Node importance evaluation contributes to the targeted protection of important nodes, which can enhance the invulnerability of complex power grids against cascading failures. Since the evaluation simply from the perspective of pure topology statistics fails to capture the electrical characteristics of power systems, the authors propose a multi-attribute node importance evaluation method based on Gini-coefficient in this study. This method considers not only the electrical characteristics of power grids when evaluation indices are built, but also the impact of the index weight method on the evaluation results. The simulations on the IEEE-39 system and the 500 kV Center China power system show that the proposed method is suitable to evaluate the node importance in power transmission grids, and can also improve the accuracy of the node importance evaluation.

In this study a new adaptive continuous wavelet transform (CWT)-based current differential protection scheme is proposed to cope with problems of conventional current differential protection in transmission lines. Current transformer (CT) saturation is a major problem for protection engineers in current differential protection systems. The mentioned problem is highlighted when an external fault occurs and causes mal-operation of differential protection. In this study, three phase currents at both ends of transmission line have been sampled and used as input signals to the proposed method using appropriate communication link. In addition, CWT with the aid of Mexican hat as a mother wavelet has been employed to extract high-frequency information of signals and detect fault occurrence and CT saturation in each phase. As a result, the proposed technique is able to prevent unwanted trip due to CT saturation during external faults. Nevertheless, simulation results confirm sensitivity and security of the proposed protection technique in facing with second internal fault of cross-country faults after a short delay.

Frequency domain spectroscopy (FDS) is widely used to diagnose the condition of transformer oil–paper insulation. However, FDS measurements are highly temperature dependent. The master curve technique is the most common method to remove the temperature effect on FDS, but the reliability of this method is largely dependent on the value of the activation energy, which can be affected by moisture content and thermal degradation degree. To understand the effects of these two factors on the activation energy, FDS measurements at different temperatures were performed on oil-impregnated pressboards with different thermal degradation degrees and moisture contents. The activation energies of the pressboards were calculated using the Arrhenius equation. Results show that they vary between 0.88 and 1.06 eV along with thermal degradation degree and moisture. The two-way ANOVA indicates that the effects of these two factors on activation energy are not significant at 95% confidence level when the degree of polymerisation (DP) changes from 415 to 1300 and when the moisture does not exceed 4%. The average activation energy (0.95 eV) can be utilised to generalise the Arrhenius equation to better eliminate the temperature effects. Finally, FDS measurements on a model transformer are performed to verify the effectiveness of the proposed technique.

This study proposes a new method for measuring the unbalance of the multiple-circuit transmission lines (MCTLs) on the same tower, which can characterise the physical natures of the line unbalance. First, the impedance and the sequence current of the MCTLs on the same tower are calculated under the consideration of the impact of ground wires. Then, the unbalance suppression methods of the optimal phase sequence arrangement and asymmetry parameter compensation are presented. Finally, the model of the six-circuit transmission lines on the same tower is simulated in PSCAD according to a real project in China. The line unbalance, the impacts of the connection patterns of ground wires on unbalance and the unbalance suppression are simulated and analysed. Simulation results show that the proposed method is applicable and convenient to accurately calculate the parameters of the line unbalance and to select the optimal way to suppress the unbalance. The work can provide the theoretical guidance for the optimal design and the operation of the MCTLs on the same tower.

This study presents a novel DC reactor-based ferroresonance and fault current limiter (DRFFCL) for stabilising ferroresonance oscillations of potential transformer (PT) in 33 kV distribution network and decreasing the amplitude of the fault current to an acceptable level. At first, the ferroresonance overvoltage is introduced and various types of overvoltage in the PT are studied. Then, the effects of the suggested DRFFCL on these oscillations are investigated. It is shown that the proposed DRFFCL not only can control the ferroresonance oscillations, but also can decrease the fault current amplitude in the case of short-circuit faults occurrence. The DRFFCL performance is simulated using MATLAB software and a scaled-down laboratory prototype is implemented and tested for the simulation results validation. The measured results are in agreement with the simulation results and clearly show the ability of the DRFFCL for controlling both the ferroresonance overvoltage and fault current.

