WO2011026250A1

WO2011026250A1 - Generator system having a generator that is directly coupled to the mains and method for overcoming mains interruptions

Info

Publication number: WO2011026250A1
Application number: PCT/CH2010/000213
Authority: WO
Inventors: Alexander Stoev
Original assignee: Ids Holding Ag
Priority date: 2009-09-03
Filing date: 2010-09-01
Publication date: 2011-03-10
Also published as: CH701746A2

Abstract

The invention relates to a generator system comprising a mains transformer (1), a mains circuit breaker (2), a generator (4) with a stator winding (4.1 ) and rotor winding (4.2), a converter (7) with a DC-link (7.3), a mains inductor (9) and a control unit (10). Said generator system comprises a voltage limiter (5) which is arranged on the secondary side of said generator. After the detection of a change in the mains voltage of a corresponding amplitude, the voltage limiter (5) is activated, thus maintaining the voltage level on the inlet of the converter (7) within the operational voltage range of the voltage limiter (5), thus no current flows into the converter (7). When the transient processes are finished, the flow lowers and the voltage limiter is deactivated. The converter (7.1) on the machine side is simultaneously reactivated. Whilst the voltage limiter (5) is activated, the converter (7.2) on the mains side (7.2) continues to supply energy to the mains in order to be able activate, together with the stator flow, the required blind current. The invention also relates to a method for operating the generator system. Said type of systems are frequently used in regenerative energy systems such as wind and hydro power plants.

Description

Generator system with direct mains-coupled generator and method

for traversing network disturbances

The invention relates to generator systems with directly grid-connected generators, in which a winding (stator), apart from start-up, switching and protective devices, connected directly to the feeding network and at least one further (rotor winding) is accessible from the outside. This applies, for example, to systems with double-fed asynchronous machines (DASM) or cascade machines. Such systems are often used in renewable energy systems, such as wind and hydroelectric power plants.

Renewable energy plants are increasingly used in areas with weak networks, ie networks with connection capacities below 200 kVA. At the same time, the demands on the availability of the installations, i. Mains faults such as voltage dips to about 5% of the nominal voltage in the range of milliseconds, single- and multi-phase short circuits must be tolerated at least for a short time and controlled in "ride through". An error shutdown of the system in the course of such an error is not permitted, on the contrary, electricity should be fed into the grid as continuously as possible.

In plants with direct grid coupling of the generator occurs in sudden change of the mains voltage to significant overcurrents in the stator and rotor circuit, which cause the network protection devices and in a DASM system exceed the permissible values for the MFU and also an impermissibly high voltage in the DC bus have as a consequence. The current increases immediately after the voltage change are limited only by the voltage difference itself and the circulating stray inductances, which already after a few milliseconds current amplitudes are reached, which are well above the overcurrent thresholds of MFU and mains side mains circuit breaker. Since generators of the order of magnitude used have main field time constants in the second range, the excited equalization processes decay only slowly, which means there can be no talk of "ride through". In order to absorb the overcurrents and the additional energy fed into the DC link, the frequency converter would have to be over-dimensioned to a multiple of the rated power, which is economically unacceptable. In addition, the mains circuit breaker can no longer effectively protect the system, since it would also have to tolerate the overcurrents that occur. The patent document EP01651865 also describes a method for controlling network disturbances. Among other things, the temperature of the generator and transmission is monitored with setpoints. This is contrary to hold that a network failure is usually in the ms range and thus a temperature monitoring of components does not bring much, because the necessary sensors detect within seconds and thus only the destruction of the parts can be determined. Also, a control of the turbine blades is not suitable to intercept a transient network disturbance, because enough energy is stored in the rotor at the onset of the fault to cause large destruction in the electronics. It usually takes 5 seconds for the rotor blades of the turbine to be twisted. This means that the response to the power failure comes way too late, because network disturbances are often only about 100 ms long. It is further described that there is a check for network problems every 0.5 seconds. Since such disturbances are usually 100 ms long, the disturbance would not even be able to be detected before it is over and the electronics have already been destroyed.

