WO2007077002A2

WO2007077002A2 - Method and system for the protection of an electricity generation facility connected to an electricity network in the presence of voltage sags in said network

Info

Publication number: WO2007077002A2
Application number: PCT/EP2006/012555
Authority: WO
Inventors: Jesus Lopez Taberna; Luis Maria Marroyo Palomo; Pablo Sanchis Gurpide; Eugenio Gubia Villabona
Original assignee: Universidad Publica De Navarra
Priority date: 2005-12-30
Filing date: 2006-12-27
Publication date: 2007-07-12
Also published as: WO2007077002A3; ES2298014A1; ES2298014B1

Abstract

Method and system for the protection of an electricity generation facility, of those which comprise a wind generator connected to an electricity network, in the presence of voltage sags in the network, the wind generator comprising a double-fed asynchronous generator formed by two windings, a winding in the rotor and a winding in the stator, of the type where, the rotor winding is connected to a rectifying-conversion circuit. The current generated in the asynchronous generator rotor is obliged to pass through an impedance connected in series to the rotor, which is designed to limit the overcurrents and overvoltages in connectors of the rectifying-conversion circuit.

Description

D E S C R I P T I O N

"Method and system for the protection of an electricity generation facility connected to an electricity network in the presence of voltage sags in said network"

Technical field of the invention

The invention relates to a method for the protection of an electricity facility, particularly of the type which comprises a wind generator, of the double-fed asynchronous type, connected to an electricity network when a voltage sag occurs in said network.

Background of the invention

Currently, wind power is on the increase and is considered, among renewable energies, as the one with most probability of becoming a real alternative to conventional, more contaminating power sources such as those derived from fossil fuels such as petrol, gas or coal.

This development, however, is checked by the problems of integration which appear when the number of wind parks increases, and in consequence the number of wind generators, connected to the electricity network. One of the most important problems is related to the behaviour of the wind generators in relation to voltage sags in the network. In order to avoid these problems in most countries with great wind development, the behaviour of the wind generators with respect to voltage sags is being regulated.

With the appearance of variable speed machines, it has been achieved that the machines suffer less mechanically from gusts of wind, that the electricity generated has less fluctuations and that the energy use of the wind is greater.

These advantages have meant that most machines installed have been this type for years.

In the main strategies used to obtain variable speed, in accordance with the electric generator used, a synchronous or asynchronous generator intervenes. The first type of generator has the drawback that all the power generated prior to its supply to the electricity network, must be converted by two electronic converters.

These converters should, therefore, be dimensioned for the full power of the wind generator, resulting expensive and voluminous. Their power losses also cause a decrease in the total output of the wind generator.

From the asynchronous generators, the most widely used is the double-fed asynchronous generator, wherein the stator is directly connected to the network, whilst the rotor is connected to the network via a converter which permits controlling the active and reactive power of the electric generator. In this case, the power which passes through the rotor is only a small fraction of that of the stator, for which reason the converters are smaller in cost and size, and generate fewer losses.

Nevertheless, the solution based on the double-fed asynchronous generator is less sensitive to the faults which may occur in the electricity network. The power converter connected to the rotor is a very vulnerable part of the system. In other words, when a fault occurs in the network, known as voltage sag, and the voltage of one or several lines drop the current which appears in said converter may reach very high values, until even destroying it.

Until recently, the typical solution consisted of deactivating the converter and connecting the system called and known as "crowbar" when it detected that the converter current was too high. The crowbar consists of a bank of very low value resistances (even short-circuit) which are connected in parallel with the rotor. In the normal situation, the crowbar is disconnected, for which reason it does not affect the normal functioning of the machine. In the event of a voltage sag occurring, the crowbar is connected during a time of between 100 and 200 msec, sufficient time so that the transitory force which the magnetic status of the machine undergoes is reduced. With this manoeuvre, the converter is completely protected but control of the machine is lost.

