FILTER EQUIPMENT

Abstract: Filter equipment in an HVDC installation has a plurality of branches tuned to different frequencies. Each branch comprises a main capcitor (e.g. C11) and an inductor (L11), wherein the main capacitor comprises a plurality of series-connected capacitor units arranged in a substantially vertical stand. The main capacitors (C11, C13, C24, C36) for at least two branches are each arranged in a separate capacitor stack in one and the same stand. The stacks are electrically and mechanically interconnected at one end by means of connection members (MF), and otherwise separated and arranged at such a large distance from one another that the required insulation distance is obtained between the stacks.

Description: Filter equipment TECHNICAL FIELD The present invention relates to filter equipment for an installation for power transmission by means of high-voltage direct current (HVDC) . The equipment has a plurality of branches tuned to different frequencies. Each one of these branches comprises a main capacitor and an inductor. The main capacitor in each branch comprises a plurality of series-connected capacitor units arranged in a substantially vertical stand. BACKGROUND ART The converters in an HVDC installation generate harmonics, and to reduce the amplitudes of these harmonics, converter stations in an HVDC installation are provided with harmonic filters both on the ac side and on the dc side of the con- verters . The filter equipment becomes large and space- demanding and makes up a considerable part of the cost and space requirement of a converter station. Filter equipment of the kind referred to here has a plura- lity of branches, each one tuned to one or more harmonics. Figure 1 shows a typical example of a known 12-pulse single- pole converter station with converters SRI and SR2 , which via a transformer TR and series-capacitors SCI and SC2 are connected to a three-phase ac network ACN. On the ac side of the station, filter equipment FAC is connected between the network and ground. On the dc side of the station, filter equipment FDC is connected between the poles PI of the station and the neutral busbar NBB of the station. The filter equipment FAC comprises four filter branches. The branch Fll is a series circuit, tuned to the 11th harmonic (550 Hz at 12-pulse operation and 50 Hz mains frequency), with a capacitor Cll and an inductor Lll as well as a surge arrester Vll as overvoltage protection device across the inductor. The branch F13 is a series circuit, tuned to the 13th harmonic, with the capacitor C13, the inductor L13 and the surge arrester V13. The branch HP24 is a series circuit, tuned to the 24th harmonic, with the capacitor C24, the inductor L24, the resistor R24 and the surge arrester V24. The branch HP36 is a series circuit, tuned to the 36th harmonic, with the capacitor C36, the inductor L36, the resistor R36 and the surge arrester V36. The last two branches have a band width adapted such that they damp the harmonics of the orders 23, 25 and 35, 37, respectively. The resistors R24 and R36 give the branches suitable quality factors. The quality factor is chosen to be considerably lower for the branch HP36 than for the branch HP24 and so low that the branch HP36 obtains high-pass characteristic and damps also higher order harmonics, for example the harmonics of the orders 47 and 49. The filter equipment FAC is shown in the figure in the form of a single-line diagram, and each one of the branches shown in the figure therefore consists of three branches, one for each phase. The filter branches have capacitive characteristic at the fundamental frequency. At least certain of the branches are therefore often dimensioned with such large capacitors that, at the fundamental frequency, they generate a reactive power which at least partially compensates for the relatively high consumption of reactive power by a conventional converter station. Certain types of converters, for example the series-capacitor-connected converters (Capacitor Commutated Converters, CCC) shown in Figure 1, however, have a low consumption of reactive power. Then all the branches can be dimensioned only in view of the harmonic damping, and the capacitors become relatively small . The filter branches may, in a known way, be designed as double-tuned filter circuits, for example according to Figure 2 which shows a double-tuned filter for the harmonics of the orders 11 and 13. The filter has a main capacitor CA in series with an inductor LA. This series circuit is series-connected to the parallel circuit consisting of the capacitor CB and the inductor LB. The main capacitor CA is dimensioned for the desired degree of reactive-power genera- tion, and this capacitor takes up the predominant part of the line voltage at the fundamental frequency. In the single-tuned branches shown in Figure 1, one of the capacitors in each branch constitutes in a known way the main capacitor of the branch and takes up the predominant part of the line voltage at the fundamental frequency. Each one of the main capacitors of the filter branches comprises