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It is necessary to restructure and expand the Danish transmission system with significant changes as a part of the green transition. The Danish Transmission System Operator, Energinet, considers the use of underground transmission cables at 400 kV and 150 kV level instead of overhead lines whenever newtransmission lines as well as reinvestment in existing transmission lines are planned in the meshed grid. The extensive use of underground cables is known to lead to measurable changes in the propagation of harmonic voltages in the grid and we have no proper explanation of why grid changes lead to destinedvariations of harmonic levels in meshed power systems today.This paper presents observations of harmonic voltage propagation in a simple meshed system representing the fundamental topologies of transmission systems having series and parallel branches and it explains the basic behaviour of the propagation in the meshed system when it undergoes topologicalchanges, in the form of replacement of overhead lines with underground cables in both series and parallel connections and when the meshed system topology changes. As a contribution, the explanation of propagation phenomena of harmonics is based on standing wave theory and verified using harmonicload flow calculations.It is shown that both a series and parallel connection with underground cables greatly affect the propagation of harmonic voltages in the system and that the phenomena can be explained by means of standing voltage waves, determined solely by the terminal condition of the lines. It is found that a series connection of overhead lines and underground cables greatly increases the harmonic voltage in the system, whereas a new parallel connection significantly lower the harmonic voltage regardless the type of line. It is also shown that these phenomena can be foreseen using the determined reflectioncoefficients for the system.

The major challenges in the electrical design process of a fully composite pylon are addressed in this paper.One of the challenges is that the electrical dimensioning of the fully composite pylon, i.e. the phase-to-phase and phase-to-ground air clearances on the pylon, should be determined. The minimum required air clearances on the pylon are calculated based on an extensive insulation coordination study. Another challenge is the proper design of insulation for the unibody crossarm and the evaluation of electric field performance. In this regard, numerous finite element analyses (FEM) of the pylon are carried out to evaluate electric field magnitudes in the different regions of interest on the fully composite pylon. What’s more, a water induced corona discharge test was performed on an equivalent full-scale phase-to-phase composite cross-arm segment in wet conditions to verify the theoretical analysis based on FEM simulation. The next challengeis the lightning shielding performance of fully composite pylon, which has been addressed by electro-goemetric model (EGM) method and by long-gap electrical discharge tests with impulse voltage. Environmental effects, including audible noise, radio noise and electromagnetic emissions from OHLs in the pylon are investigated. Additionally, material selection for the fully composite cross-arm core is evaluated by experimental methods.On the basis of theoretical analysis and experimental results obtained in the present research work, the initial electrical design of the fully composite pylon is evaluated and feasible improving measures are put forward. This is a combined paper which contains all theoretical and experimental analyses’ results in such a way as to enhance and emphasize the completeness of each other. This paper provides an important basis on the design of fully composite pylon.

The Faroese Electrical Power Company SEV is aiming for a 100% renewable electricity sector by 2030. Today, the energy production is based on thermal, wind and hydro power. The renewable production accounts for roughly 50% of the total production. The Faroe Islands have a high potential of renewable energy resources with e.g. an average annual wind speed of 10 m/s and a precipitation of up to 3000 mm/year in some places. The future energy mixture has been investigated previously by a few studies with different aspects. These studies agree that investments should mainly be made in wind turbines, solar panels and pumped storage systems, in order to reach a 100% renewable electricity sector by 2030. Storage is a very important part of the future energy mixture, since the renewable energy sources cause a varying production, and the power system is isolated; thus, it is not possible to import or export power. This paper presents an initial optimisation of the least-cost path to a 100% renewable energy mixture. Anobjective function with constraints is defined. The objective function is set to minimize the total annual costs of the systems. The costs included are investment costs and fixed and variable operation and maintenance costs. The minimisation procedure is a algorithm developed for the study. The technologies included in the optimisation are wind, solar, tidal and pumped storage. Other technologies considered, but not included in the optimisation, are the existing hydro plants and the planned biogas plant. The potential production from the different types of technologies included in the optimisation, is based on the available energy resources and technology characteristics. According to this study, the power company should invest in 85 MW of wind power, 204 MW solar power and a pumped storage system, with a pumped storage capacity of2 GWh and a generation and pumping capacity of 91 MW and 179 MW respectively. The results of this study are compared to the current RoadMap to a 100% renewable electricity sector, while the limitations of the work done are discussed and future works regarding this topic are presented.

