IMPROVEMENTS RELATING TO AC/DC CONVERTERS

Abstract: A polyphase AC/DC converter, useful as a drive for a DC motor or in a power-sharing electrical power distribution system for example, comprises a naturally-commutated, phase-controlled, polyphase thyristor bridge and control means determining the application of firing signals to the thyristors and operable in the inversion mode of the bridge to transfer conduction of load current between phases at the conventional inversion limit defined as radians advanced from the rectification limit of the bridge, but to operate an extended control range when there is zero load current at the time when the conventional inversion limit would otherwise be applied. In a DC motor drive application, a power increase of up to 15% is obtainable by specification of a higher rated DC motor voltage, and in a power-sharing electrical power distribution system a similar increase in power transmission may be obtainable.

Description: IMPROVEMENTS RELATING TO AC/DC CONVERTERS FIELD OF THE INVENTION This invention generally concerns improvements in or relating to AC/DC converters for interchanging electrical energy between AC and DC circuits. The in¬ vention will be described hereinafter with particular reference to polyphase rectifier drives for DC motors which employ naturally-commutated phase-controlled thyristor circuits to determine the armature current of the motor, but the invention is susceptible of wider application (for example in power-sharing electrical power distribution systems) and has application also to circuits employing controlled rectifier devices other than thyristors and to circuits wherein forced commu¬ tation is utilized rather than natural commutation. As employed herein the term AC/DC converter is intended to embrace DC to AC converters as well as AC to DC con¬ verters. BACKGROUND OF THE INVENTION DC motor drives are utilised in industry to serve a wide variety of purposes and commonly employ an AC to DC phase-controlled thyristor converter for controlling the motor. The converter, through the con¬ trolled switching of thyristors, changes the AC supply voltage to a controllable DC output voltage which is applied to the armature windings of the motor. By use of thyristors, commutation (that is the transfer of current from one thyristor to another and hence from one phase of the supply to the next phase) is achieved naturally in that the polyphase AC supply waveforms are responsible for turning off a conducting thyristor in much the same way as a similar circuit of diodes would behave and no special or additional circuitry is re¬ quired for the commutation process; such commutation is known in the art as natural or line commutation. Thy¬ ristor AC/DC converters are therefore relatively simple in principle, though complex circuitry is commonly employed for controlling thyristor ignition, and are widely utilised in many industrial fields. Fig. 1 of the accompanying drawings is a schematic illustration of a conventional three-phase, phase- controlled, naturally-commutated. thyristor bridge, shown typically with a DC motor load. As is well known, control circuitry (which has been omitted from Fig. 1) is employed for providing triggering signals to the thyristors in the bridge at defined time instances in the waveforms of the three phases of the AC supply to determine the signed magnitude of the DC output pro¬ vided to the motor M. Such control circuitry is commonly sophisticated and might for example employ al¬ gorithms running in microprocessors, phase locked loops, etc. as is disclosed for example in the book "Thyristor DC Drives" by Paresh C. San (published by John Wiley & Sons in 1981). The circuit of Fig. 1 is known as a full converter in that the motor terminal voltage can be reversed, in dependence upon the triggering of the thyristors, so that the thyristor converter operates in a so-called inversion mode in which power can be transferred from the motor back through the thyristor bridge and into the AC supply. By virtue of this facility, regen¬ erative braking can be accomplished in a controlled manner by means of the illustrated circuit, with the kinetic energy of a drive system coupled to the DC motor being converted into electrical energy by the motor and returned through the thyristor bridge to the AC supply. The circuit of Fig. 1 thus is useful as a controller for a DC motor which enables the motor speed and torque to be precisely determined in forward and in reverse operation of the motor, enabling the motor to be effectively and controllably braked by re¬ generation. - - The bridge arrangement shown in Fig. 1 provides for control only in two quadrants in the armature current/back emf domain. Fig. 2 of the accompanying drawings shows this domain and it will be readily appreciated by those skilled in the art that control is possible only in the first two quadrants with the cir¬ cuit of Fig. 1. To achieve full four quadrant control, dual converters in which a similar but oppositely connected thyristor bridge is additionally connected across the motor terminals are used and enable the motor current to be reversed as well as its terminal voltage. Dual converters are widely used in industry to control reversible drives in sheet metal rolling mills, in papermaking machinery, in cablemaking and wire drawing machinery etc. Current understanding of the control range of phase