GB2242792A - Reactive power generator - Google Patents

Abstract

A static VAr generator for use in controlling reactive power flow in an a.c. power system, and comprising a 6-pulse voltage-commutated inverter bridge 8 loaded at its d.c. output by a capacitor bank 4, and coupled by inductive components 7 to a three-phase a.c. supply 2. In the bridge 8 there is provided a pair of columns 9, 10; 11,12; 13, 14 of switches such as gate turn-off (GTO) thyristors for each phase A, B, C. The thyristors in each pair of columns (for example, for phase A: T1P, T1Q and T4P, T4Q) are controlled so as to conduct at intervals which are staggered symmetrically in phase with respect to the a.c. cycle by an angle theta . The resulting back-emf (eA) comprises a stepped triangular waveform having a considerably lower harmonic content than the usual square wave. Suitable choice of the angle theta enables one harmonic to be eliminated completely or two harmonics to be substantially reduced in magnitude. The use of additional pairs of GTO columns for each phase, each pair having a different stagger angle, offers scope for further reduction in the distortion of the a.c. supply due to harmonic current flow. <IMAGE>

Description

REACTIVE POWER GENERATORS This invention relates to reactive power generators of the type known as static VAr (volt-amp reactive) generators for use in controlling reactive power flow in an a.c. power system. Where the required reactive power is capacitive, such generators generate from the a.c. supply a d.c output, to which is connected a capacitive load. The a.c supply is often a three-phase supply, but this is not essential and the Invention is not so limited.

One known reactive power generator of this type, shown in Figure 1, comprises a 6-pulse voltage-commutated inverter (Graetz bridge) 1 coupled to a three-phase a.c. supply by means of bus bars 2 and, for example, a star-star three-phase transformer 3. A large capacitor bank, shown as a capacitor 4 connected at the d.c. output of the inverter bridge forms the only load and 'provides' the reactive power.

The bridge circuit 1 comprises six gate turn-off (GTO) thyristors, T1 to T6, having respective reverse parallel diodes, D1 to D6. The thyristors are arranged in three palrs or 'columns' (T1,T4; T3,T6; T5,T2), the junction of each pair being connected to a respective phase of the transformer 3. Control circuits 5 provide respective gate control signals, g1 to 96, to switch the thyristors on and off for selected intervals in a timed relationship with the a.c. supply voltage cycle to produce the required d.c.

output voltage Vdc.

Figure 2 shows typical waveforms for the generator in normal operation: (a) the three primary (bus bar) phase voltages, vA1, vB1, vCl; (b) the six gate control voltage signals, g1 to g6; (c) the three secondary phase voltages, vA2, vB2, vC2 (measured with respect to a reference potential at a point mid-way between the terminals of capacitor 4); and (d) one secondaryvphase current, SA2 the other two phase currents, 1B2 and 102, having the same waveform shape but differing in phase from the current shown by 1200 and 240 electrical.

As can be seen from Figure 2(b), the gate control signals for the pair of GTOs In each column do not overlap, and in fact are complementary, so that, for ideal switching devices, only one device can conduct at any one time. This is essential to avoid shortcircuiting of the capacitor bank.

Ideally the current drawn from each phase of the supply would be a pure sinewave of the same fundamental frequency as the a.c. phase voltage. However, in addition to the desired fundamental component, each phase current also contains unwanted harmonic components. While the second and third harmonics (and multiples) are inherently absent, the presence of a.c. harmonic currents of order 5,7,11,13,17,19, etc. often causes unacceptable distortion of the a.c. bus bar voltages.

For practical purposes, the magnitude of each harmonic current component for each phase may be taken to be equal to the same component in the back-emf presented by the bridge in that phase, divided by the corresponding commutation reactance (which is practically the same as the transformer reactance). For the circuit shown in Figure 1, the back-emf In each phase has a square waveform, as, for example, vA2 in Figure 2(c).

Known methods of reducing harmonic distortion of the bus bar voltages include: (i) adding a shunt-connected a.c. filter, as denoted 6 in Figure 1.

(ii) using a 12-pulse arrangement of two Graetz bridges, connected respectively to star and delta secondaries on the a.c. transformer. This arrangement produces cancellation of the effects of harmonics of order 5,7,17,19, etc., enabling a limited reduction in the size of the a.c.

filter. Arrangements using a higher pulse number, such as 18 or 24, are also possible but require a more elaborate and more expensive transformer.

