Discharge Characteristics of Needle Plate Electrode in Transformer Oil Under Low-frequency AC Voltage

Low-frequency transformer is the key equipment of flexible low-frequency AC transmission systems for offshore wind power. As the low-frequency voltage is applied, the transformer has significantly different insulation characteristics from those of the conventional transformer, it is necessary to study the typical characteristics of partial discharge of oil-paper insulation under such voltage. In this dissertation, the characteristics of discharge of needle plate electrodes in transformer oil under power frequency to low-frequency voltage are experimentally researched by adopting the pulse current method, and the discharge statistics, discharge atlas as well as feature parameters are analysed. With the decreasing of voltage frequency, the inception voltage (IV) value of partial discharge slightly increases, the extinction voltage (EV) phases of positive half-cycle (PHC) voltage waveform, as well as negative half-cycle (NHC) voltage waveform appear earlier, and the maximum discharge (MD), average discharge (AD), and pulse repetition rate (PRR) all show a downward trend.


Introduction
With the operation of 10 MW and above offshore wind turbines, the traditional AC and DC connection options are gradually constrained by the performance of power electronic devices, making it difficult to ensure transmission power [1,2].Fractional frequency transmission can significantly increase the AC transmission power limit by reducing the frequency and reactance of the system.This means that the output voltage of direct-drive wind turbines is boosted through the low-frequency transformer and then transmitted to an offshore booster station or via cable to land [3].This provides a competitive scheme for long-distance and high-capacity offshore wind power transmission.
For the high-voltage transmission with low-frequency, the low-frequency transformer is one of the key equipment [4].Although its major insulation still adopts the oil-paper one, the applied voltage with low frequency will result in different insulation characteristics from those of the conventional transformer.This not only affects the insulation coordination principle but also brings challenges for health assessment and fault diagnosis during the transformer operation.Thus, it is of extreme importance to study the characteristics of discharge in transformer insulation dielectric under lowfrequency voltage.
In [5], the partial discharge models are designed to study typical characteristics of discharge in transformer oil-paper insulation dielectric under AC/DC composite voltage, including needle plate, air gap, edge plate, ball plate, and inter-turn models.In [6], the effect of the ratio of AC/DC composite voltage on the inception voltage (IV) of partial discharge in oil-paper insulation is studied, and it is found that only the IV under a uniform electric field is not affected by the DC component.In [7], the effects of voltage polarity and amplitude on the partial discharge characteristics of oil-paper surface models in slightly non-uniform electric fields under composite voltage are studied, and the variation of partial discharge statistic characteristics with time and voltage amplitude is analysed.At present, the harmonic frequency is regarded as a significant factor influencing the partial discharge of transformer dielectric.In [8], it is found that the breakdown voltage of transformer insulation decreases when frequency increases.In [9], the characteristic parameters of partial discharge in transformer oil-paper insulation dielectric under fundamental frequency voltage superimposed on harmonic voltage with different frequencies, proportions, and phases are analysed.In [10], the partial discharge experiments on the air gap defect model of oil-paper insulation under DC voltage superposed with 6th, 12th, and 18th harmonic voltage.It is found that under high-frequency voltage space charges are easy to accumulate, and the number of partial discharges increases.
In summary, there is currently no research on the characteristics of insulation dielectric discharge of oil-paper systems under low-frequency voltage.In this dissertation, the typical characteristics of discharge of needle plate electrodes in transformer oil under power frequency to low-frequency voltage are experimentally researched by adopting the pulse current method, and the discharge statistics, discharge atlas as well as feature parameters are analysed.

