Lcc and mmc hybrid dc de-icing device and collaborative control method thereof

Abstract

A hybrid DC ice-melting device combining LCC and MMC and its collaborative control method are described. The AC sides of the LCC and MMC are connected in parallel, while their DC sides are connected in series. The LCC control layer acquires the DC voltage ripple, AC current harmonics, and reactive power demand of the LCC. The MMC control layer performs LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and compensates for the LCC reactive power demand. The collaborative control layer acquires the first active power output from the MMC when performing LCC DC voltage ripple suppression, the second active power output when performing LCC AC current harmonic suppression, and the reactive power output when compensating for the LCC reactive power demand. Under the condition that the first active power, second active power, and reactive power satisfy the MMC capacity constraint, the power balance between the AC and DC sides of the MMC is coordinated through the power compensation term of the active-frequency outer loop. The MMC simultaneously undertakes DC voltage ripple suppression, AC current harmonic suppression, and reactive power compensation, balancing the power on both the AC and DC sides.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC transmission and power system disaster prevention and mitigation technology, specifically involving a hybrid DC de-icing device based on the series connection of LCC and MMC on the DC side and its coordinated control method. Background Technology

[0002] With the continuous expansion of the power grid and the frequent occurrence of extreme weather events, ice disaster protection for transmission lines has become an important issue in ensuring the safe and stable operation of the power grid. DC de-icing technology is currently the mainstream solution for dealing with line icing. Among them, DC de-icing devices based on grid commutator converters (LCCs) are widely used due to their mature technology, large capacity, and relatively low cost. However, LCCs have inherent technical defects in operation: First, they absorb a large amount of reactive power from the grid during the commutation process, which can easily cause grid voltage instability or even collapse in areas with weak grid structures (weak grids); Second, their AC side current waveform is severely distorted, generating a large number of 5th, 7th, 11th, and 13th order characteristic harmonics, which seriously pollute the power quality of the grid. They usually require large and fixed passive filter banks, increasing system costs and floor space; Third, their output DC voltage has inherent ripple, which usually requires large smoothing reactors to suppress, further increasing equipment footprint and investment costs.

[0003] To overcome the shortcomings of LCC (Liquid Cylinder), Modular Multilevel Converters (MMCs), with their superior controllability and waveform quality, have been introduced into the ice-melting field. MMCs can independently control active and reactive power, and their AC-side output current has extremely low harmonic content. However, if a full-capacity ice-melting device is constructed using only MMCs, its cost and operating losses are significantly higher than those of LCCs, resulting in poor economic efficiency. In the prior art, a method and system for parallel operation control of LCCs and MMCs in a DC ice-melting device has been developed, solving the problem of insufficient rated current in existing DC ice-melting devices. A hybrid LCC-MMC DC ice-melting device topology has been developed, addressing the issues of minimum power limitations, current discontinuity when providing small currents, and relatively high harmonic content during operation of LCC-type ice-melting devices. This topology utilizes the switching of MMC operating states to filter harmonics and provide reactive power support during LCC ice-melting. Simultaneously, the LCC... The switching of the working state of the MMC hybrid DC de-icing device improves the utilization rate of the device and has the characteristics of reactive power compensation, harmonic suppression, and high de-icing current. Based on the offshore wind power DC transmission system of LCC and full-bridge MMC-STATCOM hybrid series, the LCC bears the DC bus voltage and the output DC voltage is the same as the DC bus voltage. The LCC transmits all active power, and the DC side output voltage of the full-bridge MMC is used to compensate for the DC voltage ripple of the LCC. The AC side output three-phase sine wave is used for reactive power compensation. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a DC de-icing device that combines LCC and MMC and its collaborative control method. The DC de-icing device uses a topology in which LCC and MMC are directly connected in series on the DC side. This solves the problem of unbalanced submodule capacitor voltage caused by AC / DC power imbalance when the MMC simultaneously undertakes multiple dynamic tasks such as DC voltage ripple suppression, AC current harmonic suppression, and reactive power compensation. It also addresses the problem of harmonic and reactive power pollution to the power grid generated by the LCC itself. This enables the de-icing device to operate stably, efficiently, and with high quality under constraints of low resources and high real-time performance.

[0005] The present invention adopts the following technical solution.

[0006] The present invention proposes a DC de-icing device that combines LCC and MMC, including a phase-commutating converter LCC and a modular multilevel converter MMC. The AC sides of the LCC and MMC are connected in parallel and then connected to the power grid via a transformer. The DC sides of the LCC and MMC are connected in series and then connected to the de-icing load, which is the three-phase AC transmission line to be de-iced. The DC de-icing device also includes: LCC control layer, MMC control layer, and collaborative control layer; The LCC control layer is used to obtain the DC voltage ripple, AC current harmonics, and reactive power requirements of the LCC. The MMC control layer is used to perform LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and compensate for LCC reactive power demand. The collaborative control layer is used to calculate in real time the first active power output by the MMC when performing DC voltage ripple suppression of the LCC, the second active power output when performing AC current harmonic suppression of the LCC, and the reactive power output when compensating for the reactive power demand of the LCC, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power and reactive power output by the MMC meet the MMC capacity constraints, the power compensation term of the active-frequency outer loop is used to coordinate the power balance of the AC and DC sides of the MMC.