(iii) using 'mark-space' techniques to control each GTO so that it is switched on and off at several selected times within the a.c. cycle. Selected harmonics can be reduced in this way, but high losses in the GTO snubber circuits result.

It is an object of the invention to provide a reactive power generator which produces lower harmonic currents in the a.c. supply than known generators.

According to the invention, there is provided a reactive power generator comprising a bridge circuit having an input for connection to an a.c. supply and a d.c. output connected to a capacitive load, the bridge circuit comprising two or more columns, each column comprising series-connected switching means and having a junction connected to a common phase of the a.c. supply by coupling means permitting different junction potentials, the generator further comprising means for controlling the switching means to conduct during intervals which are staggered from column to column so as to tend to suppress one or more harmonics in the phase current.

Preferably the conduction intervals are staggered symmetrically about a phase reference derived from the a.c. supply phase voltage.

The coupling means may comprise a respective inductor for each column.

Alternatively, the coupling means may include a respective auto-transformer for each pair of columns having symmetrically staggered conduction intervals, or may comprise a respective transformer for each column.

In a preferred embodiment of the invention, the coupling means comprises a single transformer common to all the columns and having a core comprising a respective limb for each column.

The switching means may comprise gate turn-off thyristors.

In accordance with one aspect of the invention, a reactive power generator for connection to a multi-phase a.c. supply, comprises, in respect of each phase, a generator as aforesaid, wherein the outputs of the generators are connected in parallel to a common capacitive load and each coupling means is coupled to a respective phase of the a.c. supply.

Two reactive power generators in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 relates to a known design of generator described above and Figure 2 shows some of its waveforms; Figure 3 is a circuit diagram of one generator in accordance with the invention for use with a three-phase a.c. supply and Figure 4 shows some of its waveforms for one phase of the supply; Figure 5 is a circuit diagram of another generator in accordance with the invention for use with a single-phase a.c.

supply and Figure 6 shows some of its waveforms; Figure 7 explains the derivation of the back-emf waveform in the circuit of Figure 5; and Figures 8 and 9 show two arrangements for coupling a bridge circuit to an a.c. transformer and suitable for use in a generator in accordance with the invention.

Figure 3 is a simplified circuit diagram of one generator in accordance with the invention. Components which are identical or similar to those in the circuit of Figure 1 have been given the same reference numerals. Bridge circuit 8 comprises twelve thyristors (GTOs), the switching of which is controlled by control circuits 5, which generate the required gate control signals etc. etc. as described below. Each GTO has a reverse parallel-connected diode as in Figure 1, but these have been omitted from the diagram for clarity. Capacitor bank 4 constitutes the output load on the bridge 8.

The GTOs in the bridge are still grouped in columns (9 to 14), but there is now more than one column for each phase of the a.c. supply. In this example, there are two columns for each phase, each column being connected to the transformer secondary by a coupling circuit 7. For example, the GTOs Tlp,T1Q,T4p,T4Q constitute the two columns (9,10) coupled to the phase A secondary voltage vA2. In this example, the coupling component 7 comprises two Inductors LAp and LAQ (assumed equal in this example) connected commonly at one end to the transformer secondary and connected respectively at their other ends to the GTO junctions in columns 9 and 10. The arrangement for and operation of the other two phases, B and C, is identical, so that only the operation of phase A will be described in detail.

In principle It is possible to provide synchronous gating signals to columns 9,10 so that the GTOs operate effectively in parallel, the inductors LAP and LAQ acting to force substantially equal current sharing between the two columns. However, In accordance with the invention a different method of gating is used.

Figure 4 shows a number of waveforms (for phase A only) related to the Figure 3 circuit ((b),(c),(d)) and their timed relationship with the primary voltage vA1 ((a)).

As previously, the gate control signals for the pair of GTOs in each column (for example g1p,g4p) are complementary so that, for ideal devices, only one device in the column can conduct at any one time. However, the gate signals for corresponding devices in columns 9 and 10, I.e. g1p and 91Q for Tlp and T1Q respectively (and similarly 9qp and 94Q for T4p and T4Q respectively) are staggered so that the conduction intervals for such corresponding devices are not coterminous.In this example, the gate signals g1p and 91Q are staggered such that the conduction invervals of GTOs Tlp and T1Q are staggered symmetrically about the positive half-cycle of the primary voltage waveform vA1. Thus, glp leads the primary voltage by angle 0, while 91Q lags the primary voltage by angle 6. The gate signals g4p and 94Q are similarly staggered. The GTO columns 11,12 for phase B and 13,14 for phase C are controlled in the same manner.

