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Construction of the 1,224-MW Tongbai project in China required development of four reversible pump-turbine units. To achieve the optimum design for this facility, contractor VA Tech Hydro employed a variety of techniques, including using computational fluid dynamics modeling to determine the profile geometry and performing numerical flow simulation of different operating points. The units now are operating reliably.
By Hu Zhongqi, Tang Yibo, Peter Nowicki, and Manfred Sallaberger
In countries with growing economies, the increasing demand for energy calls for construction of new power plants. Within a country's national grid, pumped-storage facilities are of major importance because of their ability to equalize power supply and demand. Pumped-storage plants are able to provide balancing power for and help to control the grid voltage and frequency. Furthermore, they can respond within seconds to stabilize the electrical grid during periods of rapidly changing demand. Pumped storage represents a mature technology for storing electrical energy during periods of low demand.
Many pumped-storage projects have been completed in China in recent years. For example, the 1,224-MW Tongbai project began operating in 2006. The first unit at 1,020-MW Zhanghewan began operating in May 2008, and the entire project was operational by the end of 2009.
VA Tech Hydro received a contract to supply pump-turbines and other equipment for Tongbai in December 2001. This contract required a completely new hydraulic design to meet the high demands in guarantees and mechanical design. The company employed a variety of techniques to achieve the new design for this facility. Since the units were installed, they have operated reliably.
Background on Tongbai
The Tongbai project in Zhejiang Province was built to increase generating capacity and improve load regulating capability. The development uses an existing upper reservoir and involved construction of a new lower reservoir and an underground powerhouse. The powerhouse contains four 306-MW reversible pump-turbines at a net head of 244 meters. Two penstocks each supply two units. A tailrace tunnel leads from each unit to the lower reservoir. Each tailrace tunnel is equipped with an emergency gate.
The equipment to be supplied by VA Tech Hydro included four reversible pump-turbines, including valves and motor-generators; digital electronic speed governors; additional equipment such as excitation, digital protection, the computer supervisory control and data acquisition system, and the static frequency converter; and the main transformers, including high-voltage cables and auxiliary systems. Table 1 provides a summary of technical data on the pump-turbines.
Determining the hydraulic layout of a project is a key process in the design. The hydraulic layout must define the hydraulic characteristics and main dimensions of the equipment in order to best meet the essential requirements specified by the project developer. Figure 1 shows the range of pumping head versus specific speed for several pumped-storage plants designed by VA Tech Hydro and other suppliers. To achieve good hydraulic performance and minimum overall dimensions, a high specific speed is desirable. However, an increased specific speed requires an increased submergence. Finally, the safe and proper hydraulic and mechanical operation limits the possible rotational speed of a pump-turbine.
The Tongbai pumped-storage plant is characterized by a relatively large range of gross head. In pumping mode, the ratio of the maximum head to minimum head (Hmax/Hmin) exceeds 1.2. With a nominal speed of 300 revolutions per minute (rpm) and sufficient submergence, these machines meet modern design standards. In China, pumped-storage schemes generally are required to operate during relatively large and long-lasting variations in grid frequency. These variations have to be taken into account in the hydraulic layout and design process because they enlarge the Hmax/Hmin specified for continuous operation. In pumping mode at maximum gross head, sufficient margin must be provided to avoid unstable operation; at minimum gross head, the maximum power input must be kept within the design limits of the motor-generator. A sufficient margin against inadmissible inception of cavitation within the whole head range for pumping mode must be considered when designing the runner blade profiles on the low-pressure side.
Completing the hydraulic design
To meet all the requirements of the Tongbai project, a completely new hydraulic design was prepared. For pump-turbines, VA Tech Hydro has developed a procedure that uses advanced computational fluid dynamics (CFD) methods to design and optimize components. The modules composing this design procedure are:
— Computer-aided definition of the profile geometry;
— Numerical flow simulation at different operating points; and
— Computer-aided modification of the profiles and flow channels to improve the flow field with respect to stability and uniformity and loss minimization.
The primary dimensions of the various components are defined using a database with standard profiles, combined with using simplified, one-dimensional calculation tools. In several optimization loops, these components are investigated by means of three-dimensional (3D) flow simulation methods to consider 3D flow effects. The aim of the optimization loops is to achieve optimal interaction of the flow between the stationary components and the runner in both directions of flow. Simultaneously, the component dimensions are checked with respect to the specified mechanical design criteria in view of components' required safety and service life.
The runner design process is started by means of a quick variation of parameters achieved by applying a 3D Euler code. The resulting preliminary runner profile is used for a detailed, viscous analysis of friction losses and turbulence effects. If necessary, the profile dimensions are adapted to improve the results. The design process is completed by coupled computations of the flow in the runner, together with the adjacent components. The hydraulic behavior is balanced over the whole operational range.
In the past, the design of pump-turbines primarily was focused on pump operation. Today, turbine operation also is investigated. Optimization of the blade profile is a well-balanced process covering the entire specified head and flow ranges in both operating modes. This means that special attention is paid to not only operation near optimum conditions, but also to off-design operation. The examination of vortex patterns inside the flow channels is used to assess the operational behavior at extreme conditions, such as turbine part-load or pumping near maximum head.
For pump operation, special attention is paid to avoid cavitation on the pressure and suction sides. That means the leading edge of the runner blade is carefully optimized for maximum and minimum flow. The different colors in Figure 2 indicate different levels of static pressure. The smooth and continuous variation indicates low losses in the flow field.
