Trends in the Solar Inverter Market

The solar inverter market is extremely competitive – margins are shrinking and many companies are gaining market share by gobbling up rivals. Two weeks ago, Danfoss acquired one-fifth of SMA in a stated bid to “fend off low-cost Asian rivals”. Over the last few years ABB and Power-0ne merged, Advanced Energy acquired REFusol and in 2008 Schneider purchased Xantrex. Solar inverter manufacturers are using the tried and true methods of market growth, supply chain consolidation and technology advancement by merger and acquisition.

In this competitive environment, inverter manufacturers are staying in front of the competition curve via advancements in technology. Technology trends such as DC/AC ratio optimization, increased voltages and neutral point clamped topology are a few of the means by which inverter manufacturers are helping their customers keep costs down and increase efficiency.

DC/AC Ratio Optimization – Inverter Component Stress Considerations

A relatively recent trend in the solar industry is the practice of increasing the ratio of DC input (more solar panels) to the rated AC output of a given inverter. According to Bradley Hibberd, Director of Solar Technology at Borrego Solar, “DC to AC ratio optimization is an integral part of our design process”. The driving factor, not surprisingly, is economics. The result of increasing DC input is that inverters see an increased workload. This additional demand has effects on inverter components which must be considered in the design phase.

Prior to 2009, panels were relatively expensive and the goal of designers was to wring as much DC out of them as possible. A 1:1 ratio did not work because such things as weather conditions, shading and soiling resulted in less than optimal output. Ratios of 1.10 to 1.25 were the norm. If the panels were clean, the sun shining at the right angle and the temperature optimal, the panels produced more DC than the inverter could handle and inverter controls kicked in and “clipped” the DC input to an acceptable level. Generally speaking, financial models suggested that if the ratio was more than 1.10 to 1.25 too much was being spent on panels and ROI wasn’t being optimized.

In an article published in Solar World Online, Michael Zeurcher-Martinson, a founding partner and CTO of Solectria Renewables and Jon Fiorelli, an applications engineer at Solectria, state, “With falling module prices, project financials have changed in favor of higher array-to-inverter ratios”. As a result of decreasing panel prices, array designers are now making decisions they would not have made in the pre-2009 era. Prior to 2009, for instance, it made more sense to space panels further apart to avoid shading or schedule cleanings more often. Now, however, “DC is cheap”, and the ROI equation allows for different design decisions. As a result, DC/AC ratios have increased to as much as 1.85 under certain circumstances.

According to Zeurcher-Martinson and Fiorelli, “Large array-to-inverter ratios cause the inverter to work harder for longer hours”. With more DC input, inverters operate at or near their optimum capacity for more hours per day. “In addition, most commercial three-phase inverters operate less efficiently when operating above the maximum power point voltage, resulting in greater internal heat rejection,” according Zeurcher-Martinson and Fiorelli, “…and this can cause some of the temperature sensitive components to age faster compared to a lightly loaded scenario”.

Designing with components which can handle the additional stress and not add to the heat problem, can save on costly thermal management systems. To that end, DC link capacitors with the lowest ESR will maintain the lowest hotspot temperature at a given power level. Designing with such a capacitor can save not only on thermal management but also on costly servicing due to the longer life of a low ESR capacitor.

The Power Ring Film Capacitor™, manufactured by SBE, Inc., and employed in many power conversion applications, including solar, wind and electric vehicle inverters, is specifically designed to maintain the lowest possible hotspot temperatures relative to “traditional” form factor film capacitors. To learn more, please visit SBE Technology Overview webpage.

Increased Voltages

Another technology trend which is helping the economics of solar projects is increased system voltages. In the United States, 600Vdc large commercial and utility scale systems used to be the norm. Over the last few years, more and more inverter manufacturers have released 1000Vdc inverters. Mr. Hibberd states that Borrego installed their first 1000Vdc inverter just over a year ago and that the technology is “mature, in the sense that all the associated components are UL certified or recognized”. All of Borrego’s ground mounted projects going forward will be 1000Vdc systems.

