US3437913A - Tapped regulating transformer having thyristor transfer switch means - Google Patents

Description

April 8, 1969 M. MATZL 3,437,913

TAPPED REGULATING TRANSFORMER HAVING THYRISTOR TRANSFER SWITCH MEANS Filed FBb. 28, 1967 Sheet Of 5 mm W Mm M 0M.

April 8, 1969 M. MATZL TAPPED REGULATING TRANSFORMER HAVING THYRISTOR TRANSFER SWITCH MEANS Filed Feb. 28, 1967 Sheet April 8, 1969 M. MATZL 3,437,913

TAPPED REGULATING TRANSFORMER HAVING THYRISTOR TRANSFER SWITCH MEANS Filed Feb. 28, 1967 Sheet 3 of 5 K W W M. MATZL TAPPED REGULATING TRANSFORMER HAVING April 8, 1969 Filed Feb. 28, 1 967 THYRISTOR TRANSFERSWITCH MEANS Sheet April 8, 1969 M. MATZL TAPPED REGULATING TRANSFORMER HAVING THYRISTOR TRANSFER SWITCH MEANS Sheet Filed Feb. 28, 1967 WNVENI'OR: W )UAMAM United States Patent Oflice 3,43 7,913 Patented Apr. 8, 1969 US. Cl. 323--43.5 12 Claims ABSTRACT OF THE DISCLOSURE A transfer switch to effect arcless changes between transformer taps, includes a pair of networks arranged in parallel, each network including a pair of diodes, a pair of thyristors and a forced commutation circuit composed of a thyristor, capacitor and inductor.

Background of the invention Tapped regulating transformers include tap selector means, or tap selector switches, and transfer switch means, or transfer switches. The former are used to select any desired tap among a plurality of taps associated with a tapped transformer winding, and the latter are used to establish a conductive connection between the selected tap and an outgoing line carrying the load current. The

former esta'blish current paths but do not break currents, this particular task being performed by the transfer switches.

A number of design proposals have been made to substitute static solid state switching devices for the conventional transfer switches having relatively movable confacts. The latter have been used heretofore, and are still being widely used, for performing tap-changing operations. The aforementioned design proposals proved to be quite ineffective, and it is the principal object of this invention to achieve tap-changing operations effectively by means of solid state switching devices, and to avoid the limitations and drawbacks of prior art solid state tap-changing transfer switches.

Some prior art solid state tap-changing transfer switches are predicted on performing the tap-changing operation exactly at the point of time when the load current reaches a natural current zero. This is extremely difficult to achieve. The transfer switches embodying the present invention make it possible to achieve satisfactory tap-changing operations at times relatively prior to, or after, a natural current zero.

Austrian Patent 243,935 to Siemens-Schuckertwerke Aktien gesellschaft whose effective date is Dec. 10, 1965 appears to be the closest prior art known. In the circuitry shown therein the point of time at which the switching operation must .be performed is dependent upon the phase angle between the transformer voltage and the load current. If the load is resistive, the tap changing operation can only be performed with difficulty, and only at the exact time when the load current is at a natural zero of the current wave.

The present invention makes it possible to perform tapchanging operations prior to, and after, a natural zero of the current wave, resulting in greater simplicity of the transfer switch circuitry, and in greater reliability of the switching operation.

Summary The circuitry according to this invention includes a pair of networks. A rectifier bridge having a pair of A-C terminals and a pair of D-C terminals forms part of each of the aforementioned pair of networks. Each rectifier bridge includes a pair of first thyristors each in one of the legs of the bridge. Each rectifier bridge further includes a pair of diodes each in another pair of the legs thereof. A first choke interconnects the pair of DC terminals of the rectifier bridge of each network. The circuitry further includes a pair of forced commutation means each operatively related to said rectifier bridge and each forming part of one of said pair of networks. Each of said pair of forced commutation means includes a commutating thyristor, a capacitor and a second choke connected in series, and connected to a point situated between the cathodes of the aforementioned pair of first thyristors. Each of said pair of forced commutation means further includes a pair of diodes each connecting one terminal of said second choke with the anode of one of said pair of first thyristors. The outgoing line of the transfer switch carrying the load current is connected to ,one of the aforementioned pair of A-C terminals of each rectifier bridge of each of said pair of networks.

