Direct-current power grid integrated equipment with power flow control and fault breaking functions and control method

Abstract

The invention belongs to the technical field of direct-current power grid power flow control and fault processing, and provides direct-current power grid integrated equipment with power flow control and fault breaking and a method, the direct-current power grid integrated equipment comprises a plurality of ports, a power flow controller and a thyristor branch, the number of the ports is set according to the number of incoming and outgoing lines of a direct-current bus, and the thyristor branch is connected with the power flow controller. The power flow controller comprises an energy storage capacitor and a plurality of bridge arms connected in parallel, each port is connected to the corresponding bridge arm, each bridge arm comprises an upper bridge arm and a lower bridge arm, each bridge arm is provided with a switching tube, one end of a thyristor branch is connected to the upper bridge arm, and the other end of the thyristor branch is connected to the port; when the system operates normally, power flow control of a direct-current line is carried out by controlling on-off of a switching tube, and when a fault occurs, fault current is converted to a thyristor branch. The active power flow control and the fault selective isolation of the flexible DC power grid are realized, and the power grid equipment cost and the operation loss are reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of flexible direct current (DC) grid power flow control and fault processing, and in particular relates to a DC grid integrated device with power flow control and fault disconnection and a control method. Background Art

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Flexible DC grids have the advantages of independent control of active and reactive power, no need to change voltage polarity when power flow is reversed, and the ability to power passive systems. Compared with traditional AC grids, they are more suitable for large-scale renewable energy grid connection and transmission. However, flexible DC grids currently face challenges such as difficulty in power flow control and susceptibility to DC side fault interference, which seriously restricts the further development of flexible DC grids.

[0004] In a flexible DC grid, since there is usually more than one current path between two nodes, the power flow of each DC line in it cannot be accurately controlled by the converter station. This uncontrollability may cause the DC line to overload in some operating scenarios, thereby affecting the safe operation of the flexible DC grid. The DC power flow controller can actively control the power flow of each DC line, but since it is mainly composed of fully controlled semiconductor devices, the manufacturing cost is high and the operating loss is large. In addition, when a short circuit occurs in the DC line, in order to avoid the expansion of the fault range, it is usually necessary to configure a DC circuit breaker at both ends of each DC line, and the DC circuit breaker is operated to isolate the fault. However, under the existing technical level, the DC circuit breaker contains a large number of fully controlled semiconductor devices, so the manufacturing cost is very high, and the demand for DC circuit breakers is large, and the total cost is difficult to afford. Summary of the invention

[0005] In order to solve at least one technical problem existing in the above-mentioned background technology, the first aspect of the present invention provides a DC grid integrated device with power flow control and fault disconnection, which reduces the cost and operating loss of grid equipment while realizing active power flow control and fault selective isolation of flexible DC grid.

[0006] In order to achieve the above object, the present invention adopts the following technical solution:

[0007] A DC grid integrated device with power flow control and fault disconnection comprises a plurality of ports, a power flow controller and a thyristor branch, wherein the number of the ports is set according to the number of inlet and outlet lines of a DC bus, the power flow controller comprises a plurality of bridge arms connected in parallel and energy storage capacitors connected in parallel to both ends of the bridge arms, each port is connected to a corresponding bridge arm, each bridge arm comprises an upper bridge arm and a lower bridge arm, each bridge arm is provided with a switch tube, one end of the thyristor branch is connected to the upper bridge arm, and the other end is connected to the port; when the system operates normally, the power flow of the DC line is controlled by controlling the on and off of the switch tube, and when a fault occurs, the fault current is commutated to the thyristor branch.

[0008] As an implementation method, a fast mechanical switch and a residual current switch are provided between each port and the corresponding bridge arm connection line.

[0009] As an implementation manner, the power flow controller further includes a bypass switch and a first current limiting resistor, and the bypass switch and the first current limiting resistor are connected in series and then in parallel to both ends of the energy storage capacitor.

[0010] As an implementation mode, the system also includes a common branch for each port, the common branch and the thyristor branch are connected in parallel, and the thyristor branch is provided with a first thyristor; the common branch includes a power supply side circuit breaker unit and a fault side circuit breaker unit connected in series, the fault side circuit breaker unit includes a first capacitor, a first discharge resistor and a first lightning arrester connected in parallel, the power supply side circuit breaker unit includes a second thyristor, a second current limiting resistor, a second capacitor, a second discharge resistor and a second lightning arrester, and the second capacitor, the second discharge resistor and the second lightning arrester are connected in parallel and then connected in series with the second thyristor and the current limiting resistor.

