Control system for phase-locked loop-free synchronous tracking mode of hybrid network following-constructing system

Abstract

The invention discloses a control system for a phase-locked loop-free synchronous tracking mode of a hybrid network following-construction system, and belongs to the technical field of virtual synchronous generator control, phase angle information generated by network construction type control is used for synchronization of a network following type inverter, and synchronization of the network following type inverter is realized without a phase-locked loop. According to the control technology, potential unstable factors caused by the dynamic state of the phase-locked loop can be avoided, and meanwhile, the two inverters of the following network and the constructing network have better transient performance. According to the improved scheme, an ideal voltage source on the direct current side of the inverter is replaced by a boost circuit controlled by a single current loop, and a direct current voltage loop is added to an active loop controlled by a network constructed on the alternating current side. Compared with the control technology provided by the invention, the improvement enables the control technology to have no frequency steady-state deviation and have better transient and steady-state performance.

Description

Technical Field

[0001] The present invention relates to the technical field of virtual synchronous generator control, and in particular to a control system of a hybrid grid-following-grid-building system without a phase-locked loop synchronous tracking mode. Background Art

[0002] With the large-scale penetration of new energy distributed generation into the power system, the traditional power grid is no longer the rigid power system of the past, but has become flexible and controllable, and grid-following control technology is one of the representatives. What has changed is that the inertia and damping of the system have been greatly reduced, which can easily lead to an increase in the amplitude of system frequency and voltage fluctuations. The volatility and randomness of renewable energy further aggravate this form of oscillation. In order to improve the inertia and damping of the system, the virtual synchronous generator (VSG) control technology is introduced into the power electronic converter of the distributed generation unit to improve the inertia and damping of the grid-connected system. On the one hand, the grid-following inverter has been widely verified to have reliable performance in strong power grids, but its performance in weak power grids is not ideal because of the instability caused by the phase-locked loop under low-frequency working conditions.

[0003] To overcome the stability issues corresponding to grid-following inverters, researchers have proposed several PLL-free (phase-locked loop) control strategies to eliminate the PLL from the control structure. Traditional PLL-free control strategies include: direct power control (DPC) to directly control the output active and reactive power without the need for an internal control loop. However, the DPC-based approach introduces a wide range of harmonics due to the variable switching frequency used. Voltage modulation DPC (VMDPC) is a PLL-free control strategy for grid-following inverters that has been proposed in recent years. This approach eliminates the need for PLL and avoids the requirement for an ideal grid voltage. However, the use of a bandpass filter in the VMDPC control loop to extract the basic grid voltage may have unexpected negative effects on system dynamics. In addition, the VMDPC technique requires the inverter to generate a specific amount of reactive power to deliver the rated active power to a weak grid. Although the VMDPC approach is more advanced than the traditional PLL-based approach, its performance is still not very ideal when it is connected to a grid with low impedance. Summary of the invention

[0004] The technical solution of the present invention aims at the technical problem that the existing technical solutions are too single, and provides a solution that is significantly different from the existing technologies. It mainly provides a control system for a hybrid grid-following and grid-building system without a phase-locked loop synchronous tracking mode, so that the grid-following inverter can still operate stably in a weak power grid environment and has good transient and steady-state performance.

[0005] The technical solution adopted by the present invention to solve the above technical problems is:

[0006] The hybrid grid-following and grid-building system is a control system without a phase-locked loop synchronous tracking mode. The phase angle information generated by the grid-building control is used for the synchronization of the grid-following inverter.

[0007] Specifically, the grid-forming control is divided into an active loop and a reactive loop. The active loop is obtained by subtracting the actual value from the active power reference value output by the grid-forming inverter, and then passing through an inertia damping link and an integral link to generate the phase angle information at the outlet of the grid-forming inverter.

[0008] The reactive loop is obtained by subtracting the actual value from the reactive power reference value output by the grid-type inverter, and generates the voltage information at the outlet of the grid-type inverter after passing through the inertia damping link;

[0009] The phase angle information generated by the grid-type control is used for Park transformation of the voltage and current vectors of the grid-type control.

[0010] Specifically, the grid-following control is divided into an active loop and a reactive loop. The active loop is obtained by subtracting the actual value from the active power reference value output by the grid-following inverter, and then comparing it with the d-axis current component after a proportional integral link. After the comparison, it is superimposed with the voltage d-axis component through a comparison integral link to generate a reference voltage d-axis component Ed.

[0011] The reactive loop is the reactive power reference value of the inverter output minus the actual value, which is compared with the q-axis current component after a proportional-integral link, and then superimposed with the voltage q-axis component after a proportional-integral link to generate the reference voltage q-axis component Eq;

[0012] Subsequently, Ed, Eq performs an inverse Pike transform based on the phase angle information generated by the grid-type control to obtain the modulation wave of the grid-following inverter.

