Transient stability improving method of virtual synchronous generator under low-voltage fault

Abstract

The invention aims to solve the problem that a control coordinate system and a power grid reference coordinate system are out of synchronization due to the fact that the power transmission capability of a system cannot meet the requirement of a power instruction after a low-voltage fault occurs. The invention provides a virtual power compensation (VPC) synchronous control strategy with power compensation capability, the strategy keeps synchronous with a power grid during a low-voltage fault period and still has the capability of supporting the amplitude and frequency of the voltage of the power grid, and the recovery after the fault is realized through an improved current limiting strategy. The provided VPC and current amplitude limiting strategy solves the transient instability problem of a virtual synchronous generator (VSG) under large disturbance, fault removal and recovery are achieved, and fault detection, on-line control switching and power reference value self-adaption are completely not needed any more in the process. Simulation and experiment results verify the effectiveness of the method.

Description

Technical Field

[0001] The present invention belongs to the field of microgrids, and in particular relates to a method for improving the transient stability of a virtual synchronous generator under low voltage fault conditions. Background Art

[0002] Renewable energy has the advantages of being clean, low-carbon and green, which enables renewable energy generation devices to be integrated into the power grid on a large scale and gradually form a modern power system based on synchronous generators and power electronic devices. As the main power interface unit, the grid-connected converter plays an important role in converting the electric energy generated by renewable energy into AC power and feeding it into the power grid. The virtual synchronous generator (VSG) can provide virtual inertia and virtual damping support for the system by simulating the operating characteristics of the synchronous generator, and has gradually become a current research hotspot.

[0003] However, the relevant technology of VSG is still in its infancy and still faces many stability issues. Among them, the large signal transient stability problem caused by grid voltage sag, transmission line fault, load fluctuation, etc. involves complex nonlinear analysis and is still a research difficulty. After the fault occurs, the active power reference value and the maximum power value of the VSG output determine the existence of the post-fault operating point. If the active power reference value exceeds the maximum output power value, the system will not be able to operate stably and the system will become unstable. The overcurrent capacity of VSG is much weaker than that of the synchronous generator, so the controller must set a current limiting link, which will lead to a reduction in the maximum output power of VSG, thereby weakening the transient stability and increasing the risk of system instability.

[0004] Considering the impact of current limiting on the transient stability of grid-connected converters, the current limiting of VSG will significantly affect the power output during and after the fault; compared with the current vector angle method and the direct axis (d) current priority method, the quadrature axis (q) current priority current limiting strategy provides a larger transient stability margin. The transient stability analysis of VSG equipped with quadrature priority current limiter was carried out. The results show that in addition to the instability caused by the positive feedback of the main controller, VSG may also become unstable after a large disturbance due to the failure of the internal voltage controller. Therefore, current limiting will cause the transient stability of VSG to be further reduced, which is not conducive to the improvement of transient stability.

[0005] When the VSG cannot remain stable under large disturbances, it is disconnected from the grid to avoid further deterioration. However, with the increasing penetration of power electronic devices in modern power grids, VSGs are required to provide voltage and frequency support for the grid. Therefore, VSGs should have a certain low voltage fault ride-through capability. Generally speaking, there are three main ways to improve the stability of grid-connected converters under large signal disturbances: switching the control structure to grid-following mode during faults, limiting the converter current using virtual impedance structures, and changing the power reference during faults. The scheme of switching to grid-following control during faults can achieve overcurrent suppression of the output current by directly controlling the output current. However, this requires complex parameter selection and online fault detection algorithms, and the switching between the two operating modes will bring shocks that cannot be recovered after the fault is eliminated. The virtual impedance concept is used to limit the current for the internal current reference and voltage reference. Since the current limit depends largely on the fault location and the selected virtual impedance, and the virtual impedance is greatly affected by the degree of voltage sag, it is difficult to strictly limit the current to the required value, which greatly limits its practicality in practical applications, because in practical applications, it is hoped that the converter current can be safely, reliably and maximized under any fault conditions. The transient stability of the converter can be improved by reducing the power reference value during the fault period, but this will lead to a reduction in active power transmission, which is not conducive to frequency regulation and may cause power outages.

