Due to deregulation of distribution network investment businesses, geographically divided microgrids may be owned and operated by different entities. In this study, distributed multi-area load flow (LF) calculation method that can preserve the operational independence of each entity is necessary. However, conventional fixed-point iteration-based distributed LF (DLF) algorithm may encounter convergence problems in active distribution networks integrated with high penetration of distributed generators (DGs). In this work, the authors propose a more general and novel DLF algorithm to better accommodate the integration of DGs. In this algorithm, the original non-convex LF model for multi-microgrid systems is convexified into a second-order conic LF problem and then decomposed into a regional LF subproblem using a modified alternating direction method of multipliers (ADMM). The conventional ADMM is enhanced by asymmetric decomposition and partial boundary penalty relaxation with regard to the specific LF problem structure to accelerate the convergence procedure. Numerical tests on an IEEE 69-bus distribution system and a 345-bus system show that the proposed method outperforms the fixed-point iteration-based DLF method.

This study presents a robust subsynchronous damping control (RSDC) to mitigate the subsynchronous resonance (SSR) in a practical wind power system with series capacitor compensations. Firstly, the configuration of the RSDC including the subsynchronous damping calculator and the subharmonic voltage source converter is proposed. Then, the impedance-based network model for the whole system is established and the stability criteria is introduced. Thirdly, to increase the robustness of the proposed controller, its parameters are optimised for the varying operation conditions. Finally, the RTDS-based hardware-in-loop tests with the physically implemented RSDC have fully demonstrated its effectiveness in stabilising SSR under different operation conditions and disturbing scenarios, including the switch-on of series capacitors, ramp-down of wind speed and the variation in the number of online wind turbine generators.

This study discusses a multi-period co-optimised generation and transmission expansion planning (GTEP) problem while considering a proliferation of demand side resources (DSR). Uncertain renewable energy variations and load fluctuations in the long-term planning horizon are addressed, and a system state model derived via k-means clustering algorithm is adopted to capture temporal operation features. The problem is formulated as a two-stage robust optimisation model with mixed-integer recourse, in which annual investment decisions of generation, transmission, and DSR assets are determined in the first stage and short-term operation decisions of individual system states are made in the second stage. In recognising that considering DSR deployment and the system state model brings significant computational complexity, an extended column-and-constraint-generation algorithm is adopted to effectively solve the proposed planning problem. Numerical studies show that integrating DSRs into multi-period GTEP could effectively postpone or even avoid expensive generation/transmission investment in the planning stage, and improve economic efficiency in the operation stage.

In this study, a sensitivity-based optimal reliability design (ORD) model of the AC/UHVDC (ultra-high-voltage DC) system is proposed. The model can be used to adjust the reliability parameters of the replaceable AC equipments and UHVDC components to satisfy the pre-specified system reliability while minimising the total cost. The corresponding sensitivities provide references to identify the non-coherency equipments and to adjust the reliability parameters while avoiding the repeated calculation of the system reliability. To realise the proposed ORD model, the optimal power flow model considering the unbalanced operation mode of the UHVDC is proposed, and the incremental transition rate of the UHVDC is derived, thus to quantify the system reliability indices. Combined with the sensitivity model of the UHVDC itself, the state probability is used to connect the reliability indices at the system level and the UHVDC components’ parameters, obtaining a direct solution of the corresponding sensitivities. The numerical results based on the modified IEEE reliability test system (RTS) test system are provided to validate the feasibility and accuracy of the proposed models.

