The disconnector is a high-voltage switchgear, mainly used in high-voltage circuits. It is a kind of switchgear without arc-extinguishing device, which is mainly used to disconnect the circuit without load current, isolate the power supply, and have an obvious disconnection point in the opening state to ensure the safe maintenance of other electrical equipment. It can reliably pass the normal load current and short-circuit fault current in the closing state. Because it has no special arc extinguishing device, it cannot cut off the load current and short-circuit current. Therefore, the disconnector can only be operated when the circuit has been disconnected by the circuit breaker. It is strictly prohibited to operate with load to avoid serious equipment and personal accidents. Only voltage transformer, lightning arrester, no-load transformer with excitation current no more than 2A, and no-load line with current no more than 5A can be operated directly with disconnector. In power applications, circuit breakers and disconnectors are mostly used in combination, and circuit breakers are used to switch on and off load (fault) current, and disconnectors are used to form obvious disconnection points.
The PV DC disconnector is an electrical safety device that can manually disconnect itself from modules in the solar PV system. In photovoltaic applications, DC disconnectors are used to manually disconnect the maintenance, installation or repair of solar panels. In case of installation, routine maintenance and emergency, it is necessary to isolate the panel from the AC side, so a manually operated isolating switch is placed between the panel and the inverter input. This switch is called photovoltaic DC isolation switch because it provides DC isolation between the photovoltaic panel and the rest of the system. This is an essential safety switch, which is mandatory in every photovoltaic power generation system according to IEC 60364-7-712. Photovoltaic DC disconnector is one of the most important components for the safety of the photovoltaic system. Its reliability and stability are related to the stable power generation and profit of the photovoltaic system, as well as the safe and reliable operation of the photovoltaic system. With the increase of PV installed capacity, power generation has attracted much attention. However, power investors are increasingly worried about safety issues, which frequently occur in photovoltaic power plants in recent years.
Germany, the Netherlands and other European countries require inverter manufacturers to configure built-in DC disconnectors, while the United Kingdom, India, Australia and other countries require photovoltaic systems to install external DC disconnectors. With the clarification of China's photovoltaic policy, the number of photovoltaic installations has increased year by year, especially for distributed photovoltaic systems and rooftop systems.
However, the so-called photovoltaic DC disconnectors in the market are AC disconnectors or rewired versions, and are not really DC disconnectors with arc extinguishing and high-power cut-off functions. These AC disconnectors are too deficient in arc extinguishing and power isolation from the load, which can easily lead to overheating, electric leakage and sparks, and even burn the entire photovoltaic power station.
Therefore, it is very important to select qualified photovoltaic DC disconnectors. BS 7671 stipulates that the isolation method must be provided on the DC side of the photovoltaic device, which can be provided by the isolation switch classified in EN 60947-3.
So, how to select the appropriate DC disconnector for the photovoltaic system?
1. System voltage selection
The rated working voltage of the disconnector shall be equal to or greater than the system requirements. Common ones are UL508i 600V, IEC60947-3 1000V and 1500V. Generally, the system voltage connected to single-phase inverter is up to 600V, and the three-phase series inverter or centralized inverter is up to 1000V or 1500V.
2. Number of strings to isolate
2-pole - single string, 4-pole - double string, etc.
For the built-in DC isolator, the number of MPPT of the inverter determines the pole of the DC isolator. Common series inverters have single MPPT, double MPPT and a small number of three MPPT. Generally speaking, inverters with rated power between 1kW and 3kW adopt single MPPT design; The rated power of the inverter is 3kW~30kW, and double MPPT or a small amount of three MPPT are used.
For external DC disconnectors, you can select 4, 6, 8 poles for multiple groups of solar panels or 2 poles for a group of solar panels according to different system designs.
3. Rated current and voltage of panel string
The PV DC disconnector shall be selected according to the maximum voltage and current of the panel string. If the user knows the parameters of PV inverter, especially the inverter manufacturer, in order to effectively save costs, the user can select the type according to the input DC voltage and current curve to ensure that it can be used under various weather conditions and temperatures.
BS 7671 specifies that disconnectors conforming to EN 60947-3 are suitable for photovoltaic systems. The rated value of the disconnector must consider the maximum voltage and current of the isolated PV string, and then adjust these parameters according to the safety factor specified in the current standard.
This should be the minimum rating required for the disconnector.
4. Environment and installation
The working environment temperature, protection grade and fire protection grade shall be determined according to the environment. Generally, good DC disconnectors can be used safely in the ambient temperature of - 40 ℃ to 60 ℃. The protection level of general external DC disconnector shall reach IP65;
The built-in DC disconnector shall ensure that the equipment reaches IP65. The fire rating of the enclosure box or body shall comply with UL 94V-0, and the handle shall comply with UL 94V-2.
Users can select the appropriate mode according to actual needs. Generally, there are panel installation, base installation and single-hole installation.