Method and device for power supply

Abstract

A method for modifying an effect of a disturbance in an electrical network (384) on a generator (120) connected to the network (384), the method comprising: Tracking at least one negative sequence component and one positive sequence component of a signal of the electricity network (384), Aligning at least a portion of the magnitude of the negative sequence component for feeding into the electricity grid (384) and feeding the at least one portion into the electrical grid (384) to modify the effect of the disturbance, wherein a power converter component (362) having a DC link (435) with a DC link capacitor (436) is used and wherein the power converter component (362) receives control signals from a control system (300) via a control unit (374), the control signals being based on measured conditions or characteristic operating properties; and wherein the generator (120) is a doubly fed asynchronous generator (120) and the tracking, the aligning and the injection (540) implement a current compensation (500), wherein after the alignment, the aligned signal is applied to a fault connection of a current regulator of the control unit (374), and wherein the control unit (374) controls the power converter component of the doubly fed asynchronous generator (120).

Description

BackgroundThe teachings herein relate generally to techniques for quickly compensating phase and amplitude information in an electrical signal.Many countries now require that wind turbines used as electric power generation installations remain connected to the electricity grid when the grid is in a fault condition. To remain connected during a system failure (often referred to as "low voltage ride through") may be challenging from a technical perspective. Perhaps most importantly, the phase and amplitude information of the balanced components in the line signal is made available quickly and accurately to the plant control systems. This allows the control systems to carry out correction processes on the wind turbine in good time, as a result of which the effect of large-signal transients is alleviated, and as a result prevents the wind turbine from being switched off.In general, it is a primary task of a power generation plant to regulate the positive sequence voltage. Traditional current controlled approaches implicitly attempt to remove the negative sequence current. For a wind power system using a double-fed asynchronous generator, in the presence of a fault or imbalance condition of the load, this requires a rotor-side converter to obtain a negative sequence voltage and supply a negative sequence current. Unfortunately, turbine systems may be limited in their ability to provide sufficient negative sequence voltage, current, or power. This results in system controllability degrading and repetitive deployment of protective measures (e.g., a "crowbar circuit"), thereby exposing the generator and other turbine components to repeated transients. Dynamic brake resistors may be used to shunt energy from the DC circuit, thereby limiting the activation of the crowbar and maintaining controllability.Protection of a branch or branch power circuit connected to the grid may depend on the circuit having a low impedance characteristic for negative sequence voltages. That is, it can be expected that the branch circuit is capable of supplying some current to a negative bias fault or unbalanced state. Typically, this depends on the ability of the network to withstand a fault condition. Further, in various power standards for generation facilities, in some cases, device specifications may require negative sequence current in response to negative sequence voltage. Unfortunately, traditional current management schemes typically avoid approaches to coordinated protection of branch and branch grid circuits.Traditional solutions to address these challenges have resulted in non-linear behavior systems that make it difficult to create simple models of sub-system elements for use in overall system models. The scheduling of experiments typically becomes rather complex when exhaustive scenarios are needed to attempt to cover the location of the nonlinear system.A number of resources have been directed towards facing or examining the operation of generating facilities in network disturbances. Examples include a procedure described in the paper "Vestas Handle Grid Requirements", Advanced Control Strategy for Wind Turbine s", by Bilik, et al., wherein numerous steps are performed, the first of which is disconnecting the stator of the generator from the grid.A second paper, "Transient Analysis of Twice Fed Wind Power Induction Generator Using Coupled Field-Circuit Model", by Seman et al., has studied aspects of network failures. In the approach disclosed by Seman, the rotor-side frequency converter is controlled by a modified direct torque control (DTC) control strategy.A third paper, "Comparison of Fault Ride-Through Strategies for Wind Turbines with DFIM Generators" by Dittrich et al. compares different strategies for traversing fault conditions or fault ride-through.A fourth paper, Experiences of Voltage Dip Ride Through Factory Testing of Synchronous and Twice Fed Generator Drives, by Niiranen, discloses procedures for error emulation and measurement of system fault related aspects.A number of prior art techniques for responding to disturbances in the network require the use of a Crowbar circuit. When this approach is used, the power generation plant is typically unable to appropriately respond in terms of new standards and requirements for generation systems.DE 103 54 925 A1 describes a system for compensating a voltage of a negative component in an energy system.V.B. Bhavaraju, P.N. Enjeti, in "Analysis and design of