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1.INTRODUCTIONThe insulation status of cable intermediate joint directly affects the stable operation of power grid cable system. In order to find the change of insulation status of cable intermediate joint in time, it is urgent to study a kind of online detection communication technology suitable for cable intermediate joint. The cable joint temperature online detection system is established by using ZigBee and GPRS wireless communication technology1. These methods are susceptible to the influence of the terrain environment, and the reliability needs to be improved. The monitoring and diagnosis system of underground 22.9 kV XLPE cable is built by using optical fiber sensor and single-mode optical fiber2. This method has strong anti-electromagnetic interference ability, but its application cost is high and its construction is difficult. Some researchers proposed a wireless monitoring system for medium voltage cable joints using LoRa low-power outdoor communication technology3. Similarly, some researchers have proposed a wireless sensor network based on NB-IoT and LoRa technology to detect the insulation status of the distribution network4. This technology has low power consumption, but it is not mature enough to achieve full coverage at present. Considering the particularity of the working environment of the cable intermediate joint, the PLC based on the cable body is considered to be a more reliable data communication scheme at present. Many researchers have studied the PLC and used the OFDM technology to build the communication system model5-6, but in the modeling process, they mainly focus on modulation and demodulation, channel estimation, etc. There is less research on communication coupling problem in the practical application process. By analyzing the characteristics of power line channel and the coupling principle of inductive coupler, this paper adopts inductive coupling to realize the transmission of high-frequency carrier signal. The PLC system model between cable intermediate joints is built based on OFDM. Considering the influence of inductive couplers at the transmitting end and receiving end, the relationship between different SNR and BER curve is simulated and analyzed. The simulation results show that the PLC system can realize the effective transmission of cable joint state detection data, and the PLC system with coupler can effectively reduce the BER of the system by changing the encoding method and modulation method. 2.CHANNEL CHARACTERISTICS AND COUPLING PRINCIPLE2.1Power line channel characteristicsWhen the medium-voltage (MV) power line transmits the high-frequency carrier signal between the cable intermediate joints, the channel characteristics are mainly divided into impedance characteristics, attenuation characteristics, and noise characteristics. Given the terminal voltage and current of the transmission line, taking the terminal of the transmission line as the starting point for calculating the distance, the voltage and current at the distance x from the terminal of the transmission line are: where U2 and I2 are the voltage and current at the end of the transmission line. U and I are the voltage and current from terminal x. γ is the transmission constant of the line, and Zc is the characteristic impedance of the line: where R, L, C, G are the distribution parameters of cable per unit length. α represents the attenuation constant, and β represents the phase constant. According to a large number of measured data7, the α can be approximated as: where a0, a1, k are constants. According to the attenuation model of unit length power line, the attenuation of a section of transmission line with length L can be expressed as: Since the MV power line is not directly connected with the user, the noise of the MV power line mainly comes from the relevant equipment of the distribution network and wireless interference. The overall noise of the system increases with the increase of frequency. The change of terrain and climate environment will also cause the change of noise around the cable to varying degrees, mainly including background noise and burst noise. 