Detecting and locating local degradations at an incipient stage is very important for mission-critical high-voltage rotating machines. One particular challenge in the existing testing techniques is that the characteristic of a local incipient defect is not prominent due to various factors such as averaging with the healthy remainder, attenuation in signal propagation, interference, and varied operating conditions. This paper proposes and investigates the frequency domain reflectometry (FDR) technique based on the scattering parameter measurement. The FDR result presents the object length, wave impedance, and reflections due to impedance discontinuity along the measured windings. Experiments were performed on two commercial coils with artificially created defects. These defects include turn-to-turn short, surface creepage, loose coils, insufficient end-winding spacing, and local overheating, which are commonly seen in practice. Two practical water pumps in the field were also selected for investigation. The study outcome shows that FDR can identify and locate structural and insulation degradation in both shielded and unshielded objects with good sensitivity. This makes FDR a complementary technique for machine fault diagnosis and aging assessment.

The reliability of power transformers is crucial for the safety operation of the power system. Detection of incipient faults, as well as natural aging, is the key to reduce the failure risk, which gives the operators adequate margin to perform maintenance before reaching a critical failure. Routine maintenance consists of a few testing techniques to check whether mechanical and electrical components fulfill the minimum threshold requirement. In addition, there are various advanced diagnostic testings that are capable of giving more precise condition indications of a transformer in thermal, electrical, and mechanical aspects. The reliability of transformers can thus be enhanced with the help of advanced diagnostic testings. However, in practice, it is often costly to perform the advanced testings and the effectiveness is hard to verify due to the lack of relevant cases and case studies. According to the published statistics, the failure rate is only around 0.1% - 0.2% per year. In this project, a widely accepted insulation condition diagnostic method, Dielectric Frequency Response, DFR was investigated in aspects of cost and return. Testing objects were a group of transformer bushings in three HVDC substations. Reliability is enhanced by identifying incipient bushing defects that cannot be detected by other routine testing techniques. By analyzing the cost and return of the DFR testing, the transition of the current maintenance strategy towards reliability-centered is in position.

Composite insulation systems usually exhibit nonlinear phenomena, such as voltage-dependent dielectric response and high-order current harmonics. In this article, a high-voltage system capable of performing both dielectric frequency response (DFR) and polarization/depolarization current (PDC) measurements up to 20 kV is devised. The nonlinear properties of the motor stator and power transformer insulation systems are studied. Experimental results show that the voltage-dependent effect becomes significant as the voltage magnitude approaches the nominal and frequency decreases. The third current harmonic component is found to be dominant over the other components and has a strong correlation with the decrease in the dissipation factor. By taking both the fundamental and third-harmonic components into account, more consistent voltage-dependent characteristics are revealed. These features are important indications of insulation defects that are often neglected in field diagnostic testings. The investigation of the nonlinear properties helps to enhance the condition diagnosis reliability.

Carrying out the insulation condition measurement and analysis is important for the safe operation of the submarinecable. In this study, the high-voltage frequency dielectric response is used to identify the ageing state of the AC 500 kV cross-linked polyethylene (XLPE) submarine cable. The complex capacitance and tanδ in the low-frequency area for the new, thermal ageing, electro-thermal ageing cable all increase with the testing voltage increased from 200 to 2000 V. It is founded that the dielectric response at high voltage is more likely to detect the ageing differences. Based on the carbonyl index and the dielectric loss of the XLPE specimens at different position of the insulation layer, the non-uniform ageing phenomenon of the cable insulation is presented for the thermal ageing and electro-thermal ageing cable. The loss quantity parameter and its changing behaviour with testing voltage based on the frequency dielectric response could be used to identify the ageing state. The comparison of the modified Cole–Cole model parameter under 200 and 2000 V also indicates that the higher voltage dielectric response is helpful to identify the ageing differences.

Aging of high voltage generators and motors lead to changes in the dielectric properties of the stator winding insulation system. Research on the relation between aging and dielectric response has been going on for decades. A particular challenge is that the coil insulation consists of several materials: two that strongly affect the dielectric response are the ground-wall insulation and the stress grading shield (end-corona protection, ECP), which exhibit quite different properties. The influence of the ECP on the overall dielectric response of the insulation system might dwarf the condition change of the ground-wall insulation. In this paper, we introduce a test system that can measure the dielectric response in both time and frequency domain from low to rated voltage. Measurement results of two types of custom-designed, commercial standard stator winding coils are presented, and the influence of the ECP can be observed. The validity of time-frequency domain transformation of this nonlinear system is discussed. This paper acts as a basis for the application of dielectric diagnostic testing in the field.