Three-phase four switch (TPFS) inverters are an effective solution to reduce the size and cost of the distribution STATic COMpensator (DSTATCOM). The performance of TPFS inverter is identical to three-phase four leg, three-phase split capacitor inverter topologies during balanced load conditions. During unbalanced load conditions, the absence of the neutral wire in TPFS topologies does not allow the DSTATCOM to compensate the unbalance in load currents. To overcome this limitation, a special purpose transformer is connected across the load along with the DSTATCOM. This special transformer provides a path for neutral current, reduces the cost and rating of the inverter, improve the performance under stringent unbalanced currents generated by computer loads and also improves short duration overloading capability. In the proposed work, two different TPFS topologies namely four switch one capacitor and four switch split capacitor are proposed along with two special purpose transformers which include zigzag and T-connected for three-phase four-wire distribution systems. The combination of TPFS inverter and special transformer can achieve power factor improvement, harmonic elimination, neutral current compensation and load balancing. The evaluation of the proposed method has done using simulation and experimental investigations.

Future smart distribution networks will include micro-grids (MGs) that are working in the neighbourhood. Owing to the penetration of renewable energy sources with probabilistic nature as well as deviation of load, MG may be faced with an overload or surplus power. The solution of this problem is to create and control a link between neighbour MGs for power exchange management. This study proposed a new framework for the optimal scheduling of smart reconfigurable neighbouring micro-grids (NMGs). Reconfiguration by managing the connection between MGs, through the opening and closing switches, finds the optimal structure of NMGs. Connectivity between NMGs is intended in two levels. In the first level, each MG can connect to adjacent MGs and exchange power, separately and without common area connection. In the second level, NMGs through a common area and opening or closing switches that couple them to the substrate can exchange power together. The objective of the study is to minimise load shedding of NMGs that are facing with power deficiency. The final structure selection is based on two different criteria (power loss in interconnection switches and reliability) point of view. The proposed method implemented on five NMGs system and numerical results show the effectiveness of the proposed NMGs optimal scheduling.

The first part of this two-part paper proposes tri-level transmission expansion planning (TTEP) under physical intentional attacks. In the first level, the network planner looks for an optimal transmission expansion plan to fortify the power network. In the second level, the attacker tries to maximise damages to the network. In the third level, the adverse effects of the attacks on the network operation are minimised by the network operator. Since the third level problem is a linear programming (LP) problem, the second and third levels are converted into a single-level model by using primal-dual transformation, and consequently, TTEP is converted into a bi-level programming problem. To achieve a single-level model, instead of assuming a unique attacker, a cooperative game of multiple virtual attackers (VAs) of which every VA is to maximise the damage by attacking one transmission line is introduced and modelled. The conditions that enforce Nash and Pareto equilibria of this game are derived as linear constraints which are lower level equivalent. By using this equivalence, the aforementioned bi-level TTEP model is converted into a single-level model that can be recast as a mixed integer LP problem.

The numerical results are provided in the second part.

In the second part of this two-part study, the authors numerically validate and analyse the tri-level transmission expansion planning (TTEP) which is formulated and explained in the first part. TTEP is performed aim to minimise the destructive impacts of physical intentional attacks. At first, an illustrative example (a three-bus system) is provided to show the capability of the proposed TTEP model to find the Nash equilibria and identify the Pareto equilibria of the game among virtual attackers. Then, the proposed TTEP model is applied on the Garver network and the modified IEEE 30-bus network and numerical results for several case studies are provided. The numerical results confirm the proposed model and show its significant capability in reducing the vulnerability of the power networks.

This study presents a new simple control strategy for direct driven permanent magnet synchronous generator wind turbines, using no wind speed sensor. It is shown that the fluctuation in output power, not only exists at below the rated wind speed, but also appears in above the rated speed region. Therefore, it employs power-smoothing strategies within a wide range of wind speeds and provides soft performance of the pitch angle system for above the rated speed range. In previous studies, power smoothing is mainly limited to the wind speeds below the rated value, whereas, in this study, all operating speed ranges of the turbine, including wind speeds above the rated value, are considered. Fluctuation in the output power for above the rated speed is analysed and it is shown that there is a significant level of fluctuation in the output power. Therefore, a new strategy is proposed to have more power smoothing as well as having soft performance of pitch angle system for above the rated speed region. The performance of the proposed strategy is evaluated by MATLAB/ Simulink simulation and its validity and effectiveness are verified.

