The symmetrical autotransformer may be treated in the same manner as two and three-winding transformers. This type of transformer shows to best advantage when the transformation ratio is limited and it is widely used for the interconnection of the transmission networks working at different voltages, for example 275 to 132 kV or 400 to 275 kV. The neutral point is solidly grounded, that is connected directly to earth without intervening resistance.
Earthing (Grounding) Transformers
A means of providing an earthed point or neutral in a supply derived from a delta- connected transformer may be obtained by the use of a zigzag transformer, shown grounding the 33 kV system in Figure 3.33. By the interconnection of two windings on each limb, a node of zero potential is obtained.
Due to the non-linearity of the magnetizing characteristic of transformers the current waveform is distorted and hence contains harmonics; these flow through the system impedances and set up harmonic voltages. In transformers with delta- connected windings the third and ninth harmonics circulate round the delta and are less evident in the line currents. An important source of harmonics is power electronic devices.
On occasions, the harmonic content can prove important due mainly to the possibility of resonance occurring in the system, for example, resonance has occurred with fifth harmonics. Also, the third-harmonic components are in phase in the three conductors of a three-phase line, and if a return path is present these currents add and cause interference in neighbouring communication circuits and increase the neutral return current.
When analysing systems with harmonics it is often sufficient to use the normal values for series inductance and shunt capacitance but one must remember to calculate reactance and susceptance at the frequency of the harmonic. The effect on resistance is more difficult to assess: however, it is usually only required to assess the presence of harmonics and the possibility of resonance.