In photovoltaic grid connected inverters, there are many voltage technical parameters: maximum DC input voltage, MPPT operating voltage range, full load voltage range, starting voltage, rated input voltage, output voltage, etc. These parameters have their own focus and are all useful. This article summarizes some voltage issues of photovoltaic inverters for reference and exchange.
Q:Maximum DC input voltage
A:Limiting the maximum open circuit voltage of the string, it is required that the maximum open circuit voltage of the string cannot exceed the maximum DC input voltage at the extreme minimum temperature. For example, if the open circuit voltage of the component is 38V, the temperature coefficient is -0.3%/℃, and the open circuit voltage is 43.7V at minus 25 ℃, then a maximum of 25 strings can be formed. 25 * 43.7=1092.5V.
Q :MPPT working voltage range
A :The inverter is designed to adapt to the constantly changing voltage of the components. The voltage of the components varies according to changes in light and temperature, and the number of components connected in series also needs to be designed according to the specific situation of the project. Therefore, the inverter has set a working range within which it can operate normally. The wider the voltage range, the wider the applicability of the inverter.
Q :Full load voltage range
A :Within the voltage range of the inverter, it can output rated power. In addition to connecting photovoltaic modules, there are also some other applications of the inverter. The inverter has a maximum input current, such as 40kW, which is 76A. Only when the input voltage exceeds 550V can the output reach 40kW. When the input voltage exceeds 800V, the heat generated by losses increases sharply, leading to the inverter needing to reduce its output. So the string voltage should be designed as much as possible in the middle of the full load voltage range.
Q :Starting voltage
A :Before starting the inverter, if the components are not working and are in an open circuit state, the voltage will be relatively high. After starting the inverter, the components will be in working state, and the voltage will decrease. To prevent the inverter from starting repeatedly, the starting voltage of the inverter should be higher than the minimum working voltage. After the inverter is started, it does not mean that the inverter will immediately have power output. The control part of the inverter, CPU, screen and other components work first. First, the inverter self checks, and then checks the components and power grid. After there are no problems, the inverter will have output only when the photovoltaic power exceeds the standby power of the inverter.
The maximum DC input voltage is higher than the maximum working voltage of MPPT, and the starting voltage is higher than the minimum working voltage of MPPT. This is because the two parameters of maximum DC input voltage and starting voltage correspond to the open circuit state of the component, and the open circuit voltage of the component is generally about 20% higher than the working voltage.
Q :How to determine the output voltage and grid connection voltage?
A :The DC voltage is not related to the AC side voltage, and a typical photovoltaic inverter has an AC output of 400VN/PE. The presence or absence of an isolation transformer is not related to the output voltage. The grid connected inverter regulates the current, and the grid connected voltage depends on the grid voltage. Before grid connection, the inverter will detect the grid voltage and only connect to the grid if it meets the conditions.
Q :What is the relationship between input and output voltage?
A :How was the output voltage of the grid connected photovoltaic inverter obtained as 270V?
The maximum power tracking range of the high-power inverter MPPT is 420-850V, which means that the output power reaches 100% when the DC voltage is 420V.
The peak voltage (DC420V) is converted to the effective voltage of alternating current, multiplied by the conversion coefficient to obtain (AC270V), which is related to the voltage regulation range and pulse width output duty cycle of the output side.
The voltage regulation range of 270 (-10% to 10%) is: the highest output voltage at DC side DC420V is AC297V; To obtain the effective value of AC297V AC power and the DC voltage (peak AC voltage) of 297 * 1.414=420V, the reverse calculation can obtain AC270V. The process is: DC420V DC power is controlled by PWM (pulse width modulation) after being turned on and off (IGBT, IPM, etc.), and then filtered to obtain the AC power.
Q :Do photovoltaic inverters require low voltage ride through?
A :General power station type photovoltaic inverters require low voltage ride through function.
When power grid faults or disturbances cause voltage drops at the grid connection points of wind farms, wind turbines can operate continuously within the range of voltage drops. For photovoltaic power plants, when power system accidents or disturbances cause grid voltage drops, within a certain range and time interval of voltage drops, photovoltaic power plants can ensure continuous operation without disconnection from the grid.
Q :What is the input voltage on the DC side of the grid connected inverter?
A :The input voltage on the DC side of a photovoltaic inverter varies with the load. The specific input voltage is related to the silicon wafer. Due to the high internal resistance of silicon panels, when the load current increases, the voltage of the silicon panels will decrease rapidly. Therefore, it is necessary to have a technology that becomes the maximum power point control. Keep the output voltage and current of the silicon panel at a reasonable level to ensure maximum power output.
Usually, there is an auxiliary power supply inside the photovoltaic inverter. This auxiliary power supply can usually be started when the input DC voltage reaches around 200V. After startup, power can be supplied to the internal control circuit of the inverter, and the machine enters standby mode.
Generally, when the input voltage reaches 200V or above, the inverter can start working. First, boost the input DC to a certain voltage, then invert it to the grid voltage and ensure that the phase remains constant, and then integrate it into the grid. Inverters usually require the grid voltage to be below 270Vac, otherwise they cannot function properly. The inverter grid connection requires that the output characteristic of the inverter is the current source characteristic, and it must ensure that the output phase is consistent with the AC phase of the power grid.
Post time: May-15-2024