VFD retrofit provides 3.75-MW boost to air-cooled Wyodak Power Plant
By Paul M. Fahlsing,
PacifiCorp Power Supply
Considered uneconomical when the plant was built,
technology advances eliminated VFD operating problems
The traditional benefit of installing ac variable-frequency drives (VFD) includes power savings at partial speed operation. In the case of forced-draft, air-cooled condenser fans at power stations and cogeneration facilities, this typically occurs during mild ambient temperatures. However, a VFD retrofit last year at the Wyodak Power Plant in Gillette, Wyo., resulted in significant process improvements at temperature extremes. In fact, of the 3.75-MW average capacity increase achieved from the VFD installation, only 25 percent is a result of power savings at partial speed operation.
The 330-MW Wyodak power station, operated by Pacific Power & Light Co. and Black Hills Power & Light Co., was built in 1978 and has the largest air-cooled steam condenser in the country. With the nearest river about 40 miles away, air-cooled technology allowed the Wyodak Plant to be mine mouth sited essentially on top of a seam of low-sulfur coal. Total plant water consumption is less than 300 gallons per minute (gpm), with the scrubber using about 250 gpm and the boiler about 50 gpm. A water-cooled condenser requires about 3,000 gpm for the boiler alone.
The air-cooled condenser is 164 feet wide, 360 feet long and is elevated 60 feet above the ground. Sixty-nine fans provide a forced draft of 57 million cubic feet per minute at full speed. The condenser consists of 12 fan rows with six, 21-foot diameter fans per row. One experimental row contains three 33-foot diameter fans, each providing cooling to a condenser tepee, containing finned-tube panels assembled in an equilateral triangle. Exhaust steam is condensed inside elliptical carbon-steel finned tubes.
Steam exhaust from the turbine is fed to 23 condensing groups of three fans each. Within a single condensing group, steam enters the two top parallel flow sections, called K-sections, which condense 75 percent of the steam. The remainder is fed to the bottom of a counterflow section called the dephlegmator or D-section. A vacuum is drawn on the top of the D-section to remove non-condensables.
The fans were supplied with two-speed motors, giving three control options–off, half speed or full speed. A computerized control system was provided, but plant operators were forced to use manual fan control methods. Condenser control was difficult and time consuming.
Hot and cold
The air-cooled condenser has historically limited the output of the plant at temperatures above 85 F or below 33 F. The General Electric turbine and GEA condenser were designed to operate at 6 inches of mercury back pressure (6 in. HgA) at 66 F, and with a maximum back-pressure of 15 in. Hg. While this is less efficient than a water-cooled condenser, which operates typically at 1 to 2 in. HgA, the mine mouth-sited Wyodak plant can afford to make up the difference by burning a little extra coal. The greater the difference between back-pressure and local atmospheric pressure, 25.3 in. HgA, the more kilowatts the turbine delivers per pound of steam.
During high ambient temperatures, the condenser`s capacity was reduced, requiring turbine load shedding to prevent unit tripping from a vacuum loss. When ambient temperatures fell below freezing, turbine back-pressure needed to be elevated to prevent freezing and subsequent tube damage. Operation at average back-pressures of 9 in. Hg was not uncommon during cold weather.
When the Wyodak plant was built, VFDs were considered cost prohibitive and the technology not fully developed. However, during a 1992 fan blade retrofit and testing, plant personnel became aware of possible VFD benefits. Wyodak then purchased one VFD and began testing on one fan, proving VFD benefits. Drives from six other manufacturers were then tested on a demonstration basis for six months, allowing the operators to gain hands-on experience with various manufacturers` equipment.
At the end of the test, Wyodak purchased 70 VFDs from ABB Industrial Systems Inc.–66 drives of 125 horsepower (hp) (plus one spare 125-hp drive) and three drives of 350 hp. Key selection criteria were small size and the ability to reduce harmonic distortion. Size was important because the VFDs were installed in the existing motor control center, after the previous high-speed/low-speed motor starters were removed. ABB`s ACH 500 drive is 18 inches x 39 inches x 15.3 inches for the 125-hp model.
The rectifier section of the VFDs can generate high levels of both current and voltage distortion on the electrical system. These distortions, known as harmonics, can increase heating of the conductors and transformers supplying power to the VFDs. Current distortion is the major culprit here, and ABB estimated 25-percent total harmonic current distortion at full-drive load. The supply conductors could handle the load, but the transformers, marginally sized to begin with, were not adequate. Six transformers step down from 7,200 V on the two main plant buses to 480 V for use on the condenser.
In a new VFD installation, harmonic-rated transformers would have been provided. For the Wyodak retrofit, however, new transformers would have caused the project to be uneconomical. Fortunately, a 25-percent gain in transformer rating was obtained by force ventilating the existing transformers. Still, five of the six transformers were marginally overloaded.
Two options were available to provide margin on the overloaded transformers. The first, and more expensive, was to install two additional transformers to unload the existing ones. The second option, the one implemented, was to install trap filters on 25 percent of the VFDs. This reduced total harmonic current distortion to under 20 percent, which is an acceptable level.
Other valuable features of the ABB drives include user-friendly, English operator displays, eliminating the need for a code book to communicate with the VFDs, and their ability to run on an RS-485 network. RS-485 communication has enabled Wyodak to monitor drive status and serves as a manual backup in case of a malfunction in the control room.
The VFDs were installed on the original motors` high-speed windings with current limited to full nameplate motor amperage. Output frequency from the VFD is limited between 12 and 68 Hz for forward operation and between 12 and 60 Hz for reverse operation.
All 69 VFDs were installed in two motor control houses. Since summer ambient temperature routinely exceeds 140 F, and since the VFDs generate an internal heat load of 240 kW at 97-percent efficiency, a fully redundant air conditioning system was installed. Total cooling load was 80 tons.