In the patent document EP01561275 a method for traversing power disturbances is described, wherein after recognizing such a disturbance for controlling the transient equalization processes, the stator of the generator is briefly disconnected from the mains via electronic switches. This represents a significant disadvantage because the separation short-term no reactive current can be discharged from the stator into the network.

The patent document WO 2007/057480 describes a method in which 4 load resistors can be connected to the DC link + VBUS and -VBUS by means of 4 IGBTs. This version of the connector has the significant disadvantage that not exactly the required load resistor can be connected to the DC bus in order to prevent within a narrow range that the current flow in the DC link causes a voltage increase. Moreover, this method is very expensive and expensive because multiple resistors and IGBTs are needed to dissipate the energy. The most significant disadvantage of this variant, however, is that the energy can only be destroyed after it has reached the DC link. However, this requires that all components in the converters must be able to conduct the uncontrolled energy supplied into the DC link. Now if problems arise, the converters are destroyed at the same time, because the current must flow through the converter before it can be converted into heat.

In the patent document EP1499009A1 a possibility is described, with a controlled varistor to keep the energy transients away from the converters (71; 72; 73). For this purpose, thyristors are used to place a varistor between the phases. A major disadvantage is that a fixed characteristic of a component must be applied to all different transients. In addition, there is no possibility to switch off the varistor within a certain time when the disturbing transient process has ended. The characteristics of varistors are not sharp enough, so that already energy must be destroyed before the converter would need to be protected, so that in case of emergency, the varistor can then take over the required current. This means that since this component can not be switched off when the transient is finished, significantly more energy must be burned in the varistor than would be necessary. These varistors must therefore be much larger, because an exact control is not possible.

The object of the invention is to ensure with economically justifiable effort that a generator system with sudden changes in mains voltage up to 0% of the rated voltage can feed energy into the grid continuously or with only a minor interruption. Any transient occurring should be minimized, and it should be possible to continue the feed operation under defined conditions without interruption, regardless of the direction and speed of the voltage change.

Another object is to describe a method for operating a correspondingly extended generator system.

According to the invention these objects are achieved with a generator system according to the wording of claim 1 and with a method according to the wording of claim 15. The invention will be explained in more detail below with reference to FIGS. Show it:

Fig. 1 Variable speed generator system with DASM generator with electronic voltage limiter

Fig. 2 current paths to Figure 1

Fig. 3A voltage limiter with RC combination

Fig. 3B Voltage limiter with varistor

Fig. 3C voltage limiter with resistor

Fig. 4 operating voltage range of the machine-side converter Fig. 5 current distribution between the machine-side inverter and the controlled resistance

Fig. 1 shows a variable speed generator system with DASM generator and electronic voltage limiter.

A DASM generator 4 consisting of a stator winding 4.1 and a rotor winding 4.2 and the slip rings 4.3 for tapping the rotor. The tap is connected to a voltage limiter 5. The voltage limiter 5 consists of a connector 5.1 with or without throttle and is connected to a variable resistor 5.2 and a frequency converter or, inverter 7, the latter from a machine-side inverter 7.2 (MFU), a DC link or a DC link 7.3 and a line-side converter 7.1 (NFU). The DASM generator 4 is connected to a mains transformer 1 via a stator contactor 3 and a mains circuit breaker 2.

The network-side Frequeizumformer 7.1 is connected via a mains choke 9 and a switch 8 to the mains circuit breaker 2. A control unit 10 is connected to the two frequency converters 7.1, 7.2 and the voltage limiter 5 and takes over all control and control tasks. Advantageously, the frequency converter 7 is designed as a bidirectional voltage source converter. The DC link 7.3 can be loaded with a precharge circuit 6 to a specific starting value.

The connector 5.1 optionally consists of an inductance or a coil or connecting lines. The coil is used when the voltage limiter 5 in the unit 5.2 (variable resistor) has a time problem with the voltage change (dU / dt), i. when the resistor is turned on. In this case, the coil 5.1 lowers. dU / dt and dl / dt to the required value, so that the switch ES triggers no faulty circuits.

FIG. 2 shows the distribution of the currents in the generator system according to the invention corresponding to FIG.