[0013] Specifically, the grid-forming inverter is connected in parallel with the grid-following inverter. The grid-forming inverter adopts virtual synchronous generator control, and the grid-following inverter adopts bidirectional control based on dual-loop control of an external power loop and an internal current loop.

[0014] Specifically, the control of the grid-forming inverter is consistent with that of system I; in the control of the grid-following inverter, the external power loop and the internal current loop are cascaded to realize the regulation of active power and reactive power. Except that the phase angle information generated by the grid-forming type is used instead of the phase angle information generated by the phase-locked loop for synchronization, the rest of the control features are consistent with the grid-following inverter described in system I.

[0015] As an improvement to the above solution, a boost circuit with a single current loop control is provided on the DC side of the grid-type inverter, and a DC voltage loop is added to the active loop of the grid-type control on the AC side.

[0016] Specifically, the DC side boost circuit control loop generates a modulation wave by subtracting the actual value from the current reference value at the DC voltage source through a proportional-integral link;

[0017] The AC side grid-type control active loop is obtained by subtracting the actual value from the boost circuit output voltage reference value, which is then compared with the grid-type inverter active power reference value and actual value through a proportional integral link, and then passes through a virtual inertia damping link and an integral link to obtain the phase angle information of the grid-type control.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The "System II" provided by the present invention uses a grid-type control output phase angle to achieve synchronization with the grid-type inverter, without a phase-locked loop, making the frequency dynamics significantly smoother and the fluctuation range much smaller, improving the dynamic frequency characteristics relative to "System I". In addition, it can also reduce the potential instability caused by the dynamics of the phase-locked loop.

[0020] (2) "System III" introduces the previous DC converter based on "System II". The comparison link between the active power reference value and the actual active power value in the active power control loop of the VSG is added with the DC side capacitor voltage control loop to participate in the comparison. The frequency can be adaptively adjusted so that the frequency can be controlled without steady-state error, thereby reducing the frequency steady-state deviation caused by interference in the hybrid system of grid-forming inverter / grid-following inverter, and providing better inertia and damping support for the frequency to reduce RoCoF and frequency deviation. That is, "System III" can simultaneously improve the steady-state performance and transient performance of the system frequency in the PLL-free mode.

[0021] In summary, the present invention proposes two hybrid grid-type inverter / grid-following inverter systems without a phase-locked loop (PLL-Less), which have a new phase synchronization mode under the island mode. The two proposed systems can improve the dynamic characteristics and steady-state performance of the system frequency.

[0022] The present invention will be explained in detail below with reference to the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 (a) is the traditional network-building and network-following dual-machine parallel island operation diagram with a phase-locked loop; Figure 1 (b) The new control technology for connecting two or more machines in parallel to form a network and follow the network proposed by the present invention; Figure 1 (c) Based on Figure 1 (b) The proposed improved topology and its corresponding control diagram;

[0024] Figure 2 This is a comparison chart of the effects of different control technologies in the simulation experiment of the present invention. DETAILED DESCRIPTION

[0025] To facilitate understanding of the present invention, the present invention will be described more comprehensively below with reference to the relevant drawings. Several embodiments of the present invention are given in the drawings, but the present invention can be implemented in different forms and is not limited to the embodiments described in the text. On the contrary, these embodiments are provided to make the content disclosed in the present invention more thorough and comprehensive.

[0026] like Figure 1 As shown in (a), the traditional "system I" consists of a system of a grid-connected inverter and a grid-following inverter in parallel, where the grid-connected inverter is controlled by a virtual synchronous generator (VSG), and the grid-following inverter is controlled by a bidirectional control, based on a dual-loop control of an external power loop and an internal current loop. For the grid-following inverter, the external power loop is cascaded with the inner current loop to achieve the regulation of active power and reactive power. For the grid-connected inverter, the VSG control strategy is used to provide frequency and voltage support, where the rotor motion equation is described as:

[0027]

[0028] The traditional "System I" uses a phase-locked loop to achieve synchronization, but sometimes the phase-locked loop will have a serious impact on the dynamic performance of the system.

[0029] To solve the above problems, the present invention proposes a "system II" that uses a grid-type control output phase angle to achieve synchronization with a grid-type inverter. In addition, a previous DC converter is introduced to further enhance the transient and steady-state characteristics of the system, but it also increases the complexity of the system to a certain extent. This topological structure is called "system III".

[0030] Embodiment 1: A new control technology for parallel connection of two or more machines in a grid-building and grid-following manner, in which the phase angle information generated by the grid-building control is directly used for synchronization of the grid-following inverter.