The waveform for the secondary phase voltage vAp at the GTO junction in column 9 is a square wave synchronlsed with the gate signals glp and g4p since these signals effectively short the junction point to the positive and negative d.c. output lines of the bridge alternately. The remaining junction point voltages VAQ, etc. are similarly square waves synchronised with their respective gate signals. The back-emf, eA presented by the bridge 8 to the transformer is derived as the average of the junction voltages vAP and VAQ by the circuit 7 in the manner of a potential divider, and thus comprises half the sum of vAp and vAQ, i.e. a stepped waveform having three levels (Figure 4(d)).The corresponding harmonic current for phase A comprises the harmonic component of back-emf eA divided by the total commutation inductance, which may be taken to be the transformer inductance plus the parallel combination of the coupling inductors LAp and LAQ.

The inductors LAp and LAQ provide the essential isolation between the junction points in columns 9 and 10, since the conduction intervals of, for example, Tlp and T4Q now overlap for a short period in each cycle of the a.c., effectively connecting the charged capacitor bank across the two junction points.

It can be shown by Fourier analysis that the magnitude of the nth harmonic component of the back-emf eA is equal to that in the Figure 1 circuit multiplied by a factor cos ne. The harmonic currents are thus similarly affected. The angle O can be set at any required value by the thyristor control circuits 5. The magnitude of any one harmonic current of order n can be reduced to zero by choosing O so that it equals 90"/n. For example, to eliminate the fifth harmonic (n=5), angle 6 is made 18".

Alternatively, the magnitude of two harmonics may be reduced substantially by choosing a compromise value of e. For example, to eliminate the seventh harmonic completely requires e to be approximately 130. For this value of 6, the fifth harmonic is reduced by a factor 0.43. However, if 6 is made 15 , both the fifth and seventh harmonics may be reduced in magnitude by a factor 0.26, i.e. to 26% of the magnitude they have in the Figure 1 circuit.

The principle may be extended to reduce the magnitude of a greater number of harmonic current components, by providing the GTO bridge with a greater number of parallel-connected GTO columns for each phase, with each column coupled to the transformer by a respective inductor. Iftdifferent stagger angles 6 are used for each pair of GTO columns coupled to any one phase of the a.c.

supply, then the back-emf waveform will comprise a greater number of 'steps'. It will be appreciated that as the number of steps in the back-emf waveform increases, it resembles more closely the ideal sinusoidal shape and the harmonic content decreases.

Although the generator described above with reference to Figure 3 operates from a three-phase supply, the invention is not so limited. The method of gating used to control the GTOs in each phase is clearly applicable to a single-phase bridge circuit, connected either to a single-phase a.c. supply, or to one phase of a three-phase supply. In the latter case, two further bridge circuits connected to the other phases and independently controlled are required. As an example, Figure 5 illustrates one single-phase generator in accordance with the invention.

The circuit shown in Figure 5 comprises four GTO columns (15,16,17,18) coupled by respective inductors (Lp,LQ,LR,LS) to the secondary output vA2 of a transformer (not shown) connected to a single-phase a.c. supply. The diodes across the GTOs have again been omitted for clarity. The circuit can be extended to form a three-phase generator of the type described above with reference to Figure 3.

Figure 6 shows a number of waveforms related to the Figure 5 circuit: (a) primary voltage, vA1; (b) gate control signals, 91Ps91Qs91Rs91SX for the upper GTOs in each of the columns 15,16,17,18 respectively; (c) junction voltages, Vp,vQ,vR,vS, at each GTO column; and (d) back-emf waveform, eA.

The GTO columns in Figure 5 may be viewed as grouped in two pairs : 15,16 and 17,18. The gate control signals for the GTOs in columns 15 and 16 are staggered symmetrically with respect to the phase of the primary voltage vA1 by angles " and + 1 respectively, i.e. g1p leads and 91Q lags vA1 by angle 8i. The gate control signals for the GTOs in columns 17 and 18 are staggered symmetrically with respect to the phase of the primary voltage vAl by angles -02 and +02 respectively, i.e. glR leads and g1S lags vAl by angle 02. This arrangement produces at the transformer secondary a five-level stepped back-emf waveform, eA (Figure 6(d)).

