Search

Doubly-fed variable speed pumped storage unit (VSPSU) adopts slip ring injection AC excitation mode, and its rotor side structure is complex and prone to open-phase faults. It is very important to configure specific open-phase imbalance protection to ensure its safe operation. Traditional doubly-fed motors only stay in the state monitoring stage for rotor electrical imbalance, which cannot meet the requirements of large-capacity VSPSU for fast, real-time, and high-reliability protection. Therefore, this paper proposes a VSPSU rotor open-phase imbalance protection method based on the rotation transformation fault component ratio (FCR). The fundamental frequency component and fault characteristic component of the stator current space vector are extracted by rotation transformation and FCR is calculated, and then different protection action modes are divided according to the amplitude of FCR. To realize the engineering application, a microcomputer protection algorithm suitable for the proposed protection method is further proposed, and then a complete microcomputer protection scheme is formed. Simulation and experiment results show that the proposed protection scheme has high sensitivity and fast action speed. At the same time, the proposed algorithm can be directly embedded in the existing microcomputer protection device to meet the needs of engineering applications.

Abstract The power router (PR) is a promising piece of equipment for realizing multi-voltage level interconnection and flexible power control in the future distribution power grid. In this paper, a hybrid PR (HPR) topology based on power-frequency transformer electromagnetic coupling with converters is proposed for the medium distribution power grid. The power-frequency transformer is used to undertake power transmission, voltage conversion, and other main tasks, while the power electronic converters are combined to achieve active control. Equivalent magnetic and electrical circuit models are established to help discuss the operating principle of the proposed HPR. Additionally, the power flow and control principle of the HPR in different operating conditions are analyzed, with the control system design scheme presented. The theoretical analysis results are verified by MATLAB/Simulink + Plecs simulation and a controller hardware-in-the-loop study, as well as a down-scale experimental test, indicating that the proposed HPR is flexible in active voltage support and current control.

The calculation method of the external fault short-circuit current of large units with clear mechanisms and practicality is of great value in engineering practice. Due to the special AC excitation structure and control mode, the external fault short-circuit characteristics of variable-speed pumped storage units (VSPSUs) are very different from those of conventional units. Therefore, this paper proposes a calculation method for external fault short-circuit current based on a time-domain dynamic circuit. According to the excitation and control characteristics of VSPSUs under external fault, the VSPSU under external fault is divided into two cases: continuous excitation and jumper action. Based on space vector analysis, the time-domain dynamic circuit models are established, respectively. On this basis, the analytical formula of the external fault short-circuit current of VSPSUs is derived by combining the transition boundary conditions of the two cases and the dynamic characteristics of the stator flux linkage. Finally, the comparison with PSCAD/EMTDC simulation results shows that the proposed VSPSUs’ external fault short-circuit current calculation method has high accuracy and practical value, which can meet the needs of primary equipment selection and relay protection of variable-speed pumped storage power plants.

Control rod drive mechanism (CRDM) in nuclear reactors powered by two motor generator units running in parallel, an excitation fault in one of the generators will cause the terminal current to rise in another generator, which will cause the overcurrent protection to operate before the loss-of-excitation protection, resulting in nuclear reactor shutdown. To solve the problem of mismatch between loss-of-excitation protection and overcurrent protection, this paper applies the equivalent circuit and phase diagram analysis, obtains the conclusion that the generator terminal current phase after an excitation fault is significantly different from the current phase after a phase-to-phase short circuit fault, based on this, a generator overcurrent protection criterion of compound fundamental current phase difference is constructed, which can solve the maloperation of overcurrent protection under possible excitation fault in actual engineering. Simulation results of PSCAD/EMTDC verify the effectiveness of the proposed methods under different operating modes and fault conditions.

Using salt caverns and caves to build compressed air energy storage power stations is an important development direction in the field of large-scale energy storage. With the continuous maturity of technology, compressed air energy storage generators have gradually developed into large-scale units with a single-unit capacity of 300 MW. Due to the increase of single-unit capacity, the internal short-circuit fault of the stator will cause very serious losses. In addition, the compressed air energy storage generator has complex operating conditions and is more prone to various internal short-circuit faults. In order to effectively ensure its safe and reliable operation, it is particularly necessary to study the effective main protection configuration scheme. The optimal design of the main protection configuration scheme is based on the analysis of the possible internal short-circuit faults of the generator. At present, the stator windings of large-scale compressed air energy storage generators generally adopt double-layer wave winding or double-layer tap winding structures. For these two winding structures, this paper analyzes in detail the characteristics of all fault modalities that may occur inside the large-capacity compressed air energy storage generator and designs a general algorithm that can form their respective fault sets. Finally, the application of the algorithm in the analysis of the stator fault modality in a compressed air energy storage power station is introduced.

