A direct current transmission system with reactive self-compensation and commutation failure resistance and a control method thereof

Abstract

The application discloses a DC power transmission system with reactive power self-compensation and commutation failure resistance and a control method, and relates to the technical field of high-voltage DC power transmission.The system comprises a converter, a static reactive power generator and a converter transformer.The converter adopts a converter device with active shutdown capability, and a dynamic voltage equalization branch and a lightning arrester composed of a static voltage equalization resistor and an RC element are connected in parallel at both ends of each converter device, which are used for protecting reliable operation of the device.The static reactive power generator is connected in parallel between the AC side of the converter and the converter transformer.When the voltage at the receiving end drops, the converter starts a forced commutation mode, so as to force the current to be transferred from the device to the lightning arrester, and the residual voltage generated by the lightning arrester when the current flows through the lightning arrester makes the current commutate to the next bridge arm.In the steady state, the reactive power exchange amount is changed by adjusting the tap position and cooperating with the static reactive power generator.The application effectively resists commutation failure and realizes decoupling control of active power and reactive power.

Description

Technical Field

[0001] This invention relates to the field of high-voltage direct current (HVDC) transmission technology, specifically to a HVDC transmission system and control method with reactive power self-compensation and commutation failure resistance capabilities. Background Technology

[0002] Currently, in the field of high-voltage direct current (HVDC) transmission, conventional HVDC transmission systems based on grid-commutated converters (LCCs) are widely used due to their mature technology and large capacity. However, the thyristors used in LCCs are semi-controlled devices and do not have active turn-off capability. When a fault occurs in the receiving-end AC system, causing a voltage drop, commutation failure is very likely to occur, resulting in DC power interruption and seriously threatening the safe and stable operation of the power grid.

[0003] In addition, in order to maintain commutation margin, traditional LCC receivers often use constant turn-off angle control, but this makes the DC voltage susceptible to changes in AC system voltage and reactive power, making it difficult to decouple active power from reactive power. If constant DC voltage control is used to improve power characteristics, the risk of commutation failure is further increased because a sufficient turn-off angle cannot be guaranteed.

[0004] Therefore, there is an urgent need for a DC power transmission technology that can simultaneously resist commutation failure and achieve active and reactive power decoupling control. Summary of the Invention

[0005] Purpose of the invention: To address the problems of commutation failure and strong coupling between active and reactive power that easily occur when the AC voltage at the receiving end drops in existing technologies, this invention provides a DC transmission system and control method with reactive power self-compensation and commutation failure resistance capabilities, which can improve system operation stability and decouple active and reactive power control.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is: a DC transmission system with reactive power self-compensation and commutation failure resistance capabilities, including a converter, a static var generator and a converter transformer;

[0007] The converter uses a converter device with active shutdown capability, and the two ends of the converter device are connected in parallel with a static voltage equalization resistor, a dynamic voltage equalization branch and a surge arrester.

[0008] The static var generator is connected in parallel between the AC side of the converter and the converter transformer;

[0009] When the voltage of the AC system at the receiving end drops to a preset condition, the converter starts the forced commutation mode, controls the exit valve to perform gate shutdown, forces the current to transfer to the surge arrester, and uses the residual voltage generated when the surge arrester is conducting current to cause the current to transfer from the buffer resistor branch to the new trigger valve.

[0010] During steady-state operation, the reactive power required by the converter is changed by adjusting the tap position of the converter transformer, and the reactive power output by the static var generator is changed according to the set value, thereby changing the reactive power exchange between the DC transmission system and the receiving-end AC system, so as to achieve decoupling control of active power and reactive power.

