Optical sensing in condition monitoring of gas insulated apparatus: a review

Health monitoring of gas insulated apparatus is critical to improve the reliability and to reduce the life cycle cost. Among the various methods for monitoring, optical sensing developed fast in recent years due to their highly sensitive and anti-electromagnetic interference behaviour. This study reviews the optical monitoring methods for partial discharge, gas concentration and temperature of gas insulted apparatus. In addition, the challenges of sensor development are carefully discussed here. The future prospects of optical monitoring of gas insulated apparatus are also analysed to inspire new ideas.


Introduction
High-voltage gas insulated apparatus, including gas-insulated switchgears (GISs) and gas-insulated lines (GILs), have been widely used in power transmission and distribution systems around the world [1,2].
However, due to manufacturing defects, GIS/GIL failures occur from time to time [3]. According to the Cigré surveys, the failure rates of GIS circuit breaker, GIS disconnector and GIS busbar are 0.1038, 0.1149 and 0.0645%, respectively [4]. In order to achieve early warning of the failures and protect other equipment and people, the condition monitoring technologies have to be developed.
Recently, many efforts have been devoted to investigating the monitoring systems for partial discharge (PD), gas concentration, and overheating, etc. of the gas insulated apparatus [5].
For example, ultra-high frequency (UHF) sensors and piezoelectric (PZT) sensors were developed to detect PD [6][7][8]. Gas pressure sensors were used for SF 6 leak detection [9]. However, the sensitivity of the sensors needs to be improved. In addition, many electric sensors work well in the laboratory, but are disturbed in the field, due to the strong electromagnetic interference.
In recent years, an emerging approach to solve the above problems is developing optical sensors. Compared with the conventional electric sensors, the optical sensors are immune to strong electromagnetic interference, and are highly sensitive [10]. Besides that, fast response to defects and the potential of large scale multiplexing ability make optical sensors become one of the most suitable sensors for the condition monitoring of gas insulated apparatus [11].
Many optical methods have been developed in the past decade, which attracted considerable attention from the researchers and engineers in power discipline. However, in order to better understand the principles of the new technique, additional background from optical engineering is required. However, the existing surveys of condition monitoring for gas insulated apparatus are mainly focused on the traditional electric methods [6]. Although some surveys mentioned the optical sensors as an emerging and developing technique [12], the introduction of the optical sensors was not comprehensive. There is a big research gap that needs to be filled between the condition monitoring of gas insulated apparatus and the optical sensing methods.
In this paper, the developments of optical sensors for gas insulated apparatus over recent years are reviewed in detail, wherever available, to provide some inspirations for further research on optical sensors. The paper is organised as follows: In Section 2, foundational principles of optical sensing are introduced. In Section 3, various optical measuring methods of partial discharge in GIS/GIL along with their structures, advantages and disadvantages are presented. In Section 4, optical sensing methods for the detection of SF 6 decomposition products are analysed. In Section 5, the gas leakage detection methods are discussed. In Section 6, the optical monitoring methods of GIS/GIL temperature are reviewed. In Section 7, an optical-fibre sensing system for breakdown localisation in GIL is reported. Finally, the existing problems and future research directions of optical-fibre sensors (OFSs) in condition monitoring of gas insulated apparatus are summarised in Section 8.

Foundational principle of optical sensing
The foundational principles of optical sensing are prioritised. Three important optical sensing principles are introduced, including optical interference, spectral absorption and fibre grating. Most of the optical sensors are developed for the condition monitoring of electrical apparatus, based on the following principles.

Optical interference
Optical interference is based on the superimposing of two light beams to measure the phase difference between them. Typically, an incident light beam of an interferometer is split into two paths and then recombined together to create an 'interference pattern'. In the 'interference pattern', the intensity of the interference light I (W/m 2 ) can be expressed as [13]

Spectral absorption
A substance has a selective absorption effect on light of a given wavelength, i.e. different substances absorb different amounts of light of the same wavelength, and the same substance absorbs light of different wavelengths differently. This phenomenon of light intensity attenuation is widely used in gas species identification and gas concentration detection.
The absorption satisfies the Beer-Lambert law, as shown in Fig. 2. The physical meaning is that when a bundle of parallel light passes vertically through a uniform non-scattering light-absorbing material, the absorbance is proportional to the concentration of the light-absorbing material and the thickness of the absorbing layer [15]. The relationship between light intensity and gas concentration can be expressed as [16] where I(λ) and I 0 (λ) are intensities of the incident and emitted laser, respectively, W/m 2 . c is the concentration of the gas to be detected, mol/L. L is the path length of the beam of light through the material sample, m and α(λ) is the absorption coefficient per unit distance and per unit concentration of the gas, L/(mol·m). In order to improve the sensitivity or response time, different methods are investigated based on laser absorption, including infrared (IR) and ultraviolet (UV) spectra [17].
Meanwhile, there is another kind of method based on spectral absorption, which is photoacoustic spectroscopy (PAS). PAS is mainly based on the photoacoustic effect of gas [16,18,19]. A broadband light generated by a light source is filtered to obtain the IR light of a predetermined wavelength. Then the light is absorbed by specific gas molecules. After absorbing the IR light, the molecules transit from the ground state to the excited state, resulting in an increase in temperature and gas pressure. After the light gets chopped or modulated, an acoustic signal will be generated, and the gas concentration can be back-calculated by the intensity of the sound wave detected by a microphone [18,19], as shown in Fig. 3.

