Tertiary winding working and application
In this article we will discussed about Tertiary
There are many reasons for which three winding transformer is employed . Some of the reasons are listed below.
1. If a two winding transformer has to supply an additional load which has to be insulated from the secondary windings for some reasons the three winding transformer may be used with additional load carried by tertiary winding.
2. The phase compensating devices can be supplied with three winding transformer which are not operating at either primary or secondary voltage but at some different voltage.
3. The tertiary winding can be used as a voltage coil in a testing transformer.
4. Three supply systems operating at different voltages can be interconnected using three winding transformer.
5. The three winding transformer can be used to load large split winding generators.
6. The substations requirements can be met using three winding transformer which requires a voltage different from that of primary and secondary windings.
7. The tertiary winding connected in delta reduces the impedance offered to the zero sequence currents so that a larger earth fault current flows for proper operation of protective equipment. For unbalanced load it limits the imbalance in voltage. It permits the flow of third harmonic current to reduce harmonic voltage.
Stabilization Due to tertiary winding
For unbalanced single phase load, the star-star connection offers high reactance to the flow of current. Any unbalanced load current has three components viZ. Positive and zero sequence components.
The zero sequence component on the secondary side cannot be balanced by primary currents as zero sequence currents cannot flow in the isolated neutral of star connected primary. On the secondary side the zero sequence current sets up magnetic flix in the core. The iron path is available for this flux and the impedance offered to the flow of zero sequence current is lowered. For this purpose the tertiary winding is called stabilizing winding.
Suppression of triple frequency currents by tertiary winding
In the figer the generator neutral is connected to the neutral of the primary of transformer. The neutral of the load in connected to the neutral of the secondary o the transformer. If there is a third harmonic in the wave form of the generator phase voltage, this third harmonic will appear on the secondary phase voltage and the triple frequency secondary current will flow. In the neutral wire, all the triple frequency currents for the three phases will be added and flow.
These currents will interfere with the neighboring telephone circuits. if the primary natural condition is not made , then there will be no return path for the triple frequency component of exciting current. In that case, third harmonic will appear on each phase voltage of the transformer, distorting the flux in the transformer. Now, if the secondary neutral is connected, then the triple frequency harmony currents will be developed in the secondary circuits. These currents will also interfere with telecommunication lines. If both the neutrals of primary and secondary are insulated, there will ve triple harmonics in the voltage wave of each phase of the transformer .
On the other hand, if the neutral points are not insulated, the voltage regulation of the several phases will be poor if the load is unbalanced which is the usual case with the practical loads.
Now,it is required to suppress the tiple frequency secondary currents. This can be done by providing a delta-connected transformer. The tertiary windingare connected in delta. The EMF which will be doweled in the tertiary delta due to fundamental frequency will be balanced,. The induced EMF of triple frequency will ve in time phase around the closed circuit. The KVA rating of the tertiary winding will be a very small fraction of that of the main transformer. Only due to possible short-circuit conditions, the tertiary is generally designed to have a rating about one-third of the rating of the main winding.