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Working under PWM voltage with fast voltage rise time and high frequency, medium frequency transformers may suffer from overvoltage and temperature rise in the winding. As a result, partial discharge may be triggered with high probability, threating the insulation reliability. Therefore, it is necessary to evaluate the insulation capability of medium frequency transformers with respect to partial discharge and deterioration. This paper introduces a test system based on power electronic modules that can generate repetitive pulse voltage. Then, by using this system, partial discharge inception voltage test under different temperatures and endurance lifetime test are conducted on Nomex paper, which is an insulation material usually used in medium frequency transformers. The results show the change trending of the partial discharge inception voltage of Nomex paper with temperature and indicate an accelerated aging of this material with the presence of continuous discharge.

The integration of increasing share of renewable based distributed generation in distribution networks brings great challenges to voltage control. To address this issue, this paper presents a two-stage distributionally robust chance-constrained receding horizon control algorithm. In the proposed method, the distributionally robust chance-constrained reformulation of chance-constrained voltage control is derived, which is not only accurate, but also computationally efficient. Rather than perfect knowledge about the uncertainty associated with renewable generation, the proposed method only requires partial information of the underlying probability distribution. In addition, the mechanical voltage regulation devices and the DG inverters are controlled in two stages, considering their different characteristics in voltage control. By taking into account both the current and forecasted renewable generation, the proposed method utilizes receding horizon control to determine the control actions of voltage regulation devices. The effectiveness of the proposed method is demonstrated by case studies on unbalanced IEEE-123 bus system.

In this paper, a comprehensive investigation of substation grounding soil structure effects on switching/lightning transients is presented. The full-wave computational solution Method of Moments (MoM) is adopted to solve Maxwell's equations. The significant advantage of this technique is its ability to deal effectively with substation grounding systems (SGSs) at high frequency. This evaluation is carried out considering two soil structures: (1) a uniform soil and (2) soil with a layered structure. The impacts of both soil structures on the peak transient overvoltage values are compared. To begin with, the harmonic impedance matrix (HIM) of the layered multi-terminal SGS is computed. Then, a vector fitting approach is employed to incorporate the accurate SGS behavior by relying on the obtained HIM into time domain simulations. Finally, the transient overvoltages and maximum ground potential rise (GPR) are computed. The chief contribution of this paper is the transient assessment of the two-layer multi-terminal soil structure of the large SGS taking into account the frequency dependence model of the electrical parameters of soil, which is not generally investigated and instead, treated simplistically. According to the results of the study, the lack of an exact grounding system models may lead to underestimations/overestimations of the GPR and the overvoltage values. The results demonstrate the need for the grounding modeling of SGS considering layered soil structure to be incorporated into power system transient analysis in a wide range of frequencies.

This paper established a transient electric field model of a high-voltage high-frequency (HVHF) transformer based on circuit-field coupling analysis. As the key component of power electronic transformers (PETs), the operating conditions of the HVHF transformer are determined by the converter circuit containing it. To find out the electrical stress withstood by the solid insulation, it is essential to model the transient electric field while considering the circuit model of the converter. The circuit model of the converter and the 3-D transient electric field model based on the finite element method (FEM) are built in ANSYS. The voltage and current waveforms on both primary and secondary sides of the HVHF transformer obtained from the converter model are coupled to the transient electric model as the excitations. Based on the established model, a case study of a 60 kHz HVHF transformer used for the dual active bridge (DAB) converter is investigated, and the electric field characteristics of the insulation are analyzed. The electric field distribution inside and on the surfaces of the insulation is present, and magnitude values of the highest electrical stress inside and on the surfaces of the insulation are identified. The theoretical verification is carried out based on the grid convergence index (GCI) method and it verifies the accuracy of the proposed model. This model could be used to calculate the overvoltage capacity of the HVHF transformers in various power electronics applications and it is an essential step in building the digital twin model of the HVHF transformer.

