This paper investigates the aging behavior and lifetime performance of full-size stator and rotor bars subjected to realistic inverter voltage stress. A dedicated high-voltage test setup combining an AC transformer and a bipolar square-wave generator is used to simulate voltage waveforms representative of modern inverter topologies, including voltage source inverters (VSI) and modular multilevel converters (MMC). The results enable a comparative evaluation of insulation systems used in DFIM rotor bars and CFSM stator bars under different converter technologies.

The research and development of innovative insulation systems for High Voltage Rotating Machines demand a deeper understanding and characterization of the failure mechanisms involved in the new technology. When only the electrical stress is considered, the literature describes that the breakdown of the insulation is determined by the electrical tree propagation through the material. Hence, it is important to have techniques to characterize the path of electrical tree through insulation after its rupture. Albeit some disruptive technologies are being proposed, most of the insulation still consists of mica, a carrier material, and a binder resin. Such insulation is considerably rigid, making difficult to separate one layer from the other and to follow the damage path left by the breakdown once the mica layers are broken during this process. This work describes an alternative method to characterize this electrical failure path, which consists of submitting the sample to a calcination process at high temperature before peeling it. The calcination process extracts all organic materials from insulation. In this condition, it is much easier to peel the insulation layer by layer and the carbonization path produced by electrical discharge can be clearly observed and characterized. This work illustrates this proposed technique by analyzing samples extracted from stator bars previously submitted to voltage endurance test up to electrical failure.

Converter feed synchronous generators have some important advantages in pump storage hydro power stations compared to conventional synchronous generators with fixed speed. Due to the converter voltage pulses with large du/dt and high frequency content the winding insulation is electrically and thermally stressed different than for power frequency. As known from MV motors especially the stress grading system is very sensitive regarding these voltage pulse trains. In this feasibility study the electric and thermal stress resulting of the stress grading system is analyzed via a FEM-model. This is done for different for voltage characteristics, i.e. sinusoidal and impulse voltages. The intention is to compare and assess the field strength distribution along the bar surface and the hot-spot temperature in the stress grading layer depending on the voltage signal. Finally, some conclusions and recommendations are given for converter feed windings in hydro-generators.

In this paper different kinds of electrical driven failure modes in the end-winding region, are discussed and classified. Failure types related to the stress grading system are analyzed via numerical simulation models and root causes are given for the different defects based on the calculation results. Typical pictures of various findings are presented and explained. In a last step possible diagnostic measurements to identify issues related to the stress grading are discussed and some PD-pattern are presented exemplarily. The aim of this work is to give the basis to correlate findings in the end-winding and especially the highly stressed stress grading area with their most likely cause of defect.

The ability of IEEE 1310-2012 test method to simulate the possible insulation aging mechanisms of a generator working under repetitive start-stops was studied. Real stator bars, previously aged under service by about 10,000 start-stops were characterized in the laboratory and then submitted to a limited number of thermal cycles according to IEEE 1310-2012 for comparison. The results suggest that a small number of lab thermal cycles is sufficient to create defects that did not exist in the bars aged solely under operation conditions. It is also found that the lab cycles tend to propagate the very few existing defects towards areas where such features were not present in the original condition, such as the inner portion of the stator core. Therefore, depending on how exactly the test results generated by IEEE 1310-2012 are interpreted, there is a risk that the final assessment of any set of tested bars/coils is partially influenced by such test artifacts.

In the slot portion of a generator core the installed stator bars are covered with a semi-conductive coating to prevent partial discharge (PD) activity between the main insulation surface and the grounded laminated sheets. Typically, this outer corona protection (OCP) layer ends a few centimeters outside the slot portion. To suppress PD-activity at the end of the OCP, a field grading is mandatory for machines with a rated voltage greater than approximately 6 kV. Figure 1 shows the effect of a stress grading layer on stator bars energized to high voltage. It suppresses potential creepage discharges at the OCP ends caused by field enhancement.

Fundamental research program had been started to utilize silica nanotechnology to improve the electrical and mechanical properties of generator stator winding insulation. When tested at 0.6 m long tube-like specimens with original epoxy-mica insulation system the new SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposite insulation shows an electrical lifetime curve with a smaller sloop compared to the established winding insulation resulting in substantial improvement of lifetime at rated and operational electrical stresses. As next development step original stator bars with typical design parameter for hydro generators were fabricated. Voltage endurance tests at different stress level were performed to obtain electrical lifetime curves. First results demonstrate significant longer lifetime of nanocomposite compared to the actually used insulation system. The use of SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles in epoxy-mica insulation will lead to a more efficient stator winding insulation system, which improves the specific power output on one hand and is tailored to the new highly flexible grid demands without reduction on lifetime on the other hand.

After showing in a previous work that PD measurements suffer from poor repeatability and reproducibility, the present study turns now to a more qualitative approach, investigating if a method of PD fault location along a Roebel bar length can be used to identify insulation critical areas that later on could fail under electrical stress. Typical insulation “damages” have been created on the components in advance by submitting them to both thermal-mechanical and electrical aging tests. Once such preliminary aging was done, the critical areas along the bars length were identified by mapping the PD readings on each component. Afterwards, every bar was subjected to a destructive electrical breakdown test. At last, the breakdown locations were compared with the PD mappings to search for any possible correlation between the two events. Based upon the present results, such correlation does not exist.

This paper gives an overview about comprehensive investigations and analysis on the stress grading system of a 20 kV rated voltage hydro bar. This includes relevant electrical and thermal calculation results of 3 different FEM-models to assess the efficiency of the stress grading system. Further investigations show sufficient correlation of calculation results with measurements conducted in the laboratory.

In spite of the fact that no evidence exist that PD amplitudes can somehow be used to estimate the expected "life" of generator components such as Roebel stator bars and multi-turn coils, certain portions of industry have assumed that pass-fail criteria based upon absolute PD amplitudes are meaningful enough to be used as pass-fail tests for Roebel bars and Multi-turn coils. In this context the present work evaluated the ability of expert independent laboratories to generate reproducible PD readings when performing PD tests on the very same set of bars.

Pumped-Storage Power Plants belong to energy grids systems since more than 100 years. These power plants gained on significance in recent years, as they play an important supporting role on integration of renewable energy sources in terms of frequency and voltage stability. In the past few years the development of frequency converter technology made it possible to apply variable speed generator-motors of large power input/output.This leads to unique advantages for grid stabilization due to application of variable speed technology. This development will be shown at the pumped storage power plant project Frades II in the northwest of Prtugal.

The evaluation of high-voltage groundwall insulation aged under long-term operating conditions is addressed using three sets of Roebel bars removed from large hydrogenerators after several decades of service. To cover an important cross-section of the possible operating regimes for typical hydros, the units involved in the study were selected to represent (i) the air-cooled Pumped-storage, (ii) the air-cooled base-load and (iii) the water-cooled base-load service regimes. Experimental techniques were used to permit assessment of the insulation condition under visual, electrical and morphological aspects. Contrary to what is usually assumed, the results suggest that the pumped-storage operating regime, with its repeated thermal-mechanical cycles of start-stops may create only minimum aging to the groundwall insulation while, in some particular cases, units operating at relatively low temperatures and under mild base-load conditions, may be found to exhibit a more severe insulation degradation in the long run.