HVDC transformer configuration for split PCC

Abstract

The invention relates to an HVDC transformer configuration (10, 50) for electrical power transfer to at least a first AC network and a second AC network, the configuration comprising:-three single-phase HVDC transformers, in which each phase comprises a first winding (2, 12, 22) connected to HVDC converter (s) and a second winding (4, 14, 24) connected to both the first AC network and the second AC network; -wherein the second winding (4, 14, 24) of each phase is configured as an autotransformer and comprises a tap connection (6, 16, 26) connected to a phase-shifting transformer (30) to regulate the power transfer to the second AC network.

Description

Background Art

[0001] Conventionally, an HVDC transmission system for supplying a large amount of power from one AC network at a transmitting end to an AC network at a receiving end and transmission infrastructures at both ends are planned in advance.

[0002] However, this is not entirely applicable to offshore wind farms or other renewable energy sources when the power generation is intermittent in nature, as the power received at the receiving end varies over time. Therefore, the AC network at the receiving end should be equipped to accommodate the variation in the power received at any time.

[0003] The increase in intermittent power generation increases the uncertainty and congestion problems of interconnected grids; therefore, there is sometimes a need to distribute this power into more than one AC network at the receiving AC side of the HVDC.

[0004] Furthermore, the generated power may be intended for more than one network, where there is a need to control the distribution of power from the HVDC system into more than one network, especially in liberalized markets. Normally, different AC networks are interconnected for emergency backup, synchronization and to increase the security of power supply. Therefore, in an interconnected AC transmission system, there is almost no impact on the power transmission flow into each transmission network, which is based on Kirchhoff's law. The interconnected networks can operate at the same or different voltage levels.

[0005] Normally, FACTS devices and phase shift transformers (PSTs) are employed in order to overcome the above challenges in AC power grids.

[0006] Figure 1 The architecture 100 of a known HVDC system 102 is shown connected to two separate networks 104 , 106 operating at different voltages (eg 345 kV and 138 kV) and delivering power to the separate networks 104 , 106 at the different voltages.

[0007] System 102 includes 345 kV HVDC terminal 112, 138 kV terminal 114 and step-down transformer 116 therebetween. More specifically, step-down transformer 116 is combined with a phase shifting transformer (not shown in this figure). An example of a phase shifting transformer is given in the article "Phase Shifting Transformers: Principles and Applications" by J. Verboomen et al., a conference paper in December 2005. System 101 upstream of terminal 112 converts direct current into alternating current.

[0008] Therefore, this known architecture requires the use of two sets of transformers, namely a HVDC interface transformer (usually three single-phase HVDC transformers), and a step-down transformer and a PST (usually a three-phase combination of an autotransformer and a PST transformer).

[0009] Thus, the known devices are complex and bulky and require the use of many active components.There is therefore a need for simpler systems, implementing fewer active components.

[0010] Furthermore, a three-phase combined autotransformer and PST transformer may be a challenge even for relatively low powers (e.g. 400 MW), in which case three single-phase combined autotransformers and PST transformers may be necessary, which increases the cost and footprint requirements of the installed system and also makes the system (especially the connections between the three units) even more complicated. Summary of the invention

[0011] The present invention relates to a HVDC transformer suitable for feeding power at different voltages to two separate networks from a single converter.

[0012] The invention firstly relates to a HVDC transformer or a HVDC transformer arrangement for electric power transmission to at least a first AC network operating at a first voltage (e.g. at 345 kV) and a second AC network operating at a second voltage (different from the first voltage, e.g. at 138 kV), the arrangement comprising:

[0013] - an HVDC transformer, wherein a first winding is connected or intended to be connected to the HVDC converter(s) and a second winding is connected or intended to be connected to both the first AC network and the second AC network;

[0014] - wherein the second winding is configured as an autotransformer and comprises a tapped connection connected to a phase shifting transformer to regulate power transfer to the second AC network.

[0015] In the HVDC transformer arrangement according to the invention, each phase of the phase shifting transformer may be in the same tank as one phase of the HVDC transformer.

[0016] Alternatively, each phase of the phase shifting transformer may be in a separate tank, for example in either a single phase arrangement or a three phase arrangement.

[0017] Each tank can be filled with oil.

[0018] In certain embodiments, the HVDC transformer may be configured as three single-phase transformers.

[0019] In another particular embodiment, the second winding of each phase of the HVDC transformer is provided with a tap changer to regulate reactive power and / or AC voltage between said HVDC transformer and said AC network(s).

[0020] The invention also relates to a network interconnection for supplying power to two or more of (one or more) AC networks, the network interconnection comprising a HVDC transformer arrangement according to the invention.

[0021] The invention also relates to a system comprising a HVDC transformer according to the invention, connected to a HVDC converter and to at least a first AC network operating at a first voltage, for example at 345 kV, and a second AC network operating at a second voltage, different from the first voltage, for example at 138 kV.

