Mathematical Model of Common-Mode Sources in Long-Cable-Fed Adjustable Speed Drives

Motor drive systems with long feeder cables are widely used in mining and energy sectors, such as offshore oil and gas drilling. In recent decades, to prevent issues with electromagnetic interference, power quality standards have been defined for the 2–150 kHz frequency range. In order to comply with these standards, manufacturers need to design filters to avoid exceeding the thresholds set by the standards. An important aspect that needs to be considered is the accurate modeling of currents circulating through common-mode loops. This article addresses this modeling by using a mathematical approach. The power cable’s common-mode model is extracted from the measurement data using the finite-element method via finding the effective permittivity of the PVC materials. The common-mode noise sources, including the grid side voltage through the front-end diode rectifier and the output voltage through the rear-end inverter, are both modeled by extracting the Fourier series calculations. Consequently, noise emissions circulating through different common-mode loops are analytically modeled. Through the proposed modeling strategy, the harmonic spectrum of resonances can be precisely analyzed. The results are validated through simulation and experimental data.

I. Introduction

In Today’s industrial world, adjustable speed drives (ASDs) acquire a significant share in transferring energy to different electrical and mechanical loads [1]–[3]. The drive manufacturers should comply with the requirements of the electromagnetic compatibility (EMC) in order to have the permission to enter the markets. Fig. 1 shows the typical configuration of a three-phase motor drive system. According to Fig. 1, to measure the electromagnetic interference (EMI) emissions, line impedance stabilization network (LISN) is connected at the grid side of the system. In an industrial application of three-phase ASD, power cable is used as a feeding interface between the motor and drive. However, due to the parasitic couplings between different parts of the cable, a low impedance common-mode (CM) path is created between the ground and the cable. Therefore, utilizing power cable makes the system more susceptible noise emissions, resulting in the flow of CM currents through the ground. Consequently, power cables are typically shielded to mitigate EMI emissions and the shield is connected to the ground from both sides of the cable [4]. According to Fig. 1, is the CM current going to the ground due to the parasitic couplings of the motor. Also, and are the CM currents flowing through the ground due to the parasitic couplings of the cable.