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    • 专利摘要:
    Method and system for controlling wind farms (1) connected in series to an hvdc link (8), where each wind farm (1) is connected to a bus (3) which, in turn, is connected through a step-up transformer (6) to an ac/dc rectifier station (7) that evacuates the generated energy to an hvdc link (8), where the bus (3) is connected to a capacitor bank (5) and to an electronic converter ( 4), which also comprises a plurality of alternating current links (11) connecting the bars (3) to each other at least two to two, so that it is possible to equalize the powers injected by each ac/dc rectifier station (7) to the hvdc link (8) by modifying the frequency in the busbar (3) of each wind farm (1) so that said frequency increases proportionally to the voltage in said busbar (3). (Machine-translation by Google Translate, not legally binding)
    公开号:ES2620972A1
    申请号:ES201730190
    申请日:2017-02-16
    公开日:2017-06-30
    发明作者:José Luis Rodríguez Amenedo；Santiago Arnaltes Gómez
    申请人:Universidad Carlos III de Madrid；
    IPC主号:
    专利说明:

    Method and system to control a set of wind farms connected in series to a high voltage direct current link OBJECT OF THE INVENTION
    The present invention pertains to the field of electric power transmission through an HVDC direct current link when a plurality of diode rectifier stations connected in series are used.
    A first object of the present invention is a system designed to allow control of the power contributed by a plurality of wind farms connected to a respective plurality of diode rectifier stations connected in series to an HVDC link.
    A second object of the present invention is a method of operating the previous system to control the power contributed by a plurality of wind farms connected to a respective plurality of diode rectifier stations connected in series to an HVDC link. BACKGROUND OF THE INVENTION
    In the context of offshore wind farms located on the high seas, the most viable interconnection technology for long distances is called HVDC (High Voltage Direct Current). This technology consists mainly in transmitting the electric energy generated by the wind turbines through a high-voltage continuous link from the offshore wind farm to the power grid located on the mainland. For this, at least one AC / DC grinding station is used on the side of the wind farm and a DC / AC inverter station on the land side between which the continuous link is extended.
    HVDC technology has different advantages over HVAC (High Voltage Alternating Current) technology, among which the following can be mentioned
    a) The electrical losses and voltage drops in the HVDC link are much lower than those in an HVAC link. b) In the absence of capacitive currents in the cable on an HVDC link, the capacity


    Link transmission increases significantly with respect to a linkHVACc) The alternating current networks of the wind farm and the mainland are separatedby the HVDC link, which implies an asynchronous operation of both.d) HVDC links allow higher power controllability than linksHVAC
    Within the AC / DC rectifier stations or DC / AC inverters of the HVDC links, two technologies stand out: HVDC-LCC (Line Commuted Converter), based on network-switched electronic switches (thyristors or diodes) and HVDC-VSC (Voltage Source Converter), based on self-switched electronic switches (IGBT or IGCT) arranged modularly in multilevel converters. More specifically, within the HVDC-LCC technology, the AC / DC diode rectifier stations are, for their simplicity and reliability, the most suitable option for the interconnection of offshore wind farms to HVDC links. However, in this case for the AC / DC grinding station to function properly it is necessary that the AC network on the side of the wind farm be able to keep the voltage and frequency controlled.
    The maintenance of the voltage and frequency of the AC network of the wind farm can be carried out by means of a control distributed in each of the wind turbines of the installation, as long as they are of last generation (doubly powered DFIG generators or generators with complete FULLCONVERTER converter ). In this case the wind turbines function as a voltage source, so that the active and reactive power they generate is imposed by the power transmitted by the HVDC link. However, this significantly modifies the classic wind turbine control system based on extracting the maximum power of the incident wind in the wind turbine. That is to say, this system for controlling the voltage and frequency of the AC network of the wind farm has the significant disadvantage that it involves wasting part of the wind power that affects the wind turbine.
    A more realistic option with an industrial application to control the voltage and frequency of the AC network of the wind farm is to install a capacitor bank and a current-controlled electronic converter in the input bar of the AC / DC grinding station whose purpose is to control the frequency of the voltage at this point instantly closing the reactive power balance. This solution is described in patent application P201530410 entitled "Method and system for voltage and frequency control in an isolated network", developed by the same inventors of the present application. Fig. 1


