GB2383477A

GB2383477A - 12-pulse ac to dc converter with improved total harmonic distortion

Info

Publication number: GB2383477A
Application number: GB0120897A
Authority: GB
Inventors: Walter Farrer
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-08-29
Filing date: 2001-08-29
Publication date: 2003-06-25
Also published as: GB0120897D0

Abstract

The open-circuit dc difference voltage of two standard 3-phase converters fed from 30degree displaced transformer-derived supply voltages is impressed back onto the supply transformer winding inductance's by connecting the dc outputs of the two bridges in direct parallel. This allows the dc current in one bridge to increase during a 30 degree period between firing pulses whilst the other dc bridge current reduces by the same amount, during the same 30 degree period. The sum of the bridge currents remaining constant at the dc load level. Commutations in either bridge always occur when the current is at its lowest value. This leads directly to a reduction in the distortion of the current drawn from the utility supply. Careful selection of the transformer reactance allows the current at commutation to be zero at a specific designated design point of dc voltage and dc current leading to extremely low distortion of the utility current and voltage.

Description

12-Pulse ac to dc Converter with Improved Total Harmonic Distortion This invention relates to the interconnection of standard 3-phase converter bridges and the selection of the supply transformer reactance's such that significantly reduced distortion is achieved for the utility supply currents, and hence the utility voltage distortion, compared with the traditionally used connection arrangements. Careful selection of the transformer reactance for this connection allows very low distortion levels to be achieved at specific operating points.

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) is 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 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 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 line 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 a measure of 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 current ripple over what could be achieved using a single 3-phase converter bridge.

Often the two 3-phase phase-shifted supplies 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 point at which minimum harmonic distortion occurs.

Fig. 2 shows the circuit layout for a traditional parallel connected 12-pulse system using two 3-phase converter bridges and separate isolating transformers for the required 30 deg. phase-shift and an inter-phase transformer in the dc circuit to connect the bridges to the dc load circuit.

Fig. 4. shows voltage and current waveforms in different parts of the system.

Fig. 4a. 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. 4b. shows the line currents in the"A"phase feeders to each converter bridge. i. e. The transformer secondary line currents.

Fig. 4c. shows the primary line currents in the"A"phase of the isolating transformers.

Fig. 4d. shows the"A"phase voltage and current at the Utility bus.

Pulse numbers higher than 12 are often employed in very large converter systems and are often achieved by configuring more than one 12-pulse system in either series or parallel arrangements, usually via additional inter-phase transformers or sharing reactors.

The approach in all these cases has been to ensure that each converter bridge carries its respective proportion of smooth dc current and that its commutation process and current is not influenced by the presence and operation of the other bridges.

The new and novel aspect of this invention is that the dc difference voltage that exists between two 3-phase converters is used to vary the currents in the two converters during a 30 degree conduction period such that the intantaneous current in the next device to be commutated is reduced significantly below the mean dc current level whilst at the same time maintaining constant current to the dc load circuit. The result of this current reduction at commutation results in a direct reduction of current and voltage distortion at the utility supply bus.

The objective of this approach is achieved by connecting the two converter bridges in direct parallel at their dc terminals such that the currents in the two transformers can be allowed to vary throughout their conduction period without necessarily changing the value of current in the dc load circuit. The nature of the open-circuit dc difference voltage between the two bridges is practically square wave in nature at six times the utility frequency and therefore results in approximately linear current change during a 30 deg. conduction period. The result of this is that the current level in the valve to be extinguished at each commutation may be reduced in value significantly below half that of the dc load circuit current, this being the value that would exist at commutation in the traditionally connected 12-pulse bridge arrangement. This reduction of the current at commutation results in a direct reduction of total harmonic distortion (THD) due to the fact that the duration of the commutation notch is reduced pro-rata with the initial current, at the start of the commutation, in the device undergoing commutation.

As the rate of change of current in the transformer windings is strictly determined by the open-circuit dc difference voltage of the two bridges and the transformer inductance's it is possible to define the transformer reactance such that a specific current change is achieved in a 30 deg. period at a particular phase angle of triggering. Such calculations would be readily understood by those experienced in specifying transformer and converter designs. This means that at the desired dc current and operating output voltage the current in the valve to be commutated will have reached zero whilst the current in the valve in the other bridge that is continuing to conduct will be at the dc load current level at every commutation with the result that current distortion in the utility supply will be extremely low. The transformer reactance figures required to give such performance are within the achievable practical range when designed for a 12-pulse converter arrangement on the usual 50 or 60Hz utility frequency.

This deliberate intention of using the voltage of one 3-phase converter bridge to influence the current supplied by both itself and the other 3-phase converter bridge is only possible if the bridges are directly connected at their dc terminals with low impedance connections.

