GB2397445A

GB2397445A - Power transmission circuits

Info

Publication number: GB2397445A
Application number: GB0300702A
Authority: GB
Inventors: Carl David Barker
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 2003-01-14
Filing date: 2003-01-14
Publication date: 2004-07-21
Also published as: GB0300702D0

Abstract

A circuit topology for bipolar direct current power transmission between two alternating current electrical power systems (6), comprising two equally rated dc power transmission lines (4) of opposite polarity, each of whose opposed ends are interfaced to the alternating current electrical power systems through a bipolar converter (C) which is grounded at its dc neutral point through an earthing device (5<1>) comprising at least one reactive impedance to control converter-induced harmonic currents in stray components of the transmission lines. The reactive impedance 5<1> may be an inductor, tuned circuits of inductor and capacitor and resistor in parallel and a series connection of these tuned circuits. The converter may be series connected six pulse bridges for a HVDC link.

Description

POWER TRANSMISSION CIRCUITS

Field of the Invention

This invention relates to circuit topologies for direct current power transmission schemes which interconnect two alternating current electrical power system busbars. In particular, the invention is concerned with bipolar converter circuits in which the dc power from each end of the converter is conducted via two transmission lines of equal voltage magnitude but opposite polarity during normal operation.

Background of the Invention

In high voltage electricity transmission schemes, where arrangements are made to transmit power between two electrical power systems, one option is to establish a power system interconnection which converts ac electrical power to dc electrical power, transmits the power as direct current and voltage and then converts the do power into ac power. This gives an asynchronous interconnection.

In many known direct current power transmission schemes the power conversion from ac to dc and from do to ac is performed by one or more controlled, series or parallel connected, six-pulse bridges, also referred to as Graetz bridges. In early schemes the A. controlled switching devices were mercury arc valves. However, His old valve technology has been supplanted by semi-conductor technology. The most commonly used semi-conductor device is the "thyristor" but other controlled semi-conductor devices could be utilised in a similar fashion.

In the usual configuration of a bipolar direct current transmission scheme, shown in Figure 1, the converters C at each end of the dc transmission lines 4 comprise four series-connected six-pulse Graetz bridges 1, with two of the bridges being phase shifted by 30 electrical with respect to the other two bridges. A 30 electrical phase shift is introduced in the converters C by connecting half of the bridges 1 to a transformer arranged in star 2 and the other half of the bridges to a transformer arranged in delta 3.

An earth connection 5 is provided at the converter circuit mid-point between the two transmission lines 4 in order to maintain the two transmission lines at a minimum relative voltage potential with respect to earth. In the absence of an earth connection 5 the scheme will operate but the two transmission line voltages may reach an asymmetrical condition due to either leakage currents or transient events in the scheme operation. The converter transformers 2 are connected to an electrically common, or electrically close, ac busbar 6.

In the usual bipolar configuration, Figure 1, the current harmonics generated on the do side of the converter will be multiples of twelve times the fundamental frequency of the ac system to which the converter is connected. Such circuits include, within the main current path, a lumped inductance 7 at each converter station. This inductance is provided by discrete reactors which perform several functions. The principal one is to smooth the direct current by acting as a low impedance path to direct current but a high impedance path to harmonic current. This reactor 7 can be located in the main current paw at either one or both of the high voltage terminals of the converter station, as shown, or at the mid-point between the series connected bridges.

On the ac side of the converter, harmonic currents will be injected into the system by the converter switching operation. Again, in the usual case where the converter comprises sets of two series or parallel connected six-pulse bridges, operating with a phase shift between them of 30 electrical, then the harmonic currents generated by the converter will be multiples of twelve times the ac system fundamental frequency plus or minus one (i.e., 11, 13, 23, 25 etc). It is conventional practice to limit the magnitude of the harmonic current injected into the ac system by connecting shunt harmonic filters at the converter ac terminal busbar, or to some busbar which is electrically close.

On grounds of cost and simplicity, it would be advantageous to reduce the number of series connected bridges which form the converter and reduce or even remove the need to have a reactor in the direct current path.

Summary of the Invention

In a first aspect, the invention provides a circuit topology for bipolar direct current power transmission between two alternating current electrical power systems, comprising two equally rated do power transmission lines of opposite polarity, each of whose opposed ends are interfaced to one of the alternating current electrical power systems through a bipolar converter which is grounded at its tic neutral point through an earthing device comprising at Icast one reactive impedance, thereby to control converter-induced harmonic currents in stray components of the transmission lines.

