Rethink Options for Large Drivers

Variable-speed electric motors can offer significant advantages.

By Amin Almasi, WorleyParsons Services Pty. Ltd.

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A harmonic filter is connected to the network or to a third secondary winding of the system transformer. The choice mainly hinges on cost and usually depends on the network voltage level. For 33 kV and below, the connection most often is on the network. For 110 kV and above, a third secondary winding generally is selected. In between, the decision must be made on a case-by-case basis. (The design and manufacturing of large power transformers with three secondary windings is a difficult technical challenge; only a limited number of manufacturers are capable of implementing such designs.)

To minimize the harmonics effect (particularly on the electrical network), large LCI systems usually have 12-pulse topology. Even if an LCI system has multiple pulse rectifier configurations to reduce the harmonic current level emission, the reactive power consumption of the LCI rectifier may require use of a power-factor compensation system (usually a capacitor harmonic filter). In LCI-type converters, the harmonic excitation generates a constant nominal flux in the motor air gap, which could result in train mechanical excitations.

The main issues related to the harmonic filters are:

• sizing (which requires extensive data about the entire electrical network);
• possible running without one harmonic filter rank; and
• switching a filter during normal operation and over-compensation at special operating points.

Harmonic studies should provide drive output current spectra, harmonic details (order, amplitude and phase), and how these vary with the compressor train speed. In multi-drive installations, the superimposition of individual harmonics and the sizing of harmonic filters for an entire installation require special calculations and simulations. Calculated harmonic levels must be compared against standard limits (for example, those in IEEE 519). The harmonic study contains two parts: one dedicated to calculating the electrical natural frequencies, and the other aimed at minimizing the harmonic distortion to optimize the design of the harmonic filters. The study should determine potential resonances in the entire system. Power generators usually give rise to some harmonics that could interact with VSD systems. Restrictions should be imposed on train torque ripple (usually under 1–2% peak-to-peak) to preserve the torsional stability. The THD of the line-side voltage should be within certain limits (most often 2–3%) to minimize disturbances to the other electrical loads connected to the same plant electrical network.

VSI TECHNOLOGY
LCI technology suffers from some well-known drawbacks — e.g., high torque ripple, poor power factor, relatively high losses and harmonic pollution. These disadvantages can make LCI-based variable-speed drives inadequate to reach the increasingly demanding performance required in some applications. In such cases, a VSI may provide the solution for turbocompressors and pump drivers.

Indeed, quadruple-star four-pole synchronous motor technology fed by four pulse-width modulation (PWM) multilevel VSIs is getting considerable attention. Based on today's targets for low torque ripple and low harmonic distortion (particularly low grid-side harmonic pollution), the PWM-VSI-based variable-speed drive design has been selected for several large turbocompressor projects. A cascaded multilevel converter topology usually is chosen. Each converter phase is obtained by series connecting several transistor cells. The choice of this topology makes it possible to attain some important goals like:

• voltage output (converter output to an electric motor) that approaches the sinusoidal waveform as the number of cells is increased — providing the possibility of operating the electric motor at a near-unity power factor;
• tolerance to single cell faults by implementing a faulty-cell bypass function; and
• low harmonic injection.

In fact, with LCI-based drives, having more than two supplying converters may be theoretically feasible, although this may pose commutation overlapping issues.

In the PWM-VSI technology four converters commonly are used. The decision to supply the electric motor with several (four or more) three-phase converter units naturally leads to the splitting of the stator winding into independent three-phase sets, each to be fed by a converter. The stator design needed for this purpose often is referred to as "split-phase" because it results from splitting the winding into multiple star-connected three-phase sets. The most common arrangement uses four converters; the associated electric motor design is known as quadruple-star winding. The phase currents contain harmonics of orders 5, 7, 11, 13, 17 and 19; all the resulting space harmonic fields in the electric motor air gap are very low because of the mutual cancellation effects.

Today, turbocompressors and pumps may benefit from a new electric drive option based on a VSI-fed quadruple-star 100-Hz four-pole synchronous electric motor. Compared to traditional LCI-based options, it provides particular advantages:

• torque ripple typically lower than 1–2% peak to peak;
• very low vibrations owing to the four-pole design;
• high fault tolerance due to the four-star four-converter topology; and
• high electric motor efficiency, usually above 98%.

Because of the large number of phases (12) and the four-pole design, even for high power levels the stator winding can be done with coil technology (instead of complex/expensive "Roebel" bar construction) with noticeable manufacturing and cost benefits. The 100-Hz supply frequency doesn't give excessive core losses. Stator phase currents may show fifth and seventh current harmonic distortions as a consequence of the electric motor internal electromotive force. However, these harmonic distortions don't negatively impact torque performance. The design also could be scalable to relatively high power levels (above 50 MW) by increasing the number of supplying converter units and possibly expanding the electric motor driver size.

OTHER CONSIDERATIONS
Transformers play an important role in any VSD system. Inrush current limitation requirements and protection philosophies of transformers are important.

A VSD electric motor system employs various cooling water pumps. A cooling pump's normal operating point should be as close as practical to the pump's best efficiency point (BEP). Rated cooling flows preferably should be within 20% of the BEP flow. The cooling-pump characteristic curve is very important for a trouble-free, smooth and proper operation. A cooling pump curve should exhibit the characteristic of stable continuously rising head from the rated capacity to the shutoff (preferably 10% head rise from the rated to the shutoff).

Typically, a VSI system's footprint is less than 75% of that of a comparable LCI system. In addition, it usually weighs less than 70% of a comparable LCI system.

UNDERLYING ADVANCES BOOST MOTORS
Some major developments have made large (>20 MW) electric motor drivers possible:
• better understanding of rotor dynamics, advanced balance technologies and, more importantly, use of advanced bearing options;
• progress in materials such as high-tensile steels for motor high-stressed and critical components; and
• advanced finite-element analysis, methods, e.g., for advanced electromagnetic calculations, and improved analytical approaches to predict electric motor performance parameters.

 


AMIN ALMASI is lead rotating equipment engineer at WorleyParsons Services Pty. Ltd., Brisbane, Australia. E-mail him at Amin.Almasi@yahoo.com.

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