Steep-front Impulse Research Articles

Breakdown mechanism of RTV silicone rubber coated insulators under steep-front impulse voltage

Insulators play the vital role in the power transmission, including traditional porcelain/glass insulator and silicone rubber based polymeric insulator. By coating the glass/porcelain insulators with silicone rubber, the advantages of organic and inorganic dielectrics are integrated. However, it was found that the breakdown probability of coated insulators under steep-front impulse increases significantly. It suggests the possible degradation of insulator material and might become the hidden danger of power system. Therefore, in this research, a great deal of steep-front impulse voltage tests are carried out over various types of insulators. It demonstrates that the performance degradation is introduced by RTV coating instead of intrinsic defects. Then, based on the analysis of flashover paths and breakdown locations, the breakdown mechanism of coated insulator under steep-front impulse is illustrated. Also, numerical simulation of electric field distribution is performed for verification, taking the surface arc development into account. It indicates that the development of surface arc is distorted by the coating which results in the intense local field and dielectric breakdown. Next, different controlled experiments are designed and conducted to validate the theory. Besides, the corresponding modification towards the current coating method is proposed.

Study on Steep Front Impulse Test Voltage Value of Disc Insulators with Variation in the Number of Units of Insulator String

The insulators used in power transmission and distribution networks should withstand abnormal system-generated switching impulses and natural lightning impulses. To avoid the failure of insulators in the field, the impulse puncture strength evaluation of the ceramic insulators is important. IEC 61211 standard stipulates the procedure to evaluate the ceramic disc insulators’ strength to withstand steep front impulses. In this test, the test voltage is determined by using a short standard string of five insulators. This paper presents the variation in the impulse stresses across the disc insulator units in the string due to non-linear voltage distribution under impulse flashover. This paper also correlates the test voltage values defined by the IEC standard with the highest stress on insulator units under impulse flashover of string insulators by using voltage distribution. The correlation study has been carried out with insulator strings having different arcing distances by using a different number of units.

Understanding charge trap characteristics of epoxy nanocomposite under steep fronted lightning impulse voltage

Understanding charge trap characteristics of epoxy nanocomposite under steep fronted lightning impulse voltage

Puncture phenomena and mechanism analysis of RTV coated cap and pin insulators in steep-front impulse voltage test

In recent years, more than 6 million room temperature vulcanized (RTV) silicone rubber coated cap and pin insulators with high mechanical ratings were used in ultrahigh voltage transmission lines in China to improve pollution performance. However, the unexpected puncture phenomena of RTV coated insulators were exposed during steep-front impulse voltage tests; for instance, the steep-front impulse voltage test pass rate decreased to less than 50% for 550 kN glass insulators with RTV coatings. The steep-front impulse voltage test is the most effective method used to check the insulation quality of cap and pin insulators. This unexpected phenomenon once caused serious concerns to power utilities. In this paper, several possible factors that affect the puncture of insulators were analyzed. Then, the extent of the decline in the breakdown of the 550 kN glass cap and pin insulators with and without RTV coatings were studied. The initial puncture location was then found, and the developmental process of the arc on the insulator surface in steep-front impulse voltage tests was observed with an ultra-high-speed intensified charge coupled device camera. Lastly, a breakdown mechanism is proposed. The puncture phenomena of a RTV coated insulator in a steep-front impulse voltage test is essentially an electrical breakdown of the internal insulation. The RTV coating induces the close adherence of the arc to the surface of the insulator. Such closeness changes the arc development path on the insulator surface and facilitates easy breakdown in the shed weak part. All these factors result in a significant increase in the breakdown probability of RTV coated insulators in steep-front impulse voltage tests.

