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Title:
POWER SUPPLY ARRANGEMENT FOR A SUBSTATION
Document Type and Number:
WIPO Patent Application WO/2012/034593
Kind Code:
A1
Abstract:
The present invention concerns a power supply arrangement (18) for a substation, where this power supply arrangement comprises a group of reactive voltage dividing elements (20, 22, 24) connected to a first AC power line (10) that transports AC power to or from the substation and a first transformer (26) having a primary winding (PW) and a secondary winding (SW). The primary winding is connected in parallel with at least one (24) of the voltage dividing elements and the secondary winding is coupled to the substation for providing AC power for powering devices (36) of the substation. This arrangement allows cost savings in relation to transformers, especially at high voltages.

Inventors:
JUHLIN LARS-ERIK (SE)
ASPLUND GUNNAR (SE)
Application Number:
PCT/EP2010/063615
Publication Date:
March 22, 2012
Filing Date:
September 16, 2010
Export Citation:
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Assignee:
ABB TECHNOLOGY AG (CH)
JUHLIN LARS-ERIK (SE)
ASPLUND GUNNAR (SE)
International Classes:
H02J11/00; H02M5/08
Domestic Patent References:
WO2004045046A12004-05-27
Foreign References:
US3684948A1972-08-15
FR1092040A1955-04-18
US2186486A1940-01-09
Other References:
None
Attorney, Agent or Firm:
AHRENGART, Kenneth (Intellectual PropertyIngenjör Bååths Gata 11, Västerås, SE)
Download PDF:
Claims:
CLAIMS

1. A power supply arrangement (18) for a

substation (12), said power supply arrangement

comprising:

a group (21) of reactive voltage dividing elements (20, 22, 24) connected to a first AC power line (10), said first AC power line transporting AC power to or from said substation, and

- a first transformer (26) having a primary winding (PW) and a secondary winding (SW) ,

wherein said primary winding is connected in parallel with at least one (24) of the voltage dividing elements and said secondary winding is coupled to the substation for providing AC power for powering devices (36) of the substation .

2. A power supply arrangement according to claim

1, further comprising a second transformer (34) with a primary winding coupled to the secondary winding of the first transformer and a secondary winding coupled to the substation for supplying power to devices of this substation . 3. A power supply arrangement according to claim

2, wherein the secondary winding of the first

transformer is joined to the primary winding of the second transformer via a second power line (30) and further comprising a group of reactive power

compensation elements (32) connected to the second power line.

4. A power supply arrangement according to any previous claim, wherein the power lines are three-phase power lines comprising three phase voltage conductors, and wherein there are three groups of voltage diving elements, each connected to a corresponding phase voltage conductor of the first power line, and wherein transformers of the arrangement are three phase

transformers with one primary winding and one secondary winding per phase.

5. A power supply arrangement according to claim

4, wherein the secondary windings of the first

transformer are Y-connected, thereby providing a neutral point, which neutral point is coupled to ground.

6. A power supply arrangement according to claim

5, wherein the coupling to ground includes a resistor (28) .

7. A power supply arrangement according to any of claims 1 - 6, wherein a group of reactive voltage dividing elements is connected between the first power line and ground.

8. A power supply arrangement according to any previous claim, wherein the reactive voltage dividing elements are capacitive voltage dividing elements. 9. A power supply arrangement according to any of claims 1 - 7, wherein the reactive voltage dividing elements are inductive voltage dividing elements.

10. A power supply arrangement according to any previous claim, wherein the voltage dividing elements are part of a filter for filtering away frequency components that differ from the fundamental frequency of the AC voltage of the first power line.

11. A power supply arrangement according to any of claims 1 - 9, wherein the voltage dividing elements are part of a reactive compensation device.

12. A power supply arrangement according to any previous claim, wherein the relationship between voltage dividing elements of a group connected in parallel with a primary winding of the first

transformer and the whole group is set according to the rated energy requirements of the devices in the

substation being powered.

13. A power supply arrangement according to any previous claim, wherein the relationship between voltage dividing elements of a group connected in parallel with a primary winding of the first

transformer and the whole group is set according to the reactive power rating of the group, the voltage of the first AC power line and the required active power.

14. A power supply arrangement according to any previous claim, wherein the relationship between voltage dividing elements of a group connected in parallel with a primary winding of the first

transformer provides a voltage U2 to the first

transformer that is the voltage Ul of the first power line divided by a factor of f, where f is in the range of 1.2 - 80.