Fast and accurate fault detection and localization are important topics in voltage source converter-based high-voltage direct current transmission (VSC-HVDC) grids in order to isolate the fault after the first milliseconds of occurrence. Common signal processing methods used for detecting fault traveling wave (TW) peaks like Fourier transform, S-transform and wavelet-based techniques transform the signal into the frequency domain, requiring complicated computations. In this paper, the application of mathematical morphology (MM)-based filters for detecting and locating faults in VSC-HVDC links is studied. MM-based methods analyze the signal in the time domain and detect the wave peaks accurately. Multiple MM-based filters resulted from basic MM operators are presented and used for TW-based fault study in VSC-HVDC grids. Several fault cases are applied to the CIGRE VSC-HVDC model in PSCAD, and the MM-based scripts are written in MATLAB. The impact of different sizes and types of the structuring elements on the accuracy of peak detection is analyzed. The results show the accuracy of MM filters for detecting and locating fault transient in VSC-HVDC links. The proposed method gives accurate results for both low-impedance and high-impedance faults.

Frequency and voltage stability is a challenge as power systems move towards a more renewable future. This study focuses on the power system of Suðuroy, Faroe Islands, which is in the transition towards 100% renewables. The impact of three events on the frequency and voltage responses has been simulated based on 2020, 2023, 2026 and 2030 and with different settings using a measurement validated model. These results show that additional ancillary services, provided by e.g., batteries and synchronous condensers, are required to keep the stability level at the same level as today. The isolated power system in Suðuroy (~10% of total annual demand) will be connected to the main grid (~90% of total annual demand) in the future (2026 according to the RoadMap), and thus the system has also been studied with this interconnection. According to the simulation results, the main grid contributes significantly to the power system stability in Suðuroy when the systems are interconnected. The impact of how the main grid has been represented is also analysed by conducting simulations using a detailed model of the main grid or approximated models. The results show that the suggested approximated models do not show a sufficiently accurate response compared to the detailed model; especially when batteries in the main grid are contributing with active power regulation. Therefore, new approximated models of power systems with high shares of inverter-based technologies should be developed with this consideration. Finally, the contribution from wind turbine inertia emulation is analysed and shows that the system frequency nadir can be improved with the emulated inertia feature switched on.

In this paper, control principles to perform black start services by offshore wind farms (OWFs) integrating grid-forming (GFM) control are presented. The strategy consists in exploiting a GFM battery energy storage system (BESS) to provide black start services by an OWF equipped by grid-following wind turbines. Controller modelling and operation methodology are explained. In order to show the proposed control and operation principles, the analysis is implemented on the CIGRE Working Group C4.49 benchmark, which could resemble modern large OWFs in the United Kingdom, such as Hornsea Project 1 and 2. Analysis simulations in the software PSCAD show the success of the proposed strategy.

In this study, a three-settlement electricity market including the day-ahead, risky, and real-time trades is analysed. A wind power producer faces generation uncertainty and needs to cover its real-time generation deviation unless it would be penalised. To do so, wind power producers participate in an intraday risky power market, as well as in the day-ahead and real-time markets. Mathematical program with equilibrium constraints (MPEC) is used to model the behaviour of each generation company in its profit maximisation process. The complementarity constraints describe the market clearings and constitute a bi-level optimisation problem for each company. The companies have a combination of wind and thermal power plants in their generation portfolios. Wind power uncertainty is modelled using plausible scenarios. The joint solution of all companies’ MPECs makes an equilibrium problem with equilibrium constraints (EPEC). This EPEC is linearised and solved using mixed integer linear programming techniques. EPEC solution gives the Nash equilibrium. Results show the effectiveness of the risky power trades on reducing the reserve requirements, increasing the wind power producers’ profit, and reducing the wind power spillage.

With the increasing in frequency, intensity, and side effects of grid outages caused by weather based events, the importance of resiliency is also increased. This study presents a novel multi-objective optimization problem for improving the resilience of smart grid against natural disasters in the presence of a considerable amount of line outages occurring due to the weather conditions. The proposed model considers the management of distributed energy resources (DERs), energy storages (ESs), and dynamic topology reconfiguration of the distribution network. The main goal is based on a dynamic resilient operational planning using prediction of weather condition. The objectives in the proposed approach are defined as: to minimise operational cost of distribution network from the system operator's viewpoint, to minimise the penalty costs for energy not supplied and side effects of outages, and to maximise benefits of DER-ES owners by using flexible reconfiguration and dynamic operational planning considering weather conditions. A hybrid optimization algorithm based on genetic algorithm and -constraint method using fuzzy decision maker is employed to choose the best solution from a provided Pareto optimal set. The outcome of the Pareto solution specifies the best scheduling for DERs, ESs and switching of lines during the scheduling horizon.

