Voltage-source converter-high-voltage direct current (VSC-HVDC) systems have become an attractive option for integrating remote and far-from-shore renewable energy resources to main AC grids. The desire for greater power transfer capability and the difficulty in securing right-of-way for new AC lines in many countries is also resulting in the increased use of embedded VSC-HVDC systems operating in parallel with existing AC lines. It has been stated that the control and operation of VSC-HVDC systems are of particular concern for weak grids with fewer large synchronous generation units (a highly probable case for many grids in future). If the anticipated proliferation of VSC-HVDC links continues, several aspects of system stability will be significantly impacted. This study presents an overview of the effects of VSC-HVDC control and operation on power system stability. The structure, control, control tuning, and modelling of VSC-HVDC is briefly summarised to provide context for subsequent discussion of the system dynamics. An extensive critical review of the previous research into mixed AC–DC systems incorporating VSC-HVDC is then provided including voltage stability, small and large-disturbance angle stability, high-frequency interaction, and frequency stability. Finally, recommendations are presented to guide critical future research.

Despite having more advantages, a modular multilevel converter (MMC) has a circulating current as one of its main drawbacks that augments arm current greatly. However, circulating current suppression (CCS) of the conventional MMC will deteriorate the power quality of direct current (DC)-link voltage. This study proposes the CCS by applying the three-winding transformer and a DC–alternating current CCS inverter. The mathematical models of the proposed MMC and circulating current are analysed in detail. The parameter setting of the three-winding transformer is also discussed and is set to have an equivalent arm inductance of the conventional MMC to make comparisons. The comparing results validate that the proposed MMC can exhibit much better performance under steady and dynamic conditions. The significant DC-link voltage ripple that exists in the conventional MMC does not appear. The mean-square-error values of DC-link voltage have more than 90 per cent improvement under steady and dynamic conditions. Due to this contribution, the proposed MMC can provide better DC-link power quality.

This study proposes a novel optimal phasor measurement unit (PMU) placement method for dynamic vulnerability assessment with minimum number of PMUs. PMU measurements should reflect the change of power system status as earlier as possible when a contingency occurs. Therefore, those buses which are sensitive to the change of power system status have to be equipped with PMUs to monitor the fragile areas of power systems. First, a large power system is partitioned into several coherent clusters. Next, a probabilistic vulnerability index defined as the objective function and a binary quadratic programming model are proposed for this purpose. Finally, the proposed method is tested on IEEE 9-, 39-, and 145-bus systems. Results show that this method results in a PMU configuration with minimum number of devices and high vulnerability index. Such a PMU placement is able to estimate the vulnerability of power systems.

Flexible AC transmission systems (FACTS) devices have adverse impacts on the performance of distance relays (DRs) due to the fast intervention of the controllers of these devices during power swing. Power swing blocker (PSB) as the complementary part in a DR is designed to distinguish a fault from a power swing; meanwhile its performance in compensated lines needs to be investigated. The performance of concentric PSB (CPSB) is studied in transmission lines compensated by unified power flow controller (UPFC). It is shown that fast responses of proportional-integral (PI) controllers of UPFC, reduce recorded time by CPSB significantly (it decreases by 90% of uncompensated condition) which makes DR to issue trip command during power swing condition. Detailed model of UPFC is simulated in three-machine and New England power systems to show the deficiency of CPSB in compensated condition. In order to overcome the deficiency of the CPSB in UPFC-compensated line, two remedial actions are proposed. The first method is based on modifying CPSB to solve the problem and the second method is based on modifying control loop of UPFC to prevent the problem. Both methods show their efficiency to overcome the deficiency.

Instability on direct-current (DC)-side resonance occurs if the connected converters interact with the DC network. In this study, a single-input–single-output transfer function is built to investigate the DC-side resonance of modular multilevel converter-based high-voltage DC (MMC-HVDC) system. The small signal model is developed for the DC-side resonance of an MMC-HVDC system. Furthermore, the system can be separated into two subsystems, i.e. H(s) and G(s). This study shows that both amplitudes of H(s), i.e. Abs [H(s)] and resonance peak of G(s) affect the encirclement radius of the Nyquist curve, and the encirclement radius reaches the largest at the resonance frequency. More importantly, the power transfer capability is restricted by the DC-side resonance since the more transmitted power leads to larger Abs [H(s)]. The impedance model is verified by both time-domain simulations and frequency responses imposed by impedance frequency scan in power systems computer-aided design/electro-magnetic transient design and control. Besides, an active damping control approach is introduced to suppress the resonance on DC side.

