**3.2. A fiber Fabry-Perot interferometer-based acoustic sensor**

Another novel acoustic sensor based on the fiber Fabry-Perot interferometer is demonstrated. This sensor is specifically designed to be workable in the transformer oil to monitor the partial discharges occurred inside the power transformer.

### *3.2.1. System configuration and operation principles*

The scheme of the proposed sensor system is presented in **Figure 9**, in which a CW DFB laser diode without the optical isolator was employed as the light source, as well as an in-line optical amplifier. The detection signals are obtained from a photodetector packed in laser module. A ferrule-type sensor head is constituted by inserting a single-mode fiber with a flat-cut end into a ceramic ferrule and leaving a 1-mm long space for forming an extrinsic cavity, as shown in **Figure 9**. When the sensor head is immersed into the transformer oil, the oil will permeate into the ceramic ferrule and stop before the fiber end, which forms an extrinsic cavity with two low-reflectance reflectors, the fiber end and the oil surface. The refractive index of transformer oil is estimated in the range 1.40–1.48, close to that of the fiber core, varying with the oil type and quality as well as with the oil temperature. According to the analyses in Section 2.4, the output current signal out can be expressed as

**Figure 9.** Schematic of a fiber Fabry-Perot interferometer-based acoustic sensor and structure of sensor head.

$$I\_{\rm out} \propto 2G P\_0 R \left(1 + \cos \phi\right) \tag{16}$$

where is the gain of laser diode when it is taken as an in-line optical amplifier; 0 is the optical power launched into the sensor; is an average reflectance of two reflectors; = 4FP/λ is the round-trip phase difference; and FP denotes the cavity length. When the partial discharges arise inside the power transformer, ultrasonic waves induced by discharges will propagate in the transformer oil in all directions. The energy of these induced ultrasonic waves mainly distributes in a frequency range of 50–150 kHz [14]. When some of ultrasonic waves arrive at the sensor, the acoustic pressures will modulate the phase difference by changing the oil level inside the ferrule, in turn, the cavity length FP, which, as a result, causes the signal out to change its amplitude. So that by detecting out, the partial discharges occurred in the power transformer can be monitored.

### *3.2.2. Experimental results*

experimental setup is illustrated in **Figure 7**, in which the fiber coil was placed between two

In the experiment, the AC voltage imposed on the electrodes was increased gradually and the detection signals were recorded at several specified voltages. **Figure 8(a)** shows three groups of pulse train waveforms, recorded at 1800 V, 2800 V, and 3800 V, respectively. By observing these data, it is obvious that the number of pulses or pulse density within a cycle of AC voltage

**Figure 8.** (a) Measured ultrasonic waves generated by air ionization, and (b) relationship between RMS output voltage

**Figure 8(b)** shows a set of RMS (root-mean-square) voltages measured in a cycle of AC voltage under different electric field strengths from 175 to 475 V/cm. It is obvious that the RMS voltage

Another novel acoustic sensor based on the fiber Fabry-Perot interferometer is demonstrated. This sensor is specifically designed to be workable in the transformer oil to monitor the partial

The scheme of the proposed sensor system is presented in **Figure 9**, in which a CW DFB laser diode without the optical isolator was employed as the light source, as well as an in-line optical amplifier. The detection signals are obtained from a photodetector packed in laser module. A ferrule-type sensor head is constituted by inserting a single-mode fiber with a flat-cut end into a ceramic ferrule and leaving a 1-mm long space for forming an extrinsic cavity, as shown in **Figure 9**. When the sensor head is immersed into the transformer oil, the oil will permeate into the ceramic ferrule and stop before the fiber end, which forms an extrinsic cavity with two low-reflectance reflectors, the fiber end and the oil surface. The refractive index of transformer oil is estimated in the range 1.40–1.48, close to that of the fiber core, varying with the oil type

increases proportionally with the applied electric field strength.

**3.2. A fiber Fabry-Perot interferometer-based acoustic sensor**

discharges occurred inside the power transformer.

*3.2.1. System configuration and operation principles*

parallel, copper-plate electrodes separated by 10 cm.

increases with the imposed AC voltage.

154 Optical Interferometry

and applied electric field strength.

**Figure 10** shows a set of the spectra of the sensor, measured when the sensor was placed in the air as well as was immersed into the transformer oil at different depths (>10 cm). From these results, clearly, the spectrum of the sensor at each depth is of a periodical change characteristic, and the spectral interval between adjacent dips becomes wider as the increase of immersion depth. It indicates the fact that the oil entering into the ceramic ferrule and the fiber end virtually had formed an extrinsic cavity of the fiber Fabry-Perot interferometer.

We experimentally investigated the sensor sensitivity with a setup shown in **Figure 11**, in which the sensor head was placed on a Bakelite plate table and inserted into a silicon oil droplet, here as acoustic wave couplant and also as optical reflection medium. In the experiment, a tiny, 2-cm long metal pin freely falling down from 2-cm height hit the table to generate the weak Lamb waves which propagated in the Bakelite plate and finally arrived at the sensor head in a 45-cm distance. A series of wave signals detected by our sensor is clearly shown in **Figure 12**, which demonstrates that the proposed sensor has a very high sensitivity in detection of weak Lamb waves propagating in the Bakelite plate.

**Figure 10.** Sensor output spectra measured in air (a) and in transformer oil (b–d) at different depths.

**Figure 11.** Schematic of an experimental setup for evaluating of sensor sensitivity.

An experiment was carried out for testing the sensor performances in detections of the partial discharges occurred in the transformer oil. **Figure 13** shows a set of photos on this experimental setup. One shown in **Figure 13(a)** is a photo of the hand-made sensor prototype. As shown in **Figure 13(b)**, the sensor was inserted into an 850-mm long PVC pipe filled with transformer oil and a pair of pin-type electrodes was inserted into the other end of the pipe. In the experiment, the AC voltage imposed on the electrodes was increased gradually until the discharges arose. During this process, the sensor outputs were monitored and recorded.

**Figure 12.** (a) Measured Lamb wave signals and (b) waveforms around 0.3 ms.

clearly shown in **Figure 12**, which demonstrates that the proposed sensor has a very high

sensitivity in detection of weak Lamb waves propagating in the Bakelite plate.

156 Optical Interferometry

**Figure 10.** Sensor output spectra measured in air (a) and in transformer oil (b–d) at different depths.

**Figure 11.** Schematic of an experimental setup for evaluating of sensor sensitivity.

An experiment was carried out for testing the sensor performances in detections of the partial discharges occurred in the transformer oil. **Figure 13** shows a set of photos on this experimental setup. One shown in **Figure 13(a)** is a photo of the hand-made sensor prototype. As shown in **Figure 13(b)**, the sensor was inserted into an 850-mm long PVC pipe filled with transformer

**Figure 13.** A group of photos on (a) sensor prototype and (b) experimental setup and relevant parts.

**Figure 14.** Measured partial discharge signals in transformer oil.

**Figure 14** shows a set of measured signal waveforms. The red-line trace is a power-frequency signal measured as a phase reference here, and the black-line trace is the acoustic signals measured when the discharges arose in the transformer oil. From these data, clearly, the proposed sensor can work in the transformer oil to directly detect the internal ultrasonic waves induced by discharges.

Compared with other sensors proposed in [20, 21] with similar structures, this sensor does not need the diaphragm as a reflector, which resultantly make it simpler in fabrication and more sensitive to ultrasonic waves propagating in the transformer oil.