This paper investigates the impacts of midpoint static synchronous compensator (STATCOM) on the conventional loss of field (LOF) protection of synchronous generator. Simulation studies are conducted by using the phase domain generator model available in the real-time-digital-simulator (RTDS). A realistic field circuit model is considered for the generator, which allows simulating different LOF failures in order to have a more accurate study. Obtained results show that the conventional LOF relay needs more time to detect LOF failures in presence of midpoint STATCOM. The amount of this time delay depends on the generator initial loading and the type of LOF failure.

The fault location algorithm is commonly used to measure the distance between the measurement point and the fault point in a transmission line. One of the feasible techniques is the impedance -based methods, in which the single-end fault location algorithms are widely used in distance relays with the consideration of their simple theory and efficient cost. However, most of the existing single-ended fault location algorithms are based on the equivalent RL line model. They are relatively accurate for the short transmission line case, while this may be not true in a long transmission system due to the mismatching of the RL model and the real transmission line. To overcome this demerit, a modified impedance-based fault location algorithm is developed for estimating the fault distance in this paper. The core idea of the new method is to applying the Bergeron method to compute the voltage and current at the setting point of the distance relay, which make the setting point a virtual starting point when building the distance measurement equation. Then, the voltage drop equation between the setting point and the fault point is built based on equivalent RL line model. Finally, the conventionalfault location is adopted to solve the fault distance. By this way, the new method shows a higher computation accuracy than the conventional method for the faults occurring near the protective boundary. Besides, with the coordination of the directional element, the distance relay can achieve a reliable operation along the whole protective zone. The simulation model is built in DIgSILENT/PowerFactory and the data are processed in Matlab. The results from both the conventional algorithm and the proposed new one are compared with each other and the accuracy has been investigated in simulation results. The results verify that the proposed new fault location method performs higher accuracy for remote faults than the conventional Takagi method, which can improve the reliability of the impedance -based distance protection to remote faults in a long transmission system.

Distribution networks are evolving toward the vision of microgrids, which are defined as power systems that contain both loads and Distributed Energy Resources and operate in a grid-connected or islanded mode. Correct operation of the OverCurrent protection is compromised in a typical microgrid because the fault current is significantly lower in the islanded mode compared to the grid-connected mode and consequently a single set of settings for the OC relays is not sufficient. This paper propose a communication-assisted protection scheme that is able to operate correctly in a radial Medium Voltage feeder with variable or reduced-short circuit currents. The proposed method is effective against the Single Phase to Ground fault and utilises symmetrical components of the fault current. More precisely, the Single Phase to Ground fault is indicated by evaluation of the ratio of zero-sequence current over positive-sequence current, which is a complex number. It is shown that the presence the fault in a radial feeder is associated with a non-zero magnitude of this ratio. Moreover, the closest relay placed upstream the fault will notice the highest magnitude of the ratio of zero-sequence current over positive-sequence current, while the relays placed downstream the fault detect a zero magnitude for their corresponding ratios. Protection relays calculate the magnitude of the proposed ratio and then exchange the obtained value with the adjacent upstream relays using communication. Therefore, each relay would have access to its own ratio and to the ratio provided by the adjacent relay. Finally, the relay with the highest calculated magnitude of the ratio of zero-sequence to positive-sequence current will trip, thus clearing the fault. The new protection scheme is implemented in a test Medium Voltage microgrid using PSCAD and simulation results prove its effectiveness against various Single Phase to Ground faults. The proposed method solves some of the protection issues associated with a high penetration level of Distributed Generation, as it is less sensitive to the variable short-circuit power and reduced short-circuit currents compared to the OverCurrent protection. In addition, coordination of the relays that use the proposed algorithm does not require a prior knowledge of the load or fault currents. The setting procedure of the relays that use the new protection algorithm is relatively simple in a radial feeder and a single set of settings is valid for a wide range of network conditions.

