Loading capability of transformers depends strongly on their thermal conditions which can be investigated by means of a dynamic thermal model. It is typically a set of simple differential equations representing very complex physical phenomena; however, there are several factors affecting the accuracies of the models which should be kept in mind while using these models. This study provides a review on the dynamic thermal models of power transformers from very simple to rather complex thermal models proposed in the last hundred years, the affecting factors on their accuracies, and their fields of applications. It is worthwhile to note that this study focuses on the thermal models which are simple and accurate and able to be integrated into monitoring systems.

Partial discharge (PD) detection has been proved as an effective tool for monitoring the insulation condition of high-voltage cables. However, the detection can be seriously affected by pulse-shaped interference from overhead lines (OHLs) when carried out at the cable termination of a combined cable–OHL. In this study, a novel scheme is presented for separating the PD pulses and interference in a combined cable–OHL. First, the characteristics of the currents from different pulse sources are investigated mathematically. On the basis of these characteristics, separation of multiple pulse sources can be realised by projecting the three-phase power ratios extracted from detected signals onto a ternary plot and then constructing maps using the power ratios of the defective phase. The performance of the scheme is subsequently evaluated by both numerical simulations and field applications. Furthermore, a comparison with the well-known time–frequency mapping method is provided. The results demonstrate that by using the proposed scheme, different pulse sources can be effectively identified without requiring stable pulse shapes and a better separation could be achieved compared with the conventional waveform-based approaches.

In this study, an effective energy management system (EMS) for application in integrated building and microgrid system is introduced and implemented as a multi-objective optimisation problem. The proposed architecture covers different key modelling aspects such as distributed heat and electricity generation characteristics, heat transfer and thermal dynamics of sustainable residential buildings and load scheduling potentials of household appliances with associated constraints. Through various simulation studies under different working scenarios with real data, different system constraints and user's objectives, the effectiveness and applicability of the proposed model are studied and validated compared with the existing residential EMSs. The simulation results demonstrate that the proposed EMS has the capability not only to conserve energy in sustainable homes and microgrid system and to reduce energy consumption costs accordingly, but also to satisfy user's comfort level through optimal scheduling and operation management of both demand and supply sides.

For the last few decades, there has been rising concern across the globe about depletion of conventional energy sources and the consequent energy crisis. Simultaneously, search for other energy sources, such as wind energy and solar energy, has been gaining momentum as well. Particularly, wind energy has become a viable alternative source of energy. As the wind power penetration continually increases, stability of the grid-connected doubly fed induction generator (DFIG)-based wind turbine system has become essential for wind power generation. This study presents the influence of a grid-side converter current dynamics on the small-signal stability by using the eigenvalue and participation factor technique which provides five distinct system modes, namely mechanical (M), stator electrical, converter electrical, electro-mechanical and non-oscillatory mode. In addition, flexible alternating current transmission system device has been incorporated to examine its effect on the system stability at different compensation levels. Furthermore, the sensitivity of a grid-connected DFIG system concerning system parameter variations and its impact on stability of the system modes has been investigated. Simulations have been carried out in the MATLAB/Simulink environment.

Performing optimal transmission expansion planning (TEP) in real, large-scale power systems such as the European one can be an unmanageable task, especially when long-term time scopes and multiple scenarios are considered. Project e-Highway had the daunting objective of planning the European network for the very long term and under high renewable energy penetration. The project objectives included the development of a planning methodology capable of applying optimisation to large-scale systems that are currently unmanageable in practice. This study presents this approach, which is based on simplifying the system while keeping its main features and investment drivers. The simplified system is then expanded optimally for the full time scope. Last, the original system is expanded optimally for the first time horizon taking into account the constraints imposed by the full time-scope optimisation of the simplified system. It illustrates the applicability of the method with a case study based on the European Union.

