Three major issues in designing a power electronic system are Losses, EMI (Electromagnetic interference) and Harmonics. These issues affect system cost, size, efficiency and quality and it is a trade-off between these factors when we design a power converter. In power electronics, high voltages and high currents are processed by fast switching to reduce losses which are significant sources of electromagnetic noise and it cause additional costs.
Magnetic interference in power systems is called NOISE, this is due to a magnetic field developed somewhere along the power system creating a ground potential. This potential could travel to sensitive equipment causing numerous unwanted problems, including shutting down of equipment.
Two major sources of EMI in power electronics are dv/dt and di/dt during switching times. In fact, a DC voltage of few hundred volts is chopped by a power switch in a fraction of microsecond. Thus, conducted emission is a major issue in most power electronic systems due to significant over voltage and leakage current generated by fast switching and stray components of the system.
- High di/dt may create significant over voltage in power converters due to stray inductance of current loops.
- High dv/dt may create significant leakage current in magnetic elements and electric motors due to stray capacitive coupling between windings and a frame.
So, as you know switching speed and frequency of power switches has increased dramatically. Now consider a voltage source converter with hard switches. Increasing the frequency of power switches generate high dv/dt which causes leakage currents. These leakage currents are due to stray capacitances in the converter.
It is due to a magnetic field that is developed somewhere in power system (for example a variable speed drive) between the system and the ground. Between the system and the ground they are small capacitive couplings. At high frequencies, the inductance of these capacitances is low. As a result, you can consider that, stray currents will flow through them and making leakage currents.
For example many small capacitive couplings exist in a motor drive system which may be neglected at low frequency analysis but the conditions are completely different at high frequencies.