WHAT IS CTC?

CTC consists of a group of enamelled rectangular wires, which are connected up parallel to the ends. In the group each strand (enamelled rectangular wire) successively and repetitively takes up every possible position in the whole cross section. The bunch is wrapped with oil compatible pure cellulose paper.

WHAT IS TRANSPOSING?

The enamelled wire conductors within CTC are arranged in two parallel stacks. One stack having one more conductor that the other. The conductors are continuously transposed by machine action moving them from position to position in discrete steps:

Step1: Conductor 1 is moved from the top of the left hand stack to the top of right hand stack.

Step2: The right hand stack is moved down one conductor thickness in relation to the left hand stack.

Step3: Conductor 3 is moved from bottom of right hand stack to the bottom of left hand stack.

Step4: The left hand stack is moved up one conductor thickness in relation to the right hand stack.
At the end of four transposing step, as shown in the figure, the CTC conductors have returned to the original geometry, but each conductor advanced one position clock-wise.

• INTRODUCTION OF PRECITRANS™¹ CTC:

PWIL is proud to introduce the PRECITRANS™¹ family of CTC comprising the following:

Remarks:

• The copper conductor used is Electrolytic Tough Pitch (ETP) or Oxygen Free Copper With High conductivity, for controlled low losses in electrical machines.
• The number of strands includes all numbers between 5 to 43; odd numbers can give higher cross section are in CTC. Paper tapes up to 24 wrapping is applied as external insulation over the transposed conductor bunch.
• Besides fully annealed copper conductor, PRECITRANS CTC is available with copper conductor with controlled proof stress, for increased mechanical strength for Transformer Winding. Standard Proof Stress Grade are as under:

• PRECITRANS CTC CURRENT MANUFACTURING RANGE:

• ENAMELLED RECTANGULAR WINDING WIRES USED IN CTC:

Enamelled Rectangular Winding Wires are one of the key ingredients of CTC. Wire Enamel being a critical input of Enamelled Rectangular Winding Wires is carefully chosen to give the desire properties like abrasion resistance, transformer oil resistance, temperature resistance, dielectric strength, etc.

Please refer to the following table for guidelines regarding Enamel Selection and increase in dimensions due to Enamel insulation.

Note: The above increase in dimensions can be used as a guideline. Specific requirements can be executed as per Customer request.

• DEFINITIONS:

TRANSPOSING PITCH :
Transposing pitch "P" is the distance between two transposing.

TRANSPOSING LENGTH:
Transposing length "lt" is the length measured on the CTC between two points where the stand goes from one side to the other side of the CTC stack. The transposing length is generally less than 50% of the Transposing Pitch "P".

MINIMUM WINDING DIAMETER AND TRANSPOSING PITCH:
The first property to be satisfied during the manufacture and use of the CTC is the windability on the transformers core.

Generally the length in which there is a complete transposing of one strand (called stranding pitch) should be less than the circumference of the transformer core.

The requirement is due to the flexibility of the CTC in order to avoid any damage of the CTC structure.

CTC Plan Transposing Head

CTC DIMENSIONS:

CTC DIMENSIONAL DATA
Calculation of the cable outer dimensions
Maximum dimension in axial direction:

Maximum dimension in radial direction

Where:
H = Axial cable dimension
B = Radial cable dimension
h = Axial strand dimension
b = Radial strand dimension
Kh = Maximum plus tolerance 0.10 mm for axial dimension
Kb = Maximum plus tolerance for radial dimension (values in table)
n = Number of strands in the cable
iE = Increase in dimensions due to enamel (0.13 for pva and 0.19 for pva plus epoxy)
ip = Thickness of separator between the strands stacks.
ic = Paper covering (in case of cable without paper ic/2 is the thickness of the plastic wire)

External dimensions are tested under pressure of IN/mm² or the value required by the customer.

INTERCOLUMN SEPARATOR:
An intercolumn separator between the two stacks of CTC conductors is inserted, for all stack heights more than 10 mm, unless otherwise agreed to with the user.

The standard paper thickness for this is 0.100-0.110 mm thick.

Intercolumn paper is generally inserted if:

Standard thickness of intercolumn separator paper, besides 0.100 mm, are 0.20, 0.30, and 0.40 mm

• PAPER ARRANGEMENT:

Transposed Enamelled Conductors / Strands are wrapped with multiple layers of oil resistant Kraft and/or other special papers to give high dielectric and mechanical strength to the CTC. The paper rolls can be slitted into required width tapes.

TYPES OF PAPERS:

Special papers, depending on customers requirement can be used.

Unless otherwise agreed with the customer, the insulation shall consist of at least 3 layers of 0.080 mm thick Kraft paper.

The paper covering is applied in the following arrangement:

• Papers are wound up to 8 papers in the same direction
• Above 8 papers, in groups of maximum 8 papers
• All inner layers are but lapped and staggered from 25 to 40%
• The two outermost layers are wound interlocked with about 50% overlap

Agreement with customer is required for any change in thickness of papers and the type of wrapping arrangement, in order to reach required paper insulation thickness.

• WINDING GUIDELINES FOR PRECITRANS CTC:

For optimal performance of CTC during the winding it is necessary to follow certain basic precautions.

During winding operations it is necessary to use some criteria in order to get the best winding of the Transposed Conductor on the transformer core, particularly if the CTC has a high ratio B/H.

• When the diameter core is greater than 25 times the radial dimension B, It is possible to wind the CTC leaving it free in the region between CTC and winding core (see figure a).
• In case of core with diameter less than 15 times the radial dimensions of the CTC, is possible to wind the Transposed Conductor using the following hints:
  • Distance between transformer core and the drum axis shall be multiple of the stranding pitch S.
• CTC shall be held up and guided by rollers (see figure b) in order to avoid flexions in the length between core and drum.
• In order to avoid CTC twisting, rollers shall have adjustable flanges (see figure c and adjustable flanges d).
• DRYING AND HEIGHT ADJUSTMENT PROCESSES AFTER WINDING:

PRECITRANS-Bond CTC generally gives best bonding after curing in the range of temperature between 110 and 130 degree C. In our Quality Assurance laboratory the adhesive strenght test on Epoxy bond coated conductors is done by heating for 24h at 120°C.

Many customers use the following drying cycle with great success

• During the transformer manufacturing, drying under vacuum, a pressure of 3-5 N/mm²(on the spacers of the helical coil winding) is applied and the temperature is raised to 115°C for 48-72 h. (excluding 6-8 h preheating and 10 hours of cooling periods)
• A dimensional adjustment is done at a pressure, in order to reach final dimensions
• A vapour phase drying is done at 130°C for 48hrs. (after 2-3 hrs. preheating to reach the temperature)
• Before the heat treatment, ensure that at the two ends of the strands are separated sufficiently, to avoid undesirable bonding between them.

WHERE IS CTC USED?

• Transformer coils wound with CTC
• Large Power Transformers, generally > 1 MVA
• Furnace Transformers
• Traction Loco Transformers
• Chokes
• Inductance Coils

APPLICATION OF CTC

• Transformer coil winding with CTC helps in compact design of transformers, campared to Paper insulated copper conductor.
• Paper less (netted) CTC is used in special transformers such as Furnace and Freight Loco, for improved cooling.