controlled, three-phase, six-pulse, thyristor converters has set certain limits beyond which loss of current control may be expected. In practice, these restrictions lead to motor derating through compromising the regenerative voltage span as will be explained hereinafter. It is without question that the three-phase, six-pulse, fully-controlled, naturally- commutated, phase-controlled thyristor bridge used in conjunction with separately excited direct current machines (in two quadrant and four quadrant control) has provided the mainstay of variable speed (and torque) control to industry. To achieve good control characteristics of torque (motor armature current) and speed, a complete understanding of the highly nonlinear sampled data performance of the thyristor stack is re¬ quired, and there is confusion in the mind of many users relating to the specification of motor armature voltage in that it is different for non-regenerative and regenerative applications. It is thus customary to specify armature voltages (at full speed) differently for non-regenerative and regenerative drive applications. Indeed, in Europe, this is actually de¬ fined by DIN standards (DIN 40030). Also of great significance is the variable defined as E, namely motor armature back emf. The variable E is directly proportional to speed, and is sign dependent upon direction of rotation (assuming constant field flux) . The motor manufacturer will commonly quote armature terminal voltage on the machine name plate, but the converter designer is also particularly interested in the back emf E which is of great importance. Conventionally, the control range of a naturally-commutated, thyristor, phase controlled polyphase bridge as shown in Fig. 1 has been considered to be limited to Γradians electrical, and it has been conventional to design the thyristor switching control circuitry to ensure operation within this limitation. Fig. 3 of the accompanying drawings shows the sinu¬ soidal envelope of the per phase line-to-line potential of the polyphase AC supply, and marked in Fig. 3 are the lower phase limit or rectification limit for con¬ trol of the relevant thyristor of the bridge and the upper phase limit or inversion limit spaced from the rectification limit by tr radians. The rectification limit represents the condition which would be achieved if the thyristors were simply replaced by diodes and corresponds to the cross-over point in the waveform of one phase of the AC supply with the waveform of the next adjacent phase; it can be defined as the phase value beyond which further advance would produce no in¬ crease in output power into a resistive load and, for a three phase supply, would produce an average output voltage V AVE equal to 3 /τr where V LL is the rms line-to-line voltage. The inversion limit has been de¬ ined as being ~ κ radians electrical retarded from the rectification limit, since beyond this point commu¬ tation will not successfully transfer conduction from the present phase to the next which may result in a catastrophic let-through to the load and loss of con¬ trol. Consequently, it has not been the practice to op¬ erate fully-controlled, three-phase, naturally- commu- tated, thyristor DC motor drives in an inversion mode at thyristor firing phases beyond the inversion limit. When operating in a regenerative braking inversion mode, control of a DC motor by phase control of a thyristor bridge as hereinabove described is possible only so long as the back emf of the motor (which is sign dependent upon the direction of rotation of the motor, assuming constant field flux) is no greater than the maximum voltage available at the motor terminals from the thyristor bridge. If the back emf developed by the motor exceeds the thyristor bridge output, then all possibilities for control will be lost with consequent risk of catastrophic failure as control of current is lost. The inversion limit restricts the control possibilities which are available, and in con¬ sequence of this DC motors controlled by such thyristor bridges have to be correspondingly de-rated. The conventionally set inversion limit corresponds to the need to transfer conduction between two phases of the AC supply before the commutating thy¬ ristor bias becomes such that it is no longer possible to transfer load current to the commutating thyristor in sequence in order to maintain proper control of the load current; in practice, circuit impedance necessitates a forward displacement of this limit to provide a sufficient volt-second margin to ensure reliable commutation under all reactive supply conditions. A factor of safety is therefore commonly introduced by defining a so-called inversion end stop. The inversion end stop is commonly phase advanced from the inversion limit by as much as 15 or even more. The phase displacement between the end stop and the in¬ version limit is determined in dependence upon the in¬ ductance of the supply and the amount of current to be commutated. The introduction of such an end stop effectively reduces the maximum available control range from 180° (_γradians) to for example 165°. It is the intersection of the end stop and the instantaneous voltage that sets the maximum value of motor back emf into which a controlled amount of current can be achieved. The result of these compromises is an increase in motor costs due to the non-standard requirements of the armature voltage and a further increase in