Transient Voltage Characteristics Research Articles

An Improved Suppression Method of AC Transient Overvoltage for Line Commuted Converter Based High Voltage Direct Current Considering AC-DC System Coupling

Commutation failures in line commuted converter-based high voltage direct current (LCC-HVDC) transmission systems leads to an increase in the converter bus voltage of the rectifier station, thus resulting in AC transient overvoltage in the sending-end grid. The transient overvoltage could lead to the disconnection of renewable energy generation and threaten the stable operation of the sending-end grid. However, the influences of the coupling between AC and DC systems caused by the interaction between the active and reactive power of the sending-end grid, the AC bus voltage of the rectifier station, and the DC current are ignored. The AC transient overvoltage cannot be accurately suppressed. Therefore, in this study, the transient voltage characteristics of the rectifier station under a commutation failure of the inverter station are analyzed. The influence of LCC-HVDC control on the AC bus voltage of a rectifier station through the active and reactive power of the rectifier station is analyzed. A dynamic model of the AC bus voltage of a rectifier station under an AC-DC system coupling is established. The calculation method of the command value of the DC current of the rectifier station is proposed by a predictive control model, and an improved suppression method for AC transient overvoltage is proposed. The case studies show that the accuracy and effectiveness of the suppression of AC transient overvoltage are improved by considering the coupling between AC and DC systems.

Analysis of the Degradation of Depletion-Mode GaN High-Electron-Mobility Transistors Under Reverse Pulse Electrical Stress Using the Voltage-Transient Method

The effects of reverse pulse electrical stress on the electrical and trapping properties of Depletion-Mode (D-Mode) GaN high-electron-mobility transistors (HEMTs) are investigated in this paper. The results of HEMT testing show that the device’s electrical characteristics in the OFF-state are degraded after reverse pulse electrical stress cycling, along with the gate-to-source reverse current and drain-to-source leakage current characteristics. In particular, three traps in the device are identified using the voltage-transient method, and the trapping effects in the HEMT are investigated after 70000 pulses of gate reverse electrical stress are applied. The sources of the traps identified in the study include interface traps near the surface of AlGaN or at the SiN interface (DP1), intrinsic defects such as N antisites in the GaN buffer (DP2), and traps in the gate-drain access region (DP3). Comparison with the transient voltage characteristics of the original device shows that the absolute amplitudes of the traps in the device increase after application of the electrical stress, thus indicating an increment in the trap density. It was found that DP1 remained unchanged before and after stress application, which may be the result of an inadequate surface passivation process. With regard to the other two traps, the electrical stress increases the trap density of DP2 and causes the emergence of DP3 as one new trap. These results may be useful in the design and application of AlGaN/GaN HEMTs. These results may be useful in the design and application of AlGaN/GaN HEMTs.

Measurement of zero-zone voltage for vacuum circuit breaker with oscillation zero-crossing circuit

Vacuum Circuit Breaker (VCB) with oscillation zero-crossing circuit is wildly used to interrupt high direct current, especially in the magnet quench protection system. The injection time of reverse current is the critical factor that affect the breaking result. During the current breaking period, the voltage of VCB varies in a wide range, while the arc voltage in zero-zone is very small and changes rapidly. Therefore, the conventional methods cannot meet the voltage measurement requirements. In this paper, a bi-directional zero-zone voltage measurement scheme for VCB was proposed. According to the variation characteristics of transient voltage during the breaking process, the design and simulation verification of the measurement device were completed. Then the high voltage impulse experiment and arcing measurement testing were carried out by the developed prototype of the arc voltage measurement device. The experimental results indicated that the device has fast response and high measurement accuracy, which meets the measurement requirements of VCB zero-zone voltage.

Optimal allocation of dynamic reactive power of LCC-HVDC receiving-end power grid with dynamic load

Firstly, this paper analyzes the mechanism between reactive power and transient voltage of line commutated converter HVDC (LCC-HVDC) receiving-end power grid, and the physical process in three periods is sorted out according to the time scale. The interaction between dynamic load and LCC-HVDC during AC system fault is studied. Based on this, the necessity of dynamic reactive power compensation is proposed. The simulation analysis shows that the dynamic reactive power compensation can effectively improve the transient voltage stability during fault, and can effectively prevent the diffusion of dynamic load transient instability in the heavy load area, and the compensation effect of connected to the low-voltage bus is better than that to the high-voltage bus. The research results of this paper have a significant effect on improving the transient voltage characteristics of LCC-HVDC receiving-end power grid with high proportion of dynamic load, and can guide the actual operation of the project.