Installation started in the spring of 1994. Using breakers to disconnect three fans at a time, 50 percent of the VFD retrofit was completed with the condenser on-line. While the project could have been completed entirely on-line, the remaining 50 percent of the VFDs were installed during a planned overhaul. The project took four months to complete.
The VFDs for the condenser fans are automated, controlled by the plant`s distributed control system. Manual control is provided by six ABB DPT500 remote display units. The DPT500 units, which are similar to the key pad control on the VFDs, can control VFDs individually or in groups. The remote display units also provide the operators with full alarm information.
As ambient temperatures increase, the air thins, decreasing the horsepower requirements for a fan operating at constant speed. By allowing the VFDs to over-speed 13 percent, well within the fan`s speed limit, the VFD can maintain maximum motor horsepower utilization during variations in air density. Wind conditions also can vary fan loads on a condenser such as Wyodak`s by 25 percent or more for a constant speed. The VFDs also take wind speed variations into consideration to keep the fan motors operating at maximum horsepower.
Before the VFD retrofit, the steam into the turbine sometimes needed to be throttled down during very high ambient temperatures. The condenser could not work fast enough, and the back-pressure would have exceeded 15 in. HgA.
Since the retrofit allows over-speeding and extra cooling, the net output of the plant has increased by more than 28 MW during high ambient temperatures. With VFDs, the turbine has operated at valves wide open at temperatures above 90 F.
During mild ambient temperatures, 70 F to 32 F, classic VFD benefits are achieved. Prior to VFD installation, back pressure was controlled by stepping fans from half to full speed. During step up, the fans over-blow and over-consume power. Today, power savings of up to 0.6 MW can be achieved using the VFDs to ramp up all fans at equal speeds.
The VFDs also made the development of a back-pressure optimizer possible. It is used to keep the system operating at a back-pressure setpoint that maximizes power generation. The optimizer monitors and compares fluctuations in generator output and fan power. If the generator output fluctuates more than the fan load, the fans are under-blowing. As a result, the back-pressure setpoint is reduced. If the converse is true, the setpoint is increased.
Below 33 F ambient temperature, the condenser enters a freeze protection mode. All of the K-sections are operated as a unit to control back-pressure at the desired setpoint. Condensate subcooling in each K-section drain header is monitored. If subcooling is detected, the fan speed of the respective K-section is reduced. If the condensate reaches a low alarm point, the fan group will trip off.
Prior to VFD installation, the majority of freeze damage occurred in the bottom layer of three layers of tubes in the D-section. In situations where cold weather combined with insufficient cooling, steam would hit the top header in the D-section, condense and then flow back down through the bottom tubelayer. The back-flowing water was over-cooled and could freeze, which sometimes caused the tubes to burst.
In what seems a contradiction, the problem was solved after VFD installation by speeding the fans to increase the cooling rate, rather than running slower to warm the tubes. This method of control condenses all steam prior to reaching the top tube sheet, and it prevents steam/condensate from carrying over into the lower tube levels. As a result, average back-pressure has been reduced by 3 in. HgA and net output increased by 12 MW.
Rime ice eliminated
One of the major benefits of installing VFDs is the ability to eliminate rime ice. Rime ice, a fluffy white frost, occurs when moisture freezes inside the D-section tubes. It does not cause immediate tube damage, but it does plug the tubes and cause inefficiency. Left unchecked, rime ice places the K-section at great risk of freezing.
The past control system, a 1970`s computer, utilized a control scheme that attempted to break up rime ice by fluctuating fans from high speed, to off, to half speed and back to high.
This worked poorly, so the operators spent up to 75 percent of their time during subfreezing temperatures manually controlling 69 fans.
The new system uses two thermocouples to automatically detect and melt rime ice. If the tubes are clear of rime ice, an upper level thermocouple will be at ambient temperature and a lower level thermocouple will be warm. As rime ice develops, the lower level thermocouple will cool, triggering the D-section fan to reverse and pull preheated air from above the condenser through the D-section, melting the ice. The fan then switches to forward operation.
Installation of VFDs on the Wyodak plant`s air-cooled condenser yields benefits far greater than typical power savings. For anyone examining a fan/VFD retrofit or new installation, look for potential process improvements. Of the 3.75-MW capacity increase achieved as a result of VFD installation, 75 percent is due to process improvements. Operational efficiencies should also be considered.
The actual benefits of the VFD installation were 13 percent greater than projected. The net result has been an increase in generation at an installed cost less than $400/kW. Below 85 F, the generation is achieved with no additional fuel, and the greatest VFD benefits occur at ambient temperature extremes, when power demand is greatest. z
Paul Fahlsing is a registered professional engineer for the state of Wyoming and a member of the American Society of Mechanical Engineers. Fahlsing has been with PacifiCorp Power Supply at the Wyodak Plant since 1986 and oversees all plant maintenance and system upgrades. Prior to Wyodak, Fahlsing worked as a field engineer for Dresser Industries. Fahlsing received a bachelor`s degree in mechanical engineering in 1981 from Rose-Hulman Institute of Technology.
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In the spring of 1994, the Wyodak Power Plant in Gillette, Wyo., underwent a VFD retrofit, resulting
in a 3.75-MW capacity increase, mainly attributed to process improvements.
The VFD retrofit allows Wyodak`s air-cooled condenser to maintain energy efficiencies at low, mild and high ambient temperatures.
The 69 VFDs were selected to satisfy Wyodak`s two main criteria–small size and reduced harmonic distortion capabilities.
The VFDs can be programmed remotely using the ABB DPT500 remote display units, as the author of this article demonstrates.