The resistances which form the crowbar should be chosen from a small value, to reduce the voltage applied to the converter. Nevertheless, this makes the current which circulates through them very high, reaching 3,000 A. Furthermore, the current of the crowbar means that the machine generates a peak in the torque which exceeds the nominal value from two to four times. Normally, the mechanical system is not prepared to suffer such an intensive torque surge and may break. To avoid it, an elastic seal should be added to the mechanical shaft which buffers said hit. On the other hand, as the crowbar comprises small resistances, the demagnetization or reduction time of the overcurrent is very high and, on the other hand, if the generator continues connected to the electricity network overcurrents occur in the machine. To avoid these overcurrents, the systems which comprise a crowbar disconnect the generator from the network and do not re-connect it until the voltage again reaches its nominal value.

In this way, the technique previously described protects the converted but continues without meeting the requirements currently demanded by the people in charge of the electricity network which is that of continuing working in the event of voltage sags.

In order to avoid disconnecting the generator from the network during the voltage sags, some alternatives are presented below.

Patent document JP7067393 discloses an overvoltage protection device of a variable speed power generation system. This device comprises a resistance together with a 'chopper¹ circuit in the DC bus. Indeed, what this document proposes is, in the event anomalies occur in the network voltage, deactivating the converter and connecting the resistance to the chopper circuit, in parallel with the rotor using the inverter diodes. The chopper is controlled so that the DC bus voltage remains constant, protecting in this way the converter. However, with this technique, large currents continue to arise during the first instants of the sag with the consequent torque surge. On the other hand, as the diodes of the inverter are used, other drawbacks arise, since it is necessary to oversize said diodes so that they withstand the overcurrents during the voltage sags. Furthermore, if sags arise that are not three-phase, strong overvoltages occur in the rotor which make the chopper circuit with the resistance continually connect and disconnect during the duration of the voltage sag. The device described also includes a crowbar in parallel with the rotor for security reasons.

Another alternative to the crowbar is disclosed in patent document EP1499009 which envisages a retention unit, formed by voltage-variable resistances, to limit the overvoltage which occurs in the rotor in situations of voltage sags. The retention unit is connected in parallel to the rotor circuit of the wind generator and is designed to absorb the current produced by the voltage sags in a short period of time, then allowing the current to pass through the rotor converter. Although this system envisages an alternative to the crowbar, it requires time to demagnetize the machine during which the rotor converter is deactivated. Indeed, this configuration is similar to that of the aforementioned patent document with the difference that the diode bridge of the inverter is not used, for which reason it suffers the same drawbacks.

Patent document WO2004/030199 discloses demagnetization equipment and the connection of an electronic switch between the stator and the network. The demagnetization equipment is connected in parallel to either the stator or the rotor. This demagnetization equipment is based on a variable resistance which, in the event a sharp variation is detected in the voltage network, is connected in parallel to the stator at same time as this is disconnected from the electricity network. Once the voltages of the network and the machine have been levelled out, the generator is reconnected to the network and it disconnects the variable resistance which acts as demagnetizing element. In a form similar to the first solution described (crowbar), this system demands the disconnection of the generator during voltage changes and causes the successive connection and disconnection of the demagnetizing element if single-phase and two-phase sags exist.

The previous drawbacks are the reason why the installation of the crowbar and the other alternatives explained are insufficient to resolve the current needs.

Explanation of the invention

The method for the protection of an electricity generation facility according to the invention is particularly applicable to the facilities provided with at least one wind generator connected to an electricity network, of those which comprise a double-fed asynchronous generator formed by two windings, a winding in the rotor and a winding in the stator, of the type where the rotor winding is connected to a rectifying- conversion circuit, wherein the AC generated in the generator rotor, prior to its supply to said network, first converts to DC and then inverts to produce a sinusoidal form with the frequency of the electricity network. In essence, the method according to the invention is characterized in that the current generated in the asynchronous generator rotor is obliged to pass, prior to its passage through the rectifying-conversion circuit, through an impedance connected in series to the rotor, which is designed to limit the overcurrents and overvoltages in connectors of the rectifying-conversion circuit. In accordance with these characteristics, the wind generator is not disconnected from the network at any time, enabling the overcurrent and/or overvoltage generated in the rotor circuit to be absorbed in the new impedance connected in series without the rectifying-conversion circuit being affected.