[0011] Furthermore, the dynamic voltage equalization branch connected in parallel to the converter devices in this DC transmission system is composed of a resistor and a capacitor connected in series. The surge arrester is connected in parallel with the static voltage equalization resistor, and then in parallel with the dynamic voltage equalization branch. The static voltage equalization resistor is connected in parallel across the converter devices. The static voltage equalization resistor, the dynamic voltage equalization branch, and the surge arrester are used to perform:

[0012] When the voltage drop in the AC system at the receiving end causes the converter to be unable to complete natural commutation, the forced commutation mode is activated. The exit valve performs gate shutdown, forcing the current to transfer to the surge arrester. The residual voltage of the surge arrester causes the current to transfer to the new trigger valve.

[0013] Furthermore, the surge arrester is a metal oxide resistor.

[0014] Furthermore, the control modes of the static var generator include a reactive power self-compensation mode and a decoupling control mode;

[0015] In the reactive power self-compensation mode, the static var generator provides the reactive power required for steady-state operation of the converter and converter transformer, so that the reactive power exchange between the DC transmission system and the receiving-end AC system approaches zero.

[0016] In decoupling control mode, the static var generator outputs reactive power at the rated value. By adjusting the tap position of the converter transformer, the reactive power required by the converter is changed, thereby changing the reactive power exchange between the DC transmission system and the receiving-end AC system.

[0017] Furthermore, in the decoupled control mode, when the turns ratio of the converter transformer increases, the reactive power required by the converter decreases, and the excess reactive power flows into the receiving-end AC system; when the turns ratio of the converter transformer decreases, the reactive power required by the converter increases, and the DC transmission system absorbs reactive power from the receiving-end AC system; in the above process, the active power of the DC transmission system does not change with the change in reactive power exchange.

[0018] Furthermore, the static var generator is also configured to perform harmonic suppression control;

[0019] The static var generator detects the current flowing into itself and the current flowing out of the converter, extracts harmonic components of a specific order, and generates a compensation voltage reference value to suppress harmonic current injected into the receiving-end AC system.

[0020] Further, the step of extracting harmonic components of a specific order and generating a compensation voltage reference value includes:

[0021] The current flowing into the static var generator and the current flowing out of the converter are transformed by synchronous rotating coordinates.

[0022] The current flowing into the static var generator and the current flowing out of the converter are transformed by synchronous rotating coordinates, including extracting the DC component in synchronous rotating coordinates through a low-pass filter.

[0023] Using the DC component of the current flowing out of the converter in the synchronous rotating coordinate as a reference value, and subtracting it from the DC component of the current flowing into the static var generator in the synchronous rotating coordinate, a proportional-integral controller is used to generate a dq-axis voltage reference value.

[0024] The three-phase voltage reference value is generated by inverse dq transformation to drive the static var generator to output the corresponding compensation voltage.

[0025] Furthermore, the control strategy of the converter includes a low-voltage current limiting circuit, constant DC current control, and constant DC voltage control;

[0026] During steady-state operation, the converter adopts constant DC voltage control; when the DC voltage on the rectifier side drops or the DC voltage on the inverter side rises, it switches to constant DC current control; during faults, the DC current on the rectifier side is dynamically reduced based on the DC voltage on the inverter side through a low-voltage current limiting circuit.

[0027] The present invention also provides a control method for the above-described DC transmission system, comprising:

[0028] (1) In response to the voltage drop of the AC system at the receiving end to the preset condition, the exit valve of the converter is controlled to perform gate shutdown, and the current is forced to transfer to the surge arrester. The residual voltage generated by the surge arrester is used to cause the current to transfer to the new trigger valve to complete the forced commutation.

[0029] (2) During steady-state operation, the tap position of the converter transformer is adjusted to change the reactive power required by the converter, and the reactive power output by the static var generator is adjusted according to the set value to change the reactive power exchange between the DC transmission system and the receiving AC system, so as to achieve decoupling control of active power and reactive power.

[0030] Furthermore, the control command generation process for the static var generator includes:

[0031] Based on the deviation between the reference value of the submodule capacitor voltage and the average voltage of the submodule capacitor, an active current reference command is generated through proportional-integral regulation to charge and discharge the submodule capacitor to maintain voltage equalization.

