Grid forming vector current control

Abstract

The present disclosure provides a grid-forming vector current control system configured to simulate 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 simulate the inertia of the VSM. A virtual current source is connected in parallel to the VSM.

Description

Technical Field

[0001] The present disclosure relates to a system and method for grid forming vector current control. Background Art

[0002] Traditionally, the transmission and distribution grids have a high rate of change. Conventional power generation with synchronous generators is increasingly being replaced by converter-interfaced renewable energy resources. In many studies, it has been shown that if a utility grid is equipped with a large number of renewable energy resources that do not employ any grid support control strategy, there is a risk of grid instability. 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] This grid-independent current / power control strategy has long been common in most converter-interfaced applications. However, in the last few decades, stricter regulations have been put in place, for example for photovoltaic (PV) inverters. Today, PV inverters must gradually reduce the injected active power if the grid frequency increases above the nominal value. This behavior is the first step in the direction of grid-supported power converter control. The injected active power is slowly adapted in response to the actual needs of the grid. This simple approach only provides steady-state grid support, i.e., it still relies on synchronous generators in the grid taking over transient load changes.

[0004] In parallel, more sophisticated converter control methods have been developed for isolated grids, microgrids and railway grids. These control methods are designed to provide not only steady-state grid support, but also transient grid support. Energy resources connected via converter interfaces using such control methods can completely replace conventional synchronous generators in the grid. It is possible to operate the grid without any conventional synchronous generators connected to the grid, a situation that has become common, for example, in railway grids.

[0005] In order to provide full grid support, the amount of injected power into the grid must be adapted according to the needs of the various loads connected to the grid. This is achieved by changing the behavior from a grid-independent constant power injection to one that is closer to a voltage source. Converters equipped with this control method automatically adapt and share the injected power so that they provide the actual power required by the various loads in the grid.

[0006] For reasons of backward compatibility with legacy equipment in AC power generation and distribution, the control method aims to provide "inertia" to the grid, meaning that the rate of change of the grid frequency is limited.

[0007] The combined effect of the voltage source characteristics and inertia results in characteristics very close to the behavior of a synchronous generator. This control method provides full grid support and can completely replace synchronous generators in the grid.

[0008] In general, state of the art control mechanisms can be divided into two main groups.

[0009] In the droop control method, the control law is based on the steady-state coupling of frequency (or angle) and power flow in the inductive grid. There are various implementation variants, and several ways of improving the method have been proposed in the past few years. In most implementation variants, the method creates a voltage reference that must be tracked with an additional cascaded voltage and current control loop. In this regard, references [1] and [3] are cited.

[0010] 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. The mathematical model describing the generator behavior is implemented directly in the controller. There are various implementation variants, which differ mainly in the details and complexity of the equations implemented.

[0011] To emulate a virtual generator or virtual synchronous machine (VSM), the mathematical equations of the synchronous generator are implemented in the controller to emulate the generator behavior. In the direct method (voltage source type), the machine equations are solved for the stator voltage, which is given as a reference either directly to the modulation stage or to the subsequent cascaded voltage and current control loop. In the inverse method (current source type), the machine equations are solved for the stator current, which is given as a reference to the subsequent current control loop.

[0012] However, both approaches cited above require various additional control loops in order to make the concept applicable. In particular, the cascaded voltage and current control loops are given by the direct method and provide current control including current limiting.

[0013] The current control loop and PLL track the active / reactive power reference given by the inverse method and provide current control including current limiting. Furthermore, grid synchronization and grid frequency measurement are provided.

[0014] The virtual impedance loop simulates the virtual stator impedance.

[0015] Additional components may be used to improve the performance of the virtual generator. These additional components may be at least one of the following:

[0016] Additional PLL for direct approach for synchronizing with the grid before closing the main circuit breaker (MCB) of the converter or during fault conditions; Automatic voltage regulator (AVR) to improve voltage regulation at the PCC and feed-forward gain of the grid current; and PCC voltage to improve the performance of the PI control loop. Damping winding emulation is used to improve damping of low frequency oscillations. In the simplest inverse approach, virtual inertia effects are emulated using the derivative of the measured grid frequency. In addition, low-pass filtering of the virtual impedance reference is often required to reduce high frequency noise amplification caused by the derivative used to calculate the virtual inductance.