[0007] LCC uses constant DC voltage / current control; MMC uses grid-type control.

[0008] In another aspect, this invention also proposes a collaborative control method for a hybrid LCC and MMC DC ice-melting device, comprising: The LCC control layer acquires the DC voltage ripple of the LCC; the MMC control layer performs DC voltage ripple suppression of the LCC. The LCC control layer acquires the AC current harmonics of the LCC; the MMC control layer performs AC current harmonic suppression. The LCC control layer obtains the reactive power requirement of the LCC; the MMC control layer performs compensation for the reactive power requirement of the LCC. The collaborative control layer calculates in real time the first active power output by the MMC when performing LCC DC voltage ripple suppression, the second active power output when performing LCC AC current harmonic suppression, and the reactive power output when compensating for LCC reactive power demand, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power, and reactive power output by the MMC meet the MMC capacity constraints, the power compensation term of the active-frequency outer loop coordinates the power balance of the AC and DC sides of the MMC.

[0009] Based on constant DC voltage / current control of LCC, calculate the DC voltage of LCC and its average value; and At that time, the average DC voltage output by the LCC is shown in the following formula:

[0010] In the formula, The average DC voltage output by the LCC. This refers to the peak value of the AC phase voltage. For trigger angle, As the reference angular frequency, For a specific moment; Based on the fact that the conduction angle of each thyristor is... Calculate the DC voltage ripple at different firing angles, as follows:

[0011] In the formula, This refers to the DC voltage ripple of the LCC.

[0012] The voltage difference between the de-icing voltage command value of the DC de-icing device and the DC voltage output by the LCC is obtained in real time and used as the DC voltage command value of the MMC, as shown in the following formula:

[0013] In the formula, This is the DC voltage command value for the MMC. This refers to the de-icing voltage command value for the DC de-icing device. This is the DC voltage output by the LCC; Based on the DC voltage command value of MMC and the capacitor voltage of each submodule, the number of submodules connected to each bridge arm of MMC is adjusted to control the target value of the output bridge arm voltage of MMC in order to suppress the DC voltage ripple of LCC. The number of sub-modules configured in the bridge arm is determined by the following formula:

[0014] In the formula, The total number of submodules configured for the bridge arm, The rated voltage of the capacitor built into the submodule. The average DC voltage output by the LCC. This refers to the peak value of the AC phase voltage. This refers to the DC voltage ripple of the LCC.

[0015] The target harmonic current output by the LCC is transformed into a coordinate system, converting the three-phase AC quantity into a two-phase DC quantity, which is then fed into the PI controller. The resulting dq-axis control quantity is used as the harmonic compensation quantity, as shown in the following formula:

[0016] In the formula, , For dq axis control variables , This is the commanded value for the dq axis current. , These are the parameters of the PI controller; During control, the harmonic compensation quantity output by the PI controller is reconstructed into a three-phase AC modulation signal through inverse coordinate transformation, so as to control the MMC to generate the required harmonic compensation current and suppress AC current harmonics.

[0017] MMC employs a network-based control approach and adds a voltage droop control term to the reactive power-voltage loop to obtain the AC voltage command value for MMC, as shown in the following formula:

[0018] In the formula, This is the AC voltage command value for the MMC. This is the AC voltage reference value for MMC. For grid-side voltage command, This is a reactive power instruction. This is the measured voltage on the grid side. This refers to the actual reactive power measured on the grid side. This is the voltage droop factor. This is the reactive power droop factor. For operators; Based on the AC voltage command value, the MMC outputs reactive power to compensate for the reactive power consumed by the LCC.

[0019] The satisfied MMC capacity constraint is shown in the following formula:

[0020] In the formula, The first active power output by the MMC to perform LCC DC voltage ripple suppression. The second active power output by the MMC for performing LCC AC current harmonic suppression. The reactive power output by the MMC to compensate for the reactive power demand of the LCC. This refers to the rated capacity of the MMC.

[0021] MMC employs a network-based control approach and introduces an active power compensation term into the active-frequency outer loop, satisfying the following relationship:

[0022] In the formula, This is the active power compensation term for MMC. This is the compensation adjustment coefficient; The sum of the MMC active power command value and the active power compensation term in the active-frequency outer loop before the improvement is used as the MMC active power command value in the improved active-frequency outer loop.

[0023] The present invention is also a terminal, including a processor and a storage medium; the storage medium is used to store instructions; the processor is used to perform operations according to the instructions to execute the steps of the method.

[0024] The present invention is also a computer-readable storage medium having a computer program stored thereon, which, when executed by a processor, implements the steps of the method.