Due to the pseudo-bipolar structure of the flexible DC distribution network, the short-circuit current caused by a pole-to-ground fault is weak. The fault characteristics, particularly the steady-state fault characteristics, are not prominent. Therefore, pole-to-ground fault diagnosis and protection have become more challenging. This paper theoretically studies the short-circuit current characteristics of pole-to-ground fault in the flexible DC distribution network, derives the analytical expression of the short-circuit current, and analyzes the directional characteristics. Then, a novel pilot protection scheme based on fault direction comparison is proposed. This scheme compares the positive and negative half-wave integral values of the short-circuit current fault components to judge the fault direction. The fault directions on both sides of the feeder are obtained through the pilot protection channel, thereby distinguishing internal and external faults. The simulation results show that the proposed scheme can reliably identify the faulty feeder and has an excellent ability to perform with high transition resistance. Moreover, the scheme does not require sampling synchronization, which is easy to be applied in engineering.

With the continuous increase in the proportion, renewable energy sources have brought great challenges to the traditional protection based on fundamental frequency, due to their complex short-circuit current characteristics which are different from synchronous generators. As the main protection of the tie line (outgoing line) of the renewable energy station, the differential protection will also be seriously affected in terms of its operation performance, and even the operation failure or maloperation may occur in some cases. This paper proposes a new principle of pilot protection based on cosine similarity comparison by analyzing the fault characteristics of renewable energy sources. With the full current as the main criterion and the current fault component as the auxiliary criterion, this principle comprehensively utilizes the time domain information of the current on both sides of the line to improve the protection sensitivity. At the same time, a method of capacitive current compensation for the sampling value is proposed according to the algorithm characteristics of similarity comparison, to prevent the protection maloperation in the case of external faults. The simulation results show that, compared with the existing pilot protection based on similarity comparison, the new protection principle proposed in this paper has good engineering application value by identifying internal and external faults in a more reliable and sensitive manner.

In the distribution grids, grounding fault transfer devices have been widely used to suppress the transient arc grounding fault. Based on the grounding fault transfer device, a high sensitivity faulty section location method is proposed in this paper. When an arc grounding fault occurs, the fault phase bus is earthed by the grounding fault transfer device, which can simultaneously realize arc suppression and faulty section location. There are two grounding transient processes in this method, including fault grounding and active grounding of the faulty phase bus. During the aforementioned processes, the similarity degree of the zero mode current on both directions of the faulty section varies from low to high, whereas there is almost no change in the healthy section. The ratio of S transform energy relative entropy is used to represent the level of change of the similarity degree on both directions of each section. The section with the maximal entropy ratio is determined as the faulty section. The proposed method does not need to set the threshold and presents higher sensitivity for an arc fault. Simulation proves the sensitivity and accuracy of the proposed method.

Rotor winding short circuit is a frequent fault type of generator unit. Compared with the traditional fixed-speed unit using DC excitation, the three-phase AC excitation structure of the variable speed pumped storage unit is more complicated, and the fault diagnosis of the rotor winding short circuit is more difficult. Therefore, this paper proposes an offline diagnosis method for variable speed pumped storage units based on the principle of Repetitive Surge Oscillograph (RSO). Firstly, the application scheme of the repeated pulse method in the variable speed pumped storage unit is designed. By injecting low-voltage pulses into three phases simultaneously and detecting the response curve to determine whether the unit has a short-circuit fault. Then, based on the response characteristics of different rotor winding short-circuit fault types, an identification scheme of rotor winding short-circuit fault types and faulty phases is proposed. Subsequently, the variational mode decomposition (VMD)-teager energy operator (TEO) method is used to estimate the injected pulse and characteristic curve’s initial sudden change position and then the fault position is determined by the time difference and the measured wave velocity. The simulation results show that the fault identification and fault location method proposed in this paper can be effectively applied to various rotor winding short-circuit faults of variable speed pumped storage units.

The short-circuit current calculation is of vital importance for the power system. With a high proportion of new energy sources, the setting of relay protection and the selection of electrical equipment have been severely degraded. An accurate short-circuit current calculation model of new energy source is necessary for the short-circuit calculation of power grid with new energy sources. However, due to the DFIG’s control system is a ”black box”, and an accurate fault characteristics and short-circuit current calculation models of DFIG is difficult to obtained. Currently, for the fault current of the DFIG, lots of research works is carried out under symmetrical fault or the crowbar protection is activated condition. However, under crowbar protection is not activated and unsymmetrical fault condition, there are no unified short-circuit calculation models of DFIG. In this paper, the transient process of DFIG under those condition is analyzed, and mathematical expressions are derived. Moreover, the comprehensive and unified short-circuit calculation models of DFIG have established under different low voltage ride through control conditions. The influence of GSC and slip rate of DFIGs can be considered. The test results show that the LVRT control strategies, the GSC and slip have an important influence on the DFIG’s short-circuit calculation model. The quantitative research results are of important significance for relay protection setting calculation, grid operation and control, and the selection of electrical equipment of power grid with DFIGs.

