Why should we perform Electrical Tests on the Rotating Machine Stator Winding?

Introduction:

Electrical Rotating Machines are one of the key assets in industry. A failure of a Power station Generator during operation lead to a loss of Power on the Grid and have a huge impact on the network stability and lack of Energy supply.

Likewise, MV Motors failures may lead to, in most of the time, a stoppage of production and manufacturing process in many industries, and therefore to high financial losses.

If we take a look on the root cause of failures on Generators and MV Motors, a lot of statistics studies shows that more than 50 % of the failures are caused by electrical insulation issues.

During operation of the rotating machine, the stator winding insulation system is undergoing several types of periodic or continuous stress such as electrical, thermal, mechanical, and ambient stress.

This stress has a bad influence on the integrity of the insulation and could lead to degradation and aging of the insulation components and therefor to big and irreversible damages.

Manufacturing process problems could also contribute to the enhancement of the insulation deterioration and lead to premature defects.

Many Electrical offline testing tools are performed during inspection period in order to assess the health of the stator winding insulation and monitor the degradation process.

In the other hand, Continuous monitoring is increasingly being used to provide information about the insulation status under operation conditions.

Structure of the winding bar insulation:

For machines above 2kV, Mica is considered as the key material used in manufacturing stator winding insulation, due to its temperature and partial discharge resistance capabilities.

A baking material is used with the Mica Tapes to provide tensile strength. The gaps between Mica layers are filled with special resign (mixed with Epoxy). Two main impregnation processes are known in the industry nowadays: the Resign-Rich and VPI (Vacuum Pressure Impregnation) technics. The impregnation process is crucial in terms to produce a high or medium quality of insulation.

In addition, some other semi-conductive layers are added to the groundwall insulation to provide protection against high electrical field stress and surface discharges. IPG (Inner Potential Grading), OCP (Outer Corona Protection) and EPG (End Potential Grading) layers are commonly used for machines rated 6kV and above.

The IPG layer is a semi-conductive tape applied around the conductor (Robel Bar) (mainly for machines rated 10kV and above) to homogenize and reduce the electrical field concentration at the edge of the conductors and prevents partial discharges at this area.

On the other hand, and after the main insulation is applied, the surface of the slot portion of the winding bar is coated with a semi-conductive coating (tape or paint) called OCP layer.

Due to small air gaps between the insulated bar and the laminated core (ground potential), a concentration of electrical field will occur and will lead to discharges within these gaps.

The OCP layer will harmonize this electrical filed along the slot portion of the bar and suppress, therefore, the partial discharges on the surface.

Since the end-winding area is on high potential (even an insulation is applied, due to capacitive coupling this area is on high voltage potential), a high gradient of the electrical field will appear at the end slot part where the laminated core is in ground potential, causing high discharges on the surface of the end winding portion of the bar.

To prevent this surface discharges, a semiconductive coating is applied at this portion (starting from the exit stator slot part), called EPG, to reduce the potential gradient and to smoothen the high voltage potential grading in this area.

Type of defects on Rotating Machine insulation system:

Operating environment of the rotating machine, quality of manufacturing of the insulation system and luck of maintenance are the principal factors that could affect the failure mechanism and aging process of the stator winding insulation.

The main defects on the stator winding insulation and their causes and mechanism could be described as following:

-?????? Voids and delamination could occur due to thermal stress on the insulation. Operating on high temperature will lead to the weakness of the mechanical integrity of the insulation and result on increasing the number and size of internal voids. This will cause an increasing of the partial discharge activity inside the groundwall insulation and will gradually erode the layers.

-?????? Operating on a frequent mode of starts and stops of the rotating machine will cause a high thermal cycling stress on the insulation and could lead to a premature aging of the insulation material. This will lead to high Partial discharges activity due to voids and delamination caused by the aging process, and therefore could cause to a ground fault.

-?????? High Mechanical vibration could lead to cracks and chinks on the groundwall insulation causing Partial discharge increasing activity. In the other hand, the mechanical vibration combined to a poor tightness of the stator wadges, could lead to the abrasion of the OCP layer and therefore to slot discharges.

-?????? External contamination (foreign particles on the cooling system, oil, moisture, chemicals products, etc..) play an important role on failure mechanism and could lead to faster thermal deterioration of the insulation materials, to chemical attacks and electrical tracking.

-?????? Poor manufacturing process (poor impregnation, low graphite quality of coating layer, etc..) is a main factor of premature defects on the insulation system. Early high PD activity will lead to a faster deterioration of the insulation, that may take decades to occur if the manufacturing quality was better on the beginning.

Main Electrical tests on stator winding insulation:

Different electrical test methods are used to detect eventual problems and degradation on the stator winding insulation of rotating machines. These tests could be divided in two groups: DC and AC tests.

