AC Overvoltage Research Articles

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

Transient AC Overvoltage Suppression Orientated Reactive Power Control of the Wind Turbine in the LCC-HVDC Sending Grid

Research on Energy Dissipation Technology and AC Overvoltage Suppression of Sending End in UHVDC Transmission

For suppress the amplitude of overvoltage from HVDC sending terminal, an overvoltage suppression scheme based on the principle of controllable energy dissipation was proposed. The overvoltage mechanism of a weak AC system under the fault of the DC system was analyzed, and the method and operating principle of the CSREDD were proposed. Prototype of the develop CSREDD was tested, which proved correctness about the design. The controllable energy dissipation device has been successfully applied to the ±800kV Lu-Gu UHVDC transmission project. The operation results show that the controllable energy dissipation device can effectively suppress the AC overvoltage.

Research on dual-mode fault suppression strategy of AC isolation and current limiting based on hybrid MMC

Abstract The DC side fault of the high voltage DC transmission system based on modular multilevel converters high voltage DC (MMC-HVDC) is easily affected by the double influence of AC transient electrical quantity infeed and overvoltage and overcurrent, seriously threatening the safe operation of the power grid. Aiming at the problem above, a fault suppression strategy in dual mode of isolation and current limiting is proposed. Based on the half-bridge-full-bridge hybrid MMC, the fault transient mathematical model considering AC feed and sub-module switching feed is analyzed. AC isolation module with double conduction self-resistance structure and current-limiting control module is constructed, which can be adaptively adjusted to the system’s direct current parameters. The ±500 kV double-ended hybrid MMC system model is established to verify the scheme’s effectiveness. In addition, the influence of high AC modulation parameters on the suppression effect is investigated. PSCAD/EMTDC simulation results show that AC active and reactive power feed-in peaks are reduced by 388.69 MW and 119.82 Mvar, respectively, under dual-mode control. The peak value of DC overvoltage and overcurrent is reduced by 1211.53 kV and 22.5 kA, respectively, the fault self-clearing time is shortened by 43 ms, and the fault suppression effect is remarkable. In the high AC modulation ratio operating state, the modulation parameters are positively correlated with the AC power feed and negatively correlated with the overvoltage and overcurrent of the DC system. The research results provide support for the fault protection scheme of the hybrid MMC flexible and direct power grid.

Development and application of AC section power loss protection device

—The so-called AC section power loss refers to the loss of all AC power supplies of the system on this side. After the AC section at the receiving end of the converter station loses power, it will lead to serious AC overvoltage, and the energy of the arrester will increase sharply at the same time, which will seriously affect the safety of the equipment. In view of this situation, this paper studies the action mechanism of AC section power loss protection, and gives the design scheme of AC section power loss protection device. The developed protection device has been verified by RTDS simulation test, passed the third-party detection test of a detection and Research Institute, and is running on site. The field operation is good. The device is not only suitable for conventional DC system and flexible DC system, but also for some occasions where regional stability needs to be considered.

Simulation Analysis of Overvoltage of Single Phase Grounding Fault in Paralleled 12-Pulse Rectifier System

In order to solve the problem of overvoltage of single phase grounding fault of paralleled 12-pulse rectifier system, firstly the common mode wave problem of monopole in DC side at fault is analyzed. Secondly, the transient overvoltage of the system at the AC side single phase fault is analyzed by simplifying the charging and discharging circuit of capacitances on DC side. Finally, the theory of frequency extinguishing arc is used to simulate the intermittent arc grounding overvoltage. It is proved that the capacitance on DC side also affects the AC overvoltage in the system that provides a basis for the better suppression of overvoltage in the project. The correctness of the conclusion is verified by PSCAD/EMTDC simulation software in the analysis process.

