Phase line derived cosine-wave generator and gate pulse generator for thyristor control using such generator

In accordance with the present invention, a single line-frequency sync signal is used to derive cosine-crossover type gating angle reference waves, each with additional phase shifting accounting for phase shifts introduced by the system, such as the power transformers associated with the several thyristors in the sequence. A common variable voltage control signal and comparator are used to apply the various gating signals to the thyristor multiplexer and driver circuits. A plurality of switches controlled by the firing logic circuit establishes the order of selection between the cosine reference waves to be generated.

BACKGROUND OF THE INVENTION 
The invention relates to the generation of a cosine wave reference for the 
generation of thyristor triggering pulses in static power converters. The 
cosine wave crossing timing principle is generally known in the art of 
static power converters (see "Static Power Frequency Changes" by Gyugyi 
and Pelly, John Wiley Edition 1976, pp. 279-298). The known advantage of 
using a cosine wave as a time reference is that it introduces a linear 
function between the cosine voltage and the output voltage of the 
thyristor bridge. It is known to modify a sine wave reference received 
from the line so as to generate a cosine wave which is compared with a 
variable DC control voltage for the determination of the thyristor 
conduction angle. Several problems have been met in the prior art by 
manipulating the inputted sine wave in a particular manner. Thus, in U.S. 
Pat. No. 3,983,495 the object is to obtain a sine wave to cosine wave 
conversion which is insensitive to frequency. 
An object of the present invention is to successively generate, from a 
common phase line sine wave, a series of cosine waves sequentially 
associated with the successive phase lines, each cosine wave having the 
required phase shift in the sequence of firing the thyristors, such firing 
being effected under a single and common comparator circuit. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a single line-frequency sync 
signal is used to derive cosine-crossover type gating angle reference 
waves, each with additional phase shifting accounting for phase shifts 
introduced by the system, such as the power transformers associated with 
the several thyristors in the sequence. A common variable voltage control 
signal and comparator are used to apply the various gating signals to the 
thyristor multiplexer and driver circuits. A plurality of switches 
controlled by the firing logic circuit establishes the order of selection 
between the cosine reference waves to be generated.

DETAILED DESCRIPTION OF THE INVENTION 
One conventional method of controlling the gating angle .alpha. in a six 
thyristor power converter is to generate six individual cosine voltage 
waveforms evenly spaced 60.degree. apart and to compare each cosine 
voltage to a reference voltage. As each cosine voltage in turn equals the 
reference voltage, the thyristor associated with that particular cosine 
wave is fired. The six thyristors are usually in a conventional 
three-phase rectifying bridge and the average voltage out of the bridge is 
varied by changing the gating angle .alpha.. This is accomplished by 
varying the reference voltage. Such prior art arrangement is shown in FIG. 
1. A static power converter 1, typically a six-thyristor bridge, is 
supplied with AC power through a power transformer T. The thyristors are 
gated via lines 2 by a gating pulse generator 3 firing the thyristors in 
sequence with a conduction angle established by a comparator 4. As shown 
in the prior art, the cosine wave crossing pulse timing method is used to 
establish the firing angle by reference to a direct current voltage signal 
reference (V.sub.c as shown inputted into comparator 4 of FIG. 1, and 
comparator 118 of FIG. 6). (In this respect, see "Thyristor 
Phase-Controlled Converters and Cycloconverters" by B. R. Pelly--John 
Wiley & Sons 1971, pages 229-240.) The cosine wave is derived from an 
auxiliary transformer T' having a primary winding in delta configuration 
and a six-winding equally distributed secondary. The secondary windings 
provide six reference sine waves at .pi./3 from each other, on respective 
lines 7 each applied to a corresponding wave converter 8 generating the 
intended cosine time wave for the comparator 4. Thus, six wave converters 
and six comparators are required for firing six thyristors in this 
arrangement. Gating signals GATE 1 to GATE 6 are thus derived and applied 
to the gate pulse generator 3 successively. 
Instead of generating separately six time waves, it is shown in U.S. Pat. 
No. 4,017,744 of F. O. Johnson, how with ramps, e.g., linear time 
functions, digital technique can be used to directly select from a single 
wave generator a plurality of time references, each having the proper 
phase as prescribed by the logic of the gating pulse generator. 
As shown in FIG. 2, according to the present invention, a single wave 
generator 8 is used to convert the sine wave from transformer T' into a 
cosine wave. A single output line 9 applied concurrently with the signal 
v.sub.c to a single comparator 4 causes, at the instant the threshold of 
the comparator is exceeded, the generation of a gating signal for the 
gating pulse generator 3. The wave generator 8 is controlled from lines 10 
by the logic of gating pulse generator 3. As a result, generator 8 builds 
up instantaneously one of several inherent cosine waves related to the 
sine wave inputted on line 11 from transformer T'. 