15. A power supply arrangement according to any of claims 1 - 13, wherein the relationship between voltage dividing elements of a group connected in parallel with a primary winding of the first transformer provides a voltage U2 to the first transformer that is the voltage Ul of the first power line divided by a factor of f, where f is in the range of 1.6 - 40.

16. A power supply arrangement according to any of claims 1 - 13, wherein the relationship between voltage dividing elements of a group connected in parallel with a primary winding of the first transformer provides a voltage U2 to the first transformer that is the voltage Ul of the first power line divided by a factor of f, where f is in the range of 1.6 - 8.

Description:
POWER SUPPLY ARRANGEMENT FOR A SUBSTATION

FIELD OF INVENTION The present invention relates to power supply of devices in substations. More particularly the present invention relates to a power supply arrangement for such a substation. BACKGROUND

Substations are used as interfaces between different parts of power transmission systems and as interfaces between different types of power transmission systems. The transmission system is the high voltage (HV) or extra high voltage (EHV) backbone of a power

distribution system and has a high level of reliability and availability. A substation may be connected to an AC power line and include devices such as converters, for instance for converting between AC and DC or from AC to AC. They may also or instead include

transformers. A substation is thus involved in the transfer of electrical power. However, the substation involved in this transmission of electrical power also needs to be supplied with electrical power in order to perform a number of activities, such as protection and control of

transformers and converters. There are also other devices in a substation needing such power supply, for instance cooling devices such as fluid pumps and valves. Because of the reliability requirements of the power transmission system also the power supply of the substation has to be reliable.

As the substation is transferring power, it is then of interest to tap off some of this power from the AC power line in order to operate the various devices of the substation, i.e. in order to supply these devices with power for their operation. This may be needed if there is no local power distribution system at hand or if there is such a local power distribution system at hand that has too low reliability. Furthermore, due to the importance of the substations in the electric transmission system, redundancy is often needed. There is therefore a need for two independent sources of auxiliary power, where the local power distribution system may be one and a tap off arrangement may be another. It is also possible with two separate tap off arrangements . A tapping off of power can be performed using a transformer rated for the full high voltage of the power transmission system, for instance an additional transformer connected to the AC power line. If the substation comprises a transformer employed in the transfer of power it is also possible to tap off such power using an additional auxiliary power winding on this transformer.

However, the voltage used in the power transmission system is high. In these circumstances it may not be a good solution to use an additional transformer for the high transmission system voltage, because the cost of the additional transformer in relation to the power obtained may simply be too high. The use of a

transformer for the full voltage in the substation may for a number of reasons also be less interesting. One reason is that it may not be needed for other purposes. When the substation is not feeding any network of lower order, no transformer may be needed for the main purpose of the substation of transmitting power at the high voltage level. It may also be of interest to avoid using such a transformer even when there is one present in the substation, for instance in order to keep the energy tapping technique uninfluenced by the

configuration of this transformer.

It is therefore of interest to obtain another way to tap off power from a power line than through using a transformer connected to the power line of the

transmission system.

The present invention is directed towards solving this problem.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a power supply arrangement for a substation, which taps off power from an AC power line without the use of a transformer connected to this power line.

This object is according to the present invention obtained through a power supply arrangement for a substation, where the power supply arrangement

comprises : a group of reactive voltage dividing elements connected to a first AC power line, where the first AC power line transports AC power to or from the substation, and

- a first transformer having a primary winding and a secondary winding,

wherein the primary winding is connected in parallel with at least one of the voltage dividing elements and the secondary winding is coupled to the substation for providing AC power for powering devices of the

substation .

The expression "coupled" used is intended to cover the possibility of an indirect electrical connection between two elements. There may thus be one or more elements placed between two elements defined as being coupled to each other. The expression "connected" is on the other hand intended to mean a direct galvanic connection of two entities to each other without any entity between them.

The present invention has a number of advantages. It allows the use of smaller transformers in the tapping off of power from an AC power line. In this way an economical power supply arrangement can be obtained. This is done through using reactive voltage dividing elements, which are often present for other reasons.

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 single line diagram of a first power line connected to a substation together with a power supply arrangement according to the invention connected between the first power line and the substation, and

fig. 2 schematically shows a single line diagram of one embodiment of the power supply arrangement together with the first power line and a load.

