Question

Describe the parallel operating conditions in transformers in detail. Give the tolerances of the conditions. Why does circulation flow occur? Subtract the load sharing statement for transformers connected in parallel.

Answer 1

FOR PARALLELING OF TRANSFORMERS, FOLLOWING CONDITIONS MUST SATISFY:

1) FREQUENCY RATING MUST BE SAME

AS IN THE NATION THE GRID IS INTERCONNECTED, AND ALL THE EQUIPMENTS ARE DESIGNED TO WORK FOR THE SAME FREQUENCY SO HERE ALSO FOR PARALLEL OPERATION, THE FREQUENCY SHOULD BE SAME.

2) SAME VOLTAGE RATIO AND TURNS RATIO

SAME VOLTAGE RATIO IS REQUIRED IN ORDER TO PREVENT THE FLOW OF HIGH CIRCULATING CURRENTS .

LET US CONSIDER THE VOLTAGE RATIO OF TWO TRANSFORMERS IS DIFFERENT AND THE TRANSFORMERS ARE CONNECTED TO THE SAME BUS.

NOW THE TRANSFORMERS CONNECTED IN PARALLEL HAVING DIFFERENT VOLTAGE POLARITY WITH THE SAME PRIMARY SUPPLY VOLTAGE , THERE WILL BE DIFFERENCE IN THE SECONDARY VOLTAGES.NOW AS THEY ARE CONNECTED TO THE SAME BUS THERE WILL BE CIRCULATING CURRENTS IN THE SECONDARIES AND ALSO IN THE PRIMARIES.NOW AS THE INTERNAL IMPEDANCE OF THE TRANSFORMER IS VERY LOW, A SMALL CHANGE IN VOLTAGE MAY CREATE A LARGE AMOUNT OF CIRCULATING CURRENT CREATING COPPER LOSSES.

3) PERCENTAGE IMPEDANCE SHOULD BE EQUAL.

THE CURRENT SHARED BY THE TRANSFORMERS CONNECTED IN PARALLEL IS PROPORTIONAL TO THE MVA RATINGS AND THIS CURRENTS ARE INVERSELY PROPORTIONAL TO THEIR IMPEDANCE. SO THIS IMPEDANCE IS ULTIMATELY INVERSELY PROPORTIONAL TO THEIR MVA RATINGS.THEREFORE DIFFERENT IMPEDANCE WILL CREATE THE MISMATCH IN CURRENT SHARING AND MAY CREATE IMPACT ON EACH OTHER AS THEY ARE CONNECTED TO THE SAME BUS. HENCE, THE IMPEDANCE SHOULD BE EQUAL.

4) THE POLARITY MUST BE SAME

POLARITY DETERMINES THE DIRECTION OF INSTANTENOUS EMF INDUCED INTO THE SECONDARY WINDING.IF THE POLARITY IS NOT SAME THE TRANSFORMERS ARE SAID TO BE IN THE OPPOSITE POLARITY WHICH RESULTS INTO HUGE AMOUNT OF THE FLOW OF CIRCULATING CURRENTS.

5) PHASE SEQUENCE SHOULD BE SAME

PHASE SEQUENCE SHOULD BE SAME OTHERWISE DURING EACH CYCLE, EACH PAIR SHOULD BE SHORT CIRCUITED.

** TRANSFORMERS CONNECTED IN PARALLEL, WILL SHARE THE LOAD PROPORTIONAL TO THEIR RATING. IF THE RATINGS ARE SAME, THEY WILL SHARE THE EQUAL LOAD.