The variability and uncertainty of renewable energy resources introduce significant challenges to power system operation. One particular example is the occurrence of ramp capability shortage in real-time dispatch, which can cause power balance violations and price spikes. To meet the increasing need for ramp capability, some independent system operators in the USA have led initiatives to promote the implementation of flexible ramping product (FRP). More potential FRP providers, apart from conventional generators, are being explored, among which battery energy storage (BES) appears to be a feasible option owing to its good controllability and fast responsive characteristics. This study proposes an optimisation model for a BES aggregator to optimally provide FRP in day-ahead energy and reserve markets, aiming to maximise its monetary benefits. The basic concept of FRP is first introduced, including comparisons with traditional ancillary services, pricing mechanisms, and the extensions of market models to integrate FRP. The modes and strategies for BES aggregators to participate in the electricity markets are then addressed. Case studies indicate that an aggregator can gain more profit by optimally allocating its resources among various products than only providing energy and reserves. A sensitivity analysis on several key factors is also conducted.

The fault diagnosis of transformer windings is a basis for condition maintenance. Conventional online or offline diagnosis methods require additional pulse or impulse signal. If the overvoltage signal can be regarded as the broadband excitation source for the fault diagnosis of transformer windings, the interference caused by signal injection is likely eliminated without additional pulse or impulse signals. In this study, the transient overvoltage response performance of transformer windings with short-circuit fault is presented, which could provide a new method for the diagnosis on transformer fault diagnosis. A 10 kVA, 2400 V/220 V, 50 Hz, 1-phase tapped transformer was designed, and a test platform for the fault diagnosis of transformer windings is established. The excitation signals for the fault diagnosis of the transformer windings included different lightning waves and damped oscillation waves, as indicated by the differences among actual overvoltage waveforms. The voltage and current of windings under normal and fault conditions are determined, and the frequency responses of admittance and voltage transfer function are calculated. The criterion for the fault diagnosis of transformer windings is obtained by comparing and analysing statistical indicators that reflected the differences in the frequency response of transfer function under normal and fault.

This study proposes a convex optimisation-based method that either locates all real roots of a set of power flow (PF) equations or declares no real solution exists in the given area. In the proposed method, solving the PF equations is reformulated as a global optimisation problem (GPF for short) that minimises the sum of slack variables. All the global minima of GPF with a zero objective value have a one-to-one correspondence to the real roots of PF equations. By solving a relaxed version of GPF over a hypercube, if the optimal value is strictly positive, there is no solution in this area and the hypercube is discarded. Otherwise the hypercube is further divided into smaller ones. This procedure repeats recursively until all the real roots are located in small enough hypercubes through the successive refinement of the feasible region embedded in a bisection paradigm. This method is desired in a number of power system security assessment applications, for instance, the transient stability analysis as well as voltage stability analysis, where the closest unstable equilibrium and all Type I unstable equilibrium is required, respectively. The effectiveness of the proposed method is verified by analysing several test systems.

A new islanding detection method has been developed based on the analysis of negative sequence components of the voltage at the point of common coupling, using wavelet packet transform. The binary tree classifier is used for decision-making mechanism. The proposed algorithm is able to reduce the non-detection zone to zero and is able to detect islanding within 5 ms. The developed algorithm can also discriminate between the islanding and other events in the system. The simulation results carried out by MATLAB along with Simulink toolbox are used to test the performance of the proposed algorithm in a photovoltaic generator network connected to the low-voltage grid.

