Fault isolation device and method suitable for new energy multistage boost collection system

Abstract

The invention discloses a fault isolation device and method suitable for a new energy multistage boost collection system, and the device comprises a high-voltage DC / DC converter, a current limiting reactor, and a DC damper. The direct-current damper further comprises a direct-current damper connected to the positive electrode of the high-voltage DC / DC converter and a direct-current damper connected to the negative electrode of the high-voltage DC / DC converter, the direct-current damper comprises a current-carrying branch, a transfer branch and a damping branch, the current-carrying branch realizes current passing control, and the transfer branch is connected with the damping branch. The transfer branch circuit is used for improving the voltage level of the IGBT valve group and providing a channel for fault current transfer; and the damping branch circuit realizes quick clearing of fault current. According to the invention, an economical and reliable fault isolation scheme is realized especially under the condition that the number of collection branches of a high-voltage direct-current collection side direct-current line in a new energy multi-stage boost direct-current collection system is large.

Description

Technical Field

[0001] This invention relates to the field of protection and control of DC transmission systems, and specifically to a novel fault isolation scheme for DC lines. Background Technology

[0002] The high-voltage collection side of a multi-stage boost DC collection system for new energy sources has many branches, making fault isolation difficult. When a DC fault occurs on the high-voltage collection side of such a system, the overcurrent will severely affect the safe and reliable operation of the entire system. Therefore, in the event of a DC fault, it is required to quickly disconnect the faulty line, which necessitates the use of fault isolation methods. Currently, there is limited research on fault isolation schemes suitable for multi-stage boost DC collection systems for new energy sources. The existing schemes mainly reference multi-terminal flexible DC systems, primarily including two technical solutions: the "self-clearing MMC + high-speed switch (HSS)" scheme and the "half-bridge MMC + DC circuit breaker (DCCB)" scheme.

[0003] The "self-clearing MMC+HSS" scheme achieves rapid clearing of DC fault current by locking out sub-modules and connecting the sub-module capacitor voltage to the fault circuit with reverse polarity, while also isolating the fault area with HSS. Examples include the reverse-resistance self-equalizing sub-module proposed in the literature "Bipolar DC / DC Converter Applicable to DC-Clustered Wind Farm Transmission Systems and Its Short-Circuit Current Limiting," the series dual sub-module proposed in the literature "Improved Topology of Modular Multilevel Converters and Its Application," and the hybrid sub-module combining half-bridge and T-bridge sub-modules proposed in the literature "A High-Voltage High-Power DC / DC Converter Based on a Hybrid Modular Multilevel Converter." When this scheme is applied to new energy DC-Clustered systems, the high-voltage DC / DC converters on the high-voltage collection side all use sub-modules with self-clearing capabilities for fault isolation, which significantly increases investment costs, operating losses, and control system complexity. Furthermore, during fault isolation, all DC / DC converters in the system need to be locked out, leading to an interruption of the entire system's power transmission and adversely affecting power supply reliability and operational stability.

[0004] The "half-bridge MMC+DCCB" scheme uses a DC breaker to cut off DC fault circuits, achieving rapid clearing of DC faults and ensuring uninterrupted power transmission from the intact system. For new energy DC aggregation systems, which have numerous high-voltage aggregation lines, this fault isolation scheme requires installing a DC breaker at both ends of each aggregation line, resulting in a large footprint and very high cost. Furthermore, if the commonly used hybrid DC breaker (HDCCB) is used, the metal oxide varistors (MOVs) of the HDCCB consume extremely high energy during a single fault isolation, leading to a large number of parallel MOVs and difficulties in dynamic current sharing. With the future development and manufacturing of new, low-cost circuit breakers, the economic viability and applicability of this scheme require further evaluation.

[0005] In response to this situation, based on the operating characteristics of high-voltage DC / DC converters and considering the economy and reliability of fault isolation, this invention designs a reasonable fault isolation scheme for new energy DC collection systems to ensure rapid isolation of DC line faults. Summary of the Invention

[0006] For new energy multi-stage boost DC collection and transmission systems, this invention proposes a fault isolation device and method suitable for new energy multi-stage boost DC collection systems. It designs a DC damper in conjunction with the blocking of the high-voltage DC / DC converter to achieve rapid clearing of DC fault current on the new energy side during fault isolation.