This study presents a lifetime efficiency index model for substation equipment and proposes an appropriate method for the optimal maintenance of substation equipment. Along the substation equipment lifetime, due to ageing and daily operations, the failure rate increases gradually. So it is needed to do a maintenance strategy for the aim of reduction in the failure rate and improvement in the reliability of equipment. The maintenance strategy for substation equipment typically consists of preventive and corrective maintenance. By the lifetime efficiency index, one can analyse the lifetime of the equipment under the conditions of with and without maintenance. In this study, the failure rate of the equipment is modelled by the appropriate Weibull distribution. Maintenance can keep equipment in normal condition, but it incurs cost. The maintenance strategy should be optimised for the minimum maintenance cost under the required reliability condition. In order to solve this objective problem, a cuckoo optimisation algorithm (COA) is used and results are evaluated. The results based on an example show that the proposed COA was indeed capable of obtaining higher quality solutions than other methods for the optimal maintenance.

In this study, a novel intelligent relaying scheme is proposed for fault classification and faulted phase selection using magnitude of differential power (MODP). The DP is the multiplication of difference between the magnitude of the real and estimated voltage and current phasors. The MODP for each phase is used as an input data to the data mining model known as decision tree (DT). The DT is used to generate thresholds to detect and classify the faults, since determination of robust thresholds is an important challenge in protective relay engineering. The proposed method has been tested for different operating modes of single-circuit transmission line in Simulink/MATLAB software. Simulation results indicate that the proposed method is able to detect and classify the faults with acceptable accuracy, in less than half cycle.

This study proposes a novel transverse differential protection for voltage sourced converter-high-voltage, direct current (VSC-HVDC) to clear faults on DC-line fast. The proposed scheme depends on the transient current of positive pole and negative pole at the same side. Transverse differential current can be used to distinguish DC and AC faults, novel criteria are built using the ratio of difference and sum current of bipolar. According to the variation characteristics of transient fault current, internal faults and external faults can be detected. The scheme can protect the whole transmission line without dead zone, simplify the algorithm and improve the speed of operation. A typical VSC-HVDC model in PSCAD/EMTDC is built. The tolerance to grounding resistance of the scheme is researched in simulation. The results demonstrate the feasibility and reliability of the proposed scheme. The scheme can be used as supplementary criteria for primary protection or backup protection for VSC-HVDC transmission lines.

A wide area measurement system installation cost minimisation and observability maximisation using binary gravitational search algorithm have been reported. Various methods have been employed in the past to determine the optimal locations of devices to retain the observability of the system. However, the costs of communication infrastructure (CI) from phasor data concentrator to phasor measurement units (PMUs) have not been given proper attention. To address this issue, the PMU placement problem has been formulated incorporating the cost of CI also in this study. Contingency cases have also been included to improve the reliability of the system. Besides, the effects of the presence of preinstalled PMUs have been considered. The results obtained for IEEE 14-bus, 30-bus and 118-bus systems have been compared with other methods in the literature. The test results reveal that the proposed methodology produced the PMU locations with higher observability and lesser distance path of establishing communication link compared to an existing method.

This study presents, develops and evaluates a structure preserving energy function based on the sixth-order generator model which is formulated with respect to the centre-of-inertia. Flux saturation and leakage flux effects are also included in the model. Each load is represented as voltage-dependent load. It is demonstrated that the proposed energy function satisfies the energy function required conditions. This study also derives a modified energy function from the original development which provides a simpler representation with less computational need while preserving the same accuracy. Physical interpretation of each term of the modified energy function is also discussed. To evaluate and verify the accuracy of the modified energy function, the critical clearing time using potential energy boundary surface method and the time-domain simulation results of a test system are presented. The calculation of the critical energy based on the proposed energy function differs by at least 10% (more accurate) compared with the existing energy functions.

In this study, a novel algorithm to locate regions in a distribution feeder, where power is being illegally tapped, is proposed. The basic requirements of the algorithm are voltage measuring devices located at distribution feeder nodes or transformers that can communicate data to the distribution substation. Initially, how voltage magnitude difference between successive nodes in a feeder can help identify possible locations of illegal tapping is shown. The technique is further refined by a normalised voltage double difference method, to pin-point the exact location of power theft. The algorithm does not require network parameters. Simulations are performed on the IEEE 34 node test feeder, to demonstrate the efficacy of this method.