    It shows an example of such a system. As can be seen, an installation (2) that evacuates the power generated by the wind farm (1) comprises at least one three-phase alternating current busbar (3) to which it is connected: an electronic converter
    (4) composed of self-switched current controlled switches, a bank of capacitors (5) on which the voltage control is carried out, and an elevator transformer (6) that feeds an AC / DC rectifier station (7) formed by one or more diode bridges and whose DC terminals connect to the HVDC link (8). In turn, the HVDC link (8) is connected to a DC / AC inverter station (9) that evacuates the power generated to an alternating current electrical network (10) located on the mainland.
    Although this scheme adequately solves the problem of the control of the voltage and frequency of the AC grid of a single wind farm, the high voltage of the HVDC links has led the industry to explore the possibility of connecting several AC / DC grinding stations in series in order to reduce the nominal voltage of the entire electrical switchgear. In this case, each of the AC / DC grinding stations would evacuate the power generated by a respective wind farm. This option, which from a design point of view seems the most appropriate, however implies a series of technological challenges associated with the coordinated control of facilities that have not yet been satisfactorily resolved.
    Indeed, Fig. 2 shows an example of an HVDC-LCC transmission system that has three wind farms (1) connected to three respective AC / DC grinding stations (7) formed by diode bridges that are connected in series. To control the voltage and frequency at the input of each of said AC / DC grinding stations (7), the wind farms (1) are connected to each of them through an installation (2) as described in application P201530410 mentioned above. As usual, to avoid complications related to asymmetric installations, the design of the system as a whole is done so that the three AC / DC grinding stations
    (7) are the same. This implies that all three have the same nominal DC voltage, and therefore the voltage (VHVDC) of the HVDC link (8) must be divided into three equal parts (V1DC, V2DC, V3DC).
    Therefore, in this context the operation of each of the installations (2) and the wind farms (1) must be controlled so that the AC voltages (V1AC, V2AC, V3AC) input into the respective rectifying facilities (AC / DC (7) are also equal and corresponding to their nominal AC voltage, taking into account that P = VI, as the intensity (I) that passes through each of the AC / DC rectifier stations (7) is the same, the requirement that AC voltages (V1AC, V2AC, V3AC) must be equal translates into


    that the active powers (P1, P2, P3) evacuated through each of the AC / DC grinding stations (7) must be equal. However, since the wind speed and the power produced by the wind turbines of each wind farm (1) do not have to be the same, it is not guaranteed that the distribution of active power will be balanced.
    For this reason, it is necessary to develop a control method and system capable of maintaining the controlled voltage and frequency in the wind farm's alternating current network formed by several diode rectifiers connected in series to a high-current DC link. tension. DESCRIPTION OF THE INVENTION
    The present invention solves the above problem thanks to the additional arrangement of an alternating current link that connects the busbars of each wind farm to each other at least two to two. This additional alternating current link allows an exchange of the power generated by each wind farm between the different facilities that, properly controlled, allows equalizing the evacuated powers through the respective AC / DC grinding stations.
    More specifically, it is clear that the exchange of power between the different facilities through said AC link must be carried out so that those installations whose wind farm generates more power yield part of that power to the facilities whose wind farm generates a lower power , until the power evacuated by each AC / DC grinding station is equalized. For this, it is taken into account that, as will be described in more detail below, the exchange of power between the busbars of two wind farms that are connected by an alternating current link is proportional to the relative angle formed by the voltage vectors of both bars Therefore, to ensure that the transmission of power between the facilities of the respective wind farms occurs in the correct direction, the present invention proposes to act on the angle of the tension of the busbar of that installation whose wind farm generates more power in the sense of increasing it. More specifically, since the tension in the busbar of an installation is proportional to the power generated by the wind farm connected to said installation, the frequency of the busbar voltage is increased proportionally to the voltage module. As a consequence, the angle of the tension vector in a first busbar through which more power passes becomes greater than the angle of the tension vector in a second busbar through which less power passes. A flow then occurs