Interface defect detection for composite insulators based on infrared thermography axial temperature method

Interface quality is one of the most important properties of composite insulators, and is hard to be effectively detected. Poor interface quality may lead to interface discharge or insulator fracture and hence threatens the safety of power systems. In order to detect interface defects of in-service composite insulators, 15 samples were collected from transmission lines in this paper. The dry power frequency voltage test and the steep-front impulse voltage test were conducted. The surface temperature after dry power frequency voltage test was recorded by infrared thermography. Then a method for quantitative characterization of composite insulator interface defects based on infrared thermal image axial temperature was proposed. By defining heating section according to temperature level of each sample, we summarized temperature distribution characteristics, including characteristics on location, length and maximum temperature point of heating sections. Our results show that the connection zone of composite insulators is prone to generate interface defects. Temperature analysis together with steep-front impulse voltage test results indicate that interface defects of test samples can be judged by the following three features of heating sections, namely, temperature rise (ΔT) above 80 K, ΔT above 20 K with 10 cm in length, and ΔT above 10 K with 40 cm in length.

Steep-front impulse voltage in diagnostic studies of composite insulators

Laboratory studies of composite insulators by means of steep-front impulse voltage tests are presented. The tests were performed on four groups of insulators, i.e. on newly manufactured insulators, on insulators removed from service, on insulators earlier exposed to mechanical fatigue test and on insulators with intentionally introduced technological defects. Extensive results of these investigations indicate that application of steep-front impulses of 4 kV/ns provides a better diagnostic efficiency as compared with the standard testing by means of impulses with steepness of 1 kV/ns. It is shown that the application of 4 kV/ns impulses do not cause any damage to healthy insulators, while defected insulators are identified reliably. A new methodology allowing to assess the status of internal insulation in insulators removed from service is proposed based on the performed studies.

Optimal Surge Arrester Parameter Estimation Using a PSO-Based Multiobjective Approach

In this paper, an improved self-adaptive particle swarm optimization (ISAPSO) algorithm is developed for estimating the best set of surge arrester model parameters. The purpose is to minimize the relative error between the calculated and manufacturer's measured residual voltage peak values for lightning, switching, and steep-front impulses. An objective function, which generalizes the model parameters for all impulse current types with different peak levels, is proposed. This objective function is the summation of three subfunctions that in each of them one type of impulse current with different current peak levels is considered. An efficient multiobjective approach is also applied along with the presented ISAPSO method. The proposed algorithm finds different sets of Pareto-optimal nondominated solutions for the objective functions. A fuzzy clustering technique is used to restrain the size of the repository within the desired limit. The proposed method is applied for a 150-kV TRIDELTA metal-oxide surge arrester in order to demonstrate its accuracy and effectiveness.

CONVERSION OF IMPULSE VOLTAGE GENERATOR INTO STEEP WAVE IMPULSE TEST-EQUIPMENT

This paper demonstrates the alternative measures to generate the Steep wave impulse by using Impulse Voltage Generator (IVG) for high voltage testing of porcelain insulators. The modification of IVG by incorporating compensation of resistor, inductor, and capacitor has been achieved and further performance of the modified system has been analyzed by applying the generated lightning impulse and analyzing the electrical characteristics of impulse waves under standard lightning and fast rise multiple lightning waveform to determine the effect to improve rise time. The advantageous results have been received and being reported such as increase in overshoot compensation, increase in capacitive and inductive load ranges. Such further reduces the duration of oscillations of standard impulse voltages. The reduction in oscillation duration of steep front impulse voltages may be utilized in up gradation of Impulse Voltage Generator System. Stray capacitance could further be added in order to get the minimized difference of measurement between simulation and the field establishment.