As more generators and loads are integrated by power electronic converters with complicated controls, electromagnetic transients (EMTs) simulation becomes an important tool for studying dynamic characteristics of large-scale power systems. To accelerate system-level EMT simulations, a fine-grained parallel algorithm on graphics processing units (GPU) is proposed. By decomposing the computational models of the EMT simulation into heterogeneous, homogeneous and network solution computations, the simulations are mapped into three unified GPU kernels. To incorporate control signals and non-linear features of electrical components, heterogeneous computations are formulated as layered direct acyclic graphs (LDAG) of primitive operations. An LDAG kernel is designed to carry out theses primitive operations efficiently by grouped threads. Then, homogeneous computations for state updates of electrical components are modelled as sets of fused multiply-add (FMA) operations, which are concurrently processed by an FMA kernel. Moreover, a hybrid network solution kernel is designed to solve the network equations, which can adaptively select dense or sparse solvers. Large-scale test systems are created and simulated on an NVIDIA K20x GPU. The results show that the proposed GPU-based EMT simulations are accurate and achieve 10x speedups over the CPU-based ones.

Dynamic VAR compensation (DVC) is gaining increasing attentions with growing construction of high-voltage DC systems and renewable power plants in modern power system. This study proposes a novel zoning-based heuristic planning method to reduce the sites and capacity of DVC installation. Clustering algorithm based on buses’ dynamic behaviour correlation and voltage control ability is developed to partition the system into several control zones. Then, candidate DVC sites are determined by zones, which narrow the searching space and avoid excessive installation of DVCs on adjacent buses. The voltage control index is proposed to find proper DVC installation locations by zones and capacity optimisation is then carried out to reduce investment. The proposed methodology is demonstrated on a modified New England 39-bus test system with wind farm integration. Performance comparison with conventional planning scheme confirms the advantages of the new heuristic planning scheme in practical applications.

This paper presents the design of a non-linear recurrent artificial neural network (ANN) based load frequency control (LFC) of a two-area power system interconnected via HVDC tie-line in parallel with EHVAC line. The control design is based on the combination of robust LFC and optimisation of recurrent ANN so that designed LFC scheme is suitable to handle the diverse operating conditions of power system. The various structures of the state cost weighting matrix (Q), i.e. controllability and observability aspects which affect the dynamics of the power system are used for the LFC design. The feedback gains achieved by the implementation of robust LFC using the different structures of Q are used to effectively train the recurrent ANN-based LFC design and its performance is analysed with and without system non-linearity's for 1% step load disturbance in one of the control areas to show the superiority of one design over the others. The closed-loop system eigenvalues obtained for the system ensure the system stability. Furthermore, the recurrent ANN-based LFC performance is evaluated for the diverse system operating conditions and compared with multi-layer perceptron (MLP) ANN-based and conventional PI control schemes to demonstrate the effectiveness of proposed LFC design scheme.

This study presents the results of technical and economic analysis of high-voltage direct current (HVDC) transmission system and high-voltage alternating current transmission system for bulk transmission over very long distances. The last is a point-to-point transmission that does not need reactive compensation and does not have intermediate substations, the half-wavelength transmission (HWL). Special attention is paid to reliability and ways to meet the N − 1 criteria regarding security and adequacy. Single-circuit lines instead of double-circuit ones are considered for economic and environmental reasons ensuring the same reliability when clearing the most frequent line-to-ground (or single-pole) fault. For this, single-circuit line with reserve phase for HWL and quadripolar single-circuit line for HVDC are proposed. The analysed system considered Brazilian electrical system conditions: 6000 MW transmission over 2500 km. This study highlights the advantages of applying a reserve phase for HWLs and a third converter substation increase to cope with system security with minimum cost. It is shown that the single-circuit HWL with reserve phase has an investment 30% lower than that for HVDC, resulting in a total yearly cost 15% lower.

