FIELD OF INVENTION The present invention relates to a shielding arrangement for a piece of high voltage equipment as well as to a converter station comprising a converter and such a shielding arrangement.

BACKGROUND

Electrical power transmission can be accomplished by high-voltage direct current (HVDC) and HVDC is in many cases a preferred option over AC transmission. In electrical plants there may exist a need for performing a conversion from HVDC to three-phase AC, and vice versa. A HVDC power converter comprises a number of valves, which are key components of a converter station, and the valves are typically accommodated in a valve hall. When designing a valve hall several considerations have to be taken into account. The security aspects are very important and require the valve hall to have some minimum space dimensions. For example, the air clearance between a power converter and the walls and ceiling of the valve hall within which it resides should in some cases be up to about ten meters and in others only a few meters. The dimensions of the valve hall are highly dependent on the voltage levels of the electrical power distribution network. The higher the voltage, the more distance to the surroundings is generally needed. The dimensions of the valve hall are determined by the intended application, the design of the valve structure and the adjacent structures, among other factors. However, in contrast to this, there is also a desire for the valve halls to be as small as possible. Land space is often scarce and expen sive and there is therefore a desire to keep the size of the valve halls down . Further, different countries stipulate different regulations and in some countries building permits may be difficult to obtain . Further yet, also aesthetic aspects make it more desirable to provide small and compact sub-stations, so that they affect the environment to as little extent as possible. The investment and in stallation costs, including for example material costs and labor costs, may in some countries be high and thus further yet adds to the desire to minimize the size the valve hall.

The reliability, security and safety in a high voltage application such as a power converter is of great concern . Hazards in connection with power transformers comprise for example electric discharges; power failures due to high-inten sity electric arcs may black out very large areas and are expen sive for the power companies. Protection measures, either passive or active, are therefore crucially important.

Some measures have been made in improving the high voltage assemblies for in stance with a view to increasing security and enabling designing valve halls of smaller size, without lessening the reliability requirements.

Some of these measures may be based on the use of shielding

arrangements. Shields have the function of smoothening out the electrical field around the equipment. Thereby, shields reduce the risk of corona discharges as well as the risk of electrical breakdown of the equipment.

In stead of connecting the screen directly to the HV equipment, as commonly done, one initiative is described in US 2009/ 0266605, where a screen design is used having a resistor connected between a valve and a corresponding screen . Another structure is disclosed in WO 2009/ 150 100 , where a screen design comprises a support element realized as a resistive element connected between a high voltage apparatus and a corona shield. Although the above described solution s are effective in reducing the distance between the equipment and the enclosure, there is still room for improvements.

SUMMARY OF THE INVENTION

One object of the present invention is therefore to reduce the distance between a neighboring object and a piece of the high voltage equipment.

This object is according to a first aspect achieved through a shielding arrangement for a piece of high voltage equipment spaced from a neighboring object, where the piece of high voltage equipment has a first electric potential and the neighboring object has a second electric potential, said shielding arrangement comprising a first and second shield element and at least one resistive element, where the first shield element is placed adjacent said piece of high voltage equipment and is separated from the second shield element by a gap , and the resistive element interconnects the first and second shield elements across the gap .

The object is according to a second aspect also achieved through a converter station comprising a converter for converting between alternating current and direct current and being enclosed in an enclosure. The converter comprises a number of converter valves, where at least one is provided with a shielding arrangement according to the first aspect and where the valve has a first electric potential and the enclosure is the neighboring object having the second electric potential.