TSDC Study of Space Charge Influence on PP Film by Steep Front Impulse Injection

Influence of DC and steep front impulse injection on space charge of PP by thermally stimulated discharge current (TSDC) are studied. The experimental results show that space charge of PP is almost irrelative to the rise time of pulse, but space charge has relationship with polarized field intensity and polarized time, which is similar to the theory of TSDC

A Hybrid Model of Transformer Windings for Very Fast Transient Analysis Based on Quasi-stationary Electromagnetic Fields

Based on the principle of a quasi-stationary electromagnetic field, a hybrid lumped parameter model of transformer winding under very fast transient over-voltage (VFTO) is proposed in this article. The hybrid model consists of a single-turn model combined with a two-coil model. The single-turn model can be used to calculate the inter-turn over-voltage under a steep-front impulse. The simulation time can be significantly reduced with the help of the two-coil model in the proposed hybrid model. The calculation method for the model parameters is given, and the simulation study for the voltage distribution of a practical transformer winding under a chopped-wave impulse has been performed. The simulation result has been compared with the actual data measured at the Chongqing ABB Transformer Factory to verify the effectiveness of the model. The result shows that the model can predict the voltage distribution inside a transformer winding correctly.

Streamer corona propagation of an inhomogeneous field gap in SF6 gas stressed by steep-fronted impulse

Based on experimental observations and theoretical explanation of the streamer spatial extension for an inhomogeneous field gap in SF6, a new model is developed, taking into account the probability of the appearance of an efficient electron. The model, which can help one to better understand the breakdown characteristics of the impulse-steepness dependence, relates the spatial extension of the streamer corona in an inhomogeneous field gap in SF6 gas to the steepness of applied positive impulses.

Study of ZnO arrester model for steep front wave

Measurements were performed to obtain the response of an arrester block to a steep front impulse current (1/2 /spl mu/sec), a 4/10 /spl mu/sec impulse current and a standard impulse (8/20 /spl mu/sec). As a steep front wave response model, the conventional model, the IEEE model, and a nonlinear inductance model have been studied. The method of building an arrester block model is explained in this paper. The nonlinear inductance model proposed has an accuracy of 99% in calculations of steep front impulses in block model studies. This model can be built easily and simply.

The electrical breakdown characteristics of oil-paper insulation under steep front impulse voltages

Disconnecting switch operations in gas insulated equipment cause transient voltages with risetimes as steep as 5 to 20 nanoseconds and magnitudes as high as 2.5 pu. There is very little information on the effect of these transients on oil-paper insulated equipment. There have been reports, however of transformer and bushing failures caused by these transients. The electrical breakdown characteristics of oil-paper insulation under steep front impulse were studied in this project, which was co-sponsored by the Canadian Electrical Association and BC Hydro. V/sub 50/ (50% breakdown probability voltage) breakdown data was obtained with steep front (10 ns/2500 /spl mu/s), lightning and switching impulse waveforms. Insulation breakdown voltage vs breakdown time (V-t) data and multiple impulse breakdown data were obtained with the steep front impulse waveform. The V/sub 50/ results showed that the breakdown strengths were lower for steep front impulses than for lightning impulses. The multiple impulse breakdown results showed that oil-paper insulation breakdown strength can be lower than 100 kV/mm. These results are alarming, since they suggest that oil-paper insulated equipment subjected to steep front transients will fail at voltages below the lightning impulse design level (BIL). The volt-time data had a discontinuity. The breakdown process at risetimes below about 50 ns was different from the breakdown process at risetimes above 50 ns.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Voltage-time characteristics for steep-front impulse voltages of particle-contaminated spacers in SF/sub 6/ gas-insulated switchgear

The voltage-time characteristics of spacer surfaces for steep-front impulse waves are investigated under a particle-contaminated condition in SF/sub 6/ gas. The characteristics are measured as a function of particle length, particle position, and space shapes. Flashover voltages monotonically increase in the submicrosecond region as time to flashover is shorter, and are a minimum in the 1 mu s region. Applicability of the equal voltage-time area criterion for estimating the voltage-time characteristics is discussed and the estimation is clarified. Moreover, it is demonstrated that an optimized spacer with ribs greatly improves flashover voltages in the submicrosecond region as well as in the 1 mu s and power frequency region.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Modeling of the dynamic performance of transient recorders used for high voltage impulse tests