This study presents an improved algorithm of adaptive single-phase auto-reclosing (ASPAR) for extra high voltage transmission lines with shunt reactors. It shows that the odd and even harmonic factors of the faulted phase voltage have different characteristic in different stages after breaker open. During second arc period, the odd harmonics are dominant, while the even harmonics distortion factor calculated by discrete Fourier transform is found in relatively high level after arc extinction. Hence, the calculation of ratio of even harmonic factor to odd harmonic factor (REO) is expected to determine the secondary arc extinction. The algorithm is easy to implement with small computation and no more than 20 ms delay. Moreover, the REO threshold is taken constant at unity and need not be skilfully assigned. Analysis is provided by EMTP emulation as well as filed data, and the high reliability of ASPAR has been verified.

This study discusses the technical requirements of line-commutated converter-based high-voltage direct current (LCC-HVDC) power transmission participating in a receiving end AC system's black start. The control and start-up modes of LCC-HVDC suitable for black start are discussed. Through short-circuit capacity analysis and frequency estimation, the appropriate strength of the receiving end AC system to start the LCC-HVDC is studied. A reactive power coordination strategy is proposed to reduce overvoltage caused by the HVDC filter. An innovative additional frequency control at the inverter side is proposed to suppress the frequency oscillation in the black-start procedure. This control is realised through the constant voltage at rectifier and constant current at inverter control mode. Compared with the conventional rectifier-side additional frequency control method, the inverter-side additional frequency control method requires no remote communication and is thus more practical and effective when the receiving end is in a blackout. On the basis of the analysis, the conventional LCC-HVDC is started successfully under the proposed requirements of black start and the overvoltage and frequency oscillation are also reduced effectively by the proposed control strategies, both of which are validated through electromagnetic transient simulation based on PSCAD/EMTDC software.

This study presents a novel capacitive coupling compensation scheme for 500 kV four-circuit transposed parallel transmission lines. Using this scheme, the capacitive coupling between energised phase and faulted phases in four-circuit can be fully compensated. Moreover, the secondary arc current can be limited to <10 A. On the basis of the new scheme, an optimal compensation scheme for every kind of maintenance state is also depicted. Performances of the proposed schemes are validated by real-time digital simulator.

Strategic asset management of transformer fleets in electrical power systems is a critical aspect for distribution utilities. In this study, an integrated framework for multicriteria asset management is presented. A multiobjective optimisation model is deployed, simultaneously minimising the maintenance costs and the predicted cost of unexpected failures, based on statistical estimates of failure probability and on the expected economic impact of failures. A multicriteria decision-support system is used for the selection of the recommended maintenance policy for a given time window. A criticality score for each transformer, which can assist planning engineers in defining replacement investments, is also provided. The application of the proposed framework is illustrated using the database of Brazilian power distribution utility CEMIG.

In this work, the authors have studied the interaction between electromagnetic fields which are generated by lightning and overhead line in electrical power system. For this study, the authors constructed a system of equations. In which the network topology; with all its elements (linear or not) and wave exciter lightning are taken into account. This analysis is developed in time from the general equations of transmission lines with second members, with consideration of the frequency dependence of the correction terms related to the impedance of a finite conductivity of ground as well as non-linearities loads. The simulation results presented in this study demonstrate the effectiveness of this analysis for the calculation of induced voltages generated by lightning on a grid of lines.

With the total installed capacity of small hydro generator cluster (SHGC) continues to grow rapidly, the effect of SHGC will have affected to the security and stability of the main network in China, especially to power oscillations of passageway of ultra-high-voltage AC and DC. Thus, a precise equivalent model of SHGC is proposed in this study. In the proposed model, the fifth-order model of generator is adopted as the equivalent generator model, the excitation system is simplified into proportional feedback control model and exponential load model is used as equivalent load model. In addition, Grey Wolf optimiser with chaotic local search is designed to identify the parameters of the equivalent model. Finally, the proposed equivalent technique is applied in one simulation experiment and one practical experiment. The experimental results demonstrate that the validity and accuracy of the proposed equivalent model and identification method in solving dynamic equivalent of SHGC in engineering practice.

