TRANSFORMER TEST

                     TRANSFORMER TEST

TRANSFORMER TEST

1. OPEN CIRCUIT TEST

2. SHORT CIRCUIT TEST

3. SUMPNERS TEST (BACK TO BACK TEST)

1. OPEN CIRCUIT TEST

OPEN CIRCUIT TEST ON A TRANSFORMER IS CARRIED OUT TO DETERMINE THE CORE LOSS AT RATED VOLTAGE AND FREQUENCY. ACCORDINGLY, THIS TEST IS CARRIES OUT AT RATED VOLTAGE AND FREQUENCY WITH THE INSTRUMENTS PLACED ON THE L.V SIDE AND THE HV SIDE IS LEFT OPEN CIRCUIT. 

THE NO LOAD CURRENT (Io) OF A TRANSFORMER IS 2.5% OF THE FULL LOAD VALUE, THE PRIMARY WINDING COPPER LOSS MAY BE IGNORED.

THIS TEST IS CARRIED OUT ON LV SIDE BECAUSE A RANGING RATED VOLTAGE SUPPLY AT LV LEVEL IS MUCH EASIER THEN THE HV LEVEL. ALSO, THE INSTRUMENTS REQUIRED WOULD BE CHEAP AND WOULD BE SAFE TO WORK ON LV SIDE. THE WATTMETER REQUIRED SHOULD BE A LOW PF WATTMETER AS THE NO LOAD  PF IS VERY LOW OF THE ORDER OF 0.2 PF LEG.

FROM THE TEST, WE GET:-

 Po  = NO LOAD LOSSES, IRON LOSSES

Voc = NO RATED VOLTAGE

Io   = NO LOAD CURRENT

USING THESE CAN CALCULATE:-

2. SHORT CIRCUIT TEST

SHORT CIRCUIT TEST IS CARRIED OUT ON A TRANSFORMER TO DETERMINE THE FULL LOAD COPPER LOSS. ACCORDINGLY, THIS TEST IS CARRIED OUT AT RATED CURRENT AND FREQUENCY ALTHOUGH THE REQUIREMENT OF RATED FREQUENCY IS NOT NECESSARY.

INSTRUMENTS ARE PLACED ON THE HV SIDE WITH THE LV SHORT CIRCUIT BY A VERY THICK WIRE OF LOW RESISTANCE.

IN ORDER RO CIRCULATE FULL LOAD CURRENT AT SHORT CIRCUIT, AN INPUT VOLTAGE OF 8 TO 10% IS USUALLY SUFFICIENT AND THE CORE LOSS DURING THIS TEST IS IGNORED.

MOREOVER, THE NO-LOAD CURRENT (Io) AT SUCH REDUCED VOLTAGE ALSO BECOMES NEGLIGIBLE AND CAN BE IGNORED. THIS MEANS THAT THE EXCITING PART OF THE EQUIVALENT CIRCUIT MAY NOT BE CONSIDERED DURING SHORT CIRCUIT TEST.

THE INSTRUMENTS ARE PLACED ON HV SIDE BECAUSE THE RATED CURRENT ON HV SIDE IS LOWER THAN ON THE LV SIDE. HENCE, AMMETURE WATTMETER AND INSTRUMENT TRANSFORMERS, IF AND OF LOW CURRENT RATINGS MAY BE USED. 

FROM THE TEST, THE PARAMETER VALUES WE GET:-

Pc = FULL LOAD COPPER LOSSES

Isc = FULL LOAD CURRENT

Vsc = INPUT VOLTAGE FOR FULL LOAD CURRENT ON SHORT CIRCUIT (8-10% OF V rated)

3. SUMPNER'S OR BACK TO BACK TEST

IN ORDER TO DETERMINE THE MAXIMUM TEMPERATURE RISE, IT IS NECESSARY TO CONDUCT A FULL LOAD TEST ON A TRANSFORMER. FOR SMALL TRANSFORMER FULL LOAD TEST IS CONVENIENTLY POSSIBLE, BUT FOR LARGE TRANSFORMER FULL LOAD TEST IS VERY DIFFICULT AND EXPENSIVE. LARGE TRANSFORMER CAN BE TESTED FOR DETERMINING THE MAXIMUM TEMPERATURE RISE BY THE BACK TO BACK TEST. THIS TEST IS ALSO CALLED THE REGENERATIVE OR SUMPNER'S TEST.

THE BACK TO BACK TEST ON A SINGLE PHASE TRANSFORMER REQUIRES 2 IDENTICAL TRANSFORMERS.

THE PRIMARY AND SECONDARY WINDINGS OF 2- TRANSFORMERS ARE CONNECTED IN PARALLEL AND SUPPLIED AT RATED VOLTAGE AND RATED FREQUENCY. A VOLTMETER AND AMMETER AND A WATTMETER ARE CONNECTED TO THE INPUT SIDE.

THE SECONDARY ARE CONNECTED IN SERIES WITH THEIR POLARITIES IN PHASE OPPOSITION, WHICH CAN BE CHECKED BY VOLTMETER V2. THE RANGE OF THIS VOLTMETER SHOULD BE DOUBLE THE RATED VOLTAGE OF EITHER TRANSFORMER SECONDARY.

IF VOLTMETER V2 READS ZERO, THE TWO SECONDARY ARE IN PHASE OPPOSITION AND HENCE CORRECTLY CONNECTED. IF VOLTMETER V2 READ DOUBLE OF SECONDARY RATED VOLTAGE THEN THE TWO TRANSFORMER ARE ACTING IN THE SAME DIRECTION. THIS WE NEED TO CHANGE THE CONNECTION i.e IN PLACE OF CONNECTING B TO C WE NEED TO CONNECT OR JOIN A AND B AND BAND D USED FOR THE TEST ( IN PLACE OF A AND D)

-IF THE PRIMARY CIRCUIT IS NOW CLOSED, THE TOTAL VOLTAGE ACROSS THE 2 SECONDS WILL BE ZERO. THERE WILL BE NO CURRENT IN THE SECONDARY WINDINGS ARE OPEN CIRCUIT. HENCE READING OF W1 GIVES THE IRON LOSSES OF BOTH THE TRANSFORMERS.

A SMALL VOLTAGE IS INJECTED IN SECONDARY CIRCUIT BY A REGULATING IN SECONDARY CIRCUIT BY A REGULATING TRANSFORMER Tr EXCITED BY MAINS SUPPLY. THE MAGNITUDE OF THE INJECTED VOLTAGE IS ADJUSTED TILL THE ARMATURE A2 READS THE FULL LOAD SECONDARY CURRENT. IT RESULT IN INFLOW OF FULL LOAD CURRENT THROUGH THE PRIMARY WINDINGS. THIS CURRENT WILL FOLLOW A CIRCULATOR PATH THROUGH THE MAIN BUSBARS. THE READING OF W2 WILL NOT BE AFFECTED BY THIS CURRENT, THUS IT GIVES THE FULL LOAD COPPER LOSSES OF THE 2 TRANSFORMER.