[0007] This invention is achieved using the following technical solution:

[0008] In a first aspect, this invention proposes a fault isolation device suitable for multi-stage boost-and-collection systems in new energy sources, comprising a high-voltage DC / DC converter, a current-limiting reactor, and a DC damper. The DC damper further comprises a first DC damper connected to the positive terminal of the high-voltage DC / DC converter and a second DC damper connected to the negative terminal of the high-voltage DC / DC converter. The DC damper includes a current-carrying branch, a transfer branch, and a damping branch. The current-carrying branch enables current flow control; the transfer branch increases the voltage level of the IGBT valve group, providing a channel for fault current transfer; and the damping branch enables rapid clearance of fault current.

[0009] In some embodiments, the current-carrying branch includes an ultra-fast mechanical switch and a load transfer switch connected in series, the transfer branch includes IGBT modules connected in series to form an IGBT valve group or a parallel structure of IGBT valve groups, and the damping branch includes a damping resistor connected in series.

[0010] In some implementations, the calculation formula for the damping resistor parameters is as follows:

[0011]

[0012] Among them, I R_max I represents the maximum value of the fault current transferred to the damping branch. RCB L represents the breaking value of the DC switch. F For current-limiting reactors, L s R is the equivalent inductance of MMC. s t is the equivalent resistance of MMC. set This indicates the control setting time.

[0013] In some embodiments, the device includes a three-stage fault current transfer path: a fault detection stage, in which a bipolar short-circuit fault occurs in the DC line; the fault current flows through the current-carrying branch, the DC line current rises rapidly, and the damping resistor R... damp It is in a bypassed state;

[0014] During the current transfer phase, the fault current is switched to the transfer branch: As the fault is detected and located, the IGBT valve group is shut off. At this time, the main branch current is forced to zero, providing a zero-current condition for the opening of the ultra-fast mechanical switch (UFD). The ultra-fast mechanical switch begins to open, the fault current is switched to the transfer branch, and the MMC of the high-voltage DC / DC converter at the corresponding fault location is blocked. The ultra-fast mechanical switch reaches the maximum rated opening distance, and the opening is completed.

[0015] Current damping stage: After the current transfer, the IGBT valve group of the transfer branch is turned off, and the fault current is commutated to the damping branch. Under the action of the damping resistor, it begins to decay rapidly until the decay ends. Then, the fault point is cut off by the DC switch to achieve physical isolation between the faulty line and the converter.

[0016] In some embodiments, the high-voltage DC / DC converter includes two face-to-face half-bridge MMCs connected together, and the DC-side equivalent circuit of the half-bridge MMCs includes a series structure of equivalent resistance, equivalent inductance and equivalent capacitance.

[0017] In some embodiments, the system further includes a residual current switch installed at the outlet of the high-voltage DC / DC converter, with the DC damper body configured in series on the outlet side of the high-voltage DC / DC converter.

[0018] Secondly, this invention proposes a fault isolation method applicable to multi-stage boost-and-collection systems for new energy sources, the method comprising:

[0019] In the new energy multi-stage voltage boosting and aggregation system, a DC circuit breaker is installed near the aggregation bus on each aggregation line, and a DC damper is installed near the high-voltage DC / DC converter. Current-limiting inductors are used to limit the rate of rise of short-circuit current and the peak value of short-circuit current. When any high-voltage DC aggregation transmission line fails, the MMC of the high-voltage DC / DC converter of the faulty line is blocked, and the DC damper is activated to accelerate the attenuation of the fault current on the DC / DC side. The DC circuit breaker on the high-voltage DC bus aggregation side is used to isolate the fault point from the remaining aggregation network. The high-voltage DC is stepped up to ultra-high-voltage DC by the ultra-high-voltage DC / DC converter and then transmitted over long distances through the ultra-high-voltage DC line.