[0031] like Figure 1 (b) shows that in "System II", the grid-type control is divided into an active loop and a reactive loop. The active loop is composed of the active power reference value output by the grid-type inverter minus the actual value, and then passes through an inertia damping link and then an integral link to generate the phase angle information at the outlet of the grid-type inverter. The reactive loop is composed of the reactive power reference value output by the grid-type inverter minus the actual value, and then passes through the inertia damping link to generate the voltage information at the outlet of the grid-type inverter.

[0032] The grid-following control is divided into an active loop and a reactive loop. The phase angle information generated by the grid-following control is used for the Parker transformation of the voltage and current vector of the grid-following control. The active loop is obtained by subtracting the actual value from the active power reference value output by the grid-following inverter, and then compared with the d-axis current component after a proportional integral link. After the comparison, it is superimposed with the voltage d-axis component through a comparison integral link to generate a reference voltage d-axis component E dThe reactive power loop is the reactive power reference value of the inverter output minus the actual value, which is compared with the q-axis current component after a proportional integral link. After comparison, it is superimposed with the voltage q-axis component through a proportional integral link to generate the reference voltage q-axis component E. q Then E d , E q Based on the phase angle information generated by the grid-type control, the inverse Pike transform is performed to obtain the modulation wave of the grid-following inverter.

[0033] Embodiment 2: This embodiment differs from Embodiment 1 in that:

[0034] like Figure 1 As shown in (c), in “System III”, improvements are proposed based on the above-mentioned control technology of two or more parallel machines for grid-forming and grid-following. Specifically, the ideal voltage source is replaced with a boost circuit controlled by a single current loop on the DC side of the grid-forming inverter, and a DC voltage loop is added to the active loop of the AC-side grid-forming control.

[0035] Specifically, the DC side boost circuit control loop generates a modulation wave by subtracting the actual value from the current reference value at the DC voltage source through a proportional integral link. The AC side grid-forming control active loop subtracts the actual value from the boost circuit output voltage reference value through a proportional integral link and then compares it with the grid-forming inverter active power reference value and actual value, and then obtains the phase angle information of the grid-forming control through a virtual inertia damping link and an integral link.

[0036] The rest is the same as the first embodiment.

[0037] It can be seen that system II has no phase-locked loop, while system III has a previous DC boost circuit but no phase-locked loop. The transient and steady-state characteristics of the frequency in "system III" are better than those in "system II", which is proved in the simulation verification section below. The previous DC boost circuit adopts a single current loop control strategy, which is easy to implement and simplifies the control structure. In addition, the voltage on the DC capacitor can be adjusted by a PI controller, and then the output of the outer DC voltage loop is used as the active power reference of the VSG.

[0038] One of the advantages of the "system III" of the present invention is that the comparison link between the active power reference value and the active power actual value in the active power control loop of the VSG is added to the DC side capacitor voltage control loop to participate in the comparison, which can adaptively adjust the frequency, and is also the reason why the frequency can be controlled without steady-state error. It is worth noting that PI i (i=1~6) is as follows Figure 1 The proportional-integral controller shown.

[0039] Figure 2 The simulation results of "System I", "System II" and "System III" are provided. Figure 2 The load impedance of (a) and (b) jumps from 5.01Ω, 5mH to 6.5Ω, 6mH; the load impedance of (c) and (d) jumps from 5.01Ω, 5mH to 4.5Ω, 4mH; the load impedance of (e) and (f) jumps from 5.01Ω, 5mH to 6Ω, 5.5mH; the load impedance of (g) and (h) jumps from 5.01Ω, 5mH to 5Ω, 4.5mH. Figure 2 (a), (c), (e), and (g) are the frequency dynamics of the grid-connected inverter under different grid impedance disturbances. Figure 2 (b), (d), (f), and (h) show the frequency dynamics of the grid-connected inverter under the corresponding grid impedance disturbance.

[0040] Depend on Figure 2 It can be seen that the frequency of "System I" fluctuates because the dynamics of the PLL adversely affect the frequency of both the grid-forming inverter and the grid-following inverter. The frequency dynamics of "System II" are significantly smoother and the fluctuation range is much smaller. This is because the phase angle provided by the grid-forming control is used for the Parker transformation of the grid-following inverter, not the PLL.

[0041] However, considering that the hybrid dynamic system operates in island mode without grid support, although "System II" improves the dynamic characteristics of frequency compared with "System I", the steady-state error of frequency still exists. The frequency deviation of about 0.7Hz will threaten the long-term safe operation of the system. On this basis, an improved "System III" adds a pre-stage DC boost circuit to solve this problem.

