Inverter protection system, including: self-protection, grid protection
The grid-connected inverter will only work when the AC grid is operating normally and within the predetermined operating conditions of the grid. If these conditions are not met, the grid-connected inverter is disconnected and will not output any power from the photovoltaic array. The inverter can be used to mirror the operating conditions of the grid itself. When outputting power, the MPPT software inside the inverter optimizes the output of the photovoltaic array according to the matching grid conditions. Grid interactive inverters are used in conjunction with two types of typical protection: active and passive. Both types shut down the inverter under over-frequency/under-frequency or over-voltage/low-voltage conditions. The purpose of this protection is to act as a self-protection of the inverter and to protect the power grid when extreme conditions occur. Therefore, the inverter will be disconnected when it cannot see the power grid, such as a grid collapse condition.
The inverter has a variety of protection mechanisms for problems:
①Incorrect connection: If the inverter is connected incorrectly to the PV array (such as reverse polarity), the inverter will not work and will be damaged in most cases. Although some inverters provide protection against incorrect connections, most inverter warranty coverage does not cover this type of damage.
②Temperature: The inverter is very sensitive to temperature changes, and the manufacturer will specify the operating temperature range of the inverter. Some inverters reduce the output power or shut down the inverter when the temperature rises outside the manufacturer’s specified range. Although the inverter may have overheating protection, it is very important to keep the inverter adequately ventilated and dissipate heat. Overheating can cause damage to the inverter.
③The DC voltage is too high: All grid-connected inverters have a specified voltage range for normal operation. If the DC voltage exceeds the maximum DC voltage that the inverter can withstand, some inverters will shut down to protect the circuit, but the inverter may still be damaged. Some inverters do not have similar protection.
If the grid is out of power or the grid is operating outside of predetermined parameters (such as low voltage/overvoltage, low frequency/high frequency), the grid-interactive inverter must be able to disconnect from the grid. In these cases, the inverter disconnects to avoid continuing to output power to the grid when the grid is not working.
People often worry that if there are a large number of inverters connected to the grid in an area, and the grid power supply is interrupted (such as a car hitting a pole and the cable is broken), the inverters may interact with each other, which means the voltage and frequency between each other. Become a mutual reference (that is, a single inverter will still consider the grid “operating” and continue to output power to the grid). When the grid works outside the required voltage and frequency, the passive protection will stop the inverter. In this case, the passive protection may not work. This phenomenon is called “islanding”. Therefore, in addition to passive protection, active protection is also required. “Islands” are a serious safety concern for the public grid. For example, if the grid is out of power and technicians are working, it is necessary to ensure that all grid-connected photovoltaic systems are also disconnected from the grid. The isolated island obviously poses an electric shock hazard to workers who are repairing power lines and may damage power transmission equipment.
The “islanding” problem can be dealt with through the active protection and passive protection of the inverter. Many standards require grid-interactive inverters to have two protection features at the same time. Passive protection is provided by the inverter with the ability to detect grid voltage and frequency, that is, if the inverter detects grid over-voltage/low-voltage, over-frequency/under-frequency, it will shut down. The active protection is used by the inverter to detect the unstable frequency, frequency offset or power change that may change the voltage. If any of the above conditions is found, the inverter will be shut down through the active protection.
When the condition that caused the protection action is eliminated, the inverter will synchronize with the grid again at this time, and the inverter will reconnect to the grid after a period of time, usually less than 1 min. Some countries require inverters sold in their territories to have a minimum delay from when the inverter detects that the grid is stable to when the inverter reconnects to the grid. The delay is usually programmed inside the inverter.
System balance components: system equipment other than photovoltaic arrays and inverters
In addition to photovoltaic arrays and inverters, the normal operation of the system also requires numerous other components. They are called system balance components (BoS) as a whole, and generally must comply with local and/or national codes and regulations. The system balance components are composed of components required to connect and protect the photovoltaic array and inverter, including cables, circuit breakers/disconnectors, protection devices and monitoring equipment. The key system balance components and their details are listed below:
① DC cables include the cables inside the array (the cables used to connect different components and component strings to form the photovoltaic array), the cables from the photovoltaic array to the photovoltaic combiner box (if necessary), and the cables from the photovoltaic combiner box to the inverter.
②The photovoltaic combiner box is generally only used when the photovoltaic array contains multiple parallel branches, which is located between the photovoltaic array and the inverter.
③The module junction box is located on the back of each module, and is usually used to gather the wiring of photovoltaic cells together to form a photovoltaic module.
④Usually many local standards and regulations require the use of protection and disconnecting devices, such as DC and AC main circuit breakers and isolating switches.
⑤ Lightning and surge protection.
⑥Measuring: The building already has an electric meter to measure the electricity flowing into and out of the building. According to whether the current meter meets the requirements of the system, the engineer decides whether to incorporate the original meter into the new photovoltaic system or install a new meter. The electric meter can measure total electric energy or net electric energy. The difference between the two is very important, which will be discussed later.
⑦ The AC cable connecting the inverter and the electric meter, and the AC cable connecting the electric meter and the grid.
⑧ Photovoltaic array grounding cable.
⑨Monitoring system: Most photovoltaic systems are used in conjunction with a certain type of monitoring system, so that owners can see the output of their system and quickly find problems, such as a drop in power output.