Inter-line power flow controller of multi-terminal direct current power distribution system

Abstract

The invention discloses an inter-line power flow controller of a multi-terminal direct-current power distribution system. The inter-line power flow controller comprises an LLC resonant converter unit, a neutral-point balance control circuit and a power flow adjusting unit, the LLC resonant converter unit and the neutral-point equalization control circuit jointly form an input voltage stabilization part of the inter-line power flow controller, input sides VT + and VT-are connected to a high-voltage direct-current bus to receive electric energy, and output sides VTP + and VTP-are connected with two ends of capacitors C1 and C2 which are connected in series; two input end points of the neutral-point balance control circuit are respectively connected with output sides VTP + and VTP-of the LLC resonant converter unit through internal direct-current buses P and N, and an input neutral point of the neutral-point balance control circuit is connected with a connection point of the capacitor C1 and the capacitor C2 through an internal direct-current bus O. The beneficial effects of the invention are that the device has the advantages of modular design, low port expansion cost and high degree of freedom of adjustment, solves the problem of current control coupling between lines, and achieves the independent control of the currents of different lines and the free distribution of the frequency-doubled pulse power on the lines.

Description

Technical Field

[0001] The present application relates to the field of direct current power transmission and distribution, and in particular to a line flow control circuit for a multi-terminal direct current power distribution system. Background Art

[0002] At present, with the rapid development of China's economy and the continuous optimization of its industrial structure, the corresponding emerging industries also need to constantly learn new technologies and processes in market demand and product iteration to meet the rapidly changing needs of the times. Electricity, as a clean and efficient form of energy, plays an important role in supporting modern industry and social life. Therefore, in China's industrial upgrading, it is particularly important to ensure the stability, reliability and safety of electricity supply.

[0003] The DC power flow controller is a technical device used to manage and regulate power flow in multi-terminal DC systems. Its main function is to optimize power distribution in DC power grids, improve the stability and transmission efficiency of power grids, and support the comprehensive utilization of multi-source power. Since the reactance and phase difference of the AC system do not exist in the DC system, the power flow of the line is mainly affected by the line resistance and the voltage difference between the two ends. The fundamental principle of the DC power flow controller is to control and regulate the power flow on the line by adjusting the line resistance or the voltage at both ends of the line. According to the different adjustment mechanisms, DC power flow controllers can be divided into different types: variable resistor type DC power flow controller, full power type DC power flow controller, and partial power type DC power flow controller. The variable resistor type DC power flow controller is to adjust the equivalent resistance of the line by inserting an adjustable resistor in series in the DC line and changing the resistance value of the resistor to adjust the equivalent resistance of the line, so as to achieve the purpose of controlling the system power flow. The variable resistor type DC power flow controller has the advantages of simple topology and control strategy, but its adjustment range is limited and the direction of the line current cannot be changed. At the same time, its addition will bring additional power loss to the system. The core function of the full-power DC power flow controller is to integrate the DC / DC converter into the circuit, connect its input and output to the system in parallel, and adjust the voltage ratio between the converter output and input, which is equivalent to embedding a continuously adjustable voltage source in the circuit. Its output value is V i With V k The difference between i is the converter input voltage, V kis the converter output voltage), by changing the DC voltage difference at both ends of the line, it can effectively change the power distribution in the power system, thereby achieving the purpose of power flow regulation. However, since its input and output are connected to the system in parallel, it needs to withstand the high voltage and power at the system level at the same time, so it is classified as a full-power device, which leads to relatively high requirements for equipment capacity. In a stable DC power system, there is no reactance in the DC line, and the equivalent resistance is usually only a few ohms, which leads to a significant fluctuation in the line current even when the DC bus voltage is fixed. Partial power type DC power flow controller uses this feature. Its fundamental principle is to insert an adjustable voltage source in series in the line to accurately control the line power flow. The advantage of this type of controller is that it avoids the device from bearing the high voltage and high power at the system level, reduces the design capacity of the device, and also reduces the cost.

[0004] In the traditional multi-terminal direct current (MTDC) distribution system, the line flow depends on the converter voltage and line impedance. However, when the number of system ports is large, the network is complex, and the number of lines is greater than or equal to the number of converters, simply relying on coordinated control between converters can dispatch the line flow to a certain extent, but the control freedom is not high, resulting in increased line losses on some lines, increased port expansion costs, and even line overloads that cause system outages. Therefore, the following problems need to be solved for the traditional multi-terminal direct current distribution system. First, it is necessary to study a DC power flow controller with reliable power supply and high efficiency to improve the control ability of the power flow. Secondly, in the abnormal working condition of unbalanced AC grid voltage, the output power on the DC side of the traditional multi-terminal direct current distribution system contains double frequency, which affects the power supply quality of sensitive components. Finally, the DC power flow controller also has the problem of large size and high manufacturing cost. Summary of the invention

[0005] The present application is a multi-terminal DC power distribution system line flow controller designed to solve the above technical problems.

[0006] The technical solution adopted by this application to solve its technical problem is:

[0007] A multi-terminal DC power distribution system line flow controller, the line flow controller comprising an LLC resonant converter unit, a midpoint balancing control circuit and a flow regulation unit;

[0008] The LLC resonant converter unit and the midpoint balancing control circuit together constitute the input voltage stabilization part of the line power flow controller. The LLC resonant converter unit input side V T + and V T - connected to the high voltage DC bus to receive electrical energy, its output side V TP + and VTP - connected to the two ends of the series capacitors C1 and C2, and the two input terminals of the midpoint balancing control circuit are connected to the output side V of the LLC resonant converter unit through the internal DC bus P and N respectively. TP + and V TP -, the input neutral point of the midpoint balancing control circuit is connected to the connection point of capacitors C1 and C2 through the internal DC bus O;

[0009] The power flow regulation unit is composed of n three-level topology circuits and their corresponding LC filtering devices. The two input endpoints of the n three-level topology circuits are connected to the internal DC bus P and N respectively. The input neutral point of the n three-level topology circuits is connected to the internal DC bus O and serves as the output common terminal T0. The n ports T constructed by the output of the three-level circuits i (i=1,2,...,n), together with the common terminal T0, form n independent output ports T0T of the line power flow controller i , by precisely controlling each port T0T i The voltage at both ends can be used to effectively control the line flow.