Referring to FIG. 3, six cosine waves associated with six respective 
thyristors are shown in relation to the reference voltage v.sub.c. The 
conductive periods are shown by bold lines on the curves and by reference 
to the thyristor which is "next" to be fired in sequential order at the 
instant of commutation. It appears that the successive cosine waves are as 
follows: 
V.sub.cos (2.pi.f.sub.L t+.phi..sub.1); V.sub.cos (2.pi.f.sub.L 
t+.phi..sub.2); 
V.sub.cos (2.pi.f.sub.L t+.phi..sub.6) 
where .phi..sub.2 =.phi..sub.1 +.pi./3, etc. 
These six cosine waves are usually developed on an individual basis, often 
from a transformer having a three-phase primary and a secondary possessing 
six windings from which are derived the six cosine signals. This approach 
requires six matched filters, six comparators and a complex transformer. 
With the system according to the present invention, only a single-phase 
sinusoidal synch signal is needed to generate all six cosine waves. 
Excellent filtering exist inherently in the latter system. 
Referring to FIG. 4, from the output of integrator 22, on line 25, and from 
the output of integrator 24, on line 26, two sets of switches (S.sub.1 
-S.sub.6) and (S'.sub.1 -S'.sub.6) multiplex the respective lines 25 and 
26 toward several amplifiers of different gains (L.sub.1 -L.sub.6) for the 
first set of switches, (K.sub.1 -K.sub.6) for the second set of switches, 
with a one-to-one relationship between the gains and the associated set of 
switches. All parallel lines in each set have a junction point J.sub.1 for 
(L.sub.1 -L.sub.6) and J.sub.2 for (K.sub.1 -K.sub.6). The two sets of 
switches are operated in parallel and the outputs from J.sub.1 on line 27, 
from J.sub.2 on line 28, are summed up by a summer 29 outputting the 
desired cosine signal on line 9 to the non-inverting input of an 
operational amplifier 4 used as a comparator, having a threshold defined 
by a reference signal v.sub.c applied on line 12 to the inverting input 
thereof. The generated signal is inputted on line 13 as a triggering 
signal into the gating pulse generator 3. The latter may include, as 
generally known, a ring-counter determining the sequential order of the 
thyristors and a pulse forming circuit driving the thyristors. Six gate 
signals GP.sub.1 -GP.sub.6 are applied by lines 2 to the respective 
thyristors. The two sets of six switches (S.sub.1 -S.sub.6) and (S'.sub.1 
-S'.sub.6) are controlled by pairs via lines 10 and 17 to one set, via 
lines 10 and 18 to the other set, in accordance with the logic of circuit 
3 which determines the firing of the thyristors. 
Simple trigonometry establishes that cos (.omega.t+.phi.)=cos .omega.t cos 
.phi.- sin .omega.t sin .phi.. In the light of this equation, it is seen 
that summer 29 adds two trigonometric functions derived from the 
respective integrators 22 and 24 as follows: A sin 2.pi.f.sub.L t on line 
11 becomes -A.sub.I A cos 2.pi.f.sub.L t after a 90.degree. shift by 
integration through circuit 22. Circuit 24 converts the latter function 
into -A.sub.I.sup.2 A sin 2.pi.f.sub.L t following another 90.degree. 
shift. Thus, on lines 25 and 26, the two functions cos .omega.t and -sin 
.omega.t are derived. It remains to define coefficients cos .phi. and sin 
.phi. which are a function of the particular phase .phi.. The gains 
(L.sub.1 -L.sub.6) and the gains (K.sub.1 -K.sub.6) are paired so as to 
provide six pairs of coefficients associated with the signals of lines 25 
and 26. 
The switches (S.sub.1 -S.sub.6), (S'.sub.1 -S'.sub.6) are sequentially 
controlled from lines 17 and 18 to establish the required common phase 
shift in exact correlation with the "next" thyristor in each pair by 
L.sub.1, K.sub.1 ; L.sub.2, K.sub.2 ; . . . or L.sub.6, K.sub.6. 
It appears that while on line 11 the exact instantaneous magnitude of the 
power line is known, for any pair of thyristors involved at a given time, 
a signal corresponding to one of the curves of FIG. 3 is applied to 
comparator 4. Thus, curve I is followed from t.sub.0 to t.sub.1 when the 
curve intersects v.sub.c, at which time by line 13 thyristor 1TH is fired. 
From t.sub.1 to t.sub.2, curve II is followed on line 9 and at time 
t.sub.2, thyristor 2TH is fired in response to the pulse on line 13, and 
so on. v.sub.c may vary from ReF.sub.min to ReF.sub.max causing the gating 
angle .alpha. to range from zero to .pi.. 