a number of series-connected (and possibly also parallel-connected) capacitor units which are arranged in a substantially vertical stand. One typical such stand con¬ sists of four insulator columns which support a number of capacitor racks disposed at regular intervals one above the other. Each rack comprises a metal frame which supports a number of series- and/or parallel-connected capacitor units and is electrically connected to the casings of the capaci¬ tor units arranged on the racks. Each capacitor unit com¬ prises a plurality of wound foil capacitors applied in a metal casing with one or more bushings. The bushings are the electrical terminals of the unit (alternatively, the casing may constitute one of the connections) . The stand has a high-voltage terminal at the top, and a low-voltage terminal at the bottom. Such a stand with the racks and capacitor units arranged therein will be referred to in the following as a capacitor stack. The filter equipment FDC arranged on the dc side of the station is designed in a way similar to that of the equip¬ ment FAC, but has its low-voltage terminal connected to the neutral busbar of the station. In filter equipment of this known kind, the main capacitors of the filter branches may be designed as separate capacitor stands. The equipment will then have the same number of stands as the filter has branches, and the number of insula¬ tors, base plates, high-voltage terminals, etc., will be high. Such equipment is therefore expensive and requires large ground space. Such equipment is especially unfavour¬ able at high operating voltages and in those cases where no or low demands are placed on the reactive-power generation. In these cases, the number of capacitor units per "storey" is low and the number of insulators, racks, etc., becomes large in relation to the quantity of capacitor units. It is also known to design the filter branches in the form of double-tuned or possibly triple-tuned branches. The number of main capacitors in the filter equipment is then reduced to a corresponding extent, as is the number of capacitor stands. Both the cost and the space requirement of the filter equipment are then reduced. A considerable dis¬ advantage, however, is that the quality factor for the different frequencies of the branch cannot be chosen inde¬ pendently of each other. In filter branches which have con- trollable inductors, an increase of the control ranges in relation to what is the case with single-tuned filter branches is required, and this results in an increased cost of the inductors. In a third embodiment, the main capacitors of two or more filter branches are arranged in the same stand, and one and the same rack thus supports the capacitor units which are included in different main capacitors. The fundamental- frequency voltage (direct voltage on the dc side) is largely uniformly distributed along the stacks, and the fundamental- frequency voltage difference between capacitor units arranged on the same rack is low. The harmonic-frequency voltage differences between such capacitor units may, however, be high. Since the casings of the capacitor units on the same storey are potentially connected via the racks, the maximally allowed voltage difference is limited by the insulation of the capacitor units between the bushing and the casing. This embodiment is, therefore, not useful at high voltages (actually, high harmonic voltages = harmonic current x impedance) . SUMMARY OF THE INVENTION The object of the invention is to provide filter equipment of the kind stated in the introductory part of the descrip¬ tion, which is simpler and less expensive and has reduced space requirement in relation to prior art equipment, while at the same time the filter branches for the different frequencies may be dimensioned (and possibly the inductors controlled) completely independently of each other. The invention is particularly aimed at achieving filter equip¬ ment which is suited for use at high voltages and in such installations where the need of generation of reactive power is 1ow. What characterizes a method and an installation according to the invention will become clear from the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described, in the following, in greater detail with reference to the accompanying Figures 1- 6. Figure 1 is a circuit diagram of a converter station with pieces of filter equipment according to the invention. Figures 2 shows is a circuit diagram of a double-tuned filter branch. Figure 3 shows the physical composition of the capacitor stand in filter equipment according to Figure 1. Figure 4 shows how the capacitor stand of the filter equipment and other components are arranged in relation to each other. Figure 5 shows a capacitor stand according to another embodiment of the invention. Figure 6 shows an alternative method of arranging the capacitor stand and other components in relation to each other. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 1 and 2 have already been described above. Figure 3 shows how, according to one embodiment of the invention, in filter equipment according to Figure 1, the four capacitor stacks of the equipment may be arranged in a common stand. Figure 3a and Figure 3b show the stand viewed in two directions perpendicular to each other. Figure 3c shows, viewed from above, a section through the stand in a plane immediately above the racks RAII4, RAI34, etc. The stand comprises the four capacitor stacks, each one constituting one of the four main capacitors Cll, C13, C24, C36 of the equipment. The stacks are all composed in a similar way, and as an example the capacitor stack C13 will be described. It has 18 series-connected capacitor units C13ι - C13i8- The units are applied two-by-two on metal frames or racks RA13ι - RAI39. The casing of each unit is electrically connected to the rack, which thus forms the electrical connection between the two capacitor units mounted on the racks. Each unit has a bushing, for example TE137 and TE138 of the units CI37 and C13i8- To the bushing of the lowermost capacitor unit C13]_, the low-voltage termi¬ nal LVT13 of the capacitor stack is connected. The units are interconnected in the vertical direction with the aid of conductors, for example the units C132 and CI33 with the aid of the conductor CLl3ι_. The bushing of the uppermost capa- citor unit C13i8 i s connected, by means of a conductor CL13g, to a rectangular metal frame MF which mechanically and electrically connects the upper ends of the four stacks. The racks of the capacitor stack C13 is supported by an in- sulator column which comprises the sections IS13ι - IS13ιo placed one above the other. The other three stacks Cll, C24 and C36 are, in the example shown, composed in exactly the same way as the stack C13 now described, and the components included in these stacks are designated in the same way as the components in the stack C13. The metal frame MF mechanically connects the upper ends of the four stacks, which gives the stand a construction which is very stable and resistant to wind forces and seismic forces. The frame MF also electrically connects the upper ends of the stacks. The frame is provided with one single high-voltage terminal HVT common to the four capacitor stacks . To increase the mechanical stability of the structure, the stacks are interconnected, at a suitable distance from their ends, with the aid of electrically insulated cross bars. In the example shown, thus, four insulators IS1 - IS4 are mounted between the racks RAII4, RAI34, RA244 and RA364. As mentioned above, during operation practically the whole fundamental-frequency voltage across each filter branch will be taken up over the main capacitor of the branch. This means that the potentials of those parts of the four stacks which are disposed in each horizontal plane will be almost identical, and no mentionable fundamental-frequency voltage differences will arise in the horizontal direction between the stacks. For the harmonics, on the other hand, the con¬ ditions are different. The harmonic-frequency voltages of the various stacks generally have different amplitude and frequency. The upper ends of the stacks are, of course, electrically connected to each other and harmonic-frequency voltage differences between the stacks, increasing from above and downwards, may thus arise during operation. These voltage differences are greatest at the lowermost part of the stacks. The maximum voltage along each stack is deter¬ mined by the product of the maximum harmonic current and the impedance of the capacitor at the harmonic frequency. Since in two different stacks these voltages may be in opposition to each other during part of the fundamental-frequency period, the maximum voltage difference between two stacks will be the sum of the above-mentioned stack voltages. The horizontal distance between the capacitor stacks is adapted such that the required insulation distance is obtained to take up the maximum voltage differences arising. It has been found that a capacitor stand according to the invention typically has moderate dimensions; for example in an installation for the operating voltage 400 kV, the stand has a horizontal extent of about 3 m x 3 m and a height of about 8 m. Figure 4a shows schematically the whole filter equipment FAC as viewed from above, and Figure 4b shows the equipment as viewed from the side. For simplicity's sake, the capacitor stand is shown as a rectangular block CS. Adjacent thereto, the other components of the equipment are placed. The surge arrester Vll and the inductor Lll are connected to the low- voltage terminal LVTn of the stack Cll. The surge arrester V13 and the inductor L13 are connected to the low-voltage terminal LVT13 of the stack C13. The surge arrester V24, the inductor L24 and the resistor R24 are connected to the low- voltage terminal LVT24 of the stack C24. The surge arrester V36, the inductor L36 and the resistor R36 are connected to the low-voltage terminal LVT36 of the stack C36. In filter equipment according to the invention, as is clear from the above, the different filter branches of the equip¬ ment are electrically separated from each other and may each