The currents lu, Iv and Iw from the mains transformer 1 are divided into the currents Isu, Isv, Isw of the stator and the currents liu, liv and liw of the line-side converter 7.1. The generator currents IGu, IGv, IGu of the rotor 4.2 are divided into the currents IRu, IRv, IRw through the variable resistor 5.2 and the currents lUu, lUv, lUw of the machine-side converter 7.2.

The voltages Uu, Uv and Uw are the voltages after the power transformer 1. The voltages Uru, Urv and Urw are the stresses on the machine side Inverter 7.2.

Fig. 3A shows a first embodiment of the variable resistor 5.2. At the phases U, V and W, a bridge rectifier is connected, which converts the AC voltage into a DC voltage. This DC voltage is connected to a series connection of a resistor R and an electronic switch ES with control input S, which is a capacitor C connected in parallel.

3B shows a second embodiment of the variable resistor 5.2, in which the variable resistor consists of a series circuit of a varistor V with an electronic switch ES with control input S.

Fig. 3C shows a third embodiment of the variable resistor 5.2, wherein the variable resistor consists of a series circuit of a resistor R with an electronic switch ES with control input S.

Fig. 4 shows the operating voltage ranges of the machine-side frequency converter 7.2 and the points A, B, C, D and E in which actions are initiated. The following voltage ranges are described:

UL0-UL1: Precharge area of the drive that can only be passed through during a precharge or discharge. The main contactor is switched off. The network-side and machine-side inverters (NFU and MFU) are switched off.

UL1-UL2: Operating range of the voltage limiter 5. The machine-side converter (MFU) can be synchronized and the line-side converter (NFU) operates.

UL2-UL3: Safety range to guarantee compliance with the conditions lUu = 0 and lUv = 0 and lUw = 0 when operating the voltage limiter 5. The network side (NFU) and the machine side (MFU) inverter are working. The voltage limiter realizes the safety area in action.

UL3-UL4: Normal operating range of the inverter 7. There is no risk of overvoltage within the voltage limits.

UL4-UL5: Non-permitted safety area of the DC Link. Upon detection of this operating range, voltage-reducing measures must be initiated, ie the voltage limiter is activated.

> UL5: destruction area of the inverter. If this operating range is detected, the inverter must be protected by operating the main contactor and activating the voltage limiter.

In the following, points A, B, C, D and E are described in the voltage curve as follows:

A- precharge 6 to about UL3

B- Beginning of a strong current rise caused by a network collapse

C current rise axis resulting from the network fault

D-connection point of the voltage limiter 5

E cut-off point of the voltage limiter 5

Fig. 5 shows the current distribution between the machine-side frequency converter 7.2 and the regulated resistance. The stream splits into the following two areas:

IL0-IL1: Range of allowed current through the drive

IL1-IL2: Range of the non-permitted current range for the inverter. Of the

Voltage limiter realizes this area, i. he is active in this area.

The following is a detailed description of the inventive method for operating the generator system in case of power disturbances.

According to the energy provider (E.ON Netz GmbH: Supplementary grid connection rules for wind turbines) is expected to sudden voltage changes, especially in a voltage dip, while the power-up again, the step by step. Burglaries thus represent the more critical and predominantly dominant case.

In principle, the function of a wind energy plant is explained by means of FIGS. 4 and 5, and then the inventive part is discussed.

Principle function:

Before the start of a wind turbine, the DC link 7.3 is charged up to the level UL3 by means of the precharge circuit 6. Near the point A synchronizes the Control unit 10 drives the inverters 7.1 and 7.2 to the grid and slowly increases the current (from point A) Then, according to the wind conditions up to point B, the inverter feeds power into the grid. It should be noted that the current shown in Figure 5 is not the current fed into the grid. At point B of Figure 4, there is a drop in voltage due to network problems.

