TMEIC: Pioneer of Large-capacity PV Inverter (1)

Advanced power electronics technology improves power generation capacity

2014/06/27 15:57
Kenji Kaneko, Nikkei BP CleanTech Institute

PV inverters are assembled at the rate of 400 units per month in 46B building at the Toshiba Fuchu Complex in Fuchu City, Tokyo (Fig. 1). The building is the PV inverter production base of Toshiba Mitsubishi-Electric Industrial Systems Corp (TMEIC). TMEIC constructed the building, leasing the site from Toshiba Corp. TMEIC estimates the capacity-based actual delivery for fiscal 2014 at 1.6GW. The company has the top share in the domestic (Japanese) market of PV inverters intended for medium and large-scale PV power generation systems.

The company started development and commercialization of large-scale PV inverters before the implementation of the feed-in tariff (FIT) scheme in Japan in July 2012, monitoring the trend of mega-solar (large-scale PV) power plant construction in foreign countries. The company released 250kW units in 2009 and 500kW units in 2010.

The "Solar Ware" series, which are PV inverters intended for PV power generation systems, currently have a lineup of 100kW, 175kW, 630kW, 665kW and 750kW models, in addition to the two models mentioned above. A 1,667kW model intended for the US market was developed and released in 2013. The company is also superior to its competitors in terms of its lineup.

Takes advantage of UPS technologies, production bases

The delivery volume, which was 42MW in 2011, has been increasing rapidly since the implementation of FIT in July 2012. The volume was probably much higher than 1GW in fiscal 2013, far exceeding 472MW in fiscal 2012. In line with the trend, the production volume at the Fuchu Complex has been doubling from 100 units per month to 200 and 400 units per month.

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In the production line, one worker skillfully assembles multiple parts and sends the assemblies to the next process. After completion, the PV inverters undergo a final inspection before they are ready for packaging and delivery (Fig. 2 & 3).

"We could respond to the rapid growth in the market because we had development and production bases established for the UPS (uninterruptible power supply) systems," said Naoki Fujiwara, group leader at the PV Technology Group, PV Project Promotion Office, Power Electronics Department, TMEIC.

At the Fuchu Complex, UPSes are manufactured in Building 46, which is adjacent to the building that accommodates the PV inverter production line. UPSes are used to protect servers at data centers, etc, in the event of a power failure, and the structure is very similar to that of PV inverters. PV inverters convert the direct current of solar batteries to alternating current. On the other hand, UPSes convert the alternating current of commercial power to direct current and input/output the current in/from batteries for another conversion to alternating current. TMEIC utilizes the design concept and quality of UPSes, which need to be highly reliable, in PV inverter development and production.

Leads industry through conversion efficiency improvement

At the complex, power semiconductors called IGBTs (insulated gate bipolar transistors), delivered from Mitsubishi Electric Corp, are attached to PV inverter and UPS units after the inspection process. The IGBT is a semiconductor element that controls the "switching" for quick current on/off operation and is the heart of the PV inverter and UPS systems, which convert electric current from direct current to alternating current.

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TMEIC's PV inverters retain their high market share not only because the company commercialized the products and established the production system earlier than others by taking advantage of the knowhow obtained through development of UPS technologies and UPS production. The company leads the industry having introduced its own design in the power converter circuits that incorporate the IGBT to improve conversion efficiency. This is another reason for the high market share.

Conventionally, sine wave alternating current was generated by switching two IGBTs for conversion of direct current to alternating current. Using this method, however, current loss, called switching loss, occurs, and this current loss is the major cause of the decline in the conversion efficiency of PV inverters.

Under such circumstances, TMEIC adopted the "3-level conversion circuit" (advanced multi-level inverter), where four IGBTs are used to generate sine wave alternating current (Fig. 4). According to the company, the loss is reduced due to the decrease of stress in each switching operation because the waveform is divided into three parts for conversion by four IGBTs.

The idea of this circuit design had been known for years, but commercialization was delayed because of issues regarding the materials, etc. TMEIC released the PV inverter that incorporates the "3-level" ahead of other companies in the industry in 2010. The PV inverter featured a maximum 98.6% (630kW model) conversion efficiency (Fig. 5).

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