TRANSFORMERS IN GENERAL USE

1. Star - Star YyO or Yy6
2. Delta - Delta DdO or Dd6
3. Star - Delta Yd
Delta - Star Dy
4. Star - Zigzag Yz
5. Delta - Zigzag Dz

Star-Star (Yy0 or Yy6)

Most economical connection in HV power system to interconnect two delta system and to provide neutral for grounding both of them.
Tertiary winding stabilises the neutral conditions.

In star connected transformers, load can be connected between line and neutral, only if

(a) the source side transformers is delta connected or

(b) the source side is star connected with neutral connected back to the source neutral.

Delta - Delta (Dd 0 or Dd 6)

This is an economical connection for large low voltage transformers.

Large unbalance of load can be met without difficulty.

Third harmonics are damped out as the windings are closed mesh.

But the absence of a star point is dis-advantageous in some cases.

Delta - Star (Dy or Yd)

Common for distribution transformers.

Star point facilitates mixed loading of three phase and single phase consumer connections.

The delta winding carry third harmonics and stabilises star point potential.

Delta-Star connections is used for step-up generating stations.

If HV winding is star connected there will be saving in cost of insulation.

But delta connected HV winding is common in distribution network, for feeding motors and lighting loads from LV side.

Star-Zig-zag or Delta-Zig-zag (Yz or Dz)

These connections are employed where delta connections are weak. Interconnection of phases in zigzag winding effects a reduction of third harmonic voltages and at the same time permits unbalanced loading.

This connection may be used with either delta connected or star connected winding either for step-up or step-down transformers. In either case, the zigzag winding produces the same angular displacement as a delta winding, and at the same time provides a neutral for earthing purposes.

The amount of copper required from a zigzag winding in 15% more than a corresponding star or delta winding. This is extensively used for earthing transformer.

Transformer Oil

The functions of the transformer oil are

i. to provide dielectric strength.

ii. to protect the insulation system, and

iii. cooling.

Transformer Oil

Transformer oil is a mineral insulating oil derived from crude petroleum. It is a mixture of various hydrocarbons.

Aliphatic compounds

(open chain compounds) with the general formula Cn H2n+2 and Cn H2n.

Many oils also contain certain aromatic compounds (closed chain or ring compounds) related to benzene, naphthalene and derivatives of these with aliphatic chains.

Good transformer oil must

• insulate
• prevent flash over of the exposed parts within the equipment and
• it must effectively transform the heat from the core to the radiating surface.

Transformer Oil – Characteristics

The characteristics of new transformer oil is given in I S 335 / 1983.

Characteristics	Requirement
Appearance	Clear and Transparent, Free from suspended particles
Density at 27 0 C (Max.)	0.89 g / cc
Dielectric Strength (BDV) - New - After Filtration	30 kV (rms) 50 kV (rms)
Dielectric Dissipation Factor (Tan d) At 90 0 C – ( Max.)	0.005
Neutralization Value Total Acidity (Max.) b. Inorganic acidity / alkalinity	0.03 mg. KOH / gm. Nil

Characteristics	Requirement
Kinematic Viscosity at 27 0 C (Max.)	27 cSt.
Interfacial Tension at 27 0 C (Max.)	0.04 N / m
Flash Point	140 0 C
Pour Point	-9 0 C
Corrosive Sulphur	Nil
Oxidation Stability Neutralization Value after oxidation (Max.) Total Sludge after oxidation (Max.)	0.40 mg. KOH / gm. 0.10 percent by weight
Specific resistance (Resistivity) At 90 0 C ( Min.) At 27 0 C (Min.)	30 x 10 12 ohm-cm. 500 x 10 12 ohm-cm.

TRANSFORMER OIL - QUALITY CHECKING

There is no single test to judge the quality of transformer oil. Each test is significant within its limits and is able to give only general information about the overall condition of the oil.

The most popular test is the dielectric strength test. If the dielectric strength is low, the oil is unfit for use regardless of any other condition. Good transformer oil must have 60 kV withstand voltage between two brass electrodes - 12.7 to 13 mm dia, arranged horizontally with their common axis not less than 40 mm above the bottom of the cell, with a gap of 2.5 mm.