Transient voltage mechanism and influencing factors of wind power HVDC transmission system

HVDC transmission is an important approach to delivery large-scale wind power for a long distance. When the commutation failure occurs, the voltage of sending end will fluctuate dramatically, which threatens the stable operation of wind farm. Meanwhile, different characteristics of wind turbines will further affect the transient voltage of sending end, so the impact of wind farm on the transient voltage must be considered, especially for the high proportion of wind power generation. Firstly, the sending end transient voltage characteristics are analyzed and the mechanism of transient overvoltage caused by commutation failure is revealed. Then, doubly-fed wind power generation systems with different control modes are modeled, and the dynamic reactive power response capabilities of them are studied, so as to analyze the influence of the doubly-fed induction generator (DFIG) control modes on the transient overvoltage. Finally, a simulation example is established which verifies the correctness of the theoretical analysis. The results show that when DFIG adopts the constant voltage control strategy with the quickly response speed of the converters, it has stronger reactive power output capability, and can suppress the transient overvoltage, which is beneficial to the safe and stable operation of the power grid.

Overvoltage Suppression Strategy for Sending AC Grid With High Penetration of Wind Power in the LCC-HVdc System Under Commutation Failure

With the increase of wind power installation capacity, the line commutated converter (LCC)-based high-voltage direct current (HVdc) transmission system is an effective scheme for long distance transmission of wind power. Commutation failure is a common fault for LCC-HVdc transmission system. During the commutation failure, the voltage of sending ac grid presents the “first decreased then increased” characteristic. In order to avoid the overvoltage under the commutation failure causing the wind power generation system (WPGS) to separate from the power grid, it is necessary to suppress the overvoltage of sending ac grid under commutation failure. This article quantitatively analyzes for the first time the transient voltage characteristics of the sending ac grid under commutation failure. The analysis indicates that the delay of the grid voltage detection part in the wind turbine is the significant influence factor on the overvoltage of sending ac grid. Based on the analysis results, this article proposes the delay compensation control strategy of WPGS to suppress the over voltage of the sending ac grid with high penetration of wind power under commutation failure. The effectiveness of the strategy is verified by simulation and control hardware in-loop experiment, and it is also verified that the strategy is also applicable for conventional low-voltage ride through faults, which shows that the control strategy is universal.

Numerical calculation of the lightning transient voltage distribution on converter transformer winding based a dual winding model

The insulation co-ordination of converter transformer winding has a great influence on power system security. An equivalent circuit of a 500 kV converter transformer winding structure, including a LC equivalent network circuit and an ideal transformer in parallel, was firstly established. A combined 12-pulse rectifier equivalent circuit model, mainly consisting of six single-phase converter transformer and 12 converter valves, was built in Power Systems Computer Aided Design (PSCAD) software. In certain operating states of the converter value, the transient voltage distribution was analysed when lightning strokes the DC bus of a typical converter station. The results show that the dual winding model can present the transient voltage characteristics of the converter transformer. The maximum lightning transient voltage is located at the end of the star-connected winding near the neutral point and reaches 1270 kV, and also the maximum gradient voltage occurs at the winding near the terminal point. In different operating states for the converter valve, the transient voltages of a three-phase winding behave as per the similar distributing rules. For the star-connected phase winding, the lightning transient voltage of the winding disc reaches the maximum magnitude when lightning strike occurs at the peak of the phase voltage on the valve side.

Impact Factor Identification for Switching Overvoltage in an Offshore Wind Farm by Analyzing Multiple Ignition Transients

This paper investigates the impact factors of switching transient overvoltage (SOV) in an offshore wind farm (OWF) by analyzing multiple ignitions of a vacuum circuit breaker. The statistical features of transient overvoltages, which occur under real switching operation scenarios, are analyzed based on the measurements in a laboratory platform that is built according to the configuration of an actual OWF. Next, an OWF simulation transient model is established considering high-frequency characteristics of circuit breakers, which is verified to be able to calculate the SOV accurately. Based on the developed OWF simulation model, the occurrence mechanism of SOV is discussed in detail by studying characteristics of transient voltage and current during multiple ignitions within a vacuum circuit breaker when it is switched OFF. Then, the key impact factors of switching- OFF induced overvoltage are identified, which include the inter-phase capacitance of cable in a windmill tower, the actual operating capacity of a wind turbine generator, and the single-phase inlet capacitance in terminals of a wind turbine transformer. Finally, the recommended configurations are given for the transient mitigation in an OWF.