Flashover Probability Distribution and Volt-Time Curves of Medium Voltage Overhead Line Insulation Under Combined AC and Lightning Impulse Voltages

Lightning overvoltages are considered as one of the major causes of insulation failure in medium voltage overhead lines. Therefore, the evaluation of the dielectric strength of overhead line insulation is of great importance for insulation coordination studies. This paper presents the flashover probability distributions and the volt-time characteristics of different types of insulator gaps subjected to both positive and negative polarity standard impulse (SI) and short tail lightning impulse (STLI) voltages. In practice, the transient overvoltages generated by a lightning stroke are superimposed over the power frequency AC voltage. Accordingly, the flashover characteristics of the insulator gaps under combined AC and lightning-induced overvoltages are also presented in this paper. The results indicate that superimposition of the impulse voltage on the power frequency AC voltage changes the flashover characteristics of the insulation gaps. Therefore, it is recommended that the effect of power frequency AC voltage should be considered in the risk assessment of insulation for the insulation coordination studies.

Probabilistic Risk Assessment of MV Insulator Flashover Under Combined AC and Lightning-Induced Overvoltages

The effect of lightning-induced overvoltages is more profound in overhead distribution lines due to their limited height and low insulation level. Consequently, there is a high risk of line insulation flashover when exposed to lightning-induced overvoltages. This paper presents a probabilistic method to assess the risk of insulator flashover in medium-voltage overhead lines due to lightning-induced overvoltages. In order to accomplish this, a modified Gaussian cumulative distribution function has been used to predict the probability of single-phase, two-phase, and three-phase flashover of insulators under combined ac- and lightning-induced overvoltages. The validity of the modified probabilistic model is confirmed through experiments carried out in the high-voltage laboratory. Next, Monte Carlo simulations were performed on the simplified Rusck's model to generate the distribution of peak lightning-induced overvoltages. Finally, the risk of insulator flashover is calculated based on the distributions of lightning-induced overvoltages and insulator flashover voltages. The proposed procedure could be considered beneficial to select the optimum insulation level required against lightning-induced overvoltages by distinguishing between single-phase and multiphase flashover faults.

Effect of combined AC and lightning-induced overvoltages on the risk of MV insulator flashovers above lossy ground

Soil resistivity is one of the prevailing parameters associated with the flashover of line insulation in medium voltage (MV) overhead lines due to nearby lightning strikes. This paper investigates the influence of lossy ground to determine the level of lightning-induced overvoltages and the corresponding risk of insulator flashovers. Experiments are performed in the high voltage laboratory to evaluate the flashover characteristics of a 24-kV pin-type insulator under combined AC and lightning-induced overvoltages. Based on experimental results, a modified Gaussian cumulative distribution function has been employed to estimate the probability of single-phase, two-phase and three-phase flashover of insulators based on the assumption of ground resistance close to zero. Monte Carlo simulations are performed on Rusck's and Darveniza's models to obtain the probability density function of peak lightning-induced overvoltages for both ideal and lossy grounds, respectively. The results indicate that lossy ground increases the risk of insulator flashovers. The risk indices are further aggravated with increasing soil resistivity and become significantly high for soil resistivity above 3000Ωm. The advantage of the proposed method is its simplicity and ability to distinguish between single-phase and multi-phase flashover faults with increasing soil resistivity. Thus, the proposed model can be successfully employed for insulation coordination studies of MV overhead lines.

Design and Analysis on the Turn-to-Turn Fault Protection Scheme for the Control Winding of a Magnetically Controlled Shunt Reactor

AC overvoltage emerges on dc buses of a magnetically controlled shunt reactor when a turn-to-turn fault occurs within the control winding (CtrW). Overvoltage protections are thus employed. However, this paper addresses a problem in the application of overvoltage protection against CtrW turn-to-turn faults where such a protective scheme may operate in an unacceptable time delay or even malfunction in some fault cases. It is shown that the ac overvoltage on dc buses during a turn-to-turn fault tends to be intermittent voltage spikes rather than standard sinusoidal waveform, and the fault feature may even not exist in some cases. The cause of the problem is investigated in this paper by dividing the operating state for the excitation system into three stages. It is proved that the appearance of overvoltage on dc buses is dependent on zero-crossing of the total control current. Simulation results and physical model tests have demonstrated the theoretical analysis in this paper. Moreover, a new protective scheme that is capable of detecting a 2% CtrW turn-to-turn fault by using the fundamental component in the total control current is proposed.

