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5.12 The Control System In this section I will describe the general idea of the control system for each of the two modes, normal parallel mode and stand-alone mode, see section 3.2 for a description of these modes. The main input to the regulator is an electric power reference, i.e. the amount of electric power the generator should produce. The main output of the regulator, is the fuel rate, i.e. how much fuel that will be injected in the combustion chamber. There is a maximum value, with the fuel valves fully opened and a minimum value, when the valves are open just do the flame is kept alive. The measurements that are used in the feedback loop are the speed, the power currently produced in the generator and the temperature after the turbine. In the parallel mode, the gas turbine produce heat and power at a constant level to a local heating system and a power grid, which is connected to the national power grid. If the power demand in the local power grid increases to a higher value than the maximum output of microturbine, the extra power is taken from the outer power grid. The microturbine usually runs at its optimum, full load. Since the control system does not need to meet any sudden changes, all changes can be done slowly and the thermal fluctuations inside the gas turbine are minimized. The life span of a gas turbine is closely related to how large and fast temperature changes it has to experience, e.g. at step changes and start and stop procedures. The changes are made slowly by including a rate limit function. A rate limit model has been implemented, but it only works if the signal source is differentiable, e.g. a ramp or sinusoidal. The total efficiency is optimal at a certain temperature and speed, which makes the microturbine more economical if always run at the optimum point. The control system tries therefore to produce the demanded power and at the same time keeping the temperature and speed constant at the optimal value. In the stand-alone mode the objective of the control system is to maintain a constant voltage and frequency of the local power grid regardless of the actual power load. This objective is much stricter than for the parallel mode and therefore the control system needs to be a lot faster. The faster control is achieved on the expense of higher thermal variations in the machine, thus reducing its life span. However this is a price one has to pay to achieve the objective of stand- alone power delivery. The control system consist of a normal speed regulator that tries to maintain a predefined reference speed, e.g. 63 000 rpm. Even though that speed might not be the optimal for lower power loads it is necessary to have a speed buffer if the load is sudden increased, since it is hard to increase the speed without reducing the power load for a short moment. The regulator tries to maintain the speed by controlling the fuel input to the turbine. The upper performance limit here is the limitation of the fuel injectors. There is a maximum amount of fuel that can be injected and that is based on temperature limitations of the material before and after the turbine. 6. Simulations and Verification As a part of the verification process a large number of simulations were made and some of them will be presented in this chapter. 6.1 Simulations With a complete model it is open for the user to simulate the process under any circumstances the user chooses. There are numerous parameters that must be set for each simulation case. All of the parameters have a default setting to avoid tedious work and numerical problems in case the parameter is not correctly set. Default settings can however be very dangerous, since the user can simulate the model without having checked if the correct parameter values are used. Some 42PDF Image | Modelling of Microturbine Systems
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