Technical field of the invention

The present disclosure relates to a high voltage capacitor bank that may comprise a plurality of capacitor towers. The invention further relates to a high voltage direct current (HVDC) capacitor filter arrangement for and to an electric power transmission installation for high voltage direct current comprising the capacitor filter arrangement. Background

In a capacitor bank, identical capacitors are grouped together and electrically connected in series or parallel with each other.

Capacitor banks are for example used to control the voltage that is supplied to the customer by eliminating voltage drop in a system caused by inductive reactive loads.

Another technical area where capacitor banks are used is as filter equipment in converter stations in an HVDC installation, where harmonics are generated. In order to reduce the amplitudes of these harmonics, the converter stations in an HVDC installation are provided with harmonic filters comprising capacitor banks. The filter equipment is large and space-demanding and represents a considerable part of the cost and space requirements of a converter station.

Generally, a capacitor bank comprises one or more capacitor towers. Each tower is made up of a support structure comprising substantially vertical columns of insulators which support a number of horizontal support racks at regular intervals, one above the other. Usually, a capacitor tower has four columns of insulators, but also other designs exist such as a central insulator column. On each support rack a number of capacitors are mounted, and each support rack with its capacitors is said to form a level of the capacitor tower. The number of capacitors at each level may vary and the number of levels may also vary on a case to case basis, depending on the chosen voltage level. E.g. a capacitor tower may comprise between 20 and 40 levels, and the number of capacitors at each level may be 2-8. The capacitors are electrically connected in series and in parallel. The capacitor tower is generally provided with a high voltage terminal at the top and a low voltage terminal at its base, or vice versa.

A capacitor bank for a one phase DC application may comprise one or more capacitor towers. A capacitor bank for a three phase AC application may comprise 1 , 2 or 3 towers for each phase, thus in total 1 , 3, 6 or more capacitor towers.

One consideration when designing these capacitor towers is that they must be provided with some kind of external air insulation in order to avoid corona discharges near the equipment. To this end, it is known to provide shielding in the form of corona rings. The corona rings are arranged at a distance from the tips of the bushings of the capacitors and below the capacitors for best shielding effect. Through this the tips of the bushings and the capacitors are shielded. The distance between the capacitor and the corona ring is decided by the voltage level in each case. Corona rings are usually placed at every level at the top portion of the capacitor tower, where the stress is the highest. Further down, there are corona rings at every second level and their presence becomes gradually more sparse the closer to the base you get. In Fig. 2 is shown a schematic side view of part of a capacitor bank according to prior art. The figure illustrates part of a capacitor tower and in the figure is shown five levels of capacitors 1 13 mounted on support racks 1 1 1 . Between each level there are insulators 1 15 arranged, which together form vertical columns of insulators that also have a supporting function for the support racks. At a distance from the tip 1 16 of the bushing 1 14 of the respective capacitor 1 13, there is provided a corona ring 1 17. According to prior art, the corona ring is located slightly lower than the capacitor, and the support rack that is below the capacitor is hidden by the corona ring in the view of Fig. 2. This location of the corona ring is chosen for electrical reasons, as explained above.

For high voltages, e.g. 800kV, the capacitor towers may have a height of about 20 m above ground and their mechanical structure must be designed such that they will withstand any mechanical stresses to which they may be subjected, in particular wind-load and earthquakes. However, when it is desirable to increase the voltage to ultra high voltages above 800kV, e.g. 1 100kV and even higher, the common solution is to increase the height of the capacitor towers and consequently they also become even more sensitive to wind-load and seismic load.

Examples of prior art in this technical field is found in CN 201878009U and CN 201877825U.

Summary of the invention

An object of the present disclosure is to provide a capacitor bank comprising a capacitor tower that fulfils the electrical and mechanical requirements that are necessary for high voltages, e.g. of 800kV and higher, and in particular for ultra high voltages of 1 10OkV and even higher.

