# Development and Experimental Research of VFTO Measuring Sensor

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## Abstract

**:**

## 1. Introduction

## 2. Design of VFTO Measuring Sensor

#### 2.1. Capacitive Voltage Divider

#### 2.2. Signal Conditioning Sensor

## 3. VFTO Measurement Results

## 4. Conclusions

- (1)
- The method, using the virtual break characteristic of operational amplification to obtain very large input impedance, is very effective in expanding the low-frequency measurement bandwidth of the impedance converter. Thus, the time constant of the equivalent circuit of the low-voltage port of the capacitor divider is greatly extended, and an extremely low cut-off frequency is obtained. In this research, the input impedance of the impedance transformation signal conditioning device was increased from 40 $\mathrm{G}\Omega $ to 250 $\mathrm{G}\Omega $, thereby reducing the low-frequency cut-off frequency from the original 10 mHz to 0.05 mHz. The VFTO measurement sensor can fully meet the measurement requirements of residual voltage components. Moreover, a differential circuit with a common mode disturbance rejection ratio of 53 dB is designed to effectively suppress the common mode disturbance signal caused by the transient ground potential difference in the measurement signal.
- (2)
- The transient surface electric field of the signal conditioning circuit board is simulated by COMSOL Multiphysics software. The simulation results show that the transient signal excites a large surface electric field around the wires of the circuit board. The surface electric field gradually attenuates from the center of the circuit board to the edge, which can reach 82.2 dB at the center. In the design of this signal conditioning circuit board, the smaller the wire width is, the greater the surface electric field will be excited. To avoid crosstalk between wires, the width of it should be at least greater than 10 mil. In addition, vias with too small an aperture will significantly stimulate the nearby surface electric field, so it is recommended that the aperture of vias should be at least greater than 5 mil. In addition, the amplitude frequency transfer characteristics between the input and output ports of the circuit board are also calibrated through a sweep-frequency experiment. The results show that the high cut-off frequency of the signal conditioning circuit board is about 95 MHz, which is close to the simulation results. The fluctuation in amplitude frequency transfer characteristics is less than 0.1 dB in the range of 1 MHz to 10 MHz. Therefore, it can be concluded that the signal conditioning circuit can meet the measurement requirements of fast transient components of VFTO signals.
- (3)
- Hundreds of disconnecting operation experiments are carried out on a 220 kV GIS test platform to verify the actual performance of the VFTO measuring sensor. The results show that the low-frequency performance of the new signal conditioning sensor is far better than that of the old impedance converter, and it can measure the trapped charge voltage component in VFTO well. The relative error of the measurement results is less than 2.5%, and the measurement accuracy is improved by at least 50% compared with the old sensor. The VFTO results measured by the old impedance converter are superimposed with high-frequency interference signals generated by internal circuit resonance. Although the signal is only about 500 mV at the output port of the sensor, it will cause a deviation of up to 100 kV in the measurement results after being converted into the transient overvoltage in the bus according to the voltage division ratio. After filtering to eliminate high-frequency resonance interference, we notice that the measurement results of the new and old sensors are almost identical. This shows that the new signal conditioning sensor can use the advantage of the differential circuit to effectively suppress the disturbance signal generated by internal circuit resonance, so as to obtain a more accurate measurement of VFTO amplitude. Therefore, the VFTO measurement result of the new sensor can be used to guide the insulation design of GIS primary equipment more accurately.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 3.**The Influence of frequency f and equivalent resistive load ${R}_{L}$ on the ratio of input voltage to output voltage. (

**a**) Equivalent resistive load ${R}_{L}$ changes from 1 Ω to 10 Ω; (

**b**) Equivalent resistive load ${R}_{L}$ changes from 10 Ω to 10 kΩ; (

**c**) Equivalent resistive load ${R}_{L}$ changes from 10 kΩ to 1 GΩ; (

**d**) Equivalent resistive load ${R}_{L}$ changes from 10 MΩ to 10 TΩ.

**Figure 4.**Schematic diagram of signal conditioning circuit. (

**a**) The new circuit; (

**b**) the old circuit.

**Figure 5.**The amplitude frequency characteristic and phase frequency characteristic of signal conditioning circuit.

**Figure 6.**Simulation results of surface electric field of signal conditioning circuit board under transient signal input. (

**a**) Surface electric field on the inner surface of signal conditioning sensor; (

**b**) Electric field on the surface of signal conditioning circuit board.

**Figure 7.**The response characteristics of signal conditioning circuit in the range of 1 MHz to 100 MHz simulated by COMSOL Multiphysics software.

**Figure 12.**The typical full-time waveform of VFTO measured by the new signal conditioning sensor and the old impedance converter during opening operation.

**Figure 13.**The typical full-time waveform of VFTO measured by the new signal conditioning sensor and the old impedance converter during closing operation.

**Figure 14.**The attenuation changes of trapped charge voltage within the occurrence time of two adjacent high-frequency oscillations.

**Figure 15.**Attenuation proportional coefficient of trapped charge voltage component in measurement results of the new signal conditioning sensor and the old impedance converter.

**Figure 16.**The typical micro pulse waveform of VFTO. (

**a**) The whole process waveform of micro pulse; (

**b**) the micro pulse waveform within one microsecond.

**Figure 18.**The simulation results of VFTO in time domain and frequency domain. (

**a**) Time-domain simulation results; (

**b**) Frequency-domain simulation results.

**Figure 19.**Time domain waveform of VFTO. (

**a**) VFTO time domain waveform with frequency lower than 70 MHz; (

**b**) VFTO time domain waveform with frequency higher than 70 MHz.

**Figure 20.**Frequency domain waveform of VFTO. (

**a**) Frequency domain waveform with frequency lower than 70 MHz; (

**b**) frequency domain waveform with frequency higher than 70 MHz.

Parameter | Type | Unit |
---|---|---|

High-voltage port capacitance | 0.0537 | pF |

Low-voltage port capacitance | 11.0031 | nF |

Electrode diameter | 40 | mm |

Hand-hole diameter | 50 | mm |

Hand-hole depth | 20 | mm |

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## Share and Cite

**MDPI and ACS Style**

Teng, Z.; Zhao, J.; Wang, Q.; Lu, H.; Zhang, J.
Development and Experimental Research of VFTO Measuring Sensor. *Sensors* **2023**, *23*, 264.
https://doi.org/10.3390/s23010264

**AMA Style**

Teng Z, Zhao J, Wang Q, Lu H, Zhang J.
Development and Experimental Research of VFTO Measuring Sensor. *Sensors*. 2023; 23(1):264.
https://doi.org/10.3390/s23010264

**Chicago/Turabian Style**

Teng, Zihan, Jun Zhao, Qi Wang, Haonan Lu, and Jiangong Zhang.
2023. "Development and Experimental Research of VFTO Measuring Sensor" *Sensors* 23, no. 1: 264.
https://doi.org/10.3390/s23010264