A real-time realistic framework for synchronous generator loss of field (LOF) protection studies is suggested in this paper by using the real-time digital simulator, which allows the user to realistically model the generator field supply circuit. By using such framework, the performance of LOF protection algorithms can be comprehensively evaluated in the face of the various LOF events including the short circuit and two types of the open circuit events, while the open circuit occurrence cannot be studied in the conventional simulation programs. The obtained results demonstrate that the generator dynamics are significantly different in the face of various LOF failures and then all of them must be regarded to develop the new LOF protection algorithms. By using the suggested framework, the performance of some recently proposed schemes along with the conventional impedance and the admittance relays are comprehensively evaluated. The obtained results from applying the various LOF failures show that, against the conventional relays, the new schemes cannot detect all types of LOF failures and so, they do not have enough reliability.

In a large-scale power electronic system such as a wind farm, the mutual interactions between the power converter controllers and passive components may lead to instability problems or undesired dynamic response. This paper presents an optimum parameter design procedure for the power converter controllers in a power electronic system in order to guarantee a stable operation and to guarantee an acceptable dynamic response. In the approach, first, all oscillatory modes are calculated by a multi-input multi-output (MIMO) transfer function matrix of the power system; then, a multi-objective optimization procedure based on the genetic algorithm (GA) is presented to place the modes in the desired locations in order to increase the stability margin and to improve the dynamic response. Time-domain simulations of a 400-MW wind farm in the PSCAD/EMTDC environment confirms the effectiveness of the presented design approach.

This paper presents a new method to protect the radial feeders on marine vessels. The proposed protection method is effective against Phase-Phase (PP) faults and is based on evaluation of the ratio between the negative-sequence and positive-sequence of the fault currents. It is shown that the magnitude of the introduced ratio increases significantly during the PP fault, hence indicating the fault presence in an electric network. In this paper, the theoretical background of the new method of protection is firstly discussed, based on which the new protection algorithm is described afterwards. The proposed algorithm is implemented in a programmable digital relay embedded in a Hardware-in-the-Loop (HIL) test setup that emulates a typical maritime feeder using a Real Time Digital Simulator (RTDS). The HIL setup allows testing of the new protection method under a wide range of faults and network conditions and the experimental results demonstrate its effectiveness in all scenarios conducted. The proposed protection method offers a solution to the protection challenges associated with variability of the short-circuit currents in radial feeders, advancing in this way the traditional mean of protection in maritime feeders, represented by OverCurrent (OC) relays.

The high penetration of renewable energy into distribution networks poses increasing challenges on voltage control. To address this issue, this paper presents a double-layer stochastic model predictive control algorithm to regulate voltage profile in active distribution networks. In the proposed algorithm, voltage regulation is achieved by coordination of an upper layer controller and a lower layer controller. In the upper layer, the number of operation of mechanical voltage regulation devices, including transformer with on-load tap changer and capacitor banks, is minimized in an hourly timescale. In the lower layer, the controller minimizes the active power curtailments and power losses with a control period of 5 min. The proposed double-layer stochastic model predictive voltage control utilizes not only the reactive power control, but also the active power curtailment to regulate bus voltages. In addition, mechanical voltage regulation devices and distributed generations are controlled in two different timescales. Case studies on a modified IEEE-33 bus system demonstrate that compared with traditional control and two-stage stochastic voltage control, the proposed algorithm can achieve an improvement of 8.05% and 7.43%, respectively.

Loss of excitation (LOE) relay is one of the most essential protection elements for synchronous generators in power plants. During the last few decades, several LOE detection methods have been proposed, while limited schemes such as admittance- and impedance-based ones have been adopted for industrial applications. This study investigates and compares the behavior of some practical LOE detection schemes through extensive simulation scenarios, and from the reliability, speed, and security points of view. The simulation scenarios are accomplished by using the real-time-digital-simulator, where the phase domain model of the synchronous generator is used to develop a realistic and typical power generation system. Employing such a system, different types of complete and partial LOE incidents can be applied according to IEEE Standard C37.102-2006, while the performance of any scheme can be assessed through accurate and realistic LOE scenarios.