costs as a result of the required increase in current capacity. In mainland Europe the 3 phase industrial supply is 380 yolts rms and in the UK it is 415 volts rms. According to DIN standards (in mainland Europe), an armature terminal voltage of 460 volts is defined. This value may be achieved by consideration of the fact that the maximum average (DC) output voltage obtainable from a three-phase, fully-controlled bridge at fully rectification from a 380V AC supply is: ϊ DC (average) = V AC ( ™ s) ** 513 ϊolts where throughout phase-to-phase voltages are used and allowing for supply tolerances of 10/6, this gives V DC = 460V under worst case conditions. On UK supplies, V DC is equal to 500V under worst case conditions, but it is not uncommon to find UK standards as 490, 480 and 460 volts DC, which causes confusion and non-standardisation of DC motors. OBJECTS AND SUMMARY OF THE INVENTION Accordingly, a first object of the present invention is to enable motor voltage speci ications to be maximised, particularly in the regenerative operating mode, by an observation of load conditions which when observed allow a wider control range than is currently used. More generally, the object of the invention is to improve the efficiency of AC/DC converters for inter¬ changing electrical energy between AC and DC circuits. A more particular object of the invention is to control the operation of controlled rectifier devices in a polyphase AC/DC converter bridge circuit so that commutation within the circuit is optimized during in¬ version mode operation for optimizing energy transfer between the AC and DC circuits. Yet another object of the present invention is to extend the controllability of DC motors operating in a regenerative braking mode. Another object of the invention is to improve the efficiency of power-sharing electrical power distribution systems utilizing AC/DC converters at re¬ mote ends of a DC power transmission link. The present invention resides in the realisation that the conventional inversion limit need not be ob¬ served under circumstances where zero current flows in the thyristor bridge and the load at the relevant time. Under such conditions, the inversion limit may be ignored. For a polyphase system, the extended control range can be located up to a further ^- n radians retarded from the conventional inversion limit, where p^3 represents the number of phases. By virtue of this extension of the control range, an increased inversion potential availability is obtained which enables the motor armature voltage specification to be increased and, for the same armature current, permits a typical 5% increase in power to be obtained by the use of a higher rated motor for the same rated current. According to one aspect of the present invention therefore there is provided a polyphase AC/DC converter comprising commutating controlled rectifier devices wherein the conventional inversion limit (as herein¬ before defined) for phase transfer need not be observed under conditions of instantaneous zero load current and an extended control range limit located within a further "Jr/ n radians of the conventional inversion limit may instead be observed. According to a preferred embodiment of the present invention there is provided a polyphase AC/DC converter comprising a naturally-commutated, phase- controlled, polyphase, thyristor bridge, and control means operative when in the inversion mode of the bridge to inhibit phase transfer at the conventional inversion limit (as hereinbefore defined) in response to a zero converter load current situation prevailing at the re¬ spective time, and to allow an extended control range by delaying phase transfer for a predetermined further limited period whilst maintaining control. The present invention may also be applicable to a power-sharing electrical power distribution system comprising a first naturally-commutated, phase- con¬ trolled, polyphase, thyristor AC/DC converter at a first location, said first AC/DC converter being coupled to a first polyphase AC power system and to a DC link, and a second naturally-commutated, phase- con¬ trolled, polyphase, thyristor AC/DC converter at a second location remote from said first location, said second AC/DC converter being coupled to a second poly¬ phase AC power system and to said DC link, each of said first and second converters being arranged such that the conventional inversion limit (as hereinbefore de¬ fined) for phase transfer need not be observed under conditions of instantaneous zero load current and an extended control range limit may instead be observed. Similar systems within the ambit of the present invention might employ controlled rectifier devices other than thyristors, and might employ such devices in forced commutation circuits. Other features aspects and advantages of the present invention are set forth with particularity in the appended claims and will become well understood by those possessed of relevant skills from consideration of the following specific description given with reference to the accompanying drawings. DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic representation of an AC/DC converter bridge circuit comprising controlled rectifier devices such as thyristors; Fig. 2 is a diagram illustrating the availability of control in the four quadrants of the armature current/back emf domain of a DC electric motor; Fig. 3 shows the sinusoidal