Parallel structure and control method of UPQC parallel converter based on droop control

Abstract

The application discloses a parallel structure and a control method of a UPQC parallel converter based on droop control, and relates to the field of power quality regulation of new energy power. In the parallel structure of the existing UPQC parallel converter, in order to guarantee normal operation, a central controller is used to control the parallel APFs, a reference voltage is set to force the output voltages of the parallel APFs to be the same, and a plurality of control lines are needed to connect the parallel APFs and the central controller, if the control lines are disconnected or communication is not smooth, the system cannot operate normally. The application constructs the parallel structure of the UPQC parallel converter based on droop control, collects the output voltages of the parallel APFs to obtain the output active power and the output reactive power, controls the reactive power according to the droop characteristics to keep the output voltages of the parallel APFs consistent, and adjusts the droop coefficient curve according to the proportion of the droop coefficient determined by the capacity ratio to complete the power according to the capacity ratio and ensure stable operation of the system.

Description

Technical Field

[0001] This invention relates to the field of power quality regulation in new energy sources, and particularly to the parallel structure and control method of UPQC parallel converters based on droop control. Background Technology

[0002] In recent years, under the national policy focus on ecological civilization construction, new energy power generation systems, including photovoltaic (PV) power generation systems, have received increasing attention from various countries. However, with the integration of PV, energy storage components, and nonlinear and asymmetrical loads, the instability of PV systems and other issues have made power quality problems in the power grid increasingly prominent, posing challenges to power quality control. Therefore, the PV-storage-UPQC-DVR grid connection structure is of great significance for the power quality and energy management of the future diversified power grid.

[0003] In a left-series, right-parallel UPQC structure, the series-connected Active Power Filter (APF) is connected to the grid, primarily addressing grid voltage fluctuations, while the parallel-connected APF is connected to the load, mainly responsible for load current and reactive power output. However, the grid's stability limits the utilization of the series-connected APF, while the load's inherent uncertainty necessitates the parallel-connected APF to maximize power quality output and reactive power compensation, resulting in a heavy load and reduced system stability. To address this, an effective approach is to construct a parallel structure, connecting multiple parallel-connected APFs to share the load, thus reducing the burden on individual parallel-connected APFs. Furthermore, introducing energy storage components into both the series and parallel APFs allows for more efficient current distribution among them, contributing to better reactive power output and offering greater flexibility compared to traditional UPQC structures.

[0004] In this parallel structure, since multiple parallel APFs are connected together, their outputs are interconnected. Therefore, for this structure to operate normally, there is a strict requirement: the output voltages of all the parallel APFs must be identical. Otherwise, due to their very low impedance, inconsistent output voltages will generate large circulating currents between them. One solution is to use a central controller to control these parallel APFs, forcing their output voltages to be the same by setting a reference voltage. However, this method requires multiple control lines to connect these parallel APFs to the central controller. If a control line disconnection or communication failure occurs, the system will malfunction. Summary of the Invention

[0005] The technical problem to be solved and the technical task proposed by this invention is to improve and refine existing technical solutions, and to provide a parallel structure and control method for UPQC parallel converters based on droop control, with the aim of ensuring stable system operation. To this end, this invention adopts the following technical solution.

[0006] A parallel structure based on droop control UPQC parallel converters is constructed, featuring an energy storage element. This energy storage element is located between the series-type APF and the parallel-type APF, connected in parallel with the capacitor in the UPQC structure. The series-type APF is connected to the grid, while multiple parallel-type APFs are connected to the load, and these parallel-type APFs are interconnected. This structure avoids regulation and control through a central controller, preventing issues such as control line disconnections and communication breakdowns, effectively ensuring the normal and stable operation of the system.

[0007] The control method for the parallel structure of UPQC parallel converters based on droop control includes the following steps:

[0008] 1) Collect the output voltage of each parallel APF, and obtain the actual output active power of the parallel APF by the relationship between its output voltage and reactance and load voltage; obtain the actual output reactive power of the parallel APF by the relationship between its output voltage and resistance and load voltage;

[0009] 2) In droop control, the droop coefficients of active power and reactive power are adjusted according to the capacity ratio of each parallel APF, so that their droop coefficients are also in a certain proportional relationship, thereby completing the power distribution.

[0010] 3) Combining output voltage and power distribution issues, adjust the droop characteristic curves of each parallel APF, mainly adjusting the reactive power curve, to simultaneously achieve the same output voltage and power distribution according to capacity ratio, thus completing the control of multiple parallel APFs operating in the UPQC structure.