Brief description of drawings FIG. 1 is a circuit diagram including substantially all the elements essential for switching from one tap of a tapped transformer winding to another adjacent tap thereof;

FIGS. 2a to 2d, inclusive, are circuit diagrams illustrating the consecutive steps involved in a tap-changing operation from one of the taps of the tapped trans-former winding of FIG. 1 to an adjacent tap;

FIG. 3 is a diagram illustrating the trigger currents for the constituent thyristors of the circuitry of FIGS. 1 and 2a to 2d plotted against time;

FIG. 3a is a circuit diagram of the control circuitry for triggering the thyristors included in the circuit diagram of FIG. 1 and FIGS. 2a to 2d;

FIG. 4 is a diagram illustrating the mechanical drive means for operating, i.e. closing and opening, the several switches having relatively movable contacts included in the circuitry of BIG. 1; and

FIG. 5 is a circuit diagram of a logic circuit which may be used to supplement the circuitry according to FIG. 1 and FIGS. 2a to 2d.

Descriptio of preferred embodiment of the invention Referring now to the drawings, character Tr has been applied to indicate a tapped transformer winding having two taps A and B. Actually winding Tr will have a much larger number of taps than two, but for disclosing and explaining the present invention it is not necessary to consider a larger number of taps than the aforementioned taps A and B. The presence of a relatively large number of taps calls for the application of a tap selector switch which has been deleted in FIG. 1 and FIGS. 2a to 2d. Tap selector switches are well known in the art and do not need to be disclosed in this context for a full understanding of the present invention. Reference may be had to US. Patent 3,233,049 to A. Blei btreu, Feb. 1, 1966 for Integral Selector Switch and Transfer Switch Unit for Tapped Regulating Transformers, for a full disclosure of a tap selector switch.

In FIGS. 1 and 2a to 2d reference characters 1 and 2 have been applied to indicate a pair of selector switch contacts forming part of a tap selector switch which, as mentioned above, is not shown. The outgoing line 27 may be conductively connected by current-carrying contacts 3 and 4i.e. contacts designed to carry currents, in contrast to so-called arcing contactsto either tap A or tap B. When the transformer of which winding Tr forms a part is in a stationary conditionas distinguished from a transient tap-changing condition-one of selector contacts 1, 2 and one of current carrying contacts 3, 4 establishes a direct current path from one of taps A, B to outgoing conductor 27, and the aforementioned current path carries the entire load current.

Reference characters St and St have been applied to indicate a pair of networks. Network S1 is arranged in parallel to current-carrying contacts 3 and network St is arranged in parallel to current-carrying contact 4. Both networks St and St are identical.

Reference numerals 5 and 6 have been applied to indicate a pair of disconnect switches. Disconnect switch 5 is provided to connect network St with selector switch contact 1 and tap A and to disconnect network St from parts 1 and A. Disconnect switch 6 is provided to connect network St with selector switch contact 2 and tap B and to disconnect network St from parts 2 and B. The disconnect switches are only closed during tap-changing operations and disconnect networks St S1 upon completion of a tap-changing operation. I

Network St; includes thyristors 7, 8 and diodes 11, 12. Thyristors 7, 8 and solid state diodes 11, 12 are connected to form a single-phase rectifier bridge having two A-C terminals A-C and two D-C terminals D-C. One of the diagonals of this bridge, i.e. its D-C output circuit, includes the choke 15. One end of choke 15 is conductively connected to the cathodes of thyristors 7, 8 and the opposite end of choke 15 is conductively connected to the anodes of diodes 11, 12, the electrodes of thyristors 7, 8 and those of diodes 11, 12 being shown in FIG. 1 by conventional symbols.

As is apparent from the foregoing the AC energizing circuit of the rectifier bridge including elements 7, 8, 11, 12 and 15 also includes disconnect switch 5 and outgoing line 27.

The network St includes thyristors 9, 10, diodes 13, 14 and a choke 16 forming a rectifier bridge circuit which is identical to the bridge circuit formed by parts 7, 8, 11, 12 and 15.

Network St includes forced commutation means associated with the rectifier bridge circuit formed by parts 7, 8, 11, 12 and 15. These commutation means include the two solid state diodes 21, 22, commutation thyristor 19, choke and capacitor 17. The leads to capacitor 17 may include the solid state diode 17a. When capacitor 17 is charged, its positive terminal is conductively connected to the anode of commutating thyristor 19. Diodes 21, 22 and thyristors 7, 8 are conductively connected to form a bridge. One diagonal connection of this bridge includes commutating thyristor 19, capacitor 17 and choke 25, these three parts being serially connected.

