TITLE

Improvements in electrical power converters

DESCRIPTION Field of the Invention

The present invention broadly relates to electrical power converters for converting AC power to DC power at medium to high powers, for example, 1 Mega- Watt (MW) or more. More particularly, the invention relates to the efficient use of thyristor bridge rectifiers.

Background of the Invention

It is known to construct an electrical power converter of the type which converts AC power from an AC supply network to DC power to supply a DC load, the converter comprising a phase controlled thyristor supply bridge rectifier.

In this type of use, thyristor bridge rectifiers provide good efficiency and reliability at low cost, size and mass when used to supply DC current to a DC load. However, their supply harmonics and power factor characteristics are not ideal. Thus, when used to draw power from an AC electrical network, such rectifiers can cause harmonic distortion at the point of common coupling with other consumers, thus possibly causing other equipment on the network to malfunction. Furthermore, such harmonic pollution may cause unwanted heating of transformers and generators in the supply network. Additionally, the input power factor of the rectifier is less than unity, particularly when its output DC voltage is at a low value. All of these effects dictate that the transformer or generator MVA (Mega- Volt-Amp) rating must be increased relative to an ideal MW rating.

Another limitation of the thyristor supply bridge in its simple rectifying form is that it cannot be used in reverse to supply power to the supply network, in situations where the load terminal polarity must be non-reversing.

An object of the present invention is therefore to mitigate the above mentioned disadvantageous harmonic and power factor effects. A further object is to minimise the effect of the reverse power limitation.

Summary of the Invention The invention secures the above objectives for electric power converters of the above type by means of a converter combination in which an auxiliary Pulse Width Modulated (PWM) converter is connected across a DC output filter of the thyristor supply bridge and connection of the auxiliary PWM converter is enabled either back to the main AC supply network to which the thyristor supply bridge is connected, or to a separate AC supply independent of the AC supply network to which the thyristor supply bridge is connected, or to a low voltage DC supply.

The output of a thyristor supply bridge is usually fed to a DC load via a passive DC- link filter. Accordingly, in the combination of the invention, the auxiliary PWM converter is connected across the DC link filter output terminals in parallel with the load.

The auxiliary PWM converter may be either a voltage source inverter or a matrix converter.

In the former case, the auxiliary PWM converter is connected to the main AC supply network via passive components, whose purpose is to mitigate the effects of the common mode and differential mode voltage content of the PWM carrier wave. These passive components preferably comprise a passive filter (e.g., an L-C filter) and an isolating transformer connected in series between the auxiliary converter and the main AC supply network. In cases where the load across the thyristor supply bridge produces a significant amount of regenerative power, a dynamic braking resistor may also be associated with the input of the auxiliary PWM converter (e.g., connected across the passive filter, or even located in the main AC supply network) to selectively absorb regenerative power, selection of the dynamic braking resistor being

conveniently achieved by switch means responsive to the voltage level in the main AC supply network, or to the level of regenerative power.

If the auxiliary converter is a matrix converter, it may not need to be connected to the supply network via an isolating transformer and may have sufficient inherent input filter capacity to allow its direct connection to the supply network. Nevertheless, in some circumstances, connection to the supply network via the above-mentioned passive components may be preferable.

Preferably, particularly when the auxiliary converter is a voltage source inverter, a further passive filter in the form of a capacitance may be connected to the supply network in parallel with the above-mentioned passive components.

In an alternative embodiment of the invention, the auxiliary PWM converter is arranged for connection to the separate AC supply by way of at least one passive component, this being a passive filter for connection in series between the auxiliary PWM converter and the separate AC supply.

In another alternative embodiment of the invention, the auxiliary PWM converter is arranged for connection to the low voltage DC supply by way of at least one passive component, this being a passive filter for connection in series between the auxiliary PWM converter and the low voltage DC supply.

