A direct current power flow controller and a control method thereof
Patent Information
- Application Number
- CN202211541732.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-12-02
- Publication Date
- 2026-10-09
- Estimated Expiration
- 2042-12-02
AI Technical Summary
与直流变压器型直流潮流控制器相比，串联电压源型直流潮流控制器具有潮流双向调节、额定功率低且无需承受系统级电压等优点，但是串联电压源型直流潮流控制器需要额外的交流/直流系统进行能量供给，这导致了系统结构更加复杂，拉高了设备的建设成本，同时限制了潮流控制器的安装位置
1、本发明的直流潮流控制器适用于环网式多端直流输电场合，与传统的多端口线间型直流潮流控制器相比，本发明的直流潮流控制器具有双向潮流调节能力，且仅用2+2n个开关器件就可以实现n端口直流潮流的有效控制，提高了输电网络潮流方向的控制能力以及输电线路的传输能力。
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Abstract
Description
Technical Field
[0001] This invention belongs to the technical field of multi-terminal DC transmission systems, specifically relating to a DC power flow controller and its control method. Background Technology
[0002] Multi-terminal DC transmission systems refer to DC transmission systems composed of three or more converter stations. They enable multi-source power supply and multi-point power reception, offering greater flexibility compared to two-terminal DC transmission systems. The converter stations are primarily connected in series, parallel, or hybrid configurations. Multi-terminal DC transmission systems are also one of the effective technical means to address current challenges in my country, such as large-scale renewable energy grid integration, a shortage of transmission corridors, and the need for high-capacity, long-distance transmission. Voltage source converters (VSCs) offer advantages over traditional current source converters, including reversed DC current direction without altered current-voltage polarity during power flow reversal, and the absence of commutation failure. Therefore, voltage source converters are more suitable for constructing multi-terminal high voltage direct current (VSC-MTDC) systems. Figure 1 The diagram shows the equivalent circuit of a typical ring-type four-terminal VSC-MTDC system. It assumes that VSC2, VSC3, and VSC4 are converter stations for three offshore wind farms, operating in constant power mode, while VSC1 is an onshore converter station operating in constant voltage mode. Power is transferred from VSC2, 3, and 4 to VSC1. Since power flow control can be introduced in Flexible AC Transmission Systems (FACTS) to improve the control capability of power flow direction and active and reactive power of transmission lines, DC power flow controllers can also be used in DC transmission systems to improve the directional control capability of transmission lines and the amount of power transmitted. Here, the DC transmission system does not have the reactive power and phase angle issues found in AC transmission systems; therefore, the DC power flow controller can only regulate power flow by controlling line voltage and line resistance.
[0003] When regulating power flow by controlling line resistance, a corresponding resistor is typically connected in series in the line using power electronic devices. However, the current in a DC network is generally very large, and introducing new resistance will increase the active power loss of the DC system. Therefore, although resistive DC power flow controllers have advantages such as simple architecture and flexible control, their excessive active power loss hinders their further development.