Characteristics of Insulation Diagnosis and Failure in Gas Turbine Generator Stator Windings

In order to evaluate the insulation deterioration in the stator windings of five gas turbine generators(137 MVA, 13.8 kV) which has been operated for more than 13 years, diagnostic test and AC dielectric breakdown test were performed at phases A, B and C. These tests included measurements of AC current, dissipation factor, partial discharge (PD) magnitude and capacitance. ΔI and Δtanδ in all three phases (A, B and C) of No. 1 generator stator windings showed that they were in good condition but PD magnitude indicated marginally serviceable and bad level to the insulation condition. Overall analysis of the results suggested that the generator stator windings were indicated serious insulation deterioration and patterns of the PD in all three phases were analyzed to be internal, slot and spark discharges. After the diagnostic test, an AC overvoltage test was performed by gradually increasing the voltage applied to the generator stator windings until electrical insulation failure occurred, in order to determine the breakdown voltage. The breakdown voltage at phases A, B and C of No. 1 generator stator windings failed at 28.0 kV, 17.9 kV, and 21.3 kV, respectively. The breakdown voltage was lower than that expected for good-quality windings (28.6 kV) in a 13.8kV class generator. In the AC dielectric breakdown and diagnostic tests, there was a strong correlation between the breakdown voltage and the voltage at which charging current increases abruptly (Pi₁, Pi₂).

Evaluation of the Reliability of Insulating Gases for the Bushing in Cryogenic Environment

Conventional high-voltage bushings, which adopt SF6 gas as an insulation media, cannot be directly applied in cryogenic environment due to the liquefaction of SF6 gas. In our previous study, it was shown that SF6 gas could not be used as an insulation gas in cryogenic environment, but instead, CF4 gas has shown excellent insulation properties with little liquefaction phenomena, and have a possibility to be applied for the insulation gas for high-voltage bushings in cryogenic environment. However, in order to apply CF4 gas for cryogenic bushing, it should be determined whether CF4 gas can maintain its insulation characteristics in long-term operation. In order to reveal the long-term insulation properties of various gases, which have a possibility to be applied in high-voltage bushings in cryogenic environment, the long-term breakdown characteristics of CF4 gas, SF6 gas, N2 gas, and dry-air were compared over time in a cryogenic environment. For an experiment, a high-vacuum cryostat and a cryo-cooler were prepared to prevent the evaporation of liquid nitrogen for long-term duration. An fiberglass reinforced plastics bushing was installed on the cryostat to apply high voltage up to 60 kV. A rod to plane electrode was set in a cryostat to determine the breakdown voltage of insulating gases. The lightning impulse and ac overvoltage tests have been performed and the test results were analyzed. From the test, the reduction of breakdown voltage of CF4 gas were observed as time passed by, and it was more severe in SF6 . However, stable breakdown characteristic have been shown with N2 gas. Therefore, increasing the pressure of N2 would be a viable option for increasing the breakdown voltage in a cryogenic environment.

Electrical insulation design and evaluation of 60 kV prototype condenser cone bushing for the superconducting equipment

A cryogenic bushing is an essential component to be developed for commercial applications of high voltage (HV) superconducting devices. Due to the steep temperature gradient of the ambient of cryogenic bushing, general gas bushing adopting SF6 gas as an insulating media could not be directly used due to the freezing of SF6 gas. Therefore, condenser type bushing with special material considering cryogenic environment would be better choice for superconducting equipment. Considering these circumstance, we focused on the design of condenser bushing made of fiber reinforced plastic (FRP). In case of the design of the condenser bushing, it is very important to reduce the electric field intensification on the mounted flange part of the cryostat, which is the most vulnerable part of bushings. In this paper, design factors of cryogenic bushing were analyzed, and finally 60kV condenser bushing was fabricated and tested. In order to achieve optimal electric field configuration, the configuration of condenser cone was determined using 2D electric field simulation results. Based on the experimental and the analytical works, 60kV FRP condenser bushing was fabricated. Finally, the fabricated condenser bushing has been tested by applying lightning impulse and AC overvoltage test. From the test results, it was possible to get satisfactory results which confirm the design of cryogenic bushing in cryogenic environment.