If the DC load applied to the DC-link filter output terminals associated with the thyristor supply bridge comprises a second voltage source inverter which has either a large number of parallel paths within it, or is outputting a larger than normal number of AC phases (e.g., to a ten-phase electric motor), the design and constructional features of the second voltage source inverter may be used as the basis for the design of the auxiliary PWM converter. This has the advantage of reduced manufacturing costs due to commonality of the electronic components used.

The invention further embraces an electric power system comprising:

(a) a main AC power supply network,

(b) a DC load, and

(c) an electric power converter combination for converting AC power from the main AC power supply network to DC power to supply the DC load, the converter combination comprising a phase controlled thyristor supply bridge rectifier with a DC output filter to the DC load and an auxiliary PWM converter connected across the DC output filter, the auxiliary PWM converter also having supply input means connected to power supply means comprising either the main AC supply network, an AC supply separate from the main AC supply network, or a low voltage DC supply.

The invention further includes various methods of operating an electric power converter combination constructed as described above.

In a first operational mode of the invention, the firing pulses for switching devices in the auxiliary PWM converter are inhibited, whereby when the thyristor supply bridge rectifier and the auxiliary PWM converter are connected to the main AC supply network, all the power from the main AC supply network to the load is through the thyristor supply bridge rectifier.

In a second operational mode of the invention, the thyristor supply bridge draws lagging VAr' s from the main AC supply network and the auxiliary converter operates as an inverter to inject leading VAr 's into main AC the supply network, thereby to at least partially correct the effect of the lagging VAr' s drawn by the thyristor supply bridge.

In a third operational mode of the invention, the thyristor supply bridge draws harmonic currents from the main AC supply network and the auxiliary PWM converter operates as an inverter to inject harmonic currents into the main AC supply network in anti-phase to those drawn by the thyristor supply bridge, thereby to at least partially correct an effect of the harmonic currents drawn by the thyristor supply

bridge upon the voltage drop across a supply impedance of the main AC supply network.

In a fourth operational mode of the invention, the firing pulses for switching devices in the thyristor supply bridge are inhibited and the auxiliary PWM converter operates as an inverter to inject power back into the main AC supply network.

In a fifth operational mode of the invention, the firing pulses for switching devices in the thyristor supply bridge are inhibited and the auxiliary PWM converter operates as an inverter to inject power back into a dynamic braking resistor.

In a sixth operational mode of the invention, the auxiliary PWM converter inverts while the thyristor supply bridge rectifies, thereby to avoid discontinuous conduction in the thyristor supply bridge, such that most of the power flows from the main AC supply network to the load via the thyristor supply bridge, whilst a smaller proportion of power circulates from the main AC supply network, through the thyristor supply bridge, the auxiliary converter and the common DC link filter, and back to the main AC supply network.

In a seventh operational mode of the invention, the firing pulses for switching devices in the thyristor supply bridge are inhibited and the auxiliary PWM converter operates as a PWM rectifier, whereby power flows from the main AC supply network to the load with low harmonic current distortion and with the auxiliary PWM converter controlling the supply power factor.

In an eighth operational mode of the invention, the firing pulses for switching devices in the thyristor supply bridge are inhibited and the auxiliary PWM converter is connected to the separate AC supply network, the auxiliary PWM converter operating as a PWM rectifier, whereby power flows from the separate AC supply network to the load with low harmonic current distortion and with the auxiliary PWM converter controlling the power factor.

In a ninth operational mode of the invention, the firing pulses for switching devices in the tliyristor supply bridge are inhibited and the auxiliary PWM converter is connected to the low voltage DC supply network, the auxiliary PWM converter operating as a PWM rectifier, whereby power flows from the low voltage DC supply network to the load with low impressed current distortion.

Further features of the invention will be apparent from a study of the accompanying description and claims.