As shown in FIG. 4, the SYNC signal received on line 11, is a sine wave at 
the same frequency as the three-phase AC line feeding the thyristor 
bridge, namely (A sin 2.pi.f.sub.L t), where f.sub.L is the line 
frequency. This signal is inputted into a first integrator 22 which shifts 
the SYNC signal almost exactly 90.degree. and filters out the higher 
frequency voltage spikes and notches which could cause misgating. The 
output of integrator 22 is fed via line 23 into a second integrator 24 
which shifts the SYNC signal by another 90.degree.. The outputs of 
integrators 22 and 24 are respectively inputted into adjustable gain 
circuits 51 and 52 via respective lines 25 and 26. As explained 
hereinafter, circuits 51 and 52 each contain parallel channels of 
different gains which are selected by pairs, one within circuit 51, the 
other within circuit 52. Switches S.sub.1 -S.sub.6 in block 51 and 
switches S'.sub.1 -S'.sub.6 in block 52 when appropriately controlled 
typically select corresponding gains L.sub.1 -L.sub.6 in block 51 and 
gains K.sub.1 -K.sub.6 in block 52, assuming six parallel channels 
symbolized by operational amplifiers L.sub.1 -L.sub.6 in block 51 and 
operational amplifiers K.sub.1 -K.sub.6 in block 52. In relation to a 
particular thyristor (T.sub.1 . . . or T.sub.6) which is "next" to be 
fired, a corresponding pair of switches from the respective blocks such as 
(S.sub.1, S'.sub.1), . . . (S.sub.6, S'.sub.6) are closed thereby to 
define at the output of summer 29 connected between blocks 51 ad 52 a 
particular one of six possible time waves. Whenever such a pair of 
switches are closed, gains K.sub.n and L.sub.n are introduced by blocks 51 
and 52, respectively. As a result, on output lines 27, 28 of blocks 51, 
52, signals (A.multidot.A.sub.I.sup.2 .multidot.K.sub.N) sin 2.pi.f.sub.L 
t and (A.multidot.A.sub.I .multidot.L.sub.n) cos 2.pi.f.sub.L t are 
obtained which are summed up by summer 9 to derive on line 9 at the output 
of the summer a net cosine wave V.sub.cos (2.pi.f.sub.L t+.phi..sub.n) 
which is compared with voltage v.sub.c to determine when the "next" 
thyristor is to be fired. As comparator 4 senses equality, gating is 
initiated. The next pair of switches closes and the original pair opens, 
thereby to send a new cosine wave V.sub.cos (2.pi.f.sub.L 
t+.phi..sub.n+1)=V.sub.cos (2.pi.f.sub.L t+.phi..sub.n +.pi./3) into 
comparator 4 to determine when the "next" thyristor is to be fired. 
Although the sine and cosine values have been explained in terms of 
voltages, it is understood that it is also contemplated that the SYNC 
signal be converted immediately and continuously into binary numbers so 
that all of the sine, cosine and coefficient values will be binary numbers 
treated within a processor. All addition, multiplication and logic would 
then also be handled by the processor. 
Referring again to the mathematical relationship: a sin .theta.+b cos 
.theta.=c cos (.theta.+.phi.), the constants K.sub.i and L.sub.i must be 
selected by pairs so that the desired phase shift .phi..sub.n and 
amplitude V of the resulting cosine wave is as required for each of the 
generated six cosine waves. 
Like any analog integrator, numerical integrators with constant sampling 
frequency tend to have a gain inversely proportional to input frequency. 
In such case, the value of A.sub.I would be inversely proportional to line 
frequency. This would have to be compensated for in variable line 
frequency applications. 
Heavy lines in FIG. 3 show the actual value going to the comparator (+) 
input at any instance. Thyristors are fired as shown. Gating angle can be 
varied from 0.degree. to 180.degree. by varying v.sub.c. The voltage out 
of a converter bridge operating with continuous current is approximately 
proportional to cos .alpha.. The angle .alpha. is proportional to the arc 
cos (v.sub.c). Therefore, the DC voltge out of the bridge is linearly 
proportional to v.sub.c, which, as mentioned earlier, is an advantage 
typical of cosine-crossover type gate generators. 
The constant coefficients K.sub.i and L.sub.i are calculated in order to 
take into account additional phase shifts between the SYNC signal and the 
bridge AC voltage due to transformers T and T', or other components. The 
phase shift, however, must be constant over any range of line frequencies 
at which the gate circuit has to operate. 
If implemented by analog techniques, inexpensive amplifier and analog 
switch components do exist. Analog switches could be used to successively 
change an amplifier's gain to provide the six gain constants L.sub.n. 