    of power from the first bar to the second bar. This process takes place between each pair of connected busbars so that the end result is that the power evacuated by each AC / DC grinding station is equalized and, as a consequence, that AC voltages are equalized at the input of each AC grinding station / DC.
    In a preferred embodiment of the invention, the alternating current links can be arranged so that they connect the wind farms two to two. That is, each wind farm would normally be connected to the two wind farms adjacent to it. However, it is possible that the alternating current links form a mesh that connects the busbar of each wind farm with more than one busbar of other wind farms or even, ultimately, that connects the busbar of each wind farm with each and every one of the busbars of the rest of the wind farms. In principle, it would not be necessary to close a mesh in order to balance the powers and voltages in accordance with the control procedure described in this document, although this would provide the advantage of keeping the system running even when any of the alternating current links fails.
    A first aspect of the present invention directed to a system for controlling a set of wind farms connected in series to a high voltage direct current link is defined by claim 1, whose preamble essentially corresponds to the known elements of the patent application P201530410.
    A second aspect of the present invention directed to a method for controlling a set of wind farms connected in series to a high voltage direct current link is defined by claim 4, whose preamble essentially corresponds to the known elements of the patent application P201530410. BRIEF DESCRIPTION OF THE FIGURES
    Fig. 1 shows a system for controlling voltage and frequency in an isolated network of a wind farm connected to an HVDC link in accordance with the prior art.
    Fig. 2 shows a system formed by three wind farms connected in series to an HVDC link.
    Fig. 3 shows an example of a system according to the present invention.


    Fig. 4 shows a control diagram according to the process of the present invention. PREFERRED EMBODIMENT OF THE INVENTION
    A particular example according to the present invention is described below where the different elements that make up the system and the different steps of the proposed control procedure are appreciated.
    Fig. 3 shows a set of 3 offshore wind farms (1) connected in series to a high voltage direct current link (8). Each wind farm (1) is connected to a three-phase alternating current busbar (3) which, in turn, is connected to a diode bridge AC / DC rectifier station (7) that evacuates the energy generated by the park wind (1) to a high voltage direct current link (8). The connection between the busbar (3) and the AC / DC grinding station (7) includes a lifting transformer (6). The busbar (3) is also connected to a bank of capacitors (5) and an electronic converter (4) composed of self-switched and current-controlled switches, such as a STATCOM electronic compensator. The HVDC link (8) is connected to a DC / AC inverter station (9) that evacuates the power generated to an alternating current electrical network (10) located on the mainland. This configuration is formed by several installations (2) connected to the transmission system (8) by means of the serial connection on the DC side of the diode bridge AC / DC rectifying stations (7). Likewise, the three-phase alternating current busbars (3) of each installation
    (2) are connected to the busbars (3) of an adjacent installation (2) by means of respective alternating current links (11).
    As described in patent application P201530410, the regulation of the frequency of the AC voltage in each installation (2) is carried out by orienting the AC voltage vector of the busbar (3) towards a synchronous reference system imposed by the loop of voltage control in the capacitor bank (5). Normally, as described in the previous patent application, this implies that the quadrature component of the AC voltage in the busbar (3) must be zero (Vq = 0). The previous application also demonstrates that in order to keep the AC voltage vector oriented, and therefore the frequency controlled at its nominal value, the axis current q (quadrature component) of the electronic converter (4) is used. That is, the electronic converter (4) instantly closes the reactive power balance in the AC network.


