For distribution system studies, a relevant aspect is the characterisation of the aggregate demand in a feeder. The probabilistic model of the aggregate demand is very useful for system operators or aggregators to extract information about the demand side behaviour in the operation of smart grids and microgrids. The time step used to scan the aggregate demand pattern is very important to preserve the information about the consumers’ behaviour and the related uncertainty. The conventional models of aggregate electrical demand consider an average value for a specific time step (e.g. 30 min to 1 h). In this study, a faster time step (1 min) is considered to construct a time-coupled probabilistic model of the aggregate residential demand based on Beta distributions. For a given number of aggregate loads, the parameters of the Beta distributions are found by taking into account the aggregate demand pattern variations at two successive time steps. The probabilistic model is then used to generate a number of aggregate demand scenarios. The effectiveness of the proposed scenario generation method is evaluated by using goodness of fit tests such as the Kolmogorov–Smirnov test and the average mean absolute percentage error.

Recently, an increasing use of power electronic facilities in industry has produced considerable harmonics and interharmonics in power systems. The power supply quality is therefore seriously threatened. The discrete Fourier transform (DFT) is currently the most important tool in signal analysis. However, a misapplication of DFT may result in incorrect outcome because of some inherent limitations such as spectral leakage or aliasing effect etc. The existing interharmonics even raise more difficulty in signal analysis than harmonics. To overcome this dilemma, this study develops a strategy of leakage energy allocation method for both stationary and non-stationary interharmonics identification. The proposed algorithm can regain its original interharmonics amplitude by restoring all spilled leakage energy, and also finds its individual frequency component according to the distribution of spilled leakage energy. The performance effectiveness of the proposed approach is verified using the numerical examples in terms of reliability, rapid response and high-precision performance.

With the increasing electricity power consumption and time-varying load demands, maintaining system reliability and economic operation has become a challenging task for power system operators during the scheduled outages. Nevertheless, very few publications in the current literature tackle this maintenance schedule problem by means of evolutionary algorithms, and, when they do, they focus only on generators. In the proposed model, the authors have considered coordinated maintenance scheduling (CMS) with cost minimisation and reliability maximisation as a multi-objective criterion, and fuzzy-based teaching learning-based optimisation is used as an optimisation tool for solving the formulated maintenance model. The proposed model solves the CMS along with security-constrained unit commitment (SCUC) over 1-year planning horizon and the final SCUC solution prescribes an economic and secure operation for composite power systems. The method is demonstrated on the IEEE 30-bus, IEEE reliability test system (single and three areas) and practical Indian utility 19-unit power systems, and the results are presented.

Determining the relative region where voltage sags originates is useful to identify the responsible party causing the power quality problem and monitor system or equipment operating condition. As different kinds of voltage sag will cause different sequence components to change according to the neutral point operation mode in the distribution system, a modified incremental impedance-based method for determining voltage sag source (VSS) location is proposed in this study. This study starts with the VSS direction theory based on incremental impedance, and derives location criteria for different types of VSSs according to the neutral point operation mode of the Chinese distribution network. The VSS location strategy is finally accomplished through the analyses of voltage sag's capacity for passing through the service transformer and the impact of system transient on incremental impedance calculation. The method locates various VSSs based on existing PQ measure points and has clear physics concept. Theoretical analysis, simulation and lab experiments have verified its effectiveness.

Competition in the electric power industry urges utilities to not only reduce the investment costs, but also to reasonably cut the operation and maintenance expenses as much as possible, while keeping both power quality and reliability requirements met. Reliability-centred maintenance (RCM) has been proven to be in response to this dilemma in power systems and has been yet successfully applied in various engineering contexts. This study introduces a novel approach, as of the first steps of RCM implementation in composite power generation and transmission systems, to identify the critical components for the main sake of a more focused maintenance management. Criticality evaluation is, here, concerned with components outage occurrence possibilities and cost-based consequences. A realistic market model has been implemented to account for the components outage consequences to the system different participants, that is, generation companies and distribution companies. The presented methodology is able to efficiently recognise the individual importance and contribution of each component in the cases of not only first order but also higher order contingencies. For the sake of demonstration, the proposed approach is applied to the IEEE reliability test system and IEEE 118-Bus test system, and the obtained results are discussed in detail.

