Power transformers play a vital role in power transmission systems from the energy source to the consumption center. In situ diagnostics of the performance of power transformers provides a variety of benefits to ensure reliable electricity transmission. Immunity to electromagnetic interference, high sensitivity, high insulation as well as small dimensions of optical sensing make it very attractive for applications in power transformer monitoring. This paper presents a critical review of various diagnostic methods for power transformers such as partial discharge, dissolved gases, temperature and other important sensing, and optical detection. The advantages and disadvantages of different monitoring methods are carefully discussed and assessed. Finally, the existing technical barriers and future prospects of optical monitoring methods for power transformers are presented.

Methane is one of the indicative gases in power transformer oil, and the detection of methane dissolved in oil with high accuracy is of great importance for dissolved gases analysis and fault diagnosis inside power transformers. Based on the Beer-Lambert spectral absorption law, dissolved methane detection with tunable diode laser absorption spectrum (TDLAS) is proposed in this paper for the advantages of high sensitivity and resolution. On the basis of wavelength modulation spectroscopy, a specialized TDLAS system was established. To meet the actual needs of field testing, the anti-vibration design of an integrated Herriott cell and a gas pressure (P)/temperature (T) setting are worked out. Photodetector, collimator, and Herriott cell are integrated into one component to reduce the effects of vibration. It is investigated that the temperature has little effect on the second harmonic amplitude in the range of 30 °C ~ 50 °C, and the vacuum pressure is reasonably set at about 1 kPa. Experimental results showed that the resolution of sensitivity could be reached as 6.8 mV/(μL/L), the maximum deviation was less than ±4 μL/L, and the response time is less than 5 min. In the end, field application was also carried out, proving it is a prospective online sensing technique to serve oil-immersed power transformers better.

We propose a high sensitivity optical fiber sensor for detecting the acoustic emission generated by the partial discharge in gas-insulated-switchgear (GIS). A model is established to investigate the effects of the cylinder coil on the sensitivity, the bandwidth and the detection limit. After that, the proposed sensor was modified based on the effects. The average sensitivity of the optimized acoustic emission (AE) optical fiber sensor is 32 dB higher than that of the conventional lead zirconate titanate (PZT) in the frequency range from 20 kHz to 100 kHz. Moreover, experiment results show that the detectable partial discharge initial voltage for the proposed optical system is 17.1 % lower than that for the PZT system, and the amplitude of the signal detected by the proposed optical system is 525 % higher than that detected by the PZT system.

We propose a Michelson ultrasonic sensing system for detecting the acoustic emission generated by the partial discharge in power transformer. In order to guide the sensor head design in the sensing system for high sensitivity, a theoretical model is established to investigate the effects of the sensor head dimensions on the response sensitivity. After that, an optimized sensor head is designed. In the frequency range from 80 kHz to 200 kHz, whether the PZT sensor is installed in the oil or on the tank, the average response sensitivity of the proposed sensing system is higher than that of the conventional PZT system. When the distance between the sensor head and ultrasonic source is 300 mm in oil, the average detection limit of the Michelson ultrasonic sensing system is about 0.26 Pa, which is about 18.6% of that of the PZT system. Moreover, experiment results show that the detectable partial discharge initial voltage for the proposed optical system is 21.5% lower than that for the PZT system. The enhanced sensitivity makes the Michelson ultrasonic sensing system a potential method to detect the small defects in power transformer.

Dissolved gas analysis (DGA) is widely used in monitoring and diagnosing of power transformer, since the insulation material in the power transformer decomposes gases under abnormal operation condition. Among the gases, acetylene, as a symbol of low energy spark discharge and high energy electrical faults (arc discharge) of power transformer, is an important monitoring parameter. The current gas detection method used by the online DGA equipment suffers from problems such as cross sensitivity, electromagnetic compatibility and reliability. In this paper, an optical gas detection system based on TDLAS technology is proposed to detect acetylene dissolved in transformer oil. We selected a 1530.370 nm laser in the near infrared wavelength range to correspond to the absorption peak of acetylene, while using the wavelength modulation strategy and Herriott cell to improve the detection precision. Results show that the limit of detection reaches 0.49 ppm. The detection system responds quickly to changes of gas concentration and is easily to maintenance while has no electromagnetic interference, cross-sensitivity, or carrier gas. In addition, a complete detection process of the system takes only 8 minutes, implying a practical prospect of online monitoring technology.

This paper introduces an HVDC SF6/N2 gas insulated transmission line (GIL) prototype that operates under 100 kV DC application. In the first part, the development history of GIL products over the past 50 years is reviewed. After that, the design concept of the 100 kV HVDC SF6/N2 GIL is introduced. A GIL prototype was prepared and the DC withstanding voltage test and DC superimposed lightning impulse voltage test were performed. The results show that this GIL prototype passed all the required tests, including hydrostatic test, gas tightness test, thermal cycling test, DC withstanding test, and DC superimposed impulse test, etc. The successful preparation of this product promoted a major step in researches of DC GIL and the prototype laid a solid foundation for the development of subsequent industrial products.