The load of the electric grid in increased by the connection of more renewable energy sources and less controlled power stations. The transmission operator has to decide where and when new overhead lines are required. However the operator can also chose to utilize existing lines more by increasing the transport capacity. This can be accomplished by applying dynamic line rating. In this case the conductor temperature and sag will increase but should not not violate the safety limits. Therefore a device can be installed on over-head lines to monitor the temperature and sag in real-time. Several devices are already manufactured and tested. However these devices require a reliable communication. In this thesis the feasibility and reliability of communication with Low Power Wide area Network (LoRaWAN) protocol in an overhead line environment is studied. The first objective is to determine the consequences of DLR on overhead lines. Second shielding of the electric and magnetic field in an overhead line is evaluated. Finally an antenna is chosen and a prototype is designed to evaluated its thermal behavior, low frequency magnetic shielding capabilities and communication feasibility and reliability. The first objective is to understand how the conductor temperature, the sag and magnetic field increases in respect to the transport capacity. The change in conductor temperature is examine by considering several heating gains and losses mechanisms. The sag is approached by a sagging and parabola curve to evaluate the decrease of distance to the ground. The maximum magnetic field is determined for close distances from the conductor.Two methods are examine to shield form the electric and magnetic field from the overhead lines. There-fore the materials copper, aluminum, steel and mu-metal are evaluated on their conductivity and permeability. The antenna is determined by comparing a dipole, loop and slot antenna on their complexity and expected radio noise interference. The prototype is exposed to a test setup in which the surface temperature, and magnetic field, is measured. The reliability of communication is evaluated by analyzing the amount of send and received packages.In case of an increased transport capacity the conductor temperature of an overhead line mostly increase by Joule heating. For a conductor temperature increase and concentrated weight the sag increases with negligible tents of centimeters. The maximum magnetic field was calculated and resulted in a required shielding effectiveness. The thickness of shield was determined for each material to reduce the magnetic field inside an enclosure. A slot antenna was designed and made in the prototype and compared with simulations in respect to the reflection coefficient. The surface temperature of the prototype did barely gain heat by the generated induced currents. However the prototype does reduce the magnetic field inside and confirms the theory of low frequency magnetic shielding. Furthermore the communication is tested and resulted in reliable data transfer in the case of a 0.5 cm distant from the conductor through which a current of 800 A flow. Furthermore corona discharges were generated up to 130 kV in which data still was transferred successfully

Stability and security of the grid are two vital aspects of energy supply. To keep the grid stable, it is necessary that the electricity plant can control and protect the mechanism. In the past, traditional plants could meet these requirements. Nowadays, the share of sustainable power sources in total electricity generation has become so important that these sources must keep the grid stable. A significant part of these requirements is the Low Voltage Ride Through (LVRT) capability of the sustainable energy plant. When some large loads are connected to the grid or as a result of grid faults like lightning strikes or short circuits, short–term voltage dip may occur. This low voltage dip condition plans to be simulated and the behaviours of Brushless Doubly-Fed Induction Generator (B-DFIG) should be improved in this situation. This thesis is going to realize this goal that after the improvement, when low voltage dip occurs, it is not necessary to disconnect the generator from the grid.

The thesis is organized as follows: Firstly, the B-DFIG modelling is implemented, including the steady state modelling by equivalent circuits and the dynamic modelling by differential equations. The steady and transient state characteristic of B-DFIG can be studied by equivalent circuits and dynamic models respectively. Secondly, the control algorithm is added to the power electronic converter to control the machine in a stable and responsive manner. Thirdly, simulations of both full voltage and low voltage dip conditions are done to compare B-DFIG behaviours between these two different simulation conditions. Unexpected performance like large transient current will occur under low voltage dip condition. Fourthly, the LVRT control algorithm is added to the controller of B-DFIG for improving the behaviours of B-DFIG in low voltage dip condition based on some constraints and assumptions. Finally, experiments are executed to verify this result.

