Laurent Lamarre

PDC measurements to assess machine insulation

DC testing is probably the most commonly used maintenance and diagnostic tests periodically conducted on machine stator insulation systems. With the commercial availability of more sophisticated equipment it is now possible to continuously monitor both charge and discharge current during a step voltage test, also known as polarization/depolarization current (PDC) test. This test is related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encountered in the field and the usefulness of using both the charge and discharge currents to assess the condition of the insulation system.

The use of time domain spectroscopy as a diagnostic tool for rotating machine windings

In order to assess stator winding insulation condition, a field instrument measuring the charge and the discharge current flowing through rotating machine stator winding groundwall insulation was developed at Hydro-Quebec. This instrument applies a constant DC-voltage of 1 kV during a certain time, usually 2000 s. Immediately following the charging, the stator winding is short-circuited during another period of time. The current is continuously recorded as a function of time during both steps. It is suggested that the magnitude and the shape of both curves may be indicative of the degree of aging of the insulating material. A number of measurements obtained either from generator windings at Hydro-Quebec, or from laboratory aged specimens were examined; spare bars or coils were also measured and used as reference. Measurements were conducted essentially on asphalt and epoxy bonded insulating material which are the two major stator groundwall insulating material for the 350 or so generators at Hydro-Quebec. The method was found particularly useful to detect delamination or leakage current due to moisture absorption or end-winding contamination.

Effect of pressure and length on interfacial breakdown between two dielectric surfaces

The phenomenon of interfacial AC breakdown is considered. Two pieces of EPDM (ethylene-propylene diene monomer) rubber were pressed together and a special jig was conceived to provoke arcing along the interface. The voltage at which breakdown occurred was measured and its dependence on interfacial length and pressure established. In the absence of silicone grease at the interface, breakdown voltage showed a linear dependence on pressure. With silicone grease, breakdown values were consistently higher and reached a plateau at higher pressures. The dependence on length was similar to what is observed for bulk dielectrics. While the dielectric strength curve for a silicone-greased interface decreased with length, the one with a dry interface, although it followed the same general behavior, exhibited an unexplained dip at around 4 mm.<<ETX>>

Temperature dependence of the resistance of modern epoxy mica insulation of HV rotating machines

This work presents the temperature dependence of the dielectric properties of two types of epoxy mica insulation systems. Polarization and depolarization currents, insulation resistance, capacitance and polarization index were measured over a temperature range of 20 to 100degC on whole rotating machine stator bars. An analytical relationship of the correction of the resistance as a function of temperature was derived from the experimental results.

Modeling of the Dielectric Response of a Stator Winding Insulation from a DC Ramp Test

The aging due to mechanical, thermal, electrical and environmental stresses of a stator insulation system inherently involves alterations of the material properties which are detrimental to its service operation. When these properties are deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life. To prevent such forced outages and even to improve the useful life, condition-based maintenance and diagnostic tests are periodically conducted on stator insulation system. Among the tests performed to assess the condition of winding insulation, various methods are commonly used like partial discharge measurements, hi-pot, step voltage and ramped voltage tests to name just a few. The I-V curve measured during a ramped direct voltage test, as the one obtained in a polarization/depolarization test, includes the contribution of various current components, the capacitive, the absorption, the surface leakage and the bulk conduction currents. These contributions, especially the absorption and the conduction (surface or volume) currents, are not as easy to separate in a ramped voltage test as in a polarization/depolarization test (step voltage test). A modeling of the I-V curve obtained with the ramped voltage test, allowing an easy separation of the leakage current from the absorption and the capacitive currents, is presented in this paper. This modeling is compared with a field measurement on complete stator winding and with a lab measurement on spare coil. The calculated values of the absorption and leakage currents obtained during a ramped direct voltage test are compared with the same components deduced from a polarization/depolarization test.