Power systems are currently experiencing a transition towards decarbonisation of electrical generation through large-scale deployment of renewable energy sources. These are gradually replacing conventional thermal power plants. Consequently, in case of a total/partial blackout, conventional black-start resources are not ready for operation. Offshore wind farms, with their large capacity and fast controllers, show potential as innovative black-start units. This introduces the need of a new design for the wind power system. In this paper, challenges and possible solutions in integrating black start service into offshore wind farm will be presented. A first challenge is represented by the implementation of a black-start unit. The black-start unit should be capable of forming the wind farm power island, withstanding transient phenomena due to energisation. There could be several different solutions, e.g. the integration of grid-forming converters in the wind farm design. These could be battery energy storage systems (BESSs) and/or new generation grid-forming wind turbines. The selected challenges are analysed using simulation on a wind farm, and the proposed solutions discussed. It can be concluded that a system comprised of BESS, grid-forming wind turbines and STATCOM, in combination with novel technologies such as virtual synchronous machine, soft-charging, harmonic active filtering, etc. represent a good proposal for the integration of black start into offshore wind farms.

A review of the ongoing research on black start (BS) service integrated with offshore wind farms (OWFs) is presented in this paper. The overall goal is to firstly gain a better understanding of the BS capabilities required by modern power systems. Subsequently, the challenges faced by OWFs as novel BS service providers as well as an outlook on the ongoing research which may provide solutions to these are presented. OWFs have the potential to be a fast and environmentally friendly technology to provide BS services for power system restoration and, therefore, to ensure resiliency after blackouts. As a power electronic-based system, OWFs can be equipped with a self-starter in the system in order to perform BS. The self-start unit could be a synchronous generator (SG) or a power electronic unit such as a grid-forming (GFM) converter. Preliminary BS studies performed in PSCAD/EMTDC are presented in a simplified OWF system via an SG as the self-start unit. Consequently, technical challenges during the BS procedure in an OWF benchmark system are outlined via theoretical discussions and simulations results. This is useful to understand the threats to power electronics during BS. Finally, the most relevant GFM strategies in the state-of-the-art literature are presented and their application to OWF BS is discussed.

Thanks to the solid standards and principles of design and restoration planning after a blackout, power systems in developed economies generally show a high level of resiliency. Nevertheless, this power system restoration practice stronglyrelies on conventional power plants, e.g. large thermal power plants. As future global goals aim at reducing the use of fossil fuels and lowering carbon-dioxide emissions, conventional power plants are often taken out of operation. These are characterised by slow start-up times and considerable use of fossil fuels. In this context, large offshore wind farms (OWFs) show potential as renewable-based black start (BS) service providers. These can be equipped with a self-starter, e.g. synchronous generators or innovative power-electronic-based converters, such as battery energy storage systems (BESSs) and/or grid-forming wind turbines in order to BS the system. Additionally, state-of-the-art complementary devices such as STATCOMs or synchronous condensers can help with dynamic regulation and support the OWF both in island operation and BS. In this paper, an overview of different system configurations for OWF BS is presented. Firstly, the requirements for BS from non-conventional power plants are outlined. Afterwards, the challenges faced by OWFs to fulfil these requirements are identified. Finally, different solutions for system configurationto equip OWFs for BS are proposed.

This paper presents a double-stage voltage control method based on model predictive control to address the voltage regulation in distribution network with high penetration of distributed generation. In the first stage, the operation times of transformer with on-load tap changer and switchable capacitor banks are minimized in hourly timescale. In the second stage, the controller minimizes the distributed generation curtailment while guaranteeing bus voltages within allowed limits with a control period of 1 min. In this paper, an analytical method is applied to calculate the voltage sensitivities with respect to power injections and tap changes. Numerical simulations of a modified IEEE-33 bus system are performed to validate the proposed method.

This study proposes a modal online differential fault detection and localization (MODF) scheme for VSC-MTDC cable transmission. Modal theory is used to remove the coupling between layers of two-core extruded high-voltage direct current cable. As a result, the decoupled modes are determined and the modal matrix resulted. In high frequencies, the modes have different velocities, which result in a time difference between modal initial peaks. This time delay is used to design modal fault detection and localization scheme, which can work in a variety of strategies like pilot or non-communicated protection systems, one-sided or two-sided relaying, and single-zone or double-zone relay settings. Novel modal time–distance characteristics are introduced for adjusting the zone setting of relays in the MODF scheme. Multiple faults are applied to the CIGRE VSC-MTDC model in the real-time digital simulator with a Beckhoff CX5130 controller in the hardware-in-the-loop setup for testing the proposed MODF scheme. The advantages and weak points of each strategy are analyzed and compared for the modelled faults. Hilbert–Huang transform is used for the accurate detection of modal initial peaks. It is shown that the method gives accurate and selective results for a wide range of VSC-MTDC configurations.

