Digital regulators take the strain at... Ajaure

16 July 2001

During refurbishment work at Ajaure hydro power plant in Sweden, the whole guide vane control mechanism was redesigned to prevent oil leakage into the river

Choosing individual servo motors may seem unnecessarily complicated, but there are sound reasons behind this choice of equipment for hydro power plants. For more than two decades, Swedish turbine manufacturer Nohab developed a regulating concept based on individual servo motors, one for each of the guide vanes. In most cases the servo motors were torque servo motors with a mechanical/hydraulical regulating valve on each. To ensure safe operation (especially at emergency closure) the guide vanes were modified to incorporate a self-closing tendency over the whole stroke, even without oil pressure.

The basic design principle behind individual servo motors is to handle all large forces internally, while reducing the size of the whole turbine and generator set, particularly its height, in order to reduce structural costs. Worldwide there are several turbines of this design. Most of these are large or very large, such as Ligga 3 – a 182MW Kaplan at the Luleå river, Sweden.

The Vattenfall-owned power plant Ajaure in Sweden was originally equipped with individual torque servo motors for guide vane control. Oil leakage – both internally and to the surroundings – is a major problem with this type of servo motor. The type utilised in Ajaure has the bottom of the servo motor touching the water under the head cover, hence the seal keeping the pressurised oil inside the servo was in contact with the river water on the outside. A ruptured seal would result in oil leaking out into the river and, without shutting down the turbine and emptying the spiral casing for inspection, it would not be possible to tell which of the 24 motors was leaking. There have also been problems regarding positioning accuracy and repeatability which, especially on Kaplan turbines, leads to efficiency loses.

During recent refurbishment work of Ajaure’s 4.5m diagonal turbine, which has a rated output of 80MW, Waplans developed digital regulators with proportional valves and utilised bus technology for monitoring and control. Implemented during the winter of 2000-2001, the concept aims to improve governing performance and reliability, as well as eliminate the risk of environmental impact. The unit has been in operation since March 2001.

  Mechanical considerations A totally new design concept had to be developed for Ajaure. The challenge was the limited space available. A feasibility study undertaken by the plant owner indicated that two hydraulic cylinders for each of the guide vanes were preferable. Other considerations included that:

• The guide vanes had very short shafts that had to be elongated.

• Brackets to connect the hydraulic cylinders had to be mounted on the head cover.

• The levers for the guide vanes had to be squeezed in between the stiffeners on the head cover.

• The guide vanes have a self-closing tendency over the whole stroke (even when closed) that sets some demands on the design of the hydraulic system.

• The construction of the head cover called for a two-bearing arrangement for the guide vanes.

• New upper bearing housings had to be constructed.

The study proposed that the guide vanes should be elongated with a stainless steel shaft that was to be shrunk on to them. This would involve boring larger holes in the head cover and would also require large bushings and seals with a large diameter. The diameter difference between the lower and upper seals of the guide vane would then produce a significant lift force on the guide vane that had to be handled.

Stress analyses showed relatively low stresses in the guide vane shafts, so Waplans opted to weld the elongation to the guide vane. This option had several benefits: no boring of the head cover would be needed; and the bushings (and seals) could have a smaller diameter resulting in negligible lifting force and lower friction.

For the levers, stress caused by surface pressure from the radial friction coupling was the dimensioning criteria that led to a choice of a fairly high strength material. To squeeze in the levers and to provide a fastening point for the hydraulic cylinders, extensive modification of the head cover was necessary. Calculations of the deflection of the head cover showed that there was sufficient material to allow long holes in the stiffeners for the levers and cylinders. Brackets for the cylinders were welded to the head cover, and special care was taken during the welding process to reduce stress initiation and deflections.

Hydraulic design The development of a new design concept for individual control of the guide vanes is based on basic features:

• The use of proportional valve technology rather than servo valves (today’s proportional technology has more than enough performance for turbine governing at a lower price than servo valves).

• The use of block mounted valves as far as possible to acquire a compact, easily-maintained and clean design.

• The use of standard industrial components.

Some new features were included, such as digital PID-governors (Rexroth HNC100) for each of the guide vanes, digital position transducers and bus technology for communication.

The criteria included a position accuracy of ±0.4% (which is about ±0.65 mm) and a dead time (time from a change in the position command value to a change of the actual position value) of less than 200 milliseconds measured at the output from the turbine regulator. The customer also required frequency regulation of the turbine (which, together with the required opening and closing times for the guide vanes, determines the size of pumps and accumulators).

With the block-based solution, two proportional valves with integrated control electronics are mounted on each block (there are 12 blocks in all). The blocks are supplied with oil from two redundant pumps and an accumulator set. Two-speed closing is solved hydraulically with a limit switch that controls a pilot pressure loop connected to all blocks (two-speed closing gives an extra degree of freedom when adjusting closing times to optimise the trade-off between speed and pressure rise during load rejection and emergency closure). The emergency closure is also pilot pressure controlled and is completely independent of the proportional valves. Hydraulic design and dimensioning of pumps, accumulators and so on were done in-house, whereas the choice of components was accomplished with sub-supplier Rexroth-Mecman.

The demands concerning position accuracy are easily met. High position accuracy was obtained without putting any major effort into fine tuning the digital position regulators. By operating them only with P (no I nor D) and setting the parameters alike on all regulators (although there are individual differences between the guide vanes) a position accuracy of about 0.2mm (average of the guide vanes), and in no cases larger than 0.65mm, were obtained. Even during the relatively large position change that takes place when the load is rapidly increased, position accuracy is much better than the required ±0.4%. The hardware is capable of obtaining higher position accuracy with more tuning.

Control The turbine regulator remains unchanged, although it has been reconfigured to fit to the new equipment. It sends an analogue position command value to the turbine computer (a standard PLC) and gets the actual position value back. What is completely new, however, is that the PLC in its turn communicates with the 24 position regulators with bus technology (Profibus DP).

The position regulators are completely software-controlled (NC-program) and the position control loop includes the digital regulator, the proportional valve, the hydraulic cylinders and a digital position transducer (with SSI-bus). All tuning of position transducers, regulator parameters and also evaluation of the performance is done via a laptop computer that connects via RS232 to the regulators.

Bus communication in turbine regulation is uncommon, probably because it is slower than analogue technology. However, without any major work or special components, Waplans managed to cut the dead time to about 180 milliseconds (mainly due to the profibus-protocol management in the PLC that has a cycle time of about 130 milliseconds). This is more than adequate, especially when controlling machines with large rotating inertia. As the position control loop is distributed to the position regulators, that loop has a cycle time of less than 30 milliseconds which is of high importance when it comes to regulating stability.

The turbine computer is also used to monitor the performance of the digital position regulators. There are two parallel control functions, one (drag error) sees that position accuracy is maintained during larger movements of the guide vanes, and the other (position OK) sees that position accuracy is maintained during more normal operation. Drag error is used to detect more severe failures, and if the drag error exceeds a certain value it leads to immediate shut off of the turbine.

The new regulating system has fulfilled all requirements, both technically and environmentally. The risk of oil leakage from the guide vane regulating mechanism to recipient is eliminated, while regulating accuracy has also been improved.
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