Digital recorders are becoming more commonly used in the recording of steep-front HV impulses. Although many digitizers exhibit a nonlinear deterioration in dynamic performance with increasing input signal steepness, there is no accepted method for predicting the magnitude of the resulting errors. The authors describe the development and implementation of a computer modeling technique for establishing the magnitude of these errors. The model is based on time-domain test data combined with a basic understanding of the digitizer's operating principles. The model is general in nature and can be used for the prediction of errors generated by any recorder that has systematic errors. The model was used to predict the maximum error which can occur when a particular EBS (electron-bombarded semiconductor) recorder is used to monitor HV impulse tests on power apparatus. Its use allowed for the determination of error limits for records of steep-front impulse tests which will be standardized in the near future. >

Steep front impulse flashover tests on a solid-state relay

A protective relay system, subjected to a high-voltage, steep-front impulse, has been shown to fail in the laboratory via flashover-induced current follow through on the DC power supply. Laboratory experiments were performed to characterize the response of a protective relay to steep-front impulses. These impulses may be the result of a nuclear-induced, high-altitude electromagnetic pulse. Tests were also conducted to quantify the protection offered by a surge protection package on the DC circuits of a relay system. Results from these tests were used to assess the vulnerability of a protective relay. >

Distribution transformer performance when subjected to steep front impulses

Arrester-protected pole-type distribution transformers were subjected to the combined stresses caused by the conducted steep front impulse and rated 60 Hz voltages. The results are compared with those of similar experiments on unenergized distribution transformers and with low-voltage impulse wave distribution results. It is concluded that (1) tank-mounted direct-connected arresters provide a high degree of protection to these transformers, (2) tank-mounted externally gapped or crossarm mounted arresters may not provide sufficient protection, and (3) internal transformer insulation system failures caused by the steep front impulse do not necessarily result in immediate catastrophic failures of the transformers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental determination of the effects of steep front-short duration surges on 25 kVA pole mounted distribution transformers

Nineteen standard commercial 7200 V 25 kVA distribution transformers were tested to determine vulnerability of their insulation systems to steep-front short-duration surges. These transformers were tested both with and without surge arresters using a high-voltage pulser. The open-circuit pulser test voltages were 400 kV, 500 kV, 800 kV, and 1000 kV. These voltages exhibited an approximately 60 ns risetime and approximately 2000 ns to half value. Standard lightning impulse tests for this voltage class of distribution transformers were conducted prior to the steep-front surge tests to verify insulation integrity, and were repeated following the steep-front surge tests to ascertain if an insulation failure had occurred in the tested transformer due to the steep-front impulse. Failed transformers were disassembled to evaluate failure modes. It was determined that no insulation failures occurred on any transformers tested that were protected by surge arresters mounted directly on the transformer. Unprotected transformers failed at voltages around 250 kV to 300 kV peak. The failure mode was usually an internal flashover/puncture between the first few turns and the low-voltage winding or, sometimes, an inner layer of the high-voltage winding.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental Study of the Breakdown Characteristics of a Gas-Insulated Bus

The area effect in gas-insulated systems on the breakdown voltages and V-t characteristics is studied, using a model bus section of 84 kV single-phase SF6 gas-insulated substation. The model bus, stretched along a straight line, has a number of conical spacers. The breakdowns occur at the shielding electrodes arranged near the spacers. Increasing the number of spacers means an increased chance of breakdown at these shielding electrodes. When switching impulse and ac voltage are applied to the above system including many spacers, the breakdown voltage distribution has a range somewhat lower than that of one spacer. This phenomenon is caused by the increase of weak points on the electrodes. When steep front impulse voltages are applied, the breakdown distribution of many spacers is very similar to that of one spacer, in contrast to the cases of switching impulse and ac voltage. This relative insensitivity of breakdown voltage to the system scale suggests a mechanism other than the statistics of weak points. The experimentally obtained overall V-t characteristic of this system is found to conform to the empirical formula proposed from small scale experiments.

急しゅん衝撃電圧に対する沿面放電の放電電荷量と放電機構

急しゅん衝撃電圧に対する沿面放電の放電電荷量と放電機構