[0017] However, systems according to the state of the art can be difficult to tune and require extensive commissioning efforts. There can also be interactions between the control loops due to insufficient bandwidth separation in converters with lower switching frequencies. In regular systems, approximately 15 parameters must typically be tuned in the case of unknown interactions. In strong grids, insufficient reference tracking performance often occurs. Furthermore, loss of step during faults can occur.

[0018] As just discussed, grid support control methods are becoming more and more important in various applications, such as grid-connected battery energy storage (BESS), microgrids, PV inverters, railway intertie, HVDC and UPS. Transmission system operators in various countries are discussing changing the standards for grid-connected energy resources so that grid support becomes mandatory for all grid-connected energy resources. Today's grid support control methods are mainly based on two basic concepts: the "virtual generator" concept and the droop control concept. Both methods have several disadvantages, such as high complexity, difficult tuning methods and high commissioning workload, and stability issues. In addition, the mentioned methods may not provide sufficient control performance in strong and weak grids at the same time, and therefore either readjustment or control mode switching is required during islanding events.

[0019] In this ID, a novel general grid forming / grid support control method is proposed. It is based on the simulation of the combined effect of virtual machines and parallel connected current sources. Summary of the invention

[0020] The method according to the present disclosure provides a simplified and reliable structure. It has fewer cascaded control loops, reduced complexity, integrated damping, integrated current control and simple tuning. In addition, it has good performance in both weak and strong grids, and it implements island operation. The grid support quantity can be flexibly configured using a single parameter. Due to its simplicity, the novel control method greatly reduces the commissioning effort. Due to its universal nature, it can be used in various applications with only minor modifications.

[0021] 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.

[0022] Various embodiments may preferably implement the following features:

[0023] The output signal of the droop control unit may preferably be connected to a virtual current source in order to emulate a speed regulator.

[0024] Preferably, the output signal of the droop control unit is connected to a PLL unit in order to emulate a virtual mechanical speed regulator.

[0025] The system may further include an active voltage regulator (AVR) unit.

[0026] The system is preferably deployed in at least one of the following: a microgrid converter or a photovoltaic (PV) inverter or an uninterruptible power supply (UPS), or an intertie converter, or a railway intertie converter, or a high voltage direct current (HVDC) converter, or a battery energy storage system (BESS).

[0027] Factor k VSM Can be configured to be VSM The output signal of the virtual admittance is scaled to continuously change the characteristics of the system from a voltage source to a current source.

[0028] Factor k VSM Preferably fed into at least one of a droop control unit, a virtual admittance unit or a PLL unit in order to increase or decrease the inertial response of the system to disturbances in the main grid. Preferably, at least one of the following is connected to the main grid: a microgrid converter, or a PV inverter, or a UPS, or a grid interconnecting converter, or a railway grid interconnecting converter, or a HVDC converter, or a BESS. More preferably, a microgrid converter, a PV inverter or a UPS is connected to the main grid.

[0029] Preferably, k VSM The nominal power of the VSM is scaled by: VSM Scale the output signal of the virtual admittance by k VSM Inversely scale the PLL's proportional gain and / or use k VSM Scales the droop constant.

[0030] Factor k VSMPreferably at least one of a spinning wheel factor or transient power or inertia is scaled.

[0031] 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.

[0032] The PLL unit is preferably configured to measure the voltage at a point of common coupling (PCC).

[0033] The present disclosure further relates to a method for controlling a grid-forming vector current control of a simulated virtual synchronous machine (VSM). The method includes providing a droop control unit, providing a current control unit, providing a virtual admittance unit, and providing a phase-locked loop (PLL) unit. The inertia of the VSM is simulated, and a virtual current source connected in parallel to the VSM is controlled.

[0034] Various embodiments may preferably implement the following features:

[0035] The simulation of the inertia of the VSM can preferably be realized by using a virtual admittance unit and a PLL unit.

[0036] The output signal of the droop control unit is preferably connected to a virtual current source in order to emulate a speed regulator.

[0037] The output signal of the droop control unit is preferably connected to a PLL unit in order to emulate a virtual mechanical speed regulator.

