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Legacy protection schemes face new challenges as Inverter Interfaced Distribution Generation (IIDG) significantly changes the transient fault response of the distribution grid. The performance of the protection near the IIDG side is adversely affected by Low Voltage Ride Through (LVRT) and the negative sequence current suppression control characteristics of the IIDG. The operational characteristics of the protection are very different from those of the legacy protection schemes used in the distribution grid. Traditional overcurrent protection schemes cannot meet the requirements of selectivity and sensitivity. This paper analyses the influence of the IIDG on the protection schemes used in the distribution grid. Based on the positive sequence voltage polarization impedance criterion this study proposes a polarizing impedance criterion based on the fault component of positive sequence voltage, which can adaptively follow the fault resistance variation to satisfy the requirements of grid operation. The simulation results show that: (a) the proposed criterion is immune to the adverse effects of the transient characteristics of the IIDG; and, (b) it can adaptively follow the change of fault resistance making it suitable for application in short distribution lines. Using specialized System-on-Chip technology, a new distance protection device has been developed and tested on an industrial site. Simulation results and field tests showed that the new distance protection meets the requirements of the distribution grid with IIDGs.

Large hydro-generators require a 100% protected area for stator ground protection. Because the fundamental voltage criterion has low sensitivity near the neutral point, a high sensitivity third harmonic voltage criterion is needed to ensure that the generator can accurately respond to stator ground fault. This paper analyzes the third harmonic voltage characteristics of two large hydro-generators, calculates the sensitivity of the existing third harmonic criterion, and points out that the impedance parameters of the grounding transformer should be considered. In view of the lack of sensitivity of the third harmonic criterion for large hydro-generators, this paper proposes a stator ground protection criterion based on the third harmonic voltage variation, which has zero action quantity under the generator non-fault conditions and zero braking quantity after a ground fault occurs, and thus has high sensitivity and can effectively respond to high-resistance ground faults. PSCAD/EMTDC simulation results show that the proposed method has high sensitivity at different fault locations and transition resistances, which can provide theoretical support for the improvement of stator grounding protection in practical engineering.

Since more and more distributed generators (DGs) are connected into the low voltage distribution network, the existing fault-location methods are difficult to achieve accurate fault location due to its decentralized power supply characteristics. Aiming at this problem and the shortcomings of the existing traveling-wave fault-location methods, this paper proposes an accurate fault-location method for distribution networks with DGs based on multi-terminal traveling wave. Firstly, this paper analyzes the impact of DGs on the first arrival times of the fault-generated traveling waves. Secondly, a set of fault distance data independent of traveling-wave velocity are calculated by using the traveling-wave time information of all detectors, and the faulted branch is identified based on the discrete degree of this set of data. Finally, the quartile method is used to reject the bad data, and the accurate fault distance of the fault point is obtained. Simulation results show that the proposed method can accurately locate the fault point, hardly impacted by traveling-wave velocity, and has good robustness to arrival time errors.

Due to the high concentration of electricity generation and distribution in the floating nuclear power plant grid-source direct connection system, the generator stator ground fault protection is affected by the direct connection power grid. Traditional protection methods cannot distinguish the fault direction and are difficult to guarantee the extreme safety requirements of the nuclear islands. This paper analyzes the system ground fault, points out the limitations of the existing generator stator ground protection methods, and proposes an adaptive form of generator stator ground fault protection method. Based on the phase-current differential, the method can effectively distinguish the faults inside or outside the generator, which can also form the protection criterion and action strategies according to the fault degree. It can trip the generator smoothly under slight fault conditions, ensuring the safe and stable operation of the floating nuclear power plant. Simulation in PSCAD/EMTDC proves that the proposed method can effectively realize the selective coordination of the ground fault protection for the grid-source direct connection system.

The energy internet is a multi-energy sharing system with the coordinated and optimized operation of "source-network-load-storage". The energy router (ER) is the basic equipment in the future energy internet, so the research on the routing strategy of the ER is of great significance for the energy optimization. The traditional routing strategy based on the minimum loss path (MLP) does not consider the loss cost of different distributed energy resources (DERs), so the DERs with low loss cost have the same transaction priority as the DERs with high loss cost. In addition, power exchange should not be limited to traditional centralized allocation and scheduling, but peer to peer (P2P) transactions between ERs. Therefore, a minimum loss cost path (MLCP) routing strategy that can realize P2P transactions is proposed, which can maximize the benefits and improve the priority of renewable energy by modifying marginal cost coefficient. At the same time, a scheme of congestion management based on "the principle of equal mini rate of loss" is presented, which has less loss cost and can obtain the best optimization effect under the condition of critical congestion. Effectiveness of MLCP routing strategy is proved by simulation, and the superiority of the congestion management scheme is verified by comparing with traditional schemes.