Renewable generation technologies are expected to reach unprecedented penetration levels in a number of electric power systems. The increased deployment of these renewable technologies is changing the unit commitment of the rest of generation facilities, increasing the need for cycling. As a result, operation-related issues and their costs become more relevant for an adequate analysis of generation expansion problems. In this study, the authors propose a generation expansion model including an oligopolistic market representation based on an equilibrium approach. It introduces key operation-related constraints, such as minimum stable output, start-ups and shut-downs; and short-term operating costs, such as commitment, start-up and shut-down costs. The proposed model furthermore considers the discrete nature of capacity investment decisions. The authors also propose a heuristic method for solving the arising equilibrium problem, by providing an efficient starting point to the diagonalisation process. This heuristic can lead to reductions of up to 90% in computational time. Finally, case studies are presented in order to illustrate the importance of considering both operational details and a market framework when making generation expansion planning decisions.

To mitigate the frequently over-threshold subsynchronous oscillation (FOSSO) phenomenon which occurred in Hulunbuir power plant in the northeast of China, a novel equipment based on voltage source converter (VSC), named subsynchronous oscillation dynamic suppressor (SSODS) is developed and installed. In this study, the root cause of FOSSO is revealed by analysing the response characteristics of the system. Then the control strategy of the developed SSODS is presented. Damping characteristics of SSODS are analysed by the complex torque coefficient method. To verify the effectiveness of the control strategy and the physical controller of VSC, as well as provide initially optimised controller parameters for the field tests, real-time closed-loop tests are carried out. A novel method of torsional modal frequency measure for onsite commissioning test is proposed. The operation and field verification test of SSODS is introduced, and test results demonstrate that SSODS can mitigate the new SSO phenomenon dramatically, proving the validity of SSODS.

Due to increasing contribution of renewable energy sources to the overall energy production, future control of energy consumption should achieve the principle ‘generation following demand’. Within this approach, also known as demand side management, consumers are prompted to alter demands through an incentive signal. This work addresses the use of variable price as such incentive signal to influence power consumption of private households in a controllable manner. The mathematical modelling is based on simulations of optimal power flow and includes several optimisation constraints and criteria, according to which both direct and inverse models are constructed. Therefore, the problem of power signals and price signals is considered as fully-coupled. The analysis is performed for a model, representing typical conditions and scenarios of a residential network with household customers, including solar and wind power plants. The optimisation criteria are constructed with the emphasis on distribution system operators. The results of computations, performed with simplified and extended optimisation models, show a pronounced effect on the overall power consumption, reduction of the extreme values and minimisation of the peaks. Finally, further steps are proposed to extend the model to target various optimisation constraints.

Icing experiments on three units of XP₄-160 insulator string are conducted to reveal the influence of the crystallisation effect of conductive ions during phase transition on melting water conductivity and ice flashover voltage, and to analyse the motion of conductive ions and its spatial distribution. The influence of different parameters, such as freezing water conductivity, electric field and pollution, on crystallisation effect is examined and analysed. Results of the tests indicate that the melting water conductivity of the ice layer and icicles is considerably greater than that of freezing water, particularly for a polluted insulator, because of the crystallisation effect. The electric field has a significant influence on the crystallisation effect, that is, the melting water conductivity under a non-energised condition is higher than that under an energised condition. Finally, through 35 kV insulator flashover tests, the reason for the propagation of partial arcs over icicles is determined, and the flashover process is illustrated. The crystallisation effect during phase transition changes the ion distribution in a solid ice layer, thereby increasing water film conductivity, and is also a critical reason for the decline of ice flashover voltage.