Inventive share:

The now rapidly increasing stator current ISu, ISv and ISw (FIG. 2) causes a strong current increase IGu, IGv and Igw in the rotor. Very quickly, the rotor current will exceed the allowable maximum value of the machine-side inverter 7.2. In order to prevent this, at point C by overcurrent detection or overvoltage detection, the control unit 10 to the voltage limiter 5, which then independently absorbs the entire current of the rotor. The voltage limiter 5 thus ensures that the excessively high current can not reach the inverter 7.2. The currents lUu, lUv to lUw become zero. For this purpose, the voltages UGu, UGv and UGw are regulated in the voltage range UL1-UL2, so that the machine-side converter is protected. If the control unit 10 recognizes that the transient compensation processes have been completed and the current has reached, for example, approximately 60% of the rated current, the controllable resistor 5.2 of the voltage limiter 5 is deactivated at time E and the machine-side converter is started up again. Throughout the period from D to E, the line-side converter from DC-Link 7.3 can continue to feed power into the grid to compensate reactive-current components. So can be achieved by the network disturbance in the interaction of all components passing through the wind turbine.

In a further embodiment, a rotor Crowbar can be integrated into the voltage limiter 5. In this special design, the control unit 10 is able to shut off the wind turbine targeted by overcurrent overloads of the voltage limiter. The new separate arrangement of the unit 5, it is possible to create a defined point at which damage occurs, if the coupled into the voltage limiter 5 energy for unknown reasons is much larger. In this case, 5 so-called fail-safe components are used in the voltage limiter. In case of overload, these parts are designed so that a safe short circuit is realized. This short circuit also protects the inverter from destruction and triggers the main switch 2 of the turbine, so that only in a predefined area of the turbine, a small damage is produced, which prevents a great deal of damage to the cabinet. According to the invention, the main task of the voltage limiter 5 is the control of transient high currents, which could damage the converter. The switch ES used in FIGS. 3A, 3B and 3C may preferably be formed as IGBT, GTO or MOSFETs.

With the power switch 8, the network-side inverter 7.1 (NFU) can be connected to the mains. As a result, the network-side converter can be operated independently of the switching state of the voltage limiter 5 and a continuous current flow between network and NFU can be maintained even during the period of a network fault until the DC link 7.3 is discharged. By activating the voltage limiter 5, all the currents IGu, IGv and Igw are absorbed from the rotor, so that no further energy can reach the intermediate circuit of the converter, which would lead to unacceptably high voltages. Figs. 4 and 5 show the use of the voltage limiter. A particularly advantageous design of such a variable resistor, which forms the voltage limiter 5, is shown here:

Electronic switch ES - IGBT

Capacitor C - <10mF

Resistance R depending on the turbine - <1 Ω

For a 1.5 MW 690 V turbine, the currents are for example:

IRu, IRv, IRw max - IRu.vw peak = (IUu, v, w peak) ^* 4 for 100ms =

Maximum current of the variable resistor lUu, lUv, lUw max - IUu, v, w peak = (IRu, v, w peak) / 4 =

Maximum current of the inverter

The current ratio depends on the ratio of the number of turns.

Voltage ranges of the inverter (Fig. 4):

UL0-UL1 0V - 750V DC pre-charging voltage of DC Link 7.3

UL1; 800V shutdown limit of the variable resistor

UL2: 900 V switch-on limit of the variable resistor

UL2-UL3 900V - 950V safety range to guarantee IUu, v, w = 0

UL3-UL4 950V - 1 150V normal operating range of the drive UL4-UL5: 1 150V to 1400V safety area for the DC Link

UL5-: from 1400V destructive range of the inverter

Current ranges at the inverter (Fig. 5):

IL1: IL1 = IL2 / 4, maximum current of the inverter

IL1 - IL2: IL1 - 4 ^* IL1, working range of the voltage limiter

IL2: Destruction limit of the inverter

A sudden mains voltage change can be detected directly via the detection of the mains voltage itself or indirectly via the detection of overcurrents in the stator circuit or MFU or by detection of an overvoltage in the DC link 7.3. Since overcurrents and overvoltages can have other causes, the sequence for mastering network faults also protects the system in the case of other faults. Interruption via indirect detection has the advantage that network separation actually only takes place if the fault is not countermeasures the control unit 0 and the voltage limiter 5 can be compensated. The detection of said error immediately triggers the activation of the voltage limiter 5. Simultaneous switching off of NFU (pulse inhibit) or ignition of the short-circuiter are only required if the overcurrents can not be limited by activating the voltage limiter 5 (i.e., have system-internal causes). As a result, the availability of the system and its ability to actively counteract external faults are substantially increased by means of the arrangements according to the invention.