This study presents the dynamic and steady state stability improvement of a tidal power generation system (TPGS) assisted power system model (i.e. single machine infinite bus (SMIB) system) using a thyristor controlled series capacitor (TCSC). To achieve this objective, the TCSC used in the studied model is optimally designed using quasi-oppositional harmony search algorithm (QOHSA). The complete dynamic equation of the studied power system is realised based on d–q-axis decomposition. Eigenvalue analysis is performed using the frequency-domain approach to study the steady-state stability while the time-domain-based simulation is carried out to determine the dynamic stability of the studied system. To corroborate the effectiveness of QOHSA, results yielded by this algorithm are compared with those obtained using particle swarm optimisation technique. It may be inferred from the results obtained in this study that the QOHSA tuned TCSC outperforms the other in improving the overall stability of the TPGS-based SMIB system following disturbance. Furthermore, a similar analysis is extended to the TPGS-based multi-machine power system model to validate the efficiency of QOHSA in optimal designing of TCSC for dynamic stability improvement.

Real-time visualisation of large power systems, by tracking the system states, is a challenging task as it involves processing a large measurement set to obtain the system states. This study proposes a hierarchical parallel dynamic estimation algorithm to estimate the states of a large-scale interconnected power system. The power system is decomposed into smaller subsystems, which is processed in parallel to obtain a reduced order state estimate. This information is then transmitted to the central processor, which collates the individual reduced order estimates to obtain the global estimates. Each processor uses state matrix of smaller dimension, thereby reducing the computational burden. The low-level processors utilise only a fraction of the global measurements in the proposed approach, and there is no need for any information exchange from the central processor to the low level processors, which helps in reducing the communication requirements. Moreover, detection of anomalies can also be carried out at the local processors without the need for any separate bad data detection at the central processor. IEEE 30- and 118-bus systems are used as test beds to study the proposed approach.

The development of smart grids has offered many technical solutions that can increase the reliability and resilience of distribution systems. Self-healing is an important characteristic of smart grids, as it pertains to the capability of the grid to isolate and restore the system, or part of it, to its normal operation following interruptions. This is achieved by adopting advanced monitoring and control systems and utilising all local available distributed sources. In this study, a smart self-healing optimisation strategy for smart grids is proposed. The proposed technique considers several factors, including the available power supply, system configuration, and load management. Moreover, a load prioritisation model is presented and incorporated into the proposed technique. The self-healing strategy is formulated as a mixed-integer linear programming problem, which is solved mathematically, ensuring global optimality of the solution. The strategy is tested by applying it to 16-bus and 33-bus smart grid systems. Further, the proposed formulation is utilised to solve the reconfiguration for the loss-minimisation problem for a 69-bus system. The simulation results indicate the capability of the proposed strategy in providing the optimal network configuration, optimal distributed generators output, and optimal load curtailment with remarkable accuracy and computational time.

Energising a power transformer may cause inrush current, which misleads the protection systems. Therefore, the inrush current analysis is important in designing and protecting power transformers. The non-linear behaviour of transformer core saturation makes this analysis difficult. Thus, several researches try to model the core saturation and inrush current. This study presents a new technique based on core flux analysis to develop an equivalent circuit for power transformer during inrush current. For this purpose, a new λ–i equivalent circuit is proposed for saturated core transformer by transformation of conventional v–i circuit. This λ–i equivalent circuit clearly shows the effect of parameters on transformer saturation and inrush current; and provides a powerful insight into these phenomena. Moreover, new equations are developed which can predict inrush current and core-flux envelopes. The predicted waveforms can be compared with measured transformer current to detect transformer internal-fault during inrush current, which is a challenge in transformer protection. This model and the equations are compared with the recorded inrush current waveform of a real transformer, and simulation results. These comparisons verify the efficiency of the model and accuracy of the equations.

Exponential penetration of renewable generation has led to the new issues in power distribution systems, such as reverse power flow in the feeders. In the case of sizable distributed generation during lower demand, the power tends to travel in the reverse direction. This, in turn, could cause serious operational issues such as voltage rise, extra heat in the transformers, and maloperation of protective devices. Newly emerging technologies for power-to-gas (PtG) conversion are now offering effective solutions to the problem. The proliferation of PtG facilities can lead to the deployment of integrated natural gas and power systems in the near future. PtG and gas-fired, i.e. gas-to-power (GtP), units can be utilised to address several issues in an integrated power and gas network. This study unveils the application of bi-directional energy converters within an integrated gas and power system for distribution system reverse power management (DSRPM). To that end, a new real-time algorithm is proposed for optimal joint scheduling of PtG and GtP units for DSRPM. A new index is also developed that would quantify the contribution of the PtG–GtP system to DSRPM. Numerical results using real-world data on a test system substantiate the effectiveness of the proposed algorithm.