Brief Description of the Drawings An exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which:

Figure 1 is a single-line circuit diagram showing a simple embodiment of the invention;

Figures 2 to 10 illustrate various operating modes of the circuit arrangement of Figure l; and

Figure 11 is a block diagram of a control system suitable for controlling operation of an electric power converter combination according to the invention in the context of a variable speed electric drive for an electric motor;

Detailed Description of the Preferred Embodiments Referring to Figure 1 , a three-phase AC distribution bus 1 supplies power to an AC load 2 through a converter comprising a phase controlled thyristor rectifier or supply bridge 4 and a voltage source inverter 6, the thyristor supply bridge 4 and the voltage source inverter 6 being connected by a passive DC-link filter 8, comprising in this example a capacitance 8a and an inductance 8b for each DC output terminal. In alternative embodiments, the passive DC-link filter 8 may be simplified to a single L- C filter. The AC load 2 may be, e.g., a high power electric motor requiring a multiphase supply.

For purposes of understanding the invention, the inverter 6 and the AC load 2 may be collectively thought of as a DC load connected across the DC-link filter 8 of the thyristor supply bridge 4. Other types of DC load may be substituted, such as: single

and multiple output, direct- and transformer-coupled, DC-DC converters with step- down, step-up and step-down/step-up topologies (this latter topology being otherwise known as a Buck-Boost converter); and DC-AC derivatives. The exact nature of the DC load is immaterial to the basic principle of the present invention. However, if the distribution bus 1 is a high voltage supply, e.g., 3KV or higher, the distribution voltage must be stepped down by a transformer 9 (indicated in dashed lines) because thyristor supply bridges have certain input voltage limitations.

Thus far the arrangement is known, but in accordance with the invention, an auxiliary PWM converter 10, with its own DC link filter 12, is combined with the thyristor supply bridge by connect it into the DC load circuit of the thyristor supply bridge and also into the main AC supply network 1. On the load side, the auxiliary converter 10 with its own DC-link 12 is connected across the output terminals of the DC link filter capacitance 8a. In the illustrated embodiment, the auxiliary PWM converter 10 is a voltage source inverter, whose AC supply side is connected to the main AC supply network 1 via passive components comprising an L-C filter 13 in series with an isolating (double- wound) transformer 14. Note that it will also be necessary to equip the connection of the auxiliary PWM converter 10 to the supply network 1 with a circuit breaker, shown simply by "X" in Figure 1.

If the auxiliary PWM converter 10 is a voltage source inverter, it is supplied via a transformer 14 and filter 13 because: a) a voltage source inverter is inherently a step up topology, whereas the thyristor supply bridge 4 is a step down topology and both must share the same DC link voltage, b) its PWM carrier differential mode voltage must not be applied to the supply network 1, and c) its PWM carrier common mode voltage must be isolated from the input of the thyristor supply bridge 4.

Although described above as a voltage source inverter (i.e., a group of 3 single phase H bridges or a Greatz bridge), it should be realised that in accordance with the

invention and at the discretion of the designer, the auxiliary PWM converter 10 may be either a voltage source inverter or a matrix converter.

If auxiliary PWM converter 10 is a voltage source inverter, then it is advantageous to connect it to the supply network via the passive components comprising filter 13 and transformer 14 so as to mitigate the effects of the common mode and differential mode voltage content of the PWM carrier wave produced by the auxiliary converter.

If further protection of the supply network 1 is considered desirable, a further passive filter in the form of a capacitance 16 may be connected to the supply network in parallel with the passive components 13 and 14. Another capacitance filter 17 (shown in dashed lines) may be added between the isolating transformer 14 and the supply.

However, if the auxiliary PWM converter 10 were to be a matrix converter, it would not need to be connected to the supply network 1 through an isolating transformer 14 and in fact it might have sufficient inherent input filter capacity to allow its direct connection to the supply network 1 without benefit of a separate passive filter. Nevertheless, in some circumstances, to prevent circulating current components flowing between the thyristor rectifier and the auxiliary PWM converter via the DC- link and the supply network terminals, it may be preferable to connect the auxiliary PWM converter 10 (being a matrix converter) to the supply network via the above- mentioned L-C passive filter 13 and in parallel with a capacitative passive filter 16.

