Patent Application
20090266605
The present invention relates to the field of high voltage devices and in particular to a high voltage assembly as defined in the preamble of claim 1.
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 means for performing a conversion from HVDC to three-phase AC are needed, and vice versa. A HVDC power converter comprises a number of valves, which are key components of electrical plants, and the valves are typically accommodated in a valve hall. An example of such valve is the thyristor valve, often utilized for making the conversion between DC/AC.
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 expensive 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 installation costs, including for example material costs and labour costs, may in some countries be high and thus further yet adds to the desire to minimize the size the valve hall.
The security 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-intensity electric arcs may black out very large areas and are expensive for the power companies. Protection measures, either passive or active, are therefore crucially important.
In view of the above, it would be desirable to provide an improved high voltage assembly, providing an increased security. Further, it would be desirable to provide means to enable designing valve halls of smaller size, without lessening the security requirements.
It is an object of the present invention to provide an improved high voltage structure having a higher security marginal, thereby overcoming or at least alleviating the above-mentioned drawbacks of the prior art.
It is another object of the present invention to provide an improved high voltage structure, enabling the use of valve halls a more compact design, but still with an maintained or increased security level.
These objects, among others, are achieved by a high voltage assembly as claimed in claim 1.
In accordance with the invention a high voltage assembly is provided. The high voltage assembly comprises a valve group and a shield, wherein the shield is connected to the valve group. In accordance with the invention a connection resistance is provided between the shield and the valve group. By means of the invention the impulse breakdown strength can be increased. Further, increased DC withstand levels can also be accomplished. An improved safety in HVDC valve halls can thereby be provided, without increasing the size of the valve hall within which the valves are accommodated. This improvement can further, by minor changes, be implemented in existing HVDC valves, and thereby renders the invention suitable and most attractive also to existing HVDC valves. By means of the invention, the operation reliability can be greatly improved.
In accordance with an embodiment of the invention, connection resistance is a resistor. Further, the resistance value of the resistor lies preferably within the range of 2 MΩ-40 MΩ, but preferably larger than 2 MΩ. This is a suitable range for the resistance for providing a higher security and avoiding air breakdowns. Further, such resistance values enable the use of components readily available on the market.
Further preferred embodiments of the present invention are defined in the dependent claims.
Further characteristics, advantages and objects of the invention will become apparent by reading the following detailed description.
As was explained in the introductory part of the present application, the HVDC valve hall dimensions depend directly on the air clearances required between the earthed walls of the valve hall and the different energized elements, such as the valves. In order to decrease the air clearance, the configuration of the high voltage shields is carefully designed.
The published document entitled “Inhibited electrical discharges in air”, by Anders Larsson, Uppsala University, 1997, discusses experimental results of inhibited discharge measurements. It is shown that the magnitude of the 50% disruptive voltage U50% and the SI (switching impulse) breakdown voltage was increased with 20% with a series resistor of 4 MΩ added. The published document entitled “Über ungewöhnlich groβe schalgweiten in luft bei hohen gleichspannugen”, VDE Fachberichte 12, Band 1948, Gruppe VIII, Elecktrophysik, reports on measurements made regarding breakdown phenomena. It was found that DC disruptive discharges could, under some circumstances, occur for electric field strengths below the expected values. Such anomalous discharges occurred between spherically shaped electrodes and the walls of the room, and discharges were observed already at an average electric field strength of 3 kV/cm.
The inventors of the present invention have, based on the knowledge from the above cited references, found a way to accomplish an improved performance of a HVDC system. In particular, the experimental results presented in the above cited references were the starting point for providing an improvement in an electrical plant application. More specifically, the inventors of the present invention have realized that these findings can be utilized in an innovative way for providing an improved high voltage shielding of valves.
In the figure, a wall 6 of the valve hall within which the valve structure is housed, is shown. The required distance D between the valve structure 1 and the surrounding earthed walls is in many applications within the range of about 6-10 meters, but can be smaller or larger depending on the specific application. This distance is the so called air clearance.
The present invention introduces a connection resistance 17 between the shield 15 and the valve group 13. The connection resistance 17 may for example be one or more resistors. In a preferred embodiment, the resistor or resistors have a resistance value of 4 MΩ. However, it is realized that any suitable resistance value could be chosen, for example lying within the range of approximately 2 MΩ and 40 MΩ. The connection resistance should preferably be higher than 2 MΩ. However, it is realized that the designer should choose a suitable value, which value depend on the particular application in question.
Any type of resistor can be utilized, such as for example water resistors that are able to handle high voltages. Other examples comprise wirewound resistors and metal film resistors. In an embodiment of the invention, the connection resistance 17 is variable, thereby providing adjustment means should such need arise.
As is well known, all real resistors also introduce some inductance and a small amount of capacitance, which change the dynamic behavior of the resistor from the ideal. Capacitance exists since any pair of electrical conductors that will store electrical charge when a difference of potential is applied to them constitutes capacitance. All parts of a resistor and its terminals, and all parts of a resistor in association with all its other parts have that characteristic and thus a small amount of capacitance is introduced.
In accordance with the invention, the resistance value and the corresponding capacitance of the resistor 17 is controlled and chosen in a suitable manner. In
As mentioned earlier, there are usually several valve groups 13 within a valve structure.
The operation reliability of HVDC systems can be greatly improved by means of the present invention. In particular, the impulse breakdown strength can be increased, since the resistor 17 inhibits the breakdown process. Further, the DC withstand levels can also be increased.
In accordance with the present invention, the security level of a high voltage application can be greatly increased.
In accordance with the present invention, the air clearance distance d can be reduced by approximately 10-20%, which is a substantial size reduction. This reduced air clearance requirement can be utilized for reducing the size of the valve hall. Alternatively, the reduced air clearance requirement can be utilized for providing further yet increased operation reliability of the HVDC system.
In the present application a high voltage can be considered to range from about 1 kV up to as much as 800 kV DC, but most often in the range of 50 kV-800 kV DC. Transient over voltages may occur occasionally in any system and may reach up to approximately 2-3 MV.
High voltage assemblies, in which the present invention can be utilized, comprise for example power converters or power transformers. However, it is realized that other high voltage applications may benefit from the present invention.
In summary, the present invention provides an improvement to high voltage assemblies, such as a power converter or power transformer or, in particular, a valve structure. By means of the invention the impulse breakdown strength can be increased as can the DC withstand levels. An improved safety in HVDC valve halls can thereby be provided, without increasing the size of the valve hall within which the valves are accommodated. This improvement can further, by minor changes, be implemented in existing HVDC valve halls, and thereby renders the invention suitable and most attractive also to existing high voltage assemblies. By means of the invention, the operation reliability can be greatly improved.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2006/000748 | 6/20/2006 | WO | 00 | 6/5/2009 |