[0010] The multi-terminal DC power distribution system line flow controller, the LLC resonant converter unit includes two full-bridge inverter circuits, and the LLC resonant converter unit input side V T + and V T -Through input filter capacitor C i After filtering, it is connected to the input end of the first full-bridge inverter circuit, and the output end of the first full-bridge inverter circuit is connected to the input end of the first full-bridge inverter circuit through the DC transformer T R The output end of the second full-bridge inverter circuit is coupled to the input end of the second full-bridge inverter circuit, and the output end of the second full-bridge inverter circuit is connected to the output filter capacitor C o After filtering, the LLC resonant converter unit output side V TP + and V TP -.

[0011] The multi-terminal DC power distribution system line flow controller, the midpoint balancing control circuit includes: transistors S1, S2, S3, S4, diodes D1, D2, D3, D4, clamping diode D 01 , D 02 and inductor L; the transistor S1 is connected in reverse parallel to the diode D1, the collector of the transistor S1 is connected to the internal DC bus P, and the emitter of the transistor S1 is connected to the collector of the transistor S2; the transistor S2 is connected in reverse parallel to the diode D2, and the emitter of the transistor S2 is connected to the collector of the transistor S3; the transistor S3 is connected in reverse parallel to the diode D3, and the emitter of the transistor S3 is connected to the collector of the transistor S4; the transistor S4 is connected in reverse parallel to the diode D4, and the emitter of the transistor S3 is connected to the internal DC bus N;

[0012] The two clamping diodes D 01 and D 02 Series, diode D 01 and D 02 The connection point is connected to the emitter of transistor S2 through inductor L, and diode D 01 and D 02 The connection point of the diode D is connected to the internal DC bus O; 01 The cathode of the transistor S1 is connected to the emitter of the diode D 02 The anode of is connected to the emitter of transistor S3.

[0013] The multi-terminal DC power distribution system line flow controller adopts a midpoint balance control circuit control strategy. The voltages at both ends of the capacitors C1 and C2 are U C1 with U C2 , ΔU=U C1 -U C2 , adjust switches S1~S4 according to ΔU;

[0014] When ΔU>0, that is, the midpoint voltage is low, S3 and S4 are kept off, and the voltage is adjusted by adjusting the duty cycle of S1 and S2. The conduction of S1 and S2 allows the DC bus voltage to form a loop through the inductor L and the lower bridge arm capacitor C2, so that the inductor L stores energy and charges C2 during the conduction period of S1 and S2, thereby increasing the midpoint voltage; when S1 and S2 are turned off, the inductor L releases energy to C2 through the freewheeling path of the two anti-parallel diodes S3 and S4, further increasing the midpoint potential;

[0015] When ΔU<0, that is, the midpoint voltage is too high, the midpoint position is lowered by controlling the on and off of S3 and S4. At this time, S1 and S2 are kept off. The conduction of S3 and S4 allows the DC bus voltage to form a loop through the inductor L and the upper bridge arm capacitor C1, so that the inductor L stores energy and charges C1 during the conduction cycle of S3 and S4, thereby increasing the midpoint voltage; when S3 and S4 are turned off, the inductor L releases energy to C1 through the freewheeling path of the two anti-parallel diodes S1 and S2, so that the potential of the midpoint is restored to half of the bus.

[0016] The multi-terminal DC power distribution system line flow controller, the midpoint balancing control strategy adopts voltage and current dual-loop control, the voltage outer loop is the difference between the upper and lower capacitor voltages of capacitors C1 and C2, and the inner loop current reference command is generated through the PI regulator. The current inner loop uses the PI regulator to track and filter the current I flowing through the inductor L L Generate duty cycle information.

[0017] The multi-terminal DC power distribution system inter-line flow controller adopts a pulsating power control strategy, and by adding virtual impedance in the flow regulation module, the double frequency pulsation caused by the unbalanced grid voltage is transferred to the line with low requirements on power quality, or limited to the converter with large remaining capacity and good absorption of pulsating power. The implementation process is:

[0018] S100: Select a line as the reference line, and use the PIR regulator to track the port voltage u in the voltage outer loop control d1 , the virtual impedance controller tracks the pulsating current, and the two generate the reference command value of the inner loop current; the current inner loop adopts a PI regulator, which is responsible for tracking the filter inductor current i Li To generate a corresponding duty cycle control signal;

[0019] S200: For the remaining n-1 lines, their current regulation is based on their respective power flow dispatching instructions I dref_i (i=2,3,…,n) performs the following control;

[0020] S210: by expanding the dual-loop structure of the port voltage and the inductor current of the power flow regulation unit, a special power flow control loop is added to the outermost loop;

[0021] S220: The loop described in S210 uses a PIR regulator to track the power flow dispatching instructions of each line, thereby generating a corresponding port voltage reference instruction. The power flow regulation unit port voltage loop control uses a PIR regulator to track the port voltage, and the virtual impedance controller tracks the pulsating current. The two generate a reference instruction value for the inner loop current; the power flow regulation unit inductor current loop control uses PI, which is responsible for tracking the filter inductor current i Li To generate corresponding duty cycle control signals, and thus achieve precise regulation of the power flow;

[0022] S300: Track the double frequency pulsation in the line without static error, and connect a quasi-resonant controller in parallel on the basis of the proportional-integral controller to form a quasi-PIR controller; the additional virtual impedance controller is realized by correcting the port output voltage reference command value.

[0023] The multi-terminal DC power distribution system line flow controller and the quasi-PIR controller are expressed as follows:

[0024]

[0025] In formula (1), k p is the proportionality coefficient, k i is the integration coefficient, k r is the resonance coefficient, ω cand ω1 are the cutoff frequency and the resonant frequency; the resonant frequency ω1 is set to 100 Hz to control the double frequency pulsation in the line current.

