Resonant Overvoltages Research Articles

Surge Analysis on a Long Underground Cable System

This paper investigates a basic characteristic of fault surge in a long cable system based on EMTP simulations. It is made clear from the simulation results that the sheath over-voltage is higher on a cross-bonded cable than that on a normal-bonded cable. Also, a resonant overvoltage appears when cables with shunt reactors are disconnected. It becomes clear that the period of the beat becomes long as the compensation rate increases. From simulations with earth continuity conductor (ECC), the ECC is effective to reduce an overvoltage in the case of a fault at an overhead line connected to the cable.

Interleaved winding and suppression of natural frequencies

Ideally, it is desirable to design and manufacture a transformer winding that can render all its internal resonances non-excitable. This study examines the effectiveness of an interleaved winding in achieving this goal. While investigating its effectiveness, it led to the establishment of a much desired theoretical basis that reinforces the reasons put forward in the literature to explain internal insulation failures observed in interleaved windings used in extra high voltage (EHV) transformers. Numerical calculations along with experimental verification on actual transformer windings are presented. This study reveals that most of the natural frequencies that are normally non-excitable in the line and neutral current responses of an interleaved winding have been rendered excitable in the disk-to-disk voltages, thus, providing favourable conditions for insulation overstress because of resonant overvoltages. Prevalence of such a condition is an inherent characteristic of interleaved windings.

Using an active resistive damper in hybrid active power filter to avoid resonance over-voltage

This paper presents a hybrid active power filter (HAPF) comprising an active resistive damper (ARD). The ARD consists of three resistances connected to the grid through three bi-directional electronic switches (IGBTs). The ARD can rapidly damp over voltages or over currents under resonance conditions, when the active part (inverter) of the HAPF is disconnected from the grid due to electrical or non-electrical failures. To drive the bi-directional electronic switches of the ARD, a control method is presented in this paper. The control method is based on comparison of resonance over voltage magnitude with a reference voltage (voltage protection level) to control the bi-directional electronic switches. Also the magnitude–frequency characteristics and the control method of the HAPF based on source current detection strategy are analyzed in detail in this paper. Performance of the HAPF with and without the ARD is analyzed in this paper and the results are compared with each other. Finally, the simulation results are provided to validate the proposed topology.

110- to 330-kV substation surge protection against lightning surge coming from transmission lines

A 110- to 330-kV substation protection against lightning surge coming from transmission lines is considered. It is shown that a transmission line always has a critical point, with resonance overvoltage appearing in transformer windings if a lightning strikes at this point. The transformer protection against resonance overvoltages can be implemented using ground wires around the critical point.

Resonance overvoltages in incomplete phase modes in shunt compensated power lines of superhigh voltage

It is shown under what conditions resonance overvoltage appears in shunt compensated transmission lines during single-phase autoreclosing with two phases being off duty. For transmission lines with wide-ranging parameters, we estimate the overvoltages at a disconnected phase (in the cycle of single phase autoreclosing) and at shunt reactor neutral. For resonance detuning, it is possible to use a four-leg or a controlled shunt reactor. On potentially resonant transmission lines, it is recommended to use traditional uncontrolled shunt reactors with a more exact capacity and with a high insulated neutral.

Multivariate Analysis of Transformer Resonant Overvoltages in Power Stations

In this paper, cable-transformer resonant overvoltages are studied on the low-voltage (LV) side of a 410-MVA generator step-up (main) transformer, due to ground fault and energization. The transformer is placed in a network configuration often found in Norwegian hydropower stations. The overvoltages are calculated when systematically varying the length of the high-voltage (HV) side cable and that of the cable between the main transformer LV side and the station transformer. This multivariate analysis is performed efficiently, using frequency-domain calculations and the inverse Numerical Laplace Transform. Time-domain simulations are used for analyzing the critical cases in detail. Both types of calculations make use of a blackbox, wideband model of the transformer, and novel procedures are introduced for initializing computations from correct 50-Hz initial conditions. Among the findings are that extreme overvoltages can occur on the station transformer if current-limiting reactors are placed in series with the cable between the main transformer LV side and the station transformer.

Physical mechanisms for reduction of the breakdown voltage in the circuit of a rod lightning protector with an opening microswitch

The effect of air gap breakdown voltage reduction in the circuit with an opening microswitch is substantiated from the physical point of view. This effect can be used to increase the efficiency of lightning protection system with a rod lightning protector. The processes which take place in the electric circuit of a lightning protector with a microswitch during a voltage breakdown are investigated. Openings of the microswitch are shown to lead to resonance overvoltages in the dc circuit and, as a result, efficient reduction in the breakdown voltage in a lightning protector-thundercloud air gap.

