Abstract:
A wind turbine generator is disclosed herein. In a described embodiment, the wind turbine generator comprises an electrical generator  101  configured to generate AC signals, a plurality of power converters  110,112,111  operated by a gating signal with each power converter configured to convert the AC signals from the electrical generator  101  into fixed frequency AC signals. The wind turbine generator further comprises a controller configured to enter a fault mode when a grid voltage falls outside an acceptable threshold, and during the fault mode the controller is configured to provide a reactive current reference dependant on a grid voltage distant from the wind turbine generator. A method of controlling a wind turbine generator is also disclosed.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a wind turbine generator (WTG) and/or a method. 
     BACKGROUND OF THE INVENTION 
     In each country there is a grid code which sets out the various standards that a power generator must meet to connect to the grid. The same applies to wind power. There are various requirements, one of which is to support the grid voltage during a grid fault. For example when the voltage at the point where the wind farm connects to the grid (point of common connection PCC) drops below 0.9 p.u. (90% of the nominal voltage) the wind turbines might be expected to inject a certain amount of reactive current. 
     For wind farms, this requirement is often met using capacitor banks and static synchronous compensators (STATCOMs), especially if the turbine is not a variable speed wind turbine. It has been seen that even for variable speed wind turbines certain amount of compensation equipment (STATCOMs and capacitor banks) are required. However, this compensation equipment is expensive and adds to the cost of Balance of Plant (BoP) equipment and which is required over and above the wind turbine generators (WTGs). Further, complex control strategy is required to meet the reactive power requirement at the PCC using the additional compensation. 
     In some cases the wind turbine controller may enter a fault mode when a low grid voltage condition occurs at the turbine terminals or in the grid. The fault mode may include injecting reactive current into the grid to support the grid voltage. However due to the significant impedances in the system external to the WTG, when the WTG starts injecting a reactive current the terminal voltage changes relative to voltages further along into the grid i.e. the actual voltage, and thus such a compensation from the turbine may not be accurate or reliable. Also there may be a problem where the WTG gets stuck in the fault mode leading to tripping of the WTG eventually. The same can be explained when the turbine is absorbing reactive power due to a high voltage event in the grid. 
     SUMMARY OF THE INVENTION 
     In general terms the invention proposes that the individual WTGs provide a better reactive current injection during a grid fault condition. This may have the advantage that (1) the grid support from a given wind farm is improved (2) the compensation requirement is lowered for each wind farm, thus reducing the installation cost and/or (3) it may also allow the WTG to reliably exit the fault mode once the voltage is improved. 
     In a first specific expression of the invention, there is provided a wind turbine generator comprising an electrical generator configured to generate AC signals, a plurality of power converters operated by a gating signal, each power converter configured to convert the AC signals from the electrical generator into fixed frequency AC signals, and a controller configured to enter a fault mode when a grid voltage falls outside an acceptable threshold, and during the fault mode the controller is configured to provide a reactive current reference dependent on a grid voltage distant from the wind turbine generator. 
     The grid voltage distant from the wind turbine generator may be estimated based on a relationship including a terminal voltage of the wind turbine generator. Preferably, the relationship also includes a terminal reactive current of the wind turbine generator and impedance external to the wind turbine generator. Further, the relationship may include impedance of a turbine transformer. Even further, the relationship may deduct/add a voltage drop across a WTG transformer and a voltage drop across an effective external impedance as seen by the wind turbine generator, from the terminal voltage of the wind turbine generator. Specifically, the effective external impedance may be an effective impedance of a wind farm. 
     Advantageously, the impedance of the wind farm may be determined based on the operational status of a plurality of transmission links with the wind farm or it may be determined at the predesign stage of the wind farm. 
