WO2010018264A1

WO2010018264A1 - Electric power transmission system with a dc link

Info

Publication number: WO2010018264A1
Application number: PCT/ES2009/000410
Authority: WO
Inventors: Ramón Manuel BLASCO GIMÉNEZ; Antonio Correcher Salvador; Emilio GARCÍA MORENO; Francisco José MORANT ANGLADA; Eduardo Quiles Cucarella
Original assignee: Universidad Politecnica De Valencia
Priority date: 2008-07-31
Filing date: 2009-07-31
Publication date: 2010-02-18
Also published as: WO2010018264A4; ES2358816A1; ES2358816B1

Abstract

The invention relates to an electric power transmission method and system with a DC link, including electric generators (1) connected to an AC collector or bus (4) either directly or via a wiring system (2) consisting of electronic power systems and/or transformers and the corresponding electric lines (3) thereof, said system also including reactive power compensators (7) and filters (6). The AC collector (4) is connected using transformers (8) to a non-controlled rectifier or converter (9a), such that the offset angle between voltage and current is minimum, as is the reactive power requirement thereof.

Description

Electric power transmission system with direct current link

The described invention consists of a system of transmission of electric power in direct current that joins two or more buses of alternating current and its associated control method. It is of special relevance in the sector of transport and distribution of electrical energy, especially for cases where the production plants are very far from the distribution and consumption areas, as is the case of wind farms located on the high seas.

Problem to solve

The coupling to electricity transmission networks of large generating facilities located over long distances is particularly complex. This coupling is especially important in wind farms of very high capacity. Offshore facilities with more than 1 GW of installed power are currently being projected in the North Sea; Some of these parks are located more than 40 km from the coast, so that the transmission of high-voltage direct current (HVDC) energy is the most feasible technological solution. At these power levels, the most appropriate technology consists in the use of rectifier-inverter systems based on self-switched thyristors. At lower powers it is possible to use HVDC-Light systems, with voltage source converters (VSC); However, the use of systems with line switched thyristors (LCT) is the most feasible solution when the installed power exceeds 500-600 MW.

The typical configuration consists of the connection of the different generators, directly or through electronic power systems, to a common collector or alternating bar. Said alternating bar is connected to a converter that rectifies the alternating current and delivers it to a direct current transmission line. The direct current line is connected in turn to the alternating transmission network through an inverter. It is necessary to control the voltage variations in the alternating line to which the generators are connected and for this a synchronous or static compensator (or STATCOM) and / or different reactive power compensation filters and banks are used. The proposed systems known so far consider that different characteristic parameters (effective value, frequency) of the voltage at the connection point of the generators (collector) must be kept constant. Therefore, the control of the transmitted power is carried out by means of trigger pulses applied to each of the converters. Generally, the rectifier converter controls the current and the inverter converter controls the voltage of the continuous link, getting a control of the power transmitted by means of the firing impulses applied to each of the converters. It is also possible to control the voltage by the rectifier and the current by the inverter.

On the other hand, there is no knowledge of any system that exploits the capacity of injection of harmonic and reactive currents of the generators for the minimization of the filtering requirements of the rectifier.

The coupling of alternating current networks through high-voltage direct current links is justified, in most cases, for economic reasons. In other cases, the need to couple two or more asynchronous AC buses justifies said connection. The invention described herein proposes a new control method and a new configuration of the energy transmission system by which the voltage, frequency and harmonic content in the collector is modified to control the voltage and / or current (and , therefore, the power) of the continuous comment link, resulting in a simpler and cheaper method when it comes to its implementation than those known to date.

State of the art

In the state of the art, references to electric power transmission systems by high-voltage direct current and its associated control method are known, such as those found in WO 2006/35018 A2, WO 2006/034250 A1, WO 02 / 063758 A1, WO 00/62409 A1, WO 2006/094952 A1, WO 2006/037412 A1 or WO 2007/027141 A1. In other documents, such as WO 2007/033619 A1 and WO 2007/033620 A1, only the control method is described. For its part, GB 2423650 A describes a connection system of a single generator of variable voltage and frequency to power supply networks, preferably of fixed voltage and frequency, using a rectifier-inverter; This invention would constitute an alternative for the connection, through electronic power systems, of each of the generators to the collector mentioned above.