A Phase Selection Method for Wind Power Integration System Using Phase Voltage Waveform Correlation

The fault characteristics of the wind power system, which are reflected in the weak feed features, variation of system impedance, frequency deviation, and non-sinusoidal waveform, will cause the inadaptation of traditional phase selectors to a wind power integration system. In this paper, a new phase selection principle based on the waveform correlation of transient voltage is proposed, which is valid for relay protection of intermittent solar or wind power plants. First, the characteristics of transient voltage of various fault types are deduced according to their boundary conditions. And on this basis, the correlations between the fault transient voltages of every two phases can be acquired. With the differentiated waveform correlations of different fault types, the fault phase selection criterion based on correlation coefficient is formed accordingly. Because it depends on the waveform characteristics of fault transient voltages, the new phase selector is not influenced by the wind power system's fault characteristics and it can select the correct fault phase quickly with high sensitivity. The proposed novel phase selection criterion is verified by the PSCAD-based simulation results and field fault recording data of the wind power integration system.

A Study on the Tolerance Band of Voltage Drop during Motor Startup for Refineries and Chemical Plants with Isolated Power Systems

Refineries and chemical plants with isolated power systems that have a limited power supply are more susceptible to voltage changes from disturbances compared to power systems connected with a power company. Furthermore, most loads in such cases are induction motor loads, and therefore, transient voltage characteristics when starting a high-capacity motor must be examined. In general, high-capacity motors are customized appropriately to the load performance curve by the manufacturer during the construction of an industrial plant. Subsequently, when complying with the voltage drop permitted by international standards during the design process, power supply equipment such as transformers and generators is overdesigned. Therefore, a novel analysis is necessary on standards for startup and constraint voltage drops, as well as on identifying the voltage drop limitations for starting high-capacity motors in refineries and chemical plants with isolated power systems. In this study, field tests on an industrial plant were conducted,, and simulations modeled under conditions identical to those of the field test system were performed using the general-purpose program ETAP in order to compare the results.

Conducted noise and transient voltage in a subsea cable

This article examines the characteristics of transient voltage in long-distance cables in the subsea environment. The effects of changes in temperature and pressure (caused by changes in the ocean depth) on the transient voltage in cables were quantitatively analyzed. To interpret the characteristics of transient voltage, the transmission line was equalized as a π-type circuit, and the effects of inductance, resistance, and capacitance (L, R, and C) according to the changes in temperature and pressure were mathematically modeled. Thus, we confirmed that the effect of temperature on transient voltage generated in cables is approximately 10-fold greater than the effect of pressure. Furthermore, using a fast Fourier transform analysis, we verified that the rate of change in the conducted noise caused by transient voltage generation is 0.001 dB/°C. Thus, through this study, we were able to quantitatively identify the effects of the subsea environment on conducted noise generation in cables.

Electrical Parameters and Conducted Noise on Subsea Environments

This study examines conductive noise in long distance cables in a subsea environment. The inverters currently used in subsea plants generate conducted noise, which is the cause of damage and insulation failure of electric motor bearings. In particular, it is important to analyze the characteristics of transient voltages for different subsea temperature and pressure changes. To analyze the transient voltages, the effects of L, R, and C for different temperature and pressure changes were investigated. Among the analysis factors of L, R, and C, the capacitance C was identified as the factor that influences conductive noise the most. As a result, we analyzed the impact of the electrical components (L, R, and C) on the noise in the design stage of a long-distance cable, focusing on the variables of pressure, and presented the guidelines for designing a subsea plant model.

Spatial dynamics of thermal overload in silicon diodes and dynistors

A modified model of injection and thermal processes is proposed for the description of spatial dynamics of pulsed current overload in ultra-high-power silicon switches. The results of numerical analysis demonstrate an unusual phenomenon of dynamic depletion of plasma density in a narrow layer of the injection channel with anomalously high electric field and heat evolution. This layer is observed when the current density increases above several kA/cm2 and temperature gradients on the order of ∼5 × 105 K/cm develop. The moment of absolute minimum in the plasma density coincides with the appearance of a sharp peak on the transient voltage characteristics, which is a precursor of irreversible thermal breakdown.

Impedance Derivation and Wave Propagation Characteristics of Pipe-Enclosed and Tunnel-Installed Cables

This paper has proposed an impedance formula of pipe enclosed and tunnel-installed cables of which the thickness of the pipe and tunnel wall is finite. Thus, it is composed of a two-layered conductor (i.e., the first medium is the finite thickness pipe or the finite thickness tunnel wall, and the second medium is the outer conductor to infinite). The impedance by the formula developed in the paper shows a quite different characteristic from that calculated by an existing impedance formula which was derived from the assumption of an infinitely thick pipe. Wave propagation and transient voltage characteristics are investigated using the proposed formula, and a difference from those by the existing formula is made clear. The proposed impedance formula has made it possible a detailed and more accurate analysis of wave propagation and transient overvoltage characteristics on pipe-enclosed and tunnel-installed cables.