The present invention provides a high voltage capacitor bank, comprising at least one capacitor tower, said capacitor tower comprising

-a plurality of support racks for supporting capacitors,

- a plurality of insulators arranged between the support racks,

- a plurality of capacitors distributed on the support racks and electrically connected, - a plurality of corona rings, located at a distance from the capacitors, characterised in that at least one corona ring is located such that an imaginary plane, extending in a direction parallel to the at least one corona ring, extends through the corona ring and through an adjacent capacitor.

Through this location of a corona ring and an adjacent capacitor in relation to each other, there will be at least a partial overlapping of the location of the corona ring and the location of the capacitor in the vertical direction, which means that the combined area of the corona ring and the capacitor that will be exposed to wind forces blowing essentially horizontally, i.e. in a direction essentially perpendicular to the vertical capacitor tower, will be smaller than the combined area of the two when they do not overlap, as in prior art when the corona ring is located underneath the capacitor. As a consequence, the wind load and the mechanical stress due to wind forces hitting the capacitor tower will be reduced as compared to prior art. This will make it possible to build higher capacitor towers, which may be used for high voltages, e.g. over 800kV, or ultra high voltages of 1 100kV and even higher. However, nothing prevents the use of the capacitor bank also for lower voltages. Generally, high voltage may be defined as 36kV and above, and the capacitor bank may be used for any high voltage installation.

Since the corona ring is normally arranged parallel to the support rack and parallel to the main direction of extension of a capacitor mounted on the support rack, this means that the imaginary plane in other words may be described as extending parallel to the capacitors and perpendicular to the capacitor tower. The capacitors, support racks and corona rings can generally be said to be horizontally oriented in relation to the vertical capacitor tower.

According to one embodiment, the capacitor tower is a vertical structure and a corona ring and an adjacent capacitor may have a respective vertical location such that the corona ring is completely overlapped by an adjacent capacitor in the vertical direction. This has the advantage of reducing the area exposed to wind load even more, since the combined area of the corona ring and the adjacent capacitor is no more than the area of the largest one of the two, which is usually the end area of the capacitor.

According to another embodiment, the imaginary plane extending through the corona ring may be a centre plane of the corona ring and this centre plane may coincide with a centre plane of an adjacent capacitor. This would result in a symmetrical arrangement of the corona ring and the adjacent capacitor, around a common plane, which is advantageous from an air flow point of view and will further reduce the wind load.

According to yet another embodiment, an upper surface of said adjacent capacitor is level with an upper tangent to the at least one corona ring, which tangent is parallel to said imaginary plane. According to one feature, the capacitors may be connected in series along two parallel circuits in the at least one capacitor tower.

According to a further feature, the capacitor bank may comprise 2-6 capacitors on each support rack. The number of capacitors may be adapted to the particular requirements for each case. E.g. there may be 2, 4 or 6 capacitors in each rack. However, even higher numbers of capacitors are foreseen.

The number of capacitor towers in the capacitor bank may vary. It is foreseen that the number of towers may be in the range from one to eight.

According to another feature, the capacitor bank may comprise at least two capacitor towers that are mechanically and electrically connected at a respective top end by means of a top structure. This will provide for a mechanically strong structure.

Generally, the capacitor bank may comprise electric terminals for connecting a top of the capacitor bank to a higher voltage than a base of the capacitor bank, or vice versa.

According to yet another feature, the capacitor bank may comprise three capacitor towers that are mechanically and electrically connected at a respective top end by means of a top structure. This provides for an even more stable capacitor bank.

The disclosed capacitor bank may be used both for AC and DC applications.

According to one embodiment, each capacitor tower may comprise 39 levels of support racks with 4 capacitors on each support rack. Such a capacitor bank is suitable for HVDC with voltages of 1 10OkV and even higher.

A corresponding embodiment for an 800kV installation may comprise 29 levels with 4 capacitors at each level.

The present disclosure further provides a high voltage direct current capacitor filter arrangement comprising a capacitor bank according to any one of the claims defining a capacitor bank.

Finally is provided an electric power transmission installation for high-voltage direct current, comprising a capacitor filter arrangement.

In the context of the present disclosure, the word capacitor should be interpreted as a capacitor including its bushing or bushings.