an active power filter for balancing unbalanced loads," in IEEE Transactions on Power Electronics, vol. 8, no. 4, pp. 640-647, Oct. 1993, doi: 10.1109 / 63.261037 describe an active filter for load balancing.US 2005 / 0 207 190 A1 describes a power supply system having a phase locked loop (PLL) with a blocking filter for synchronous reference frame sequence separation.What is needed is a technique to keep a generation plant, such as a wind turbine, coupled to the electricity grid during low voltage periods or periods with grid signal instability, the technique coordinating the voltage, current and power characteristics to account for the grid states of the positive and negative sequence systems to extend the small signal linearity and cause a reduction in the self-protection transients.Brief DescriptionAs shown in the above publications, the response of the system controller to the unbalanced grid states led to the application of the "crowbar" to the rotor circuit.A method for altering an effect of a fault in an electricity grid on a generator coupled to the grid is disclosed, the method including tracking at least one negative sequence component and one negative sequence component of a signal of the electricity grid, aligning at least a portion of the magnitude of the negative sequence component for feeding to the electricity grid, and feeding the at least a portion to the electricity grid for altering the fault.A power converter component having a DC link with a DC link capacitor is used, and the power converter component receives control signals from a control system via a control unit, wherein the control signals are based on measured states or characteristic operating characteristics.The generator is a double-fed asynchronous generator and the tracking, the alignment and the feeding implement current compensation, wherein after the alignment an application of the aligned signal to a fault connection of a current regulator of the control unit takes place and wherein the control unit controls the power converter component of the double-fed asynchronous generator.There is also disclosed an apparatus for changing an effect of a fault in an electricity grid on a generator coupled to the grid, the apparatus including a controller for tracking at least one negative sequence component and one negative sequence component of a signal of the electricity grid, aligning at least a portion of the magnitude of the negative sequence component for feeding to the electricity grid, and feeding the at least a portion to the electricity grid for changing the fault.The apparatus also comprises a power converter component comprising a DC link with a DC link capacitor, the power converter component receiving control signals from a control system via the control unit and the control signals being based on measured states or characteristic operating characteristics; wherein the generator is a double-fed asynchronous generator and the tracking, the alignment and the feeding implement current compensation, wherein after the alignment an application of the aligned signal to a fault connection of a current regulator of the control unit takes place and wherein the control unit controls the power converter component of the double-fed asynchronous generator.The features and advantages of the present invention will be apparent to and understood by those skilled in the art from the following detailed description and drawings.DRAWINGSReferring now to the drawings, wherein like elements are numbered the same throughout the several drawings: FIG. 1 shows aspects of a control system for a wind turbine, FIG. 2 shows an example topology for a double-fed asynchronous generator (DFIC), FIG. 3 shows an example topology for a DPIG with a Crowbar circuit, FIG. 4 shows an example topology for a DPIG with a dynamic brake, FIG. 5 shows a flow chart illustrating an example method for current compensation, FIG. 6 shows signal inputs to the DPIG, FIG. 7 shows relationships of the negative sequence components, FIG. 8 is a block diagram illustrating aspects of signal analysis and determination of compensation current, FIG. 9 shows a further exemplary embodiment for determining aspects of the signal analysis and the determination of the compensation current, FIG. 10 is a block diagram illustrating aspects of effects on the compensation current for use as the feed current, FIG. 11 illustrates aspects of the ability to supply negative sequence current and positive sequence current and reactive sequence current. FIG. 12 shows the response of a wind turbine according to the prior art to a simulated transient event, FIG. 13 shows the response of a wind turbine making use of the teachings herein to the simulated transient event; and FIG. 14 shows an example topology for a synchronous machine system with conversion of the entire energyDetailed DescriptionThe teachings herein enable determination of the values of the negative sequence voltage in the event of a fault and, with the exception of short transient protection measures, a Crowbar circuit at the beginning of the fault allows a reaction that allows control of positive sequence and negative sequence currents flowing into the fault state. Limits for the positive and negative sequence current depend on system parameters, such as the system states, the system performance and the topology of the converter installation. The resulting controllability can be used to improve the response of the generator in unbalanced voltage states of the grid.The teachings herein provide techniques for altering the effects of a fault in an electrical grid on a generation plant (e.g., a wind turbine). The techniques for altering the effect of the fault make it possible to maintain a connection to the electrical grid during