Description of the invention

The invention described herein refers to an electric power transmission system with direct current link consisting of several electric generators connected to a collector, directly, or through electronic power systems, and using current transformer lines and transformers. alternate, if it were necessary, with some filters and reactive power compensators. The alternating current collector is connected by one or several transformers to an uncontrolled converter or rectifier, so that the angle of displacement between voltage and current is minimal, as well as its reactive power demand, without the need of the transformer tap changer of the rectifier and with a significant reduction or even elimination of harmonic filters and reactive power compensation systems (eg STATCOM or Synchronous Compensator).

The uncontrolled rectifier, on the other hand, feeds a direct current line that connects to the alternating transmission network through an inverter, by means of one or several transformers.

The invention also relates to a method of control and transmission of electrical energy in an electrical energy transport network, whereby the voltage, frequency and harmonic content of the alternating bus where the generators are located are modified to control the voltage and / or the current of the direct current link of an electrical energy transmission system in an electrical energy transport network. Said network consists of electric generators connected to a collector or bus of alternating current, directly or through a connection system consisting of electronic power systems and / or transformers, with their corresponding power lines, with filters and power compensators reactive The collector or bus of alternating current is connected, by one or several transformers, to an uncontrolled converter or rectifier, so that the angle of displacement between voltage and current is minimal, as well as its reactive power demand, without the need of the changer of shots. In said method, the uncontrolled rectifier feeds a direct current line that is connected to the alternating current receiver network through an inverter, adapting the voltage by one or more transformers.

In another configuration, a method of control and transmission of electrical energy in an electrical energy transport network is proposed, whereby the voltage, frequency and harmonic content of the alternating bus where the generators are located are modified to control the voltage and / or the current of the direct current link of an electrical energy transmission system in an electrical energy transport network. Said network consists of electric generators connected to a collector or bus of alternating current, directly or through a connection system consisting of electronic power systems and / or transformers, with their corresponding electric lines, with filters and reactive power compensators, and where the collector or AC bus is connected, by one or several transformers, to a controlled converter or rectifier, so that the tap changer of the transformers is eliminated and the systems are reduced or eliminated of filtering and reactive compensation. In said alternative method, the controlled rectifier feeds a direct current line that is connected to the alternating current receiver network through an inverter, adapting the voltage by one or more transformers.

Description of the figures Figure 1 represents a standard high voltage direct current (HVDC) transmission system according to the state of the art known until now.

Figure 2 illustrates an energy transmission system with direct current link and uncontrolled rectifier according to what is described in the present invention.

Figure 3 shows a system that uses several uncontrolled converters in the same location or in geographically separated locations, which evacuate their power in the same high-voltage direct current line.

In these figures, the enclosed numerical references correspond to the following elements:

I. Generators 2. System connecting the generators to the alternating network.

3. AC power lines that connect the generators to the AC bus or collector (4).

4. Bus or alternating collector of the generating network of alternating generator.

5. Dynamic reactive compensator (synchronous compensator or STATCOM).

6. Filter banks.

7. Compensators (capacitor banks).

8. Transformer.

9. Rectifier. 9a. Uncontrolled rectifier.

10. High voltage direct current line.

I I. Investor.

12. Transformer.

13. AC bus of the receiving AC power grid. 14. Local charges. Description of a preferred embodiment of the invention

As already mentioned, the invention described consists of a system of transmission of electric power in direct current that links two or more AC buses and their associated control method. The coupling of alternating current networks through direct current links is justified in most cases, as previously noted, for economic reasons. In other cases, the need to couple two or more asynchronous AC buses justifies said connection.

The installations of transmission of electrical energy by means of direct current lines are, in general, bidirectional, that is, the electrical energy

(product of the continuous voltage and current) can be transmitted in both directions. The inversion of the direction of transmission of the energy is achieved, in the majority of cases, by reversing the direction of circulation of the continuous comment, since the inversion of the polarity of the voltage presents important technical disadvantages.

In those cases where DC systems require only one-way power transmission, it is possible to replace one or more of the converters with uncontrolled rectifiers, so that the voltage and / or current of the DC link It can be controlled by variations in voltage, frequency, reactive power, and / or harmonic content of the corresponding AC buses.

