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Considerations in choosing large Variable Frequency Drive

Advances in high-speed electric motor technology along with improvements in the cost and the performance of VFD (variable frequency drive) systems make direct coupling of a gearless electrical motor to a pump worth considering for many services requiring large VFDs. Brushless synchronous motors with two-pole rotors often suit high performance duties. Special applications may benefit from other options such as induction electric motors.

When using a VFD, full power is available instantly over the entire site ambient temperature range and train speed range (including startup). The number of successive and cumulative start/stop and load cycles generally isn't critical.

Variable-speed electric motors in the upper-megawatt power ranges (say, over 20 MW) usually have energy efficiencies exceeding 97% over the entire useful speed range (typically 70–105% of the rated speed). In a combined-cycle power plant, the electric VFD's efficiency generally is 15–25% better than that of typical heavy-frame gas turbine VFDs. In addition, some of today's electric motors don't need scheduled maintenance for periods of up to 6 years of continuous operation and even after that don't require replacement of costly parts.

Large VFDs always are custom engineered for an application, allowing, e.g., a pump to be optimized in capacity and speed for the process, rather than being limited by a given gas turbine rating. The rotor design and overall features of the motors closely match those of electrical generators; design and manufacturing of large (over 100 MW) generators is well established, and numerous units are operating successfully. However, motors are variable speed while generators usually are constant speed, and motors suffer from oscillating shaft torques during operation (particularly when starting).

When designing large high-speed electric motors, mechanical and dynamic problems must be solved carefully. Mechanical stresses, vibration level, losses and cooling restrictions can limit the capacity and the maximum speed of a large electrical motor.

In any high-speed VFD application, mechanical excitations, electrical pulsations, rotor dynamics issues, balance problems and mechanical-dynamic considerations in general are of paramount importance in ensuring a smooth-running rotating train over the entire speed range and during all normal and transient operations. Also, prior to ordering, it's essential to know the behavior of the train during any electrical fault conditions (the most severe probably being a short circuit at the electric motor terminals). variable frequency drive-fed electric motors continuously produce some small torque oscillations over the entire speed range. So, the design phase should include careful analysis of the effects of such torque pulsations, along with other excitations, particularly torsional ones.

A large electric motor requires a complex and heavy rotor assembly. For example, the assembly can weigh 6–35 tons for 20–120-MW units. Balancing such a rotor assembly is an extremely difficult job. (Some expensive assemblies actually have been scraped after many unsuccessful attempts to balance them.) Coarse balancing of an electric motor rotor usually gets to within 0.015–0.03 mm of mass center offset; final balancing for some high-speed electric motors, for example, may require getting to within around 0.002 mm of mass center offset. Advanced systems such as active magnetic bearings also could be used to further improve the VFD.

Most VFD systems rectify alternating current (AC) to direct current (DC) and invert DC to variable frequency AC. For a variable frequency drive system with a rated output of over 60 MW, two popular and field-proven inversion options are a load commutation inverter (LCI) and a gate commutated turn-off thyristor (GCT). Other options, such as a voltage source inverter (VSI), may not be mature enough for rating above 60 MW. A grey area where both VSI and LCI technologies are feasible exists between 30 MW and 60 MW.

Today, LCI technology is the most popular variable frequency drive converter system. It's a mature technology; disadvantages and solutions to minimize its problems are well known. It commonly is teamed with dual-star two-pole synchronous motors with supply frequencies between 50 and 80 Hz.

If the electric power supply is interrupted (for example, due to a temporary problem in a generator or a power transmission malfunction), the pump or other driven equipment will decelerate rapidly and may trip a protection system (e.g., for lubrication oil low-pressure or anti-surging). This may prevent the unit from re-accelerating when power is restored. So, all protection system issues deserve detailed study.

The main issues for the variable frequency drive converters are:
  • size (very important);
  • redundancy of the equipment;
  • control system details (alarms, diagnostics, reliability, etc.);
  • guarantees for disturbance "ride through" capability;
  • harmonic mitigation, harmonic filter and torque oscillation; and
  • converter cooling-system requirements.
To provide "ride through" capabilities — a standard feature for an LCI converter with synchronous motor VFD — a secure uninterruptible power supply should back up the power to the control system. The arrangement and layout of the converter system should prevent a domino effect (i.e., the loss of one part shouldn't disturb other parts as far as practical).

Like any nonlinear system, a frequency converter produces harmonic currents. Therefore, conducting a harmonic study (and usually providing a harmonic filter package) makes sense. The analysis should look at the complete electrical network (including variable frequency drive) over the entire frequency spectrum, calculating the voltage total harmonic distortion (THD) under all system operating and upset conditions. Usually, a network short circuit when the system is under no-load (or the minimum load) conditions constitutes the worst case.

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 VFD output current spectra, harmonic details (order, amplitude and phase), and how these vary with the compressor train speed. In multi-VFD 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 variable frequency drive 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.

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 VFDs inadequate to reach the increasingly demanding performance required in some applications. In such cases, a VSI may provide the solution for pump VFDs.

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 VFD design has been selected for several large pump 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 VFDs, 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, pumps may benefit from a new VFD 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 VFD size.

Transformers play an important role in any variable frequency drive system. Inrush current limitation requirements and protection philosophies of transformers are important.

A variable frequency drive 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.

Some major developments have made large (>20 MW) VFDs 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.

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