[0004] When regulating power flow by controlling line voltage, some scholars have proposed using DC transformers to fine-tune the DC port voltage. For example, Professor D. Jovcic and others from the University of Aberdeen, UK, proposed a DC power flow controller composed of a low-voltage circuit, a high-voltage circuit, a centrally grounded energy storage capacitor, and a resonant inductor (D. Jovcic and B. T. Ooi, "Developing DC Transmission Networks Using DC Transformers," in IEEE Transactions on Power Delivery, vol. 25, no. 4, pp. 2535-2543, Oct. 2010.). This scheme can be used to connect DC ports of different voltage levels, improving the flexibility of DC transmission systems. However, DC transformers need to withstand system-level DC voltages, requiring high insulation levels. When used alone for power flow control, it is often not economical, therefore this scheme is generally not adopted for power flow control. Another approach is to connect an adjustable voltage source in series with the transmission line to change the DC voltage and thus control the DC power flow. For example, Dr. Wu Wen et al. from Beijing Jiaotong University proposed a combined topology with series input and parallel output (Wu Wen, Wu Xuezhi, Jing Long, Wei Menghang, Jiang Jiuchun. Modular multi-port DC power flow controller suitable for multi-terminal DC transmission systems [J]. Journal of Electrical Engineering, 2019, 34(03):539-551.), which realizes port expansion and improves system redundancy. Compared with DC transformer type DC power flow controllers, series voltage source type DC power flow controllers have advantages such as bidirectional power flow regulation, low rated power and no need to withstand system-level voltage. However, series voltage source type DC power flow controllers require additional AC / DC systems for energy supply, which leads to a more complex system structure, increases the construction cost of the equipment, and limits the installation location of the power flow controller. The third approach is to connect an adjustable voltage source to adjacent DC transmission lines to jointly control the power flow of the transmission line. Power balance can be achieved through power exchange between the lines without obtaining energy from the external power grid. This is also known as an inter-line type DC power flow controller. Summary of the Invention
[0005] In view of the shortcomings of the prior art, the purpose of this invention is to provide a DC power flow controller and its control method to solve the problems mentioned in the background art.
[0006] The objective of this invention can be achieved through the following technical solutions: A DC power flow controller is used in a multi-terminal flexible DC transmission system. The DC power flow controller includes n adjustable voltage sources, which are connected in series in n adjacent DC lines. None of the adjustable voltage sources require external power supply to provide power or absorb their power. When all currents in the DC power flow controller are in the forward direction, the DC power flow controller contains n complementary power flow branches. Each power flow branch consists of a coupling inductor control switch T1, a coupling inductor L1, and a line capacitor C. i Switching transistor D i2 Switching transistor Q i1 They are connected in series, and the energy between the line capacitors is transferred between the power flow branches through the inductor L1; When all currents in the DC power flow controller are reversed, the DC power flow controller also contains n complementary power flow branches. Each power flow branch is connected to a coupling inductor L2, a coupling inductor control switch T2, and a switching transistor D. i1 Switching transistor Q i2 Line capacitance C i They are connected in series.
[0007] Preferably, the DC power flow controller has both forward and reverse line operation capabilities, providing bidirectional power flow control.
[0008] Preferably, when the line running in the forward direction transfers energy to the line running in the reverse direction, the energy in inductor L1 is transferred to inductor L2 by turning off the coupling inductor control switch T1 and closing the coupling inductor control switch T2, and then the energy is transferred from inductor L2 to the corresponding line capacitor.
[0009] Preferably, the coupled inductors control switches T1 and T2 and switching transistor Q. i1 Q i2 When all three switches are disconnected and bypass switches S1, S2, and S3 are closed, the power flow controller is bypassed and does not participate in DC power flow regulation.
[0010] Preferably, the control of DC power flow at n ports is achieved through 2+2n switching transistors.
[0011] Preferably, the control method of the DC power flow controller includes the following steps: Step 1: Collect the DC current of the lines containing n-1 adjustable voltage sources; Step 2: Set the current reference value; Step 3: Subtract the current reference value from the collected transmission line current. The difference is processed by a PI circuit and logic operation to obtain the PWM switching signals of the n line switching transistors, so that the n power flow branches work in a complementary conduction state, maintaining the transmission line current at a given value.
[0012] The beneficial effects of this invention are: 1. The DC power flow controller of the present invention is applicable to ring network multi-terminal DC transmission applications. Compared with the traditional multi-port line-to-line DC power flow controller, the DC power flow controller of the present invention has bidirectional power flow regulation capability, and only 2+2n switching devices are needed to achieve effective control of n-port DC power flow, thereby improving the control capability of power flow direction of the transmission network and the transmission capability of the transmission line. Attached Figure Description
[0013] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.
