Sohail Ansari

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Transformer Polarity Test

The Importance of Polarity : An understanding of polarity is essential to correctly construct three-phase transformer banks and to properly parallel single or three-phase transformers with existing electrical systems. A knowledge of polarity is also required to connect potential and current transformers to power metering devices and protective relays. The basic theory of additive and subtractive polarity is the underlying principle used in step voltage regulators where the series winding of an autotransformer is connected to either buck or boost the applied line voltage.
Transformer Polarity refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half-cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to
the way the leads are brought out of the transformer.
Bushing Arrangement: The position of the High Voltage Bushings is standardized on all power and instrument transformers.
The rule is this: when facing the low voltage bushings, the Primary Bushing H1 is always on the left-hand side and the Primary Bushing H2 is on the right-hand side (if the transformer is a three-phase unit, H3 will be to the right ofH2).
Distribution Transformers are Additive Polarity and the H1 and X1 bushings are physically placed diagonally opposite to each other. Since H1 is always on the left, X1 will be on the right-hand side of a distribution transformer. This standard was developed very early in the development of electrical distribution systems and has been adhered to in order to prevent confusion in the field when transformers need to be replaced or paralleled with existing equipment.


In situations where the secondary bushing identification is not available or when a transformer has been rewound, it may be necessary to determine the transformer polarity by test. The following procedure can be used :

  • The H1 (left-hand) primary bushing and the left-hand secondary bushing are temporarily jumpered together and a test voltage is applied to the transformer primary. The resultant voltage is measured between the right-hand bushings. If the measured voltage is greater than the applied voltage, the transformer is Additive Polarity because the polarity is such that the secondary voltage is being added to the applied primary voltage. If, however, the measured voltage across the right-hand bushings is less than the applied primary voltage, the transformer is Subtractive Polarity.

Note: For safety and to avoid the possibility of damaging the secondary insulation, the test voltage applied to the primary should be at a reduced voltage and should not exceed the rated secondary voltage.

polarity test
In the figure shown above, if the transformer is actually rated 480 – 120 volts, the transformer ratio is 4:1 (480 / 120 = 4).
Applying a test voltage of 120 volts to the primary will result in a secondary voltage of 30 volts (120 / 4 = 30). If transformer is subtractive polarity, the voltmeter will read 90 volts (120 – 30 = 90). If the voltmeter reads 150 volts, the transformer is additive polarity (120 + 30 = 150). The red arrows indicate the relative magnitude and direction of the primary and secondary voltages.

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Difference between Power Transformer & Distribution Transformer

  1. Power transformers are used in transmission network of higher voltages for step-up and step down application (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.  Whiile Distribution transformers are used for lower voltage distribution networks as a means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and are generally rated less than 200 MVA.
  2. Power transformer is used for the transmission purpose at heavy load, high voltage greater than 33 KV & 100% efficiency. It also having a big in size as compare to distribution transformer, it used in generating station and Transmission substation .high insulation level.The distribution transformer is used for the distribution of electrical energy at low voltage as less than 33KV in industrial purpose and 440v-220v in domestic purpose. It work at low efficiency at 50-70%, small size, easy in installation, having low magnetic losses & it is not always fully loaded.
  3. The main difference between power and distribution transformer is distribution transformer is designed for maximum efficiency at 60% to 70% load as normally doesn’t operate at full load all the time. Its load depends on distribution demand. Whereas power transformer is designed for maximum efficiency at 100% load as it always runs at 100% load being near to generating station.


O.C Test & S.C. Test of Single Phase Transformer

The O.C test and S.C. test of single phase transformer is done in order to find all the parameter of transformer. It is also done to find the iron loss and cu loss occuring in x mer at any load.


Open Circuit Test (O.C. Test)

The experimental circuit to conduct O.C test is shown in the figure.

Fig 1. Experimental circuit for O.C. test
       The transformer primary is connected to a.c. supply through ammeter, wattmeter and variac. The secondary of transformer is kept open. Usually low voltage side is used as primary and high voltage side as secondary to conduct O.C test.
       The primary is excited by rated voltage, which is adjusted precisely with the help of a variac. The wattmeter measures input power. The ammeter measures input current. The voltemeter gives the value of rated primary voltage applied at rated frequency.
       Sometimes a voltmeter may be connected across secondary to measure secondary voltage which is V= Ewhen primary is supplied with rated voltage. As voltmeter resistance is very high, though voltmeter is connected, secondary is treated to be open circuit as voltmeter current is always negligibly small.
      When the primary voltage is adjusted to its rated value with the help of variac, readings of ammeter and wattmeter are to be recorded.

The observation table is as follows

V= Rated voltage
W= Input power
I= Input current = no load current

       As transformer secondary is open, it is on no load. So current drawn by the primary is no load current Io. The two components of this no load current are,

I= Isin Φo
I= Icos Φo
where  cos Φo = No load power factor
And hence power input can be written as,
W= VIcos Φo
The phasor diagram is shown in the Fig. 2.

Fig. 2
       As secondary is open, I= 0. Thus its reflected current on primary is also zero. So we have primary current I=Io. The transformer no load current is always very small, hardly 2 to 4 % of its full load value. As I= 0, secondary copper losses are zero. And I= Iis very low hence copper losses on primary are also very very low. Thus the total copper losses in O.C. test are negligibly small. As against this the input voltage is rated at rated frequency hence flux density in the core is at its maximum value. Hence iron losses are at rated voltage. As output power is zero and copper losses are very low, the total input power is used to supply iron losses. This power is measured by the wattmeter i.e. Wo. Hence the wattmeter in O.C. test gives iron losses which remain constant for all the loads.

