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Power electronic transformers (PETs) play an important role in AC/DC hybrid distribution networks. However, the fault probability of a PET is higher because of the use of many power electronic devices. The short-circuit protection of a PET is relatively mature, but studies on its open-circuit faults are limited to insulated gate bipolar transistors (IGBTs) in the cascaded H-bridge (CHB). Therefore, other types of open-circuit faults result in dead zones in the fault location process. In this paper, the open-circuit fault characteristics of switch elements at different positions of a PET are analyzed, and a protection circuit is designed for the possible overvoltage caused by the open-circuit fault in the CHB. Moreover, the fault location methods of switch element open-circuit faults are proposed. The proposed methods could adapt to the diversity of control modes, power directions and load levels. The correctness and effectiveness of the methods are verified via simulation tests. This work provides a basis for the implementation of fault-tolerant control and protection of PETs.

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.

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.

In order to improve the quality of the reactive power compensation current injected via winding taps on the primary windings of the transformer, this paper presents an improved winding tap injection distribution transformer with LCL-type static synchronous compensator (DT-STATCOM) and its control strategy. The winding tap injection method can make full use of the leakage inductance and spare capacity of the distribution transformer and reduce the voltage stress. Based on the winding tap connection, the system mathematical modeling is analyzed to build the equivalent circuit of DT-STATCOM. In consideration of the frequent changes in operating point, a Cascaded Passivity Based Control (CPBC) with additional integral action is developed for LCL-type DT-STATCOM. The step-by-step stability analysis and the principles for selecting injection dampings of the CPBC controller are presented. Under the proposed CPBC, the coupling terms in d -axis and q -axis of LCL filter can be attenuated significantly, and a fast dynamic response is achieved with low-current overshoot. Moreover, the method has a strong robustness against DT-STATCOM parameters deviation. Finally, a laboratory prototype of 800V DT-STATCOM with LCL filter is used to test the feasibility and effectiveness of the proposed structure and CPBC control strategy.

In recent years, there have been situations in which the zero-sequence protection of the transformer has been incorrectly operated due to the converter transformer energizing or fault recovery. For converter transformers, maloperation may also occur. However, there is almost no theoretical research on the zero-mode inrush currents of converter transformers. This paper studies the characteristics of the zero-mode inrush currents of the converter transformers, including the relationship between the amplitude and attenuation characteristics of the zero-mode inrush currents of converter transformers, and their relationship with the system resistance, remanence, and closing angle. First, based on the T-type equivalent circuit of the transformer, the equivalent circuit of the zero-mode inrush current of each transformer is obtained. On this basis, the amplitude relationship of the zero-mode inrush currents of different converter transformers is obtained: the zero-mode inrush current of the energizing pole YY transformer becomes larger than the YD transformer, the energized pole YD becomes greater than the YY transformer, and the YY transformer zero-mode inrush current rises from 0. It is also analyzed that the sympathetic interaction will make the attenuation of the converter transformer zero-mode inrush current slower. The system resistance mainly affects the initial attenuation speed, and the later attenuation speed is mainly determined by the converter transformer leakage reactance. Finally, PSCAD modeling and simulation are carried out to verify the accuracy of the theoretical analysis.

Recent applications of modular multilevel converter (MMC) have posed various problems such as ac/dc grid faults, asymmetric operation conditions, ac grid voltage excessive harmonics, and ac circulating currents, which deteriorate the system's performance and also threaten the safe operation. In this article, an improved comprehensive control architecture of MMC is proposed to maintain a stable voltage control, eliminate harmonics in ac-side and circulating currents, and dc-bus voltage ripple under ac/dc grid faults and harmonic operation conditions. The proposed comprehensive control consists of improved hierarchical voltage control and repetitive controller-based arm current control. The improved hierarchical voltage control, including a novel negative-sequence current injection and dc-circulating current injection-based leg-averaging control and their seamlessly switching handling, enables the MMC to operate under various operating conditions, thereby producing the arm current reference for inner-loop control. The proposed repetitive controller-based arm current control can recognize the control of multifrequency components of arm current without separating positive-, negative- and zero-sequence current components of ac-side currents and circulating currents. Finally, the simulation and experimental studies of the three-phase MMC were carried out in detail, and the findings demonstrate the efficiency and merits of the proposed control method.

The DC distribution network based on the hybrid modular multilevel converter (MMC) has the ability of active current-limiting control, which plays an active role in fault clearing of distribution network. However, the fault characteristics of the DC distribution network become complicated because of the active current-limiting control. It brings new problems to the fault detection and judgment of relay protection. According to the structural features of the hybrid MMC, the start-up strategy and control objectives of the active current-limiting control are analyzed in this paper. The improvement suggestions are put forward to maintain the operational safety of the power grid, as well as meet the application requirements of the relay protection. Moreover, The characteristics of the short-circuit current of the DC distribution network are analyzed based on the active current-limiting control. Meanwhile, the influences on the relay protection are estimated. Finally, the effectiveness of the proposed current-limiting control strategy and the correctness of the fault characteristic analysis results are verified by simulation tests.

Aiming at the difficulty of CT polarity checking of MMC-HVDC converter station transformer, a method for CT polarity checking based on waveform comparison and instantaneous power theory is proposed. The transient current during the process of the converter transformer and the converter valve charging is analyzed, and the polarity of the zero-sequence differential protection and the longitudinal differential protection is checked by comparing the crossing transient current on both sides of the protection based on the longest common subsequence (LCSS) distance. At the same time, the instantaneous active power during the converter valve deblock is used to check the polarity of directional protection. This method can not only check the polarity of longitudinal differential protection and zero-sequence differential protection, but also can check the polarity of directional protection, and the reversed phase of CT can be found out. Moreover, it can withstand a certain degree of ratio error and phase error of CT. The simulation verifies the effectiveness and applicability of the method.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The transients of the coupling capacitor voltage transformer (CCVT) have an impact on the performance of distance relays and result in the overreach problem. Based on the equal transfer process of transmission lines (ETPTL) theory, the virtual digital CCVT can be applied to reduce the impact of the CCVT transients and improve the operation speed of distance relays. The virtual digital CCVT is required to have the same transient characteristics as the actual CCVT. This paper proposes a simple and universal design method of virtual digital CCVT. Based on the CCVT transient characteristic matching and the ETPTL theory, this paper proposes an improved numerical distance relay algorithm to prevent the overreach problem of distance relays without sacrificing the operation speed of distance relay under the internal fault conditions. An extensive performance evaluation using PSCAD/EMTDC simulation corroborates the effectiveness of the proposed distance relay under different fault conditions. Finally, a prototype device based on the proposed distance relay is developed in laboratory and evaluated through a hardware in loop test using real-time digital simulator (RTDS).

In recent years, zero-sequence overcurrent protection misoperated frequently when the high-impedance transformers are energized without load. This paper studies the characteristics of zero-mode inrush current of the high-impedance transformers, including amplitude and attenuation. Firstly, the corresponding relation of “winding arrangement – flux distribution – equivalent circuit parameters” of transformer was clarified. On the basis of analyzing single-phase and three-phase inrush current, the analytical expression and equivalent circuit of zero-mode inrush current were proposed. Then, according to the different winding arrangement, the different parameters of hollow inductance, self-leakage inductance and saturated mutual inductance of the high-impedance transformers and ordinary transformer were explained and calculated. Further, based on the different parameters and analytical expression, the zero-mode inrush current characteristics of high-impedance transformers were analyzed. The correctness of characteristic analysis of high-impedance zero-mode inrush current was verified via digital simulations and dynamic physical simulations tests.

This paper proposes a distribution static synchronous compensator (DSTATCOM) integrated with a Y-y connection transformer for reactive power compensation. A cascaded DSTATCOM is connected to a group of taps on the primary windings of the transformer to form an integrated structure. The proposed structure can achieve a flexible connection point voltage selection to achieve a compromise between the voltage rating and current rating. Therefore, the initial investment, voltage stress and size of the compensation system can be reduced. The winding currents distribution and compensation capacity constraint are analyzed in detail to determine the attainable compensation range. To improve the dynamic response performance of the system, a feedforward compensation control strategy is introduced in this paper. To prevent the transformer windings from overcurrent, a reactive reference current calculation method is also presented. Finally, both simulation and experimental results are provided to verify the effectiveness of the proposed system.

The analysis of maloperation cases indicates that mal-operations of the transformer differential protection caused by sympathetic inrush usually occur under the combined action of sympathetic inrush and current transformer (CT) saturation. The process and characteristics of the mal-operation caused by sympathetic inrush are analyzed. According to the wave characteristics of sympathetic inrush that firstly increases to maximum gradually and then decays slowly, a new sympathetic inrush identification method is proposed, based on the calculation of the fundamental component increment of transformer differential current. The new method can identify sympathetic inrush quickly and reliably and adopt effective measures accordingly before sympathetic inrush makes CT saturated. The simulation and dynamic simulation experiment verifies the validity and feasibility of the method.

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 presents a fast distance relay for series compensated transmission lines based on the R–L differential-equation algorithm using the theory of equal transfer process of transmission lines. The measuring distances based on the proposed algorithm can fast approach the actual value of fault distance when a fault occurs in front of the series capacitor. When a fault occurs behind of the series capacitor, the fault loop, including the series capacitor, does not match the R–L transmission line model, so the measuring distances fluctuate severely. Based on this, the relative position of the fault with respect to the series capacitor can be judged effectively according to the fluctuation range of the measuring distances, and the accurate fault location can be obtained fast. A variety of PSCAD/EMTDC simulation tests show that the new relay has fast operating speed and high accuracy when applied to the long series compensated transmission lines.

The measuring accuracy and the measurement stability of conventional distance relay will be influenced by complex and remarkable harmonic components due to the large capacitance of the line when it is used for a long line. Correspondingly, the tripping speed will be delayed to some extent. To solve this problem, a fast distance relay for long transmission lines is presented, which is on the basis of the differential equation algorithm using π transmission line model and the theory of Equal Transfer Process of Transmission Lines (ETPTL). The shortcomings of π model differential equation algorithm due to the impact of high frequency components can be overcome by using a low-pass filter. The problem resulting from the difference between the transfer feature of the voltages used by the distance protection and that of the currents due to the transient characteristic of coupling capacitor voltage transformers (CCVT) can be solved by using virtual digital CCVT. Then, the new distance relay can trip quickly by re-structuring the voltage at the fault point and iterative calculations. A variety of ATP simulation tests show that the new relay has fast tripping speed and high reliability when applied to the long transmission lines.

For the construction of the smart grid, protection needs to be accurately measured, and the current transformer needs to correctly transfer the transient process. The transient process of nonlinear ferromagnetic elements mainly includes the transformer inrush current and CT saturation. Differential protection misoperation is rarely caused by a single factor. It involves the coupling effect between the transformer inrush current and CT saturation. In this paper, the magnetizing inrush current and sympathetic inrush current and the CT saturation are studied from the aspects of principle analysis and digital simulation. Based on an accident, the coupling between the transformer inrush and CT saturation and its impacts on the differential protection are analyzed. Finally, methods to this problem are discussed.

When faults or disturbance occurs in AC/DC hybrid complex power grid, there have happened many generator, transformer and transmission line differential protection maloperation accidents caused by current transformer (CT) saturation, which seriously threaten the safe and stable operation of grid. As the analytical analysis of CT transient characteristics is very complex, it is urgent to develop the CT model which can accurately simulate the transient characteristics of CT in practical use. In this paper, different class (P, PR and TPY) protection CTs in practical use are taken to do physical test. Based on the test results and genetic and simulated annealing algorithms (GASA), the Lucas model parameters of practical CT are identified, and the simulation model is built in PSCAD which can simulate the transient characteristics of different class CT in practical use. The dynamic test verifies the accuracy and validity of the model in different typical power system transient conditions.

When a switching-on transformer is energized, complex sympathetic inrush will flow through adjacent operating transformers, which may cause the protection misoperation in power system. At present, the mathematical analysis of sympathetic inrush is usually based on the model that the operating transformer without load, which may cause engineering error. In this paper, the analytical expressions of flux linkages, the source current and load current for the integration of switching-on transformer and operating transformer with possible load and power-source are presented. The models for simulation are established in PSCAD. The influencing factors and key characteristic of the complex sympathetic inrush are obtained through simulation. The dynamic physical simulation tests verify the conclusion.