The invention has a number of advantages. It allows the distance between the piece of high voltage equipment and the neighbouring object to be reduced. Thereby the space surrounding the high voltage equipment may be more efficiently used. Moreover, this is done through further increasing the voltage withstand capability. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will in the following be described with reference being made to the accompanying drawings, where fig. 1 schematically shows a grounded enclosure in the form of a valve hall comprising high voltage equipment in the form of a converter,

fig. 2 schematically shows a valve of the converter being shielded by a shielding arrangement according to a first embodiment, and

fig. 3 schematically shows the valve being shielded by a shielding arrangement according to a second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The present invention concern s high voltage equipment in high power applications. The high voltage equipment may be a High Voltage Direct Current (HVDC) equipment operating at a high voltage such as at 320 kV and above. The equipment may furthermore be enclosed in an enclosure that has a different electric potential than the voltage at which the piece of equipment operates. It is for in stance possible that the enclosure is grounded while the piece of equipment may operate at a voltage level of +

HO OkV or - 1100 kV or some high voltage therebetween . The equipment may for instance be a converter, converting between AC and DC such as a current source converter (CSC) or a voltage source converter (VSC) .

Moreover, a voltage source converter may be provided as a modular multilevel converter (MMC), where a number of cascaded converter submodules are used for forming an AC waveshape. Fig. 1 schematically shows one such exemplifying HVDC converter 10 enclosed in an enclosure 12, which enclosure in this case is a building in the form of a valve hall comprising grounded walls, i.e. walls having an electric zero potential. The enclosure is one example of an object close to at least one piece of the high voltage equipment.

The converter 10 comprises a number of valves. In the example given in fig. 1 the HVDC converter 10 is illustrated as comprising four valves 14, 16, 18 and 20. As an example, the valves may be in stalled hanging from the valve hall ceiling and fixed to the ceiling via suspending in sulators 26. It should be realized that the way that the valves are being placed in the enclosure is not central and that they may as an exemplifying alternative be placed on a supporting structure on the valve floor. There is also a shield structure comprising a plurality of shielding arrangements 22 for providing shielding against corona discharges. The shielding arrangements are provided for covering exposed surfaces of the valves in order to avoid possible corona discharges or electrical breakdown between the enclosure 12 and the valves 14, 16, 18 and 20 . The

exemplifying HVDC valves 14, 16, 18 and 20 shown in figure 1 comprise in total nine sides with exposed surfaces being protected by shielding arrangements 22, eight lateral sides facing each other at opposing surfaces and one lower side facing away from the ceiling. Each such exposed surface is thus protected by a shielding arrangement against corona discharges and electrical breakdown from the valve to the enclosure, such as to a wall or to the floor. In the figure also the distance d between one such shielding arrangement 22 and the wall 12 is indicated. Each of the valves 14, 16, 18 and 20 may be made up of a number of series- connected switches, or as a number of cascaded submodules, where such a switch may be made up of a switching element like an In sulated Gate Bipolar Tran sistor (IGBT) or Integrated Gate-Commutated Thyristor (IGCT) with anti-parallel diode. A submodule may be realized as one or two strings of switches, where each string is connected in parallel with an energy storages element such as a capacitor or a battery. Moreover, the voltage at which a valve operates, for instance the DC level of the valve, may be con siderable. Also overvoltages due to lightning strikes and switching events in the system are critical for the insulation . At least one of the outermost valves 14 and 20 at each end of the structure may thus have a con siderable voltage potential difference between itself and the enclosure.

The shielding arrangements 22 comprise shield elements in the form of screen s at a distance from the valve element. A screen may also have shape that stretches around any edges or corners of the physical valve shape.

Such screen s may be necessary in order to protect a part of the converter 10 , such as a valve, from any corona discharge and electrical breakdown from the HV part to the enclosure. As stated earlier, the voltages are high and therefore the distance d between the screen arrangement 22 and the enclosure 12 normally has to be high in order to safeguard that no corona discharges or electrical breakdown occurs. Aspects of the present invention are directed towards providing a screen arrangement that allows smaller distances d between the arrangement and the wall and thus allows a reduction of the size of the enclosure.