The auxiliary PWM converter 10 can operate in both rectifying and inverting modes, and for economy may be built from the same types of modules as the voltage source inverter 6. Typically, auxiliary PWM converter 10 and the voltage source inverter 6 employ IGBTs, GTOs, or their respective derivatives, operating with either asynchronous or synchronous PWM carrier frequencies.

Note that the input voltage of the auxiliary PWM converter 10 must be lower than that of the thyristor supply bridge 4 and its transformer VA rating will be much lower than the rating of the thyristor supply bridge. Consequently, an auxiliary converter with a

relatively small VA rating can have a significant effect upon a system having a large VA rating.

The minimum VA rating for the auxiliary PWM converter as a voltage source inverter is realised when a Greatz bridge is used. An increased rating is achieved when three H bridges are employed. Further parallel-connected arms containing the auxiliary PWM converter components of Figure 1 can be employed to further increase the VA rating.

Typically, the supply voltage for the auxiliary PWM converter is 50-70% of that applied to the thyristor supply bridge 4.

The invention provides the benefits of both phase controlled thyristor and PWM equipment whilst suffering few of their disadvantages. As always, the result will be a compromise and will benefit from an optimisation process.

As will now be shown, the invention can operate in a number of different modes, to give greatly enhanced flexibility of operation as a power supply. For each mode, a diagram illustrates the power flow in a converter supplying the DC load comprising the inverter 6 and the AC load 2. The thyristor supply bridge 4 is shown as having a transformerless input, but all the operating modes are equally applicable to a transformer-fed system. Two or more operating modes can occur simultaneously, as specified below.

(a) In most circumstances, Modes 2 and 3 as specified below, can occur simultaneously.

(b) Mode 4 inherently combines the benefits of Modes 2 and 3, without suffering the disadvantage of Mode 1.

(c) Mode 6 inherently combines Modes 1 and 4, and can also achieve the benefits of Modes 2 and 3.

(d) Mode 7, like Mode 4, inherently combines the benefits of Modes 2 and 3, but does not suffer the disadvantage of Mode 1.

(e) Modes 5, 8 and 9 are specialised modes of operation in which the auxiliary converter 10 does not take power from, or supply power to, the main AC supply network 1.

MODE 1 - THYRISTOR RECTIFIER

This mode is illustrated in Figure 2, the power flow being through the thyristor supply bridge 4 and the voltage source converter 6 to the AC load 2, with firing pulses being inhibited for the switching devices in the auxiliary PWM converter 10 so that it merely acts as a diode rectifier. In this mode, the thyristor supply bridge 4 provides maximum efficiency and reliability, at lowest cost, size and mass, however, its harmonics and power factor are non-ideal, as indicated at the left of the power flow arrow. The non-ideal power input conditions dictate that the generator MVA rating must be increased relative to an ideal MW rating and supply voltage distortion may be significant. The thyristor supply bridge cannot sustain inversion, other than during operation with a thyristor firing delay angle sufficiently large to cause reversal of the

DC output terminal voltage of the supply bridge 4, which may be employed for overcurrent protection. The thyristor supply bridge 4 can employ variable thyristor firing delay angle control as a means of achieving DC link voltage control, but such control worsens supply harmonics and power factor.

It should be noted that the auxiliary PWM converter 10 acting as a supply bridge will inherently bypass the rectification action of thyristor supply bridge 4 when the firing delay angle of the thyristor supply bridge is increased beyond the condition at which its DC output voltage equals the DC output voltage of the auxiliary PWM converter 10, as produced by its diode rectifier action. Hence, when it is desired to operate the thyristor supply bridge 4 at such firing delay angles, it is necessary to disconnect the auxiliary PWM converter 10 from the supply network by opening the circuit breaker X. For example, circuit breaker X must be open when using firing delay angle control

as a means of limitation of inrush current experienced by the DC link filter 8 during start-up.

Although firing pulses for the switching devices in the auxiliary PWM converter 10 are inhibited in this mode, in all other modes the PWM firing pulses in converter 10 are enabled and thereby serve to remedy the above-mentioned disadvantages and provide other benefits.