[0032] Based on the deviation between the reactive power reference value and the actual value, a reactive current reference command is generated through proportional-integral regulation; the actual value is the reactive power exchange between the converter station and the AC system.

[0033] The voltage reference values ​​for reactive power compensation and harmonic compensation are superimposed, and the drive signal for the static var generator is generated through a preset modulation strategy.

[0034] Beneficial effects: Compared with the prior art, the DC transmission system and control method provided by the present invention, which has reactive power self-compensation and commutation failure resistance capabilities, establishes the physical basis for forced commutation by using converter devices with active shutdown capability, and connecting each converter device in parallel with a dynamic voltage equalization branch composed of static voltage equalization resistors and RC elements and a surge arrester.

[0035] When the AC system voltage at the receiving end drops to a preset condition, the system can actively control the exit valve to perform gate turn-off, forcing the current to transfer to the surge arrester branch. The residual voltage generated by the current flowing through this branch serves as an additional commutation voltage source, facilitating a smooth transfer of current to the new trigger valve. This mechanism overcomes the limitation of traditional semi-controlled devices relying on the AC grid voltage crossing zero for natural commutation, physically eliminating the risk of commutation failure due to voltage drops and significantly improving the operational stability of the DC transmission system in weak AC grid environments.

[0036] On the other hand, this invention achieves decoupling of active and reactive power through the coordinated control of the static var generator (SVA) and the converter transformer tap position. During steady-state operation or reactive power regulation, the SVA outputs reactive power according to a set value. At this time, the reactive power demand of the converter itself is changed by adjusting the tap position of the converter transformer. Since the output of the SVA is relatively constant, the change in the reactive power demand of the converter is transformed into a change in reactive power exchange between the system and the receiving-end AC system. The active power is mainly determined by DC voltage and current and is not affected by the reactive power flow changes caused by tap adjustment.

[0037] This decoupling control strategy of the present invention enables the system to flexibly adjust its reactive power support or absorption capacity to the AC system while maintaining stable active power transmission, thus solving the technical problem of strong coupling between active and reactive power under traditional constant DC voltage control. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the topology of a DC power transmission system according to an embodiment of the present invention;

[0039] Figure 2 This is the steady-state equivalent circuit diagram of the receiving end according to an embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the converter valve branch circuit according to an embodiment of the present invention;

[0041] Figure 4 This is a waveform diagram of the valve voltage and current in the natural commutation mode according to an embodiment of the present invention;

[0042] Figure 5 This is a waveform diagram of the forced commutation mode valve voltage and current according to an embodiment of the present invention;

[0043] Figure 6 This is a block diagram of the LCC-SHCC DC system control according to an embodiment of the present invention;

[0044] Figure 7 This is a block diagram of the SVG harmonic suppression control strategy according to an embodiment of the present invention;

[0045] Figure 8 This is a block diagram of the receiving-end HCC multi-mode coordinated control strategy according to an embodiment of the present invention;

[0046] Figure 9 This is the steady-state equivalent circuit diagram of the reactive power self-compensation mode according to an embodiment of the present invention;

[0047] Figure 10 This is the steady-state equivalent circuit diagram of the decoupling control mode according to an embodiment of the present invention;

[0048] Figure 11 This is a waveform diagram of the steady-state operating electrical quantities according to an embodiment of the present invention;

[0049] Figure 12 This is a comparison diagram of the current waveforms before and after SVG harmonic compensation in an embodiment of the present invention;

[0050] Figure 13 This is a diagram showing the distribution of harmonic content in the grid-connected current according to an embodiment of the present invention;

[0051] Figure 14 This is a graph showing the active power step dynamic response characteristics of an embodiment of the present invention.