In accordance with a variant of the invention, the series impedance consists of an inductance permanently connected to the generator rotor, which means the current is obliged to pass through said inductance both in normal conditions and if a voltage sag occurs in the electricity network.

According to another characteristic of the invention, the method further comprises the step of connecting in parallel, in the event of a voltage sag occurring, in the AC bus of the converter and/or with the AC side of the rotor, power dissipation elements designed for the evacuation of the magnetic power generated during the first instants of the voltage sag.

In accordance with another variant of the invention, the series impedance is connected to the generator rotor when a voltage sag occurs, while it is short- circuited in normal conditions. In accordance with another variant of the invention, the rectifying-conversion circuit consisting of a converter, in the event of a voltage sag occurring, imposes a new setpoint current which is the result of adding a new term to the setpoint current, called demagnetizing current, which generates a flow in the rotor winding opposite the free flow, the free flow being that which is not caused by the direct component of the stator voltage, consequently reducing the voltage in converter connectors.

According to another characteristic of the invention, the demagnetizing current is proportional to value of the free flow ψ_sι of the generator stator, estimated as the difference between the value of the magnetic flow in the generator stator ψ_s and the value of the stator flow associated to the direct component of the stator voltage, called forced flow ψ_sf .

In accordance with another aspect of the invention, a system is disclosed for the protection of an electricity generation facility of the type which comprises at least one wind generator connected to an electricity network which in turn comprises a double-fed asynchronous generator with two windings, one in the rotor and another in the stator, of the type where the rotor is connected to a rectifying-conversion circuit, in turn connected to the electricity network.

In essence, the system is characterized in that the system comprises an impedance connectable in series between the winding of the asynchronous generator rotor and said rectifying-conversion circuit, designed to limit the overcurrents and overvoltages in connectors of the rectifying-conversion circuit in the presence of a voltage sag in the network.

According to a variant of the invention, the series impedance consists of an inductance which is permanently connected between the generator rotor winding and the rectifying-conversion circuit. In accordance with another characteristic of the invention, the system further comprises power dissipation elements, connectable in parallel in the event of a voltage sag occurring, with the DC bus of the converter and/or with the AC side of the rotor.

In accordance with another variant of the invention, the series impedance comprises a resistance and means of short-circuit, designed to short-circuit said resistance in normal conditions and to connect it in the event of a voltage sag occurring in the network.

In accordance with another characteristic of the invention, the means of short-circuit comprises a relay or a controlled switch.

According to another characteristic of the invention, the system comprises a converter which, governed by a control unit, imposes a predetermined rotor current called setpoint current, and in that the control unit comprises an auxiliary module which incorporates a first unit for the estimate of the value of the stator flow; a second unit for the estimate of the stator flow associated to the direct component of the stator voltage, called forced flow, in the event of a voltage sag occurring in the network; a third unit, which calculates the difference between the values of the stator flow and the previously estimated forced flow; a fourth unit, multiplier, which multiplies the value of the previously calculated difference by a K2 factor for the production of the demagnetizing current; and a fifth unit, for the sum of the value of the setpoint current and the value of the previously calculated demagnetizing current.

In accordance with another characteristic of the invention, the K2 factor is less than 1.

Brief description of the drawings The attached drawings schematically illustrate an electricity generation facility, as well as different variants of a system according to the invention. In said drawings:

Fig.1, is a schematic representation of a conventional electricity generation facility; Fig.2, is a schematic representation of the facility of Fig.1 which incorporates a known protection system;

Fig.3, is a schematic representation of the facility of Fig.1 which incorporates a protection system according to the invention;

Fig.4, is a block diagram of a variant of the method according to the invention; and

Fig.5a and 5b are different schematic representations of two variants of the protection system according to the invention.

