Učinski izmjenjivač visoke djelotvornosti s posebnim istosmjernim međukrugom

In case of medium voltage (several tens up to hundred volts on DC-side) solar inverter applications, a DC-to-DC converter for voltage level adaptation is required in series of the DC-to-AC inverter. This leads to a two-stage concept with accumulation of the losses. In our case a concept was chosen where the efficiency of each stage is maximized by using the best topology. The given requirements make the application of a non-isolated design imperative to avoid additional transformer losses. In this paper a 60V–120V DC (input) to 230V AC (output) / 1kW converter with minimal conversion losses is derived. A simple modification in the inverter\u27s output section leads to a significant improvement of the losses in the inverter system. Only three additional components (two diodes and one inductor) are necessary to optimize the inverter\u27s power stage. The topology presented here shows a remarkable improvement of the switching losses and significantly reduced EMC. It is well-suited for solar power inverter applications.U slučajevima srednjenaponskih razina ulaznog napona (nekoliko desetaka do stotinu volti na istosmjernoj strani) izmjenjivača napajanih iz fotonaponskih ćelija, zahtjeva se istosmjerni pretvarač spojen u seriju s izmjenjivačem, za prilagodbu naponskih razina. Takav dvostupanjski pristup uzrokuje povećanje gubitaka. U ovom je slučaju odabran pristup kod kojeg se djelotvornost svakog stupnja pretvorbe maksimizira uporabom najpovoljnije topologije. Zadani zahtjevi čine obveznom uporabu istosmjernog pretvarača bez galvanskog odvajanja, da bi se izbjegli dodatni gubici u transformatoru. U ovom članku izveden je pretvarač snage 1 kW, ulaznog istosmjernog napona 60 V–120 V, izlaznog izmjeničnog napona 230V, s minimalnim gubicima pretvorbe. Jednostavna prilagodba u izlaznom stupnju izmjenjivača dovodi do značajnog smanjenja gubitaka u izmjenjivaču. Za optimiranje izlaznog stupnja izmjenjivača potrebne su samo 3 dodatne komponente (2 diode i prigušnica). Ovdje prikazana topologija pokazuje značajno smanjenje sklopnih gubitaka i smanjuje elektromagnetske smetnje. Posebno je pogodna za primjenu kod izmjenjivača napajanih iz fotonaponskih ćelija

Advanced analytics for process analysis of turbine plant and components

This research investigates the use of an alternate means of modelling the performance of a train of feed water heaters in a steam cycle power plant, using machine learning. The goal of this study was to use a simple artificial neural network (ANN) to predict the behaviour of the plant system, specifically the inlet bled steam (BS) mass flow rate and the outlet water temperature of each feedwater heater. The output of the model was validated through the use of a thermofluid engineering model built for the same plant. Another goal was to assess the ability of both the thermofluid model and ANN model to predict plant behaviour under out of normal operating circumstances. The thermofluid engineering model was built on FLOWNEX® SE using existing custom components for the various heat exchangers. The model was then tuned to current plant conditions by catering for plant degradation and maintenance effects. The artificial neural network was of a multi-layer perceptron (MLP) type, using the rectified linear unit (ReLU) activation function, mean squared error (MSE) loss function and adaptive moments (Adam) optimiser. It was constructed using Python programming language. The ANN model was trained using the same data as the FLOWNEX® SE model. Multiple architectures were tested resulting in the optimum model having two layers, 200 nodes or neurons in each layer with a batch size of 500, running over 100 epochs. This configuration attained a training accuracy of 0.9975 and validation accuracy of 0.9975. When used on a test set and to predict plant performance, it achieved a MSE of 0.23 and 0.45 respectively. Under normal operating conditions (six cases tested) the ANN model performed better than the FLOWNEX® SE model when compared to actual plant behaviour. Under out of normal conditions (four cases tested), the FLOWNEX SE® model performed better than the ANN. It is evident that the ANN model was unable to capture the “physics” of a heat exchanger or the feed heating process as a result of its poor performance in the out of normal scenarios. Further tuning by way of alternate activation functions and regularisation techniques had little effect on the ANN model performance. The ANN model was able to accurately predict an out of normal case only when it was trained to do so. This was achieved by augmenting the original training data with the inputs and results from the FLOWNEX SE® model for the same case. The conclusion drawn from this study is that this type of simple ANN model is able to predict plant performance so long as it is trained for it. The validity of the prediction is highly dependent on the integrity of the training data. Operating outside the range which the model was trained for will result in inaccurate predictions. It is recommended that out of normal scenarios commonly experienced by the plant be synthesised by engineering modelling tools like FLOWNEX® SE to augment the historic plant data. This provides a wider spectrum of training data enabling more generalised and accurate predictions from the ANN model