[0026] The multi-terminal DC power distribution system line flow controller, the virtual impedance control link uses a second-order bandpass filter to maximize the extraction of the VSC DC side output current i when the grid voltage is unbalanced d The pulsating component i h , where the second-order bandpass filter transfer function is:

[0027]

[0028] In formula (2), k0 is the gain of BPF, which is generally taken as 0.707, and the pulsation frequency of ω0 is taken as 100Hz; then multiply it by the impedance coefficient Z SV , we can get the transfer function of the virtual impedance control link:

[0029] G sv (s) = Z sv g BPF (s)(3)

[0030] Combining equations (2) and (3), we can get the output voltage u of the line power flow controller port with the virtual impedance controller added: d The expression is:

[0031] u d (s)=G(s)I dref (s)+Z eq (s)i d (s)(4)

[0032] Where:

[0033]

[0034] In formula (5), G(s) is the closed-loop transfer function of the outermost loop; Z eq (s) is the equivalent impedance of the controller, Z0(s) and Z x (s) are their own equivalent impedance and additional virtual impedance respectively. sv By setting a suitable impedance coefficient in (s), the controller equivalent impedance Z can be changed without affecting G(s). eq (s), thereby changing the impedance difference of each line and controlling the pulsating power distribution.

[0035] The multi-terminal DC power distribution system inter-line flow controller provides efficient flow control in a multi-line system by selecting a site based on a sensitivity analysis method and then distributing and installing the site. The set of lines that meet the inter-line flow controller site selection principle is λ. On the basis of clarifying the inter-line flow controller installation principle, the site selection under normal operating conditions is planned step by step. The specific steps are:

[0036] P100: Systematically screen all lines in the entire power grid, and then identify and determine the target set λ consisting of those lines suitable for installing power flow controllers;

[0037] P200: Refer to the performance index in the static safety analysis of AC power grid to define the IDCPFC configuration index Q in DC power grid, as shown in the following formula:

[0038]

[0039] In the formula ——The current transformation ratio of each branch k mi The average value of the absolute value of sensitivity;

[0040] ——The current transformation ratio of each branch k mi The standard deviation of the absolute value of sensitivity;

[0041] The formula (6) comprehensively considers the following two aspects:

[0042] First, consider the sensitivity of the line power flow controller to the response of all branch currents. The higher the sensitivity of the line power flow controller to the branch current change, that is, The larger the value is, the more significant its regulatory effect is at that position.

[0043] Secondly, in order to eliminate the reliance on a single variable To evaluate the performance of the line flow controller in the current regulation of the entire line, there is a limitation of "shielding effect". It is necessary to consider whether the influence of the line flow controller on the current of each branch is balanced. For this purpose, another parameter is introduced. In guarantee Under the premise of being larger, the influence of the line power flow controller on the current of each branch is more balanced, that is, The smaller it is, the better its adjustment performance is;

[0044] Finally, the configuration index Q value is used to determine the optimal access position of the line power flow controller. When the Q value reaches the maximum, it can be concluded that the overall performance of the line power flow controller in the DC power grid power flow regulation is optimal. At this time, the corresponding position of the line power flow controller is its optimal installation position.

[0045] The multi-terminal DC power distribution system line flow controller, the LLC resonant converter unit adopts a full-bridge LLC circuit or a half-bridge LLC circuit.

[0046] At least one of the above technical solutions adopted in the embodiments of the present application can achieve the following beneficial effects:

[0047] (1) Aiming at the application scenarios that require line power reversal regulation, the problems of complex structure and insufficient control flexibility in the current power flow controller topology are analyzed, and a line power flow controller for a multi-terminal DC distribution system is proposed. The controller has the advantages of modular design, low port expansion cost and high degree of adjustment freedom. Through the cooperation of partial power auxiliary converters, the current control coupling problem between lines is solved, and independent control of currents of different lines is realized.

[0048] (2) A pulsating power distribution control strategy is proposed to address abnormal conditions such as unbalanced AC grid voltage in multi-terminal DC distribution systems. By adding virtual impedance for double-frequency pulsation at each port of IDCPFC, the double-frequency pulsating power can be freely distributed on the line to reduce the impact on sensitive loads. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings:

[0050] Figure 1 It is a schematic diagram of the topological structure of the line power flow controller of the multi-terminal DC distribution system;

[0051] Figure 2 It is the schematic diagram of LLC topology;

[0052] FIG3( a ) is a schematic diagram of a typical switching mode P of IDCPFC when the current direction is positive;

[0053] Figure 3(b) is a schematic diagram of a typical switching mode O of IDCPFC when the current direction is positive;

[0054] Figure 3(c) is a schematic diagram of a typical switching mode N of IDCPFC when the current direction is positive;

[0055] FIG4( a ) is a schematic diagram of a typical switching mode P of IDCPFC when the current direction is negative;

[0056] FIG4( b ) is a schematic diagram of a typical switching mode O of IDCPFC when the current direction is negative;

[0057] Figure 4(c) is a schematic diagram of the typical switching mode N of IDCPFC when the current direction is negative;

[0058] Figure 5(a) is a schematic diagram of the typical switching mode P of IDCPFC when the current direction is different;

[0059] Figure 5(b) is a schematic diagram of the typical switching mode O of IDCPFC when the current direction is different;

[0060] Figure 5(c) is a schematic diagram of the typical switching mode N of IDCPFC when the current direction is different;

[0061] Figure 6 It is a schematic diagram of the topological structure of the midpoint balancing control circuit;

[0062] Figure 7 It is the control strategy diagram of the midpoint balance control circuit;

[0063] Figure 8 It is the pulsating power balancing control strategy for the power flow regulation module;

[0064] Fig. 9 This is the control block diagram of the virtual impedance controller. DETAILED DESCRIPTION

[0065] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.

