A two-stage current-limiting control method for a half-bridge MMC converter

Abstract

This invention discloses a two-stage current-limiting control method for a half-bridge MMC converter. The method is as follows: S1. The protection element detects a fault; S2. All distributed DC transformers at the wind turbine terminals are blocked; S3. The MMC converter on the low-voltage side of the centralized DC transformer DCT0 switches to the first-stage current-limiting control mode; S4. The positive and negative voltages of the lines at the output of the distributed DC transformers at the end of each wind turbine group are collected; S5. Based on the protection zone selection and pole selection results, the method switches to the corresponding second-stage current-limiting control. This invention, by changing the control mode of the half-bridge MMC converter, can avoid overcurrent blocking of the MMC converter arm, which is beneficial for protecting power electronic equipment and ensuring the safe and stable operation of the power system. Simultaneously, the proposed current-limiting control method can attenuate the fault current to a low level under different fault conditions, thereby enabling the isolation of the faulty line using disconnect switches, reducing the construction cost of offshore wind power, and meeting the requirements for lightweight offshore wind power construction.

Description

Technical Field

[0001] This invention belongs to the field of power systems and relates to the field of control technology for fault current limiting of MMC converters. Specifically, it relates to a two-stage current limiting control method for a half-bridge MMC converter in an offshore wind power full DC system. Background Technology

[0002] Currently, my country's offshore wind power is mainly focused on near-shore projects less than 50km from the shore, but the site selection for near-shore wind farms is becoming increasingly tight. Far-offshore wind power can be consumed locally, and it offers high-quality wind energy, large single-unit capacity, and does not occupy land resources, which is conducive to the construction of new power systems and is expected to become the main battleground for future offshore wind power development.

[0003] To adapt to long-distance, high-capacity transmission scenarios, offshore wind power is showing a development trend of "from AC transmission to DC transmission" and "from centralized converter to distributed converter". Offshore wind power all-DC systems based on DC transformer step-up structures are beginning to emerge.

[0004] However, due to the technical difficulties and cost issues in constructing offshore converter platforms, lightweight offshore equipment is required. On the other hand, DC systems have significant low inertia, resulting in rapid fault development and propagation, and the power electronic equipment has weak current tolerance, making it highly susceptible to damage in the event of a fault. Therefore, while minimizing the weight of offshore equipment, effectively suppressing the impact of fault currents on the equipment and system is of significant engineering importance for protecting the safe and stable operation of the equipment and system. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a two-stage current limiting control method for a half-bridge MMC converter. This method can quickly start and switch to the first-stage current limiting control mode within 0.1ms, and can switch to the second-stage current limiting control mode after the protection output based on the fault type. While limiting the fault current, it ensures that the MMC converter does not experience overcurrent blocking, greatly guaranteeing the safe and stable operation of power electronic equipment and systems. Under this control method, faults can be isolated using DC disconnect switches, reducing the construction cost of offshore wind power and meeting the lightweight requirements of offshore platform construction.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A two-stage current limiting control method for a half-bridge MMC converter includes the following steps:

[0008] Step 1: Collect information from the protection components at the line protection installation location to identify the fault occurrence;

[0009] After a fault occurs, the protection element is activated, indicating that a fault has occurred;

[0010] Step 2: Lock down all distributed DC transformers at the wind turbine cluster ends;

[0011] Step 3: The MMC converter on the low-voltage side of the centralized DC transformer DCT0 is switched to the first-level current limiting control mode;

[0012] The first-level current limiting control controls the DC current I collected in the circuit. dc With DC current reference value I dcref The difference signal is processed by a PI circuit to obtain a reduction coefficient k, which is then multiplied by the bridge arm voltage reference value of the MMC converter to achieve continuous adaptive adjustment. The specific calculation formula is shown in equation (1).

[0013]

[0014] Among them, K p and K I These are the proportional coefficient and integral coefficient, respectively; s is the Laplace operator; and k is output limited to [0,1].

[0015] The reference value of the bridge arm voltage after switching to current limiting control is shown in equation (2).

[0016] u′ p =ku p (2)

[0017] u′ n =ku n

[0018] Among them, u p u n These are the reference values ​​for the upper and lower bridge arm voltages before the current limiting control is activated, u′ and u′, respectively. p u′ n These are the reference values ​​for the upper and lower bridge arm voltages after the current limiting control is activated;

[0019] The number of MMC converter submodules deployed at this time is shown in equation (3).