In general, the two groups of tests are needed for an overall investigation and health assessment of the insulation. DC tests are simpler to set and are focusing on the conductivity of the insulation material, therefore it is sensitive to cracks, moister and particles contamination. In the other hand, AC tests are closer to operation conditions in terms of electrical field stress. Consequently, besides conducting properties of the insulation, AC tests can spot dielectric losses and capacitance properties of the insulation, as well as partial discharges and corona activities, which makes these group of tests more sensitive to all kind of insulation defects and gives wider and deeper analysis outputs than DC tests.

The most used DC and AC tests could be described as following: (this is not an exhausted list of tests, but these measurements are the most known and used around the world)

-?????? DC Insulation Resistance test: It is the most used test for all types of rotation machines and consist of measuring the ohmic resistance of the insulation between the conductor and the ground wall for each phase (or for the three phases together if the star point of the motor is not accessible). The IR test is very easy to set and indicates only contamination (moisture) and serious problems. Due to different behavior of different currents circulating on the insulation when a DC voltage is applied (Charging current, polarization current, conduction current) the IR value is depending on the applied time. The Polarization Index (PI) is calculated as the ratio between IR obtained at 10mn and IR obtained at 1mn, and it is as an indication of surface contamination and/or dryness of the insulation. The IR is also temperature dependent and must be corrected at a reference temperature for any comparison matter. The test is totally covered on IEEE Std-43 / 2014 standard.

-?????? DC Leakage Test: Called also DC ramp test, this test is applied for stator winding insulation for 2.3 kV rotating machines and above. It is based on applying a controlled DC voltage on the insulation (linearly increased at a constant rate) and plotting the current measured at the end of each voltage step, till reaching the Maximum test voltage (usually chosen in the range of 1.25 to 1.5 of 1.7 x Vph-ph). The insulation problems that may be spotted by DC leakage current test include surface contamination, moisture absorption, delamination, cracks, fissures, and uncured resin. In order to analyze the results, the curves of the three phases are compared together or to previous measurements on the same machine made at the same conditions (humidity, temperature, pressure, etc.). Some patterns of the curves are proposed to indicate certain insulation defects.

-?????? Dissipation Factor/Power Factor test: This AC test is used to measure the overall dielectric losses inside the groundwall insulation (losses due to conductivity, polarization, and partial discharges). It is considered as a relevant test for stator winding insulation (and used as a quality control test after bars manufacturing). An increasing of the DF value during the time could indicates a increasing of the voids number inside the insulation, aging, delamination and thermal deterioration. Since the DF values are depending on the number of voids inside the insulation (losses due partial discharges and therefore these losses are voltage dependent) the Tip-up test (plotting the DF according to the applied voltage) (Fig.? ) is used to indicate any failure processes are happening. The comparison of the curves could be done between the three phases or according to the time, with previous measurements done on the same conditions of temperature, humidity, and pressure.

-?????? Partial Discharge Test: PD test is one of the most advanced and important tests on winding stator insulation. The relevance of this test is coming from the fact that PD activity on the insulation is a symptom of different kind of defects (voids, delamination, cracks, aging, surface discharges, etc..).

Partial Discharge Test on Rotating Machine Insulation:

As per IEC60270 standard, a Partial discharge is a localized dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high voltage stress.

In Rotating machine insulation, based on Mica (bounded with Epoxy), Partial discharges are symptoms of several types of defects, including aging and degradation of the insulation material.

The Mica is very resistance to internal partial discharges and acts as a barrier to prevent radial electrical trees along the insulation and therefore early breakdown. In the other hand, slot discharges and end winding discharges (surface discharges) are a very common type of insulation failure on rotating machines.

Due to poor manufacturing of semiconductive coating and outer corona protection layer, to external contamination and to mechanical impacts that may damage these layers (loose of wedge tightness with vibration of the bars), surface discharges could appear and might lead to a high failures risk if protective measurement are not took at time.

PD test is a very sensitive test and could detect the surface discharges defects at their preliminary stage (where other tests like Dissipation Factor and DC leakage couldn’t).

PD test is based on measuring the pulse current generated by PD activity using a capacitive coupler. Two kinds of PD measurement is used: Online PD, used mainly for monitoring purpose, where the measurement is taken during the operation of the generator or the motor, and Offline PD test, used for diagnostics purpose, where the measurement is taken during the maintenance period of the machine and the windings need to be energized with external HV source.

The PRPD analyzing is the best tool used to identify the origin of the PD activity. It is based on the visualization of PD pulses according to the phase cycle of the AC test voltage.

Typical PRPD patterns deliver a valuable information on the PD source and can be used as a base to identify the PD activity root cause.