If commutating thyristor 19 is triggered while one of the thyristors 7 or 8 is carrying current, i.e. a load current i;,, a current impulse i opposite to load current i is produced. This causes a commutation of the load current from its path including one of thyristors 7 or 8 to another path including either of the two diodes 21, 22, choke 25, capacitor 17 and commutating thyristor 19. This process of commutation has been illustrated in FIGS. 2a and 2b and will be considered below more in detail.

Network S1 includes identical commutating means as network St The former comprise the solid state diodes 23, 24, choke 26, capacitor 18 and commutating thyristor 20.

Each capacitor 17 and 18 must be associated with an appropriate auxiliary power supply (not shown in FIG. 1) from which it is being charged. Such an auxiliary power supply may, in turn, be energized by the transformer which includes the tapped winding Tr, or the auxiliary power supply may be independent, or separate, from the above referred-to transformer. The power supply for charging each capacitor 17 and 18 must be sufliciently decoupled from the commutating circuitry.

If forward voltage is applied to a thyristor too soon after cessation of current flow, the thyristor will go again into the conduction state thereof. The turn-off time of a thyristor is defined as the shortest interval between the time when forward current reaches zero and the time the thyristor is able to block reapplied forward voltage without turning on. Turn-off time is measured under specified conditions of current, voltage and temperature. The choke 15 in the network St and the choke 1-6 in the network St preclude a change of the charge of capacitors 17 and 18, respectively, at a rate not compatible with the turn-off characteristics, or the turn-off time, of the particular thyristors.

Another characteristic of thyristors is the maximum permissible rate of increase in forward current amplitude. The choke 25 in network St and the choke 26 in network St limit the rate of rise of current through commutating thyristor 19 and commutating thyristor 20, respectively, to permissible values.

If it is intended to disconnect the outgoing line 27 from tap A of winding Tr and to connect the outgoing line 27 with tap B of winding Tr, the following steps, or operations, are performed in their proper sequence.

As long as the outgoing line 27 is conductively connected to tap A of winding Tr, tap selector contact 21 and current carrying contact 3 are closed, disconnect switches 5 and 6 are open and tap selector contact 2 and currentcarrying contact 4 are likewise open. Thus the load current by-passes disconnect switch 5 and network S1 and its constituent rectifier bridge, i.e. it flows directly from tap A and contact 1 through current-carrying contact 3 to the outgoing line 27.

In order to change from tap A to tap B selector contact 2 and disconnect switch 6 are closed, and thyristors 7 and 8 are triggered. This is illustrated in FIG. 2a, assuming that thyristor 7 is current carrying.

Thereafter current-carrying contact 3 is opened and disconnect switch 6 is closed. The load current i;, then flows through the network St the thyristors 9 and 10 of network St still remaining in the non-conductive state thereof, i.e. not yet being triggered at this point of time.

Then the trigger pulses for thyristors 7 and 8 of network St are removed, commutating thyristor 19 is triggered and thyristors 9 and 10in network S1 are triggered. Triggering of thyristors 19, and 9 and 10 occurs substantially simultaneously, i.e. within 1-2 usec.

The discharge current i of capacitor 17 causes a commutation of the load current i from its current path shown in FIG. 2a to a new or another current path shown in FIG. 2b. This shift of one current path to the other current path occurs within a few microseconds. The aforementioned other current path includes diode 21, choke 25, capacitor 17, and commutating thyristor 19. Thyristor 7 does not carry current any longer. As indicated before the action of the forced commutation means, including capacitor 17 and chokes 15, 16 and 25 is of sufficient duration to exceed the turn-off time of thyristor 7 so that the latter is capable of blocking reapplied forward voltage without turning on.

FIG. 2b illustrates in a somewhat diagrammatic fashion the condition resulting from the above operational steps. There may be fractional currents through diodes 1'1 and 12, and these currents have been neglected in FIG. 2b. At the point of time to which FIG. 2b refers a circulating current i cir. may have been established, i.e. a current from network St to network St prevailing during the turn-off time of thyristor 7. This circulating current has been neglected in FIG. 2b.

The sum total of the inductances of chokes 15, 25 and 16 and the leakage inductance of the circuitry shown preclude a sudden change of the rate of current flow even though the thyristors 9, 10 of network 'St have been triggered.

