Grid-forming vector current control

Abstract

The present disclosure provides a grid-forming vector current control system configured to emulate a virtual synchronous machine (VSM). The disclosed system includes a droop control unit, a current control unit, a virtual admittance unit, and a phase-locked loop (PLL) unit. The virtual admittance unit and the PLL unit are configured to emulate the inertia of the VSM. A virtual current source is connected in parallel with the VSM.

Description

[Technical field]

[0001] The present disclosure relates to systems and methods for grid forming vector current control. [Background technology]

[0002] The rate of change in the power grid has traditionally been high. Traditional power generation using synchronous generators is increasingly being replaced by converter-interfaced renewable energy resources. Many studies have shown that there is a risk of grid instability when the power system is equipped with a large amount of renewable energy resources that do not employ any grid-assisted control measures. The reason for the instability is that such resources do not adapt their power injection into the grid according to the actual power demand in the grid.

[0003] Such grid-agnostic current / power control strategies have been common in most converter interface applications for a long time. However, in the past decades, stricter regulations have been put in place for e.g. PV inverters. Today, PV inverters must gradually reduce the injected active power if the grid frequency increases above nominal. This behavior is the first step in the direction of grid-assisted power converter control. The injected active power is slowly adapted according to the actual needs of the grid. Such a simple method only provides steady-state grid assistance, i.e. it still relies on synchronous generators in the grid taking over transient load changes.

[0004] On the other hand, more advanced converter control methods are being developed for insular grids, microgrids and railway grids. These control methods aim to provide not only stationary grid support but also transient grid support. Converter-interfaced energy resources with such control methods can completely replace conventional synchronous generators in the grid. It is possible to operate grids where no conventional synchronous generators are connected, a situation that is already common, for example, in railway grids.

[0005] In order to provide full grid support, it is necessary that the amount of power injected into the grid is adapted according to the demands of the various loads connected to the grid. This is achieved by changing the behavior from a constant power injection that is grid agnostic to a behavior that is closer to a voltage source. Converters equipped with such a control method automatically adapt and share the injected power to provide the actual power required by the various loads in the grid.

[0006] For reasons of backward compatibility with legacy equipment in AC generation and distribution, the control methods aim to provide "inertia" to the grid, which means that the rate of change of the grid frequency is limited.

[0007] The combined effects of voltage source characteristics and inertia result in characteristics that closely resemble the behavior of a synchronous generator. Such a control method can provide full grid assistance and completely replace a synchronous generator in the grid. Summary of the Invention [Problem to be solved by the invention]

[0008] Generally, state of the art control mechanisms may be classified into two main groups. In the droop control approach, the control law is based on a steady-state coupling of frequency (or angle) and power flow in the inductive grid. Various implementation variants exist and several methods have been proposed in recent years to improve on this method. In most implementation variants, this method creates a voltage reference that must be tracked using additional cascaded voltage and current control loops. See references [1] and [3] in this regard.

[0009] In the virtual generator approach described for example in [2], the control law is based on the principle of emulating the behavior of a synchronous generator. A mathematical model describing the behavior of the generator is directly realized in the controller. Various implementation variants exist, differing mainly in the level of detail and the complexity of the equations realized.

[0010] To emulate a virtual generator or virtual synchronous machine (VSM), the mathematical equations of a synchronous generator are realized in the controller to emulate the behavior of the generator. In the direct method (voltage source type), the mechanical equations are solved for the stator voltages that are given as reference either directly to the modulation stage or to subsequent cascaded voltage and current control loops. In the inverse method (current source type), the mechanical equations are solved for the stator currents that are given as reference to the subsequent current control loops.

[0011] However, both of the above approaches require various additional control loops to make the concept applicable. In particular, the cascaded voltage and current control loops provided by the direct method provide current control, including current limiting.

[0012] The current control loop and PLL track the active / reactive power reference given by the inverse law and provide current control including current limiting. Grid synchronization and grid frequency measurement are also provided.

[0013] The virtual impedance loop may emulate a virtual stator impedance. Additional components may be used to improve the performance of the virtual generator. The additional components may be at least one of the following:

[0014] Additional PLLs for the direct method to synchronize with the grid before closing the converter's main circuit breaker (MCB) or during a fault, automatic voltage regulators (AVRs) to improve voltage regulation at the PCC, and feed-forward gains for grid current and PCC voltage to improve the performance of the PI control loop. Attenuator winding emulation is used to improve the damping of low-frequency oscillations. In the simplest inverse method, virtual inertia effects are emulated with the derivative of the measured grid frequency. Furthermore, low-pass filtering of the virtual impedance reference is often required to reduce the amplification of high-frequency noise caused by the derivative used to calculate the virtual inductance.

