What is TMEIC's '3rd-Generation PV Inverter'? (1)

With advantages of both central-, string-type PV inverters

2019/08/17 17:45
Kenji Kaneko, Nikkei BP Intelligence Group, CleanTech Labo

"We suggest the third-generation PV inverters, which incorporate advantages of string-type PV inverters, while following the trend of central-type units."

Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC) unveiled the "Solar Ware U," a new PV inverter model, at a solar power exhibition in Yokohama City July 10, 2019. Vice President Naotada Sawada, Renewable Energy & New Technology Division, highlighted the original features of the new product.

TMEIC released the "Solar Ware U" in May 2019 in overseas markets prior to its release in Japan. The product is highly evaluated in regions including North America, India and Vietnam, and the company has already received orders for a total output of approximately 2GW, indicating a strong start. A new Solar Ware U featuring domestic specifications was unveiled at the exhibition (Fig. 1).

Fig. 1: Actual model exhibited at exhibition in Yokohama City in July. Special model painted with latest Spiderman illustration (not for sale). (source: Nikkei BP)

Central type and string type

When the "central-type PV inverter" is used, DC current from solar panels is collected and converted to AC current using a large-capacity PV inverter for connection to the power grid. Central-type PV inverters are traditionally used in designing mega (large-scale) solar systems. In the case of an approximately 2MW mega solar power plant, for example, four 500kW units or two 1MW units are used.

In contrast, when the "string-type PV inverter" is used, DC current from solar panels is converted to AC current by a small-capacity PV inverter of several tens of kilowatts, and the AC current is collected for grid connection. DC current from each string (series circuit of solar panels) is input to a small PV inverter. In the case of an approximately 2MW mega solar power plant, 34 60kW units are used, for example.

String-type PV inverters made abroad, including in China and Taiwan, have been increasing their share in the domestic and overseas mega solar markets over the past few years, taking advantage of cost competitiveness realized by mass production and so forth.

On the other hand, the cost competitiveness of central-type PV inverters was improved by increasing the output per unit. Vice President Sawada indicated that there is an increasing trend in capacity when he mentioned "the trend toward using central-type units."

In 2012, when the feed-in tariff (FIT) scheme was started, 500kW units were the mainstay products of TMEIC. After 2015, however, the company released new models with higher capacity, starting from 1MW units and then increasing the capacity to 2.5MW, and then to 3.2MW. These mega-class units are now used by large projects that are connected to extra-high-voltage transmission lines.

The number of installed units is reduced when the output per one unit of PV inverter is increased, making it possible to integrate peripheral equipment and reduce the construction man-hours, contributing to the lowering of construction cost.

As explained above, string-type units of about several tens of kilowatts and central-type units with a large mega-class capacity co-existed and competed in the market of PV inverters intended for mega solar power plants.

Maximum output of 4.5MW realized by connecting 6 units

Under such circumstances, TMEIC announced that it aims to develop a "third-generation PV inverter" with the advantages of the two systems and actively promote the sale of the products as the future flagship model. The company calls the new model with the new concept the "modular PV inverter."

The domestic model of the "Solar Ware U" unveiled on July 10 comes in two types: one with a rated unit output of 675kW and one with a rated unit output of 750kW. The units are installed independently or by connecting up to six units. The optimum output is realized by changing the number of units to be connected according to the conditions of each site (Fig. 2).

Fig. 2: Four modular PV inverters connected in this example (source: TMEIC)

Specifically, when six 750kW modules are connected, the maximum output will total 4.5MW. Because the maximum output of conventional TMEIC PV inverters was 3.2MW per unit, the maximum output realized by connecting the "Solar Ware U" units will exceed this total by 1.3MW. This means that the "large capacity," which is the advantage of central-type PV inverters, was further advanced by the new models.

Meanwhile, which of the "advantages of the string type" was incorporated in the new products and how?

Flexibility and scalability added

Many responses from users point out the fact that string-type PV inverters have an advantage in terms of flexibility and scalability compared with central-type PV inverters, according to TMEIC. Because string-type PV inverters feature small capacity, the total output can be changed flexibly by adjusting the number of PV inverters of the same model, which is convenient to add to and expand the equipment.

In contrast, in the case of central-type large-capacity PV inverters, the models vary depending on the output, even if they belong to the same series. In Japan, when the PV inverter model is altered, "the certification needs to be renewed" even if the output to the grid remains the same, entailing increased restrictions in capacity for expanding the existing PV inverter or adding modules.

The new models incorporated the "modular system" to solve the issues of central-type PV inverters. The total output can be increased without changing the PV inverter models by changing the project, for example, from installing four 750kW modules (totaling 3MW) to six 4.5MW modules (Fig. 3).

Fig. 3: Example of structure consisting of six PV inverter modules (source: TMEIC)

In other words, "cost reduction by capacity increase," the advantage of central-type PV inverters, and "flexibility and scalability," the features of string-type PV inverters, were realized at the same time by incorporating the "modular system."

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