Auto-connected hexagon transformer for a 12-pulse converter

A 12-pulse converter of lower rating and minimal current harmonics is obtained with an auto-connected regular hexagon transformer including a delta winding in its center.

FIELD OF INVENTION 
The invention relates to AC-to-DC and to DC-to-AC static converters. 
BACKGROUND OF THE INVENTION 
It is known from U.S. Pat. No. 4,876,634 to implement a multiphase AC/DC 
converter system with an auto-connected transformer having two identical 
hexagon secondaries pertaining to separate DC branching sides formed with 
tappings from the apices thereof. 
It is known from U.S. Pat. No. 4,876,634 to create an 18-pulse converter 
system with an auto-transformer having tappings distributed about an 
irregular hexagon wherein three windings are disposed triangularly thereon 
and have midtaps for the three primary lines, tappings being derived for 
the six secondary outputs from the apeces of the six windings. It is also 
shown, there, to have a regular hexagon for a 18-pulse converter, but at 
the expense of boosting the AC power supply to compensate for the regular 
succession of windings on the hexagon. 
It is also known from U.S. Pat. No. 4,255,784 to establish a 12-pulse 
converter system with a three-phase transformer, having a delta primary 
and a regular hexagon for the secondary, wherein six windings are 
distributed regularly, each having two tappings of opposite polarities for 
the secondary outputs. 
In all cases of multiphase converter systems operating on basic three-phase 
AC lines, there is a problem of harmonics on the AC lines as well as of 
rating for the transformer. 
SUMMARY OF THE INVENTION 
It is now proposed to maximize the efficiency of a 12-pulse AC/DC converter 
based on a regular secondary hexagon by choosing the auto-transformer 
approach solution, the three AC lines being directly connected at 120 
degree phase shift anywhere to the hexagon and a delta winding being 
coupled centrally thereof with all the windings. 
As a result, a 12-pulse static converter is obtained having maximum 
reduction of harmonics and an improved overall transformer rating.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1 which is taken from U.S. Pat. No. 4,876,634, two 
hexagon-type auto-transformers TNF1 and TNF2 are associated with the 
three-phase AC lines (1,2,3) to generate on lines 1',2',3' for transformer 
TNF1 and lines 1", 2", 3" for transformer TNF2 two six-pulse rectifier 
systems. For the purpose of disclosing FIG. 1, the afore-stated U.S. 
Patent is hereby incorporated by reference. 
FIG. 2 is also taken from the incorporated-by-reference U.S. Pat. No. 
4,876,634. Instead of two transformers, a 12-pulse converter is obtained 
with a dual shift autotransformer. The three AC lines are going to 
midpoints I, H, G of three windings W'1 (between endpoints D,E), W'2 
(between windings A,F) and W'3 (between windings B,C), respectively 
coupled to main windings W1, W2 and W3. These windings form a polygon, as 
shown in FIG. 3, sustaining tow pairs of three outputs (1', 2', 3') and 
(1", 2" and 3"). There outputs, like those of FIG. 1, are connected to two 
rectifier bridges BR#1 and BR#2 which as shown in FIG. 2 are mounted 
across the DC output terminals TN and TP opposed to the three-phase AC 
input lines; conversely, if the converter is a DC-to-AC converter. The 
equivalent conventional double-wound transformer, for a unity voltage 
ratio, is rated at about 15% of the total output of DC power. 
FIG. 3 is also coming from the afore-stated incorporated-by-reference U.S. 
patent. 
Referring to FIG. 4, a 12-pulse converter is shown (taken from U.S. Pat. 
No. 4,255,784) which is characterized by a true hexagon comprising six 
identical windings regularly distributed on the side of an hexagon for the 
secondary and a delta winding (ABC) for the primary (AC lines X, Y, Z). 
The rectifier lines are derived from twelve tappings taken symmetrically 
by pairs on each winding, one (T1, T2, T3, . . . ) for the positive side, 
the other (T1', T2', T3', . . . ) for the negative side. The U.S. Pat. No. 
4,255,784 is hereby incorporated by reference. Referring to FIG. 5, by 
comparison with FIG. 4, the auto-connected hexagon converter according to 
the present invention is shown to differ therefrom in that the three AC 
lines (X,Y,Z) are directly connected to points A, B, C, respectively, 
which are at 120 degree phase shift on the regular hexagon. The hexagon is 
similar to the hexagon of FIG. 4. For the sake of clarity, each side of 
the hexagon is shown having one central winding and two lateral windings 
symmetrically disposed (W1 and W1', W1" ; W2 and W2',W2"; . . . ) thereby 
accounting for the tappings (T1 and T1'; T2, and T2"; . . . in FIG. 4) of 
lines 1, 2, . . . shown in FIG. 5. Also, for the purpose of 
simplification, the rectifiers (R1, R1'; R2, R2'; in FIG. 4) have not been 
shown in FIG. 5, but they are provided as well known in the prior art 
regarding static AC-to-DC or DC-to-AC converters. In contrast to the 
setting of the hexagon of FIG. 4, the hexagon of FIG. 5 is auto-connected. 
This means that there is no separate primary winding for magnetically 
coupling currents derived from the AC lines with currents flowing in the 
secondary windings proper forming the hexagon. AC lines X, Y, Z of FIG. 4 
are in FIG. 5 directly applied to the hexagon, although at 120 degree 
phase shift, and they are connected through inductors L1, L2 and L3. 
Current IL1 from line Z goes to nodal point A between windings W1 and W1" 
for the first line, whereas current IL2 of line Y goes to nodal point B 
between W3 and W3". Similarly, for line X the AC line nodal point will be 
between W5 and W5". The same could be obtained by choosing the opposite 
series of nodal points : between W1' and W1, W3'and W3, W5' and W5. 
Another possibility of auto-connecting lines X, Y, Z to the hexagon would 
be to use three apeces of even number (or of uneven number) successively. 
According to the present invention, in addition to such an auto-connected 
hexagon transformer, a delta winding such as would be used for the primary 
winding in 10 FIG. 4 is now placed in the center of the hexagon 
(delta-connected windings WA, WB and WC) as a dead winding. By combining 
auto-connection and adding such tertiary winding, a 12-pulse converter 
system is obtained having clearly superior characteristics. First, the 
transformer rating has by about 42%. Moreover, the input current 
distortion will be reduced to less than 3% as explained hereinafter. It is 
observed, thus, that by auto-connection the AC output open circuit voltage 
is 11.53% higher than the AC input voltage. This increase in voltage is 
highly desirable and it leads to an open circuit DC voltage which is 1.56% 
times the AC line input voltage. A most favorable consequence of this is 
to make it possible to insert AC line reactances (L1, L2 and L3 on FIG. 5) 
which will have the effect of reducing quite significantly the harmonic 
currents on lines X, Y, Z. At the same time, this will allow a full DC 
output voltage to be generated, in relation to the available AC line input 
voltage. In this regard, the choice of 120 degree inputs at A, B, C, shown 
in FIG. 5, while very easily applied, will also raise the effective 
voltage. Moreover, since three of the diodes, or SCR's, (placed at 
junction points 1, 5 and 9) become directly connected to the source and 
the transformer rating is lower than if lines X, Y, Z were connected to 
the apeces. 
Returning to the presence of a tertiary winding at the center of the 
hexagon, the circulating currents in windings WA, WB and WC allow a free 
flow of the third and even the triple harmonic currents, a serious 
improvement which is coming on top of the other advantages mentioned 
earlier.