[0015] However, state-of-the-art systems can be difficult to tune and require high commissioning efforts. Due to insufficient bandwidth separation in converters with lower switching frequencies, there can also be interactions between control loops. In conventional systems, typically about 15 parameters must be tuned with unknown interactions. In strong grids, a lack of reference tracking performance often occurs. Furthermore, loss of synchronization during disturbances can appear.

[0016] As already discussed, grid-assisted control methods are becoming increasingly important in various applications such as grid-connected battery energy storage (BESS), microgrids, PV inverters, railway interties, HVDC and UPS. Transmission system operators in various countries are discussing changing the standards for grid-connected energy resources so that grid assistance becomes mandatory for all grid-connected energy sources. Today's grid-assisted control methods are mainly based on two basic concepts, namely the "virtual generator" concept and the droop control concept. Both of these methods have several drawbacks such as high complexity, difficult tuning techniques with high commissioning efforts, and stability issues. Furthermore, the above-mentioned methods may not provide sufficient control performance simultaneously in strong and weak grids, and therefore may require retuning or control mode switching during islanding events. [Means for solving the problem]

[0017] In this ID, we propose a novel generic grid formation / grid assistance control method, which is based on emulating the combined effect of a virtual machine and a parallel connected current source.

[0018] The method according to the present disclosure provides a simplified and reliable structure, with fewer cascaded control loops, reduced complexity, integrated damping, integrated current control, and simple tuning. Moreover, it has good performance in both weak and strong grids and allows island control. The amount of grid support can be flexibly configured with a single parameter. Due to its simplicity, the novel control method significantly reduces the commissioning effort. It can be used in various applications with only minor modifications due to its generic nature.

[0019] The invention is specified in the independent claims. Preferred embodiments are defined in the dependent claims. The system is directed to a grid forming vector current control system configured to emulate a virtual synchronous machine (VSM). In particular, the disclosed system comprises a droop control unit, a current control unit, a virtual admittance unit, and a phase locked loop (PLL) unit. The virtual admittance unit and the PLL unit are configured to emulate the inertia of the VSM. A virtual current source is connected in parallel to the VSM.

[0020] Various embodiments may preferably implement the following features. The output signal of the droop control unit may preferably be connected to a virtual current source to emulate a speed governor.

[0021] Preferably, the output signal of the droop control unit is connected to a PLL unit to emulate a virtual mechanical governor.

[0022] The system may further comprise an active voltage regulator (AVR) unit. The system is preferably deployed in at least one of a microgrid converter, or a photovoltaic (PV) inverter, or an uninterruptible power supply (UPS), or a grid intertie converter, or a railway grid intertie converter, or a high voltage direct current (HVDC) converter, or a battery energy storage system (BESS).

[0023] Coefficient k VSM is the output signal of the virtual admittance k VSM The system may be configured to change its characteristics continuously from a voltage source to a current source by proportionally scaling with

[0024] Coefficient k VSMis preferably fed to at least one of a droop control unit, a virtual admittance unit or a PLL unit to increase or decrease the inertial response of the system to disturbances in the main grid. Preferably, at least one of a microgrid converter, or a PV inverter, or a UPS, or a grid intertie converter, or a railway grid intertie converter, or a HVDC converter, or a BESS is connected to the main grid. More preferably, a microgrid converter, a PV inverter or a UPS is connected to the main grid.

[0025] Preferably, k VSM is the output signal of the virtual admittance k VSM Scaling the PLL proportional gain by k VSM Scaling inversely with and / or the droop constant k VSM Scale the VSM nominal power by proportionally scaling with

[0026] Coefficient k VSM preferably scales at least one of the spinning wheel coefficient or the transient power or the inertia.

[0027] The droop control unit may include a fast current source.The AVR unit may be configured to keep the magnitude of the voltage at the point of common coupling (PCC) constant.

[0028] The PLL unit is preferably configured to measure the voltage at the point of common coupling (PCC).

[0029] The present disclosure further relates to a method for controlling a grid forming vector current control emulating a virtual synchronous machine (VSM), the method including providing a droop control unit, providing a current control unit, providing a virtual admittance unit, and providing a phase-locked loop (PLL) unit, inertia of the VSM is emulated and a virtual current source connected in parallel with the VSM is controlled.

[0030] Various embodiments may preferably implement the following features. The emulation of the inertia of the VSM may preferably be achieved using a virtual admittance unit and a PLL unit.

[0031] The output signal of the droop control unit is preferably connected to a virtual current source to emulate a speed governor.