FIG. 2c illustrates the path of the circulating current i cir. due to the difference in the voltage of taps A and B which flows during a change from tap A to tap Band FIG. 2d shows the path of the current from tap B to outgoing line 27 just prior to completion of the operation of changing from tap A to tap B when network S1 is still carrying current. FIGS. 2c and 2d are self-explanatory.

When the tap-changing operation is completed disconnect switch 6 is open, current-carrying contact 4 is closed and thus the current path from contact 2 and tap B to outgoing conductor 27 does not any longer include network Sig.

The phase angle of the load current 1}, at the point of time of changing taps plays an important part in regard to the time required for performing a. tap-changing operation, the maximum capacitor voltage and the amplitude as well as the polarity or direction of the circulating current due to the difference of the potential of taps A and B.

FIGS. 2a to 2d were drawn assuming that the load is a substantially ohmic resistance and that tap -A is positive relative to tap B.

The circuitry of FIG. 1 operates in a similar fashion in case of inductive loads and of capacitive loads.

In FIG. 3 the thyristor trigger pulses J are plotted against time t. Thyristors 7 and 8 are triggered at t and thyristors 9, 10 and 19 are triggered at 2 This applies for a change from tap A to tap B. The trigger pulses of thyristors 7, 8, 9, 1'0 and of commutating thyristor 19 have been indicated by applying to them the reference signs J J zs, zit) and m- The reference characters included in brackets in FIG. 3 refer to the trigger impulses for a change from tap B to tap A, and FIG. 3 clearly indicates the required timing thereof.

Thyristors 7 and 8, and 9 and 10, respectively, may be triggered by series or sequences of shorter pulses, indicated in FIG. 3 by dotted lines.

It will be noted from FIG. 3 that the trigger pulses 1 and (1 are of relatively short duration, this being an essential requirement for the proper operation of the circuitry of FIG. 1. Trigger pulse 1 is applied to commutating thyristor 19 when it is desired to change from tap A to tap B, and trigger pulse (I is applied to commutating thyristor 20 when it is desired to change from tap B to tap A.

When the current in network St becomes zero, all thyristors forming part thereof are blocking. Then disconnect switch 5 is being opened. Thereupon currentcarrying contact 4 is closed and disconnect switch 6 is opened. This establishes a direct current path from tap B to outgoing line 27. Since both disconnect switches 5, 6 are open at the end of a tap-changing operation, networks St and S1 are no longer directly connected to taps A and B.

FIG. 3a shows the circuitry for triggering the con stituent thyristors of the tap-changing circuitry of FIG. 1. Reference character K has been applied in F IG. 3a to generally indicate a bistable circuit. Bistable circuit K may be operated by a change-over switch U having two limit positions and including relatively movable contacts. In one of its limit positions change-over switch 'U causes circuit K to assume one of its two stable states, and in the other of its limit positions change-over switch U causes circuit K to assume the other of its two stable states. The bistable circuit K includes transistors 50, 51, silicon diodes 58, 59 and resistors 52 to 57. Since bistable circuits and their mode of operation are well known in the art, and since the bistable circuit K of FIG. 3a is such a prior art circuit, a detailed description thereof is not deemed necessary.

The circuits generally indicated by reference characters JG1 and JG2 are arranged to both sides of bistable circuit K and are intended to generate the trigger pulses for thyristors 7 and 8, and 9 and 10, respectively, shown in FIG. 3 in dotted lines, and to generate the short trigger pulses I and (I for thyristor 9 and for thyristor 20, which pulses are also shown in FIG. 3. The trigger-pulsegenerating circuits JG1 and JG2 of FIG. 3a are well known prior art circuits and, therefore, do not require a detailed description. Reference characters 60, 61 have been applied to indicate a pair of transistors and reference characters 68 and '69 have been applied to indicate a pair of unijunction transistors. Reference characters 66,

6 67, 72 and 73 have been applied to indicate capacitors, reference characters 74, 75,84, and 86 to 91 have been applied to indicate diodes, reference characters 7881 have been applied to indicate transformers and reference char acters 62-65, 70, 71, 76, 77, '82 and '83 have been applied to indicate resistors.

The letters P and N have been applied to indicate a pair of terminals connected to battery 102 having positive and negative terminals A rectifier 101 energized by an insulating transformer may be substituted for battery 102.