[0011] As an implementation mode, the system further comprises a plurality of diode branches, one end of each diode branch is connected to a corresponding port, and the other end of each diode branch is connected to a thyristor branch.

[0012] In order to solve the above problems, the second aspect of the present invention provides a flexible DC grid flow control method considering fault segmentation, based on the DC grid integrated device with flow control and fault disconnection described in the first aspect, which reduces the grid equipment cost and operating loss while realizing active flow control and fault selective isolation of the flexible DC grid.

[0013] In order to achieve the above object, the present invention adopts the following technical solution:

[0014] When receiving the power flow control command issued by the flexible DC grid, each switch tube is periodically turned on and off according to the corresponding control strategy, so as to control the energy storage capacitor voltage and DC line current to reach the control voltage reference value and the grid power flow control command value respectively;

[0015] When a short circuit fault occurs in a DC line, the location of the fault point is determined, and the action timing of isolating the corresponding fault port is determined according to the location of the fault point. The fault is isolated according to the action timing of isolating the corresponding fault port.

[0016] As an implementation method, when the system operates normally or processes power flow control, the fast mechanical switch and the residual current switch are closed.

[0017] As an implementation mode, when a short circuit fault occurs in a DC line, determining the location of the fault point, determining an action sequence of isolating a corresponding fault port according to the location of the fault point, and isolating the fault according to the action sequence of isolating the corresponding fault port specifically includes:

[0018] When a short circuit fault occurs in the DC line, the first thyristor is turned on, and all the lower bridge arm switch tubes and the upper bridge arm switch tubes connected to the corresponding fault port are turned on, the first capacitor is charged by the fault current, and the voltage across the capacitor gradually rises until it reaches the residual voltage value of the first lightning arrester;

[0019] When the current of the fast mechanical switch becomes zero, the fast mechanical switch is controlled to start opening;

[0020] After the fast mechanical switch action is completed, the second thyristor is turned on, the second capacitor is charged via the first capacitor, the fault current and the charging current of the second capacitor flow through the first thyristor at the same time, the charging current amplitude is adjusted to be greater than the fault current amplitude by controlling the first discharge resistor, and the first thyristor is locked;

[0021] After the first thyristor is locked, the voltage across the second capacitor gradually rises until it reaches the action condition of the second lightning arrester, and the voltage of the second thyristor becomes zero, completing the fault isolation.

[0022] As an implementation method, the control strategy is specifically as follows: one of the ports adopts energy storage capacitor voltage control, and the other ports adopt DC line current control.

[0023] As an implementation mode, when one port adopts energy storage capacitor voltage control, the duty cycle of the switch tube corresponding to the port is controlled according to the deviation between the energy storage capacitor voltage measurement value and the energy storage capacitor voltage reference value; when the remaining ports adopt DC line current control, the duty cycle of the switch tube corresponding to the port is controlled according to the deviation between the DC line current measurement value and the DC line current reference value.

[0024] The beneficial effects of the present invention are:

[0025] 1. When the system of the present invention operates normally, the flow of the DC line is controlled by controlling the on and off of the switch tube. When a fault occurs, the fault current is commutated to the thyristor branch, which can control the flow of the DC line connected to all ports, thereby avoiding overload of the DC line and improving the flexibility of the flexible DC grid operation; at the same time, the DC line faults connected to all ports can be isolated to avoid the expansion of the fault range;

[0026] 2. The number of ports of the present invention is set according to the number of inlet and outlet lines of the DC bus; the number of ports is set according to the number of inlet and outlet lines of the DC bus;

[0027] 3. The device of the present invention is mainly composed of thyristors and only requires a small number of fully controlled semiconductor devices, so the manufacturing cost is relatively low.