Hybrid cascaded multilevel converter (HCMC) is a newly developed modular multilevel converter (MMC) with DC fault isolation capability. It is consisted of wave-shaping circuit (WSC) and direct switch (DS). This study studies the parameter design and operating zone of HCMC employing asymmetrical square-wave modulation at the DS. Analytical method of determining the asymmetrical component is derived through the energy balancing between AC side and DC side. Method of determining the required additional full-bridge cells is then deduced. By analysing the energy fluctuation flowing through the WSC, method of dimensioning the cell capacitors is proposed. Size of cell capacitance and insulated gate bipolar transistor current ratings for HCMC and traditional half-bridge MMC are compared. Feasible operating zone of the HCMC is studied and the overall design process of HCMC is summarised. Semiconductor cost, cell capacitance and arm inductance of HCMC and other DC fault tolerant MMC topologies are compared. It is shown that HCMC is able to isolate DC fault current with significantly lower cell capacitance, lower number of power semiconductors and no arm inductance compared with other MMC topologies. Detailed simulations on PSCAD/EMTDC confirmed the theoretical studies.

An improved control methodology for off-grid, three-phase induction machine working as a single-phase generator is presented in this study. A three-phase machine made to work as a single-phase generator has advantages of lower size and cost than their single-phase equivalent in almost all ratings except in the lower fractional kilowatt (kW) ranges. The three-phase windings are configured as isolated two equivalent phases in the proposed system for this purpose. The machine uses photovoltaic (PV)-assisted single-phase inverter to one of its two phases for variable excitation requirement to cater dynamic loads at different rotor speeds besides having a fixed capacitor to provide bulk excitation. A storage battery is used along with the PV panels to improve the reliability of the scheme during low or no insolation periods. A stator-flux-oriented control is implemented for regulating the terminal voltage within specified limit at a fixed frequency. The control strategy ensures fixed load voltage and fixed frequency even with perturbations of load or rotor speed. Different simulation results, verified by suitable experiments using a 1 kW, 415 V laboratory prototype, confirm the suitability of the proposed concept.

This study proposes the generalised unified power flow controller (GUPFC) model in the hybrid current power mismatch Newton–Raphson formulation (HPCIM). In this model, active power, real and imaginary current components are injected at the terminals of series impedances of GUPFC. These injected values are calculated during the iterative process based on the desired controlled values and buses voltage at the terminals of GUPFC. The parameters of GUPFC can be calculated during the iterative process and the final values are updated after load flow convergence. Using the developed GUPFC model, the original structure and symmetry of the admittance and Jacobian matrices can still be kept, the changing of Jacobian matrix is eliminated. Consequently, the complexities of the computer load flow program codes with GUPFC are reduced. The HPCIM load flow code with the proposed model is written in C++ programming language. Where, the SuperLU library is utilised to handle the sparse Jacobian matrix. The proposed model has been validated using the standard IEEE test systems.

Aiming at the problems that static var compensator cannot suppress the voltage flicker and the cost of static synchronous compensator is expensive, a three-layer hybrid reactive power compensation scheme was proposed in this study based on the expert-decision, differential-slope voltage regulation, and the expert-decision subsystem combining with fuzzy self-tuning proportional–integral (PI) control. By switching in a dynamic load which can always change the bus voltage, the whole system was simulated under the much reactive power shortage and the less shortage. The simulation results verify the effectiveness of the proposed coordinated control method in the reactive compensation and the suppression of the voltage flicker.

The intermittency of renewable energy and load uncertainty in the combined cooling, heating, and power (CCHP) system operation is considered in this study. The authors propose a robust optimisation scheduling method to attenuate the disturbance of uncertainty, and derive the day-ahead scheduling decision under two strategies including electrical load tracking and thermal load tracking. Also, the budget of uncertainty is introduced to alleviate the conservatism of robust optimisation. Moreover, a minimax regret non-linear formulation is constructed to describe the performance of the system and different realisations of uncertainty. To solve the problem, the authors develop a hybrid solution method, which is composed of a two-stage Lagrangian relaxation iterative algorithm and an improved cross entropy algorithm. Simulation results demonstrate that the minimum of the maximum regret can be obtained, and the conservatism of robust optimisation is significantly reduced by properly setting the level of budget of uncertainty. As a result, the validity and effectiveness of the proposed robust optimisation model and algorithm are confirmed.