In the position of change-over switch U shown in FIG. 3a a negative voltage is applied across resistor 54 to the base of transistor 50. In that position of change-over switch U transistor 50 conducts and transistor 51 blocks the pas-' sage of current. Hence point C has a positive and point D has a negative bias. If change-over switch U is opened, i.e. moved from one of the limit positions thereof to an intermediate position, transistor 50 still remains conductive since there is a closed circuit including the base of transistor 50, resistor 56 and diode 58. However, when change-over switch U is at its right limit position (as seen in FIG. 3a), transistor 51 is made conductive, point D has a positive bias, and this causes blocking of transistor 50. Point C turns negative and this establishes an additional current path for transistor 51 including the base thereof and resistor 57 and diode 59.

The change of the bistable circuit K from one of its stable states with change-over switch U in its right limit position to the other of its stable states with change-over switch U in its left limit position occurs in an analog fashion.

As soon as transistor 50 blocks the current flow, transistor 60 in trigger pulse generator JG1 becomes conductive. This, in turn, initiates charging of capacitor 66 across resistor 62. When the voltage across capacitor 66 reaches the breakdown voltage of unijunction transistor 68, capacitor 66 is discharged across resistor 82 and across the primary winding of transformer 80 connected in parallel to resistor 82. The cycle of charging and discharging of capacitor 66 repeats itself, i.e. it is periodic. The pulses generated in the two secondary windings of transformer :80 are the trigger pulses I and I for triggering thyristors 7 and 8 (see also FIG. 3).

Transformer 78 is designed in such a way that the time-voltage-area of the traces of the pulses generated by transformer 78 is very small. In other words, the core of transformer is readily saturated and, therefore, the pulses generated by it are of extremely short duration. When transistor 50 is blocked, a short current pulse 1 is generated in the secondary winding of transformer 78 which is applied to thyristor 20 for triggering the latter.

When the core of transformer 78 is fully saturated there is a tendency of a drastic increase of the energizing current of transformer 78. In other words, the energizing current of transformer 78 may reach short-circuit current proportions. Resistor 76 and capacitor 72 form a shunt across the primary winding of transformer 78, precluding the flow of excessive currents therein.

It will be understood that when changeover switch U is moved from one to the other of its limit positions, impulse generator JG2 is caused to operate in an analog fashion as described above in connection with impulse generator I G1. Impulse generator I G2 generates the trigger pulse 1 and J for thristors 9 and 10 and the trigger pulses I for thyristor 19. Impulse generator JG2 is operative during the periods of time in which impulse generator I G1 is inoperative, and vice versa.

Referring now to FIG. 4, reference character U has been applied to indicate the change-over switch also shown in FIG. 3a and whose function has been described in connection with FIG. 3a. FIG. 4 further shows movable main contacts or current-carrying contacts 3 and 4 whose function has been described above in considerable detail in connection with FIG. 1. Each movable current-carrying contact 3, 4 includes a knife blade pivoted at 3 and 4, re-

spectively which may engage with, or be separated from, a fixed arcuate current carrying contact 3a and 4a, respectively. The knife blades of current carrying contacts 3 and 4 are conductively connected to outgoing line 27 and the fixed or stationary current carrying contacts 3a, 4a are conductively connected to selector switch contacts 1 and 2, respectively. FIG. 4 further shows the two disconnect switches 5 and 6 also shown in FIG. 1 and described in connection therewith. Each disconnect switch 5, 6 includes a blade contact pivoted at 5' and 6', respectively, and conductively connected to network St and St respectively. Each disconnect switch 5, 6 further includes a fixed arcuate contact 5a, 6a conductively connected to selector switch contact 1 and tap A and to selector switch contact 2 and tap B, respectively (see also FIG. 1). The knife blades forming part of disconnect switches 5, 6 and of current-carrying contacts 3, 4 and the knife blade forming part of changeover switch U are mounted on, or tied to, an insulating tie rod Sa which, in turn, is slidably mounted in a slide bearing B. Tie rod Sa may be moved selectively to the right and left to two limit positions thereof defined by abutments or dogs E and E integral with tie rod Sa and adapted to selectively engage the end surfaces of slide bearing B. Tie rod Sa may be operated to the left and right by means of a motor Sp and a linkage Sb, both diagrammatically indicated in FIG. 4. The linkage includes preferably over-center spring means to move tie rod Sa with a snap action to either of its two limit positions.