[0020] n′ p =kn p (3)

[0021] n′ n =kn n

[0022] Where, n p n n These represent the number of MMC converter submodules deployed in the upper and lower bridge arms before the current limiting control was implemented, n′. p n′ nThese represent the number of MMC converter sub-modules activated in the upper and lower bridge arms after the current limiting control is activated; by reducing the number of activated MMC converter sub-modules, capacitor discharge is reduced, thereby suppressing the rapid increase of current in the early stage of a fault.

[0023] Step 4: Collect the positive and negative voltage signals of the lines at the output of the distributed DC transformer at the end of each wind turbine group. In preparation for switching to Level 2 current limiting control;

[0024] Step 5: Switch to the corresponding second-level current limiting control based on the protection zone selection results;

[0025] For the second-level current limiting control, the main reason is that the fault circuits under different fault types are different, and the strategies for limiting the fault current are also different. Therefore, it is necessary to communicate with the protection system. Specifically, different control strategies are determined based on the fault type information, and the voltage signals of different lines are also determined as control input signals based on the fault line selection.

[0026] The control strategies corresponding to the fault types are as follows:

[0027] In the event of a bipolar fault, the voltage supported by the capacitors of the MMC converter submodule is made equal to the sum of the voltages of the positive and negative capacitors on the faulty line's fan side (i.e., the difference between the positive and negative voltages of the line at the end fan outlet). Because u′ p +u′ n =k(u p +u n )=kU dc ,at this time,

[0028]

[0029] In the formula, U dc The rated DC voltage of the collection line;

[0030] In the event of a positive fault, the voltage supported by the capacitors of the MMC converter submodule is made equal to the voltage of the negative capacitor on the turbine side of the faulty line (i.e., the absolute value of the negative voltage at the outlet of the terminal turbine). Because u′ p +u′ n =k(u p +u n )=kU dc ,at this time,

[0031]

[0032] In the event of a negative pole fault, the voltage supported by the capacitor of the MMC converter submodule is made equal to the voltage of the positive capacitor on the turbine side of the faulty line (i.e., the positive voltage of the line at the outlet of the terminal turbine). at this time,

[0033]

[0034] Under the second-level current limiting control, the reference value of the MMC converter arm voltage can be calculated according to equation (2), and the number of sub-modules put into the MMC converter can be calculated according to equation (3).

[0035] Compared with the prior art, the present invention has the following advantages:

[0036] 1. By coordinating with protection systems, different control strategies are switched according to the fault type, effectively avoiding overcurrent in the MMC converter arm under fault conditions, and strongly ensuring the safe and stable operation of power equipment and systems.

[0037] 2. By improving the control of the half-bridge MMC converter, the initial fault current is suppressed, and the fault current can decay naturally. This allows the use of disconnect switches instead of DC circuit breakers or full-bridge MMC converters with fault self-clearing capabilities to isolate faults in the topology, reducing the construction cost of offshore wind power and meeting the requirements for lightweight construction of offshore platforms. Attached Figure Description

[0038] Figure 1 This is a power transmission system model diagram applicable to the method of the present invention.

[0039] Figure 2 yes Figure 1 Distributed DC transformer model in the system.

[0040] Figure 3 yes Figure 1 The system uses a centralized DC transformer model.

[0041] Figure 4 This is a flowchart of the method of the present invention.

[0042] Figure 5(a) shows the positive voltage waveform of the line at the DCT2 outlet.

[0043] Figure 5(b) shows the negative voltage waveform of the line at the DCT2 output.

[0044] Figure 6 It is the fault circuit in the case of a bipolar fault.

[0045] Figure 7 This is a control block diagram of the method of the present invention.

[0046] Figure 8 It is the waveform of the reduction coefficient k during the control process.

[0047] Figure 9(a) shows the arm current waveform of the MMC converter under bipolar fault.

[0048] Figure 9(b) shows a partially magnified waveform of the MMC converter arm current under bipolar fault.

[0049] Figure 10(a) shows the positive current waveform at the installation point of the protection for the collection line 1 under a bipolar fault.

[0050] Figure 10(b) shows a locally magnified waveform of the positive current at the installation point of the protection of line 1 under a bipolar fault. Detailed Implementation

[0051] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.

