Discussion on Anti-interference of high frequency

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Discussion on Anti-interference of high-frequency protection in substation

Abstract: This paper systematically analyzes all kinds of electromagnetic interference existing in substations at present, and discusses the anti-interference measures that should be taken for high-frequency protection

key words: Discussion on Anti-interference of high-frequency protection

1 introduction

high frequency protection is line pilot protection with transmission line carrier channel as communication channel. At present, with the increase of capacitance and system voltage, various electromagnetic interference phenomena are more serious. As the transmission line is a part of the high-frequency channel, the voltage caused by the breaker operation, short-circuit fault and lightning stroke of the high-voltage system may interfere with the high-frequency transceiver, resulting in the malfunction of the high-frequency protection. Therefore, it is very important to understand all kinds of interference sources and take corresponding anti-interference measures

2 interference source

(1) operation of high voltage disconnector and circuit breaker. These operations may cause attenuated shock waves with multiple frequency components on the bus or line. The bus (or the connection between electrical equipment) is equivalent to an antenna, which radiates the energy of transient electromagnetic field to the surrounding space, and is directly coupled to the secondary circuit through the measuring equipment connected to the bus or line. The frequency of electromagnetic interference generated by circuit breaker operation is generally 0.1 ~ 80MHz, and the duration of each series of electromagnetic interference wave is 10 μ s~10ms。

from the theoretical analysis and measured data, the following laws can be obtained: ① the amplitude of transient electromagnetic field increases with the increase of voltage level, and the dominant frequency decreases with the increase of voltage level. ② Compared with disconnector operation, the amplitude of transient electromagnetic field caused by circuit breaker operation is small, the dominant frequency is high, and the total number of pulses is small. ③ Compared with the slow disconnector, the transient repetition frequency produced by the fast disconnector is lower and the duration is shorter. The slow disconnector may generate tens of thousands of pulses in one operation, while the fast disconnector only generates dozens of pulses

(2) lightning strike lines, structures and control buildings. When lightning strikes outdoor lines or structures directly, large current will flow into the grounding. When the shielding layer of the secondary cable is grounded at different grounding points, the transient current flowing through the shielding layer will be generated due to the existence of ground resistance, so as to induce interference voltage in the core line of the secondary cable, and the overvoltage induced by the line will also be introduced into the secondary circuit through the measuring equipment. The interference voltage generated in the secondary circuit of the lightning substation can be as high as 30KV, and its frequency can reach several megahertz

(3) system short circuit fault. The system short-circuit fault, like the lightning strike frame, will cause the rise of ground potential, thus causing interference voltage in the secondary cable. When the high-voltage bus in the substation is single-phase grounded, the interference voltage generated on the core line of the secondary cable can range from dozens of volts to nearly 10000 volts, and the frequency of the transient interference voltage is about kHz to hundreds of kHz

(4) close to the high-voltage line is affected by its power frequency electromagnetic field. This is very obvious for the electronic beam display equipment to produce electromagnetic interference. In outdoor substations, high-voltage lines or busbars will produce power frequency electromagnetic fields. Generally speaking, the higher the voltage level is, the greater the electric field is, but the magnetic field decreases on the contrary

(5) partial discharge produces electromagnetic radiation with high frequency, which may cause electromagnetic interference in the circuit of electronic equipment

(6) switch operation in the secondary circuit. Due to the existence of inductive load, fast transient pulse interference is generated at the signal power port and control port of the secondary circuit. Due to the extensive use of electromagnetic appliances, medium frequency oscillation transient voltage will be generated when the secondary circuit works

3 anti interference measures

(1) add series capacitors at the entrance of the channel. The principle of high-frequency locking protection is that when the signal on the opposite side of the line is received and the signal on the opposite side stops, a pair of contact signals for protection action are given by the "receiving output". In this process, the high-frequency signal has an interruption of about 5ms, which will be used as the criterion for outlet action. When a 220kV line belonging to Guangzhou Baiyun power supply bureau had an out of area fault, the protection malfunction was caused by interruption due to interference. In order to prevent similar situations, 0.1 should be connected in series in the cable core at the entrance of the channel μ F capacitor can effectively play an anti interruption role and eliminate the discharge tube in the output of the line filter of Ybx series transceiver

(2) the device is reliably grounded. Since the grounding of the substation is not the actual equipotential surface, there will be potential differences between different points. When a large grounding current is injected into the grounding, there may be a large potential difference between the points. If the same connected circuit is grounded at different points of the substation at the same time, the ground potential difference will jump into the connected grounding circuit, causing undue ground diversion. In some cases, the earth voltage that does not exist in the primary system may also be introduced into the detection circuit of the relay protection device, or the fund protection device that they have financed in China may refuse to operate or misoperate in the process of failure due to shunt. Therefore, for the microcomputer protection device, the protection screen must be reliably grounded, and the high-frequency protection should also be equipped with grounding copper bar or copper stranded wire (wire diameter not less than 100mm2) according to the requirements of the Ministry, To ensure the reliable judgment of the device in case of failure

(3) limit the influence of Overvoltage on the device. In order to prevent overvoltage caused by lightning strike, a suitable capacitor can be connected in parallel at the entrance of the channel. Because the capacitor has the property that the voltage at both ends cannot change suddenly, when the overvoltage caused by electrostatic induction occurs, the shunt capacitor must be charged first. It shows the infinite possibility of artificial intelligence in various industries in the future... With the charging process, the secondary side voltage will slowly rise. Because the electrostatic induction overvoltage generally occurs for a short time, the overvoltage has disappeared before the voltage at both ends of the shunt capacitor (i.e. the secondary side voltage) rises high enough, which can greatly limit the voltage's infringement on the high-frequency transceiver

(4) when the signal stops at the high-frequency position, add a manual relay to delay the closing contact. When the no-load line is manually switched on frequently, due to the distributed capacitance of the line, a large capacitive current will be generated, which sometimes reaches the starting value of the high-frequency protection. At this time, it will cause the high-frequency protection to malfunction, causing the line to fail to close the circuit breaker. In order to prevent this kind of phenomenon, the circuit breaker protection device at the power transmission side can delay the position stop signal slightly, so that the position stop signal can be delayed and stopped. Therefore, a pair of normally closed contacts (time-lapse disconnection, instantaneous closure) of the manual relay should be connected into the position stop signal circuit of the device, and the device should be locked for high-frequency protection

(5) in the phase to phase coupling mode, the high-frequency differential connection must be reliably grounded

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4 conclusion

in addition to the above situations, in order to ensure the normal and reliable operation of high-frequency transceiver, we must also pay attention to several points in the on-site commissioning: ① the test equipment must be zeroed and the gear must be selected correctly. ② The adjustable rheostat in the commissioning plug-in should use a non inductive tool. ③ When the channel is adjusted, the transceiver value is accurately recorded for many times. ④ The power amplifier tube of the component in the debugging power amplifier plug-in cannot work under the overload state. ⑤ The impedance of the high-frequency cable is matched with that of the transceiver. (end)

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