What is the purpose of using a neutral transformer? And Why is tertiary winding used in neutral transformers?
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| How Harmonic Excluded in Neutral Transformer [figure 1] |
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| How Harmonic Excluded in Neutral Transformer [figure 2] |
Introduction
We know that a delta connection does not have a common point through which fault current and Harmonic can flow.So this is a problem with delta winding transformers.
Fault current comes to the delta winding transformer and cannot be excited or grounded as there is no common point then Fault current burns the windings. Show this purpose neutral transformer used.
Neutral transformer
The neutral transformer creates a low reactance path for fault current exits or grounded. Also one more thing to do, any harmonic component comes in line helps to exclude harmonic component.The tertiary winding of the neutral transformer creates a low reactance path and excludes the third harmonic component.
Tertiary winding
Tertiary winding looks like figure 1.Basically we know that if there are high and low percentage of reactance paths in the same circuit, fault current always tries to flow in the low percentage of reactance path.The percentage reaction of the neutral transformer is 35% of the reaction of the power transformer.
Tertiary winding works
So the fault current will always flow towards the neutral transformer.And how tertiary winding exclude harmonics this are.The third harmonic current is mainly responsible for the distortion of the supply voltage.
As the neutral current enters the fault current for the low reactance path, the harmonic component is added to the winding with the fault current.Saturates the core very quickly when it comes to harmonic component winding.
Elemination of Harmonic
This saturation is essential to protect the main flux waveform from distortion.High magnetic flux lines form when the harmonic component enters the core.The oil and tank body in the transformer create a high reluctance path .
High Flux line is reduced by oil and tank body results reflect the voltage waveform .And for which the main flux becomes distortionless and the voltage waveform becomes harmonic component less and sinusoidal. Distortionless low voltage waveform is created due to high reluctance of oil and tank body.
It is need to know elaborately , input current with harmonic component when entering in primary of neutral transformer the i/p current created flux in core the created flux produce many flux line in core most number of flux line are loss in oil (for cooling cooling and insulation purpose) and tank due to high reluctance (flux not travel high reactance path) so many of harmonic component are neutralize.
Neutral transformers only use delta connected primary and secondary winding , Star connected not use neutral transformers because Star connected systems have proper ground arrangement.
In case some harmonic component try to enter a power transformer even if the neutral transformer is still working ,the differential relay breaks the supply from load the transformer goes to shutdown condition.
Conclusion
For all these reasons the oil test of the neutral transformer is done regularly and the oil change is done regularly. Now day the Transform Oil Test uses help of some embedded systems that use alarm systems for regular oil maintenance to schedule oil tests at regular intervals.
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