In a second aspect, the invention provides a bipolar ac-to-dc or dc-to-ac power converter circuit topology for interfacing bipolar dc power transmission lines to an ac power system busbar, in which me converter comprises an even number of serially connected Gractz bridges, there being the same number of bridges on each side of a dc neutral point of the converter, the bridge(s) on each side of the dc neutral point being phase shifted with respect to each other, the dc neutral point being connected to ground through an earthing device comprising at least a reactive impedance, thereby to enable operation of the converter and the power transmission lines with symmetrical direct voltage.

In comparison with the prior art, placing of the reactive impedance in the converter's connection to ground enables reactance in the direct current path to be reduced or eliminated and also facilitates a reduction in the total number of Graetz bridges required, for example from four to two. However, the ambit of the invention is not restricted to one Graetz bridge on each side of the do neutral point - there may be more than one Gractz bridge on each side of the tic neutral point if the particular power transmission scheme being considered should require such.

The earthing device may comprise one or more circuit elements connected in series, the one or more circuit elements being selected from the group comprising: (a) at least one inductor, (b) at least one tuned circuit comprising an inductor and a capacitor, and (c) at least one damped tuned circuit comprising an inductor, a capacitor and a resistor.

Further aspects of the invention will be apparent from the following description and the appended claims.

Brief Description of the Drawings

Exemplary embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure l illustrates me circuit topology of a known bipolar direct current transmission scheme; Figure 2 illustrates an alternative topology for a bipolar direct current transmission scheme in accordance with the invention; and Figures 3 to 8 illustrate various alternative dc neutral earthing devices which may be used in conjunction with the circuit topology of Figure 2. .... :.

15.. :.

Detailed Description of Embodiments of the Invention In the present invention, the circuit topology of a bipolar direct current transmission scheme is modified such that the number of series connected bridges may be reduced À . by a factor of two. In the usual case of four seriesconnected six-pulse bridges, symmetrically arranged either side of the earth connection, the invention will reduce A. this to two six-pulse bridges, one either side of the dc earth connection, see Figure 2. À A Shown in Figure 2 are converters C comprising two six-pulse converter bridges l, one connected to a transformer arranged as a three-phase star 2 and the other connected to a transformer arranged as a three-phase delta 3, resulting in a 30 electrical phase shift, as in the prior art. The do current is transmitted via the transmission lines 4. An earth connection 5 is provided at the converter circuit mid-point between the two transmission lines 4 in order to maintain the two transmission lines at a minimum relative voltage potential with respect to earth. The converter transformers are connected to an electrically common, or electrically close, ac busbar 6.

In Figure 2, the two series connected six-pulse bridges symmetrically arranged around the dc neutral earth connection will each generate harmonic voltages with respect to earth which are multiples of six times the fundamental ae system frequency. These harmonic voltages will cause harmonic currents to flow in the transmission lines and due to the low impedance shunt path created by the conductor stray capacitance to earth the majority of the harmonic current will return to the converter do neutral earth connection. Due to the phase shift between the two sixpulse bridges those harmonies which are multiples of twelve (i.e., 12, 24, 36 etc) will cancel whereas those harmonies which are multiples of six but not twelve (i.e., 6, 18, 30 ete) will add.

The converters will also produce harmonies on the ae side of the system. These can be limited by the usual filtering techniques.

To limit the magnitude of the harmonic current which flows through the shunt stray components of the transmission conductor, a de neutral earthing device is provided in the form of a reactive impedance 5 i in series with the earth connection of the converter.

This impedance could be in the form of an inductor L as shown in Figure 3.

Alternatively, a tuned circuit could be employed. À.e

If the neutral earth connection incorporates a circuit element consisting merely of an À . inductance, this will be effective in holding the mean dc voltage at the midpoint at À A practically zero, but in normal operation the two six-pulse converter bridges will generate moderate voltages at harmonic orders 6, 18, 30, etc., thereby causing substantial currents at these frequencies in each de neutral earth connection circuit element. Equal halves of these currents will also flow in the two converter bridges and therefore in the two de transmission lines. This may cause unacceptable interference to nearby telephone lines and therefore a tuned circuit may be preferred.

In the case of a tuned circuit, this could be a parallel-connected inductor L and capacitor C as shown in Figure 4, or a parallel-connected inductor L, capacitor C and resistor R as shown in Figure 5, giving a damped tuned circuit. A plurality of such tuned circuits, connected in series, could also be used as part of the neutral earthing device, as shown in Figures 6 and 7.

