Transformerless inverter configuration used in solar photovoltaic projects

This article describes the construction of transformerless inverters used in today's commercial and utility PV installations. It examines how power integrators and utilities can unlock new capabilities by integrating multiple inverters directly into the grid or with just a medium-voltage transformer. Finally, the article details the many benefits of reduced system complexity and maximized power generation efficiency.

The new generation of transformerless technology reduces system complexity for power integrators and utilities, maximizing the power transmission capacity of the two most common large-scale PV installation projects: direct connection of building inverters and utility installations for grid-connected power generation and transmission.

While solar PV power is becoming increasingly competitively priced, it is critical for the industry to continue to enhance performance, improve efficiency, and reduce costs. Improving the quality and performance of large investment equipment is one way to continue to increase returns, and the performance and efficiency of inverters are just as important as PV modules and arrays. When it comes to large-scale PV system design, power integrators and utilities are moving away from traditional inverter equipment and beginning to choose state-of-the-art transformerless inverter technology to reduce system complexity and maximize power delivery. It is indeed necessary to take a closer look at how transformerless inverter technology is helping to change the competitive landscape by affecting system design, efficiency, and balance of system (BoS) costs.

This new technology eliminates the need for transformers on the low-voltage three-phase grid by using a separable two-pole +600 and -600 VDC battery array for direct conversion. This configuration not only improves power generation efficiency, but also eliminates the need for the inverter transformer traditionally required, reducing the associated balance of system (BoS) costs and avoiding unnecessary line attenuation associated with single-pole configurations. This technology also brings additional benefits to large commercial or utility installations for power integrators and utilities. For example, a typical commercial project of 1 to 2 MW requires one to eight inverters with the connection point on the low-voltage side of the building entrance transformer, and each inverter requires a separate, custom isolation transformer - even if the transformer is integrated with the inverter. A truly transformerless inverter design supports direct connection without any additional transformer equipment, custom modifications, and balance of system costs. For utility installations with medium-voltage transformer connection points between 5 and 12.7 kV, multiple transformerless inverters can be consolidated into a standard medium-voltage transformer of appropriate size. The transformer can be placed anywhere in the electric field, but it is most suitable to be close to the inverter.

Transformerless inverter technology and two-pole array configuration

Solar PV systems that use transformerless inverter technology generate power without any transformer between the PV modules and the loads—typically high voltage alternating current (HVAC) equipment and commercial fluorescent lighting. Although some manufacturers claim transformerless technology, in reality, their products still require an isolation transformer between the inverter and the load. They simply integrate the inverter into an inverter box or sell them separately. True transformerless inverters convert and transmit power directly from the inverter to the attached load. This is due to the bipolar ±600 VDC array configuration. Power integrators and utilities can benefit from improved system performance and reduced system balance costs:

Higher efficiency

Reduce the size and number of equipment and wires

Reduce material and installation construction costs

To illustrate these advantages, let’s look at the two most common large-scale PV installations: direct inverter connections to local grids in the U.S. and utility installations that connect power to the grid.

Direct-to-grid PV inverters for commercial rooftop installations

A 1-megawatt commercial rooftop system with a connection point on the low-voltage side at the entrance to the facility requires one to four grid-tied PV inverters. With traditional inverters, each must be paired with a separate or custom isolation transformer—whether the transformer is integrated with the inverter or not. As a result, the power supply is immediately reduced because isolation transformers are typically only 98% to 99% efficient, and they can reduce efficiency by up to 2%.

Traditional inverters limit the design of PV inverter systems due to their large size and weight. A system design using two 500 kW inverters requires mounting the inverters on the ground due to the large size and weight of the inverter/transformer combination. Even if the isolation transformer can be separated from the inverter, the lower output voltage and multiple windings required for each inverter will limit the distance of separation due to the expensive wiring costs caused by such installation due to the lower voltage and higher current.

Stability issues when integrating inverters are also a concern. Traditional inverter designs often use large undamped triangular filters. When many devices are placed in parallel or the inverter is set up on a long transmission line, these filters may cause system instability. Moreover, if the inverters are placed in parallel in the same box, each 500 kW

With the inverter driven by four smaller 125 kW units, the system would be susceptible to electrical disturbances and would present multiple points of failure for the entire PV system.

In contrast, a true transformerless inverter is fixed directly to the building's entrance, or even to a distribution panel of sufficient size. Because there is no isolation transformer, the additional 1% to 2% energy efficiency gained from the PV module power goes directly to the load, which means a minimum of 5 kW of additional output for free at 500 kW. In addition, the direct conversion to a usable voltage, rather than the lower AC voltage of a single-pole inverter, reduces the AC current by more than half, thereby reducing the cost of wires on the AC side.