With the deepening of PV poverty alleviation, many problems began to appear. For example, in some remote areas, when construction workers are connected to the grid, they find that the grid voltage is always high, which not only triggers frequent voltage failure alarms, but also causes inverter shutdown protection, which seriously affects the photovoltaic revenue.
For such problems, manufacturers often provide solutions from the inverter side, such as relaxing the protection voltage range (for different regions, the factory AC voltage is relaxed to 160-300 AC adjustable). Although this method can solve the inverter protection shutdown problem, but because of the output grid voltage is too high, will always cause damage to some household appliances.
It should be said that this type of phenomenon is very common. Many forum posts have also conducted professional technical analysis. However, many practitioners are still confused. Here, I want to do a simple combing through some analogies to help everyone avoid the above situation from the power plant design source.
We all know that the photovoltaic grid-connected system is the process of converting DC power into AC power through an inverter and transmitting it to the grid. If the grid is compared to the sea, the inverter can be seen as a trickle, and the grid connection is like trickling into the sea. The AC cable connected to the grid is the confluent bed.
In the process of grid-connected power generation in some remote areas or in weak grid areas, the output voltage of the inverter has to be raised due to the influence of line impedance (the riverbed is narrow and there are many obstructions), and the river water level increases to form a high water potential. Can flow to the sea to ensure that the AC power flow to the power grid efficiently (river flow into the sea). However, this can lead to two types of problems: First, the output voltage is higher than the inverter's own protection voltage value, so that the inverter error and perform protective shutdown; Second, the transformer capacity of the grid point is small (that is, "the sea water storage The quantity is insufficient, this is the reason that many places restrict the connection capacity to be about 30%). It is easy to access the grid due to the overloaded electricity, raising the grid voltage (the storage capacity of the reservoir is insufficient, overflowing).
In fact, the above two conditions are the two main reasons that cause the grid voltage to be too high, that is, the capacity of the grid connection point is too small, the capacity of the load consumption is insufficient, or the line impedance is weakly highlighted by the grid. So, how can we solve the above problems?
There is no doubt that the first is to increase the size of the cable and reasonably select the grid point; the second is to increase the capacity of the transformer and improve the "storage capacity." Among them, the rational choice of grid point and capacity expansion transformer is very easy to understand, for example, the nearest transformer selection grid point is the most common way to choose the grid point, and the capacity expansion transformer is to increase the capacity of the transformer. In this way, only the size of the cable is left. Using an image metaphor, it is to enlarge the riverbed and clear the silt near the sea to significantly reduce the middle resistance of the river.
In addition, there is another situation worth mentioning, that is, when multiple devices are connected to the network, if they are concentrated on one phase, it is easy to raise the phase voltage (similar to a number of rivers sinking into a narrow river bed, causing water overflow). This biases the grid and causes similar high grid voltages. Therefore, when multiple grid-connected units at the same grid point are proposed, the equipment should be distributed as evenly as possible on three phases.
The above is intended to give everyone an image of the cause of overvoltage faults in the power grid through simple analogy. However, the ultimate goal is still to hope that in the construction of photovoltaic power plants, these possible hidden dangers can be evaded at the beginning of design, so as to improve design efficiency and help PV to alleviate poverty.
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