Fig. 6a, 6b, 6c, 6d, 6e, 6f and 6g are respective graphics of the evolution of the main electrical variables which arise in the wind generator of the example given according to the current state of the art in the event of a three-phase voltage sag;

Fig. 7a, 7b, 7c, 7d, 7e, 7f and 7g are respective graphics of the evolution of the main electrical variables which arise in the wind generator of the example given according to the current state of the art in the event of a two-phase voltage sag;

Fig. 8a, 8b, 8c, 8d, 8e, 8f and 8g are respective graphics of the evolution of the main electrical variables which arise in the wind generator of the example given according to the invention in the event of a three-phase voltage sag; and

The Fig. 9a, 9b, 9c, 9d, 9e, 9f and 9g are respective graphics of the evolution of the main electrical variables which arise in the wind generator of the example given according to the invention in the event of a two-phase voltage sag.

Detailed description of the drawings

The method for the protection of an electricity generation facility which as example of embodiment is explained below, is applied to a facility which comprises a wind generator 1 connected to an electricity network 8 which comprises a double- fed asynchronous generator 11 formed by two windings, a winding in the rotor 13 and a winding in the stator 12, of the type where the rotor winding 13 is connected to a rectifying-conversion circuit 2, all as indicated in Fig.1.

The rectifying-conversion circuit 2 is where the AC generated in the generator 1 rotor, prior to its supply to the electricity network 8, first converts to DC and then inverts to produce a sinusoidal form with the frequency of said electricity network 8, typically 50 Hz.

Fig. 2 schematically represents a conventional protection system, which is known as crowbar 9, designed to short-circuit the rotor circuit of the wind generator 1 when a voltage sag occurs in the network 8. In accordance with this solution, during the time periods wherein the crowbar 9 is connected in parallel to the rotor circuit, the generator 1 is disconnected from said network 8.

The method for the protection of electricity generation facilities according to the invention is characterized in that when a voltage sag occurs in the network 8, the current generated in the asynchronous generator rotor 11 is obliged to pass, prior to its passage through the rectifying-conversion circuit 2, through an impedance 3 connected in series to the rotor as is represented in Fig. 3. Said impedance 3 is designed to limit the overcurrents and overvoltages in connectors of the rectifying- conversion circuit 2.

The mission of said impedance 3 is to limit the overcurrents and overvoltages in any case of voltage sag, if a fault or reduction in the electricity network 8 occurs due to a short-circuit in just one of the lines, which is known as a single-phase sag, in two of the lines, called two-phase sag, or in the case of a short- circuit occurring in the three lines, which is known as three-phase sag.

In the state of the art, such as the example represented in Fig.1, if a voltage sag appears in the electricity network 8, the current which circulates through the rotor is limited by the converter and by the leak inductance 10 of the generator, not represented, whose value is normally very low. In order to reduce the value of this current, the electricity generation facilities of Figs.4, 5a and 5b incorporate a protection system which comprises an impedance 3 fitted in series between the rotor and the converter. In this way, the current will be mainly limited by this new fitted impedance 3. The impedance 3 of the protection system according to the invention, should not be confused with the filter inductance 10 which is conventionally connected in series to the generator 1 rotor, whose objective is that of helping the current control loop of the converter of the rectifying-conversion circuit 2 and reducing the overvoltages due to reflections. The value of these inductances 10 is low, typically 100 μH. Contrary, the value of the impedance 3 of the system according to the invention is much greater, in the order of mH, and their presence pursues a very different purpose as previously explained.

In a variant of the aforementioned method, represented in Fig. 5a, the series impedance 3 consists of an inductance 31 which is permanently connected between the rotor winding 13 of the generator and the rectifying-conversion circuit 2. The fact that the impedance 3 consists of an inductance 31 , offers the advantage that this may be left disconnected in normal conditions since its presence does not affect the generator 1. Due to the fact that the series impedance 3 consists of an inductance 31 , the small filter inductance 10 necessary to attenuate the effects of other reflections can also be dispensed with.

