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Transformer Riddle No.29 - Tertiary winding as station supply
Can a tertiary winding on a power transformer be used for station supplies? if so how is its connection to the auxillary transformer. what other use does it have other than 3rd harmonic suppression and compensation for unbalanced loads.
Author : Hatisari Kujinga - From: Zimbabwe
Sun, August 23rd, 2009 - 19:11
Yes,The most common reason for the addition of a third winding to a three-phase transformer is the provision of a delta-connected tertiary winding. Other reasons for doing so could be as follows:
-To limit the fault level on the LV system by subdividing the infeed that is, double secondary transformers.
-The interconnection of several power systems operating at different supply voltages.
-The regulation of system voltage and of reactive power by means of a synchronous capacitor connected to the terminals of one winding.
The number of turns, and hence rated voltage, of any tertiary winding may be selected for any convenient value. Thus the tertiary terminals may be brought out for supplying any substation auxiliary load, dispensing with the need for any separate auxiliary transformer. In the case of large transmission autotransformers, which must of necessity be star/star connected, a common use of the tertiary winding is for connection of system compensation equipment.
Although any auxiliary load may be quite small in relation to the rating of the main transformer, the rating of the tertiary must be such as to carry the maximum circulating current which can flow as a result of the worst system unbalance. Generally this worst unbalance is that condition resulting from an autotransformer often has a delta-connected tertiary winding to reduce third-harmonic voltages, to permit the transformation of unbalanced three-phase loads, and to enable the use of supply-station auxiliary load or power-factor improvement equipment. The tertiary winding must be designed to accept all of these external loads as well as the severe short-circuit currents and forces associated with three phase faults on its own terminals or single line-to-ground faults on either the primary or secondary terminals. If no external loading is required, the tertiary winding terminals should not be brought out except for one terminal to ground one corner of the delta during service operation. This eliminates the possibility of a three-phase external fault on the winding.
Author : Hamid - From: Iran
Fri, October 9th, 2009 - 07:32
With everything Ok , the only risk of connecting the tertiary to Station auxilliariesv are :

( a ) The loading of the other secondary  winding are to be reduced otherwise the feding winding will be overloaded .

(b) The trnasformer may fail due to any fault ( voltage or current )  in the load connected to the tertiary

( c ) Besides

(a) Power factor correction
( b ) Unbalaced load
( c ) Harmonic suppresson
( d ) Connecting to Auxilliary loads    

the tertiary winding can also be used for
( e) electrical isolation
( f ) Feed back system for voltage regulation  
( g) Higher Zero sequence impedence
Author : R K Mohapatra - From: India
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