In addition to the core components such as PV modules and inverters, PV DC switches also play a vital role in the smooth operation of PV power systems, which have experienced frequent outbreaks of quality problems in recent years. The reliability and stability of PV DC switches are not only related to the safe and reliable operation of PV distribution systems, but also to the stable power generation and revenue of the PV industry. Therefore, good power distribution products are especially important.
In 2014, the Australian PV DC switch recall fiasco made power plant investors see the importance of PV DC switches. In China, fires in power stations caused by the quality of switches have also occurred from time to time.
The main problems with these DC switches are.
High contact impedance causes overheating and even fires; The switch cannot be turned off properly and the switch handle stays in the 'OFF' state; Incomplete cut-off causes sparks; As the allowable operating current is too small, it is easy to cause overheating and damage to the switch interrupter chamber or even deformation of the shape.
There are also many so-called DC circuit breakers on the market today are not true DC breakers, but are improved from AC circuit breakers. PV systems generally disconnect voltage and current are relatively high, in case there is a ground fault, the high short-circuit current will pull the contacts together, thus creating an extremely high short-circuit current, up to thousands of amps (depending on the different products). In particular, it is common in PV systems to have multiple parallel panel inputs or multiple independent panel inputs, so that it is necessary to cut off multiple parallel panel DC inputs or multiple independent panel DC inputs at the same time, these occasions will require higher interrupting capability of DC switches, these improved DC circuit breakers used in PV systems can be very risky.
Most materials used in any AC circuit breaker are manufactured using some design suitable for AC circuits, which means that it is a 50/60 Hz sine wave regardless of whether the load voltage is 230 volts (AC) or 400 volts (AC). When switching on and off AC power, it is important to note that the voltage is characterized by the fact that it must pass through 0 volts, so although the situation varies from load to load, the current will gradually disappear on its own - this means that even if the circuit breaker is switched on and off at the peak of the power supply, and there is an arc formed between the contacts, the process of reducing the power supply voltage to 0 volts means that the voltage of the load will also tend to zero The arc is extinguished. However, the DC load voltage always remains constant, and the power between the contacts is always constant unless the load goes to zero. If the load is 500 volts (DC), 25 amps, then it is now, one second later, one minute later, one hour later, 500 volts, 25 amps - it remains constant. Unlike AC, all DC loads have a constant load between the DC switch contacts throughout the shutdown process; DC does not cross the 0 volt level unless there is a system power failure (or other error).
1. Try to choose a big brand, especially one that has passed international certification
PV DC circuit breakers are mainly certified in Europe IEC60947-3 (common European standard, followed by most countries in Asia Pacific), UL 508 (common American standard), UL508i (for PV systems with DC switches American standard), GB14048.3 (common domestic standard), CAN/CSA-C22.2 (common Canadian standard), VDE0660 .
At present, international brands have all the above certifications, such as IMO of the UK and SANTON of the Netherlands, while most domestic brands have only passed the general standard IEC60947-3.
Arc extinguishing effect is one of the most important indicators to assess the DC switch. Real DC circuit breakers have special arc extinguishing devices and can be switched off with load. Generally, the real DC circuit breaker structure design is special, the handle and contacts are not directly connected, so the on/off time is not directly rotate the contacts and disconnect, but there is a special spring for connection, when the handle rotates or moves to a specific point triggers all the contacts "sudden disconnection", thus producing a very rapid On and off action, so that the arc time duration is relatively short.
Generally, the arc of the first-tier international brands of photovoltaic DC switches lasts a few milliseconds to be extinguished, such as IMO's SI system claims to be in 5 milliseconds to extinguish the arc. While the general AC circuit breaker improvement over the DC circuit breaker arc lasts more than 100 milliseconds.
The general PV system voltage may reach 1000V (600V in the US), and the current to be disconnected varies according to the brand and power of the module, and whether the PV system is connected in parallel with multiple strings or independently (multiple MPPT). The voltage and current of the DC switch is determined by the string voltage and parallel current of the PV array to be disconnected. PV DC circuit breakers are selected with reference to the following experience:
Voltage = NS x VOC x 1.15 (Equation 1.1) Current = NP x ISC x 1.25 (Equation 1.2) where
NS - number of panels connected in series
NP- Number of battery packs connected in parallel VOC- Panel open circuit voltage ISC - Panel short circuit current 1.15 and 1.25 are empirical factors Generally DC switches from big brands can disconnect 1000V system DC voltage and are even designed to disconnect 1500V DC input. Large brands often have high power series of DC switches, such as ABB's PV DC switches with hundreds of amps series of products, IMO focus on DC switches for distributed PV systems and can provide 50A, 1500V DC switches. While some small manufacturers can generally only provide 16A, 25A DC switch, its technology and process is difficult to produce high-power PV DC switch.
Generally, DC switches of big brands have various models to meet the needs of different occasions, including external, built-in, terminals in series and parallel to meet the multiple MPPT inputs, with and without lock, and more to meet a variety of installation methods such as base mounting (installed in the bus bar and distribution cabinet), single-hole and panel mounting, etc.