a fault condition of the system and to bring about a suitable behaviour for the connection point to the grid. An example of the system fault includes low voltage periods in at least a portion of the electricity grid. The asymmetry of the phase voltages caused by system errors typically leads to states of negative sequence voltage and negative sequence current.As discussed herein, the electrical grid includes an electrical signal that is a three-phase electrical signal. It should be appreciated, however, that consideration of a three-phase input signal is for convenience and illustration purposes only and does not limit the teachings herein. The teachings herein may also be applied to other polyphase or polyphase input signals, for example.As used herein, the terms "fault", "grid fault", "fault", "system fault", "transient", and other similar terms refer generally to any event that causes faults in the input signal from the electricity grid. Examples of events that may cause a disturbance in the grid signal (e.g., a fault on an electricity grid) are well known and will not be discussed further herein. Generally, and as discussed herein, it is assumed that the mains signal includes a three phase signal having balanced components at particular frequencies. When different generation facilities contribute to the network signal and when different phenomena including transient events may occur, the symmetric components of the network signal may inevitably degrade or vary to some extent. For example, the symmetric components may form harmonic frequencies or phase shifts, both of which may complicate efficient operation of control systems and other aspects of the performance of the network. Generally, and as used herein, the three-phase signal is assumed to include positive sequence system components, negative sequence system components, and zero sequence system components. Each of the components includes frequency information, phase information, and magnitude information.In other words, the voltages in the phases become unbalanced during a typical perturbation. As an example, there may be a phase-to-phase or biphasic short while the remaining phase maintains a system-specific voltage. In such cases, the network signal comprises negative sequence and positive sequence components.The actual response of a generator control system to a given fault condition depends upon the goals for the operation of the generation system. To assist in fault recovery on the faulty lines, for example, a suitable response is to supply a current of sufficiently large magnitude (above normal values) to the electrical grid. This approach gives the protection devices carrying the fault current the ability to trigger fault control at a protection device location closest to the fault, thereby reducing the unnecessary overhead of a higher system or backup protection.The teachings herein are effective to minimize fault and maintain connection to the electricity grid by regulating the voltage of the subsystem and responding to the voltage of the subsystem. In a typical embodiment, this means that the teachings herein cause a response to the negative sequence voltage in such a way as to maintain the small signal linearity using a method that properly controls an amount of negative sequence current. The negative sequence current is converted (or "aligned") and then injected into the electricity grid. The current injection effects a regulation of the system to be maintained and reduces the severe effects of transients on system components. Furthermore, this approach provides linear characterization of system behavior and supports system design and analysis, enabling system operability and performance in response to a wide variety of scenarios and specifications.As discussed herein, "small signal linearity" refers to the use of a signal that is within the range of a controller. That is, small signal linearity refers to a signal that is within a signal range that allows the regulator to maintain operation based on its design. Maintaining the small signal linearity allows continued operation of the system as disclosed herein and prevents activation of protection measures such as actuation of the crowbar circuit 410.Furthermore, the use of current injection (also referred to as "current compensation") helps various schemes of branch circuit protection and voltage compensation. This is vital to the design of a robust generation system because the rules applied in branch circuit protection and voltage sustaining are widely distributed and have been recognized by those skilled in the art. To enable a plurality of different power generators to exist on a single grid, aspects of power injection may be controlled and matched to different standards and plant specifications.To relate the teachings of the present invention, an overview of aspects of components for generating electrical energy using a wind turbine will now be discussed. Referring to FIG. 1, an example embodiment of aspects of a wind energy system 350 is shown.In this embodiment, a rotor 106 includes a plurality of rotor blades 108 coupled to a rotating hub 110 and collectively form a propeller (e.g., a 30 meter propeller). This propeller is connected to a gear unit 118 which is in turn coupled to a generator 120. The generator 120 is a dual-feed asynchronous generator 120 (also known in the art as a "wound rotor" and referred to herein as a "DFIG 120"). A tachometer 352 is also coupled to the generator 120 and allows monitoring of the speed of the generator 120.The generator 120 is typically coupled to a stator synchronization switch 358 via a stator line 354 and is also connected to a power inverter component 362 via a rotor line 356. Stator line 354 enables three-phase power to be output