2.2Coupling principle analysisThe carrier equipment needs to couple the carrier signal to the overhead line or cable line through the coupling equipment. Capacitive coupler is mostly used for overhead lines, while an inductive coupler is mostly used for cable lines. Clamp inductive coupling equipment is a coupling method based on the principle of electromagnetic induction, which can realize data communication under the condition of an uninterrupted power supply. Compared with the injection inductive coupler, it is more conducive to realize the effective transmission of cable intermediate joint state detection data. The coupler is clamped on the cable line, and the three-phase cable core and shielding layer of the cable passes through the coupler magnetic ring. There is an air gap at the magnetic core interface, which is equivalent to a loosely coupled transformer. The coupler needs to be equivalent to a mutual inductance model for analysis. The mutual inductance M in Rogowski coil is adopted and corrected considering the influence of air gap8-9. The corrected expression of mutual inductance M, self-inductance L1of primary coil, and self-inductance L2of secondary coil can be expressed as: where l1 is the length of the magnetic core loop, a is the inner radius of the coupler, b is the outer radius of the coupler, h is the height of the coupler, μfe is the permeability of the magnetic ring, μ0 is the vacuum permeability, δ is the length of a single air gap, N1is the number of coil turns on the primary side, N2 is the number of coil turns on the secondary side, L0 is the distributed inductance per unit length of the cable, and x is the length of the cable, l = l1 + 2δ. 3.PLC SYSTEM MODELDue to the limited length of a single cable, a line usually needs multiple cable joints to extend the power supply length. The conventional data communication method has certain limitations, using the transmission line as the carrier signal transmission medium is expected to meet the practical application requirements. The structure diagram of the PLC system between cable intermediate joints is shown in Figure 1. The carrier equipment at the transmitting end modulates and amplifies the status data of the cable intermediate joint, and couples the high-frequency carrier signal to the cable shielding layer and the three-phase cable core through the high-frequency cable connection clamp inductive coupler. At the receiving end, the coupling reception is carried out through the principle of electromagnetic induction, connected with the high-frequency cable, and transmitted to the carrier equipment for demodulation processing, so as to obtain the status data of the cable intermediate joint at the transmitting end. The cable shielding layer outside the inductive coupler is grounded to form a loop. At present, the carrier equipment mostly adopts the multi-carrier modulation technology based on OFDM. In order to ensure the effective transmission of state data between cable intermediate joints, the simulation modeling based on OFDM modulation and demodulation technology is considered, and the inductive couplers at the transmitting end and receiving end are modeled to build the simulation model of the PLC system between the cable intermediate joints. 3.1PLC system model based on OFDMThe block diagram of the PLC system between cable intermediate joints based on OFDM is shown in Figure 2. At the transmitting end, the carrier equipment receives the state data of the cable intermediate joint to be transmitted, takes the 44-bit binary data as a frame, and expands the frame length through coding. After channel mapping and serial-parallel conversion, the PN training sequence is introduced to transform the data by IFFT. Then set the guard interval, add the cyclic prefix and parallel-serial conversion, convert the discrete digital signal into continuous analog signal through DA conversion, and transmit it to the power line through the coupling protection circuit after up-conversion. At the receiving end, after coupling protection circuit, down-conversion and AD conversion, the signal is changed back to the discrete digital signal, the cyclic prefix is removed, the subcarrier is demodulated by FFT, channel estimation and channel compensation are carried out to overcome signal distortion, and then the state data of cable intermediate joint at the transmitting end is obtained after de-mapping and decoding. The simulation model of the PLC system designed in this paper will not include the process of signal conversion and up-down conversion, and the discrete complex signals will be transmitted in the channel. After parallel-serial conversion at the transmitting end, the real and imaginary part waveforms of the signal are shown in Figure 3. 