Field testing and data analysis of impact impedance of transmission line tower composed of weathering angle steel

Weathering steel material has good corrosion resistance and mechanical properties. Compared with galvanized steel tower, weathering steel has longer operation life and lower operation and maintenance cost. With the pilot operation of weather resistant angle steel tower on the transmission line, its electrical performance has been widely concerned. The impact impedance of the tower is an important parameter to evaluate the lightning strike back performance of the tower. The rust layer formed on the surface of the weathering steel material will cause large lap resistance, which may increase the lightning strike back risk of the tower. In this paper, the impulse impedance test of a 220kV weathering angle steel tower in Fuyang, Anhui Province of China is carried out, and the circuit calculation of the test results is carried out to verify the rationality of the test data. The results show that: the zero frequency resistance of 220kV weathering angle steel tower is 1.027Ω, and the low frequency inductance is 311.94μH. The impact impedance results of the tower not only depend on the size, material and model of the tower, but also related to the impact waveform; the test results are in good agreement with the circuit simulation results.


Introduction
Hot dip galvanizing is often used for anti-corrosion of transmission line towers.Special procedures of rust removal, pickling and galvanizing are required for hot-dip galvanizing, which has large investment and heavy pollution.At the same time, galvanizing increases the manufacturing cost of iron tower.In the severe corrosion environment such as ocean, acid rain and industrial atmosphere, the service life of zinc coating is generally about 10 years, which is shorter than the service life of the tansmission line.In the process of line operation, it is still necessary to brush anti-corrosion paint and other means for anti-corrosion maintenance [1].Due to the high cost and the environmental pollution of galvanizing process, and the need for maintenance in the later stage of galvanized steel tower, the local corrosion of tower steel leads to very high safety risks, and consumes a lot of human and economic costs.Therefore, in order to improve the energy conservation and environmental protection of transmission lines, it is urgent to find new anti-corrosion materials to replace galvanized steel.On this premise, the weather resistant steel has been used as the transmission tower material.After the surface of weathering steel is corroded, a dense protective film will be formed on the surface to prevent the material from longitudinal deep corrosion, which makes it have excellent corrosion resistance.The atmospheric corrosion resistance of weathering steel is 2 ~ 8 times of that of ordinary carbon steel, which can avoid the current pickling and hot galvanizing processes.The weather resistant steel material not only improves the service life of the tower, but also reduces the operation and maintenance cost and industrial pollution [2].
The transmission tower is made of multi sections, and the rust layer on the surface of the weather resistant angle steel tower also causes higher contact resistance.The contact resistance may affect the impact impedance of the tower, thus affecting the lightning strike back level of the tower.At present, there is no specific research on impact impedance of weathering angle steel tower.For the impulse impedance measurement of tower, researchers have carried out a lot of experimental research.Jondan first regarded the tower as a vertical cylinder and proposed the first formula for calculating the wave impedance of the tower [3].Later, Breuer et al. and Kawai measured the impact impedance of the actual tower using the reflection method and the direct method respectively [4] [5].
In this paper, a 220kV weathering angle steel tower in Fuyang, Anhui Province of China is tested by using impulse voltage generator to simulate lightning stroke.The current injected into the tower and the potential rise data of tower body are measured respectively.Finally, the impulse impedance of the weathering angle steel tower is calculated, and the circuit calculation of the test results is carried out to verify the rationality of the test data.

Field test arrangement
The transmission line of a 220kV high-speed railway traction station in Fuyang, Anhui Province was tested.The ground wire and transmission line were not installed in the weather resistant angle steel tower.The tower model is 2E2-SZ2 tower, the total height of the tower is 43m, the nominal height is 27m, and the foot distance is 6.88m.The photos of the test site are shown in Fig. 1.The impulse voltage generator is used to transmit the voltage shock wave on site, and the lightning voltage waveform of 1.2/50 μs with the amplitude of 456kV is used in the test.The high voltage end of the punch generator is connected with the ground wire cross arm on the top of the tower through the injection line, and the four legs of the tower are connected with the low voltage end of the generator.All wiring is overhead insulated.At the same time, the insulating support rod is installed on the cross arm of the middle conductor of the tower, and the injection line is fixed away from the tower.The Current Transformer (CT) is used to measure the current injected into the tower at the connection between the injection line and the cross arm of the tower.At the same time, the HV Mete is used to connect the tower top and the tower foot to measure the potential signal of the tower body.In order to prevent the measurement signal from electromagnetic interference, all the measured data are transmitted to the computer terminal by optical fiber.The test circuit is shown in Fig. 2 and the tower body electric potential is shown in Fig. 3.

