Three-phase Short Circuit Research Articles

Dynamics of a Turbo-generator Driven by DC Motors During Off-line Three-phase Sudden Short Circuit

This article reports the responses of a turbo-generator driven by separately excited DC motors when the former is subjected to a sudden short-circuit test. Such setups are usually arranged in test rigs of generator manufacturers and electrical machines laboratories, where DC motors are used as the prime mover. Sudden short-circuit tests on turbine generators offer a short-circuit current spectrum that can be used to extract operational impedances and time constants of the generator. The sets of differential equations representing the DC motors, coupling system, and generator are appropriately solved together to obtain the responses. The results show the machine and system dynamics under damped and undamped shaft coupling. Transient parameters of the generator are extracted in each case and compared. The influence of speed departures on shaft torques and current waveforms are also studied. These results suggest that short-circuit tests can be performed at rated voltages if the couplings can be designed so as to absorb the high short-circuit torques developed during sudden short-circuit tests.

Research and Application of Phase-Sensitive Protection in Load Control Center

The malfunction of short-circuit protection became increasingly serious in the fully mechanized face casused by large power equipment, more kind of loads and long distance power. According to the field requirements, the electrical characteristics between motor starting and three-phase short-circuit are compared, the phase-sensitive protection circuit combining current signal amplitude with power factor is designed, the protection data is tested and the protection characteristic is analyzed. The phase-sensitive protection circuit has been uesd in KJZ-4000-10（6）/3.3 flameproof and intrinsically safe load control center. The circuit is stable, sensitive and reliable. It has been proved that the short-circuit protection of power in underground is reliably realized.

Determination of Synchronous Generator Parameters from Time-variant Analytical Load-rejection Curve Fitting

In this article, the decoupled property of the recently developed so-called hybrid model is used to derive time-variant analytical waveforms of the phase voltage and the field current following the load-rejection test. In the proposed identification approach, these time-variant responses are fitted to actual data using the asymptotic weighted least squares estimator along with a Newton-type optimization algorithm to compute the 4-pole 1.5-kVA, 208-V, 60-Hz laboratory synchronous generator parameters. In order to assess the robustness of the proposed identification scheme more thoroughly, the synchronous generator estimated parameters were successfully applied at the cross-validation stage using the symmetrical three-phase short-circuits test data.

Application of SMES Unit in Improving the Performance of an AC/DC Power System

This paper investigates the use of a superconducting magnetic energy storage (SMES) unit to improve the performance of an ac/dc power system. In this context, investigations have been conducted on a large turbine-generator unit connected to a high-voltage direct current (HVDC) system. The impact of HVDC converter station faults on the torsional torques induced in turbine-generator shafts with and without using an SMES unit is elaborated. Faults considered are fire-through, misfire, short circuit across the inverter station, flashover, and a three-phase short circuit in the ac system. These investigations are studied using an electromagnetic transient program power system simulation/electromagnetic transients including DC (PSCAD/EMTDC) and the results are presented in the form of typical time responses as well as harmonic analysis.

A New Approach to Multifunctional Dynamic Voltage Restorer Implementation for Emergency Control in Distribution Systems

The dynamic voltage restorer (DVR) is one of the modern devices used in distribution systems to protect consumers against sudden changes in voltage amplitude. In this paper, emergency control in distribution systems is discussed by using the proposed multifunctional DVR control strategy. Also, the multiloop controller using the Posicast and P+Resonant controllers is proposed in order to improve the transient response and eliminate the steady-state error in DVR response, respectively. The proposed algorithm is applied to some disturbances in load voltage caused by induction motors starting, and a three-phase short circuit fault. Also, the capability of the proposed DVR has been tested to limit the downstream fault current. The current limitation will restore the point of common coupling (PCC) (the bus to which all feeders under study are connected) voltage and protect the DVR itself. The innovation here is that the DVR acts as a virtual impedance with the main aim of protecting the PCC voltage during downstream fault without any problem in real power injection into the DVR. Simulation results show the capability of the DVR to control the emergency conditions of the distribution systems.