envelope of the per- phase line-to-line potential of a polyphase AC supply; Fig. 4 shows the sinusoidal envelope of one phase of a three phase AC supply; Fig. 5 is a schematic showing of an exemplary em¬ bodiment of the present invention; and Fig. 6 is a schematic showing of the voltages associated with a DC motor. SPECIFIC DESCRIPTION OF THE INVENTION Reference should now be made to Fig. 4 of the accompanying drawings wherein the sinusoidal envelope shown in full line represents the line-to-line po¬ tential for one phase of a 3-phase AC supply to a thy¬ ristor bridge as in Fig. 1 and the broken line represents the preceding phase. To understand the reason for the conventional differences in specified armature voltage, it is necessary to explore the reasoning in more detail. Consider a portion of the 3 phase supply as illustrated in Fig. 4; since all phases are identical, only one need be considered remembering that phase-to- phase potentials are shown. The rectification limit is defined as the maximum phase advance of the firing point beyond which the output voltage (DC) will not in¬ crease and the thyristor bridge becomes in effect a diode bridge, thus VDC - _ \ VAC RMS ~ τ and the inversion limit is defined as the maximum phase retard allowed such that commutation (transfer from phase to following phase) during inversion can always occur, failure to commutate being a fault condition. Put simply, if conduction is occurring on the phase illustrated, then unless conduction is transferred to the following phase before the inversion limit, then the conducting phase will swing positive and initiate large amounts of uncontrolled current. Since the 3 phase supply is always inductive at supply frequencies, it requires time (volt seconds) to reduce the phase current from its instantaneous value of armature current to zero, whilst at the same time increasing the incoming phase current from zero to the armature current value assuming that the armature current is substantially constant. Hence in practice, it is necessary for safe operation to define an inversion end stop which is phase advanced from the in¬ version limit rather than let the maximum phase retard be the inversion limit. Whilst the theoretical control range can be seen to be 180° (Tr) electrical, a typical endstop can be positioned 15° advanced, thus giving a control range of 165°. With the thyristor bridge operating in its inversion mode and the DC motor subject to regenerative braking, for continuity of control the transfer of con¬ trol from the broken line phase to the full line phase would conventionally be effected at the cross-over point of the two phases or, as aforementioned, shortly therebefore, this point representing the conventional inversion limit. In accordance with the present invention this conventional procedure is followed except when load current is zero at the time when the thyristors in the bridge would conventionally commu- tate to transfer control, in which case thyristor con- trol transfer is not effected at the conventional in¬ version limit, but is delayed for up to a further Tr/g radians. The solution to the prior art problems is thus to identify the largest firing angle which will produce a controlled flow of current however small. It will be well understood by those knowledgeable in the art of thyristor phase-controlled polyphase con¬ verters that the ability to maintain control of a motor is dependent inter alia upon the controllability of current in the thyristor bridge. If, for example, the back emf E of the motor exceeds the maximum instantaneous potential available from the thyristor bridge (as represented by the level E- shown in Fig. 4), then controlled motor braking is not possible. In practical terms this problem could arise, for example when a motor rated at 400V is used on a 380V supply.- The resulting back emf E when the motor is in regenerative mode will provide the motive force for un¬ controlled current flow which could be large and destructive. As shown in Figure 6 the armature of the motor has resistance R and inductance L which will pro¬ duce an IR voltage drop, and thus the back emf E pro¬ duced by the motor will be less than the rated armature voltage. If the magnitude of the back emf of the motor is less than the peak inversion voltage output of the thy¬ ristor bridge as represented by the level E, shown in Fig. 4 for example, then control of armature current is possible in either continuous or discontinuous con¬ duction modes. The rationale for transfer from one phase to the next in order to maintain control of in¬ version in a braking motor is determined by the suit¬ ability of consecutive phases to establish controlled current against the back emf of the motor. When the magnitude of the motor back emf is at the level E shown in Fig. 4 then discontinuous current can be pro¬ duced by the thyristor bridge and control of the motor is achievable. As previously mentioned herein, various different techniques are currently available to those skilled in this art for determining the generation of the six fir¬ ing signals for the thyristors in the three-phase thy¬ ristor bridge of the kind shown in Fig. 1 and for en¬ suring transfer of conduction from one phase to the next at the conventional inversion limit, and apparatus for achieving this is available from SSD Limited of Littlehampton, West Sussex, England. Implementation of the present invention