Wide area measurement system relies on phasor measurement unit (PMU) data to monitor, protect, and control high-voltage transmission networks. However, errors in instrument transformers (ITs) located at the inputs of a PMU can significantly degrade its output quality. This study proposes two methodologies for voltage and current transformers calibration using PMU data. The first method calibrates ITs using one good quality voltage measurement located at a tie-line. This method tolerates errors in both the ITs (which are to be estimated) as well as the PMUs. The second method attains the same objective as the first one, with the additional constraint that some portion of the data is unusable. Thus, the second method can be used even when the incoming data is intermittent.

Thermostatically controlled loads (TCLs) have been shown to be able to provide regulation service in ancillary service markets. In this work, an online optimisation model of TCLs was established to track the automatic generation control signal. In the optimisation model, the TCLs are regulated with different control commands, and the cluster-based control structure of the TCLs is proposed for implementation. To solve the optimisation problem, a mapping between the temperature setpoint variation and the on/off state of TCLs was established. Hence, the problem was transformed into a 0–1 non-linear programming, which can be solved by the binary dynamic multi-swarm particle swarm optimisation with cooperative learning strategy (DMS-PSO-CLS). Simulation results demonstrate that the binary DMS-PSO-CLS algorithm is an efficient method to solve the optimisation problem. It is promising to control the TCLs individually to serve for the frequency regulation in power grid.

Multi-order stochastic subspace identification (MOSSI) has been extensively used to estimate the electromechanical oscillation modes from probing, ambient and ringdown data. It has been validated with good performances, while the computational burden is still a major obstacle. This study develops a fast iterative MOSSI (FSSI) approach for computational enhancement of MOSSI in mode estimation. In the proposed approach, an initial cluster of eigenvalues is formulated through FSSI with repetitive calculations (RCs), and electromechanical oscillation mode separation (EOMS) is utilised to discriminate the electromechanical modes. The RCs within the FSSI is calculated through changing the model order successively over the defined range given by a mean of singular values based order determination strategy. Additionally, the proposed approach is highly reliable against prevalent measurement noises owing to RCs and the EOMS. The performance of the proposed method is evaluated in Kundur's two-area test system by comparing with MOSSI, Prony and autoregressive moving average exogenous. Its applicability for both the ringdown and ambient data is also demonstrated with the phasor measurement units field-measurement data from the China Southern Power Grid. The results confirmed the accuracy, robustness and efficiency of the proposed approach for oscillation mode estimation.

In this paper, a novel framework is proposed in order to evaluate impacts of the uncertain models of the system components on the voltage regulation problem of the medium-voltage distribution systems. The investigation focuses on the model uncertainty associated with voltage dependency of loads, power factor of loads, thermal dependency of lines, shunt admittances of lines and internal resistance of substation transformer. To this end, firstly, voltage constraints are managed using a centralised voltage control algorithm (VCA) by relying on the simplified models of the system components. The system loads and lines as well as the substation transformer are then modelled with the uncertain variables which are bounded in the predefined ranges. Monte Carlo (MC) simulations are used to create wide series of scenarios that cover the possible values that the parameters of the system components can take due to their uncertain nature. The model uncertainty impacts on the voltage regulation problem are finally evaluated by the load flow calculations considering the scenarios created by the MC simulations and the set-point obtained by the VCA. The proposed investigation brings useful information regarding the possible deviations that the node voltages can have due to the uncertain models of the studied components.