MODE 2 - ACTIVE POWER FACTOR CORRECTION

In this mode, illustrated in Figure 3, the power flows as shown by the arrows are into the load 2 from the main AC supply network 1 via the thyristor supply bridge 4 and the inverter 6 and back into the supply network 1 from the load via the inverter 6 and the auxiliary converter 10. Here, the thyristor supply bridge 4 draws lagging MVAr' s from the supply network 1 and the auxiliary converter 10 operates as an inverter to inject leading MVAr's into the supply network, so enabling partial, full, or overcorrection of the effect of the associated thyristor supply bridge 4. In this way, for example, loads causing leading or lagging power factor on the supply network 1 can be fully or partially compensated, as desired, insofar as this is within the capacity of the auxiliary converter 10 in combination with the converter 6.

In the case of a generator supplying power through the supply network 1 to an electric motor as load 2, this mode would enable a reduction in generator size, mass and cost, or increase the propulsive power available from a given generator rating.

Unity power factor can easily be achieved at loads below 50% power, using the minimum size auxiliary converter configuration (Greatz bridge) mentioned above. Hence, this mode is especially relevant to operation where the supply network is provided with limited generator capacity. As load is increased, power factor reduces but is still better than in existing equipment. A larger leading MVAr capacity can be provided by changing the auxiliary converter 10 to an H bridge type, or by employing

parallel-connected switching arms, but this may not be the most cost effective use of such a circuit topology.

MODE 3 - ACTIVE FILTRATION OF ELECTRICAL SUPPLY TO LOAD AND/OR OF HARMONIC DISTORTION ON SUPPLY NETWORIC

In Figure 4, the Mode 3 power flow from the main AC supply network 1 through the supply bridge 4 and the converter 6 is associated with reversible power flows for harmonic injection through the auxiliary converter 10, which operates in both inverting and rectifying modes to provide active filtration of the current harmonics generated by the thyristor supply bridge 4 and/or, active filtration of pre-existing voltage harmonics on the supply network 1. For example, if the load 2 is an electric motor, Mode 3 enables reduction of generator size, mass and cost, or enables an increase in the effective power available from the supply network.

For a given active filtration capacity, this mode of operation of the invention would be more competitive than a stand-alone low voltage (LV) inverter-based system, due to its integration with the load's converter power circuit 6 and controls. This system would be much smaller than a competing medium voltage (MV) passive filter and the effects of supply impedance variation upon the tuning of resonant circuits would be less problematic, due to the reduction in capacitance of filters and the ability to employ active damping of reactances associated with the point of common coupling on the main AC supply network 1.

MODE 4 - REGENERATION INTO SUPPLY NETWORK

As shown in Figure 5, auxiliary converter 10 operates as an inverter in Mode 4 and power is fed back into the main AC supply network 1. For example, this mode is advantageous if the load 2 is an induction motor being used regeneratively as a brake and is thereby generating power, which may be fed into the network 1 to help supply other loads. Note that this operating mode does not inject significant harmonics into the supply network 1, because it inherently incorporates the function of Mode 3. Additionally, this operating mode does not inject significant MVAr 's into the supply network 1 because it inherently incorporates the function of Mode 2, thereby enabling

control of the power factor in the supply network, say between 0.9 lagging and 0.9 leading, including unity power factor.

MODE 5 - REGENERATION INTO A LOCAL AC CONNECTED DYNAMIC BRAKING RESISTOR

This mode is shown in Figure 6 and is closely related to Mode 4 (Figure 5), in that if the circumstances are such that one or more loads elsewhere on the main AC supply network 1 were to be taken off-line by protective devices or for some other reason, the supply network 1 might not be capable of absorbing the regenerated power from load (motor) 2, again indicated by the power flow arrow. Hence, in Figure 6, a dynamic braking resistor 20 is connected to the AC input terminals of the auxiliary PWM converter 10 to absorb motor regenerated power in the above-mentioned event of removal of load from the network. In this particular embodiment, the braking resistor is connected across the filter 13, but could be located within the AC supply network 1. Additionally, the resistor 20 is connected through a switch 21 with the aim of reducing unnecessary dissipation of power in the resistor when regenerative power is not being produced. Operation of the switch can be automatic, e.g., by sensing an excessive voltage increase on the supply network 1, or by detecting production of regenerative power.

