Smooth torque production is one of the most important quality criteria for PWM-VSI-fed drives operation. In an ideal case merely additional torque harmonics of the switching frequency and of its multiples occur compared to sinusoidal power supply operation of a drive. At switching frequencies in kHz-range they are absorbed by the inertia of the machine and do not deteriorate the drive performance. Although additional low order torque harmonics may occur due to parasitic effects during the PWM operation. Low order torque harmonics are particularly undesired, because they may cross the mechanical resonance frequency of a drive train. New low order torque harmonic sources are described in the paper and compensation methods are proposed for them.

Sensorless winding temperature monitoring of PWM inverter-fed electrical machines may be made by a superposition method. A small dc offset current superimposed on the AC current may be used to measure winding resistances during machine operation. To obtain acceptable results, the dc voltage component in the inverter output must be measured with sufficient accuracy. Previous investigations showed that the referenced DC voltage offset is distorted by parasitic effects. A deeper knowledge of these parasitic effects is necessary. The paper describes a suitable correction method. Experimental results verify the proposed method.

A new source for the DC current offset in the PWM inverter-fed loads is found. A significant irregularity in dead time voltage compensation due to a very small offset in phase current measurement is the cause for it. In the phase currents and voltages, high DC offsets appear at low current magnitudes, which lead to a first torque harmonic in the drive. Recommendations how to avoid or compensate this effect are given in the paper.

The origin for the low power quality at the output of a 132 MW wind farm has been investigated. A resonant circuit formed by the wind farm components is found to be responsible for a high level of the 701 current harmonic. The structure of the investigated wind farm is similar to those of offshore wind farms with AC cable connection. The same resonance problem is expected to occur in offshore installations with long high voltage AC cables. Various compensation strategies are proposed and analyzed.

Based on previous investigations it is shown that the accuracy of online stator resistance monitoring of AC-machines fed by PWM inverters when using the superposition method suffers from dead time effects. A new and simple correction procedure is proposed to achieve high measurement accuracy without using temperature sensors. No additional hardware is required for this method. Experimental results are presented.

A possible reason for a low power quality at the output of a 132 MW wind farm is investigated. A resonant circuit formed by the wind farm components is found to be responsible for a high level of the dominating 7th current harmonic. Various compensation strategies are proposed and analyzed.

A new source for the DC current offset in the PWM inverter-fed loads is found. A significant irregularity in dead time voltage compensation due to a very small offset in phase current measurement is the cause for it. In the phase currents and voltages, high DC offsets appear at low current magnitudes, which lead to a first torque harmonic in the drive. Recommendations how to avoid this effect are given in the paper.

Current harmonic production of a 5 MW offshore wind turbine prototype is investigated on an onshore test site. On the PCC an atypical high level of the sixth harmonic current was measured. The evaluation of the existing configuration for future extensions with other wind turbines requires the investigation of this effect. Therefore the existing technical structure with medium voltage cable connection and parallel double-tuned harmonic filter is analyzed. The sixth harmonic generation through asymmetrical loads is shortly introduced. It is shown, that the reason for the high level of sixth current harmonic is not a series-resonance, but an undesired additional effect caused by the double-tuned filter, which damps the uneven harmonics. Measurements confirm the theoretical approach.

Harmonic production of a 132 MW on-land wind park is investigated. It is shown, that the main reason for current harmonics is the earth capacitance of a long high-voltage cable, which forms a small impedance for grid-specific voltage harmonics and causes harmonic current flow from the grid into the wind park. Measurements confirm the theoretical approach