Index Terms — Brushless Doubly-Fed Induction Generator, Low Voltage Ride Through, security, stability

In a cordless kitchen system, the appliances do not make use of a power cord. The system consists of a power channel and a communication channel. There are various resonant topologies which can be implemented in the power channel[9]. The chosen topology should meet the functional as well as non-functional equirements of the system. Presently, a series resonant topology is used in the system. The topology is easy to understand, to implement and could meet the functional requirements like the amount of power and high efficiency of the system easily. But, it is difficult to meet the non-functional requirements like overvoltage protection, EMI and simple control of the system. This master’s thesis is mainly focused on design and realization an inductive power transmitter which addresses the following research questions:
• EMI: The present inductive power transmitter is based on a series resonant inverter which has a disadvantage of high dV/dt’s across the inverter bridge and the transmitter coil. Furthermore, ZVS of the power switches can be lost at low duty cycles or low loads. Both mechanisms causes high dV/dt’s in the system, which results in increase in common mode noise problems.
• Overvoltage: The present series resonant topology with the calculated circuit components and given load conditions (coupling factor (k) and load resistance (RL)), is observed to be operating in the capacitive mode of operation, when the load is disconnected/removed. Therefore, disconnection of the load causes overvoltage on the transmitter as well as the receiver coil. The overvoltage, in the absence of ’expensive’ protective means, can damage the system components and the system may fail to operate.
• Simple control: The present system is a 4th order system. Due to weak coupling between the transmitter and the receiver coils, there exists multiple operating points where the voltage gain is the same. Therefore, it is difficult to find an optimal operating point for the control loop. Also, as mentioned in previous point, the load disconnection results in capacitive mode of operation of the system. To protect the system the control loop should take care that the system operates in the desired inductive mode of operation, as fast as possible and without causing significant losses.
The report proceeds with an introduction about the cordless kitchen project. It is followed by the description and the analysis of the present system. Next, the detailed analysis of other possible topologies and selection of the alternative topology is described. It is followed by the description about the new proposed model. The report concludes with the conclusion and a paper publication. The paper is published during the Wireless Power Congress arranged by WEKA FACHMEDIEN GmbH in July-2017.

In the world of DC railway trains nowadays, asynchronous-traction machine with traction controllers, i.e. an inverter with variable output frequency, is implemented. This controller changes the incoming DC voltage into an AC voltage of a variable frequency and RMS amplitude. The disadvantages of switching inverters, compared to a pure sine wave source, are the harmonics they inject in both the output as in the input of the inverters connection. At the incoming power line, it is necessary to damp these harmonics, in order to avoid resonance issues or issues regarding other systems, for instance, train detection. For this purpose of harmonic and transient filtering, generated within as well as outside the train, an LC filter is utilised.
However, this LC-filter has also influence on stability. Due to the resulting impedance to current variation in the inductance of the LC-filter, the power flow dynamics towards the train are decreased. When applying a certain amount of power, the voltage over the capacitance can become unstable very quickly. In this thesis, a graphical user interface simulation model is made to simulate these stability phenomena in Simulink in order to find the optimum size of the LC-filter in a train on the Dutch DC railway.

The simulations are achieved with a model representing a generalised train. The model is simple to modify, and consists of the important parts for determining the stability of the system.

Different simulations have been carried out, in order to examine the effect certain parameter variations have on the stability. Two systems have been considered, a constant power controlled system and a system where the motors had no controlling regime. An important factor is the value of the capacitance and inductance. A larger filter inductor results in an unstable system. Likewise, implementing a higher value capacitance will cause the system to be more stable.
Simulations have shown that a motor controller with a simple constant power control regime is unstable with normal values for the inductance and capacitance. However a damping branch can solve this problem up to an extent. A system without any controlling regime has proven to be more stable with smaller capacitance.

By utilising the model presented in this thesis, the stability of the system, consisting purely of a controlled constant power load or as a non-controlled load, can be investigated, and the impact of the LC-filter can be determined.

At the end of a power transmission line, the cable should be terminated carefully, otherwise it will create problems during the operation. A special equipment called termination is designed to tackle down the problems from cable ends. The main functional part of a termination is a piece of rubber with a cone shape. This rubber cone has a semi-conductive layer which is connected with ground potential. By doing this, the cone smoothens the concentric equipotential lines at the cable end, so the electric field stress there can be decreased. The cone in the termination is called stress cone. However, insulation degradation on the termination stress cone due to water contamination is often found in its operation. This, in the end, will cause operation failures in the power network. This research aims to investigate the influences brought by water contamination on the stress cone, and discuss possibilities of detecting the presence of water content by using electrical methods. By using 3D FEM simulations, the changes of electric field distribution, especially the filed along the oil-rubber interface, were studied. Based on the simulation results, we built up a small scale test setup to observe how water contamination behaves on the oil-rubber interface, and measured the changes of partial discharge signals and tanδ values. As conclusions, we discussed the electrical ways of detecting the presence of water contamination on the stress cone, and gave suggestions for the future work.

