In the 1990s, as the application of grid-connected photovoltaic systems continued to increase humidity, some people studied ways to make the installation time shorter and simpler. Many manufacturers have developed pluggable connection methods, which are now widely used.
The plug is positive and negative, which reduces the risk of errors during the series connection of components. Usually there is a locking mechanism, which is difficult to separate by pulling the cable.
Many module manufacturers provide modules with cables and plugs, which are connected to the junction box on the back of the photovoltaic module (see Figure 3). Cables and plugs can also be purchased separately, so that the cables can be configured according to the length of the actual installation. For some installation situations, longer cables are usually required.
The selection of cables should meet the output current and output voltage of the photovoltaic array and minimize the voltage drop. Typical cable specifications include: 2.5mm2, 4mm2 and 6mm2. These specifications are suitable for most systems. The AC cables and grounding cables used in small grid-connected photovoltaic systems are the same as the building cables.
PV combiner box
If the photovoltaic array is composed of multiple parallel component strings, the cables from each component string can be connected to a photovoltaic combiner box, sometimes called an array junction box or a DC combiner box. Even when the array has only one component string, the photovoltaic combiner box can be used to link the output cables of the photovoltaic array and the cables to the inverter, especially when the diameter of the array cable is large and the interconnection cables of the internal components are displayed.
If there are multiple parallel module strings, the photovoltaic combiner box is convenient to connect the positive and negative cables drawn from different module strings, and lead a pair of positive and negative cables to interconnect with the inverter (through the DC main circuit breaker/isolating switch )
Component junction box
In some standard photovoltaic modules, the wire connection is enclosed in a module press box attached to the module. The junction box includes bus terminals, which can be installed with cables or conduits. In this way, the cable can be directly installed into the terminal of the junction box. On some photovoltaic modules, the module junction box is permanently sealed on the back of the module.
Circuit breakers and fuses
Fuses and circuit breakers are often used for overcurrent protection of photovoltaic systems. Generally, DC fuses or DC circuit breakers are used on the array access side of the inverter, and AC fuses and AC circuit breakers are used on the grid side of the inverter. The specifications of photovoltaic systems may require the use of DC fuses, and DC fuses are very different from AC fuses used in conventional electrical appliances. The breaking of DC current is more difficult, so the fuse used in DC appliances should be selected according to the DC rating. It is very important to ensure this. Most DC fuses can be used in AC appliances (the manufacturer will generally specify), but DC fuses are more expensive. Similarly, AC circuit breakers are not compatible with DC circuits, and vice versa. It is worth noting that the operation of DC fuses and DC circuit breakers in photovoltaic systems is very different from AC fuses and AC circuit breakers in AC systems, because photovoltaic is a current-limited power supply (that is, the maximum current of the array is its short-circuit current Isc) .
In the AC system, due to the very large current under fault conditions, the fuses and circuit breakers will quickly blow/open, and the current (Isc) of the photovoltaic system under fault conditions is not much higher than the normal operating current (Imp) , Branch fuses and circuit breakers may not be able to disconnect the circuit. Therefore, there is an urgent need for photovoltaic system installers to be qualified to work on electrical systems (especially photovoltaic systems) and to be familiar with over-current protection and local regulations for installation.
The fuse is used to protect the conductors in the circuit from being damaged by excessive current and reduce the risk of fire caused by overheating of the conductor. The fuse usually consists of a small section of conductor installed in an insulating housing, the capacity of the conductor is sufficient to carry the load current, and the fuse will open the circuit under a fault condition. The fuses include rewiring fuses and high-breaking capacity (HRC) tube fuses.
Some people think that rewiring fuses are not enough to protect the circuit system: tube fuses should be used. Fuses generally have a current rating, that is, the current that passes before the fuse is blown. A circuit breaker is a mechanical device that opens a circuit under fault conditions: when the fault is eliminated, this switch can be manually operated to close the circuit and restore the current. When the current exceeds the rated value of the fuse or circuit breaker, the device will be activated (disconnected) to prevent the current from flowing.
Fuses and circuit breakers can be used not only as fault (over) current protection, but also as breaking devices, as described below. Fault current protection is used on the DC side of the system (photovoltaic arrays, sub-arrays, and module strings) and the AC side of the system (usually there are circuit breakers on the grid side of the photovoltaic system). Overcurrent protection requirements and ratings vary from country to country, so it’s important to consult national regulations
Photovoltaic main circuit breaker/disconnector
The circuit breaker/disconnector isolates the equipment from the grid and the power supply, and cuts off the power in the circuit. Both the DC side and the AC side of the inverter should be equipped with a circuit breaker/isolating switch, and the DC side should use a DC circuit breaker.
Lightning and surge protection
The need or need for lightning and surge protection depends on system requirements and local regulations. If required, this type of protection may be required on both the DC side (lightning protection from the photovoltaic array) and the AC side (lightning protection from the AC grid) of the inverter.
This type of protection must be based on the recommendations of the manufacturer of the protection equipment to determine its exact location. Usually the wires placed in the photovoltaic array are already close to each other. In addition, if the inverter is remotely monitored through a modem, the protection equipment should be connected to the telephone line of the line-side modulation regulator.
Photovoltaic systems can use data loggers to measure and record system performance information. The recorded information includes time, system power, system voltage and system temperature. Then, this information may be sent to a central website maintained by the inverter manufacturer, or displayed on the website of a school or company, or displayed in other places.