In the variant represented in Fig.5b, the series impedance 3 consists of a resistance 32 which further comprises means of short-circuit 34 designed to short- circuit said resistance 32 in normal conditions, and to connect it in the event of a voltage sag occurring in the network 8. In this last variant, it is necessary to have available said means of short- circuit 34, since the connection of the resistance 32 in normal conditions would affect the normal functioning of the generator 1. This fact is partly due to the power dissipation of the resistance 32, in the form of heat, which would decrease the output of the generator 1. Said means of short-circuit 34 may consist, in a known way, of a controlled relay switch 33. In other non-represented variants, said controlled relay or switch 33, which comprises the means of short-circuit 34, may consist of either an electronic relay or switch of the bipolar, IGBT, GTO or similar type.

On the other hand, as illustrated in Fig. 4, the control of the rotor converter 21 typically incorporates a current loop which enables imposing the desired rotor current or setpoint current 4b, which is calculated to generate the desired active and reactive powers. This is achieved by comparing this setpoint current 4b with the real current 4a and making the converter 21 apply the voltage Vr required to cancel out its difference.

In a variant of the method according to the invention, as well as the addition of a series impedance 3 and taking advantage of the fact that the rotor converter is susceptible to controlling the rotor current, the control of the rotor converter is modified so that in the event of a voltage sag occurring, it imposes a new setpoint current 4b' which is the result of adding a new term to the setpoint current 4b called demagnetizing current 4c, which generates a flow in the rotor winding 13 opposite the free flow, the free flow being that which is not caused by the direct component of the stator voltage, consequently reducing the voltage in converter connectors 21. Said demagnetizing current 4c makes all or a large part of the overvoltage occur in the assembly formed by the leak inductance of the generator 1 rotor, the filter inductance 10 and the series impedance 3 so that, in the event of voltage sags, the voltage which appears in converter connectors 21 of the rectifying-conversion circuit 2 is reduced. In accordance with this variant, the value of the impedance 3 necessary for the system protection can be significantly reduced.

In an option of the method of said demagnetizing current 4c, which added to the power reference current, setpoint current 4b, determines the value of the new setpoint current 4b', is proportional to the value of the free flow ψ_s! of the generator stator 1. The value of said free flow ψ_s! is estimated as the difference between the value of the magnetic flow in the generator stator ψ_s and the value of the stator flow associated to the direct component of the stator voltage, called forced flow ψs_f.

Fig. 4 is a schematic representation of a system to perform the aforementioned variant of the method. This system comprises an impedance 3, connectable in series between the rotor winding 13 of the asynchronous generator 11 and the converter 21 of said rectifying-conversion circuit 2, designed to limit the overcurrents and overvoltages in connectors of the rectifying-conversion circuit 2 in the presence of a voltage sag in the electricity network 8. Furthermore, the system is provided with a control unit 7 for the governing of the converter 21 which imposes a predetermined rotor current called setpoint current 4b. Said control unit 7 comprises an auxiliary module 70 which comprises different units: a first unit 71 for the estimate of the value of the stator flow ψ_s; a second unit 72 for the estimate of the value of the stator flow associated to the direct component of the stator voltage, called forced flow ψs_f, in the event of a voltage sag occurring in the network; a third unit 73 which calculates the difference between the values of the stator flow ψ_s and the previously estimated forced flow ψ_sf; a fourth unit 74, multiplier, which multiplies the value of the difference previously calculated by a K2 factor for the production of the demagnetizing current 4c; and a fifth unit 75, for the sum of the value of the setpoint current 4b and the value of the previously calculated demagnetizing current 4c.

As detailed in Fig.4, the third unit 73 comprises a comparer 73b which is in charge of producing the difference between the values of the stator flow ψ_s and the forced flow ψ_sf previously estimated in the first and second units 71 and 72, respectively. The result of this difference provides the value of said free flow ψ_sι of the generator stator 1 , which is then multiplied by a K1 constant (Lm/Ls, where Lm is the mutual inductance of the generator 1 and Ls is the inductance of the generator stator 1 ) to obtain the value of the free flow ψ_ri of the generator rotor. The value obtained is multiplied by a K2 constant less than 1 in the fourth unit 74, provided for said purpose with a multiplier 74b. The value which results is proportional to the free flow but with changed sign and it determines the value of the demagnetizing current 4c. This demagnetizing current 4c is constituted, in the aforementioned variant, by a 90° current before the voltage induced by said free flow ψ_sι of the generator 1. This simple implementation of the system proposed for the embodiment of the method according to the invention optimizes the reduction in the voltage seen by the converter in the event of only having inductances.