3.2Inductive coupler modelThe inductive coupler is the main coupling equipment for the carrier signal access to the cable line of MV distribution network. The coupling performance will directly affect the data transmission efficiency of the whole system. The mutual inductance of the inductive coupler is related to the internal and external radius of the coupler, the number of winding turns, the width of the air gap, and the magnetic permeability of the core material. In order to briefly analyze the mutual inductance model of the inductive coupler, the resistance of the primary and secondary coils is ignored. Suppose the selfinductance of primary coil and secondary coil is L1 and L2 respectively, mutual inductance between coils is M, carrier equipment is equivalent to a voltage source Us and internal impedance Rs, and load impedance is RL. By de-coupling, it can be further equivalent to a type T equivalent circuit. Type T equivalent circuit of inductive coupler at the transmitting end, as shown in Figure 4. According to Kirchhoff’s voltage law: After Laplace transform simplification, we get: The type T equivalent circuit of the inductive coupler at the receiving end is similar to that at the transmitting end, which can also be simplified by Laplace transform: Because the discrete digital signal needs to be transmitted in the simulation channel, the continuous data needs to be discretized by transforming from S domain to Z domain. The C2d function is provided in MATLAB, which can realize the discretization of the model and make the data transfer discrete complex signals through the coupler. 3.3Power line channel modelPower line channel modeling mainly includes the transmission line model, the multipath model, the autoregressive model, and the FIR filter model. The transmission line model needs to calculate the path length, attenuation amplitude and time delay by using cable parameters and load impedance, so as to determine the transfer function of a specific network. Due to the limitation of experimental conditions, the calculated results cannot be compared with the actual measurement results; The multipath model is mostly used in the bus-organization of MV distribution network, which reflects the multipath reflection attenuation caused by a large number of branch lines, and is not suitable for the point-to-point data communication between cable intermediate joints studied in this paper; The autoregressive model needs to measure a large number of amplitude frequency response data for statistical analysis, needs measured data and has a large amount of calculation workload. Due to the limitations of laboratory conditions choose to use the FIR filter model for modeling. Xing10 considers that the power line channel is a limited impulse response channel. By comparing the measured frequency response curve of MV power line channel with that of FIR filter model, it can be seen that the 120 order FIR filter model can accurately reflect the attenuation characteristics of the channel. Therefore, based on the data10, this paper establishes the FIR filter model of power line channel attenuation, and uses additive Gaussian white noise to simulate the noise of power line channel. 4.SIMULATION AND RESULT ANALYSISIn order to verify that the PLC technology can realize the effective communication of cable intermediate joint detection data, the PLC system between cable intermediate joints based on OFDM is built by using SIMULINK platform. Considering the influence of inductive couplers at the transmitting end and receiving end, the relationship between SNR and BER curve is analyzed. The simulation adopts M =10.38 × 10–6 H, L1 = 31.14 × 10–6 H, L2 = 3.46 × 10–6 H, Rs = 75Ω, and RL = 50Ω. 4.1Channel encodingChannel encoding is a common error control method in the communication system. RS encoding and convolutional encoding are respectively added to the simulation of the PLC system between cable intermediate joints. When QPSK modulation is adopted, the relationship between SNR and BER curve of the system under different channel coding modes is simulated and analyzed. The code type of RS encoding is (15, 11). The convolutional encoding adopts encoding efficiency of 1/2, constraint length is 7, generated polynomial is [171, 133], and Viterbi hard decision decoding method is adopted for decoding. The influence curve of channel encoding mode on system BER is shown in Figure 5. The simulation results show that when the SNR is low, the encoding mode has little influence on the reliability of the PLC system, and the BER of convolutional encoding is higher than that of RS encoding. With the increase of SNR, the BER of the PLC system with RS encoding and convolutional encoding decreases to zero faster than that without encoding. Under the same SNR, convolutional encoding has a stronger anti-interference ability than RS. It can be seen that adding channel encoding to the PLC system between cable intermediate joints can better overcome the time-varying attenuation characteristics of medium voltage power lines and improve the communication stability of the system. 