GB2383695A

GB2383695A - 12-pulse ac to dc converter with automatic minimisation of total harmonic distortion

Info

Publication number: GB2383695A
Application number: GB0122430A
Authority: GB
Inventors: Walter Farrer
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-09-17
Filing date: 2001-09-17
Publication date: 2003-07-02
Anticipated expiration: 2021-09-17
Also published as: GB0122430D0; GB2383695B

Abstract

The open-circuit dc difference voltage of two standard 3-phase converters 1 fed from 30 degree displaced transformer-derived supply voltages together with the voltage developed by an "Automatic Voltage Compensation Circuit" 10 in series with each of the two 6-pulse converters, is impressed back onto the supply transformer winding inductance's. This produces complimentary current variation in the two bridges, during each 30 degree period between bridge firings, whilst the sum of the currents is constant at the dc load value. The "Automatic Voltage Compensation Circuits" adjusts its voltage until the current variation is equal to the dc load current value such that commutations in both bridge's always occur when the current is at zero, or a low value. Such operation results in very low distortion of the current drawn from the utility supply. Careful selection of the transformer reactance allows a more simple construction of the switches associated with the Automatic Voltage Compensation Circuit.

Description

12-Pulse ac to dc Converter with Automatic Minimisation of Total Harmonic Distortion This invention relates to the interconnection of two standard 3-phase converter bridges together with additional means at their dc output which ensure that the resulting 12-pulse ac-to-dc controlled rectifier may automatically achieve very low Total Harmonic Distortion (THD) of the currents drawn from the utility source over the full operating range of output dc voltage and current.

Low distortion of the currents drawn from the utility can be extremely important in some applications which have to operate at specific conditions when the capability of the utility supply is very low, such as only one generator running in a ships engine room, or when the utility is provided from a weak source, such as long feeder lines. Under both these examples the source inductance of the utility is large and gives rise to distortion at the utility connection from the commutation process within the converter bridges. This distortion (notching) can be harmful to other users and equipment and must be kept within certain limits.

The standard 3-phase converter bridge is well known and is used for the controlled rectification of ac current to dc current. Facility is provided to connect each of the input phases to the positive or negative dc terminals of the bridge via valves with unidirectional current carrying properties such as Thyristors, Gate Turn Off devices (GTO's) or Integrated Gate Controlled Thyristors (IGCT's). The standard 3-phase converter is often referred to as a "Gratz Bridge"or"Controlled Rectifier"or just"Thyristor Bridge".

By phase controlled triggering of the valves in the required sequence selected sections of the line to line voltage at the bridge ac terminals are connected through to the two dc terminals such that a regular pattern of pulses, at 6 times the supply frequency, appear at the dc output with a mean voltage related to the phase-angle of triggering. A standard 3-phase thyristor converter is depicted in Fig. 1.

The distortion that occurs in the utility bus supply lines feeding converter circuits is principally due to the commutations that occur in the bridge circuits. As the source impedance of the utility bus is mainly inductive, as is that of any interposing transformers between the bus and the converter bridges, the current cannot be instantly established or terminated. This means that when a valve is triggered to connect a phase to a dc terminal of a bridge then the current will grow in that valve at a controlled rate determined by the voltage and inductance in the circuit, whilst the previously conducting valve to that dc terminal, fed from another phase, will decay current at a similar rate. i. e. the open circuit line to line voltage at that instance is supported by the total circuit inductance on the a. c. side of the bridge and the dc terminal of the bridge will be at the midpoint voltage of the two phases.

The depth of the notch at the utility bus will depend on the ratio of the utility source inductance to the total circuit inductance of the two phases involved in the commutation, including additional transformer inductance's in the bridge supply lines, as a function of the open-cicuit line to line voltage difference at that instance. The duration of the notch will be

determined by the rate of change of current and the level of current in the device to be commutated.

