ABSTRACT: Along with uprating the voltage class of the switchgear the electromagnetic compatibility (EMC) between the high-voltage components and the low-voltage smart devices becomes increasingly prominent, and the breaking of the circuit breaker inside the medium-voltage switch cabinet is regarded as the source of the most severe electromagnetic interference, because it could lead to the failures in the secondary control devices, therefore it is significant to research the electromagnetic interference on secondary smart devices caused by breaking the switchgear. The oscillation mode of synthetic test circuit is used to perform breaking test of 10kV switch cabinet and the acquisition of the output current signal of CT and the input current signals of smart control devices and bay smart units are carried out simultaneously. The time-frequency analysis on the acquired signals is performed to obtain the frequency band and the energy distribution situation of the interference signals. Test results show that the breaking of the switchgear leads to the transient electromagnetic interferences on CT’s secondary side, smart control devices and bay smart units; the interference frequency band during the breaking and arcing process is mainly within the range from 15.625 MHz to 62.5 MHz; the maximum amplitude of the interference is four times as large as its normal value.

KEY WORDS: switch cabinet; electromagnetic interference; synthetic test; wavelet analysis; characteristic energy; frequency distribution

0Introduction

With the development of smart grids, the intelligentization of equipment has become the main direction of today's power grid development. The high level of secondary control and protection equipment has increased the sensitivity and vulnerability of secondary intelligent equipment to transient interference.The switchgear is an important switchgear for protection and control in the power distribution system. Its internal structure is complex. The secondary device is close to the high-voltage circuit breaker. It is more susceptible to transient high-frequency interference caused by high-voltage switch operation, and the low-voltage part is mutually Crosstalk can also occur between the two, which can affect the display and other functions. In severe cases, the switchgear will be mistaken, refused or even destroyed. Therefore, it is of practical significance to study the electromagnetic interference of the secondary intelligent device when the switchgear is opened.Foreign countries have conducted more research on the electromagnetic interference caused by the operation of switches (isolation switches, circuit breakers), focusing on high-voltage grade air insulated substation (AIS), gas insulated switchgear (GIS). Transient electric and magnetic fields generated by the switching operation are measured and analyzed;It is found that the rising edge of transient electromagnetic pulse generated by the operation of the isolating switch and circuit breaker in AIS and GIS can reach ns level, the maximum electric field strength in space exceeds 10 kV/m, the maximum magnetic field strength exceeds 200 A/m, and the dominant frequency of switching operation. At 0.5~120 MHz, the circuit breaker operates with a smaller transient amplitude and a higher dominant frequency than the isolating switch. Domestic research started late, with the focus on 500 kV substation. The maximum field strength of the isolated switch operation measured in the literature is about 19 kV/m, and the maximum field strength during circuit breaker operation is about 15 kV/m.

For the electromagnetic interference problem of medium voltage switchgear, foreign research focuses on re-ignition, but domestic research is less; the high-frequency current caused by the secondary side of CT on the re-ignition of 20 kV switchgear circuit breaker in [14-15] The measurement and analysis were carried out to obtain the high-frequency current frequency of 1~3 MHz caused by reignition; the electromagnetic interference signal caused by repeated heavy breakdown of the 40 kV switchgear circuit breaker was measured in [16], and the last time And the waveform of the second re-ignition of the last time is analyzed. The analysis shows that the frequency range of the interference current reaches 200 MHz, and the frequency range of the radiated electric field reaches 750 MHz. Although the arc reignition has a large interference to the secondary side, the circuit breaker is High-frequency interference will also occur to the secondary intelligent equipment during the arc-breaking process. At present, the electromagnetic interference research on the opening and closing of the switchgear is not directly measured by the secondary side interference current due to the harshness of the field test. The result is not Visually reflecting the strength of the interference, it is necessary to study the current interference in this case.

In this paper, the oscillating synthetic test loop is used to carry out the synthetic breaking test under different conditions for the electromagnetic interference problem of 10 kV switchgear, and the current output of the CT output terminal, the intelligent control device and the interval intelligent unit during the breaking process are collected; The time-frequency characteristics of the signal are analyzed, and the frequency band and energy distribution of the interference signal are obtained, which can provide reference for the improvement of the anti-electromagnetic interference performance of the switchgear.