Experimental model and circuit
The needle plate model is used to form the extremely non-uniform electric field, large electric field strength is prone to occur near the tip electrode, which is beneficial for studying and analyzing the mechanism process and influencing factors of streamer and electrical tree in transformer oil.This model is adopted to simulate the typical point discharge in the low-frequency transformer, and the model structure and the experiment model are shown in Figure 1 and Figure 2, respectively.The model consists of a carbon steel needle with a 50 μm curvature radius of the needle tip and a circular plate electrode with a 60 mm diameter, and then the needle tip is set 3 mm to the flat electrode.The experiment circuit is designed as shown in Figure 3, and some equipment is shown in Figure 4.
In Figure 1-Figure 4, the power supply provides voltage with large amplitude to the metal electrode in the transparent acrylic cylinder via the protective resistor.Both the needle tip and circular plate electrodes are immersed in the transformer oil dielectric, and the circular plate electrode is grounded by connecting to the metal disc at the bottom of the cylinder.After applying voltage with a large amplitude, the electrode will produce partial discharge measured by the pulse current method.When the voltage frequency decreases, the maximum discharge (MD) in PHC and NHC shows an overall downward trend.The MD in PHC decreases from 12028.16 pC at 50 Hz to 9715.23 pC at 17Hz, which is down by 19.2%;The MD in NHC decreased from 331.14 pC to 80.13 pC, which is down by 75.7%.The MD of some frequency bands has slightly increased, but it is still significantly lower than that of the fundamental frequency.The average discharge (AD) in PHC and NHC also shows a downward trend.The AD of PHC decreased from 4945.69 pC at 50 Hz to 3091.11 pC at 17 Hz with a decrease of 21.1%;The AD of NHC decreased from 47.68 pC to 27.07 pC with a decrease of 51.61%.The AD of some frequency bands has slightly increased, but it is still significantly lower than that of the fundamental frequency.The pulse repetition rate (PRR) in PHC and NHC shows a decreasing trend.The PRR of PHC decreased from 6.25 times/s at 50 Hz to 4 times/s at 17 Hz with a decrease of 36.68%.The PRR of NHC decreased from 13.11 times/s to 6.84 times/s with a decrease of 47.8%.The PRR of some frequency bands has slightly increased, but it is still significantly lower than that of the fundamental frequency.(a) 50 Hz (b) 17 Hz Figure 6.PRPD at 50 Hz and 17 Hz The q mean -p and n-p spectra at different frequencies are shown in Figure 7 and Figure 8. Figure 7 shows that the shape of PHC and NHC in the q mean -p spectrum at each frequency is similar, which has the lower rectangle superimposed on the upper triangle.As the frequency decreases, the proportion of the rectangle part increases but the triangle part tends to blur.In addition, the vertical distribution range (VDR) of PHC decreases from 0-12 nC at 50 Hz to 0-7.5 nC at 17 Hz, the VDR of NHC decreases from 0-80 pC to 0-50 pC, and the horizontal distribution range (HDR) of NHC reduces from 200 °-300 ° to 200 °-260 °.
As shown in Figure 8, the shape of PHC and NHC in the n-p spectrum remains consistent at each frequency, presenting a triangular pattern.As the frequency decreases, the triangle tends to blur.Moreover, the VDR of PHC decreases from 0-15 times at 50 Hz to 0-10 times at 17 Hz, the VDR of NHC decreases from 0-20 times to 0-10 times, and the HDR of NHC reduces from 200 °-300 ° to 200

Conclusion
In this paper, the characteristics of the typical discharge of the needle plate electrode in the transformer oil dielectric under fundamental frequency to low-frequency voltage are experimentally researched by adopting the pulse current method.The following conclusions are drawn from the results of the research study presented in this paper.
1) Some typical characteristics of needle plate electrode discharge at fundamental frequency and low-frequency voltage are similar, that is, the discharge of PHC is significantly larger than that of NHC, and the discharge distributes with peak characteristics near the voltage peak.As the frequency decreases, the peak characteristic trends to blur.
2) For the discharge statistics analysis, when the voltage frequency decreases, the value of IV increases slightly, the phases of EV in PHC and NHC advance gradually, and the MD, AD, and PRR all show a downward trend.
3) For the discharge spectra, when the voltage frequency decreases, the shapes remain consistent, but the distribution ranges will change.As the frequency decreases from 50 Hz to 17 Hz, for the q meanp spectrum, the VDRs of PHC and NHC decrease by 21.1% and 43.2%, respectively, and the HDR of NHC decreases by 40%.For the n-p spectrum, the VDRs of PHC and NHC decrease by 33.3% and 75%, respectively, and the HDR of NHC decreases by 30%.

Figure 5 .
Figure 5. Discharge statistic analysis2.3.Discharge spectra and feature parametersThe phase-resolved partial discharge pattern (PRPD) at 50 Hz and 17 Hz is shown in Figure6.The discharge of PHC is significantly larger than that of NHC, and the discharge of both PHC and NHC happens near the voltage peak, but the discharge distribution of PHC at 50 Hz has a more obvious peak characteristic.As the frequency decreases, the MD of PHC decreases from 12 nC at 50 Hz to 6 nC at 17 Hz.