Oil Pressure Monitoring for Sealing Failure Detection and Diagnosis of Power Transformer Bushing

Power transformer bushings withstand great electrical and mechanical stress during high voltage and high current working conditions. Sealing failure poses a great threat to the long-term and reliable operation of the bushing and power transformer; however, the criterion to evaluate the sealing status of a bushing caused by mechanical problems is still lacking. In this paper, a transformer bushing model is established to gain theoretical insight into the relationship between temperature and pressure of a compact multilayer bushing. To evaluate the bushing mechanical status, different sealing conditions are tested based on the temperature and pressure monitoring within the physical 110 kV bushing. The results show that mechanical sealing failure can be diagnosed when the fluctuation of the oil pressure value exceeds the theoretical curve in steady state by 3 kPa. With different reliability coefficients, gas leakage and oil leakage are available to be further determined. The primary and auxiliary criteria based on oil pressure and its gradient are proposed to evaluate comprehensively the actual sealing condition of the bushing, and a wireless oil pressure module is developed at the bottom valve, which is quite beneficial to field online application. It is promising to extend the online mechanical monitoring and diagnosis to oil-immersed power equipment.

How to Seal Hydraulic Fracturing Boreholes in the Large-Size HDR Rocks under HTHP Conditions

The hydraulic fracturing (HF) is a key technique to enhance the permeability and heat production of hot-dry-rock (HDR) geothermal reservoirs. Normally, laboratory HF tests should be preconducted to understand the HF characteristics of HDR samples. However, in the laboratory test, sealing failure between boreholes and injection pipes always limits the experimental efficiency and data accuracy, especially for the HF tests under high-temperature and high-pressure (HTHP) conditions. Traditional sealing methods, such as rubber and cement sealing, are easy to be failed because of their poor load and/or thermal bear performance under HTHP conditions. Therefore, in this study, we proposed a novel HTHP seal by using wedge-buckled copper components and steel rings. The sealing efficiency was verified by successfully conducting the HF tests of HDR rocks with a dimension of φ200×400 mm under various high temperatures ranging from 100°C to 400°C. As expected, the unfavorable factors such as HTHP and high injection pressure could be turned into favorable ones during the introduced seal method. By this investigation, we expect to provide some sealing solutions for researchers when conducting HF tests under HTHP environments.

Study on the Unbalanced Curl Seal Failure of the Magnetorheological Fluid Sealing Device of the Hydraulic Turbine Main Shaft under Different Speed Abrupt Conditions

The fluid flow in the runner of a hydraulic turbine has serious uncertainties. The sealing failure of the magnetorheological (MR) fluid sealing device of the main shaft of the hydroturbine, caused by a sudden change in speed, has always been a difficult topic to research. This study first derives the MR fluid seal pressure and unbalanced curl equations of the hydroturbine main shaft, and then analyzes the seal pressure and friction heat under different rotational speed mutation conditions through experiments. After verification, the temperature field and magnetic field distribution of the MR fluid sealing device of the main shaft of the hydraulic turbine are obtained via numerical calculation. The results show that the external magnetic field affects the magnetic moment of the magnetic particles in the MR fluid, resulting in a significant change in frictional heat, thereby reducing the saturation of magnetic induction intensity of the MR fluid. This results in a decrease in the sealing ability of the device. The size and abrupt amplitude of the main shaft of the hydraulic turbine, and friction heat is positively correlated reducing the sealing ability of the device and causing sealing failure. Based on our results, we recommend adding the necessary cooling to the device to reduce the frictional heat, thereby increasing the seal life of the device.

Study on Preapplied Annulus Backpressure Increasing the Sealing Ability of Cement Sheath in Shale Gas Wells

Summary Annulus pressure buildup (APB) problems in shale gas wells seriously affected on the safety and efficient exploitation of shale gas all around the world. The sealing failure of the cement sheath on interfaces caused by periodically changed fluid pressure in casing during hydraulic fracturing is treated as the main reason for APB in shale gas wells. Many methods are put forward to solve the APB problem in the field, and fortunately, the preapplied annulus backpressure (PABP) method shows an excellent utility. In this paper, an analytical model is established to explain the mechanism of the PABP method increasing the sealing ability of the cement sheath. The residual strain of the cement sheath and radial stress on interfaces are considered to analyze the factors that affect the effectiveness of the PABP method. In addition, based on the field data, an experimental device is established to test the validity of the PABP method and to certify the accuracy of the analytical model established in this paper. The analytical results show that the thickness of the casing has little effect on radial stress on interfaces. The outer diameter of the casing and the thickness of the cement sheath can temperately affect the radial stress. The elastic modulus of the cement sheath and the formation rock can significantly affect the radial stress. The higher elastic modulus of the cement sheath can dramatically increase the radial stress on interfaces. On the contrary, the higher elastic modulus of formation rock will induce smaller radial stress on the interfaces. In the field, the number of newly added shale gas wells with APB problems has dramatically decreased by using the PABP method. The work in this paper can be significantly useful for researchers and engineers to reduce the APB in shale gas wells.

Experimental study on application characteristics of zeolite/water stuffing for the nanofluidic packer rubber

Aiming at sealing failure problem of packer rubber during well testing and completion, a new type of “nanofluidic packer rubber” is developed. The nanofluidic packer rubber is composed of honeycomb matrix skeleton encapsulating nanofluidic system as stuffing. Taking ZSM-5 zeolite/water nanofluidic system as an example stuffing for the nanofluidic packer rubber, the application properties are studied by means of experiment. Ten loading/unloading cycles are carried out on the pretreated zeolite/water stuffing at different loading rates and system temperatures on a pressure-volume characteristic test bench. The impact law of loading rate and system temperature on repeatable practicability, pressure threshold, and deformation capacity of the stuffing are obtained and the influence mechanisms are discussed. Results show that the zeolite/water stuffing works stable and repeatable after the first three loading/unloading cycles. The loading rate has lifting effects on throughput capacity, pressure threshold and deformation capacity when system temperature is under 75°C. With the increase of system temperature, pressure threshold decrease, and throughput capacity and deformation capacity increase. All the application characteristics found in zeolite/water stuffing are favorable for improving the working performance of packer rubber. This work provides theoretical and data support for the application of the nanofluidic packer rubber.

Experimental study on the thermal damage characteristics of cement stone

Aiming at the problem of cement ring sealing failure during deep high-temperature shale gas exploitation, comprehensively considering the influence of the characteristics of multi-cluster fracturing of multiple horizontal wells and formation temperature, the cementing cement the southwest region is taken as the research object. After exposure to different temperatures (95?C and 135?C) and for different times (5, 10 and 20 times), axial and triaxial tests with different confining pressures (0, 5 MPa, 15 MPa and 30 MPa) were carried out. The research shows that: (1) the stress-strain curve of cement stone after heat treatment can be divided into four stages: compaction, elastic, yield and post-peak stage. As the confining pressure increases, the compaction phase disappears, the yield phase increases, and we see the transition from brittle to ideal plasticity after the peak; (2) as the temperature and number of thermal cycles increase, the cohesive force decreases significantly, and the internal friction angle shows a slight increase. The elastic modulus and the peak strength decreased.