[0066] The technical solutions provided by various embodiments of the present application are described in detail below in conjunction with the accompanying drawings.

[0067] In order to facilitate the understanding of the various embodiments of the present application, Figure 1 A multi-terminal DC power distribution system line flow controller is shown, and the line flow controller includes an LLC resonant converter unit, a midpoint balancing control circuit and a flow regulation unit.

[0068] like Figure 2 As shown, the LLC resonant converter unit and the midpoint balancing control circuit together constitute the input voltage stabilization part of the line power flow controller. The LLC resonant converter unit input side V T + and V T - connected to the high voltage DC bus to receive electrical energy, its output side V TP + and V TP - connected to the two ends of the series capacitors C1 and C2, and the two input terminals of the midpoint balancing control circuit are connected to the output side V of the LLC resonant converter unit through the internal DC bus P and N respectively. TP+ and V TP -, the input neutral point of the midpoint balancing control circuit is connected to the connection point of capacitors C1 and C2 through the internal DC bus O.

[0069] The power flow regulation unit is composed of n three-level topology circuits and their corresponding LC filtering devices. The two input endpoints of the n three-level topology circuits are connected to the internal DC bus P and N respectively. The input neutral point of the n three-level topology circuits is connected to the internal DC bus O and serves as the output common terminal T0. The n ports T constructed by the output of the three-level circuits i (i=1,2,...,n), together with the common terminal T0, form n independent output ports T0T of the line power flow controller i , by precisely controlling each port T0T i The voltage at both ends can be used to effectively control the line flow.

[0070] The multi-terminal DC power distribution system line flow controller, the LLC resonant converter unit includes two full-bridge inverter circuits, and the LLC resonant converter unit input side V T + and V T -Through input filter capacitor C i After filtering, it is connected to the input end of the first full-bridge inverter circuit, and the output end of the first full-bridge inverter circuit is connected to the input end of the first full-bridge inverter circuit through the DC transformer T R The output end of the second full-bridge inverter circuit is coupled to the input end of the second full-bridge inverter circuit, and the output end of the second full-bridge inverter circuit is connected to the output filter capacitor C o After filtering, the LLC resonant converter unit output side V TP + and V TP -.

[0071] The multi-terminal DC power distribution system line flow controller, the LLC resonant converter unit includes two full-bridge inverter circuits, and the LLC resonant converter unit input side V T + and V T -Through input filter capacitor C i After filtering, it is connected to the input end of the first full-bridge inverter circuit, and the output end of the first full-bridge inverter circuit is connected to the input end of the first full-bridge inverter circuit through the DC transformer T R The output end of the second full-bridge inverter circuit is coupled to the input end of the second full-bridge inverter circuit, and the output end of the second full-bridge inverter circuit is connected to the output filter capacitor C o After filtering, the LLC resonant converter unit output side V TP + and V TP -.

[0072] (1) Working principle

[0073] The power flow regulation module is recorded as Part i(i=1,2,…,n), draws power from the input voltage regulator composed of LLC resonant circuit and midpoint balance circuit, and adjusts its output voltage by controlling the on and off of the switch element. This operation is equivalent to connecting an adjustable voltage source in series on the relevant line, thereby changing the port voltage U i The difference between (i=1,2,...,n) and the reference voltage U0 is used to achieve precise control of the current in each branch and effective management of the power flow of the entire system.

[0074] The current flowing into the IDCPFC terminal is defined as positive, and it is stipulated that when the IDCPFC is connected to the system, the input voltage regulation part composed of the LLC resonant circuit and the midpoint balance circuit must first ensure the constant voltage across the capacitors C1 and C2, that is, U C1 =U C2 =U C ,(U C Represents half of the LLC resonant converter output voltage. LLC resonant converter output capacitor C o Half of the voltage, that is, U C1 =U C2 =U C =0.5U Co )This is a prerequisite for the flow regulation module to perform flow regulation.

[0075] When the current direction is positive: Figure 3 shows the three working modes of IDCPFC when the line current is positive. When the three-level topology circuit in module part1~part3 is in the P state, the voltage of each output port of IDCPFC is U T0T1 =U C1 , U T0T2 =U C1 , U T0T3 =U C1 , as shown in Figure 3(a); when the three-level topology circuit in module part1~part3 is switched to the O state, the output voltage of each port of IDCPFC is U T0T1 =0,U T0T2 =0,U T0T3 =0, as shown in FIG3(b); when the three-level topology circuit in modules part1 to part3 is in the N state, the output voltage of each port is U T0T1 =-U C2 , U T0T2 =-U C2 , U T0T3 =-U C2 , as shown in Figure 3(c). Further expansion shows that when the current direction of the controlled circuit is positive, since the voltages across C1 and C2 are equal, both are U CBy controlling the state change of the three-level topology circuit in each module, U can be obtained at each output port of IDCPFC. T0T1 =m1U C , U T0T2 =m2U C , U T0T3 =m3U C Adjustable DC voltage (where m i The output voltage of each module and the single capacitance value U c When the midpoint equalization circuit is working normally, the voltages across capacitors C1 and C2 in the equalization circuit are equal, both are U C , that is, both are half of the LLC resonant converter output voltage, and this value is fixed; the output voltage of each module is set according to the usage requirements, and the output voltage is divided by the voltage of a single capacitor, and the ratio obtained is the m of the line. i . ), the power flow control of the line can be achieved by adjusting m1~m3.

[0076] When the current direction is negative: Figure 4 shows the three working modes of IDCPFC when the direction of the controlled line is negative. When the three-level topology circuit in module part1~part3 is in the P state, the output voltage of each port of IDCPFC is U T0T1 =-U C1 , U T0T2 =-U C1 , U T0T3 =-U C1 , as shown in Figure 4(a); when the three-level topology circuit in module part1~part3 is in the O state, the output voltage of each port of IDCPFC is U T0T1 =0,U T0T2 =0,U T0T3 =0, as shown in FIG4(b); when the three-level topology circuit in modules part1 to part3 is in the N state, the output voltage state of each port is U T0T1 =U C2 , U T0T2 =U C2 , U T0T3 =U C2 , as shown in Figure 4(c). Further expansion shows that when the current direction of the controlled circuit is negative, by controlling the change of the state of the three-level circuit in each module, U can be obtained at each output port of IDCPFC. T1T0 =-m1U c , U T2T0 =-m2U c , U T3T0 =-m3U cThe adjustable DC voltage can control the power flow of the line by adjusting m1~m3.