Multi-microgrid is an integrated system of microgrids, distributed generations, and battery energy storage system (BESS). As the significant equipment in microgrid, BESS can perform multitasking, such as load management and peak shaving. This study mainly focuses on the energy consumption scheduling of multi-microgrid considering the optimisation of BESS capacity. Energy management with BESS optimisation is studied by considering the cost of distributed generations, cost of BESS, and bidirectional energy trading. The optimisation problem is tackled from two different aspects: an individual-oriented optimisation and a coalition-based optimisation. In the first approach, each microgrid is optimised individually with a non-cooperative game; while in the second approach, the joint optimisation of all microgrids is formulated through cooperation among multi-microgrids. In order to achieve the optimal energy consumption strategy and BESS capacity, distributed algorithms for two formulations are presented, which combine particle swarm optimisation and interior point method. Simulation results show that both approaches can contribute to peak shaving and reducing the daily cost of multi-microgrid.

The power flow is driven by the voltage drop in the dc network. In long-distance bulk power transmission cases, the voltage drop across the network might be severe with some node voltages susceptible to load variation. To tackle this problem, a voltage drop compensation method is proposed. A component called voltage compensator (VOLTCOM) is integrated inside the network to modify the network topology and thereby reduce the voltage drop and node voltage variation. From a system perspective, the characteristic of the VOLTCOM is designed and the compensation principle is explained. Then a scheme to lay out VOLTCOMs in the network is given with the objective to minimise the power rating of VOLTCOMs needed. The simulation results show that the load is transmitted with less voltage drop. The voltages remain in the stipulated range even when the grid reaches the planned operating status limits. Also, the VOLTCOM auto-regulates its output without communications with converter stations. The techno-economic assessment demonstrates the economy of the method compared to enlarging conductor cross-section. As an alternative to enlarging conductor cross-section, the proposed method provides an innovation worth consideration in network design.

This study is dedicated to establish an adaptive primary droop control of interlinking converter (IC) for the islanded operation of hybrid AC–DC microgrids (MGs). In this practise, the amount of transferred power through the IC is controlled such that all of the resources in both AC and DC MGs are in equal loading per cents. To do so, two different notions are contemplated. In the first one, the IC is controlled such that the aforementioned control objective is satisfied for all possible values of transmission lines impedance angles. To this end, a new algorithm is developed within which the IC estimates the impedance angle based on the local measurements in AC MG and then adapts itself. In the second outlook, the uncertainties in association with the renewable energy resources are considered and their effects are explored on IC performance. To lessen the negative impacts of uncertainties and fluctuating power generation of renewable resources, the control of IC is updated based on the instantaneous power generation in AC and DC MGs. Accordingly, the main control objective is effectively pursued. Performance of the proposed adaptive droop control is evaluated on a test MG. Results are discussed in depth.

Due to the intermittent output power of distributed generations (DGs) and the variability of loads, voltage fluctuation and three-phase power imbalance easily occur when hybrid single/three-phase microgrid operates in islanded mode. To address these issues, the power exchange unit (PEU) and energy storage unit (ESU) are added into hybrid single/three-phase microgrid, and a power coordinated control method with frequency support capability is proposed for hybrid single/three-phase microgrid in this study. PEU is connected with three single-phase microgrids to coordinate power exchange among three phases and provide frequency support for hybrid microgrid. Meanwhile, a power coordinated control method based on the droop control is proposed for PEU to alleviate three-phase power imbalance and reduce voltage fluctuation of hybrid microgrid. Besides, ESU is injected into the DC-link to buffer the system power fluctuation when DGs or loads suddenly change. The hierarchical control method is proposed for ESU to stabilise the DC-link voltage and reasonably allocate the net exchanged power of PEU. Considering the collaboration between PEU and ESU, the state of charge of ESU is proposed to adjust the power transmission coefficient of PEU. The validity of the proposed control method has been verified by simulation and experiment.

This study presents a supervisory model predictive control (MPC) scheme for coordinated active power control of a wind farm with a centralised battery energy storage system (BESS). The control target is wind turbine generator (WTG) and BESS coordination to respond to transmission system operator power dispatching, along with equipment ageing deceleration. To alleviate the WTG mechanical fatigue and extend the BESS expected lifetime, the twist angle variation and an index introduced by a weighted ampere-hour (Ah) throughput model are considered in the cost function. The damage equivalent load and weighted Ah throughput are used for the equipment ageing evaluation. Simulations and comparisons are conducted to demonstrate the effectiveness and applicability of the proposed MPC-based supervisory controller design.