The value of increased voltages is realized in infrastructure savings and efficiency improvements. Although 1000Vdc-rated BOS equipment is generally more expensive than 600Vdc equipment, those costs are more than offset by the effect on the overall system of moving to the higher voltage. Higher array voltages mean fewer strings, connections and terminations, reduced cabling, less system losses, decreased inverter cost for a given capacity and increased energy harvest. Jim Morgenson is the director of business development for power plant solutions at SMA America. In a white paper entitled “The Commercial Promise of 1,000 Vdc PV Design” Morgenson writes: “With approximately 40% BOS wiring savings and up to 2% efficiency improvement, the economic value [of going from 600 Vdc to 1,000 Vdc] can top $100,000 per MWdc ($0.10/W). This is exactly why 1,000 Vdc systems have been the preferred standard in Europe for several years.

Will voltages go even higher? At the most recent Solar Power International and Intersolar trade shows, there was talk of 1500V systems. According to Hibberd, it’s a bit too soon. “1500V systems are interesting but not near term given the lack of UL certified associated components, switches and combiner boxes in particular,” says Hibberd.

SBE’s Power Ring Film Capacitors™ and integrated capacitor/bus solutions are currently being used in 1000Vdc solar inverters for exactly the reasons stated above – they can handle heavy workloads and last longer than conventional film capacitors. SBE offers the most voltage efficient solution for those ready to tackle the next generation 1500V architectures.To learn about how SBE’s solutions are employed in solar inverters, please visit SBE Solar Applications webpage.

Neutral Point Clamped Topology (NPC)

Economics is also driving a move towards NPC. Traditional inverters for alternative energy utilize two level architectures. However, three level neutral point clamp (NPC) topologies are becoming more popular. While the three level approach requires use of more individual active elements, it offers significant advantages including reduced IGBT losses since only half the voltage is switched, higher output voltage using the same switch voltage ratings as a two level inverter, and better fidelity of the output waveform. As a result, less copper is required for the interconnections, efficiency is improved, and output filtering requirements are reduced. Mr. Hibberd agrees, “There is significant savings in the fact that there is much less copper connecting the array to the inverter.” The three level approach is particularly attractive as operating voltages in increasing from 600V to 1000V and even to 1500V.

Inverter designer Fred Flett, of Jentec International, has the following goals in mind for transformer-less inverters:

  1. Maximum power point tracking to provide the maximum amount of power output by tracking the input voltage form the solar array. Here the input voltage requires dynamic adjustment by means of a boost stage at the input for example to smooth out periods of fluctuating irradiation
  2. High throughput efficiency of the power inverter in the order of >99%
  3. Low harmonic distortion
  4. Good EMC properties
  5. Surge capability to withstand output loads such as electric motor start-up
  6. Low leakage current for safety
  7. Paralleling capability
  8. Reliability; 10 years of lifetime.

Mr. Flett states that NPC addresses all of the above criteria. “This inverter topology has the advantages of no internal reactive power flow, a three level inverter output and a low leakage current,” adds Mr. Flett.

The advantages of the NPC approach can only be harvested to their full extent through careful system design. According to Mr. Flett, “imbalances between the two legs of the DC link can result in neutral current flows which are not desirable.”

The SBE’s UL-recognized integrated capacitor and bus solution offers the ability to achieve near perfect impedance balance over the full harmonic range of interest. This essentially eliminates neutral current flow in the inverter topology rather than using cumbersome control schemes to manage the imbalance which reduces efficiency.

As the merger and acquisition market saturates, advancements in technology will be how inverter manufacturers gain and maintain market share. DC/AC optimization, increased voltages and NPC are three leading candidates. SBE offers a full range of UL recognized, Power Ring Film Capacitor™ and integrated capacitor and bus solutions, which are currently being used by inverter manufactures to achieve these goals.