Fault localization in electrical power systems: A pattern recognition approach

Electrical power system is one of the most complex artificial systems in this world, which safe, steady, economical and reliable operation plays a very important part in social economic development, even in social stability. The fault in power system cannot be completely avoided. In this paper, we developed a method to resolve fault localization problems in power system. In our researches, based on real-time measurement of phasor measurement units, we used mainly pattern classification technology and linear discrimination principle of pattern recognition theory to search for laws of electrical quantity marked changes. The simulation results indicate that respectively study on the phase voltage, positive sequence voltage, negative sequence voltage, phase current, positive sequence current, negative sequence current of single-phase grounding faults and the positive sequence voltage, positive sequence current of three-phase short circuit faults, the pattern classification technology and linear discrimination principle are able to quickly and accurately identify the fault components and fault sections, and eventually accomplish fault isolation. In the study of electrical power systems, pattern recognition theory must have a good prospect of application.

Modelling and analysis of switching overvoltages caused by short circuits in MV cables connected with overhead lines

In this paper, overvoltage transients are investigated and mitigated when a three-phase short-circuit occurs in cable lateral connected with medium voltage (MV) overhead line and fuse is operated. The transients are studied using EMTP-ATP simulation environment and the need of metal oxide varistors (MOVs) is investigated. The cable laterals consist of sub-laterals and their effect on reducing transients is also investigated. The surge arresters are installed at the junction of overhead line and cable and at far end of the cable near short-circuit point. The different mitigation methods are tested and optimal protection scheme to reduce overvoltages is proposed on the basis of surge arresters locations and sub-lateral connections.

Fault Currents of Doubly-Fed Induction Generator Under Symmetrical Fault Condition

In wind power systems, the fault ride-through capability of doubly-fed induction generators (DFIGs) has become a focus of study. The analysis and computation of fault currents are of great significance for implementing DFIG fault ride-through. This paper systematically analyses dynamic responses of DFIG stator and rotor currents when three-phase short circuit occurs at stator terminals. A new approach, in which the DFIG under fault condition is equivalent to a rotor excited induction machine running in a steady-state and a squirrel-cage induction machine subjected to a three-phase short circuit, is proposed to gain the fault currents mathematical expressions. The two equivalent machines represent the particular solution and the general solution of DFIG differential equations in physical meaning, respectively. Moreover, the method of space vector analysis is introduced to achieve analytical expressions of the fault currents. The maximum rotor and stator short circuit currents, as well as the times when they appear, are demonstrated. Main components of the short circuit currents and their relations are presented. Finally, simulations on a 2 MW DFIG system are carried out, and prove to be in close agreement with results from mathematical analysis.

H∞ Robust Controller Design for an Induction Generator Driven by a Variable-Speed Wind Turbine

This paper presents the modeling and robust controller design design for a wind-driven induction generator system. a robust controller for the static synchronous compensator (STATCOM) and the variable blade pitch in a wind energy conversion system (WECS) is designed to be controlled voltage and mechanical power. This controller leading to satisfactory damping characteristics achieved for the closed loop system. Effects of various system disturbances on the dynamic performance have been simulated, and the results comparison with output feedback controller reveal that the proposed controller is effective in regulating the load voltage and stabilizing the generator rotating speed for WECS. The nonlinear simulation was conducted and that comparison with the above linear simulation shows that the simulations carried out for small changes in system inputs is sufficiently accurate. For review performance against large disturbances from a symmetrical three-phase short circuit at infinity bus bar has been used and the results show robust controller design as well as fluctuations resulting from the short circuit is damped.

Output-feedback IDA stabilisation of an SMIB system using a TCSC

Interconnection and damping assignment (IDA) passivity-based control (PBC) is currently a well-known viable alternative for solving regulation control problems of a wide class of nonlinear systems. However, a distinctive feature that, in spite of its appearance under several applications, has not been exhaustively exploited, is the flexibility that this technique exhibits for designing output-feedback controllers (OFCs). The purpose of this article is to illustrate this attractive characteristic by approaching the (practically important) case study given by the improvement of the transient stability properties of power systems. The particular system composed by a synchronous generator connected to an infinite bus via a thyristor controlled series capacitor is considered. Two OFCs are presented, one that does not involve the unmeasurable state and another that, although including this state, presents some input-to-state stability properties that allow for establishing a sort of separation principle concerning an observer-based structure for the closed-loop system. The advantages of both controllers are illustrated by numerical simulations when a three-phase short circuit at the generator bus is induced.

