Anti-interference Technology of Multi-type Data Acquisition and Transmission Based on HPLC

With the development of the social economy, electricity consumption increases rapidly. A large amount of real-time electricity consumption data plays an important role in analyzing the power grid business. Monitoring power consumption performance through electricity consumption data is also an important indicator to improve power grid performance. By analyzing the characteristics of HPLC channels, such as capacitance characteristics, frequency loss characteristics, signal-to-noise ratio characteristics, line icing, and noise interference, the critical values of data transmission distortion caused by the characteristics of each channel were summarized. On this basis, it is concluded that suitable filters and amplifiers should be selected to improve the signal-to-noise ratio and the transmission quality of HPLC signals. Therefore, an anti-interference technology for data transmission based on HPLC is proposed. Finally, the application advantages and existing problems of HPLC technology in three-stream integration data acquisition and transmission are further explored.


Introduction
The integration of three flows in the power system, that is, the simultaneous transmission of power flow, information flow, and business flow in a power line, is the inevitable trend of the power system towards intelligence, digitization, and informatization [1].Traditionally, power flow is transmitted through transformers, switches, and other equipment, while information flow and business flow are transmitted through dedicated communication equipment.With the development of power line carrier communication technology, power lines can also transmit information flow and business flow at the same time, to achieve the goal of three-in-one.
As a high-speed power line communication technology, broadband power line carrier (HPLC) technology uses power lines as communication media for data transmission and communication [2].As the power line is the most popular and the most extensive coverage of physical media, the use of power lines to transmit data information has great convenience.Without rewiring, all electrical appliances connected to the power line can form a communication network for information interaction and communication [3].This method is simple to implement and easy to maintain.It can effectively reduce operating costs and the expenditure on building new communication networks.It has become the main communication method for smart grids, energy management, smart home, photovoltaic power generation, electric vehicle charging, and other applications [4] [5] [6].
Aiming at this problem, considering the influence of various external factors, this paper analyzes the reliability index of data transmission from the two directions of channel characteristics and transmission loss characteristics, simulates the high-frequency acquisition system of electricity information based on Compared with the traditional communication transmission channel, the HPLC channel has the characteristics of high complexity and strong time variability when the HPLC channel is used for information transmission.In the transmission reliability verification, the channel is analyzed from the characteristics of capacitance, frequency loss, signal-to-noise ratio, and power system noise interference to verify the reliability of HPLC channel information transmission.

Capacitance characteristics
The combination of HPLC and the combined filter forms a high-pass filter, which plays an important role in eliminating the interference noise in the HPLC transmission process.It is necessary to add a coupling capacitor before the combined filter.For different HPLC transmission lines, the loaded capacitance values are different, as shown in Table 2.

Circuit
Permittance (pF) 110 kV 10, 000 220 kV 5, 000, 10, 000 330 kV 5, 000, 7, 500 500 kV 5, 000, 7, 500 On different HPLC lines, different capacitance values are loaded, resulting in different degrees of line loss.In the signal coupling circuit of HPLC communication, a capacitive voltage transformer needs to be introduced to protect the equipment and strengthen the communication ability.The composition of HPLC analog channel communication equipment mainly includes two substations, a wave trapper, coupling capacitor, combined filter, carrier wave machine, and transmission line.The HPLC channel can enable the transmission of electric energy, data acquisition and transmission, and communication between the two substations.The transmission characteristics of the line can be monitored by the coupling capacitor, combined filter, carrier, etc., which reflects the reliability and timeliness of the line data transmission.The larger the coupling capacitance value is, the smaller the impedance is, and the lower the data transmission reliability becomes.

Frequency loss characteristics
The frequency loss of the HPLC transmission channel refers to the loss value of the transmission line.The loss value of some lines is expressed by A and its unit is dB, which is expressed by Equation ( 1): 1 where L denotes line length (km); f denotes frequency;  value is related to the voltage level of the HPLC line.The  value of the voltage level is shown in Table 3.

Signal-to-noise ratio
The signal-to-noise ratio (S/N) is an important indicator of the reliability of transverse transmission, and its value is equal to the difference between the useful signal level and the noise level.The HPLC transmission noise level is determined by the transmission bandwidth, as shown in Equation (3): where B 1 represents the transmission bandwidth of the band; Bf2 represents the transmission bandwidth of the 2 band.
In the HPLC communication channel, if the frequency band of the signal transceiver filter is wider, the noise level is greater, resulting in a lower signal-to-noise ratio, which is not conducive to signal transmission.The data transmission rate is related to the transmission bandwidth and signal-to-noise ratio.The transmission rate is different under different bandwidth and signal-to-noise ratio conditions.In severe weather conditions, S/N ≥ 30 dB should be used to minimize data transmission interference.