Dynamical modelling of a flexible motorised momentum exchange tether and hybrid fuzzy sliding mode control for spin-up

A space tether is a long cable used to couple satellites, probes or spacecrafts to each other or to other masses, such as a spent booster rocket, space station, or an asteroid. Space tethers are usually made of thin strands of high-strength fibres or conducting wires, which range from a few hundred metres to several kilometres and have a relatively small diameter. Space tethers can provide a mechanical connection between two space objects that enables a transfer of energy and momentum from one object to the other, and as a result they can be used to provide space propulsion without consuming propellant. Additionally, conductive space tethers can interact with the Earth's magnetic field and ionospheric plasma to generate thrust or drag forces without expending propellant. The motorised momentum exchange tether (MMET) was first proposed by Cartmell in 1996 and published in 1998. The system comprises a specially designed tether connecting two payload modules, with a central launcher motor. For the purposes of fundamental dynamical modelling the launcher mass can be regarded as a two part assembly, where the rotor is attached to one end of each tether subspan, and the other side is the stator, which is attached to the rotor by means of suitable bearings. Both the launcher and the payload can be attached to the tether by means of suitable clamps or bearing assemblies, dependent on the requirements of the design. The further chapters in this thesis focus on a series of dynamical models of the symmetrical MMET syste, including the dumbbell MMET system, the solid massless MMET system, the flexible massless MMET system, the solid MMET system and the discretised flexible MMET system. The models in this context have shown that including axial, torsional and pendular elasticity, the MMET systems have a significant bearing on overall performance and that this effect should not be ignored in future, particularly for control studies. All subsequent analyses for control applications should henceforth include flexible compliance within the modelling procedure. Numerical simulations have been given for all types of MMET models, in which, accurate and stable periodic behaviours are observed, including the rigid body motions, the tether spin-up and the flexible motions, with proper parameter settings. The MMET system's spin-up control methods design and analysis will henceforth be referenced on the results. For the non-linear dynamics and complex control problem, it was decided to investigate fuzzy logic based controllers to maintain the desired length and length deployment rate of the tether. A standard two input and one output fuzzy logic control (FLC) is investigated with numerical simulations, in which the control effects on the MMET system's spin-up are observed. Furthermore, to make the necessary enhancement to the fuzzy sliding mode control, a specialised hybrid control law, named F$\alpha$SMC is proposed, which combines fuzzy logic control with a SkyhookSMC control law together, then it is applied for the control of motorised space tether spin-up coupled with an flexible oscillation phenomenon. It is easy to switch the control effects between the SkyhookSMC and the FLC modes when a proper value of $\alpha$ is selected $(0<\alpha<1)$ to balance the weight of the fuzzy logic control to that of the SkyhookSMC control, and the hybrid fuzzy sliding mode controller is thus generated. Next, the simulations with the given initial conditions have been devised in a connecting programme between the control code written in $MATLAB$ and the dynamics simulation code constructed within $MATHEMATICA$. Both the FLC and the hybrid fuzzy sliding mode control methods are designed for the control of spin-up of the discretised flexible MMET system with tether-tube subspans, and the results have shown the validated effects of both these control methods for the MMET system spin-up with included flexible oscillation. To summarise, the objectives of this thesis are, firstly, to propose a series of new dynamical models for the motorised momentum exchange tethers; secondly, to discuss two types of control methods for the spin-up behaviour of a flexible motorised momentum exchange tether, which include a fuzzy logic control and a hybrid fuzzy sliding mode control. By the weight factor $\alpha$, fuzzy logic control and SkyhookSMC controllers can be balanced from one to each other, and there is observed difference for each of the elastic behaviour in the MMET system involving these MMET systems with different controllers - FLC($\alpha = 1$), F$\alpha$SMC($\alpha = 0.5$) and SkyhookSMC($\alpha = 0.0$). The results state the control effects for FLC, F$\alpha$SMC and FLC, which lead to stable spin-up behaviour with flexible oscillations