Rethink Options for Large Drivers

Variable-speed electric motors can offer significant advantages.

By Amin Almasi, WorleyParsons Services Pty. Ltd.

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Advances in high-speed electric motor technology along with improvements in the cost and the performance of variable speed drive (VSD) systems make direct coupling of a gearless electrical motor to a turbocompressor or pump worth considering for many services requiring large drivers. 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 an electric motor driver, 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 drive's efficiency generally is 15–25% better than that of typical heavy-frame gas turbine drivers. 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 electric drives always are custom engineered for an application, allowing, e.g., a turbocompressor 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).

MOTOR ISSUES
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 electric motor drive 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). VSD-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 variable-speed electric motor driver.

LCI TECHNOLOGY
Most variable-speed electric-drive systems rectify alternating current (AC) to direct current (DC) and invert DC to variable frequency AC. For a VSD 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 VSD 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 turbocompressor 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 VSD 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 drive — 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 VSD converter) 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.

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