Winning on penalties1 December 2002
High availability was the first requirement when the Grossraming plant in Austria was refurbished. The solution was model testing, redundancy and standardised equipment
THE turbine sets first installed at the Grossraming hydro power plant in Austria were originally made by Voith in Heidenheim for a power station in South America. The planned delivery was shortly before the outbreak of the Second World War and when war broke out the delivery was cancelled.
The sets were reconsidered later, when plans for stations at two stages on the river Enns were combined into a single plant with a net head of over 20m. This was the head for which the machine sets had been designed, so they could be employed at the new station. The two units at the Grossraming power station were completed and the turbine generators at last put into service in 1950 and 1951.
By the end of the 1990s the turbine sets had to be replaced. The refurbishment was planned after one unit suffered a breakage in the runner blades in the area of the outer journal bearing, involving serious damage around the runner. At that time temporary repairs were made - the broken blade was screwed to the hub and the machine set ran as a propeller-type water turbine pending completion of a new installation.
When the output from the second machine set started to deteriorate, owner Ennskraft Aktiengesellschaft decided to call for tenders for complete replacement of the electro-mechanical equipment. In June 2000, supply of the mechanical equipment for the two turbines was entrusted to andritz.
The contract scope comprised supply of the two vertical-axis Kaplan turbines - each with 4.9m runner diameter and a maximum output of 36MW - along with turbine control equipment. The generators, supplied va-tech-hydro Elin, are linked directly to the turbine shaft.
Start-up of the two new machine sets are taking place a year apart. The first was started on 27 March 2002, and the second set is due to go on-line in March 2003.
Design, development and test
The turbines for Grossraming were developed in close co-operation between Andritz and the customer, Ennskraft. In designing the turbine, essential components were modelled in three dimensions, using the Pro/Engineer design and engineering system. This allows kinematic sequences, such as collision tests or assembly of parts, to be simulated quickly. A useful aspect of the system is that it allows immediate data transfer between the hydraulic development, design and strength calculation functions. Strength assessments can be carried out quickly, as can optimisation of the hydraulic shape.
During the design process regular meetings were held to ensure each sub-assembly was optimised to respond to customer requirements. The three-dimensional computer models were essential communication aids
On receipt of the tender documents on 15 October 1999, it became clear that there would be heavy penalties if efficiency guarantees were not met. Although the major components had been modelled using the software, the influence of the remaining components (draft tube, volute and stay ring) on efficiency could not be assessed, so Andritz decided to conduct a model test, using a runner model that had been employed before. Time was tight: to meet the offer deadline the first test results had to be available within less than two months. This required extensive efforts from the Astro model testing laboratory (also sited in Graz) and Andritz, but the test results allowed the company to give competitive efficiency guarantees.
Following some other optimisations on the first draft tube cone and on the runner casing in January 2000, work on the model tests was suspended until a definite order was placed. The contract was awarded on 15 June 2000.
In the course of the contract negotiations, the prototype diameter was increased, compared with that in the offer, and some parts of the model turbine had to be remanufactured. Andritz tried to obtain optimum compatibility with the very unusual draft tube and used flow numerics to design three model runner blade sets. In doing this, the company succeeded in achieving the development goal over the entire load range: this was done by shaping the short transition piece to the existing draft tube liner to optimise the velocity distribution after the runner.
These results were submitted to the customer during a demonstration test in December 2000. The guarantees concerning the efficiency and cavitation behaviour were entirely fulfilled. Using a five-blade runner with a 40% hub, Andritz succeeded in increasing the output from 28MW to 36MW with excellent cavitation properties and an increase in the flow rate and efficiency.
Turbine control systems
The process-technology unit between the hydraulic governor and digital controls includes the software, the hydraulic power amplification and the servo motors, plus digital, drift-free speed and position recording.
In order to achieve and maintain efficiencies as guaranteed, precise control is required, using flexible, digital turbine controls in 32-bit micro-processor technology, powerful communication interfaces and very good industrial compatibility.
