Richard Emery looks at the various aspects of the coil manufacturing process in order to provide a better understanding to those operating and maintaining these key pieces of equipment.
In any industry unplanned downtime can be costly, but for those operating high voltage (HV) generators and motors, unexpected failures can cause huge disruption and financial losses due to lost production that can mount up very rapidly. It is therefore important to employ a system of maintenance, testing and monitoring to ensure that any signs of impending failure are highlighted at the earliest opportunity.
The next step is to ensure that once an issue is raised, the equipment in question is scheduled for maintenance in a properly planned operation so as to minimise any disruption. This planning stage will also include what maintenance actions will be required and which suppliers will be required to complete the repair. In some cases this will involve the replacement of some, or all, of the high voltage coils within the equipment - and it is here more than ever that the decision making process could affect the future reliability and performance of the machine in question for years to come.
The manufacture of high voltage coils is a very precise science and the design and construction methods will vary between suppliers, as will the testing procedures, which form the bulk of the proof the client has of the quality of the finished product. Obviously, when repairs are unplanned, speed is crucial in getting the equipment back up and running, but this cannot be achieved to the detriment of quality, which has much longer lasting implications.
Key to the performance and reliability of HV equipment is the insulation system
Key to the performance and reliability of HV equipment is the insulation system, which is made up of insulating tape and resin, which seals the winding and prevents moisture ingress. The design and construction of the insulation depends on the application and it is important that the site survey conducted by the repair shop gathers all the necessary information and measurements to ensure an effective and reliable repair is completed.
The more common method of insulation is Vacuum Pressure Impregnation (VPI), which is very competitive in terms of cost when compared to B-stage fully cured coils, but may not be appropriate or even possible for some applications. VPI requires the taped coils to be inserted into the stator core, connected and braced in position before lowering the complete assembly into the VPI tank. The vacuum applied to the tank removes the air and moisture, replacing them with a resin which is then cured to form an encapsulated solid mass.
This process can only be applied to those machines that can be removed from site and are small enough to fit inside the vacuum tank. One other disadvantage is that it is very difficult and much more expensive to effect a coil repair to a machine that has VPI insulation, due to the solid mass of resin which makes removal and replacement of a single coil impractical. The alternative is to use individually cured, B-stage resin rich coils, which can be transported to site to allow the repair or complete rewind to be completed.
The use of B-stage resin coils is predominant in the rewinding of larger machines but this method also delivers benefits for smaller machines. The construction of the B-stage coils allows the resin to flow to the end of the winding and bond the layers of insulating tapes to produce a more homogenous insulation and sealing of the winding. In most cases the use of modern insulating materials will improve both the dielectric and thermal performance when compared to those used by the OEM during original manufacture.
In fact, approximately 90% of the insulating performance is achieved before the curing process takes place and many machines can enter service without the curing process being completed. This will take place over a period of three to six months under normal operating conditions, with no adverse effects to performance.
In addition, each B-stage resin coil can be independently tested before being installed, giving the repair centre and the client peace of mind that every coil has passed the numerous tests required. In contrast, the machines undergoing repair using VPI coils can only be fully tested once all the coils have been installed and the VPI process completed.
Quality assurance is crucial
Of course, quality assurance (QA) is crucial and there are several standards which must be met, especially for the testing of HV coils, which is carried out both in-house and by external, independent, laboratories. Starting with the basic raw materials and right through the manufacturing process, a series of continuous checks are in place to ensure every coil is produced to the exact design specification.
For every job a set of wooden formers are created to replicate the stator slots to allow the shape and fit of the coils to be checked quickly and efficiently. In addition, go, no-go gauges are used to check the coil dimensions against the stator specification. These gauges are quicker and more accurate than using micrometers, which can introduce errors through a simple misreading of the scale.
At Sulzer, at any point in the manufacturing process where the shape of the coil could have changed, an additional check is made to ensure the coil is still to specification. After each application of a press or heat, the coils are placed back into the formers to be certain that the finished coil will fit exactly during final assembly.
In addition, if there is any perceived change in the materials used in the insulation system, then a sample coil will be subjected to voltage endurance testing in order to prove the insulation properties and reliability are unchanged. Suppliers have been known to change specifications of their materials without providing such information to the end users and so periodic sampling is conducted to ensure the overall quality of the finished product is maintained.
The electrical testing of the completed coil includes the Tan δ, which is a measure of the integrity of the slot wall insulation, where a lower figure indicates a better quality coil. At Sulzer Coil Manufacturing in Birmingham, the figure achieved is always less than half the international standard, with an aim to realise less than one third of the standard. This can only be done by the continuous attention to detail which is encouraged throughout the manufacturing process.
Further testing is carried out on sample coils by independent laboratories, including the thermal endurance test, which is carried out at 30kV for 500 hours, a simple enough task for a high quality coil. However, a more difficult test is done at 35kV for 250 hours but once again the attention to detail pays off, and all the coils manufactured by Sulzer at its Birmingham Service Centre have also passed this standard.
For those involved in the operation and maintenance of large HV rotating machines, there are many choices when looking to repair or rewind such equipment. The key to a successful project is ensuring that those involved will be able to deliver a high quality product, precisely, quickly and with the necessary support to ensure a timely completion.
Richard Emery is Head of Technical Services at the Sulzer Birmingham Service Centre in the UK.