Low grid power generation due to photovoltaic system

After installation of the PV system, the user is most concerned about the amount of power generated, because it is directly related to the user's return on investment. There are many factors that affect power generation, such as components, inverters, cable quality, installation orientation, azimuth, tilt angle, dust and shadow shielding, component and inverter ratio system solutions, line design, construction, grid voltage, and so on. Factors are possible. This series of articles will discuss various factors based on actual cases. This article focuses on the impact of power quality on the system.

The power quality of the power grid includes: voltage deviation, current deviation frequency deviation, voltage fluctuation or flicker, three-phase unbalance, temporary or transient over-voltage, waveform distortion, voltage dips and so on.

1, the grid voltage over range

The voltage and frequency of the grid are not constant and will change with changes in load and power flow, and the output voltage of the inverter follows the grid voltage. However, when the grid is abnormal, inverters are required to stop the supply of electricity, the standard given by the National Energy Administration.

AC output side over/under voltage protection

When the voltage of the AC output terminal of the inverter exceeds the allowable voltage range of the power grid, the inverter is allowed to disconnect the power supply to the grid, and an alarm signal should be issued when the inverter is disconnected. The response time to abnormal voltages other than high-power inverters should meet the requirements of Table 1. The inverter should start up normally when the grid voltage returns to the allowable voltage range.

Note 1: The maximum off-network time refers to the time from the occurrence of an abnormal state until the inverter stops supplying power to the grid.

Note 2: For inverters with low voltage ride through, priority is given to low voltage ride through.

Table 1

In the following two situations, the grid voltage will be high: First, near the step-down transformer, in order to ensure that the voltage far away from the transformer is normal, taking into account the line voltage loss, the transformer output voltage will generally be pulled high; second, photovoltaic users The side can not digest, transport to a farther place to increase the voltage, causing the inverter output side voltage is too high, causing the inverter protection shutdown. At this time, there are three methods: First, increase the output cable diameter, because the cable is thicker, the lower the impedance; Second, the mobile inverter close to the grid point, the shorter the cable, the lower the impedance, the third is to manually adjust the inverter voltage The range, but cannot be adjusted too high, more than 270V may damage the user's other electrical equipment.

2, voltage fluctuations, flicker and harmonics

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, and gantry milling machines, and some electric arc furnace plants. Between the opening and closing of the equipment, the electric energy changes very drastically. The power grid is too late to adjust, and the voltage is short at 320-460V. Between the changes, accompanied by a large number of harmonics, the presence of harmonics and unbalanced negative sequence components in the power grid will lead to active power output fluctuations of the photovoltaic system, and the higher the grid voltage distortion rate, the smaller the output active power of the photovoltaic system; The current is distorted, and the higher the voltage distortion rate of the grid, the greater the output current THD of the photovoltaic system.

Power harmonics are divided into voltage harmonics and current harmonics. The ratio of voltage harmonics and fundamental waves is much smaller than that of current harmonics and fundamental waves. The main factor affecting voltage harmonics is the spike current that occurs instantaneously in the load. Because there is a resistance value in the power supply line, a voltage drop occurs when the current flows. This resistance is connected in series with the load, which causes the instantaneous waveform of the load voltage waveform to appear as a harmonic. Current harmonics are generated by the characteristics of the load, and all non-linear loads can generate harmonic currents.

The MPPT function of the PV inverter, how large the input power of the components is, and how large the output power of the inverter is. The output power of the inverter is related to the voltage and current. When the voltage of the power grid fluctuates violently, the capacity of the inverter is limited, which may cause frequent restart of the photovoltaic inverter. In severe cases, the power device in the inverter is also caused. Over-expander, electrolytic capacitor burst open.

Installation of photovoltaic systems in places with poor grid quality requires real-time monitoring and addition of equipment for power quality correction, such as active power filter APF, static reactive power generator SVG, etc., and real-time monitoring of these equipment and power grid parameters. .