[0022] The invention also relates to a method of implementing a HVDC transformer or system according to the invention for:

[0023] - Supply power to different AC networks;

[0024] - and / or transfer power between two networks.

[0025] A method for supplying power to at least a first AC network and (one or more) second AC network(s) through a network interconnection or system according to the invention comprises providing power:

[0026] - to the first AC network;

[0027] - and via said phase-shifting transformer to said second AC network. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] - Figure 1 shows the architecture of a known HVDC system connected to two separate networks;

[0029] - Figure 2 is a first embodiment of a transformer arrangement according to the invention;

[0030] - Figure 3 is a variation of the transformer arrangement according to the invention;

[0031] - Figure 4 is a second embodiment of a transformer arrangement according to the invention. DETAILED DESCRIPTION

[0032] The invention is described in the context of a three phase network, but may also be applied to other numbers of phases. In particular and unless otherwise indicated, references to windings will refer to any phase, explanations given for one phase applying to the other phases.

[0033] Figure 2 A first embodiment of an HVDC transformer arrangement 10 according to the invention is shown. It is intended for connection to an HVDC converter and / or for electric power transmission to at least two AC networks, or for exchanging power between at least two networks, for example at least a first AC network operating at a first voltage, for example at 345 kV, and at least a second AC network operating at a second voltage different from the first voltage, for example at 138 kV.

[0034] Each phase comprises a primary winding 2 (or a separate winding) (12, 22, respectively) and a secondary winding (or autotransformer winding) 4 (14, 24, respectively); the secondary winding 4 (14, 24, respectively) is itself configured as an autotransformer and comprises a tap connection 6 (16, 26, respectively) connected to the phase of a phase shifting transformer (PST) 30. For each phase, the voltage supplied to the PST depends on the position of the tap on the corresponding secondary winding. A first network (operating at a first voltage, for example at 345 kV, not shown in the figure) is connected to the terminals 41, 141 and 241 of each secondary winding, and a second network (operating at a second voltage different from the first voltage, for example at 138 kV, not shown in the figure) is connected to the terminals 301, 302 and 303 of the PST.

[0035] The terminals 23, 123, 223 are used to connect or be connected to a HVDC converter (not shown in this figure or in any of the subsequent figures).

[0036] Terminals 21, 121, 221 may be connected together to form an AC network side star neutral point for grounding.Terminals 22, 122, 222 may be connected together to form an HVDC converter side star neutral point for high impedance grounding when a star-star transformer connection is used.

[0037] Alternatively, when a star-delta transformer connection is used, 22, 122, 222 together with terminals 23, 123, 223 form a delta connection to make a connection to a HVDC converter.

[0038] For each phase, the part (secondary winding) used as step-down transformer is preferably combined with the phase of the HVDC transformer in the same tank 5 (15, 25 respectively). Each tank can be filled with oil.

[0039] The three-phase PST 30 may be a symmetrical type single core phase shifter (but other types are possible). It may be placed in a separate box 35, in which case the system utilizes only 4 boxes 5, 15, 25, 35, instead of a bulky solution of 6 boxes (one box for each phase of the HVDC transformer and one box for each phase of the combined step-down and phase shifting transformer).

[0040] According to this embodiment, an HVDC transformer 2-6 (respectively 12-16, 24-26) is used at a first voltage (eg 345 kV) with an autoconnection feeding a second network side at a second voltage lower than the first voltage (eg 138 kV).

[0041] Connections from single-phase transformers to phase-shifting transformers can include through-busbars or common cable connections to ease installation and reduce site footprint.

[0042] In this first embodiment, all units can be of conventional type, for example the HVDC converter transformer can implement conventional windings 2, 12, 22 and 4, 14, 24 and cores. The phase shift transformer 30 is a separate standalone unit and can be tested completely independently of the converter transformer. According to this first embodiment, the step-down function is moved from the PST to the HVDC transformer, which optimizes the material and its size.

[0043] Figure 3 A variation of the first embodiment is shown which comprises a tap changer to adjust (see arrows 61, 62, 161, 162, 261, 262) the position of any of the taps, for example to regulate the voltage and / or reactive power of one of the AC networks. Figure 2 The same reference numerals in the drawings refer to the same or similar elements, and reference is made to the above description.

[0044] The other aspects and advantages described above in conjunction with the first embodiment apply to this variation.

[0045] Figure 4 A second embodiment of a HVDC transformer arrangement 50 according to the invention is shown.

[0046] In this embodiment, the active components of the HVDC transformer are the same as those already described above and are Figure 2 and Figure 3 The same reference numerals in the drawings refer to the same or similar elements.

[0047] But each phase of both step-down transformer and PST are incorporated into HVDC transformer. Connection between single phase units for PST operation can be achieved through common bushings and / or plug-in cable bushings.