Fibre Bragg grating (FBG)
An FBG is typically made by utilising the photosensitivity of the fibre material and forming a spatial phase grating by exposing the fibre core to UV light. The sensing principle is to obtain external information by modulating the centre wavelength of the fibre grating [20]. FBGs, also called reflection gratings or short-period gratings, are commonly used. FBG is a kind of narrowband reflection filtering passive device with excellent performance. When a broadband source is an incident on an FBG, narrowband light of a component near a specific wavelength (Bragg wavelength) will be reflected, as shown in Fig. 4.
According to the coupled-mode theory, the Bragg wavelength meets the Bragg condition where λ B is the centre wavelength of FBG, n eff is the effective refractive index and Λ is the grating period. When the external conditions cause temperature or stress variation of FBG, the effective refractive index and the grating period will be affected by them [21]. Therefore, it is possible to monitor the change of the temperature or strain by recording the change in the Bragg wavelength of the reflected light, according to (4). The relationship between the shift of the Bragg wavelength Δλ and temperature variation ΔT or strain variation Δε is as follows: where α f is the thermal expansion coefficient of the optical fibre, 1/K. ξ is the thermal-optical coefficient of the optical fibre, 1/K. P e is the strain optic coefficient. In the FBG sensor design, different sensing objects such as PD, acoustic, gas, force, wind, humidity etc. are converted into the temperature or strain variation with sensing objects [21][22][23][24][25][26][27][28]. Then, the sensing parameters can be calculated based on the measured Bragg wavelength.

PD detection with optical methods
Gas insulated apparatus has a complex structure that internal defects may happen during the processes of manufacturing, transferring, and installing [29]. The worldwide in-service experiences have shown that the main defects in GIS/GIL are metal pollution on spacer surface, metal protrusion on electrode, free metal particles, spacer void and loose shields lead to floating electrode defect, as shown in Fig. 5 [30]. These defects may induce PD, which is one of the main causes of dielectric strength deterioration of the gas insulated apparatus. Thus, monitoring the PD signal at an early stage is required.     Under the operating voltages, PD commonly accompanies a range of physical phenomena, such as transient current, electromagnetic (EM) radiation and mechanical vibration. By effective detection of these signals, PD in the GIS can be reflected [31]. At present, the conventionally used PD detection methods include pulse current method, ultra-high frequency method and ultrasonic method [32,33]. However, electric sensors are mainly used in these methods, which are subject to EM interference on site. Therefore, many efforts have been made to develop novel optical sensors for PD detection in recent years, including vacuum photomultiplier tube (PMT), silicon photomultiplier (SiPM) sensor, fluorescent fibre sensor and optical-fibre ultrasonic sensor.

Vacuum PMT
The working principle of PMT is to receive photons and to convert optical signals into electrical signals through an optical converter. The spectral range of the optical signals generated by PD in SF 6 gas is roughly 300-500 nm, which is mainly visible light [34]. And the PMTs are extremely sensitive detectors of light in the range. Thus, the PMT can be used in PD detection [35]. A photo of a typical vacuum PMT is displayed in Fig. 6.
Kaufhold et al. reported the optical measurements of PD by a PMT in a scaled-down model of GIS. And they compared the sensitivity of a PMT with that of the conventional electrical PD sensors. In his experimental platform, a needle defect was set on the conductor, as shown in Fig. 7. The results indicated that, if the PMTs were assembled in gas insulated substations between each pair of two insulating spacers, the discharge, which is smaller than 1 pC could easily be detected. In addition, the PMT could detect glow (pulseless) discharges, which cannot be detected by the conventional PD detection systems [36].
In addition to metal protrusions defect, the vacuum PMT was used in the detection of PDs caused by surface metal particles on GIS spacer by Li et al. [37]. The experimental results showed that the signal measured by the PMT was consistent with that of the UHF.
De Maria et al. installed a PMT on an actual 420 kV GIS to monitor the PD. The experimental results indicated that the PMT was very sensitivity for free moving particle defects and fixed defects on the spacer surface. This work also confirmed the feasibility of the application of PMT on site. De Maria et al. thought that PMT could be recommended for GIS PD measurements on-site [38].
In addition, many other researchers have used vacuum PMTs for PD detection in SF 6 [39][40][41][42]. And these PMTs were all highly sensitive to PD signals. However, these studies were designed for specific experimental samples in the laboratories. Since these researches were not applicable to PD monitoring on-site, they were not described in detail here.
Although these vacuum PMTs are sensitive, and even have a higher signal-to-noise ratio than electrical PD detection techniques. They are very large in size and need to be installed inside GIS/GIL. These shortcomings may limit the application of the vacuum PMT.