Further features and advantages of the invention will also become apparent from the following detailed description of embodiments.

Brief description of the drawings

A detailed description of the present invention and embodiments thereof, given as examples only, will now be made with reference to the accompanying schematic drawings, i which: Fig. 1 is a side view of an embodiment of a capacitor bank according to the present invention,

Fig. 2 is a schematic side view of part of a capacitor bank according to prior art, Fig. 3 is an enlarged side view of part of the capacitor bank shown in Fig. 1 , Fig. 4 is a top view of part of the capacitor bank shown in Fig. 1 , with the top structure removed,

Figs. 5a - 5e illustrate variants of the present invention, and

Fig. 6 is a perspective view of an embodiment of a capacitor bank according to the present invention.

Detailed description

Fig. 1 shows an embodiment of a HV capacitor bank comprising a capacitor tower 1. The capacitor bank may be used for DC or AC.

The top 4 of the capacitor tower 1 of the capacitor bank is connectable to a high voltage, e.g. 1 10OkV, and the base 6 of the tower is connectable to a lower voltage, e.g.

170kV. However, vice versa is also possible. Usually, a capacitor tower of this type is located at a certain distance above ground, as illustrated in Fig. 1 , on insulating supports stands 7, which in the illustrated example would be four, as seen in Fig. 6. The capacitor tower 1 comprises a plurality of support racks 1 1 that carry the capacitors 13, which in the illustrated case are four capacitors on each support rack, as is more clearly seen in Fig. 4. The support racks are, in the illustrated embodiment, configured as a metal frame on which the capacitors are mounted. Since the capacitor tower 1 is standing upright from the ground as a vertical structure, the support racks 11 can be described as being horizontal and extending in a direction perpendicular to the capacitor tower. The main orientation of the capacitors 13 is also horizontal and perpendicular to the vertical capacitor tower.

Each support rack 1 1 with its capacitors 13 can be described as forming a level L of the capacitor tower. Thus the capacitor tower comprises a plurality of levels, distributed at regular intervals in the vertical direction, one above the other. In the capacitor tower illustrated in Fig. 1 , the levels are simply indicated as to L _n , since the actual number n of levels is decided depending on the particular requirements of each individual installation. As an example, for an 1 10OkV capacitor bank there may be 39 levels with 4 capacitors on each level. The capacitors are arranged in pairs, and these pairs face opposite directions. In the shown example, each capacitor 13 comprises two bushings 14. In the tower, the capacitors are connected in series by means of conductors connecting said bushings, and along two parallel circuits on opposing sides of the tower. This can also be seen in Fig. 4. If the capacitor bank comprises more than one capacitor tower, as in the embodiment illustrated in Fig. 6 showing three towers 1 , 2, 3, the towers are connected in parallel, which results in six circuits in parallel with 78 capacitors each. However, also other configurations are conceivable by varying the number of capacitors at each level, varying the number of levels, etc. The number of capacitor towers may also vary depending on the requirements from case to case

Between every support rack 1 1 , there are insulators 15 arranged such that a column of insulators stretches from the base 6 of the capacitor tower to its top 4. In the illustrated embodiment there are four insulators 15 at each level and consequently the tower 1 comprises four insulator columns. The insulators have a height corresponding to the desired distance between two support racks 1 1 . These insulator columns form part of the support structure of the capacitor tower, in combination with the support racks. The support racks may be mounted on the insulators, as in the illustrated embodiment.

The capacitor bank further comprises corona rings 17 at a plurality of levels of the tower 1 . The corona rings 17 are mounted on supports connected to the support racks 1 1 , at a distance from the outermost tips 16 of the capacitor bushings 14. Contrary to prior art corona rings, the corona rings 17 according to the present invention are not placed lower than the capacitors, when seen from the side of the tower. Instead, the corona rings 17 are placed such that they at least to some extent overlap the capacitors, when seen in a horizontal direction, i.e. seen in a direction essentially perpendicular to the essentially vertical capacitor tower 1 . In other words, their vertical locations overlap. This may also be described by defining an imaginary plane A which extends in a direction that is parallel to the corona rings 17 and forming a horizontal plane, and which imaginary plane A extends through both the corona ring 17 and an adjacent capacitor 13, at a capacitor tower level provided with corona rings. This is further illustrated in Figs. 5a - 5e.

