DC Ramp testing of Electric Power Generator Stator Windings Umbertomilano@hotmail.com The DC Ramp test has been performed over 5400 times at Reclamation and only 4 failures occurred under test as the test is usually terminated prior to failure. Those 4 failures had to do with instances where one would never want to put a unit back in service. The test current is so low and the voltage so controlled that core damage simply does not occur. In discussing the DC Ramp test please refer to figure 1. When performing a DC Ramp test, the voltage is raised in a linear 1 kV/minute ramp fashion (16.667 volts/second) and the resultant current is plotted against the voltage. This voltage ramp produces a capacitive charging current that is a constant dc current. The absorption current under these test conditions becomes a linearly increasing ramp. Any deviation from this ramp is then either a current through the insulation (conduction current) or a current over the surface of the insulation (leakage current). The shape of the plot allows one to determine the quality of the insulation. The initial current response provides information regarding the condition of the grading treatment. The slope of the current ramp provides information as to the absorption current and the insulation related constants. Deviations in the slope provide information regarding the leakage and absorption current and thus provide insight regarding insulation condition. Another property of the curve often referred to as snaking (refers to the shape of the curve). The snaking indicates that the capacitance of the insulation is increasing. This occurs at the upper test voltage levels and is the result of considerable internal ionization and internal conductive byproducts of the aging processes. This ionization and the conductive byproducts at the higher test voltages effectively short out several layers of insulation thereby increasing the capacitance and thus produces the snaking effect. Ionization is the release of electrons from various molecules within the insulation system in a high electric field. These free electrons and the conductive contamination essentially short out several layers of insulation. The insulation basically becomes thinner and; the thinner the insulation the higher the measured capacitance of the insulation. In essence this is similar to blowing up a balloon. As we increase the air pressure (voltage) the balloon surface thickness becomes thinner and the balloon capacity to hold air (capacitance) increases. Figure 2 shows insulation that is severely aged due to conductive byproducts and ionization. Figure 3 is indicative of insulation that has high conduction currents due to damage in a single coil or in a couple of coils. A similar plot with a more gradual increase in conduction current is usually related to conduction current that is wide spread and would be typical of an aging winding ….similar to what is seen in photo 1. Figure 1: Showing the various components of DC Ramp testing current components. This DC Ramp test result shows quality equipment test results for an aging insulation. The initial curvature below 5 kv is typical of quality grading treatment and shows the grading treatment is not contaminated. The green tag refers to the capacitive charging current. The blue tag identifies the absorption current component of the total current. The red tag identifies the leakage or conduction current. In an aged winding that is not contaminated this red tag component is typically the conduction current that is through the groundwall insulation. The insulation system condition is related to the conduction current plot. The shape, the voltage level at which the conduction current becomes prominent, the current level and the conduction current magnitude are all important in assessing the insulation system. Figure 2 above shows snaking due to internal conductive deterioration and ionization of a very aged winding. This observation when so pronounced indicates the winding is at or nearing end of life. In some instances, when this is found in a fairly new winding, a corona probe test can be performed to determine if the problem is limited to only one or 2 coils or is occurring in general in the majority of coils operating at or near line potential. If the DC Ramp test on each phase has the same degree of snaking the deterioration is not limited to only one or 2 coils, however, a corona probe test can be performed to locate the coils that are in danger of failing. In that case the, those few bars can be replaced to extend the winding life considerably longer or perhaps long enough to schedule an outage or to procure a replacement winding. (Turbo Alternator replacement can be procured in a very short time, however, hydro units having hundreds of bars take considerably longer to obtain a replacement winding.) Figure 3 shows conduction current due to conductive paths within the coil/bar indicative of unacceptable insulation quality. The current magnitude is very high and in indicative of serious winding damage. In this case water during the brazing process traveled into the insulation and worked down inside the insulation and into to the insulation beneath the grading treatment. It usually gets in through/between the strand insulation. Further, contrary to common belief epoxy and epoxy mica insulation is not water proof (it may be water resistant but it is not water proof.) Best to keep the brazing operations to induction brazing and use appropriate measure to prevent or minimize water ingress. The water will and does react with the epoxy to cause the insulation tapes to become soft and flexible and thus lose their insulating qualities. Water causes the epoxy to swell and breakdown. This type of response is also typical of damaged bars (cracks, physical damage, production issue, etc.).