In differential protection, the methods to correct amplitude and phase deviation of currents in primary and secondary winding of the Y/Delta-connected transformer will weaken the harmonic characteristics. Researchers have proposed the inrush restraint criterion of the primary inrush current directly. However, the helping effect of circulating current in the delta winding may cause symmetrical inrush current or waveform distortion, and the reliability of the restraint criterion based on the typical magnetizing inrush current characteristics will be affected. This paper reveals the generating mechanism, the helping effect and the influencing factors of the circulating current.

The Floating nuclear power plant grid is composed of power generation, in-station power supply and external power delivery. To ensure the safety of the nuclear island, the in-station system adopts a special power supply mode, while the external power supply needs to be adapted to different types of external systems. Because of frequent single phase-ground faults and various fault forms, the fault line selection protection should be accurate, sensitive and adaptive. This paper presents a fault line selection method in cooperation with multi-mode grounding control. Based on the maximum united energy entropy ratio (MUEER), the optimal wavelet basis function and decomposition scale are adaptively chosen, while the fault line is selected by wavelet transform modulus maxima (WTMM). For high-impedance faults (HIFs), to enlarge the fault feature, the system grounding mode can be switched by the multi-mode grounding control. Based on the characteristic of HIFs, the fault line can be selected by comparing phase differences of zero-sequence current mutation and fault phase voltage mutation before and after the fault. Simulation results using MATLAB/Simulink show the effectiveness of the proposed method in solving the protection problems.

Because the penetration level of renewable energy sources has increased rapidly in recent years, uncertainty in power system operation is gradually increasing. As an efficient tool for power system analysis under uncertainty, probabilistic power flow (PPF) is becoming increasingly important. The point-estimate method (PEM) is a well-known PPF algorithm. However, two significant defects limit the practical use of this method. One is that the PEM struggles to estimate high-order moments accurately; this defect makes it difficult for the PEM to describe the distribution of non-Gaussian output random variables (ORVs). The other is that the calculation burden is strongly related to the scale of input random variables (IRVs), which makes the PEM difficult to use in large-scale power systems. A novel approach based on principal component analysis (PCA) and high-dimensional model representation (HDMR) is proposed here to overcome the defects of the traditional PEM. PCA is applied to decrease the dimension scale of IRVs and eliminate correlations. HDMR is applied to estimate the moments of ORVs. Because HDMR considers the cooperative effects of IRVs, it has a significantly smaller estimation error for high-order moments in particular. Case studies show that the proposed method can achieve a better performance in terms of accuracy and efficiency than traditional PEM.

Zero-sequence differential protection is one of the foremost protections for EHV and UHV power transformers due to the advantages of high sensitivity and fast operation speed. The precondition of its correct operation is the correct CT polarity, but the checking of its CT polarity requires large zero-sequence current. However, there is almost no zero-sequence current during normal operation of transformer, which makes it difficult to carry out CT polarity checking of zero-sequence differential protection. To solve this problem, a new checking method of CT polarity for zero-sequence differential protection is proposed in this paper. First, calculate the similarity (S and F) between the self-produced zero-mode inrush current (3I 0p ) and the current of neutral point (3I 0n ). Second, compare the similarity (S and F) with the setting values determined in this paper. Third, assume different error types of CT polarity and reverse the corresponding CT current. Fourth, recalculate the similarity (S and F) in different error types and get the the maximum value of S and the corresponding F (S max and F m ). Finally, repeat step 2 and the error type corresponding to the S max and F m can be identified. This method is verified by digital simulation in this paper. The results show that this method can accurately identify all error types of CT polarity under different working conditions.

The traditional security-constrained optimal power flow (SCOPF) model under the classical N-1 criterion is implemented in the power industry to ensure the secure operation of a power system. However, with increasing uncertainties from renewable energy sources (RES) and loads, the existing SCOPF model has difficulty meeting the practical requirements of the industry. This paper proposed a novel chance-constrained preventive SCOPF model that considers the uncertainty of power injections, including RES and load, and contingency probability. The chance constraint is used to constrain the overall line flow within the limits with high probabilistic guarantees and to significantly reduce the constraint scales. The cumulant and Johnson systems were combined to accurately approximate the cumulative distribution functions, which is important in solving chance-constrained optimization problems. The simulation results show that the model proposed in this paper can achieve better performance than traditional SCOPF.

