String inverters replace central inverters in MW-scale PV installations

String inverters make fewer demands on installers than large central inverters. They also enable more flexible investment planning. The power density of the latest devices is over 800 kilowatts per cubic meter.

Ever more power in ever less space: that’s the philosophy of Delta Energy Systems. And indeed: the power density of Delta inverters is growing continuously. The technology group started its transformerless series in 2009 at 96 kilowatts per cubic meter. Within eight years, the research and development has increased the power density of the inverters more than eight fold. The latest M88H inverter presented at Intersolar achieves 815 kilowatts per cubic meter and will be delivered starting in 2017.       

From Delta’s perspective, the relationship between size and performance is crucial, rather than the built-in performance per kilogram. The new inverter with 88 kilowatts output power is even smaller than the RPI M50A with 50 kilowatts. It measures just 64 centimeters in height, is 61.5 cm wide and 27.5 cm deep. The size difference becomes even more apparent when you compare the dimensions of the M88H with the older Solivia 15 TL, which came onto the market around the end of 2009 and only has 15 kilowatts of output power (see image). The external dimensions are comparable, but the M88H has almost 6 times the output power and includes a terminal box in addition to the power module. The power module itself is about one third smaller than the Solivia 15 TL.

Product manager Patrick Schahl in front of the new M88H (left), which is clearly more compact than the Solivia 15 TL from 2009

Development of the power density of Delta inverters since 2009

Full power up to 40 degrees Celsius

An aluminum housing protects the device against humidity and corrosion. The inverters provide the full output power at ambient temperatures of - depending on the model – up to 40 to 48 degrees Celsius at full power supply. The electronics will only throttle performance if this temperature level is exceeded. At 60 degrees Celsius, the maximum operating temperature is considerably higher for all devices.

Like all inverters from Delta Energy Systems, the new M88H has the protection grade IP65 and is approved for outdoor installation. The advantages of the inverter include the fact that the fan assembly can be removed quickly and easily for cleaning or removal, without having to switch off the inverter.

Cost advantages compared to central inverters

The general market trend confirms Delta Energy System’s strategy of using string inverters in larger solar parks, opening more leeway in facility planning and minimizing loss of earnings in the event of partial facility breakdowns. Furthermore, every string can be monitored individually.

Investors should also be interested in this trend: market data shows that the investment costs for string and central inverters are relatively identical. However, the installation and operating costs differ significantly.

A central inverter needs a solid foundation and a large crane on-site. The crane rent alone, is around 2,000 euros per day. Normally, an experienced crane driver is needed, as well, as an asphalt road on the facility site. Since central inverters also feed into the medium voltage, a fitter is also required who is qualified to connect such high voltages. The security guidelines are also more extensive for central inverters. For example, a separate DC terminal box is prescribed which must be installed between each 100 kilowatts of module output and the central inverter. And that, in turn, increases the costs.

The M88H string inverter, however, is supplied with all components required on the DC side, even the terminal box is already integrated with the device. Due to its relatively light weight, only a small van and a lightweight lifting device are required for transportation to and lifting at the installation location.

Lower repair costs

In the event of damage, a string inverter is superior to a central inverter. If a central inverter with an output of one megawatt fails, it will cost the facility owner about 500 euros per day, assuming potential energy production of 10,000 kilowatt hours. For a small inverter with an output of 88 kilowatts and 880 kilowatt-hours, this would only cost 44 euros – i.e. less than a tenth. The risk of a high cost failure is significantly lower.

Technical faults in central inverters always require a specialist who must be available on short notice and may not necessarily live nearby, and also requires heavy equipment for the installations. A string inverter, on the other hand, can be replaced on site quickly and easily by any local electrician. A string inverter is also significantly lighter. An RPI M50A weighs 74 kg, for example. For the M88H, the power model, which can be disconnected from the wiring box, only weighs 68 kg, while the entire device weighs approximately 84 kg.