[0006] Therefore, the key problem to be solved by the present invention is to study a method that can improve the power angle stability under low voltage faults, enhance the low voltage fault ride-through capability of VSG, and improve the current limiting control strategy to achieve post-fault recovery. Summary of the invention

[0007] The purpose of the present invention is to provide a method for improving the transient stability of a virtual synchronous generator under low voltage faults, based on a virtual power compensation (VPC) strategy to improve the power angle stability under low voltage faults, and an improved current limiting control strategy to improve the fault recovery capability.

[0008] In order to achieve the above-mentioned purpose, the present invention adopts the following technical scheme.

[0009] First by Figure 1 The mathematical expression of the traditional VSG can be obtained as follows: Among them, P set and Q set are the active and reactive power reference values ​​of VSG, P e and Q e are the instantaneous active and reactive power output by VSG, respectively. The calculation formula is as follows: E dq and I dq The PCC voltage and current are obtained after Park transformation, J and D p are the virtual inertia coefficient and droop coefficient of the active power circuit, K q is the integral coefficient of the reactive power circuit. n and V0 are the rated frequency and amplitude of the output voltage, ω and δ are the frequency and virtual power angle of the active power controller output, respectively. It is the voltage reference amplitude generated by the reactive power controller.

[0010] In order to prevent overcurrent from damaging the electronic components in the VSG, it is necessary to add a current limiting link in the inner loop control, such as Figure 1 (b) shown in which I 1d and I 1q They are the d-axis and q-axis components of the inverter bridge output current, respectively. In VSG control, there are two commonly used current limiting strategies, namely current reference current limiting and voltage reference current limiting with virtual impedance. In order to retain the d-axis and q-axis current dynamics for fault recovery, the specific current limiting measures are designed as follows: In the formula, and is the output of the voltage loop, A m is the amplitude of the voltage loop output. lim is the maximum current amplitude allowed to be output. The final output and integration link of the voltage loop are limited, and their limit values ​​are (-I lim ,+I lim ). dref and I qref It is the reference current output after limiting, which can generate the final voltage reference value e after passing through the current loop dref and e qref .

[0011] Similar to the power angle of a traditional synchronous generator, if the phase angle of the VSG active power controller output is defined as θ VSG , the phase angle of the power grid is θ g , then the phase angle difference between the VSG control coordinate system and the grid coordinate system can be defined as the virtual power angle (virtual power angle, VPA) δ, δ satisfies the following relationship: δ=θ VSG -θ g (4)

[0012] Under large disturbances, the fundamental reason for VSG transient instability is that the maximum transmission power P of the system after the low voltage fault eMAXLess than the power reference P set , resulting in the loss of synchronization between the control coordinate system of the VSG and the grid coordinate system, and the absence of a stable operating power angle, resulting in transient instability. To this end, the present invention proposes a VPC strategy to enhance the transient stability of the VSG.

[0013] The basic idea of ​​VPC is to introduce the q-axis current component of the VSG output, that is, I q The specific control block diagram is as follows Figure 2 During normal operation, I q is about 0 and does not affect the power tracking capability of the VSG active loop. During a fault, I q The feedback loop can play the role of virtual power compensation to prevent the VSG system from being damaged due to the maximum output power P eMAX <Power setting value P set This results in a failure in synchronization with the power grid.

[0014] After the introduction of virtual power compensation, the active power control equation of VSG is shown in (5): After further simplification and equivalence, we can get (6) and Figure 3 The active power control equation after the introduction of VPC is (7) Among them, L -1 represents the inverse Laplace transform, P e ′ is the newly defined active output power in the synchronous control including VPC, K p and K i is the proportional integral (PI) parameter in VPC. From (6), we can see that the introduction of VPC can play a role in virtual power compensation.