one be dimensioned (and, where applicable, controlled) optimally and independently of one another. The equipment may be used without difficulty for the highest operating voltages occurring. At the same time, the number of insula¬ tors, racks, base plates, high-voltage terminals, etc., are greatly reduced compared with the corresponding prior art equipment, and this without reducing the resistance of the equipment to mechanical stresses. The ground requirement and the cost of the equipment are also considerably lower. Figure 5 shows an alternative embodiment of a capacitor stand according to the invention. The embodiment makes use of the fact that the voltage differences arising between the stacks are zero in the uppermost part of the stand and greatest at the bottom. Therefore, the stacks are placed at the greatest distance from each other at the bottom, and the stacks are inclined inwards towards each other such that the distance is lowest at the top, where the stacks are held together by the metal frame MF. As in the example described above, the capacitor stand comprises four stacks. Of these, the stacks C13 and C36 are shown in the figure. The stacks rest on four base plates which are each arranged in a corner of a square. The base plates FD13 and FD36 are shown in the figure. The stack C13 and the stack Cll (not shown) are inclined to the left in the figure and the stack C36 and the stack C24 (not shown) are inclined to the right. Further, the stacks Cll and C13 are inclined inwards towards each other, as are the stacks C24 and C36, whereby the four stacks will constitute side edges of a regular four-sided pyramid. To make possible simple mounting, the stacks are suitably designed rotatable around horizontal axes or joints in the base plates (not shown in the figure) in such a way that each stack can be mounted with the stack vertically, whereupon it is rotated to its final position and is connec¬ ted at its upper end to the other stacks. The embodiment according to Figure 5 gives increased resis¬ tance to mechanical stresses. Alternatively, the stand according to Figure 5 may be provi- ded with transverse bindings of the kind shown in Figure 3 to further increase the mechanical stability of the stand. Figure 6 shows an alternative embodiment of filter equipment according to the invention, Figure 6a shows the equipment as viewed from above and Figure 6b a perspective view of the equipment. In the same way as described above, the equipment comprises four filter branches, two branches for the 11th and 13th harmonics and two branches with high-pass charac¬ teristic tuned to the harmonics of the orders 24 and 36. As the examples described above, the capacitor stand has four capacitor stacks Cll, C13, C24 and C36, which at their upper ends are mechanically and electrically interconnected by means of a metal frame MF, which in turn is connected to a high-voltage terminal HVT. In its upper part, where the voltages are highest, the capacitor stand is provided with corona shields, for example the shield designated SH. In addition, the filter branches comprise the inductors Lll, L13, L24 and L36, the surge arresters Vll, V13, V24 and V36 as well as the resistors R24 and R36. In addition, the filter branches are connected to ground with the aid of the conductors GC11, GC13, GC24 and GC36. In these conductors, current transformers CTll, CT13, CT24 CT36 are connected for measurement of the currents in the filter branches. The equipment is surrounded by a fence F with a gate G. In the two filter branches Cll and C13 , the protective vol¬ tages of the surge arresters are higher than in the other two branches. It is therefore advantageous, as in the equip- ment according to Figure 6 , to arrange these two branches diagonally opposite to each other and utilize the larger insulation distance between the branches provided by this location of the branches. One problem with filter equipment of the kind referred to here is the often annoyingly high sound level from the capa¬ citors. It is known to reduce the sound level by arranging sound-absorbing boxes on each rack, which surround the capacitor units placed on the racks. This is a complicated and costly solution. Another method of reducing the sound level is to provide the capacitor units with internal damp¬ ing means, for example in the way described in Swedish patent application 9402178-9 (corresponding to US Patent No. 5,563,763) . In equipment according to the invention, the filter branches are completely separate. The sound level is often highest for the 11th to 13th harmonic filters. For these, the internal damping means may then be designed with a narrow band width and such a high degree of damping that additional damping measures, for example in the form of the above-mentioned sound-absorbing boxes, become superfluous. The embodiments of the invention described above are only an example. Thus, the invention may be applied also to other types of filter equipment than those described above, for example in filter equipment on the direct-voltage side, in equipment with another number of