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
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"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.