Commutation failure (CF) may be caused by AC fault in AC/DC interconnected system, and it will affect the performance of distance protection. The transient characteristic of equivalent current of power-frequency of the dc–ac inverter is first analysed by introducing the concept of quadrant. Then, the effect of CF on distance protection is studied in detail with consideration of the connection mode of distance relay. The analysis results show that both the fault position and fault resistance contribute to the effect of CF on the performance of distance protection significantly. Furthermore, a novel countermeasure based on the amplitude of second harmonic is put forward, and a concept of ‘successive trip’ by using the change of measured reactance is also presented. Simulation results based on PSCAD/EMTDC prove the correctness of these conclusions.

Transient excitation boosting control (TEBC) is a cost-effective emergency control technique to improve the power system transient stability. However, it is found in this study that the conventional TEBCs have some drawbacks and may be ineffective or even worsen the system stability in some cases. To overcome these problems, a novel wide-area measurement system (WAMS)-based TEBC (WTEBC) is proposed in this study. The proposed control law is as follows: boosting the excitation of the critical machines (CMs) in the critical swing and stopping their excitation boosting as the rotor angles of the CMs drop below a threshold after the critical swing. The implementations of the above control law in the special protection system are also provided. The simulation results from the 4-machine system and the China Southern power grid prove the superiority of the WTEBC over the conventional TEBCs. Furthermore, it also indicates that the stabilising effect is quite significant as long as the round-trip WAMS delay is <0.3 s, which further confirms the feasibility of the WTEBC in the practical engineering.

This study presents a new methodology for modelling and handling with aggregated harmonic loads and linear loads in the harmonic power flow analysis. This method, so-called variable load method, can be used in balanced and unbalanced systems and for deterministic and stochastic studies. The main types of harmonic loads and their participation coefficients inside the facility are estimated by inspection or based on usual values. In addition, a new type of load model that represents the load variation and minor loads is also defined based on field measurements, in order to build a more adherent harmonic load representation. A stochastic analysis, based on Monte Carlo simulations, is also performed. This sequence of steps makes possible to evaluate if the harmonic distortion has a significant probability of exceeding the limits set by standards because of load variations. This analysis is performed jointly with the bootstrapping method for obtaining the probability density functions of the voltage harmonic distortions. Tests on a real plant show that the proposed approach is able to represent measured data with high precision.

This study deals with the idea that comprehensive knowledge representation should be established for fault diagnosis. Sufficient grid fault information including the network topology and protection knowledge are used with a diagnostic algorithm. In this way, the fault diagnosis programme not only facilitates accurate judgment of fault sections for which many kinds of information are available but also optimises knowledge to simplify the fault diagnosis method. Petri nets are used for logical reasoning on the basis of knowledge representation, which can be used to judge fault elements accurately even when the protective relays and circuit breakers malfunction. It was proved through experimentation here that this method meets the requirements of real-world diagnosis. The programme can be used as an interface to the self-healing mechanism of a smart grid. This study also posits that the smart grids should be constructed on the basis of knowledge representation for every subsystem.

In order to increase the power transfer and providing the optimum utilisation of the power system capability, application of flexible alternating current transmission system (FACTS) devices technology, such as static synchronous compensator (STATCOM), has been attracted, in recent years. However, the STATCOM changes the voltage and current of the power system that disturb the operation of the impedance-based synchronous generator loss of excitation (LOE) protection. Then, this study proposes a new flux-based LOE protection method, in the presence of STATCOM, that is based on the local variation of the flux linkage of the generator available at the location of relay. The mentioned relay is independent of other generator protection relays without any coordination. In order to show the satisfying operation of the proposed method, some simulations have been carried out in the Matlab/Simulink software. A single-machine infinite bus and a three-machine infinite bus systems have been used in simulations. The results show that the presence of STATCOM has no important effect on the performance of proposed relay opposite to a negative offset relay with two protective zones (the widely used impedance method) which is affected by STATCOM operation.