from a stator (not shown) of generator 120, and rotor line 356 enables three-phase power to be output from a rotor (not shown) of generator 120. Stator synchronization switch 358 is connected to system power switch 376 via system line 360. With particular reference to the power converter component 362, the generator 120 is connected to a rotor-side filter 364 via the rotor line 356. The rotor-side filter 364 is connected to a rotor-side inverter 366. The rotor-side inverter 366 is typically coupled to a line-side inverter 368, which is also coupled to both a line-side filter 370 and a line contactor 372. In example embodiments, the rotor-side inverter 366 and the grid-side inverter 368 are configured for a normal operating mode in a three-phase two-level pulse width modulation (PWM) arrangement using insulated gate bipolar transistor (IGBT) switching devices (not shown). The rotor-side converter 366 and the grid-side converter 368 are connected via a DC link 435 in which a DC link capacitor 436 is located.The power inverter component 362 also includes a control unit 374 for controlling the operation of the rotor-side inverter 366 and the grid-side inverter 368 as described in more detail herein. It should be appreciated that in typical embodiments, the control unit 374 is configured as an interface between the power conversion component 362 and a control system 300. When referred to herein as a "conduit", it should be further appreciated that this refers to any communication or transmission connection that includes one or more conductors or conduits defining or forming an electrical path, a communication path, or other type of path.In typical embodiments, the line contactor 372 is connected to an inverter power switch 378 via a line 388. The inverter power switch 378 is also connected to the system power switch 376 via the system line 360. It should be appreciated that the output lines of inverter power switch 378 and the conductors of system line 360 may be connected in any manner known in the art, including by wiring respective conductors (e.g., the conductors of the respective power phase) together using a current conversion technique.The system power switch 376 is connected to a transformer 380 which is connected to a line power switch 382. The grid power switch 382 is connected to a portion of a medium voltage distribution grid via a grid line 384.In operation, the energy generated by the rotating rotor 106 in the generator 120 is provided to a power grid via a dual path. The dual path is provided by the stator line 354 and the rotor line 356. On the rotor line 356 side, the power of a sinusoidal three-phase alternating current (AC) is converted into direct current (DC) power by the power conversion component 362. The rotor-side filter 364 is typically used to compensate for the rate of change of the PWM signals of the rotor-side inverter 366, and the grid-side filter 370 is used to compensate for harmonic currents in the PWM signals of the grid-side inverter 368. The converted power from the power converter component 362 is combined with the power from the stator of the generator 120 to provide three phase energy at a frequency that is maintained substantially constant, e.g., at a value of 60 Hz AC current. The power conversion component 362 compensates for the frequency of the three-phase energy from the rotor of the generator 120 or adjusts the frequency to changes. An obvious example is changes in the speed of the hub 110. It should be appreciated that the stator synchronization switch 358 synchronizes the three-phase power from the stator of the generator 120, which is combined with the three-phase power output from the power inverter component 362.The power switches in the wind energy system 350, including the inverter power switch 378, the system power switch 376 and the grid power switch 382, are configured to disconnect the associated lines, e.g., when the current flow is excessive and may damage the components of the wind energy system 350. Other protection components including the line contactor 372 are also present that can enable a disconnection operation by opening a switch (not shown) for each of the lines of the line-side line 388.It should be appreciated that the wind energy system 350 may be modified for operation in conjunction with various power supply systems, etc. Generally, the wind energy system 350 generates energy as is known in the art. It should also be appreciated that the aspects of the wind energy system 350 as discussed herein are illustrative only and are not limiting to the system.The power conversion component 362 receives control signals from, e.g., the control system 300 via the control unit 374.The control signals are based on, among other things, measured conditions or characteristic operating characteristics of the wind energy system 350, as described herein. Typically, the control signals provide control over the operation of the power converter component 362. For example, feedback from tachometer 352 in the form of the measured speed of generator 120 may be used to control the conversion of the output power from rotor line 356 to maintain an adequate and balanced three phase power output. Other feedback from other sensors may also be used by the control system 300, including, for example, the feedback of the stator and rotor line voltages and currents to control the power converter component 362. Using various forms of feedback information and, for example, switch control signals, control signals for the stator synchronization switch and control signals (trip signals) for the system power switch may be generated in any known manner.Referring to FIG. 2, aspects of a typical topology 400 are shown for the dual-powered asynchronous generator (DPIG) 120 