The five voltage levels in the waveform ((e),(f),(g),(h),(j)) are determined by the relative values of the inductors Lp,LQ,LR,LS, which effectively form a potential divider connected between the positive and negative d.c. output lines of the bridge. Figure 7 illustrates the effective circuits which give rise to the back-emf voltage step levels (e) to (j) shown in Figure 6(d). Each of the inductors Lp,LQ,LR,LS is considered to be connected to either the positive d.c. output line (+Vdc/2) or the negative d.c. output line (-Vdc12) of the bridge, according to which GTO in its column is switched on for the duration of the step.

If the parallel combination of inductors Lp and LR has the same value as the parallel combination of inductors LQ and Ls, then the step (g) (Figure 6(d)) will be at OV. Further, if Lp=LQ and LR=LS, then the steps (f) and (h) (Figure 6(d)) will occur at a fixed proportion g of Vdc/2 above and below the OV level. It can be easily shown that in this case:

Fourier analysis gives the harmonic reduction factor in this case as: Eg cos n61 + (1-g) cos nO2] (1) and if the four inductors are equal, g = 0.5.

Since the values of three variables (gs 1s62) can now be chosen, it is possible to eliminate three harmonics.

Alternatively, two harmonics may be eliminated and a further two reduced In magnitude.

In the case of a single-phase bridge circuit the a.c.

harmonics produced are of order 3, 5, 7, 9, etc. The lower order harmonics are most suitably eliminated or reduced in magnitude by the staggering technique of the present invention since they have significant amplitude. The higher order harmonics have lower amplitude and may be suppressed by the use of conventional tuned filters.

To eliminate the harmonics of order 3, 5 and 7 in the single-phase bridge of Figure 5, equation (1) above is set equal to zero for n = 3, 5 and 7. Solving the three simultaneous equations gives: g = 0.618 81 = 18C 82 = 540 The relative values of the four inductors may then be chosen to give the required value of g.

The use of a greater number of GTO columns offers scope for further reduction in the distortion of the a.c. supply due to harmonic currents. For example, in a bridge having six GTO columns per phase, in which three stagger angles joe1, +62, +83 are used, the harmonic reduction factor becomes: 91 cos n01 + (g2-g1) cos ni2 + (1-92) cos n33 (2) where g1 and 92 are dependent, as g previously, upon the relative values of the coupling inductors and determine the 'intermediate' step-levels in the back-emf waveform.

Equation (2) gives a maximum of five degrees of freedom to reject particular harmonics. If the six coupling inductors have equal values, then g1 = 0.333 and 92 = 0.667. However, the use of as many as six GTO columns per phase is not preferred since it calls for an undesirably high current rating of the a.c. transformer.

Whilst 'symmetrical' staggering of the gate control signals, i.e. where theslead of 91P equals the lag of glQ' for each pair of GTO columns, as described in the two examples above, is preferred, complete symmetry of the step-level transitions in the back-emf waveform is not essential to achieve a reduction in the harmonic currents. For this reason, it is possible also to realise a bridge circuit in accordance with the invention which has an odd number (other than one) of GTO columns per phase.

In all cases, the waveform transitions must be symmetrical about the 900 and 2700 points of the a.c. cycle, otherwise even order harmonics will be produced. However, the waveform transitions may be skew-symmetrical about the 0 and 1800 points of the a.c.

cycle. In the case of an even number of GTO columns the back-emf level will not change within + 1 of the 0 and 1800 points, whereas in the case of an odd number of GTO columns there will be a transition at these points.

The ability to tailor the back-emf waveform provides a significant advantage in that the substantial reduction in the harmonic currents generated by the GTO bridge permits the use of a smaller a.c. filter than would otherwise be possible. In some applications, the filter may be omitted altogether.

Advantageously, in a generator in accordance with the invention, each GTO in the bridge circuit experiences only two switching transitions within each cycle of the a.c. waveform, achieved by switching at half cycle intervals. This aspect of the invention substantially reduces the losses which occur in the GTO snubber circuits. A further advantage is derived from the operation of several GTOs effectively in parallel, which avoids the problems of matching the stored-charge characteristics which arise in generators where series-connection of devices is needed to achieve a particular VA rating.