Voltage source converter-based multi-terminal high-voltage direct current (VSC-MTDC) systems are starting to be commissioned. However, concentrated integration of large-scale wind power demands stronger robustness against power fluctuation and system disturbances to increase the reliability of the whole system. This study proposes a perturbation observer-based robust passivity-based control (PORPC) for VSC-MTDC systems connected to an offshore wind farm to meet the demands. The aggregated effect of system nonlinearities, parameter uncertainties, unmodelled dynamics and external disturbances includes grid faults and time-varying wind power output is estimated by a linear perturbation observer and fully compensated by a passive controller, thus no accurate VSC-MTDC system model is required. PORPC attempts to regulate DC voltage and reactive power at the rectifier side, as well as active power and reactive power at the inverters side connected to an offshore wind farm. Besides, a DC-link voltage droop controller is introduced so as to provide immediate response to the grid unbalance situation. Moreover, a noticeable robustness against parameter uncertainties can be achieved as no accurate system model is needed. Case studies are carried out to compare the performance of PORPC to other typical approaches. Finally, a hardware-in-the-loop test is undertaken via dSPACE which validate its implementation feasibility.

In order to achieve adequate time for emergency control against transient instability, a scheme to predict the post-fault rotor angle trajectory based on the grey Verhulst self-memory model is provided. The proposed scheme is model free and only uses wide-area measurement system data to provide timely, reliable information about the transient rotor angle stability. Moreover, the scheme is model-parameter-adaptive and takes full consideration of the non-linearity as well as uncertainty of power systems. Firstly, the grey Verhulst self-memory prediction model is presented to cope with the strong non-linear and non-autonomous nature of the power system. Secondly, the equal dimension and new information data model as well as the rolling prediction method are adopted to improve adaptability and robustness of prediction. Investigations with the IEEE-39 bus system and NCE China power system indicate that the proposed prediction scheme gives better prediction performance compared with the other two prediction methods, i.e. grey Verhulst prediction and auto-regressive prediction. Furthermore, combined with equal area criterion based on the severely disturbed generator pair, the proposed prediction scheme is conducted to detect transient instability. Simulation results indicate that the proposed prediction scheme is effective for prediction of transient stability.

The most convenient solution to link faraway significant renewable energy sources (RESs) is the voltage-source converter multi-terminal high-voltage DC systems (MT-HVDC). However, to maintain system stability and continuity of supply, a rigid and fast fault locating technique is required. This study proposes a novel inherent travelling waves based short-circuit DC fault identifier, which accurately identifies both of the fault location and faulty pole in multiple numbers of cables in MT-HVDC system using a single current sensor. Both of a discrete wavelet examiner and a fuzzy-neural pattern recogniser precisely spot the faulty line and fault location based on the mutual effects of short-circuit initiated travelling waves between lines belonging to the same loop. A software toolbox is structured to illustrate the adequacy of the proposed artificial intelligence technique. This method is valuable to MT-HVDC administration centres, particularly those concerned with long-distance RES.

To improve the automation level of distributed generation, a hierarchical optimisation strategy is proposed in this study. The strategy consists of day-ahead dispatch and scheduling implementation by power control. The energy management framework about the multi-agent system is also designed. Given the collaborative gaming process between microgrid and distributed network, a day-ahead dispatch is used to minimise the general expenses. Moreover, considering security constraints, the secondary control strategy is proposed to realise the precise control of the active power, which is adaptive to voltage inconsistency. Besides, the consensus algorithm is utilised to trace the dispatch target of tie-line power by monitoring power deviation at the point of common coupling. Finally, a series of simulation verifies the effectiveness of the method proposed. The influence of communication delay is also discussed.