The stator single-phase grounding fault location of large generator can provide guidance for rapid maintenance of faults. A third harmonic potential distribution based stator ground fault accurate location method is proposed. The current location methods mainly take coil as location unit and may have location error. However, accurate locate to the slot number is more meaningful for troubleshooting. In addition, the third harmonic potential induced by adjacent slots has larger phase difference, and has more distinct fault characteristics than the fundamental potential. In this paper, the fault location optimization model is constructed based on the analysis of the distribution characteristics of the third harmonic potential. The exhaustive solution of the location model is realized by artificially setting multiple reference points, and finally the reference point closest to the real fault point is obtained. The proposed method can accurately locate the fault slot number. PSCAD simulation verifies that the proposed method is robust against the transition resistance error and has higher accuracy than the existing methods.

The frequent occurrence of ground faults in distribution networks seriously affects the power supply reliability, and the accurate fault section location method is the foundation of automatic distribution systems. Since there are location blind areas in distribution networks with the similarity characteristics distortion of the zero-mode current waveforms, the application of the existing location methods based on the zero-mode current similarity degree are limited. However, when a ground fault occurs in the location blind areas, the polarity relationship characteristics of the zero-mode current remains unchanged. But the location method only based on polarity relationship is seriously affected by the reverse connection of the CTs. Maximum mean discrepancy (MMD) method can comprehensively reflect the waveform similarity degree and polarity relationship between zero-mode currents. Based on the current relationship of the fault zero-mode equivalent circuit, it is concluded that the MMD value of the fault section is larger than that of the normal section in the fault line. On this basis, a small-current ground fault section location method for distribution networks based on MMD is constructed. Simulation results show that the proposed method can effectively eliminate the location blind areas, which can accurately locate the fault section under different line types, fault points and fault conditions. Moreover, it has strong robustness, which is less affected by arc faults and measurement noises.

Safe and reliable generator circuit breaker (GCB) failure protection can ensure effective isolation between generator and system under different fault conditions to avoid generator damage. In this paper, aiming at the existing problems of the GCB failure protection criterions based on current quantities, the voltage quantities are combined to constitute the improved GCB failure protection. Through fault sequence network analysis, it is concluded that there are voltage phasor differences between the two sides of GCB for the normally separated phases, while the values are zero for the rejected phases. On this basis, a GCB failure protection criterion based on the voltage phasor difference is proposed for the generators equipped with PTs at the two sides of GCB. After a generator fault occurs, in the process of de-excitation, the terminal voltage drops quickly except GCB three-phase rejection because of the voltage support from the system side. Moreover, under the GCB three-phase rejection situation, the loads cannot be effectively shed and no terminal voltage increase is caused compared with other GCB operation situations. Thus, the protection criterions based on the voltage variation are proposed to distinguish GCB three-phase rejection situation, including the voltage changing rate criterion and the increased voltage mutation criterion. They are used for generators equipped with only one terminal PT. Simulation results of PSCAD/EMTDC verify the effectiveness of the proposed methods.

During the energization of transformer with high-impedance (HT), the zero-sequence overcurrent protection (ZSOCP) often malfunction with the inrush current of primary winding (iP-inrush) shows a large unbalance. In order to clarify the cause of the misoperation and prepare for a solution, the imbalance characteristic is studied in this paper. The imbalance is visually displayed by dynamic phasor diagram. Through the method of numerical simulation, it can be obtained that the peak value of the zero-sequence current always appears near characteristic opening and switching angles (COSAs). At this time, the two phases are deep saturated and one phase is not saturated. The iP-inrushof the saturated phase is large, and that of the unsaturated phase is numerically the same as the circulating current (iD). According to the parameters characteristics of HT, the iDis small, so iPpresents a large imbalance. The research conclusions provide a theoretical basis for zero-mode inrush current suppression.

Under the development trend of AC/DC hybrid power grids, converter transformers are indispensable as AC/DC conversion equipment, whose safety and stability are particularly important. There have been many serious accidents where the zero-mode inrush current(abbreviated as ZMIC) of high-impedance transformers caused the zero-sequence backup protection of the transformer and even the upper-level line to malfunction. As a type of high-impedance transformer, the converter transformer may cause the existing zero-sequence protection(abbreviated as ZSP) to malfunction due to the ZMIC generated during no-load operation. However, its winding connection form and no-load operation method are different from ordinary high-impedance transformers. There may be sympathetic interaction between multiple transformers. So, it is necessary to analyze the ZMIC separately.Through the existing ZMIC equivalent circuit of converter transformer, the amplitude relationship and attenuation characteristics of four bipolar converter transformer ZMIC are clear, which lay the foundation of the following protection scheme.The fact that ZMIC may cause the zero-sequence overcurrent protection to malfunction is found through PSCAD (Power Systems Computer-Aided Design) simulation. An inrush current identification scheme based on the station area information of the relationship between zero-mode inrush fundamental wave and harmonic amplitude is proposed. Considering the system resistance, closing angle, CT saturation, and fault conditions, the effectiveness of the proposed scheme and its superiority compared with the existing inrush current identification methods are verified.