[0025] The beneficial effects of this invention, compared with the prior art, include at least the following: Based on the topology of LCC and MMC connected in series on the DC side, this invention designs a multi-objective cooperative control strategy. This strategy not only allows the MMC to bear part of the DC voltage, but more importantly, through precise control of the MMC, it enables it to dynamically and adaptively achieve multiple functions such as reactive power demand compensation for the LCC, AC side harmonic mitigation, and DC side ripple suppression. Crucially, thanks to the effective suppression of DC ripple by this control strategy, the large-volume DC-side smoothing reactor originally required for the LCC can be eliminated, thereby significantly reducing the system footprint and equipment investment. This deep cooperative control method transforms the MMC from passively bearing voltage to actively improving system performance. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the topology of the LCC-MMC hybrid DC ice melting device described in this invention; Figure 2 This is a comparison diagram of the DC-side output voltage waveform of the ice-melting device before and after voltage ripple suppression is applied in an embodiment of the present invention; Figure 3This is a waveform diagram of the AC side current before MMC current harmonic suppression and its FFT analysis in an embodiment of the present invention; Figure 4 This is a waveform diagram of the AC side current after MMC current harmonic suppression and its FFT analysis in an embodiment of the present invention; Figure 5 This is a dynamic response waveform diagram of reactive power and active power at the grid connection point under the conditions of reactive power compensation function activation and grid voltage drop in an embodiment of the present invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of this invention. The embodiments described in this application are merely some embodiments of this invention, and not all embodiments. Based on the spirit of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of this invention.

[0028] The topology of the DC ice-melting device that combines LCC and MMC proposed in this invention is as follows: Figure 1 As shown, the DC de-icing device includes: LCC, MMC and the line to be de-iced. The AC sides of LCC and MMC are connected in parallel and then connected to the power grid via a transformer. The DC sides of LCC and MMC are connected in series and then connected to the de-icing load. The de-icing load is the three-phase AC transmission line to be de-iced. The three-phase AC transmission line is connected into the de-icing load through a switching operation.

[0029] The DC de-icing device also includes: LCC control layer, MMC control layer, and collaborative control layer; The LCC control layer is used to obtain the DC voltage ripple, AC current harmonics, and reactive power requirements of the LCC. The MMC control layer is used to perform LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and compensate for LCC reactive power demand. The collaborative control layer is used to calculate in real time the first active power output by the MMC when performing DC voltage ripple suppression of the LCC, the second active power output when performing AC current harmonic suppression of the LCC, and the reactive power output when compensating for the reactive power demand of the LCC, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power and reactive power output by the MMC meet the MMC capacity constraints, the power compensation term of the active-frequency outer loop is used to coordinate the power balance of the AC and DC sides of the MMC.

[0030] Among them, LCC adopts constant DC voltage / current control; MMC adopts grid-type control.

[0031] This invention proposes a parallel AC-side design, which connects the LCC and MMC to the power grid in parallel on the AC side. Through precise control of the MMC, the characteristic harmonics of the AC current generated by the LCC can be dynamically compensated, thereby improving the quality of the grid-connected current. At the same time, the MMC adopts grid-type control, which can provide reactive power support to the system under fault conditions such as grid voltage drops, thereby enhancing system stability.

[0032] This invention proposes a DC-side series design, where the LCC and MMC are connected in series on their DC sides to jointly output the de-icing voltage. To suppress DC voltage ripple on the LCC, the MMC outputs a compensation voltage with the same amplitude but opposite phase to the LCC. Simultaneously, the AC side of the MMC is controlled to achieve power balance between the AC and DC sides, ensuring stable capacitor voltage in the submodule. Furthermore, with the LCC and MMC connected in series on their DC sides, in the event of a DC fault, the LCC can force a phase shift to reduce the DC voltage to a negative value, blocking the fault current path without the need for expensive DC circuit breakers.

[0033] This invention proposes a DC output port design that effectively suppresses the inherent ripple of the LCC by controlling the DC side of the MMC. Compared with traditional ice melting systems, this technology does not require a series smoothing reactor, significantly reducing the size of the equipment and facilitating transportation and deployment.

[0034] The collaborative control strategy proposed in this paper is based on grid-type control. By introducing a power compensation term into the active frequency outer loop, it achieves AC / DC power balance and ensures the normal operation of the MMC. In addition, a voltage droop term can be added to the reactive voltage loop, which innovatively enables the de-icing system to support the grid while ensuring that the de-icing voltage does not drop under grid fault conditions.

[0035] This invention also proposes a collaborative control method for a DC ice-melting device that combines LCC and MMC, specifically including the following steps: Step 1: The LCC control layer performs constant DC voltage / current control of the LCC and obtains the DC voltage ripple of the LCC.

[0036] Specifically, step 1 includes: Step 1.1: Based on the constant DC voltage / current control of LCC, calculate the DC voltage of LCC and its average value.

[0037] First, we analyze and calculate the DC-side output voltage and its ripple of the LCC. If the DC side is a large inductive load, the current flowing through the thyristor is... There will be no discontinuity within the conduction angle. At this time, the DC-side output voltage of the LCC is the difference between any two phase voltages, and the conduction angle of each thyristor is constant. Taking the conduction period of the thyristor in the upper arm of phase A as an example, the DC voltage of the LCC is calculated as follows: (1) In the formula, DC voltage This refers to the peak value of the AC phase voltage. For trigger angle, As the reference angular frequency, For a specific moment; Simplifying the above equation further, we get: (2) Integrating equation (2) and taking the average is as follows:

[0038] Calculated and At that time, the average DC voltage output by the LCC is shown in the following formula: (3) In the formula, This represents the average DC voltage output by the LCC. Step 1.2, based on the fact that the conduction angle of each thyristor is... Calculate the DC voltage ripple under different firing angles.