[0020] Compared with the prior art, the beneficial technical effects achieved by this invention are as follows: an economical DC damper is designed, which, in conjunction with the blocking of the high-voltage DC / DC converter, enables rapid clearing of DC fault current on the new energy side during fault isolation, and has certain advantages in both economy and breaking performance; an economical and reliable fault isolation scheme is realized; especially for the case of multiple DC line collection branches on the high-voltage DC collection side in a multi-stage boost DC collection system for new energy, a new type of DC line fault isolation is realized. Attached Figure Description

[0021] Figure 1 Typical structural diagrams of a multi-stage boosting and collection system for new energy; (1a) system structure, (1b) wind farm structure, (1c) photovoltaic farm structure;

[0022] Figure 2 This is a structural diagram of a fault isolation system applicable to a multi-stage boost and collection system for new energy sources, according to an embodiment of the present invention.

[0023] Figure 3 This is a structural diagram of the DC damper of the present invention.

[0024] Figure 4 A diagram showing the specific installation location of the RCB.

[0025] Figure 5 The following are schematic diagrams of the fault current transfer paths at different stages of a specific embodiment of the fault isolation device for a new energy multi-stage boosting and collection system of the present invention: (5a) fault detection stage, (5b) current transfer stage, and (5c) current damping stage.

[0026] Figure 6 This is a schematic diagram of the current transfer path when considering the most severe fault situation in the DC damper parameter design of the present invention, (6a) fault detection stage, (6b) current transfer stage.

[0027] Figure 7 This is a schematic diagram of the fault isolation configuration for a high-voltage collection line fault according to an embodiment of the present invention. Detailed Implementation

[0028] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings, but this is not intended to limit the present invention.

[0029] Figure 1 A typical structural diagram of a multi-stage boosting and collection system for new energy is shown. As shown in Figure (1a), the DC power output from the photovoltaic array of power station n is connected to the medium-voltage DC / DC converter via a boost circuit. The AC power output from the wind turbine of power station 1 is rectified and then boosted to medium-voltage DC by the medium-voltage DC / DC converter and connected to the medium-voltage collection bus. At the outlets of the wind farm and the photovoltaic power station, the voltage is boosted to high-voltage DC by the high-voltage DC / DC converter and sent out via high-voltage lines to the high-voltage DC collection bus. Finally, the DC voltage is boosted by the ultra-high-voltage DC / DC converter and converted into high-frequency high-voltage AC by the inverter DC / AC. The DC power from the photovoltaic power station and the wind farm is transmitted over long distances via ultra-high-voltage DC lines. Figure (1b) shows the wind farm and Figure (1c) shows the photovoltaic power station. The high-voltage DC system (shown in the dashed box on the right in Figure (1a)) usually adopts a pseudo-bipolar structure, and the system grounding method adopts a transformer star-side neutral point grounding scheme through a large resistor. When a single-pole ground fault occurs in the system, it is assumed that the bridge arm submodule capacitor has no discharge path, and only the faulty DC line-to-ground capacitor forms a discharge circuit through the grounding point. Since only the distributed capacitor of the faulty line discharges to the fault point, the peak fault current is small, and there is no risk of overcurrent. Therefore, this invention mainly analyzes the two-pole short-circuit fault of the high-voltage collection and transmission line, which poses a greater hazard. Since this invention mainly studies the fault isolation method on the high-voltage DC side, in order to simplify the analysis, the medium-voltage DC transmission port of the new energy power station (as shown in the dashed box on the left in Figure (1a)) is equivalent to a DC voltage source.

[0030] Example 1

[0031] Figure 2 This invention illustrates a fault isolation device suitable for a multi-stage boost and collection system for new energy sources, comprising a high-voltage DC / DC converter and a current-limiting reactor L. F A DC damper connected to the positive terminal of the high-voltage DC / DC converter and a DC damper connected to the negative terminal of the high-voltage DC / DC converter.

[0032] High-voltage DC / DC converters typically employ two half-bridge MMCs connected face-to-face, with a centralized transformer used to change the voltage gain in the intermediate AC stage. While traditional half-bridge MMCs cannot achieve fault current self-blocking, a high-voltage DC / DC converter composed of two face-to-face connected half-bridge MMCs achieves fault current self-blocking by blocking the primary and secondary sides. However, the fault current on both sides of the faulty line needs to be dissipated using the line's equivalent resistance and the on-state resistance of power electronic devices, making the fault current clearing process lengthy. During normal operation, the equivalent circuit viewed from the DC side is considered as an equivalent resistance R. s Equivalent inductance L s and equivalent capacitance C s The three components are connected in series. Current-limiting reactor L F It is installed at the output end of the high-voltage DC / DC converter to limit the rate of rise of the short-circuit current and the peak value of the short-circuit current.

