Variable frequency drives
In the case of variable frequency drives (VFDs) either applied as stand-alone drives on applications including ESPs, compressors or as components within a common DC bus system based drilling package the two basic PQ problems the same, namely :
- Harmonic voltage distortion
- Common mode voltage
AC VFDs – Harmonic voltage distortion
The negative effects of harmonics are acknowledged in many technical papers (see Resources section for more information) and do not require any explanation here other than to state that they generally fall into two basic categories:
- Excessive heating caused by additional I2R losses, iron losses, skin effect, etc. in cables and equipment (e.g. generators, transformers and motors).
- Voltage distortion resulting from harmonic currents, at the various frequencies, passing through the system impedances and leakage inductances of the power system, disrupting or destroying susceptible equipment.
Harmonic voltage distortion is essentially ‘pollution’ of the supply voltage and is ‘seen’ by all equipment connected to the power system.
Most conventional variable AC and DC speed drives offshore are ‘6 pulse’ (i.e. one three phase rectifier). All rectifiers, when fed with sinusoidal voltages, draw non-sinusoidal or ‘non-linear current’ from the supply (Fig 7) and are hence termed ‘non-linear loads’. When the supply voltage is distorted and/or imbalanced, uncharacteristic harmonic currents and voltage are also drawn from the supply.
Fig 9 shows the relationship between the Uthd, line voltage and current waveforms and Ithd on a 560kW VFD. Note the high frequency components in the line voltages.