Variable frequency drives (VFD) are increasingly being used in oil fields to operate pump jacks. Significant harmonic distortions are expected for such applications. Compared to traditional harmonic problems, the oil-field harmonic problem is quite unique since the harmonicproducing VFDs are scattered all over a field. It is the cumulative effect of individual sources that causes unacceptable harmonic distortion in a system. This paper presents a COURSE-sponsored project to develop methods for assessing the cumulative harmonic effect of distributed VFDs and for designing effective harmonic mitigation measures.
Variable frequency drives (VFDs) are increasingly being used in oilfields due to their high efficiency and flexibility. A typical application is to drive the pump jack motors distributed across a field. For new oilfields or existing fields with expansions, a significant portion of the electric loads can be VFDs. This situation is particularly true in Alberta where many oilfield expansion projects are either in progress or to be pursued.
VFDs are harmonic-producing loads, that is, they generate currents or voltages that have frequencies of multiples of 60Hz. A typical waveform of the current drawn by a VFD is shown in Figure 1. This harmonics polluted waveform is quite different from the sinusoidal waveform seen in normal electrical systems. The harmonic currents are very disruptive to the operation of variable speed drives and can damage sensitive electronic equipment connected in the system. They also cause electromagnetic interference with telecommunication and computer equipment. International and national standards have been established to limit harmonics in power systems and to define compatibility levels based on which electrical apparatuses are designed [1,2]. As a result, it is essential to reduce the harmonic distortion levels in oilfield distribution systems. A reliable and cost effective solution to this problem could mean sizable savings in investment costs, and reductions in production losses for Alberta's oil and gas industry. Unfortunately, there are no readily available solutions to this problem.
In response to industry needs, the COURSE program has approved a research project to undertake strategic research into this emerging problem. The objective is to determine the most reliable and cost-effective means to mitigate harmonic distortions in oilfield electrical systems. A guideline on the assessment and designing of harmonic filtering schemes will be developed. This paper provides an overview on the project and presents some related results
Harmonic distortion in electrical distribution systems is not a new phenomenon. Considerable knowledge has been accumulated over the past twenty years on the assessment and mitigation of harmonic distortions. What makes the oilfield harmonic problem unique is the distributed nature of the harmonic sources: the harmonicproducing VFDs are scattered all over a field. It is the cumulative effect of individual sources that causes unacceptable harmonic distortion in the system. How to assess the cumulative harmonic effect on the oilfield distribution systems and how to design appropriate mitigation measures is a new area of research. Central to this subject are the following two questions:
