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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.

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.

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

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

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

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

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.

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.

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

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

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

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

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

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

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

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

To improve the accuracy of semi-physical simulation, this paper mainly discussed interpolation and extrapolation algorithms for switching events compensation of cascaded static synchronous compensators (STATCOM). The moment of the switching events was captured by Field-programmable gate array (FPGA). Two linear interpolations and one extrapolation were adopted to compensate the error caused by multiple switching events during one simulation step. In addition, a linear interpolation was also applied for time delay compensation in hardware in loop test. Both rapid control prototyping and hardware in loop tests were performed to verify the effectiveness of the interpolation algorithms. Experimental results indicated that the time delay was almost eliminated and the output waveform was improved after compensation.

Due to the diversity of the user on-site power quality and the rated power limitation of the distribution static synchronous compensator (D-STATCOM), it might be impossible for the power quality index of users to fully meet related requirements. Therefore, in order to customize power quality, this paper proposes a comprehensive evaluation model of power quality based on the electricity price penalty factor. Limited by the capacity, an optimal compensation algorithm with the goal of minimizing the penalty for the user’s power quality is introduced, and solved by Lagrange multiplier method. The optimal algorithm presents different weighting coefficients in the cost index. The simulation results verify the effectiveness of the proposed optimal compensation strategy.

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.

At present, most power system fault diagnosis methods are modeled for single element and have poor adaptability to network changing. In this paper, an adaptive fault diagnosis model that considers the network topology is proposed. Firstly, the topological information fusion model is established based on the power system topology description, which uses the incident matrix to represent the mapping relationship between elements and protective devices. Then, the topology transformation rules are established to deal with the cross-range characteristics of secondary backup protection, and based on which the whole reasoning process is given. Finally, the case studies of the IEEE 14-bus system demonstrate the efficiency and adaptability of the proposed method.

In order to solve the problems that the conventional back-up protection for HVDC (High Voltage Direct Current) transmission lines fail to respond to internal faults quickly, an improved back-up protection scheme was proposed in this study. If voltage and current signals at both sides of the DC line are filtered by the low-pass filter and the cut-off frequency is low enough, then the distributed parameter model of the line can be equivalent to Π model. Based on the Π model, the filtered differential current of the non-fault line is proved to be always smaller than the maximum capacitive current. According to the above analysis, the new differential protection criterion was implemented. A ±800kV HVDC transmission system was built to test the performance of the new protection scheme. Comprehensive test studies show that the proposed method not only can detect internal faults and select the fault lines correctly and quickly, but also respond to internal faults with high transition resistance.

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.

Conventional protection of double-circuit lines on the same tower trip and reclose the two circuits independently after transmission line faults. The possibility of tripping all lines is high when inter-circuit faults occur. Large amount of load will be rejected and electrical link between two sides will be disconnected. It has negative effect on the stability of the power system. The paper proposes a novel tripping and reclosing strategy which regards the double circuits as a whole. It can decrease the possibility of tripping all lines and reclosing to permanent faults. According to the proposed strategy, the double circuits can transfer into the Quasi-Three-Phase Operation (QTPO) which remain at least one line of each phase after faults. The paper researches and analyses the feasibility of QTPO. A typical model of double circuit lines on the same tower is built in PSCAD/EMTDC. Some important electrical indexes which can reflect the performance of QTPO are analyzed by simulation. The results show that QTPO is feasible. The proposed scheme is benefit to the stability of system and satisfied the requirement of power industry operation.

Traditional distribution networks have been faced with more and more challenges in recent years. To provide a system-level solution to achieve regional energy optimization scheduling, distribution generation connection, power quality improvement, custom power requirements and so on, this paper proposes a hierarchical system of regional energy interconnection distribution network based on power router technology for the newly incremental distribution network, which may be a transition formation between the traditional and future intelligent distribution networks. The proposed regional energy interconnection distribution network is divided into three different layer based on different functions. The first layer is centralized energy optimization layer to achieve original energy optimization scheduling. The second layer is distributed autonomous layer to connect different AC/DC voltage levels. The third layer is energy interface layer to provide multiple interfaces with plug and play functions. Based on the function and voltage level of different layers, structures of power routers used in different layers are also given.

To decrease the initial investment of the compensation system in distribution networks, this paper proposes a cascaded distribution static synchronous compensator (DSTATCOM) integrated with D-y connection distribution transformer for reactive power compensation. The cascaded H-bridge DSTATCOM is connected to a group vertex-taps on the primary windings of the D-y connection distribution transformer, which the connection voltage range is chose from zero to the rated ac line voltage in theory. The proposed structure can achieve a suitable connection voltage selection based on the actual reactive power compensation requirements, thus, the voltage stress, initial investment and size of the compensation system can be greatly reduced. The equivalent model is built and the winding currents distribution is also analyzed. Based on the results of winding current distribution analysis, the compensation capacity constraint is deduced. Finally, both simulation and experimental results demonstrate the effectiveness of the proposed system.

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.

Urban power grid is a typical load center. According to the operational characteristics and structure of urban power grid, a complete set of urban grid operation risk index system where the calculation methods of each affiliated index is given, is proposed from the point of equipment direct loss, grid structure integrity failure, grid security margin and power grid load loss. On this basis, the paper makes use of Analytic Hierarchy Process (AHP) to synthesize the affiliated index and draw the total risk index of power system to assess the risk level of power grid. Taking the Polish power grid as an example, the paper calculates and analyzes each affiliated index and the total risk index based on a preset contingency, and verifies the feasibility of the proposed risk assessment index system and methods. And the indices computed are for use by operators and operational engineers in the control room to assess system security levels as a function of existing and near-future network conditions.

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.