[0052] Figure 15 This is a waveform diagram of reactive power change when the transformer turns ratio increases according to an embodiment of the present invention;

[0053] Figure 16 This is a timing diagram of the tap position and turn-off angle during the transformer ratio reduction process according to an embodiment of the present invention;

[0054] Figure 17 This is a waveform diagram of reactive power change when the transformer turns ratio decreases according to an embodiment of the present invention;

[0055] Figure 18 This is a waveform comparison of the fault response characteristics of SHCC and HCC in an embodiment of the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0057] Combination Figures 1-3 As shown, this embodiment provides a DC transmission system with reactive power self-compensation and commutation failure mitigation capabilities. The DC topology of this system includes a converter, a static var generator, and a converter transformer.

[0058] The converter uses a converter device with active turn-off capability, such as a reverse blocking integrated gate commutator thyristor (RB-IGCT), which is used to realize subsequent forced commutation operation.

[0059] A static var generator is connected in parallel between the AC side of the converter and the converter transformer to dynamically provide or absorb reactive power and suppress harmonics.

[0060] The converter transformer is connected between the static var generator and the AC system. Its valve-side winding is connected to the converter, and its grid-side winding is connected to the receiving-end AC bus.

[0061] Combination Figure 3 As shown, the surge arrester MOV and the first resistor (static equalizing resistor) After being connected in parallel, it is then connected in parallel with a dynamic voltage equalization branch composed of RC elements. This dynamic voltage equalization branch composed of RC elements includes capacitors connected in series. Second resistor Simultaneously, a static voltage equalization resistor is connected in parallel across the converter device. Based on this structure, the static voltage equalization resistor ensures voltage balance of the converter device in the off-state; the RC dynamic voltage equalization branch limits the rate of voltage change (dv / dt) during the converter device's turn-on and turn-off processes; and the surge arrester constitutes a critical energy transfer channel during forced commutation. When the receiving-end AC system voltage drops to a preset condition, the converter initiates forced commutation mode, controls the exit valve to perform gate turn-off, forces the current to transfer to the surge arrester, and utilizes the residual voltage generated by the surge arrester to transfer the current to a new trigger valve.

[0062] The following further combines Figure 4 and Figure 5 The physical mechanism of forced commutation is analyzed. Under natural commutation mode, such as... Figure 4 As shown, the transfer of current from the exit valve (e.g., VT4) to the entry valve (e.g., VT6) depends entirely on the AC line voltage. The provided commutation voltage satisfies the following voltage integral equation during the commutation process:

[0063]

[0064] in, To trigger the angular angle, To change the phase angle, This is the effective value of the phase voltage on the AC side. For the shut-off angle, It is direct current. This is the commutation inductor. When a serious fault occurs in the AC system, causing a significant voltage drop... Significantly reduced; if the original trigger angle is maintained, the turn-off angle will be reduced. The value will shrink drastically or even become negative, causing natural commutation to fail.

[0065] In the forced commutation mode of this embodiment, such as Figure 5 As shown, when a voltage drop to a preset threshold is detected, the control system immediately sends a shutdown signal to the exit valve. Because the converter device has an active shutdown capability, the exit valve quickly closes, forcing the current that was originally flowing through the exit valve to instantly transfer to the parallel metal oxide resistor. The nonlinear volt-ampere characteristic of the metal oxide resistor causes it to generate a relatively high residual voltage when current flows through it. At this point, the voltage balance equation of the commutation circuit becomes:

[0066]

[0067] in, For the forced commutation duration, This represents the residual voltage of the metal oxide resistor. This refers to the reactance of the converter transformer referred to the valve side. Therefore, it can be seen that even at AC line voltage... The residual voltage generated by the metal oxide resistor when the voltage drops to extremely low levels or even below zero due to a fault. As an additional commutation voltage source, it can still maintain a sufficient commutation voltage area (Volt-second product), forcing the current to smoothly complete the transfer from the exit valve through the MOV to the new trigger valve. This invention adopts an "active shutdown + residual voltage commutation" mechanism, which fundamentally breaks through the dependence of traditional semi-controlled devices on grid voltage and effectively resists the risk of commutation failure.

