Full-controlled converter with active commutation capability and modulation method thereof

Abstract

The application discloses a full-controlled converter with active commutation capability and a modulation method thereof, and belongs to the technical field of high-voltage direct current transmission. The converter comprises three phase units, each phase unit comprises a upper bridge arm and a lower bridge arm connected in series, each bridge arm comprises a plurality of full-bridge sub-modules, a plurality of full-controlled devices and a bridge arm inductor. The reference levels of the full-bridge sub-modules in the upper and lower bridge arms of the phase unit are obtained according to the bridge arm reference direct current voltage and the alternating current side voltage of the phase unit; when the bridge arm commutates, the commutation voltage is obtained according to the bridge arm inductor and the bridge arm current change rate of the bridge arm participating in commutation, the output voltage of the full-bridge sub-modules in the bridge arm participating in commutation is the corresponding reference level minus the commutation voltage, the bridge arm current is transferred between different bridge arms, and the active commutation is completed. The application can reduce the dependence of the commutation process on the natural commutation condition of the alternating current power grid, improve the commutation reliability, and take into account the active and reactive power regulation capability and the compact operation requirement of the converter.

Description

Technical Field

[0001] This invention belongs to the field of high voltage direct current transmission technology, specifically relating to a fully controlled converter with active commutation capability and its modulation method. Background Technology

[0002] High-voltage direct current (HVDC) transmission technology, with its advantages of low transmission losses, minimal line corridor occupation, and the ability to achieve asynchronous grid interconnection, has become the preferred solution for long-distance power transmission, renewable energy grid integration, and regional grid interconnection. It is also a crucial technological means supporting the energy transition strategy. As the core equipment of HVDC transmission systems, converters directly affect system operating characteristics. Current projects primarily utilize thyristor-based grid-commutated converters (LCCs) and voltage source converters (VSCs) based on fully controlled devices.

[0003] Traditional LCCs (Limited-Level Converters) have advantages such as low construction cost, large transmission capacity, and mature technology, making them the most widely used transmission type in actual high-voltage direct current (HVDC) transmission projects. However, because the thyristor devices used in LCCs rely on the AC grid for commutation voltage, they are prone to commutation failure during AC faults, easily leading to significant power deficits and threatening the security of the receiving-end grid. Furthermore, the unidirectional flow of thyristor current prevents LCCs from independently achieving active and reactive power regulation, and they lack the capability for grid connection and passive network power supply to renewable energy generation systems such as wind farms and photovoltaic power plants. Modular multilevel converters (MMCs), as typical voltage source converters, offer advantages such as four-quadrant operation, active and reactive power decoupling, and good voltage and current quality. However, the large number of power submodules and the energy storage capacitors and power semiconductor devices within them make MMC HVDC transmission systems large, heavy, and expensive, making it difficult to meet the compactness and lightweight requirements of converters in scenarios such as offshore wind power HVDC aggregation and transmission, and flexible interconnection of high-density urban power grids.

[0004] Currently, some scholars have proposed a three-level self-commutated current source converter (SCC) composed of fully controlled devices, which avoids the risk of commutation failure and has advantages such as high power density and power decoupling. To meet the flexibility and adjustability requirements of engineering applications, SCCs often employ SPWM and SVM modulation in scenarios such as reactive power compensation in the medium and low voltage fields. However, both of these modulation methods inevitably result in zero-level shoot-through of the bridge arm current, leading to large fluctuations in the converter's output DC voltage and requiring DC smoothing reactor values ​​to reach the "H" level, making it difficult to promote to high voltage applications. Research has proposed a fundamental frequency modulation method to eliminate the zero level, making the DC voltage characteristics similar to those of a traditional LCC. However, its modulation ratio is small, and the modulation freedom is limited, necessitating the search for a more flexible system coordination control strategy to expand the power operating range. Other researchers have modified dual-voltage source MMCs by connecting submodules with power devices and energy storage inductors in parallel to obtain modular multilevel current source converters. Direct control of the arm current is achieved by switching submodules. However, to avoid the risk of discontinuous arm current, additional thyristor freewheeling and parallel capacitors are required in the submodules for voltage equalization. This results in low power density and complex control strategies, making it difficult to apply in DC applications. In conclusion, there is an urgent need to find new converter topologies with high capacity, high power density, flexible control, and the ability to withstand commutation failure risks. Summary of the Invention

[0005] To address the problems in the prior art, this invention provides a fully controlled converter with active commutation capability and its modulation method.