4.2Channel mappingWhen RS coding condition is selected for channel coding mode, the relationship between SNR and BER curve of the system under QPSK, BPSK, and 16-QAM mapping is simulated and analyzed. The influence curve of channel mapping mode on system BER is shown in Figure 6. The simulation results show that different modulation modes have little effect on the BER of the PLC system at low SNR. With the increase of SNR, the BER curve of BPSK decreases faster. When the SNR is 18dB, the BER of BPSK decreases to 10-4. When the SNR is 23dB, the BER of QPSK decreases to 10-4. The BER curve of 16-QAM decreases slowly, and the BER can be reduced to 10-4 only when it is about 27dB. BPSK modulation has the best BER performance, but the spectrum efficiency is low and the data transmission rate is slow. Choosing different modulation modes in the PLC system can meet the different communication needs of the system. In practical application, it is necessary to select appropriate modulation modes according to the system to improve the communication performance of the system. Assuming that the communication coupling problem is not considered, the BER of QPSK, BPSK is close to 10-4 at about 14dB, and the BER of 16-QAM is close to 10-4 at about 20dB. Considering the communication coupling problem, the BER of each modulation mode is significantly increased, and a higher SNR system is required to achieve a lower BER. Therefore, the modeling of the inductive coupler is an essential step in building the PLC system model between cable intermediate joints. The existence of a coupler makes the system have a certain coupling attenuation, which will affect the BER of the PLC system. 5.CONCLUSIONIn this paper, the PLC system model between cable intermediate joints is built based on OFDM. Considering the influence of inductive couplers at the transmitting end and receiving end, the relationship between different SNR and BER curve is simulated and analyzed. The simulation results show that:
The PLC system with coupler can reduce the BER and improve the transmission performance of the system by changing the encoding mode and modulation mode, which provides a new idea for the effective transmission of online detection data of cable intermediate joint. ACKNOWLEDGMENTSThis work is supported by the Science and Technology Project of China Southern Power Grid Company (No.030600KK52210023). REFERENCESWenzhi, C., Xiaohui, H., Zhendong, G. and Chengrong, L.,
“The design of temperature monitoring system for power cable joint,”
IEEE Int. Conf. on Condition Monitoring & Diagnosis, 671
–6
(2012). https://doi.org/10.1109/CMD.2012.6416235 Google Scholar
Park, S.-W., Yeo, C. I., Kim, H. J., Kang, H. S., Seo, I. -J. and Park, H. -J.,
“Fiber optic sensor network for a monitoring and diagnosis system of the underground 22.9K XLPE cable,”
Optical Fiber Communications Conf. and Exhibition (OFC), 1
–3
(2021). Google Scholar
Voigt, S., Lee, C.-K., Joung, J.-M., Kurth, S. and Roscher, F.,
“Flexible multi sensor monitoring system for medium voltage cable joints,”
Smart Systems Integration (SSI), 1
–4
(2021). Google Scholar
Zhuang, T., Ren, M., Gao, X., Dong, M., Huang, W. and Zhang, C.,
“Insulation condition monitoring in distribution power grid via IoT-based sensing network,”
IEEE Transactions on Power Delivery, 34
(4), 1706
–14
(2019). https://doi.org/10.1109/TPWRD.61 Google Scholar
Ma, Z., Gholamzadeh, A., Tang, B., Dang, S. and Yang, S.,
“MATLAB based simulation of the efficiency of the complex OFDM on power line communication technology,”
Fourth Int. Conf. on Instrumentation & Measurement, Computer, Communication & Control, 374
–8
(2014). Google Scholar
Khusnutdinov, E. R. and Potapchuk, N. K.,
“Modeling in a narrow-band communication system of a power transmission line multiplexing method with orthogonal frequency division of channels,”
in Int. Conf. on Electrotechnical Complexes & Systems (ICOECS),
142
–4
(2021). Google Scholar
Zimmermann, M. and Dostert, K.,
“A multipath model for the powerline channel,”
IEEE Transactions on Communications, 50
(4), 553
–9
(2002). https://doi.org/10.1109/26.996069 Google Scholar
Ferković, L., Ilić, D. and Leniček, I.,
“Influence of axial inclination of the primary conductor on mutual inductance of a precise Rogowski coil,”
IEEE Transactions on Instrumentation & Measurement, 64
(11), 3045
–54
(2015). https://doi.org/10.1109/TIM.2015.2444254 Google Scholar
Ata, K., Ashtiani, S. J. and Akmal, A. S.,
“A wideband, sensitive current sensor employing transimpedance amplifier as interface to Rogowski coil,”
Sensors & Actuators A Physical, 256 43
–50
(2017). https://doi.org/10.1016/j.sna.2017.01.018 Google Scholar
Xing, Z. M.,
“Research and Application on the Medium Voltage Broadband Power Line Communication on the Basis of OFDM,”
North China Electric Power University, Hebei, Master’s Thesis,
(2006). Google Scholar
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