One way of reducing the distance is through the shielding arrangement comprising a breakdown inhibiting resistance, for in stance in the form of a breakdown inhibiting resistor. A breakdown inhibiting resistor acts as a current limitation device during a partial electrical discharge. As a result, the voltage withstand level of the valve arrangement is increased which in turn allows a more compact design . Through the use of such a resistor the impulse breakdown strength of the shielding arrangement may be increased. Further, increased DC withstand levels can also be

accomplished. Improved safety in an HVDC valve hall can thereby be provided, without increasing the size of the valve hall within which the valves are accommodated. Thereby also the operation reliability of the valve can be greatly improved.

The problem with a breakdown inhibiting resistor is that a relatively high withstand voltage is needed also between the two screens. Accordingly, the design of such screen system needs to incorporate the electric stress around such resistors and the electrical field between the shields.

Aspects of the invention provide a feasible design in which such a breakdown inhibiting resistance can be implemented either through at least one resistive element in the form of spacers or through a foam .

A first embodiment of a shielding arrangement 22 comprising a

breakdown inhibiting resistance is schematically shown in fig. 2.

Fig. 2 schematically shows a shielding arrangement that comprises a first shield element or screen 26 adjacent and in electrical contact with the valve 18 and a second shield element or screen 28 separated from the first shielding element or screen 26 by a gap G. In the figure the gap G is bridged by two spacers 30 and 32, which spacers are resistive and thus form a resistive connection between the two shields 26 and 28 . The spacers thus form two resistive elements connected in parallel between the shield elements 26 and 28 . These spacers have the dual purpose of providing mechanical support and stability and the breakdown inhibiting resistance between the shielding elements. Moreover, it can be seen that the second shield element 28 is aligned with and covers the first shield element 26 along the whole length of the first shield element 26. It can also be seen that in case the valve 18 comprises a corner, for instance between a vertical and horizontal surface, then the first and second shield elements each comprise a curved section aligned with each other for shielding and covering this corner.

The spacers 30 and 32 may be made of a non-conducting polymer, such as polyethylene (PE) or polyurethane (PU) comprising a conducting filler such as carbon black. Alternatively they may be realized through a semiconducting material such as Silicon (Si) or Gallium Arsenide (GaAs) . Thereby a resistance formed by the two parallel spacers is provided between the two shielding elements 26 and 28 . This is combined with providing mechanical support for the structure. It may here be realized that it is possible with more spacers in the gap G, which gap may otherwise be an air gap or it may be filled by a dielectric surrounding the spacers.

A second embodiment of the shielding arrangement is shown in fig. 3. The shielding elements 26 and 28 and the gap G are in this embodiment provided in the same way as in the first embodiment. However, in this case the gap G is filled by foam 34 that is used to attach the shields 26 and 28 to each other. The foam 34 may in this case also be of a semi-conductive material or a non -conducting polymer comprising particles of conductive or semi-conductive material. As an alternative to a foam a solid material may be used also in this second embodiment.

The suggested screen design with integrated resistors would enable significant reduction of the cost of large HVDC in stallations such as valve and DC halls.

Moreover, the screen design aims at reducing the air clearance d between a valve and the valve hall wall, which allows the footprint and the cost of the buildings to be reduced. Through the use of two screens the withstand voltage when using a breakdown inhibiting resistor is increased.

With an increased withstand voltage, the distance to ground can be reduced which will directly impact the size of the building.

The functionality of the resistor when connected between a valve and a single screen is briefly described here: Under normal condition s and without any electrical discharge at any of the HV screens, the outer screen takes the same potential as the HV part, i.e. as the valve and the inner screen, to which it is connected via the resistor. The outer screen will therefore act as any ordinary HV screen . As soon as a corona discharge occurs at the outer HV screen , the current drawn by the discharge is limited by the resistor which frustrates the discharge. This can result in as much as 35% higher withstand level which in theory would reduce the needed clearances tremendously. It is possible that in a practical situation the voltage withstand level increase is limited to 5- 10 %. However, it should be noted that a 5- 10 % increment of the withstand level would reduce the clearances significantly more than 5- 10 % due to the nonlinear behavior of the withstand level and gap length at high voltages.