     Preferably, the acceptable threshold relates to a predetermined WTG high voltage limit, low voltage limit, and/or voltage imbalance limit. The reactive current reference may also depends on one or more of: rated maximum reactive current rating, reactive current/voltage gain (K); Active power ripple, Reactive power ripple, Active power delivery, Reactive power delivery, Grid support requirements, Active power priority, Reactive power priority, Current Limit, Voltage limit, DC-link voltage ripple, DC-link voltage limit, Torque ripple, Converter voltage limit, Turbine supervision limits, Temperature of chopper resistors etc. 
     The wind turbine generator may further comprise a voltage transducer to measure the grid voltage distant from the wind turbine generator and a transmission link to convey a signal from the transducer to the controller. 
     In a second specific expression of the invention, there is provided a method of controlling a wind turbine generator comprising: determining whether to enter a fault mode, while in the fault mode, estimating a voltage at a location distant from the wind turbine generator, and providing a reactive current based on the estimated voltage. 
     Estimating the voltage may comprise measuring a terminal voltage of the wind turbine generator voltage and deducting or adding the voltage drop across impedances to the distant location. The voltage drop may be determined based on the reactive current and a determination of the impedance of a transformer and a transmission network between the wind turbine generator voltage and the distant location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only, an example embodiment described below with reference to the accompanying illustrative drawings in which: 
         FIG. 1  is a schematic diagram of a full scale converter based WTG; 
         FIG. 2  is an equivalent circuit of the WTG in  FIG. 1  according to an embodiment; 
         FIG. 3  is a graph of the reactive current to be injected versus Grid Voltage in  FIG. 2 ; 
         FIG. 4  is an equivalent simplified circuit of the WTG in  FIG. 1  according to another embodiment; 
         FIG. 5 : is a simplified circuit of the WTG in  FIG. 4 ; 
         FIG. 6  is a graph of the reactive current to be injected versus the Grid Voltage in  FIG. 4 ; 
         FIGS. 7 to 9  are graphs of simulation results for the WTG; and 
         FIG. 10  is a flowchart illustrating operation steps implemented by the embodiment of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     A WTG generally includes a set of blades, a generator, transformer and a power converter. The turbine blades are rotated about a shaft by the wind and are mounted in a nacelle on a tower relatively high off the ground to ensure good airflow. The electrical generator is driven by the rotating shaft to produce typically alternating current (AC) voltage which varies in magnitude and frequency according to the wind speed. A gearbox may be used to step up the slow rotational speed of the shaft to a high rotational speed suitable for operating the generator. The generator may either be synchronous or asynchronous. The power converter converts and transfers power from the generator to the grid as described below. 
       FIG. 1  shows an electrical system of the wind turbine according to an embodiment. The electrical system includes a generator  101 , a power converter  102  and a main transformer  105 . 
     The first power converter  102  includes a generator-side converter  110  and a grid-side converter  111  connected via a direct current (DC) link  112 . The DC-link  112  includes a DC-link capacitor  113 . 
     The generator  101  converts mechanical energy to electrical energy having AC (alternating current) voltage and current (collectively referred to as “AC signals”), and provides the generated AC signals to the generator-side converter  110 . The AC signals from the generator have a variable frequency, due to varying wind. The generator-side converter  110  converts or rectifies the AC signals to a DC (direct current) voltage and a DC current (collectively know as “DC signals”) towards the DC-link  112 . A load dump  114  is provided to control the DC voltage on the DC-link  112  by absorbing any excess amount of active power provided by the generator  101  due to a grid fault or a similar event. The load dump comprises a power dissipation element and a switch in series, and connected across the DC-link of the converter. The grid-side converter  111  converts the DC signals from the DC-link  112  into fixed frequency AC signals for a power grid. The transformer  105  transforms the grid voltage to a suitable level on the low voltage side according to the drive train system hardware design. The power output of each grid-side converter  111  is fed to the power grid through main transformer  105 . The transmissions lines may be connected directly to the grid or, if in a wind farm, join with other transmission lines at a point of common connection (PCC) before connecting to the grid. Filters  115  in the form of capacitors, inductors and resistors may be located between the common output point of each grid-side converter  111 , and the power grid. Normally, a passive filter carries two branches: a main harmonic filter branch (capacitor banks) to absorb switching harmonics and a resonance branch (capacitors, small chokes and small resistors) to damp resonance phenomenon. The filter may also include other components such as contactors, fuses, sensors etc. 