These are the components that form the basis of the concept for the new high-economy generation of turbine controls as used in Grossraming. The ATC 2005, (digital Andritz Turbine Control) system has a great number of control types as standard features (speed and primary control, with adaptive parameterisation), in order to guarantee optimum control quality at any machine operating point.
The ATC has been developed in response to customers' demands for ever-shorter start-up times. This led to consideration of a multi-user workplace structure. Networking the hardware components via an Ethernet hub allows several start-up engineers to work simultaneously on the same control.
An engineering tool with a database allows viewing of all changes and work carried out on the digital turbine control, or it can be made verifiable at any time by 'versionising'.
A basic requirement for ease of handling of the start-up and programming tools is a progressive operating system such as Linux. This has the advantage of a multi-screen solution (one that Windows screens do not have). This means that the engineering environment, trend display and a host operating system (eg. Windows) can be operated in parallel on one computer.
The engineering tool has a continuous database structure and this makes it possible to use one computer as 'server' but with full functionality, or to give one or several computers access to the same database via TCP-IP. This leaves the data structure consistent and always up to date. There is also a routing option: a link can be established via a serial connection RS232 to the first ATC 2005 and via CAN Bus on to the operating panel or a second ATC2005.
With special drivers on the control it is possible to transfer control variables to the engineering computer at a precise cycle time and with a time stamp (event-controlled) for precise analysis. All data are recorded continually, along with any action of the start-up engineer, and stored in a so-called runtime database. Thus a complete analysis of the trend data can be made retroactively and at any time in the event of a failure. This feature is enabled by the 'master computer' function of the APROL engineering tool.
Redundancy for availability
As a run-of-river station, Grossraming is one of the most important power stations in the chain of stations along the river Enns and much value is attached to its full availability at all times. The customer's wish to have an economical, good value for money, yet highly reliable turbine control system was met by using a redundant control system.
The digital turbine control was designed as a 'hot standby'. It has 100% redundancy up to the servo valves - ie. both computers are identical, have the same hardware and software packages, and can work independently.
The two controllers have separate sensors. When control passes from one controller to the other, the switch is 'bumpless' whatever the operating mode, because the signals are run in parallel. It is of no importance whether the changeover is manual or event-controlled.
The turbine controller is provided with an efficiency optimisation feature that adjusts the turbine operating point and the adjacent systems (the other turbine and the weir sections) to maintain flow rate. This automatically minimises any loss of efficiency due to changes in the hydraulic supply (flow, contamination). The strategy applied is based on the reduced output principle, which automatically balances inconsistent conditions during an optimisation process, for example changes in the head. If conditions are near-constant, the optimisation process can be time-controlled.
Extra features can be incorporated as modules in the controller software, thanks to the use of C-Code as a standard high-level program language under the APROL engineering tool. This integration is simple and easy, and special functions can be programmed and implemented quickly and safely.
During the overhaul, the mechanical-hydraulic turbine governor (pressure level 2MPa) was replaced by a new unit in a modern module and control block construction with a new pressure tank. With regard to flexibility, functionality and breadth of use of hydraulic units, some innovative thinking was done, and also taken into consideration in the design and construction phase.
The hydraulic governor was to be used as an optimised element within the complete turbine control system. It has a simple modular design, focusing on ease of service and access, adapted function mode and high reliability. With many products available, nearly all the customer's wishes were fulfilled and using standard equipment meant the operator needed a small spare parts stock, aiding high availability.
The control block, which replaces a component structure, can be manufactured economically using high-tech machine tools. Logic-based operation ensures high functional reliability and can be used for a variety of control options. Maintenance intervals have become longer and longer because the hydraulic components have a growing life. Precise control valves with internal position control, fail-safe function and flow rates exceeding 1000l/min provide high control quality and fulfil very high safety requirements.
The large number of design considerations is reflected in these logic valves. Cartridge valves (2/2-way-type) with lift limitation on the front side and externally adjustable orifice valves saves time in start-up operations.
The success of modern turbine control systems is due to consistent standardisation: it improves operation and reduces the cost of fabrication.
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