During nonsevere fault conditions, the crowbar protection is not activated and the rotor windings of a doubly fed induction generator (DFIG) are still excited by the ac/dc/ac converter. In these cases, the dynamic response of a rotor-side converter (RSC) has a large influence on the fault current characteristics of the DFIG. In this paper, an analysis method for the fault current characteristics of the DFIG under nonsevere fault conditions is proposed. First, the dynamic response of the RSC is analyzed on condition that the external power control loop is shut down and the reference signals of the inner rotor current control loop are kept constant. Then, the simplified calculation models of the rotor fault current are established according to different design principles of the inner rotor current controller. Based on it, the fault characteristics of the stator current are studied and the analytical expressions of the stator fault current are obtained. Finally, digital simulation results validate the analytical results. The proposed analysis method is applicable for the study of fault current characteristics of DFIG with different control strategies for low-voltage ride through. The research results are helpful to the construction of adequate relaying protection for the power grid with penetration of DFIGs.

Stator grounding fault is very common to the large generator, and the accurate fault location can greatly reduce the time of corrective maintenance. However, this problem can be very hard since the structure of the winding is very complex. In this paper, a fault location method for large generator stator grounding considering armature reaction is proposed. Firstly, the fundamental potential distribution of the stator winding and the influence of armature reaction on the measured voltage are analyzed, and the mathematical relationship among fault position, fundamental potential, fundamental voltage and grounding resistance is then constructed. On this basis, the process of the fault location can be established. Finally, the accuracy and efficiency of the proposed method are verified by extensive experiments performed on PSCAD/EMTDC.

The no-load closing of transformer in the medium-voltage ship power system will cause a large inrush current and threaten the security of the system. This paper aims to provide reference for the pre-magnetization technology of the transformer in medium-voltage ship. Firstly, the basic principles of the inrush current and the series small-capacity transformer based pre-magnetization technology for the transformers are analyzed. Based on the analysis, the typical medium-voltage engineering ship and power generation ship are taken as examples to perform the simulation of the series small-capacity transformer based pre-magnetization scheme, and the result shows that the pre-magnetization technology can suppress the inrush current effectively. Besides, the factors of the switch closing time and the number of generators operating in parallel are considered in the research and their effects of on the inrush current as well as the transient voltage drop are analyzed. Finally, the influence of the pre-magnetization scheme on the differential protection is studied and the operation suggestions are given.

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 presents a stator ground protection scheme based on the fault current flowing in the breakdown channel. It trips only if the fault current exceeds the fault current tolerance capacity of the generator. Otherwise, it will just emit alarm signals. Thereby, the unnecessary trip-out of large generator can be avoided. The stability of the power system and the generator security can then be ensured. To achieve the scheme, methods of calculating the fault current in real time are proposed for three common generator neutral grounding methods based on the zero-sequence equivalent circuits. In addition, a ground fault location algorithm is presented which can automatically identify the faulted coil in a large generator with multicircuit stator winding. It greatly reduces the workload of repair. An implementation scheme is given on the basis of the subharmonic injection protection device. Simulation results show the validity of the protection scheme and the accuracy of the fault location algorithm.

The overreach of the distance protection caused by CCVT is still a serious problem for high-speed line protections. Based on the theory of Equal Transfer Process of Transmission Lines (ETPTLs), a new high-speed distance relay scheme is proposed in order to overcome above problem. The solution is to make the three-phase voltages and currents at the relay location and the voltage at the fault point have the same transfer links by virtue of a new design. Three major steps of the new method are demonstrated: re-structuring of the voltage at the fault point, the virtual digital transfer method and solving the R–L differential equation. A variety of ATP simulation tests show that the new method effectively reduces the transient error caused by CCVT and improves the operating speed by a series of technical countermeasures including three major steps, iterative calculations of the fault distance and an inverse time delay setting criterion. The distance measuring error is within 5% at approximately 15 ms after fault occurrence, which is superior to various adaptive protection algorithms based on CCVT transient error estimation or source impedance ratio (SIR).