    On the other hand, the effective value of the AC voltage on the busbar (3) is set by the active power generated by the wind turbine set (1) connected to each grinding station (7). As the active power of each wind farm
    (1) it depends on the wind speed, in the absence of the alternating current links (11) it can happen that the AC voltage levels in the busbar (3) of each installation (2) are different. As mentioned earlier, this implies an unequal distribution of tensions between the three AC / DC grinding stations (7), which is not admissible for the reasons described above.
    In the present invention, it is proposed to make a power distribution between the different AC / DC grinding stations (7) through the control of the power flow through the alternating current links (11) by means of the angle control (θ) of the tension vectors in each busbar (3). In fact, the active power (P12) transmitted between two busbars (3) connected by an alternating current link (11) depends on the module of the AC voltages (V1, V2) on the busbars (3) and the relative angle (θ12) between them according to the following equation (1):
    = ∙ without (-) (1)


    In principle, when there are several wind farms (1) each controller will have an arbitrary axis of position reference θ arbitrary. Therefore, to control the transfer of power through the links (11) so that it flows from those facilities (2) that evacuate more power to those that evacuate less power, a coordinated control of the frequency of each busbar (3) regulated by the respective electronic converters (4). This control is based on modifying the frequency in the busbar (3) of each wind farm (1) so that said frequency increases proportionally to the voltage module in said busbar (3). This is represented by the block (17) of Fig. 4, which will be described later. In this context, note that if each installation (2) is controlled so that Vq = 0, then the voltage module on the busbar (3) will coincide with the direct component Vd of said tension.
    For example, suppose you start from a situation in which all wind farms
    (1) generate the same active power. If, for example, the wind speed increases in the area of the wind turbines of the first wind farm (1) and as a consequence increases the power (P1) injected into the busbar (3) of the first installation (2),


    Automatically there would be an increase in voltage (V1) in the corresponding busbar (3) of said first installation (2). As a consequence of this voltage increase (V1), the proposed control method would implement an increase in the frequency of rotation of the reference synchronous axis in the tension vector (V1) of said busbar (3) of the first installation (2) . Because of this, the axis of the first installation (2) would advance to the axis of the second installation (2), and therefore a gap (desf1 -θ2) would occur between the angles of the respective voltage vectors (V1, V2) in the busbars (3) of the first and second installations (2). A power (P12) would flow from the busbar (3) of the first installation (2) to the busbar (3) of the second installation (2). As a consequence of the evacuated power (P12), the voltage (V1) on the busbar (3) of the first installation (2) would decrease and, thanks to the control implemented in said installation (2), this would imply a decrease in the frequency in said busbar (3). Simultaneously, as a consequence of the power (P12) received, the voltage (V2) on the busbar (3) of the second installation (2) would increase and, thanks to the control implemented in said installation (2), this would imply an increase of the frequency in said busbar (3). The result is that the voltages (V1, V2) of the busbars
    (3) of the first and second installation would be approaching until a balance is reached in which the powers (P1, P2) transmitted by each AC / DC grinding station (7) are also balanced. It is easy to deduce that this process would take place simultaneously between the second installation (2) and the third installation (2), producing a power transfer (P23). The final result of the whole process would be the achievement of a balance in all powers (P1, P2, P3) and voltages (V1, V2, V3), thus ensuring proper functioning of the HVDC link (8).
    Fig. 4 shows a preferred embodiment of the control system proposed in the present invention for an installation (2) in order to guarantee the control of the voltage and frequency of a set of wind farms (1) connected in series to an HVDC link (8).
    In the scheme of Fig. 4 the following notation is used:
    V: tension in the busbar (3) of the installation (2).Va, Vb, Vc: phases of the tension in the busbar (3) of the installation.Vq, Vd: components d-q of the tension in the busbar (3) of the installation.ω: frequency of the tension in the busbar (3) of the installation.θ: angle of the tension vector on the busbar (3) of the installation.iG: intensity injected by the wind farm.