The value of the magnetic flow of the generator stator, ψ_s can be determined from the currents in the stator and in the rotor. In the event of voltage sag, the phase of said forced flow ψs_f is delayed 90° with respect to the voltage and its module can be calculated using the following expression:

wherein i_s is the stator current; ω_s is the frequency of the network voltage; R_s is the stator resistance; V₅ is the stator voltage; and ψ_sf is said forced flow.

In the event of asymmetric voltage sages (single-phase and two-phase) the network voltage contains an inverse component which makes a flow associated to said component appear in the asynchronous stator generator 11. To reduce the voltage in converter connectors 21 it is then necessary that the demagnetizing current also opposes said flow. In these circumstances, the expression previously presented for the unit 72 is no longer valid, since it does not take into account the inverse component of the network 8. A possible option is to separate the two components, direct and inverse, which compose the network 8 voltage, using filtration techniques which are well known in the literature.

The value of the free flow ψ_s] of the generator stator 1 is estimated as the difference between the value of the stator flow ψ_s and the value of the previously estimated forced flow ψ_sf. Said free flow ψ_s, is that which exists in the generator 1 and is not caused by the direct component of the stator voltage. Multiplying said free flow ψ_sι of the generator stator 1 by a constant K1 (Lm/Ls, where Lm is the mutual inductance of the generator 1 and Ls is the inductance of the generator stator 1 ) the value of the free flow ψ_ri of the generator rotor is obtained which is that which multiplying in turn by a constant K2 by the fourth unit 74 provides us with the demagnetizing current 4c.

The demagnetizing current 4 induces an additional flow in the rotor opposite said free flow ψ_sι, so that the voltage induced in converter connectors 21 is reduced. Introducing the sufficient quantity of demagnetizing current 4c, it is possible to exceed the maximum admissible voltage of the converter 21. If this is the case, the converter 21 is protected, its deactivation, and consequently, the disconnection of the generator 1 of the electricity network 8, not being necessary. Example of application:

As an example of application, below we describe the behaviour of a 1.5MW wind generator 1 with asynchronous coiled rotor generator 11 during the different types of voltage sags which may arise in the event that a crowbar 9 with variable resistances is used, one of the typical techniques in the state of the art, and in the case whereof one of the options proposed in the invention is used. Said generator 1 has the following characteristics:

In all the cases described below, the converter of the rectifying-conversion circuit 2 of said electricity generation facility work with a continuous bus voltage of 1200 V.

Protection against a crowbar 9 with variable resistances:

In the event of a voltage sag, when the rotor current or the voltage of the DC bus exceed a certain level (which in the case of the example is 1 ,130 A or 1 ,300 V respectively), the crowbar 9 will be activated, short-circuiting the rotor by a variable resistance in time. Figs. 6a to 6g represent the evolution of the variables in the event of an 80% three-phase voltage sag. Fig. 6a depicts the effective value of the network voltage, observing the appearance of the voltage sag at instant t=0.25s. Next, Fig.βb, 6c and 6d show the evolution of the effective value of the current in the stator, in the rotor and in the converter respectively. At the point when the voltage sag occurs, the three currents begin to rapidly rise. When the rotor current exceeds the preset value, the control unit 7 activates the crowbar 9 and the rotor current the begins to circulate through the crowbar 9. Generally, whilst the crowbar 9 is active the converter is deactivated and its current is cancelled out, as is observed in Fig. 6d, in this way protect the converter from overcurrents of up to 3,500 A which arise in the rotor. In Fig. 6e we can observe the value of the resistance imposed by the crowbar 9. Conventionally, a pre-programmed resistance of a duration of around 100ms is usually made to follow the crowbar 9. Once this time has passed, the crowbar 9 is deactivated, stops leading, and the rotor current then starts recirculating through the converter which is reactivated. On the other hand, Fig.βf shows the evolution of the voltage of the DC bus of the converter and Fig. 6g shows the evolution of the generator 1 torque, where it is observed that in the time where the crowbar 9 is activated, a peak torque is produced (torque surge) 2.5 times over the nominal torque of the generator 1.