DETAILED DESCRIPTION OF THE INVENTION

The environment in which the power supply arrangement according to the present invention may be provided will now first be described followed by a description of a power supply arrangement according to one embodiment of the invention.

The invention concerns the tapping off of power from a power line in a power transmission system to a

substation using a group of reactive voltage dividing elements. Large reactive voltage dividing elements are normally present in relation to for instance High

Voltage Direct Current (HVDC) stations. They may as an example be used in order to generate reactive power.

Therefore the present invention is particularly suited for being employed in relation to an HVDC substation. It should however be realized that the invention is not limited to HVDC substations.

In fig. 1 there is shown a single line diagram of environment in which a power supply arrangement

according to the invention may be provided. In fig there can be seen a first AC power line 10 being connected to the AC side of a converter 16 for

conversion between AC and DC. This converter 16 also has a DC side to which a DC power line 14 is connected. The converter 16 is here a part of a substation 12, which substation 12 is indicated through a dashed box. In the drawing there is also a power supply arrangement 18 according to the invention, which power supply arrangement 18 is connected to the first AC power line 10 as well as to the substation 12. The first AC power line 10 transports AC power to or from the substation 12. The functioning of the arrangement 18 is to tap some of this AC power from the first power line 10 and use this tapped-off power for operating devices of the substation 12. In the exemplifying environment the substation 12 is furthermore used for conversion between a high AC voltage and a high DC voltage. The first AC power line 10 may therefore be a power line in a high voltage AC power transmission system, while the DC power line may be a part of a high voltage direct current power transmission system (HVDC) .

As can be seen in fig. 1, the first AC power line 10 is directly connected to the converter 16. This means that in the exemplifying environment shown in fig. 1, no high voltage transformer is needed for the converter. It lacks a transformer between converter 16 and first AC power line 10. However, it should be realized that this is no requirement, but the power supply

arrangement can be used also with substations that include such transformers in the path between the first AC power line 10 and the DC power line. It should here be realized that it is also possible that other types of converters are used instead of the converter 16, such as AC/AC converters. It should thus be realized that the power supply arrangement 18 according to the present invention is not limited to be used in the type of environment shown in fig. 1. From the foregoing discussion it is also evident that the substation may be a part of or an interface to a power transmission system. Fig. 2 shows a single line diagram of a power supply arrangement 18 according to a first embodiment of the invention being connected between the first power line 10 and a load 36, where the load 36 may be one or more of the devices in the substation of fig. 1. In fig. 2 there is a first group 21 of reactive voltage dividing elements connected to the first power line 10. They are also connected to ground and therefore connected between the first power line 10 and ground. As an example the group 21 here includes a first, second and third reactive voltage dividing element 20, 22 and 24 connected in series between the first power line 10 and ground. It should here be realized that the number of elements shown are only three in order to better describe the invention. The group 21 may and normally does include many more elements. In this first

embodiment of the invention the reactive voltage dividing elements are all capacitors. In alternative variations of the invention, they may be reactors. The power supply arrangement 18 also comprises a first transformer 26 that is rated for a lower voltage than the transmission system voltage used on the first AC power line. The first transformer 26, has a primary winding PW connected in parallel with at least one of these voltage dividing elements, which may be in parallel with one of them. In this first embodiment it is connected to only one such element, which element is the third element 24 closest to ground. This is the most practical element to use because the potential of the series connection will be lowest here. The first transformer 26 has a secondary winding SW. In this first embodiment of the invention the secondary winding SW has a first end coupled to ground and a second end connected to a second power line 30, which second power line 30 in turn leads to a second transformer 34 and more particularly to a primary side of the second transformer 34. The second transformer 34 also has a secondary side, which in turn is connected to the load 36. In this way it can be seen that the secondary winding SW of the first transformer 26 is coupled to the substation for providing AC power for powering devices in the substation, which devices are

exemplified by the load 36. It can also be seen that the primary side of the second transformer 34 is coupled to the secondary winding SW of the first transformer 26 and the secondary side of the second transformer 34 coupled to the substation for supplying power to the devices of this substation.