When the outgoing valve current reaches zero the commutation is over and the voltage at the dc pole of the bridge reverts to that of the newly connected a. c. line which, if the dc current is almost constant, will be very close to the open-circuit line voltage. The a. c. line current therefore consists of steps of current at each commutation with approximately constant magnitude between commutations. The sum of the area of the notches in the utility voltage is directly related to the Total Harmonic Distortion (THD) and is thus seen to be proportional to the current in the device under commutation. The current at commutation will of course be equal to the dc current for a single 3-phase converter bridge.

At higher powers it is usual to use more than one 3-phase converter connected so as to give a higher pulse ripple frequency and smaller pulse amplitude at the dc load terminals.

This is achieved by supplying the bridges with separate phase-shifted supplies derived via isolating transformers. Two such 3-phase bridges with 30 electrical degrees of phase shift between their supplies will yield a rectifier system with 12 pulses per cycle. The isolation provided by the transformers allows the designer to configure the bridges either in series or in parallel. In the series case, rated at half voltage and full current for each bridge, then patently each bridge carries the same current as the dc circuit value at all times. In the parallel case, rated at full voltage and half current for each bridge, the traditional approach is to apply the two bridge output voltages, which are not instantaneously the same value, via an inter-phase transformer or sharing reactors in the bridge dc outputs to summate their contributions to a common dc load without interference with the individual performance of each bridge. ie each bridge is constrained, by the impedance of the interphase transformer or sharing reactors, to carry a constant value of dc current equal to one half of the total dc current.

Both the traditionally used series and the parallel arrangements operate with identical performance for the same load, source and triggering conditions and provide a factor of four reduction in dc load current ripple over what could be achieved using a single 3-phase converter bridge.

Often the two 3-phase phase-shifted supplies of a 12-pulse converter arrangement are derived from two separate secondary windings on one transformer, the traditional star/delta secondary arrangement. For simplicity this invention will be described for the case when separate transformers are used for each bridge. However the invention applies to the use of dual secondary transformers with the provisor that secondary side inductance values, rather than the total primary to secondary terminal values, become important in determining the conditions required to achieve minimum harmonic distortion.

It has been shown (Patent Application Number 0120897. 4) that if two standard 3-phase converters, A and B, with transformer derived voltage supplies such that converter A supplies are 30 degrees displaced from the supplies of converter B and the converters A and B are connected in direct parallel at their dc terminals then the current provided at the dc terminals of bridge A will vary from a minimum to a maximum in a 30 degree period between bridge triggerings whilst that of bridge B will vary from maximum to minimum. In the next 30 deg interval the growth of current is in bridge B whilst the reduction occurs in bridge A.

Furthermore it has been shown that this current change may be made equal to the operating dc load current at a specific operating point by judicial choice of the transformer leakage

inductance. Such conditions give zero current in the device under commutation, whilst still maintaining the constant current to the dc load circuit, and result in very low current and voltage distortion at the utility supply.

The driving force for this current variation in the individual bridges is the open-circuit difference voltage being applied to the inductances of the transformer windings that are conducting at the time. This voltage is approximately square-wave at six times the frequency of the utility, thus the voltage which drives the current-change reverses polarity every 30 degrees.

Fig. 2 shows the open-circuit output voltage of the two separate converters for a firing angle of alpha = 90 deg, which corresponds to zero dc output voltage, together with the difference voltage for this condition. For other firing angles the difference voltage is somewhat less square and of lower amplitude, as shown, and would thus results in proportionally lower current change in the bridges during a 30 deg interval.

This invention provides a means for automatically augmenting the volt-seconds of this difference voltage over the 30 deg intervals such that, in the steady state, the current change produced is always sufficient to ensure that zero current is reached in a device by the time it is due to be commutated.