[0032] The output signal of the droop control unit is preferably connected to a PLL unit to emulate a virtual mechanical governor.

[0033] Coefficient k VSM is the output signal of the virtual admittance k VSM By proportionally scaling with , the system characteristics may be changed continuously from a voltage source to a current source.

[0034] The invention will now be described with reference to the accompanying drawings. [Brief description of the drawings]

[0035] [Figure 1a] FIG. 1 shows a schematic diagram according to the prior art. [Figure 1b] FIG. 1 shows a schematic diagram according to the prior art. [Figure 2a] FIG. 1 shows a schematic diagram according to the present disclosure. [Figure 2b] FIG. 1 shows a schematic diagram according to the present disclosure. [Figure 3a] FIG. 1 shows a schematic block diagram according to one embodiment of the present disclosure. [Figure 3b]FIG. 1 shows a schematic block diagram according to one embodiment of the present disclosure. [Figure 4] FIG. 1 shows the basic structure of the proposed grid-assisted scheme. [Figure 5a] 13 shows a modification of a virtual machine. [Figure 5b] 13 shows a modification of a virtual machine. [Figure 5c] 13 shows a modification of a virtual machine. [Figure 6a] An equivalent model of the proposed control scheme is shown. [Figure 6b] An equivalent model of the proposed control scheme is shown. [Figure 7] 4 shows a vector diagram for a transient state. [Figure 8] FIG. 13 is a block diagram illustrating the use of coefficient kVSM to adjust the amount of grid assistance. [Figure 9] FIG. 1 illustrates the system's response to a disturbance and its clearance in different settings. [Figure 10] FIG. 1 illustrates the system's response to a disturbance and its clearance in different settings. [Figure 11a] FIG. 2 shows a schematic block diagram according to another embodiment of the present disclosure. [Figure 11b] FIG. 2 shows a schematic block diagram according to another embodiment of the present disclosure. [Figure 12] FIG. 1 shows an alternative implementation of the proposed control concept. [Figure 13] 1 is an equivalent circuit of the control concept with an optional active attenuator in parallel with the current source. [Figure 14] 14 shows an example of the realization of an active attenuator branch according to FIG. [Figure 15] FIG. 1 shows a block diagram according to one embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Figure 1a shows a scheme of a control system according to the prior art. It employs a virtual impedance and a VSM. The outputs of both the VSM and the virtual impedance are fed into cascaded voltage and current controls. Figure 1b shows a more detailed view of the virtual generator control approach. As can be seen from the schematic diagram, this is a rather complex structure with many parameters that must be adjusted and adapted to each other.

[0037] In the following, the terms block and unit may be used interchangeably and are not meant to be exclusive.

[0038] FIG. 2 shows a schematic of a vector current control system according to the present disclosure. In particular, FIG. 2a shows a virtual synchronous machine (VSM) with parallel connected current sources, and FIG. 2b shows its equivalent circuit diagram. The system emulates the combined effect of a virtual machine, whose nominal power and amount of grid support can be independently scaled online with a single input / parameter and parallel connected current sources. Thus, the system may be remotely controlled. For certain aspects, e.g., fault handling, it may be preferable to change the nominal power and amount of grid support within the application code.

[0039] More specifically, the disclosed system according to the exemplary embodiment emulates the proposed behavior using a control block as shown in Figure 3, the calculation of which is described below. Figure 3a shows the point of common coupling (PCC) V pcc A phase-locked loop (PLL) block is fed by the voltage of ref The output of the PLL is a speed control block having a V pcc and the virtual synchronous machine voltage V vsm The speed control output, virtual admittance output, and reference current I ref is then processed in the current control block to produce the voltage V out Output.

[0040] FIG. 3b further includes a parameter k as an input to at least one of the PLL block, the speed control block, and the current control block. VSM This may control the amount of grid support for the main grid. VSM can also be used to smoothly change the character of the control scheme from voltage source to current source, and also supports a mix of both characters. During a grid disturbance, a smooth switchover to robust current control can be beneficial to keep in sync with the grid. The control mode may not be changed.

[0041] The system may also include an active voltage regulator (AVR). The system according to an exemplary embodiment emulates the combined effects of virtual inertia, attenuator windings, virtual stator impedance, AVR, and droop governor. The virtual inertia and attenuator windings are emulated using standard PLL blocks.

[0042] In contrast to conventional virtual machine implementations, active power set point tracking and droop governor are conceptually realized using fast current sources. Therefore, there is no performance degradation on strong grids that can often be seen when using VSM control. Thus, good performance on weak and strong grids is achieved. The method supports unplanned islanding and island control.