[0048] Relative to the combination Figure 2 and Figure 3 The difference of the disclosed embodiment is that Figure 2 and Figure 3 The phase shift transformer 30 in Figure 4 The transformer is split into three single-phase cores 32 (132, 232 respectively), each of which is placed in the same box 5 (15, 25 respectively) as the corresponding single-phase converter transformer.

[0049] As with the previous embodiment, this embodiment saves cost and space. Multiple connections 52 may be required between different units, but this can be done with ordinary cable connections, such as ordinary bushings and / or plug-in cable bushings, particularly if these cable connections are relatively low voltage connections.

[0050] The invention can be implemented in a method for transmitting electric power to one, two or more AC networks.In a variant, power between two different AC networks can also be transmitted by means of a HVDC transformer according to the invention.

[0051] In the prior art, separate converter transformers are connected to a combined phase shifting / autotransformer.

[0052] According to the present invention, the autoconnection is now moved to the converter transformer. An autoconnection winding is used instead of a single AC line winding in the converter transformer. Figure 2 and Figure 3 Windings 2 and 4 in the converter transformer are shown. Winding 2 may be rated for full power from the converter. Winding 4 is an autocoupled winding: it may be split into two sections with a tap at point 6 for connection to a phase shifting transformer. This removes the step-down function from the phase shifting active components, the converter transformer having one input connection from the converter and two output terminals: one for connection to the HVAC network and the other to a separate phase shifting transformer operating to a different AC system voltage. The relevant section of the autocoupled winding may be rated for power transfer to the associated AC network.

[0053] Figure 4 The embodiment of achieves the same functionality, but the active components for the phase shifting function are moved to or placed inside the box of the converter transformer. Figure 2 There are 4 boxes in total, three single-phase converter transformers and a three-phase phase-shifting transformer. Figure 4 Only three boxes are utilized. The connections between the three boxes are associated with phase shifting active components.

[0054] Any embodiment of the HVDC transformer according to the invention can be connected to the HVDC converter and to at least a first AC network operating at a first voltage (e.g. at 345 kV) and to a second AC network operating at a second voltage (different from the first voltage, e.g. at 138 kV). The invention also comprises a configuration of the HVDC transformer according to the invention, the HVDC transformer being connected to the HVDC converter and to at least the first AC network and to the second AC network, the first AC network operating at a first voltage (e.g. at 345 kV) and the second AC network operating at a second voltage (different from the first voltage, e.g. at 138 kV).

[0055] The new solution proposed by this invention saves transformer costs (because a smaller number of boxes and materials etc. are used), allows a smaller footprint to install them and easier connections between different units.

[0056] In terms of process, the invention offers the possibility to split the power flow between different networks as required.

[0057] With the HVDC transformer according to the invention, power can be provided or supplied to the first AC network operating at a first voltage, for example at 345 kV, and to the second AC network operating at a second voltage (different from the first voltage, for example at 138 kV).

[0058] Using the HVDC transformer according to the invention:

[0059] - power may be provided or supplied to said first AC network operating at a first voltage (eg at 345 kV), and to said second AC network operating at a second voltage (different from the first voltage, eg at 138 kV);

[0060] - Alternatively, power may be transferred from one of said first and second AC networks to the other network to another network.

[0061] The present invention may be implemented, for example, in a network interconnect as disclosed in WO2022 / 117489.

Claims

1. A HVDC transformer arrangement (10, 50) for electric power transmission to at least a first AC network and a second AC network, the arrangement comprising: - three single-phase HVDC transformers, wherein each phase comprises a first winding (2, 12, 22) connected to the HVDC converter(s) and a second winding (4, 14, 24) connected to both the first AC network and the second AC network; - wherein each of the second windings (4, 14, 24) is configured as an autotransformer and comprises a tap connection (6, 16, 26) connected to a phase shifting transformer (30) to regulate power transfer to the second AC network.

2. The HVDC transformer arrangement according to claim 1, wherein: Each phase of the phase-shifting transformer is in the same box (5, 15, 25) as one phase of the HVDC transformer.

3. The HVDC transformer arrangement of claim 1, wherein: Each phase of the phase shifting transformer is in a separate box (35), for example in either a single phase arrangement or a three phase arrangement.

4. A HVDC transformer arrangement according to any one of claims 1 to 3, wherein: The second winding (4, 14, 24) of each phase of the HVDC transformer is equipped with a tap changer to regulate reactive power / AC voltage between the HVDC transformer and the second AC network.

5. A network interconnection for supplying power to at least a first AC network and (one or more) second AC network(s), comprising a HVDC transformer arrangement as claimed in any one of claims 1 to 4.

6. A method for supplying power to at least a first AC network and (one or more) second AC networks via a network interconnection according to claim 5, comprising providing power: - to the first AC network; - and to the second AC network via the phase-shifting transformer (30).

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