1Test setup and method

1.1Test arrangement

The synthetic breaking test arrangement of the switchgear is shown in Figure 1. It mainly includes the current source and the voltage source. The two are used to generate high current and high voltage respectively. Before the switch cabinet TB is turned off, the current source is input to generate the power frequency current. The actual current of the line before the analog switchgear is opened; about 0.5 ms before the zero-crossing of the switchgear, the input voltage source generates a transient recovery voltage (TRV) applied to both ends of the fracture, and the line is generated when the analog switchgear cuts off the current. TRV; the test process satisfies the principle of equivalence of circuit breaker synthesis test [17]. Both ends of a and b are connected to the switch cabinet TB. Since only the single-phase breaking test can be carried out, the C phase of the switch cabinet is connected to the main circuit during the test. Figure 2 is a schematic diagram of the internal wiring of the switch cabinet, and the secondary side of the CT includes the protection side. And the measuring end, the measuring end is connected with the intelligent control device (monitoring the internal environment of the switchgear, such as temperature and humidity) to form a measuring current loop, and the protective side is connected with the interval intelligent unit (for line protection tripping)

CCB—closing circuit breaker, initial state is divided; ACB—auxiliary circuit breaker, initial state is combined; TB—switch cabinet, initial state is combined; Rog—Rogowski coil, measuring main loop current; R0—frequency modulation resistor; C0— FM capacitor (R0, C0 adjust the amplitude and frequency of TRV together); RCVDT - RC capacitor, measure the voltage across TB; SG - ignition ball gap.

Fig. 1  Experimental setup of synthetic breaking

The protection current loop, which is the core component of the secondary intelligent control part of the switchgear, uses the shunt to collect the input current of the two and the secondary side current signal of the CT. The acquisition points 1~4 are shown in Fig. 2.

The shunt is a non-inductive resistor with a resistance of about 0.1 Ω and an inductance of nH. The shunts with similar parameters are connected to the collection point shown in Figure 2. Since the resistance is small, it will not affect the normal operation of the CT. The shunt voltage signal is collected for the 500 MHz P6139B probe; to reduce the influence of the RF interference signal and the spurious parasitic signal on the measurement loop, and improve the accuracy of the measurement results, all the collected signals are passed through the fiber isolation sampling system into the DPO4054 four-channel 500 MHz. Bandwidth digital oscilloscope.

1.2Test parameters

The circuit breaker in the switchgear is a ZN63A-12 vacuum circuit breaker with a CT ratio of 400:1; the rated voltage of the switchgear is 12 kV, and the rated current is 2 kA;since this test mainly studies the electromagnetic interference when the switchgear is opened. It is not necessary to evaluate the breaking capacity of the circuit breaker, so it is only necessary to simulate the normal breaking rated load current; the specific parameters of the test setting are shown in Table 1.

1.3Test methods and procedures

1) The basic waveform under ambient noise.

In the case of no current, close CCB, disconnect ACB and TB, collect the current signal of CT protection side and measurement end, as the basic waveform under ambient noise, eliminate the mechanical collision of switch contacts and the interference caused by environmental noise.

2) Waveform acquisition under normal working conditions of the switchgear.

In order to eliminate the interference of the introduction of ACB to the test results, the waveform of the switch cabinet under normal working conditions is collected. After the normal operation of the switchgear is the input of the current source, the TB remains closed without any action, but in order to protect the current source capacitor and related equipment, the current needs to be interrupted as soon as possible, so the ACB breaks the current when the first half wave crosses zero. The current waveform is acquired under this condition as the basic waveform under normal operation.

3) Waveform acquisition in the open state of the switchgear. In order to observe the electromagnetic interference of the TRV on the secondary side separately, the simulation of the actual line current of the switchgear (ie, the switchgear is in the open state) is divided into two cases: 1 input current source according to the set parameters (CCB is closed, Disconnect ACB and TB), do not apply a voltage source for the breaking test, and 2 add a voltage source for the breaking test. At the same time, in order to compare the acquired waveform with the normal working condition, it is necessary to satisfy the principle of single variable, that is, in the case of breaking, the ACB is still set in the first half-wave action of the current, and the time advance switch cabinet TB 0.5 ms (the initial time of the ACB is consistent with the normal operation), and the two jointly break the current. Under these two conditions, a repeated test was performed to collect a current signal as a test waveform in the case of a switchgear opening. The above signals are collected by the fiber optic isolation acquisition system and the DPO4054 four-channel 500 MHz bandwidth digital oscilloscope. The oscilloscope is set to high resolution mode, and the points are collected and saved at the maximum storage depth (data volume is 106). The sampling frequency is 500 MHz. The data was processed using Matlab, and the measurement results of the current signals were all converted to the results under a 100 mΩ shunt.

