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.
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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- 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.
- 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.
- 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.
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.
The experimental circuit to conduct O.C test is shown in the figure.
|Fig 1. Experimental circuit for O.C. test|
The observation table is as follows
Vo = Rated voltage
Wo = Input power
Io = Input current = no load current
Im = Io sin Φo
Ic = Io cos Φo
where cos Φo = No load power factor
And hence power input can be written as,
Wo = Vo Io cos Φo
The phasor diagram is shown in the Fig. 2.
... Wo = Pi = Iron losses
Calculations : We know that,
Wo = Vo Io cos Φ
cos Φo = Wo /(Vo Io ) = no load power factor
Once cos Φo is known we can obtain,
Ic = Io cos Φo
and Im = Io sin Φo
Once Ic and Im are known we can determine exciting circuit parameters as,
Ro = Vo /Ic Ω
and Xo = Vo /Im Ω
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|
... 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 = Isc2 R1e = copper loss
... R1e =Wsc /Isc2
while Z1e =Vsc /Isc = √(R1e2 + X1e2)
... X1e = √(Z1e2 – R1e2)
We know that,
From O.C. test, Wo = Pi
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.
From S.C. test we get the equivalent circuit parameters referred to primary or secondary.
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.
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:
Measure the voltage between the primary and the secondary terminals. The following requirements shall be fulfilled depending on the vector group of the transformers :