    As can be seen, this scheme is almost identical to that represented in Fig. 5 of patent application P201530410 entitled "Method and system for voltage and frequency control in an isolated network" except for the introduction of the correction block (17 ) in the reference of the frequency of the wind farm (1).
    5 More specifically, as in the previous patent application, the scheme of Fig. 4 comprises a block 12 of transformation of instantaneous tensions of the three phases of the tension in the busbar (3) to components dq in a synchronous system, a block 13 for generating the synchronous axis by integrating the frequency signal, and a
    10 block 14 for regulating the orientation of the tension vector to the synchronous reference axis by reactive current injection. Additionally, in the present invention a block (17) is added consisting of producing an increase in the reference frequency as a consequence of the increase that occurs in the tension of the busbar (3) when the power injected by the wind farm increases ( one). Specifically,
    This increase is proportional to the module of the tension in the busbar (3) according to a constant called Kdroop. Note that, in this specific case in which the control is carried out in such a way that the component q of the tension in the busbar (3) (Vqref = 0), the module of the tension in the busbar (3) is zeroed ) will match the component Vd.

    权利要求:
    Claims (4)
    [1]
    1. System for controlling a set of wind farms (1) connected in series to a high voltage direct current link (8), where each wind farm (1) is connected to a busbar (3) which, at its once, it is connected through an elevator transformer (6) to a diode bridge AC / DC rectifier station (7) that evacuates the energy generated by the wind farm (1) to a high voltage direct current link (8 ), where the busbar (3) is connected to a capacitor bank (5) and an electronic converter (4), characterized in that it also comprises a plurality of alternating current links
    (11) that connect the busbars (3) of each wind farm (1) to each other at least two to two, so that it is possible to match the powers injected by each AC / DC rectifier station (7) to the direct current link at high voltage (8) by modifying the frequency in the busbar (3) of each wind farm (1) so that said frequency increases proportionally to the voltage module in said busbar (3).
    [2]
    2. System according to claim 1, wherein the plurality of alternating current links (11) form a mesh connecting the busbar (3) of each wind farm (1) with more than one busbar (3) of other wind farms (one).
    [3]
    3. System according to claim 2, wherein the plurality of alternating current links (11) form a mesh connecting the busbar (3) of each wind farm (1) with each and every busbar (3) of the rest of wind farms (1).
    [4]
    Four. Method to control a set of wind farms (1) connected in series to a high voltage direct current link (8), where each wind farm (1) is connected to a busbar (3) which, in turn, It is connected through an elevator transformer (6) to a diode bridge AC / DC rectifier station (7) that evacuates the energy generated by the wind farm (1) to a high voltage direct current link (8), where the busbar (3) is connected to a bank of capacitors (5) and an electronic converter (4), and which also comprises a plurality of alternating current links (11) that connect the busbars (3) to each other. each wind farm (1) at least two to two, characterized in that it comprises the step of matching the powers injected by each AC / DC rectifier station (7) to the high voltage direct current link (8) by

    the modification of the frequency in the busbar (3) of each wind farm (1) so that said frequency increases proportionally to the voltage module in said busbar (3).

     FIGURES 


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    同族专利:
    公开号 | 公开日
    WO2018150068A1|2018-08-23|
    ES2620972B2|2017-12-26|
    引用文献:
    公开号 | 申请日 | 公开日 | 申请人 | 专利标题
    GB2397445A|2003-01-14|2004-07-21|Alstom|Power transmission circuits|
    US20120175962A1|2011-01-11|2012-07-12|Converteam Technology Ltd.|Power Collection and Transmission Systems|
    ES2584535A1|2015-03-27|2016-09-28|Universidad Carlos Iii De Madrid|Method and system for voltage and frequency control in an isolated network |
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    ES201730190A|ES2620972B2|2017-02-16|2017-02-16|Method and system to control a set of wind farms connected in series to a high voltage direct current link|ES201730190A| ES2620972B2|2017-02-16|2017-02-16|Method and system to control a set of wind farms connected in series to a high voltage direct current link|
    PCT/ES2018/070106| WO2018150068A1|2017-02-16|2018-02-14|Method and system for controlling a group of wind farms connected in series to a high-voltage dc link|
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