SiPM sensor
Recently, the development of silicon solid-state photoelectric technology has driven the advancement of SiPM sensor. With the advantages of high quantum efficiency, high responsivity, broad response spectral range, high EM-interference immunity, and small size, the SiPM sensor is a highly attractive alternative to the conventional vacuum PMT.
In 2017, Ming Ren et al. firstly introduced the SiPM sensor to PD detection in GIS [43]. The experimental platform and the SiPM sensor in this study are shown in Fig. 8. Three PD models, which named (a) needle defect in the HV electrode; (b) floating metal particle defect and (c) surface metal defect on spacer were used in the experiment. And the basic performances of SiPM in PD detection were investigated. The experimental results showed that the SiPM-based PD detection almost has the identical capability to vacuum PMT in responding to notably weak PD without additional signal amplification.
In 2018, the research group further worked out a doublespectral SiPM sensor for a refined PD diagnosis in GIS [44]. The proposed double-spectral PD sensor presented good performances in acquiring PRPD patterns in UV, visible and full bands simultaneously. And the SiPM sensor could achieve the identical sensitivity to PMT detection as well as HFCT for the different PD types in SF 6 . Typical pulses detected by SiPM sensor, HFCT and vacuum PMT in his experiment are shown in Fig. 9.
Regrettably, the above experiments were carried out under a scale-down model of GIS. The transmission distance and angle of the optical signal were ideal. For the actual gas insulated apparatus, the detection performance of the SiPM sensor still needs to be further confirmed.

Fluorescent fibre sensor
In addition to the above two sensors, fluorescent fibre can also be used for PD optical detection in gas insulated apparatus. The working principle of fluorescent fibre for PD detection is that: the fibre core of the fluorescent fibre is mixed with a trace of fluorescent material that is selectively absorbent of certain wavelengths for optical signals. When the PD-produced light illuminates the fluorescent fibre, the light can be absorbed by the fluorescent molecules in the fibre core. The electrons of the fluorescent molecules leap from the ground state to the excited state. When excited-state electrons return to the ground state, they often release energy (light electroluminescence) in the form of rays of light (fluorescence). The fluorescent signal spreads in the fibre and is consequently detected [45]. The diagram of a fluorescent fibre detecting a weak fluorescent signal is shown in Fig. 10.
In 1989, Muto applied the fluorescent fibre on PD detection. His test device was similar to a section of a GIS. A 1-m-long fluorescent optical fibre with a transmission loss of more than 1000 dB/km was installed in the pipe. And it was connected by a microlens with a conventional optical fibre installed outside the test device. When a high voltage of 10 kV was applied to the electrode to generate PD, the electric-discharge light was converted into a red light by the fluorescent optical fibre and successfully detected by a photodiode in the remote detection instrument [46].
Mangeret et al. also introduced the detection effect of fluorescent fibre on needle electrode discharge in SF 6 . The results showed that the fluorescent fibre could successfully detect the discharged optical signal, but the researchers thought that the detection threshold must be further improved [47].
Since the detection sensitivity of fluorescence fibre is positively related to its length, the detection threshold could be improved if long fibre can be arranged in a limited space. Therefore, Li et al. [48] reported a novel sensor that combined fluorescent optical fibre (1.22 m) and UHF sensor, which can solve the problem of mounting a long fluorescent optical fibre into GIS. The photo of the integrated sensor was shown in Fig. 11. In the experiment, a metallic protrusion with a length of 26 mm was adopted as the defect to produce electric field concentration on the GIS conductor. And the defect was located on the conductor opposite to the sensor. The experimental results showed that the PD signals first appeared in the fluorescent fibre channel when the applied voltage reached 53 kV. There were no UHF signals at this time. The PD signals appeared in both optical and UHF channels when the applied voltage reached 69 kV. The optical method has a higher sensitivity than the UHF method.
Recently, Li et al. further confirmed that fluorescent fibre has a high sensitivity for PD detection. They found that the amplitude of the optical signal has a linear relationship with the discharge quantity of the floating defect, in contrast to the UHF method. And the amplitude or the number of the optical signals induced by PD decreases clearly with the increase in the detection distance [49].
Based on the above reviews, it can be found that all of the above three optical sensors can be used for PD detection directly in GIS. These sensors have shown high sensitivity in laboratory tests for needle defects or surface floating defects. Even if the sensing unit is not facing the position of the PD source, the optical signal can be detected. And this view has been confirmed through simulation studies [50]. However, the light is hard to transmit to the sensor from spacer void defects or floating electrode defects inside the shield; the effect of the three methods is challenged.

Optical-fibre ultrasonic sensor
The use of fluorescent fibres, PMTs, etc. for optical detection requires the sensors to be installed inside the GIS, which increases the difficulty of practical application. Researchers have developed the optical-fibre ultrasonic sensors for PD detection in gas insulated apparatus. Since the ultrasonic signal can pass through the enclosure and spacer, the optical-fibre ultrasonic sensors can be flexibly installed on the outside of the enclosure of GIS/GIL.
In 2001, Zargari and Blackbum proposed a Michelson interferometer with a non-invasive fibre sensor for the detection of ultrasonic produced by PD within an SF 6 chamber [51]. The optical sensor was a coil of single-mode fibre wrapped around the SF 6 chamber, as shown in Fig. 12. And a needle-plane electrode arrangement was used as a discharge source to simulate the PDs. The experimental results showed that the optical-fibre ultrasonic sensor has a detection sensitivity of at least 20 pC in tests. In addition, by increasing the length of the sensing arm of the OFS its sensitivity could be increased.
As the above sensor was wrapped around the SF 6 chamber, it is hard to relocate the sensor on another GIS. A smaller Michelson interferometer-based fibre sensor was developed by Pang et al. [52]. In order to improve the sensitivity, the optical fibre was wound around the elastic cylinder with a high Poisson ratio, as shown in Fig. 13a. In his experiment, the sensor was attached to the GIS enclosure, as shown in Fig. 13b. And the PD was generated by putting a small metal chip in the GIS. This experimental work was to verify the feasibility of the sensor head and the sensitivity of the acoustic emission of PD in real applications.   [53]. The performance of the optical-fibre ultrasonic sensor was compared with a conventional piezoelectric (PZT) sensor in the experiment based on a 126 kV GIS. The results indicated that, for the small free metal particle defects, the times of PD detected by the proposed optical system were obviously larger than that detected by the PZT system. Moreover, the amplitude of the signal detected by the proposed AE optical system was 525% higher than that detected by the PZT system, as shown in Fig. 14.
Moreover, many researchers have also reported many noninvasive interferometric optical-fibre ultrasonic sensors for PD detection [54][55][56][57]. Although these optical sensors have not been tested in the actual gas insulated apparatus, the feasibility can be predicted.

