Short-circuit Current Characteristics Research Articles

A Backup Protection Based on Compensated Voltage for Transmission Lines Connected to Wind Power Plants

Due to the influence of complex control, the short circuit current provided by wind power generation units will exhibit new characteristics, such as limited amplitude and controlled phase, affecting the performance of traditional protections. This paper analyzes the work principle and defects of one kind of multiphase compensated distance protection. Drawing on the idea of fault identification by multiphase compensated distance protection, novel pilot protection using the compensated voltage on both sides is proposed. The proposed method has a strong anti-fault resistance ability. The test results indicate that the proposed method can distinguish the internal faults and external faults reliably, unaffected by the new characteristics of short circuit current.

Analysis and research on short-circuit current characteristics and grid access faults of wind farms with multi-type fans

As the grid-connected capacity of a large number of wind farms continues to increase, their short-circuit current will have a great impact on protection. Therefore, accurate short-circuit current calculation is beneficial for correct protection action evaluation. Considering the issues related to the diversity of wind turbine types in wind farms and the inability to accurately simulate the failure transient process in previous studies, this research focuses on investigating the impact of control strategies during faults on the transient process of wind turbines. In this paper, the single-machine equivalent model of doubly-fed induction generators (DFIG) and permanent magnet synchronous machine (PMSM) is established by considering the types of wind turbines, and the calculation method of short-circuit current of hybrid wind farms is further proposed by using the classification aggregation equivalent effect method. The accuracy of short-circuit current calculation proposed in this article was verified on RTDS, and a power grid fault analysis method suitable for wind farm connection was proposed based on the characteristics of equivalent circuits of DFIG and PMSM during fault steady-state.

Protection for AC transmission line between DFIG wind farm and MMC station

The characteristics of power electronic controlled sources are reflected at both ends of the AC line applied to the integration of long-distance wind farms into the MMC-HVDC converter station, and the fault characteristics of the system have undergone fundamental changes. For the doubly-fed wind power AC transmission line with a modular multilevel converter, the short-circuit current characteristics provided by the power supply at both ends of the line lead to the inadaptability of longitudinal differential protection. In view of the significant difference in the complexity of the short-circuit current component provided by the doubly-fed wind power and the converter at both ends of the AC transmission line, a line pilot protection idea based on the complexity of the current component is proposed. The complexity of the current component is characterized by singular entropy theory, and then the singular entropy algorithm and its scheme for pilot protection are constructed. A refined electromagnetic transient model of MMC-HVDC for doubly-fed wind farms is built in PSCAD for simulation verification. The simulation results show that the proposed method can quickly and reliably identify various types of internal and external faults, and solve the problem of incorrect action of traditional protection in this scenario. In addition, the scheme is not affected by fault type, location, or transition resistance.

Analysis of Short-Circuit Fault Current Characteristics on AC Side of Wind-PV-Storage Integrated Grid Connected System

The penetration rate of Wind-Soloar-Storage integrated grid-connected systems in the power grid is getting higher and higher. However, the mechanism of its influence on the short-circuit current is still not deeply understood. This paper analyzes the low-voltage ride-through and reactive power compensation strategies of a Wind-PV-Storage integrated grid-connected system; the short-circuit process of the AC side is studied, and the voltage sag degree and limiting link affect the steady amplitude and phase of the current. Through establishing the Wind-PV-Storage integrated grid connected system model in PSCAD/EMTDC, theoretical analysis and verification are carried out on the influence of factors such as the collection line inside the station and different capacity distribution proportions of the station on the short-circuit current.

Research on Pilot Protection for Tie Line of Renewable Energy Sources

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.

Study on Short-circuit current interruption characteristics of Double-break fast vacuum circuit breaker within the minimum arcing time

It is significant for circuit breakers to expel a short-circuit fault in a short time for the new power grid with high proportional photovoltaic or wind power plants to achieve high transient operating stability. Compared with a traditional circuit breaker, a fast vacuum circuit breaker (FVCB), operated by an electromagnetic repulsion actuator, has an extremely shorter opening time and lower deviation in their opening times. This potentially contributes the FVCBs to interrupting a short-circuit fault current in the first half-wave when introducing a controlled switching technology. Meanwhile, it is beneficial to improve the short-circuit current breaking capacity of the FVCBs by adopting series-connected double-break circuit breaker technologies. This paper aims to explore short-circuit current interruption characteristics of a double-break FVCB within the minimum arcing time. We performed a series of short-circuit current interruption tests in a Weil synthetic test circuit. Two series-connected FVCBs and independent single-break FVCBs were used. Three different kinds of opening velocities were adopted for each of the FVCBs. Results showed that the increase in opening velocity resulted in both a decrease in the minimum arcing time and the critical arc extinguishing contact gap, whether for the single-break or the double-break interruption tests. The ratio of the shared voltage on each of the series-connected FVCBs was about 54 (for the break on the high-voltage side) and 9 (for the break on the low-voltage side). Taking the break with a shorter opening time as a break in the high-voltage side could reduce the minimum arcing time and the critical arc extinguishing contact gap. At the same opening velocity, the sum of the critical arc extinguishing contact gap of the two breaks in double-break interruption tests was significantly higher than twice the critical arc extinguishing contact gap in the single-break interruption tests. This could be caused by the fact that in double-break interruption tests, after the breakdown of one break, even if the break restored insulation, could not share the transient recovery voltage (TRV) before the break broke down. In addition, the sudden drop of TRV without breakdown occurred for the break on the high-voltage side might correlate with the micro-discharge of the vacuum gap. The interaction of the charged particles with the electrode surface would be responsible for the formation and evolution of the micro-discharge in some successful interruption cases.