[0043] The complexity of the control scheme and associated commissioning effort is very low. Due to the elimination of the cascaded voltage control loops and by reusing the PLL to emulate inertia and damping, the number of parameters can be reduced from over 15 in the conventional approach to only 7 parameters with clear meaning and fewer interactions. Therefore, the system can also be more easily tuned.

[0044] Further advantages of the proposed concept are the possibility to configure the amount of grid support with a single parameter, simplifying the provision of grid support in applications with low energy storage, such as UPS in data centers, PV with low or no storage, EV charging with buffer batteries, non-sensitive loads, etc., and smooth transitions in and out of operation at maximum current without losing synchronisation, thus improving fault handling capabilities.

[0045] The system provides explicit current control and limiting and an explicit PLL for synchronization with the grid before closing the MCB and to prevent loss of synchronization during a fault. The PLL acts as a virtual swing equation with virtual admittance, eliminating the need for synchronization using power balance.

[0046] 4-6 show a more detailed overview of the proposed grid-assisted control scheme and its respective equivalent circuit.

[0047] The control scheme of Figure 4 is conceptually shown in Figures 2a and 2b. The control scheme of Figure 2a can be separated into two functional parts, as indicated by the dotted lines: a governor-less VSM, i.e., the spinning wheel or inertia, respectively, and a current source connected in parallel at the terminals of the virtual synchronous machine. The behavior of the virtual synchronous machine is emulated using a PLL. The current source operates in parallel. It ensures a steady-state power balance between the grid and the spinning wheel, so it acts simultaneously as a fast power setpoint tracker and governor. The inertial response to the active power is guaranteed by a virtual admittance. In steady state, the full power flowing to the grid is injected by the current source. The spinning wheel injects power only during transients.

[0048] 4 shows a system according to one embodiment having a droop control unit 1, a current control unit 2, a virtual admittance unit 3, and a phase-locked loop (PLL) unit 4. The virtual admittance unit 3 and the PLL unit 4 are configured to emulate the inertia of a VSM, and a virtual current source 5 is connected in parallel to the VSM. In this embodiment, the output signal of the droop control unit 1 is connected to the virtual current source 5 to emulate a speed governor.

[0049] The system according to the preferred embodiment will now be described in more detail. The simplest form of conventional state-of-the-art realization of a virtual synchronous machine is based on the equations of motion. The goal of virtual synchronous machine control is to make the converter behavior at the Point of Common Coupling (PCC) equal to that of a synchronous machine.

[0050] Figures 5a to 5c are equivalent circuits of the components shown in Figures 1a and 1b, i.e., virtual machine variants of the current technology. In particular, Figure 5a shows a converter, Figure 5b shows a VSM without virtual inductance, and Figure 5c shows a VSM with virtual inductance.

[0051] The following derivation is based on a converter with an inductive output filter for reasons of simplicity, however the control method proposed herein is suitable for converters with any type of output filter structure such as L, LC or LCL.

[0052] The equation of motion is given by the voltage source V C In the case of a VSM without virtual inductance, the converter voltage V C is equal to the back electromotive force (back emf) of the virtual machine. Converter filter inductance L C is equal to the stator inductance of the virtual machine, and the PCC voltage V pcc is equal to the stator voltage of the virtual machine. Converter voltage V C (virtual back emf) is the rotational frequency ω to emphasize its equivalence with the virtual rotor frequency. rThe PCC voltage (virtual stator voltage) is pcc The equations of motion look like this:

[0053]

number

[0054] In the quasi-steady state, the power flow is a virtual back-emfV C and the converter filter reactance X C =ω N *L C PCC voltage V pcc is given by the angular difference between

[0055]

number

[0056] There is a direct dependence on the power flow across the transmission line components, reactances and angular differences, i.e.

[0057]

number

[0058] Therefore, the equation of motion is as follows:

[0059]

number

[0060] In a second step, the state-of-the-art VSM implementation calculates the virtual reactance X according to Fig. 5c. vs =ω N *L vs The virtual reactance slightly changes the equation. The converter voltage V in phasor notation C is calculated according to the following:

[0061]

number

[0062] The virtual back emf of the machine is V v (Rotational frequency ω r and angle θ r ) with a virtual reactance X vs In terms of behavior in PCC, the virtual machine now v =X c +X vs and the power flow equation is

[0063]

number

[0064] and the equation of motion becomes:

[0065]

number

[0066] Virtual inductance L vs is the converter filter inductance L C Note that the inductances of the virtual machine 111 are in series with the PCC and therefore add to the total virtual stator inductance of the virtual machine seen by the PCC.