Conventional distance protection elements are predominantly based on voltage and current phasors. As a result, those elements require the half-cycle/full-cycle time window to estimate the accurate voltage and current phasors. Using information in the transients of voltages and currents is a possible way to improve the distance protection operation speed. To break free of phasor limitations, this paper presents a novel incremental quantities distance protection algorithm based on instantaneous values of voltage and current. Firstly, the instantaneous voltage distribution along the AC transmission line is analysed. Then, the principle of the incremental quantities distance protection is presented. The influence of fault angle on the presented algorithm is also analysed and a solution is proposed to deal with the relative small fault angle conditions. Finally, an extensive simulation results based on PSCAD/EMTDC simulation verify the performance of the presented incremental quantities distance protection.

For floating nuclear power plants, the inrush current produced under no-load closing situations of the working transformer (T) is necessary to be suppressed. Since the pre-magnetization technology of connecting small capacity pre-magnetized transformer (Pre-T) in series is simple to realize and can be used as the standby power source, it has a great application prospect. In this paper, the inrush current peak value which may be generated during each switch-closing step is analytically analyzed, and the suppression mechanism is concluded that: In the first two steps, the inrush current is suppressed because of the high equivalent resistance and leakage reactance of the windings of Pre-T. During the pre-magnetization process, the actual steady-state flux of T is gradually established. When its amplitude and phase are pre-synchronized with the expected steady-state flux of the power source, it is switched into the system without any inrush current generated. On this basis, guiding functions are constructed to select the best parameters of Pre-T and the switch-closing control strategies. Based on the optimal parameters, the simulation model is built by MATLAB/Simulink software. Simulation results show that the proposed method has a prominent inrush current suppression effect without closing angle control. It has high robustness under the conditions of typical remanent flux in the iron core and certain dispersion of the circuit switches. Moreover, it significantly accelerates the differential protection operation speed when the transformer is closed under internal faults.

The phase sequence arrangements of multi-circuit overhead lines on the same tower, commonly used in extremely high voltage/ultra-high voltage (EHV/UHV) transmission lines, directly affects the symmetry of power transmission systems, which influence the operation of power grid and relay protection. It is difficult to achieve complete transposition of four-circuit overhead lines on the same tower because of their complex structure, and it is a practical and effective way to select a proper phase sequence arrangement to improve the symmetry of the line parameters. This paper presents an evaluation index based method for evaluating the symmetry of four-circuit overhead lines on the same tower and selecting the optimal phase sequence arrangement. First, the unbalance degrees of the single-sequence component and composite-sequence component are defined as the basic evaluation index of the symmetry of the parameters and obtained by the calculation of the line sequence parameters. Furthermore, the unbalance degree comprehensive evaluation index is constructed by the weighted sum based on the ranking of basic evaluation indexes. The weight distribution factors of various unbalance degrees are calculated according to the exponential (0,2) scale algorithm. The unbalance degree comprehensive evaluation index is used to evaluate the symmetry of transmission line parameters and select the optimal phase sequence arrangements of four-circuit overhead lines on the same tower. Finally, the optimal phase sequence arrangements for the different towers with typical structures, such as vertical, horizontal, d-type and e-type towers are obtained by using the above analysis method and verified by simulation of PSCAD/EMTDC.

The conventional transverse differential protection has some drawbacks, such as low sensitivity, long successive operation zone, and being significantly influenced by operation mode when applied in high-voltage and asymmetric power systems. To overcome these drawbacks, an innovative integrated transverse differential protection scheme is proposed in this paper. This scheme includes three parts: faulty phase selection, faulty line selection, and tripping and reclosing strategy. The phase-selection criterion regards the two circuits as a whole and selects only the faulty phase. The criterion of line-selection distinguishes the faulty line with the faulty phase based on the transverse impedance ratio. Tripping and reclosing strategy decreases the number of operations of breakers and achieves quasi-three-phase operation as far as possible. Results of studies with PSCAD/EMTDC and on a dynamic physical model show that the proposed protection scheme can improve the accuracy of fault discrimination, decrease the successive operation zone, and maintain more transmission power after the fault.

This paper proposes a distance relay in time domain based on the R-L differential-equation algorithm which is suitable for wind power integration system. However, the conventional R-L differential-equation algorithm cannot detect the correct fault direction if the fault point is very close to the relay location. Therefore, this paper mainly focuses on this problematic conditions and proposes a solution. The instantaneous value equivalent model of DFIG is established and characteristics of the transient electromotive force (EMF) of DFIG after the faults are also analyzed, which shows that the EMF of DFIG has the inertia within a short time after the faults. The memory voltage drop and actual voltage drop on equivalent impedance from relay location to back-side and opposite-side equivalent power sources are introduced in this paper to detect the fault direction. Meanwhile, the characteristics of the memory voltage drop and actual voltage drop under forward and reverse fault conditions are deduced, respectively. With the differentiated correlation coefficient between the memory voltage drop and actual voltage drop under different fault conditions (forward and reverse fault), the fault direction identification criterion is formed accordingly. Finally, an extensive performance evaluation using PSCAD/EMTDC simulation corroborates the effectiveness of the proposed method. Results show that the proposed method can detect the fault direction quickly with high sensitivity during zero-voltage fault conditions.