Optimized heat dissipation

In order to achieve the performance specified in the data sheet, there must be sufficient distance between the devices. The research division at Delta works with simulation tools to calculate the minimum distance between individual inverters. Thermal images created using infrared cameras show what device spacing is required so that, on the one hand, emitted heat can escape, and on the other hand, no warm air is drawn into neighboring inverters.

The new inverter achieves a maximum efficiency of 98.8 percent, the EU value is 98.5 percent in the data sheet. With price pressure on the inverter market, every percentage point of efficiency counts. The M88H was developed specifically for use in commercial solar farms.

Input voltages up to 1,000 volts

The M88H has two MPP trackers and operates with input voltages of up to 1,000 volts DC. A protective device protects the device up to 1,100 volts DC open-circuit voltage. In M88H, two boosters operate that lift the input voltage according to the requirement of both MPP trackers at an optimum voltage level.

On the AC-side, the M88H can be connected to 400 V or 480 V mains. With a mains voltage increase of 80 V, the lower AC power reduces heat-induced cable losses with the same cable cross-section and cable length by about 4%. Conversely, you can reduce the cable gauge because the currents are lower, thereby saving costs for the cables. 

The M88H is designed to withstand power surges up to 30% of the nominal voltage. In this way, the voltage drops in the cables between the mains connection point and the inverter are better taken into account.

Touch-safe surge arresters are integrated with the terminal box. A mechanical DC circuit breaker ensures that the inverter can also be separated under charge from the solar modules. Since transformerless devices such as the M88H does not have a galvanic isolation between the inverter and the photovoltaic modules, the device continuously monitors the residual current.

The power modules are equipped with particularly robust construction elements made of silicon carbide (SiC). This material is extremely hard and enormously wear-resistant. The very good thermal conductivity is maintained even at high temperatures, and the material expands only minimally in the event of temperature fluctuations. Silicon carbide therefore plays an important role in reducing energy losses and improving the efficiency of the inverter. Above all, the integrated ceramic material increases the reliability of the inverter – and for the facility operator the most important thing is that the inverter is feeding into the power grid.

Taking national regulations into account

Close and continuous collaboration with certification companies during product development ensures that the M88H meets the national power connection guidelines of the major European operators. Thanks to preconfigured country settings, the inverter can be configured on the display in a few quick steps and then operated. Different password-protected access levels protect individual access rights. To capture and evaluate operating data, the new inverter has two RS485 interfaces. It is possible to communicate via a Delta-specific or Sunspec protocol.

The M88H terminal box meets the requirements of protection class II, as required by the safety standards of many European countries.

Perfect for large solar installations

Regardless of whether solar panels are installed on a private home or an industrial plant – from Delta’s perspective there are no size restrictions when using string inverters. For example, inverter RPI M50A was installed in Scandinavia's largest solar park with 50 MW output power in the Danish region of Vandel. The devices are installed under the solar modules, protecting them from direct sun and rain.    

Solar park near Vandel, Denmark with RPI M50A inverters.

In the port town of Port-Saint-Louis-du-Rhône, near Marseille, the largest industrial rooftop solar system in France went online with 6.7 megawatts in the Summer of 2016. For this, Delta provided 112 of its RPI M50A inverters that convert the energy from around 25,000 solar modules each at 270 watts. In order to work as efficiently as possible, the inverters are arranged into four groups to minimize the cable length and thus cable losses.

The solar plant in Port-Saint-Louis-du-Rhône in Marseille produces enough energy for the company and the city. 112 M50A inverters from Delta are deployed here.

Delta Ako Energy Park with 180 Delta RPI M20A / RPI M50A inverters

Los Angeles rooftop PV system with 164 Delta M88H (88 kVA) inverters

China PV plant with 300 Delta RPI M50A inverters


Delta Energy Systems sees string inverters in multi-megawatt facilities as a very promising option. Not only does the company continue to drive technical development, it also provides technical support and trains installers who work with Delta devices. Experience gained in the field show that Delta is right: the physical limitations of the technology are far from being achieved. In the future, Delta will continue to work on reducing weight, improving efficiency, and developing ever more compact inverters.