This study investigates the insulation performance of the new environmental friendly insulation gas C₆F₁₂O mixed with N₂ and CO₂ under the AC frequency voltage under 0.10–0.30 MPa. Firstly, the mixing ratio of C₆F₁₂O in the mixed gas is discussed by the gas phase state equation. The breakdown voltage of C₆F₁₂O gas mixtures under quasi-uniform electric field and the initial voltage of partial discharge in the highly non-uniform field are measured by the experimental method, and the insulation characteristics of SF₆ gas mixtures are compared. The self-recovery performance of the gas mixture is analysed as well. Experimental results show that the mixing ratio of C₆F₁₂O is lower than 7% under 0.10 MPa, such that the operation requirements of the equipment can be satisfied. The breakdown voltage of 3% C₆F₁₂O gas mixtures is the same as that of 10% SF₆ gas mixtures. The partial discharge inception voltage of 6% C₆F₁₂O gas mixtures is close to that of 10% SF₆ gas mixtures. After 100 times of breakdown, the breakdown voltage of C₆F₁₂O gas mixtures does not decrease. Compared with N₂, the breakdown voltage of C₆F₁₂O mixed with CO₂ shows a more dispersive trend.

This study proposes a simple control algorithm for LCL-filter-based distribution static compensator (DSTATCOM) for power quality improvement. The proposed algorithm is able to operate DSTATCOM in both current control mode (CCM) and voltage control mode (VCM) by controlling point of common coupling (PCC) voltage. However, in the conventional method, controlling both filter current and PCC voltage is required to operate DSTATCOM in CCM and VCM, respectively. The achieving of CCM and VCM operations with only PCC voltage controlling method reduces the computational burden on the controller since it eliminates the necessity of filter current sensors. For smooth reference tracking, the interfacing LCL-filter parameters are designed based on the switching dynamics of hysteresis controller (HC) and harmonic order to be compensated by DSTATCOM. The total interfacing inductance value required in LCL DSTATCOM is low when compared with conventional DSTATCOM (L-filter-based DSTATCOM); therefore, the required dc-link voltage is reduced. An adaptive dc-link voltage control has been implemented to improve LCL-DSTATCOM performance in terms of switching voltage stress and efficiency. To show the efficacy of the proposed algorithm, simulation and experimental studies have been performed.

Faults in power systems cause voltage sags, which, in turn, provoke large current peaks in grid-connected equipment. Then, a complete knowledge of the inverter behaviour is needed to meet fault ride-through capability. The aim of this study is to propose a mathematical model that describes the behaviour of the currents that a three-phase inverter with RL filter injects to a faulty grid with symmetrical and unsymmetrical voltage sags. The voltage recovery process is considered, i.e. the fault is assumed to be cleared in the successive zero-cross instants of the fault current. It gives rise to a voltage recovery in different steps (discrete voltage sag), which differs from the usual model in the literature, where the voltage recovers instantaneously (abrupt voltage sag). The analytical model shows that the fault-clearing process has a strong influence on the injected currents. Different sag durations and depths have also been considered, showing that there exist critical values for these magnitudes, which provoke the highest current peaks. The analytical study is validated through simulations in MATLAB^TM and through experimental results.

In modern power distribution networks, voltage fluctuations and violations are becoming two major voltage quality issues due to high-level penetration of stochastic renewable energies (e.g. wind and solar power). In this study, a hybrid control strategy based on power inverters for voltage regulation in distribution networks is proposed. Firstly, a decentralised voltage control is designed to regulate voltage ramp-rate for mitigating voltage fluctuations. As a beneficial by-product, the var capacity from the inverters become smoothed. Then, a distributed voltage control is developed to fairly utilise the var capacity of each inverter to regulate the network voltage deviations. Furthermore, once there is a shortage of var capacity from inverters, on-load tap changers control will supplement to provide additional voltage regulation support. The simulation results on IEEE 33-bus distribution network with real-world data have validated the effectiveness of the proposed voltage regulation method.