POLARITAS TRAFO

PENDAHULUAN

Fasor dan polaritas adalah dua alat penting dan berguna dalam proteksi sistem daya.

Keduanya membantu dalam pemahaman dan analisis dari hubungan, operasi, dan pengujian

relay-relay dan sistem. Selain itu, konsep-konsep ini penting dalam memahami kinerja sistem

daya selama kedua operasi normal dan abnormal. Dengan demikian, pengetahuan teoritis dan

praktis suara fasor dan polaritas adalah sumber daya mendasar dan berharga.

The polarity of a device with mutual inductance designates the relative

instantaneous current directions of such device's winding leads. Leads of primary and

secondary windings are said be of the same polarity when instantaneous current entering the

primary winding lead results in instantaneous current leaving the secondary winding lead as

though the two leads were a continuous circuit. In the case of two windings wound around the

same core in parallel, for example, the polarity will be the same on the same ends: A sudden

(instantaneous) current in the first coil will induce a voltage opposing the sudden increase

(Lenz's law) in the first and also in the second coil, because the inductive magnetic field

produced by the current in the first coil traverses the two coils in the same manner. The

second coil will therefore show an induced current opposite in direction to the inducing

current in the first coil. Both leads behave like a continuous circuit, one current entering into

the first lead and another current leaving the second lead.

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 depends on which direction coils are wound around the core

(clockwise or counterclockwise) and how the leads are brought out, see in figure 1.

Figure 1. Direction coils are wound around the core

TRANSFORMER POLARITY

Transformer polarity refers to the relative direction of the induced voltages between

the high voltage terminals and the low voltage terminals. Two methods are commonly used to

denote which terminals present the same relative polarity. A dot may be used, or an

alphanumeric designation. Alphanumeric designations are typically in the form H1 for

primaries, and for secondaries, X1, (and Y1, Z1, if more windings present). These markings may

be found on transformer cases beside terminals, winding leads, nameplates, schematic and wiring

diagrams. 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.

More often, transformer polarity is shown simply by the American National Standards Institute

(ANSI) designations of the winding leads as H1, H2 and X1, X2. By ANSI standards, if you face the

low-voltage side of a single-phase transformer (the side marked X1, X2), the H1 connection will

always be on your far left. If the terminal marked X1 is also on your left, it is subtractive polarity. If

the X1 terminal is on your right, it is additive polarity see in figure 2.

Additive polarity is common for small distribution transformers. Large transformers, such as

GSUs at Reclamation powerplants, are generally subtractive polarity. It is also helpful to think of

polarity marks in terms of current direction. At any instant when the current direction is into a

polarity marked terminal of the primary winding, the current direction is out of the terminal with the

same polarity mark in the secondary winding. It is the same as if there were a continuous circuit

across the two windings.

Figure 2. Alphanumeric Designation by ANSI present the same relative polarity

POLARITY TEST

Polarity test 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. Polarity is a convenient way of starting how leads

are brought out. If you want to test for polarity, connect the transformer as shown in figure 3.

A transformer is said to have additive polarity if, when adjacent high and low-voltage terminals are

connected and a voltmeter placed across the other high- and low-voltage terminals, the voltmeter

reads the sum (additive) of the high- and low-voltage windings. It is subtractive polarity if the

voltmeter reads the difference (subtractive) between the voltages of the two windings. If this test is

conducted, use the lowest AC voltage available to reduce potential hazards. An adjustable ac

voltage source, such as a variac, is recommended to keep the test voltage low.

Figure 3. Wiring of Polarity Test

Simply polarity test :

Additve polarity : V3 = V1+V2

Substractive polarity : V3 = V1-V2

Example below :

If the transformer is actually rated 480 -120 volts, the transformer ratio is

4:1 (480/120 = 4). 480 volt as primary voltage, 120 volt as secondary voltage. 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 (V3) will read 90 volts (120-30 = 90).

If the voltmeter (V3) reads 150 volts, the transformer is additive polarity (120 + 30 = 150).

