The IPST Conference

The terms of [A] represent the magnetic coupling (flux linkages) between windings, i.e. between coils on the same core leg. In addition to the representation of coupling between windings of the same phase, intra-phase coupling effects can also be included.

This paper investigates the sensitivities of results of EMTP-type simulation to intra-phase entries of [A]. A method for obtaining [A] that includes intra-phase coupling is developed and verified by comparing simulation results with laboratory measurements. The percent error between measured and simulated short-circuit currents that result from implementing a full and simplified [A] was quantified. A correlation of the size of the elements representing intra-phase coupling in [A] with transformer MVA size is investigated and valuable conclusions and recommendations are made.

Some of the protection-related problems are inadequate coordination, energization with generators out of synchronism, transfer tripping, protection against formation of unintentional islands, overvoltages, etc.

This paper discusses transient studies made for the connection of an Independent Power Producer (IPP) to a 138 kV connection point. The results show the overvoltages that can result from out-of- step reclosing after a line-to-line fault with ground connection on the transmission line. Solutions are suggested to prevent sustained overvoltages with revised under- and overvoltage settings. The proper choice of surge arresters is discussed as well.

If the IPP is connected to a distribution system of 13.8 kV, 25 kV or 34.5 kV, the grounding of the neutral of the transformer at the connection point (point of common coupling) will influence the current distribution between the utility and IPP. This will affect the sensitivity of the neutral protection relays.

The scope of this paper is to analyze the overvoltages due to harmonic resonance at the no-load energization of a prospective 400kV-50Hz double circuit submarine cable line. ATP-EMTP analyses are carried out to predict the possibility of occurrence of low-order harmonic resonance; a phenomenon more liable to happen in case of EHV cables connected to low short circuit power networks.

The occurrence of resonances is sought for in the frequency domain and overvoltages are calculated in the time domain for the no-load energization of the EHV double circuit cable line. A sensitivity analysis is carried out varying the switching instants of line circuit breakers and the network short circuit power, while load and operating configurations are taken as a parameter. The effect of shunt reactor saturation voltage on the harmonic overvoltages is also investigated.

Operational and design countermeasure are proposed.

The proposed method takes into account the actual geometry of the slots and the number of turns in each coil and uses an offline procedure to obtain the magneto-motive force (MMF) distribution due to each winding for a unit injection of current. This MMF along with the air-gap geometry information is used to calculate the flux linkages, and hence the self and mutual inductances of the windings. Thus in contrast with earlier approaches, it is able to calculate the inductances of the machine when the windings are arbitrary distributed.

Since this model is developed for real-time digital simulator, it has the unique feature of being a tool to test the relays designed to protect the synchronous machines from internal faults.

The paper presents a successful hardware implementation for IEC 61850 messaging on a real time simulator and discusses the key design criteria. The software required to configure the IEC 61850 will also be addressed along with the advantages in using the IEC 61850 protocol. One of the biggest advantages is brought about by the realization of the IEC 61850-9-2 sampled values, removing the need for amplifiers as the standard interface to protection devices. Sampled values of the voltage and current signals can be sent via Ethernet, making it even more practical to perform testing on a protective relaying scheme rather than just individual devices.

The complete network is modeled with SimPowerSystems with ARTEMIS real-time plug-in and is simulated in real-time on a RT-LAB InfiniBand PC-cluster composed of 3 dual-CPU dual-core Opteron PCs.

The network model includes the HVDC control and protection systems as well as the synchronous machine regulators and power stabilizers. It also includes typical fault simulation capability like HVDC DC faults, thyristor misfires and AC faults. This model is excellent to study the complex interactions between an HVDC link and AC network under normal and transient conditions.

The real-time simulation is controlled and monitored with a TestDrive interface from Opal-RT. This interface, based on LabView, permits easy monitoring and control of the complete system and enables Python-based scripting for automated tests. The proposed simulator can be interfaced with external equipments and controllers by direct reconfiguration of a FPGA I/O card with Xilinx System Generator blockset.

The results presented were obtained with a new simulation tool which was specifically build for railway networks design. This development was lead in parallel by CEGELY and SEMALY.

This tool is composed of three modules:

- Mechanical calculations. Speed, position and mechanical power vs. time are calculated,

- Electrical calculations. Single or dual AT-feeding systems can be simulated. Contact wire to rail and rail to earth potential and currents in each conductor and at the pantograph of a train are calculated. Some other data such as power factor at the substation are also available.