used in electric power generation with wind turbines. The control system 300 typically measures the voltage, current, speed, and position of the rotor 106 and provides control of switching of both the rotor-side inverter 366 and the grid-side inverter 368.Referring to FIG. 3, aspects of the typical topology 400 are shown including a system protection device referred to as a crowbar circuit 410. In typical embodiments, the crowbar circuit 410 is a fully controllable short circuit device. In other embodiments, the crowbar is realized by a control action of the rotor-side inverter 366. In these embodiments, the rotor-side inverter 366 is typically configured to limit operations of the crowbar circuit once control is restored.Because the wind energy system 350 is sensitive to disturbances in the network and because it typically requires the performance requirements of the network that wind energy systems 350 remain connected to the network 384 during each disturbance (i.e., to meet the requirements for low voltage ride-through (LVRT) driving), crownbar circuits 410 have typically been used as a protection measure. An embodiment of the crowbar circuit 410 is illustrated in FIG. 3. In some embodiments, the crowbar circuit 410 is integrated into the rotor-side inverter 366. Regardless of where the crowbar circuit 410 is located and regardless of the structure of the crowbar circuit 410, it protects the rotor-side inverter 366 when the crowbar circuit 410 is active by preventing a large current from flowing into the DC capacitors 436. When the crowbar circuit 410 is active, the rotor-side inverter 366 is inhibited from controlling the DFIG 120.Referring to FIG. 4, aspects of the typical topology 400 are shown including another system protection device, an arrangement of a dynamic brake 700. In this embodiment, the dynamic brake 700 is included across the DC link 435. In typical embodiments, the dynamic brake 700 includes a fully controllable short circuit device that is arranged in series with a resistor. The dynamic brake 700 typically prevents overvoltage of the DC link 435 and thus protects the rotor-side inverter 366 and the grid-side inverter 368. By using the dynamic brake 700, the crowbar circuit 410 is activated less frequently, so that the controllability of the rotor-side converter 366 during the grid fault is improved. Further, in a second approach and by using the dynamic brake 700 with a sufficiently large nominal power, the crowbar circuit 410 may be removed or modified. However, this second approach will typically require an embodiment of the dynamic brake 700 that requires a costly and costly implementation thereof. One skilled in the art will appreciate that one can address this problem in accordance with the teachings herein that can be used to reduce the capacity of the dynamic brake 700 and activate the crowbar circuit 410.If the network 384 has a single ended fault, the crowbar circuit 410 will often operate, so that the DFIG 120 will often not be controlled. This problem makes it very difficult to control the active and reactive power output to the grid 384 during the fault situation. This problem may be addressed according to the teachings herein that enable current compensation.Current compensation 500 may be described generally as including three steps, as illustrated in FIG. 5. In a first step, the voltage signals of the positive and negative sequence systems and the voltage of the DC intermediate circuit 435 are tracked. Among other things, voltage tracking 510 enables modeling of the negative sequence voltage that cannot be recorded for a given operating state. In a second step, the excess negative sequence voltage signal is transformed into a rotor current signal, and the negative sequence voltage is oriented for application to XY controllers within the control unit 374. After voltage alignment 520, current compensation 500 requires applying the aligned signal at the fault connection to the XY current regulator in control unit 374, thereby effectively adjusting the feedback and compensating for some of the mains fault signal.Prior to power injection 540 and typically (but not necessarily) after voltage alignment 520, acquisition of system parameters 530 is completed. Example system parameters include parameters such as system states, system performance, and topology. Knowledge of the system parameters allows constraints, constraints and goals for power injection to be determined in a manner that is consistent with the goals of the user. In this way, at least a portion of the amount of the negative sequence component can be used for feeding, wherein the at least a portion is within a predetermined range. In typical embodiments, the predetermined range takes into account the various system parameters and an objective to maintain small signal linearity.A power feed 540 minimizes the effect of the grid fault on the rotor inverter controller, thereby reducing the frequency of activation of system protection devices such as the crowbar circuit 410 as well as the required capacity of the dynamic brake 700. Consequently, a crossing of voltage interruptions by the wind energy system 350 is supported.Referring to FIG. 6, a relationship between the power sources is illustrated. In FIG. 6, power line 384 provides a negative sequence line voltage signal (V S(neg) and a negative sequence line current signal (I s,neg) to DPIG 120. Similarly, the rotor-side inverter 366 provides a negative sequence rotor voltage signal (V r(neg) and a negative sequence rotor current signal (I r,neg) to the DPIG 120. These designations are also used in Figure 7.In FIG. 7, a variation of the relationships of FIG. 6, aspects of the negative sequence voltages in the wind energy system 350 are shown. In addition to the