Current transients in the small salient-pole alternator during sudden short-circuit and synchronisation events

Small salient-pole machines, in the range 30 kVA to 2 MVA, are often used in distributed generators, which in turn are likely to form the major constituent of power generation in power system islanding schemes or microgrids. In addition to power system faults, such as short-circuits, islanding contains an inherent risk of out-of-synchronism re-closure onto the main power system. To understand more fully the effect of these phenomena on a small salient-pole alternator, the armature and field currents from tests conducted on a 31.5 kVA machine are analysed. This study demonstrates that by resolving the voltage difference between the machine terminals and bus into direct and quadrature axis components, interesting properties of the transient currents are revealed. The presence of saliency and short time-constants cause intriguing differences between machine events such as out-of-phase synchronisations and sudden three-phase short-circuits.

Long-term stabilisation of grid connected wind farms with FACTS controllers

The increasing power demand has led to the growth of new technologies that play an integral role in shaping the future energy market. Keeping in view of the environmental constraints, grid connected wind turbines are promising in increasing system reliability. This paper presents the impact of FACTS controllers on the long-term dynamic stability of power systems connected with wind farms based on doubly fed induction generator systems. The stability assessment is made first for a three-phase short circuit without the FACTS controllers in the power network and then with the FACTS controllers. The long-term dynamic simulation results yield information on: 1) the impact of faults and slow wind speed changes on the performance of wind driven induction generators; 2) the change in controllable parameters of the FACTS controllers following the faults and wind speed changes; 3) transient reactive power ratings of FACTS controllers for enhancement of rotor speed stability of induction generators and angle stability of synchronous generators.

Analysis on the Short Circuit Current of a Low Voltage Direct Current(DC) Distribution System using PSCAD/EMTDC

In this paper, we analyzed the short circuit current of a low voltage direct current distribution system. For the analysis, we performed the modeling of the low voltage direct current distribution system with a 6-pulse three-phase thyristor rectifier using the PSCAD/EMTDC, surveyed impedance of sources, transformers and distribution lines to run a simulation. A result of the simulation is that short circuit currents of the direct current distribution system with the rectifier decreased due to a thyristor-ON-resistance(Ron). But in case of the low thyristor-ON resistance, output fault current of the rectifier increased over three-phase short circuit current of an AC power system without a rectifier by regular ratio of the rectifier. Because the output fault current of the rectifier can increase over interrupting the capacity of circuit breakers, studying short circuit currents of a low voltage direct current distribution system with a rectifier is necessary for introducing the direct current distribution systems.

Closed solution of the transient skin effect in induction machines

The transient skin effect is solved for the rectangular open and semi closed slots containing a massive conductor. The solution is applied to the Joule losses and to the analytical calculation of the air gap torque and the stator currents of the induction machine exposed to the three-phase short circuit. The calculation of the Joule power losses under transient conditions has been accomplished, especially, for induction machines of the middle- and high-power range. This becomes important in dynamically controlled induction machines, especially when torque cycles of approximately 100 ms or less occur. Never before have the copper losses been evaluated completely under these transient conditions. For the calculation of the losses, the direct proportionality of the losses between torque and rotor bar current can be used. Furthermore, the results of the transient skin effect can find application in the operational behavior of the induction machine. The consideration of the transient skin effect of the rotor is necessary for calculating currents and torques. Until now, state of the art has only provided an analytical solving of the skin effect free model. Aiming to apply the given rotor geometry, this paper derives an approach which includes the transient skin effect, applying it on the three-phase short circuit of the idle running induction machine.

A Real Application of Measurement-Based Load Modeling in Large-Scale Power Grids and its Validation

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Load model is one of the most important elements in power system simulation and control. Based on more than 20 years of practice using load characteristic recorders in the measurement-based load modeling, this paper proposes the Expectation Composite Load Model to predict unseen data. The methods for load bus classification and parameter identification are also provided. The generalization capability of the proposed load modeling is validated by the measured load dynamics and system dynamics during two three-phase short circuit tests of the NE power grid in China. In order to evaluate the measurement-based load model, the Probabilistic Collocation Method (PCM) is applied to quantitatively analyze uncertainties of the simulation responses raised by the parameters. </para>

H&lt;SUB align=right&gt;&amp;infin; controller for stability enhancement in VSC HVDC system with parallel AC lines