[0042] It can be seen from all the sub-figures that after being subjected to the same disturbance as the first two systems, the frequency in “System III” can return to the equilibrium point of 50Hz. The results show that the previous DC converter can simultaneously reduce the rate of change of frequency (RoCoF), reduce the lowest frequency point (deviation), and achieve near-zero steady-state error control of the frequency.

[0043] It should be emphasized that in the three systems described, except for the only phase-locked loop in "System I" and the only pre-stage DC boost circuit in "System III", the system parameters are the same. This "method of controlling variables" can verify the effectiveness of the proposed system. Therefore, the simulation results show that the proposed "System II" can improve its dynamic characteristics, but cannot solve the problem of frequency steady-state deviation, which can be further solved by the proposed "System III". "System III" can also mitigate RoCoF and frequency deviation.

[0044] The present invention proposes a new phase-locked loop-free (PLL-Less) hybrid grid-type inverter / grid-following inverter system with a new phase synchronization mode in island mode. The two proposed systems can improve the dynamic characteristics and steady-state performance of the system frequency. The main contributions are:

[0045] 1) The proposed “System II” can improve the dynamic frequency characteristics relative to “System I”. In addition, it can also mitigate the potential instability caused by the phase-locked loop dynamics.

[0046] 2) The invented "System III" can alleviate the frequency steady-state deviation caused by interference in the grid-connecting inverter / grid-following inverter hybrid system and provide better inertia and damping support for the frequency.

[0047] 3) Both "System II" and "System III" do not need PLL to achieve phase lock when operating in the island mode. This is a new mode. Especially for the proposed "System III", it can simultaneously improve the steady-state performance and transient performance of the system frequency in the PLL-free mode. The simulation verifies the superiority of the transient performance and steady-state performance of the improved system.

[0048] The present invention is also applicable to the determination of the frequency motion trajectory of a grid-connected inverter under any other control technology.

[0049] The above is an exemplary description of the present invention in combination with the accompanying drawings. It is obvious that the specific implementation of the present invention is not limited to the above-mentioned method. As long as such non-substantial improvements are made by adopting the method concept and technical solution of the present invention, or the concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode is characterized by: The phase angle information generated by the grid-type control is used for synchronization with the grid-type inverter.

2. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to claim 1 is characterized in that: The grid-forming control is divided into an active loop and a reactive loop. The active loop is obtained by subtracting the actual value from the active power reference value output by the grid-forming inverter, and then passing through an inertia damping link and an integral link to generate the phase angle information at the outlet of the grid-forming inverter. The reactive loop is obtained by subtracting the actual value from the reactive power reference value output by the grid-type inverter, and generates the voltage information at the outlet of the grid-type inverter after passing through the inertia damping link; The phase angle information generated by the grid-type control is used for the Park transformation of the abc three-phase current into the dq axis current components in the grid-type control.

3. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to claim 2 is characterized in that: The grid-following control is divided into an active loop and a reactive loop. The active loop is obtained by subtracting the actual value from the active power reference value output by the grid-following inverter. After a proportional integral link, it is compared with the d-axis current component. After the comparison, it is superimposed with the voltage d-axis component through a comparison integral link to generate a reference voltage d-axis component Ed. The reactive loop is the reactive power reference value of the inverter output minus the actual value, which is compared with the q-axis current component after a proportional-integral link, and then superimposed with the voltage q-axis component after a proportional-integral link to generate the reference voltage q-axis component Eq; Subsequently, Ed, Eq performs an inverse Pike transform based on the phase angle information generated by the grid-type control to obtain the modulation wave of the grid-following inverter.

4. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to claim 1 is characterized in that: The grid-building inverter is connected in parallel with the grid-following inverter. The grid-building inverter adopts virtual synchronous generator control, and the grid-following inverter adopts dual-loop vector control, which is based on dual-loop control of outer loop voltage and inner loop current.

5. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to claim 4 is characterized in that: In the grid-following inverter control, the outer power loop is cascaded with the inner current loop to achieve the regulation of active power and reactive power.

6. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to any one of claims 1 to 5, characterized in that: A boost circuit with a single current loop control is set on the DC side of the grid-type inverter, and a DC voltage loop is added to the active loop of the grid-type control on the AC side.

7. The control system of the hybrid network-building system without phase-locked loop synchronous tracking mode according to claim 6 is characterized in that: The DC side boost circuit control loop generates a modulation wave by subtracting the actual value from the current reference value at the DC voltage source through a proportional-integral link; The AC side grid-type control active loop is obtained by subtracting the actual value from the boost circuit output voltage reference value, which is then compared with the grid-type inverter active power reference value and actual value through a proportional integral link, and then passes through a virtual inertia damping link and an integral link to obtain the phase angle information of the grid-type control.

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