This paper describes an improved transformer winding tap injection distribution static synchronous compensator (WTI-DSTATCOM) for medium-voltage reactive power compensation. The cascaded multilevel converter based DSTATCOM is connected to the special-designed winding taps on the primary windings of the transformer instead of the conventional point of common coupling. The voltage stress for DSTATCOM to handle is reduced. The winding tap injection (WTI) method can make full use of the spare capacity of the transformer and obtain a flexible connection voltage for DSTATCOM. The compensation mechanism and winding current distribution after currents injection are analyzed by phasor diagrams under steady state. A nonlinear passivity-based control (PBC) algorithm is designed for inner-loop current control and a three-layer voltage balancing control strategy is applied to balance the dc capacitor voltage. The results obtained from the MATLAB/Simulink simulations and a down-scaled laboratory prototype experiment of 800 V verify the feasibility and effectiveness of the proposed WTI-DSTATCOM system with PBC algorithm in reactive power compensation.

This paper proposes a novel adaptive passivity-based control (PBC) of cascaded multilevel converter based D-STATCOM integrated with distribution transformer for medium voltage reactive power compensation. A cascaded multilevel converter-based D-STATCOM is connected to a group of taps on the primary windings to form an integrated structure. The power transformer is used as coupling transformer at the same time. The structure is helpful to achieve a flexible connection point voltage and improve the capacity utilization of the transformer. The winding currents distribution and compensation capacity constraint are analyzed in detail by steady-state phasor diagrams. A nonlinear passivity-based control is designed to achieve reference currents tracking. Furthermore, an adaptive control is also presented to attenuate the effects of parameters perturbation on the performance of the D-STATCOM. Finally, down-scaled prototype experimental results are provided to verify the steady and dynamic performances of the integrated D-STATCOM with PBC for reactive power compensation.

When a fault occurs in the power grid, voltage-source converter high-voltage direct current (VSC-HVDC) connected offshore wind power plants (OWPP) will enter the fault ride-through operation state under the control of wind turbines and modular multilevel converters. The adverse effects of its complex fault current characteristics on power grid protection are highlighted. According to the topological structure and control strategies of OWPP, a theoretical calculation method for OWPP’s fault current under symmetric and asymmetric faults is proposed. On this basis, this paper comprehensively studies the applicability of distance protection on the tie line of OWPP, analyzes the existing problems, and gives corresponding engineering solutions. The correctness of the analysis conclusions is verified by digital simulation results.

The multi-energy interconnected energy system (MEIES) consists of multiple energy hubs (EHs) connected through the energy router (ER). To realize the optimal operation of the MEIES, this paper proposes an energy optimization and routing control strategy for the neotype MEIES based on ER using the lower and upper layers. In the lower layer, the multi-energy conversion device and storage device in the energy local area network (E-LAN) are modeled in detail, and the minimum cost electricity-heat-gas-cool multi-energy optimization strategy is realized. In the upper layer, the electric, heating, and natural gas network's energy flow is normalized by strict mathematical derivation in the energy wide area network (E-WAN). The generalized energy loss model of a multi-energy network with the unified mathematical equation is obtained. Then, based on the loss model and fully considering the differences of national conditions and policies, the unified energy routing control strategy of minimum loss local absorption (MLLA) in the monopoly market and of minimum loss multipath transmission (MLMT) in the competitive market are proposed for different decision-makers. Moreover, two kinds of ER transactions of competitive market priority ranking methods are proposed to achieve the expected incentive goal by transferring the energy loss of the transaction from one to another, which are the priority determined by the quotation and transaction volume. Finally, the feasibility and effectiveness of the overall strategy are verified by simulation and comparison.

Since distribution networks have multiple branches, complex topologies and increasing penetration of the distributed energy resources (DERs), the accurate fault location is difficult to realize. The existing traveling wave fault location methods are strongly affected by the arrival time errors. To overcome the problems mentioned above, a multi-terminal traveling wave fault location method is proposed for active distribution networks based on residual clustering. Firstly, the traveling wave arrival times are utilized to construct a minimized optimization model for each section. The objective optimization function represents the minimization of the sum of squared errors (MSSE), and the global optimal solutions reflect the wave velocity and the fault distance. Subsequently, the particle swarm optimization algorithm (PSO) is used to solve the above optimization models, and the section with the minimum MSSE is judged as the faulty section. Finally, the density-based spatial clustering of applications with noise (DBSCAN) algorithm is utilized to group the residuals of the faulty section to identify the bad data, which are affected by the arrival time errors. And the normal data remained are applied to reconstruct the optimization model and calculate the optimal solution of the fault distance. Thus, the fault location results can be corrected. Simulation results and field tests indicate that the proposed method has high fault location accuracy, strong robustness to time errors and high adaptability for active distribution networks.