Advantages and disadvantages
In this section, four types of optical PD sensing methods of gas insulated apparatus are reviewed. Among them, the PMT, SiPM sensor and the fluorescent fibre sensor monitor the PD actions by detecting the light signals. And the optical-fibre ultrasonic sensor is designed to be sensitive to acoustic signals generated by PD.
In terms of a light defect, the PMT is currently mainly used for laboratory research due to its larger size. As a new technology, the SiPM sensor is still costly, and also only used in laboratory tests. Although both sensors have high sensitivity to many defect types, the SiPM sensor is considered to be more popular in the field application as a result of the small size. In general, the fluorescent fibre has the advantages of insulation, low cost, small size, and anti-electromagnetic interference, so it is a very suitable optical method for PD monitoring by light in gas insulated apparatus onsite. However, all three sensors require internal installation, reducing the flexibility of PD detection. And they are not suitable for spacer void defect detection, due to the light is blocked.
Compared with the first three sensors, the biggest advantage of the optical-fibre ultrasonic sensor is that it can be mounted externally. In addition, the sensor is very sensitive to free metal particle defects, even if it has not discharged yet. But for metal protrusion defects, spacer surface metal particle defects or floating potential defects, the sensitivity of the optical-fibre ultrasonic sensor should be further improved.
For a concise comparison, the advantages and disadvantages of these novel optical sensors for PD detection in gas insulated apparatus are summarised in Table 1.

Gas decomposition products detection with optical methods
As we are known, under the action of PD or high temperature, SF 6 molecules may decompose. Besides that, the compositions and formation rates of SF 6 decomposition products (such as SO 2 F 2 , SOF 2 , SO 2 , CF 4 , SOF 4 , SF 4 , HF, H 2 S etc.) are related to the discharge insulating defect types, so that the internal status of gas insulated apparatus can be identified by the composition and concentrations of SF 6 decompositions.
Although the PD action cannot be responded quickly by gas decomposition products detection, sensing of SF 6 decomposition products are essential for GIS/GIL to distinguish whether there are possible faults.
At present, the most used optical methods for gas decomposition product detection in gas insulated apparatus are Fourier transform IR spectrometry (FTIR), PAS, differential optical absorption spectroscopy (DOAS), non-dispersive IR absorption spectroscopy (NDIR) and Raman spectroscopy.

Fourier-transform IR spectroscopy
FTIR is a technique for obtaining the IR absorption spectra of gases [58], which is based on the principle of applying Fourier transform to the IR light after interference. When the laser passes the gas samples, the resulting interference spectrum will change accordingly due to the absorption of energy at certain frequencies.
By Fourier transform technology, each frequency on the spectrum will be converted into a corresponding light intensity. The composition and concentration can be detected based on the different characteristics of the spectrum [18]. This technology has wide use in gas sensing; it has the advantage of high sensitivity and good stability.
The detection of SF 6 decomposition products by using FTIR could date back to the 1990s. The effect of H 2 O and O 2 on the  One problem that should be paid enough attention to the application of the FTIR technique is the existence of crossinterference between the absorption spectra of multi-gases. In order to detect each composition accurately, it is important to avoid the overlapping parts of the spectrum. Thus, figuring out the absorption peaks of typical SF 6 decomposition components should be done at first. Zhang et al. [60] conducted a series of experiments based on white-cell, and summarised the typical absorption peaks of SF 6 decomposition components; the specific information is shown in Table 2.
Another problem of the FTIR is the spectrum of SF 6 may overwhelm the spectrum of the decomposition products. In order to amplify the signals submerged by the SF 6 IR absorption section, Ren [61] increased the optical path length of the white-cell to 20 m and the two-dimensional correlation spectroscopy was used. Two important decomposition components of SOF 4 and CF 4 were effectively identified. Meanwhile, SOF 2 , SO 2 F 2 , SF 4 , SOF 4 , SO 2 , CO and CF 4 were detected.
Moreover, in order to establish the correspondence among fault type, severity and FTIR test results, Zhang et al. [62] analysed the volume fraction trends of SOF 2 , SO 2 F 2 , SO 2 and CO with different PD time and different PD quantity, which could be a reference to judge the severity of the PD faults.
The minimum detection limit for sensors based on FTIR technology is related to the length of the optical path. And almost all kinds of SF 6 decomposition products can be detected. However, if FTIR sensors want to achieve online monitoring in gas insulated apparatus, its gas cell and quantitative algorithm have to be customised [17].