     It should be noted that the electrical system described with reference to  FIG. 1  is only an example of the electrical configuration of the wind turbine and only the main components are shown to illustrate the embodiments. The present invention should not be limited to the exact electrical system configuration shown in  FIG. 1 . Other electrical configurations are possible. For example while one power converter is shown, parallel converters may be provided depending on interleaving/redundancy requirements. Also, many components in the electrical system of the wind turbine may not be shown in  FIG. 1 . For example, the electrical system may include filters between the generator  101  and the power converters. Also, there may be switches arranged at various locations for connecting or disconnecting certain components of the turbine, and there may be transducers or transformers at various locations for measurement or metering. The converter may be placed on rotor circuit rather than being placed on stator circuit. 
     Under normal conditions a Power Plant Controller (PPC) monitors the AC signals at each WTG in the wind farm, the grid, the functional status of all WTGs and transmission links. According to a control strategy, the PPC issues an active power reference command Pref. and a reactive power reference command Qref for a given WTG to deliver a desired AC signal. Each WTG controller  116  comprises a generator-side converter controller, and a grid-side converter controller for controlling the generator-side converter  110  and the grid-side converter  111 , respectively. These controllers could be on the same piece of controller hardware. The grid-side converter controller may also control a load dump switch of the load dump  114 . 
     According to one embodiment shown in  FIG. 2 , during a low voltage grid fault condition, each WTG may switch to a fault mode. The fault mode may relate to a local fast control algorithm which operates independently of the PPC based on locally measured parameters at the WTG level. For example, the grid side converter may be operated so that the WTG appears as a voltage controlled reactive current source  200 . The i r  is dependent on the AC measured voltage v lvt    202  of the WTG, measured between the grid inductor or choke  204  and the transformer  206 . This location is defined as the low voltage terminal of the WTG. 
     The same may be explained for a high voltage grid fault condition. 
     The control characteristic between the voltage and the current is shown in  FIG. 3 . When the voltage exceeds 0.9 p.u., the WTG controller will exit  300  the fault mode and the PPC will take over control of the WTG. The numbers shown in  FIG. 3  are exemplary and may change at various location/country/site. 
     When in the grid fault mode, the i r  is generally delivered as per  FIG. 3 . However when the voltage approaches 0.9 p.u. the current is scaled  302  back towards a zero value which is effected when the voltage approaches 0.9 p.u. The reactive current/voltage gain (K) in this case is 2, although this may depend on whether the WTG is connected to a strong or weak grid system. 
     Using the plot in  FIG. 3  on the WTG of  FIG. 2 , voltages which are distant to the low voltage terminal of the WTG, i.e. further along the circuit towards the PCC, will be lower due to the voltage drop across the various impedances as compared to the WTG terminal voltage. In  FIG. 3  since K=2, when the v lvt  is above 0.4 p.u. the WTG will be delivering an i r  of less than 1 p.u. Even at the PCC, due to the significant voltage drop across the wind farm transformers and other impedances in between, the voltage will be lower than what it could potentially be if the WTG were delivering a higher i r , which it is capable of doing. This can be understood from  FIG. 2  i.e. since the current injection is dependent on the turbine LV voltage, the turbine terminal voltage changes with the amount of injected reactive current. This is dependent of the impedance seen. This leads to reduced injection from the turbine. Also, when the grid voltage is close to 0.9 p.u. the turbine sometimes remains in grid fault mode for a significantly longer time and eventually trips. The seemingly obvious solution would be to increase K, but since wind farms may also be connected to a weak grid system, this would lead to voltage instability. Another solution would be to dynamically set K, but this would need real time interaction with the power plant controller and it would require significant coordination efforts which requires sufficient communications speed. 
