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Co-ordinated control ensures quality assured energy supply

21st February 2013


Niclas Krantz describes different aspects on the process towards the goal of 'quality assured energy supply'

The concept of quality assured energy supply refers to the power plant designed and tuned from a holistic perspective in which the operating conditions in all parts are considered. This is necessary to achieve high availability and good performance.

Solvina has specialised in system integration of power turbines, generators, boilers and new complex control systems in new and existing plants, and uses complementary methods to take the necessary approach towards high efficiency and stability.

This article focuses on the possibilities with dynamic simulation as a tool to formulate requirements and control principles, perform control tuning and verify requirements at commissioning.

Design requirements

The basic requirement of a power plant is to maintain stability in all normal operating conditions. In addition, different disturbances, either internal or external, should cause a minimum of plant trips and outages. In a design process it is important to define the different operating conditions and events thoroughly in order to get the right design criteria.

The requirements should contain both steady state and dynamic requirements. The dynamic requirements must be defined carefully in order to enable final verification. One example of dynamic requirement is island operation (isolated grid). It is not enough just to require island operation capability, the quantitative aspects must be defined, such as frequency drop after a load change, time constants etc. Other requirement might concern the ability to withstand a turbine trip, short circuits in the power grid, or thunderstorms.

These requirements affect the whole plant and have to be defined on plant level and transferred to detail requirements for control systems, turbine, boiler, generator, etc.

Dynamic control design and tuning

With simulation models, the whole plant can be studied in normal and extreme operating conditions. For a given operating condition, analysis can be made on the whole system, the boilers, steam system, turbine frequency/power control and generator voltage control.

The benefit of using dynamic simulator technology is that it can be used through the whole project from the design phase to commissioning and training. Extreme operating conditions and faults can be studied carefully. This kind of use requires careful validation of models. There are often good possibilities to improve the models gradually and finally validate them against the actual plant during commissioning.

The user interface of a simulator can be designed with the authentic operating screens in the actual plant. The following example describes the work to implement a superior control system for a pulp mill steam system. Similar can of course be carried out for example in combined cycle plants.

The example simulator is developed for a sulphate pulp mill in Sweden. The pulp manufacturing process contains major consumers of steam and hot water. To address these needs, high-pressure steam is produced in the process recovery boiler, but also when necessary, in an extra boiler fired with leftover bark or oil. The high-pressure steam is reduced to the desired pressure level through turbines, valves, or a steam accumulator. Steam consumers are placed directly on the steam branches. Some of them are condensers used to produce hot water for process purposes. Excess vapour is, in the first place, condensed to get the feed water back to the boilers. However this process is energy inefficient since the thermal energy leaves the plant with the cooling water. If the maximum condenser capacity is reached, steam is blown out into the open air.

The work was initiated when new turbines were purchased and the steam control system was modernised. In order to get the best possible plant performance a simulator was developed. The simulator was used to develop an efficient control strategy and to perform tuning before commissioning. Control optimisation was performed in two steps.

Primary control loops were tuned to handle all the operational conditions set up in the requirements. The events included turbine trips, island operation, instant changes in steam consumption, etc.

With the objective to increase the economy in the process by reducing the vapor surplus, increasing electricity generation, and optimising steam and hot water production, a new superior control system was developed and implemented with excellent results.

The results of these efforts were stable operation with very few outages and large amounts of energy savings. In addition, the commissioning and the plant startup to full production became very short. Major parts of the control system changes could also be made without plant shutdown, since the control was already optimised in the simulator. Furthermore the simulator was used for operator training so that the plant staff knew what to expect when the new systems were started. The payback time for the simulator and engineering was estimated to a few months.

Commissioning

The commissioning is valuable for verifying that the requirements have been met. Using a combination of measurements testing normal operation conditions, and simulation for the most extreme events, it is possible to get a good picture of the performance.

A successful method and test equipment has been developed for certifying and optimising operation in island operation. 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 eliminates the risk for network (customer) disturbances, unit trips and drastically improves the possibility to perform proper tuning of the control systems. This is an efficient way to reach stable island operation, and to verify the island operation requirements. More than 50 island operation tests have been carried out by Solvina since year 2000 on steam turbines, hydro power turbines, gas turbines and combined cycle power plants.

The simulator technology is well suited for operator training. In some industry branches, training has been carried out with simulators for many years, for instance in the nuclear power industry. With the new tools available on the market, the simulator training can be made available for a wider range of companies. If a simulator has been developed in the engineering process, it can be used also for training. Specific applications can also be developed only for training and at a reasonable price.

Enter 29 or at www.engineerlive.com/ipe

Niclas Krantz is managing director, Solvina AB, Västra Frölunda, Sweden. www.solvina.se







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