[0039] Analysis shows that for the thyristor of the upper bridge arm of phase A... On-time, if the trigger angle < , DC side output voltage at = and = The maximum value is obtained at point , in = + and = + The minimum values ​​are obtained at the following locations: (4) If trigger angle > , DC side output voltage at = + and = + The maximum value is obtained at point , in = + and = + The minimum values ​​are obtained at the following locations: (5) Based on equations (4) and (5), the following can be calculated respectively. < as well as > DC voltage ripple at time They are respectively: (6) Equation (3) represents the average value of the DC-side output voltage. To make it greater than 0, the firing angle is... Should be smaller Based on this, analyzing equation (5), it is easy to see that the DC voltage ripple will change with the firing angle. It increases as it grows.

[0040] Step 2: The MMC control layer performs LCC DC voltage ripple suppression.

[0041] Specifically, step 2 includes: Step 2.1: Obtain the voltage difference between the de-icing voltage command value of the DC de-icing device and the DC voltage output by the LCC in real time, and use it as the DC voltage command value of the MMC, as shown in the following formula: (7) In the formula, This is the DC voltage command value for the MMC. This refers to the de-icing voltage command value for the DC de-icing device. This is the DC voltage output by the LCC; By decomposing the measured DC voltage output of the LCC, the following relationship is obtained: (8) Step 2.2: Based on the DC voltage command value of MMC and the capacitor voltage of each submodule, adjust the number of submodules connected to each bridge arm of MMC, and control the target value of the output bridge arm voltage of MMC to suppress the DC voltage ripple of LCC.

[0042] The DC voltage command of the MMC is input to the modulation module of the MMC. Since it is the DC port voltage, the DC voltage command of the MMC corresponds to the common mode component of the voltage of each phase upper and lower bridge arm of the MMC. The MMC executes the modulation strategy, adjusts the number of sub-modules connected to each bridge arm, and outputs the target bridge arm voltage, thereby achieving the control target of ripple elimination.

[0043] Considering that the MMC needs to filter out the DC voltage ripple introduced by the LCC while delivering de-icing power, higher requirements are placed on the voltage output range of its bridge arms. The number of sub-modules configured in its bridge arms is determined by the following formula: (9) In the formula, The total number of submodules configured for the bridge arm, This refers to the rated voltage of the capacitor built into the submodule.

[0044] When the DC output voltage of the LCC meets the ice-melting requirements, this invention uses the MMC to solve the DC voltage ripple problem. Based on the topology of the LCC and MMC connected in series on the DC side, it makes full use of the controllability of the MMC on the DC side, so that it adjusts its own voltage output at the DC port according to the fluctuation of the output voltage of the LCC DC port, thereby smoothing out the ripple of the DC side output voltage of the DC ice-melting device.

[0045] DC voltage ripple has a significant adverse impact on the de-icing process: First, ripple causes fluctuations in the DC current during de-icing, leading to unstable Joule heat power and a reduction in average de-icing efficiency. Second, periodic current fluctuations induce alternating thermal and electromagnetic forces on iced lines, potentially causing mechanical fatigue at the ice-conductor interface, posing a risk of uneven ice shedding and line tripping. Third, large voltage ripple peaks can cause additional electrical stress on the line insulation, posing a safety hazard in long-term operation. Therefore, suppressing DC voltage ripple is crucial for achieving efficient, safe, and stable de-icing.

[0046] Since the LCC and MMC adopt a DC-side series structure, the minimum voltage of the LCC is determined under the requirement of maintaining the minimum firing angle (usually not less than 0.15~0.2 pu). The MMC outputs the DC voltage base component to make up for the difference between the average value of the LCC and the total requirement. The MMC also actively outputs a compensation voltage that is in phase and frequency with the LCC ripple. The sum of the output voltages of the LCC and MMC meets the de-icing voltage requirement.

[0047] Step 3: The LCC control layer acquires the AC current harmonics of the LCC; the MMC control layer performs AC current harmonic suppression.

[0048] Because the conduction angle of each thyristor in the LCC is only Since the current flowing through it is a direct current with a fixed duty cycle, the AC side current will contain a large number of harmonics, mainly the 5th (negative sequence), 7th (positive sequence), 13th (negative sequence), and 15th (positive sequence). Considering the parallel topology of the LCC and MMC AC sides, we can consider controlling the AC side of the MMC to generate a potential of the same sequence and order, thereby exciting a current of the same sequence and order, which cancels out the inherent harmonics of the LCC AC side.

[0049] Specifically, step 3 includes: Step 3.1: Perform coordinate transformation on the target harmonic current output by the LCC, convert the three-phase AC quantity into a two-phase DC quantity, and then send it to the PI controller. Use the obtained dq axis control quantity as the harmonic compensation quantity.