Digital recording techniques for electrical insulation measurements

The problems of measuring the characteristics of transient voltage and current waveforms occurring in high-voltage power systems are discussed, and the analog recorders that have been used in the past are reviewed. The theory of operation of digital recorders is described, and sources of recording errors are examined. Applications of digital oscilloscopes to measuring switching surges and partial discharges and to high-voltage impulse testing are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The Ageing of Electrical Insulation at Cryogenic Temperatures

This paper presents some theoretical and experimental evidence that, under certain conditions, the transient voltage characteristics of electrical insulation impregnated with liquid nitrogen become the limiting factor in determining the normal operating stress. The life of the insulation is assumed to obey the well known life equation: (stress)n x time = conatant. It will be shown that the operating stress will depend upon the switching and lightning impulse strengths of the insulation, if the exponent n is above a critical value.

ChemInform Abstract: IMPEDANCE OF SILVER OXIDE‐ZINC CELLS

AbstractÜber 100 AgO/Zn‐Zellen werden über lange Zeiträume gelagert und die Impedanz‐ und Spannungsänderungen in Abhängigkeit von der Temperatur und der Lagerungsweise (open circuit, trickle Charge, ?oat Charge) während und am Ende der Lagerungsperiode aufgezeichnet.

Impedance of Silver Oxide‐Zinc Cells

Over 100 sealed cells were subjected to prolonged periods of storage over a range of temperatures and storage modes including open circuit, trickle charge, and float charge. Impedances of these cells were monitored throughout and to the end of the storage period at which point their transient voltage characteristics were observed at the onset of discharge. Results revealed that the impedances of these cells tended to increase with time on stand and that the magnitude of the impedance rise was dependent primarily on temperature and to a lesser degree on storage mode. Typical values for 50 A‐hr cells were usually less than 100 mohm at time zero (immediately after activation) and from 1 to 30 ohm after 6–10 months of storage. Transient voltages of these cells were noted to drop sharply during the first msec of discharge and then to rise and reach a stabilized value during the following few seconds. Magnitude of the initial drop as well as the stabilized voltage values were found to be related to impedance but not in a linear manner. Magnitude and duration of the low transient voltages may be unacceptable in some applications of these cells. Cause for the impedance variations is attributed to changes that occur at the positive electrode. Results provide new and useful information for designers and users of power systems.

On the investigation of voltage surges in Van de Graaff generators

This paper presents some results of calculations and model measurements concerning transient voltages occurring in the stack of Van de Graaff generators, subsequent to the spark-overs between the terminal and the earthed tank. The equations giving the transient voltage characteristics and some of their applications are described. The values of capacitances proposed in an earlier work [8] for protection against transients are calculated by a computer for special cases.

Power-System Transients Caused by Switching and Faults

This paper summarizes the results of an investigation of transient voltages on power systems caused by switching and faults. The transient voltages on power systems as measured by the “klydonograph” are reviewed and compared with the flash-over values of transmission-line insulation. It is shown that the higher values of transient voltages are produced by intermittent arcs. In part I, the various theories for the production of transient voltages of high magnitude as a result of intermittent arcs are reviewed and extended in order to obtain the highest voltages on typical polyphase systems with the range of natural frequencies and attenuation factors that are encountered in practice. Previous studies are of limited scope and apply principally to the case of an arcing ground on an ungrounded system. The present study shows broadly the range of transient voltages which may be produced with intermittent arcs and applies to switching operations as well as arcing grounds. A typical transmission system is studied with the aid of the a-c network calculator. One of the principal variable factors in this study is the method of system grounding and this includes a range of both resistance and reactance between the limits of a solidly grounded system and an ungrounded system. The study is carried out for four different conditions, namely: (1) arcing grounds, (2) de-energizing an unfaulted line section, (3) de-energizing a line section with a fault on one phase, and (4) de-energizing a line section with a fault on two phases. The results of this study are presented in graphical form in part II and show many interesting properties of systems with respect to the method of grounding, and the characteristics of transient voltages for the different switching and fault conditions. It is the authors' opinion that the transient voltages due to. faults and switching deserve more attention than they have received within the last few years.