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A Discussion of the Solution for High-voltage Safety Hazards on DC Side in Residential PV System

Growing solar photovoltaic demand

In the past 2019, global solar photovoltaic installed capacity has also grown tremendously. Taking the European Union as an example, according to data from Solar Power Europe, 2019 is the best year for solar photovoltaic development in the European Union, with 16.7 GW of new installations, an increase of 104% (8.2 GW of new installations in 2018). Spain increased its installed capacity by 4.7 GW in 2019, making it the EU’s largest solar market, followed by Germany (+4 GW), the Netherlands (+2.5 GW), France (+1.1 GW) and a surprised Poland (the installed capacity quadrupled in 2019, reaching 784 MW by the end of 2019, and the EU’s incremental photovoltaic capacity reached 131.9 GW, an increase of 14% from last year (the total installed capacity at the end of 2018 was 115.2 GW). As it continues to rise, some security risks are gradually emerging.

Increasing concern for the safety of photovoltaic systems

For photovoltaic systems, there are two major safety risks, "DC high-voltage risk" and "rescue risk." In the current mainstream household string type system, the line voltage of the whole string of components can generally reach 600V ~ 1000V. A DC switch is configured in the photovoltaic inverter, which is used to segment the DC voltage on the photovoltaic inverter side to ensure the safety of the photovoltaic inverter side. However, the DC voltage of the entire string of photovoltaic modules is eliminated. Through the power generation principle of photovoltaic modules, once there is a light with enough energy on module, current will be generated. The high voltage in the DC cable is still in application. And due to the loose contact of the photovoltaic module connector, poor contact, moisture on the wire, insulation rupture and other reasons, it is easy to cause DC arcing and fire. On the other hand, when a fire accident occurred, the high voltage on the DC side that had always interfered the firefighters’ work. Firefighters have difficulty dealing with the rescue at the scene and can only control the fire from a distance. With the rising indicators of global photovoltaic installations, countries must find solutions to ensure the safety of families.

Forerunner of safety regulations

The National Electrical Code (NEC) originally proposed the requirement for rapid photovoltaic shutdown in 2014, and updated this standard in 2017. The relevant requirements are as follows:

According to the requirements of the regulations, by implementing rapid shutdown at the component level, when a danger occurs, the DC high voltage on the roof is eliminated, the “rescue risk” in the photovoltaic system is solved, and the personal safety of the rescuers is guaranteed. The industry is currently discussing revisions for the NEC v.2020 and hopes to further address potential security issues.

However, in practice, to check whether the array drops to 80 V or lower within 30 seconds, the inspector must board the roof and insert the voltmeter into the connector to test. This is unreasonable for the operation, maintenance and inspection of photovoltaic facilities on civil buildings. Is there a solution to fundamentally eliminate the security risks of DC high voltage?

Application of MLPE component-level power electronics technology

Module-Level Power Electronics (MLPE) component-level power electronics technology is currently the main method to achieve rapid shutdown at the component level, and micro-inverters are one of them. The working principle of the micro-inverter is to subdivide the DC input in the photovoltaic system and achieve energy conversion for each photovoltaic module. At present, the mainstream photovoltaic modules in photovoltaic systems have an open circuit voltage Voc of less than 60V. That is to say, the DC voltage of a single-chip photovoltaic module meets the requirements for safe operation, so there is no hidden danger of high voltage on the DC side.

In addition, for the safety protection on the AC side, the micro inverter meets the requirements of island protection. Taking VDE-AR-N 4105: 2018 6.5.3 as an example, the inverter will cut off the AC output within 2s when an island situation occurs. In other words, in an emergency, as long as the grid-side circuit breaker is opened, the AC-side output of the micro-inverter will drop to zero within 2s.

It can be seen that the micro-inverter eliminates the risk of high-voltage on the DC side in terms of working principle, functionally meets the safety requirements on the AC side, and can guarantee the personal safety of emergency rescue personnel.

New generation of micro inverters from TSUN

After nearly two years of design and development, TSUN has launched four second-generation micro-inverters such as M350 based on the first-generation micro-inverters.

The second-generation micro-inverter has been redesigned to increase AC output power. For each photovoltaic module, the maximum output power of the micro inverter can reach 350W, which can meet the use of more models of photovoltaic modules on the market.

In addition, the second-generation micro-inverter has been optimized for the enclosure, increasing the density of the cooling teeth, and increasing the surface area of the enclosure by about 25%, which greatly improves the heat dissipation capacity of the micro-inverter. As one of its key components, the electrolytic capacitor will double its lifetime, while the operating temperature is reduced by 10°C. It can be seen that the improvement of heat dissipation performance will greatly increase the MTBF of the second-generation micro-inverter.

Finally, the second-generation micro-inverter has optimized the DC-side wiring method. A DC cable was added from the original board-end DC terminal. After optimization, engineers can more easily access photovoltaic modules during installation and reduce damage to the DC terminals.