[0011] Compared to the control method using a central controller in a parallel structure, this method, which distributes power among several parallel APFs by droop control to control the output voltage of each parallel APF, eliminates circulating currents between them, as voltage variations affect output power. This method eliminates dependence on a central controller and the control lines connecting the devices, reducing the possibility of unexpected events. Droop control allows for the proportional allocation of active and reactive power based on the capacity of each parallel APF by adjusting the droop coefficient, enhancing the system's ability to control power quality. The flexible power allocation capability of droop control allows for the reasonable increase or decrease of the number of parallel APF modules according to the actual UPQC requirements, improving the utilization rate of each parallel APF and enabling plug-and-play functionality. This significantly improves system flexibility and alleviates the pressure on the original single parallel APF structure.

[0012] As a preferred technical approach: In step 2), during droop control, active power and reactive power are automatically allocated according to capacity ratio, which needs to meet the following conditions:

[0013]

[0014] In the formula, P i Q i These are the active power and reactive power of each parallel APF;

[0015] m i n i These are the droop factors for active power and reactive power, respectively. This allows active and reactive power to be automatically allocated according to capacity ratios.

[0016] As a preferred technical means: In step 3), in order to simultaneously achieve the same output voltage and power distribution according to capacity ratio, the droop characteristic curve of reactive power needs to meet the following conditions:

[0017] V = -nQ + b

[0018] In the formula, V is the output voltage, Q is the reactive power, and n is the reactive power droop coefficient.

[0019] b is the output voltage when the reactive power is 0. Simultaneously, the output voltage is the same and the power is distributed according to the capacity ratio.

[0020] Beneficial effects: The parallel structure and control method of this scheme can eliminate the circulating current generated between the parallel APFs; this method eliminates the dependence on the central controller and the control lines connecting the two devices, reducing the possibility of accidents; through droop control, the active and reactive power can be proportionally allocated according to the capacity of each parallel APF by adjusting the droop coefficient, enhancing the system's ability to control power quality; due to the flexible power allocation capability of droop control, the number of parallel APF modules can be reasonably increased or decreased according to the actual situation of UPQC, improving the utilization rate of each parallel APF, achieving plug-and-play functionality of the parallel APF modules, greatly improving the system's flexibility, and alleviating the pressure on the original single parallel APF structure. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the parallel structure in this invention.

[0022] Figure 2 This is a schematic diagram of the droop characteristic curves of the reactive power of each parallel-type APF in this invention.

[0023] Figure 3 This is a schematic diagram of the control flow in this invention.

[0024] In the diagram: 1. Energy storage element; 2. Series-connected APF; 3. Parallel-connected APF; 4. Capacitor. Detailed Implementation

[0025] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings.

[0026] like Figure 1As shown, a parallel structure of a UPQC parallel converter based on droop control is illustrated. Taking a UPQC structure with energy storage element 1 and parallel APF3 as an example, it includes energy storage element 1, a single series APF2, and three parallel APF3s. Energy storage element 1 is located between the series APF2 and the parallel APF3s and is connected in parallel with capacitor 4 in the traditional UPQC structure to reasonably distribute the current of the parallel APF3s. The series APF2 is connected to the grid terminal and is responsible for solving problems such as grid voltage fluctuations. The three parallel APF3s are connected to the load terminal and are connected in parallel with each other. They are mainly responsible for load current and output reactive power and can also play a role in distributing the load. When the microgrid is in grid-connected mode, for power quality management, the three parallel APF3 units are responsible for providing harmonic and reactive power compensation, while the series APF2 units absorb active power from the energy storage element 1 to offset the harmonic voltage penetration or voltage surges / dips in the distribution network. Therefore, the distributed power generation is not affected by the distribution network. The parallel APF3 units can also serve as interface circuits for the active and reactive power output of the load. The DC-side energy storage element 1 provides energy to supplement the microgrid's energy. When the microgrid is in off-grid mode, the series APF2 units do not function, completing the seamless switching of the microgrid from off-grid to grid-connected mode.

[0027] like Figure 3 As shown, the control method for the parallel structure of a UPQC parallel converter based on droop control includes the following steps:

[0028] S1) Collect the output voltage of each parallel APF3, and obtain the actual output active power of the parallel APF3 through the relationship between its output voltage and reactance and load voltage; obtain the actual output reactive power of the parallel APF3 through the relationship between its output voltage and resistance and load voltage;

[0029] S2) Due to the drooping characteristic, the active power P of each parallel APF3 is... i With the frequency f of the output voltage i Reactive power Q i With output voltage U iThere is a certain relationship: the frequency of the output voltage decreases as active power increases and vice versa, while the output voltage decreases as reactive power increases and vice versa. Based on this characteristic, the output voltage of each parallel APF3 can be kept consistent by controlling the reactive power, thus solving the operation problem of multiple parallel APF3 structures. However, since the capacities of each parallel APF3 may be inconsistent, the power distribution between them needs to be considered. In droop control, for the distribution of active and reactive power, the droop coefficients of active power and reactive power need to be adjusted according to the capacity ratio of each parallel APF3, so that their droop coefficients also have a certain proportional relationship, as shown in the following formula:

[0030]

[0031] This is how the allocation is completed;

[0032] In the formula, P i Q i These are the active power and reactive power of each parallel APF3;

[0033] m i n i These are the droop coefficients for active power and reactive power, respectively.