Application of the transfer function to recognition of resonance overvoltages in transformer windings

Power transformer insulation systems are subjected to many stresses during normal operation due to lightning and switching. If the spectrum of incoming surge voltage matches the winding one, a corresponding resonance will be excited. Therefore external transients occurring in power systems might trigger internal overvoltages of a large maximum value in transformer windings. Overvoltages having such a character have been the main cause of many power transformer failures. This paper presents a novel approach to the identification of sensitive zones in the transformer windings based on measurements of overvoltages inside the windings and the frequency dependences of admittance of the power transformer. The frequency characteristics of the transformer winding may determine those regions in the frequency spectrum. The presented methodology might be used both for design optimization and diagnostics of power transformers.

Study of Transformer Resonant Overvoltages Caused by Cable-Transformer High-Frequency Interaction

Power transformers can fail from dielectric stresses caused by electromagnetic transients. In this paper, we focus on a special phenomenon where excessive overvoltages arise due to resonance. This situation can take place when a transformer on the high-voltage side is connected to a cable and the low-voltage side is unloaded. Very high overvoltages can then result on the low-voltage side from transient events that cause a weakly attenuated overvoltage on the cable with a dominant frequency matching a resonance peak in the transformer voltage ratio. Laboratory tests on a 11-kV/230 V distribution transformer show that a step voltage excitation on a 27-m cable produces 24-p.u. overvoltage on the open low-voltage side. The voltage waveforms are accurately reproduced by a black-box model obtained from frequency sweep measurements. Simulations show that overvoltages as high as 43 p.u. could occur with the most unfavorable cable length. It is further shown that the following situations can lead to high overvoltages on an unloaded transformer low-voltage side: 1) ground fault initiation at the far cable end, 2) cable energization from a busbar with several other cables connected, 3) cable energization from another cable with the same length, and 4) capacitor bank energization at the far cable end.

Resonance Overvoltages in Medium-Voltage Multilevel Drive Systems

Simulations and field tests indicate that unacceptable motor-terminal overvoltages and waveform shapes can occur in the normal operating range with high-output-voltage multilevel drive systems. These waveforms (if unattended) can result in premature motor insulation failures. A case study of an 11-kV multilevel system is presented. Simulations and calculations confirm the theory of resonance overvoltages. Different solution possibilities are analyzed. Further simulations and investigations are performed to determine the optimal carrier frequency. Test results confirm that the proposed modification has the desired effect, with waveforms well within limits stipulated by international standards. Simplified equations and recommendations are provided to determine suitable application solutions. Further simulated resonance case studies are presented, considering the effect of the system configurations, motor size, and cable length. Suitable carrier-frequency selection methods are presented to solve the problem.

Internal Winding Failure Due to Resonance Overvoltage in Distribution Transformer Caused by Winter Lightning

In this paper, we describe the internal winding failures of the no. 3 distribution transformer at Katayamazu substation caused by winter lightning. Almost the same internal winding part of the transformer was damaged in January 1997 and November 2001. Therefore, detailed investigations of the cause of failures were carried out. From the investigations on the measurement of the frequency characteristics of transformer windings and detailed lightning surge analysis used by Electromagnetic Transients Program (EMTP), it was found that resonance overvoltages were generated by resonance phenomena between the surge waveform passing through the transformer and the natural frequency characteristics of the transformer winding. This finding was used to improve the winding form of tap windings and install a surge protection device between tap windings. After the improvement of winding structures, it was clearly shown that the internal stress of tap windings was reduced and the breakdown probability of the damaged part was significantly reduced.

Simulation of resonance over-voltage during energization of high voltage power network

Under network equipment outage conditions, the system has to be restored to normal operating conditions as fast as possible to ensure the supply security and avoid prolonged interruption of power supply. In the development of the restoration strategy under different contingencies, a wide range of issues are needed to be studied. For different contingencies, it is important that appropriate approaches, tools and study models are selected and used to meet different objectives. The extraordinary system conditions during restoration require special electromagnetic transient analysis with different system modelling methods and system parameters. All these are very different from those studies conducted for normal operating conditions. Field tests of various restoration arrangements also play an important role in calibrating the study models and parameters in order to verify the study results. With all these studies, CLP Power is able to establish practical and reliable system restoration plans.