The calculation of power flow through electrical networks has been utilized commonly in transmission networks as the first calculation tool to assess their steady state conditions. However, the current introduction of renewable micro-sources (many of them single-phase connected) and the implementation of microgrids makes the calculation of power flow in distribution networks a more valuable and desirable tool.

This project presents the development of a software tool to calculate power flow, losses, voltages and currents of large-scale unbalanced distribution networks. The project has consisted of four main sections: modelling of the distribution network, prototype implementation using MATLAB, C-code implementation targeting large-scale networks and testing of the performance and reliability of the program.

The modelling of the elements of the distribution network has been performed under the phase domain (ABC frame) and its validity has been verified using a MATLAB prototype and the IEEE distribution test feeders. After verifying the models, a program has been written using C to improve performance. The PETSc library has been used to solve the nonlinear and linear problems required.

Finally, a test network of 452745 three-phase nodes has been created and used as input for the program, with the objective of testing the software.

As the scale and complexity of power systems increase, simulating them in efficient and accurate ways continues to
be a challenge in power systems engineering. Electromagnetic Transient (EMT) and Transient Stability (TS) simulation are the two main dynamic power system simulation methods. To simulate large and complex power systems in sufficient detail without sacrificing execution time, one of the idea is to perform a co-simulation that couples EMT and TS simulator. Although several attempts have been made to couple both simulator, only on rare occasions do these hybrid EMT-TS simulators couple two or more industry-adopted simulation tools.

The objective of this thesis is to to develop and study the benefits and limitations of the Electromagnetic Transient – Transient Stability co-simulation based on PowerFactory and PSS/E, both of which are among the most extensively used simulation tools in industry and academia alike. With regards to the objective, the EMT-TS co-simulation using PowerFactory and PSS/E has been developed. Then, several tests are performed to evaluate the function of each composing part of the EMT-TS co-simulation, and to test the integration between all its component. Next, The developed co-simulation is applied to study cases and the results are compared to a monolithic EMT simulation to evaluate its accuracy and execution time. Furthermore, the effect of TS and EMT time step to the accuracy and execution time of EMT-TS co-simulation have also been investigated.

The study case results show that the developed EMT-TS co-simulation has not been beneficial yet in terms of accuracy and execution time. Although the active power result shows a similar tendency with the monolithic EMT result, the difference between both are visible. The difference between both are more prominent in the reactive power result. The total execution time of the developed co-simulation in the study cases are in the range of 23-24 minutes, significantly larger than the total execution time obtained from the monolithic EMT simulation which is around of 12s. Also, it is found that reducing the TS time step from 0.02 s to 0.01 s slightly increases the total simulation time from 23 to 26 minutes. However, it does not contribute a significant improvement on the accuracy of the developed EMT-TS co-simulation. The result obtained from reducing the EMT time step to $ 25 \mu s$ is the same with the result obtained using $ 50 \mu s$ EMT time step. Moreover, the reduction of the EMT time step significantly increases the total simulation time from 23 to 42 minutes. The developed co-simulation still has a lot of room for improvement and further developments in this topic might increase its performance.

It is expected that DC grids will evolve into large meshed networks, which will have multiple DC voltage levels. Two DC grids operating at different voltage levels could be interconnected using a DC-DC converter. The role of the DC-DC converter in meshed DC networks is similar to the role of a transformer in traditional AC systems. The DC-DC converter could take a range of functions in DC grids. The DC-DC converter should prevent the propagation of DC faults and should also have the ability to isolate DC faults. Because of its high cost, the DC-DC converter cannot be used as DC breaker in meshed DC grids, but it can be strategically placed in the DC grid to provide protection zone separation. In case multiple HVDC (High Voltage Direct Current) lines need to be interconnected, a multiport DC-DC converter should be used. The DC hub is a multiport DC/DC converter which consists of an inner AC circuit and AC/DC converters interfacing each HVDC line. The main goal of the DC hub is the interconnection of multiple HVDC lines operating at different voltage levels and with different DC grid configurations.