[0077] When the current directions are different: When the current directions of the controlled lines are different, I T1 Take the reverse direction as an example. At this time, the total current I T0 The direction of I0 is uncertain. Assuming that the direction of I0 is still negative, the three operating modes of IDCPFC can be shown in Figure 5. The analysis is as follows: When the current I T1 The current is reversed, and U can be obtained at each output port of IDCPFC by controlling the switch state of the three-level topology circuit in the control module part1~part3. T0T1 =-m1U c , U T0T2 =m2U c , U T0T3 =m3U c Extending to any port IDCPFC, we know that as long as the voltage across the capacitors C1 and C2 of the midpoint balance control circuit is kept stable, no matter what the direction of the current in each controlled line is, the flow regulation module Part in IDCPFC can be adjusted. i The operation mode is used to control the voltage at both ends of each controlled line, and the positive and negative directions of the current in each line are adjusted by the adjustment ratio m. i The positive and negative of the circuit remain consistent, and each circuit can be freely adjusted, freely reversed, and decoupled from each other.

[0078] (2) Midpoint balance control circuit control strategy

[0079] When IDCPFC is connected to a multi-terminal DC distribution system for power flow regulation, it draws power directly from the DC system through the LLC resonant converter, and then uses the midpoint balancing control circuit to control the zero-sequence current flowing into the midpoint, thereby stabilizing the voltage of the two DC side support capacitors C1 and C2 to 1 / 2U C , providing stable voltage input for each power flow regulation module, the midpoint balancing control circuit topology is as follows Figure 6 shown.

[0080] The multi-terminal DC power distribution system line flow controller, the midpoint balancing control circuit includes: transistors S1, S2, S3, S4, diodes D1, D2, D3, D4, clamping diode D 01 , D 02and inductor L; the transistor S1 is connected in reverse parallel to the diode D1, the collector of the transistor S1 is connected to the internal DC bus P, and the emitter of the transistor S1 is connected to the collector of the transistor S2; the transistor S2 is connected in reverse parallel to the diode D2, and the emitter of the transistor S2 is connected to the collector of the transistor S3; the transistor S3 is connected in reverse parallel to the diode D3, and the emitter of the transistor S3 is connected to the collector of the transistor S4; the transistor S4 is connected in reverse parallel to the diode D4, and the emitter of the transistor S3 is connected to the internal DC bus N;

[0081] The two clamping diodes D 01 and D 02 Series, diode D 01 and D 02 The connection point is connected to the emitter of transistor S2 through inductor L, and diode D 01 and D 02 The connection point of the diode D is connected to the internal DC bus O; 01 The cathode of the transistor S1 is connected to the emitter of the diode D 02 The anode of is connected to the emitter of transistor S3.

[0082] The multi-terminal DC power distribution system line flow controller adopts a midpoint balance control circuit control strategy. The voltages at both ends of the capacitors C1 and C2 are U C1 with U C2 , ΔU=U C1 -U C2 , adjust switches S1~S4 according to ΔU;

[0083] When ΔU>0, that is, the midpoint voltage is low, S3 and S4 are kept off, and the voltage is adjusted by adjusting the duty cycle of S1 and S2. The conduction of S1 and S2 allows the DC bus voltage to form a loop through the inductor L and the lower bridge arm capacitor C2, so that the inductor L stores energy and charges C2 during the conduction period of S1 and S2, thereby increasing the midpoint voltage; when S1 and S2 are turned off, the inductor L releases energy to C2 through the freewheeling path of the two anti-parallel diodes S3 and S4, further increasing the midpoint potential;

[0084] When ΔU<0, that is, the midpoint voltage is too high, the midpoint position is lowered by controlling the on and off of S3 and S4. At this time, S1 and S2 are kept off. The conduction of S3 and S4 allows the DC bus voltage to form a loop through the inductor L and the upper bridge arm capacitor C1, so that the inductor L stores energy and charges C1 during the conduction cycle of S3 and S4, thereby increasing the midpoint voltage; when S3 and S4 are turned off, the inductor L releases energy to C1 through the freewheeling path of the two anti-parallel diodes S1 and S2, so that the potential of the midpoint is restored to half of the bus.

[0085] like Figure 7 As shown, the multi-terminal DC power distribution system line flow controller, the midpoint balancing control strategy adopts voltage and current dual-loop control, the voltage outer loop is the difference between the upper and lower capacitor voltages of capacitors C1 and C2, and the inner loop current reference command is generated through the PI regulator. The current inner loop uses the PI regulator to track and filter the current I flowing through the inductor L L Generate duty cycle information.

[0086] (3) Pulsating power control strategy

[0087] In order to ensure the stable operation of the system while taking into account the reliability of DC load power supply, so that it can simultaneously realize the scheduling of DC power flow and the transfer and distribution of pulsating power, by adding virtual impedance in the power flow regulation module, the double frequency pulsation caused by grid voltage imbalance is transferred to the line with low requirements on power quality, or limited to the converter with large remaining capacity and better absorption of pulsating power.

[0088] The multi-terminal DC power distribution system line flow controller adopts a pulsating power control strategy, and by adding virtual impedance in the power flow regulation module, the double frequency pulsation caused by the unbalanced grid voltage is transferred to the line with low requirements on power quality, or limited to the converter with large residual capacity that can better absorb the pulsating power. The implementation process is as follows Figure 8 As shown in the figure: for n controlled branches, the power flow control of all lines can be achieved by tracking the power flow instructions of n-1 lines. The power flow regulation instructions are calculated in an appropriate manner according to the characteristics of the DC power grid and DCPFC.