[0050] If the target to be suppressed is a positive-sequence harmonic current, the coordinate transformation matrix is: (10) If the target to be suppressed is a negative-sequence harmonic current, the coordinate transformation matrix is: (11) In the formula, , These are the coordinate transformation matrices for the positive-sequence and negative-sequence harmonic currents, respectively. For harmonic order; Taking the 5th negative sequence current harmonic as an example, the coordinate transformation process is as follows:

[0051] In the formula, , This represents the dq component of the 5th negative sequence current harmonic. This represents the amplitude of the fifth current harmonic. After obtaining the direct current through coordinate transformation, the dq-axis control quantity obtained by the PI controller is shown in the following formula: (12) In the formula, , For dq axis control variables , This is the commanded value for the dq axis current. , These are the parameters for the PI controller.

[0052] Step 3.2: During control, the harmonic compensation quantity output by the PI controller is reconstructed into a three-phase AC modulation signal through inverse coordinate transformation to control the MMC to generate the required harmonic compensation current.

[0053] Performing an inverse coordinate transformation on the dq-axis control quantity reconstructs it into a three-phase AC control quantity. The inverse coordinate transformation matrices for the positive and negative sequence components are as follows: (13) (14) In the formula, The inverse coordinate transformation matrix for the positive-order components. is the inverse coordinate transformation matrix for the negative-order components.

[0054] Step 4: The LCC control layer obtains the reactive power requirement of the LCC; the MMC control layer performs compensation for the reactive power requirement of the LCC.

[0055] Specifically, the MMC adopts a grid-based control and adds a voltage droop control term in the reactive-voltage loop to obtain the AC voltage command value of the MMC; based on the AC voltage command value, the MMC outputs reactive power to compensate for the reactive power demand of the LCC.

[0056] During operation, the LCC absorbs a significant amount of reactive power from the power grid, an inherent characteristic determined by its operating mechanism based on grid voltage commutation. The reactive power demand of the LCC is directly related to its operating conditions, and its theoretical value can be accurately described by the following formula: (15) In the formula, For the reactive power requirements of LCC, The active power consumed by the LCC For ideal no-load DC voltage, This is the actual DC voltage.

[0057] If the firing angle and commutation overlap angle are taken into account, a more precise expression can be given as: (16) In the formula, Line voltage, It is direct current. For trigger angle, This is the commutation overlap angle.

[0058] The above analysis and calculations show that LCCs continuously absorb reactive power under any non-ideal no-load conditions, and this amount can reach more than 50% of the active power. This huge reactive power demand can cause a significant drop in grid voltage in areas with weak grid structures, and in severe cases, it can lead to voltage instability accidents, greatly limiting the application of traditional LCC de-icing devices in remote mountainous power grids.

[0059] To solve this technical problem, this invention utilizes the superior reactive power regulation capability of the MMC to achieve dynamic compensation for the reactive power consumed by the LCC through a collaborative control layer. As a fully controlled converter, the amplitude and phase of the AC side output voltage of the MMC can be independently and rapidly adjusted, thereby precisely controlling the reactive power exchanged with the grid. The reactive power output of the MMC is: (17) In the formula, The reactive power output of the MMC. This is the grid voltage. This is the AC voltage output by the MMC. For the angle of attack, To connect the reactor.

[0060] By controlling Amplitude realization of Continuous adjustment. In the collaborative control layer of this invention, the reactive power consumed by the LCC is monitored in real time. This reactive power command is used as the reactive power command for the MMC. The amplitude reference value of the AC voltage output by the MMC is adjusted through a PI controller, causing the MMC to generate reactive power equal to but opposite in nature to the LCC's consumption. This control strategy makes the MMC equivalent to a high-performance static synchronous compensator, capable of meeting the LCC's reactive power demand in real time and accurately, thus enabling the entire hybrid ice-melting system to exhibit purely active power characteristics to the power grid. This technical solution fundamentally eliminates the reactive power impact of the LCC on the power grid, significantly improving the system's operational stability and voltage support capability under weak grid conditions, and providing key technical guarantees for the reliable application of DC ice-melting technology in various power grid environments.

[0061] Considering the potential for sudden voltage drops in the grid during the operation of the ice-melting system, this invention requires the ice-melting device to provide reactive power support to the grid while maintaining a constant DC-side ice-melting power, in order to improve the system's voltage stability. To this end, a voltage droop control element is introduced into the MMC control architecture. Traditional control strategies generally employ grid-following control, which often adds a power outer loop outside the current inner loop to coordinate AC and DC-side power, but lacks the ability to support the grid. With the introduction of grid-following control into the MMC, a voltage droop control term can be added to the reactive power-voltage loop to automatically provide reactive power support to the grid when the voltage drops. When the system detects a grid voltage drop, this control element will automatically increase the amplitude of the MMC's AC-side output voltage, thereby increasing its reactive power output to the grid and achieving active support for the grid connection point voltage.