[0067] 6a and 6b show an equivalent model of the control scheme according to an exemplary embodiment. Here, the equation of motion is not directly realized as in the conventional approach. Furthermore, the effect of the PLL with virtual admittance is compared with the equation of motion. The angle θ r is no longer calculated by the equations of motion, it is the output of the PLL by the PLL equations.

[0068]

number

[0069] Calculating the derivative of equation (9) gives:

[0070]

number

[0071] For small angle differences (which is the case for reasonable PLL tuning), the sine function can be approximated as:

[0072]

number

[0073] Its derivative is:

[0074]

number

[0075] Therefore, the set of equations can also be formulated as follows:

[0076]

number

[0077] By comparing equation (11) with the original equation of motion (1), the PLL gain for the formal equivalence of the two equations may be:

[0078]

number

[0079] The governor power will be replaced by a parallel current source in the following later steps. Thus, the proportional gain of the PLL emulates the attenuator winding effect, and the integral gain combines the angle difference with the increase or decrease in frequency, emulating the self-synchronization principle of a synchronous machine.

[0080] Contrary to the equation of motion, the PLL does not implicitly couple the power flow with the angle difference, because it is the internal PLL angle θ r The coupling between the power flow and the angle difference must be explicitly guaranteed by means of an additional current reference for the current controller. This reference is created with a virtual admittance equal to the inverse of the virtual stator impedance.

[0081]

number

[0082] The stator impedance is assumed to be of RL type. If the admittance is realized in the dq reference frame, it can be written as:

[0083]

number

[0084] Current reference i v,dq is the virtual admittance and the virtual back-emf voltage V aligned in the d direction of the PLL reference frame. v (V v =V N +j*0) and PCC voltage V pcc and the voltage difference between the input and output terminals (this is shown in FIG. 7, which shows a vector diagram for the transient state).

[0085]

number

[0086] Alternatively, it is possible to use complex numbers and realize the admittance according to the following formula:

[0087]

number

[0088] At steady state, R v <<ω N *L v Assuming that, the admittance simplifies to:

[0089]

number

[0090] This leads to the steady state current criterion:

[0091]

number

[0092] PCC voltage V pcc is the nominal voltage V N Therefore, the virtual stator reactance (X v =ω N *L V ) and the effective current in the d direction is given by

[0093]

number

[0094] and the associated power flow (assuming fast current tracking) is:

[0095]

number

[0096] Thus, the present disclosure ensures equivalence between the quasi-steady state power flow governed by the equations of motion and the power flow generated by the PLL+virtual admittance combination.

[0097] The virtual admittance may be realized with a full dynamic model to ensure passivity and damping of high frequency resonances. However, simplified admittance models (neglecting the s*L term) are also possible, as are more complex models that allow additional filtering functions. Even asymmetric admittances are possible. The minimum requirement is to realize a coupling between the q voltage and the active current.

[0098] A combination of special settings of PLL gain and virtual admittance was shown to make it possible to emulate the effects of inertia, damper windings, and stator impedance of a virtual synchronous machine using conventional control structures used for vector current control.

[0099] The droop block is further explained below. Unlike the original VSM realization, the proposed control method only realizes a spinning wheel without a governor for the mechanical input power. Therefore, the injected power from the spinning wheel will be zero in the steady state (i v,d =0), angle θ r is θ pcc and align.

[0100] The governor power is replaced by a current source that ensures a steady-state power balance. In addition to the criteria coming from the virtual admittance, an active current criterion is added:

[0101]

number

[0102] which leads to the injected power:

[0103]

number

[0104] The main difference is that this current reference is fed directly to the current controller and no filtering is done on the equations of motion. Setpoint changes feed through directly and are tracked much faster compared to traditional VSM implementations.

[0105] For reasonable load conditions of the spinning wheel and reasonable virtual stator reactance, the angle difference θ r -θ pcc is small, and cos(θ r -θ pcc ) is close to 1. In that case, V pcc,d Division by a constant V N can be replaced by division by

[0106] Additionally, an Active Voltage Regulator (AVR) can be added to keep the PCC voltage magnitude constant. The AVR block can be of PI, or integrator (I) type, or proportional type (P) only. It is the current reference i that is used to regulate the voltage magnitude. qref Form.

[0107]

number

[0108] However, since the virtual admittance already realizes the proportional gain AVR effect as follows, the AVR block can also be omitted.