2test results

2.1Main circuit test parameters

According to the parameters set in Table 1, the current and TRV of the main circuit are measured by a Rogowski coil and a RC capacitor. The main loop current peak is 2.1 kA; the transient recovery voltage TRV peak is 9.517 kV, and the recovery voltage rise rate is 0.24 kV/μs.

2.2Signal waveforms in different states

The signal waveform measured when the current is not flowing is shown in Figure 3. As can be seen from the figure, the waveforms collected have a large amplitude near zero time when there is no current. After many tests, it is found that CCB is closed. At the same time, similar waveforms will be collected (shown in Figure 4-6). The analysis found that this is mainly due to the noise interference caused by contact contact and jitter when CCB is closed. The interference is not eliminated in the interference signal test. , analysis can not be considered.

3Spectrum analysis of signal waveforms

3.1Wavelet analysis

Fast Fourier Transform (FFT) is only suitable for analyzing stationary signals, and can not perform time-frequency analysis and insensitivity to singularity. This paper uses wavelet transform to analyze signals. This method is a kind of window. A time-frequency localization analysis method in which the (area) size is fixed but the time window and the frequency window can be changed, that is, having a higher frequency resolution and a lower time resolution in the low frequency portion and a higher frequency portion in the high frequency portion. Time resolution and lower frequency resolution [18], so it can better compensate for the deficiencies of FFT, and is used for the detection and analysis of local transient mutation signals in the normal signal.

In wavelet analysis, the selection of wavelet basis is very important; the signal analyzed in this paper is a transient pulse signal, and the selected wavelet base has good sensitivity to transient signals, that is, it has better localization ability, so the wavelet base should be With tightness, regularity, and sufficiently high vanishing moments, and to reduce the frequency aliasing produced by the filter, the wavelet base should also have orthogonality. The Daubechies series wavelets have been proved to have good orthogonality, tight support, arbitrary order vanishing moments, and sensitivity to irregular signals [18-19], which can be well applied to the analysis and detection of the abrupt signal in the stationary state, considering wavelets comprehensively. According to the characteristics of db2~db10, through the actual effect analysis, the measurement signal of this paper is analyzed by db5 wavelet. Specific steps are as follows: The original signal is discretized to obtain a sampling sequence. If the original signal is recorded as A, the wavelet decomposition of the signal can be expressed as

Where: Aj is the low frequency part signal; Dj is the high frequency part of the signa

The frequency range corresponding to the nth layer detail coefficient is                  

The frequency range corresponding to the nth layer approximation coefficient is                 

The energy expressions defining the high frequency reconstruction sequence and the last low frequency reconstruction sequence are as follows:                                                

Where: dkj is the kth component of the j-th layer high-frequency wavelet reconstruction sequence, j is the decomposition layer number; a0k is the last layer of low-frequency wavelet after wavelet decomposition

Reconstruct the kth component of the sequence.

The energy of the wavelet decomposition coefficient sequences of the above layers are normalized to form a characteristic energy vector, and the characteristic energy is used to reflect the percentage of the total energy of the signals in different frequency bands [20].                                                      

3.2 The influence of the opening and closing of the switchgear on the current of the measuring terminal of the transformer

 The time domain waveform of the output current of the CT measuring terminal is shown in Fig. 4(a), 5(a), and 6(a). In the waveform, the area 1 (breaker arcing process) and zone 2 (arc zero crossing phase) A high frequency abrupt signal is generated, so the analysis focuses on the two parts. It can be seen from Fig. 4(a) and 4(c) that when ACB is operated under normal working conditions, there is no obvious interference component in the area, indicating that the ACB breaking arcing process has little effect on the secondary side current of the switchgear CT. It is mainly related to the long distance between the ACB and the CT. Therefore, increasing the distance between the secondary equipment and the breaker break can reduce the high-frequency interference. When the switchgear is opened, the area in Figure 5(a) and 6(a) appears. The obvious spike abrupt signal indicates that the interference of the arcing process is caused by the switchgear. In the arc zero-crossing stage, large interference jamming signals appear in Figures 4(a), 5(a), and 6(a). Figure 5(a) is more serious than Figure 4(a), indicating that the ACB breaking arc crosses zero. At the stage, high-frequency interference is generated in the secondary equipment of the switchgear. When the ACB and the switchgear are interrupted, the interference situation increases. The high-frequency interference in Figure 6(a) is more serious than that in Figure 5(a), indicating the TRV The input causes the high-frequency interference signal to increase significantly, which is consistent with the actual situation.