MODE 6 - CIRCULATING CURRENT MODE

In this mode, illustrated in Figure 7, the auxiliary converter 10 inverts while the thyristor supply bridge 4 rectifies. Most of the power flows from the supply network 1 to the DC load (i.e., inverter 6 plus motor 2) via the more efficient thyristor supply bridge 4, but a smaller proportion of power circulates from the supply network I ₅ through the thyristor supply bridge 4, the common DC link filter 8 and the auxiliary converter 10, back to the supply network.

This circulating current mode avoids discontinuous conduction in the thyristor supply bridge 4, providing the benefits of reduced disruption of the control of the thyristors in bridge 4. Such disruption results from discontinuous current through the thyristors and consequent sporadic resumption of thyristor conduction at the mid-point in each

half-cycle of the thyristor supply bridge's AC line current waveform. As known to the skilled person, this sporadic resumption of thyristor conduction may also cause forward recovery failure of the thyristor.

MODE 7 - PWM RECTIFICATION

Before operation in this mode, the supply bridge DC link filter 8 (Figure 8) must be charged to working voltage, just as is necessary for operation in Mode 1. In Mode 7, the auxiliary converter 10 operates as a rectifier, with the power flow being from the main AC supply network to the load via the auxiliary converter 10. The firing pulses of the thyristor supply bridge 4 will normally be suppressed. All power flow is via the PWM bridge, with low harmonics and controllable power factor, say 0.9 lagging to 0.9 leading, including unity power factor. This enables reduction (if desired) of size, mass and cost of a generator feeding the supply network, or an increase in the effective power available from a given generator rating. This mode is especially relevant to situations where generator capacity is limited, such as on board ships or at isolated sites.

MODE 8 - OPERATION FROM SEPARATE AC SUPPLY

Mode 8 operates identically to Mode 7, except that as shown in Figure 9, a separate LV AC power supply IL is used to supply power to the load through the auxiliary converter 10, rather than the main AC supply network 1. Such an LV AC supply could be produced by an AC generator driven by a prime mover, such as a diesel engine or a gas turbine.

Figure 9 assumes a transformerless system. However, this entails the use of an additional line filter 22 to deal with the PWM carrier harmonics from the auxiliary converter 10. Such a filter 22 would probably need to be more complex than shown, comprising inductive and capacitative components, as known. Alternatively, auxiliary converter 10 could be isolated from the supply IL by inserting a transformer between the supply IL and the L-C filter 13, as in Mode 7. In fact, strict quality of power supply requirements for an LV AC supply would make such transformer- isolation highly desirable.

MODE 9 - OPERATION FROM LV DC SUPPLY

This mode of operation, as shown in Figure 10, is similar to Mode 8, except that the separate LV power supply is a DC source, such as a battery 30, a fuel cell or a DC generator driven by a prime mover. It will be apparent that the use of a DC/AC converter to supply AC power to the auxiliary converter 10 would detract from this arrangement's simplicity and introduce inefficiencies. Hence, the insertion of an isolating transformer between the DC supply line and the filter 13 is not an advantageous modification and therefore an additional line filter 22 ' will be required, similar to the filter 22 in Figure 10.

In the absence of an isolating transformer, it is essential that the LV DC supply's transient response to the PWM carrier common mode voltage of the auxiliary PWM converter 10, is carefully managed. The LV DC power supply is unlikely to be able to accommodate significant ripple current at PWM carrier frequency and the input filter 22 ¹ must take this into account.