By way of example, it is possible to mention the coupling to electric power transmission networks of large generating facilities, in which the energy transmission will always be unidirectional. The transmission of the energy generated by direct current links is economical when the generating installation is of great power and is located a great distance from the electrical transmission network.

This is the case of wind farms of very high capacity located at relatively large distances from the coast. These types of installations have the characteristic of a unidirectional and variable energy transmission, in which the use of direct current energy transmission systems represents the most advisable alternative from a technical and economic point of view.

A typical system of these characteristics, known in the state of the art, is the one that appears in figure 1. In it a number of generators (1) connected directly or through electronic power systems (2) have been represented, together with the corresponding transformers if necessary, to a common alternating bar or manifold (4) by means of the corresponding alternating current lines (3). Said collector (4) is connected to a controlled converter or rectifier (9) directly or by means of one or more transformers (8). The controlled rectifier (9) rectifies the alternating current and delivers it to a direct current transmission line (10). This direct current line (10) is connected to the receiving alternating network (13) through an inverter (11), directly or through the use of one or more transformers (12).

The corresponding filters (6) are included on both sides of the transmission system, which attenuate the level of harmonics produced by the controlled rectifier (9), as well as compensators (7), which provide the reactive power necessary for the operation of controlled rectifier (9). In this way, the filters (6) and the compensators (7) act as elements of the filter banks, which are switched on or off in a staggered manner according to the level of reactive power necessary for the operation of the controlled rectifier (9). In many cases, said filters (6) and the compensators (7) are complemented by synchronous or static compensators (STATCOM) (5), so that the stepwise variation of the reactive power produced between the connection of a bank and the next one is compensated by reactive power producing elements of lower nominal power, but with continuous variation thereof. There is also an undetermined number of local loads (14), such as those associated with the rectifier or those presented by each wind turbine.

The systems proposed so far consider that the voltage, frequency and harmonic content of the alternating current network at the connection point of the generators (collector) must be kept constant. This is the most common case when two alternating buses are connected through a continuous link and the transmission of energy in either of the two directions is intended.

In these cases, the control of the transmitted power is carried out by means of the trip impulses applied to each of the converters. Generally, the rectifier converter controls the current flowing through the DC link and the inverter converter controls the DC link voltage.

The present invention consists of a control method whereby the voltage, frequency and harmonic content of the alternating bus where the generators are located, is modified in order to control the voltage and / or the current of the direct current link and to reduce the Need for filters (6) and reactive compensation (5 and 7). Figure 2 shows the proposed continuous power transmission system. Similar to the configuration of Figure 1, in Figure 2 a number of generators (1) connected directly or through electronic power systems (2), together with the corresponding transformers if necessary, have been represented to a bar of alternating common or collector (4) by means of the corresponding lines of alternating current (3). Likewise, there may be reactive power compensation banks (7), filters (6) and compensators (5).

Unlike the state of the general technique, the alternating current bus (4) is connected, by means of a transformer (8), to an uncontrolled converter or rectifier (9a). Obviously, if the generators (1) change the collector voltage (4) continuously, the transformer sockets (8) are superfluous. In fact, the tap changer of the transformer (8) is used to maintain the firing angle of the controlled rectifier valves (9) in a certain range in steady state and thus reduce the reactive power demand of said rectifier (9) . With the use of an uncontrolled rectifier (9a), the firing angle is always zero degrees and the reactive power demand is minimal, without the need for the tap-changer. Again, the proposed system entails reducing costs and increasing the robustness of the system as a whole.

The instantaneous voltage of the alternating current bus (4) and, therefore, the effective voltage, is controlled by acting on the generators (1), either directly, modifying its magnetizing current, or acting on the electronic power systems (2 ) that connect the generators to the alternating bus (4).

The voltage variation in the alternating bus (4) is ideally reflected proportionally in a variation in the rectified voltage in the continuous bus (10), through the uncontrolled rectifier (9a). In this way, the voltage in the continuous bus can be controlled, by adjusting the corresponding voltage of the alternating bus. In this case, the inverter (11) should be controlled to ensure the current circulation to achieve the transmission of the desired power.

