Power conditioner for variable-frequency drives in offshore oil fields

Abstract

The proliferation of variable-frequency ac motor drives (VFDs) as submersible pumps in oil exploration has aggravated many power-quality problems on the distribution network, for which VFDs are themselves susceptible to maloperation. The distortion in the input voltage waveform is a major issue, since the offshore power system cannot be regarded as a stiff source, and thus, distortions in the voltage waveform become more severe, which disturbs other loads connected at the point of common coupling (PCC). The oil fields require the operation of many parallel-connected VFDs on the same bus. Such a system significantly increases the power-quality problems on the distribution network. In this paper, an active power conditioner which is connected in series between the source and the PCC, and its dc bus shared with one of the VFDs connected to the bus, is proposed to mitigate a majority of power-quality problems for the whole bus. The proposed single power conditioner is sufficient to block the harmonic currents flowing toward the source, balance the currents from the source side, protect the VFDs on the bus against sag/swell at PCC, protect preinstalled passive filters connected with VFDs against series and parallel resonances, and maintain regulated balanced voltages at PCC. Investigations are carried out to validate the use of the proposed topology of a power conditioner to safeguard the distribution system and a set of VFDs against major power-quality problems. The proposed power conditioner employs a new control scheme based on an indirect current control for its effective operation under varying load conditions on different VFDs connected to the same bus.