Although the majority of variable speed drives on vessels are now AC VFD (variable frequency drives) some DC SCR drives still exist. The common PQ problems encountered are:
Note : 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 (due to VFDs), 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 vessel’s supply voltage and is ‘seen’ by all equipment connected to the power system.
Most conventional variable AC VFD 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 hence are 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 1 illustrates the voltage distortion despite the load current being relatively sinusoidal. The maximum total harmonic voltage distortion (Uthd) show is 19.2%. The maximum Uthd during the site visit was 26.3%. The high Uthd was due to the switching of the SCRs.