[0089] Therefore, first select a certain line as the reference line, and the voltage outer loop control uses the PIR regulator to track the port voltage u d1 , the virtual impedance controller tracks the pulsating current, and the two generate the reference command value of the inner loop current; the current inner loop adopts a PI regulator, which is responsible for tracking the filter inductor current i Li To generate a corresponding duty cycle control signal.

[0090] For the remaining n-1 lines, their current regulation is based on their respective power flow dispatch instructions I dref_i (i=2,3,…,n). By expanding the voltage-current dual-loop control structure, a dedicated power flow control loop is added to the outermost loop. This loop uses a PIR regulator to track the power flow dispatching instructions of each line, thereby generating the corresponding port voltage reference instructions. The voltage loop control uses a PIR regulator to track the port voltage, and a virtual impedance controller to track the pulsating current. The two generate the reference instruction value of the inner loop current; the current inner loop uses a PI regulator to track the filter inductor current i LiTo generate the corresponding duty cycle control signal, thereby achieving precise regulation of the power flow.

[0091] Since it is necessary to track the double frequency pulsation in the line without static error, it is necessary to connect a quasi-resonant controller in parallel on the basis of the proportional-integral controller to form a quasi-PIR controller; the additional virtual impedance controller is realized by correcting the port output voltage reference command value.

[0092] Implementation steps:

[0093] S100: Select a line as the reference line, and use the PIR regulator to track the port voltage u in the voltage outer loop control d1 , the virtual impedance controller tracks the pulsating current, and the two generate the reference command value of the inner loop current; the current inner loop adopts a PI regulator, which is responsible for tracking the filter inductor current i Li To generate a corresponding duty cycle control signal;

[0094] S200: For the remaining n-1 lines, their current regulation is based on their respective power flow dispatching instructions I dref_i (i=2,3,…,n) performs the following control;

[0095] S210: by expanding the dual-loop structure of the port voltage and the inductor current of the power flow regulation unit, a special power flow control loop is added to the outermost loop;

[0096] S220: The loop described in S210 uses a PIR regulator to track the power flow dispatching instructions of each line, thereby generating a corresponding port voltage reference instruction. The power flow regulation unit port voltage loop control uses a PIR regulator to track the port voltage, and the virtual impedance controller tracks the pulsating current. The two generate a reference instruction value for the inner loop current; the power flow regulation unit inductor current loop control uses PI, which is responsible for tracking the filter inductor current i Li To generate corresponding duty cycle control signals, and thus achieve precise regulation of the power flow;

[0097] S300: Track the double frequency pulsation in the line without static error, and connect a quasi-resonant controller in parallel on the basis of the proportional-integral controller to form a quasi-PIR controller; the additional virtual impedance controller is realized by correcting the port output voltage reference command value.

[0098] like Fig. 9 Shown: G d is the transfer function of the innermost loop inductor current loop PI controller; G PIR_U (s), G PIR_I (s) are the transfer functions of the capacitor voltage loop and the outermost power flow control loop, respectively. The multi-terminal DC distribution system line power flow controller and the quasi-PIR controller are expressed as follows:

[0099]

[0100] In formula (1), k p is the proportionality coefficient, k i is the integration coefficient, k r is the resonance coefficient, ω c and ω1 are the cutoff frequency and the resonant frequency; the resonant frequency ω1 is set to 100 Hz to control the double frequency pulsation in the line current.

[0101] The multi-terminal DC power distribution system line flow controller, the virtual impedance control link uses a second-order bandpass filter to maximize the extraction of the VSC DC side output current i when the grid voltage is unbalanced d The pulsating component i h , where the second-order bandpass filter transfer function is:

[0102]

[0103] In formula (2), k0 is the gain of BPF, which is generally taken as 0.707, and the pulsation frequency of ω0 is taken as 100Hz; then multiply it by the impedance coefficient Z SV , we can get the transfer function of the virtual impedance control link:

[0104] G sv (s) = Z sv g BPF (s)(3)

[0105] Combining equations (2) and (3), we can get the output voltage u of the line power flow controller port with the virtual impedance controller added: d The expression is:

[0106] u d (s)=G(s)I dref (s)+Z eq (s)i d (s)(4)

[0107] Where:

[0108]

[0109] In formula (5), G(s) is the closed-loop transfer function of the outermost loop; Z eq (s) is the equivalent impedance of the controller, Z0(s) and Z x (s) are their own equivalent impedance and additional virtual impedance respectively. sv By setting a suitable impedance coefficient in (s), the controller equivalent impedance Z can be changed without affecting G(s). eq (s), thereby changing the impedance difference of each line and controlling the pulsating power distribution.

[0110] (4) Site selection based on sensitivity analysis

[0111] IDCPFC can provide efficient power flow control between different system ports, so its application in multi-line systems is particularly important. However, the coupling relationship between the system terminal voltage and the IDCPFC output voltage increases the difficulty of determining the optimal configuration position.

[0112] The multi-terminal DC power distribution system inter-line flow controller provides efficient flow control in a multi-line system by selecting a site based on a sensitivity analysis method and then distributing and installing the site. The set of lines that meet the inter-line flow controller site selection principle is λ. On the basis of clarifying the inter-line flow controller installation principle, the site selection under normal operating conditions is planned step by step. The specific steps are:

[0113] P100: Systematically screen all lines in the entire power grid, and then identify and determine the target set λ consisting of those lines suitable for installing power flow controllers;

[0114] P200: Refer to the performance index in the static safety analysis of AC power grid to define the IDCPFC configuration index Q in DC power grid, as shown in the following formula:

[0115]

[0116] In the formula ——The current transformation ratio of each branch k mi The average value of the absolute value of sensitivity;

[0117] ——The current transformation ratio of each branch k mi The standard deviation of the absolute value of sensitivity;

[0118] The formula (6) comprehensively considers the following two aspects:

[0119] First, consider the sensitivity of the line power flow controller to the response of all branch currents. The higher the sensitivity of the line power flow controller to the branch current change, that is, The larger the value is, the more significant its regulatory effect is at that position.