[0062] In summary, the AC voltage command value of MMC satisfies the following relationship: (18) In the formula, This is the AC voltage command value for the MMC. This is the AC voltage reference value for MMC. This refers to the grid-side voltage command of the MMC. This refers to the reactive power command of the MMC. This is the measured voltage on the grid side. This refers to the actual reactive power measured on the grid side. , This is the voltage droop factor. This is the reactive power droop factor. For operators.

[0063] Based on the AC voltage command value, the MMC outputs reactive power to compensate for the reactive power consumed by the LCC.

[0064] Step 5: The collaborative control layer calculates in real time the first active power output by the MMC when performing LCC DC voltage ripple suppression, the second active power output when performing LCC AC current harmonic suppression, and the reactive power output when compensating for LCC reactive power demand, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power, and reactive power output by the MMC meet the MMC capacity constraints, the power balance of the AC and DC sides of the MMC is coordinated through the power compensation term of the active-frequency outer loop.

[0065] The MMC control layer simultaneously performs LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and LCC reactive power compensation, which affects the power balance on the AC and DC sides within the MMC, leading to submodule capacitor voltage imbalance. Therefore, to control the power balance on the AC and DC sides of the MMC, this invention proposes a collaborative control layer that acquires in real-time the first active power output by the MMC during LCC DC voltage ripple suppression. The second active power output by performing LCC AC current harmonic suppression And the reactive power output to compensate for the reactive power demand of the LCC. The first active power output by the MMC Second active power and reactive power Under the constraint of MMC capacity, the power balance of the AC and DC sides of MMC is coordinated through the power compensation term of the active-frequency outer loop.

[0066] The MMC capacity constraint is satisfied, as shown in the following formula: (19) In the formula, This refers to the rated capacity of the MMC.

[0067] In traditional solutions, the active power command value is usually a fixed value, which cannot reflect the dynamic changes in DC-side power when the MMC performs tasks such as ripple suppression and harmonic suppression. This leads to power imbalance between AC and DC sides and increased voltage fluctuations in the submodule capacitors. Therefore, this invention introduces an active power compensation term of the MMC into the active-frequency outer loop, satisfying the following relationship: (20) In the formula, This is the active power compensation term for MMC. The first active power output by the MMC to perform LCC DC voltage ripple suppression. The second active power output by the MMC for performing LCC AC current harmonic suppression. To compensate for the adjustment coefficient, the value range in the example is 0.4~0.8.

[0068] The active power command value of MMC in the active-frequency outer loop before improvement With active power compensation item The sum of these values ​​serves as the MMC active power command value in the improved active-frequency outer loop. The improved active-frequency outer loop is based on following... and The dynamically changing active power command value The phase angle is quickly adjusted so that the active power on the AC side of the MMC can respond promptly to changes in the power on the DC side, thereby achieving power balance between the AC and DC sides of the MMC. The first active power output from the MMC performing LCC DC voltage ripple suppression will cause phase angle lead / lag, and the second active power output from the MMC performing LCC AC current harmonic suppression will cause frequency fine-tuning. This can be addressed by introducing... MMC can sense changes in DC power in advance and adjust the AC output power accordingly, thereby maintaining the power balance between AC and DC sides and solving the technical problem of how MMC can maintain stable operation when performing multiple tasks.

[0069] According to the law of conservation of energy, power imbalance on the AC / DC side will cause changes in the total energy of the MMC submodule capacitors, leading to an overall shift in the capacitor voltage of each submodule. Simultaneously, due to differences in the switching states, loss characteristics, and control precision of each submodule, the net energy accumulation of each submodule differs under power imbalance conditions, causing the voltage difference between submodules to widen cycle by cycle. This may eventually trigger overvoltage / undervoltage protection actions in the submodules, or even cause the MMC to shut down. Therefore, maintaining AC / DC power balance also effectively ensures the equalization of capacitor voltages in each submodule.

[0070] In this embodiment, the present invention designs the parameters and constructs the topology of the DC de-icing device. Based on the electrical parameters of the target de-icing circuit, the de-icing voltage that the device should output is determined. On this basis, a main circuit topology in which the LCC and MMC are directly connected in series on the DC side is constructed, and the DC voltage values ​​undertaken by the two are reasonably allocated, thereby determining the configuration scheme of the MMC submodule.

[0071] A three-layer control system architecture is established. The first layer is configured as the LCC control layer, which adopts a constant DC voltage or constant DC current control strategy. The second layer is configured as the MMC layer, with the control objectives being LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and compensation for LCC reactive power demand. The third layer is configured as the collaborative control layer, with the control objective being to coordinate the power balance between the AC and DC sides of the MMC.

[0072] The system is started and, while ensuring a stable ice-melting power output on the DC side, the collaborative control function of the MMC is activated. By real-time monitoring of the DC voltage ripple, characteristic subharmonic current, and reactive power demand generated by the LCC, the MMC is controlled to generate corresponding compensation voltage, compensation current, and reactive power: ripple compensation is achieved by adjusting the number of submodules to output a voltage with the same amplitude but opposite phase as the voltage ripple; active filtering is achieved by outputting a current with the same amplitude but opposite phase as the harmonic components; and dynamic reactive power compensation is achieved by injecting capacitive reactive power into the grid.