This study investigates the frequency behaviour in the Nordic power system (NPS) utilising phasor measurement units (PMUs) from different locations in the system. Different metrics are selected or proposed to quantify the system frequency response in case of major disturbances and of daily normal operations. Based on the collected PMU data, there are on average 17.5 major, generator disconnection related, disturbances per year. The maximum absolute rate of change of frequency exceeds 0.1 Hz/s for 20% of these disturbances. The average frequency nadir falls to 49.7 Hz, and the average time to nadir is about 8.7 s. If wind turbines were to be requested to provide inertia support to the system, these four metrics indicate how often, how fast, for how much and for how long the wind turbines should act. Moreover, during daily normal operations, the system has in general longer duration of over-frequency (f > 50.1 Hz) than under-frequency (f < 49.9 Hz). These frequency deviations are observed to occur around the hourly clearing of the Nord Pool Spot market and, especially during the hours between 05:00 and 08:00. In order to improve the frequency quality, the Nordic system operators tested and implemented an automatic frequency restoration reserve (aFRR) in addition to the existing manual one. Based on a frequency evaluation, this aFRR is needed most during 4:45–08:15 and 22:00–23:30 o'clock in 2012 in the NPS.

The interaction between controllers of doubly fed induction generators (DFIGs) and fixed series compensations may cause a new type of subsynchronous resonance (SSR), namely subsynchronous control interaction. To mitigate this emerging issue, a novel suppression method, embedding subsynchronous notch filters (SNFs) into DFIG converter controllers, is proposed in this study. By using the impedance-model-based analysis and a quantitative location-dependent performance index, the best location is identified to insert SNFs into the controllers of both rotor-side and grid-side converters. Two specific SNF schemes are recommended for practical use, and a design procedure is developed to tune their parameters. As a case study, they are then applied to a practical series-compensated wind-farm system that suffered from SSR. Both impedance analysis and time-domain simulations have been conducted to investigate their performance. The results verified that they can defuse the interaction between DFIG controllers and series compensation; therefore, successfully eliminating the risk of unstable SSR under all possible operating conditions. The proposed SNF schemes are easy to design and implement, robust to changeable operating conditions and would not affect the normal dynamics of DFIGs. So, they are of great potential in addressing practical SSR issues.

Given that currently available lightning protection measures cannot effectively protect overhead lines from lightning strikes, this study develops an active and fast arc-extinguishing lightning protection gap to address this issue. When a power frequency arc is formed within the protection gap due to a lightning strike, the lightning current immediately ignites the device, while strong airflow is synchronously produced. In this manner, the follow-up power frequency arc can be extinguished within an arc duration that is considerably shorter than the action time of relay protection; thus, relay protection can be avoided, and the lightning trip-out of overhead lines can be effectively prevented. A test of insulation coordination for impulse flashover and a test of power frequency arc extinction are performed to validate the effectiveness of the device. The arc-extinguishing test proves that the strong airflow can extinguish the arc within 0.4 ms. The devices have been widely applied in China and have been extremely successful.

This study proposes a novel approach for estimating the power system-wide inertial frequency response (IFR) using strategically allocated phasor measurement units (PMUs). First, a method is presented to identify generator clusters that form aggregated sources of inertial response. Then, the collective dynamics of each cluster are synthesised using PMUs placed at key network buses. These buses are identified as the inertial centres which accurately represent the centre of inertia motion. The proposed synthesis process ultimately yields IFR estimates at zonal and global (system-wide) scales. Simulation results demonstrate the proposed methodology's ability to accurately estimate IFR under several operating conditions, network topology changes, and disturbances.

From an environmental perspective, petroleum-based aged oils removed from power transformers are source of several pollutants and therefore cannot be disposed of without due care. The degradation of oil in in-service transformers is due to various factors concurrent with the operation of the units over several years. The present study proposes a new strategy to rejuvenate used mineral oils by combining centrifugation, dehydration and sorption with four different adsorbents: activated carbon (ACH), silica gel (SG), magnesium oxide (MO) and activated bentonite (AB). The process of regeneration proposed in this study resulted in a level of restoration that saw the used oil take on the characteristics of new oil (colour, dissipation factor, resistivity, permittivity, acid number). The results also showed that the optimum form of the re-refined base oil can be attributed to a 10% (w/w) quaternary mixture of the adsorbents, itself comprised of 1% ACH, 6% SG, 1% MO and 2% AB. The anticipated benefits are reduced risk of dielectric breakdown blamed for over 75% of extra high-voltage (EHV) power transformer failures and extended transformer life expectancy by retarding the solid insulation aging processes.