[0014] Figure 1 This is the equivalent circuit diagram of a four-terminal VSC-MTDC system; Figure 2 This is a diagram of a four-terminal flexible DC transmission system incorporating the present invention; Figure 3 This is a schematic diagram of the circuit topology of the DC power flow controller of the present invention; Figure 4 This is a block diagram of the control strategy of the DC power flow controller of the present invention; Figure 5 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it is running in the forward direction. Figure 6 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it is running in the forward direction. Figure 7 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it is running in the forward direction. Figure 8 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it operates in reverse. Figure 9 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it operates in reverse. Figure 10 This is a diagram showing the operating modes of the DC power flow controller of the present invention when it operates in reverse. Figure 11 This is a waveform diagram of the line current of the DC power flow controller when all line currents are in the positive direction in this invention. Figure 12 This is a waveform diagram of the capacitor voltage of the DC power flow controller when all line currents are in the positive direction in this invention; Figure 13 This is a waveform diagram of the inductor current of the DC power flow controller when all line currents are in the forward direction in this invention. Figure 14This is a waveform diagram of the line current of the DC power flow controller when a power flow reversal occurs in this invention; Figure 15 This is a waveform diagram of the capacitor voltage of the DC power flow controller when power flow reversal occurs in this invention; Figure 16 This is a waveform diagram of the inductor current of the DC power flow controller when a power flow reversal occurs in this invention. Detailed Implementation
[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.
[0016] This invention proposes a four-terminal flexible DC transmission system including the present invention, such as... Figure 2 As shown, VSC1 is a constant voltage station, and VSC2, 3, and 4 are constant power stations. This invention is connected in series at the DC port of VSC1 to realize the transmission of power between lines 2, 3, and 4.
[0017] The topology diagram of the DC power flow controller of the present invention is shown below. Figure 3 As shown. It includes: three adjustable voltage sources, each connected in series in three adjacent transmission lines, requiring no external power supply; the DC power flow controller, when all currents are in the forward direction, includes three complementary power flow branches, each branch consisting of a coupling inductor control switch T1, a coupling inductor L1, and a line capacitor C. i Switching transistor D i2 Switching transistor Q i1 The power flow branches are connected in series, and the energy between the line capacitors is transferred through inductor L1. When all currents of the DC power flow controller are running in reverse, each power flow branch is connected to coupling inductor L2, coupling inductor control switch T2, and switching transistor D. i1 Switching transistor Q i2 Line capacitance C i The three adjustable voltage sources are connected in series. The DC power flow controller operates in both forward and reverse directions. When the forward-running line transfers energy to the reverse-running line, the energy in inductor L1 is transferred to inductor L2 by turning off the coupling inductor control switch T1 and closing the coupling inductor control switch T2. Then, inductor L2 transfers energy to the corresponding line capacitor. None of the three adjustable voltage sources require external power supply or absorb its power.
[0018] The DC power flow controller of this invention can control the current magnitude on three lines of a power line. The control strategy block diagram is shown below. Figure 4As shown, the current on two transmission lines is collected and compared with a given reference value I. ref The difference is subtracted, and after passing through a PI controller and logic operations, the PWM switching signals for the three line switches are obtained. This enables the three power flow branches to operate in a complementary conduction state, maintaining the transmission line current at a given value. First, the first switch Q is turned on. 11 Under the action of the adjustable voltage source C1, the inductor current I L1 Increase, such as Figure 5 As shown; after a period of time, Q is turned off. 11 At the same time, close the switch Q. 21 Under the action of adjustable voltage source C2, the inductor current I L1 Decrease, such as Figure 6 As shown; after a period of time, Q is turned off. 21 At the same time, close the switch Q. 31 Under the action of adjustable voltage source C3, the inductor current I L1 Increase, such as Figure 7 As shown. In this way, power P is transferred from adjustable voltage sources C1 and C3 to adjustable voltage source C2, so that the line current is maintained at a given current reference value I. ref .