Modern day overhead transmission lines are taking a giant leap in modernization, with the change in power generation from fossil fuels to renewable sources such as solar power, hydro power and wind power. The renewable generation needs to be connected to a large scale high voltage transmission grid. In Europe alone, 28.000 km of 400 kV transmission line is needed by 2020 to fulfil the aim of providing 20% of Europe’s energy from green energies. It means that more than 100.000 new pylons will be needed [1]. For this reason, the next generation of overhead line is introduced, by developing new design pylons that are easier to erect, less costly, smaller and environmentally more feasible than the old ones, which is important to get public acceptance. In this regard, a fully composite pylon for 400 kV overhead transmission lines is presented with a new innovative design concept shown in Figure 1. The integration of insulators in the cross-arm design is the prominent feature of the fully composite pylon in comparison with conventional towers. The unibody cross-arm of the pylon is expected to have 30 degree inclination and all conductors are fixed to the top of the cross-arm by cable clamps or special high-quality insulation sections. Thus, the configuration of phase conductors on the cross-arm is in the form of diagonal and differs from other widely used configurations in overhead transmission lines i.e. horizontal, delta and vertical configurations. Unlike traditional steel lattice towers, the composite pylon does not provide access to ground potential due to its non-conductive materials. Therefore, the lightning shielding of pylon and conductors requires a ground potential access to shield wires by utilizing a ground cable inside the hollow cross-arm and pylon body.Efficient design of a lightning shielding system for the fully composite pylon is one of the major challenges in the electrical design of the pylon which also has to be considered in terms of mechanical and material designs. The other challenges are determination of air clearances and design of shed profiles on the unibody cross-arm which are discussed in [2] and [3], respectively. In this paper, the weaknesses and strengths of the preliminary assigned shielding angle for the pylon are investigated and subsequently, the shielding failure rate of the line is derived from the electro-geometric model (EGM).

In this article, measurements of a 245 kV 3x1x630 mm2 armoured and unarmoured cable is presented along with a description of the data processing of the measured quantities. The results of the measurements show that the losses in the armoured cable are larger than the losses in the unarmoured cable. The measurements also show that the equivalent phase resistance of the cable increases as a function of the current.

For more than 20 years it has been a consistent plan by all Danish governments to turn the Danish power production away from fossil fuels towards renewable energy. The result today is that 37% of the total Danish power consumption was covered by mainly wind energy in 2013 aiming at 50% by 2020. Another consequence is the public way of generally thinking green which have led to a national decision of undergrounding not only all of the Danish distribution system but also the future transmission system. These issues initiate the infrastructure constructions of the transmission system i.e. a large amount of overhead lines over 100 kV as well as the new planed transmission lines will be undergrounded; the transfer capacities will be enlarged by upgrading the interconnections. Large amounts of reactive power components will be placed in the system partly for cable compensation and partly for voltage security. Consequently, the ramping speed of the transits in the main corridors via the interconnections is foreseen to be increased. The voltage control based on the present system is thus becoming a challenging objective. This survey paper presents some of the existing AVC systems i.e. system structures, objectives, constraints, algorithms for optimal power flow and some special functions in particular systems, which inspires the concept design of a Danish AVC system to address the future challenges of voltage control. In the concept, the Danish AVC design is based on a centralized control scheme. All the buses are monitored where the voltage magnitudes are maintained continually. The loss minimization including switching cost is the objectives of the AVC system. The reactive power reserves as constraints are taken into account to limit the regulation capabilities of generators. The Day ahead and short-term forecast is to be implemented to reduce the working load of the real time system, as well as to minimize the numbers of switching times of the discrete components. The fallback controllers are to be installed in selected substations to enhance the system reliability and the robustness in case the substation loses the telecommunications to the control center. RPCs will be integrated to the AVC system as normative regulators in the later stage. Distributed generation units can be organized as virtual power plants and participate in voltage control at transmission level. Energinet.dk as the Danish TSO will implement the first stage of the AVC system by 2016.