The Navier-Stokes solver VA Tech Hydro applied for the flow simulation in turbines and pump-turbines is Ansys CFX. This commercial CFD package is well-established in the field of turbomachinery and provides a fully viscous method for solving the Reynolds-averaged Navier-Stokes equations. The system of equations is closed by a turbulence model for the solution of the viscous quantities. For the Tongbai simulations, the well-tried k-ε turbulence model was applied to solve the viscous terms. General grid interfaces allow nonmatching grids to be connected, and multiple frames of reference and sliding meshes provide time-averaged or transient rotor-stator interaction.
For the Tongbai pump-turbines, Tongbai Pumped Storage Power Company requested not only extensive CFD flow calculations but also model testing to demonstrate that the specified and guaranteed hydraulic main performance data will be met. Consequently, a homologous model with a scale of 1:11.93 was designed, manufactured, and optimized. The test conditions were such that for normal operation, a minimum test head of at least 60 meters was achieved.
In a first step, the hydraulic main performance characteristics of the basic design were checked. By means of minor modifications of the hydraulic profile, it was possible to achieve optimum operating conditions with respect to maximum power, guaranteed weighted efficiency, and cavitation pattern in pump mode.
The final tests were verified by the customer's representatives. These tests comprised not only verification of the main hydraulic guarantees, but also the check of wicket gate torques, pressure pulsations in the draft tube and at the runner, hydraulic thrust, and performance characteristics in the four quadrants as the basis for calculation of various transient conditions occurring on the prototype units.
General design concept of the pump-turbines
The developer of Tongbai desired a design that would minimize the size required for the civil structure. Thus, the pump-turbines for Tongbai are designed so they can be dismantled and removed through the generator pit. The turbine-generator set is equipped with two radial guide bearings for the generator and one radial guide bearing for the pump-turbine shaft. The thrust bearing is combined with the lower generator guide bearing and supported on a bracket below the generator.
The distributor is equipped with a regulating ring connected to two servomotors that are operated with a nominal oil pressure of 64 bars.
A spherical valve, also equipped with two servomotors, is located upstream of the unit and connected to the penstock. A pipe with an access door connects the spherical valve to the spiral case.
The hydraulic governor and inlet valve control are fed with pressure from separate pressure oil supply units, each equipped with two accumulator tanks. To start the unit in pump operation, the water in the runner chamber is depressed using pressurized air. The high-pressure air pipe is fastened to the upper part of the draft tube cone. To accelerate the rotor to nominal speed, the unit is driven by a static frequency converter.
A main aspect in the design of hydraulic turbines and especially pump-turbines is the technical coordination between the design of mechanical parts and the civil structure. A good vibration behavior of a unit — among other things — can only be achieved if the hydraulic forces from the unit are transmitted into the concrete in the most direct and efficient way. For example, the pressure forces acting on the bottom ring are transmitted into the concrete not only via the stay ring but also via the freely accessible upper draft tube cone.
Embedded parts: Stay ring and spiral case
The stay ring, together with the spiral case, is the main supporting structure of the pump-turbine. It was split into two parts and welded together on-site. The stay ring is fixed into the concrete with hydraulically prestressed anchors.
To optimize the wall thickness, the spiral case is equipped with gussets between the single sections. With these gussets having a slightly larger wall thickness than the surrounding spiral case plates, the stresses at the transition between spiral case plate and stay ring are reduced. Consequently, the wall thickness of the spiral case sections can be reduced. VA Tech Hydro has applied for a patent for this solution.
Wicket gate bearings
In pump-turbines, the wicket gates are loaded with highly dynamic forces, especially during phases of transient operation. These dynamic excitation forces may produce unwanted vibrations. The gaps normally existing in the wicket gate bearings have a negative influence on this behavior.
To avoid such bearing gaps, the lower and middle wicket gate bearings for the units at Tongbai are provided with prestressed Teflon bearings. This type of bearing can increase the lifetime of the bearing and reduce vibration. The bearing consists of two conical bushings. The inner bushing includes a reinforced Teflon foil. The bushings are telescoped together, which results in a radial deformation. This radial deformation can be adjusted to produce the necessary radial prestress.
Each reversible pump-turbine shows regions of possible instability. It is the goal of the hydraulic design engineer to develop a hydraulic shape of the runner that places the region of instability outside of the normal operating range. During model tests, the Tongbai pump-turbine was analyzed regarding hydraulic instability, especially in the region of synchronization and shut-off. The analysis at the layout stage with a transient simulation did not show any danger of instability for the defined transient operations.
During commissioning, this analysis was verified by the synchronization and no-load tests. No instability occurred neither during shut-off nor during no load operation and synchronization.
Pressure pulsation and shaft runout
During stationary operation, the pressure pulsation between the guide vane and runner is about 0.7 bar at full load. This meets the expectations of the developer and the supplier.
The preferred way to measure bearing housing vibration is determining the vibration velocity. The vibration of the Tongbai bearing is in the specified operation range on a level of very good hydraulic machines.
For the Tongbai pumped storage plant, Andritz VA Tech Hydro developed and optimized a new pump-turbine. All components were carefully analyzed by means of the modern tools of flow simulation. A thorough analysis of the interaction of the components ensures an optimal design of the new hydraulic profile and an improved hydraulic performance.
Model tests were used to verify the results of the numerical analysis and fine tune the profiles. A comparison of the measured quantities in the prototype with the results of the model tests and measurements of other pump-turbines show good agreement. The predictions in the design phase have been met very well.
Hu Zhongqi is chief engineer with of the 1,224-MW Tongbai pumped-storage plant. Tang Yibo, PhD, formerly manager of the technical department with Tongbai Pumped Storage Power Company Ltd., is now department chief of technical information with Shanghai Electric Power Company. Peter Nowicki is head of the technical department hydraulic turbines with VA Tech Hydro Escher Wyss in Germany. Manfred Sallaberger, PhD, is responsible for the hydraulic design of radial machines with VA Tech Hydro in Switzerland.