THE VPI PROCESS

12.1 INTRODUCTION TO VACUUM PRESSURE IMPREGNATION SYSTEM (VPI)

12.2       HISTORY

        

DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year 1956. Vacuum Pressure Impregnation has been used for many years as a basic process for thorough filling of all interstices in insulated components, especially high voltage stator coils and bars. Prior to development of Thermosetting resins, a widely used insulation system for 6.6kv and higher voltages was a Vacuum Pressure Impregnation system based on Bitumen Bonded Mica Flake Tape is used as main ground insulation. After applying the insulation coils or bars were placed in an autoclave, vacuum dried and then impregnated with a high melting point bitumen compound. To allow thorough impregnation, a low viscosity was essential. This was achieved by heating the bitumen to about 180°C at which temperature it was sufficiently liquid to pass through the layers of tape and fill the interstices around the conductor stack. To assist penetration, the pressure in the autoclave was raised to 5 or 6 atmospheres. After appropriate curing and calibration, the coils or bars were wound and connected up in the normal manner. These systems performed satisfactorily in service provided they were used in their thermal limitations. In the late 1930’s and early 1940’s, however, many large units, principally turbine generators, failed due to inherently weak thermoplastic nature of bitumen compound.

Failures were due to two types of problems:

a.      Tape separation

b.      Excessive relaxation of the main ground insulation.

            Much development work was carried out to try to produce new insulation systems, which didn’t exhibit these weaknesses. The first major new system to overcome these difficulties was basically a fundamental improvement to the classic Vacuum Pressure Impregnation process. Coils and bars were insulated with dry mica flake tapes, lightly bonded with synthetic resin and backed by a thin layer of fibrous material. After taping, the bars or coils were vacuum dried and pressure impregnated in polyester resin. Subsequently, the resin was converted by chemical action from a liquid to a solid compound by curing at an appropriate temperature, e.g. 150°C. this so called thermosetting process enable coils and bars to be made which didn’t relax subsequently when operating at full service temperature. By building in some permanently flexible tapings at the evolutes of diamond shaped coils, it was practicable to wind them without difficulty. Thereafter, normal slot packing, wedging, connecting up and bracing procedures were carried out. Many manufacturers for producing their large coils and bars have used various versions of this Vacuum Pressure Impregnation procedure for almost 30 years. The main differences between systems have been in the types of micaceous tapes used for main ground insulation and the composition of the impregnated resins. Although the first system available was styrenated polyester, many developments have taken place during the last two decades.

Today, there are several different types of epoxy, epoxy-polyester and polyester resin in common use. Choice of resin system and associated micaceous tape is a complex problem for the machine manufacturer.

         Although the classic Vacuum Pressure Impregnation technique has improved to a significant extent, it is a modification to the basic process, which has brought about the greatest change in the design and manufacture of medium-sized a.c. industrial machines. This is the global impregnation process. Using this system, significant increases in reliability, reduction in manufacturing costs and improved output can be achieved. Manufacture of coils follows the normal process except that the ground insulation consists of low-bond micaceous tape. High-voltage coils have corona shields and stress grading applied in the same way as for resin-rich coils, except that the materials must be compatible with the Vacuum Pressure Impregnation process. Individual coils are inter turn and high-potential-tested at voltages below those normally used for resin-rich coils because, at the un- impregnated stage, the intrinsic electric strength is less than that which will be attained after processing. Coils are wound into slots lined with firm but flexible sheet material. Care has to be taken to ensure that the main ground insulation, which is relatively fragile, is not damaged. After inter-turn testing of individual coils, the series joints are made and coils connected up into phase groups. All insulation used in low-bond material, which will soak up resin during the impregnation process. End-winding bracing is carried out with dry, or lightly treated, glass-and/or polyester-based tapes, cords and ropes. On completion, the wound stator is placed in the Vacuum Pressure Impregnation tank, vacuum-dried and pressure-impregnated with solvent less synthetic resin. Finally, the completed unit is stoved to thermo set all the resin in the coils and the associated bracing system.

        After curing, stator windings are high-potential-tested to the same standard. Loss-tangent measurements at voltage intervals up to line voltage are normally made on all stators for over 1kv. A major difference between resin-rich and vacuum pressure impregnation lies in the importance of this final loss-tangent test; it is an essential quality-control check to conform how well the impregnation has been carried out. To interpret the results, the manufacturer needs to have a precise understanding of the effect of the stress-grading system applied to the coils. Stress grading causes an increase in the loss-tangent values. To calculate the real values of the ground insulation loss-tangent, it is necessary to supply from the readings the effect of the stress grading. For grading materials based on the materials such as silicon carbide loaded tape or varnish, this additional loss depends, to a large extent upon the stator core length and machine voltage.

            VPI is a process, which is a step above the conventional vacuum system. VPI includes pressure in addition to vacuum, thus assuring good penetration of the varnish in the coil. The result is improved mechanical strength and electrical properties. With the improved penetration, a void free coil is achieved as well as giving greater mechanical strength. With the superior varnish distribution, the temperature gradient is also reduced and therefore, there is a lower hot spot rise compared to the average rise.