Photoacoustic spectroscopy
PAS is a spectral absorption method based on photoacoustic effect and IR absorption spectroscopy [63]. It has the advantages of high sensitivity, and high detection speed [16,64].
In order to perform real-time online monitoring of SF 6 decomposition components under the faults of PD action, Tang et al. [65] designed an SF 6 decomposition products detecting device based on PAS. The comparison between the device and the gas chromatography indicated that the changing trend of the gas of each component has a good consistency. The detection error of each component gas was basically unchanged after 96 h, and the maximum relative error was about 8%. In his further study, the minimum detection limit of SOF 2 reached 4.6 ppm, and the average error was 5.9% [66].
Obviously, PAS could be used for SF 6 decomposition products sensing effectively. For the purpose of further enhancing the performance of the sensing system, a number of related researches have been done. One of the effective ways is to improve their components. Varga et al. [67] designed a dual-channel H 2 S concentration measuring system. And the researchers used PAS in combination with a tunable diode laser spectrometer (TDLS) at 1.57 μm to measure H 2 S in natural gas process streams, based on a differential resonant PA cell designed by Miklós et al. [68] (shown in Fig. 15). The results of laboratory tests have been reported; the minimum detection limit was 0.5 ppm.
Another meaningful effort is that Wu et al. [69] developed a quartz enhanced PAS (QEPAS) sensor for H 2 S sensing. The sensor employed an acoustic micro-resonator. This fibre-amplifierenhanced QEPAS sensor for H 2 S achieved a sensitivity of 0.89 ppm at 1 s integration time.
In general, PAS has the advantages of high sensitivity and high detection speed. However, due to the presence of a microphone, this detection method is susceptible to environmental noise interference. Also, PAS is susceptible to multi-gas crossing sensitive effects.

Differential optical absorption spectroscopy
Absorption spectroscopy based on the narrowband laser is a sensitive technology to identify gas components [15,[70][71][72][73]. Normally, the spectral intensity in UV spectra is much higher than that in IR spectra. Particularly, there is almost no cross-sensitive problem among multi-gases in UV-DOAS (UV-DOAS), so it has a good application value the SF 6 decomposition product sensing.
Among the SF 6 decomposition products, SO 2 , H 2 S, and CS 2 have absorption characteristics in UV spectroscopy. Some gases have been investigated in other field-based on UV-DOAS. SO 2 is one of the important decomposition products of SF 6 . Degner et al. [74] designed a novel measuring system for chemical analysis with the use of LED-light sources. The system can be used in a strong electromagnetic field or at high temperature, which has a good prospect for in-situ utilisation. The sensing system realised a minimum detection limit of 1 ppm for SO 2 .
In order to achieve more accurate and faster detection of SO 2 gas generated by SF 6 decomposition in GIS/GIL, Zhang et al. [75] improved the detection limit of SO 2 to 12 ppb per metre by the partition of DOAS. And the concentration of H 2 S is obtained by converting H 2 S to SO 2 . This system was supposed to be applied for the fault diagnosis for GIS. H 2 S is another important decomposition product of SF 6 , but there are few reports on the detection of H 2 S gas based on DOAS before. Zhang et al. [76] established a UV-DOAS platform to detect H 2 S and CS 2 , and the detection system is shown in Fig. 16. By the use of the detection system, the UV absorption spectra of H 2 S and CS 2 are obtained and realised the quantitative measurements of two gases.
UV-DOAS has the advantages of high selectivity, high stability and high sensitivity. What's more, the cost of the detection system is lower than the systems based on FTIR and PAS. The studies on SF 6 decomposition gas sensing based on UV-DOAS are limited to

Other optical sensing technologies
NDIR is a technology based on the single-beam dual-wavelength detection method [77]. Yuan et al. [77] presented a high sensitivity SF 6 gas sensing system based on NDIR. Its measuring range was up to 0-50 ppm, and the accuracy was 0.1 ppm. It could detect SF 6 gas in GIS. Some companies such as Siemens, Maihak, Varian, UNOR designed a series products based on NDIR to detect H 2 S, SO 2 , CS 2 [78]. After long-term use, the SF 6 detecting system based on NDIR will have problems such as aging of the light source, optical path interference and detector noise, which make the system unstable and increases the measurement error. Laser Raman spectroscopy (LRS) is a scattering technique, which is based on the Raman Effect. LRS deals with the structure and properties of a substance by directly measuring the Raman scattered light from the laser irradiation [79,80]. The advantages of the LRS are long-term stability of detection, short detection time and high sensitivity. Irawan et al. [81] introduced a sensing system based on LRS. The rates of degradation of SF 6 and production of gaseous by-products being shown to be proportional to the charge transported by the corona, which indicates LRS is able to detect SF 6 decomposition products and get used in-situ.