This paper presents the modal analysis of different HVDC cable bundles. Using modal theory, specific transformation matrices are presented, and the modal equivalent circuits are illustrated. The behavior of the resulting modes is studied in a wide frequency range, the impact of different cable parameters and burial configurations on the modal characteristics are analyzed. A concept for online fault localization in HVDC cable transmission based on the difference between the modal velocities and consequently the arrival times of the modal traveling wave (TW) initial peaks is presented. In this method, modal voltages and currents are used instead of direct measurements and the proposed concept uses one-sided measurement without needing to detect the second TW reflections. Multiple faults are applied to CIGRE HVDC models in PSCAD/EMTDC and the modal analysis is obtained in MATLAB. The impact of different sampling frequencies, fault resistance, fault location and different grounding schemes on the proposed modal fault location principle is analyzed. The theoretical results show the accuracy of the proposed modal fault localization concept for different HVDC cable bundles.

A proposal for a fully composite pylon has been designed to meet the requirements of compact structure and elegant appearance for new-generation 400 kV transmission towers, able to save lines corridors and reduce visual impact. Correspondingly, a method of external down-lead has been proposed to bring grounding potential to the shield wires. Based on this design, in this paper, lightning overvoltage level of the fully composite pylon is investigated by using transient simulation tools and backflashover performance is evaluated through Monte Carlo method. An equivalent distributed parameter model of the pylon is established, with the stray capacitances of phase conductors calculated in advance. Monte Carlo method is used to simulate the randomness of lightning current waveform in the nature. The overvoltage levels are simulated in PSCAD to determine the occurrence of backflashover. The backflashover rate (BFOR) is estimated by accumulate the probability of backflashover after repeated random sampling of lightning current. Simulating results show that the BFOR is 0.074 flashes/100 km·year and the overvoltage level calculated becomes lower in the modified model considering stray capacitances. Effects of both the lightning peak current and the tail time on the probability of backflashover occurrence is also summarized.

Combined faults occurring between different voltage levels in overhead lines present a challenge for distance protection. Previous work has shown that such faults most often appears as single-phase-to-ground (SPTG) faults in a normal type of overhead line. However, it is not obvious that distance relays will identify and select the correct fault loop according to being similar to SPTG, as all six fault loops get excited when combined faults occur. A study where two distance relays of different manufactures are tested using transient replay and secondary test equipment is presented, in order to reveal which fault loops are activated and whether a safe trip for combined faults happens.

Correct setting of protection relays are of major importance to the reliable operation of the power system. The root cause of two consecutive blackouts is analysed and shown to origin from a combination of several minor errors which all can be related to an insufficient/wrong setting of the distance relays and their optional functions together with insufficient testing when putting into operation. The learning lessons are discussed and outlined in order to avoid future blackouts having similar origin.

Participation factor analysis is an interesting feature of the eigenvalue-based stability analysis in a power system, which enables the developers to identify the problematic elements in a multi-vendor project like in an offshore wind power plant. However, this method needs a full state space model of the elements that is not always possible to have in a competitive world due to confidentiality. In this paper, by using an identification method, the state space models for power converters are extracted from the provided data by the suppliers. Some uncertainties in the identification process are also discussed and solutions are proposed, and in the end the results are verified by time domain simulations for linear and nonlinear cases with different complexities, no matter which domain (phase or dq) is used.

Some couplings exist between the positive- and negative-sequence impedances of a voltage-sourced power converter especially in the low frequency range due to the nonlinearities and low-bandwidth control loops such as the phase-locked loop. In this paper, a new numerical method based on the harmonic transfer function for analysis of the linear time periodic systems is presented, which is able to handle these couplings. In a balanced three-phase system, there is only one coupling term, but in an unbalanced (asymmetrical) system, there are more couplings, and therefore, in order to study the interactions between these couplings a matrix-based method should be used. No information about the structure of the converter is needed and elements are modeled as black boxes with known terminal characteristics. The proposed method is applicable for both power quality (harmonic and interharmonic emissions) and stability studies, which is verified by simulations in this paper.