           In order to minimize the overall cost of the machine & to reduce the time cycle of the insulation system vacuum pressure Impregnated System is used. The stator coils are taped with porous resin poor mica tapes before inserting in the slots of cage stator, subsequently wounded stator is subjected to VPI process, in which first the stator is vacuum dried and then impregnated in resin bath under pressure of Nitrogen gas.

            The chemical composition of our resin type and its advantages are explained in the later sections. Now let us discuss the various stages involved in VPI process for resin poor insulated jobs.

VPI process is done in the VPI camber. For higher capacity stators of steam turbine or gas turbine generator stators, horizontal chamber is used where as vertical chamber is used for smaller capacity systems such as Permanent Magnet Generator (PMG), coil insulation of  small pumps and armature of motors etc..,

  Vacuum Pressure Impregnation of resin poor insulated jobs:

VPI process for a stator involves the following stages.

1.      Preheating

2.      Lifting and shifting

3.      Vacuum cycle

4.      Vacuum drop test

5.      Heating the resin

6.      Resin admission.

7.      Resin settling

8.      Pressure cycle

9.      Aeration.

10.  Post curing cycle

11.  Cleaning

12.3.1 General instructions before VPI process:

The jobs that are entering tank for Vacuum Pressurised Impregnation shall not have any oil based coatings. Any such, rust preventive/ corrosion preventive viz., red oxide etc., shall be eliminated into the tank. Jobs shall be protected with polyethylene sheet for preventing dust or dirt on jobs, till it is taken up for impregnation. Resin in the storage tank shall be stored at 10 to 12°C and measured for its viscosity, viscosity rise. Proper functioning of the impregnation plant and curing oven are to be checked by production and cleared for taking up of job for impregnation.

12.3.2 Pre heating:  

The foremost stage of VPI, the completed stator is placed in the impregnation vessel and kept in an oven for a period of 12 hours at a temperature of 60 deg. Six thermocouples are inserted at the back of the core to measure the temperature. The temperature should not exceed to 85 deg .Smaller stator can be inserted directly into the impregnation chamber. The job is to be loaded in the curing oven and heated. The temperature is to be monitored by the RTD elements placed on the job and the readings are logged by production. The time of entry into the oven, time of taking out and the temperature maintained are to be noted. Depending on convenience of production the jobs can be preheated in impregnation tank by placing them in tubs.

The impregnation tubs used for impregnation of jobs are to be heated in the impregnated tank itself, when the jobs are preheated in the curing oven

12.3.3 Insertion of tub with job into the impregnation tank:

The wound stator is lifted and shifted into the tub. By the time, the preheating of job is completed, it is to be planned in such a way that the heating of tub and tank heating matches with the job. This is applicable when the job is heated in the curing oven separately. The preheated job is to be transferred into the tub by crane handling the job safely and carefully with out damage to the green hot insulation the tub is then pushed in the 140 tank furnace or also called as vacuum tank, after which the lid is closed and the tank furnace was heated to 60 +/- 3 deg      The warm tub with job is inserted into impregnation tank by sliding on railing, in case of horizontal tank. The thermometer elements are to be placed at different places on the job. The connection for inlet resin is to be made for collection of resin into tub. After ensuring all these lid of the impregnation tank is closed. In case of vertical tank the job along with tub is slinged and inserted carefully into impregnation tank without damage to insulation

12.3.4    Vacuum cycle:

             The pre heated job will be placed in the impregnation chamber by a hydraulic mechanism. The vessels are kept clean and the resin available in the vessel is wiped out. Methylene and traces of resin should not be allowed on the inner side of the tank. Now the vacuum pumps are all  switched on  and a vacuum pressure of about 0.2 mb is maintained for about 17 HRS, after which the wound stator is subject to vacuum drop test.

12.3.5 Vacuum drop test: 

This drop test is important phase, all the vacuum pumps are switched off for about 10 mins, and the vacuum drop is measured and it is checked whether it exceeds 0.06mb, if it exceeds 0.06mb then it is subject to repetition of vacuum cycle for another 6 to 8 hrs, else it is sent to the next cycle

12.3.6 Drying the job in vacuum

            The job is to be dried under vacuum. Drain out the condensed moisture/ water at the exhausts of vacuum pumps for efficient and fast vacuum creation. Also check for oil replacement at pumps in case of delay in achieving desired vacuum.

12.3.7 Heating the resin in the storage tank

            The completion of operations of drying and the heating of the resin in the storage tank are to be synchronised. The heating of resin in the tank and pipeline is to be maintained as at preheating temperature .i.e. the temperature is maintained at 60+/- 3 deg ,including pipeline

12.3.8 Admission of resin into impregnation tank

                  The resin is allowed into the impregnation tank tub if required from various storage tanks one after the other, such that the difference in pressure fills the tank, up to a level of 100mm above the job generally, after which the resin admission is stopped. After 10mins of resin settling the tank is to be pressurised by nitrogen. While admitting resin from storage tanks pressurise to minimum so that nitrogen will not affect resin to spill over in tank.