It will be noted that change-over switch U includes two fixed arcuate contacts U in addition to its movable blade contact which is connected to, and operated by, tie rod Sa. The structure of FIG. 4 performs the switching operations described in connection with FIG. 1 when tie rod Sa is moved from the left to right, and vice versa. It will be apparent from the geometry of the structure of FIG. 4 that both disconnect switch 5 and 6 close simultaneously preparatory to a tap-changing operation before one of the current-carrying contacts 3 or 4, respectively, is opened, that both current-carrying contacts 3, 4 can never be closed at the same time, and that disconnect switches 5, 6 open only subsequent to closing of one of the current-carrying contacts 3, 4. FIG. 4 further shows that the change from network St, to the network St and vice verse, by means of changeover switch U is effected at a point of time when current-carrying contacts 3, 4 are in their open positions, i.e. at a point of time when the knife blades of currentcarrying contacts 3 and 4 have parted from, or not yet engaged, their cooperating fixed arcuate contacts 3a, 4a.

It is desirable to provide the transfer switch circuitry of FIG. 1 with means making it possible to minimize the circulating current i which occurs during each tap-changing operation on account of the fact that there is a difference of the voltage at taps A and B. These means make it also possible to use commutating thyristors 19 and 20 having a relatively small rating relative to that of thyristors 7 and 8, and 9 and 10, respectively. The aforementioned end can be achieved by the addition of a logic circuitry to that of FIG. 1, which logic circuitry tranmits a trigger pulse only at such times when the instantaneous difierence in voltage between taps A and B is relatively small. This condition is achieved when the trigger pulse is transmitted a relatively short time prior to a natural current zero, or a relatively short time after a natural current zero, preferably in the interval of time of 15 el. deg. prior to a natural current zero and 15 el. deg. after a natural current zero.

FIG. 5 shows diagrammatically a logic circuit intended to be applied jointly with the transfer switch circuitry of FIG. 1 to minimize the magnitude of circulating currents i cir. occurring during tap-changing operations. In FIG. 5 reference character EB has been applied to indicate a rectifier bridge having a pair of terminals BB and BB intended to be connected to taps A and B of the transformer winding Tr of FIG. 1. Thus the same instantaneous voltages prevail between terminals BB and BB as between taps A and B. The D-C terminals BB and EB; of rectifier EB are connected into a circuit which includes Zener diode Z and ohmic resistor R. Reference character U has been applied to indicate the voltage drop along ohmic resistor R. The circuitry of FIG. 5 further includes logic element D which is a NOT gate and logic element D which is a NOR gate. These logic elements do not need to be described in detail in this context, since they are means which are well known in the art. (See, for instance, Richard B. Hurley, Transistor Logic Circuits, John Wiley & Sons, Inc. 1961 and The Digital Logic Handbook 1966-1967 Edition, Digital Equipment Corporation, Maynard, Mass).

Zener diode Z breaks down whenever the voltage between terminals EB, and EB, exceeds the breakdown voltage of Zener diode Z. As a result, there will be a flow of current in the circuit including Zener diode Z and resistor R. The voltage drop along resistor R, or a portion thereof, is applied to the input terminal e; of NOR gate D As long as the voltage U or a portion thereof, is being applied to NOR gate D the tap-changing impulse K thereof will be zero. The tap-changing signal B is fed to the input of NOT gate D whose output is fed to terminal e of NOR gate D If the tap-changing signal B is being given, the output of the NOT gate D and the input at the terminal e of the NOR gate D will be zero, and the output signal K of the NOR gate will only occur when the input at its terminal e is zero, i.e. when the difference of the voltage between terminals BB and BB of rectifier BB is less than a predetermined maximum voltage.

I claim as my invention:

1. In a tapped regulating transformer having transfer switch means made up of thyristors and diodes ar anged in a pair of networks adapted to be connected in parallel to adjacent taps of a transformer winding, the combination of:

(a) a pair of networks each including a rectifier bridge having a pair of A-C terminals and a pair of D-C terminals, said bridge further including a pair of first thyristors each in one pair of the legs thereof and a pair of diodes each in another pair of the legs thereof, and a first choke interconnecting said pair of D-C terminals of said bridge;

(b) a pair of forced commutation means each operatively related to said bridge and each forming part of one of said pair of networks, each of said pair of forced commutation means including a commutating thyristor, a capacitor and a second choke connected in series and connected to a point situated between the cathodes of said pair of first thyristors, each of said pair of forced commutation means further including a pair of diodes each connecting one terminal of said second choke with an anode of one of said pair of first thyristors; and

(c) an outgoing line conductively connected to one of said pair of A-C terminals of each said rectifier bridge of each of said pair of networks.