Converter protection, including bridge differential protection and valve short-circuit protection, frequently malfunctions during the disturbance from an AC system, such as the short-circuit fault and the inrush current, which severely threatens the safe and stable operation of an AC/DC hybrid power system. This paper analyzes the cause of such maloperation based on mechanism analysis and simulation tests. It is found that saturation of the current transformer in the delta winding of the YD converter transformer during the disturbance of the AC system is the main reason for this sort of maloperation. To solve this problem, this paper proposes a countermeasure by introducing the zero-sequence circulating current as the restraint current of the bridge differential protection. The time difference between the increase of the zero-sequence circulating current and the rise of the bridge differential current is utilized to determine whether to apply the proposed countermeasure. Simulation tests indicate that this countermeasure can improve the reliability of the bridge differential protection during the disturbance in the AC system, without affecting the sensitivity and operation speed of the bridge differential protection for the internal fault. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

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.

Modern power grids, especially high-voltage grids, are increasingly integrated with wind farms. The correct action of distance protection for the wind farm tie line plays an important role in the safe and stable operation of the power system. However, based on the fault characteristics of wind farms based on the doubly fed induction generator, considering crowbar action and weak infeed properties, this paper reveals some serious defects of conventional distance protection. The result shows that, due to the influences of mixed data window and damped current component with near-rotor-speed frequency, the fault distance calculation result obtained by the fundamental-component-based distance protection is inaccurate. To tackle this problem, a transient-based distance protection countermeasure along with a faulted phase selection technique is proposed. Specifically, the transient phase voltage characteristic for different fault types is analyzed to select the faulted phase. Considering the effect of high-frequency components, pre-fault voltage compensation, and transducer characteristics, a distance calculation procedure including a low-pass filter, the reconstruction of voltage at the fault point, and an iterative fault distance calculation is proposed. The comprehensive performance of the proposed method is validated by the simulation case studies using the PSCAD/EMTDC software. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The centralized substation protection (CSP) system is one of the novel protection systems in a smart substation. The configuration mode of the CSP system is quite different from that of the traditional protection systems. According to the configuration mode and the operational characteristics of the CSP system, the overall reliability models are established in this paper. Then, the equivalent reliability models of the CSP system in a bay are also established. A novel approach is used to compute the reliability indices of the CSP system based on fault tree and the Monte Carlo method. The proposed method also takes into account the fault-tolerant function and repair characteristic of the CSP system. Simulation results show that reliability of the CSP system is much better than that of the traditional protection system. The results of this study will be helpful for engineering the CSP system in a smart substation. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

In the transient process of power grid faults, the transferring distortion of current transformer (CT) can seriously affect relay protection performance. Under these conditions, it is difficult to analyze the ferromagnetic characteristic of the magnetizing branch in the transient equivalent circuit of CT. The Jiles-Atherton hysteresis model (J-A model), which is widely used in digital simulations, can accurately describe the hysteresis and saturation process of the core characteristics; however, to acquire the parameters of the J-A model of current transformers in practical use is still a challenging problem. In this paper, physical tests based on a practical CT and parameter identification are presented to solve the problem. The basic hysteresis loops of P, PR, and TPY class of practical current transformers are obtained through physical tests. Thus, the J-A model parameters are identified using a hybrid genetic/simulated annealing algorithm, based on which transient simulation models of different class CTs are constructed. The effectiveness of the proposed method is verified via dynamic physical simulation tests. A typical accident is analyzed based on these models.

As a key device of the energy Internet, the power router can realize efficient allocation and management of electric energy. The DC transformer is an important part of the power router and is the core equipment for energy control. In this paper, the operating status and balance control of the isolated modular multilevel dc-dc converter (IMMDCC) in the case of asymmetric bridge arm inductance are studied. Firstly, the working principle of IMMDCC and the model of transmission power are analyzed. The common mode current loop and differential mode current loop of the IMMDCC are established by analyzing the arm current. On this basis, the working principle of IMMDCC and the imbalance of capacitor voltage caused by the asymmetry of arm inductance are analyzed. A control method for changing the duty ratio is proposed to solve the problem of capacitor voltage imbalance.

