Qiaohua Wang

Processing of partial discharge ultra-high frequency signals from a true size transformer

Quite a few methods have been proposed to detect partial discharge in power transformers, such as dissolved gas analysis (DGA), pulse current method, ultrasonic wave method and ultra-high frequency (UHF) method. UHF method, is widely used in condition monitoring recently for the sake of its wider spectrum and higher sensitivity. This paper presents a PD detection experiment inside a true size transformer which was rebuilt to install 9 UHF sensors. Four types of PD sources were placed into in the transformer. The pattern and intensity of PD can be changed by adjusting PD voltage and insulation distance. Signals detected by the sensors were decomposed and reconstructed by methods of Continuous Wavelet Transform (CWT) and Multi-Resolution Analysis (MRA) based on different wavelets. The time-frequency characteristic and de-noising performance of different transforms applying to PD signals were compared and the result shows that different transforms reflect advantages in different respects. A new method is proposed to extract PD signals from noises.

Temperature effect on space charge dynamics in LDPE/MgO nanocomposite under DC stress

A pulsed electro-acoustic method (PEA) of space charge measuring system was developed, which could be used to investigate space charge distribution in solid dielectric at a temperature high up to 383 K. With the measuring system, space charge dynamics in the composite of low density polyethylene (LDPE) and nano-MgO which is expected to be used as an insulation of HVDC plastic insulated cable was investigated from room temperature to 353 K in detail. It is shown that the amount of space charge decreases significantly at a higher temperature in all samples including the LDPE sample. However, it should be pointed out that the amount of space charge in samples with nano-MgO is lower than that in LDPE and the shape of space charge in the composites varies with temperature. In order to know space charge decay rate, space charge in all samples after being pre-stressed for 1800s was measured and it is found that the decay rate increases significantly as temperature increasing.

Space charge measurement system using high-frequency pulse generator

In pulsed electroacoustic method (PEA), averaging technique is necessary for the signal process to acquire waveform with higher S/N (signal to noise). It could be obtained by repeatedly applying the pulse voltage to the sample. The suitable minimum number of times for averaging depends on the measurement conditions and characteristics of equipment. For general measurement system, the number of times above 100 is necessary. This means the frequency of repeated pulse voltage should be higher enough to complete the data averaging in a short period. However, the frequency is low for conventional pulse generator used in general measurement system, leading to significant delay during data averaging. To overcome this disadvantage, a high-frequency pulse generator was design based on high voltage transistor switch in this paper. This pulse generator could output high voltage pulse with basis frequency up to 3 kHz and the pulse width in 5 to 10 nanoseconds. Moreover, a burst of 100 pulses can be issued on the basis frequency and the minimum pulse interval could be as low as 1 μs. Measurement with high-frequency pulse could reduce the time delay during data averaging effectively. Further more, a stimulation of the PEA system with extremely high-frequency pulses was accomplished indicating the huge influence of stray capacitance and inductance to the system.

Investigation of space charge at the interface between the insulation of cable and its accessory

According to the Maxwell-Wagner model, there is much amount of space charge at the interface between two different insulations. The more difference of ratios between dielectric constant and conductivity of both insulations, the higher amount of space charge emerges at the interface. And the higher amount of space charge stores at the interface, the more distortion of electrical field occurs. Therefore it is one of most important factors to considering the compatibility of dielectric constant and conductivity of cable and accessory insulations in polymer insulated HVDC power cable. There are many ways to evaluate the compatibility, such as calculating the ratios of dielectric constant and conductivity in both insulations at various temperatures. However, most of the ways are indirect and it is very difficult to satisfy the engineers designing cable accessory insulation. In the paper, composite samples of silicon and/ or ethylene-propylene-diene (EPDM) rubbers with LE4253 were prepared, respectively. Space charge at the interface between rubber and LE4253 were measured with pulse-acoustic method at various temperatures. It is found that character of space charge at the interface is the same in both composite samples. However, the amplitude of space charge at the interface between SIR/LE4253 is much greater than that at the interface between EPDM/LE4253 and it goes upwards quickly as the temperature arises. At the same time space charge at the interfaces in both composite samples moves distinctly towards both electrodes. In addition, the character of space charge in both kinds of rubbers varies at different vulcanizing temperatures. At last it is indicated that not only the space charge suppression of accessory insulation, but also the space charge at the interface in the composite sample should be considered as designing cable accessory.

Charge carrier transportation in the composite of Nano-MgO and cross-linking polyethylene

Space charge behavior of the composite of polyethylene and nano-MgO was investigated with pulsed electroacoustic method (PEA). It is found that space charge was well suppressed as the content of nano-MgO is less than 2.0%wt. Also it could be seen that bi-polar injection occurs in the composite as electrical stress increases to 60 kV/mm. Comparing to the amount of space charge in the short-circuited pure cross-linking polyethylene, it is very low in the composite. It is more interesting that semiconductive electrode has a great effect on space charge in both cross-linking polyethylene and the composite. In order to know space charge injection threshold, dc conduction was measured at various electrical stresses. And it is found that nano-MgO increases the threshold.