In this paper, a state-space model for a voltage source converter based high voltage direct current (VSC HVDC) transmission link in parallel with an AC transmission line is developed using dq-coordinate system. A robust H∞ control design for the converter and inverter of the VSC HVDC system is proposed to maintain the DC voltage and control the active and reactive power at the converter stations. The H∞ control for the converter control systems is realised using output feedback control obtained by measuring the converter side DC voltage and reactive power, and the inverter side active and reactive power and satisfying the H∞ norm of the closed loop transfer function matrix. Simulation results with a large number of disturbances like torque changes and three-phase short circuits at the converter and inverter buses demonstrate the effectiveness of the designed controller and robustness with respect to the variations of the system operating conditions.

界磁巻線開放時および短絡時の直流試験法による界磁過渡特性を考慮した同期機の等価回路定数算出法

It has been reported that the calculated values of field transient behavior in a synchronous machine differ considerably from the measured values. This discrepancy is caused by the use of equivalent circuit constants in standardized tests provied by JEC2130 and IEC60034-4, in which the mutual leakage reactance between the damper and field windings is not accounted for. The authors have been studying a method for calculating equivalent circuit constants for the accurate simulation of transient behavior including the field winding side, by means of a standstill test with a small-capacity DC power supply (DC decay testing method). The authors have previously presented a calculation method using operational impedances with the field windings opened, shorted, and inserted with an external resistance, obtained by the DC decay test. This paper presents a new method in which the external resistance used in our previous method is no longer needed. Instead, the field winding impedance is determined based on its invariability against slip. The validity of the new method is demonstrated by comparing the calculated and measured values of the armature and field currents during a sudden three-phase short-circuit using 10kVA-200V-31.9A-4P-50Hz test machines.

Fuzzy risk index for power transformer failures due to external short-circuits

A novel methodology to assess the risk of power transformer failures caused by external faults, such as short-circuit, taking the paper insulation condition into account, is presented. The risk index is obtained by contrasting the insulation paper condition with the probability that the transformer withstands the short-circuit current flowing along the winding during an external fault. In order to assess the risk, this probability and the value of the degree of polymerization of the insulating paper are regarded as inputs of a type-2 fuzzy logic system (T2-FLS), which computes the fuzzy risk level. A Monte Carlo simulation has been used to find the survival function of the currents flowing through the transformer winding during a single-phase or a three-phase short-circuit. The Roy Billinton Test System and a real power system have been used to test the results.

Dynamic model for power systems with multiple FACTS controllers

Computer simulation and analysis of power systems are necessary for both planning and operation. This requires an appropriate mathematical model of the system that includes many inter-related linear/nonlinear differential and algebraic equations. Such mathematical model is also needed for the design of globally coordinated controllers to improve power system dynamic performance and stability. This paper presents a procedure for finding comprehensive dynamic models of power systems fitted with shunt and/or series connected FACTS controllers such as STATCOM, SSSC, and UPFC. In this procedure, individual components of a power system are modeled using appropriate frame of references. Then all related equations are transformed to a common network frame of reference and tied to each other through the Y-matrix of the transmission network. The procedure is tested on the Western System Coordinating Council (WSCC) test system including FACTS controllers by performing computer simulations of the system for three-phase short circuit faults. MATLAB/Simulink software package is used for the simulations.

相互漏れリアクタンスを考慮した制動巻線付き同期機の等価回路定数算出法

The mutual leakage reactance between the d-axis damper and field windings is ignored in the conventional d-axis equivalent circuit. It has been pointed out that the calculated value of the field transient current differs considerably from the measured value when this reactance is not taken into account. A method to determine this reactance has been reported previously, but this method has problems concerning measurement precision. The authors have previously presented a calculation method for equivalent circuits, adapting a DC decay testing method, using two synchronous machines of the same specifications (one with damper winding, the other without). Yet, this method is not practical because of the use of two machines. This paper presents a calculation method for equivalent circuit constants taking into account the mutual leakage reactance to accurately represent the field transient current using a single machine. The proposed method determines equivalent circuit constants by calculating the physically correct d-axis transient reactance from the operational impedances when the field winding is shorted and when the field winding is shorted with an external resistance. The validity of the proposed method is demonstrated by comparing the measured values with the calculated values of field and armature currents at sudden three-phase short-circuit. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(2): 71–78, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20891