In view of the operating characteristics for voltage sags of AC side of the power electronic transformer(PET), a low-voltage ride through(LVRT) strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety. During the asymmetric voltage sag, the negative sequence current of PET is eliminated to ensure the symmetry of the injected current. According to the degree of positive sequence voltage sag, the reactive current injection is provided to assist in voltage recovery. According to the PET active power condition before the voltage sag, the level and direction of which are maintained as far as possible without exceeding the limit, for which the disturbance to the AC and DC grids is reduced. Finally, the effectiveness of the proposed LVRT strategy is verified by simulation model.

This paper proposes a fault location method for distribution lines based on coupling parameter identification using two terminals data. In view of the problems of insufficient deployment of on-site phase selectors and inaccurate line parameters in medium voltage distribution network, the line coupling parameters are identified by characteristic loop voltage equation with coupling parameters based on pre-fault PMU recording data. On this basis, the corresponding fault distance percentage is calculated by genetic algorithm using the recorded data of fault transient process of three-phase synthetically. The weighting coefficients are constructed by the signal energy of current fault components and the percentage of fault distances of three phases is fused to determine the fault location. The method can locate the fault point without the accurate parameters of the measured line and the phase selector. The simulation results verify the effectiveness of the method.

Power Electronic Transformer(PET) can realize flexible control of AC/DC hybrid distribution network and efficient absorption of new energy. During a grid fault, if PET can adopt a superior low-voltage ride-through(LVRT) strategy, the operational safety of PET itself and the stability of AC/DC hybrid grid can be improved significantly. Moreover, PET exhibits obvious controlled characteristics during LVRT operation, which brings difficulties for grid fault analysis and relay protection setting calculation. Therefore, it is urgent to study the LVRT strategy and fault equivalent model of PET, so as to provide guarantee for the safe and stable operation of the system and lay a theoretical foundation for grid fault analysis. In this paper, a LVRT strategy adapted to bidirectional power exchange of PET is proposed for the purposes of maintaining the system stability, assisting the system voltage recovery and protecting PET safety, based on which the fault current analytical expression of PET is obtained by deducing and analyzing the control model in the bidirectional rotating coordinate system. And the corresponding short-circuit current equivalent calculation model of PET is established. Finally, the correctness of the proposed LVRT strategy and the short-circuit calculation equivalent model is verified by simulation.

The large-scale inverter interfaced generators (IIG) integrated into power system shows more and more unique fault features. There are adaptability issues when traditional distance protections are used in such system. Aiming at the adaptability problem of distance protection configured on the tie line, a simulation model is built according to the local grid and the distance protection algorithm in protection device, and the influence of IIG on the distance protection of different principles in the actual protection device is comprehensively analyzed. The research results show that, due to the fault characteristics of IIG, the IIG side distance protection is prone to refuse to operate and misoperation, and the system side distance protection is prone to reverse misoperation. The research results of this paper provide reference for the engineering application and performance improvement of distance protection.

The main issue of power system alarm processing is to interpret the raw alarm messages to summarized information for dispatchers in decision making. The existing analysis for alarm processing is performed element by element, and the process of establishing the logical relationship is both complex and time consuming. A novel analytic method for power system alarm processing employing topological description is proposed in this paper. The topological description of the elements, protections and circuit breakers is established, and based on which the expected states of the protective devices are analyzed and the optimizing function for alarm processing is then formulated. The fault scenario can be well interpreted according the best solution of the optimization problem. The testing results of the IEEE 14-bus system show that the proposed method can quickly determine the fault element, even with abnormal operation of the protective devices.

In recent years, the zero-mode inrush current of high-impedance transformer with built-in high-voltage winding (T-Hin), which has large amplitude and decays slowly, causes the misoperation of zero-sequence overcurrent protection. Compared with magnetizing inrush current, the waveform of zero-mode inrush current is inconsistent and irregular, and few researches have proposed the mathematical analysis as well as the improved protection using waveform characteristics. In this paper, the mathematical expression of transformer zero-mode inrush current is derived. Further considering the parameter differences, the zero-mode inrush current of T-Hin is larger, which tends to cause the misoperation. The mathematical waveforms fit well with the recorded waveforms. Both recorded waveforms and mathematical waveforms in various conditions prove that the second harmonic ratio (the ratio between the second harmonic and first harmonic) of zero-mode inrush current is significant. Based on the above analysis, a criterion based on the second harmonic ratio restraint of zero-mode inrush current is proposed. If the second harmonic ratio exceeds the setting value, it is considered that the inrush current is generated and sends a signal to restrain the protection. The theoretical setting value of the proposed criterion and the practical engineering method for determining the setting value are obtained.

