A new energy grid-connected converter negative feedback enhancement control method with additional active power loop

Abstract

The application discloses a new energy grid-connected converter negative feedback enhancement control method with an additional active power loop. The method collects q-axis current, phase-locked loop (PLL) angle frequency output and direct current voltage outer loop output in a grid-connected converter control system as input signals of an additional stabilizer, and sets a direct current isolation link to extract q-axis current disturbance component and PLL angle frequency output disturbance component. Transfer function calculation based on the input signals can obtain additional compensation direct current voltage, which is fed forward to a direct current voltage control loop. The strategy can enhance the active power negative feedback effect of the grid-connected system under a weak power grid, inhibit local positive feedback effect caused by PLL dynamics, and improve the small disturbance stability of the grid-connected converter under a weak power grid.

Description

Technical Field

[0001] This invention relates to the field of new power systems, specifically to a negative feedback enhancement control method for new energy grid-connected converters that constructs an additional active power loop. This method aims to improve the dynamic characteristics of the control interaction loop in grid-connected converter systems, thereby improving the stability level under small disturbances and enhancing the operational performance of new energy grid-connected systems with high-inertia energy storage synchronous condensers in weak grid conditions. Background Technology

[0002] With the increasing penetration rate of new energy sources, grid-connected converters with grid-following control have become the mainstream interface for connecting new energy power plants in modern power systems. Due to the spatial distribution characteristics of new energy resources, new energy power plants typically need to be connected to the main grid via long-distance transmission lines. Therefore, the AC grid adjacent to a new energy power plant usually has two characteristics: 1) high grid impedance; 2) insufficient inertia support. Under weak grid conditions, grid-connected converters for new energy are susceptible to a series of stability issues, including weakly damped oscillations, voltage and frequency stability problems. To address the stability challenges under weak grid conditions, high inertia energy storage synchronous condensers (HIESSCs) can provide both voltage and frequency support simultaneously, and have become an important technological development direction for improving the dynamic characteristics of new energy power plants. However, the integration of HIESSCs may alter the grid-side impedance characteristics, thereby reshaping the interaction between various control loops of the new energy source and raising uncertainty about the applicability of traditional control strategies for grid-connected converters based on grid-following control. Furthermore, a grid-connected system with a HIESSC-based converter is essentially a complex multi-closed-loop control system. Therefore, designing an additional damping controller capable of coordinating various internal control loops remains a significant challenge. Currently, scholars both domestically and internationally have conducted related research, as evidenced by the following published literature:

[0003] [1] J. Fang, X. Li, H. Li, and Y. Tang, “Stability improvement for three-phase grid-connected converters through impedance reshaping inquadrature axis,” IEEE Transactions on Power Electronics, vol. 33, no. 10, pp. 8365–8375, Oct. 2018.

[0004] [2] Gao Lei, Lü Jing, Ma Junchao, et al. Instability analysis and stability control of grid-connected inverters based on circuit equivalence [J]. Journal of Electrical Engineering, 2024, 39(8): 2325-2341.

[0005] Reference [1] proposes a stability enhancement method for voltage source grid-connected converters based on qq channel impedance reshaping. It uses current reference value and modulation signal to replace steady-state quantity, thereby improving the adaptability of the strategy. However, this method does not adequately consider the impact of other internal control loops. This neglect may exacerbate dq axis coupling and even eventually induce system instability. Reference [2] proposes an active damping adaptive suppression measure combining notch filter and virtual resistor and applies it to offshore wind turbines. However, the above suppression methods using bandpass filter or notch filter can only suppress a specific frequency. When the external environment or grid operation mode changes, it is easy to cause the resonant frequency to drift, making the damping effect of the notch filter worse or even fail. Summary of the Invention

[0006] To address the aforementioned shortcomings of existing technologies, this invention proposes a negative feedback enhancement control method for new energy grid-connected converters that constructs an additional active power loop. This method aims to improve the dynamic characteristics of the control interaction loop of the new energy grid-connected converter system, thereby enhancing the stability level under small disturbances from a mechanistic perspective and improving the operational performance of new energy grid-connected systems with high-inertia energy storage synchronous condensers under weak power grid conditions.

