The design of the protection system is essential to ensure the safe operation of the photovoltaic system. There are many potential causes of photovoltaic system failure. Natural threats such as lightning, floods, or high winds can damage system components or cause the system to operate in dangerous conditions. In typical cases, national regulations require photovoltaic systems to be equipped with many types of protection for safety reasons: overcurrent protection, lightning and surge protection, and a series of disconnection methods are common requirements. The national regulations will state what kind of protection methods are needed, and these regulations must be followed.
① Overcurrent protection
The over-current protection of all circuits (including AC and DC) is designed to protect components and cables to prevent damage due to over-current or short-circuit. The overcurrent protection capacity is determined by the type of equipment used and the maximum current that can safely flow through the circuit components. National regulations clearly indicate how over-current protection should be designed and rated. In the United Kingdom and Australia (and most of the world), all capacity configurations are based on the array short-circuit current. The following are important reasons why the array short-circuit current is used:
·Photovoltaic module is a device with limited current, that is, the highest current it can output is ISC.
·Under the given temperature and irradiance conditions, the short-circuit current is the maximum current that the photovoltaic module can output.
·The maximum current output by the photovoltaic array is the sum of the short-circuit currents of the branches in the descending column.
Since the short-circuit current changes with the temperature and irradiance of the photovoltaic module, these two important factors should be considered in the design of the photovoltaic system.
②Fault current protection
It is important to note that photovoltaic modules need to limit the current flowing in the opposite direction so as not to damage the modules. This is called the “maximum series fuse rating”, “reverse current rating” or “overcurrent protection rating” and is described in the component data sheet. If an internal fault occurs in one branch, current from other branches may feed into the faulty branch. If this current exceeds the maximum series fusing rating, the component may be damaged. This problem generally only occurs on arrays with multiple branches, that is, the sum of the currents of all non-faulty branches exceeds the maximum series fusing rating. In order to prevent this problem, branch fuses or miniature circuit breakers are usually used, and these devices must be rated according to the DC photovoltaic system.
In order to determine whether fault current protection is required, the designer needs to know the short-circuit current of the array and the maximum series fusing rating of the component. Usually national regulations specify what kind of protection is needed, so national regulations should always be consulted. For example, in the United Kingdom, when the component rated reverse current (Ir) is less than the number of all branches minus the short-circuit current of 1 and then multiplied by 1.25, it is required to install a branch fuse, as shown below: Ir<{Isc (number of branches- 1) ×1.25}. As a rule of thumb, branch fuses are required in photovoltaic arrays with more than 4 branches, and the fuses used must meet the following requirements:
·The fuse must be suitable for the positive and negative branch cables.
·The fuse must be rated according to DC calibration.
·The rated value of the fuse voltage must be Voc × the number of components in the branch × 1.15.
·The fuse trigger current (Itrip) should be less than 2×Isc or less than 2× the current carrying capacity of the branch cable (the minimum value should be used).
·If the system does not have a branch circuit fuse, the minimum rated current of the branch circuit cable should be Isc × (number of branches-1) × 1.25.
National regulations and regulations must be consulted.
③Lightning strike and surge protection
National codes and regulations usually include lightning protection and surge protection, and they can vary greatly in different regions of a country. For example, in the Australian/New Zealand standard AS/NZS 1768:2007, when the photovoltaic array supplies power to important loads (such as communication relay stations), or if the rated capacity of the photovoltaic array is greater than 500w, a lightning arrester is required. Lightning arresters are usually found in commercial photovoltaic inverters. In systems where this is not the case, it is recommended to use metal oxide varistors (MOV) as protection. According to AS1768, MOV should be selected according to the continuous operating voltage greater than 1.3 times the array open circuit voltage and the maximum discharge current greater than 5kA. If lightning protection is required, it needs to be integrated into the photovoltaic system. How to achieve integration should be carried out in accordance with national norms and regulations.
④Ground
Local standards and regulations specify the grounding system. Array installation structure and photovoltaic array should be considered separately. The grounding of the installation structure is usually used for lightning protection and provides a path for fault currents. Whether the array conductor is required to be grounded and in what manner is a more complicated matter. It is necessary to comply with the recommended methods of the following local standards and regulations.
⑤Mechanical protection
Photovoltaic arrays are usually placed as high as possible to prevent shielding and interference from the outside, so they usually bear high levels of wind loads. If so, the supporting structure of the photovoltaic array should comply with local building codes, national standards or regulations regarding wind loads.
⑥Array protection
Array protection depends on local regulations and varies from country to country. Since grid-connected photovoltaic systems have no other DC current sources (that is, no batteries), fault current protection is usually not required in grid-connected photovoltaic systems without batteries. However, national regulations generally require the installation of load-breaking devices between the photovoltaic array and the inverter. This is called the main circuit breaker/isolator for the photovoltaic array. The device is usually near the inverter and requires a blocking capability so that the photovoltaic array can be safely disconnected during maintenance work. The breaker/isolation device must be calibrated according to the DC rating; it is unsafe if only the capacity of the breaker/isolation device is calibrated according to the equivalent AC value. The safety voltage and current parameters and whether the switch must break one or more DC lines can be found in the relevant specifications. For example, the Australian/New Zealand standard AS/NZS 5033:2005 requires a lockable switch for breaking the two-terminal circuit (bipolar), and a rated voltage of 1.2 x Voc.
⑦ Sub-array protection
For several reasons, the array may be divided into several sub-arrays, for example, the two parts of the array are installed in separate different areas. If this happens, local regulations may require the configuration of sub-array protection, and this protection will specify the trigger current of the sub-array over-current protection, which is usually related to the sub-array’s short-circuit current. Local regulations may also require sub-arrays to be equipped with load disconnecting equipment to isolate the sub-arrays from other parts of the system.
⑧Ultra-low voltage (ELV) section
Another reason why the array is divided into sub-arrays is that the PV module branch is divided into ELV segments. Most photovoltaic arrays work at a DC voltage of 120-500V. In order to make the array safer and reduce the risk of electric shock, the photovoltaic array may need to be divided into V, less than 120V ELV sub-array. Generally, national regulations pay special attention to ELV segmentation. For example, in the Australian/New Zealand standard AS/NZS 5033:2005, each branch of the ELV array is not required to be equipped with fault current protection or breaking devices, but the low voltage (LV) array must have The appropriate method of breaking into ELV segments.