This study deals with a developed thyristor based bridge-type fault current limiter (TBFCL), in which the main fault current limiting function is relegated to a limiting circuit controlled by a silicon controlled rectifier (SCR). Since SCR is a reliable power electronic switch and can be found in high ratings, the proposed TBFCL can be utilised in higher ratings in comparison with other proposed BFCLs. The TBFCL limits fault current in two steps; (i) by a fully controlled DC reactor during transient fault interval (ii) by a SCR based current limiting circuit during steady state fault interval. The TBFCL has a bridge-type DC reactor controlled by an isolated gate bipolar transistor and a current limiting circuit in parallel with the bridge controlled by a thyristor. Upon fault occurrence, the DC reactor limits increasing rate of the fault current, while the generated over voltage activates auto-triggering control circuit of the SCRs. Therefore, the fault current is limited through the SCR current limiting path and the DC reactor path is interrupted. Analysis of the proposed TBFCL is presented in detail. Simulation and experimental results verify the performance of the TBFCL.

The main focus of this study is on reserve market settlement in an electricity market with marginal pricing scheme as the pricing mechanism. Following a day-ahead market clearing, the marginal prices are found using the appropriate Lagrange multipliers as the bi-products of the optimisation process. The payments to energy and reserve providers can then be found based on these prices. However, it is hard to find an appropriate settlement scheme that provides the fair and justifiable consumer payments based on marginal pricing scheme considering the actual reserve functions in power systems. In this study, a reserve marginal pricing that is based on a proper reserve marginal price definition and considers the free market requirements and accounts for the lost opportunity of selling the convertible products is proposed. Also, a value-based reserve market settlement scheme is proposed in this study that is justifiable for customers and best suits the actual reserve functions in the system. In the proposed settlement scheme, the reserves are eliminated one after the other and the loss of load (LL) and generation is then calculated for market participants in different contingencies via a novel optimal power flow that finds the fair and optimal LLs and generations. The reserve market is then settled based on the LLs and generations in different contingencies. The proposed reserve pricing and settlement schemes are tested on a 3-bus system, a 6-bus system, and the 24-bus reliability test system to show the effectiveness of the proposed method.

The reliability analysis of protection systems in smart substations helps to improve their designed functions, whose difficulty lies in the complex configuration with various components. To avoid the dimension disaster, the state spaces of the protection systems may be aggregated to several lumped states. When the states do not satisfy the balance criterion, it is newly found that the lumped states not following the Markov process introduce notable error to the short-term reliability. To avoid this, an equivalence algorithm based on the semi-Markov process (SMP) is newly proposed. It is derived that the residence time distributions of the lumped states are the weighted averages of the distributions of the states to be lumped. The kernel matrix of the SMP, the instantaneous probability, and its sensitivity, are derived from the transfer rates and distributions. The numerical results validate accuracy of the proposed algorithm, which shows that the traditional model underestimates the availability. It underestimates sensitivity of the time source (TS), while overestimates those of the others. The probabilities of the proposed algorithm are more sensitive to the TS, while those of the traditional model are more sensitive to the merging unit and the ethernet switch.

Modern smart grid (SG) should efficiently accommodate the renewable resources such as wind energy under different operation conditions. High level of wind energy penetration into SG results in operation constraints to avoid any tripping out of wind generators from the grid during fault conditions. Therefore, following the fault occurrence, the utility has to restore power as quickly as possible. This requires accurate and efficient fault location algorithms to have appropriate action for rapid service restoration. This study presents two fault location algorithms taking into account the dynamic behaviour of wind farm generation. In the first conventional algorithm, fault location is estimated using the recorded voltage and current signals at one end of the line. While second algorithm is two-end fault location based on the synchronised voltage and current signals from the line ends. Calculations are carried out in time domain within half cycle using movable windows for recorded signals. The obtained results of two-end algorithm are superior and more accurate compared with the conventional one-end fault location algorithm. This is attributed to SG facilities with modern communication links and data synchronisation technology. The proposed algorithms are tested through grid and wind farm simulations to justify their effectiveness.

