Which Solar Power Inverter is Better? String or Central?

Which Solar Power Inverter is Better? String or Central?

By: Adam Baker

Pros and cons on solar power inverters from an instrumentation viewpoint

I’m just going to go right out and say it:string inverter central inverter central inverters are better. But they’re often overlooked for string inverters. Here’s why.

 

The appeal of PV string inverters

From the prospective of a plant owner, there are three 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.

 

Disadvantages of PV string inverters

I would argue that 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, I 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.

Communications

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. I recently completed a project using Huawei string inverters. The communications to their 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, Huawei 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. My 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. But I would argue that 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, I can tell you that reliability gets more difficult with more devices, and the monitoring trade offs are not to be taken lightly.

Less risk

The more string inverters you have, the more failure points your 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.

 

What it all boils down to

Ultimately, from an instrumentation standpoint, the same capabilities exist whether you’re running on string or central inverters; from plant control, to ramping, to reactive power controls. It really just comes down to your thoughts on how much it will cost to maintain inverters over the long-haul, and the associated risk. Just don't fall for the first pass cost for string inverters as being a blessing. The hidden costs just move to other areas of the project budget, and will come as an unpleasant surprise later when they appear where you weren't expecting them.

I’d be happy to help you determine which solar power inverter is right for your utility-scale project from an instrumentation standpoint, or help you set up instrumentation on an already-existing site.

 

Adam Baker - Affinity EnergyAdam Baker is Senior Sales Executive at Affinity Energy with responsibility for providing subject matter expertise in utility-scale solar plant controls, instrumentation, and data acquisition. With 23 years of experience in automation and control, Adam’s previous companies include Rockwell Automation (Allen-Bradley), First Solar, DEPCOM Power, and GE Fanuc Automation.

Adam was instrumental in the development and deployment of three of the largest PV solar power plants in the United States, including 550 MW Topaz Solar in California, 290 MW Agua Caliente Solar in Arizona, and 550 MW Desert Sunlight in the Mojave Desert.

After a 6-year stint in controls design and architecture for the PV solar market, Adam joined Affinity Energy in 2016 and returned to sales leadership, where he has spent most of his career. Adam has a B.S. in Electrical Engineering from the University of Massachusetts, and has been active in environmental and good food movements for several years.