[0007] The technical solution of this invention is implemented as follows:

[0008] A method for enhancing negative feedback control of a new energy grid-connected converter by constructing an additional active power loop is proposed. First, the q-axis current i in the control system of the new energy grid-connected converter is collected. cq Phase-locked loop angular frequency output and DC voltage outer loop output i cdref Then, the q-axis current disturbance component ∆i is extracted respectively. cq and phase-locked loop angular frequency output disturbance component Based on the DC voltage outer loop output i cdref q-axis current disturbance component ∆i cq and phase-locked loop angular frequency output disturbance component The additional compensation DC voltage ∆u is calculated. dc-com It is fed forward to the DC voltage control loop, which increases the negative feedback of the system's active power and improves the small disturbance stability of the new energy grid-connected system under weak grid conditions.

[0009] Furthermore, the q-axis current disturbance component ∆i cq The following formula is used to calculate:

[0010]

[0011] Where: s is the Laplace operator; For q-axis current DC isolation; T w1 It is the time constant of the q-axis current DC isolation link.

[0012] Furthermore, the phase-locked loop angular frequency output disturbance component The following formula is used to calculate:

[0013]

[0014] in: For phase-locked loop angular frequency output DC isolation stage; T w2 It is the time constant of the DC isolation link of the phase-locked loop angular frequency output.

[0015] Furthermore, the additional compensation DC voltage ∆u dc-com It is calculated according to the following formula;

[0016]

[0017] Where: C dc U represents the DC bus capacitance value of the grid-connected converter for new energy sources. dc0 This refers to the rated DC bus voltage; K d This is the proportional gain.

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

[0019] This invention focuses on a grid-connected system of a renewable energy grid-connected converter containing a HIESSC (High-Intensity Grid-Connected Converter). It fully considers the interactions of multiple control loops and the effects of d-q axis coupling, calculating the additional compensation DC voltage in the renewable energy grid-connected converter control system to enhance the system's active power negative feedback and effectively improve the small-disturbance stability of the renewable energy grid-connected system containing a HIESSC under weak grid conditions. Furthermore, in the calculation of the additional compensation DC voltage, the reference value of the d-axis current of the renewable energy grid-connected converter is used instead of the steady-state value of the d-axis current, effectively improving the adaptability of the proposed negative feedback enhanced control strategy. Attached Figure Description

[0020] Figure 1 This is a block diagram of the topology of a new energy grid-connected system containing a high-inertia energy storage synchronous condenser.

[0021] Figure 2 This is a block diagram of the control system for a grid-connected converter for new energy sources.

[0022] Figure 3 The block diagram for the negative feedback enhancement control of the new energy grid-connected converter with an additional active power circuit is provided for this invention.

[0023] Figure 4 Simulation waveforms showing the oscillation suppression effect of the system using the negative feedback enhancement control strategy based on an additional active power loop proposed in this invention. Detailed Implementation

[0024] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0025] This invention is used to improve the small disturbance stability of new energy grid-connected systems containing HIESSC under weak grid conditions. Figure 1 This is a structural diagram of a renewable energy grid-connected system including a HIESSC. The system consists of a 100MW renewable energy grid-connected converter and a HIESSC operating in parallel at the Point of Common Coupling (PCC). The renewable energy grid-connected converter is connected to the grid via a power converter, filter, and step-up transformer T1; the HIESSC is connected to the grid via step-up transformer T2. Figure 2 This is a block diagram of a grid-connected renewable energy converter control system. A typical control strategy for a grid-connected renewable energy converter is to use a phase-locked loop (PLL) to obtain the phase of the PCC voltage, thereby achieving synchronization and decoupling control with the grid. The outer loop is a DC voltage outer loop to maintain the stability of the DC bus voltage; the inner loop is a current inner loop to ensure rapid current regulation.

[0026] This invention proposes a negative feedback enhancement control method for renewable energy grid-connected converters by constructing an additional active power loop. It calculates the additional compensating DC voltage in the renewable energy converter control system, increasing the system's active power negative feedback effect to improve the small disturbance stability of grid-connected converters in weak power grids. For example... Figure 3 As shown, this method acquires the q-axis current i in the grid-connected converter control system. cq Phase-locked loop angular frequency output and DC voltage outer loop output i cdref As an input signal for the additional stabilizer, a DC isolation loop is set up to extract the q-axis current disturbance component and the phase-locked loop angular frequency output disturbance component. Furthermore, transfer function calculations are performed on these input signals to obtain an additional compensating DC voltage, which is then fed forward to the DC voltage control loop. This strategy can enhance the active power negative feedback effect of the grid-connected system under weak grid conditions, suppress the local positive feedback effect dynamically introduced by the PLL, and enhance the small disturbance stability of the grid-connected system under weak grid conditions.