would only require the detection of zero current at the conventional inversion limit (as modified by source impedance of the three phase supply) commutation point whereupon up to a further τr/g radians of control range would be made available by appropriate modification of the apparatus. In a microprocessor based embodiment of the invention for example, an additional conditional path need only be added to the algorithms running in the microprocessor to delay phase transfer when appropriate circumstances are sensed. In an alternative and well known conventional apparatus manufactured by SSD Limited aforementioned, the terminal voltage of a naturally-commutated, thy¬ ristor, phase-controlled, polyphase bridge such as is shown in Fig. 1 is controlled by converting a control variable (phase angle) representing a point in time relative to the AC supply into a firing event by comparison with a synchronised time variable in the form of an analogue ramp voltage proportional to phase. Phase displacement is defined to be zero at the rectification limit, and in a three phase implementation there are three time variables (ramps) which are displaced by -TΓ/ ? from each other and each span "rr radians before ramp reset at the inversion limit. The control variable (phase angle) is compared with the synchronised time variables and, when equal, the thyristors are fired on a per phase basis. Since phase synchronous timings are always available in such " an apparatus, the situation prevailing at the conventional phase transfer point at the inversion lirait can readily be made to influence the phase transfer timing in order to modify the conventional apparatus to comply with the principles of the present invention. Fig. 5 of the accompanying drawings schematically illustrates such a generally conventional arrangement incorporating modifications to give effect to the teachings of the present invention. As described above, firing signals to the six thyristors of a three- phase, six-pulse, thyristor converter are developed in dependence upon the relationship between a control voltage and the instantaneous level of ramp voltage waveforms generated in synchronisation with the phases of the three phase supply. The control voltage and the ramp voltage waveforms are conventionally generated, except in that, in accordance with the present invention, a modification is effected when, at the time when phase commutation would conventionally occur at the inversion limit, the load current in the thyristor converter is zero, the modification under these cir¬ cumstances being such as to extend the range of up to r/g radians. The monitoring of load current and the requisite modification can readily be effected by any convenient means as will be well appreciated by those possessed of the relevant skills. For example in a microprocessor controlled system, the control range between the rectification limit and the conventional inversion limit would be determined by a hardware/software counter. To modify such a system for operation in accordance with the teachings of the present invention, an extended count period need only be provided with the software determining when the ex¬ tended range would be applied as a junction of instantaneous load current. It is to be noted that whilst the conventionally utilised control range for such a polyphase regenerative converter as hereinbefore described is ~~- radians electrical, namely the angular displacement between the rectification and inversion limits, and the present invention proposes in certain circumstances to increase this control range, the increase being fromir radians to 7τ_'/6 radians for a three-phase system, it is nonetheless to be considered within the scope of the invention to maintain a control range of Tτbut to phase shift the control range so as to extend the inversion limit. Thus, the present invention does not require that one limit of the control range be the rectification limit, and only requires that access to the extended control range be allowed when at the con¬ ventional inversion limit as modified by end stop con- siderations the load current was observed to be insig¬ nificantly low or zero. Whilst the invention has been described herein with particular reference to thyristor converters for DC motors, as previously mentioned herein it will be appreciated by those skilled in the art that the in¬ vention has wider application. For example, the in¬ vention can be applied to analogous situations with loads other than DC motors. Furthermore, the invention may have application to high voltage DC power trans¬ mission applications as, for example, in the load sharing arrangement currently practiced between the electricity distribution authorities in France and the United Kingdom. Power transmission capabilities in such relatively long distance transmission systems are limited firstly by the size and rating of the cable and secondly by the number of variables (such as frequency, phase compatibility, etc.) to be accommodated and, for optimal capacity, DC transmission is employed in preference to AC and a dual thyristor (or equivalent) converter is employed with one thyristor stack at each end of the transmission line for making the requisite conversions between AC and DC. The present invention can be applied to such converters in such applications and could enable up to 15/6 more power to be transmitted over the same line.