2. A tapped regulating transformer as specified in claim 1 including trigger means for triggering simultaneously said pair of first thyristors in each of said pair of networks.

3. A tapped regulating transformer as specified in claim 1 including trigger means for establishing series of con secutive pulses of short duration for triggering simultaneously said pair of first thyristors in each of said pair of networks.

4. A tapped regulating transformer as specified in claim 1 including trigger pulse timing means for first triggering said pair of first thyristors in one of said pair of networks and thereafter triggering said commutating thyristor in said one of said pair of networks and said pair of first thyristors in the other of said pair of networks.

5. A tapped regulating transformer as specified in claim 1 including a pair of first trigger means for selectively triggering substantially simultaneously said pair of first thyristors in each of, said pair of networks, each of said pair of first trigger means generating series of consecutive pulses of short duration, and a pair of second trigger means for selectively triggering said commutating thyristor in each of said pair of networks, each of said pair of second trigger means generating pulses of short duration of the same order as one single pulse of said series of consecutive pulses.

6. A tapped regulating transformer as specified in claim 1 including a pair of first trigger means for selectively triggering substantially simultaneously said pair of first thyristors in each of said pair of networks;

switching means operatively related to said first network and to said second network for rendering one of said pair of first trigger means sequentially effective and ineffective in regard to said pair of first thyristors in one of said pair of networks and for rendering the other of said pair of first trigger means effective in regard to said pair of first thyristors in the other of said pair of networks substantially at the time when said switching means render one of said first pair of trigger means ineffective in regard to said pair of first thyristors of one of said pair of networks; and a pair of second trigger means for selectively triggering said commutating thyristor in each of said pair of networks substantially at the time when said switching means render one of said pair of first trigger means ineffective in regard to said pair of first thyristors in one said pair of networks. 7. A tapped regulating transformer as specified in claim 1 including trigger means for triggering said pair of first thyristors in each of said pair of networks and for triggering said commutating thyristor in each of said pair of networks, and logic circuitry for limiting the time when said trigger means are allowed to effect a tap-changing operation to time intervals in the order from el. degrees prior to a natural current zero of the load current flowing in said outgoing line to 15 e1. degrees after said natural cur-rent zero of said load current flowing in said outgoing line.

8. A tapped regulating transformer as specified in claim 1 including trigger means for triggering said pair of first thyristors in each of said pair of networks and for triggering said commutating thyristor in each of said pair of networks, logic circuitry for limiting the time when said trigger means are allowed to effect a tap-changing operation, said logic circuitry including a rectifier having an A-C input circuit, a D-C output circuit including a serially connected Zener diode and a resistor, a NOR gate having a first input under the control of a voltage drop established along said resistor and a second input, said second input being under the control of a NOT gate.

9. A tapped regulating transformer as specified in claim 1 including a pair of disconnect switches for connecting one of said pair of A-C terminals of said rectifier bridge of each of said pair of networks to one of a pair of taps of a transformer winding and for disconnecting one of said pair of A-C terminals of said rectifier bridge of each of said pair of networks from one of said pair of taps of a transformer winding, a pair of current-carrying contact means each connected to shunt one of said pair of disconnect switches and the other of said pair of A-C terminals of said rectifier bridge of each of said pair of networks, and common operating means for said pair of disconnect switches and for said pair of current-carrying contact means for substantially simultaneously closing said pair of disconnect switches, precluding simultaneous closing of said pair of current-carrying contact means and causing opening of one of said pair of current-carrying contact means subsequent to closing of said pair of disconnect switches.

10. In a tapped regulating transformer having transfer switch means made up of thyristors and diodes arranged in a pair of networks adapted to be connected in parallel to adjacent taps of a transformer winding the combination of (a) a first rectifier bridge having a pair of A-C terminals and a pair of D-C terminals, said first rectifier bridge further including a pair of thyristors each in one pair of the legs thereof and a pair of diodes each in another pair of the legs thereof, and said first rectifier bridge further including a choke interconnecting said pair of D-C terminals thereof;

(b) a second rectifier bridge having a pair of A-C terminals and a pair of D-C terminals, said second bridge further including a pair of thyristors each in one of a pair of the legs thereof and a pair of diodes each in another pair of the legs thereof, and said second rectifier bridge further including a choke interconnecting said pair of D-C terminals thereof;