고압전동기 고정자 권선의 절연열화 평가

To assess the insulation deterioration of stator windings, diagnostic and AC breakdown tests were performed on the eleven high voltage (HV) motors rated at 6kV. After completing the diagnostic tests, the AC overvoltage test was performed by gradually increasing the voltage applied to the stator windings until electrical insulation failure occurred, to obtain the breakdown voltage. Stator winding of motors 1, 3, and 8 failed at above rated voltage at 14 kV, 13.8kV, and 16.4kV, respectively. The breakdown voltage of three motors was higher than expected for good quality windings in 6kV motors. Based on deterioration evaluation criteria, the stator winding insulation of eleven HV motors are confirmed to be in good condition. The turning point of the current, Pi2, in the AC current vs. voltage characteristics occurred between 5kV and 6kV, and the breakdown voltage was low between 13.8kV and 16.4kV. There was a strong correlation between the breakdown voltage and various electrical characteristics in diagnostic tests including Pi2.

6.6 kV 전동기 고정자 권선의 절연진단과 절연파괴 특성

To assess the condition of stator insulation, nondestructive and overpotential tests were performed on four high voltage motors. The stator windings under these tests have nominal ratings of 6.6 kV. After completing nondestructive tests, the AC overvoltage applied to the stator windings was gradually increasing until insulation failure in order to obtain the breakdown voltage. No. 1, No. 2, No. 3 and No. 4 of 6.6 kV motors failed near rated voltage of 18.4 kV, 19.8 kV, 19.7 kV and 21.7 kV, respectively. The breakdown voltage of four motors was higher that expected for good quality coils(14.2 kV) in 6.6 kV motors. Almost all of failures were located in a line-end coil at the exit from the core slot. The breakdown voltages and the types of defects showed strong relation to the stator insulation tests such as in the case of AC current, dissipation factor(<TEX>$tan{\delta}$</TEX>) and partial discharge magnitude.

Design and installation of extra high voltage cryogenic dielectric test facilities for the superconducting electric equipment

Superconducting electric equipments have a lot of advantages over the conventional devices such as improvements in overall efficiency, size reduction, high current carrying capability, and also environment-friendly products. Owing to these advantages, many research institutes have been tried to develop commercial superconducting devices, which could be applicable to the high voltage and high current electric networks. But up to now, most of the research works to develop commercial products were delayed because it was not easy to acquire reliable high voltage insulation properties and optimum insulation design skills considering extremely low temperature environments. Furthermore, it was difficult to determine optimum high voltage insulation level due to the lack of high voltage cryogenic dielectric test facilities. Thus, in order to establish the cryogenic dielectric technology regarding insulating design, cryogenic dielectric test skills, and to implement more reliable high voltage superconducting devices, high voltage cryogenic dielectric test facilities should be prepared for extra high voltage superconducting electric equipments. Recently, cryogenic dielectric test facilities were installed including 3m long 2m wide cryostat system with bushing, and 1.6MVA lightening impulse generators, 400kVA AC overvoltage test system in Korea. In this paper, design and installation of high voltage cryogenic dielectric test system including very big size FRP cryostat and high voltage bushing were minutely introduced. And some test results to develop high voltage bushings for liquid nitrogen cryostat in order to implement extra high voltage level bushings were illustrated. These test facilities will be fully utilized for the optimization of insulation design for extra high voltage superconducting electric equipments.