The dielectric response measurement is an important technique to assess the properties of insulation materials. However, the widely used approach with contact between samples and electrodes can in some cases limit the accuracy of the measurement. In this paper, an easily fabricated design is introduced and used to perform non-contact measurements. Air-reference measurements, comparing the sample to an air-gap for improved calibration, are used for all measurements. Results obtained by contact and non-contact methods, and with the feedback of electrometer locked and unlocked are compared. The effect of the pressure applied by the electrode is also investigated for both non-contact and contact measurements. Results show that the non-contact method can be an alternative to reduce contact problems between the sample and electrodes. For air-reference measurements, the impedance measurement instrument should be forced to use the same reference component for the air and sample measurements. Results obtained by the non-contact measurements are less sensitive to the pressure compared to that by contact measurements. 

Insulation condition is an essential aspect for the operational reliability of high voltage rotating machines in power plants and industrial applications. Insulation resistance (IR) and line-frequency dissipation-factor / power-factor (tanδ) measurement are often performed for the assessment of stator insulation condition. These measured values need to be normalized to a reference temperature (e.g. 40 °C) for comparison and trending and this is traditionally achieved by multiplying the results with a certain factor. However, this correction could be subject to error for an individual devicesince the correction factors recommended by various standards are average values of a certain number of machines at different conditions. In addition to that, insulation condition also has some influence on the temperature dependent property. With the introduction of Dielectric Frequency Response, DFR and Polarization/Depolarization Current, PDC as more advanced insulation diagnostic methods, with proper modelling, temperature correction can be done based on the insulation condition of an individual device and thus accuracy is considerably improved. In this paper, the background of DFR and its superiority in temperature correction are introduced. After that, the numerical Fourier and Inverse Fourier Transformation algorithm is applied to correct the time domain measurement (IR and PDC).

Failure of HV transmission transformers can cause extensive damage, interruption of power supply and financial losses. Therefore, techniques capable of detecting potential faults at an early stage are of high importance to the reliable operation of the power grid. The failure mechanisms reported by CIGRE show that winding deformation, insulation breakdown and ancillary component failures (such as bushings and tap changers) are the main causes of transformer failure. Nowadays, sophisticated diagnostic methods used to reveal transformer's condition are being investigated by various utilities and research groups and the outcome of this investigation is very promising. These new methods include dielectric frequency responses (DFR) for insulation diagnosis, sweep frequency response analysis (SFRA) for electro-mechanical integrity assessment and dynamic resistance measurement (DRM) for on-load tap changer testing. In this paper, basic theories and advantages of these diagnostic methods are briefly introduced. Also, some measurement examples obtained in the field are discussed to demonstrate the advantages of these methods.

The presence of moisture in a transfomer needs to be monitored throughout its service life. Moisture deteriorates transformer insulation by decreasing both electrical and mechanical strength. High moisture content accelerates solid insulation aging, reduces the breakdown strength and makes the transformer vulnerable to the overload conditions due to high temperature spot bubbling. In addition to that, partial discharge can occur in a high voltage region becuase of the moisture disturbance. Traditional indirect estimation of the moisture concentration in the solid insulation of power transformers includes testing the oil samples as well as measurements of the insulation resistance and loss tangent (50/60 Hz) of the transformer.  However, these methods usually give limited information and may lead to wrong conclusions. Direct measurement is not viable and may not be representative to take a paper sample from the surface insulation because moisture distribution is not homogeneous along the insulation geometry. Dielectric Frequency Response, DFR was introduced more than 20 years ago and has been thoroughly evaluated and proven. Several documents have been published summarizing the research work and field tests all over the world. DFR is a practical non-intrusive and non-destructuve technique for moisture condition assessment in power transformers, a breakthrough compared with traditional methods. Scientists, researchers and utility operators have shown great interest in the development and application of DFR technique. In this paper, the limitation of traditional methods is presented at first. Later, a comprehensive review of DFR method demonstrates its advantages over traditional methods. Finally, latest research about the mathematical model, temperature correction and test voltage is included to answer common questions regarding the application of DFR method.