We will be discussing an example of the generator failure mechanism which can lead to a quick (months!) insulation breakdown. Do you wish to know that it is?

Please join us on Thursday 04 May 2023, 11am Brazil time (3pm UK time). We will go through details and RCA of 2 generator breakdowns, with hope that it will help you to prevent such situation in the future.

Please join the training using the link below:  
https://meet.zoho.eu/UDSxeI82x6

NOTE:
Please remember to register before the meeting, as number of participants is limited. Chrome and Firefox do not require downloading any other add-ons or apps, therefore these are preferable web browsers. Other web browsers may require downloading additional files. Please check with your IT Department.

O post FREE WEBINAR: Generator failure – Root Cause Analysis (RCA) of 2 generators with insulation breakdown apareceu primeiro em Nishi.

“Partial Discharge” and “Corona” are terms commonly reported in the electrical and maintenance fields, they are commonly confused as being similar terms. They have in common that they do not immediately cause a complete path in the dielectric for the electric current.

According to the IEEE, a partial discharge is an incomplete or “partial” discharge that occurs between insulators and also between conductor and insulator. In contrast, a full discharge is one that crosses the space between conductors, causing what is known as an electrical fault, by forming a conductive bridge.

Corona, on the other hand, occurs when the gases adjacent to an energized conductor ionize and produce normally visible discharges. Thus, partial discharges inside the insulating wall cannot be called corona.

Visit our Predictive Maintenance page and contact our team for more information on partial discharge monitoring and predictive diagnostics. Evaluate the advantages of unscheduled shutdowns, reducing costs in equipment maintenance and increasing the efficiency of the production process.

*Illustrative images taken from the IEC 60034-27-2 standard.

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To ensure the loss levels and yields of these machines, the generators are designed with coils formed by multiple rectangular conductors in parallel, insulated from each other (“strands”) and transposed along their length, which we call “Roebel” transposition. Each parallel conductor is designed with a small cross section because in this way the eddy currents are reduced (Fujita, Kabata – 2005).

The commonly used transpositions are 360º and 180º. Studies show that in the 360º transposition the losses by circulating current reach close to zero.

Access our blog and read more NISHISAN tips on corrective maintenance services and characteristics of electrical machines. Contact our team and ask all your questions about different coil and bar manufacturing projects for turbogenerators and hydrogenerators.

O post Roebel transposition – its importance in large machines apareceu primeiro em Nishi.

The failure of the insulator is usually the result of a gradual deterioration, over time, which we call aging. Aging is basically the result of different stresses: thermal; thermo-mechanical; mechanical; electrical; and environmental, which act simultaneously in operation. The insulating system can change its behavior as a function of the various types of “over stresses” when compared with the behavior of normal aging.

The insulation system in large machines is one of the most important parts because of the high manufacturing and maintenance costs. Around 40% of the failures in large machines are due to the high-voltage insulating systems, according to research.

The study of the behavior of insulating material subjected to different stresses during its operation is very important to improve the design/process and the quality of the insulating system and thus avoid early aging.

One of the most common tests for approval in coil manufacturing is the VET – “Voltage Endurance Test”, according to IEEE 1043 (test method) and IEEE 1553 (acceptance criteria). This test simulates an accelerated aging through thermal and electrical stress simultaneously, in a test room, with 4 (four) prototype coils. Only after its approval the process is approved and released for coil manufacturing.

Contact our team and get to know the techniques to evaluate the insulation system and which is the best coil manufacturing project/process for your equipment.

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The principle of an electrical machine is that the conduction of electric current is the basic condition for its operation. In order to conduct this current continuously through the various coils, there are electrical connections that connect one coil to another, both in the rotor and in the stator. These connections are made by soldering or brazing.

In brazing, the connection established between two metallic elements is made by means of an addition metal, which is in the liquid state and whose fusion temperature is higher than 450ºC and lower than the fusion temperature of the base materials, and the base metals do not participate in the constitution of the joint.

Welding, on the other hand, is a weak brazing, in which the melting temperature of the filler metal is below 450ºC. Normally, the filler metal penetrates by capillarity between the surfaces of the parts to be joined. Uniform heating of the surfaces should be promoted, because with the heating, the base material molecules get farther apart and the filler metal in liquid state penetrates between these molecules.

The goal to be achieved in the connections is that in this bridge, which connects the coils, there is no bad contact generating a high ohmic resistance value and consequent heat losses and Joule effect. The mechanical part is also important, because in addition to the Joule effect (electrical losses) there are centrifugal forces and thermal movements that generate stress in these connections, reasons that cause wear and malfunctions. Predictive and preventive maintenance is essential to identify the integrity of these connections and thus avoid unscheduled failures in them.

As an illustration, in an 8 160 hp, 13 800 V, 1 800 rpm motor, the client had a history of failures in the damping cage and requested a detailed inspection of the equipment for preventive evaluation and intervention, considering the need for high reliability of the equipment for operation. Through tests on the rotor circuit we concluded that the connections were intact, which enabled the return in operation, restoring peace of mind to the customer.

winding rotor connections

Inspection of electrical connections of a high voltage wound rotor	Wound rotor test to evaluate electrical connections

In another case, a 66 760 kVA, 13 800 V, 257.14 rpm hydro generator, a partial winding was performed on the stator winding with bar change. Since this generator had already failed due to problems in the brazing of the series connections of the bars, as a preventive measure, a general evaluation of all the connections was made before releasing the machine for operation.

Brazing process on electrical connection	Test to evaluate problems in the brazing of serial connections of the bars

Request a quote for a preventive evaluation of your equipment and count on our team of specialists in electrical machines!

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The aging cycle of a machine is directly associated with the deterioration of the insulating material of its winding. We have already covered in previous issues that controlling the temperature of the machine is fundamental to avoid premature aging, because as we said, temperature is usually the predominant factor for the aging of insulating materials.

The most common practice to monitor the temperature is through the installation of temperature sensors (usually RTD type PT 100) that are in direct contact with the coils inside the slots. The installation of these sensors is usually in strategic regions of the winding, where the objective is to find the hottest point and control its limit. Remember that the greatest source of heat is in the copper losses in the coils (Joule effect).

As the sensors are contact sensors, they need to be protected by an enclosure. There will always be a temperature gradient between the inside of the copper and the temperature sensor. To adjust the temperature limits in the protection and alarm system, this difference must be taken into account.

Check out more Nishisan tips in our blog and contact our team for a quote and to clarify your questions about motors and generators.

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In the insulating system of the coils we have the deterioration of the main phase x internal ground wall and the contact surfaces between coil x core or between coils of different phases in the region we call. The internal discharges are monitored through specific equipment, mainly measuring partial discharges. Surface discharges, on the other hand, can be monitored by equipment or also through visual inspections.

The coils, when manufactured, with the objective of avoiding these surface discharges, have a corona suppressor system in the grooves and in their exit region. The material applied in the groove is conductive and in the output a semi-conductive with specific characteristics that soften the rapid rise of the electric field when there is a loss of reference to ground. These 2 materials (conductive and semi-conductive) are electrically connected, a region called the corona sleeve.

Over time of operation and during routine maintenance it is a practice, when visually observing the appearance of corona in these regions, to correct these points of the corona suppressor system.

The tip and warning we want to leave is that this action must be done with great care, because it seems that it is only a dark “paint job” and only its repainting does not guarantee the success of the operation, on the contrary, it may even make it worse. As we have already pointed out, its function is electrical, and the premises of smoothing the electric field must be maintained to achieve the goal of avoiding discharges in this region.

Contact our team and clarify your doubts about corona suppressor system and make an evaluation of your case with us.

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