[0015] Due to the application of the above technical solution, the present invention has the following characteristics: 1. The present invention proposes a VPC synchronization control strategy with power compensation capability, which can achieve synchronization with the power grid during low voltage faults and still have the ability to support the voltage amplitude and frequency of the power grid. 2. The present invention proposes an improved current limiting strategy, which retains the dynamic characteristics of the d-axis and q-axis currents and enhances the fault recovery capability. 3. The VPC strategy proposed in the present invention solves the problem of transient instability of VSG under large disturbances and realizes fault removal and recovery. In this process, fault detection, online control switching and power reference value adaptation are no longer required. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 : System configuration diagram of VSG in the present invention;

[0017] Figure 2 : The VPC block diagram proposed in the present invention;

[0018] Figure 3 : The simplified VPC block diagram of the present invention;

[0019] Figure 4 : Response of the VSG with VPC when the grid voltage drops to 0.5 pu in the present invention (a) voltage and current response (b) active and reactive power response diagram;

[0020] Figure 5 : When the grid voltage drops to 0.5 pu in the present invention, the response of the traditional VSG control is (a) voltage and current response and (b) active and reactive power response diagram; DETAILED DESCRIPTION

[0021] The technical solution will be clearly and completely described below in conjunction with the preferred examples and drawings of the present invention. It should be understood that the preferred examples are only for illustrating the present invention, not for limiting the scope of protection of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the scope of protection of the present invention.

[0022] The present invention provides a method for improving the transient stability of a virtual synchronous generator under low voltage faults. The q-axis current feedback based on the VSG output is used to achieve virtual power compensation and avoid transient instability under large disturbances. In combination with an improved current limiting strategy, the fault recovery capability of the VSG is enhanced, and a more excellent transient performance is obtained.

[0023] The invention mainly includes: the implementation of virtual power compensation strategy and the current limiting strategy to maintain the current dynamics. The implementation process of the invention scheme is as follows:

[0024] Step S10: Using the q-axis current output by the VSG and combining it with a PI controller, the VSG output power is compensated in the control to ensure that there is still a power angle that can operate stably under large disturbances.

[0025] Step S20: Limit the integral link of the voltage inner loop and the final output, and the limit value is (-I lim ,+I lim ), and then perform the limiting process again. The processing method is shown in Figure 1 (b).

[0026] Step S30: Introduce VPC into the active power control loop, as follows Figure 2 As shown in the figure, the VSG output power is virtually compensated in the control. Figure 1 The limiting measure in (b) is placed in the inner loop, which realizes the current limiting protection of VSG under large disturbances.

[0027] A schematic diagram of a VSG structure with VPC compensation and current limiting measures is shown in Figure 1 As shown in the figure, VPC is implemented as follows Figure 2 As shown, Table 1 shows the parameters used for VSG.

[0028] Table 1 shows the parameters used by VSG

[0029] according to Figure 1 The module was built in RT-Lab for semi-physical simulation verification, and the parameters were consistent with those in Table 1.

[0030] Figure 4 The time domain response after using VPC is shown, because VPC can prevent the VSG from becoming unstable under large disturbances due to the inability of the power output capacity to meet the power reference value by performing virtual power compensation on the control. Figure 4 (a) and (b) show that when the voltage temporarily drops to 0.5pu, the output voltage, active power and reactive power of the VSG can remain constant without oscillation. The active power is basically maintained at 0.4pu, the reactive power is basically maintained at 0p.u., and the output current is maintained at the maximum limit value. When the voltage recovers to 1.0pu, the VSG takes about 0.5s to desaturate and then automatically returns to the initial state. The recovered VSG can respond normally to power regulation requirements, which shows that the VSG after fault recovery has the same ability to simulate synchronous generator operation as before the fault. It is further proved that during non-fault periods, the introduction of VPC does not change the steady-state operating point of the VSG. Figure 5 The time domain response of VSG in traditional current limiting mode is shown. After the grid voltage temporarily drops from 1.0pu to 0.5pu, the output current of VSG reaches the limit value. At this time, the system works in current saturation mode. Since the power output capacity of VSG does not meet the requirements of power reference value at this time, and there is no virtual power angle that can be used for stable operation, the system is in an oscillating and unstable state. Figure 5 (a) and (b) show that during the voltage transient process, the voltage amplitude, the active power and reactive power output by the VSG remain oscillating, and the VSG also absorbs energy from the grid, indicating that the system has become unstable. After the voltage returns to 1.0 pu, the traditional VSG cannot exit the saturation state and therefore cannot return to its original operating state. Obviously, when the voltage drops to 0.5 pu, the traditional VSG cannot be stable. gSAGRepresents the voltage amplitude after the grid drops, that is, it is impossible to meet P set >1.5I lim V gSAG It cannot operate under the condition of 1.0pu, nor can it return to the original operating state after the voltage recovers to 1.0pu.

