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A principle to select DC isolator switch for a PV system
With rising installed PV capacity, the amount of power generated draws a lot of attention. However, power investors are increasingly worried about safety and security issues, which in recent years frequently occurred to photovoltaic power plants. The PV DC switch is one of the most important parts for the PV system safety, whose reliability and stability relate to the stable generation and profit of photovoltaic systems, as well as the safe and reliable operation.
Germany, the Netherlands and some other European countries require inverter manufacturers to configure with built-in DC isolator switch, while countries like the UK, India, and Australia stipulate that the PV systems must be installed with external DC isolator switch. With the clarity of the Chinese PV policies, the number of PV installs rises every year, especially that of distributive PV systems, and also the rooftop systems are getting more popular. However, the so-called DC isolator switch for PV in the market are AC switches or rewired versions, not real DC isolator switch with functions of arc extinction and high power cut. These AC switches are far too deficient in arcing extinction and power isolation with loads, easily leading to overheating, leakage and sparks, or even burning down of entire photovoltaic power plants.
Therefore, the selection of qualified photovoltaic DC isolator switch will be crucial. BS 7671 states that a method of isolation must be provided on the DC side of a PV installation and this can be provided by a switch-disconnector as classified under EN 60947-3. This is also covered by “Guide to the installation of PV systems” .How to size a right DC switch for the photovoltaic system?
Here are some steps on selecting the DC isolator:
1．Sizing of the Voltage of System
The rated operational voltage of the switch should be equal to or greater than the requirements of the system. Common are to meet the UL508i 600V, IEC60947-3 1000V and 1500V. Typically the system voltage connected to single-phase inverter is up to 600V, three-phase string inverters or centralized inverter up to 1000V or 1500V.
2．Number of strings to be isolated
2 Pole - Single string, 4 Pole - Two string, etc.
For built-in DC isolator switch, the number of MPPT’s of the inverter determines the poles of a DC switch. Common string inverters are with a single MPPT, dual MPPT’s and a few, triple MPPT’s. Generally speaking, an inverter, power rating at 1kW ~ 3kW, is designed with single MPPT; 3kW ~ 30kW with dual MPPT’s or few triple.
For external DC isolator switch, you can choose 4 Pole, 6 Pole, 8 Pole for multi-string solar panels or select 2 Pole for one string of solar panel, based on the different system design.
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