Active transfer arc-suppression device is a kind of arc suppression device which has been widely used in the distribution network. When a single-phase grounding fault occurs, the bus of faulty phase is actively grounded metallicity in the substation to limit the voltage of faulty phase, so as to realize arc extinction. In this paper, an accurate fault location method is proposed for the distribution network equipped with active transfer arc-suppression device. According to the variation characteristics of the fault traveling wave signals before and after arc-suppression device operation, the fault location formula based on the traveling wave velocity difference of different modes and the fault location formula based on the mutation of aerial-mode traveling wave are combined to obtain the final fault location method that is not affected by ground-mode wave velocity. The proposed method expands the function of active transfer arc-suppression device, makes it have additional fault location ability, and greatly improves the economy of the device. Simulation verifies that the proposed method is not affected by fault conditions, soil resistivity and extreme fault. The absolute error is within 50 m in all cases, which has high reliability.

For floating nuclear power plants, the internal power system adopts the generator–grid directly connected structure. Generator ground fault diagnosis enables the protective device to adaptively determine the generator tripping control mode based on the fault degree, and shorten the troubleshooting time by fault location. To accurately calculate the ground fault current under different system operating modes, a real-time grounding capacitance parameters calculation method without injection devices is proposed based on third harmonic voltage. Using the fault component principle, the ground fault current is calculated including the fundamental and the third harmonic components. Through fuzzy membership function, the ground fault degree function is established including the ground fault current and the system overvoltage. It comprehensively reflects the damage degree of both the generator operating security and the system insulation security. According to the 60° phase-band distribution characteristic of the stator windings, a fault coil location method based on the windings’ electric potential distribution characteristics is proposed. PSCAD/EMTDC simulation results show that the proposed method has high computational accuracy.

Abstract Overhead transmission line faults are mostly single‐phase grounding faults, and to ensure the power supply reliability of photovoltaic power plants in the cases of tie line transient faults, a feasible solution is to employ the single‐phase reclosing. However, for the single‐phase reclosing of the tie line, photovoltaic power source may face the over‐voltage problem during the non‐full phase operation, which will seriously affect the success rate of the single‐phase reclosing. This paper, according to different non‐full phase operation control strategies of photovoltaic power source, establishes the corresponding sequence network analysis models, and proposes the theoretical calculation method of non‐full phase operation voltage, which takes account of different neutral point grounding modes of step‐up transformer and different load levels. Meanwhile, the closing impulse problem of the single‐phase reclosing is discussed. The correctness of the theoretical method is verified by digital simulation results. On this basis, the applicable scope and application suggestions of the single‐phase reclosing are given. The research conclusions can provide important guidance and reference for the application of single‐phase reclosing technology in tie line of renewable energy power plant.

Inrush current with high amplitude is generated when the transformer is energized. On the one hand, it will have a negative impact on the safety of the transformer itself, and even cause the relay protection to malfunction. On the other hand, it may reduce operation speed of the protection when there is slight fault because of the protection restraint criteria. Both aspects will affect grid security. Based on the generation mechanism of inrush current, this paper proposes an inrush current reduction strategy that combines prefluxing and controlled switching technology. By constructing an equivalent magnetic circuit model of the large-capacity three-phase transformer with a universal core structure, the analytical formulas of the magnetic flux at each stage of implementing the strategy are obtained, and then the parameters design method of this strategy is proposed. The accuracy of the theoretical analysis of magnetic flux is verified through the accurate simulation. Compared with common inrush current reduction strategies, this strategy can reduce the inrush current to 0.5 times rated current below in any situation where the residual flux is unknown and the “core flux equalization” effect are not obvious, avoiding the problem of residual flux measurement. Finally, in the case of the transformer differential protection, the influence of this strategy on the protection is analyzed from the perspective of theory and simulation, and it shows that it can improve the performance of various protections effectively.