In this first embodiment of the invention there is furthermore a group of reactive power compensation elements 32, here in the form of a series connection of reactive power compensation elements 32 connected to the second power line 30. The series connection has a first end connected to the second power line 30 as well as a second end, which second end will be described in more detail later. This series connection of reactive power compensation elements could be provided through capacitors connected in series and selectably connected into the series connection in order to provide a variable reactive compensation circuit.

The first and second power lines 10 and 30 shown in fig. 2 are furthermore three-phase power lines, which means that they will according to the first embodiment of the invention have three phase voltage conductors, i.e. three conductors intended for carrying phase voltages. This also means that in this first embodiment there will be one group of voltage dividing elements between one phase voltage conductor of the first power line 10 and ground as well as one series connection of reactive compensation elements connected to each phase voltage conductor of the second power line 30.

This also means that the first transformer 26 is in fact a three phase transformer having three primary windings and three secondary windings. Also the second transformer 34 is a three-phase transformer having three primary windings on the primary side and three secondary windings on the secondary side. In this first embodiment of the invention the primary windings of both the first and second transformers are delta connected, which is indicated with a D in the second transformer, while the secondary windings are Y- connected, indicated with a Y in the second

transformer.

In fig. 2 the first transformer 26 is shown as only having a single primary winding PW and a single secondary winding SW even though it is a three-phase transformer. This is done in order to clarify the relationship between the primary windings and the voltage dividing elements to which they are connected.

Since the secondary windings of the first and second transformers are Y-connected, they both have a neutral point. These neutral points are here furthermore coupled to ground, where the neutral point of the secondary windings of the second transformer is directly connected to ground, while the neutral point of the secondary windings of the first transformer is connected to ground via a resistor 28. This is shown in fig. 2 through marking the junction between the first end of the secondary winding SW of the first

transformer 26 and the resistor 28 and indicating two other connections leading to this junction. These two other connections then lead to the second ends of the other secondary windings. It should here be realized that this is just one example of how the first

transformer may be connected. Several other possible configurations exist.

Also the second ends of the three groups of reactive power compensation elements are connected to each other. This is in fig. 2 also indicated through the second end of the shown group of reactive compensation elements being connected to a junction. Two conductors are here shown as connected to this junction. These two conductors are intended to indicate that the second ends of the two other groups of reactive elements associated with the other phase voltage conductors are connected to this point, which in turn is grounded. In this way the reactive compensation elements are Y- connected with isolated neutral point.

As can also be seen in fig. 2, the first power line 10 has a first voltage Ul, which is divided down to a second voltage U2 by the third voltage dividing element 24. This is in turn transformed into a third voltage U3 by the first transformer 26. Finally the third voltage U3 is transformed to a fourth voltage U4 by the second transformer 34.

The functioning of the power supply arrangement 18 according to the first embodiment will now be described in more detail .

It is of interest to supply the devices of a substation with power through tapping off power from the first AC power line. This may be of interest for a number of reasons, such as the absence of a power distribution system or utility grid in the vicinity of the substation or if there is a local power distribution system present, which however is unstable. The power supply arrangement may here be a main or a redundant power supply

arrangement .

As mentioned earlier, this tapping off of power can be done using a transformer connected to the first AC power line 10, i.e. a transformer rated to the voltage of this first AC power line 10. In the exemplifying environment provided here the first AC power line 10 is provided in a high voltage AC system where the first voltage Ul may as an example be 400 kV. Typically this voltage may be in an interval of 300 - 800 kV. At these voltage levels such a transformer is big and also expensive. Furthermore, in the example given in fig. 1, the substation 12 lacks such a main transformer. Thus, there is no transformer involved in the transmission of power from the first AC power line 10 to the DC power line 14. There is thus no transformer placed between the first AC power line 10 and the converter 16. This means that the costs involved with providing a separate transformer for supply of power to the devices of the substation are high and therefore an alternative solution is desired. In the exemplifying power supply arrangement of the first embodiment this is solved through providing the group 21 of reactive voltage dividing elements 20, 22, 24, which divide the first voltage by a factor f in order to obtain a second voltage U2 and connecting the first transformer to this second voltage U2 in order to supply the required power. The second voltage U2 may here be a voltage that is set according to the power supply requirements of the substation 12. It has to be high enough to be able to supply the energy required. This means that the relationship between the voltage dividing elements of the group connected in parallel with the primary winding of the first transformer and the whole group 21 is set according to the rated energy requirements of the devices in the substation that are to be powered. This means that in the first embodiment the relationship between the third voltage dividing element 24 and the whole group 21 is set according to the reactive power rating, Mvar rating, of the group 21 and here the rating of the total capacitance of the series-connected group, the size of the voltage Ul and the required active power. The factor f may here be in the range of 1.2 - 80. This range may be even further limited to the range of 1.6 - 40 and selected according to the power supply requirements. It may be even further limited to the range of 1.6 - 8 and thus as an example have any of the values of 1.6, 3.1, 5.7 or 8 in order to provide a second voltage U2 of 50 kV, 70 kV, 130 kV or 245kV when the first voltage Ul is 400 kV. These specific voltages are suitable because they are standard values often used.

