Harmonics become a larger issue every year. Electrical suppliers are looking for ways to maximize their system’s capability as electrical systems become ever more loaded. Harmonic injection into the power grid by “nonlinear” sources has become the target of electric suppliers in an effort to reduce the expansion required to meet these demands and related costs.

The Institute of Electrical and Electronics Engineers (IEEE)-519 standard provides guidelines to both users and suppliers of electricity. It says, in short, that users can only generate or inject so much harmonic distortion back into the electrical grid and that they must remain below the limits laid out in the standard. The electrical supplier must supply a clean and consistent supply.

Standard six-pulse converters used in most variable speed drives (VSDs) create harmonics (e.g. fifth, seventh, etc.) that can generate anywhere from 30% to 50% current distortion. Twelve and 18-pulse rectifiers are an improvement over standard six-pulse rectifiers, but they do not meet IEEE-519 standards, and they still inject harmonic distortion back into the grid.

The active rectifier used in the Active Front End (AFE) VSD creates cancelling harmonics that effectively eliminate those that would be created in standard 6/12/18-pulse VSDs. AFE variable speed drives use two power sections. An active rectifier is used to minimize the amount of harmonic distortion reflected to the power grid by drawing nearly sinusoidal current from the power grid.

AFE converter technology has been around for many years and is well proven. Its use in the oil field has been very limited due to excessive cost. In recent years the technology has become more affordable as both pressure from electrical suppliers and the number of applications for the technology increase.

AFE benefits

An AFE drive offers several benefits over an 18-pulse harmonic reduction drive: energy savings, less harmonic distortion, and sometimes initial cost. While the advertised performance specifications may show only a slight improvement using AFE over 18-pulse, the AFE advantage can be significant in the field.

For 18-pulse drives to achieve their stated reduction levels, the power entering the drive system must be balanced to within levels not practical in the real world. Eighteen-pulse systems provide satisfactory results when the drive load is at full load conditions. However, some suppliers cannot provide an 18-pulse VSD below 520 kilovolt-ampere, yielding sub-par performance for all wells below that requirement. Additionally, even when the VSD system is optimally sized at startup, today’s electric submersible pump (ESP) applications require systems to perform over extended flow ranges as the well is unloaded (e.g., from 2,000 b/d to 200 b/d). As the well declines, the VSD system will be operating at less than full load, affecting the ability of the standard 6/12/18 pulse VSDs to reduce harmonics. The AFE VSD system technology performs equally well in balanced and unbalanced conditions.

Eighteen-pulse drives often meet IEEE-519 requirements for total harmonic distortion current (THD) but frequently fail within individual harmonic orders, particularly the 23rd and 25th.

The Summit AFE drive offers enhanced performance not only under ideal conditions but also under conditions that exist in many field installations. AFE drives employ active “filtration” to compensate for harmonics generated by nonlinear current of the drive at any given moment. This is accomplished in the same way as noise-canceling headphones. The harmonics are read at the

input, and an equal but opposite waveform is injected that effectively reduces the harmonics to the specified levels regardless of typical imbalances in input voltage.

IEEE-519 is a buffer between the electrical customer and the supplier. Although IEEE-519 can benefit the customer in peripheral ways (primarily longer electrical equipment run-life), the cost is borne almost solely by the customer. But AFE technology has benefits not directly related to harmonic reduction.

The Summit AFE drive guarantees less than 5% THDwith all individual harmonic orders within IEEE-519 requirements under all maximum short circuit and maximum demand load current at the point of common coupling or I/Isystems. This can lead to considerable savings in electrical charges and fines.

The dual converter, filter, and expanded DC bus topology provides for a robust front end. The AFE drive, while not bulletproof, can tolerate a greater range of input power disruptions. Every power interruption leads to an ESP shutdown, lost run-time, and lost production.

Power considerations

More and more electric companies charge their commercial accounts not only on the basis of kilowatt hours used; they also charge penalties for power factor (reactive power) and peak demand. Reactive power or kilovolt amps reactive is the actual current flow in the system that cannot be used by electrical equipment. Although unusable, the system is still required to carry this additional current load, requiring larger transmission wires and transformers throughout the system. Reduced power factor increases the cost for utilities to supply power. Likewise, peak demand requires the utility to provide the capability to meet that peak demand even though the normal demand for power may be much less.

Both controlled and uncontrolled multipulse converters provide approximately 0.92 power factor at full load. Both types of converters suffer from reduced power factor when operated at frequencies less than 60 Hz and even worse when operated in less than full load conditions. Controlled converters can reach levels of 0.60 power factor.

The Summit AFE drive has a unity power factor under all operating conditions, which is an important consideration in areas where commercial and industrial electricity rates or fines are based in part on power factor. A specific power factor also may be selected (leading or lagging) to aid in improving overall power factor in the field.

Improving THD

The addition of legacy VSDs, transformers, and other equipment to a field degrades total harmonic distortion performance, eventually resulting in increased shutdowns, utility company charges/penalties, etc. AFE drives are the only IEEE-519 compliant offering on the market other than deploying 24-pulse systems. Deploying an AFE Drive will meet IEEE-519 standards for that system and improve the total field THD.