[0034] S3) Finally, considering both output voltage and power distribution issues, to simultaneously achieve identical output voltages and power distribution proportional to capacity, it's necessary to adjust the droop characteristic curves of each parallel-connected APF3. Furthermore, since active power does not significantly affect the output voltage amplitude, the primary adjustment focuses on the reactive power curve. For example... Figure 2 As shown, assuming the required output voltage of all parallel APF3s is U, and the droop coefficient of a certain parallel APF3 is 1, its droop characteristic curve equation is V = -Q + b. To achieve an output voltage of U, the reactive power allocated to it is bU. Let's take a capacity of... For a parallel-connected APF3, based on the above steps regarding power allocation according to capacity ratio, the droop factor of this parallel-connected APF3 is 2, and the allocated reactive power should be... Therefore, its droop characteristic curve needs to be adjusted to V = -2Q + b to ensure that the output voltage of each parallel APF3 is the same and automatically distributed according to the capacity ratio. Similarly, when the relationship between reactive power and output voltage amplitude meets the following conditions:

[0035] V = -nQ + b

[0036] This will solve the above problems and enable control of multiple parallel APF3 parallel structures running in the UPQC structure.

[0037] In the formula, V is the output voltage, Q is the reactive power, n is the droop coefficient of reactive power, and b is the output voltage when the reactive power is 0.

[0038] In summary, after constructing the parallel structure of the UPQC parallel converter with droop control, the output voltage of each parallel APF3 is first acquired to obtain its actual output active and reactive power. Then, based on the droop characteristics, the reactive power is controlled to keep the output voltage of each parallel APF3 consistent. Finally, considering the power distribution among the parallel APF3s, the droop coefficient ratio is determined according to the capacity ratio, and the droop coefficient curve is adjusted in conjunction with the output voltage to achieve the desired control of the structure.

[0039] above Figure 1-3 The parallel structure and control method of the UPQC parallel converter based on droop control shown are specific embodiments of the present invention, demonstrating the outstanding substantive features and significant progress of the present invention. According to actual use needs, equivalent modifications in shape, structure, etc. can be made to it under the guidance of the present invention, all of which are within the protection scope of this solution.

Claims

1. A parallel system of UPQC parallel converters based on droop control, characterized in that: Construct a UPQC structure with an energy storage element (1), wherein the energy storage element (1) is located between the series APF (2) and the parallel APF (3) and is connected in parallel with the capacitor (4) in the UPQC structure; the series APF (2) is connected to the grid terminal, and multiple parallel APFs (3) are connected to the load terminal, and each parallel APF (3) is connected in parallel with each other; The parallel APF uses droop control, and its control method includes the following steps: 1) Collect the output voltage of each parallel APF (3), and obtain the actual output active power of the parallel APF (3) through the relationship between its output voltage and reactance and load voltage; obtain the actual output reactive power of the parallel APF (3) through the relationship between its output voltage and resistance and load voltage; 2) In droop control, the droop coefficient of active power and the droop coefficient of reactive power are adjusted according to the capacity ratio of each parallel APF (3) so that their droop coefficients are also in a certain proportional relationship, thereby completing the power distribution. 3) Combining the output voltage problem and the power distribution problem, adjust the droop characteristic curve of each parallel APF (3). The main adjustment is the reactive power curve, so as to achieve the same output voltage and the power distribution according to the capacity ratio at the same time, and complete the control of multiple parallel APF (3) parallel structures operating in the UPQC structure.

2. The parallel system of UPQC parallel converters based on droop control according to claim 1, characterized in that: In step 2), during droop control, active and reactive power are automatically allocated according to capacity ratios, which requires the following conditions to be met: In the formula, , These are the active power and reactive power of each parallel APF (3); , These are the droop coefficients for active power and reactive power, respectively.

3. The parallel system of UPQC parallel converters based on droop control according to claim 2, characterized in that: In step 3), to simultaneously achieve the same output voltage and power distribution according to capacity ratio, the droop characteristic curve of reactive power must satisfy the following condition: In the formula, V is the output voltage, Q is the reactive power, and n is the reactive power droop coefficient. b is the output voltage when the reactive power is 0.

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