variables introduced in FIG. 6, the sum (X s,ne g+X r,ne g) denotes the negative sequence impedance for the DPIG 120. FIG. 7 shows that by feeding an additional negative sequence current into the DPIFG 120, the required negative sequence voltage for the rotor-side converter 366 can be reduced. This is important because the rotor-side inverter 366 has limited ability to withstand both positive sequence and negative sequence voltages. A reduction in the negative sequence voltage allows an additional positive sequence voltage to be applied to the electricity grid 384. Thus, it is possible to improve the control of the positive sequence current and to control the positive sequence power and negative sequence power.When a negative sequence current is fed into the DPIG 120, the current I r,neg with respect to the rotational direction of the negative sequence system typically has a phase shift of 90° before the negative sequence voltage of the network 384. Pre-running before the negative sequence voltage of the network 384 causes negative sequence reactive power to be consumed. By using various tools not discussed herein, it is possible to measure the magnitude and phase of both the positive sequence voltage and the negative sequence voltage in the signal of the network 384. Once these quantities are known, the appropriate phase and amount for aligning at least a portion of the amount of the negative sequence component for feeding to the electricity grid 384 may be determined.To discuss this with more depth or from another approach, certain conventions are now introduced that are considered useful for discussion of aspects of the various signals. As used herein, the term "instrument signal" voltage (v si g n ai) includes the subsystem voltage, the subsystem voltage, and the phase angle, respectively, for the subsystem and subsystem components, a voltage (v nee d) refers to the measured amount of the subsystem voltage, a voltage (v a ii owed) refers to a voltage margin remaining after the subsystem voltage has been sufficiently accounted for, a voltage (V sh0 rtfaii) refers to the non-durable amount of the subsystem voltage, a current (I comp) refers to the compensation current, which is associated with the shortfall voltage and is available to the current feed 530, while a current (I compxy) refers to the symmetrical components of the compensation being split and rotated, the current (I compxy) being added to the error nodes of the current regulator.Referring to FIG. 8, aspects of current compensation 500 and signal analysis are shown. In FIG. 8, an input signal is typically evaluated in the control unit 374. Typically, a difference between the DC link voltage 601 and the magnitude 602 of the positive sequence voltage is determined to define an allowable voltage margin (V anowed) 603 available for supporting (and compensating for) harmonic voltages. Assuming that transformations between the stator circuits and the rotor circuits are handled appropriately, the difference between the actual negative sequence voltage demand 604 and the possible voltage margin (Vaired) 603 (which is set to positive values) constitutes the value of the shortage voltage (V sh0Etfall) 605, or the negative sequence voltage cannot be taken. The grid impedance, expressed as a reactance, is used to convert the shortage voltage (V sh orffaii) 605 into a current amount, thus providing a compensation current signal 606 (or simply a compensation current 606).Referring to FIG. 9, another embodiment of the signal analysis for current compensation 500 is shown. FIG. 9 illustrates an embodiment for processing considered as an approximation of the embodiment shown in FIG. 8. As is the case with the embodiment shown in FIG. 8, a processing unit, typically the control unit 374, enables the determination of the voltage demand NEED 604 and the possible voltage PERMITTED 603 and determines the shortage voltage 605. The shortfall voltage 605 is converted to a compensation current COMP 606.In each embodiment, the compensation current signal 606 (a scalar magnitude) in the counter system reference system must be placed between the X axis and the Y axis. The resulting vector must also be re-aligned with the reference system of the subsystem. These X-axis and Y-axis components are added to the error connections of the X-Y current regulator following the realignment step.FIG. 10 illustrates aspects of the stress alignment step 520. In FIG. 10, the compensation current 606 is measured and rotated to add the appropriate contributions to the error nodes of the current regulator as a pair of signals (comp XY) 607.In another embodiment (not shown), the negative sequence feed current (comp XY 607) is determined by detecting the saturation state of a regulator configured to control the negative sequence current. The detector (not shown) may be used to change the value of a scalar compensation term, which is then measured and rotated in a manner similar to the embodiment described above with reference to FIG. 10. With this approach, comp XY 607 can be controlled to maintain the linearity of the current regulators.Increasing the negative sequence current (I r,neg) fed decreases the negative sequence required inverter voltage (v r>ne g), as shown in FIG. 11. This reduction is shown from C to D. Accordingly, the available positive sequence inverter voltage (v r,p os) is increased from A to B. If the ability of the system to control a positive sequence current (I r, pos) is increased from A to B, this also applies to the ability to feed positive sequence power and negative sequence power.The total current, which contains both the negative sequence current (I r,neg) and the positive sequence current (I r, pos), should not exceed the total current capacity I r, max (shown between B and D) of the rotor-side converter 366. Because the total current capacity I r, maxof the rotor-side inverter 366 is known, one can calculate the maximum negative sequence current (point D) and the positive sequence current / reactive current (point B) that the system 350 can feed into the grid. The minimum negative sequence current (point C) which the converter must feed in in order to make the system fully controllable is also given when the negative sequence current I r, pos is zero (point A). Accordingly, the maximum (point D) and the minimum (point C) form a power range for the system 350.One benefit achieved by the teachings herein is a technique for reducing the current in the rotor-side inverter 366. Reference is made to the method shown in FIG. 9. Another benefit is to provide a greater variety of conditions under which active / reactive power demand may be followed by a higher level controller (e.g., a turbine controller or a wind farm controller). A further benefit is an increased ability to feed active power into the grid or to increase reactive power feed into the grid. Accordingly, a generation system that makes use of the teachings herein is equipped to more accurately replicate the behavior of traditional synchronous generators during transients in the network and events with negative sequence voltage.In an evaluation of the preceding exemplary embodiments, simulations were carried out. Two separate representations are provided. First, a response to a simulated transient is shown in FIG. 12. In Figure 13, aspects of the same simulated event are shown. However, in Figure 13, performance was improved according to the teachings herein. Note that the signals shown in the simulation diagrams are denoted with reference to FIGS. 12 and 13 as follows: "Cb_gate" refers to a gate signal of a crowbar event, "1_dscmvmagn" and "1 _dscmvmagp" are signals of the negative and positive sequence voltage levels, "Iapcc" and "Ibpcc" denote the inverter currents at the point of common connection (pcc). The phase-to-phase error was applied on the power side of transformer 380 between phase A and phase C. "Vca" and "Vab" indicate a phase-to-phase voltage at pcc.The simulated transient event (in which phase A and phase C are shorted) is a phase-to-phase perturbation with a signal decrease to 0 volts. The short circuit was applied at the 0.6 second and eliminated at the 1.0 second.Referring to FIG. 12, the results for a prior art wind energy system 350 are shown. In FIG. 12, the wind energy system 350 experiences multiple and sustained actuations of "crowbar events" caused by the control action of the rotor-side inverter 366. The crownbar events continue over the duration of the network fault. Referring to FIG. 13, the assertion of the crowbar event gate signal (Cb_gate) provides an indication that the system controllers are applying control. After a few Crowbar actuations, the control unit 374 has recovered current control and regulated the system to the desired values. Note that control was recovered within a comparatively short interval.The above description is directed to feeding positive positive positive positive power and reactive power to the network. However, one skilled in the art will recognize that this invention also applies to receiving positive positive power and reactive power subsystem from the network 384, as required. By supplying a negative sequence current to the network 384, the system 350 is equipped to control the active and reactive power consumed according to a user's needs.Note that the above description relies on embodiments using the DFIG 120. However, one skilled in the art will appreciate that the teachings herein may also be applied to systems for converting total energy, as shown in FIG. 14.FIG. 14 shows a system topology 600 for overall energy conversion. The total energy conversion system topology 600 includes a generation side inverter 466 and a grid side inverter 368. The generation-side inverter 466 and the grid-side inverter 368 are connected via a DC link 435. A total power generator 220 powers the generation-side inverter 466. It should be appreciated that the grid side inverter 368 connected to the grid via a transformer is similar in many respects to the rotor side inverter 366 connected to the grid via the DFIG 120. This is particularly important in view of the ability to supply a current to the grid, and aspects thereof may be interchangeable. Accordingly, these example inverters are not limiting to the teachings herein.Further components may be included as described above or known in the art. Filter 370 is also included in this illustration, for example.Typically, the teachings herein are practiced in this embodiment only for the line-side inverter 368. Injection of the negative sequence current by the line side inverter 368 into the network 384 increases the ability of the total power generator to control the positive sequence current. Accordingly, the ability to feed positive positive positive or negative positive positive positive positive sequence power is increased. Knowing the current carrying capacity of the grid side inverter 368 for the overall energy conversion system, the possible operating range for feeding a negative sequence current and a positive sequence current to the grid 384 may be determined. The user can select the setting aspects of the power feeding within the range according to preference or need.One skilled in the art will appreciate that the current compensation techniques 500 may be embodied in a variety of ways. The current compensation 500 may be monitored, for example, by the operation of software or firmware. Typically, the software and firmware are implemented in the control unit 374 to enable rapid adjustment of the