[0068] During steady-state operation or reactive power regulation, the reactive power required by the converter transformer is changed by adjusting the tap position. This, combined with the reactive power output of the static var generator (SVA) at a set value, alters the reactive power exchange between the DC transmission system and the receiving-end AC system, thus achieving decoupled control of active and reactive power. Specifically, the SVA control modes include reactive power self-compensation mode and decoupled control mode. In reactive power self-compensation mode, the SVA provides all the reactive power required for steady-state operation to the converter and converter transformer, causing the reactive power exchange between the DC transmission system and the receiving-end AC system to approach zero. At this time, the system exhibits a pure active load and does not impose a reactive power burden on the AC system.

[0069] Combination Figure 9 and Figure 10 Further explanation. In decoupling control mode, the static var generator outputs reactive power at its rated value. By adjusting the tap position of the converter transformer, the reactive power required by the converter is changed, thereby altering the reactive power exchange between the DC transmission system and the receiving-end AC system. The mathematical principle is as follows: the reactive power absorbed by the converter... It can be represented as:

[0070]

[0071] in, This is the no-load DC voltage of the inverter station. This is the actual DC voltage. Because... ,and With transformer turns ratio They are inversely proportional, therefore under constant DC voltage control, and Keep constant, Only follow (i.e., ratio) (Change). When the turns ratio of the converter transformer increases, Reduce the reactive power required by the converter. Reduced; due to the static var generator operating at its rated value Fixed output, excess reactive power ( The current will flow into the receiving-end AC system. Conversely, when the transformation ratio decreases, As the system increases, it absorbs reactive power from the receiving-end AC system. During this process, active power... Determined solely by DC voltage and current, and unaffected by changes in reactive power exchange, this achieves complete decoupling of active and reactive power. This decoupling control allows DC transmission systems to participate in grid voltage support as an independent reactive power regulation mechanism without impacting active power transmission plans.

[0072] Furthermore, the static var generator (SVA) is also configured to perform harmonic suppression control. The SVA extracts specific harmonic components by detecting the current flowing into and out of the converter and generates a compensation voltage reference value to suppress harmonic currents injected into the receiving-end AC system. Specifically, in conjunction with... Figure 7 and Figure 8 As shown, the process of extracting specific harmonic components (such as the 11th and 13th harmonics) and generating compensation voltage reference values ​​for characteristic harmonics (e.g., the 11th and 13th harmonics) generated by a 12-pulse converter includes: transforming the current flowing into and out of the converter using a synchronous rotating coordinate system; extracting the DC component in the synchronous rotating coordinate system using a low-pass filter; using the DC component of the current flowing out of the converter in the synchronous rotating coordinate system as a reference value, subtracting it from the DC component of the current flowing into the converter in the synchronous rotating coordinate system, and then generating a dq-axis voltage reference value using a proportional-integral controller; and generating three-phase voltage reference values ​​through an inverse dq transformation to drive the static var generator to output the corresponding compensation voltage. For example, for the 11th negative-sequence harmonic, a negative-sequence 11th-order synchronous rotating coordinate transformation matrix is ​​used. It can convert the three-phase time-varying 11th harmonic current into a DC current in the dq coordinate system. and This facilitates zero steady-state error tracking by the PI controller. Compared to traditional proportional resonant (PR) control, this harmonic suppression strategy based on a synchronous rotating coordinate system offers better dynamic performance and robustness.

[0073] The control strategies for converters include low-voltage current limiting, constant DC current control, and constant DC voltage control. For example... Figure 6 As shown, during steady-state operation, the converter employs constant DC voltage control to maintain a constant DC bus voltage. When the DC voltage on the rectifier side drops or the DC voltage on the inverter side rises above a certain threshold, it automatically switches to constant DC current control to prevent overcurrent or overvoltage. During faults, the low-voltage current limiting link (VDCOL) dynamically reduces the rectifier side DC current command based on the inverter side DC voltage, alleviating commutation pressure and accelerating fault recovery. Because the converter has active turn-off capability, constant DC voltage control eliminates the need for a large turn-off angle margin as required by traditional LCCs, thereby improving the power factor and active power transmission capacity during steady-state operation.