[0006] The technical solution of the present invention is as follows:

[0007] In a first aspect, the present invention discloses a fully controlled converter with active commutation capability, comprising three phase units. Each phase unit includes an upper bridge arm and a lower bridge arm connected in series. The DC-side terminals of the upper bridge arms of all phase units are interconnected, and the DC-side terminals of the lower bridge arms of all phase units are interconnected. The AC-side terminals of the upper and lower bridge arms in each phase unit are connected to the AC-side of the corresponding phase unit. Each bridge arm includes multiple full-bridge sub-modules, multiple fully controlled devices, and a bridge arm inductor connected in series. The fully controlled devices are used to control the complementary half-wave conduction of the upper and lower bridge arms of each phase unit.

[0008] Specifically, the reference level of the full-bridge submodule in the upper and lower bridge arms of the phase unit is obtained based on the reference DC voltage of the bridge arm and the AC side voltage of the phase unit. When the bridge arm is turned on, the full-bridge submodule outputs a multi-level stepped wave with the reference level as the modulation target. When the bridge arm is commutated, the commutation voltage is obtained based on the bridge arm inductance and the bridge arm current change rate of the bridge arm participating in the commutation. The output voltage of the full-bridge submodule in the bridge arm participating in the commutation is the corresponding reference level minus the commutation voltage, so that the commutation voltage acts on the bridge arm inductance, controls the transfer of bridge arm current between different bridge arms, and completes active commutation.

[0009] Furthermore, when the bridge arm is turned off, the full-bridge submodule continuously outputs a preset level to reduce the voltage stress on the fully controlled devices in the corresponding bridge arm.

[0010] Secondly, the present invention also discloses a modulation method for the aforementioned fully controlled converter, comprising the following steps:

[0011] The upper and lower bridge arms of each phase unit are controlled to conduct complementary half-waves. Based on the reference DC voltage of the bridge arm and the AC side voltage of each phase unit, the reference level of the full-bridge submodule in the upper and lower bridge arms of each phase unit is obtained according to Kirchhoff's laws. When the bridge arm is on, the reference level is used as the modulation target, and the full-bridge submodule is controlled to output a multi-level stepped wave by the nearest level approximation modulation. When the bridge arm is off, the full-bridge submodule is controlled to continuously output a preset level.

[0012] When the fully controlled converter is located on the inverter side, within a 60° electrical angle after the bridge arm to be turned on, the commutation voltage is obtained based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned on. The output voltage of the full-bridge submodule in the bridge arm to be turned on is the reference level minus the commutation voltage, so that the bridge arm current is transferred from the original conducting bridge arm to the bridge arm to be turned on. When the fully controlled converter is located on the rectifier side, within a 60° electrical angle before the bridge arm to be turned off, the commutation voltage is obtained based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned off. The output voltage of the full-bridge submodule in the bridge arm to be turned off is the reference level minus the commutation voltage, so that the bridge arm current is transferred from the bridge arm to be turned off to the next conducting bridge arm, thereby completing active commutation on the inverter side or rectifier side.

[0013] Further, obtaining the commutation voltage based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned on includes: multiplying twice the bridge arm inductance by the preset bridge arm current change rate of the bridge arm to be turned on to obtain the commutation voltage; obtaining the commutation voltage based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned off includes: multiplying twice the bridge arm inductance by the preset bridge arm current change rate of the bridge arm to be turned off to obtain the commutation voltage.

[0014] Furthermore, the lead of the initial conduction phase of the upper bridge arm relative to the AC side voltage phase of its corresponding phase unit is defined as the phase shift angle; by adjusting the phase shift angle, the active power and reactive power output ratio of the fully controlled converter can be adjusted; the phase shift angle, the DC current of the fully controlled converter, the reference DC voltage of the bridge arm, the bridge arm inductance, and the AC side voltage of the corresponding phase unit satisfy the bridge arm energy balance constraint relationship.