Detection methods for novel gases
Recently, in order to minimise the emission of greenhouse gases, some novel gases (such as C 4 F 7 N/CO 2 and C 4 F 8 /CO 2 ) are focused on. These novel gases may replace SF 6 in the future [82]. Therefore, it also has great importance for on-line monitoring and fault diagnosis of GIS/GIL filled with those gases.
In 2018, Zhang et al. investigated the discharge decomposition characteristics and decomposition mechanism of C 4 F 7 N. It was found that the decomposition of mixed gas mainly produced CF 4 , C 2 F 6 , C 3 F 8 , CF 3 CN, C 2 F 4 , C 3 F 6 and C 2 F 5 CN, among which the relative contents of C 2 F 6 , CF 4 and CF 3 CN were relatively high [83]. Fortunately, the optical sensing methods introduced in Sections 4. 1-4.4 are also applicable to the detection of the above gases. For example, Shiling detected CF 4 gas concentration in high-voltage electrical equipment by NDIR, the test concentration points were 0, 200, 400, 600, 800, 1000, the result was acceptable [84]. Zhang et al. designed a sensing system based on FTIR to monitor the C 4 F 7 N gas mixture, the detection limit was about 100 ppm [85]. Luo et al. introduced a system based on PAS for CF 4 sensing, the detection limit was 5.5226 ppm with excellent stability. To sum up, optical sensing methods are also applicable for monitoring alternative gases and their decomposition products [86].

Advantages and disadvantages
In this section, the main optical sensing methods for gas decomposition product sensing are reviewed. All of them are based on the principle of spectral absorption. Among them, sensors based on FTIR and UV-DOAS are worked in the way of monitoring the absorbed light by comparing the light intensity in the presence and absence of gas absorption. The concentrations of gases detected by a sensor based on PAS are calculated by measuring the intensity of the acoustic signal generated by the photoacoustic effect.
The sensors based on FTIR and UV-DOAS have the advantages of high sensitivity and short response time in gas sensing. FTIR can detect almost all types of decomposition products of SF 6 , but less products can be detected by UV-DOAS than FTIR. The lower limit of detection of FTIR is generally ppm level, while the lower limit of detection of UV-DOAS can reach ppb level. Also, FTIR has cross-sensitive problem of absorption lines among multi-gases, so it requires much more attention in utilisation. In addition, from the perspective of measuring system cost, UV-DOAS is cheaper than FTIR.
In terms of PAS sensing, it offers high sensitivity, good selectivity and short response time. Similar to FTIR, PAS also could detect almost all types of SF 6 decomposition products. Also, there have been some commercial applications for gas sensing available nowadays, which are widely used in environmental monitoring, industrial emission monitoring and so on. Thus, PAS sensing has a promising prospect to be used for real-time and high sensitivity gas sensing in GIS/GIL. However, external noise is a major problem affecting the applicability of the sensor. So how to eliminate the influence of external noise needs to be solved.
Based on the above reviews, the advantages and disadvantages of these optical gas decomposition product detection methods for GIS/GIL are summarised in Table 3.

Gas leakage detection with optical methods
The on-site operation experience shows that SF 6 in GIS will leak when there are faults happen, such as cracking of GIS explosionproof membranes and chamber cracking due to improper setting of expansion joints (especially in areas with large temperature differences between day and night). Moreover, some gas decomposition products, e.g. HF, H 2 SO 4 , and SOF 2 , are very corrosive. These gases may cause damage to the metal surface or rubber sealed coil, and eventually may cause leaks of SF 6 in trace amounts. The leakage of SF 6 to a certain extent, will reduce insulation performance and pollute the ambient air and, thus the environment. Therefore, it is necessary to rigorously monitor the leakage of SF 6 .
In order to detect the occurrence of SF 6 leakage more accurately, the researchers conducted a number of related studies. The results showed that the traditional SF 6 leakage sensors mainly Fig. 16 Schematic diagram of the UV-DOAS system [76]  detecting the gas density basing on density relays, but the sensitivity and selectivity are relatively low. In recent years, researchers have proposed an optical detection method, based on the PAS (the basic principle has been introduced in Section 2). For example, Gondal et al. [87] developed a novel photoacoustic spectrometer system for in-situ detection of SF 6 leaks in low concentrations in 2002. The core device of the sensing system is a high-quality factor resonant photoacoustic cell and a continuous wave line tunable CO 2 laser. His experimental results showed that the minimum detection limit could reach 3.5 ppb. In 2009, Xu et al. [88] developed a similar sensing system for SF 6 gas leakage detection based on the IR absorption spectrum. The detection sensitivity could reach 0.12 ppb [89].
Recently, in order to further improve the detection capability, Sampaolo et al. [90] designed a gas-leak optical sensing system based on mid-IR quartz enhanced PAS (QEPAS) technique. The schematic diagram of the system is shown in Fig. 17. Among the PAS methods, QEPAS was demonstrated to be the most sensitive technique. In his experiment, the QEPAS scanned the absorption line centred at 947.93 cm −1 . Initially, the minimum detection limit was 2.75 ppb with an integration time of 1 s. In his further research, he achieved a minimum detection sensitivity of 0.05 ppb in 1 s [91].
Above all, gas leakage detection based on optical methods has the merits of high sensitivity and selectivity, short response time. However, all studies were carried out in the lab, how to integrate these sensing systems with gas insulated apparatus to achieve multi-point measuring is still unclear. In addition, the system costs should be reduced to be affordable.

