The Lower Colorado River Authority (LCRA) operates six hydroelectric powerhouses and dams in central Texas. At one of the six powerhouses, 110-MW Marshall Ford, LCRA battled problems with condensation forming on the generator stator coolers. By installing new butterfly valves, electric actuators, and digital controllers on the three units ’ cooling water systems, LCRA can control temperatures and thus eliminate condensation.
What caused the condensation
A variation in the pressure and flow of the water used to cool the generators as well as in humidity in this area of Texas created condensation in the equipment.
The project ’s lake, Lake Travis, is formed by the 266-foot-tall Mansfield Dam. Water stored in Lake Travis is used to supply water for municipal, industrial, and irrigation needs to generate electricity. The project also provides flood control.
The lake level can fluctuate 20 to 30 feet in a normal year and more than 70 feet under drought conditions. This fluctuation results in a variation in both the pressure and flow of the water used to cool the generators in the powerhouse. However, the temperature of the water in not affected. The cooling water system withdraws water deep in the lake, so water temperature remains in the 55- to 65-degree Fahrenheit range, even with the level fluctuations.
The project is located on the edge of the Texas Hill Country and the Coastal Plains. Warm humid air from the Gulf of Mexico can cause humidity to vary, sometimes daily, from 20 to near 100 percent. With changes in the weather, condensation formed so heavily on the generator stator coolers that it literally seemed like water was raining off the cooler tubes inside the generator! Needless to say, this was an undesirable operational condition.
Attempts to solve the problem
In the early years of operation, the plant was staffed with a control room and auxiliary operator. To prevent condensation during operation, the auxiliary operator manually opened the cooling water valves upon start up, then periodically adjusted them to maintain a sufficient temperature rise. At the time, the units were started and stopped daily for irrigation releases, peaking generation, and provision of voltage ampere reactive (var) support to the transmission system via synchronous condensing. While periodic adjustment worked fine once the machine was warm, condensation was a problem for a period after start up until the generator warmed up.
In the early 1970s, the operations staff was reduced to one man per shift. Air-operated butterfly valves were installed in the generator cooling system; the manual valves were left in the system for making periodic adjustments. However, the one operator was tied to control room duties and thus was not available to make the adjustments to the valves. And, maintenance personnel were only available intermittently – and not always available on a set schedule – to make the needed adjustments.
Over the years, LCRA staff made various attempts to resolve the condensation problem. For example, staff installed electric strip heaters to keep the generators warm while off line. Also, they attempted sealing all openings in the generator housings to minimize the induction of additional humid air. While these minor changes helped, condensation was a continuing problem.
As the original generator asphalt coils aged, it became difficult to maintain an acceptable polarization index (PI) of 2 or higher on the stator coils, especially with the intermittent moist generator air.
The way in which LCRA operated the generators also was changing. When not required to generate/release water, the units were being run less for provision of var support and more for spinning reserve. This operational change caused even more of a condensation problem. In the var mode, the generation excitation is operated at a high setting, which, in turn, produces enough heat to keep the unit above the condensation point. However, in the spinning mode, the generator excitation is loaded very low, so there is minimal heat produced.
Finding the answer
In 1999, LCRA began a major refurbishment of the Marshall Ford powerhouse. As part of the refurbishment, the generators were uprated to 34 megavolt amperes (MVa) from 25 MVa by rewinding the stator. And, LCRA decided to replace the generator coolers and piping system owing to corrosion and leaks. The replacement included new butterfly valves and electric actuators. This presented an ideal opportunity to resolve the generator condensation problem.
Initially, LCRA planned to control the new valves and actuators with the programmable logic controllers (PLC) installed in 1990 for unit remote control. Upon further investigation, the utility realized this would require some complex PLC programming.
As an alternative, LCRA ’s engineer and control technicians began investigating the idea of using a stand-alone valve controller. After looking at several of the new digital controllers, they chose to use a Honeywell UDC 3000 Universal Digital Controller. This controller accepts multiple set-points and can utilize two digital and two analog inputs.
The controller was instrumented to monitor a thermocouple placed on the hot air (inlet) side of each of the two coolers for each unit. These two thermocouples are connected to provide one temperature input point to the controller. The controller uses this temperature input as the monitored point to keep the air inside the generator above the temperature that will cause condensation.
The controller is set for two operating modes. A digital (dry contact) input indicates if the unit is operating in the spinning reserve or generating mode. In the spinning reserve (synchronous condenser) mode, the valve control set-point is 140 degrees Fahrenheit (F). In generating mode, the temperature set-point is 110 degrees F. These set-points are the temperatures the air needs to be inside the unit in order to not condense the moisture out of the air. The set-points also ensure the generator receives proper cooling. LCRA established these temperature set-points through years of operating experience.
The controller has a “feed forward ” function connected to an analog MW input from the generator controls. When the controller is in the generating mode, it uses changes in the analog MW signal to help anticipate increases or decreases in cooling requirements. As the analog MW signal changes, the controller ramps the valves further open or closed to meet this anticipated need for more or less cooling water to the unit.
If the digital controller ever fails, it also has a built-in manual feature that allows the valve actuators to function in a full open or full close mode.
The stand-alone valve controllers provide many pre-programmed features, such as temperature set-points, operational ranges, and feed forward rates, which can be set without spending lengthy programming time.
– By James E. Guenther, Dam and Hydro Maintenance Coordinator for LCRA, P.O. Box 8, Buchanan Dam, TX 78609; (1) 512-793-3003; E-mail: [email protected]
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