Alternatively, it is possible to measure the direct current at the output of the rectifier (9) and modify the voltage of the continuous bus (10) to obtain a certain current in the continuous bus (10) according to the method mentioned above, that is, varying the instantaneous voltage of the alternating current bus (4). In this case, the inverter (11) should be controlled to ensure the level of voltage in the continuous bus (10) to achieve the transmission of the desired power. At the same time, it is possible to vary the frequency of voltages and currents of the alternating bus (4) to achieve a continuous variation of the reactive power delivered by the filters, so that the transient connection and disconnection of filters would be limited and losses could be optimized, continuously controlling the reactive energy balance. The frequency and voltage control of the alternating bus (4) could be carried out jointly. The proposed control method also contemplates the use of voltage waveforms and harmonic currents in the collector, which is equivalent to the use of non-sinusoidal waveforms. In this way, it is possible to collaborate in the filtering of harmonics produced by the rectifier, and even to eliminate the filter banks used for this purpose. The proposed control method could complement, if deemed necessary, the method proposed in WO 2006/035018 A2, and thus achieve the advantages of said control method, among others, a reduction in thermal and stress stress of the different elements of the system and a decrease in the risk of switching faults in the inverter. The proposed method is especially indicated, among other cases, for use in large wind farms located a great distance from the transmission network (for example, offshore wind farms), in which the voltage, power and frequency of the different wind turbines is controlled by electronic converters.

With this control method, the control of the rectifier can be ignored, and replaced by an uncontrolled rectifier. The advantages obtained from this method of operation are, among others:

• Elimination of trip circuits and rectifier control.

• Elimination of synchronous compensator (STATCOM).

• Elimination of the need for the tap changer on the rectifier side and, possibly, on the inverter side.

• Natural operation in the firing angle that produces the minimum demand for reactive power and harmonic content in the current.

• Increase the efficiency of the rectifier, since the diodes have less conduction losses than the thyristors. • Reduction of rectifier costs.

• Reduction or elimination of reagent filters on the rectifier side.

• Reduction or elimination of harmonic filters on the rectifier side.

• Rectifier weight reduction.

The exposed advantages are especially relevant, for example, in large offshore wind farms, in which an increase in the robustness of the rectifier, to the For a decrease in their weight, they represent important economic benefits in the implementation and operation of the wind farm.

On the other hand, with the proposed control method, the reactive power provided by the filters (6) and compensators (7) can be continuously adjusted, and the losses produced in lines (3), (10) and transformers (8) can be minimized. ), (12), by means of the variation of the voltage and the frequency of the collector (4), either independently or in a coordinated manner. In this way, the power of the required synchronous compensator (5) could be reduced or eliminated completely. Again important economic advantages are achieved, as well as in the robustness of the transmission system as a whole, having managed to eliminate a point of critical failure.

In the same way, the operation at relatively high frequencies allows the reduction of losses in lines and transformers. It is foreseeable that the losses in the rectifier will increase as the frequency of the collector increases, so that it is also possible to adjust the frequency within a range so that the overall losses are minimized.

Below are two examples of using this method.

Example 1

Use of the control method and the transmission system with uncontrolled rectifier and transformer without tap-changer in wind farms with wind turbines based on asynchronous double-feed induction generators.

This type of generators is characterized by the direct connection of the stator to the network, while its control is carried out by means of AC / DC / AC converters that connect the generator rotor with the alternating current network. They have the great advantage that the nominal power of the electronic converters used is a fraction of the nominal power of the set.

The stator voltage of the generators, which are directly connected to the collector (or through one or more transformers), can be easily reduced by flow weakening techniques. Said voltage reduction allows controlling the current flowing through the direct current link.

As a disadvantage, it is that the operation at reduced flow necessarily implies a reduction in the maximum power that the generator can deliver. However, said reduction of flow and tension necessarily occurs when the energy present in the wind is reduced and, consequently it is necessary to reduce the energy transmitted by the continuous link.

The voltage reduction range produced by the wind turbines is not generally excessively large, but this small variation margin is sufficient to control the current through the direct current link. In this case, the DC voltage would be controlled by the inverter converter.

As an additional advantage, the iron losses of the generators and transformers would be reduced when operating at a voltage lower than the nominal voltage. In addition, the operation at reduced voltage in the stator also allows the reduction in the rotor voltage, with which the range of operating speeds of the turbine is extended, being able to extract more energy at low wind speeds.