Arm current control is a promising candidate for modular multilevel converter (MMC) due to it can effectively regulate ac-side current and control the ac component of circulating currents simultaneously. The conventional arm current controller designed in frequency domain based on classical control theory have to design the separate controller for each frequency reference signal. As a result, the controller structure is complicated. And limited by the bandwidth of the arm current compound controller, the control performance will be affected by the grid-side voltage harmonics. Model predictive control (MPC) is an optimal control method in time domain. It not only has fast dynamic response, but also can track multi-band composite signals with high accuracy. To address the above issues of traditional arm current control, this paper proposes a model predictive arm current control strategy. Combined with phase-shifted carrier pulse width modulation (PSC-PWM), the computation burden of the presented method can be independent of the number of cascaded sub-modules (SMs). Simulation and experimental results reveal the proposed MMC integrated control strategy has fast dynamic response with small tracking errors even under non-ideal grid conditions.

The short‐circuit current characteristics fed by the renewable energy source are quite different from those of the synchronous generator, which may lead to the phase selection error or failure of the traditional fault phase selection component, and then affect the operating reliability of the distance protection and the automatic reclosing. This study analyses the existing issues in the application of traditional phase selection components to the tie‐line of renewable energy power, according to the fault characteristics of different renewable energy sources such as photovoltaic power source, direct‐drive wind turbine and doubly‐fed wind turbine. Furthermore, the sequence voltage expressions in the case of different types of faults, which takes account of the influence of the renewable energy type and the transition resistance, are theoretically derived. Hereby, a novel phase selection method by using sequence‐voltage phase comparison and phase‐voltage amplitude comparison is proposed. This method is applicable to the tie line of different types of renewable energy power station, and possesses the superiorities of high reliability and strong tolerance towards large fault transition resistance. Digital simulation results verify the favourable performance of the proposed method.

Abstract 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.

When a fault occurring in the tie line of photovoltaic power station is cleared by the protection three-phase trip, photovoltaic power source (PVS) will form an unplanned island with the local load. However, since the voltage regulation and frequency control capability of PVS is generally poor, it is very difficult to satisfy the synchronous reclosing conditions, eventually resulting in an automatic reclosing failure. According to the islanding control strategy of PVS and the principle of phase-locked loop (PLL), a vector relationship model of PVS port actual voltage, port calculation voltage, grid-connected point voltage and so on before and after reclosing is established. Based on this, the influences of reclosing phase difference, amplitude difference and frequency difference on reclosing impulse voltage and current are analyzed, and a synchronous check reclosing strategy applicable to the tie line of PVS is proposed, which can effectively improve the success rate of the three-phase reclosing in tie line. Digital simulation results verify the correctness of the proposed reclosing strategy.

A probabilistic load flow (PLF) is an effective tool that helps describe the uncertainty of power system operation. However, when confronting random variables with non-Gaussian distributions and highly discrete characteristics, existing PLF methods have difficulty balancing efficiency and accuracy. Therefore, a novel approach based on bivariate dimension reduction (BDR) and the Johnson system is proposed herein. BDR is used to estimate the moments of output random variables (ORVs). Because BDR considers the joint effect of input random variables, it significantly reduces the estimation error for high-order moments in particular. In addition, a strategy to improve BDR efficiency is proposed. The Johnson system is used to obtain the probability distributions of ORVs as it has better adaptability and accuracy than the series expansion method. Case studies including comparisons between this approach and others found in the literature were conducted, and the results obtained showed that the proposed method has better performance than previous approaches.

Floating nuclear power plants contain sensitive loads of nuclear reactors. After equipment faults, fast and efficient power supply recovery should be realized. To realize the unified analysis of system topology and power flow distribution, a power supply recovery strategy based on Petri nets is proposed. Considering that systems of different voltage levels cannot be connected instantaneously, a two-stage power supply recovery mode is adopted. Emergency power supply is put in first, and then the whole network is reconstructed. In the network reconstruction process, load transfer is realized through switching the transformation to redistribute the load of each switchboard and adjust the power output of each power source. Corresponding to the Petri net model, the above process is similar to the dynamic transmission process of a token in each library by firing the transition. Therefore, the topological model of system is constructed based on the Petri net, and a power flow analysis is proposed through its dynamic updating mechanism. The objective function of the network reconstruction is established by integrating load recovery amount, switch operation cost and generator operation efficiency, and the optimal switching state combination scheme that satisfies the system constraints is obtained by the multi-population genetic algorithm (MPGA). Simulation results show that the proposed method can provide complete power supply recovery.

Rotor winding short circuit faults are common faults for variable-speed pumped-storage generator-motors (VSPSGM). At present, the exciting rotor fault protection of VSPSGM is simple and has low sensitivity. It can only act when the instantaneous value of the rotor phase current reaches three times the rated current. Therefore, it is difficult to cover some rotor winding short-circuit faults with weak fault characteristics. It is urgent to study a novel rotor winding short-circuit-fault protection method for VSPSGM. In this paper, a protection method that combines the stator and rotor currents with different frequencies is proposed. The characteristics of the stator and rotor currents before and after the fault is analyzed by using Clark transformation. On this basis, a specific protection criterion is constructed based on the discrete integral operation, which is easy to implement and not affected by the change of rotor speed. Then, the calculation method of the protection setting is proposed, considering the effect of unbalanced voltage and sensor measurement error. Simulation results show that the proposed method can reliably realize the protection of rotor winding faults. It has faster protection action speed than other methods in the same field. The protection coverage rate is over 90%.