[0074] Finally, regarding the generation of the drive signal for the static var generator (SVA), the control command generation process includes: generating an active current reference command based on the deviation between the submodule capacitor voltage reference value and the average voltage of the submodule capacitor through proportional-integral (PI) regulation to charge and discharge the submodule capacitors and maintain voltage equalization; generating a reactive current reference command based on the deviation between the reactive power reference value and the actual value through PI regulation; and superimposing the reactive power compensation voltage reference value and the harmonic compensation voltage reference value, and generating the drive signal for the SVA through a preset modulation strategy. This multi-objective collaborative control architecture ensures that the SVA maintains internal energy balance while performing reactive power compensation and harmonic suppression, achieving stable and reliable operation of the system-level functions.

[0075] Example 2: To verify the effectiveness of the technical solution of the present invention in practical engineering applications, this example uses the PSCAD / EMTDC electromagnetic transient simulation platform to build a 500kV / 2kA level 12-pulse LCC-SHCC DC transmission system model for verification. This model fully includes the converter, static var generator, converter transformer, and its supporting control system. The simulation parameters are set according to the CIGRE Benchmark standard test model, aiming to intuitively demonstrate the actual operating effect of the technical solution through simulation data of three typical operating conditions: steady state, dynamic, and fault.

[0076] First, the steady-state operating characteristics of the system are verified. Under the condition that the static var generator is in reactive power self-compensation mode and performing harmonic suppression control, the waveforms of various electrical quantities of the system are as follows: Figure 11 As shown in the figure. Simulation results show that the DC voltage at the receiving end stabilizes at around 1.0 pu, the DC current remains at 1.0 pu, and the lead firing angle stabilizes at around 28.5°.

[0077] More importantly, the active power Pac on the AC side remained stable at 0.975 pu, while the reactive power Qac on the AC side consistently hovered around 0 pu. The reactive power output of the static var generator (SVA) was 1.02 pu. This fully verifies that the SVA can provide all the reactive power required for steady-state operation of the converter and converter transformer, bringing the reactive power exchange between the DC transmission system and the receiving-end AC system close to zero. Simultaneously, regarding the characteristic harmonics generated by the converter, the SVA detects the current flowing into and out of the converter, extracts specific harmonic components, and generates a compensation voltage reference value, effectively suppressing the harmonic current injected into the receiving-end AC system.

[0078] like Figure 12 and Figure 13As shown, the current waveform flowing into the AC system after compensation exhibits a smooth standard sinusoidal shape, with a total harmonic distortion (THD) of approximately 1.16%, far below the 3% limit specified in the IEEE 519 standard. The content of each harmonic is effectively suppressed, demonstrating the effectiveness of the harmonic suppression strategy based on synchronous rotating coordinate transformation in engineering applications.

[0079] Next, the decoupling control characteristics of active and reactive power were verified. In decoupling control mode, when the DC current reference value at the sending end jumps from 1.0 pu to 0.8 pu at t=5s and recovers at t=6s, the system dynamic response curve is as follows: Figure 14 As shown in the data, regardless of whether the active power experiences a negative step or a positive recovery, the reactive power exchange quantity Qac between the receiving-end grid and the converter station only fluctuates slightly and does not change significantly with the change in active power Pac, indicating that the system maintains good decoupling characteristics during dynamic adjustment.

[0080] Furthermore, when the static var generator outputs reactive power at its rated value, the reactive power required by the converter is changed by adjusting the tap position of the converter transformer, thereby changing the reactive power exchange between the DC transmission system and the receiving-end AC system.

