By Oleg V. Vasilev
A long-term program to retool the complex and upgrade 22 generators of the 23 units is under way at the 2,639.5 MW Volzhskaya hydropower plant in Russia. This facility, owned by RusHydro, is on the lower Volga River in the southeastern European portion of Russia.
The first unit at this plant began operating in 1958, and the plant was fully operational in 1961. The plant was in need of an upgrade so the company could replace the obsolete and worn-out generating and auxiliary equipment.
Work is ongoing, with implementation that began in 2005 and all work expected to be complete in 2020. As a result of this RUB10 billion (US$312 million) program, reliable operation of the plant will increase and additional generating capacity will be available.
Volzhskaya is the largest plant in the 10 GW Volga-Kama Cascade and is also the largest hydro plant in Europe. When the plant was originally built, 15 of the units were 115 MW, four 120 MW, three 125.5 MW and one 11 MW. The 11 MW unit is not scheduled for modernization at this time. Overall, the upgrade and modernization activities will increase rated capacity by 203.5 MW.
In 2011, OJSC Power Machines began producing replacement generators.
RusHydro is replacing 22 generators at Volzhskaya with new generators being manufactured based on the mounting dimensions of the existing generators. This article covers primarily the work being performed to replace the generators. As of October 2015, five of the generators had been replaced. All of this work is expected to be complete by 2021.
The design process for the upgrade includes using modern technology to reproduce pre-stressed conditions in the core. By creating differences in stator core and frame temperatures, Power Machines was able to obtain improved stator core shape, which reduced unit vibration.
This operation is accomplished in the following way: The key bars are fastened to the frame by using temporary ties, then the core is stacked and pressed, whereupon the core is heated. In doing so, by this design value, the core displaces externally in a radial direction relative to the frame.
In this position, the key bars are welded to the stator wings. When cooled down, the core appears to be expanded, while the frame is compressed. This permits lowering stress levels that occur in the core during generator operation. Thus, stator core “waviness” is reduced.
The new design of the stator attachment to the foundation prevents the stator from being turned due to torque both under the rated- and emergency-conditions. It also permits the stator to expand during heating. For this purpose, the radial pins are placed in sole plates, which permits stator core expansion during heating.
|At RusHydro’s 2,639.5 MW Volzhskaya hydropower plant in Russia, while atop one of the 22 generators being rehabilitated, engineers from RusHydro and OJSC Power Machines discuss progress on the plant’s modernization.|
In the existing generator design, the sole plate was welded directly to the stator frame lower ring with the help of pins. Due to welding heat effects, stator frame lower ring buckling (warpage) occurred. The warpage prohibited successful radial pin placement during the assembly process. In the new design, the sole plates are fixed to each other using pins. This allows successful radial pin placement prior to welding them to the stator frame.
In a departure from the usual practice of using a pieced stator frame assembly, the assembly was welded together to form a ring. This type of design for this portion of the generator does not use joint plates. This allows reduced stator frame weight and requires about 2.5 months to construct. That provides for fewer hours of labor to manufacture compared to the usual practice, which could add several hours for production considerations.
The generator’s rotor spider central part has also undergone changes. In the existing design, manufacturers used induction shrink fitting to attach the rotor spider. In the new design, the generator shaft is not an integral component part and consists of two parts. The upper portion is the extension shaft, and the lower section is the intermediate shaft. Both parts are bolted to the rotor spider central part. This design change facilitates the part’s assembly, which makes manufacturing more cost-effective.
In the new process, rotor arm design is fundamentally changed.
New generators feature customized box-type arms constructed using new designs that call for rotor arms attached to the central part by means of joint plates. This design avoids imparting stress to metals when using hot keying procedures to create threads for fastening elements.
Before this design change, existing generators used I-shaped arms attached to the rotor spider central part by means of tapered studs. Using tapered studs this way is labor-intensive because during the manufacturing process, technicians must drill holes and then they must ream joint holes in the central part’s discs that allow them to install tapered studs.
Moreover, the use of tapered studs restricts the execution of the rotor rim hot keying procedure because during the hot keying process, impermissibly high stresses are placed on the tapered studs.
Volzhskaya generator rotor arms and rim laminations are identical to those used at the 2,372.5-MW Zhigulyovskaya hydroelectric facility, also part of the Volga-Kama Cascade.
By using rotor arms design similar to the project, Volzhskaya’s generator rotor design placed a restraint on the rotor rim height, and this dimension should not be changed. But, instead of reducing the rotor rim height, engineers decided to decrease the rotor rim’s mass by use of a false ventilation package – a package of conventional ventilation channels for the passage of air.
|OJSC Power Machines personnel work on a generator in the Volzhskaya hydroelectric plant’s powerhouse as part of owner RusHydro’s rehabilitation program for the facility.|
Updated pole coil brazing technology includes using silver-bearing brazing alloy to improve pole coil manufacture, as opposed to the old method that used welded special-purpose electrodes on the coils.
New ventilation design for generators applies a one-side, closed-circuit ventilation system. This ensures the lowest aerodynamic resistance at the inlet to the rotor arms, which excludes the possibility of oil vapor and brake dust ingress into the space occupied by the generator’s active components.
This is attained by mounting a closure on the lower surface of the arms. As a result, the air flow from the cold air chamber is directed along the path with the lowest resistance between the upper-bracket arms, to the space between the rotor arms.
Modern environmental requirements call for asbestos-free materials used to improve generator reliability during generator construction. Asbestos-free materials are used in the brake shoes, which are fastened to the generator brakes, and in the turn-to-turn insulation of the poles.
The guide-bearing bushing and segments are fitted with holes that create directional flow of oil into the friction zone, thus improving the ordered motion of oil in the guide-bearing oil reservoir and permit reduced oil vapor beyond the oil reservoir through the use of installed shields that calm the surface of the oil and prevent spattering.
Using polytetrafluoroethylene-coated thrust-bearing pads and guide-bearing segments makes it less difficult to manufacture these components, and using this material improves its operation. By using PTFE-coated material, without raising the generator’s rotating parts by applying brakes/jacks, generator assembly and startup is considerably accelerated and simplified. For example, operations for installation are reduced to two weeks.
Using U-shaped oil coolers in the oil reservoir improves oil cooler reliability and makes its assembly and operation more convenient. Additionally, their application means the oil reservoir structure is a single, undivided unit. The U-shaped oil coolers also elicit increased heat removal indices compared to the current design.
The basic design solutions listed above not only ensure obtaining specified electromagnetic parameters, they also provide the hydro generator operational reliability and convenience.
Sensors placed on the generator monitor the temperature in the segments of the thrust bearing, guide bearing, oil, and the stator winding. In addition, there are oil level sensors and vibration sensors of the stator core, thrust bearing and guide bearing.
Oleg Vasilev is an engineer for OJSC Power Machines in generator design.