In the first embodiment of the invention the first transformer 26 then transforms the second voltage U2 to an intermediate voltage, the third voltage U3, which in the first embodiment is a voltage of 10 kV. This voltage is also a standard local power distribution system voltage. This means that standard transformers operating on the exemplifying second voltages U2 mentioned above and the exemplifying third voltage U3 of 10 kV are provided. These are readily available and a good selection when economy is of importance. It should here be mentioned that the value of the third voltage U3 is a mere example. This particular value is no requirement. The third voltage U3 may for instance be in the range of 3 - 40 kV and thus be a fraction of the first voltage Ul, and the relationship that may be in the range of 1/266 - 1/8.

The second transformer 34 then transforms the third voltage U3 to the fourth voltage U4, which is a voltage required for the devices of the substation. This fourth voltage U4 can as an example be 400 V, which is a type of voltage normally provided in a power distribution system or utility grid. During the supply of power the group of reactive compensation elements 32 may also be selectively connected to the second power line 30 in order to perform reactive compensation of the load 36 and perhaps also voltage control.

The resistor 28 is here used for limiting the current in case of faults and may be omitted if this

functionality is not desirable.

In this way economical power supply is provided. It could seem like the costs are not lowered through the described solution, since transformers are still used together with added reactive voltage dividing elements and thereby the number of elements used in the power supply seem to increase. However this is in reality a chimera. Reactive voltage dividing elements are normally present for other reasons, such as for being parts of reactive

compensation devices or filters, for instance filters for filtering away harmonic components of the AC voltage of the first power line 10. This means that the additional reactive voltage dividing elements are actually obtained at no extra cost while allowing a smaller transformer to be used because of the voltage division. The reactive voltage dividing elements are thus put to dual use, i.e. to both act as voltage dividing element and filter element. The reactive voltage dividing elements may thus be part of a filter for filtering away frequency components that differ from the fundamental frequency of the AC voltage of the first power line.

There are a number of variations that are possible to make of the present invention apart from those already mentioned. The reactive power compensation elements were above described as being connected in a Y- connection in relation to the second power line. It should be realized that each group may here instead be connected in a delta-configuration. It is in fact possible that reactive compensation is not needed and therefore these elements may furthermore be completely omitted . It is also possible to omit the second transformer. If for instance the required power is so low that a low second voltage can be used, for instance a voltage of 1 kV in relation to the example of 400 kV, then it is possible to use only one transformer that directly converters from the second voltage to the fourth voltage .

It should also be realized that the primary windings of each transformer may be Y-connected instead and/or the secondary winding of each transformer may be delta connected instead. The invention may in fact be

implemented in a single-phase system, in which case neither delta- nor Y-connections are used. The devices that need to be supplied with power can include control and protection computers for the valves. However the devices requiring most energy are normally devices used for cooling such as fans and pumps .

It was above mentioned that the invention is

advantageous when the substation is a substation lacking a transformer operating at the voltage of the first AC power line. However, the power supply

arrangement may also be combined with a transformer used in the transmission of power over this first AC power line without tapping off power from this power transmission transformer. This may be of interest when the configuration of the power supply transformer is to be different from the configuration of the power transmission transformer. The power transmission transformer may in some instances require to have a delta-connection on the side facing the converter, which may give rise to problems when also tapping off power for power supply. This is avoided with the present solution, since the first transformer used in the power supply arrangement can be configured

independently of the configuration of such a power transmission transformer.

From the foregoing discussion it is evident that the present invention can be varied in a multitude of ways. It should for instance be realized that other voltage and potential levels than the above described may be used. It is also evident that the reactive voltage dividing elements may be reactors instead of

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

following claims.