(0) a first forced commutating circuit including a first commutating thyristor operatively related to said first rectifier bridge;

(d) a second forced commutating circuit including a second commutating thyristor operatively related to said second rectifier bridge;

(e) a first disconnecting switch for connecting one of said pair of AC terminals of said first rectifier bridge to a first transformer winding tap and for disconnecting said one of said A-C terminals of said first recti fier bridge from said first transformer winding tap;

(f) a second disconnecting switch for connecting one of said pair of A-C terminals of said second rectifier bridge to a second transformer winding tap and for disconnecting said one of said A-C terminals of said second rectifier bridge from said second transformer winding tap;

(g) an outgoing line conductively connected to the other of said pair of A-C terminals of said first rectifier bridge and conductively connected to the other of said pair of A-C terminals of said second rectifier bridge;

(h) conductor means forming a first current path for establishing a direct connection between said first transformer winding tap and said outgoing line bypassing said first disconnecting switch and said first rectifier bridge, said first current path including a first separable current-carrying contact means;

(i) conductor means forming a second current path for establishing a direct connection between said outgoing line by-passing said second disconnecting switch and said second rectifier bridge, said second current path including a second separable current-carrying contact means;

(j) common operating means for said first disconnecting switch, said second disconnecting switch, said first current-carrying contact means and said second current-carrying contact means for moving said first disconnecting switch and said second disconnecting switch simultaneously to the open positions thereof and to the closed positions thereof, for keeping said first current-carrying contact means engaged when said second current-carrying contact means is sepa rated, for keeping said second current-carrying contact means engaged when said first current-carrying contact means is separated, for allowing selective engagement of said first current-carrying contact means and of said second current-carrying contact means only upon opening of both said first disconnecting switch and said second disconnecting switch and for causing separation of said first current-carrying contact means and separation of said second current-carrying contact means preparatory to closing of said first disconnecting switch and said second disconnecting switch; and

(k) pulse generating means under the control of said common operating means for sequential application of trigger pulses to and removal of trigger pulses from said pair of thyristors in said first rectifier bridge and said pair of thyristors in said second rectifier bridge and said first commutating thyristor and said second commutating thyristor.

11. A tapped regulating transformer as specified in claim 10 wherein said first commutating thyristor is connected with the cathode thereof to the cathodes of said pair of thyristors in said first rectifier bridge and connected in series with a first capacitor and a first choke, the anodes of said pair of thyristors of said first rectifier bridge being connected to said first choke by the intermediary of a pair of diodes, and said first capacitor being connected to a first auxiliary capacitor charging DC power supply, and wherein said second commutating thyristor is connected with the cathode thereof to the cathodes of said pair of thyristors in said second rectifier bridge and connected in series with a second capacitor and a second choke, the anodes of said pair of thyristors of said second rectifier bridge being connected to said second choke by the intermediary of a pair of diodes, and said second capacitor being connected to a second auxiliary capacitorcharging D-C power supply.

12. In a tapped regulating transformer having transfer switch means made up of thyristors and diodes arranged in a pair of networks adapted to be connected in parallel to adjacent taps of a transformer winding in combination, a pair of networks each including a four terminal single-phase rectifier bridge and a commutating circuit, said bridge of each of said pair of networks including a first pair of diodes each in one of a pair of legs of said bridge and a pair of thyristors each in another of a pair of the legs of said bridge, said bridge of each of said pair of networks further including a choke interconnecting points situated between the cathodes of said pair of thyristors and the anodes of said pair of diodes, said commutating circuit of each of said pair of networks including said pair of thyristors each in one of a pair of legs of said bridge and a second pair of diodes each connected with the anode thereof to an anode of one of said pair of thyristors, said commutating circuit of each of said pair of networks further including a series arrange ment of a commutating thyristor, a capacitor and a choke conductively interconnecting a point situated between the cathodes of said pair of thyristors and the cathodes of said second pair of diodes, and said commutating circuit of each of said pair of networks further including an auxiliary capacitor-charging D-C power supply.

References Cited UNITED STATES PATENTS 3,340,462 9/ 1967 Ebersohl 323-435 3,349,314 10/1967 Giannamore 321-43 3,358,218 12/1967 Biihler 323-435 JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner.

US. Cl. X.R. 307-252; 321-45

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