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.).