High-voltage inverter refers to series stacked high-voltage inverter, by using multiple single-phase three-level inverters connected in series to output variable-voltage variable-voltage AC power. The main circuit mainly consists of three-phase or single-phase rectifier bridge, smoothing capacitor, filter capacitor, IPM inverter bridge, current limiting resistor, contactor and other components.
DC-LINK for Inverter Industry
DC Link Capacitor is a very important filtering and energy storage device in voltage source high voltage inverter, thus the design of DC link parameters of power unit directly affects the performance of the whole high voltage inverter, from one side, it can be seen that the selection of capacitor will play a decisive role in the life of the inverter.
2.1 Calculation of DC-side capacitance parameters
Theoretically, the larger the capacitance of the filter capacitor will be better, but considering the volume cost factor, it is impossible to choose too large, the middle DC filter capacitor is mainly chosen from the perspective of limiting voltage fluctuations. The specific capacity (uF) can be calculated using the following empirical formula
Formula (2-1)
ω is the input voltage angular frequency of the inverter,Udis the DC bus voltage,
I0is effective value of output current,K is the allowable DC voltage fluctuation factor,taking the value of 0.01-0.1.
Case
A model of high-voltage inverter, rated voltage 10kV, power 4675kW, each phase consists of 8 cascaded power units with rated voltage 690V, the same structure of the main circuit of power devices and identical parameters, the current flowing through each cascaded unit is the same size as the phase current flowing into the motor. Therefore, the output current RMS1 of a single cascade unit is 311A, 50Hz at fundamental frequency, and the output-side power factor is cosφ = 0.87, which gives the rectifier-side DC output voltage Ud
Ud= 1.414*U=1.414*690 975V
Therefore, when the input three-phase line voltage is 690V, the maximum voltage of rectifier output is 975V, considering the voltage fluctuation of 10%, the maximum output voltage can reach 1073V, the filter capacitor withstand voltage should be selected above this voltage, and there is a certain amount of gold detection. The film capacitor with withstand voltage ≥1100V (or 3 series connection of 400V and 450V electrolytic capacitors) should be selected.
DC LINK capacitors, at present, there are mainly two kinds of metalized film capacitors and aluminum electrolytic capacitors. Metallized film capacitor is a new type of DC capacitor made by winding metallized film material, in which organic polymer material
is the insulating medium and the metal plating layer on the high fraction 4 is the electrode: while aluminum electrolytic capacitor is a DC capacitor made by winding aluminum foil, in which the aluminum oxygen layer of the anode foil is the insulating medium, the unoxidized anode foil is the positive electrode, and the cathode foil and the electrolyte impregnated on the electrolytic paper is the negative electrode. The design process, material, and electrical principle of the two types of capacitors are very different, mainly in: film capacitor has strong heavy current capability, high single withstand voltage, and long life span: electrolytic capacitor has small through-current capability, low single withstand voltage, and short life span. The above characteristics bring a big difference in the use of both capacitors, and the capacity of electrolytic capacitors should be chosen to be larger to meet the requirements of high ripple current.
Film Capacitor
Using the formula (2-1), k takes the value of 0.03-0.04, such as the above case, DC LINK capacitor withstand voltage selected 1100VDC or 1200VDC, each unit of capacitance is generally selected multiple direct parallel connection, the total capacity of the model with 4874uf-6769uf. Generally,"1A output current corresponds to 15-30uF capacity".
Electrolytic capacitor
Using the formula (2-1), k takes the value of 0.01, such as the above case, DC Link capacitor withstand voltage of 400VDC or 450VDC, the capacitors of the unit generally choose 3 series and more connected, the total capacitance of the capacitor value is about 20306uf. Domestic and foreign famous inverter manufacturers basically comply with the principle of "1A output current corresponds to 50-80uF capacity"