[0038] Factor k VSM By using k VSM The output signal of the virtual admittance is scaled to continuously change the characteristics of the system from a voltage source to a current source. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The present invention will be described with reference to the accompanying drawings.

[0040] Figure 1a and Figure 1b shows a schematic diagram according to the prior art,

[0041] Figure 2a and Figure 2b shows a schematic diagram according to the present disclosure,

[0042] Figure 3a and Figure 3b shows a schematic block diagram according to an embodiment of the present disclosure,

[0043] Figure 4 shows the basic structure of the proposed grid support scheme,

[0044] Figure 5 a to Figure 5 c shows a virtual machine variant,

[0045] Figure 6 a and Figure 6 b shows the equivalent model of the proposed control scheme,

[0046] Figure 7 Vector diagram depicting the transient situation,

[0047] Figure 8 is to show the use of factor k VSM To adjust the grid support capacity,

[0048] Fig. 9 and Fig.10 is a diagram showing the system response to a fault and its clearing under different settings,

[0049] Fig.11a and Fig.11b shows a schematic block diagram of another embodiment according to the present disclosure,

[0050] Fig.12 shows an alternative embodiment of the proposed control concept,

[0051] Fig.13 is the equivalent circuit of the control concept with an optional active damper in parallel with the current source,

[0052] Fig.14 Shown according to Fig.13 Implementation method of the active damper branch,

[0053] Fig.15 A block diagram according to an embodiment of the present disclosure is shown. DETAILED DESCRIPTION

[0054] Figure 1a A scheme of a control system according to the prior art is shown. It employs a virtual impedance and a VSM. The outputs of both the VSM and the virtual impedance are fed into a cascaded voltage and current control. Figure 1b is a more detailed view of the virtual generator control method. As can be seen from the schematic, it is a rather complex structure with many parameters that must be tuned and adapted to each other.

[0055] Hereinafter, the terms block and unit may be used interchangeably and are not intended to be exclusive.

[0056] FIG2 schematically depicts a vector current control system according to the present disclosure. In particular, Figure 2a shows a virtual synchronous machine (VSM) with current sources connected in parallel, while Figure 2bIts equivalent circuit diagram is shown. The system simulates the combined effect of a virtual machine and a parallel connected current source, the nominal power and grid support of which can be scaled independently and online using a single input / parameter. Therefore, the system can be remotely controlled. For some aspects, such as fault handling, it is preferable to change the nominal power and grid support within the application code.

[0057] In more detail, the disclosed system according to an exemplary embodiment simulates the proposed behavior using a control block as shown in Figure 3. The calculation thereof will be discussed below. Figure 3a The voltage V at the point of common coupling (PCC) pcc The phase-locked loop (PLL) block fed by the reference frequency f ref The output of the PLL is connected to the V pcc and the voltage V of the virtual synchronous machine vsm The speed control output, virtual admittance output and reference current I ref This is then processed in the current control block, which outputs a voltage V out .

[0058] Figure 3b In addition, the parameter k VSM As an input to at least one of the PLL block, the speed control block and the current control block. Thus, the grid support amount of the main grid can be controlled. VSM It can also be used to smoothly change the characteristics of the control scheme from voltage source to current source, which also supports a mix of both characteristics. It can be beneficial to smoothly switch to stiff current control during a grid fault to maintain synchronization with the grid. The control mode does not have to be changed.

[0059] The system may also include an active voltage regulator (AVR). The system according to an exemplary embodiment emulates the combined effects of virtual inertia, damping winding, virtual stator impedance, AVR and droop governor. The virtual inertia and damping winding are emulated using a standard PLL block.

[0060] In contrast to conventional VSM implementations, active power setpoint tracking and droop governor are conceptually implemented using fast current sources. As a result, there is no performance degradation in strong grids, which is often observed when using VSM control. As a result, good performance is achieved in weak and strong grids. The approach supports unplanned islanding and islanding operation.

[0061] The complexity of the control scheme and the related debugging work is very low. Due to the removal of the cascade voltage control loop and the reuse of the PLL to emulate inertia and damping, the number of parameters can be reduced from more than 15 in the conventional method to only 7 parameters with clear meaning and less interaction. Therefore, it is also easier to tune the system.