[0120] Secondly, in order to eliminate the reliance on a single variable To evaluate the performance of the line flow controller in the current regulation of the entire line, there is a limitation of "shielding effect". It is necessary to consider whether the influence of the line flow controller on the current of each branch is balanced. For this purpose, another parameter is introduced. In guarantee Under the premise of being larger, the influence of the line power flow controller on the current of each branch is more balanced, that is, The smaller it is, the better its adjustment performance is;

[0121] Finally, the configuration index Q value is used to determine the optimal access position of the line power flow controller. When the Q value reaches the maximum, it can be concluded that the overall performance of the line power flow controller in the DC power grid power flow regulation is optimal. At this time, the corresponding position of the line power flow controller is its optimal installation position.

[0122] Considering the topological structure and working principle of IDCPFC, the following IDCPFC site selection principles are proposed:

[0123] 1) IDCPFC should be deployed in the DC ring network to maximize its control flexibility.

[0124] 2) When the current on the line can be fully controlled by the converter alone, there is no need to install additional IDCPFC, avoiding unnecessary cost and complexity.

[0125] 3) The converter at the IDCPFC installation node has at least two outgoing lines, and ensures that the IDCPFC controls at least two branch flows simultaneously.

[0126] The multi-terminal DC power distribution system line flow controller, the LLC resonant converter unit adopts a full-bridge LLC circuit or a half-bridge LLC circuit.

[0127] The above is only an embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.

Claims

1. A line flow controller for a multi-terminal DC power distribution system, characterized in that: The line power flow controller includes an LLC resonant converter unit, a midpoint balancing control circuit and a power flow regulation unit; The LLC resonant converter unit and the midpoint balancing control circuit together constitute the input voltage stabilization part of the line power flow controller. The LLC resonant converter unit input side V T + and V T - connected to the high voltage DC bus to receive electrical energy, its output side V TP + and V TP - connected to the two ends of the series capacitors C1 and C2, and the two input terminals of the midpoint balancing control circuit are connected to the output side V of the LLC resonant converter unit through the internal DC bus P and N respectively. TP + and V TP -, the input neutral point of the midpoint balancing control circuit is connected to the connection point of capacitors C1 and C2 through the internal DC bus O; The power flow regulation unit is composed of n three-level topology circuits and their corresponding LC filtering devices. The two input endpoints of the n three-level topology circuits are connected to the internal DC bus P and N respectively. The input neutral point of the n three-level topology circuits is connected to the internal DC bus O and serves as the output common terminal T0. The n ports T constructed by the output of the three-level circuits i (i=1,2,...,n), together with the common terminal T0, form n independent output ports T0T of the line power flow controller i , by regulating each port T0T i The voltage at both ends is used to control the line flow.

2. The line flow controller of a multi-terminal DC power distribution system according to claim 1, characterized in that: The LLC resonant converter unit includes two full-bridge inverter circuits. The LLC resonant converter unit input side V T + and V T -Through input filter capacitor C i After filtering, it is connected to the input end of the first full-bridge inverter circuit, and the output end of the first full-bridge inverter circuit is connected to the input end of the first full-bridge inverter circuit through the DC transformer T R The output end of the second full-bridge inverter circuit is coupled to the input end of the second full-bridge inverter circuit, and the output end of the second full-bridge inverter circuit is connected to the output filter capacitor C o After filtering, the LLC resonant converter unit output side V TP + and V TP -.

3. The line flow controller of a multi-terminal DC power distribution system according to claim 1, characterized in that: The midpoint balancing control circuit includes: transistors S1, S2, S3, S4, diodes D1, D2, D3, D4, clamping diode D 01 , D 02 and inductor L; the transistor S1 is connected in reverse parallel to the diode D1, the collector of the transistor S1 is connected to the internal DC bus P, and the emitter of the transistor S1 is connected to the collector of the transistor S2; the transistor S2 is connected in reverse parallel to the diode D2, and the emitter of the transistor S2 is connected to the collector of the transistor S3; the transistor S3 is connected in reverse parallel to the diode D3, and the emitter of the transistor S3 is connected to the collector of the transistor S4; the transistor S4 is connected in reverse parallel to the diode D4, and the emitter of the transistor S3 is connected to the internal DC bus N; The two clamping diodes D 01 and D 02 Series, diode D 01 and D 02 The connection point is connected to the emitter of transistor S2 through inductor L, and diode D 01 and D 02 The connection point of the diode D is connected to the internal DC bus O; 01 The cathode of the diode D is connected to the emitter of the transistor S1. 02 The anode of is connected to the emitter of transistor S3.

4. The line flow controller of a multi-terminal DC power distribution system according to claim 3, characterized in that: The line power flow controller adopts the midpoint balance control circuit control strategy. The voltages at both ends of the capacitors C1 and C2 are U C1 with U C2 , ΔU=U C1 -U C2 , adjust switches S1~S4 according to ΔU; When ΔU>0, that is, the midpoint voltage is low, S3 and S4 are kept off, and the voltage is adjusted by adjusting the duty cycle of S1 and S2. The conduction of S1 and S2 allows the DC bus voltage to form a loop through the inductor L and the lower bridge arm capacitor C2, so that the inductor L stores energy and charges C2 during the conduction period of S1 and S2, thereby increasing the midpoint voltage; when S1 and S2 are turned off, the inductor L releases energy to C2 through the freewheeling path of the two anti-parallel diodes S3 and S4, further increasing the midpoint potential; When ΔU<0, that is, the midpoint voltage is too high, the midpoint position is lowered by controlling the on and off of S3 and S4. At this time, S1 and S2 are kept off. The conduction of S3 and S4 allows the DC bus voltage to form a loop through the inductor L and the upper bridge arm capacitor C1, so that the inductor L stores energy and charges C1 during the conduction cycle of S3 and S4, thereby increasing the midpoint voltage; when S3 and S4 are turned off, the inductor L releases energy to C1 through the freewheeling path of the two anti-parallel diodes S1 and S2, so that the potential of the midpoint is restored to half of the bus.

