The same photovoltaic system, components, inverters, cables and other components are completely consistent, and the installation tilt angle and azimuth angle are also completely consistent. If they are installed in the same place, is it about the same amount of power generation? In fact, the amount of photovoltaic power generation has a great relationship with the grid voltage. Compared with areas where the voltage is extremely unstable and the area where the voltage is stable, the amount of power generation may vary by 10% to 20%.
This article discusses the impact of the grid voltage on the system. The voltage of the grid includes: low voltage, high voltage, and rapid voltage fluctuations. All three situations will affect the power generation of the system.
1 Low grid voltage
In a photovoltaic system, no matter how large the components are installed, the maximum output current of the inverter cannot be exceeded, because once it is exceeded, it will cause the inverter to overheat and blow up. In the case of the rated voltage of the power grid, this maximum output current can reach the rated output power, but if it is lower than the rated voltage, it cannot output at full load.
Such as 17kW inverter, the maximum output current is 27.4A, under the rated voltage of 400V, the maximum output power is 27.4 * 400 * 1.732 = 18.98kW, which can meet 1.1 times overload, but if the grid voltage is relatively low, only 340V , Then the maximum output power of the inverter is 27.4 * 340 * 1.732 = 16kW, at this voltage, even if the power of the component is no longer large, it can not output at full load.
2 The grid voltage is too high
There are two situations that will cause the grid voltage to be too high. One is near the step-down transformer. In order to ensure that the voltage far away from the transformer is normal, considering the line voltage loss, the output voltage of the transformer is generally raised; the second is The user side of photovoltaic power generation cannot digest it, and the voltage must be increased when it is transmitted to a remote place. There is impedance in the line and there is voltage loss. The voltage on the output side of the inverter must be increased before it can be sent to the grid.
There are three aspects to the loss of power generation caused by the increase in voltage. One is the overvoltage protection of the inverter, which causes the inverter protection to shut down. At this time, the power will be lost. The second is the inverter overvoltage and load reduction. This is a new technology adopted by the inverter in places where the grid voltage is too high. When the grid voltage rises to a certain level, the inverter actively reduces the power to ensure the inverter. The generator is not off-grid, which can reduce the loss of shutdown, but there is also a certain amount of power loss.
The third is the loss of efficiency. The voltage of the power grid increases, and the DC bus voltage should also increase. For example, the AC voltage of 400V, the DC bus voltage is about 610V, the rated voltage of the general inverter is within this voltage range, if the string voltage is 600V The PWM duty ratio is close to 1. At this time, the DC conversion part of the inverter has the highest efficiency. If the AC voltage rises to 460V, the DC bus voltage needs to rise to about 700V. If the string voltage is still around 600V, the PWM accounts for The air ratio is 0.86. At this time, the efficiency of the DC conversion part of the inverter will decrease, about 90%.
There are usually three methods to reduce the high voltage of the grid: one is to increase the output cable diameter, because the thicker the cable, the lower the impedance; second, move the inverter closer to the grid connection point, the shorter the cable, the lower the impedance, and third, manual adjustment Inverter voltage range, but can not be adjusted too high, more than 270V may damage other electrical equipment users.
3 Severe voltage fluctuations
Photovoltaic inverters deliver electrical energy to the grid, and the quality of the grid will also affect the inverter. In some mechanical processing plants, there are high-power equipment such as driving, welding machines, gantry milling machines, etc., and some electric arc furnace factories. Between the start-up and shutdown of the equipment, the electrical energy changes very drastically, the power grid is too late to adjust, and the voltage is within 320-460V for a short time Changes between, accompanied by a large number of harmonics.
The MPPT function of the photovoltaic inverter, the input power of the component is as large as the output power of the inverter, and the output power of the inverter is related to the voltage and current. When the grid voltage fluctuates sharply, the inverter's ability to adjust Limited, will lead to confusion of MPPT tracking function, can not find the maximum power point, resulting in loss of power generation. In severe cases, the photovoltaic inverter may be restarted, or even the inverter may be damaged, resulting in loss of shutdown power.
to sum up
The loss of power generated by AC voltage is actually very large, so when exploring a photovoltaic site, the voltage stability of the power grid is also an indispensable part. If the voltage is stable, it can be installed; if the voltage is not very stable, you should consider whether to add a voltage regulator and other devices, otherwise it will affect the system's power generation.
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