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June 16, 2014

In order to meet with the requirements of the existing IEEE 286-2000 and the new upcoming IEC 60034-27-3 standard for dielectric dissipation factor testing (also known as tangent delta testing) on rotating machine stator windings and individual bars, Power Diagnostix made a re-design of the existing ICMflex, so called ICMflex GRC "Guard Ring Control".

Both standards mentioned above apply to rotating machinery with a rated voltage of 6 kV and higher and describe the test procedures and evaluation criteria for tangent delta testing of individual stator bars and complete assembled stator windings. The testing as described applies to the common insulation system techniques in use, such as the resin rich and global vacuum pressure impregnated systems. However, the standards are not applicable for non-impregnated stator bars, also called green coils.

The main purpose of tangent delta and tangent delta tip-up testing is determining the overall condition of the stator winding or bar's slot section also called ground wall insulation. The test results mainly show the performance in terms ionization losses versus the applied voltage of the ground wall insulation under the slot's conductive outer layer, acting as the ground electrode to the stator core.

Usually, the stator bars with a rated voltage of 6 kV and higher are provided of a field grading junction, consisting of a semi-conductive material, e. g. silicon carbide, at the slot-exit area. In order to grade the surface potential were the stator bar leaves the grounded core. Depending on the rated voltage, this high resistance material with non-linear resistive voltage characteristic needs to be overlapped for a certain length with the slot's linear resistive layer applied to the ground wall. Based on such design, the tangent delta measurement, which is intended for the ground wall insulation only, may be affected by the resistive losses of the field grading junction. For complete assembled stator windings, the contribution of the losses generated by the semi-conductive material in the overall tangent delta level cannot be neglected, and, hence, the regular ICMflex can still be used. However, as the evaluation criteria are specified for the slot section of individual bars only, guarding techniques shall be applied in order to minimize the influence of the field grading junction's losses on the overall tangent delta value.

Several techniques have been studied during the past such as the foil wrap methods and the slot-end gap method. Nowadays, with automatized bar manufacturing processes, it is practically not feasible to make an interruption at the slot exit and to provide the semi-conductive material after the tangent delta test, since this will cause tremendous delays and, in worst case, essential failures affecting the integrity of the insulation system. Besides this, both standards recommend performing the measurement on the end product, i. e. the individual bar provided with the field grading junction, and, hence, using both techniques as mentioned above are not an option. The only technique left is the driven guard method.

The new GRC (Guard Ring Control) option provides such driven guard inputs on the ICMflex' digital tangent delta bridge. Beside this new feature, the instrument still keeps its versatility by enabling the simultaneously updated partial discharge and tangent delta measurement results with the highest precision. This flexible unique concept strongly reduces the (re)winding and testing times in rotating machine manufacturing companies and service groups and can even be used in quality assurance labs for non-destructive evaluation of the ground wall performance of individual stator bars.

The ICMflex GRC is optimized for the capacitance range of stator bars and for smaller asynchronous induction motors up to Roebel bars for larger synchronous turbo generators. Furthermore, the embedded voltage divider for up to 30 kVRMS comes with a DAkkS (former DKD) calibration certificate.

The instrument is battery operated (up to 10 hours) and equipped with a fiber optic link for communication with the computer via USB. The advanced software provides manual and automatic record modes, even a step-by-step guidance structure with customized report after completing the test sequence. The data tables, phase resolved PD-patterns, different charts, and graphs can be exported into common spreadsheet processing packages such as MS Excel.