This is achieved by adding a voltage source, developed on a capacitor, in series with the dc output of each of the standard 3-phase converter bridges and switching the polarity of this voltage source in synchronism with the 30 degree periods between the firing pulses of the two converters. The value of the capacitance is chosen such that maximum operating current for a 30 degree period will only cause a voltage change much smaller than that of the open-circuit difference voltage of the two converters. Such an arrangement of a 12-pulse ac to dc controlled rectifier including the switches and capacitors in the bridge output circuits is shown in Fig. 3. It may be noted that as the current from the dc terminal of the converter is always unidirectional then the switches may have unidirectional current carrying properties.

When the open circuit voltage of one converter is greater than the other converter during a specific 30 degree firing period then the current will increase in the first converter and will decrease in the second converter due to this difference voltage plus the sum of the voltage present on the two capacitors being applied across the transformer inductances that are in circuit at that particular instance. If the current in the first converter reaches the value of the dc load current before the end of the 30 degree period then the current in the second converter will have reached zero and these conditions will be maintained until the end of the 30 degree firing period. During this time the capacitor in series with the first converter will continue charging for the full period whilst the capacitor in series with the second converter will cease to charge when its current reached zero. The result of this additional charging inreases the voltage on the capacitor but reduces the nett voltage in the circuit such that the growth of current in the transformer inductances during the next 30 degree period will be somewhat less rapid. Eventually the capacitor voltages will stop increasing when the nett voltage in the circuit is just sufficient to grow the current from zero to the dc load current level in a 30 degree firing interval. At this condition all commutations in both bridges occur when the current in the device undergoing commutation has reached zero with the consequence that current and voltage distortion at the utility bus will be very low.

By using switches with the capability of supporting both positive and negative voltage it is possible to develop the necessary voltage on the capacitor to achieve the required rate of change of current regardless of whether positive or negative voltage is required on the capacitors. However it is possible to select the reactance of the transformer such that voltage of only one polarity is required on the capacitors for all operating dc load voltage and dc load current ranges, including regenerative operation of the bridges. Such a design allows the use of simpler unidirectional current-carrying switches, which may have only unidirectional voltage supporting properties, such as transistors, insulated gate bi-polar transistors (IGBT's), integrated gate turn-off thyristors (IGCT's) and gate turn-off thyristors (GTO's). In fact some of the switches can be plain diodes. Fig. 4 shows a specific embodiment of the switch/capacitor arrangement for such a design where the capacitor voltage is always zero or greater in the direction shown for all operating conditions of the dc load. In this specific embodiment two of the switches in each automatic voltage compensator are plain diodes whilst the other two switches are IGBT's. The pair of IGBT switches in one compensator are both closed during the same periods whilst the pair of IGBT switches in the other compensator are both open during those same periods and both close when the ones in the first group become open circuit. With such a sequence the voltages of the two capacitors are additive and the total subtract from the open-circuit difference voltage of the two converters to produce a reduced rate of change of current in the two converters.

According to the present invention there is provided a 12-pulse ac to dc controlled rectifier comprising two standard 3-phase converter bridges fed with 30 electrical degrees displaced transformer derived supplies the dc output of each 3-phase converter bridge being connected in series with an automatic voltage compensating circuit before connection to the common dc load circuit, each automatic voltage compensating circuit consisting of two series pairs of semiconductor switches connected in parallel with each other and with a capacitor, the input connection to the automatic voltage compensating circuit being the centre point of the first pair of series connected switches whilst the output connection is the centre point of the second pair of series connected switches and the switch sequence is such that the output dc current of the converter is steered in opposite directions through the capacitor in the alternate 30 degree periods defined by the firing pulses of the two converters, with the result that in the steady state the sum of the open-circuit difference voltage between the two converters plus the voltages developed on the capacitors within the two automatic voltage compensating circuits are impressed on the transformer inductances in circuit and produce, in a 30 degree period between firing pulses, a change of current equal to the magnitude of the dc load current such that the current in the next device in the converter bridges to be commutated is reduced to zero, or a low value, at the next commutation.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which :- Fig. 1. Shows the Standard 3-phase Converter Bridge.

