The requirements on today's power systems are very high. Customer require high availability, the market deregulations result in a broader operation range of power system, and the power production is becoming more diversified with distributed generation like wind power etc. These factors require careful design and integration of components in the system in order to maintain stable operation. This article focuses on methods to improve turbine control in order to achieve stable frequency.
When installing new equipment in existing plants and industries it is of great importance to carry out careful system integration. New machinery must work with the old processes and boilers, and control systems must be tuned to work properly in the systems where it is installed.
The need for careful formulation of requirements during the tender process, an intense dialog between the plant staff and the supplier engineers, and careful commissioning to ensure the correct performance are all very important measures to achieve requested functionality.
Systems integration
Solvina has specialised in system integration of power turbines, generators, boilers and new complex control systems in new and existing plants. The use of simulators has shown to be very successful in the integration of new turbines and other process and power equipment. Dynamic simulation can be used to formulate requirements and control principles, and tuning can be optimised.
One simulator application is performance tests and tuning using a hard-ware-in-the-loop (HWIL) simulator in commissioning. Successful method and test equipment has been development for certifying and optimising operation in isolated networks, island operation, of industries and power stations.
The test equipment, called SolvSim Power Station, is connected to the governor and simulates an island network, so that the governor acts as in real island mode while the machine still runs synchronised to the main grid.
The plant and the simulator are running together in real-time in closed loop operation. This method eliminates the risk for network (customer) disturbances, unit trips and drastically improves the possibility to perform proper tuning of the control systems.
More than 50 island operation tests have been carried out since year 2000 on steam turbines, hydro power turbines and combined cycle power plants.
During the tests it has repeatedly been shown how the power supply and industrial productivity can be vastly improved by using careful system integration and control system tuning. Common problems that have been discovered during testing have been:
- Malfunctioning valves.
- Improper valve linearisation.
- Bad tuning of control parameters.
- Erroneous control design.
- Badly dimensioned transducers.
- Inadequate design criteria.
Examples
In order to ensure island operation capability of a new power station with three gas turbines and a steam turbine in combined cycle, Solvina was engaged to perform tests of the turbine governors using the HWIL method. The tests of the three identical gas turbines rendered good governing capability - but only for two of the three turbines! It was detected that one fuel valve was malfunctioning.
A hysteresis, caused by mechanical backlash, little enough not to have shown up during the just finished commissioning but big enough to collapse an island network, was found (see Fig. 4).
During optimisation of a turbine governor at a pulp and paper mill with co-generation, verifying tests with the HWIL method showed that the turbine governor, optimal for one point of operation, was insufficient to run the isolated network in stable operation at other power output levels.
After some examination the results showed that the reason for this was a faulty linearisation of the steam valves (Fig. 6).
A test of operation at a real island network would not have revealed this serious defect.
Neither would a stability simulation have found this problem since the linearisation was not implemented as intended. Now the problem could be easily solved by tuning the linearisation function.
Turbines are complex machines and involves many technical disciplines during a project.
The described test methods have shown great value for both customers and OEMs and simplifies the communication about technical requirements during the whole project.
Enter 20 or XX at www.engineerlive.com/ipe.
Sven Granfors and Niclas Krantz, Solvina are with Solvina AB, Västra Frölunda, Sweden. www.solvina.se
