Basics of photovoltaic cells

What problems can arise when the DC current component is injected into the grid? How to suppress it?

In recent years, the photovoltaic industry has developed rapidly, the non-isolated photovoltaic grid-connected power generation system has developed rapidly, and the photovoltaic grid-connected capacity has increased significantly. Therefore, the impact of the injection of the DC current component into the AC grid has attracted more and more attention, and the problem of injection of the DC current component into the grid has become the current One of the hotspots of research.

For single-phase non-isolated photovoltaic grid-connected power generation inverter system, DC current injection includes common mode DC current injection and differential mode DC current injection. The injection of common mode DC current (ie common mode current) is generally caused by the stray capacitance of the photovoltaic array to the ground. The circuit topology and control methods discussed in Section 6.3 solve the problem of common mode current in non-isolated photovoltaic power generation systems, while The main reasons for differential mode DC current injection are:
①The difference in the parameters of the power switching device makes the impedance, turn-on and turn-off process and other characteristics not completely consistent, resulting in a certain difference in the charging and discharging time.
② The pulse modulation width sent out by the controller is unbalanced and the gate drive circuit has different degrees of delay, which causes inconsistent actions of the power switching devices.
③ The dead time set by the voltage-type inverter to prevent the bridge arm from passing through also affects the action process of the power switching device; the current limiting protection measures that instantaneously block the drive pulse such as intelligent drive circuits will also cause DC current components.
④The influence of the offset error of the current controller measurement element. The zero-point drift error of the current sensor and the offset error of the sampling circuit lead to asymmetry between the positive and negative half-cycles of the sampling current, causing the problem of DC injection.
⑤ There is a current controller error caused by a DC component in the reference current, resulting in a DC component in the inverter output current.

The main hazards of DC current components injected into the grid are as follows:
①The amount of DC current changes the working point of the transformer or the transformer, which may lead to one-way saturation of the transformer, a surge of excitation current, waveform distortion, excessive loss, overheating of equipment and shortening of equipment life, which may cause system subversion and other problems in severe cases
②The direct current will aggravate the corrosion degree of the metal, resulting in the electric corrosion of the substation grounding grid.
In recent years, the problem of DC current injection in photovoltaic grid-connected power generation systems has received increasing attention from Chinese and foreign governments. The United States, Japan, the United Kingdom, China and other countries have formulated relevant standards for DC current injection of photovoltaic grid-connected power generation systems. Among them, IEEE Std.929-2000 stipulates that the DC component of the grid-connected current of the photovoltaic system must be less than 0.5% of the rated current of the system, Japan stipulates that it does not exceed 1% of the rated output current of the inverter, and the United Kingdom stipulates that the DC current component does not exceed 5mA. The Australian AS4777.2 standard stipulates that the larger of 0.5% of the system rated current and 5mA is not exceeded, and the State Grid Corporation of China stipulates that the DC current component does not exceed 0.5% of the inverter output current rating.

There are many methods to suppress the DC current component, which are basically divided into two categories: passive suppression methods and active suppression methods. Passive suppression methods mainly include the use of power frequency transformer isolation method, DC blocking capacitor method, etc. Active suppression methods mainly include parallel transformer detection method, two-stage RC filter detection method, critical saturable reactor detection method, self-correcting DC link current reconstruction method, and integral action detection method.

1.Passive suppression method
① Adopt the power frequency transformer isolation method As shown in Figure 6.31, the output of the photovoltaic grid-connected inverter is connected to the power grid through a power frequency transformer. Since the power frequency transformer has an electrical isolation function, no DC current component will be injected into the power grid. The disadvantage is that the system cost increases, the weight and volume are large, the transformer loss is large, and the conversion efficiency is low.

Common frequency transformer isolation method

②DC blocking capacitor method
The DC blocking capacitor method includes the DC side blocking capacitor method and the AC side DC blocking capacitor method. The DC side blocking capacitor method is implemented by a half-bridge inverter circuit , as shown in Figure 6.32. Since the DC capacitor forms a capacitor bridge arm, the current flowing into the grid must pass through one of the DC capacitors regardless of whether it is positive or negative. Therefore, the half-bridge inverter circuit has the unique property of suppressing the DC current component. The disadvantage is that the voltage of the input DC bus is twice that of the full-bridge inverter circuit, which leads to a higher selection of withstand voltage of the power switching device, an increase in switching loss, and a decrease in conversion efficiency. relatively low.

Half-bridge inverter circuit

The AC side DC blocking capacitor method is shown in Figure 6.33(a) and (b). Figure 6.33 (a) Use AC capacitors for DC blocking. Since there will be a voltage drop on the AC capacitor, in order to reduce the voltage drop, the AC capacitor capacitive reactance at the power frequency is bound to be greatly reduced, which will lead to AC The capacitance value is large, resulting in an increase in the size and cost of the inverter. Figure 6.33 (b) uses DC electrolytic capacitors for DC isolation. Since the electrolytic capacitors can be made large in capacity and not large in size, the problems existing in AC capacitors are solved. Active suppression can also be performed by using DC electrolytic capacitors for DC blocking. It is real-time sampling of the small forward bias DC voltage caused by the injection of DC current across both ends of the DC electrolytic capacitor. Figure 6.33(c) The negative feedback compensation method shown suppresses the DC current component. Since electrolytic capacitors cannot withstand back pressure, a Schottky diode with low on-voltage drop is used for negative voltage clamping. In order to prevent the forward breakdown of the electrolytic capacitor, multiple common diodes are used in series, and the series voltage drop is less than the withstand voltage of the electrolytic capacitor, but far greater than the forward bias DC voltage that the capacitor under normal conditions bears. The disadvantage of this method is that once the reverse negative pressure occurs and the Schottky diode or the series diode is turned on, it will cause additional DC components, and will also cause distortion of the current injected into the grid, resulting in an increase in current harmonics.

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Figure 6.33 AC side DC blocking capacitor method