[0033] Figure 3 This invention illustrates a DC damper for a fault isolation device applicable to a multi-stage boost-and-collection system for new energy applications. Its structure includes a current-carrying branch, a transfer branch, and a damping branch. The current-carrying branch includes an UltraFast Disconnector (UFD) and a Load Commutation Switch (LCS), connected in series. The transfer branch comprises a group of multiple IGBT modules forming an IGBT valve group, with each IGBT module connected in series. Multiple IGBT modules in the transfer branch can be connected in parallel after being connected in series. This branch increases the voltage level of the IGBT valve group, providing a path for fault current transfer. Furthermore, the power electronic components required for this branch design are significantly reduced compared to the transfer branch of a hybrid DC circuit breaker. The damping branch includes a set of damping resistors R. damp Similarly, each damping resistor R damp These circuits are connected in series, and this branch is used to accelerate the decay of fault current, achieving rapid clearance of fault current. Additionally, a residual current breaker (RCB) is used to open / close residual currents with smaller amplitudes.

[0034] Compared with MOV, damping resistor technology is more mature, easier to manufacture, and more economical, making it highly valuable for engineering applications.

[0035] Figure 4 The installation locations of the residual current switch (RCB) and the DC damper on the high-voltage DC / DC converter side are shown. The RCB is installed at the outlet of the high-voltage DC / DC converter, and the DC damper body is configured in series on the outlet side of the high-voltage DC / DC converter.

[0036] Figure 5 The following are illustrated specific embodiments of the fault isolation device of the present invention applicable to multi-stage boost-and-collection systems for new energy, showing fault current transfer paths at different stages, including:

[0037] (1) Fault detection stage: This stage is when a short circuit fault occurs between two poles of the DC line; the fault current still flows through the current-carrying branch, and the DC line current rises rapidly. At this time, the damping resistor R damp It remains in a bypass state and will not cause additional power loss to the normal operation of the new energy multi-stage boost DC collection system. The current flow path during the fault detection phase is shown in Figure (5a).

[0038] (2) Current transfer stage: This stage involves the fault current being transferred to the transfer branch. As the fault is detected and located, the IGBT valve group is shut off. At this time, the main branch current is forced to zero, providing a zero-current condition for the opening of the ultra-fast mechanical switch (UFD). The UFD begins to open, and the fault current is transferred to the transfer branch. At the same time, the MMC of the high-voltage DC / DC converter at the corresponding fault location is blocked. The UFD reaches its maximum rated opening distance, and the opening is completed. The current flow path during the current transfer stage is shown in Figure (5b).

[0039] (3) Current damping stage: After the current transfer, the IGBT valve group of the transfer branch is turned off, and the fault current is switched to the damping branch. Under the action of the damping resistor, it begins to decay rapidly. When the DC current decays to a certain value, the fault point can be cut off by the DC switch to achieve physical isolation between the fault line and the converter. The current flow path in the current damping stage is shown in Figure (5c).

[0040] Specifically, the damping resistor R damp The selection of the damping resistor must meet certain constraints. The first step in parameter designing the damping resistor value is to determine the maximum value of the fault current. Considering the most severe fault scenario, the fault occurs at the head end of the transmission line; this serves as a prerequisite for determining the maximum fault current. Figure 6 As shown, R L =0 and L L =0, meaning the fault occurs at the beginning of the line, and the resistance and inductance of the line are 0, so the resistance and inductance of the line are not considered. The fault current flow path during the two-pole short-circuit fault stage is shown in Figure (6a). Assuming that a two-pole short-circuit fault occurs at the beginning of the DC line at time t0, according to Kirchhoff's Voltage Law (KVL), the fault current at time t during the two-pole short-circuit fault stage is as shown in Equation (1):

[0041] (1)

[0042] (2)

[0043] Where I0 and U0 are the DC current and voltage at the converter station outlet at the moment before the fault occurred, respectively, and L F For a current-limiting reactor, C s For the submodule capacitor of MMC, L s R is the equivalent inductance of MMC. s σ is the equivalent resistance of the MMC, σ is the attenuation coefficient, which describes the attenuation characteristics of the transient component; ω is the frequency coefficient, which describes the frequency and periodic characteristics of the transient component; A is the amplitude coefficient, which describes the amplitude characteristics of the transient component; β is the initial phase, which represents the phase shift of the transient component at the initial moment.

