MANUFACTURING AND DESIGN OF INSULATION SYSTEM FOR AIR COOLED TURBO GENERATOR

Electrical insulating materials are defined as materials that offer a large resistance to the flow of current and for that reason they are used to keep the current in its proper path i.e. along the conductor. Insulation is the heart of the generator. Since generator principle is based on the induction of e.m.f in a conductor when placed in a varying magnetic field. There should be proper insulation between the magnetic field and the conductors. For smaller capacities of few KW, the insulation may not affect more on the performance of the generator but for larger capacities of few MW (>100MW) the optimization of insulation is an inevitable task, moreover the thickness of insulation should be on par with the level of the voltage, also non homogenic insulation provisions may lead to deterioration where it is thin and prone to hazardous short circuits, also the insulating materials applied to the conductors are required to be flexible and have high specific (dielectric) strength and ability to withstand unlimited cycles of heating and cooling.

            Keeping this in view among other insulating materials like solids gases etc liquid dielectrics are playing a major role in heavy electrical equipment where they can embedded deep into the micro pores and provide better insulating properties. Where as solid di-electrics provide better insulation with lower thickness and with greater mechanical strength. So the process of insulation design which has the added advantage of both solid and liquid dielectrics would be a superior process of insulation design. One such process which has all the above qualities is the VPI (vacuum pressurised impregnation) process and has proven to be the best process till date. 

2.1 Drawbacks of Early VPI Process:      

DR. MEYER brought the VPI system with the collaboration of WESTING HOUSE in the year 1956. It 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, the widely used insulation system for 6.6kv and higher voltages was a VPI system in which, Bitumen Bonded Mica Flake Tape is used as main ground insulation. The bitumen is heated up to about 180°C to obtain low viscosity which aids thorough impregnation. 

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 used is in the type 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.

2.2 Advantage of present resin poor VPI process:

                         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.

3 Introduction to various parts of a Generator:

                        The manufacturing of a generator involves in manufacturing of all the parts of the generator separately as per the design requirements and assembling them for the operation. It is worth knowing the parts of the Turbo Generator. Usually for larger generators the assembling is done at the generator installation area in order to avoid the damage due to mechanical stresses during transportation, also this facilitates easy transportation. Let us have a view about various parts of a turbo generator. Parts of a turbo generator:

1. Stator

2. Rotor

3. Excitation system

4. Cooling system

5. Insulation system

6. Bearings

           

3.1     STATOR:

3.1.1   STATOR FRAME

The stator frame is of welded steel single piece construction. It supports the laminated core and winding.  It has radial and axial ribs having adequate strength and rigidity to minimise core vibrations and suitably designed to ensure efficient cooling.  Guide bards are welded or bolted inside the stator frame over which the core is assembled. Footings are provided to support the stator foundation.

3.1.2 STATOR CORE

             The stator core is made of silicon steel sheets with high permeability and low hysteresis and eddy current losses. The sheets are suspended in the stator frame from insulated guide bars.

            Stator laminations are coated with synthetic varnish; are stacked and held between sturdy steel clamping plates with non-magnetic pressing fingers, which are fastened or welded to the stator frame.

           In order to minimize eddy current losses of rotating magnetic flux which interacts with the core, the entire core is built of thin laminations. Each lamination layer is made of individual segments.

          The segments are punched in one operation from electrical sheet steel lamination having high silicon content and are carefully deburred.  The stator laminations are assembled as separate cage core without the stator frame.  The segments are staggered from layer to layer so that a core of high mechanical strength and uniform permeability to magnetic flux is obtained.  On the outer circumference the segments are stacked on insulated rectangular bars, which hold them in position.

To obtain optimum compression and eliminate looseness during operation the laminations are hydraulically compressed and heated during the stacking procedure.  To remove the heat, spaced segments are placed at intervals along the core length, which divide the core into sections to provide wide radial passages for cooling air to flow.

The purpose of stator core is

1.                  To support the stator winding.

2.                  To carry the electromagnetic flux generated by rotor winding.

So selection of material for building up of core plays a vital role.

3.1.3   STATOR   WINDING:

                      The stator winding is a fractional pitch two layer type, it consisting of individual bars.  The bars are located in slots of rectangular cross section which are uniformly distributed on the circumference of the stator core.

                 In order to minimize losses, the bars are compared of separately insulated strands which are exposed to 360.degrees transposing

                 To minimize the stator losses in the winding, the strands of the top and bottom bars are separately brazed and insulated from each other.