Generally, the enclosure, body material or handle of DC switches are plastic, which have their own flame retardant characteristics and can usually meet UL94 standards. The enclosure or body of a good quality DC switch can meet UL94 V-0 standards, while the handle generally meets UL94V-2 standards.
Secondly, for DC switches built inside the inverter, if there is an external handle for switching, the protection level of the switch is generally required to meet at least the test requirements for the protection level of the whole machine. String inverters (generally less than 30kW power level) that are currently used more in the industry generally meet the IP65 protection level of the whole machine, which requires a higher requirement for the built-in DC switch and the sealing of the panel when the machine is installed. For external DC switches, if installed outdoors, they are required to meet at least IP65 protection level.
II. Selection steps for DC switches in PV systems
The selection of PV DC switch is generally based on the initial estimation of key parameters and ensures sufficient margin. The output power of the PV panel itself in the PV system is affected by weather, ambient temperature, inverter power point tracking, etc. Secondly, the PV inverter itself has input power limitation and protection, maximum allowable input voltage, as well as current limitation and protection; finally, the rated shutdown capacity of the DC switch itself and the ambient temperature are also related.
When the environment is certain the output of the panel (battery array) is affected by the power tracking on the DC side of the inverter, and as the voltage rises, the ability of the DC switch to turn on and off the current decreases (thermal effect). In general, the DC switch used is capable of interrupting the actual output voltage and current of the panel, weather, environmental stability, inverter power tracking, etc. all need to be taken into account.
The simplest approach is to select a DC switch (both external and internal) that meets the requirements of Equations 1.1 and 1.2 above. Since it is impossible for the panel to operate at maximum short circuit voltage and open circuit current, external DC switches generally use the maximum power point voltage and current for reference.
Voltage = NS xVMP x 1.15 (Equation 1.3)
Current = NP x IMP x 1.25 (Equation 1.4) For the built-in DC switch, it may also be limited by the maximum input voltage and current of the inverter itself, so in order to reasonably save system cost, the DC switch is generally based on the actual output voltage and current of the battery panel, and not higher than the protection voltage and current of the inverter selected for use. In general, the DC switch's on-off V-1 curve needs to envelop the inverter's allowable input DC voltage and current curve. In general the selection of a PV DC switch can be based on the following steps.
The maximum allowable voltage of the selected DC switch needs to be able to meet the PV system voltage requirements. Generally, the maximum system voltage is 600V for single-phase inverters and 1000V for three-phase string or power plant inverters.
If it is a built-in DC switch, it has a relationship with the number of independent MPPT of the inverter because it is integrated inside the inverter. Common inverters are single MPPT, dual MPPT, and there are a few grid-connected inverters with three MPPTs on the market. The number of independent MPPT of the inverter determines the number of independent pass section pairs of the built-in DC switch selected.
If it is an external DC switch, it may be related to the designed system networking. You can choose an external DC switch with pass-through for multiple strings of battery panels or choose an external DC switch that can only pass-through for one string of battery panels.
If you know more about the construction of PV inverters, especially when inverter manufacturers choose the built-in DC switch, you can choose the DC voltage and current curve by studying the inverter in and out of the DC voltage and current curve in order to effectively save the cost of the whole inverter, to ensure that the selected DC switch curve envelops the DC switch and current curve of the inverter input in various weather environments and temperatures.
The working temperature, protection and fire rating are determined according to the environment of use. The common protection level of external DC switch is IP65, and the built-in DC switch is installed to ensure that the whole machine passes IP65. the fire protection level is generally UL94V-0 for the shell or body material, UL94V-2 for the handle. the installation methods are panel mounting, base mounting, single hole mounting, etc.
If the above 1-4 conditions are met, the next need to select the specific model from the preferred brand. Generally, the specifications of the specific model are segmented by current, and it is difficult to find an exact match for the DC switch. Generally, a certain margin is left to choose a DC switch with specifications higher than the theoretical calculation. Commonly there are 16A, 25A and 32A, and some large manufacturers will have 40A, 50A, 600V or 1000V rated voltage levels.
With the clarity of domestic PV policy, the year by year increase in new installations of PV power plants, and especially the current start of distributed PV systems, the installation of rooftop PV systems for individual households will become increasingly popular. The PV DC switch will be the most important component for the security of these systems. The reliability and stability of the PV DC switch is directly related to the stable power generation and revenue of the PV system, which directly affects the safe and reliable operation of the PV system, so it will be crucial to choose a qualified PV DC switch.
At present, the so-called PV DC switches in the domestic market are mostly AC switches or their improved products, not DC switches with safe arc extinguishing and high power cutoff that are really applied in PV systems. These AC switches are far from adequate in terms of arc extinguishing capacity and rated load cut-off power, which can easily lead to overheating, leakage and sparks, and even burn down the entire solar power plant in serious cases.But In Portable DC Solar Generator, DC switches is not a must, a small switch can work well.