branches than four (e.g. three branches) , at other pulse numbers than 12 (e.g. 6), in other types of converters than the series-capacitor-connected converters described above. In, for example, filter equipment with three branches, the lower parts of the three capacitor stacks may be placed on base plates located in the corners of a triangle and incli¬ ned towards each other so as to form a tetrahedron. The invention provides the greatest advantages when the filter branches are single-tuned, but the invention may also be applied to equipment where one or more of the branches are double-tuned or possibly triple-tuned. It is then the main capacitors of these multiple-tuned branches that are arranged in stacks in the manner described above. In an alternative embodiment, filter equipment according to the invention may be designed with two double-tuned branches, one branch for the harmonics of the 11th and 13th orders and one branch with high-pass characteristic and tuned to the 24th and 36th harmonics. These branches then have one main capacitor each, which each form a capacitor stack. These two stacks can then be arranged to form a capacitor stand together with an auxiliary stack. The three stacks are then placed, for example, vertically and each in a respective corner of a triangle. The auxiliary stack then only serves the function of giving the stand the necessary mechanical strength and stability and may, for example, consist of an insulator column. In the event that it is made of metal, the necessary electrical insulation must be arranged between the auxiliary stack and the high-voltage terminal of the stand. Alternatively, the three stacks now mentioned may be arranged inclined towards each other so as to form a tetra¬ hedron. In the embodiments of the invention described above, in each branch the main capacitor is placed nearest to the high- voltage terminal of the branch. Even if this location is advantageous, the capacitor may, in principle, within the scope of the invention, be placed in some other location, for example nearest to the connection of the branch to ground or a neutral bar. Further, in the embodiments of the invention described above, the number of racks is the same in the different branches. Alternatively, however, the branches may have a different number of racks between themselves. The metal frame described in the embodiments described above, which mechanically and electrically connects the upper parts of the capacitor stacks, may alternatively consist of, for example, a frame of glass-fibre-reinforced plastic, which provides the mechanical connection, in combi¬ nation with copper conductors which provide the required electrical connections.

Claims: CLAIMS 1. Filter equipment for an installation for power trans¬ mission by means of high-voltage direct current (HVDC) , which equipment has a plurality of branches (Fll, F13 , HP24, HP36) tuned to different frequencies, wherein each branch (e.g. Fll) comprises a main capa¬ citor (Cll) and an inductor (Lll) , and wherein the main capacitor (e.g. Cll) in each branch comprises a plurality of series-connected capacitor units (Cllι-Clli8) arranged in a substantially vertical stand, characterized in that the main capacitors (Cll, C13 , C24, C36) for at least two branches are arranged in one and the same stand, with each main capacitor in a separate capacitor stack, the stacks are electrically and mechanically inter¬ connected at one end, and that the stacks are otherwise separated and arranged at such a large distance from one another that the required insula- tion distance is obtained between the stacks. 2. Filter equipment according to claim 1, characterized in that the upper ends of the stacks are mechanically and elec¬ trically interconnected. 3. Filter equipment according to claim 2, characterized in that the stacks are interconnected by means of a frame of metal (MF) . 4. Filter equipment according to any of claims 2-3, characterized in that the upper ends of the stacks have a common high-voltage terminal (HVT) . 5. Filter equipment according to any of the preceding claims, characterized in that electrically insulating transverse connections (IS1-IS4) between the stacks are arranged at a distance from the ends of the stacks. 6. Filter equipment according to any of the preceding claims, characterized in that stacks are arranged inclined in relation to each other in such a way that the distances between the stacks decrease in a direction towards that end where the stacks are connected to each other. 7. Filter equipment according to claim 6, characterized in that the lower ends of the stacks are arranged on base plates (FD13, FD36) at a distance from each other and that the mutual distances of the stacks decrease in a direction towards the upper ends of the stacks, where the stacks are connected to each other.

Inventors: ANDERSSON LARS, HOLMGREN TOMMY, ANDERSSON, LARS, HOLMGREN, TOMMY
Applicants: ANDERSSON LARS, ASEA BROWN BOVERI, HOLMGREN TOMMY, ANDERSSON, LARS, ASEA BROWN BOVERI AB, HOLMGREN, TOMMY
Application year: 1997
Main classification: H02M1
IPC codes: H02J3