Likewise for the various values of K.sub.n. Two integrating amplifiers, 
two gain amplifiers (for the K.sub.n and L.sub.n constant) a summing 
amplifier and a comparator would require a minimum of six amplifiers to 
implement the circuit. This compares favorably with the six comparators 
used in the conventional approach. Thus, the gain switching logic can be 
handled by a single PROM (programmable read-only memory). This 
implementation is shown by FIG. 5. 
FIG. 5 can be understood by reference to FIG. 4. Three solid state devices 
IH4010 are used to form switches (S.sub.1 -S.sub.6) and (S'.sub.1 
-S'.sub.6). Each of these solid state devices embodies four FET switches, 
controlled by respective lines (C.sub.1 -C.sub.4), (C.sub.5, C.sub.6, 
C.sub.1, C.sub.2) and (C.sub.3 -C.sub.6) in accordance with the logic of a 
decode logic circuit 30, which is a PROM, type IM5600, located within 
gating pulse circuit 3. Comparator 4 clocks a 3-bit counter 31 which in 
turn controls the decode logic circuit 30 and another decode logic circuit 
32, the latter to generate the thyristor gating pulses. The control 
signals from circuit 30 are C.sub.1 -C.sub.6, and also signals PS (for 
sine wave select, e.g., lines 26 and 28) and PC (for cosine wave select, 
e.g., lines 25 and 27). Signals PS and PC control FET devices 33 and 34. 
As shown in FIG. 5, operational amplifier 35 is associated with (S'.sub.1 
-S'.sub.6) on the sine wave side, while operational amplifier 36 is 
associated with (S.sub.1 -S.sub.6) on the cosine wave side. These 
amplifiers are inserted in circuit with resistors R.sub.x and R.sub.s to 
provide a gain in accordance with the value of R.sub.x, e.g., with the 
selection made by (C.sub.1 -C.sub.6). Thus on line 28 at the output of 
operational amplifier 35 is derived a wave V representing .+-.R.sub.s 
/R.sub.x SIN, while with operational amplifier 36, the wave is .+-.R.sub.s 
/R.sub.x COS. 
In the preceding, it has been assumed that on lines 23, 25, 26, 27, 28 and 
10 signals having the form of a cosine wave were treated to determine when 
the "next" thyristor is to be fired. It is within the scope of the present 
invention to convert the signals of lines 11, 23 and 26 directly and 
continuously into binary numbers, so that all the sine, cosine and 
coefficient values would be binary numbers. This could be achieved by data 
processing within a microprocessor which would handle addition, 
multiplication and other logic operations as earlier stated. 
Referring to FIG. 6, the cosine wave generator of FIG. 4 is shown connected 
in circuit within a gate pulse generator of the type disclosed in either 
U.S. Pat. No. 4,017,744 of F. O. Johnson, or U.S. Pat. No. 4,028,609 of R. 
L. Detering. For the purpose of describing the preferred embodiment of the 
invention, the Johnson and the Detering patents are hereby incorporated by 
reference. The SYNC sine wave signal of line 11 is shifted by 90.degree. 
by integrator 22 and fed into block 51. It is shifted again by 90.degree. 
when passed through integrator 24, then fed into block 52. Switches 
S.sub.1 -S.sub.6 of block 51, and switches S'.sub.1 -S'.sub.6 of block 52 
are FET devices controlled from respective lines 17 and 18, as explained 
hereinafter, so that a particular pair of switches S.sub.1 -S'.sub.1 . . . 
S.sub.6 -S'.sub.6 is closed at any given time and summer 29 provides at 
the output on line 9 one selected time wave reference V.sub.cos (2.pi. 
f.sub.L t+.phi..sub.n) as explained earlier; such time wave reference 
being shifted by .pi./3 from the preceding as well as from the subsequent 
time wave derived by switch selection in the sequence of firing of the 
thyristors. Comparator 118 is responsive to the time wave reference of 
line 9 from summer 29 and to a reference voltage v.sub.c determining the 
firing angle of the thyristors. Comparator 118 via line 13 controls a 
multivibrator 30 which generates a triggering pulse on line 129. A ring 
counter 20 actuated by the triggering pulses transfer states and causes 
via line 75 a distributor 77 to logically define the multiplexing relation 
affecting firing of the "next" thyristor to be fired. This occurs by line 
2, driver circuit 77' and firing line 78 to the control electrodes of 
thyristors T.sub.1 -T.sub.6. The pulse of line 29 is carried by line 19 
onto driver circuit 77', whereby a firing pulse is triggered on a 
corresponding one of lines 78. At the same time, ring counter 20 causes 
the distributor 77 to select by lines 10 and 17, 18 an appropriate pair of 
FET devices (S.sub.1, S'.sub.1) . . . (S.sub.6, S'.sub.6) within blocks 
51, 52, thereby preparing the formation of a cosine wave on line 9 at the 
output of summer 29 to match the phasing of the lines with respect of the 
"next" thyristor to be fired.