1) Analysis of electromagnetic interference during the arcing phase. In order to determine the frequency band distribution of the high-frequency interference signals in Fig. 4(a) and 5(a), wavelet denoising is first performed on the waveform of the region one, then the wavelet decomposition is performed using the wavelet function db5, and the characteristic energy of the reconstructed sequence is calculated. And the coefficient of variation, the results are shown in Table 2. It can be seen that the characteristic energy variation of the frequency band where d3 is located is the largest, which is 19.35, which corresponds to the maximum modulus value of the detail signal d3 after wavelet reconstruction, indicating that the interference frequency band caused by the breaking arc process of the switchgear is mainly 31.25~62.5 MHz. The characteristic energy of d4 and d5 is slightly increased, indicating that the frequency band of this part of the breaking process does not increase much; although d1 and d2 also increase, their increase is mainly caused by the randomness of environmental interference.

2) Analysis of electromagnetic interference in the arc zero crossing phase.

The wavelet function db5 is used to perform 6-layer wavelet decomposition on the region 2, and the characteristic energy is calculated. The results are shown in Table 3. Under normal operation and breaking without voltage source, the characteristic energy of d4 is the largest, indicating that the high frequency interference caused by arc zero crossing is mainly distributed in the frequency band of the detail coefficient d4 is 15.625~31.25 MHz, and the frequency band 31.25~62.5 MHz also has a large distribution; This is quite different from the result of Zone One, mainly related to the complex changes in the arc zero crossing. When the arc crosses zero, although the current peak is close to zero, there is still a large amount of post-arc current formed by electrons and ions between the arc gaps, and the arc voltage of the fracture is abruptly changed, so that the transient interference frequency band is widely distributed; after the voltage source is added, The energy of the frequency band of the detail signals d1 and d5 is obviously increased, indicating that the interference caused by TRV is mainly distributed in the frequency band: 125~250 MHz, 7.8125~ 15.625 MHz; this also reflects that the high frequency interference caused by TRV is serious, so the actual line needs to be TRV takes restrictive measures.

3.3 The influence of the opening and closing of the switchgear on the current of the protection terminal of the transformer

CT secondary side measurement end and protection end, the two coil structure and function are different, the protection side range is large, and its output current enters the interval intelligent unit; it is necessary to analyze the interference side of the protection side, analysis method and The foregoing is consistent. Wavelet decomposition is performed on the CT protection side signal, and the characteristic energy and variation coefficient are calculated. It is found that the interference caused by the opening and closing arcing process of the switchgear to the CT protection side is the same as that of the measurement end, and the signal frequency band is mainly distributed at 31.25~62.5 MHz; The high-frequency interference caused by the zero-crossing phase of the arc is mainly distributed in the frequency band 15.625~31.25 MHz; the band distribution caused by TRV is mainly concentrated in: 125~250, 7.812 5~ 15.625 MHz, which is consistent with the conclusion of the measuring end. Note that when the CT is not saturated, the switching cabinet is disconnected and the interference between the CT measuring end and the protective side is the same.
3.4 Influence of switchgear opening on input current of intelligent control unit

 The output current of the CT measuring end is input into the instrument room intelligent control device through the cable room. During the transmission of the cable, the current may be exposed to the radiation interference caused by the breaking arc of the switch cabinet and the crosstalk between the wires; In the case, the input current of the intelligent control device is analyzed. The input current time domain waveform is shown in Figure 4(b), 5(b), and 6(b). The FFT of the region 1 and region 2 signals in the waveform is calculated, and the power spectrum is calculated. It is found that the switch cabinet works differently. In the state, the input current of the intelligent control device is significantly reduced compared with the high-frequency interference of the output current of the CT measuring terminal, and the frequency change components are all concentrated below 100 MHz, which indicates the arc-breaking process of the switchgear and the radiation interference of the cable during the arc zero-crossing phase. The crosstalk between the wires is negligible. At the same time, since the line inductance effect has attenuating the high frequency signal during the conduction process [16], the high frequency component of the input intelligent control device is greatly reduced, and the waveform is improved.
In order to further determine the radiation interference of the intelligent control device after the TRV is added, the 6-wavelet decomposition is performed on the waveform of the region 2 in Fig. 6(a) and 6(b) using the wavelet function db5, and the characteristic energy and the coefficient of variation are calculated. Table 4 shows. The high-frequency variation coefficient in the table is far less than 1, indicating that the high-frequency component of the input current waveform of the intelligent control device is greatly reduced compared with the output current waveform of the CT measurement terminal, which further indicates that the radiation interference of the TRV on the intelligent control device is not large; The good shielding effect of the secondary instrument room and the rationality of the wire arrangement indicate that the high frequency interference is mainly the conduction coupling interference of the CT.

