Analysis of the output voltage of the medium power inverter

       Analysis of the output voltage of the medium power inverter
　　As we all know, the input voltage range of the photovoltaic inverter will affect the power generation of the system. Generally speaking, the wider the input voltage range, the greater the flexibility of the series and parallel connection of the components. More power generation. So is there any relationship between inverter output voltage and power generation?
　　There are two types of inverter output, one is directly integrated into the low-voltage power grid, the other is first connected to the step-up transformer and then combined into the medium-voltage power grid. The single-phase voltage of my country's power grid is 220V, and the three-phase is 380V. This is the rated voltage of the user-side equipment. Taking into account the line voltage drop and the voltage drop of the transformer itself after being loaded, the rated voltage of the transformer output is three-phase 400V and single-phase 230V. Therefore, the output voltage of the inverter, the single camera is close to the device side, so it is generally standard 220V, and there is also a standard 230V. The three cameras are generally close to the transformer side, so they are generally rated at 400V.
　　National Grid Regulations:
　　8kW and below can be connected to 220V, 8-400kW can be connected to 380V, 400kW-6MW can be connected to 10kV, and 10MW can be connected to 35kV.
　　Therefore, photovoltaic power stations below 400kW can be directly connected to the 380/220 low-voltage power grid. If the power station capacity exceeds 400kW and is connected to the medium-voltage power grid, medium and high-power power stations, medium-power string inverters and high-power centralized inverters are generally used, and there are many kinds of output voltages. The common centralized inverter is 315V. , 360V, 400V, etc., string inverters include 480V, 500V, 540V, 800V, etc. The rear stage of the inverter is connected to a step-up isolation transformer. There is no corresponding standard for this voltage, so which voltage Okay?
　　1 Centralized inverter
　　High-power power station inverters have output voltages of 315V, 360V, 400V, etc., and input MPPT voltage ranges are 480-820V, 525-820V, and 600-820V. From the standpoint of the efficiency of the inverter itself, the higher the voltage, the lower the current, and the higher the efficiency of the 400V inverter. However, considering the power generation capacity of the system, the 315V inverter has a wide voltage range and longer power generation time, and the total power generation will be higher. The 400V inverter has a rated input voltage of about 680V.
　　Why the lower the output voltage of the centralized inverter, the wider the MPPT voltage range of the photovoltaic inverter. Many people don’t know how this value comes from. In fact, it is determined by two aspects. The withstand voltage value of the IGBT determines the highest voltage of the MPPT. ; If you choose a 1200V tube, you must first leave a 100V margin for the IGBT, plus the overvoltage of the busbar 100~200V, so the working voltage of the inverter is generally lower than 1000V~900V, taking into account the temperature characteristics of the components, The inverter MPPT voltage is generally lower than the maximum operating voltage of 50V to 150V, so the maximum MPPT voltage is generally 820V to 850V. The inverter AC output voltage determines the minimum voltage of the MPPT. According to related principles, DC input voltage = AC output voltage * 1.414 + PWM dead zone voltage loss + inductance loss, such as 315V AC output, DC minimum voltage = 315*1.414+35= 480V, such as 400V AC output, the lowest DC voltage = 400*1.414+35=600V.
　　2 string inverters
　　The string inverter has a secondary structure, the front stage is DC/DC-BOOST boost, and the latter stage is DC/AC inverter. The voltage range of MPPT is not affected by the output voltage. In more than 400KW power stations, the inverter The latter stage is connected to a step-up isolation transformer, so the inverter voltage can be varied. At present, there are 4 voltages in China, such as 480V, 500V, 540V, and 800V. The three types of 480V, 500V, and 540V are connected to the 1000V system, and 800V is connected to 1500V. system. How to choose these voltages?
　　From the perspective of machine cost, the higher the output voltage of the inverter of the same power, the lower the current and the lower the cost. From this point of view, it seems that the higher the voltage, the better, but considering the efficiency of the previous step-up, as well as the life of the device and the flexibility of the component configuration, a voltage of 480V is more suitable.
　　The front-stage DC/DC-BOOST circuit of the string inverter needs to boost and stabilize the DC voltage to a certain value. This is called the DC bus voltage before it can be converted to AC. The specific calculation is the same as that of a centralized inverter, such as 480V. For output, the DC bus voltage should be around 710V, 500V output, the DC bus voltage should be around 740V, 540V output, and the DC bus voltage should be around 800V. However, the series voltage of modules is generally not so high and needs to be adjusted by the circuit. Inverters generally use PWM to adjust. There is a term called duty cycle, which is equal to the series voltage of the modules/DC bus voltage. The duty cycle has a great relationship with efficiency. , The larger the duty cycle and the smaller the voltage difference, the higher the efficiency.
　　480V AC voltage is also a common voltage in Europe and the United States. Many AC switches, cables, lightning protectors and other devices are designed according to 480V, leaving sufficient margin, and 500V and 540V systems can only use 480V. The same device, relatively speaking, has a smaller margin and lower safety. The higher the voltage stress, the shorter the life of the inverter's power devices IGBT, capacitors, busbars and other devices. Therefore, if the inverter with 480V output is selected, the power generation will be higher and the safety performance will be better.