The component values of a tuned circuit employed in the earth connection should be chosen to resonate at the 6 harmonic of local fundamental frequency in each converter. The impedance of the dc neutral earthing device will then ideally tend to infinity at the 6' harmonic, so that 60' harmonic currents in the converters and the dc transmission lines will also be small. If desired, this harmonic filtering process can be extended to the harmonics at orders 18, 30, etc., by adding further appropriately tuned circuit elements to the dc neutral earthing device, the further circuit elements being connected in series.

In fact, a combination of all of the above devices could be used, as shown purely by way of example in Figure 8, in which an inductor L, an L- C tuned circuit, and two or more L-C-R tuned circuits are connected in series.

During operation of a converter according to the invention, there should normally be zero or very small dc currents flowing in the do neutral earthing device, except briefly during major transients. This state of affairs can be obtained by appropriate operation of the converter controls, as in known arrangements. Peak transient voltages in the do neutral earthing device can be limited by surge arresters. Hence, the cost of the earthing device components will be relatively small.

The magnitude of the impedance required in the neutral earthing device is related to the magnitude of the voltage ripple as measured at the converter high voltage terminals with respect to earth. By selection of the appropriate value of neutral impedance and considering the impedance contribution of the converter transformers and the stray shunt and series components of the transmission conductor, the magnitude of the transmission line voltage ripple can be controlled. As the neutral impedance is in the earth connection, but the tic current flow is through both six-pulse bridges, the only do current that will flow through the neutral impedance is that due to imbalance of the direct current control of the bridges on either side of the dc earth connection. Hence, an advantage of the invention's power transmission circuit topology is that depending on system requirements, it is now possible to substantially reduce or even eliminate the need for discrete reactors in the direct current circuit.

In the present invention the six-pulse bridges are electrically parallel on the ac side, as in the known arrangement. In the particular case of one six-pulse bridge either side of the earth connection men, with the bridges phase shifted by 30 electrical, the major harmonic currents injected into the ac system remain multiples of twelve times the ac system fundamental frequency, plus or minus one (i.e., 11, 13, 23, 25 etc) .

Whereas the above description has focussed on the use of six-pulse converters, there is no reason why me invention could not be used in connection with other converter types, for example 24-pulse converters in a 24-pulse HVDC transmission scheme. The harmonic orders mentioned above would of course be doubled and the transformers which connect the converter bridges to the ac busbar would be configured differently, as known. Additionally, there would be a large reduction in the size of the circuit elements in the dc neutral earthing device. a

A À À Àe.e

Claims

À À t. I ::e::e CLAIMS

1. A circuit topology for bipolar direct current power transmission between two alternating current electrical power systems, comprising two equally rated do power transmission lines of opposite polarity, each of whose opposed ends are interfaced to one of the alternating current electrical power systems through a bipolar converter which is grounded at its dc neutral point through an earthing device comprising at least one reactive impedance, Hereby to control converter-induced harmonic currents in stray components of the transmission lines.

2. A bipolar ac-to-dc or dc-to-ae power converter circuit topology for interfacing bipolar de power transmission lines to an ae power system busbar, in which the converter comprises an even number of serially connected Graetz bridges, there being the same number of bridges on each side of a de neutral point of the converter, the bridge(s) on each side of the de neutral point being phase shifted with respect to each other, the de neutral point being connected to ground through an earthing device comprising at least a reactive impedance, thereby to enable operation of me converter and the power transmission lines with symmetrical direct voltage. .....

20,' :.

3. A circuit topology according to claim l or claim 2, in which there is one Graetz bridge on each side of the dc neutral point.

4. A circuit topology according to any one of claims l to 3, in which the earthing....

device comprises an inductor. À À

5. A circuit topology according to claim 4, in which the dc neutral earthing device comprises a plurality of inductors connected in series.

:} :. : 2','tt'' ', 2.''.

6. A circuit topology according to any one of claims 1 to 3, in which the earthing device comprises an inductor and a capacitor connected in parallel to form a tuned circuit.

7. A circuit topology according to claim 6, in which the dc neutral earthing device comprises a plurality of tuned circuits connected in series.

8. A circuit topology according to any one of claims 1 to 3, in which the earthing device comprises an inductor, a capacitor and a resistor connected in parallel to form a damped tuned circuit.

9. A power conversion circuit topology according to claim 8, in which the earthing device comprises a plurality of damped tuned circuits connected in series.

10. A power conversion circuit topology according to any one of claims I to 3, in which the earthing device comprises a plurality of circuit elements connected in series, the plurality of circuit elements being selected from the group comprising: (a) at least one inductor, (b) at least one tuned circuit comprising an inductor and a capacitor, and:. .. À .e

(c) at least one damped tuned circuit comprising an inductor, a capacitor and a À ..

resistor. À À À e. À À À À .