[0109]

number

[0110] As shown above, the coefficient k VSM The coefficient k may be used. VSM is the output signal of the virtual admittance k VSM The system may be configured to change its characteristics continuously from a voltage source to a current source by proportionally scaling with

[0111] Coefficient kVSM may be fed to at least one of the droop block, virtual admittance block or PLL block as described throughout this specification to increase or decrease the inertial response of the system to disturbances in the main grid to which the microgrid converter, PV inverter, UPS or the like is connected. Preferably, at least one of the microgrid converter, or PV inverter, or UPS, or grid intertie converter, or rail grid intertie converter, or HVDC converter, or BESS is connected to the main grid. More preferably, the microgrid converter, PV inverter or UPS is connected. Where k VSM is the output signal of the virtual admittance k VSM Scaling the PLL proportional gain by k VSM Scaling inversely with and / or the droop constant k VSM The nominal power of the VSM may be scaled by proportionally scaling with

[0112] k VSM may also be used to adjust or scale the amount of grid assistance. VSM is continuous and ranges from 0 to 1. It is possible to use this factor to scale the nominal power of a virtual machine that can be thought of as operating in parallel with a current source by a single factor. k VSM The connections and inputs are shown in Figure 3b. The equation is:

[0113]

number

[0114] Referring to FIG. 4, a modified block diagram of the droop block, the virtual admittance block and the PLL block is shown in FIG.

[0115] The kth filter used in the PLL block to keep the PLL loop stable.VSM It is important to saturate the inverse of to a maximum value. Too high a proportional gain in the PLL loop can lead to instability, a well-known effect of standard vector current control in a PLL.

[0116] It should be noted that adjusting the amount of grid assistance by scaling the nominal power of the VSM is a unique feature of the control structure of this disclosure. The nominal power of the parallel current sources is not affected, and therefore the control structure can be scaled to any number of grids, even if k VSM is set to zero and still operate with conventional current regulation and maximum power capability.

[0117] This feature may not be realized in conventional VSM implementations. In a standard VSM, the power transfer capability is linked to the nominal power of the VSM. The power flow is controlled by advancing the angle of the virtual back emf in the VSM, and the voltage difference is applied across the virtual stator reactance. Reducing the nominal power of the VSM leads to an increase in the reactance, and as a result, an increased angle difference is required to transfer the same amount of power. Since the maximum angle difference is limited to 90°, the power transfer capability decreases with the nominal power of the VSM. As a result, the nominal power of the VSM cannot be independently adjusted with conventional control approaches. Applications other than those disclosed herein may also be possible.

[0118] A possible application of resynchronization during a failure event and subsequent resynchronization after the failure is cleared is shown in Figure 9. (Similar to standard VSM behavior) VSM Without reducing the coefficient, heavy oscillations will be caused after the fault is cleared because the synchronization between the grid and the VSM is lost while the PCC voltage is close to zero.

[0119] In contrast, during LVRT events, VSM If the coefficient is reduced to zero, a smooth resynchronization is achieved and no oscillations between the converter and the grid after fault clearance are observed. This can be seen from the graph in Figure 10, which shows the behavior of the proposed control system. VSMDue to the temporary reduction in current, the control characteristic is a conventional robust current control scheme with controllable and favorable behavior during a fault event. It is possible to inject reactive current or apply any other predefined current reference during the LVRT event.

[0120] Grid support may also be operated at a reduced nominal power, for example 10-20%, in applications with only low energy storage capability, such as UPS systems or PV with low or no energy storage. A certain percentage of the nominal power may be reserved for grid support, while the remaining power requirements of the application are handled with conventional current control.

[0121] Figures 11a and 11b show another preferred embodiment of the present disclosure as a schematic block diagram. In particular, Figure 11a relates to an alternative implementation having a virtual machine governor instead of a current source governor. This is achieved, among other things, by connecting the output signal of the droop control block to a PLL block. This emulates a virtual machine governor. Similar to the control scheme as shown in Figure 3a, the coefficient k VSM may also be fed to a PLL block and / or a speed control block and / or a virtual admittance block. Description of features corresponding to the first embodiment will be omitted.

[0122] Figure 12 shows an alternative configuration for the scheme proposed in Figure 4. Instead of using a current source as the governor, a virtual machine governor can be implemented that varies the power flow by advancing or retarding the rotor angle of the VSM. This is achieved by removing the direct connection of the droop block to the current controller. Instead, the governor power reference P ref An additional input is needed to the PLL block to turn it into a virtual rotor angle shift.

[0123] 12 shows a system according to an embodiment having a droop control unit 1, a current control unit 2, a virtual admittance unit 3, and a phase-locked loop (PLL) unit 4. The virtual admittance unit 3 and the PLL unit 4 are configured to emulate the inertia of a VSM, and a virtual current source 5 is connected in parallel to the VSM. In this embodiment, the output signal of the droop control unit 1 is connected to the PLL unit 4 to emulate a virtual machine governor.