Short Circuit Modelling and Analysis of PV Inverters in Large Solar Farms

The short circuit behavior of solar farms are different from conventional generating stations. These generating resources arestatic in nature and have a rich power electronic interface with a grid, limiting these solar farms’ short circuit capabilities.The solar inverter voltage versus short circuit current characteristics is modeled to supply the fault current within inverterdesigned ratings. In this research paper, a large number of solar power investors are grouped to pool their power into thegrid. Short circuit studies are carried out for a 500 MW solar farm with string inverters rating of 3125 kW per IEC 60909.The protective relaying coordination is performed as per IEEE C37.90 and IEC 60255-1 relaying standard.

Experimental study on short-circuit current characteristics of a photovoltaic system with low voltage ride through capability under a symmetrical fault

Large number of photovoltaic (PV) power plants connected to a power grid can bring significant impacts to fault currents and the operation of protection systems. In this paper, short-circuit current characteristics of a PV system with low voltage ride through (LVRT) capability under a symmetrical fault is studied. PV system short-circuit experiments with different voltage dips at high and low output power levels are designed and conducted. The experiment results provide useful and valuable references for researches of PV system short-circuit current characteristics, modeling and PV system short-circuit current contribution to a power grid. Then, based on the experiment results, steady-state values, peak values and steady state short-circuit currents harmonic characteristics at different conditions are obtained and analyzed. It is found that, the DC component amplitude of a PV system short-circuit current is almost negligible compared with that of the power frequency component, which is different from conventional synchronous generator short-circuit current. Meanwhile, based on the LVRT control strategy and experimental results, the expression of steady-state short-circuit current RMS value is derived and verified, which provides an important basis for short-circuit current calculation of power system with large scale of PV plants.

Research on Single-phase Power Supply in Short-circuit Test of Transformer on Magnetic-force Coupling Field

The short-circuit test is a test of the comprehensive technical ability of transformer manufacturing. The three-phase power supply is usually used for short-circuit test of transformer. To investigate the feasibility of using a single-phase power supply in the short-circuit test of transformer, in this paper, we modeled a 200 kVA power transformer in three dimensions, using the finite element method to calculate the short-circuit current and short-circuit electromagnetic force of the windings under different excitation modes when the three-phase short-circuit occurs in the low-voltage windings. Analyze and compare the short-circuit current and short-circuit electromagnetic force distribution characteristics of windings under different working conditions. The results show that when the three-phase short-circuit condition occurred in the low-voltage windings, regardless of the three-phase excitation or single-phase excitation exerted on the high-voltage windings, the short-circuit current and the short-circuit electromagnetic force of the voltage zero-crossing closing phase is basically the same. This proves that a single-phase power supply can be used instead of a three-phase power supply for short-circuit tests. the analysis result has certain reference significance for improving the short-circuit test of transformer.

Photovoltaic Device Application of a Hydroquinone-Modified Conductive Polymer and Dual-Functional Molecular Si Surface Passivation Technology.

In the last decades, the conductive polymer PEDOT:PSS has been introduced in Si-based hybrid solar cells, gaining noticeable research interest and being considered a promising candidate for next generation solar cells which can achieve both of low manufacturing cost and high power conversion efficiency. This study succeeded in improving the electrical conductivity of PEDOT:PSS to 937 S/cm through a simple process of adding hydroquinone (HQ) to the pristine PEDOT:PSS solution. The results also showed that the addition of HQ to PEDOT:PSS(HQ-PEDOT:PSS) could not only dramatically improve the conductivity but also well-sustain the work function characteristics of PEDOT:PSS by promoting the formation of more continuous conductive-PEDOT channels without removing the insulating PSS. In this report, we reveal that the application of the HQ-PEDOT:PSS to the Si/PEDOT:PSS HSC could significantly improve the short-circuit current and open-circuit voltage characteristics to increase the power conversion efficiency of the HSCs compared to the conventional approaches. Moreover, we also treated the Si surface with the organic monomer, benzoquinone (BQ) to (1) passivate the excess Si surface defect states and (2) to improve the properties of the Si/PEDOT:PSS interface. We show that BQ treatment is able to dramatically increase the minority carrier lifetime induced by effective chemical and field-effect passivation in addition to enhancing the wettability of the Si surface with the PEDOT:PSS solution. As a result, the power conversion efficiency was increased by 10.6% by introducing HQ and BQ into the fabrication process of the Si/PEDOT:PSS HSC.