[0052] like Figure 1 As shown, a simulation model of an offshore wind power all-DC collection and transmission system is presented. The rated DC output voltage of the wind turbine rectifier is ±5kV, and the rated power of the wind turbine group is 100MVA. The transformation ratios of the distributed DC transformers (DCT1~DCT4) are ±5kV / ±60kV, and the transformation ratio of the centralized DC transformer (DCT0) is ±60kV / ±400kV. The line length from the wind turbine group to the centralized DC transformer is 15km.

[0053] like Figure 2 As shown, the structural topology of distributed DC transformers (DCT1~DCT4) is presented.

[0054] like Figure 3 As shown, the structural topology of a centralized DC transformer (DCT0) is presented.

[0055] When a bipolar low-resistance (0.1Ω) short-circuit fault occurs at the midpoint of collection line 1, the fault occurrence time is counted as time 0. The method provided by this invention can accurately and quickly activate and limit the fault current, ensuring that the MMC converter does not experience current blocking during the entire fault isolation process. For example... Figure 4 As shown, it includes the following steps:

[0056] Step 1: Collect information from the protection components at the line protection installation location to identify the fault occurrence;

[0057] After a fault occurs, the protection element is activated, indicating that a fault has occurred;

[0058] Step 2: Lock out all distributed DC transformers (DCT1~DCT4) at the wind turbine cluster ends;

[0059] Step 3: The MMC converter on the low-voltage side of the centralized DC transformer DCT0 is switched to the first-level current limiting control mode;

[0060] The distributed DC transformer has a single active bridge structure. A blocking signal is applied to all IGBTs in it. Since the switching frequency of the single active bridge is very high, it is considered to block immediately after a fault is detected.

[0061] If no control measures are taken in the initial stage of a fault, the submodule capacitors of the MMC converter will rapidly discharge to the fault point, causing severe overcurrent. Therefore, after the fault is detected, the MMC converter quickly switches to the first-level current-limiting control. In equation (1), K p Take 0.0005, K I Take 0.000001 as the reference value for DC current I. dcref When k is set to 0, it adaptively adjusts as the DC fault current changes.

[0062] The current limiting control method of this invention essentially reduces capacitor discharge by reducing the number of MMC converter sub-modules in operation, thereby suppressing the rapid increase of current in the early stage of a fault.

[0063] Step 4: Collect the positive and negative voltages of the lines at the output of the distributed DC transformers (DCT2, DCT4) at the end of each wind turbine group. In preparation for switching to Level 2 current limiting control;

[0064] Since the high-voltage side of DCT0 is a diode uncontrolled rectifier structure, no current will flow into the high-voltage side of DCT0 as at the fault point. Furthermore, after DCT1 to DCT4 are blocked, the fan will not experience a current flow into the fault point. Therefore, the fault area at this time is a passive system, and the line voltage relies on capacitor support, eventually decaying to 0. The positive and negative voltage waveforms at the outlet of the faulty line DCT2 are shown in Figures 5(a) and 5(b), respectively.

[0065] Step 5: Switch to the corresponding second-level current limiting control based on the protection zone selection results;

[0066] The key point of the method of this invention is the current limiting control strategy. Therefore, it does not focus on how the protection completes the fault selection area and pole selection, but only uses the protection output signal to communicate with the control system to achieve control and protection coordination.

[0067] The relay protection device determined that a bipolar fault had occurred in the collection line 1, and the control system switched to the corresponding second-level current limiting control according to the faulty line and the fault type.

[0068] The faulty circuit at this time is as follows Figure 6 As shown. When the transition resistance is low, loop 1 and loop 2 can be considered decoupled, and the fault current flowing through the MMC converter arm is related to the fault loop it belongs to; however, when the transition resistance is large, there will be a strong coupling relationship between the two loops. The number of MMC converter submodules that are cut off will affect the charging and discharging direction of the MMC converter submodule capacitor and the wind turbine side capacitor, which is not conducive to eliminating the fault current, and may even lead to overcurrent in the MMC converter arm in severe cases.

[0069] Therefore, if the voltage supported by the capacitor of the MMC converter submodule is equal to the voltage difference between the positive and negative terminals of the line at the end of the collector line 1 fan outlet, that is... This ensures that no large fault current will occur in the circuit, thus preventing overcurrent in the MMC converter arm. calculate Make k follow And change.