Computer simulation and analysis of electric and temperature fields of HVDC cables

HVDC cables play a more and more important role in interconnecting national grids and HVDC flexible power system. This paper deals with the calculation of electric and temperature fields in HVDC cables. The calculation of electric and temperature in an HVDC cables is far more complex than the equivalent case in AC cables. This is due to the fact that the conductivity of the cable insulation materials is temperature and field dependent, due to this fact that electric fields under dc voltage may be time-dependent. The field distribution in an HVDC cable may be of a capacitive or resistive nature. Electric field distribution is also influenced by temperature field. This even may lead to an instability that causes breakdown of the cable. The calculation and analysis of the temperature field distribution and its influence on the electric fields is carried out.

Effect of nano-filler grain size on space charge behavior in LDPE /MgO nanocomposite

The dielectric properties of nanocomposite can be affected by the concentration, surface modification and grain size of the nano-filler. This paper presents the pulsed electro-acoustic (PEA) measurement results of space charge distribution in pure low-density polyethylene (LDPE) and LDPE/MgO nanocomposite with different nano-filler size. The results show that a large amount of heterocharge accumulates near both electrodes in pure LDPE. After nano-MgO is adopted, the amount of heterocharge apparently decreases, and homocharge is observed and increases when the grain size of nano-MgO increases. Besides, under the suitable concentration of 1wt%, smaller MgO of 30 to 100nm has a greater effect on suppressing the space charge accumulation than that of 500nm. This is probably because nano-MgO filled in LDPE can reduce the potential barrier of interfaces between nanocomposite and electrodes effectively. Furthermore, the potential barrier decreases with the increasing of nano-MgO size.

Investigation of cable and its accessory insulation in extruded HVDC cable

Polymer insulated HVDC cable and its accessories are one of the most important equipments in flexible DC power transmission and space charge inhibition is always a problem to solve by science and engineering of electrical insulation. Since the 1980s, there are lots of papers on space charge in cables reported, and there are few papers on electric insulation of accessory. Most of researchers and engineers focus on how to suppress the space charge in cable insulation. However, there is much amount of space charge at the interface between cable insulation and accessory insulation considering there is a big difference of ratios between dielectric constant and conductivity of cable and accessory insulation. And this space charge might distort the local electric field near the interface. Based on the consideration above, both of silicon rubber (SIR) and ethylene-propylene-divinyl (EPDM) rubbers with the same nano-filler of aluminum (Al2O3) were investigated at various temperatures. It is found that the conductivity of SIR varies more sensitively than that of EPDM rubber. In order to know which rubber matches cable insulation better, three kinds of commercial cable insulation (A, B and C) have been taken into account. It is found that EPDM with nano-filler of alumina matches A the best.

High field conduction behavior of nanocomposite of low-density polyethylene and Ag nanoparticles

High field conduction of the composite of nano Ag/low-density polyethylene (LDPE) and pure polyethylene is studied, as well as the effect of the contents of nano Ag on high field conduction. Schottky effect, Poole-Frenkel effect, space-charge-limited conduction (SCLC), and ion hopping conduction are used to fit the results. It is pointed out that both space-charge-limited conduction and Poole-Frenkel effect do not fit well for all of the samples. However, it is found that Schottky effect fits well for nano Ag/LDPE composites, and the ion hoping distance of the nanocomposite calculated from the experience data shows that ion hopping conduction fits for the composite of nano Ag/LDPE.

Study on space charge behavior of oil-paper insulation under AC-DC combined stress

The space charge within oil-impregnated pressboard under AC and AC-DC combined stress were observed with a fast space charge detecting system based on the automatic equipartition phase shift (AEPS) principle. The phase resolution is about 1.79°, i. e. 201 equal division during whole period. Space charge accumulation is not distinct under AC stress at 15 kV/mm. The homocharge accumulates near ground electrode and the density decreases toward the top electrode. It may be associated with the charge injection from electrode. The amount of injected charge and the transfer distance are both depended on the electric field. Space charge accumulation is significant under AC-DC combined stress with the equal component at 15 kV/mm. The negative space charge distribution is dominant within oil-impregnated pressboard during whole period. It is mainly caused by the disruption of balance for electric field and polarization time under AC-DC stress. The polarity reversal, i.e. the voltage crossing 0 V for periodic stress, must be considered when the DC component exceeds a certain range. The space charge accumulated within oil-impregnated pressboard can distort the electric field distribution.