Calculation Method of DC Link Filter Capacitor of Voltage Type Inverter

Although the rectifier circuit can be used to convert AC power into DC power, this DC voltage or current in a three-phase circuit contains voltage or current ripples with a frequency six times the power frequency. In addition, the inverter circuit of the frequency converter will also generate ripple voltage or current due to output and carrier frequency, which in turn affects the quality of the DC voltage or current. Therefore, in order to ensure that the inverter circuit and the control circuit can obtain high-quality DC voltage or current, the DC voltage or current must be filtered to reduce the pulsation of the voltage or current.

The DC link refers to the filter circuit inserted between the DC power supply and the inverter circuit. The difference in its structure will have different effects on the performance of the converter: if it adopts an inductive structure, the input current ripple is small, similar to the current source property; if it adopts a capacitive structure, the input voltage ripple is small, similar to the voltage source property.

For voltage-type inverters, the output of the rectifier circuit is a DC voltage, and the DC intermediate circuit filters the voltage through a large electrolytic capacitor; for current-type inverters, the output of the rectifier circuit is a DC current, and the intermediate circuit filters the current through a large inductor.

l Calculation of electrolytic capacitors in the DC intermediate circuit of a three-phase inverter

Three-phase input and rectified voltage waveform

The three-phase input line voltage 220V and the rectified voltage waveform are shown in Figure 2.

In Figure 2, Ua, Ub, and Uc are the three-phase input phase voltages of the three-phase three-wire system; uc is the capacitor voltage, and ur is the voltage after rectification without adding a capacitor.

1.3 Analysis process

1.3.1 Calculation of Rectified Voltage

For three-phase three-wire inverters with a 220V input line voltage (hereinafter referred to as 220V series), U=220V; for 440V series, U=440V.

1.3.2 Calculation of equivalent resistance

For the convenience of calculation, for an inverter with an output power of P, the DC input impedance is equivalent to a pure resistor R, then

1.3.3 Analysis of the Capacitor Charging and Discharging Process

Since the rectified DC voltage fluctuates, assuming that the fluctuation range of ur is a%, then

Assuming that the circuit is already in steady state, the voltage across the capacitor is shown in Figure 2. At t2, the capacitor voltage reaches its maximum value. Since the power supply voltage is less than the capacitor voltage, the capacitor begins to discharge. At t3, when the power supply voltage drops to the minimum value, the capacitor voltage is still greater than the power supply voltage, and the capacitor continues to discharge. At t4, the power supply voltage is just equal to the capacitor voltage, and the power supply charges the capacitor. At t4, the capacitor voltage is equal to UPV (1-a%).

1.3.4 Calculation process

As shown in Figure 2, the discharge time of the capacitor is tf=t4-t2

Example: Take a three-phase 220V series 2.2kW transformer as an example to calculate the electrolytic capacitor required for its DC intermediate circuit.

Given that U=220V, UPN=310V, f=50Hz, and assuming a=5,

Substituting the above data into formula (9), we get C>1036.56μF

Considering the actual capacitance value, you can choose to use three 470μF electrolytic capacitors in parallel.

2 Calculation of electrolytic capacitors in the DC intermediate circuit of a single-phase inverter

The value of the line voltage of the single-phase input here is still the same as that of the three-phase input mentioned above.

Because the voltage waveform after rectification of the three-phase inverter is a six-pulse wave; while the single-phase inverter has only two wave heads.

For single-phase 220V series 0.4kW series inverter

Therefore, for the single-phase 220V series 0.4kW inverter, three 220μF electrolytic capacitors are selected for parallel use.

3 Experimental Results

(1) When the three-phase input 220V series 2.2kW inverter is at a carrier frequency of 14.5 kHz, full load, and three 470μF electrolytic capacitors are connected in parallel in the DC link, the maximum and minimum values ​​of the capacitor voltage are 312V and 299V respectively, the average value is 305V, and the ripple factor is about 4.26%;

(2) For a single-phase 220V series 0.4kW inverter, under the conditions of a carrier frequency of 14.5kHz, full load, and three 220μF electrolytic capacitors in parallel in the DC link, the maximum and minimum values ​​of the capacitor voltage were measured to be 308V and 294V respectively, with an average value of 301V, and a ripple factor of approximately 4.65%.

4 Conclusion

The design of the inverter hardware circuit should consider the use of high-performance components while paying more attention to minimizing costs. The capacitance value selected based on experience generally leaves a large margin, which indirectly increases the cost. The algorithm for electrolytic capacitors in the DC intermediate circuit in this article is feasible and reliable in practical applications. Through theoretical calculations, designers can select corresponding filter capacitors according to different voltage levels and different power loads. This algorithm has been applied in practice and has achieved certain economic benefits.