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Transformer Riddle No. 24 – Essentiality Tertiary winding in Power Transformer

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  • #315
    R K Mohapatra

      Whether a Tertiary winding is essential in a 3 phase 3 leg power transformer and not in 5 leg configuration ?
      If the tertiary winding is unloaded , whether it should be grounded or kept open ?

      Whether the Inrush current reduces by providing a Tertiary winding in Power transformer ?

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    • Author
    • #1610

        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. Advantages 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. Disadvantages 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.

        R K Mohapatra

          The corner grounding may provide a path for flow of the Zer Sequence current through the HV neutral , which can be sensed by providing a a protection circuit . For 3phase core type power trfr, when a corner of tertiary is grounded , the other 2 phases experience a situation like Neutral shifting occurs . This will give rise to a unhbalanced violtage to the other 2 phases to some extent . Is ir preferred to leave the tertiary Delta winding floating in isolation so that zero seqiuence curremnt flows in the loop of the delta tertiary winding . For the sasme rason , the inrush currnt may be reduced in Isolated Tertiary conditiion rather than a grounded Teritiary condituion .

          R K Mohapatra

            Conventionally the High voltage winding is kept away from the core and the low voltage winding is kept closer to the core . when HV winding is energised due to larger cross section area bett the HV winding and core , the inrush current is lower as stated above and by many other authors . But in some transformers where the volatge ratio is very high or when the LV current is very high , the LV winding is kept outside and the HV winding is kept inside , closer to the core . Whether the in-rush current will also be low ? Is it also correct to presume that the high voltage winding have more no of turns and the enegisation is required to be fed by Ampere turns . HV winding owing to having more turns will draw lesser amperes ?


              The amount of inrush-current relate to ampere-turn of each side of transformer winding which are equal approximately. Physical characteristics of wound coil such as Far or Close to core (coil diameter) or length of energizing coil influence on coil inductance (X) that formed to inrush current amount and its variations; therefore we must zone on actual equivalent of coil reactance and resistance in transformer energizing conditions for control of inrush currents. Of course, the existing of linked tertiary winding to both of primary or secondary winding (with regard to relevant resistance) can be help to faster decaying of inrush currents.


                About your experiance please note to probably nature of inrush current , are you sure about frequently switching?

                R K Mohapatra

                  I can just mention my expeirence with a transformer which has Primary 11 kV and Secondary 430 volts , 30 kAmp The tappings are on Primary for variation of secondary volrtage from 380 volt to 450 violt ( VFVV configuration ) . The trfr tripped while charging at normal tap . We changed to higher turn tap and charged the transformer with a time gap of 5 minutes. this has led us belief that the increase in turns has reduced the current at chraging . I will like to hear similar experiences .

                  R K Mohapatra

                    The system is in operation and the switching is infrequent .

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