[0028] Advantages of additional aspects of the present invention will be given in part in the following description, and in part will become obvious from the following description, or will be learned through practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0030] Figure 1 It is a topological structure diagram of a flexible DC power grid power flow control system considering fault segmentation provided by an embodiment of the present invention;

[0031] Figure 2 is a line current control block diagram provided by an embodiment of the present invention;

[0032] Figure 3 is a block diagram of energy storage capacitor voltage control provided by an embodiment of the present invention;

[0033] Figure 4 It is the current simulation waveform of each port in the power flow control process provided by the embodiment of the present invention;

[0034] Figure 5 This is a simulated waveform of current at each port during the fault isolation process provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0035] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0036] It should be noted that the following detailed descriptions are all illustrative and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0037] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit exemplary embodiments according to the present invention. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0038] In the present invention, terms such as "connected" and "connection" should be understood in a broad sense, indicating that the connection can be fixed, integral or detachable; it can be directly connected or indirectly connected through an intermediate medium. Relevant scientific research or technical personnel in this field can determine the specific meaning of the above terms in the present invention according to specific circumstances, and they should not be understood as limiting the present invention.

[0039] like Figure 1 FIG. 1 shows a basic topology diagram of a flexible DC grid power flow control system considering fault segmentation. The system includes multiple ports, an embedded power flow controller (E-CFC) and a thyristor branch T m The number of ports is set according to the number of input and output lines of the DC bus, and the ports can be expanded according to the number of input and output lines of the connected DC bus;

[0040] The power flow controller includes a storage capacitor and a plurality of bridge arms connected in parallel, each port is connected to a corresponding bridge arm, each bridge arm includes an upper bridge arm and a lower bridge arm, each bridge arm is provided with a switch tube, one end of the thyristor branch is connected to the upper bridge arm, and the other end is connected to the port;

[0041] When the system is operating normally, the flow of the DC line is controlled by controlling the on and off of the switch tube. When a fault occurs, the fault current is commutated to the thyristor branch T m .

[0042] The above-mentioned equipment is mainly composed of thyristors and only requires a small number of fully-controlled semiconductor devices, so the manufacturing cost is relatively low.

[0043] The power flow controller also includes a bypass switch and a first current limiting resistor R dc , the bypass switch and the first current limiting resistor R dc After being connected in series, they are connected in parallel to both ends of the energy storage capacitor.

[0044] Wherein, a fast mechanical switch (UFD) and a residual current switch (RCB) are arranged between each port and the corresponding bridge arm connection line.

[0045] The system further comprises a first common branch and a second common branch shared by each port, wherein the first common branch is provided with a first thyristor T m, the second common branch includes a source side circuit breaker unit (SBU) and a fault side circuit breaker unit (FBU) connected in series;

[0046] The fault side circuit breaker unit (FBU) is composed of three parallel components, including a first capacitor C c , the first discharge resistor R c and the first arrester A c ;

[0047] The power side circuit breaker unit (SBU) includes a second thyristor T d , the second current limiting resistor R d1 , the second capacitor C d , the second discharge resistor R d2 and the second arrester A d , where the second capacitor C d , the second discharge resistor R d2 and the second arrester A d After connecting in parallel with the second thyristor T d and the second current limiting resistor R d1 Series connection.

[0048] The system further comprises a plurality of diode branches, one end of each diode branch is connected to a corresponding port, and the other end of each diode branch is connected to the thyristor branch.

[0049] The embedded power flow controller (E-CFC) realizes active control of the line power flow during normal operation; and when a short-circuit fault occurs, it is responsible for commutating the fault current to the thyristor branch T m .

[0050] The power flow control strategy of E-CFC is as follows Figure 2 and Figure 3 As shown, when performing flow control, one port uses Figure 3 The energy storage capacitor voltage control method shown in the figure, all other ports use Figure 2 The line current control method shown.

[0051] If port x uses Figure 3 The energy storage capacitor voltage control method shown is based on the capacitor voltage measurement value U c And capacitor voltage reference value U cref The deviation of the switch tube S corresponding to port x ux , S dx Similarly, if port y adopts Figure 2 The line current control method shown is based on the line current measurement value I y and line current reference value I ref,y The deviation of the switch tube S corresponding to port y uy , Sdy The duty cycle is controlled.

[0052] Thyristor branch T m Used to carry the fault current during UFD opening process;

[0053] When the system is working normally, the flexible DC grid does not require the system to provide active control function of the power flow. Therefore, the states of each switch in the system are shown in Table 1. In Table 1, ON represents the on or closed state, and OFF represents the locked or open state.