[0081] Specifically, such as Figure 15 As shown, when the transformer ratio gradually increases, the turn-off angle decreases from 19.5° to 2.1°, the reactive power required by converter 100 decreases, and the excess reactive power flows into the receiving-end AC system, causing Qac to gradually increase from 0 pu to 0.17 pu; conversely, as... Figure 16 and Figure 17 As shown, when the transformer turns ratio decreases, the turn-off angle increases to 28.5°, and the system absorbs reactive power from the receiving-end AC system, causing Qac to drop to -0.155pu. Throughout the tap changer adjustment process, the active power Pac remains stable around 0.975pu, without changing with the amount of reactive power exchange.

[0082] This simulation result strongly confirms that the control strategy of achieving complete decoupling of active and reactive power through tap adjustment and fixed output of static var generator is feasible.

[0083] Finally, the system's fault response characteristics and commutation failure resilience were verified. To highlight the technical advantages of this invention, a comparative test was conducted between the SHCC system employing a forced commutation mechanism and the HCC system using the traditional CIGRE control strategy. The test covered six typical fault conditions: 0.1H single-phase grounding, 0.05H single-phase grounding, single-phase metallic grounding, 0.1H three-phase grounding, 0.05H three-phase grounding, and three-phase metallic grounding, with a fault duration of 100ms for each condition.

[0084] like Figure 18 As shown, under all test conditions, the bus voltage drop of the SHCC system was significantly smaller than that of the HCC system, and the voltage recovery speed was faster after the fault was cleared. For example, under the most severe condition of a three-phase metallic ground fault, the SHCC system did not prevent commutation failure, and compared with the HCC system, the bus voltage of the SHCC system recovered faster after the fault was cleared.

[0085] Meanwhile, the DC current and active power of the SHCC can quickly recover to their rated values ​​after the fault is cleared, and its active power deficit area is significantly smaller than that of the HCC system. Most importantly, during the aforementioned six fault periods, the reactive power exchange Qac between the SHCC system and the receiving-end AC system remains near 0pu.

[0086] This fully demonstrates that when the AC system voltage at the receiving end drops to the preset condition, the converter initiates the forced commutation mode, controls the exit valve to perform gate shutdown, forces the current to transfer to the surge arrester, and utilizes the residual voltage generated by the surge arrester to cause the current to transfer to the new trigger valve. This mechanism can effectively resist commutation failure and ensure the safe and stable operation of the system in a weak AC power grid environment.

[0087] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. For example, the specific selection of converter devices, the topology of the buffer resistor branch, the harmonic extraction algorithm of the static var generator, and the threshold setting in the control strategy can all be adaptively adjusted or equivalently replaced according to actual engineering needs. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this invention should be included within the scope of protection of this invention. Therefore, the scope of protection of this invention should be determined by the scope of the claims.

Claims

1. A DC transmission system with reactive power self-compensation and commutation failure mitigation capabilities, characterized in that, This includes converters, static var generators, and converter transformers; The converter uses a converter device with active shutdown capability, and the two ends of the converter device are connected in parallel with a static voltage equalization resistor, a dynamic voltage equalization branch and a surge arrester. The static var generator is connected in parallel between the AC side of the converter and the converter transformer; When the AC system voltage at the receiving end drops to a preset condition, the converter starts the forced commutation mode, controls the exit valve to perform gate shutdown, forces the current to transfer to the surge arrester, and uses the residual voltage generated when the surge arrester is conducting current to cause the current to transfer from the buffer resistor branch to the new trigger valve. During steady-state operation, the reactive power required by the converter is changed by adjusting the tap position of the converter transformer, and the reactive power output by the static var generator is changed according to the set value, thereby changing the reactive power exchange between the DC transmission system and the receiving-end AC system, so as to achieve decoupling control of active power and reactive power.

2. The DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 1, characterized in that: In a DC transmission system, the dynamic voltage equalization branch connected in parallel with the converter device is composed of a resistor and a capacitor connected in series. The surge arrester is connected in parallel with the static voltage equalization resistor, and then in parallel with the dynamic voltage equalization branch. The static voltage equalization resistor is connected in parallel across the converter device. The static voltage equalization resistor, the dynamic voltage equalization branch, and the surge arrester are used to perform: When the voltage drop in the AC system at the receiving end causes the converter to be unable to complete natural commutation, the forced commutation mode is activated. The exit valve performs gate shutdown, forcing the current to transfer to the surge arrester. The residual voltage of the surge arrester causes the current to transfer to the new trigger valve.