       FIGS. 7 to 9  show simulation results for a DFIG type WTG but both DFIG and Full Scale Converter based WTG behave like a current source during the fault mode in a grid low voltage event. This may be similar to what might be expected for the WTG in  FIG. 2 . In  FIG. 7 , when the voltage at the PCC is reduced (as shown by plot  702  of  FIG. 7  which is voltage at the PCC and plot  700  corresponds to WTG terminals voltage), the voltage at the WTG terminal is also reduced.  FIG. 8  illustrates a plot of the reduced WTG terminal voltage  800  (as shown in  FIG. 7 ), voltage at the PCC and a reference threshold voltage  802 . In this particular case, as per the requirements, when the voltage at the WTG terminal is lower than 0.85 p.u., the Fault Mode is turned ON, and the WTG injects reactive power and reduces the active power. However if the WTG terminal voltage remains below 0.9 p.u., it remains in the fault mode. It is worth noting that the voltage at the PCC is below 0.85 p.u. when the voltage at the WTG terminal crosses 0.85 p.u. 
     The above effect is illustrated in  FIG. 9 . With reduction in the WTG terminal voltage, the WTGs slips into the LVRT mode where the active power is reduced and the reactive power is increased. The maximum reactive current is available when the voltage at the WTG terminals is between 0.2 p.u. to 0.5 p.u as per an embodiment. After that, it starts reducing and follows a slope curve of 50% for the reactive current injection against the WTG terminals voltage. During the LVRT mode, Q reference  (Q ref ) from the PPC is not followed and WTGs are generating the reactive power depending on the WTG terminal voltage. When the WTG terminal voltage exceeds 0.85 p.u., the WTG is out of the LVRT mode and starts following the Q reference  from the PPC, and active power is ramped up to the requested power. 
     An alternative is shown in  FIG. 4  according to another embodiment and  FIG. 10  illustrates the steps for providing reactive current. Similar to the other embodiment, a PPC monitors the AC signals at step  1000  at each WTG in the wind farm, the grid, the functional status of all WTGs and transmission links  404 . Also, if a low voltage grid fault condition is detected, each WTG switches to a fault mode as illustrated at step  1002 . However, in this case, the i, current source  400  is dependent on an AC voltage  402  distant from the WTG terminals as illustrated in  FIG. 4 . For example it may depend on the voltage at the PCC vpcc rather than vM. Since the distant voltage is much less affected by the current injection and the impedances, this voltage will be more stable and suitable for current injection. This may have the advantage that more reactive current is supplied by each WTG under low voltage conditions, and thus more accurate/reliable injection may be expected in the grid during the fault. The same can be explained for the high voltage grid fault event. 
     In order to determine v pcc , it may be possible to measure it directly, although in view of the requirement for fast response, it may be more desirable to estimate it based on the WTG terminal voltage, and this is illustrated as step  1003  of  FIG. 10 . A simplified circuit of the WTG of  FIG. 4  is shown in  FIG. 5 . As seen by the grid side converter, the main voltage affecting components are the turbine choke inductance  504 , the turbine transformer inductance  506  (or more generally, the turbine transformer impedance), the combined inductance of the wind farm transmission network  508  including the grid transformer, and the grid inductance  510 . Thus, the voltage drop across the turbine transformer inductance  506  and the combined inductance of the wind farm transmission network  508  can be obtained or deducted from the turbine terminal voltage to estimate the v pcc . The positive and negative sequence PCC voltage can be determined per equations 1 and 2:
 
{right arrow over ( v   pcc   + )}={right arrow over ( v   lvt   30 )}−( x   tr   +x   pk )*{right arrow over ( i   r   + )}  (1)
 
{right arrow over ( v   pcc   − )}={right arrow over ( v   lvt   31 )}+( x   tr   +x   pk )*{right arrow over ( i   r   − )}  (2)
 
where
 
     {right arrow over (v pcc   + )} Approximate/estimated positive sequence voltage at the PCC 
     {right arrow over (v pcc   − )} Approximate/estimated negative sequence voltage at the PCC 
     {right arrow over (v lvt   + )} Actual positive sequence voltage at the low voltage (LV) side of the WTG 
     {right arrow over (v lvt   − )} Actual negative sequence voltage at the low voltage (LV) side of the WTG 
     i r   +  Positive sequence reactive current injected/absorbed by WTG 
     i r   −  Negative sequence reactive current injected/absorbed by WTG 
     x tr  Reactance offered by the nacelle transformer 
     x pk  Reactance offered by the wind farm. This can be determined in real-time by PPC according to status of the transmission links. This is the equivalent value for one turbine. This could also be determined during the electrical predesign stage of the wind farm. 