[0054] Table 1 Summary of the switch status of the integrated device during normal operation

[0055]

[0056]

[0057] Flow Control:

[0058] When the flexible DC grid issues a power flow control command, the integrated device enters the power flow control mode, and the states of the switches in the integrated device are shown in Table 2.

[0059] Table 2 Summary of switch states of integrated equipment during power flow control

[0060] Switch Name Switch Status <![CDATA[S u ]]> according to Figure 2 and Figure 3 The control mode is periodically in the ON and OFF state <![CDATA[S d ]]> according to Figure 2 and Figure 3 The control mode is periodically in the ON and OFF state <![CDATA[S dc ]]> OFF UFD ON RCB ON <![CDATA[T m ]]> OFF <![CDATA[T d ]]> OFF

[0061] At this time, the switches S of the E-CFC in the integrated device u1 ~S un , S d1 ~S dn according to Figure 2 and Figure 3 The control mode shown is periodically turned on and off, so as to control the energy storage capacitor voltage and line current to reach the control reference value U cref ,I ref,k Among them, U cref is the default value, I ref,k Obtained according to the power grid power flow control instructions.

[0062] Through the above solution, the power flow of the DC lines connected to all ports can be controlled, thereby avoiding overload of the DC lines and improving the operational flexibility of the flexible DC grid.

[0063] Fault Isolation:

[0064] When a short circuit fault occurs in the DC line, assuming that the fault point is located in the DC line connected to port P1, the integrated device starts to isolate the faulty port P1, and the action sequence is as follows:

[0065] (1) Turn on the thyristor branch T m And the switch S in E-CFCu1 , S d1 ~S dn At this time, the fault current is commutated to the thyristor branch T m ; At this time, the capacitor C in FBU c The fault current will charge the arrester, and the voltage at both ends will gradually rise and reach the arrester A. c The residual voltage value.

[0066] (2) When the current of UFD1 becomes zero, UFD1 is controlled to start opening, and its action time is usually considered to be 2 milliseconds.

[0067] (3) After the UFD1 action is completed, the thyristor T in the SBU d Apply a high level to control its conduction. At this time, the capacitor C in SBU d will pass through capacitor C c Charging, thyristor branch T m The fault current and capacitor C will flow through d The charging current is controlled by the current limiting resistor R d1 The resistance value adjusts the charging current amplitude to be greater than the fault current amplitude. m The current will pass through zero, thus blocking the thyristor branch T m .

[0068] (4) Waiting for the thyristor branch to be locked T m After that, the capacitor C d Under the action of the system voltage, the voltage at both ends will gradually increase until it reaches the arrester A. d The action condition is to wait for the current flowing through the thyristor T d When the current becomes zero, it will meet the blocking condition and then block, and the fault port current will continue to flow through the FBU and gradually decrease to zero. At this point, the fault isolation process of the integrated device is completed.

[0069] Through the above solution, the DC line faults connected to all ports can be isolated to prevent the fault scope from expanding.

[0070] Simulation Verification

[0071] A three-terminal flexible DC grid simulation model was established in PSCAD / EMTDC software to verify the power flow control and fault isolation performance of the integrated device proposed in the present invention. The simulation results are shown in Figure 2. Figure 4 and Figure 5 Set the start time of the flow control function to 4.0 seconds, and the flow control target is: i p2 、i p3 The load currents are 1.0kA and 0.5kA respectively; the fault occurs at 5.0 seconds, the fault port is P3, so the fault port current is ip3 .

[0072] Depend on Figure 4 It can be seen that the integrated device can reliably control the power flow according to the requirements of the power grid. Figure 5 It can be seen that the fault port current i p2 It decays to zero a few milliseconds after the fault occurs, indicating that the integrated device can reliably interrupt the fault current.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A DC grid integrated device with power flow control and fault disconnection, characterized in that: It includes multiple ports, a power flow controller and a thyristor branch. The number of the ports is set according to the number of inlet and outlet lines of the DC bus. The power flow controller includes multiple bridge arms connected in parallel and energy storage capacitors connected in parallel to both ends of the bridge arms. Each port is connected to a corresponding bridge arm. Each bridge arm includes an upper bridge arm and a lower bridge arm. Each bridge arm is provided with a switch tube. One end of the thyristor branch is connected to the upper bridge arm, and the other end is connected to the port. When the system is operating normally, the flow of the DC line is controlled by controlling the on and off of the switch tube. When a fault occurs, the fault current is commutated to the thyristor branch.

