A brief discussion on the feasibility study of applying wireless sensor networks to substations

Abstract: This paper introduces various interferences existing in substations and the direct sequence spread spectrum technology used in wireless sensor networks, and demonstrates the feasibility of applying wireless sensor networks in high electromagnetic interference environments such as substations.

0Introduction

At present, the automation of substation systems is becoming an irreversible trend, and the importance of its monitoring and communication systems is becoming increasingly prominent. Existing substation measurement and control systems mostly use wired communication, but the disadvantages of wired communication are obvious, such as the complexity of laying transmission lines, difficulty in inspection and maintenance, and the susceptibility of long-distance transmission lines to electromagnetic interference. Wireless communication has the advantages of reliable operation, flexible installation, and low cost, especially when real-time monitoring of substation information is required, wireless communication has great advantages.

The existing wireless communication methods mainly include IEEE802.11b/g, Bluetooth, ZigBee, GPRS/GSM, etc. ZigBee technology has become the preferred wireless communication technology in the real-time monitoring system of substations due to its many advantages such as high security, fast response time, low system resource occupation, low cost and low energy consumption. ZigBee technology is specially developed for wireless sensors. The application research of wireless sensor networks in substations is still in its infancy. Its research focus is mainly on distribution network automation and online monitoring of temperature and electric energy. However, the research on the impact of high-intensity electromagnetic environment of substations on wireless sensor network communication is relatively lacking. Therefore, this paper studies the interference of substations and the modulation technology of wireless sensor networks, and demonstrates the feasibility of the application of wireless sensor networks in substations.

1 Electromagnetic interference in substations

The substation has a complex electromagnetic environment, so various typical electromagnetic interference sources must be analyzed in detail. Typical electromagnetic interference sources in substations include: 50Hz power frequency electromagnetic field; pulse magnetic field caused by short circuit at equipment outlet; corona discharge; electrostatic discharge; partial discharge; air breakdown arc; SF6 gap breakdown arc; vacuum gap breakdown arc, etc. Among them, power frequency electromagnetic field and pulse magnetic field have little effect on wireless signals.

1.1 Electrostatic discharge and partial discharge

When two objects with different static potentials are in direct contact or induction of electrostatic field, the electrostatic charge between the two objects is transferred. When the energy of the electrostatic field reaches a certain level, the phenomenon of breaking through the medium between them and discharging is called electrostatic discharge. When the field strength generated by the applied voltage in the electrical equipment is sufficient to cause discharge in the insulation area, but no fixed discharge channel is formed in the discharge area, this discharge phenomenon is called partial discharge. Both are breakdowns with small insulation gaps and small energy discharges.

The radiation interference generated by these two types of discharges is within a few hundred kHz, with low energy and fast decay, so it will not affect wireless communication.

1.2 Corona discharge and air breakdown discharge

Power conductors may generate free discharge corona in the surrounding space under the action of high voltage and strong electric field. Mechanical damage on the surface of the conductor, polluted particles, or water droplets and dust near the conductor will cause the curvature of the conductor surface to change, so that the point gradient reaches the breakdown medium of the air medium. Therefore, the generation of corona is almost inevitable in the actual operation of the power system.

It can be seen from FIG1 that the radiation signal of the corona discharge is mainly concentrated in two envelopes near 78 MHZ and 180 MHZ, and the maximum signal strength is only -40 dBmW.

As shown in Figure 2, the electromagnetic field bandwidth generated by the breakdown of the air gap is relatively wide, mainly concentrated below 600 MHZ, and the intensity of the interference signal is very small, only -35dBmW even near the frequency of 580:MHZ.

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1.3 Switching operation interference

When the primary equipment such as circuit breakers and disconnectors in the substation are switched on or off or switch fault currents, due to the presence of inductive loads, the extinguishing and reigniting of the arc generated when the switch contacts are opened may cause attenuated oscillation waves containing multiple frequency components on the bus or line, radiating the energy of the transient electromagnetic field to the surrounding space through the bus or the connection between the equipment, forming a radiated pulse electromagnetic field. The equipment operation interference mainly includes the radiation signals generated by the breakdown of the SF6 gap and the breakdown of the vacuum gap.

As shown in Figure 3.4, the interference signals generated by SF6 gap breakdown discharge and vacuum gap breakdown discharge cover a wide frequency band, and the intensity of the electromagnetic signal is relatively strong in the entire frequency band. In the 2.4 GHz frequency band, the intensity of the electromagnetic signal is about -40 dBmW.