Design and Photoelectric Performance of Perfect Solar Absorber Based on GaAs Grating

In recent years, solar energy has received extensive attention as a clean and renewable energy. We present a perfect broadband solar absorber based on tungsten and semiconductor GaAs in this paper. The structure of GaAs grating-GaAs film-W substrate has been proposed. And the finite time domain difference method (FDTD) has been used for the numerical simulation of the model. Broadband absorption has been realized in the 500–1,850 nm, by adjusting the parameters of geometry to excite high-efficiency surface plasmon resonance. The absorption spectrum of the structure can be changed by adjusting the geometric parameters to meet different needs. The proposed absorber has incidence insensitive (0–60°) and high short-circuit current characteristics. The structure is simple and easy to manufacture, and has superior photoelectric properties to be application in photothermal conversion, collection and utilization of solar energy.

Design of Grating Type GaAs Solar Absorber and Investigation of Its Photoelectric Characteristics

In recent years, as a renewable clean energy with many excellent characteristics, solar energy has been widely concerned. In this paper, we propose an ultra-broadband solar absorber based on metal tungsten and semiconductor GaAs structure. A multilayer metal semiconductor composite structure composed of W-Ti-GaAs three-layer films and GaAs gratings is proposed. The finite difference time domain method is used to simulate the performance of the proposed model. High efficiency surface plasmon resonance is excited by adjusting the geometric parameters, and the broadband absorption of up to 2,350 nm in 500–2850 nm is realized. The spectrum of the structure can be changed by adjusting the geometric parameters to meet different needs. The proposed absorber has good oblique incidence characteristics (0–60°) and high short-circuit current characteristics. The geometry of the absorber is clear, easy to manufacture, and has good photoelectric performance. It can realize solar energy collection, light heat conversion, high sensitive sensing and other functions.

Comparative Investigations of Inverter Short-Circuit Fault and Winding Terminal Short-Circuit Fault in Open-End Winding Five-Phase PM Machine System

The open-end winding multi-phase permanent-magnet (PM) machine driven by full-bridge inverter is a potential choice for safety-critical application fields. Based on the open-end winding multi-phase PM machine system, comparative investigations of the inverter short-circuit (ISC) fault and winding terminal short-circuit (WSC) fault are carried out to indicate the special characteristics of the ISC in aspects of short-circuit (SC) current characteristics and post-fault performances. The analytical prediction of the SC current under ISC fault is investigated based on the results of WSC conditions. The maximum SC current estimation, SC current rms value variation, and torque harmonic component distribution under the two fault conditions are further investigated. The results can be used for ISC fault diagnosis, PM demagnetization evaluation, and post-fault performance prediction of ISC fault. The post-fault hardware reconfiguration method is further provided to alleviate the effect of ISC fault and reduce the difficulty of compensation.

Improved Short-Circuit Current Calculation of Doubly Fed Wind Turbines With Uninterrupted Excitation

With the enlarging scale of a doubly fed induction generator (DFIG) connected to a power system, the influence of short-circuit current on the system relay protection could not be ignored. Setting and configuring relay protection would be affected by an imprecise short-circuit current calculation. However, some existing studies only consider the condition that the input is the Crowbar and the rotor excitation is blocked. China's new network standard requires the output reactive support current of a DFIG and will change the characteristics of short-circuit current. To solve this problem, on the basis of analyzing the characters of the transient equivalent potential of a DFIG, the transient model of a DFIG with uninterrupted excitation is provided. Based on the characteristics of a non-abrupt change of flux linkage and the requirement of a new grid standard reactive support current, the short-circuit current calculation method of a DFIG with uninterrupted excitation is put forward. Based on the real-time digital simulator (RTDS), a digital-analog experimental platform containing the actual control unit of the DFIG converter is founded, the proposed short-circuit current root mean square (RMS) value calculating method is validated.

Adaptive analysis of main transformer protection in hybrid wind farms

With the development of wind power technology, wind farms have gradually formed a mixed operation pattern dominated by DFIG and PMSG. By analyzing the short-circuit current characteristics when a fault occurred in the main transformer protection zone of the wind farm and the hybrid wind farm is connected to the grid, the possible impacts of the short-circuit current of the hybrid wind farm on the protection of the transformer is studied. The results show that after mixing different unit types, when the fault occurs in the main transformer protection zone of the wind farm, the short-circuit current contains the second harmonic component, and its attenuation speed is related to the number of PMSG, the number of DFIG, the fault location, and the severity of the fault. Under the influence of above factors, the blocking time of the second harmonic blocking protection will be prolonged, thereby increasing the operating time of the ratio differential protection.