[0031] From the above results, it can be concluded that the present invention proposes a virtual power compensation strategy to compensate the power transmission capacity of the system under large disturbances in the control, ensuring that the power transmission capacity of the system after the fault can meet the power command in the control, and then there is a virtual power angle that can operate stably to prevent transient instability of the VSG. The proposed VPC strategy is simple and easy to implement, without the need for fault detection, online control switching, and power reference value adaptation.

[0032] The above embodiments are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and they cannot be used to limit the protection scope of the present invention. The general principles defined in this article can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown in this article, but should conform to the widest scope consistent with the principles and novel features disclosed in this article. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for improving transient stability of a virtual synchronous generator under low voltage fault, characterized in that: A method for improving the transient stability of a virtual synchronous generator (VSG) under low voltage fault is provided; based on a virtual power compensation (VPC) strategy, the power angle stability under low voltage fault is improved; And an improved current limiting control strategy to improve fault recovery capability; specifically comprising the following steps: Step 1: Using the quadrature axis (q) current output by the VSG and combining it with a proportional integral (PI) controller, the VSG output power is compensated in the control to ensure that there is still a power angle that can operate stably under large disturbances; Step 2: Limit the integral link of the voltage inner loop and the final output. lim The maximum current output by VSG is used as the subsequent inner loop limit value (-I lim ,+I lim ), and then perform the limiting process again. The processing method is shown in (1); The direct axis (d) and q axis outputs of the voltage loop are and I dref is the d-axis current reference value output by the controller after limiting processing, I qref is the q-axis current reference value output by the controller after the limiting process, and Am is the signal amplitude output by the controller after the limiting process; Step 3: Introduce VPC into the active power control loop and add the current limiting strategy to the dual loop, so as to achieve virtual compensation of VSG output power in control and current limiting protection under large disturbances.

2. The method for improving transient stability of a virtual synchronous generator under low voltage fault according to claim 1 is characterized in that The virtual power compensation method is specifically as follows: After the introduction of virtual power compensation, the active power control equation of VSG is shown as (2): After further simplification and equivalence, we can get (3) The active power control equation after the introduction of VPC is (4) Among them, L -1 represents the inverse Laplace transform, I q is the q-axis current component of the VSG output, P e ′ is the newly defined active output power in the synchronous control including VPC; P set is the active reference value of VSG, P e is the instantaneous active power output by VSG, and the calculation formula is as follows: E dq and I dq The PCC voltage and current are obtained after Park transformation, J and D p are the virtual inertia coefficient and droop coefficient of the active power circuit, ω n is the rated frequency of the output voltage, ω and δ are the frequency and virtual power angle of the active power controller output, respectively, K p and K i is the PI parameter in VPC; from (3), it can be seen that the introduction of VPC can play a role in virtual work compensation.

3. The method for improving transient stability of a virtual synchronous generator under low voltage fault according to claim 1, characterized in that: The improved current limiting measures are described as follows: In order to prevent overcurrent from damaging the electronic components in the VSG, a current limiting link needs to be added to the inner loop control. In VSG control, there are two commonly used current limiting strategies, namely current reference current limiting and voltage reference current limiting with virtual impedance. In order to retain the d-axis and q-axis current dynamics for fault recovery, the specific current limiting measures are designed as shown in (1); where, and is the output of the voltage loop, A m is the amplitude of the voltage loop output; I lim is the maximum current amplitude allowed to be output. The final output and integration link of the voltage loop are limited, and their limit values ​​are (-I lim ,+I lim );I dref and I qref It is the reference current output after limiting, and after passing through the current loop, it can generate the final d-axis and q-axis voltage reference values ​​e respectively. dref and e qref .

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