wind energy system 350. However, this is for illustrative purposes only and does not limit the embodiments disclosed herein.A method and apparatus are provided that provide a response to the negative sequence power demand during a fault of the network system connected to an energy generating plant, such as a wind turbine, and facilitate tracking components in the network signal, aligning at least a portion of the signal, and feeding the aligned portion. The controlled injection of the negative sequence current provides for an extension of the small signal control response and also enables changes in the apparent impedance of the mains connection of the power converter installation.While the invention has been described with reference to an exemplary embodiment, those skilled in the art will recognize that various modifications may be made and equivalents may be substituted for the elements of the invention without departing from the scope of the invention. Furthermore, numerous changes may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.List of reference characters106 Rotor 108 Blade 110 Hub 118 Transmission unit 120 Generator 220 Total energy generator 300 Control system 350 Wind turbine 352 Tachometer 354 Stator line 356 Rotor line 358 Stator synchronization switch 360 System line 362 Power converter component 364 Rotor-side filter 366 Rotor-side converter 368 Grid-side converter 370 Nez-side filter 372 Grid contactor 374 Control unit 376 System power switch 378 Converter power switch 380 Transformer 382 Grid power switch 384 Electricity grid 388 Grid-side line 400 Typical topology 410 Crowbar circuit 435 Direct current link 436 Direct current link capacitor 466 Generation-side converter 500 Current compensation 510 Voltage tracking 520 Voltage alignment 530 Acquisition of 540 Parameters 600 Current feed 601 System topology 602 Direct current link voltage 603 Positive sequence voltage magnitude 604 Allowable voltage distance 605 (V a n owe d) 606 actual negative sequence voltage demand (v ne ed) 606 shortage voltage (V shor tfaii) 607 compensation current compensation current signal fault nodes of the current regulator as a pair of signals 700 (comp XY) dynamic brake

Claims

A method of varying an effect of a fault in an electricity grid (384) on a generator (120) connected to the grid (384), the method comprising: tracking at least one negative sequence component and one negative sequence component of a signal of the electricity grid (384); aligning at least a portion of the magnitude of the negative sequence component for feeding to the electricity grid (384); and feeding the at least one portion to the electricity grid (384) to vary the effect of the fault, wherein a power converter component (362) having a DC link (435) with a DC link capacitor (436) is used, and wherein the power converter component (362) receives control signals from a control system (300) via a control unit (374), the control signals being based on measured states or characteristic operating characteristics; and wherein the generator (120) is a double-fed asynchronous generator (120) and the tracking, the alignment and the feeding (540) implement a current compensation (500), wherein after the alignment an application of the aligned signal to a fault connection of a current regulator of the control unit (374) takes place and wherein the control unit (374) controls the power converter component of the double-fed asynchronous generator (120).The method of claim 1, wherein the negative sequence component and the positive sequence component each comprise phase information, frequency information and magnitude information.The method of claim 1, wherein the tracking includes detecting the fault of the network (384).The method of claim 1, further comprising obtaining parameters from the electricity grid (384), the generator (120), and / or a control system for the generator (120) prior to the feeding.The method of claim 1, wherein the altering of the effect includes increasing the small signal linearity for a subsystem network state and / or a counterpart system network state.The method of claim 1, wherein the altering includes reducing actuation of system protection devices (410, 700).The method of claim 1, wherein parameters for the at least one portion are selected to maximize the active power injection and / or the reactive power injection.The method of claim 1, wherein the injecting includes simulating a synchronous generator.An apparatus for changing an effect of a fault in an electricity grid (384) on a generator (120) connected to the grid (384), the apparatus comprising: a controller (374) for tracking at least one of a negative sequence component and a positive sequence component of a signal of the electricity grid (384), aligning at least a portion of the magnitude of the negative sequence component for feeding to the electricity grid (384), and feeding the at least a portion to the electricity grid (384) for changing the effect of the fault; a power converter component (362) having a DC link (435) with a DC link capacitor (436), wherein the power converter component (362) receives control signals from a control system (300) via the control unit (374), and the control signals are based on measured states or characteristic operating characteristics; and wherein the generator (120) is a double-fed asynchronous generator (120) and the tracking, the aligning and the feeding (540) implement a current compensation (500), wherein after the aligning the aligned signal is applied to a fault connection of a current regulator of the control unit (374), and wherein the control unit (374) controls the power converter component of the double-fed asynchronous generator (120).

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