Electrical Designing : Selection of MCCB for inverter circuit

Causes of distorted waveform current

Distorted waveform current can be caused by CVCF units with thyristors and transistors used as computer power supply units, various rectifiers and VVVF units for induction motor control for meeting the recent trend toward energy conservation. These units are used to make DC power using the semiconductor switching function or further make the target AC power from the DC power. Generally, a large capacitor is connected for smoothing after a rectifier circuit, and, therefore, pulsed charging current to the capacitor flows to the power supply side every half cycle. Load current generated by superposition of high-frequency current by chopped frequency on the fundamental frequency flows to the load side because the voltage is chopped by higher harmonics in the process of conversion to AC power. Below is described the selection for the VVVF inverters which will be developed as the main control method for widely-used induction motors. There are two VVVF inverter control methods, PAM (Pulse Amplitude Modulation) and PWM (Pulse Width Modulation). The harmonic content generated varies depending on the method. To reduce the harmonic content in the input current, according to Tables 5. 18 and 5. 19 , it is effective to add a DC reactor (DCL) or AC reactor (ACL). In the output current waveform shown in Fig. 5. 20, the harmonic content in the case of PWM is higher.

Selection of MCCB

For the power supply side of inverter, at first, select MCCB recommended by the inverter manufacturer. I f the manufacturer does not recommend any MCCB, correct the relationship between MCCB rated current IMCCB and the load current I as shown below in consideration of changes in characteristics and temperature rise due to distortion of load current waveform.

Thermal magnetic (bimetal) and electronic (RMS value detection) type MCCB use current RMS value detecting systems and ensure correct protection against overload even at current with distorted waveform. It is better to select one of these types.

Note (1) Since hydraulic magnetic MCCB may considerably change in characteristics depending on waveform distortion, it is recommended to use thermal magnetic MCCB.