Claims: CLAIMS : 1. A polyphase AC/DC converter comprising commutating controlled rectifier devices, wherein the conventional inversion limit (as hereinbefore defined) for phase transfer need not be observed under conditions of instantaneous zero load current and an extended control range limit located within a further ^ - radians of the conventional inversion limit may instead be observed, p?>3 denoting the number of phases. 2. A polyphase AC/DC converter comprising a naturally-commutated, phase-controlled, polyphase thy¬ ristor bridge, and control means operative when in the inversion mode of the bridge to inhibit phase transfer at the conventional inversion limit (as hereinbefore defined) in response to a zero converter load current situation prevailing at the respective time, and to allow an extended control range by delaying phase transfer for a predetermined further limited period whilst maintaining control. 3. A polyphase AC/DC converter according to claim 2, wherein the predetermined further limited period does not exceed τ/ 2 - radians, p^3 denoting the number of phases. 4. A polyphase AC/DC converter according to any pre¬ ceding claim, which is adapted for operation within a predetermined fixed control range advanced from the conventional or extended inversion limit, the arrange¬ ment being such that in situations where the conventional inversion limit is observed the control range is defined between the rectification limit (as hereinbefore defined) of the converter and the conventional inversion limit, and in situations where the extended inversion limit is observed the control range is determined by the extended inversion limit. 5. A polyphase AC/DC converter according to claim 4 wherein said predetermined fixed control range comprises ~ ~ ~ radians. 6. A polyphase AC/DC converter substantially as herein described with reference to the accompanying drawings. 7. A polyphase AC/DC converter according to any pre¬ ceding claim, adapted for use as a drive for a DC motor with the phase transfer deferment facility of the con¬ verter effective under regenerative conditions of the DC motor. 8. A polyphase AC/DC converter according to claim 6 in combination with a DC motor. 9. A polyphase AC/DC converter according to any of claims 1 to 5 incorporated into a power-sharing electrical power distribution system. 10. A power-sharing electrical power distribution system comprising a first naturally-commutated, phase-controlled, polyphase, thyristor AC/DC converter at a first location, said first AC/DC converter being coupled to a first polyphase AC power system and to a DC link, and a second naturally-commutated, phase- controlled, polyphase, thyristor AC/DC converter at a second location remote from said first location, said second AC/DC converter being coupled to a second poly¬ phase AC power system and to said DC link, each of said first and second converters being arranged such that the conventional inversion limit (as hereinbefore de¬ fined) for phase transfer need not be observed under conditions of zero load current and an extended inversion limit may instead be observed.

Inventors: SLATTERY DANIEL T, WATSON JEREMY DANIEL MCKENDRIC, SLATTERY, DANIEL, T., WATSON, JEREMY, DANIEL, MCKENDRICK
Applicants: SSD LTD, SSD LTD.
Application year: 1986
Main classification: H02M7
IPC codes: H02M7