Figs. 7a to 7g represent the evolution of the different electric variables in case of a two-phase voltage sage of 80% depth. In this case, in addition to overcurrents and torque surges appearing similar to the previous case, in Fig, 7g we can clearly see how unlike the previous case, wherein the crowbar 9 is activated just once, said crowbar 9 should be connected and disconnected successively throughout the voltage sag to the converter of the rectifying-conversion circuit 2. This means that the converter will be deactivated throughout the voltage sag losing control of the generator 1. This behaviour, as described in the background of the present invention, is not desired for the stability of the electricity network 8.

Protection by an impedance 3 connected in series between the rotor winding 13 and the rectifying-conversion circuit 2: In this case, the generator 1 is protected by an impedance 3 connected in series which consists of an inductance 31, of 1mH, and a resistance 32, of 0,7 Ω, connected in each one of the phases between the rotor 13 windings of the generator 1 and the converter of the rectifying-conversion circuit 2. The inductances 31 are permanently connected whilst the resistances 32 are short-circuited in normal functioning by a switch 33 and are only connected when the rotor current or the voltage of the DC bus exceeds a certain level (1,400 A or 1,300 V respectively). Furthermore, in the event of a voltage sag occurring, a demagnetizing current 4c is made to circulate through the rotor to reduce the flow of the generator 1 and, in this way, reduce the voltage which appears in converter connectors 21 of the rectifying- conversion circuit 2.

Figs δa to 8g show the evolution of the different electrical variables of the generator 1 in the event of an 80% three-phase voltage sag occurring. Fig.δa represents the effective value of the electricity network 8 voltage. At instant t=0.25s the three-phase voltage sag occurs which makes the voltage drop to 20% of its nominal value. Figures Fig 8b and 8c show the evolution of the effective value of the current in the stator and in the rotor (which in this case is the same as in the converter), respectively. As in the figures described in the previous case, at the time when the voltage sag appears, the currents start to rapidly rise. When the rotor current exceeds the preset value, the control unit 7 connects the resistances 32. The inductances 31 , together with the resistances 32 when they are connected, are capable of checking the growth of the currents, achieving that the rotor current does not exceed 1400 A, the value which can be withstood by the converter in a transitory form. In this way, the converted continues active at all times.

Figs 8d and 8e show the effective voltage in the inductances 31 and the resistances 32. These voltages oppose the growth of the current and help the converter maintain control thereof. As can be seen in Fig. 8e, the resistance 32 only remains connected during the first instants of the voltage sag.

The evolution of the voltage in the DC bus of the converter is shown in Fig. 8f. Fig. 8g shows how the peak in the torque (torque surge) which occurs in the first instants of the voltage sage is lower, in this case, 1.25 times the nominal torque.

Figs. 9a to 9g are similar to the aforementioned figures except that they correspond to a two-phase sag with a depth of 80%. It can be observed how the solution based on the invention, unlike in the case of using a crowbar 9, also functions with two-phase sags, maintaining the currents and the voltages at values which involve no danger for the different system components.

Claims

C L A I M S

1.- Method for the protection of an electricity generation facility, of those which comprise at least one electric generator (1) such as a wind generator connected to an electricity network (8), in the presence of voltage sags in said network (8), the wind generator (1) comprising a double-fed asynchronous generator

(11 ) formed by two windings, a winding in the rotor (13) and a winding in the stator

(12), of the type where which the rotor winding (13) is connected to a rectifying- conversion circuit (2), wherein the AC generated in the generator (1) rotor, prior to its supply to said network (8), is first converted in DC and then inverts to obtain a sinusoidal form with the frequency of the electricity network (8); characterized in that the current generated in the asynchronous generator rotor (11 ) is obliged to pass, prior to its passage through the rectifying-conversion circuit (2), through an impedance (3) connected in series to the rotor, which is designed to limit the overcurrents and overvoltages in connectors of the rectifying- conversion circuit (2).