In contrast to the patent document EP01651865B1, the method according to the invention is able to detect the network disturbances in a few ps and to initiate countermeasures.

In contrast to the patent document EP1499009A1, the method according to the invention is able to initiate the necessary protective measures already after 0.5 ms following a power line interruption, and when the conditions for protection have been removed, the regulating element can be switched off again after 0.5 ms. This is only possible with a thyristor after the current falls below the self-holding current. Since the varistor must have a characteristic in which always flows electricity, because otherwise in case of emergency, the required power can not be accepted, a deletion of the current is not possible, so that the thyristor turns off significantly later and is not directly controlled by the transient. An essential component according to the invention consists in preventing the complete flow of current into the converters when overcurrents occur. As a result, all components of the converter can be made smaller.

In contrast to patent document EP1965075A1, not only the destructive energy of the transient processes is introduced into the converter, in order then to derive it again from the converter by conversion into heat. Furthermore, the dissipation energies can be regulated and can not be derived in digital only 6 steps. According to the invention, any necessary resistance can be set and thus the transient can follow exactly. This is to prevent that the converters have to be designed for the transient currents because they are up to 5 times higher than the rated currents and thus cause immense costs in the converters.

The inventive idea is to electronically controlled burn exactly the transient energy that can destroy the converter. For this purpose, a real control loop is set up, which compares the setpoint with the actual value.

This in turn is not possible according to patent document EP1965075A1, because only a limiting characteristic can be switched on or off.

In contrast to the solution presented in the patent document EP01561275A2, the system disturbance is regulated by the method according to the invention and thus no separation of the stator from the network is carried out. This helps to stabilize the grid more quickly because larger reactive currents are possible immediately after the fault has occurred

In the event of mains interruptions, a magnetic reversal must take place in the generator. After a network fault, the excessively high current in the rotor circuit is derived from the machine-side inverter (MFU) 7.2, in that the then applied input voltage of the generator-side inverter is kept electronically regulated below the otherwise normal operating voltage. In addition to the derivation of too large currents, the generator is re-magnetized so that it adapts to the new voltage in the network. One advantage of this arrangement is that the stator winding of the generator does not have to be disconnected from the grid during resynchronization, and mains supply via the generator can take place continuously after fault entry. The requirement in various network standards to feed electricity directly into the grid at the level of the rated system current after the occurrence of faults can thus be fully met. It is e.g. refer to the following network standards:

E N Netz GmbH: Supplementary grid connection rules for wind turbines,

Status 2003/2006;

- National Grid UK, Gn ^'d code GCV 4 (2008); EDF France grid code.

In the event that the voltage increases dU / dt or current increases dl / dt are so great that malfunction can be triggered in the variable resistor 5.2, the connector 5.1 may include a throttle, which reduces the increases. With these two combinations for the connector 5.1 and the 3 design variants for the resistor 5.2 a total of 6 design variants for the inventive arrangement.

Since the voltage limiter 5 is arranged spatially separated from the inverter, even the inverter can be protected if it comes to destruction in the voltage limiter. The already described fail-safe function of the voltage limiter in this case triggers the protection mechanisms of the generator system, i. the main switch 2 is opened.

Surprisingly, it is found that in case of unexpected overloads, the provided 'fail-save' construction of the voltage limiter outside the inverter further protects it by a defined short circuit and trigger the necessary safety equipment such as the main switch. Possibly destroyed components can cause no damage due to the spatial separation from the inverter in the inverter, which is particularly advantageous.

It is essential to the invention that it is ensured with economically justifiable expenditure that the generator system is fed with sudden mains voltage changes continuously or with only insignificant interruption of energy into the grid. The feed operation is continued under defined conditions in the shortest time after the voltage change, regardless of the speed of the voltage change or the current change.

Such systems are often used in renewable energy systems, such as wind and hydroelectric power plants.

