EP0254946B1 - A single phase auto transformer - Google Patents

Description

• The present invention relates to a single phase auto transformer, and relates particularly to an improvement for a single phase auto transformer having a tap winding and an exciting winding wound on an iron core side leg.
• In view of an electric power transmission line stability and a protective relay specification, the smaller the transformer impedance variation due to the tap position change over, the better.
• In one conventional single phase auto transformer having a small impedance variation due to tap position change over, a tap winding and an exciting winding thereof are wound on an iron core leg (side leg) other than one (main leg) on which a shunt winding and a series winding thereof are wound, and the exciting winding and the shunt winding are connected in parallel so that a change in current distribution flowing through the shunt winding and series winding caused by a change in tap position is prevented, in other words no change of magnetic flux density distribution at the iron core main leg portion is caused by the tap position change over, and a change in impedance at the iron core main leg portion is prevented even when a tap position is changed over. As a result, a change in impedance due to the tap position change over is limited to that at the side leg portion where the tap winding and the exciting winding are wound and a total impedance change of the transformer is rendered comparatively small.
• The extent of the total impedance change of the conventional single phase auto transformer is limited to that of the ratio of voltage adjustment by the tap winding, i.e, when the voltage adjustment range of the single phase auto transformer is l0%, the total impedance of the transformer varies upto in an order of l0%.
• An object of the present invention is to provide a single phase auto transformer having a small impedance variation when tap position of the tap winding thereof is changed over.
• A single phase auto transformer of the present invention including a shunt winding and a series winding wound around a first iron core leg and a tap winding and an exciting winding wound around a second iron core leg is characterized in that said exciting winding is divided into first and second exciting winding units (16a, l6b) connected in series, said first exciting winding unit (16a) being arranged around said second iron core leg (12) inside said tap winding (17) and said second exciting winding unit (16b) being arranged around said second iron core leg (12) outside said tap winding (17).
• Number of turns of the first and second exciting winding units is preferably the same, however the number of turns of one exciting winding unit may be between 20-80% of the total required number of turns for the exciting winding of the single phase auto transformer depending upon the required winding structure of the tap winding.
• One embodiment of the present invention will be described in the following with reference to the accompanying drawings.
• Fig. 1 schematically shows one embodiment of a single phase auto transformer of the present invention.
• Fig. 2(a) shows a winding arrangement at the side leg illustrated in Fig. 1.
• Fig. 2(b) shows magnetic flux density curves for three tap positions at the side leg portion corresponding to the winding arrangement shown in Fig. 2(a) and a conventional winding arrangement.
• Fig. 3 schematically shows transformer impedance curves of the present invention shown in Fig. l and the conventional single phase auto transformer with respect to three tap positions.
• Fig. l is a diagram showing the arrangement of the windings of a single-phase auto transformer according to the embodiment of the present invention. In Fig. l: reference numeral l0 designates an iron core having three legs; numeral ll the main leg of the iron core l0; and numeral l2 the side leg of the iron core l0. Numerals l3, l4 and l5 designate a ternary winding, a shunt or common winding and a series winding, respectively, all of which are wound in the recited order on the main leg ll. Numeral l6a designates one of the two divided exciting windings, as will be referred to as a "first exciting winding". Numeral l6b designates the other of the two divided exciting windings, as will be referred to as a "second exciting winding". Numeral l7 designates a tap winding. These individual windings are wound on the side leg l2 such that the first exciting winding l6a is at the inner most side, the tap winding l7 at the intermediate and the second exciting winding l6b is at the outer most side. In other words, the tap winding l7 is sandwiched between the two divided exciting windings l6a and l6b.
• The first exciting winding l6a and the second exciting winding l6b are connected in series with each other to provide a series circuit, with which is connected in parallel the shunt winding l4 to provide a parallel circuit, with which is connected the series winding l5.
• In the auto transformer of the present embodiment the voltage of the secondary side, in that intermediate voltage side, is changed over. As indicated above, the series winding l5 and the shunt winding l4 are connected in series and one terminal of the tap winding l7 is drawn out as a secondary terminal u and the other terminal thereof is connected to a common juncture of the series winding l5, the shunt winding l4 and the second exciting winding unit l6b.
• In the single-phase auto transformer thus constructed, the current distribution in the shunt winding l4 and the series winding l5 is never changed when the tap position of the tap winding l7 is changed over, because the shunt winding l4 is connected in parallel with the series circuit of the exciting windings l6a and l6b, as a result, no change in magnetic flux density, thus no impedance variation at the main leg portion occurs.
• Moreover, since the exciting winding is divided into the first exciting winding l6a and the second exciting winding l6b, between which is sandwiched the tap winding l7 to limit the impedance variation of the single phase auto transformer because substantially the half of the exciting winding is arranged outside the tap winding l7, the maximum magnetic flux density at the side leg portion is reduced to substantially the half of the conventional single phase auto transformer having non-divided exciting winding.
• As indicated previously, impedance change at the main leg portion does not occur so that the transformer impedance variation is controlled by the impedance between the exciting winding and the tap winding at the side leg portion.
• Figs. 2(a) and 2(b) are a diagram showing the arrangement of the windings at the side leg l2, as shown in Fig. l, and a diagram showing the characteristics of the interlinking magnetic flux density in the corresponding positions. In Fig. 2(a), the same portions as those shown in Fig. l are designated at the identical reference numerals. In Fig. 2(b), a curve 20 represents the magnetic flux density in the case of the tap position at the highest voltage; a curve 22 represents the magnetic flux density in the case of the tape position at the center; and a curve 2l represents the magnetic flux density in the case of the tap position at the lowest voltage. Here:

N_E1:

Number of turns of the first exciting winding l6a;

N_E2:

Number of turns of the second exciting winding l6b;

D_E1:

Width (cm) of the first exciting winding l6a;

D_G1:

Size (cm) of the gap Gl between the first exciting winding l6a and the tap winding l7;

D_T:

Width (cm) of the tap winding l7;

D_G2:

Size (cm) of the gap G2 between the tap winding l7 and the second exciting winding l6b;

D_E2:

Width (cm) of the second exciting winding l6b;

R_E1:

Average radius (cm) of the first exciting winding l6a;

R_G1:

Average radius (cm) of the gap Gl; R_T: Average radius (cm) of the tap winding l7;

R_G2:

Average radius (cm) of the gap G2;

R_E2:

Average radius (cm) of the second exciting winding l6b;

I:

Current (A) flowing through each of the exciting windings l6a and l6b;

f:

Frequency (Hz) of the current I;

h:

Height (cm) of each of the windings l6a, l6b and l7; and

P:

Reference capacity (VA) of the transformer.

Then, the % impedance voltage %V_Z1 between the tap winding l7 and each of the exciting windings l6a and l6b of the present embodiment shown in Fig. 2(a) is generally expressed by the following equation (l):


Here, Δ ₁ is expressed by the following equation (2):

• For comparison, % impedance voltage of the conventional single phase auto transformer having non-divided exciting winding, in that, schematically the second exciting winding unit is eliminated, is examined. In Fig. 2(b), a curve 23 represents the magnetic flux density when the tap position is at the highest voltage position, a curve 25 represents the magnetic flux density when the tap position at the center, and a curve 24 represents the magnetic flux density when the tap position at the lowest voltage position. Here:

N_E:

Number of turns of the exciting winding;

D_E:

Width (cm) of the exciting winding;

D_G:

Size (cm) of the gap between the exciting winding and the tap winding;

D_T:

Width (cm) of the tap winding;

R_E:

Average radius (cm) of the exciting winding;

R_G:

Average radius (cm) of the gap between the two windings; and

R_T:

Average radius (cm) of the tap winding.

Then, the % impedance voltage %V_Z2 between the tap winding and the exciting winding of the conventional single phase auto transformer is generally expressed by the following equation (3):



Here, Δ ₂ is expressed by the following equation (4):

• When assuming that the specifications of the embodiment of the present invention and the conventional single phase auto transformer are the same, in that
  
  ${\text{N}}_{\text{E1}}{\text{+ N}}_{\text{E2}}{\text{= N}}_{\text{E}}\text{.}$

  

  
  
  Assuming further for example that,

  

  
  and
  although Δ ₁ and Δ ₂ vary depending upon insulation distances between windings, however take for example,
  
  ${\text{}}_{\text{Δ1}}{\text{/}}_{\text{Δ2}}\text{= 1.6}$

  

  .
  
• Since f, I, and P are the same for the equations (l) and (3), the ratio of the two % impedance voltages is expressed by the following equation (5):

  

  
  In other words, the impedance at the side leg portion is lowered to about 40% in the structure of the present embodiment, in which the exciting winding is divided into two windings sandwiching the tap winding inbetween, than in the structure of the conventional one in which the exciting winding is not divided into two. Since the absolute impedance value at the side leg portion is thus reduced, the change in impedance, when the tap position is changed over is naturally reduced.
• Fig. 3 shows transformer impedance variation of the present embodiment and the conventional single phase auto transformer with respect to tap positions. A curve 26 represents transformer impedance of the present embodiment and a curve 27 represents that of the conventional single phase auto transformer. In both curves, the minimum transformer impedance appears at the center tap position and the impedance gradually increases when the tap position moves away from the center position. As seen from Fig. 3, the transformer impedance variation of the present invention shown by the curve 26 is controlled smaller than that of the conventional single phase auto transformer shown by the curve 27.

Claims (5)

1. A single phase auto transformer including first and second iron core legs (ll, l2), a shunt winding (l4) and a series winding (l5) connected in series and wound around said first iron core leg (ll) and a tap winding (l7) and an exciting winding wound around said second iron core leg (l2), said exciting winding being connected in parallel with said shunt winding (l4) characterized in that said exciting winding is divided into first and second exciting winding units (l6a, l6b) connected in series, said first exciting winding unit (l6a) being arranged around said second iron core leg (l2) inside said tap winding (l7) and said second exciting winding unit (l6b) being arranged around said second iron core leg (l2) outside said tap winding (l7).
2. The single phase auto transformer according to claim l wherein said tap winding (l7) is connected in series with said shunt winding (l4).
3. The single phase auto transformer according to claim l wherein said series winding (l5) is arranged outside said shunt winding (l4).
4. The single phase auto transformer according to claim 3 further comprising a ternary winding (l3) wound around said first iron core leg (l) inside said shunt winding (l4).
5. The single phase auto transformer according to claim l wherein the number of turns of said first exciting winding unit (l6a) and said second exciting winding unit (l6b) is substantially the same.

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