Calculation of commutation spikes and harmonics on offshore platforms

The AC-supply voltage waveforms on offshore oil platforms can be heavily distorted during drilling operations. The harmonic and commutation distortion is caused by the drilling rectifiers which drive large DC motors. The authors examine a method of analysing these disturbances, especially the commutation spikes which appear on the high-voltage busbars (the point of common coupling) and are spread throughout the platform supplies. The analytical method uses proprietary CAD software and equivalent-circuits to analyse the waveforms using a time-domain approach. The tool thus developed is used to show directly how the drilling and platform parameters will affect the waveshapes. The effects of firing angle, rectifier rating, platform short-circuit levels and cable capacitance amongst others are discussed. It is also shown that variable-speed drives will have similar effect. The consequence of sharing generation between platforms is illustrated, as is the use of damper circuits to reduce the commutation spikes