CONTROL SYSTEM FOR MODES 1 TO 9

Figure 11 shows simplified control systems for both the thyristor supply bridge 4 and the auxiliary PWM converter 10, the latter being employed in accordance with the invention to enhance the performance of the known thyristor supply bridge. Considered per se, the control scheme for the thyristor supply bridge 4 is of a generally known type, in which a current transducer 36 on the AC input line to the thyristor supply bridge 4 provides a current feedback signal 33 to a current regulator 32. The thyristor supply bridge 4 is regulated thereby so as to satisfy the overall requirements of a set of functions in the Variable Speed Drive (VSD) regulator 40, these functions being represented by the current regulator reference signal 37.

The auxiliary PWM converter control system necessary for the invention may be a virtually self-contained add-on enhancement to the above known type of control system, and may comprise a digital controller with frequency-specific DQ regulator loops or a sliding mode control. Alternatively, other known techniques could be used.

The control system for the supply bridge 4, comprising VSD regulator functions 40 and current regulator function 32, may in fact have sufficient performance to allow its adaptation to facilitate implementation of the enhanced control of the auxiliary converter 10. Alternatively, additional dedicated regulators may be used. For the purposes of the present embodiment, it is assumed that the VSD regulator functions 40 are shared, but that the auxiliary PWM converter 10 has a dedicated current regulator 34. However implemented, it is essential that the thyristor supply bridge controls and auxiliary PWM converter controls are fully integrated with each other in order to allow the combined power system to be operated in the nine modes described above.

Similarly to the control scheme employed for the thyristor supply bridge 4, the auxiliary PWM converter 10 employs an AC line current transducer 38 that provides a current feedback signal 35 to a current regulator 34, which regulates the auxiliary PWM supply converter 10 in order to satisfy the overall requirements of a set of functions in the VSD regulator 40, these functions being represented by the current regulator reference signal 39.

The VSD regulator must be set up to apportion load between the thyristor supply bridge 4 and auxiliary PWM supply converter 10, according to the required operating mode. The basic characteristics of the thyristor supply bridge 4 current control loop,

36, 33, 32 and the auxiliary PWM converter 10 current control loop 38, 35, 34, are equivalent and both converters 4 and 10 are able to feed power to the load 2 as demanded by respective reference signals 37 and 39. In the event that load power reverses, i.e., is regenerative, the VSD regulator 40 must be able to output a reverse polarity reference 39 to the auxiliary PWM supply converter 10.

An AC line voltage transducer 48 is linked to the supply line of the thyristor supply bridge 4 and provides a phase reference signal 50 for processing in feedback processor 46. The feedback processor 46 also receives the current feedback signal 33 from the same AC supply line and decomposes it into fundamental "real",

fundamental "reactive" and harmonic components. Reference signals 41, 43 and 45 are then generated according to the required system operating mode, .

Signals 41 and 43 are circulating current trimming signals and may be employed to adjust the contributions of thyristor supply bridge 4 and auxiliary PWM converter 10 to load current. The adjustments may be such as to force a circulating current to flow in the main AC supply network 1, from thyristor supply bridge 4, via DC link filter terminals 8 and 12 and the auxiliary PWM converter 10. This is clockwise circulating power flow. Alternatively, this circulation may be regulated by the VSD regulator 40. The harmonic content of the circulating current may also be regulated, if desired, by means of the feedback processor 46.

Harmonic and MVAr reference signals 45 are employed to enable the auxiliary PWM converter 10 to source and sink fundamental and harmonic currents, at whatever angular displacement from the phase reference signal 50 is appropriate for the required operating mode.

In Mode 1, Figure 2, thyristor supply bridge reference signal 37 is generated by conventional VSD regulator functions and the auxiliary PWM converter reference signals 39, 43, 45 are set to zero.

In Mode 2, Figure 3, thyristor supply bridge reference signal 37 is generated by conventional VSD regulator functions and the feedback processor 46 outputs the auxiliary PWM converter reference signal 45, this being an AC supply network fundamental frequency sine wave with a leading power factor.

In Mode 3, Figure 4, thyristor supply bridge reference signal 37 is generated by conventional VSD regulator functions and the auxiliary converter reference signal 45 is again derived by the feedback processor 46, signal 45 being a series of AC supply network harmonic frequency sine waves.