This paper proposes a disturbance observer-based internal model control for the three-phase LCL-type grid-connected inverter. The proposed control scheme is designed in dq-frame, and the compound controller includes an internal model control (IMC) feedback part and a feedforward compensation part for the disturbances by using a disturbance observer (DO). The attractive advantages of this structure are that the lumped disturbances including the strong coupling effects of LCL filter, model mismatched and external disturbances can be suppressed effectively, and the parameters of the two controllers can be designed separately with easy implementation. The simulation and experimental results demonstrate that the proposed method has a better lumped disturbance rejection property than that of PI method in controlling three-phase LCL-type inverter.

High short-circuit impedance transformer is widely used because of its advantage in limiting short-circuit current. Among the two types of high short-circuit impedance transformers commonly used, high-voltage winding built-in type frequently occurs zero-sequence over-current protection maloperation during no-load closing, while another type of low-voltage series reactance transformer is rarely reported. The main difference between them lies in the winding structure. However, the existing transformer simulation models can hardly reflect the difference. Through research, the equivalent circuit model which can reflect the difference of winding structure through flux linkage analysis is established, the method of building simulation model of high impedance transformer based on EMTDC/PSCAD is put forward, and the accuracy of the model is verified through dynamic simulation test, which provided a powerful tool for the research of high impedance transformer. Finally, the difference in inrush current characteristics of different high-impedance transformers are compared, and the reasons why the built-in transformer is easy to malfunction is explained.

This paper proposes a hybrid UHVDC system dedicated for long-distance power transmission and back-to-back interconnection of the receiving region power grids. The receiving terminal of the UHVDC system is formed by the LCC and MMC in cascaded connection. The system combines the merits of hierarchical connection and asynchronous interconnection, which optimizes the network structure of the receiving system and makes the power distribution of the receiving region grids more flexible. The topology of the system is introduced in detail. The control strategy and the DC fault clearing strategy are proposed. Finally, the UHVDC system is built in PSCAD / EMTDC. The simulation verifies the effectiveness of the control strategy, the DC fault clearing strategy and the re-starting strategy. The results indicate that this kind of UHVDC system has a good application prospect in large-capacity long-distance power transmission to the asynchronous regional power grids.

In allusion to fault location failure on wide area travelling-wave-based fault location methods caused by the indeterminate shortest transmission path of initial travelling wave and the inaccurate travelling wave velocity, this study proposed a novel fault location method for complex transmission grids based on the time difference of arrival of wide area travelling wave. Firstly, the shortest transmission path in the loop network is determined online by using the time difference of arrival of the initial travelling wave measured at both ends of the fault line. Then, the travelling wave velocity and accurate fault distance are calculated by fusing arrival time of wide area initial travelling waves via the least-square estimation. The phase current travelling wave is used for the proposed method, and the S transform is used for detecting the arrival time of phase current travelling wave at the Nyquist frequency. The proposed method avoids the phase-mode transformation and the matching between the actual fault condition and the pre-determined shortest transmission path. The proposed method can also exactly calculate the fault distance without obtaining the travelling wave velocity. The performance of the proposed method is verified by the power systems computer aided design/electro magnetic transients in DC system (PSCAD/EMTDC) simulation results.

Inverse time protection is a typical backup protection for large generator, and the suitable setting value is critical to ensure the security of both the generator and power system. The existing protection setting methods recommended by the standard or specification cannot meet the coordination relationship of the protection, and the determined operating time may exceed the generator's affordability. A polynomial optimal fitting based inverse time protection for large generator is proposed in this paper. Firstly, the operation characteristic of the protection is determined according to the heating process of the generator under abnormal conditions. The least square method is then used to fit the coefficients in the characteristic expression. Finally, the proposed method is tested by actual generator parameters, and the results demonstrate that the protection can achieve the optimal matching between the operation characteristic and the generator capacity curve. Besides, the protection can operate in a time extremely close to the maximum ability of the generator, which means to provide more effective support for the power system while ensuring the generator safety.

© 2019 IEEE. Passivity-based control (PBC) relies on an accurate mathematical model and thus its performance will be degraded by the weak robustness against parameters uncertainties, modeling error, and external disturbances. Moreover, it cannot achieve zero tracking error of the steady-state current under parameter uncertainties and modeling error. This paper proposes a novel disturbance-observer-(DO) based PBC (DO-PBC) for static synchronous compensator (STATCOM) to achieve better stability and dynamic performances against disturbances. A DO that has been introduced into the PBC current loop is used to compensate system disturbances, which can improve the robustness of the control system and eliminate the steady-state tracking error. Moreover, the proposed DO-PBC provides faster responses in handling various kinds of disturbances. Then, the detail design process, stability and robustness analysis, and parameters tuning method are investigated and presented. Also, the proposed method is simple to be implemented by the separation principle. The performance comparisons among the proportional integral, the conventional PBC, and the proposed DO-PBC are carried out to show the effectiveness of the proposed method against disturbances and the precise current tracking, via simulation tests and experimental tests based on a down-scale laboratory prototype experiment of 380 V STATCOM.