Fiber optic voltage sensor for 420 kV electric power systems

We present an optical fiber voltage sensor for 420 kV electric power lines. The sensor exploits the converse piezoelectric effect of quartz and measures the voltage by a line integration of the electric field. The alternating voltage is partitioned to a series of four cylinder-shaped quartz crystals, which are embedded in polyurethane resin within a 3.2-m long insulator tube of fiber reenforced epoxy. The alternating piezoelectric deformations of the crystals are sensed by a common elliptical-core dual-mode fiber, which is wound onto the circumferential crystal surfaces. The fiber is interrogated using low coherence interferometry. We determine the dielectric design of the sensor from a numerical analysis of the electric field distribution within and in the vicinity of the sensor. We experimentally verify the dielectric reliability under ac overvoltages up to 520 kV root mean square (rms) and lightning and switching impulse voltages up to 1425 and 1050 kV, respectively. Further, we investigate the sensor performance including accuracy, dynamic range, bandwidth, and temperature dependence.

Enhancement of continuous operation performance of HVDC with self-commutated converter

This paper describes a converter control scheme to enhance continuous operation performance of HVDC system with voltage source type self commutated (SC) converter. The continuous operation of SC converter can be achieved by suppressing converter overcurrents and AC overvoltages in the case of AC line faults including phase failure. Development of a new control scheme has been accomplished by experiment using an AC/DC power system simulator in CRIEPI and the EMTP (Electro Magnetic Transients Program) analysis. At first, we show that converter overcurrents and AC overvoltages are caused by lack of control information to static limiter and insufficient control performance to negative-sequence current of SC converter. Next, an adequate converter control scheme is proposed which is composed of an ACR's (automatic current legulator) adjustable dynamic limiter to suppress the converter overcurrents, and the limit logic of SC converter to suppress AC overvoltages during phase failure.

Fiber-optic voltage sensor for SF_6 gas-insulated high-voltage switchgear

We present an optical-fiber voltage sensor for 170-kV gas-insulated high-voltage switchgear. The sensor is based on the converse piezoelectric effect of quartz. The full voltage is applied to a cylinder-shaped quartz crystal. The resulting alternating piezoelectric deformation of the crystal is sensed by an elliptical-core dual-mode fiber, which is wound onto the circumferential crystal surface. The fiber is interrogated by low-coherence interferometry. We address the dielectric design of the sensor and verify its dielectric reliability under ac overvoltages and lightning impulse voltages. We then investigate the sensor performance, including accuracy, dynamic range, bandwidth, and temperature dependence.

Flashover voltage characteristics of contaminated station insulators under temporary ac overvoltages in the ac system connected with a frequency converter station

AbstractAccording to the recent analysis results of temporary ac overvoltage in the ac system connected with a frequency converter station, large‐magnitude over‐voltages were confirmed to occur under some special system conditions.Most of the station insulators currently used cannot withstand such overvoltages according to an evaluation based on the data obtained earlier.The necessity of tests to be done to evaluate such performance more accurately was recognized. Both power frequency and switching impulse overvoltage flashover tests were made on contaminated insulators by the method well simulating the natural wetting condition.Switching impulse flashover voltage with the waveshape having a long wavefront time of 2 ms can be well correlated with the flashover voltage characteristics of temporary ac overvoltage. Higher flashover voltage characteristics were obtained by a clean fog test method compared with those obtained by equivalent fog test method.

交直変換所交流側がいしの短時間過電圧に対する汚損特性の研究

According to the recent analysis results of temporary ac overvoltage in the ac system connected with a frequency converter station, large-magnitude over-voltages were confirmed to occur under some special system conditions. Most of the station insulators currently used cannot withstand such overvoltages according to an evaluation based on the data obtained earlier. The necessity of tests to be done to evaluate such performance more accurately was recognized. Both power frequency and switching impulse overvoltage flashover tests were made on contaminated insulators by the method well simulating the natural wetting condition. Switching impulse flashover voltage with the waveshape having a long wavefront time of 2 ms can be well correlated with the flashover voltage characteristics of temporary ac overvoltage. Higher flashover voltage characteristics were obtained by a clean fog test method compared with those obtained by equivalent fog test method.