Fig. 2. Shows the open-circuit output voltage of the two 3-phase converter bridges together with the difference voltage.

Fig. 3. Shows the invention in which two standard 3-phase converter bridges (1) are connected via the automatic voltage compensation networks to the common dc load to produce a 12-pulse ac to dc controlled rectifier.

Fig. 4 Shows a specific embodiment of the invention using switches with only unidirectional current-carrying and voltage supporting properties.

Fig. 5. Shows voltage and current wavefonns in various parts of the 12-pulse controlled rectifier constructed as described in the present invention and shown in Fig. 4.

Fig. 6. Shows the voltage on the automatic voltage compensation capacitor for different operating levels of dc voltage and dc current.

Fig. 7 Index of numbered items in Figures.

Referring to the drawings the standard 3-phase converter (1), or Gratz Bridge, shown in Fig. 1. comprises six unidirectional current carrying controlled valves (4) connected as shown between the ac supply terminals (5) and the two dc output terminals (2 and 3). Means for the controlled triggering of the valves in the required sequence is provided.

In the specific embodiment shown in Fig. 4. the negative dc terminal (3) of each standard 3-phase converters (1) are connected directly together and to one end of the dc load circuit.

The positive dc terminal (2) of each converter is connected to the input terminal of the automatic voltage compensation network (10) associated with that converter, whilst the output terminal of the automatic voltage compensation network is connected directly to the positive terminal of the common dc load circuit. The sequence of switching in each of the automatic voltage compensation networks is such that the current is steered so that it changes direction in the compensation capacitor during alternate 30 degree firing periods. The triggering of the IGBT switches in the automatic voltage compensators is such that if the first converter current is conducting through the diodes such as to charge the capacitor positive then the current from the second converter will be conducting through the IGBT switches in the second compensating circuit in a manner to discharge its capacitor, the switch conditions and the current direction are reversed in the following 30 degree switching period. The switch signals may be derived directly from the signals firing the thyristors in the two converters. The load circuit is assumed to consist of series inductance, of value very much

greater than the transformer leakage inductances, resistance and back emf. Each bridge is supplied at its ac terminals (5) with separate supply transformers (6 and 7) of one half the total KVA rating requirement and provide the appropriate 30 degrees of phase-shift between the two bridge supply voltages. Other transformer vector groupings which give the required 30 degree phase shift at the bridge ac input terminals (5) would suffice, including a single dual secondary transformer arrangement in place of the two separate transformers. The primaries of the separate supply transformers are fed from the common utility supply (8) whilst the utility is supplied by the source generator (9) which has internal source inductance (Lg Henries per phase). The leakage inductance of the separate transformers is selected such that with zero voltage on the two compensation capacitors the current change in each bridge in a 30 degree interval between sequential triggerings is equal to the maximum operating dc current at the maximum dc operating voltage, which this design assumes is achieved at an operating firing angle of alpha = 30 degrees, this current change being driven by the

difference of the open circuit output voltage that the two separate bridges would produce at that firing angle acting upon the leakage inductances of the two transformer windings in series that feed each of the dc output terminals. The open-circuit dc difference voltage is Vpk/2, where Vpk is the peak of the line to line voltage at the utility bus (8), when the operating firing angle alpha = 90 degrees and which corresponds to zero voltage at the dc output. At other firing angles the difference voltage is reduced as sin (alpha) so the current change may easily be determined at other firing angles.

Fig. 5a. shows the dc current in the load (IdcLoad), the voltage impressed on the load (VdcLoad) and the current contribution by each of the standard 3-phase converter bridges.

Fig. 5b. shows the line currents in the"A"phase feeders to each converter bridge. i. e. The transformer secondary line currents.

Fig. 5c. shows the primary line currents in the"A"phase of the isolating transformers.

Fig. 5d. shows the"A"phase voltage and current at the Utility bus.