Abstract 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.

Fault location in a power distribution network is a challenging task due to the presence of multilayer branches and short line lengths. Existing fault-location methods generally require measurements at both ends of each branch, which requires a large number of measuring points. The placement of measuring points at branch terminals is an approach that can be used to reduce the number of measuring points. Such a measuring point layout allows the existing fault-location methods for power distribution networks to determine fault points after identifying faulted branches. However, these methods fail to locate a fault if the faulted branch cannot be correctly identified. This paper proposes a traveling-wave-based fault-location method for branched power distribution networks without requiring faulted branches to be identified. In the proposed method, by using the first arrival times of the fault-generated traveling waves detected at the substation and each branch terminal, the computational fault time difference (CFTD) is defined. By calculating the value of CFTD, the fault point is directly searched out. Finally, the quartile method is used to eliminate the impact of the arrival-time error on the fault-location accuracy of the proposed method. The simulation results verify the high accuracy, traveling-wave velocity stability, and strong arrival-time error robustness of the proposed method.

Modern power systems are equipped with comprehensive protective devices to remove the fault section, and the fault diagnosis problem is to interpret the alarms of the protective devices and estimate the fault section. To deal with the uncertainty and temporal constraint of the alarms, a novel fault diagnosis model based on the temporal constrained fuzzy Petri nets (TCFPNs) is proposed in this paper. The truth degree and the timing contribute of the alarms are introduced into the graphic model of the TCFPNs, and the matrix algorithm, considering both the fuzzy reasoning and temporal reasoning, is carried out to obtain the fault probability as well as the time point constraint of each candidate section. The developed approach is performed on different test systems for case studies, and the results demonstrate the feasibility, efficiency, and fault tolerance of the method.

In electrical machinery, the rotor windings’ internal short-circuit faults are addressed by the instantaneous over-current protection of the power electronic excitation device, which has low sensitivity and has difficulty meeting the safety requirements. In this paper, a rotor windings’ internal short-circuit fault protection method is proposed based on the harmonic characteristics of the circulating current between stator branches. The magnetomotive force distribution of the short-circuit coils in the rotor windings is theoretically deduced, and the characteristic frequencies of the circulating current between stator branches are analyzed. On this basis, the protection criterion of the rotor windings’ internal short-circuit fault is constructed by using the harmonic component of the circulating current. Then, an analytic model of the variable-speed pumped storage unit is established based on the multi-loop method, and the finite element method is used to verify the correctness of the proposed modeling method. An actual large variable-speed pumped storage unit is taken as an example, and the possible faults under different slip ratios are simulated. In the simulation results, the stator branch circulation has the obvious characteristic frequency harmonic components, which is consistent with the theoretical analysis. It verifies the effectiveness of the proposed protection method. Finally, it is analyzed and verified that the proposed protection has a strong maloperation prevention ability under other kinds of faults.

Single-phase ground faults frequently occur at the generator stator windings. To realize reliable arc suppression, a dual-frequency active arc-suppression strategy has been widely considered. However, the existing method needs an inverter with large capacity, which is large in volume and high in cost, causing limited engineering application prospects. To reduce the capacity, a novel hybrid flexible neutral grounding method for large generators is proposed in this paper. It combines the dual-frequency active arc-suppression strategy with the arc-suppression coil-grounding mode. Thus, most of the fundamental components in the fault current are compensated by the arc-suppression coil, and the rest are compensated by the active arc-suppression device. The capacity of the inverter can be greatly reduced under the premise of reliably realizing arc suppression. To identify transient and permanent ground faults after arc suppression, the phase-angle relation between the third-harmonic voltage and current variation at the generator neutral point is used to form the fault-type identification criterion. The third-harmonic quantities can avoid the blind area at the neutral point caused by fundamental quantities. Simulation and experimental test results verify the effectiveness of the proposed method.

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.

When single-phase arc grounding fault occurs in the neutral ungrounded distribution system, the fault can be transferred by artificially grounding the faulty phase of the busbar, which can effectively improve the fault handling capability of distribution system. The theoretical analysis shows that the characteristic of transient zero-mode current is obviously different between grounding fault and fault transferred. That is to say, in the two grounding processes, the directions of the transient zero-mode current of the fault line are opposite and the direction of the transient zero-mode current of the non-fault line is identical. So, the fault line can be determined based on this theory. The zero-mode current of each line is analysed by wavelet packet decomposition, and the feature band is selected according to the point of maximum energy. The correlation coefficient polarity between two reconstructed data, each is extracted from the first quarter of power cycle of grounding process in respective feature band, is used to determine the criterion for fault line selection. This method only uses its own line information to choose fault line without any other line information. Simultaneously, number of simulation results shows that the method is accurate and reliable.