[0044] Substituting the fault detection delay time t1 (typically 3ms) and the parameters of the converter station and current-limiting reactor into equations (1) and (2), we obtain the fault current at time t1, which is the initial current of the fault current being transferred to the transfer branch, denoted as I. 1。

[0045] The fault current then flows through the transfer branch, waiting for the UFD to trip at time t2 before being switched to the damping branch. The fault current flow path during this stage is shown in Figure (6b). According to KVL, the fault current i during t1~t2 can be obtained. dc_Q (t):

[0046] (3)

[0047] Substituting the UFD tripping time (typically 2ms), i.e., t-t1=2ms, into equation (3) yields the fault current at time t2, which is the maximum value I of the fault current transferred to the damping branch. R_max This current is used as the basis for the subsequent design of damping resistor parameters.

[0048] Next, the fault current decays rapidly under the action of the damping resistor. At time t3, when the DC current decays to a certain value, the fault point can be cleared by a DC switch. According to KVL, the fault current i during t2~t3 can be obtained. R (t) is:

[0049] (4)

[0050] Assume the fault current occurs within a set time t. set The decay time to the DC switch breaking value I is typically within tens of milliseconds. RDS (Generally several hundred amperes), according to equation (4), the damping resistance R can be calculated. damp for:

[0051] (5)

[0052] Then set the DC switch breaking value I according to the actual situation. RCB The value of controls the set time t set Within the system's allowable time range, the damping resistance R is calculated according to equation (5). damp The range of values ​​for the damping resistor is then determined. Within this range, various factors are considered to select the appropriate damping resistor value. Theoretically, the damping resistor R... damp The smaller the value of R, the slower the fault current decays, the longer the fault isolation time, which in turn affects the subsequent system recovery process. damp It should not be too small. And the damping resistor R... damp The larger the value of R, the higher the peak voltage, which in turn requires a higher withstand voltage from the IGBT valve group in the transfer branch. This increases the number of IGBTs connected in series and consequently raises the cost of the DC damper. Therefore, the damping resistor value should not be too large. Thus, R... damp The selection of the value can be based on two perspectives: the system's requirements for fault isolation speed and the upper limit of IGBT overvoltage tolerance.

[0053] Example 2

[0054] Utilizing the fault isolation device applicable to multi-stage boost-and-collection systems of new energy sources according to the present invention, a reliable and practical fault isolation method for such systems is designed. Specifically, this includes: further designing a reasonable fault isolation configuration scheme for the high-voltage collection side of the multi-stage boost-and-collection DC system of new energy sources, namely, installing a DC-DC converter (DCCB) near the collection bus and a DC damper near the high-voltage DC / DC converter on each collection line, such as... Figure 7 The circuit on the left side of the high-voltage DC collection bus is shown. Inductors L1 / L2 / L3 / L4 are current-limiting inductors, used to limit the rate of rise and peak value of the short-circuit current. Line 1, Line 3, and Line 4 represent the high-voltage DC collection outgoing lines, while Line 4 represents the ultra-high-voltage DC outgoing line. When any high-voltage DC collection outgoing line fails, the high-voltage DC / DC converter on the faulty line is blocked, and a DC damper is simultaneously engaged to accelerate the attenuation of the fault current on the DC / DC side, thereby ensuring rapid system recovery and significantly improving the system's power supply reliability. The high-voltage DC bus collection side can utilize a DC-DC converter (DCCB) to isolate the fault point from the remaining collection network, thus ensuring selective fault isolation. Figure 7 The circuit on the right side of the medium- and high-voltage DC collecting bus indicates that the high-voltage DC is stepped up to ultra-high-voltage DC by the ultra-high-voltage DC / DC converter, and then transmitted over long distances through the ultra-high-voltage DC line.

[0055] When any DC collection line fails, the DC-DC converter blocker (DCCB) can be used to isolate the fault point from the remaining collection network. Simultaneously, the high-voltage DC / DC converter of the faulty line is blocked, and a DC damper is engaged to achieve millisecond-level rapid clearance of the fault current, thereby ensuring rapid system recovery and significantly improving the system's power supply reliability.

