# On-Line Partial Discharge Monitoring System for Switchegears Based on the Detection of UHF Signals
## 3 Hardware Circuit Design Scheme
### Detection Circuit Design  
As shown in Figure 4, the detection module is mainly composed of an ultra-high frequency sensor, a preamplifier circuit, a logarithmic detection circuit, and a signal follower circuit. The core of the detection circuit is the logarithmic detection circuit. The logarithmic detector is cascaded by several demodulating logarithmic amplifiers. The amplitude of the input signal and the output signal is logarithmic, which can help us compress the dynamic range and make the RF signal into a baseband signal [17; 18]. Due to the low amplitude of the UHF signals, it is difficult to obtain them by direct measurement. Therefore, it is necessary to design a preamplifier prior to the logarithmic detector circuit. The preamplifier is designed using the AD8099 ultra-low distortion high-speed operational amplifier from Analog Devices. Figure 5 shows the simulation of the amplifier circuit using NI Multisim 14.0 software. In Figure 5, the UHF signal is fed into the preamplification circuit from the 'Input' node. This circuit is a typical gain circuit with gain G = 1 + RL1/RG1. A set of capacitors is connected to the pos  
Figure 4: Frameworks of the circuit. Figure 3: Antenna sensor physical diagram. of the power supply at the same time; the large capacitor is 10 uF, and the small capacitor is 0.1 uF, which can filter out the noise caused by the power supply. The logarithmic detection circuit is a logarithmic processor, which is designed around the Analog Devices AD8313 log amplifier demodulation chip. The function of the logarithmic detection circuit is to extract the envelope waveform of the UHF signal, which will greatly reduce the sampling rate of the system. The working frequency of AD8313 is 0.1 to 3.5 GHz. It can maintain its high-precision consistency for signals in the frequency range of 0.1 to 2.5 GHz, and it can convert the radio frequency signal into the corresponding amplitude DC output. Figure 6 is a Multisim simulation diagram based on the AD8313 logarithmic amplifier circuit. As the frequency of the UHF signal generated by PD is between 300 MHz and 1500 MHz, we need to filter out the invalid signal below 300 MHz. The relationship between cut-off frequency and capacitance resistance is shown in Equation (2). \[f_{3 dB}=\frac{1}{2\times\pi\times C\times\mathrm{R}} \tag{2}\]  
Since the impedance value adopted by the system is 50 \(\Omega\), where R = 50 \(\Omega\), the value of capacitance \(C\) is determined by \(C_{1}\) and \(C_{2}\). The relationship between \(C\), \(C_{1}\), and \(C_{2}\) is given by Equation (3). \[C=\frac{C_{1}\times C_{2}}{C_{1}+C_{2}} \tag{3}\]  
Figure 5: Preamplifier simulation circuit. Figure 6: Logarithmic amplification simulation circuit. It can be seen from Equations (2) and (3) that the invalid electromagnetic wave signal below 300 MHz can be filtered out by using a 20 pF capacitor for \(C_{1}\) and \(C_{2}\). Since the output impedance of the logarithmic amplifier circuit is large and the input impedance of the data collector is small, if the data collector is directly used to collect the input signal, the loss of the input signal will be serious and the data collector cannot collect the normal signal, so a voltage follower circuit needs to be designed. Like the preamplifier, the AD8099 is used to design the voltage follower. From the gain equation 1 + RL1/RG1, the gain G = 1 when RL1 is at 0 resistance. In order to validate the detection circuit, this paper uses the double exponentially decaying oscillatory signal expressed in Equation (1) to simulate the UHF signals generated by partial discharges in switchgear cabinets. In this paper, four groups of UHF signals with different parameters are simulated as partial discharge signals of the switchgear cabinet, and the outputs are shown in Figure 7 after conditioning of the detector circuits, in which the red waveforms indicate the original signals without signal conditioning, and the black waveforms indicate the conditioned signals.