In Fig. 3, illustrating the top five levels of a capacitor tower, the corona rings 17 are illustrated as being located such that their underside is essentially level with the underside of the adjacent capacitor 13. As can be seen in Fig. 4, each corona ring 17 is arranged at a distance from the tip of two adjacent capacitors 13. As can also be seen in Fig. 4, the corona ring is made up of two ring halves. Accordingly, at each level where there are corona rings, there are two corona ring halves, one for each pair of capacitors. However, the corona rings may as well be designed as complete rings, i.e. not divided in ring halves. Further, the corona rings may have any rounded shape, and need not be circular.

At the upper portion of the capacitor tower, there are corona rings at each level, as can be seen in Fig. 1 . Then the presence of corona rings decreases gradually towards the base 6 of the capacitor tower 1 . There may be corona rings at every second level, and further down at every third level until there are no corona rings at all close to the base 6 where the stress is lesser and the risk for corona discharges has diminished. As can be seen in the very simplified figures 5a-5e, the corona rings and the capacitors may overlap to a smaller or higher degree. Generally, this may be described in terms of an imaginary plane A that extends in a direction parallel to the corona rings, which imaginary plane A extends through the corona ring 17 and through the adjacent capacitor 13. In Figs. 5a and 5e, only part of the corona ring 17 overlaps the capacitor 13 when seen in a horizontal view. In Fig. 5b, the underside 17a of the corona ring is in the same plane as the underside 13a of the capacitor, as has already been described. Consequently, the corona ring is completely overlapped by an adjacent capacitor, when seen in a side view. Naturally, this type of overlapping may also occur in relation to the upper side of the respective capacitor and the corresponding corona ring as shown in Fig. 5d, such that the upper side 17b of the corona ring is in the same plane as, i.e. level with, the upper side 13b of the adjacent capacitor. Or to be more precise, a tangent T to the upper side 17b of the corona ring is in the same plane as the upper side 13b of the capacitor. In Fig. 5b is shown how the tangent T to lower side 17a of the corona ring is in the same plane as the lower side 13a of the capacitor. In Fig. 5c, the corona ring and the capacitor are arranged symmetrically in relation to a horizontal plane, corresponding to the imaginary plane A, such that a centre plane of the capacitor coincides with a centre plane of the corona ring.

With regard to Fig. 4, it should be noted that this figure schematically illustrates the capacitor tower of Fig. 1 as seen from above. In this figure the tips 16 of two levels of capacitors 13 can be seen, namely the uppermost level of capacitors 13 with their corona ring 17, and the lowermost level of capacitors 13 whose tips protrude beyond the tips of the capacitors at the other levels, as can also be seen in Fig. 1 . The lowermost levels usually will have no corona ring. In Fig. 4 can also be seen how the capacitors are connected in series at each side of the tower, thus forming two parallel circuits.

Fig. 6 shows an embodiment of a HVDC capacitor bank comprising three capacitor towers 1 , 2, 3. All of these three capacitor towers would have the same configuration as has been described above in relation to the one capacitor tower. The three towers are mechanically connected at their tops 4 by a top structure 5 comprising a metal frame. The top structure 5 also connects the upper ends of the three towers electrically. The top structure is normally provided with one single high voltage terminal 21 that is common to the three capacitor towers. The top structure also provides for a top corona shield 27. At the base 6, the three towers are connected to a lower voltage, by a single common low voltage terminal 23. Alternatively, the top terminal can be a low voltage terminal and the base terminal can be a high voltage terminal. In a capacitor bank where the higher voltage is connected at the base, the arrangement of the corona rings would be reversed, i.e. there would be corona rings at each level close to the base and gradually fewer corona rings towards the top. The invention shall not be considered limited to the illustrated embodiments, but can be modified and altered in many ways, as realised by a person skilled in the art, without departing from the scope defined in the appended claims.