A key benefit of this system is provided by the integration of the control functions of the thyristor supply bridge 4 and the auxiliary PWM converter 10, whereby the current harmonics drawn by the thyristor supply bridge in its supply line simply form the active filtration reference signal by virtue of the current feedback 33, with the auxiliary PWM converter 10 performing the active filter function. In prior art systems, active filters measure voltage harmonics and employ complex algorithms from which a filter current reference signal is derived. Even in the case where an active filter is dedicated to the filtration of current harmonics that are drawn by a particular piece of equipment, it is conventional to provide the active filter with its own feedback transducers and regulator system. A typical active filter also requires its own DC bus filter and supporting voltage control system. The active filter concept of the present invention can be far simpler than other systems, provided that the primary source of harmonic pollution on the AC supply network is integrated with the filter function.

If additional sources of harmonic pollution outside the integrated thyristor supply bridge must be filtered, the equipment described can typically be configured to perform that function without requiring additional hardware, the exception being the case where the external source of harmonic pollution dominates over that of the integrated thyristor supply bridge. In most cases, the regulator would be required to provide active damping of resonant modes associated with auxiliary PWM converter carrier wave filters and various impedances connected to the supply network 1.

In Mode 4, Figure 5, reference signal 39 for the auxiliary PWM converter 10 is generated by conventional VSD regulator functions and the thyristor supply bridge reference signal 37 is set to zero.

In Mode 5, Figure 6, reference signal 39 for the auxiliary PWM converter 10 is again generated by conventional VSD regulator functions and the thyristor supply bridge reference signal 37 is set to zero. The control strategy must manage the AC- connected dynamic braking resistor 20 voltage waveform in the event that the AC

supply network 1 is disconnected from the AC input terminals of the auxiliary PWM converter 10.

In Mode 6, Figure 7, the feedback processor 46 identifies the AC line current harmonics that are associated with DC link current ripple (and potential DC link current discontinuity). From this information, opposing reference signal components 41, 43 are generated for summation into thyristor supply bridge and auxiliary PWM converter current regulators 32, 34, respectively. By this means the thyristor supply bridge current increases by a predetermined value of amp. seconds integral (Coulombs) per half cycle, while the PWM supply bridge current is reduced by a corresponding amp. seconds integral. Whereas the thyristor supply bridge currents are heavily filtered by the DC link filter reactor 8, the auxiliary PWM converter currents closely correspond with the circulating current trim reference signal 43. The balanced amp. second integrals result in the presence of the circulating current, described previously.

In Mode 7, Figure 8, the referencing for the auxiliary PWM converter is similar to that for an industry standard back-to-back PWM voltage source inverter/variable speed drive.

In Mode 8, Figure 9, control is like Mode 7, but the quality of power supply on the

LV AC supply network IL may detract from the effectiveness of the auxiliary PWM converter 10 if the source impedance on that network is particularly high, unless the harmonics appearing on the supply are taken into account during the design process.

As is well known, Greatz bridges comprise six arms of power semiconductor devices and H-bridges comprise four arms of power semiconductor devices, with three such H-bridges being used in a three phase system. When dealing with three-phase AC supplies, all arms in these bridges are utilised. However, when dealing with DC supplies, only four out of six of the arms in a Greatz bridge, or only two out of the three H-bridges need be in operation. Following from the above, it will be understood that in Mode 9, Figure 10, control is similar to that in Modes 7 and 8, except that when using a Greatz bridge, only 4 out of 6 arms are operational, and that when using

aii H-bridge, only two out of the three bridges are operational. The auxiliary PWM converter design strategy must ideally take into account equalisation of switching device duty cycles and optimise the carrier frequency common mode voltage components.

As already mentioned, some of the operating modes may be merged. For example, supply harmonics and power factor may be corrected simultaneously, as in Modes 2 and 3. However, when modes are merged, compromises will be required between converter functionality in different modes. For example, ability of the auxiliary PWM converter to cancel supply harmonics will conflict with its ability to compensate for extreme power factor excursions on the supply network.