[0062] A further advantage of the proposed concept is that it is possible to construct the grid support quantity with a single parameter to simplify providing grid support features in applications with low energy storage (such as UPS in data centers, PV with low or no storage, EV charging with buffer batteries, insensitive loads, etc.), and to smoothly transition into operation or out of operation at maximum current without losing steps, thereby improving fault handling capabilities.

[0063] The system provides explicit current control and current limiting as well as explicit PLL to synchronize with the grid before closing the MCB and prevent loss of synchronization during faults. The PLL together with the virtual admittance acts as a virtual swing equation, thereby precluding the use of power balancing for synchronization.

[0064] Figures 4 to 6 A more detailed overview of the proposed grid support control scheme and the corresponding equivalent circuit is shown.

[0065] Figure 4 The control scheme is conceptually Figure 2a and Figure 2b Depicted in. Figure 2a The control scheme can be divided into two functional parts: a VSM without any regulator, i.e., a spinning wheel or an inertia, respectively, as indicated by the dashed lines; 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 works in parallel. It acts as a fast power setpoint tracker and speed regulator at the same time, as it ensures a steady-state power balance between the grid and the spinning wheel. The virtual admittance ensures an inertial response with respect to the active power. In steady state, the current source injects the full power flowing to the grid. The spinning wheel injects power only during transients.

[0066] Figure 4 A system according to an embodiment is shown, which has 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 the 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 the speed regulator.

[0067] The system according to the preferred embodiment will now be described in more detail.The simplest form of conventional state of the art implementation of a virtual synchronous machine is based on the swing equation.The goal of the virtual synchronous machine control is to make the behavior of the converter at the point of common coupling (PCC) equal to that of the synchronous machine.

[0068] Figure 5 a to Figure 5 c is the equivalent circuit of the corresponding component, that is, Figure 1a and Figure 1b A variant of the state-of-the-art virtual machine described in . In particular, Figure 5 a shows the converter, Figure 5 b shows the VSM without virtual inductance, and Figure 5 c shows the VSM with virtual inductance.

[0069] For simplicity reasons, the following derivations are based on a converter with an inductive output filter. However, the control method proposed herein is suitable for converters with any type of output filter structure, such as L, LC or LCL.

[0070] The swing equation converts the voltage source V C The frequency variation of is coupled with the active power flowing out of or into the machine. In the case of a VSM without virtual inductance, the converter voltage V C Equal to the back-emf of the virtual machine. Converter filter inductor L C is equal to the stator inductance of the virtual machine, and the PCC voltage V pcc Equal to the stator voltage of the virtual machine. Converter voltage V C (virtual back-emf) has a rotation frequency ω r , to highlight its equivalence with the virtual rotor frequency. The PCC voltage (virtual stator voltage) has a rotation frequency ω pcc . The swing equation looks like this:

[0071]

[0072]

[0073] In the quasi-steady state, the power flow is determined by the voltage across the converter filter reactance X C =ω N *L C Virtual back-emf V C and PCC voltage V pcc The angle difference between them is given.

[0074]

[0075] There is a direct dependence of power flow, reactance and angle difference across the transmission line, i.e.

[0076] The swing equation thus becomes

[0077]

[0078]

[0079] In the second step, according to Figure 5 c. The current state-of-the-art VSM implementation has introduced a virtual reactance X vs =ω N *L vs The virtual reactance changes the equation slightly. The converter voltage V in phasor notation C It is calculated according to the following formula:

[0080] V C =V v -jX vs I c (6)

[0081] The virtual back-emf of the machine is designated as V v (with a rotation frequency ω r and angle θ r ) and has been moved to virtual reactance X vs Regarding the behavior at PCC, the virtual machine now has X v =X c +X vs The total virtual stator reactance is , and the power flow equation changes to

[0082]

[0083] And the swing equation becomes

[0084]

[0085]

[0086] It should be noted that the virtual inductance L vs and the converter filter inductor L C in series, and therefore adds to the total virtual stator inductance of the virtual machine seen by the PCC.