The quality of output current of L C L -type voltage source inverter (VSI) is degraded by the grid voltage distortion, the dead time effect, and the parameter mismatches. This paper proposes a novel fractional order harmonic disturbance observer-based control (FOHDO) for the three-phase L C L -type inverter to suppress all the dc- and ac-component periodic disturbances. Firstly, a harmonic disturbance observer based control for L C L -type inverter is proposed with the internal model knowledge of the periodic disturbances signals. Subsequently, the FOHDO based on Lagrange interpolating polynomial fractional order delay approximation is put forward to enhance the adaptability of the grid frequency. Then, the stability and robustness performance of the proposed FOHDO for L C L -type inverter are evaluated, which also involves a detailed parameter design process. Finally, the effectiveness and performance of the proposed FOHDO method are verified by both experimental and simulation results.

The new emerging idea of energy router and energy blockchain network (EBN) is gaining remarkable attention due to various applications and innovative solutions for efficient energy trading. However, the implementation of the EBN raises multiple challenges such as lack of power loss consideration and congestion management, insufficient computing, and higher transaction costs. This paper introduces a new energy trading approach to counter energy trading challenges effectively and perform optimal energy trading. The proposed systematic design was divided into four steps which execute (i) forming trading pairs through a two-stage Stackelberg game model, (ii) checking security by using the characteristics of directional power flow, (iii) congestion management through the minimum loss multi-path power transmission scheme, (iv) forming smart contracts, and settling transactions. The performance of the proposed framework is verified using 13 nodes EBN implementation, where the results obtained demonstrate accurate energy trading, balanced trading prices, even in congestion conditions.

This paper presents an approach to single line-to-ground fault location in a power distribution system based on the time lag of different modal components . It is a single-ended method applicable to unsynchronized measurements. The fault location formulation is derived by analyzing the traveling wave propagation characteristic in a distributed parameter model. To tackle the key problem of ground-mode velocity estimation, an iterative velocity method based on prior knowledge is first proposed. Since the practical measurements are not ideal because of sensor accuracy and nonequivalent transient response, fuzzy neural network is utilized to process traveling wave data acquired at multiple measurements to get an error-tolerant effect. Furthermore, electromagnetic transient simulations for a 34-bus test feeder system under various conditions of fault resistances, inception angles, and distributed generator access reveal the robust estimation of fault location. The performance of the proposed method is also validated by simulations with noisy and noise-free measurements. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

This paper studies the sympathetic interaction between transformers. The influencing factors of the sympathetic inrush current, including the system impedance, closing angle and the initial remnant flux of the added transformer, the system voltage, and the neutral grounding mode of the transformers, are analyzed in detail through dynamic simulation. Based on this, a method for identifying the sympathetic inrush current is proposed based on the essential curve feature of the magnetizing currents of the transformers during the sympathetic interaction, taking advantage of the information sharing between substations. The results from dynamic simulation and field-recorded data show the validity of the proposed method. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Summary Because of the capacity of satisfying the quality of service requirements of each traffic service in smart distribution networks, Ethernet passive optical network (EPON) technology is one of the primary technologies for establishing communication systems in smart distribution networks. This paper presents an improved weighted interleaved polling with adaptive cycle time (IW-IPACT) algorithm according to the quality of service requirement of different traffic classes, especially the requirement of the new protection and control technique. During a polling cycle, a virtual weighted buffer queue that meets the bandwidth requirements of the EPON was constructed. Thus, strict priority scheduling was used for the expedited forwarding service, and the weighted fair queuing scheduling was applied for the other services. In addition, to evaluate the real-time performance of the IW-IPACT algorithm, the upper bound for the packet delay of each traffic class was derived by using the network calculus method. The evaluation results indicate that the bandwidth utilization of the EPON was improved by using the IW-IPACT algorithm. In addition, the calculus and OPNET simulation results also show that the EPON can meet the real-time requirements of relay protection. Copyright © 2016 John Wiley & Sons, Ltd.

The special fault current characteristics of inverter interfaced dis -tribution generator (IIDG) make the conventional fault analysis methodbased on synchronous generator not applicable to a power grid withsuch IIDGs. In order to address this issue, according to the specialfault current characteristics of IIDG, the problems of conventional faultanalysis method when it is applied for the power grid with IIDGs arepointed out firstly. Based on it, a novel fault analysis method of thepower grid with IIDGs is proposed. Lots of simulation cases showthat the proposed fault analysis method has high calculation accuracy,which can meet the requirements of short-circuit calculation of thepower grid and setting calculation of relay protection.