2 Spread Spectrum Technology of Wireless Sensor Networks

2.1 ZigBee Protocol

The framework of the ZigBee protocol for wireless sensor network applications is based on the IEEE802.15.4 standard, which defines the physical layer and media access layer of ZigBee. IEEE802.15.4 defines two physical layer standards, namely the 2.4GHz physical layer and the 868-15MHz physical layer. Both physical layers are based on direct sequence spread spectrum (DSSS) technology, mainly completing functions such as energy detection, link quality indication, channel selection, and data transmission and reception. The wireless sensor network outputs a 2.4GHz ISM band direct sequence spread spectrum signal with an output power greater than -17dBm and an operating frequency band of 2.405 to 2.480GHz.

2.2 Direct Sequence Spread Spectrum Technology

Spread spectrum is a process that uses random codes that are irrelevant to information to expand the modulated spectrum width to a much wider bandwidth than the original modulated signal through modulation. Commonly used spread spectrum technologies include frequency modulation, hybrid spread spectrum and direct sequence spread spectrum. Wireless sensor networks use direct sequence spread spectrum technology.

The direct sequence spread spectrum system uses a pseudo-random (PN) sequence with a high code rate to spread the signal spectrum at the transmitting end, and uses the same PN sequence to despread the signal at the receiving end to restore the original signal. [page]

3. Impact of Substation Interference on Sensor Networks

The electromagnetic interference of substations is mainly divided into two parts: the low-frequency part of 0~300MHz and the same-frequency bandwidth of 2.4~2.5GHz.

1) The frequency band of low-frequency interference generated by corona discharge and air breakdown is far away from the working frequency band of wireless sensor networks (2.4 GHz), and the intensity is less than -40 dBmW, which can be processed by a low-pass filter. Therefore, it has little effect on the wireless communication of wireless sensor networks.

2) The electromagnetic interference generated by the SF6 gap breakdown discharge and the vacuum gap breakdown discharge also has strong signals in the 2.405GHz~2.485GHz frequency band. When the gap breakdown voltage is about 15KV, the electromagnetic intensity reaches -40dBmV. The breakdown voltage at the substation site may be higher, and the electromagnetic intensity is also higher, so it will have a certain impact on wireless communication. However, the impact of co-frequency interference on wireless sensor network communication is very small, which can be explained in two aspects:

① Direct sequence spread spectrum technology used in wireless sensor networks. The anti-interference ability of direct sequence spread spectrum technology is due to the fact that the receiver multiplies the spread signal with the spread spectrum code again to restore the original signal. At the same time, the interference signal is also multiplied by the spread spectrum code at the receiving end to widen its frequency band. The interference signal energy is also dispersed over a very wide frequency band. In this way, there is only a small part of the interference signal energy in the 2.405GHz~2.485GHz frequency band. Therefore, the co-frequency noise has little interference with the wireless sensor network communication.

②SF6 gap breakdown discharge and vacuum gap breakdown discharge produce transient electromagnetic interference, which can only last for a short time, so the interference to the wireless sensor network is also instantaneous. When the transient electromagnetic interference ends, the wireless sensor network returns to normal.

In addition to electromagnetic interference, there is also multipath interference that cannot be ignored in the substation. Since a large number of metal equipment and columns in the substation are easy to reflect radio frequency signals, the signal received by the receiving end includes refracted or reflected signals from multiple different transmission paths, thus causing multipath interference. Multipath will cause signal fading, phase shift and decomposition, which will have a great impact on wireless systems that use signal energy as a judgment standard. However, direct sequence spread spectrum technology has great advantages in resisting multipath interference, which depends largely on the periodic correlation characteristics of the pseudo-random sequence used in spread spectrum communication. Because the random sequence has a sharp autocorrelation similar to white noise, despreading at the receiving end can effectively suppress the interference of multipath signals, thereby achieving the purpose of improving the signal-to-noise ratio and communication quality. The standard DSSS receiver automatically selects the waveform signal with the largest amplitude through a better correlator, and locks it with synchronization, thereby reducing multipath interference. Therefore, the direct sequence spread spectrum technology applied in wireless sensor networks can effectively suppress multipath interference.

4 Conclusion

The direct sequence spread spectrum technology used by wireless sensor networks can effectively suppress the high-intensity electromagnetic interference in substations, and also has a good inhibitory effect on the multipath interference that may be generated in substations. Therefore, the application of wireless sensors in substation automation is feasible and is also an inevitable trend in the development of power system monitoring systems.