[0073] The system performance was verified through simulation analysis. The focus was on observing the DC side voltage ripple suppression effect, the change in AC side current harmonic distortion rate, and the improvement of the power factor at the grid connection point. This confirmed that the ice-melting device proposed in this invention effectively improves the power quality and operational stability of the power grid while performing the ice-melting function.

[0074] Figure 2 For ripple suppression effect, by Figure 2 As can be seen, after ripple suppression is initiated in 7.5s, the voltage ripple drops from 1.15kV to 0.28kV, with a ripple suppression rate as high as 75%.

[0075] Figure 3 and Figure 4 The comparison of current harmonic suppression effects is shown. Figure 3 and Figure 4 In the graph, the horizontal axis represents frequency (Hz), and the vertical axis represents the percentage of harmonic amplitude in the fundamental frequency amplitude. The fundamental frequency (50Hz) amplitude was 3601A before suppression and 3614A after suppression. The total harmonic distortion (THD) of the current decreased from 22.69% before suppression to 2.75% after suppression. The power quality meets the grid connection conditions, and the 5th, 7th, 11th, and 13th current harmonics are effectively suppressed.

[0076] Figure 5 The waveforms show the reactive and active power during the ice melting period. After 3 seconds, the MMC (Mechanical Management Control) initiates reactive power compensation for the LCC (Limited Capacitor). Between 5 and 10 seconds, the grid voltage drops to 0.8 pu. Figure 5 As can be seen, after the reactive power compensation is activated, the de-icing device no longer absorbs reactive power, and after the grid voltage drops, the de-icing system feeds back 15 Mvar of reactive power to the grid to provide voltage support. Throughout the entire de-icing process, the de-icing power remains stable at 1.3 MW, maintaining normal de-icing operation.

[0077] This disclosure can be a system, method, and / or computer program product. A computer program product may include a computer-readable storage medium having computer-readable program instructions loaded thereon for causing a processor to implement various aspects of this disclosure.

[0078] Computer-readable storage media can be tangible devices capable of holding and storing instructions for use by an instruction execution device. Computer-readable storage media can be, for example—but not limited to—electrical storage devices, magnetic storage devices, optical storage devices, electromagnetic storage devices, semiconductor storage devices, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of computer-readable storage media include: portable computer disks, hard disks, random access memory (RAM), read-only memory (ROM), erasable programmable read-only memory (EPROM or flash memory), static random access memory (SRAM), portable compact disc read-only memory (CD-ROM), digital multifunction disc (DVD), memory sticks, floppy disks, mechanical encoding devices, such as punch cards or recessed protrusions storing instructions thereon, and any suitable combination of the foregoing. The computer-readable storage media used herein are not to be construed as transient signals themselves, such as radio waves or other freely propagating electromagnetic waves, electromagnetic waves propagating through waveguides or other transmission media (e.g., light pulses through fiber optic cables), or electrical signals transmitted through wires.

[0079] The computer-readable program instructions described herein can be downloaded from computer-readable storage media to various computing / processing devices, or downloaded via a network, such as the Internet, local area network, wide area network, and / or wireless network, to an external computer or external storage device. The network may include copper transmission cables, fiber optic transmission, wireless transmission, routers, firewalls, switches, gateway computers, and / or edge servers. A network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards them to the computer-readable storage media in the respective computing / processing device.

[0080] Computer program instructions used to perform the operations of this disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-dependent instructions, microcode, firmware instructions, status setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages ​​such as Smalltalk, C++, etc., and conventional procedural programming languages ​​such as the "C" language or similar programming languages. The computer-readable program instructions may execute entirely on the user's computer, partially on the user's computer, as a standalone software package, partially on the user's computer and partially on a remote computer, or entirely on a remote computer or server. In cases involving a remote computer, the remote computer may be connected to the user's computer via any type of network—including a local area network (LAN) or a wide area network (WAN)—or may be connected to an external computer (e.g., via the Internet using an Internet service provider). In some embodiments, electronic circuitry, such as programmable logic circuitry, field-programmable gate arrays (FPGAs), or programmable logic arrays (PLAs), is personalized by utilizing the status information of the computer-readable program instructions to implement various aspects of this disclosure.

[0081] 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 protection scope of the claims of the present invention.

Claims

1. A DC ice-melting device combining LCC and MMC, comprising a commutated converter (LCC) and a modular multilevel converter (MMC), characterized in that, The AC sides of LCC and MMC are connected in parallel and then connected to the power grid via a transformer. The DC sides of LCC and MMC are connected in series and then connected to the de-icing load, which is the three-phase AC transmission line to be de-iced. The DC de-icing device also includes: LCC control layer, MMC control layer, and collaborative control layer; The LCC control layer is used to obtain the DC voltage ripple, AC current harmonics, and reactive power requirements of the LCC. The MMC control layer is used to perform LCC DC voltage ripple suppression, LCC AC current harmonic suppression, and compensate for LCC reactive power demand. The collaborative control layer is used to calculate in real time the first active power output by the MMC when performing DC voltage ripple suppression of the LCC, the second active power output when performing AC current harmonic suppression of the LCC, and the reactive power output when compensating for the reactive power demand of the LCC, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power and reactive power output by the MMC meet the MMC capacity constraints, the power compensation term of the active-frequency outer loop is used to coordinate the power balance of the AC and DC sides of the MMC.