2.- Method for the protection of an electricity generation facility according to claim 1 , characterized in that the series impedance (3) consists of an inductance (31 ) permanently connected to the generator (1) rotor, which means that the current is obliged to pass through said inductance (31 ) both in normal conditions and in the event of a voltage sag occurring in the electricity network (8).

3.- Method for the protection of an electricity generation facility according to claim 1 , characterized in that it further comprises the step of parallel connection, in the event of a voltage sag occurring, with the DC bus of the converter and/or with the AC side of the rotor, power dissipation elements designed for the evacuation of the magnetic power which is generated during the first instants of the voltage sag.

4.- Method for the protection of an electricity generation facility according to claim 1 , characterized in that the series impedance (3) is connected to the generator (1 ) rotor when a voltage sag occurs, while it is short-circuited in normal conditions.

5.- Method for the protection of an electricity generation facility according to any of the previous claims, characterized in that, in the event of a voltage sag occurring, the rectifying-conversion circuit (2) consisting of a converter (21) imposes a new setpoint current (4b¹) which is the result of adding to the setpoint current (4b) a new term, called demagnetizing current (4c), which generates a flow in the rotor winding (13) opposite the free flow, the free flow being that which is not caused by the direct component of the stator voltage, consequently reducing the voltage in converter connectors.

6.- Method for the protection of an electricity generation facility according to any of the previous claims, characterized in that the demagnetizing current (4c) is proportional to the value of the free flow ψ_s) of the generator (1 ) stator, estimated as the difference between the value of the magnetic flow in the stator (12) of the generator ψ_s (71) and the value of the stator flow associated to the direct component of the stator voltage, called forced flow ψ_Sf (72).

7.- System for the embodiment of the method according to claim 1, particularly applicable to an electricity generation facility, of the type which comprises at least one wind generator (1 ) connected to an electricity network (8), the wind generator (1 ) comprising:

- a double-fed asynchronous generator (11) with two windings, one in the rotor (13) and another in the stator (12), of the type where the rotor winding (13) is connected to a rectifying-conversion circuit (2), in turn connected to the electricity network (8), characterized in that the system comprises an impedance (3) connectable in series between the rotor winding (13) of the asynchronous generator (11) and said rectifying-conversion circuit (2), designed to limit the overcurrents and overvoltages in connectors of the rectifying-conversion circuit (2) in the presence of a voltage sag in the network (8).

8.- System according to claim 7, characterized in that the series impedance (3) consists of an inductance (31 ) which is permanently connected between the generator rotor (13) winding and the rectifying-conversion circuit (2).

9.- System according to claim 7 or 8, characterized in that it further comprises power dissipation elements, connectable in parallel in the event of a voltage sag occurring, with the DC bus of the converter and/or with the AC side of the rotor.

10.- System according to claim 7, 8 or 9, characterized in that the series impedance (3) comprises a resistance (32) and means of short-circuit (34), designed to short-circuit said resistance (32) in normal conditions and to connect it in the event of a voltage sag occurring in the network (8).

11.- System according to claim 10, characterized in that the means of short- circuit (34) comprise a relay or a controlled switch (33).

12.- System according to any of claims 7 to 11 , characterized in that it comprises a converter (21 ) which, governed by a control unit (7), imposes a predetermined rotor current called setpoint current (4b), and in that the control unit comprises an auxiliary module (70) which incorporates a first unit (71 ) for the estimate of the value of the stator flow (ψ_s); a second unit (72) for the estimate of the stator flow associated to the direct component of the stator voltage, called forced flow (ψ_sf), in the event of a voltage sag occurring in the network; a third unit (73), which calculates the difference between the values of the stator flow (ψ_s) and the previously estimated forced flow (ψs_f); a fourth multiplying unit (74), which multiplies the value of the previously calculated difference by a K2 factor for the production of the demagnetizing current (4c); and a fifth unit (75), for the sum of the value of the setpoint current (4b) and the value of the demagnetizing current (4c), previously calculated.

13.- System according to the preceding claim, characterized in that K2 is less than 1.