3.5 The effect of circuit breaker breaking on the input current of the interval intelligent unit

 The same method is used to analyze the input waveform of the interval intelligent unit, and the results similar to those of the intelligent control device are obtained. It shows that the switching process of the switchgear and the high-frequency interference generated by the TRV have little effect on the current conduction process of the protection side. Neglected; at the same time, the inductive effect of the cable also has a certain attenuation effect on high frequency components. It is further illustrated that increasing the distance between the breaker break and the secondary smart device can attenuate high frequency interference.

4 Interference mechanism and electromagnetic interference suppression measures

4.1 Analysis of interference mechanism

 The arcing process of the switchgear can be illustrated by Figure 7. When the breaker contacts are separated, an arc will be generated between the contacts. When the voltage across the breaker contacts is lower than the dielectric breakdown voltage, the arc is extinguished and the current is Cutting off, that is, only a single arcing phenomenon occurs; and when the voltage across the contacts of the circuit breaker is higher than the dielectric breakdown voltage, an arc is generated between the contacts (ie, re-ignition once), the circuit is re-conducted, and the capacitor C is discharged. High-frequency current, when the current crosses zero, the arc is extinguished, and the circuit breaker breaks overvoltage again. If the above process occurs repeatedly, there will be multiple re-ignition. In [14-16], the re-ignition phenomenon of the switchgear is analyzed. The high-frequency current of the reignition process has a large amplitude and the main frequency is low, about several kHz~several MHz; single arcing and Compared with repeated reignition, the interference current pulse frequency is relatively high, reaching tens of MHz to hundreds of MHz, but its amplitude is small and the duration is short, mainly concentrated near the current zero crossing.

Although the single-arc spark has a small amplitude of interference compared with multiple re-ignition, its frequency is very high, and it is likely to generate a high over-voltage on the inductive load, so the switchgear breaks a single arc. The interference situation should not be ignored. It can be seen from the above analysis that the electromagnetic interference of the secondary intelligent device is mainly caused by the conduction process during the breaking process of the switchgear, so the focus is on the analysis of the conducted interference mechanism.

Figure 8 is the equivalent circuit of high-frequency interference transmitted to the secondary circuit via CT [21], where: Ci, Li are current source capacitance and inductance; C1, C2 are ground-to-ground stray capacitance; Z1 is current source loop impedance CT is the transformer casing capacitor; CIN and C2N are the parasitic capacitance between the primary side, the secondary side and the Faraday shield of the transformer; Z2, ZL and ZD are the impedance, load impedance and connection of the secondary loop cable respectively. Ground impedance. The secondary loop interference current calculated according to the graph is

In steady state, i is the main circuit current of the current source, without obvious fluctuation, and basically does not generate high-frequency conduction interference; when the circuit breaker turns off the arc, due to the burning of the arc, a high-frequency interference current signal is generated, the current Can be determined by the arc voltage of the circuit breaker TB and its fracture equivalent capacitance Cu

The secondary circuit interference current i1 is directly related to the arc voltage of the circuit breaker fracture. In order to observe the change of the arc voltage during the arcing, the arc voltage waveform is measured, and the waveform is shown in Fig. 9. It can be seen that the arc voltage is basically stable during arcing, and there is no large fluctuation, and the arc voltage appears to fluctuate significantly near the arcing moment and the current zero point, especially near the zero point, the amplitude is greatly increased, and multiple transient processes occur. The transient process is bound to generate electromagnetic interference, and the high-frequency interference current is generated in the secondary circuit by CT coupling, which is consistent with the position of the high-frequency signal of the CT secondary circuit obtained by the test, indicating the transient of the arc voltage of the circuit breaker. The change process is the root cause of large interference when the current crosses zero; while the circuit breaker has a high TRV when the switch is zero-crossing, it is necessary to take corresponding measures to suppress it to reduce high-frequency interference. At the same time, the time domain waveform can be observed. In the arcing process of the circuit breaker, the position of the interference signal (Zone 1) changes slightly, which is mainly related to the dispersion of the inherent opening time of the circuit breaker. The circuit breaker is a longitudinal magnetic vacuum arc extinguishing mode, but the operating mechanism is a spring mechanism, thereby causing a large dispersion time, which causes a change in the position of the interference signal.