Temperature detection with optical methods
Some defects may be caused in the manufacturing process and onsite installation in gas insulated apparatus. For example, the insufficient of the tightening torque of the bolt and the defect of contact springs, the contact resistance of the contactors of gas insulated apparatus increases. Under the operating current, the local temperature of the current conductor increases continuously, which may cause the metal parts or solid insulation inside GIS to burn out (as shown in Fig. 18). Even worse, this may lead the metal parts to melted, change the original electric field distribution and cause a discharge failure. Therefore, monitoring the temperature of key areas inside the gas insulated apparatus (such as contacts and connectors) is necessary to ensure their operational reliability.
The traditional method of monitoring overheating in gas insulated apparatus by measuring loop resistance cannot be used for online monitoring. Even if the loop resistance is detected abnormal, it cannot determine the fault location. For the relatively longer GIL devices, temperature measurement is also a useful method of detecting hot spots caused by contact defects. Therefore, it is of great significance to provide a new temperature monitoring method for gas insulated apparatus through optical monitoring means. The optical temperature monitoring methods currently applied to gas insulated apparatus mainly includes optical-fibre distributed temperature sensing (DTS), FBG temperature sensing and IR temperature sensing.

Optical-fibre DTS
The optical-fibre DTS technology using the Raman effect was initially developed in 1980s. And the principle of the DTS is as following: Thermal affects the internal structure and locally change the way light travels in the fibre. When the light injected into the fibre travels along, the amplitude of a part of the scattered light called 'Raman scattering' is directly related to the temperature of the scattering position. The position of the temperature measurement point can also be determined accurately by measuring the round-trip propagation time of the scattered light [92]. This is illustrated in Fig. 19.
In the 1990s, the world's longest gas insulated transmission line was being constructed in Nagoya-City, Japan. In order to detect the hot spots caused by contact defects in the GIL, Araki et al. [93] proposed an application of a fibre-optic temperature distribution sensor for locating contact defects based on the technique of R-OTDR (Raman-Optical Time Domain Reflectometry). The sensing fibre was placed on the surface of the enclosure to reflect the temperature distribution of the conductor inside. And the experimental results indicated that the distributed temperature sensor was capable of achieving a temperature resolution of 1°C and a spatial resolution of 1 m over a distance of more than 10 km. After actual operational testing, the fibre-optic temperature distribution sensor was proved useful for the detection of contact defects. The temperature distribution through the line could be measured with only one fibre laid on GIL.
A similar study was carried out by Miyazaki et al. [94]. A distributed temperature monitoring system was established to monitor the distributed enclosure temperature of six phases of a 275 kV GIL, in the same GIL project (the Shinmeika-Tokai line) as the literature above. The schematic diagram of the GIL distributed temperature monitoring system was shown in Fig. 20. By introducing an optical switch to change the measuring fibres, only one unit is required to monitor all the six phases of the GIL and tunnel. The optical fibres were attached to the surface of GIL by aluminium and protection tapes. Similarly, the accuracy of the optical-fibre temperature sensing system was 1°C and the distribution was shown at 1 m intervals.
As mentioned above, the optical-fibre DTS is primarily used to measure the GIL enclosure temperature, because it can easily achieve DTS over several kilometres. However, the cost is relatively high, and it is rarely used to monitor temperature in GIS.

FBG temperature sensing
For GIS temperature monitoring, due to the advantages of low cost, anti-electromagnetic interference and high resolution, FBG temperature sensing is a good choice. The basic principle of this method has been introduced in Section 2.
As we have known, the FBG temperature sensing is a very mature technology, and its temperature accuracy can reach to 0.1°C (even better). The method has been widely used in aviation, coal mines, oil wells and other fields. In power field, some researchers used FBG temperature sensing system to monitor GIS temperature.
For example, Xie et al. [95,96] reported the application of a high sensitivity FBG temperature sensor in on-line monitoring of GIS bus temperature. By fixing the pre-stretched FBG to the aluminium alloy substrate, the sensitivity of the temperature sensor reached 30 pm/°C. The temperature monitoring system was installed on a 110 kV GIS bus and operated well.
Whether it is optical-fibre DTS or FBG temperature sensing, they both reflect the internal temperature distribution or defects by measuring the enclosure temperature of gas insulated apparatus. The advantage of this kind of non-intrusive approach is that the sensor is easy to be installed and the sensor components do not affect the internal insulation. However, the temperature increasement on the enclosure is much lower than that on the conductor, according to the simulation results, the stable temperature difference between the conductor and the enclosure can reach to 30°C [97]. This may result in a decrease of the overheat sensing sensitivity. Besides that, this method is susceptible to the external environmental interference.

IR temperature sensing
Some researchers developed optical methods that directly detect the internal conductor of gas insulated apparatus, which can be used for temperature measurement in specific areas. IR temperature sensing works by focusing the IR energy emitted by an object onto a photodetector, and the photodetector converts that energy into an electrical signal, which is proportional to the IR energy emitted by the object. Because the emitted IR energy of an object is proportional to its temperature, the electrical signal provides an accurate reading of the temperature of the object that it is pointed at.
Obviously, the IR temperature sensing can be used for monitoring the GIS internal temperature directly, such as conductor, contacts, etc.
Cong et al. [98] developed an IR temperature sensor to an online temperature monitoring system for contact temperature in GIS. As shown in Fig. 21, a quartz window on the side of the GIS enclosure was installed, which allowed IR frequencies to pass through it to an IR sensor. And the researchers applied a 70-100 μm thick low-temperature IR radiation coating on the surface of the GIS contact to improve the surface emissivity of the metal conductor. The experimental results indicated that the IR temperature sensing system had a measurement accuracy of 1°C.

