1- Tertiary winding in shell-type transformer
Under certain conditions, the third-harmonic component of the phase voltage of star/star-connected three-phase shell-type transformers or banks of single-phase transformers may be amplified by the line capacitances.
This occurs when the HV neutral is earthed, so that third-harmonic currents may flow through the transformer windings, returning through the earth and the capacitances of the line wires to earth. The amplification occurs only when the capacitance of the circuit is small as compared to its inductance, in which case the third-harmonic currents will lead the third-harmonic voltages almost by 90, and they will be in phase with the third-harmonic component of the magnetic fluxes in the transformer cores. The third-harmonic component of the fluxes therefore increases, which in turn produces an increase in the third-harmonic voltages, and a further increase of the third-harmonic capacitance currents. This process continues until the transformer cores become saturated, at which stage it will be found the induced voltages are considerably higher and more peaked than the normal voltages, and the iron loss of the transformer is correspondingly greater. In practice, the iron loss has been found to reach three times the normal iron loss of the transformer, and apparatus has failed in consequence. This phenomenon does not occur with three-phase core-type transformers on account of the absence of third harmonics.
Therefore the delta connection can be useful in shell-type transformers as described above too. Also the circulating third-order harmonic currents flowing in the neutral can cause interference with telecommunications circuits and other electronic equipment as well as unacceptable heating in any liquid neutral earthing resistors, so this provides an added reason for the use of a delta connected tertiary winding. Any decision to omit the tertiary winding from a star/star-connected transmission transformer would only be taken following careful consideration of the anticipated third-harmonic current in the neutral, the third-harmonic voltage at the secondary terminals and the resultant zero sequence impedance to ensure that all of these were within the prescribed values for the particular installation.
2- Grounding of tertiary winding
The Corner-grounded delta systems are not recommended for new installations because more suitable and reliable systems are available today, it is encountered today for several reasons:
- Nearly all low voltage systems in the past were supplied from transformers with delta-connected secondaries. Grounding one of the phases provided a means of obtaining a grounded system. In this way, a grounded system could be obtained at a minimum of expense where existing delta transformer connections did not provide access to the system neutral.
- The recommended practice for most systems involves grounding one conductor of the supply.
- Possibly, customers wanted to avoid installing equipment ground fault protection as required by the NEC on solidly grounded wye electrical services.
- This system could result in the use of less expensive equipment, since two-pole switches and a neutral could be used for three-pole applications.
Corner-grounded delta systems have several advantages and disadvantages, as listed below.
Corner-grounded delta systems:
- Stabilize voltages of the ungrounded phases to ground.
- Reduce the generation of transient overvoltages.
- Provide a method for protecting electrical distribution systems when used in combination with equipment grounding.
Due to its disadvantages, the corner-grounded delta system has little reason for modern day use:
- The system is unable to supply dual-voltage service for lighting and power loads.
- It requires a positive identification of the grounded phase throughout the system.
- A higher line-to-ground voltage exists on two phases than in a neutral-grounded system.
- Most electrical distribution equipment manufactured in North America is not rated for use on this system.
- Fault switching (opening) is much more severe for the clearing device, and ratings may be greatly reduced.
3- Transformer building and inrush current level
Generally the magnetizing inrush is more severe when the saturation flux density of the core is low. Designers usually work with flux densities of 1.5 to 1.75 tesla. Transformers operating closer to the latter value display lower inrush currents. The maximum inrush current is influenced by the cross-sectional area between the core and the winding which is energized. Higher values of the inrush current are observed when the inner (having smaller diameter) winding is energized first. It is approximated, that for transformers with oriented core steel, the inrush current may reach 5-10 times the rated value when the outer winding is switched-in first, and 10-20 times the rated value when the inner winding is energized first. Due to the insulation considerations, the lower voltage winding is usually wound closer to the core, and therefore, energizing of the lower voltage winding generates higher inrush currents.