     The voltage determined from these expressions may not be the actual voltage at PCC but a voltage closer to actual PCC voltage. The use of this voltage may lead to better injection from the turbine as compared to using simply the voltage at the turbine low voltage terminal. Thus, it may be desirable to estimate a voltage from a location distant from the WTG and provide a reactive current based on the estimated voltage as illustrated at step  1004  of  FIG. 10 , but the precise actual location of that particular voltage in the line profile, may not be known or determined precisely. For the expressions above, the resistive drop is neglected. However if resistive component is significant then resistive drop may be taken into consideration. 
     Thus x tr , x pk  and other impedances are stored by the WTG controller. x pk  may be periodically updated by the PPC but it normally does not change very often. Referring to equation 1, an initial estimate of i r   +  should be determined in advance based off the measured v pcc  with using the voltage control characteristic in  FIG. 6 . 
     On the same lines, the initial value of the negative sequence PCC voltage can be estimated leading to an initial estimate for i r   − . In a symmetrical grid fault condition, the positive sequence reactive current could be determined based on  FIG. 3 or 6  and the negative sequence reactive current is substantially zero. But under an asymmetrical fault condition, the final value for the positive and negative sequence reactive current are also dependent on at least one of the following; Active power ripple, Reactive power ripple, Active power delivery, Reactive power delivery, Grid support requirement, Active power priority, Reactive power priority, Current Limit, Voltage limit, DC-link voltage ripple, DC-link voltage limit, Torque ripple, Converter voltage limit, Turbine supervision limits, Temperature of chopper resistors etc. 
     In simpler implementations it may be possible to estimate for a particular installation a correction factor to apply v lvt  to estimate v pcc . 
     In  FIG. 3 , the K=2 but its value can be higher or lower. For example for K=10 the  FIG. 3  is modified to  FIG. 6 . 
     The control characteristic in  FIG. 6  is also modified by providing a small positive sequence current i r   +  when v pcc =0.9 p.u. to force the WTG controller out of fault mode. If the i r   +  is too low when v pcc  approaches 0.9 p.u. it may never get past the threshold and thus never break out of fault mode. Under some grid codes the WTG would be expected to trip after 60 seconds in fault mode, which is undesirable in such cases as it may lead to wider instability. 
     Also the peak available i r   +  may not be 1 p.u. With certain faults, particularly unbalanced faults, the grid side converter may not be capable of delivering as much reactive current. The WTG controller will be able to determine in real-time the maximum possible i r   +  which may depend on the relative level of negative and/or zero sequence voltage. 
     As well as low voltage, if the voltage rises above 1.1 p.u. the WTG can react as shown in  FIGS. 3 &amp; 6  by absorbing reactive current. When a voltage imbalance occurs, the current injection or absorption can be implemented on each phase independently. Thus the present invention can be useful in those cases as well to ensure that the estimated distant voltage e.g. v pcc  is more effectively used by the WTG control for fault handling. 
     While embodiments of the invention have been described in detail, many variations are possible within the scope of the invention as claimed as will be clear to a skilled reader. For example while a full scale turbine was described, the invention may also be applied to Doubly Fed Induction Generator (DFIG) type WTG. 
     One or more embodiments may have the advantage that:
         Less dynamic compensation may be required;   Very fast reaction time to grid voltage conditions; and/or   Can be implemented in existing hardware with simple software update.