The mechanism of eliminating harmonic resonance with controllers by over-voltage in the power systems

The phenomenon of anti-symmetrical bifurcation of periodic solutions occurring near an integral manifold is the intrinsic cause resulting in harmonic resonance over-voltage in power systems. On the basis of this conclusion, the principle of eliminating harmonic resonance by using an anti-bifurcation approach is presented, which means that the theoretical basis of every measure to eliminate resonance is unified firstly from a point of view of basic theory. Our discussion models depend on a class of non-linear control model produced by over-voltage in the power systems. Using the direct Lyapunov technique, a complete theoretical proof is given in accordance with the measure of eliminating resonance by connecting non-linear resistors in series to the neutral point of PT and applying the feedback control law. It comprises the action of the parameters of the resistors to eliminate resonance and the actual process of eliminating resonance, that is, to go against the bifurcation process which forces the big harmonic solutions to retreat to the integral manifold gradually and disappear eventually, by using the non-linear controllers. This ensures that the intrinsic cause of resonance is eliminated thoroughly.

Transient and Harmonic Overvoltages Related to Transformer and Transmission Line Interaction During Black Start Energization

Restoration of the power grid following a blackout is a subject of major concern for all power utilities. Such a black start typically begins by starting small gas turbine or hydro generators and using them to backfeed the high-voltage grid, energizing one piece of the system at a time. However, the energization of high-voltage transmission lines and large transformers is a challenge, since this can result in a combination of transient and sustained harmonic resonant overvoltages which may cause damage to equipment and/or lead to surge arrester failure. This paper addresses the transient and harmonic overvoltages occurring during black start procedures, discusses the development of Electromagnetic Transient Program (EMTP) models, and presents simulation results. The results of EMTP simulation using more complex models compare more favorably to the event records than the results using simpler models. Based on comparisons with measurements, modeling requirements and some guidelines necessary for obtaining accurate results are provided.

Analysis and control of temporary overvoltages for automated restoration planning

The paper presents a new approach that assists power system restoration planning with regards to control of temporary overvoltages (TOVs) due to transformer energization. The frequency-domain analysis in combination with analytical calculations is applied to find an optimum resonance condition. The analytical equations are based on sensitivity analysis for single frequencies and are extended to total sensitivity calculation. After an introduction of the phenomenon of overvoltages during power system restoration and harmonic resonance overvoltages, basic principles of their control are explained. Also, an algorithm is introduced for the results evaluation of harmonic resonance studies where it is necessary to perform simulations. The objective is to find a minimum time after which the simulation runs of temporary overvoltages can be terminated using short-time Fourier transform. Then the new algorithms are presented and demonstrated for a partial of 39-bus New England test system.

Analysis and Control of Temporary Overvoltages for Automated Restoration Planning

The paper presents a new approach that assists power system restoration planning with regard to control of temporary over-voltages (TOV) due to transformer energization. The frequency domain analysis in combination with analytical calculations is applied to find an optimum resonance condition. The analytical equations are based on sensitivity analysis for single frequencies and are extended to total sensitivity calculation. After an introduction to the phenomenon of overvoltages during power system restoration and harmonic resonance overvoltages, basic principles of their control are explained. An algorithm is introduced for the results evaluation of harmonic resonance studies where it is necessary to perform simulations. The objective is to find a minimum time after which the simulation runs of temporary overvoltages can be terminated using short-time Fourier transform. The new algorithms are presented and demonstrated for a partial of 39-bus New England test system.

New power transformer model for the calculation of electromagnetic resonant transient phenomena including frequency-dependent losses

A new power transformer model for the accurate predetermination of transient resonant processes is introduced in this paper. The accurate representation of the losses is the most important feature of the new model since it has been developed for the accurate determination of maximal stresses during resonance phenomena within transformers. The new equivalent circuit only contains constant lumped parameters. It represents very accurately the frequency variation of all impedances (real and imaginary parts) of the transformer, including mutual magnetic coupling. This allows carrying out transient calculations in the time domain directly using programs for the electromagnetic transients calculations like EMTP. Several laboratory measurements on transformer winding models have been performed in order to verify the agreement of measured and calculated values of resonant overvoltages.

変電所近距離雷撃時に誘発される変圧器共振現象の送電鉄塔用避雷器による防止

変電所近距離雷撃時に誘発される変圧器共振現象の送電鉄塔用避雷器による防止

Resonant Overvoltage Phenomena Caused by Transmission Line Faults

Resonant overvoltages in EHV transformers can be initiated by transmission line faults. A series of field tests were performed on the 151 mile long Kammer-Marysville 765 kV line in which a sequence of single line to ground faults were applied to one outside phase at 4 different locations. Fault location criteria and fault initiation procedures are presented. Oscillograms recorded the 3 phase response at both terminals of the line; one is transformer terminated, the other is open-ended. A frequency spectrum analysis of the waveforms is provided showing transient frequencies of sufficient magnitude and duration near the EHV transformer's resonant frequency to stimulate internal winding resonant overvoltages. Digital computer studies are presented and agree with the field tests quite favorably.

Resonant Overvoltage Phenomena Caused by Transmission Line Faults

Resonant Overvoltage Phenomena Caused by Transmission Line Faults