The system described would not require a transformer with tap changer, although in certain cases it may require sufficient filtering to mitigate the harmonics produced by the rectifier.

With the reactive power generation capacity of the current wind turbines, the value of the necessary reactive banks could be reduced compared to those used with the continuous transmission systems that use a controlled rectifier. These banks could perhaps be eliminated with the proper design and / or appropriate selection of wind turbines.

Example 2

Use of the control method and transmission system with uncontrolled rectifier and transformer without tap changer in wind farms with wind turbines based on synchronous generators.

Wind turbines based on synchronous generators are connected to the network through AC / DC / AC (or AC / AC) converters of nominal power equal to that of the generator. This type of converters implies a greater expense than in the double-feed asynchronous generators, however, they provide, among other advantages, a greater flexibility in the control, a greater range of operating speeds of the turbine and a better behavior before transients in The electricity network.

This type of wind turbines can use the described control method to regulate the current flowing through the continuous link. In the same way, the rapid response of this type of wind turbines allows the wind turbines themselves to be able to control the currents that circulate in the alternating and high-voltage continuous installations during faults and transients. Since the converters of the wind turbines can carry out the constant voltage regulation, it is possible to eliminate the changes of sockets of the different transformers. In addition, converters of this type of wind turbines can provide a certain level of reactive power and even harmonic currents, so that the nominal values of the harmonic and reactive filters can be lowered or even eliminated.

On the other hand, it is possible to modify, in a simple way, the frequency of the alternating side, so that it is possible to reduce the losses and, at the same time, regulate the reactive power produced by the different filters.

Claims

1. - Electric power transmission system with direct current link consisting of electric generators (1) connected to an AC collector or bus (4), directly or through a connection system (2) consisting of systems power electronics and / or transformers, with their corresponding power lines (3), with some filters (6) and reactive power compensators (7), characterized in that the collector or alternating current bus (4) is connected, by one or several transformers (8), to an uncontrolled converter or rectifier (9a), so that the angle of displacement between voltage and current is minimal, as well as its reactive power demand.

2. - Electric power transmission system with direct current link according to claim 1 characterized in that the uncontrolled rectifier (9a) feeds a direct current line (10) that connects to the alternating current receiver network (13 ) through an inverter (11), adapting the voltage by one or several transformers (12).

3. - Method of control and transmission of electrical energy in an electrical energy transport network, whereby the voltage, frequency and harmonic content of the alternating bus where the generators are located are modified to control the voltage and / or The current of the direct current link of an electric power transmission system in an electric power transport network, which consists of electric generators (1) connected to an alternating current bus or bus (4), directly or through a connection system (2) consisting of electronic power systems and / or transformers, with their corresponding power lines (3), with filters (6) and reactive power compensators (7), characterized in that the collector or bus of alternating current (4), it is connected, by one or several transformers (8), to an uncontrolled converter or rectifier (9a), so that the angle of travel between Minimum voltage and current, as well as your reactive power demand, without the need for the tap-changer.

4. - Method of control and transmission of electric energy in an electric power transport network according to claim 3 characterized in that the uncontrolled rectifier (9a) feeds a direct current line (10) that is Connects to the alternating current receiver network (13) through an inverter (11), adapting the voltage through one or more transformers (12).

5. - Method of control and transmission of electrical energy in an electrical energy transport network, whereby the voltage, frequency and harmonic content of the alternating bus where the generators are located are modified to control the voltage and / or The current of the direct current link of an electric power transmission system in an electric power transport network, which consists of electric generators (1) connected to an alternating current bus or bus (4), directly or through a connection system (2) consisting of electronic power systems and / or transformers, with their corresponding power lines (3), with filters (6) and reactive power compensators (7), characterized in that the collector or bus of alternating current (4), it is connected, by one or several transformers (8), to a controlled converter or rectifier (9), so that the tap changer of the transformers (8) and the filtration systems (6) and reagent compensation (7) are reduced or eliminated.

6. - Method of control and transmission of electrical energy in an electrical energy transport network according to claim 5, characterized in that the controlled rectifier (9) feeds a direct current line (10) that is connected to the receiver network of alternating current (13) through an inverter (11), adapting the voltage by one or several transformers (12).