Underground cable performance indices such as maximum cable temperature and ampacity are non-continuous functions of the configuration parameters such as depth and width of various trench layers. In this respect, existing traditional gradient-type methods cannot be used to optimise such performance indices. This study presents an efficient methodology for optimising power cable thermal performance with respect to configuration parameters involving cable spacing, depth of burial and size of backfill. The new methodology integrates the powerful features of the finite elements (FEs) technique coupled with the flexibility and effectiveness of the particle swarm optimisation (PSO) algorithm in order to handle various geometrical parameters in the complex surrounding operating environment. The introduced methodology is tested using a commercial FE simulation package used in conjunction with developed PSO code. The integrated methodology can be employed to minimise the maximum cable temperature, minimise installation cost or maximise cable ampacity. Practical applications are presented for 15 kV cables, which demonstrate the usefulness and versatility of the presented methodology. Notable improvements have been achieved by optimising the cable trench configuration parameters. For example, the cable ampacity was maximised, optimising the cable spacing, barrier depth and backfill thermal conductivity, which resulted in an appreciable increase of 4.5%.

After a single-phase high-impedance fault (HIF) for low-resistance grounded distribution network (LRGDN) occurs, fault current level is very low and cannot be detected by conventional overcurrent relays. In this study, composite power of each feeder which is composed of complex power and its conjugate is defined, and a HIF protection method based on the composite power of each feeder is presented. For a sound feeder, the phase angle of the composite power is equal to 0, and its amplitude is approximately 0. However, for the faulty feeder, the phase angle of the composite power is π, and its amplitude is much greater than 0. Base on the above significant difference, the protection criterion for HIFs in the LGRDN can be obtained. Theory analysis and simulation results show that the proposed method does not need to check the polarities of the zero-sequence current transformers and is able to remove faulty feeder reliably. Besides, the protection can accurately operate for HIFs in the case of noise interference and unbalanced current. The experimental results based on RTDS also show the efficiency of the proposed method.

In power system, the control performance deteriorates due to the penetration of the fluctuating renewable generation if no advanced control strategy is implemented. In this study, the authors investigate a class of thermostatically controlled loads – air conditioners (ACs) – to provide fast frequency regulation service in system contingencies by switching ON/OFF state based on the system frequency deviation in a decentralised way. First, an estimation of the steady-state aggregate power of a population of heterogeneous ACs is derived corresponding to the ambient temperature, and the power overshoot caused by a safe protocol (SP) is discussed analytically, both of which are verified by simulations. Then, a decentralised frequency-based control strategy is designed and a multi-stage frequency responsive model is implemented implicitly. An SP with the timer is introduced in the strategy in order to eliminate the power oscillations during the load pick-up period. Finally, the proposed strategy is incorporated into the primary control of the power system to provide fast frequency regulation service in power contingencies. Comparing with other decentralised strategies, the proposed method can obtain a lower-frequency deviation and the smoothest demand trajectory without long-term power oscillations when a power contingency occurs. Meanwhile, the comfort of the consumers is guaranteed during the demand response.

Oil-immersed bushing is one of the most important accessory equipment of a transformer, whose insulation status is crucial. Moisture is an important factor that affects the insulation status of oil-immersed bushing. It has a very important significance to estimate the moisture content of oil-immersed bushing accurately. Here, the 72.5 kV bushing samples with different moisture contents were made. Afterwards, the frequency domain dielectric spectrum (FDS) of samples were tested at 15°C. The Havriliak–Negami equation was modified based on the analysations of complex planes, then it was used to analyse the experimental results. Moreover, the characteristic parameters and method to estimate moisture content were obtained and testified. Finally, the field application of the proposed estimation method was also discussed. The results of the studies shown here indicate that the moisture content of oil-immersed bushing can be estimated based on the proposed method effectively; however, in order to estimate the moisture content of field oil-immersed bushing, the elimination of temperature effects should be considered.

This study presents the case study of a substation in-service power transformer referred to the on-site partial discharge (PD) detection and evaluation methods. An original methodology for simultaneous application of three methods is proposed: electrical, acoustic and ultra-high frequency. Transformer is powered by the power grid and no external generator is required according to the proposed methodology. Furthermore, several possibilities of applying these concurrent measurements and benefits of such solution in terms of result interpretation, interference resistance and on-site measurement applicability are indicated. The proposed methodology allows for a fast, accurate and secure PD diagnostics in a power transformer under on-site operating conditions. In the described example, a failure as a surface PD in the vicinity of the first phase of high voltage winding output is detected. The research results provide the basis for further works on a diagnostic expert system, which exploits automatic detection and evaluation of the PD phenomenon in power transformers under normal operating conditions.