[0056] It should be noted that although the present invention has been shown and described with reference to specific exemplary embodiments thereof, those skilled in the art should understand that the present invention is not limited to the above embodiments, and all modifications to the present invention fall within the scope of protection of the present invention.

Claims

1. A fault isolation device suitable for multi-stage boosting and collection systems of new energy sources, characterized in that, include: The system includes a high-voltage DC / DC converter, a current-limiting reactor, and a DC damper. The DC damper further comprises a first DC damper connected to the positive terminal of the high-voltage DC / DC converter and a second DC damper connected to the negative terminal of the high-voltage DC / DC converter. The DC damper includes a current-carrying branch, a transfer branch, and a damping branch. The current-carrying branch enables current flow control, the transfer branch increases the voltage level of the IGBT valve group to provide a channel for fault current transfer, and the damping branch enables rapid clearance of fault current.

2. The fault isolation device for a multi-stage boost and collection system of new energy sources according to claim 1, characterized in that, The current-carrying branch includes an ultra-fast mechanical switch and a load transfer switch connected in series; the transfer branch includes an IGBT valve group composed of IGBT modules connected in series and parallel; and the damping branch includes a damping resistor connected in series.

3. The fault isolation device for a multi-stage boost and collection system for new energy sources according to claim 2, characterized in that, The calculation formula for the damping resistor parameters is as follows: ; Among them, I R_max I represents the maximum value of the fault current transferred to the damping branch. RCB L represents the breaking value of the DC switch. F For current-limiting reactors, L s R is the equivalent inductance of MMC. s t is the equivalent resistance of MMC. set This indicates the control setting time.

4. The fault isolation device for a multi-stage boost and collection system for new energy sources according to claim 1, characterized in that, The device includes a three-stage fault current transfer path: During the fault detection phase, a two-pole short-circuit fault occurs in the DC line: the fault current flows through the current-carrying branch, the DC line current rises rapidly, and the damping resistor R... damp It is in a bypassed state; During the current transfer phase, the fault current is switched to the transfer branch: As the fault is detected and located, the IGBT valve group is shut off. At this time, the main branch current is forced to zero, providing a zero-current condition for the opening of the ultra-fast mechanical switch (UFD). The ultra-fast mechanical switch begins to open, the fault current is switched to the transfer branch, and the MMC of the high-voltage DC / DC converter at the corresponding fault location is blocked. The ultra-fast mechanical switch reaches the maximum rated opening distance, and the opening is completed. Current damping stage: After the UFD trips, the IGBT valve group of the transfer branch is shut off, and the fault current is switched to the damping branch. Under the action of the damping resistor, the DC current begins to decay rapidly until the decay ends. Then, the fault point is cut off by the DC switch to achieve physical isolation between the faulty line and the converter.

5. The fault isolation device for a multi-stage boost and collection system for new energy sources according to claim 1, characterized in that, The high-voltage DC / DC converter includes two face-to-face half-bridge MMCs connected together. The DC-side equivalent circuit of the half-bridge MMC includes a series structure of equivalent resistance, equivalent inductance and equivalent capacitance.

6. The fault isolation device for a multi-stage boost and collection system of new energy as described in claim 1, characterized in that, The system also includes a residual current switch, which is installed at the outlet of the high-voltage DC / DC converter, and the DC damper body is configured in series on the outlet side of the high-voltage DC / DC converter.

7. The apparatus according to any one of claims 1 to 6 implements a fault isolation method suitable for a multi-stage boost-and-collection system of new energy sources, characterized in that, The method includes: In the new energy multi-stage boosting and aggregation system, a DC circuit breaker is installed near the aggregation bus on each aggregation line, and a DC damper is installed near the high-voltage DC / DC converter. The current-limiting inductor is used to limit the rate of rise of the short-circuit current and the peak value of the short-circuit current. When any high-voltage DC aggregation transmission line fails, the high-voltage DC / DC converter of the faulty line is blocked, and the DC damper is activated to accelerate the attenuation of the fault current on the DC / DC side. The DC circuit breaker is used on the high-voltage DC bus aggregation side to isolate the fault point from the remaining aggregation network.

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