Fig. 5 shows the operating voltage and currents at various points in the circuit at one operating point and it can be seen that the distortion of the current and voltage at the utility bus is very low. At other operating points the automatic voltage compensators adjust the capacitor voltage, and hence the rate of change of current, so that the waveforms of the converter output current retain the triangular form shown in Fig. 5a but with an amplitude equal to the new dc load current value with the result that all commutations in the two converters are carried out at zero current and the low distortion at the utility bus is maintained. Fig. 6 shows how the voltage on the capacitors of the automatic voltage compensators vary as the output dc voltage is varied from rated positive voltage to rated negative voltage (ie both positive and negative power flow) for full rated current (1600amps), for a mid-range value (lOOOamps) and for a low current (200amps). The distortion of the current and voltage at the utility supply is maintained at a low value throughout these operating variations by the automatic adjustment of the automatic voltage compensating circuits.

Fig. 7. Index of numbered items in Figures: 1 Standard 3-phase Converter Bridge: 2 Positive dc output terminal of standard 3-phase converter bridge.

3 Negative dc output terminal of standard 3-phase converter bridge.

4 Unidirectional current carrying valve such as Thyristor, Gate Turn-Off Thyristor (GTO), Integrated Gate-Controlled Thyristor (IGCT), or combination of devices to give phase-controlled rectifier characteristics.

5 Connections for three input supply phases to standard 3-phase converter bridge.

6 3-phase Transformer connected wye/wye with total leakage inductance per phase of Ltx 7 3-phase Transformer connected wye/delta, with total leakage inductance per phase of Ltx, to provide 30 degrees of phase-shift.

8 Utility 3-phase supply Bus: To the 12-pulse converter arrangement shown and to other users.

9 Utility supply source voltage generator with source inductance Lg per phase.

10 Automatic voltage compensation circuit.

11 Resistive component of the common dc load.

12 Inductive component of the common dc load.

13 Back emf component of the common dc load.

Claims

CLAIMS 1. A 12-pulse ac to dc controlled rectifier comprising two standard 3-phase converter bridges fed with 30 electrical degrees displaced transformer derived supplies the dc output of each 3-phase converter bridge being connected in series with an automatic voltage compensating circuit before connection to the common dc load circuit, each automatic voltage compensating circuit consisting of two series pairs of semiconductor switches connected in parallel with each other and with a capacitor, the input connection to the automatic voltage compensating circuit being the centre point of the first pair of series connected switches whilst the output connection is the centre point of the second pair of series connected switches and the switch sequence is such that the output dc current of the converter is steered in opposite directions through the capacitor in the alternate 30 degree periods defined by the firing pulses of the two converters, with the result that in the steady state the sum of the open-circuit difference voltage between the two converters plus the voltages developed on the capacitors within the two automatic voltage compensating circuits are impressed on the transformer inductances in circuit and produce, in a 30 degree period between firing pulses, a change of current equal to the magnitude of the dc load current such that the current in the next device in the converter bridges to be commutated is reduced to zero, or a low value, at the next commutation.
2. A 12-pulse ac to dc controlled rectifier as claimed in Claim 1 wherein the automatic voltage compensating circuits are only capable of supporting unidirectional voltage and each series pair of semiconductor switches consists of one plain diode and one active switch such as an IGBT or similar device with unidirectional voltage supporting properties.
3. A 24-pulse ac to dc controlled rectifier comprising two 12-pulse ac to dc controlled rectifiers as claimed in Claim 1 or Claim 2 connected in Series arrangement.
4. A 24-pulse ac to dc controlled rectifier comprising two 12-pulse ac to dc controlled rectifiers as claimed in Claim 1 or Claim 2 and connected in Parallel via either interphase transformer or sharing reactors to a common dc load.
5. Any multi-pulse ac to dc controlled rectifier in which the 12-pulse ac to dc controlled rectifier of Claim I or Claim 2 forms a constituent part.
6. A 12-pulse ac to dc controlled rectifier substantially as described herein with reference to Figures I to
7.