...          W= Pi  = Iron losses
Calculations : We know that,
W= VIcos Φ
cos Φo = W/(Vo I) = no load power factor
Once cos Φo is known we can obtain,
Ic  = Icos Φo
and        I= Isin Φo
Once Ic  and Iare known we can determine exciting circuit parameters as,
R= V/I  Ω
and        X= V/Im   Ω

Key Point : The no load power factor cos Φo is very low hence wattmeter used must be low power factor type otherwise there might be error in the results. If the meters are connected on secondary and primary is kept open then from O.C. test we get Roand Xo with which we can obtain Rand Xknowing the transformation ratio K.
Short Circuit Test (S.C. Test)

In this test, primary is connected to a.c. supply through variac, ammeter and voltmeter as shown in the Fig. 3.

Fig. 3 Fig 1. Experimental circuit for O.C. test
       The secondary is short circuited with the help of thick copper wire or solid link. As high voltage side is always low current side, it is convenient to connect high voltage side to supply and shorting the low voltage side.
       As secondary is shorted, its resistance is very very small and on rated voltage it may draw very large current. Such large current can cause overheating and burning of the transformer. To limit this short circuit current, primary is supplied with low voltage which is just enough to cause rated current to flow through primary which can be observed on an ammeter. The low voltage can be adjusted with the help of variac. Hence this test is also called low voltage test or reduced voltage test. The wattmeter reading as well as voltmeter, ammeter readings are recorded. The observation table is as follows,
       Now the current flowing through the windings are rated current hence the total copper loss is full load copper loss. Now the voltage supplied is low which is a small fraction of the rated voltage. The iron losses are function of applied voltage. So the iron losses in reduced voltage test are very small. Hence the wattmeter reading is the power loss which is equal to full load copper losses as iron losses are very low.

...           Wsc = (Pcu) F.L. = Full load copper loss
Calculations : From S.C. test readings we can write,
Wsc = Vsc Isc cos Φsc 
...            cos Φsc = Vsc Isc /Wsc = short circuit power factor
Wsc = IscR1e = copper loss
...             R1e =Wsc /Isc2
while        Z1e =Vsc /Isc = √(R1e2 + X1e2)
...             X1e = √(Z1e2 – R1e2)

       Thus we get the equivalent circuit parameters R1e, X1e and Z1e. Knowing the transformation ratio K, the equivalent circuit parameters referred to secondary also can be obtained.
Important Note : If the transformer is step up transformer, its primary is L.V. while secondary is H.V. winding. In S.C. test, supply is given to H.V. winding and L.V is shorted. In such case we connect meters on H.V. side which is transformer secondary through for S.C. test purpose H.V side acts as primary. In such case the parameters calculated from S.C. test readings are referred to secondary which are R2e, Z2e and X2e. So before doing calculations it is necessary to find out where the readings are recorded on transformer primary or secondary and accordingly the parameters are to be determined. In step down transformer, primary is high voltage itself to which supply is given in S.C. test. So in such case test results give us parameters referred to primary i.e. R1e, Z1e and X1e.
Key point : In short, if meters are connected to primary of transformer in S.C. test, calculations give us R1e and Z1e if meters are connected to secondary of transformer in S.C. test calculations give us R2e and Z2e.
Calculation of Efficiency from O.C. and S.C. Tests

We know that,
From O.C. test, W=  P
From S.C. test,  Wsc = (Pcu) F.L.

Thus for any p.f. cos Φ2 the efficiency can be predetermined. Similarly at any load which is fraction of full load then also efficiency can be predetermined as,

where       n = fraction of full load

where       I2= n (I2) F.L.

Calculation of Regulation

From S.C. test we get the equivalent circuit parameters referred to primary or secondary.

       The rated voltages V1, V2 and rated currents (I1) F.L. and (I2) F.L. are known for the given transformer. Hence the regulation can be determined as,

where I1, I2 are rated currents for full load regulation.
For any other load the currents I1, I2 must be changed by fraction n.
...    I1, I2 at any other load = n (I1) F.L., n (I2) F.L.

Key Point : Thus regulation at any load and any power factor can be predetermined, without actually loading the transformer
In this way we can find all the parameters of single phase transformer. If u have any query related to this topic then u should comment below or message me on fb.

Transformer Vector Group Test

Vector Group test is done in order to know the phase displacement between the high voltage and low voltage winding.


Without earthing the winding neutral points, interconnect one phase of HV winding (say 1U ) to the corresponding phase of LV winding 2U and apply a balanced 3 phase low voltage to the HV winding. The phase sequence of the supply should be the same as the specified phase sequence of the transformer winding. The phasor will show you how to measure the voltages between any terminal.

Connections for Dy11 and Dy1 transformers and the corresponding vector groupings are given in figure:

Vector Gropu Test

Measure the voltage between the primary and the secondary terminals. The following requirements shall be fulfilled depending on the vector group of the transformers :

conditionIn this way you can measure the vector group of any 3 phase transformer. If u have any query then contact me or give a reply below.