2. The DC grid integrated device with power flow control and fault disconnection according to claim 1, characterized in that: A fast mechanical switch and a residual current switch are arranged between each port and the corresponding bridge arm connection line.

3. The DC grid integrated device with power flow control and fault disconnection according to claim 1, characterized in that: The power flow controller further includes a bypass switch and a first current limiting resistor. The bypass switch and the first current limiting resistor are connected in series and then in parallel to two ends of the energy storage capacitor.

4. The DC grid integrated device with power flow control and fault disconnection according to claim 1, characterized in that: The system also includes a common branch for each port, the common branch and the thyristor branch are connected in parallel, and the thyristor branch is provided with a first thyristor; the common branch includes a power supply side circuit breaker unit and a fault side circuit breaker unit connected in series, the fault side circuit breaker unit includes a first capacitor, a first discharge resistor and a first lightning arrester connected in parallel, the power supply side circuit breaker unit includes a second thyristor, a second current limiting resistor, a second capacitor, a second discharge resistor and a second lightning arrester, and the second capacitor, the second discharge resistor and the second lightning arrester are connected in parallel and connected in series with the second thyristor and the current limiting resistor.

5. The DC grid integrated device with power flow control and fault disconnection according to claim 1, characterized in that: The system further comprises a plurality of diode branches, one end of each diode branch is connected to a corresponding port, and the other end of each diode branch is connected to the thyristor branch.

6. A control method for a DC grid integrated device with power flow control and fault disconnection according to any one of claims 1 to 5, comprising the following steps: When receiving the power flow control command issued by the flexible DC grid, each switch tube is periodically turned on and off according to the corresponding control strategy, so as to control the energy storage capacitor voltage and DC line current to reach the control voltage reference value and the grid power flow control command value respectively; When a short circuit fault occurs in a DC line, the location of the fault point is determined, and the action timing of isolating the corresponding fault port is determined according to the location of the fault point. The fault is isolated according to the action timing of isolating the corresponding fault port.

7. The control method of the DC grid integrated device with power flow control and fault disconnection according to claim 6, characterized in that: When the system is operating normally or handling power flow control, the fast mechanical switch and the residual current switch are closed.

8. The control method of the DC grid integrated device with power flow control and fault disconnection according to claim 6, characterized in that: When a short circuit fault occurs in the DC line, the location of the fault point is determined, the action sequence of isolating the corresponding fault port is determined according to the location of the fault point, and the fault is isolated according to the action sequence of isolating the corresponding fault port, specifically including: When a short circuit fault occurs in the DC line, the first thyristor is turned on, and all the lower bridge arm switch tubes and the upper bridge arm switch tubes connected to the corresponding fault port are turned on; the first capacitor is charged by the fault current, and the voltage across the capacitor gradually rises until it reaches the residual voltage value of the first lightning arrester; When the current of the fast mechanical switch becomes zero, the fast mechanical switch is controlled to start opening; After the fast mechanical switch action is completed, the second thyristor is turned on, the second capacitor is charged via the first capacitor, the fault current and the charging current of the second capacitor flow through the first thyristor at the same time, the charging current amplitude is adjusted to be greater than the fault current amplitude by controlling the first discharge resistor, and the first thyristor is locked; After the first thyristor is locked, the voltage across the second capacitor gradually rises until it reaches the action condition of the second lightning arrester, and the voltage of the second thyristor becomes zero, completing the fault isolation.

9. The control method of the DC grid integrated device with power flow control and fault disconnection according to claim 6, characterized in that: The specific control strategy is: one of the ports adopts energy storage capacitor voltage control, and the other ports adopt DC line current control.

10. The control method of the DC grid integrated device with power flow control and fault disconnection according to claim 9, characterized in that: When one port adopts energy storage capacitor voltage control, the duty cycle of the switch tube corresponding to the port is controlled according to the deviation between the energy storage capacitor voltage measurement value and the energy storage capacitor voltage reference value; when the remaining ports adopt DC line current control, the duty cycle of the switch tube corresponding to the port is controlled according to the deviation between the DC line current measurement value and the DC line current reference value.

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