[0087] Figure 6 a and Figure 6 b shows an equivalent model of the control scheme according to an exemplary embodiment. Here, the swing equation is not directly implemented as in the conventional method. In addition, the effect of PLL together with virtual admittance is compared with the swing equation. Angle θr It is no longer calculated by the swing equation, but rather it is the PLL output according to the PLL equation:

[0088]

[0089]

[0090] Calculating the derivative of equation (9) yields:

[0091]

[0092]

[0093] For small angle differences (which is the case with reasonable PLL tuning), the sine function can be approximated by sin(θ r -θ pcc )≈(θ r -θ pcc ), and its derivative becomes s.sin(θ r -θ pcc )≈ω r -ω pcc .

[0094] Therefore, the system of equations can also be expressed as follows:

[0095]

[0096]

[0097] By comparing equation (11) with the original swing equation (1), the PLL gain that is equivalent to these two equations can be obtained as

[0098]

[0099] The speed regulator power will be replaced by a shunt current source in a later step below. Thus, the proportional gain of the PLL emulates the damping winding effect, and the integral gain combines the angle difference with the increase / decrease in frequency, emulating the self-synchronization principle of a synchronous machine.

[0100] In contrast to the swing equation, the PLL does not implicitly couple the power flow to the angle difference, since it only describes the internal PLL angle θ r The coupling of the power flow and the angle difference must be ensured explicitly by means of an additional current reference for the current controller. This reference is created with the virtual admittance equal to the inverse of the virtual stator impedance, i.e.

[0101] Y v (s) = Z v (s) -1 (13)

[0102] The stator impedance is assumed to be of RL type. If the admittance is implemented in the dq coordinate system, it can be described as follows

[0103]

[0104]

[0105] Current reference i v，dq is created from the matrix multiplication of the virtual admittance and the voltage difference between: the virtual back-emf voltage V aligned to the d-direction of the PLL coordinate system v (V v =V N +j*0), and PCC voltage V pcc (This is Figure 7 , which depicts a vector diagram of a transient situation).

[0106] i v，d =Y v，dd (s)·(V pcc，d -V N )+Y v，dq (s)·V pcc，q (16)

[0107] i v，q =Y v，qd (s)·(V pcc，d -V N )+Y v，qq (s)·V pcc，q (17)

[0108] Alternatively, it is also possible to work with complex numbers and implement the admittance according to:

[0109]

[0110] In steady state and assuming R v <<ω N *L v , the admittance is simplified to

[0111]

[0112] And thus leads to the following steady-state current reference:

[0113]

[0114]

[0115] If the PCC voltage V pccis equal to the nominal voltage VN, the injected q-axis current can be ignored. Therefore, in the virtual stator reactance (X v =ω N *L V ) and the coupling between the active current in the d direction can be described as

[0116]

[0117] where the relevant power flow (assuming fast current tracking) is

[0118]

[0119] Thus, according to the present disclosure, equivalence between the quasi-steady-state power flow governed by the swing equation and the power flow created by the PLL+virtual admittance combination is ensured.

[0120] Virtual admittance can be implemented using a full dynamic model to ensure passivity and damping of high frequency resonances. However, a simplified admittance model (neglecting the s*L term) is also possible, as more complex models allow additional filtering functions. Even asymmetric admittance is possible. The minimum requirement is to implement the coupling between the q voltage and the active current.

[0121] It has been shown that a combination of special settings for the PLL gain and the virtual admittance allows emulating the effects of the inertia, damping winding and stator impedance of a virtual synchronous machine with conventional control structures for vector current control.

[0122] In the following, the droop block is further described. Unlike the original VSM implementation, the proposed control method implements only the spinning wheel without a regulator to achieve the mechanical input power. Therefore, in the steady state (i v，d =0), the injected power from the spinning wheel will be zero, and the angle θ r With θ pcc Alignment.

[0123] The governor power is replaced by a current source that ensures steady-state power balance. In addition to the reference from the virtual admittance, an active current reference is created according to the following equation:

[0124]

[0125] This results in the following injected powers:

[0126] P=V pcc，d ·i d，ref =V pcc cos(θ r -θ pcc )·i d，ref =P set +K g·(ω r -ω set ) (25)

[0127] The main difference is that the current reference is fed directly to the current controller, without filtering the swing equation. Set point changes are fed directly and noticed much faster than in conventional VSM implementations.