Progress in DC Testing of Generator Stator Windings: Theoretical Considerations and Laboratory Tests

The aging due to mechanical, thermal, electrical, and environmental stresses of a stator insulation system inherently involves alterations of the material properties that are detrimental to its service operation. When these properties are deteriorated to the point where the material can no longer operate safely under normal stress conditions, it implies that it has reached the end of its useful life. To prevent such forced outages and improve the useful life, condition-based maintenance and diagnostic tests are periodically conducted on stator insulation systems. Among the tests performed to assess the condition of winding insulation, various methods are commonly used, such as partial discharge measurements, hipot, step voltage, and ramped voltage tests (RTs), to name just a few. DC testing, where the current is continuously monitored, whether during a step voltage test, such as the polarization/depolarization test, or during a linearly increasing voltage test, such as the RT, is commonly used in the field in order to assess the quality of stator winding insulating systems of large synchronous rotating machines. Both of these tests are related to the dielectric response of the insulation system. This paper presents theoretical considerations on the dielectric response of the various types of machine winding insulation systems encounter in the field as well as laboratory results on individual bars and coils.

Influence of rise time on dielectric parameters extracted from time domain spectroscopy in the context of generator stator insulation

The low-frequency dielectric response of generator stator winding insulation materials was calculated from high voltage time domain spectroscopy measurements. These results were used to calculate the theoretical response after the application of a voltage function consisting of a ramped voltage followed by a constant level voltage. The rate of rise of the ramped portion of the voltage function was shown to influence the well known and much used polarization index (PI) and insulation resistance (IR) parameters.

Impact of humidity on dielectric response of rotating machines insulation system

The study of dielectric response is a significant method to characterize the electrical properties of insulating materials and a useful tool for condition assessment of the insulation systems of rotating machines. Time and frequency domain measurements can be used to determine the dielectric response. Polarization and depolarization current measurements are usually used as a time domain measurement while in frequency domain the capacitance and dissipation factor of the samples can be measured over a broad range of frequencies. The dielectric response of machine insulation is affected by variety of parameters such as degree of aging, contamination and the environmental conditions such as temperature and humidity. In this work, time and frequency methods are used to determine the impact of humidity on the dielectric response of different insulation technologies used for stator insulation of large rotating machines. The measurements were conducted on bars and coils in laboratory conditions as a comparison base and then the dielectric response of the samples was monitored in an environmental chamber during two different humidification periods. The samples were selected to cover the most common materials and technologies used in operating rotating machines including asphalt; epoxy and polyester based insulating systems. The results showed considerable change in the dielectric response when the samples were exposed to moisture. Further studies demonstrated that the change in dielectric response is mostly attributable to the end-winding and stress-grading layer.

Study on the effect of humidity on dielectric response and partial discharges activity of machine insulation materials

Operational and environmental conditions can affect the dielectric response of the insulation system of large rotating machines. Humidity is one of the well-known environmental parameters which could affect dielectric parameters such as insulation resistance, polarization index and dissipation factor up to a certain degree. It also could affect partial discharges occurring in rotating electrical machines with open ventilated cooling system. In order to investigate the effect of humidity on the characteristics of different insulation systems, one asphalt coil and one epoxy bar were subjected to high humidity for a certain period of time and the humidity effect on time and frequency domain dielectric responses were studied. Also partial discharge measurements were conducted on two samples to study the possible correlation between partial discharges and changes in other parameters such as insulation resistance and polarization index due to exposure to high humidity.

The use of transient current for the evaluation of the condition of rotor and stator insulation systems of large synchronous rotating machines

Results related to the measurement of transient currents recorded after a DC voltage step followed by a short-circuit are reported for both rotor and stator windings for a number of hydro and turbo synchronous rotating machines, motors and generators, in order to assess the quality of the insulating system. Measurements were conducted in the field for windings from different technology yielding different time domain response. Besides reckoning the aging of the insulation, other features such as moisture absorption, end-winding contamination and incomplete curing of the insulating resin (followed by post-curing) were detected in several cases