Under the existing 3/2 connection mode, the incompletion between the configuration of the protection and transformer leads to the existence of the dead zone. When the fault occurs in the dead zone, line protection and breaker failure protection are generally matched with each other to achieve the complete isolation of the fault. This approach will make the scope of the circuit breaker to expand, and even leads to unnecessary outage of the normal bus. So it seriously affects the safe and stable operation of the system. In view of the above problems, this paper analyzes the features of the dead zone under different protective CT configuration mode, and provides a method for dead zone fault isolation and fast recovery of error cutting element. By introducing circuit breaker opening and closing state information, the method identify dead zone fault and the breaker failure effectively. At the same time, based on the information sharing in the station area, the circuit breaker can be quickly recovered by using the reclosing function of the circuit breaker, which can significantly improve the safe and stable operation level of the system.

This paper proposes a comprehensive compensation strategy for DSTATCOM which is suitable for both reactive power compensation and harmonic elimination in distribution network. The DSTATCOM is connected with the transformer taps on the high voltage (HV) windings and the transformer adopts the Dyn11 connection group. Firstly, according to the converter model, the decoupled control strategy is presented for the independent control of the active and reactive current components. Then, the detection method of reactive command current and harmonic command current is analyzed. Conventional PI controller and quasi-PR controller are adopted respectively for the control of the reactive current and harmonic current. Simulation test results based on MATLAB/SIMULINK show that the proposed control strategy has a good performances.

This paper presents a novel cascaded multilevel converter (CMC)-based distribution-type STATCOM integrated with distribution transformer for medium voltage application. The star-connected, n-level CMC is connected to the winding taps on the primary windings of the transformer to achieve voltage stress reduction and maintain a flexible connection voltage level. Taking full use of the spare capacity of distribution transformer, a compromise between cascaded cells count and the voltage stress is achieved. The CMC-based DSTATCOM is controlled as a controlled current source to inject compensation current to the primary windings through the taps. A modified decoupled current control is proposed to control the system. Simulation model and scale-down prototype are developed to verify the performance of the proposed integrated structure. Simulation and experimental results demonstrate the good performance of the proposed structure for reactive power compensation.

In the typical HVDC power system, the current transformers (CTs) between the Y-delta converter transformer and the converter bridge is located in the delta-winding of the transformer. Under such condition, the measuring current of these CTs includes the zero-sequence circulating current of the delta-winding. In the case of the adjacent transformer energization or the fault of AC system, the zero-sequence circulating current in the delta-winding maybe very large containing a great amount of dc component. Thus the CTs will be saturated, resulting in the misoperation of the relevant protection in HVDC system, such as the bridge current differential protection. This sort of misoperation seriously threatens the safe and sound operation of the HVDC power system. In this paper, the misoperation cause is analysed in detail via the PSCAD/EMTDC-based simulation. Based on this, a method is proposed to prevent this sort of misoperation by utilizing the zero-sequence circulating current as the restraint quantity.

This paper analyses the mechanism of the sympathetic inrush current of the converter transformer of the HVDC transmission system. The impacting factors of the sympathetic inrush current, including the system impedance, the closing angle and the initial remnant flux of the incoming transformer, and the operating condition of the HVDC system and so on, are analysed utilizing on the PSCAD/EMTDC soft. Based on this, this paper proposed two identification methods for the sympathetic inrush current and the simulation proofs the validity of these methods.

According to the theory of equal transfer process of transmission lines (ETPTL), the voltages along the line and the current through the line transformed by the same linear transferring link still satisfy the original distributed parameter-based transmission-line model. It is proved that the voltage signals near power frequency along the line follow the law of linear distribution. On the basis of the aforementioned analytical results, an ETPTL-based new current differential protection scheme for long transmission lines by using restructured instantaneous value was presented. The primary voltage and current experience equal transfer processes and are transformed to be virtual instantaneous values before they are used by differential protection. According to the instantaneous voltage value of the transmission line, the instantaneous values of the capacitive current can be calculated. After removing this current, the instantaneous value-based differential criterion can be implemented. The results of Alternate Transients Program simulation test results show that the proposed technique has highly reliability, fast speed, excellent performance under high-resistance earth-fault conditions, and phase selection ability.

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.