Advantages and disadvantages
In the above optical temperature measurement methods applied to gas insulated apparatus, based on the optical-fibre DTS, the distributed temperature can be obtained, which is suitable for temperature monitoring of long-distance GIL. However, the method is not suitable for GIS because its spatial resolution is (usually 1 m) worse than FBG sensing method (1 cm). The cost of the distributed system is also higher than that of FBG system. FBG temperature sensing is a mature and simple method, which has the advantages of high sensitivity and low cost. However, it can only achieve quasi-distributed monitoring.
Both the DTS and FBG only monitor the enclosure temperature of gas insulated apparatus, which reduces their fault sensitivity.
In terms of the IR temperature sensing, this method enables non-contact but direct measurement of the inner conductor temperature. However, its measurement range is extremely limited, and the installation is inconvenient.
The characteristics of the above temperature sensing methods in gas insulated apparatus were summarised in Table 4.

New optical distribution measurement system for GIL discharge localisation
Despite the fact that the significant efforts have been devoted, there are still missing areas which future investigations should be concerned. For example, breakdown localisation is a challenge for long-distance GIL.
Recently, an optical distribution measurement system for GIL breakdown detection of GIL was proposed to overcome the high cost, low spatial resolution of the electrical discharge localisation systems [99]. The system was based on the phase-sensitive optical time-domain reflectometer (phi-OTDR) and the optical-fibre Michelson interferometer. In the system, the Michelson interferometer provided a trigger signal since it had a better sensitivity and a higher frequency response, and the phi-OTDR Fig. 20 System configuration of enclosure surface temperature monitoring [94] Fig. 21 Installation of IR temperature sensor [98]  localised the ultrasonic vibration induced by a discharge. The sensing system of merged phi-OTDR and Michelson interferometer was displayed in Fig. 22.
The principle of the Michelson interferometer was discussed before. Thus, only the theory of the phi-OTDR is introduced here. A coherent laser pulse is generated by an ultra-narrow linewidth laser and modulated by an acousto-optic modulator. Then, the laser pulse is sent along an optical fibre, and Rayleigh scattering sites within the fibre cause the fibre to act as a distributed interferometer with a gauge length approximately equal to the pulse length. In order to increase the Rayleigh scattering signal, the input laser is magnified by an erbium-doped fibre amplifier. The intensity of the reflected light is measured as a function of time after the transmission of the laser pulse. Strain of the sensing fibre is caused by acoustic induced by the GIL breakdown. And the changes in the reflected intensity of successive pulses from the same region of fibre are caused by the strain of that section of fibre.
Experiment indicates that the sensing distance of the system reached 1 km and the localisation accuracy of the fault was smaller than 15 m. The proposed system is non-intrusive, has better spatial resolution, lower cost, and is greatly simplified as only one fibre is needed to be mounted to the outer surface of the GIL tank.

Suggestions for future works
This paper reviewed the development of optical methods in the field of condition monitoring for gas insulated apparatus. Despite all these researches, there are still many areas that require further investigation: (i) For PD detection, major works done inside laboratories with artificial defects, which are usually larger than those in actual conditions. It is important to further improve the sensitivity of optical sensors for weak discharge detection, especially for the optical-fibre ultrasonic sensors. In addition, the development of distributed optical-fibre ultrasonic sensing systems should be focused on. For example, the sensitivity of the phi-OTDR technique may be further improved to detected PD. Then, the PD in a GIS substation can be detected by using only one fibre. (ii) For gas sensing in GIS/GIL, the performances of response time, sensitivity and detection accuracy have been greatly improved. However, most of the long-path optical gas cells currently have the problem of large size. According to the Beer-Lambert law, the detection limit is proportional to the length of gas cell, which makes it difficult to reduce the volume of gas cells. Considering the price of the gas cells and optical devices, how to achieve distributed gas online monitoring is an urgent problem. In order to increase the optical path length while reducing the volume of gas cells, hollow core photonic crystal fibre (HC-PCF) used for gas cell should be taken into consideration. Since the hollow HC-PCF has the unique structure of its micro-structured hollow core [16,17,100], it can be utilised as a gas cell to reduce the cell size and be applied to distributed gas measurement. However, this method is still in the research stage, how to get the gas flow into the hollow core need more attention.
(iii) In order to improve the sensitivity of overheat temperature, the fibre needs to be attached to the conductor directly. The challenge is how to extract the fibre to an outside enclosure for detection.
One of the possible solutions is by using a post insulator coupling with an optical fibre. Moreover, distributed optical-fibre sensing technologies need be focused on. The distributed sensing methods may be used to monitor the enclosure temperature distribution continuously in GIL. However, the performances of the distributed optical-fibre sensing technologies are required to be further improved. The distributed sensing technology of optical frequency domain reflectometer (OFDR) should be concerned. Compared to the phi-OTDR, the OFDR can achieve higher spatial resolution. Theoretically, there is a spatial resolution of 10 cm over a distance of hundreds of metres for OFDR [9]. When the sensing distance is reduced to tens of meters, the spatial resolution can even be increased to several tens of micrometres. Meanwhile, further researches are needed to improve the sensing distance of OFDR.