Solve the Problem of Inverter and Grid Connection Point Voltage from Multiple Angles
When the distributed photovoltaic power generation is full and the capacity ratio reaches a certain proportion, it is easy to cause the problem of voltage overrun. Voltage overruns not only affect power quality issues but also limit the penetration of photovoltaics in the distribution network. In response to voltage overruns, power grid companies have issued corresponding grid-connected technical specifications for photovoltaic power generation systems. There are also solutions in the industry, such as adjusting the voltage at the grid-connecting point or adding new reactive power compensation devices through inverter control solutions. In this article, we will popularize the voltage problems of photovoltaic and grid-connected points for everyone.
01 Is a grid-tied inverter a voltage source or a current source?
First, we need to understand the type of inverter power supply, which helps to understand the impact of the grid on the inverter. There is no doubt that the grid-connected inverter is a power generation device and belongs to a current source. The characteristic of the current source is that the internal resistance is infinite, and the output current is controlled by the internal algorithm of the device. The voltage and frequency are determined by the external circuit (grid). The characteristics of the current source require that the current source cannot be opened (the grid cannot fail), and the current source can be used in parallel.
The characteristic of the voltage source is that the internal resistance is zero and the output voltage is constant. The current and its direction are determined by the voltage source and the external circuit. The characteristics of the voltage source require that the voltage source cannot be short-circuited.
The operation strategy of the grid-connected inverter is to rely on the rigid support of the voltage and frequency provided by the large power grid. At this time, the load fluctuation, voltage and frequency disturbance in the power grid are all borne by the large power grid, and the distributed power supply does not need to consider the voltage and frequency regulation. .
The output voltage of the photovoltaic grid-connected inverter is determined by the grid. When the grid voltage exceeds the inverter's operating voltage range, the inverter will fail and shut down. When the grid voltage is within the inverter's operating voltage range, the inverter will work normally.
The power grid has certain requirements for the power quality output by different forms of inverters, and there are certain differences between different specifications and different inverters.
02 The difference between class A and class B inverters
Class A inverters are closely related to the requirements of the public grid, and class B inverters are mainly used in distributed photovoltaic power generation systems. It has the characteristics of low grid-connected voltage, not close connection with the public grid, and small impact on the grid.
03 Common solutions to grid-connected voltage problems
a. Inverter power control
The inverter can adjust and control the active power and reactive power output by the photovoltaic during the grid connection, and can adjust the voltage of the grid connection point by controlling the power.
b. Reactive power compensation device
The importance of reactive power compensation to the power system has been paid more and more attention, and reactive power compensation equipment should be used reasonably. It plays a very important role in adjusting the grid voltage, improving the quality of power supply, suppressing harmonic interference, and ensuring the safe operation of the grid.
c. Configure energy storage
1) Energy storage can solve the problem of three-phase voltage imbalance.
2) Through the fast response capability of power-type energy storage, the voltage problem of voltage flicker and sag can also be compensated.
3) Energy storage can perform reactive power compensation and improve power factor without affecting photovoltaic active power output.
With the increase in the proportion of distributed power sources, the power grid should make overall planning for distributed power sources and power grids, calculate the carrying capacity of distribution networks at all levels, strengthen distribution network group measurement, group control and group adjustment technology, on-load capacity regulation, voltage regulation and distribution. key technologies such as electricity. We look forward to the introduction of unified standards for the power grid, which can solve the specification requirements for a large number of distributed power sources, and work together to build a new power system.
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