Claims

Patent claims:

1. Generator system with a doubly fed three-phase generator, comprising a doubly fed three-phase generator (4) with a grid-coupled primary winding (4.1) and at least one secondary winding (4.2), a frequency converter (7) in the secondary circuit, which consists of a machine-side converter (7.2.) and a grid-side There is a converter (7.1) and a control unit (10), connected to a three-phase network (1), characterized in that a voltage limiter (5) is connected to the generator on the secondary side in such a way that if the current through the machine-side converter (7.2) or If the voltage on the DC link (7.3) is too high, the entire secondary-side current can be diverted in such a way that there is no longer any danger to the machine-side converter (7.1) because no current flows into the converter.

2. Generator system according to claim 1, characterized in that in the event of an overcurrent or overvoltage, the control unit (10) sends control signals to the voltage limiter (5) in order to activate it.

3. Generator system according to one of claims 1 - 2, characterized in that the voltage limiter (5) is designed so that in the event of a fault, no protective devices are activated, which trigger a separation of the generator (4) from the network (1) or to switch off the frequency converter (7) lead.

4. Generator system according to one of claims 1 - 3, characterized in that the voltage limiter (5) can be used to remagnetize the generator (4) to the current size of the mains voltage (1) in the event of mains voltage drops, without the mains having to be disconnected .

5. Generator system according to one of claims 1 - 4, characterized in that a rotor crowbar can be connected in parallel to the voltage limiter (5), which causes the system to be switched off in a targeted manner if the fault persists for so long that components are thermally or electrically overloaded . >

6. Generator system according to one of claims 1 - 5, characterized in that with regard to the operating voltage of the converter (7) on the DC link (7.3) different operating ranges are defined in which defined actions are to be initiated to protect the converter.

7. Generator system according to one of claims 1 - 6, characterized in that with regard to the rotor currents (IG) there is a first range A as the maximum operating current range of the converter (7) and a second range B as the non-permitted operating current range of the converter (7), where In area B, the converter can be protected against overcurrent by activating the voltage limiter (5).

8. Generator system according to one of claims 1 - 7, characterized in that in one embodiment the voltage limiter (5) is formed by a controllable resistor (5.2), which consists of a series connection of an electronic switch (ES) and a resistor (R). , with a capacitor (C) being connected in parallel to this series connection.

9. Generator system according to claim 8, characterized in that in the voltage limiter (5) the electronic sensor (ES) can be modulated to adjust the resistance value and the capacitor keeps the voltage across the resistor (R) in the operating voltage range (UL1 - UL2), so that no current flows into the converter (7).

10. Generator system according to one of claims 1 - 7, characterized in that the voltage limiter (5) is formed by a controllable resistor (5.2) which consists of a series connection of an electronic switch (ES) and a varistor (V).

1 . Generator system according to one of claims 1 - 7, characterized in that the voltage limiter (5) is formed by a controllable resistor (5.2) which consists of a series connection of an electronic switch (ES) and a resistor (R).

12. Generator system according to one of claims 8 - 11, characterized in that the electronic switch (ES) is preferably implemented by an IGBT or a GTO or a MOSFET.

13. Generator system according to one of claims 1 - 12, characterized in that the voltage limiter (5) contains a connector (5.1) which is designed as a coil.

1 . Generator system according to one of claims 1-13, characterized in that the voltage limiter (5) is arranged outside the DC link (7.3) of the converter (7) in order to prevent damage to the converter (7) in the event of destruction.

15. A method for operating a generator system according to one of claims 1 - 14, characterized in that a sudden reduction in the mains voltage is detected by constant measurement of the mains voltage, and that a sudden reduction in the mains voltage results in an excessive current in the primary and secondary circuits of the generator or an operating voltage on the DC link (7.3) that is too high would cause destruction and that destruction can then be avoided by activating the voltage limiter (5) by reducing the operating voltage on the DC link and keeping current peaks away from the inverter (7). .

16. The method according to claim 15, characterized in that the network-side converter (7.1) continues to feed electricity into the network (1) and thus at least part of the reactive current of the generator is compensated for during the duration of the network fault.