- Thermal calculations. The temperature of the contact wire is calculated.

The work presented here deals with the electrical aspect and is divided into two parts. In the first part, three different line models are compared. The goal of this part is to quantify the error made on the calculated currents and voltages using one of the three models (Carson, Dubanton, Escane). In the second part, the influence of different parameters is studied in order to quantify the results sensitivity to input data variations.

Parameters such as earth resistivity, rail to earth impedance value (along the line or at the substation) and rail to earth conductance are studied. These studies were made on the 1x25 KV single power supply system. A special attention is given to the rail to earth potential calculation, which must not exceed 50 Volts. This work has also been validated with dual power supply (2x25 Autotransformers system)

This work investigates the impact of wind parks on the distribution network power quality. The field measurement results give important indications about the real effects of the integration of large interruptible renewable energy sources within the power system.

This paper presents the characterization of transients resulting from faults in power systems using the Discrete Wavelet Transform (DWT). This characterization will aid in the development of a method for localize and find faults in a power system. The analyzed case study is obtained by simulations of different fault locations.

The fault initiation causes current spikes that can be used to detect the fault and identify the faulty feeder. The spikes are reduced by the fault resistance and the measurements indicates 1 kΩ as an upper limit for detecting the fault, while the phasor analysis indicates 5 kΩ as an upper limit.

The propagating waves can thus be involved in directional procedures in lines or in cables. Under certain conditions the faulted feeder detection can be assumed also in mixed networks, built up partly of cables and partly of lines.

Then the way to assure the directional decision is to exploit data recorded in transient regime, where an apparent phase difference is more important than actual phase difference in steady state. This is a consequence of the way the transient regime develops, beginning just after the fault inception with phase opposition between the faulted and the sound current residuals and finishing in steady state with a slight phase advance of the faulted residual over all the sound ones.

A CSR with a detailed control system is modeled along with a typical EHV system in PSCAD/EMTDC environment. The study includes the effectiveness of filters introduced in the tertiary of the reactor transformer in controlling the harmonics generated during partial conduction of thyristors. The transient and steady state performance of the CSR system for varying system conditions is studied and the same is compared with the conventional practice. The paper presents and discusses the results of the study.

The purpose of this study was to identify those towers at which back-flashover is more likely to occur than at other towers along the line route. Replacement of one insulator string of a 110-kV duplex insulator by a surge arrester is shown to be a successful mitigation technique to reduce the back-flashover rate of those 110-kV lines.

This paper shows how overvoltages can be produced during energization of the filter, which may cause flashovers at the filter reactor by means of digital simulations of switching transients. The system components including the filter circuit are represented in detail using ATP-EMTP. Both filters are fed by a relatively long 33-kV cable. The saturation characteristic of the filter reactors plays an important role in initiating repetitive high transient voltages. The neutral point of the filters is isolated, since the neutral of the feeding 33-kV system is not grounded.

The overvoltages across the reactor obtained by a statistical analysis of switching surges are far higher than the amplitude of the withstand test voltage performed at the factory. Severe overvoltage stress of the filter reactor may also happen, when the filter will be re-energized shortly after it has been switched off, so that the filter capacitors remain charged. In this case, the highest overvoltage of 83.9 kV is expected across the filter reactor.

Typically, recovery of a system operation starts from small generator(s), which power should be given to a remote larger power plant to start up it. During this process, various nonstandard combinations of sources, lines and transformers can occur, that can be unstable and exhibit very high level of transient temporary overvoltages. The main cause of that is a ferroresonant oscillation between source, line capacity and switched power transformers with saturation. The transient behaviour with temporary overvoltages can endanger hv equipment and it was considered that surge arresters are the most sensitive. Therefore a criterion of satisfactorily low and permissible thermal stressing of surge arresters was established as the criterion for acceptability of a given step of black-start process.

The problem is in the paper demonstrated on the real event - black-start experiment realized in czech power system. The ferroresonance was predict by EMTP simulation during preparation of the test and it really occured.

The authors analysed various configurations of sources, lines and transformers by EMTP simulations on appropriate EMTP models and prooved them for transient ferroresonance - its magnitude and duration. They looked for some rules for appearing of resonances and temporary overvoltages and found some regularity, which will be presented in the paper. Then the authors created models of real network configurations for blackstarts and prooved them by simulation. Permissible black-start scenarios were specified on basis of the criterion of acceptable energy absorption of surge arresters then verified as feasible and implemented to plans of system recovery.