3. A DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 1, characterized in that, The control modes of the static var generator include reactive power self-compensation mode and decoupling control mode; In the reactive power self-compensation mode, the static var generator provides the reactive power required for steady-state operation of the converter and converter transformer, so that the reactive power exchange between the DC transmission system and the receiving-end AC system approaches zero. In decoupling control mode, the static var generator outputs reactive power at the rated value. By adjusting the tap position of the converter transformer, the reactive power required by the converter is changed, thereby changing the reactive power exchange between the DC transmission system and the receiving-end AC system.

4. A DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 3, characterized in that: In decoupling control mode, when the turns ratio of the converter transformer increases, the reactive power required by the converter decreases, and the excess reactive power flows into the receiving-end AC system; when the turns ratio of the converter transformer decreases, the reactive power required by the converter increases, and the DC transmission system absorbs reactive power from the receiving-end AC system. During the above process, the active power of the DC transmission system does not change with the change in reactive power exchange.

5. A DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 1, characterized in that: The static var generator is also configured to perform harmonic suppression control; The static var generator detects the current flowing into itself and the current flowing out of the converter, extracts harmonic components of a specific order, and generates a compensation voltage reference value to suppress harmonic current injected into the receiving-end AC system.

6. A DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 5, characterized in that, The step of extracting harmonic components of a specific order and generating a compensation voltage reference value includes: The current flowing into the static var generator and the current flowing out of the converter are transformed by synchronous rotating coordinates, including extracting the DC component in synchronous rotating coordinates through a low-pass filter. Using the DC component of the current flowing out of the converter in the synchronous rotating coordinate as a reference value, and subtracting it from the DC component of the current flowing into the static var generator in the synchronous rotating coordinate, a proportional-integral controller is used to generate a dq-axis voltage reference value. The three-phase voltage reference value is generated by inverse dq transformation to drive the static var generator to output the corresponding compensation voltage.

7. A DC transmission system with reactive power self-compensation and commutation failure resistance capability according to claim 1, characterized in that, The control strategy of the converter includes a low-voltage current limiting circuit, constant DC current control, and constant DC voltage control. During steady-state operation, the converter adopts constant DC voltage control; when the DC voltage on the rectifier side drops or the DC voltage on the inverter side rises, it switches to constant DC current control; during faults, the DC current on the rectifier side is dynamically reduced based on the DC voltage on the inverter side through a low-voltage current limiting circuit.

8. A control method for a DC transmission system with reactive power self-compensation and commutation failure resistance capability as described in any one of claims 1-7, characterized in that, include: In response to the voltage drop of the AC system at the receiving end to a preset condition, the control converter's exit valve is turned off, forcing the current to transfer to the surge arrester. The residual voltage generated by the surge arrester's current flow causes the current to transfer to the new trigger valve, thereby completing the forced commutation. During steady-state operation, the tap position of the converter transformer is adjusted to change the reactive power required by the converter, and the reactive power output by the static var generator is adjusted according to the set value to change the reactive power exchange between the DC transmission system and the receiving-end AC system, so as to achieve decoupling control of active power and reactive power.

9. The control method according to claim 8, characterized in that, The control command generation process for the static var generator includes: Based on the deviation between the reference value of the submodule capacitor voltage and the average voltage of the submodule capacitor, an active current reference command is generated through proportional-integral regulation to charge and discharge the submodule capacitor to maintain voltage equalization. Based on the deviation between the reactive power reference value and the actual value, a reactive current reference command is generated by proportional-integral regulation. The actual value is the reactive power exchange between the converter station and the AC system. The voltage reference values ​​for reactive power compensation and harmonic compensation are superimposed, and the drive signal for the static var generator is generated through a preset modulation strategy.

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