[0128] For reasonable load conditions and reasonable virtual stator reactance of the spinning wheel, the angle difference θ r -θ pcc is small, and cos(θ r -θ pcc ) is close to unity. In that case, dividing by V pcc，d can be divided by the constant V N replace.

[0129] In addition, an active voltage regulator (AVR) can be added to keep the amplitude of the PCC voltage constant. The AVR block can be a PI, or integrator (I) type, or just a proportional type (P). It creates a q current reference i for regulating the voltage amplitude. qref .

[0130] i qref =G avr (s)·(|V pcc |-V N ) (26)

[0131] However, the AVR block can be omitted because the virtual admittance already implements the proportional gain AVR effect using the following equation

[0132]

[0133] As already indicated above, the factor k can be used VSM . Factor k VSM Can be configured to be VSM The output signal of the virtual admittance is scaled to continuously change the characteristics of the system from a voltage source to a current source.

[0134] Factor k VSM can be fed into at least one of the droop block, virtual admittance block or PLL block, as described throughout the specification, in order to increase or decrease the inertial response of the system to disturbances in the main grid to which the microgrid converter, PV inverter, UPS, etc. are connected. Preferably, at least one of the following is connected to the main grid: a microgrid converter, or a PV inverter, or a UPS, or a grid interconnecting converter, or a railway grid interconnecting converter, or a HVDC converter, or a BESS. More preferably, a microgrid converter, a PV inverter or a UPS is connected. Wherein, k VSMThe nominal power of the VSM can be scaled as follows: VSM Scale the output signal of the virtual admittance by k VSM Inversely scale the PLL's proportional gain and / or use k VSM Scales the droop constant.

[0135] k VSM It can also be used to adjust or scale the grid support. VSM is continuous and ranges from 0 to 1. With this factor, it is possible to scale the nominal power of a virtual machine, which can be considered to be running in parallel with the current source, by a single factor. VSM The connections and inputs are Figure 3b These equations are as follows:

[0136] Y virt =k VSM ·Y virt，N (28)

[0137] M=k VSM ·M N (29)

[0138]

[0139]

[0140] refer to Figure 4 , the modified block diagram of the droop block, virtual admittance block and PLL block is in Figure 8 Shown in.

[0141] It is important to make the k used in the PLL block VSM The reciprocal saturates to a maximum value to keep the PLL loop stable. Excessive proportional gain of the PLL loop can lead to instability, which is a well-known effect of standard vector current control using a PLL.

[0142] It should be noted that adjusting the grid support amount by scaling the nominal power of the VSM is a unique feature of the control structure of the present disclosure. The nominal power of the parallel current source is not affected, and thus even if k VSM Set to zero, the control structure can also continue to operate with conventional current control and at maximum power capability.

[0143] This feature may not be achievable with conventional VSM implementations. For a standard VSM, the power transfer capability is tied 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 a voltage difference is imposed on the virtual stator reactance. Reducing the nominal power of the VSM results in an increase in reactance, and therefore an increase in the 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. Therefore, in conventional control methods, the nominal power of the VSM cannot be adjusted independently. Other applications besides those disclosed herein may also be feasible.

[0144] Possible applications during a fault event and subsequent resynchronization after the fault has cleared are Fig. 9 Without reducing k VSM In the case of a factor (similar to the standard VSM behavior), severe oscillations are triggered after the fault is cleared because there is a loss of synchronism between the grid and the VSM when the PCC voltage is close to zero.

[0145] In contrast, if k VSM The factor decreases to zero during the LVRT event, a smooth resynchronization is achieved, and no oscillations between the converter and the grid are observed after the fault is cleared. This can be seen from Fig.10 As can be seen from the diagrams in the figure, these diagrams show the behavior of the proposed control system. VSM Due to the temporary reduction of , the control characteristic has become a conventional rigid current control scheme with a controllable and preferred behavior during the fault event. It is possible to inject reactive current or impose any other predefined current reference during the LVRT event.