For the case of Electric Arc Furnace (EAF) compensation, the new real time digital simulation has made it convenient and easy to demonstrate the flicker improvement factor provided by the actual VSC controls under realistic system conditions.

The paper compares results from off-line and real time RTDS simulations, as well as from field measurements taken at a full scale ABB SVC Light® installation. The good correlation between simulation results and field measurements give confidence to the digital modeling.

The results obtained using an accurate cable line model that takes into account the semiconductive layers in the cable formation and a less accurate one that disregards such layers are compared and discussed. Although the analysis shows that the differences in the propagation of transients using the mentioned models are not strong, the advanced models is in any case to prefer in order to avoid possible misleading results.

However, the simulation time can be greatly reduced if the simulator's hardware is custom configured. This paper is an effort to introduce and develop a reconfigurable hardware simulator using FPGAs to simulate power systems that contains power electronic switching elements. The simulation time-step for this simulator is less than 100ns. A new time domain model for power electronic switching elements is also presented in this paper. The new model is very convenient for real-time simulation purposes due to its simplicity and the elimination of numerical oscillations associated with other models.

This paper describes a study made in an actual 500 kV AC transmission system considering two alternatives in order to limit switching surge overvoltages during line energization: the use of metal oxide surge arresters at both line ends and along the line and the use of synchronized closing of circuit breakers.

Digital simulations have been made with PSCAD/EMTDC. The degree of shunt compensation is considered as independent parameter.

This paper focuses on protection impacts of distributed generation and on the possibility of applying a novel simulation environment for these studies. The environment is described and some motivation for building such system is discussed. An example case is studied using the simulation environment. Some observations are made based on the results.

Although, a number of insulation coordination studies have discussed controlling over-voltages due to lightning and switching in Gas Insulated Switchgear (GIS) and transformers, not many deal with the additional complexity raised when cables are used to connect overhead lines to the new equipment.

The paper reports on aspects of work being done within the CIGRE Working Group on Insulation Coordination. (C4.301). An introduction is provided to the sensitivities which affect the magnitude of fast front transients occurring in a typical arrangement including overhead-lines connected to GIS and transformers using short cable sections. This considers factors including lightning, cable length, tower footing impedance and substation running arrangements. Generic sensitivities will be examined for a 400kV circuit, using studies carried out in EMTP to establish the nature of these over-voltages within the substation and the effectiveness of control methods to mitigate their impact.

Based on this fact this paper deals with investigations to evaluate the transient behaviour of a close combined 400 kV urban system. Numerical tools are basically a modern method to estimate transient stresses, help to optimize the insulation coordination and can simulate various circuit states. Thus, temporary faults can be reduced or avoided at all.

One of the major aspect of this work is the close connection of an overhead line, a cable section and a substation related to the 400 kV system. Additionally attention was paid to the transformers and the secondary side of the system, a 110 kV urban distribution network.

A number of calculations were carried out to get an overview of the transient stress caused by lightning or switching in numerous network nodes of the substation. Of additional interest have been different circuit states at the 400 kV GIS and the transient behaviour at the arrestors. Amplitudes and the energy consumption at the arrestors were taken into account.

These actual investigations were carried out to get useful information's about the transient stress in this important Viennese 400 kV substation, which was officially put into operation in May 2006. As an output of these investigations the results influence the strategy in running the network.

Regarding high-fault currents, KEPCO (Korea Electric Power Corperation) plans to use CLR (Current Limit Reactors) on UHV transmission lines or the bus in substations to limit the magnitude of fault currents. CLRs make a significant contribution to reduce the severity of the TRV (Transient Recovery Voltage) experienced by the feeder and bus circuit breakers when clearing feeder faults. Based on the conclusions of investigating actual circuit breaker failures while performing this duty, the mitigation of the TRV associated with the reactors is described.

This study describes the underlying principles of current breaking, insulation recovery, the occurrence of TRV and its analysis methods (2-parameter and 4-parameter methods) with regard to the rise in fault currents and TRV using EMTP [5][9]. Through the analysis of real fault cases, measurements which can be readily implemented in the field are closely studied and suggested. Also this paper describes two models ; one is Short Line Fault (SLF), the other is Bus Terminal Fault (BTF) [5][6]. The TRV of bus circuit breakers are in three positions in the 345kV substation when a three-phase short circuit fault occurs. The other case shows the TRV of two bus circuit breakers in four positions when a three-phase short circuit fault occurs and in two positions when a one-phase line-ground fault occurs.