Noise interference characteristics of the power system
Power system noise interference refers to the interference characteristics of the power line, mainly including corona, switch closure, circuit breaker, and insulator discharge.When the bandwidth is 4 kHz, the reference value of the corona is shown in Table 4 [11].The longer closing time of the switch and the circuit breaker will have a serious impact on the transmission.The discharge of the insulator is divided into porcelain bottle discharge and gap discharge.It has the characteristics of large interference intensity and long duration.It is essential for line transmission and sometimes cannot be eliminated.

Anti-interference technology of data transmission based on HPLC
The effects of HPLC capacitance characteristics, frequency loss characteristics, signal-to-noise ratio, and power system noise interference on the channel are analyzed.In terms of capacitance characteristics, due to the capacitance characteristics of the power line, high-frequency signals will be transmitted in the power line, while low-frequency signals will propagate in the air around the power line.This feature can be used to improve the reliability and stability of HPLC signal transmission.In terms of frequency loss characteristics, the higher the frequency is, the greater the loss of the signal in the transmission is.Therefore, it is necessary to select the appropriate frequency range to ensure that the signal can be transmitted in the power line.In terms of signal-to-noise ratio, power line communication will be affected by power system noise, which will affect the transmission quality of HPLC signals.Therefore, it is necessary to select the appropriate filter and amplifier to improve the signal-to-noise ratio of the signal, thereby improving the transmission quality of the HPLC signal.In terms of power system noise interference, power line communication is also affected by power system noise interference, which will affect the transmission reliability of HPLC signals.To solve this problem, OFDM technology can be used to improve the transmission reliability of HPLC signals [12].
OFDM system transmits data information by using multiple overlapping and orthogonal subcarriers.Because of the outstanding anti-interference ability of the OFDM system and the advantages of highspeed data transmission, it has been widely used in the field of HPLC application [13].
The number of OFDM subcarriers is represented by N, x (n) is the data of X (k) after IDFT, and its Equation ( 4) is: where 0 ≤ n ≤ N-1.
To avoid the inter-symbol interference (ISI) of OFDM, the cyclic prefix (CP) is usually inserted into the time domain signal x (n), and the signal obtained by parallel/serial processing is transmitted through the PLC channel.The received signal y (n) is (5): ⊗ ℎ    5 where   denotes the channel impulse response in the time domain;   denotes channel noise; ⊗ represents convolution.

Conclusion
This paper mainly introduces the research on data acquisition and transmission reliability of three-inone in-power line carrier communication based on HPLC technology.Firstly, the principle and advantages of HPLC technology and its application in power line carrier communication are introduced, including improving data transmission rate and reliability, reducing noise interference, and so on.Then, the HPLC channel is analyzed from the characteristics of capacitance, frequency loss, signal-to-noise ratio, and power system noise interference, and the reliability of information transmission is verified.Finally, the advantages and challenges of HPLC technology in practical applications were discussed, and future research directions were proposed.Further research is needed as follows: the characteristics and influence mechanism of various interference sources in power line carrier communication systems to design more reliable channel and anti-interference algorithm; new HPLC acquisition and processing technology to improve the acquisition and processing accuracy; the application scenarios and scalability of HPLC technology to meet the development needs of different applications and power systems; the integration of HPLC technology and other communication technologies to achieve more intelligent power system management and control.The three-in-one data acquisition and transmission based on HPLC technology is an effective power line carrier communication scheme, which has an important practical application value.

Table 4 .
Corona reference values under different voltage levels and weather conditions

Table 3 .
values corresponding to different lines  represents the line terminal attenuation value 5.7 dB frequently.The line loss calculated by Equation (2-2) is the total loss value of the transmission.The K value of HPLC transmission lines with different voltage levels is different, resulting in different losses.Under the same transmission distance, the transmission loss of HPLC with a high voltage level is relatively small and the transmission reliability is high.
where A represents the partial loss value of the line, which can be obtained by Equation (2);  represents the number of bridge combinations (i.e., high-frequency resistors, coupling capacitors, combined filters and cables on both sides of the substation bus);  represents the number of intermediate line carrier and line branches;  represents the number of branch line carrier machines;  represents the cable attenuation coefficient (available cable description);