In this thesis, the operation of a DC hub based on the MMC (Modular Multilevel Converter) converter was investigated. A three-port MMC-based DC hub was simulated using Matlab/Simulink. An important contribution of this thesis is a comparison of different DC hub topologies which could be used for the interconnection of multiple HVDC lines. Several designs for the inner AC circuit are suggested and the advantages and disadvantages of each recommended design are described in detail. Furthermore, a methodology is presented for the clearing of AC faults within the DC hub and the protection of the system during the fault. An AC fault within the DC hub should be cleared by opening the AC circuit breaker of the faulted port. A methodology is also presented for the safe connection or disconnection of additional ports from the DC hub. Any port should be easily connected or disconnected from the DC hub without affecting the operation of the other ports. The DC hub should also offer connection points to allow the interconnection of more ports. Finally, the operation of the DC hub is investigated when generation or consumption units are connected to its inner AC circuit. In case the DC hub is used for the interconnection of multiple offshore HVDC cables, the loads of the offshore platform or a nearby offshore wind farm could be directly connected to its inner AC circuit.

With the concepts analysed in this thesis, HVDC network system designers will be able to select the most suitable DC hub topology for the interconnection of multiple HVDC lines, use a methodology for the clearing of AC faults within the DC hub, use a methodology for the safe connection or disconnection of ports and understand the design limitations of the DC hub when generation or consumption units are connected to its AC side.

Focusing on real life commercial applications, this thesis presents the theoretical and practical control requirements of an application board, which exploits flexible bidirectional DC-DC converters, to power up a unit of multi USB type-C ports. Emphasis is given in the power management between the connected devices since the power flow is bidirectional.
The USB type-C interface allows power management between the devices and offers the possibility of using the Li-ion batteries of the end-devices as a backup power source in case of a blackout or source failure. At the beginning of this research, the appropriate topology of the converter is defined based on the requirements of the application. The power stage and the system control is fully simulated to prove the application concept. The final stage is the design and implementation of the real application board to extract useful results regarding the viability and feasibility of producing such an integrated solution.
The results arising from the application board prove the real application use. This research topic is setting the foundations for a further research on future applications of the USB type-C interface, in commercial or domestic buildings which are powered by a DC network.

Nowadays there is a movement towards the creation of HVDC grids. The massive integration of renewable energy sources, the operation of the current grid close to its limits and the need to facilitate the power trade are some of the reasons driving the research on HVDC grids. Towards HVDC grids, there are still some obstacles and technical limitations to be confronted. To address these problems, the idea of a DC hub is introduced. A DC hub is a multiport DC-DC converter, which serves as an actively controlled DC node. The basic parts of the DC hub are the ports, where AC/DC converters are used and the intermediate AC link. Until now there is no standardization of HVDC grids and thus, if different projects need to be interconnected a DC-DC conversion is necessary. A DC hub can offer solutions in the connection of HVDC lines with different voltage levels and different configurations. Moreover, it can facilitate the power exchange between any two HVDC lines connected in the DC hub. This thesis deals with the development of a DC hub model and its study under transient phenomena. More specifically, the ability of the DC hub to aid in the protection of HVDC grids is examined.
The first step to address this problem is to propose the most suitable multiport DC-DC converter topology. A literature review over different DC-DC converter topologies took place. The advantages, disadvantages and the ability to upgrade into a multiport DC-DC converter are examined. The most promising is the front-to-front topology where either a transformer or an LCL filter is used. Although the LCL DC hub offers small footprint and fault ride-through capability, its efficiency depends on the level of active power flow and its expandability is limited. In the current thesis, the front-to-front topology with an intermediate AC transformer is studied.
Before studying the DC fault response, the basic attributes of the DC hub must be acquired. A three-port DC-DC converter is designed where the state of the art modular multilevel converter is used in every port. The model operation is validated through step reference changes in the AC voltage, DC voltage, and active power. Moreover, the control role distribution is suggested, where the port with the higher power rating operates in AC voltage control mode. Being one of the main attributes, the connection/disconnection of a line without disturbing the operation of the DC grid is presented. For a smooth transition, the suggested steps are presented. The next step is to study the natural DC fault response of the DC hub. In the case of pole-to-ground faults, a design methodology is presented in order to acquire fault ride-through capability. Finally, the pole-to-pole fault contingency study leads to useful conclusions. A design methodology of adding large phase reactors between the converter and the transformer is suggested in order to limit the large fault currents.
The main contributions of this thesis are the following. First, a DC hub average model, using three modular multilevel converters in front-to-front configuration is provided. Moreover, the DC hub response to line connection/ disconnection is studied and steps are suggested for a smooth transition. Finally, after studying the natural DC fault response of the DC hub, the design requirements are provided in order to acquire fault ride-through.