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.

During the inrush period, multiple misoperation accidents of differential protection of transformers happened, which seriously threatened the safe and stable operation of power grid. In the traditional malfunction analysis, only the inrush current process is considered, and the electromagnetic transient interaction between the transformer and other non-linear ferromagnetic components such as CT is hardly considered. Here, the variation rule of inrush current waveform characteristic is analysed, and the influence rule of secondary load and type of CT on inrush current second harmonic proportion and dead angle is simulated by using the simulation Lucas model of industrial practical CT (P/PR). The results show that the second harmonic proportion of the inrush current and the dead angle of the waveform depend on the saturation angle of the transformer core. The larger the saturation angle is, the smaller the second harmonic proportion is, and the larger the dead angle of the waveform is. The second harmonic proportion of the inrush current becomes larger, and the dead angle of sympathetic inrush becomes smaller after CT transfer. The dead angle of magnetising inrush waveform depends on the secondary load resistance. It may increase or decrease..

With renewable power sources and new topology structures being widely introduced into the power system, the current local information-based power generation-side protection cannot fully guarantee the protection performance and the safety coordination with the power grid. This paper proposes an improved hierarchical protection system on the power generation-side. The proposed system takes advantage of the fusion of multi-information provided by the system, station and local layers. The system layer provides the information such as system voltage control and power regulation demand, so that the generation-side protection and control system can adapt to the system operation mode and power regulation demand. The station layer realizes the coordination of the protection principle and action strategy among the related units through information interaction with the power grid, the automatic control system and the local layer protection. The local layer introduces the condition monitoring information and more abundant protection information to enhance the protection performance and master the generation units’ safety condition. To illustrate the hierarchical protection system construction method and actual application mode, the multi-information fusion-based comprehensive local layer protection method and the multi-generators information fusion-based hierarchical protection method are taken as examples. Case analysis shows that the proposed methods can reflect more slight internal fault forms and can adaptively determine the protection action characteristic and tripping strategy according to the system operating conditions and other generators’ fault conditions, which effectively improve the protection sensitivity and coordination capability. To provide reference and inspiration for follow-up research, the hierarchical protection system construction mode, communication technology and research approaches of new protection methods are further pointed out.

Traditional robust optimization methods use box uncertainty sets or gamma uncertainty sets to describe wind power uncertainty. However, these uncertainty sets fail to utilize wind forecast error probability information and assume that the wind forecast error is symmetrical and independent. This assumption is not reasonable and makes the optimization results conservative. To avoid such conservative results from traditional robust optimization methods, in this paper a novel data driven optimization method based on the nonparametric Dirichlet process Gaussian mixture model (DPGMM) was proposed to solve energy and reserve dispatch problems. First, we combined the DPGMM and variation inference algorithm to extract the GMM parameter information embedded within historical data. Based on the parameter information, a data driven polyhedral uncertainty set was proposed. After constructing the uncertainty set, we solved the robust energy and reserve problem. Finally, a column and constraint generation method was employed to solve the proposed data driven optimization method. We used real historical wind power forecast error data to test the performance of the proposed uncertainty set. The simulation results indicated that the proposed uncertainty set had a smaller volume than other data driven uncertainty sets with the same predefined coverage rate. Furthermore, the simulation was carried on PJM 5-bus and IEEE-118 bus systems to test the data driven optimization method. The simulation results demonstrated that the proposed optimization method was less conservative than traditional data driven robust optimization methods and distributionally robust optimization methods.

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.

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.

During non-severe fault conditions, crowbar protection is not activated and the rotor windings of a doubly-fed induction generator (DFIG) are excited by the AC/DC/AC converter. Meanwhile, under asymmetrical fault conditions, the electrical variables oscillate at twice the grid frequency in synchronous dq frame. In the engineering practice, notch filters are usually used to extract the positive and negative sequence components. In these cases, the dynamic response of a rotor-side converter (RSC) and the notch filters have a large influence on the fault current characteristics of the DFIG. In this paper, the influence of the notch filters on the proportional integral (PI) parameters is discussed and the simplified calculation models of the rotor current are established. Then, the dynamic performance of the stator flux linkage under asymmetrical fault conditions is also analyzed. Based on this, the fault characteristics of the stator current under asymmetrical fault conditions are studied and the corresponding analytical expressions of the stator fault current are obtained. Finally, digital simulation results validate the analytical results. The research results are helpful to meet the requirements of a practical short-circuit calculation and the construction of a relaying protection system for the power grid with penetration of DFIGs.

