【ShiShuo Design】Come and see how to design the best front end for UHF partial discharge online monitoring system~

When partial discharge occurs, a signal with a wide frequency range is generated, so there are 4 partial discharge detection technologies for different frequency ranges. Ultrasonic detection technology is for the frequency range of 20 kHz to ~200 kHz, high-frequency current transformer (HFCT) detection technology is for the frequency range of 3 MHz to ~30 MHz, transient earth voltage (TEV) detection technology is for the frequency range of 3 MHz to ~100 MHz, and ultra-high frequency (UHF) detection technology is for the frequency range of 300 MHz to ~1500 MHz. UHF detection technology has high detection sensitivity and is widely used in partial discharge online monitoring systems for gas insulated switchgear (GIS), transformers and ring main units (RMUs).

The simulated PD signal time domain waveform is shown in Figure 1, and the frequency domain waveform is shown in Figure 2. According to Figure 2, the PD signal with the highest energy is in the frequency range below 1 GHz. When the pulse rise time is less than 300 ps, ​​more energy is distributed in the higher frequency range.

In the modern complex electromagnetic environment, there are many wireless interference signals between UHF PDs with operating frequencies between 300 MHz and 1500 MHz. In order to eliminate this interference, customers generally choose sub-bands between 300 MHz and 1.5 GHz to capture PD pulses. Under normal circumstances, the wireless communication signal of GSM around 900 MHz will be the largest interference signal. One way to solve this problem is to use a band-stop filter (BRF) to suppress signals from 800 MHz to 1000 MHz. The typical sub-band division scheme is shown in Table 1. Of course, the sub-band division is flexible, and customers can adjust it according to the actual electromagnetic environment.

Table 1. Typical UHF PD sub-band division scheme

According to the sub-band division in Table 1, we only retain the corresponding energy spectrum components of the PD signal spectrum shown in Figure 2, and then perform an inverse fast Fourier transform (IFFT) to study what the time domain waveform will look like after the corresponding filtering. The filtered time domain waveform is shown in Figure 3. According to Figure 3, after filtering, the PD pulse peak value will decrease. After filtering, the PD pulse rise time will increase and the fall time will decrease. After filtering, among all waveforms, the full frequency band has the largest peak, followed by the band stop band and the low pass band. The high pass band has the smallest peak, but the PD pulse can still be captured.

The first stage of this front end is the RF gain block ADL5611. The ADL5611 has a low noise figure (NF) of 2.1 dB and a high P1dB of 21 dBm, which provides a high dynamic range. The ADL5611 has a gain of 22 dB and its gain is extremely flat within the UHF PD operating frequency of 300 MHz to 1500 MHz, with a gain ripple of less than 0.4 dB. All these features make the ADL5611 very suitable for UHF PD detection applications.

The second stage is an LC-based 300 MHz to 1500 MHz bandpass filter (BPF) that provides out-of-band interference rejection.

The third stage uses two single-pole four-throw (SP4T) RF switches HMC7992 to implement the band selection circuit. The first RF path is a DC to 800 MHz low-pass path, the second RF path is a 1 GHz high-pass path, the third path is a band-stop path from 800 MHz to 1 GHz, and the fourth path is a straight-through path. Depending on the different RF path selections, customers can choose different RF bands to capture PD pulses in a band with no interference or minimal interference. The HMC7992 has a low insertion loss of 0.6 dB, a high isolation of 45 dB, and a high P0.1dB of 33 dBm.

Stage 4 is a 300 MHz to 1500 MHz BPF, the same BPF used in stage 2, which further provides out-of-band interference suppression.

The last stage is the RF logarithmic detector ADL5513, which converts the UHF PD signal into a low-frequency signal of tens of MHz. Therefore, an ADC with a sampling rate of 40 MSPS or 65 MSPS can be used to convert the analog PD signal into a digital signal. For PD detection applications, the main characteristics of the RF detector required are response time and dynamic range. The ADL5513 has a response time as low as 20 ns and a dynamic range as high as 80 dB, so it is very suitable for PD detection applications. The RF logarithmic detector AD8318 is also suitable for PD detection applications. Compared with the ADL5513, it has a faster response time but a slightly smaller dynamic range.

Figure 6 shows the S parameters of the through path from the first-stage input to the input port of the last-stage ADL5513. The figure shows that the gain is about 14 dB, the gain flatness is better than 2 dB, and the input return loss is better than –8 dB over the full frequency band from 300 MHz to 1500 MHz.

Figure 7 shows the response curve of the output voltage to the power of the input CW signal at the center frequency of the PD at 900MHz. Two channels were measured using input power. According to the test results, the entire signal chain has a linear response in the input power range of –75 dBm to –5 dBm. The performance consistency between channels is also very good.

Figure 8 shows the output waveform measured when a 900 MHz continuous wave signal pulse is input. The signal power is –75 dBm, the pulse width is 5 μs, and the pulse period is 10 μs. According to the waveform, when the signal power is as low as –75 dBm, the signal-to-noise ratio of the output signal is still considerable.

This article shows how to use the ADI signal chain to build a UHF PD detection board. This complete reference design allows users to flexibly select different frequency bands to eliminate interference in complex electromagnetic environments. It also meets the requirements of the Chinese Q/GDW11059.8-2013 standard.

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