4.2 Electromagnetic interference suppression measures

It can be seen from the time domain waveform that the maximum amplitude of the high-frequency transient interference current caused by the opening of the switchgear is close to 4 times of the peak value of the normal operating current, especially after the introduction of the TRV, although the process time is short. However, the overvoltage generated in the long run will cause damage to the secondary control protection equipment. Therefore, it is necessary to take measures to suppress the high frequency interference; transient high frequency interference caused by the breaking current (load or fault) of the switchgear Some suppression, starting from the hardware system and the external environment to suppress high-frequency interference [22].

1) Shielded. Further optimize the shielding measures of the outer casing of the switchgear to reduce the interference of space radiation. In addition, in order to minimize the amplitude of the core common mode interference voltage and the differential mode interference voltage, the shielding layer of the shielded cable can be selected as a double-ended grounding method [3, 23].

2) Filtering. It mainly includes ferromagnetic ring, low-pass filter and decoupling circuit [22]. For conducted interference, a decoupling capacitor can be placed at the entrance of the secondary intelligent device to suppress high frequency interference, and a ferrite magnetic ring can also be connected in the secondary circuit. For the internal crosstalk of the controller, isolation measures such as isolation transformers and surge absorbers can be used on the controller hardware. Software filtering can be used in software, such as setting filter algorithm and watchdog circuit [11, 24].

3) The elimination method. It is a compensation method for suppressing electromagnetic interference. After detecting the interference current signal on the main circuit (busbar), after the transformation process, the cancellation signal with the amplitude of the interference signal and the opposite phase is injected into the secondary circuit of the CT to cancel An interference signal [25] coupled to the secondary loop by a transformer. 4) Add an overvoltage suppression device. It can be seen from the above analysis that after the introduction of TRV, the secondary side interference signal increases significantly. Therefore, in actual operation, appropriate measures must be taken to suppress the TRV of the circuit breaker breakage, such as installing a lightning arrester [26] and installing a shunt resistor at the fracture. Etc., this not only suppresses high-frequency interference, but also prevents re-ignition of the fracture and avoids causing greater high-frequency interference.

5 Conclusion 

1) The arc-breaking process of the switchgear and the arc zero-crossing phase all produce large electromagnetic interference to the transformer output current, and the maximum value of the interference current amplitude is close to 4 times of the normal condition.

2) At the moment of the circuit breaker and the arc zero-crossing phase, due to the transient change of the arc voltage of the fracture, the output current of the CT protection side and the measurement terminal are more obvious interference pulses. When the CT core is not saturated, two The interference situation is the same. The interference band distribution at the time of the splitting is mainly at 31.25~62.5 MHz, and the zero-crossing frequency band is mainly distributed at 15.625~31.25 MHz.

 3) TRV has a great influence on the current and protection current of the transformer. The amplitude of the interference current increases significantly. The high frequency component increases from 125 to 250 MHz. The increase of the frequency band of 7.812 5~15.625 MHz is also obvious, indicating the transient. The overvoltage has a great influence on the secondary intelligent equipment. In actual operation, it is necessary to take measures to suppress the overvoltage of the fracture.

4) The overall shielding effect of the control room of KYN28A-12 switchgear is better. The high-frequency interference generated by the breaking process on the intelligent control device and the interval intelligent unit is mainly the conduction coupling through CT, and the radiation interference of the main circuit affects it. Smaller.

5) The input current of the intelligent control device and the interval intelligent unit is reduced compared with the output current of the CT. This is mainly due to the inductance effect of the transmission cable, which causes the high-frequency signal to be attenuated during the propagation process. Also related to the reasonable layout of the cable.

 6) For the high-frequency electromagnetic interference caused by the opening of the switchgear, measures such as filtering, shielding and cancellation can be adopted to suppress the suppression effect. Further, the suppression effect needs further verification. At the same time, the electromagnetic interference characteristics of the switchgear under different types of fault conditions and the electromagnetic interference mechanism of the arc plasma itself need further research and analysis.

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  By: JUCRO Electric (Focus on Vacuum Interrupter, Vacuum circuit breaker, Vacuum contactor and Switchgear) 

  2018.11.7