reasons to choose string inverters over central). In addition, this string inverters don’t have built-in HMIs on the inverter to provide diagnostics. This means O&M can’t do any significant diagnostics if communications go down.
Anti-islanding - Another important point to bring up is anti-islanding. Our experience with the coordination of anti-islanding with string inverters has been problematic. Out of an abundance of caution, most owners add an additional recloser to the site so that they can meet utility requirements for anti-islanding. At the end of the day, when plant cost estimators compare initial costs, string inverters are the cheaper, obvious option. The additional work created by smaller inverters is a larger, hidden cost.
The advantages of PV central inverters – Yes, central inverters themselves are initially more expensive in $/Watt. however the little maintenance they require greatly outweighs the problems that come with string inverters. In brief, the advantages of a central solar power inverter include:
Diagnostics – Central inverters have a whole lot more diagnostics, which means they will warn you before faulting. Warning alerts give you the ability to schedule onsite techs to fix the problem before the fault actually occurs.
Reliability – Central inverters are engineered for reliability more than cost savings. The largest market for string inverters has been residential or commercial rooftop systems, and the installation environment is fairly well controlled. Today's central inverter is designed to be installed in the toughest environments (118 degrees in the Arizona desert is a miserable existence, but the inverters don't seem to mind...) so the thermal management capabilities are more robust than required for most installations.
Monitoring - Monitoring a lot of devices is expensive. For a control standpoint, sending commands to 33 string inverters (vs. just one central inverter) is slower. As I already mentioned above, central inverters have double the data points than a single string inverter, but since it takes 33 string inverters to make up one central inverter, you’re really only looking at 28 inverters per MW x 5 MW x 50 tags is 7,000 string inverter data points vs (5) 1MW inverters @ 120 points totaling 600 points to monitor. The management of screen real estate alone is a trick. Having designed a monitoring screen for a site with 520 central inverters, reliability gets more difficult with more devices, and the monitoring trade offs are not to be taken lightly.
Less risk - The more string inverters at a site, the more failure points the site has. Fewer inverters just means fewer places O&M techs have to visit and complete their work. There's fewer devices to wrangle when starting the site up, and the issues around connectivity are a lot easier to troubleshoot with a few inverters on Ethernet vs. a hundred inverters on a multi-drop serial bus.
Micro Inverter Configuration
Advantages of Micro Inverters – The core advantage of using micro inverters is that theoretically can yield more solar electricity.
The reason for this is that there are slight differences in voltages between solar panels. When solar panels are in a string the voltage is reduced to the voltage of the lowest voltage panel in the string.
If a solar system is facing multiple angles, meaning some panels are facing south, some east, and some west, then micro-inverters are the way to go. Or, if you have shading issues from trees or a large chimney, again micro-inverters would be best. In these situations, the solar panels will be producing different amounts of electricity at different times of the day, but micro-inverters will ensure you harvest all of the energy, while with a standard inverter you will lose some of this production. With solar panels all facing one direction on your system, and you have marginal shading issues, then your best option is a standard inverter. You’ll get about the same production, without paying the higher cost.
To throw more confusion in this, Optimizers are an option for standard inverters as well, which function very similarly to a micro-inverter. With an optimizer, you still have a standard inverter, but you also have optimizers for each individual panel combating production differences.
There are other aspects to consider as well. Micro-inverters typically have 25 year warranties while a standard inverters typically have 5 or 10 year warranties. The reliability of micro-inverters was in questions several years ago, but the technology now has caught up with the industry and the long warranties on the micro-inverters shows the confidence the manufacturers have in their products.
Micro-inverters and the add-on optimizers both offer an additional perk in system monitoring as well. With either of these devices, you have the ability to track the production of each individual panel, while with a standard inverter you only can track the production of the whole system.
If you were to expand your system in the future, micro-inverters are simple to add one at a time. However, with a standard inverter, it would be more costly to add another full unit. To sum it all up, micro-inverters are definitely a value-add, but are only recommended if you have panels facing multiple orientations or you have shading issues. Otherwise, the less expensive standard inverter is usually more cost effective.
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