Moreover, the larger the resistance is, the better the withstand voltage increase in principle gets. A large resistor will however create a large voltage drop between its terminations, i.e. between the inner and outer screen , and this large voltage drop may result in a breakdown over the resistor.

When such a breakdown occurs, the benefit of the resistance is lost. The problem is accordingly to incorporate the resistance in a design and have it to withstand high enough voltage, without having it to use up a lot of space. All the extra volume the resistance design requires will affect the total volume reduction of the HVDC installation . Con sequently a design is here presented where the withstand voltage over the resistor is maximized while using minimal space in a very compact solution.

According to aspects of the invention a double screen solution is proposed. The inner screen or first shielding element 26 of the HV parts 18 may be seen as corresponding to a known construction. The idea is to add an additional outer screen or second shielding element 28 that connects to the inner screen via the resistive spacers 30 and 32. The spacers will act as breakdown inhibiting resistors. In absence of any discharge, both screens 26 and 28 will be at the HV potential. When a discharge occurs at the highly stressed outer screen, a current will start to run through the resistors 30 and 32, which will prevent the discharge to proceed into a breakdown to ground. The air gap G between the screens will have a very homogeneous field distribution which maximizes the electrical withstand and keeps the resistors engaged through the discharge event. In that way, the resistor value can be chosen high, which improves the breakdown inhibiting effect of the resistance.

The resistance needed for obtaining a breakdown inhibiting effect is dependent of the voltage level and the screen design, but may for instance be set in the range 100 kil - 12 ΜΩ.

A compact design with quasi-homogeneous electric field distribution is very sensitive to local field enhancement due to particles and dust. A more compact design in the valve hall is associated with a higher risk of electrical breakdown of the insulating gas. Besides the improved withstand voltage, described above, a breakdown inhibiting resistor may mitigate the risk of particles in the insulation system. Particles in compact HVDC systems will give rise to corona discharges and anomalous DC breakdowns which has been shown to occur at unexpectedly low voltage levels. Studies have shown that a large series resistance prevent anomalous DC

breakdown, which otherwise would limit a more compact design. It might be possible to retrofit an extra shield with resistors to existing designs with just a slight modification . This would be a very useful tool to mitigate breakdown risks for the most stressed and critical points in existing valve halls or DC halls.

An obvious example would be to add such extra screens connected with resistors onto the sides and bottom of the valves. That would reduce the needed clearance to the walls, each other and ground. Also other critical points for insulation in a valve hall design should be considered.

It can also be seen that the inclusion of first and second shield elements separated by the gap in turn forms a capacitor, the capacitance of which may be set based on the areas of the shield elements, the distance between them and any filler used in the gap . Furthermore, the capacitance and the resistance forms a time constant t corresponding to how fast the capacitance is charged. The time constant t, which may be in a range of 5 - 5 ms, corresponds or is proportional to RC, where R is the resistance of the spacers or foam and C is the capacitance of the capacitor. Moreover, it is also possible that, the capacitance and the resistance are decoupled and may thus be chosen individually in order to optimize the withstand voltage of the insulation for each design case.

The invention has a number of advantages. It allows the footprint and cost of high voltage equipment such as HVDC installations to be reduced. By reducing the needed air clearance for insulation, significantly advances can be made in this field. Incorporating a breakdown inhibiting resistor into the design is one route to realize a more compact solution. As was shown above it is possible to use a filler material to fill up the void between the shield elements instead of the spacers. This filler material may be a solid or foamed material. If the resistivity of the material can be tuned appropriately, the foam/ solid would create a well-defined and homogenous volume between the screens. Preferably, the material should also be mechanically strong to support the outer screen .

The neighboring object was above exemplified by an enclosure in the form of a valve hall. It should be realized that the neighboring object is in no way limited to such an object. In fact the neighboring object does not have to be an enclosure but can be separate object close to a part of the high voltage equipment. Such a neighboring object may as an example be provided outdoors.

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways.

It shall consequently be realized that the present invention is only to be limited by the following claims.