Test data results
The data of positive and negative polarity impulse voltage are collected in this test.When the peak value of positive polarity is 456kV impulse voltage, the tower voltage signal and current signal flowing through the tower are obtained, as shown in Fig. 4. The peak voltage drop of tower body is 5246V, and the peak current flowing into the tower is 61.9A.At the same time, the wavelength difference between the voltage waveform and the current waveform is large, and there is oscillation, which is caused by the reflection phenomenon of current wave in the tower.No matter how the polarity of impulse voltage is, the wavelength and pole of voltage waveform and current waveform do not correspond, so it is unreasonable to analyze in time domain.In this paper, the impact impedance of tower is analyzed in frequency domain.

Data Analysis
In this paper, Fourier transform is used to process voltage and current waveforms, and the results in frequency domain are obtained for calculation.The calculation formula is as follows: Where u t is the voltage waveform of the tower body and i t is the current waveform injected into the tower.
(2) Where R is the zero frequency resistance of the tower and L is the tower inductance.It can be seen from the above formula that the impulse impedance of the tower is related to the impulse waveform [6].At the same time, it is necessary to ensure that the data are complete waveforms when analyzing in frequency domain.According to the formula (1), Matlab is used to process the data.Due to the different data length of voltage and current, the frequency resolution is inconsistent.After FFT processing, the frequency of F i t is selected, which is consistent with the voltage frequency resolution.The final impedance Z data results at zero frequency and low frequency are shown in Table 1 According to formula (2), the zero frequency resistance R is 1.027 Ω and the low frequency inductance L is 311.94 μH.The simulation circuit takes the actual resistance, inductance and capacitance into account, and sets the impulse voltage generator according to the actual situation.Since the voltage measuring line needs to have the phenomenon of wave refraction and reflection, the current value is taken as the comparison object.As shown in Fig. 6, the de-noising actual current waveform and simulation current waveform are respectively shown, and the results are basically consistent.

Conclusion
Based on the field testing and data analysis of impact impedance of transmission line tower composed of weathering angle steel, the following conclusions are obtained: (1) The polarity of impulse voltage has little effect on the response of tower.
(2) For the weathering angle steel tower of 220kV transmission line, the zero frequency resistance R is 1.027 Ω, and the low frequency inductance L is 311.94 μH.According to the actual lightning current and tower grounding resistance setting, the lightning resistance level of weathering angle steel tower can be analyzed.It is hoped that the test results can provide data support and reference for lightning protection of weather resistant angle steel tower.
(a) Photos of UAV at test site (b) Panoramic photos of test site Fig.1 Photos of test site

Fig. 2
Fig. 2 Test loop Fig. 3 Tower body electric potential (a) Positive voltage waveform (b) Positive current waveform Fig. 4 Data under positive impulse voltage For the negative 456kV impulse voltage, the tower voltage signal and current signal flowing through the tower are obtained, as shown in Fig. 5.The negative impulse voltage makes the tower potential rise to 5186V, and the same voltage level and different polarity impulse voltage make the tower body voltage decrease the same.The amplitude of current waveform is 60.22A, ignoring the initial high frequency oscillation.The polarity of impulse voltage has little effect on the response of tower.(a) Negative voltage waveform (b) Negative current waveform Fig. 5 Data under negative impulse voltage (a) Test current waveform (b) Simulation current waveform Fig. 6 Comparison of test and simulation current waveforms