2. The DC ice-melting device combining LCC and MMC according to claim 1, characterized in that, LCC uses constant DC voltage / current control; MMC uses grid-type control.

3. A collaborative control method for a DC ice-melting device combining LCC and MMC, applicable to the DC ice-melting device combining LCC and MMC as described in claim 1 or 2, characterized in that, The LCC control layer acquires the DC voltage ripple of the LCC; the MMC control layer performs DC voltage ripple suppression of the LCC. The LCC control layer acquires the AC current harmonics of the LCC; the MMC control layer performs AC current harmonic suppression. The LCC control layer obtains the reactive power requirement of the LCC; the MMC control layer performs compensation for the reactive power requirement of the LCC. The collaborative control layer calculates in real time the first active power output by the MMC when performing LCC DC voltage ripple suppression, the second active power output when performing LCC AC current harmonic suppression, and the reactive power output when compensating for LCC reactive power demand, based on the LCC firing angle and DC current. Under the condition that the first active power, second active power, and reactive power output by the MMC meet the MMC capacity constraints, the power compensation term of the active-frequency outer loop coordinates the power balance of the AC and DC sides of the MMC.

4. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 3, characterized in that, Based on constant DC voltage / current control of LCC, calculate the DC voltage of LCC and its average value; and At that time, the average DC voltage output by the LCC is shown in the following formula: In the formula, The average DC voltage output by the LCC. This refers to the peak value of the AC phase voltage. For trigger angle, As the reference angular frequency, For a specific moment; Based on the fact that the conduction angle of each thyristor is... Calculate the DC voltage ripple at different firing angles, as follows: In the formula, This refers to the DC voltage ripple of the LCC.

5. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 3, characterized in that, The voltage difference between the de-icing voltage command value of the DC de-icing device and the DC voltage output by the LCC is obtained in real time and used as the DC voltage command value of the MMC, as shown in the following formula: In the formula, This is the DC voltage command value for the MMC. This refers to the de-icing voltage command value for the DC de-icing device. This is the DC voltage output by the LCC; Based on the DC voltage command value of MMC and the capacitor voltage of each submodule, the number of submodules connected to each bridge arm of MMC is adjusted to control the target value of the output bridge arm voltage of MMC in order to suppress the DC voltage ripple of LCC. The number of sub-modules configured in the bridge arm is determined by the following formula: In the formula, The total number of submodules configured for the bridge arm, The rated voltage of the capacitor built into the submodule. The average DC voltage output by the LCC. This refers to the peak value of the AC phase voltage. This refers to the DC voltage ripple of the LCC.

6. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 3, characterized in that, The target harmonic current output by the LCC is transformed into a coordinate system, converting the three-phase AC quantity into a two-phase DC quantity, which is then fed into the PI controller. The resulting dq-axis control quantity is used as the harmonic compensation quantity, as shown in the following formula: In the formula, , For dq axis control variables , This is the commanded value for the dq axis current. , These are the parameters of the PI controller; During control, the harmonic compensation quantity output by the PI controller is reconstructed into a three-phase AC modulation signal through inverse coordinate transformation, so as to control the MMC to generate the required harmonic compensation current and suppress AC current harmonics.

7. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 3, characterized in that, MMC employs a network-based control approach and adds a voltage droop control term to the reactive power-voltage loop to obtain the AC voltage command value for MMC, as shown in the following formula: In the formula, This is the AC voltage command value for the MMC. This is the AC voltage reference value for MMC. For grid-side voltage command, This is a reactive power instruction. This is the measured voltage on the grid side. This refers to the actual reactive power measured on the grid side. This is the voltage droop factor. This is the reactive power droop factor. For operators; Based on the AC voltage command value, the MMC outputs reactive power to compensate for the reactive power consumed by the LCC.

8. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 3, characterized in that, The satisfied MMC capacity constraint is shown in the following formula: In the formula, The first active power output by the MMC to perform LCC DC voltage ripple suppression. The second active power output by the MMC for performing LCC AC current harmonic suppression. The reactive power output by the MMC to compensate for the reactive power demand of the LCC. This refers to the rated capacity of the MMC.

9. The coordinated control method for the DC ice-melting device combining LCC and MMC according to claim 8, characterized in that, MMC employs a network-based control approach and introduces an active power compensation term into the active-frequency outer loop, satisfying the following relationship: In the formula, This is the active power compensation term for MMC. This is the compensation adjustment coefficient; The sum of the MMC active power command value and the active power compensation term in the active-frequency outer loop before the improvement is used as the MMC active power command value in the improved active-frequency outer loop.

10. A terminal, comprising a processor and a storage medium; characterized in that: The storage medium is used to store instructions; The processor is configured to operate according to the instructions to perform the steps of the method according to any one of claims 3-9.

11. A computer-readable storage medium having a computer program stored thereon, characterized in that, When executed by a processor, the program implements the steps of the method according to any one of claims 3-9.

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