It is possible to omit the a.c. transformer and connect the bridge to the a.c. supply via the coupling Inductors, in which case the inductors provide the necessary commutation inductance.

If two three-phase generators in accordance with the invention are connected in the known 12-pulse arrangement, i.e.

connected respectively 'to star and delta secondaries of the a.c.

transformer, it is known that the harmonics of orders 5,7,17,19, etc. will vanish. The GTO columns in the two bridges may then be controlled, as described previously, to eliminate and/or reduce the remaining harmonics, i.e. those of orders 11,13,23,25, etc.

Alternative arrangements for coupling the bridge circuit to the a.c. transformer will now be described with reference to Figures 8 and 9. The arrangements to be described are specifically intended for use with a bridge circuit having four GTO columns per phase, but they can be extended or otherwise modified for use where there is a different number of GTO columns.

The coupling arrangement shown in Figure 8 comprises three auto-transformers 19,20,21, i.e. transformers comprising a single, but tapped, winding. If the tapping points on each of the three transformers are at the winding mid-points, then the back-emf will have the form shown in Figure 6(d), i.e. like that produced when the four inductors Lp ,LP,LR and LS (Figure 5) have the same value.

Alternatively, if the transformer tappings are made 'off-centre', then the height of the steps In the back-emf waveform may be non-uniform, the arrangement then being equivalent to four inductors of not all the same value. However, the use of four equal inductors is preferred; giving a 'smoother' back-emf waveform.

The use of auto-transformers gives a substantial reduction in the effective circulating currents between GTO columns, and hence in the required ratings of the GTOs and their associated diodes.

Figure 9 shows a further coupling arrangement In which the component 22 comprises effectively four identical transformers (23,24,25,26), each having a turns ratio N2/N1=4. This arrangement is equivalent to the four equal inductors In the Figure 5 circuit.

The back-emf waveform can be tailored by making the turns ratios of the four transformers not all the same, the arrangement then being equivalent to four inductors of not all the same value.

The transformers 23, 24, 25, 26 may be wound on a common core having four limbs, one for each column, so that component 22 comprises a single transformer common to all the columns, with the attendant advantages of compactness and a reduced total iron volume.

Whereas in the described embodiments gate turn-off (GTO) thyristors are used, it will be appreciated that the invention is not so limited and other types of electronic or opto-electronic switching devices may be used if preferred.

Claims (12)

1. A reactive power generator comprising a bridge circuit having an input for connection to an a.c. supply and a d.c. output connected to a capacitive load, the bridge circuit comprising two or more columns, each column comprising series-connected switching means and having a junction connected to a common phase of said a.c.

supply by coupling means permitting different junction potentials, the generator further comprising means for controlling said switching means to conduct during intervals which are staggered from column to column so as to tend to suppress one or more harmonics in the phase current.

2. A generator according to Claim 1, wherein the conduction intervals are staggered symmetrically about a phase reference derived from the a.c. supply phase voltage.

3. A generator according to Claim 1 or Claim 2, wherein said coupling means comprises a respective inductor for each column.

4. A generator according to Claim 2, wherein said coupling means includes a respective auto-transforl-ner for each pair of columns having symmetrically staggered conduction intervals.

5. A generator according to Claim 1 or Claim 2, wherein said coupling means comprises a respective transformer for each column.

6. A generator according to Claim 1 or Claim 2, wherein said coupling means comprises a single transformer common to said columns, the single transformer having a core comprising a respective limb for each column.

7. A generator according to any preceding claim, wherein said switching means comprise gate turn-off thyristors.

8. A reactive power generator for connection to a multi-phase a.c. supply, comprising, in respect of each phase, a generator in accordance with any preceding claim, wherein the outputs of the generators are connected in parallel to a common capacitive load and each said coupling means is coupled to a respective phase of the a.c. supply.

9. A three-phase reactive power generator according to Claim 8, further Including a three-phase transformer for coupling said generators to said a.c. supply.

10. A reactive power generator for connection to a three-phase a.c. supply, substantially as hereinbefore described with reference to Figure 3 and Figure 4 of the accompanying drawings.

11. A reactive power generator for connection to a single-phase a.c. supply, substantially as hereinbefore described with reference to Figure 5 and Figure 6 of the accompanying drawings.

12. A reactive power generator according to Claim 10 or Claim 11, with further reference to Figure 8 or Figure 9 of the accompanying drawings.