5. The line flow controller of a multi-terminal DC power distribution system according to claim 4, characterized in that: The midpoint balancing control strategy adopts voltage and current dual-loop control. The voltage outer loop is the difference between the upper and lower capacitor voltages of capacitors C1 and C2. The inner loop current reference command is generated through the PI regulator. The current inner loop uses the PI regulator to track and filter the current I flowing through the inductor L. L Generate duty cycle information.

6. The line flow controller of a multi-terminal DC power distribution system according to claim 1, characterized in that: The line power flow controller adopts a pulsating power control strategy. By adding virtual impedance in the power flow regulation module, the double frequency pulsation caused by the unbalanced grid voltage is transferred to the line with low requirements on power quality, or limited to the converter with large residual capacity and good absorption of pulsating power. The implementation process is: S100: Select a line as the reference line, and use the PIR regulator to track the port voltage u in the voltage outer loop control d1 , the virtual impedance controller tracks the pulsating current, and the two generate the reference command value of the inner loop current; The current inner loop uses a PI regulator to track the filter inductor current i Li To generate a corresponding duty cycle control signal; S200: For the remaining n-1 lines, their current regulation is based on their respective power flow dispatching instructions I drefi (i=2,3,…,n) performs the following control; S210: by expanding the dual-loop structure of the port voltage and the inductor current of the power flow regulation unit, a special power flow control loop is added to the outermost loop; S220: The loop in S210 uses a PIR regulator to track the power flow dispatching instructions of each line, thereby generating a corresponding port voltage reference instruction. The power flow regulation unit port voltage loop control uses a PIR regulator to track the port voltage, and a virtual impedance controller to track the pulsating current, and the two generate a reference instruction value for the inner loop current; The inductor current loop control of the power flow regulation unit adopts PI, which is responsible for tracking the filter inductor current i Li To generate corresponding duty cycle control signals, and thus achieve precise regulation of the power flow; S300: Track the double frequency pulsation in the line without static error, and connect a quasi-resonant controller in parallel on the basis of the proportional-integral controller to form a quasi-PIR controller; the additional virtual impedance controller is realized by correcting the port output voltage reference command value.

7. The line power flow controller of a multi-terminal DC power distribution system according to claim 6, characterized in that: The quasi-PIR controller is expressed as: In formula (1), k p is the proportionality coefficient, k i is the integration coefficient, k r is the resonance coefficient, ω c and ω1 are the cutoff frequency and the resonant frequency; the resonant frequency ω1 is set to 100 Hz to control the double frequency pulsation in the line current.

8. The line flow controller of a multi-terminal DC power distribution system according to claim 6, characterized in that: The virtual impedance control link uses a second-order bandpass filter to extract the VSC DC side output current i when the grid voltage is unbalanced to the maximum extent. d The pulsating component i h , where the second-order bandpass filter transfer function is: In formula (2), k0 is the gain of BPF, which is generally taken as 0.707, and the pulsation frequency of ω0 is taken as 100Hz; then multiply it by the impedance coefficient Z SV , we can get the transfer function of the virtual impedance control link: G sv (s) = Z sv G BPF (s)(3) Combining equations (2) and (3), we can get the output voltage u of the line power flow controller port with the virtual impedance controller added: d The expression is: u d (s) = G(s)I dref (s) + Z eq (s)i d (s) In equation (4): In formula (1), G(s) is the closed-loop transfer function of the outermost loop; Z eq (s) is the equivalent impedance of the controller, Z0(s) and Z x (s) are their own equivalent impedance and additional virtual impedance respectively. sv By setting a suitable impedance coefficient in (s), the controller equivalent impedance Z can be changed without affecting G(s). eq (s), thereby changing the impedance difference of each line and controlling the pulsating power distribution.

9. The line power flow controller of a multi-terminal DC power distribution system according to claim 1, characterized in that: The line flow controller provides efficient flow control in a multi-line system by selecting a site based on the sensitivity analysis method and then distributing and installing it. Assuming that the set of lines that meet the line flow controller site selection principle is λ, the site selection of the line flow controller under normal operating conditions is planned step by step on the basis of clarifying the line flow controller installation principle. The specific steps are: P100: Systematically screen all lines in the entire power grid, and then identify and determine the target set λ consisting of those lines suitable for installing power flow controllers; P200: Refer to the performance index in the static safety analysis of AC power grid to define the IDCPFC configuration index Q in DC power grid, as shown in the following formula: In the formula ——The current transformation ratio of each branch k mi The average value of the absolute value of sensitivity; ——The current transformation ratio of each branch k mi The standard deviation of the absolute value of sensitivity; The formula (6) comprehensively considers the following two aspects: First, consider the sensitivity of the line power flow controller to the response of all branch currents. The higher the sensitivity of the line power flow controller to the branch current change, that is, The larger the value is, the more significant its regulatory effect is at that position. Secondly, in order to eliminate the reliance on a single variable To evaluate the performance of the line flow controller in the current regulation of the entire line, there is a limitation of "shielding effect". It is necessary to consider whether the influence of the line flow controller on the current of each branch is balanced. For this purpose, another parameter is introduced. In guarantee Under the premise of being larger, the influence of the line power flow controller on the current of each branch is more balanced, that is, The smaller it is, the better its adjustment performance is; Finally, the configuration index Q value is used to determine the optimal access position of the line power flow controller. When the Q value reaches the maximum, it can be concluded that the overall performance of the line power flow controller in the DC power grid power flow regulation is optimal. At this time, the corresponding position of the line power flow controller is its optimal installation position.

10. The line power flow controller of a multi-terminal DC power distribution system according to claim 1 or 2, characterized in that: The LLC resonant converter unit adopts a full-bridge LLC circuit or a half-bridge LLC circuit.

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