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.

When a fault occurs in a power system, fault section estimation is the primary premise for troubleshooting and power recovery, and an effective fault diagnosis system will play a big role in decision making. However, the topology information is not well employed in existing fault diagnosis methods, and it is complex and time consuming to analyze the relationship between the protective devices and the sections. In this paper, a novel analytic method which employs topology description is proposed for fault diagnosis. The topology descriptions of the sections and the protective devices are firstly established according to the network structure, and based on which the operating logic and the cooperative relationship of the protective devices can be easily analyzed by matrix operation. Considering the factors of logic error and communication error, the fault diagnosis problem is formulated as an integer programming problem and can be solved by intelligent algorithm. The case studies of different power systems show that the proposed method can quickly identify the fault section, even with the abnormal operation or error alarm of protective devices.

Current studies on preventive condition-based maintenance of power transmission and transformation equipment mainly focus on mid-term or long-term maintenance, and cannot meet the requirements of short-term especially temporary maintenance. In order to solve the defects of the present preventive maintenance strategies, according to the engineering application and based on risk-cost analysis, a short-term maintenance strategy is proposed in this manuscript. For the equipment working in bad health condition, its active maintenance costs and operation risk costs are evaluated, respectively. Then the latest maintenance time is calculated in accordance with the principle that its operation risk costs are no higher than active maintenance costs. Utilizing the latest maintenance time, the best maintenance time is calculated by setting the maximum relative earnings of postponing maintenance as the target, which provides the operation staffs with comprehensive maintenance-decision support. In the end, different cases on the IEEE 24-bus system are simulated. The effectiveness and advantages of the proposed strategy are demonstrated by the simulation results.

Current differential protection is the main protection of transmission lines which include multi-circuit lines on the same tower, and whose sensitivity and reliability of differential protection is mainly affected by the distributed capacitive current. For the four-circuit line on the same tower, due to the influence of coupling between the loop road, the distributed capacitance current increases significantly when compared with ordinary lines, affecting the sensitivity of the current differential protection, especially for different voltage levels throughout the four-circuit lines on the same tower. The relationship of the electrostatic coupling between the circuits is more complex, and increases the difficulty of the compensating the distributed capacitance current. This paper is based on the electrostatic coupling principle of four-circuit lines on the same tower, establishes the distributed parameter model of four-circuit transmission lines on the same tower, and discusses the effect of circuit operation mode on the compensation of capacitance current differential protection when different faults occur on the complex four-circuit transmission lines on the same tower. A new compensation scheme suitable for capacitive current compensation is proposed. Simulation results show that this capacitive current compensation scheme can effectively improve the performance of current differential protection.

In order to solve the problems of the existing wide-area backup protection (WABP) algorithms, the paper proposes a novel WABP algorithm based on the distribution characteristics of fault component current and improved Dempster/Shafer (D-S) evidence theory. When a fault occurs, slave substations transmit to master station the amplitudes of fault component currents of transmission lines which are the closest to fault element. Then master substation identifies suspicious faulty lines according to the distribution characteristics of fault component current. After that, the master substation will identify the actual faulty line with improved D-S evidence theory based on the action states of traditional protections and direction components of these suspicious faulty lines. The simulation examples based on IEEE 10-generator-39-bus system show that the proposed WABP algorithm has an excellent performance. The algorithm has low requirement of sampling synchronization, small wide-area communication flow, and high fault tolerance.

The series capacitor compensation is one of the key technologies in the EHV and UHV long distance power transmission lines. This paper analyzes the operation characteristics of the main protection combined with the engineering practice when the transmission line overcompensation due to the series compensation system is modified and analyzes the influence of the transition resistance and the system operation mode on the current differential protection. According to the simulation results, it presents countermeasure on improving the sensitivity of differential current protection.

Natural disasters may be of significant impact on overhead transmission lines and cause communication outage related to pilot protection. This paper aims at reconstructing communication channels and maintaining functions of pilot-wire differential protections after the main channel fails. With the development of smart grids as well as new communication technologies, wireless sensor networks (WSNs) have been potential means for realizing reconstructed communication channels (RCCs) without further installation. For a reliable design, system arrangement and the communication structure were presented. Theoretical planning of sensor nodes was formulated, which enjoys advantages such as high reliability, cost optimization, and capacity of satisfying the connectivity of the communication network. To meet the need of time delay, a novel routing protocol for WSNs was proposed with three stages including route establishment, route discovery and route maintenance, which ensured the directional propagation of data packets. Practical experiments and simulation results indicate that the proposed RCC scheme can satisfy time delay of protection relaying in emergency communication channel, as well as guarantee the connectivity of networks when some WSN nodes are damaged.