This study presents an efficient power flow method for analysing distribution systems. The proposed method utilises efficient quadratic-based (QB) models for various components of distribution systems. The power flow problem is formulated and solved by a backward/forward sweep (BFS) algorithm. The proposed QBBFS method accommodates multi-phase laterals, different load types, capacitors, distribution transformers, and distributed generation. The advantageous feature of the proposed method is robust convergence characteristics against ill conditions, guaranteeing lower iteration numbers than the existing BFS methods. The proposed method is tested and validated on several distribution test systems. The accuracy is verified using OpenDSS. Comparisons are made with other commonly used BFS methods. The results confirm the effectiveness and robustness of the proposed QBBFS at different conditions.

Polyimide (PI) film is widely used as the insulation between coils in generator, where partial discharge (PD) may occur to cause the insulation breakdown. Fluorination is considered as an effective method for surface modification of polymer materials to change its electrical properties. In this study, to study the effect of fluorination on the PD behaviour of PI film used as magnet wire insulation, the samples of magnet wire were covered by PI film. Samples were fluorinated in a reaction kettle using the F₂/N₂ mixture for different time. The non-fluorinated and fluorinated samples were subjected to partial charges under alternating electric field. A wideband detection system was used to detect PD waveform and record discharge phenomenon. The experimental results indicate that the direct fluorination has great effects on the surface charge accumulation and the decrease in PD intensity. The magnet wire fluorinated for 45 min shows the highest PD inception voltage, the lowest intensity of successive discharges and the smallest number of discharge pulses.

As microgrid system is still in the stage of demonstration application, microgrid system has to satisfy more stringent requirements for grid connection: 1. In order to ensure the security of grid, the output power of microgrid system is less than the factory load, not sending power to the grid. 2. The power fluctuations of microgrid system must meet the demands of National Standard. In order to improve the output characteristics of microgrid system applied in industrial park to meet the requirement of grid connection, an energy management strategy based on the ultra short term power prediction and feed-forward control is proposed. Through the ultra short term power prediction, the output power of microgrid system is effective controlled to track the factory load, not sending power to the grid. Through feed-forward control of battery energy storage systems (BESS), the power fluctuations of microgrid system are smoothed to meet the demands of National Standard. On the basis of real time digital simulator (RTDS) and energy management controller, the hardware in the loop (HIL) experimental platform is built to verify the effectiveness of algorithm. The security and stability operation of MW microgrid demonstration project verified the validity of the energy management strategy.

The use of electrical energy storage (EES) and demand response (DR) to support system capacity is attracting increasing attention. However, little work has been done to investigate the capability of EES/DR to displace generation while providing prescribed levels of system reliability. In this context, this study extends the generation-oriented concept of capacity credit (CC) to EES/DR, with the aim of assessing their contribution to adequacy of supply. A comprehensive framework and relevant numerical algorithms are proposed for the evaluation of EES/DR CC, with different ‘traditional’ generation-oriented CC metrics being extended and a new CC metric defined to formally quantify the capability of EES/DR to displace conventional generation for different applications (system expansion, reliability increase etc.). In particular, specific technology-agnostic models have been developed to illustrate the implications of energy capacity, power ratings, and efficiency of EES, as well as payback characteristics and customer flexibility (that often also depend on different forms of storage available to customers) of DR. Case studies are performed on the IEEE RTS to demonstrate how the different characteristics of EES/DR can impact on their CC. The framework developed can thus support the important debates on the role of EES/DR for smart grid planning and market development.