Furthermore it is most practical for the case of two bridges, ie a 12-pulse converter

arrangement, where the practical range of achievable transformer reactances result in appropriate current changes in the 30 degree periods.

For higher pulse numbers the required transformer reactances are generally not achievable in power transformers at the usual utility frequencies. However, as the larger power, higher pulse number, systems may be achieved using combinations of 12-pulse parallel bridge arrangements the advantages of the invention can be derived for these higher pulse number systems. Each constituent 12-pulse arrangement being arranged as described in this invention.

Whilst the utility harmonic performance at the optimum specific design point of a particular output dc voltage and dc current will be excellent the performance at other values throughout the voltage and current control range will be less good but better than that achieved with the conventional 12-pulse converter arrangement using the inter-phase transformer or sharing reactors.

According to the present invention there is provided a 12-pulse ac to dc controlled rectifier comprising two standard 3-phase converter bridges with their positive dc terminals connected together and with their negative dc terminals connected together such that the total impedance of these connection, both resistive and inductive, is significantly less than that of the series impedance of the transformer that supplies the a. c. terminals of the bridges and that the transformer inductance has been specified such that a current change in the transformer secondary line connections to the bridges that are conducting during the period of successive commutations be of the order of the operating dc load at that operating dc voltage due to the difference voltages that exist in the loop comprising the windings of the two supply transformers and the common connections at the dc terminals of the parallel bridges and the common primary phase connections of the two transformers at the utility bus.

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 use of two standard 3-phase converter bridges in the conventional known parallel arrangement to produce a 12-pulse ac to dc controlled rectifier.

Fig. 3. Shows a specific embodiment of the invention in which two standard 3-phase converter bridges (1) are connected directly together to produce a 12-pulse ac to dc controlled rectifier.

Fig. 4 Shows voltage and current waveforms in various parts of the conventional known parallel 12-pulse controlled rectifier shown in Fig. 2.

Fig. 5. Shows voltage and current waveforms in various parts of the 12-pulse controlled rectifier constructed as described in the present invention and shown in Fig. 3.

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. 3. two of the standard 3-phase converters (1) are connected in direct parallel with low impedance connections in order that the dc current from each bridge may rise and fall in alternate 30 degree periods such that the sum of the two bridge currents equals the constant dc current required by the dc load and that commutation of the valves in each bridge always takes place when the current is at its minimum. 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 provided by the source generator (9) which has internal source inductance (Lg Henries per phase). The leakage impedance of the separate transformers is selected such that the current change in each bridge in a 30 degree interval between sequential triggerings is equal to the operating dc current at the designated dc operating voltage, this current 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.

The waveforms have been shown for the optimally designed operating point where the THD is at a minimum which, for caparison purposes, is the same load and triggering conditions as displayed for the traditionally connected 12-pulse system shown in Fig. 2 and its voltage and current waveforms shown in Fig. 4. It is seen that the operation of the two systems are completely different in nature and that the new connection results in much improved harmonic performance at the utility.

Claims

CLAIMS 1. A 12-pulse ac to dc controlled rectifier comprising two standard 3-phase converter bridges with their positive dc terminals connected together and with their negative dc terminals connected together such that the total impedance of these connection, both resistive and inductive, is significantly less than that of the series impedance of the transformer that supplies the a. c. terminals of the bridges and that the transformer inductance has been specified such that a current change in the transformer secondary line connections to the bridges that are conducting during the period of successive commutations be of the order of the operating dc load at that operating dc voltage due to the difference voltages that exist in the loop comprising the windings of the two supply transformers and the common connections at the dc terminals of the parallel bridges and the common primary phase connections of the two transformers at the utility bus.
2. A 12-pulse ac to dc controlled rectifier as claimed in Claim 1 wherein the balanced voltage and the requisite 30 degrees phase-shift between the ac inputs to the two standard 3-phase converter bridges is provided by two separate transformers with other winding arrangements.
3. A 12-pulse ac to dc controlled rectifier as claimed in Claim 1 and Claim 2 wherein the two bridge supply transformers are replaced with a dual secondary transformer with the required balanced voltage and the requisite 30 degrees of phase-shift.
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 or Claim 3 connected in Series arrangement.
5. 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 or Claim 3 and connected in Parallel via either interphase transformer or sharing reactors to a common dc load.
6. Any multi-pulse ac to dc controlled rectifier in which the 12-pulse ac to dc controlled rectifier of Claim 1 or Claim 2 or Claim 3 forms a constituent part.
7. A 12-pulse ac to dc controlled rectifier substantially as described herein with reference to Figures 1 to 6.