What is auto transformer?equivalent circuit Advantages Disadvantages
3 three phase auto transformer working principle pdf advantages disadvantages variac positive negative zero sequence impedance transformer equivalent circuit
What is auto transformer ?
auto-transformer diagram
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| auto transformer variac diagram |
combined with the 'common' winding, forms the 'series common' winding between the high voltage terminals.
The advantage of using an auto-transformer as opposed to a two winding transformer is that the auto-transformer is smaller and lighter for a given rating.The disadvantage is that galvanic isolation between the two windings does not exist, giving rise to the possibility of large overvoltages on the lower voltage system in the event of major insulation breakdown.
Equivalent circuits of three phase auto-transformer
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Figure(a): Equivalent circuits of three phase auto-transformerpositive negative and zero sequence impedance equivalent circuit |
Three-phase auto-transformer banks generally have star connected main windings, the neutral of which is normally connected solidly to earth. In addition, it is common practice to include a third winding connected in delta called the tertiary winding, as shown in Figure (b).
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Positive Sequence Equivalent Circuit:
The positive sequence equivalent circuit of a three-phase auto transformer bank is the same as that of a two- or three winding three winding transformer. The star equivalent for a three-winding transformer, for example, is obtained in the same manner, with the difference that the impedances between windings are designated as follows |
| Positive Sequence Equivalent Circuit |
Where:
Zsc-t = impedance between ‘series-common’ and tertiary windings.
Zsc-c = impedance between ‘series-common’ and ‘common’windings.
Zc-t = impedance between ‘common’ and tertiary windings
When no load is connected to the delta tertiary, the point T is open-circuited and the short-circuit impedance of the transformer becomes Z L+Zh=Zsc-c, similar to the equivalent circuit of a two-winding transformer, with magnetising impedance neglected; see Figure (c).
Zero Sequence Equivalent Circuit:
The zero sequence equivalent circuit is derived in a similar manner to the positive sequence circuit, except that, as there is no identity for the neutral point, the current in the neutral and the neutral voltage cannot be given directly. Furthermore, in deriving the branch impedances, account must be taken of an impedance in the neutral Zn, as shown in Equation, where Zx, Zy and Zz are the impedances of the low, high and tertiary windings respectively and N is the ratio between the series and common windings. |
zero sequence impedance of transformer
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Figure (d) shows the equivalent circuit of the transformer bank. Currents ILo and IHo are those circulating in the low and high voltage circuits respectively. The difference between these currents, expressed in amperes, is the current in the common winding. The current in the neutral impedance is
three times the current in the common winding.
Special Conditions of Neutral Earthing
With a solidly grounded neutral, Zn=0, the branch impedances Zx, Zy, Zz, become ZL, ZH, ZT, that is, identical to the corresponding positive sequence equivalent circuit, except that the equivalent impedance ZT of the delta tertiary is connected to the zero potential bus in the zero sequence network. When the neutral is ungrounded ZT= ∞, and the impedances of the equivalent star also become infinite because there are apparently no paths for zero sequence currents between the windings, although a physical circuit exists and ampere-turn balance can be obtained. A solution is to use an equivalent delta circuit (see Figure (e)), and evaluate the elements of the delta directly from the actual circuit. The method requires three equations corresponding to three assumed operating
conditions. Solving these equations relates the delta impedances to the impedance between the series and tertiary windings as follows:
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Special Conditions of Neutral Earthing |
With the equivalent delta replacing the star impedances in the autotransformer zero sequence equivalent circuit the transformer can be combined with the system impedances in the usual manner to obtain the system zero sequence diagram.
Advantages of auto transformer:
- For the same VA rating an autotransformer requires less copper, less iron and hence low exciting current, low ohmic loss and less weight as compared to a two winding transformer
- For the same material used, an autotransformer as compared to a 2-winding transformer gives higher output, has higher efficiency, lower leakage impedance and hence better voltage regulation.
Disadvantages auto transformer :
- They are used for a voltage ratio less than 2. If the ratio differs far from unity then the economic advantages decrease.
- A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output)
- The short circuit current in Autotransformer is more than that in two winding transformer
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