The power company requires reduced-voltage starting to limit its line voltage dip. What type of starter is best for various-sized motors ranging from 200 hp to 2000 hp? See how a comparison of autotransformer and solid-state technologies drives the decision process.
Water pumping stations can be electrical challenges, especially when constructed in a residential location. Take the new Spring Mountain-Durango pumping facility in Las Vegas, for example. The length and voltage of the utility's power feeder circuit to the pumping station was a major problem. The utility required reduced voltage (RV) starters to limit the effects of voltage dip on other users of the feeder when the pumping facility's 2000 hp motors started. This is a normal requirement because a typical motor will draw about six times its full-load current on an across-the-line (full voltage) start. Since the full-load current of the 2000 hp motors was around 270A, an across-the-line start would be expected to draw 1620A at 4.16 kV, or 540A on the utility company's 12.47kV side of the transformer.
Which type of RV starter is best? RV motor starting methods include primary impedance, part-winding, wye-delta, autotransformer, and solid-state. For single-winding, medium-voltage (MV) squirrel cage induction motors, autotransformer/solid-state units are commonly used.
Primary-impedance starters achieve current reduction by inserting a resistor or reactor directly in series with the motor during starting. They're a sensible choice for smaller horsepower motors. Once you get over 200 hp, autotransformer RV starters also become a viable option because they cost almost the same as primary-impedance starters yet offer more features.
With part-winding or wye-delta starters, you need motors with special windings. This means a more expensive motor with a more involved design. Therefore, we decided not to use such starters on this project.
Features of the autotransformer. An autotransformer RV starter is connected so the motor is on the secondary of an autotransformer during starting. (See the simplified diagram in Fig. 1, in original article). The autotransformer has taps, to limit the voltage, applied to the motor at 50%, 65%, or 80% of full-voltage. Because the line current varies as the square of the impressed voltage, these same taps equate to 25%, 42%, and 64% of the full-voltage value of line current. The latter values reveal the advantage of the autotransformer over the primary impedance starter. Those same taps on a primary-impedance starter equate to 50%, 65%, and 80% of the full-voltage value of the line current.
Let's refer to Fig. 1 again. To start the motor, the 'M' and two-pole neutral 'S' contactors are closed. The motor feeds through the autotransformer, from the 65% tap. After a preset time, the 'S' contactor opens and the 'R' contactor closes, delivering full voltage to the motor. In newer autotransformer starters equipped with solid-state motor protection relays, the transition from the 'S' to 'R' contactor can be triggered based on current rather than time.
Advantages of the autotransformer starter include its lower relative cost and simplicity. For a 2000 hp, 4.16kV motor, an autotransformer starter costs about 66% of a similarly sized solid-state starter. Features now found with autotransformer starters include solid-state motor protection relays and vacuum contactors. While these starters may have complex equipment, they operate on a concept much simpler than solid-state starters.
Disadvantages of the autotransformer starter include its non-continuous acceleration and inflexibility. Acceleration is non-continuous because the torque developed by the motor is practically constant during the initial starting period and then changes to another value after the transition period. With the typical three taps, the autotransformer starter was historically the most flexible of reduced voltage starters until the advent of the solid-state starter. However, its flexibility pales in comparison to the solid-state starter.
Characteristics of the solid-state starter. In the last 10 to 15 years, the solid state-starter has become the workhorse of the industry for RV starting at low-voltage (up to 600V). However, MV models have only been available in the last five to seven years. Solid-state type starters (see Fig. 2, on page 44 of original article, for simplified diagram) use back-to-back thyristors for each line to the motor. These six thyristors control power to the motor. The power adjusts by not completely turning on the thyristors during starting. In other words, only a portion of the 3-phase sinusoidal wave is supplied to the motor during start.
Because of these control features, the big advantage of the solid-state starter is the large number of starting characteristics. The standard soft-start mode simply ramps the voltage from a preset initial torque value to 100% during a user-selected time of 0 sec to 30 sec.
Another available control mode is a start based on current limitation. In this mode, you select how much you want to limit the current (between 50% and 600%), and the duration (between 0 sec and 30 sec). If you try to limit the current to a level lower than required to start the motor, the motor won't start.
But there is much more flexibility using a solid-state starter than an autotransformer starter with its three taps. For example, we were pretty certain the 2000-hp motors would not start on 150% of full-load current, which would be the 50% tap on an autotransformer starter. We did expect them to start on 384% of full-load current, which would be the 80% tap on an autotransformer starter. However, we didn't know if they would start at 254% of full-load current, which would be the 65% tap.
With a solid-state starter, we can attempt a start at 300%, and move either up (to 310%) or down (to 290%) as necessary. With an autotransformer starter, the jump would be all the way to 384%.
Other available operating modes include kickstart, soft stop, and pump control options. The last option starts a pump motor on a curve rather than a straight line ramp. This causes the hydraulic system to react as if there were a closed discharge valve behind the pump, opening during starting. We did not use that starting mode on this project because of hydraulic reasons. For the Spring Mountain-Durango pumping facility, a pump control valve is required on each pump anyway, so there's no need for the pump control option.
There's another advantage in using a solid-state starter: Larger sized units (over 1000 hp) take up less floor space than their autotransformer counterpart. That was an important factor for this project because we installed the electrical equipment in a building on the roof of the pump station. The smaller this building, the less detraction to the neighborhood.
The major disadvantage of the solid-state starter is its higher relative cost. For larger sizes, these units cost 150% of an autotransformer starter. Another disadvantage is complexity. Finding experienced maintenance workers capable of troubleshooting a thyristor problem is much more difficult than finding someone to change out an autotransformer. For smaller solid-state units (200 hp), finding a competent maintenance staff is not difficult.
We recently began testing one of the 2000 hp motors and found it would start at the 250% current limiting setting on the starter. At this setting, the motor reached full speed in 10.8 sec, with a maximum recorded current of 510A. (The normal full-load current is 270A.) We're considering using the standard soft-start ramp mode and exploring what time period is best for minimal starting current. Because of the flexibility in these solid-state starters, we can find the optimum starting mode that will satisfy the power company while minimizing stress on electrical and mechanical components.
For the Spring Mountain-Durango pumping station, the solid-state starter's additional benefits outweigh its additional cost. But, this certainly isn't true for all projects. First, the starter's ability to limit starting current as much as possible is very important-enough to convince the local power company to quickly provide 12.47kV service to the location. This helps maintain our fast-track construction schedule. Second, the starter's smaller size helps reduce the size of the electrical building. This, in turn, reduces the visual impact of the pump station on the neighborhood.
With continual advances in semiconductor technology, lower-cost thyristors for use at 5kV are almost a certainty. Thus, solid-state starters for MV applications may become as economical as the autotransformer starter.