[0146] In applications with only low energy storage capabilities, such as in UPS systems or in PV with low or no energy storage, grid support can also be operated at a reduced nominal power (e.g., 10-20%). A certain percentage of the nominal power can be reserved for grid support, while the remaining power demand of the application is handled using conventional current control.

[0147] Fig.11a and Fig.11b Another preferred embodiment of the present disclosure is shown as a schematic block diagram. In particular, Fig.11a An alternative embodiment is provided with a virtual mechanical speed regulator instead of a current source speed regulator. This is achieved in particular by connecting the output signal of the droop control block to the PLL block. Thus, a virtual mechanical speed regulator is simulated. Figure 3a The control scheme shown in VSMIt may also be fed into the PLL block and / or the speed control block and / or the virtual admittance block. Descriptions of features corresponding to the first embodiment will be omitted.

[0148] Fig.12 Shows about Figure 4 An alternative configuration of the scheme proposed in . Instead of using a current source as a speed regulator, a virtual mechanical speed regulator can be implemented that changes 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 PLL block requires an additional input, which converts the speed regulator power reference P ref becomes a virtual rotor angular displacement.

[0149] In other words, Fig.12 A system according to an embodiment is shown, which has 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 the 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 mechanical speed regulator.

[0150] The following applies to both the embodiment and the control method, respectively. d,ref2 Connecting a current source in parallel to the VSM, the current source can be manipulated independently of the VSM. It is still possible to use a factor k VSM To adjust the nominal power of the VSM. However, as the nominal power P of the VSM N,VSM Decrease, P set Only available in +-P N,VSM In order to utilize the full power capability of the converter, a current source is required to input i d,ref2 To operate, the current source input is independent of the nominal power of the VSM.

[0151] Optionally, an active damping branch (e.g., a virtual damping resistor) may be connected in parallel to the current source.

[0152] i ad，dq =G ad，dq (s)·V pcc，dq (33)

[0153] This active damping branch usually uses a bandpass filter so that it is only effective within a certain frequency range. The difference from the virtual stator impedance introduced earlier is that the active damping branch directly multiplies the PCC voltage without calculating the PCC voltage and the virtual back-emf voltage V vTherefore, it creates a current in another direction than the virtual stator impedance (i.e., for the virtual resistor), which is in phase with the PCC voltage. The active damping branch is not affected by the factor k introduced in Section 2.2.4. VSM The effect of the nominal power of the VSM is scaled, so if the VSM effect is set to zero, it is also active. The active damper branch can potentially be used in two embodiments, namely the variant using a current source speed regulator and the variant using a virtual mechanical speed regulator.

[0154] Fig.13 The equivalent circuit of the control concept using an active damper in parallel with a current source is depicted. Fig.14 is a detailed view of an embodiment of an active damper branch.

[0155] Optionally, two PLL blocks can be implemented. The current controller can be run in the created coordinate system using a fast PLL to improve performance, while the response of the VSM is simulated using a slow PLL. Instead of a dq coordinate system based approach using a PI current control block, the concept can also be implemented using any other current control method, such as current control based on proportional resonance (PR) control, state feedback control, LQR or MPC.

[0156] The implementation of the control method is not limited to the synchronous dq coordinate system, but it can also be implemented in any other coordinate system, such as a stationary α-β coordinate system, or a phase-oriented abc coordinate system, or any other suitable coordinate system. The control method proposed according to the embodiment described above is not limited to a three-phase system. It can also be implemented for a single-phase system.

[0157] In case of operation under asymmetric conditions, the following measures can be taken to adapt the system to this situation.

[0158] 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 current and use a specific control scheme for each sequence. Improved negative sequence current control can be achieved using a proportional resonance (PR) block tuned at the second harmonic in the dq coordinate system.

[0159] Zero back-emf can be simulated for the negative sequence component current response of a VSM with different parameters for positive and negative sequence virtual admittance. Harmonic current control using proportional resonance (PR) blocks can be tuned to specific harmonic frequencies or their corresponding frequencies in the dq coordinate system.

[0160] According to the present disclosure, the proposed control method greatly reduces the complexity of grid forming vector current control while improving the response to faults or irregularities in the system.

[0161] Fig.15 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) through a droop control unit 1 , a current control unit 2 , a phase-locked loop (PLL) unit 4 and a virtual current source 5 .