In November 2008 the Danish government decided that all overhead lines below 400 kV should be replaced by underground cables. This is due to a demand of reducing the overall visibility of the transmission system, sometimes referred to as the beautification of the transmission system. The agreement furthermore included to reinforce some of the existing 400 kV transmission lines. This is due to both increasing wind penetration and power flow between Scandinavia, Germany and possible future connections. As a part of reinforcing the 400 kV transmission system in Jutland, Denmark, the Danish TSO (Energinet.dk) is in the process of constructing a new gas insulated substation (GIS) in Revsing. As a part of this process, new Eagle pylons will replace some of the existing Donau pylons. The new Eagle type pylon is meant to reduce the visual impact of transmission lines. The reliability of the substation in Revsing is of great importance as it is part of the 400 kV systems backbone between Sweden, Norway, Germany and the offshore windfarms in Horns Rev. The design of the insulation coordination for GIS must therefore be studied carefully. During a disconnector operation in GIS, very fast transient (VFT) may generate overvoltages (VFTO) inside the enclosure. Because the gas insulated system must be viewed as non-self-restoring, it is important to ensure that the voltage inside the GIS does not exceed the insulation strength. This must therefore be accounted for, when conducting an insulation coordination study of a GIS.This article describes how the VFT phenomenon occurs inside the GIS and how it may generate overvoltages. This includes an explanation of how it is generated, what causes it and why it is so fast. A schematic consisting of the surge impedances from the manufacturer of the GIS is simulated and compared to the same model with a number of capacitances added (representing the corresponding component). These added capacitances were not modelled by the manufacturer, but were added in order to further increase the level of detail. This is important as VFT may also be generated by circuit breaker (BRK) operations, a ground switch or due to a fault [1]. A detailed model is more likely to detect a VFT generated by e.g. a fault than a simplified model. It is shown via simulations in a EMTP software, how the level of modelling detail affects the results. As may be seen from the simulation results, there is a significant difference between the voltage characteristics when simulating the GIS with and without the added components. The difference is approximately 1.5 p.u., and is apparent 35 ns from when the disconnector operation is performed. It is of interest to investigate what the primary cause of this difference is. A further analysis of this difference, lead to a closer look at how the breaker is represented in the model from the manufacturer and the model with the added components. A comparison of the two models revealed the importance of detailed modelling, especially all of the capacitances which are present in the GIS for e.g. circuit breaker, disconnector (DS) and spacers.

This paper presents some of the results obtained from the electrical measurements on a 99.7 km, 150 kV three-phase AC cable, connecting 215 MW offshore wind farm Horns Rev 2, located in Denmark west coast, to Denmark's 400 kV transmission network. The measurements were performed at nominal voltage. The cable was energised from the grid side with the receiving end open. An 80 Mvar shunt reactor was installed in the middle of the cable and it was energised together with the cable. The use of nominal voltage allows to present and to verify some of the phenomena that are common in long HVAC cables, but unusual in overhead lines. In this paper is made a characterization of the cable and the power transmission system in which the cable is installed, followed by a description of the measurement equipment and of the test setup/procedures. A theoretical explanation and presentation of different analysed phenomena is conducted. The cases analysed in this paper are: Zero-missing phenomenon, Ferranti effect, energisation transient, effect of the cable's connection in the busbar voltage and cable disconnection. For all the phenomena described in the paper measurement data are presented and it is verified if the obtained results are in accordance with the theory and also with simulations performed in PSCAD/EMTDC. With the exception of the cable disconnection, for all the remaining cases introduced in this paper the measurements confirmed the theoretical expectations. Depending on the cable disconnection sequence, an overvoltage may appear in one of the phases, this overvoltage depends on the mutual inductance between the shunt reactor phases and it is not shown in the simulations. The measurement's data files and extra information about the cable and power transmission system can be received by contacting the first author of this paper.