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, thus reducing the cost of wires on the AC side.

Without a transformer, the inverter is smaller and lighter, giving power integrators greater freedom in installation and overall system design. While installing a traditional inverter on the roof of a five-story building might be cost prohibitive due to weight restrictions and required reinforcement, designers can install a transformerless inverter on the roof of a commercial building (rather than in the basement) and connect directly to a mounting panel on the fifth floor. Not only does this eliminate the need for expensive five-story DC wiring, it also shortens the length of AC wiring and reduces the associated costs.

Lastly, multiple inverters can be connected in parallel without a transformer, and the power can be used directly to achieve stable performance. Transformerless inverter technology uses a much larger power optimizer (Line Reactor) and smaller triangle filter capacitors. These smaller triangle filter capacitors are also buffered by a series resistor, thereby improving the stability of the control system and reducing the interaction between parallel inverters. A 500 kW inverter with a single engine design can also reduce the number of parts, thereby improving the reliability of the entire system.

Figure 1. Commercial installation. a) New two-pole system connection; b) Traditional single-pole system connection

Parallel inverters used in utility installations

The same principles apply to utility-scale installations. However, most utility-scale installations involve large grounded PV arrays with many inverters that quickly step up to medium voltage (4160 to 13.8 kV). In addition, traditional inverters require a separate isolation transformer to pair with each inverter, which accounts for up to two efficiency points in unnecessary losses.

In a 1 MW module, one to four traditional inverters can be placed on a single pad, each with a medium voltage connection. Medium voltage connections are expensive, and electricians performing this work require a higher level of training and certification. A larger equipment pad or utility enclosure is required. If the farm has a tracker, a separate transformer is required to power those trackers. Balance of system equipment, material, and installation costs can quickly add up.

Conventional inverters also detect islanding conditions through utility line self-interference, such as various VAR generation. When many inverters are connected in parallel, this interference can generate VAR beat frequencies between all inverters, and the resulting false trips will shut down the field. Multiple conventional inverters and their large delta capacitors can also create instability and absorb large harmonic currents.

These issues can be avoided with transformerless inverter technology. Transformerless inverters can be connected in parallel to separate windings of a medium voltage transformer. Each set of inverters requires only a separate, standard medium voltage transformer of 1000, 1500, 2000 or 2500 kVAR. This opens up many possibilities for site configuration. Since the current is lower than that of traditional inverters, there is more flexibility in the way the inverters and transformers are placed.

About half the size of traditional inverters, transformerless inverters convert directly to higher voltages, which reduces the required floor space, shipping and lifting equipment costs (plus incremental equipment pad or utility enclosure construction costs), and the size and number of connected windings. In addition, a standard power distribution board connected to the transformerless inverter can power the tracker without the need for a separate transformer. With fewer transformers, there are fewer reactive components in the system, resulting in the most stable operation. In addition, each inverter is automatically and independently addressed via Ethernet, eliminating any interference issues.

In addition, the fully passive anti-islanding technique does not disturb the utility voltage with VAR deviations, nor does it set up other transients on the route, thus enabling efficient, smooth and stable power supply, all for relatively low installation costs.

Figure 2. Common field connected inverters. a) New bipolar connection with fewer transformers; b) Traditional unipolar connection with one transformer per inverter

Unlock new capabilities in commercial and utility installations

Power integrators and utilities can unlock new capabilities by integrating multiple transformerless inverters directly into the grid or medium voltage. The resulting maximum power generation and high efficiency gains will continue to drive solar photovoltaic power generation and alternative energy into the mainstream. At the same time, new photovoltaic system designs have achieved unprecedented flexibility and cost savings, which has a far-reaching and wide-ranging impact on power integrators and utilities. Currently, many organizations have adopted transformerless inverter technology, and this new configuration is changing the face of the industry.

Conclusion

By leveraging transformerless inverter technology, power integrators and utilities can reduce the complexity of PV systems and maximize power delivery, whether directly connected to the grid in commercial installations or connected to medium voltage in utility installations. In addition, transformerless inverter technology can reduce the size of PV system installations and reduce system balance costs, thus reversing the trend. The new trend emphasizes the levelized cost of electricity (LCOE), and the new generation of transformerless inverters discussed in this article can significantly reduce LCOE, and this can be achieved by simply providing direct conversion - a topic worthy of future research.