[0162] Other aspects, features, and advantages will be apparent from the foregoing summary and from the following description, including the drawings and claims.

[0163] Although 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 illustrative or exemplary rather than restrictive. It will be understood that changes and modifications may be made by a person 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.

[0164] Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single unit may fulfill the functions of several features recited in the claims. The terms "substantially", "approximately", "approximately" and the like related to an attribute or value also precisely define the attribute or precisely define the value, respectively. Any reference signs in the claims should not be construed as limiting the scope.

[0165] Reference List

[0166] [1] J. Rocabert, A. Luna, F. Blaabjerg, and P. Rodríguez, “Control of Power Converters in AC Microgrids,” in IEEE Transactions on Power Electronics, Vol. 27, No. 11, pp. 4734–4749, November 2012

[0167] [2] S. D'Arco and J. A. 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

[0168] [3] Y. Sun, X. Hou, J. Yang, H. Han, M. Su, and J. M. 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 simulate a virtual synchronous machine (VSM), the system comprising: Droop control unit (1); A current control unit (2); Virtual admittance unit (3); as well as Phase-locked loop PLL unit (4), The virtual admittance unit (3) and the PLL unit (4) are configured to simulate the inertia of the VSM by setting the PLL gain. wherein a virtual current source (5) is connected in parallel to the VSM, and Among them, the factor k VSM is configured to be VSM The output signal of the virtual admittance is scaled to continuously change the characteristics of the system from a voltage source to a current source.

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

3. The system according to claim 1, wherein: The output signal of the droop control unit (1) is connected to the PLL unit (4) to emulate a virtual mechanical speed regulator.

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

5. The system according to any one of claims 1 to 3, wherein: The system is deployed in at least one of the following: a microgrid converter, a photovoltaic PV inverter, an uninterruptible power supply UPS, a grid interconnection converter, a railway grid interconnection converter, a high-voltage direct current HVDC converter, or a battery energy storage system BESS.

6. The system according to claim 1, wherein: The factor k VSM is fed into at least one of the droop control unit (1), the virtual admittance unit (3) or the PLL unit (4) in order to increase or decrease the inertial response of the system to disturbances in the main grid.

7. The system according to claim 6, wherein: The system is deployed in at least one of the following: a microgrid converter, a photovoltaic PV inverter, an uninterruptible power supply UPS, a grid interconnected converter, a railway grid interconnected converter, a high-voltage direct current HVDC converter, or a battery energy storage system BESS, and wherein at least one of the following is connected to the main grid: the microgrid converter, the PV inverter, the UPS, the grid interconnected converter, the railway grid interconnected converter, the HVDC converter, or the BESS.

8. The system according to claim 6, wherein: The system is deployed in at least one of the following: a microgrid converter or a photovoltaic PV inverter or an uninterruptible power supply UPS, or a grid interconnected converter, or a railway grid interconnected converter, or a high-voltage direct current HVDC converter, or a battery energy storage system BESS, and wherein the microgrid converter, PV inverter or UPS is connected to the main grid.

9. The system according to claim 1, wherein: k VSM The nominal power of the VSM is scaled by: VSM Scaling the output signal of the virtual admittance by k VSM Scaling the proportional gain of the PLL inversely, or by k VSM Scale the droop constant.

10. The system according to claim 1, wherein: The factor k VSM At least one of a spin wheel factor or a transient power or the inertia is scaled.

11. The system according to any one of claims 1 to 3, wherein: The droop control unit (1) comprises a fast current source.

12. The system according to claim 4, wherein: The active voltage regulator AVR unit is configured to keep the amplitude of the voltage at the point of common coupling PCC constant.

13. The system according to any one of claims 1 to 3, wherein: The PLL unit (4) is configured to measure the voltage at the point of common coupling PCC.

14. A method for controlling a grid-forming vector current control of a simulated virtual synchronous machine (VSM), the method comprising: - 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), By setting the PLL gain to simulate the inertia of the VSM, controls a virtual current source connected in parallel to the VSM, and Among them, the factor k VSM By using k VSM The output signal of the virtual admittance is scaled to continuously change the characteristics of the grid forming vector current control system from a voltage source to a current source.

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

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

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