Fault phase selection method applied to tie line of renewable energy power stations

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.

Study on characteristics and calculation method of VSC-HVDCshort circuit current under three-phase AC short circuit

VSC-HVDC has been widely used at home and abroad, but there is no clear conclusion about the characteristics and calculation method of the short-circuit current contributed by VSC-HVDC during the AC Short-Circuit Fault near the converter station. The calculation methods in the field of short-circuit current engineering have not been unified, and the adaptability and error of different methods have not been analyzed in detail. Through electromagnetic transient simulation, this paper analyzes the transient characteristics of VSC-HVDC contribution short-circuit current, and combs the key influencing factors of VSC-HVDC contribution short-circuit current. Finally, taking a VSC-HVDC system in China as an example, the adaptability of the commonly used short-circuit current algorithm is analyzed. It is verified that the existing engineering algorithms have made great progress compared with the original. However, compared with the electromagnetic transient refined simulation, there is still a large error in the calculation results of individual nodes under some control modes / fixed values.

Efficient Estimation and Characteristic Analysis of Short-Circuit Currents for MMC-MTDC Grids

Short-circuit current calculation and its characteristic analysis are the foundation of selecting circuit breakers and designing protection systems. First, a companion circuit-based method is presented to fast calculate the short-circuit current of asymmetric bipolar modular multilevel converter-based multiterminal direct current (MMC-MTDC) grids. With the reduction of the whole MTDC grid into an equivalent RLC circuit, the short-circuit current of the grid is efficiently and accurately solved by companion circuit method instead of the numerical integration. In addition to the efficiency, the proposed method can readily adapt to various types of MTDC grids and all kinds of dc faults. Second, unique short-circuit current characteristics of a bipolar MTDC grid following different types of dc faults are theoretically analyzed. A fault type discrimination method is then proposed for bipolar MMC-MTDC grids with dedicated metallic return. Finally, intensive numerical results of two MTDC grids validate the accuracy, adaptability, and efficiency of the presented dc grid short-circuit current calculation method as well as the correctness of the short-circuit current characteristic analyses.

Probabilistic Short-Circuit Current in Active Distribution Networks Considering Low Voltage Ride-Through of Photovoltaic Generation

The probabilistic short-circuit current (PSCC) of photovoltaic generation is determined by the voltage sag at the grid connection and the low voltage ride-through of the unit during the grid fault, while the voltage sag and the low voltage ride-through are both stochastic. According to short-circuit current characteristics and Thevenin equivalent circuit in the transient state, this paper analyzes the changing trajectory of the equivalent internal potential of photovoltaic generation during the period of low voltage ride-through. To deal with the uncertainty of low voltage ride-through, the probability density function by the maximum entropy method is used to characterize the stochastic process of low voltage disconnection of photovoltaic generation. Based on the fault information, the calculation method of PSCC in active distribution network considering the uncertainty of low voltage ride-through of photovoltaic generation is proposed. Two cases of 9-node and 34-node distribution network are used to verify the effectiveness of the method. The obtained PSCC results can reasonably reflect the actual short-circuit current level of the active distribution network.

Short Circuit Current Characteristics Analysis and Improved Current Limiting Strategy for Three-phase Three-leg Inverter under Asymmetric Short Circuit Fault

Three-phase three-leg inverter is one of the most popular topologies for inverters. When short-circuit fault occurs, the inverter is generally switched to the current-controlled mode from voltage-controlled mode to limit the fault currents. In this situation, the inverter becomes equivalent to a symmetric, three-phase, positive sequence current source. However, the voltage limiting may occur under asymmetrical fault. In this paper, a closed-loop model based on time-domain is originally developed as a means to investigate the fault current characteristics under asymmetrical fault. Based on the proposed model, a more intuitive insight into fault currents is presented as follows. 1) The positive sequence fundamental component of three-phase currents can still track the symmetric current references. 2) The magnitude of fault phase currents (ignoring the harmonics) will vary between 1 and $\sqrt 3 $ times of magnitude of current limiting references. Based on the conclusion, although the magnitudes of fault phase currents can be reduced by adopting smaller current limiting references, the actual magnitudes of fault phase currents will still vary in a range depending on the load impendence of healthy phase. Therefore, the reliable action of the protection devices is greatly affected and the selective protection cannot be realized. Furthermore, in order to achieve completely controllable fault currents under asymmetrical fault, two alternative current limiting strategies are presented with reverse current limiting references, and with current limiting references based on the introduced phase angle regulation. The theoretical results are validated by simulations and experimental results.