[0027] The specific calculation for the additional compensation DC voltage is as follows:

[0028] A1) Collect the q-axis current i of the new energy grid-connected converter control system. cq As an input signal for an additional stabilizer;

[0029] A2) Extract the input signal i according to the following formula. cq The disturbance component ∆i cq :

[0030]

[0031] Where: s is the Laplace operator; For q-axis current DC isolation; T w1 It is the time constant of the q-axis current DC isolation link;

[0032] A3) Collect the angular frequency output of the phase-locked loop (PLL) in the control system of the new energy grid-connected converter. As an input signal for an additional stabilizer;

[0033] A4) Extract the input signal according to the following formula perturbation components :

[0034]

[0035] in: For phase-locked loop angular frequency output DC isolation stage; T w2 It is the time constant of the DC isolation link of the phase-locked loop angular frequency output;

[0036] A5) Collect the outer loop output of DC voltage in the control system of the new energy grid-connected converter. cdref As an input signal for an additional stabilizer;

[0037] A6) Calculate the additional compensation DC voltage ∆u in the outer loop of the DC voltage in the control system of the new energy grid-connected converter according to the following formula. dc-com ;

[0038]

[0039] Where: C dc U represents the DC bus capacitance value of the grid-connected converter for new energy sources. dc0 This refers to the rated DC bus voltage; K d This is the proportional gain.

[0040] Description of the effects of this invention:

[0041] Figure 4 Simulated waveforms of PCC voltage, output current, and active / reactive power of the grid-connected converter for new energy sources are presented when SCR=1.52. As can be seen from the figure, the reactive power Q... c Maintaining at 0.0 pu, at 3 seconds, the active power output P of the new energy grid-connected converter... cWhen the active power was increased to 0.90 pu, the system began to oscillate and lose stability. Then, at 4s, the negative feedback enhancement control strategy for the renewable energy grid-connected converter based on the additional active power loop proposed in this invention was applied, and the system oscillation was effectively suppressed, and the system returned to stability. At 5s, the active power of the renewable energy grid-connected converter was further increased to the rated power, and the system still remained stable. Simulation results show that the strategy proposed in this invention can effectively improve the stability margin of the system and enhance the active power output capability of the renewable energy grid-connected system.

[0042] Therefore, the negative feedback enhancement control strategy for new energy grid-connected converters based on additional active power loop proposed in this invention can effectively improve the dynamic characteristics of the system control interaction loop and enhance the safe and stable operation capability of new energy grid-connected systems containing HIESSC.

[0043] Finally, it should be noted that the above examples of the present invention are merely illustrative and not intended to limit the implementation of the invention. Although the applicant has described the present invention in detail with reference to preferred embodiments, those skilled in the art can make other variations and modifications based on the above description. It is impossible to exhaustively list all possible implementations here. All obvious variations or modifications derived from the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for enhancing negative feedback control of a new energy grid-connected converter by constructing an additional active power loop, characterized in that: First, collect the q-axis current i in the control system of the new energy grid-connected converter. cq Phase-locked loop angular frequency output and DC voltage outer loop output i cdref Then, the q-axis current disturbance component ∆i is extracted respectively. cq and phase-locked loop angular frequency output disturbance component ; Based on the DC voltage outer loop output i cdref q-axis current disturbance component ∆i cq and phase-locked loop angular frequency output disturbance component The additional compensation DC voltage ∆u is calculated. dc-com It is fed forward to the DC voltage control loop, which increases the negative feedback of the system's active power and improves the small disturbance stability of the new energy grid-connected system under weak grid conditions.

2. The negative feedback enhancement control method for a new energy grid-connected converter with an additional active power loop as described in claim 1, characterized in that: The q-axis current disturbance component ∆i cq The following formula is used to calculate: Where: s is the Laplace operator; For q-axis current DC isolation; T w1 It is the time constant of the q-axis current DC isolation link.

3. The negative feedback enhancement control method for a new energy grid-connected converter with an additional active power loop as described in claim 1, characterized in that: The phase-locked loop angular frequency output disturbance component The following formula is used to calculate: in: For phase-locked loop angular frequency output DC isolation stage; T w2 It is the time constant of the DC isolation link of the phase-locked loop angular frequency output.

4. The negative feedback enhancement control method for a new energy grid-connected converter with an additional active power loop as described in claim 1, characterized in that: The additional compensation DC voltage ∆u dc-com It is calculated according to the following formula; Where: C dc U represents the DC bus capacitance value of the grid-connected converter for new energy sources. dc0 This refers to the rated DC bus voltage; K d This is the proportional gain.

Images