[0124] The following applies to both the embodiment and the control method, respectively: The inputs to the current controller i, which can be operated independently of the VSM d,ref2 A current source may be connected in parallel to the VSM by using the coefficient k VSM However, it is still possible to adjust the nominal power of the VSM using the reduced nominal power P N,VSM So, P set is +-P N,VSM otherwise the VSM may be overloaded due to the increased virtual reactance. In order to utilize the full power capability of the converter, the current source input i d,ref2 It is required to operate in

[0125] Optionally, an active damping branch (e.g., a virtual damping resistor) may be connected in parallel to the current source. Thus,

[0126]

number

[0127] Such active damping branches usually employ band-pass filters so that they are effective only in a particular frequency range. The difference with the virtual stator impedance introduced earlier is that it is the PCC voltage and the virtual back-emf voltage V vThe active damping branch is multiplied directly with the PCC voltage without calculating the voltage difference between the stator impedance and the PCC voltage. It therefore generates a current in a different direction than the virtual stator impedance, i.e. in the case of a virtual resistor, the damping current is in phase with the PCC voltage. The active damping branch is multiplied directly with the PCC voltage without calculating the voltage difference between the stator impedance and the PCC voltage. VSM The active attenuator branch is not affected by the scaling of the VSM nominal power in and is therefore active even when the VSM effect is set to zero. The active attenuator branch can potentially be used in both embodiments, i.e., the variant with the current source governor and the variant with the virtual machine governor.

[0128] Figure 13 shows an equivalent circuit of the control concept using an active attenuator in parallel with a current source, and Figure 14 shows a detailed diagram of an implementation of the active attenuator branch.

[0129] Optionally, two PLL blocks can be implemented: the current controller can operate in a reference frame created with a fast PLL to improve performance, while the response of the VSM is emulated with a slow PLL. Instead of a dq-frame based approach with a PI current control block, this concept can also be implemented with any other current control method, such as proportional resonant (PR) control, state feedback control, LQR or MPC based current control.

[0130] The implementation of this control method is not limited to the synchronous dq reference frame, but can be implemented in any other reference frame, such as a fixed alpha-beta reference frame, a phase-oriented abc reference frame, or any other suitable reference frame. Also, the proposed control method according to the above embodiments is not limited to three-phase systems. It can also be implemented for single-phase systems.

[0131] In case of operation in asymmetric conditions, the following measures may be taken to adapt the system accordingly.

[0132] The positive / negative / zero sequence separator block can be used to extract and separate the positive / negative / zero sequence components of the PCC voltage and converter currents and use specific control schemes for the individual sequences. Improved negative sequence current control can be achieved with the Proportional Resonant (PR) block tuned to the second harmonic frequency in the dq reference frame.

[0133] Zero back emf may be emulated for the negative sequence component current response of the VSM with different parameters for the positive and negative sequence virtual admittances. Harmonic current control with proportional resonant (PR) blocks can be tuned to specific harmonic frequencies or their counterparts in the dq reference frame.

[0134] According to the present disclosure, the proposed control method significantly reduces the complexity of grid forming vector current control while improving the response to disturbances or irregularities in the system.

[0135] 15 is a schematic diagram of a grid forming vector current control system according to the present disclosure. The system is configured to emulate a virtual synchronous machine (VSM) by a droop control unit 1, a current control unit 2, a phase locked loop (PLL) unit 4, and a virtual current source 5.

[0136] Other aspects, features, and advantages will become apparent from the above summary, and from the following description, including the figures and claims.

[0137] While the present invention has been illustrated and described in detail in the drawings and the foregoing description, such illustration and description are to be considered as illustrative and not restrictive. It will be understood that changes and modifications may be made by those skilled in the art within the scope of the following claims. In particular, the present invention encompasses further embodiments having any combination of features from the different embodiments described above and below.

[0138] Moreover, in the claims, the term "comprising" does not exclude other elements or steps, and the indefinite articles "a" or "an" do not exclude a plurality. A single unit may fulfill the functions of several features recited in a claim. Words such as "essentially," "about," "approximately," etc. in connection with an attribute or value specifically define that attribute or that value, respectively, precisely. Any reference signs in the claims should not be construed as limiting the scope.

