Condition assessment of power transmission devices such cable systems is essential for their reliable operation. For example, diagnostics of oil-filled cables is important for transmission system operators to provide an indication of remaining lifetime. This aspect is also relevant for cable systems with extruded insulation. There are mature technologies that allow both continuous monitoring and also periodic assessment of the condition of a cable system. Temperature measurement on the cable surface and partial discharge sensors are examples of such monitoring systems and partial discharge measurements are established tools for condition assessment of cable accessories as well.
Most outdoor terminations these days are filled with an insulating fluid, typically silicone oil, and a diagnostic of this fluid therefore offers another means of overall assessment, apart from PD measurement. The key parameters in this regard are moisture content, dielectric properties and dissolved gases. This INMR contribution by Jan Debus of Brugg Cables in Switzerland describes methods to assess terminations used on cables with extruded insulation and which are the interfaces to other components of the power grid. These can be classified into indoor terminations for connection with transformers and gas-insulated switchgear or outdoor terminations for connection to an overhead line or switchyard.
Design of Fluid-Filled Outdoor Terminations
A typical fluid filled outdoor cable termination is shown in Fig. 1. Much the same design is used for voltages Un = 60 kV to 500 kV. The main component of this accessory is the stress control element, which uses a semi-conductive electrode made of silicone rubber or EPDM for geometric field grading. The outer insulator can be porcelain or a composite housing, depending on customer preference.
As mentioned, silicone oil is the main insulating fluid in use these days. A certain chain length of silicone molecules is necessary to avoid migration into the silicone rubber of the stress cone. The consequence is a relatively high viscosity if the stress cone is made from silicone. But there are also terminations still under operation that are filled with mineral oil in spite of the fact that environmental impact of leaks has caused the shift in technology towards the silicone oil, which is also non-flammable.
Analysis of Dissolved Gases
Dissolved gas analysis (DGA) is a well-established technology for assessing condition of power transformers and also widely used for evaluation of old oil-filled paper (LPOF) type cables. In both cases, the insulating fluid is typically mineral oil, as also used in the past in outdoor terminations for extruded high voltage cable. The aim of a DGA is detection of partial discharges in the insulating medium so as to prevent failure of the termination. Hot spots that can sometimes occur in power transformers are highly unusual in a termination since any heat losses can easily be removed due to the large surface area relative to oil volume. The basic procedure in a dissolved gas analysis consists of the following:
• Taking sample from the termination;
• Extracting the gas from the oil;
• Analyzing the gas sample by computer tomography;
• Interpreting findings according guidelines of the manufacturer.
Convective flow causes a homogenous mixture of the oil during operation and a sample taken from the lower base plate of the termination can be therefore be regarded as representative. Sampling using a probe is theoretically possible during service but typically has to be done offline for reasons of safety. It is obviously important to avoid any contamination of the sample. There are a number of possible procedures for extracting gases from the oil and each procedure might yield somewhat different results. As a result, it is recommended to always use the same laboratory. Moreover, what is also important in such assessment, apart from absolute value, is the trend.
There is no standard available that could be used for interpretation of results. Therefore, the experience of the accessory manufacturer is necessary to properly assess fluid condition based on gas content. The limits, given in Table 1 are based on such experience and are classified into A, B and C. As long as gas content stays below the values in A, no further maintenance action is required. Findings in the range of B, suggest that further tests or a repetition of testing after a shorter than normal interval are recommended. Reaching the limits given in C generally implies that immediate action must be taken.
The main gases for such evaluation are hydrogen (H2) and acetylene ethyne (C2H2). H2 indicates that partial discharges of low energy are occurring inside the termination. If C2H2 is detected as well, it could be concluded that there have been partial discharges of higher energy or even some electric arcing taking place inside the fluid. The presence of methane and propane are further indicators of internal electrical activity in the insulating fluid, mainly in relation to the mineral oil in an oil-filled cable or transformer. However, evaluating the condition of a termination on a polymeric XLPE cable is not always possible due to the fact that both of these gases are generated during cable production as by-products of the cross-linking process. These gases could therefore remain in the insulation if the cable is not properly de-gassed after extrusion and migrate during operation into the insulating fluid of the termination