[0019] The technical solution of the present invention will be illustrated below through two specific examples: Implementing the above example requires using Figure 1 The four-terminal ring network flexible DC transmission system shown in the figure operates in constant voltage mode with VSC1 controlled at 10kV, while VSC2, 3, and 4 operate in constant power mode, injecting power of P2=18MW, P3=7MW, and P4=12MW into the system, respectively. The parameters of the DC transmission line are shown in Table 1.
[0020] Table 1 Line parameters of the three-terminal ring network DC transmission system In Example 1, let the current I of line 12 be... 12 The given value I 12_ref The current I of line 13 is 800A. 13 The given value I 13_ref It is 1500A. Figure 11 The current waveforms for lines 12, 13, and 14 are shown in the figure. Figure 12 The voltage waveforms of adjustable voltage sources C1, C2, and C3 are given in the figure. Figure 13 The current waveform of inductor L1 is shown in the figure. As can be seen from the above waveform, the DC power flow controller of the present invention can achieve the predetermined control target, and the voltage of the adjustable voltage source can be stabilized at a constant value, while the current in the inductor also remains constant.
[0021] In Example 2, let the current I of line 12 be... 12 The given value I 12_ref The current I of line 13 remains at 800A. 13 The given value I 13_ref It is -800A. Figure 14 The current waveforms for lines 12, 13, and 14 are shown in the figure. Figure 15 The voltage waveforms of adjustable voltage sources C1, C2, and C3 are given in the figure. Figure 16 The current waveform of inductor L1 is shown in the figure. As can be seen from the waveform, the DC power flow controller can still operate normally when power flow reversal occurs, verifying the bidirectional power flow control capability of the power flow controller and its wide applicability; at the same time, the capacitor voltage and inductor current stabilize at a constant value after a brief fluctuation. In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.
[0022] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.
Claims
1. A DC power flow controller, used in a multi-terminal flexible DC transmission system, characterized in that, The DC power flow controller includes n adjustable voltage sources, which are connected in series in n adjacent DC lines. None of the adjustable voltage sources require external power supply to provide power or absorb its power. When all currents in the DC power flow controller are in the forward direction, the DC power flow controller contains n complementary power flow branches, and each power flow branch is coupled to an inductor to control a switch. Coupled inductor Line capacitors Switching transistor Switching transistor The power flow branches are connected in series, with inductors connecting them. To transmit energy between line capacitors; When all currents in the DC power flow controller are reversed, the DC power flow controller also contains n complementary power flow branches, each of which is coupled to an inductor. Coupled inductor control switch Switching transistor Switching transistor Line capacitors It is formed by connecting them in series; The DC power flow controller has both forward and reverse line operation capabilities, providing bidirectional power flow control. When the line running in the forward direction transfers energy to the line running in the reverse direction, the control switch is activated by turning off the coupling inductor. Closed-loop coupling inductor control switch Inductor Energy is transferred from the inductor Up, then by inductor Energy is transferred to the corresponding line capacitor; The control of DC power flow across n ports is achieved through 2+2n switching transistors.
2. A DC power flow controller according to claim 1, characterized in that, The coupling inductor controls the switch , With switching transistor , All are disconnected, and the bypass switch is also disconnected. , , When both are closed, the power flow controller is bypassed and does not participate in DC power flow regulation.
3. A DC power flow controller according to claim 1, characterized in that, The switches on n different branches are complementary and conduct.
4. A DC power flow controller according to claim 1, characterized in that, The adjustable voltage source is a capacitor.
5. A DC power flow controller according to claim 1, characterized in that, n is a positive integer greater than or equal to 2.
6. A DC power flow controller according to any one of claims 1 to 5, characterized in that, The control method of the DC power flow controller includes the following steps: Step 1: Collect the DC current of the lines containing n-1 adjustable voltage sources; Step 2: Set the current reference value; Step 3: Subtract the current reference value from the collected transmission line current. The difference is processed by a PI circuit and logic operation to obtain the PWM switching signals of the n line switching transistors, so that the n power flow branches work in a complementary conduction state, maintaining the transmission line current at a given value.