[0139] List of citations [1] J. Rocabert, A. Luna, F. Blaabjerg and P. Rodriguez, "Control of Power Converters in AC Microgrids," in IEEE Transactions on Power Electronics, vol. 27, no. 11, pp. 4734-4749, Nov. 2012 [2] S. D'Arco and JA Suul, "Virtual synchronous machines -- Classification of implementations and analysis of equivalence to droop controllers for microgrids," 2013 IEEE Grenoble Conference, Grenoble, 2013, pp. 1-7 [3] Y. Sun, X. Hou, J. Yang, H. Han, M. Su and JM Guerrero, "New Perspectives on Droop Control in AC Microgrid," in IEEE Transactions on Industrial Electronics, vol. 64, no. 7, pp. 5741-5745, July 2017

Claims

1. A grid-forming vector current control system configured to emulate a virtual synchronous machine (VSM) A control system, A droop control unit (1); A current control unit (2); A virtual admittance unit (3); A phase locked loop (PLL) unit (4); A virtual current source (5), The virtual admittance unit (3) and the PLL unit (4) are configured to emulate the inertia of the VSM; The current control unit (2) receives an output signal from the virtual admittance unit (3); The PLL unit (4) outputs a signal to each of the droop control unit (1) and the virtual admittance unit (3); The virtual current source (5) is connected in parallel to the VSM; A grid forming vector current control system, wherein a coefficient kVSM is configured to change the characteristic of the system continuously from a voltage source to a current source by proportionally scaling the output signal of the virtual admittance unit (3) with kVSM.

2. 2. The system of claim 1, wherein the output signal of the droop control unit (1) is connected to the virtual current source (5) to emulate a speed governor.

3. 2. The system of claim 1, wherein an output signal of the droop control unit (1) is connected to the PLL unit (4) to emulate a virtual machine governor.

4. The system of any one of claims 1 to 3, further comprising an active voltage regulator (AVR) unit.

5. The system of any one of claims 1 to 4, wherein the system is deployed in at least one of a microgrid converter, or a photovoltaic (PV) inverter, or an uninterruptible power supply (UPS), or a grid intertie converter, or a railway grid intertie converter, or a high voltage direct current (HVDC) converter, or a battery energy storage system (BESS).

6. The system of any one of claims 1 to 5, wherein the coefficient kVSM is supplied to at least one of the droop control unit (1), the virtual admittance unit (3) or the PLL unit (4) to increase or decrease the inertial response of the system to disturbances in the main grid.

7. The coefficient kVSM is supplied to at least one of the droop control unit (1), the virtual admittance unit (3) or the PLL unit (4) to increase or decrease the inertial response of the system to disturbances in the main grid; 6. The system of claim 5, wherein the at least one of the micro-grid converter, the PV inverter, the UPS, the grid intertie converter, the rail grid intertie converter, the HVDC converter, or the BESS is connected to the main grid.

8. The coefficient kVSM is supplied to at least one of the droop control unit (1), the virtual admittance unit (3) or the PLL unit (4) to increase or decrease the inertial response of the system to disturbances in the main grid; The system of claim 5 , wherein the microgrid converter, the PV inverter, or the UPS is connected to the main grid.

9. The system of any one of claims 1 to 8, wherein the kVSM scales the nominal power of the VSM by proportionally scaling the output signal of the virtual admittance unit (3) with kVSM, by inversely scaling the proportional gain of the PLL with kVSM, and / or by proportionally scaling a droop constant with kVSM.

10. The system of any one of claims 1 to 9, wherein the coefficient kVSM scales at least one of a spinning wheel coefficient or a transient power or the inertia.

11. A system according to any one of the preceding claims, wherein the droop control unit (1) comprises a fast current source.

12. The system of claim 4 , wherein the active voltage regulator (AVR) unit is configured to keep a voltage magnitude at a point of common coupling (PCC) constant.

13. The system according to any one of claims 1 to 12, wherein the PLL unit (4) is configured to measure a voltage at a point of common coupling (PCC).

14. 1. A method for controlling a grid forming vector current control emulating a virtual synchronous machine (VSM), the method being performed by a grid forming vector current control system, comprising: configuring the grid forming vector current control system; The above-mentioned configuration includes: - providing a droop control unit (1), - providing a current control unit (2), - providing a virtual admittance unit (3), and providing a phase locked loop (PLL) unit (4), emulating the inertia of the VSM by the virtual admittance unit (3) and the PLL unit (4); controlling a virtual current source connected in parallel to the VSM by the droop control unit (1); The coefficient kVSM scales the output signal of the virtual admittance unit (3) proportionally with kVSM, thereby continuously changing the characteristics of the system from a voltage source to a current source.

15. The method of claim 14 , wherein an output signal of the droop control unit is connected to the virtual current source to emulate a speed governor.

16. 16. The method according to claim 14 or 15, wherein an output signal of the droop control unit is connected to the PLL unit to emulate a virtual mechanical governor.

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