This study analyses the effect of parallel lines on the impedance seen by the traditional ground distance function. Different pre-fault load flows, system impedances and fault resistances are considered, as well as the possible connections of the parallel line (in parallel, in open-circuit or short-circuited at both line ends). Cases with and without compensation for the mutual impedance of the parallel line are considered, as well as cases with and without adaptive change of relay settings. In general, the results are highly dependent on the connection of the parallel line. System impedances have more influence on the results than the pre-fault load flow. On the other hand, the maximum fault resistance that can be seen by the distance function is moderately influenced by the effect of the parallel line. Thus, this exhaustive analysis shows the relative importance of each relevant factor (pre-fault load flow, system impedances, fault resistances, parallel line connectivity, and possible use of compensation for mutual impedances). A similar analysis about this topic was not available in the literature, and this knowledge is useful for the recommendations about the setting of the traditional ground distance function by considering the effect of parallel lines.

The reliability analysis of a power system-wide area measurement system (WAMS) is presented based on a Markov graph theoretic approach. Initially, a phasor measurement unit is considered wherein a state-space model is used to denote state transitions due to various component failures. This model is then expanded to include different common cause failures and component redundancies. The approach is extended to a regional WAMS and further the impact of the nature and type of communication network of the complete WAMS on the overall system reliability is investigated. The evolution of the overall reliability due to the ageing of individual components is also studied. Finally, sample calculations are presented for the WAMS system currently under consideration in India.

This study proposes a fault diagnosis method for extracting and classifying the fault features of switchgears in accordance with the monitoring data of three-phase voltage and current, temperature, humidity, and flashing from the smart breaker and sensors. The fault features are calculated using multivariate multiscale sample entropy (MMSE) for the data mining of multivariate monitoring process. The similarity measure of the composite delay vectors of multivariate time series in MMSE is improved by introducing a cloud model to soften the similar tolerance criterion. The modified MMSE is defined as multivariate multiscale cloud sample entropy (MMCSE). The MMCSE features of switchgear monitoring data can be achieved in different time scales in describing various switchgear faults. Subsequently, a classification method based on fuzzy support vector machine (FSVM) is further adopted to identify different types of switchgear faults using the MMCSE features. In addition, a dropping semi-normal membership cloud model is applied to modify the uncertainty quantification of the relationship among fault samples in FSVM. The effectiveness of the proposed method is validated with the monitoring data in a 10 kV switchboard.

This study presents two different methods under uniform and non-uniform pollution layer in order to measure and calculate the leakage current (LC) of silicone rubber insulators. Experimental test for evaluating the LC analysis of polluted insulator have been done in a laboratory clean fog chamber. The electric field and potential distributions were obtained from finite element method software for 3D models. The mathematical background and circuit theory are described in details by a section of insulator and using the extended form factor formula. The surface conductivity used in the calculations was extracted from the measured LC after wetting rate. LC characteristics under 1 : 1, 1 : 2, 1 : 5 and 1 : 10 ratios of top to bottom surface salt deposit density on polymeric insulators are studied. To verify the proposed models of this study, the results of experimental data and two other approaches are compared with together before dry-band formation. Moreover, a dynamic LC model under uniform pollution layer has been introduced and extended in order to calculate the LC when the formation of dry-bands along the insulator surface occurs. The dynamic model is drawn from experimental data and measured surface conductivity.

The impact that renewable energy sources interfaced by power electronics have on power systems becomes more important as their share in the generation mix increases, thus requiring detailed analyses that take into account their dynamics and controllers. In this study, the impact of photovoltaic (PV) power plants on the power system of northern Chile is analysed. The studied plants employ a controller that allows power converters to interact with the grid like virtual synchronous generators, and their model includes the dynamics of the plant and converter controllers, as well as the dc and PV system. The presented analysis, which comprises modal analysis and time-domain simulations of large disturbances, evaluates the impact of these plants with respect to PV plants based on a conventional converter controller. Tests and validations of the proposed models and controllers are carried out for an actual PV plant connected to the power system of northern Chile, and for a higher PV penetration case. The results show the ability of PV plants formed by virtually synchronous power converters to limit frequency excursions induced by large power imbalances, and to mitigate power oscillations of the synchronous machines in the system.