[0070] The specific control block diagram of the present invention is as follows: Figure 7 As shown in the figure, this example assumes a time definite delay (2ms after the fault occurs) switching, and the reduction coefficient k changes throughout the control process as follows: Figure 8 As shown:

[0071] Under the proposed control method, the number of sub-modules put into the MMC will gradually decrease to 0 as the line voltage decays. The bridge arm does not experience current throughout the fault, and the bridge arm current waveforms are shown in Figure 9(a) and Figure 9(b). When the number of sub-modules put into the MMC is 0, the fault line current naturally decays. The fault line current waveforms during the entire isolation process are shown in Figure 10(a) and Figure 10(b). Finally, the fault line is isolated under the condition that the MMC converter does not experience current blocking.

Claims

1. A two-stage current limiting control method for a half-bridge MMC converter, characterized in that, Includes the following steps: Step 1: Collect information from the protection components at the line protection installation location to identify the fault occurrence; After a fault occurs, the protection element is activated, indicating that a fault has occurred; Step 2: Lock down all distributed DC transformers at the wind turbine cluster ends; Step 3: The MMC converter on the low-voltage side of the centralized DC transformer DCT0 is switched to the first-level current limiting control mode; The first-level current limiting control collects the DC current of the line. With DC current reference value The difference signal is processed by a PI circuit to obtain the reduction coefficient. k and reduce factor k Multiplying this by the reference value of the bridge arm voltage of the MMC converter achieves continuous adaptive adjustment. The specific calculation formula is shown in equation (1). (1) Among them, K p and K I These are the proportionality coefficient and the integral coefficient, respectively, and s is the Laplace operator. k The output is limited to [0,1]. The reference value of the bridge arm voltage after switching to current limiting control is shown in equation (2). (2) in, u p , u n These are the reference values ​​for the upper and lower bridge arm voltages before the current limiting control is activated. , These are the reference values ​​for the upper and lower bridge arm voltages after the current limiting control is activated; The number of MMC converter submodules deployed at this time is shown in equation (3). (3) in, n p , n n These represent the number of MMC converter submodules deployed in the upper and lower bridge arms before the current limiting control was implemented. , These represent the number of MMC converter sub-modules activated in the upper and lower bridge arms after the current limiting control is activated; by reducing the number of activated MMC converter sub-modules, capacitor discharge is reduced, thereby suppressing the rapid increase of current in the early stage of a fault. Step 4: Collect the positive and negative voltage signals of the lines at the output of the distributed DC transformer at the end of each wind turbine group. , This is to prepare for switching to the second level of current limiting control; Step 5: Switch to the corresponding second-level current limiting control based on the protection zone selection results; For the second-level current limiting control, the main reason is that the fault circuits under different fault types are different, and the strategies for limiting the fault current are also different. Therefore, it is necessary to communicate with the protection system. Specifically, different control strategies are determined based on the fault type information, and the voltage signals of different lines are also determined as control input signals based on the fault line selection. The control strategies corresponding to the fault types are as follows: In the event of a bipolar fault, the voltage supported by the capacitors of the MMC converter submodule should be equal to the sum of the voltages of the positive and negative capacitors on the faulty line's fan side, i.e., the difference between the positive and negative voltages of the line at the end fan outlet. ,because ,at this time, In the formula, The rated DC voltage of the collection line; In the event of a positive fault, the voltage supported by the MMC converter submodule capacitor is made equal to the absolute value of the negative capacitor voltage on the faulty line's fan side, i.e., the negative voltage at the end fan outlet. ,because ,at this time, In the event of a negative pole fault, the voltage supported by the capacitor of the MMC converter submodule is made equal to the voltage of the positive capacitor on the fan side of the faulty line, i.e., the positive voltage of the line at the outlet of the terminal fan. ,at this time, Under the second-level current limiting control, the reference value of the MMC converter arm voltage is calculated according to equation (2), and the number of sub-modules put into the MMC converter is calculated according to equation (3).

2. The two-stage current limiting control method for a half-bridge MMC converter according to claim 1, characterized in that, The centralized DC transformer DCT0 adopts a half-bridge MMC converter structure on the low-voltage side and a diode uncontrolled rectifier structure on the high-voltage side; all distributed DC transformers at the wind turbine group end adopt a single active bridge structure.

3. The two-stage current limiting control method for a half-bridge MMC converter according to claim 1, characterized in that, The sampling frequency of the current at the line protection installation point described in step one is 10kHz.

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