THE DOT IN TRANSFORMER

Since transformers are essentially AC devices, we need to be aware of the phase

relationships between the primary and secondary circuits. Using our SPICE example from

before, we can plot the waveshapes (Figure 4 below) for the primary and secondary circuits

and see the phase relations for ourselves :

spice transient analysis file for use with nutmeg: transformer v1 1 0 sin(0

15 60 0 0) rbogus1 1 2 1e-12 v2 5 0 dc 250 l1 2 0 10000 l2 3 5 100 k l1 l2

0.999 vi1 3 4 ac 0 rload 4 5 1k .tran 0.5m 17m .end nutmeg commands:

setplot tran1 plot v(2) v(3,5)

Figure 4. Plot The Waveshapes

It would appear that both voltage and current for the two transformer windings are in-

phase with each other, at least for our resistive load. This is simple enough, but it would be

nice to know which way we should connect a transformer in order to ensure the proper phase

relationships be kept. After all, a transformer is nothing more than a set of magnetically-

linked inductors, and inductors don’t usually come with polarity markings of any kind. If we

were to look at an unmarked transformer, we would have no way of knowing which way to

hook it up to a circuit to get in-phase (or 180o out-of-phase) voltage and current.

Figure 5. Ambiguous

As a practical matter, the polarity of a transformer can be ambiguous like figure 5

above. Since this is a practical concern, transformer manufacturers have come up with a sort

of polarity marking standard to denote phase relationships. It is called the dot convention,

and is nothing more than a dot placed next to each corresponding leg of a transformer

winding, show in figure 6 below.

Figure 6. The Dot Convention

A pair of dots indicates like polarity, show in figure 6. Typically, the transformer

will come with some kind of schematic diagram labeling the wire leads for primary and

secondary windings. On the diagram will be a pair of dots similar to what is seen above.

Sometimes dots will be omitted, but when “H” and “X” labels are used to label transformer

winding wires, the subscript numbers are supposed to represent winding polarity. The “1”

wires (H1 and X1) represent where the polarity-marking dots would normally be placed.

The similar placement of these dots next to the top ends of the primary and secondary

windings tells us that whatever instantaneous voltage polarity seen across the primary

winding will be the same as that across the secondary winding. In other words, the phase shift

from primary to secondary will be zero degrees.

On the other hand, if the dots on each winding of the transformer do not match

up, the phase shift will be 180o between primary and secondary, like this. See in figure 7.

Figure 7. The dots are not match

Out of phase: primary red to dot, secondary black to dot. Of course, the dot convention only

tells you which end of each winding is which, relative to the other winding(s). If you want to

reverse the phase relationship yourself, all you have to do is swap the winding connections

like this, In phase: primary red to dot, secondary red to dot. See in figure 8.

Figure 8. The dots are match

The phase relationships for voltage and current between primary and secondary

circuits of a transformer are direct: ideally, zero phase shift. The dot convention is a type

of polarity marking for transformer windings showing which end of the winding is which,

relative to the other windings.

The circuit polarity signs '+' and '-' indicate the relative polarities of the induced

voltages in both coils, i.e. how an instantaneous (sudden) magnetic field traversing the

primary and secondary coils induces a voltage in both coils. The instantaneous polarities of

the voltages across each inductor with respect to the dotted terminals are the same. The

circuit arrows indicate example applied and resultant relative current directions. The '+' and '-'

polarities in the diagram are not the voltages driving the currents. The instantaneous

directions of the current entering the primary inductor at its dotted end and the current

leaving of the secondary inductor at its dotted end are the same. Subtractive polarity transformer designs are shown in the upper circuit diagrams. Additive polarity transformer

designs are shown in the lower circuit diagrams. Show in figure 9.

Figure 9. Subtractive and Additive Polarity in The Circuit Diagram

Thanks for read and good luck, be success. By Muhammad Kurniawan

Reference :

1. http://namasayariyandi.blogspot.co.id/2011/03/phasor-dan-polaritas.html

2. http://electrical-engineering-portal.com/understanding-transformer-polarity

3. http://www.allaboutcircuits.com/textbook/alternating-current/chpt-

9/phasing/#02138.png

4. https://www.idc-

online.com/technical_references/pdfs/electrical_engineering/Transformer_Polarity.pd

f

5. http://www.feca.com/HDS_BASIC.pdf

6. https://en.wikipedia.org/wiki/Polarity_%28mutual_inductance%29#Transformer_win

dings

7. http://electrical-engineering-portal.com/identifying-the-primary-and-secondary-

phasor-polarities-of-transformer-polarity-test