[0015] Furthermore, the energy balance constraint relationship of the bridge arm is such that the energy accumulation of the energy storage capacitor of the full-bridge submodule is zero during the conduction interval of the bridge arm in one power frequency cycle.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0017] 1) This invention adopts a topology architecture that combines module cascading and device series connection. By setting multiple cascaded full-bridge sub-modules, multiple series fully controlled devices, and bridge arm inductors in each bridge arm, while maintaining a bridge arm structure suitable for high-capacity power transmission, the number of cascaded modules and energy storage capacitors is reduced by half compared to the MMC solution. At the same time, no DC capacitor is required, thereby reducing the number of power modules and energy storage components used in the converter, reducing the equipment size and weight caused by a large number of sub-modules and energy storage capacitors, and achieving higher power density and lightweight level. This is beneficial for balancing high-capacity power transmission capacity and the compact requirements of the converter.

[0018] 2) This invention controls the switching on and off of the bridge arms using series-connected fully controlled devices and adjusts the output voltage of the full-bridge submodule during bridge arm commutation. This allows the full-bridge submodule to actively generate a commutation voltage acting on the bridge arm inductor, controlling the transfer of bridge arm current between different bridge arms. This eliminates the commutation process's reliance on the AC grid for natural commutation voltage, thus reducing the risk of commutation failure under abnormal AC grid conditions in traditional LCCs. Simultaneously, by adjusting the phase shift angle to change the phase relationship between AC side current and AC side voltage, the ratio of active and reactive power output can be flexibly adjusted, expanding the converter's power operating range. Furthermore, by cascading submodules to output multi-level stepped waves according to a reference level, decoupling of AC and DC voltages is achieved, improving the DC voltage waveform quality. Therefore, while retaining the characteristics of current source converters suitable for high-capacity, high-power-density operation, this invention achieves active commutation and power regulation capabilities similar to VSCs. Compared to LCC schemes, it offers advantages such as AC / DC decoupling, flexible active and reactive power allocation, and resistance to commutation failure.

[0019] 3) The modulation method described in this invention actively generates commutation voltage through a full-bridge submodule and uses the bridge arm inductance to control the bridge arm current to transfer according to a predetermined change process. This allows the current in the bridge arm to gradually build up from zero when it is turned on and gradually decrease to zero before it is turned off, thereby achieving zero-current turn-on and turn-off of the bridge arm and reducing the operating losses during the switching process of fully controlled devices. At the same time, this topology does not require the use of reverse-resistance devices with bidirectional withstand voltage capability, and when the power transmission direction is fixed, a full-bridge submodule structure combining fully controlled devices and uncontrolled diodes can be used, thereby reducing the need for high-performance power devices and further reducing the system construction cost. Attached Figure Description

[0020] Figure 1 A topology diagram of a fully controlled converter with active commutation capability provided in an embodiment of the present invention;

[0021] Figure 2This is a schematic diagram of the bridge arm state when the upper bridge arm of phase a of the fully controlled converter located on the inverter side is turned on in an embodiment of the present invention.

[0022] Figure 3 This is a schematic diagram of the conduction signal of the fully controlled device connected in series in phase a of the fully controlled converter located on the inverter side in an embodiment of the present invention, as well as the waveforms of the AC side voltage, the upper arm voltage, and the AC side current.

[0023] Figure 4 This is a schematic diagram of the switching state of the full-bridge sub-modules of the fully controlled converter located on the inverter side under the sub-module capacitor voltage balancing strategy in an embodiment of the present invention.

[0024] Figure 5 The above diagram shows the simulated waveforms of the output level of the full-bridge submodule and the inductor voltage of the bridge arm in the upper bridge arm of the fully controlled converter located on the inverter side in this embodiment of the invention.

[0025] Figure 6 This is a simulation waveform diagram of the upper arm current and AC current of phase a of the fully controlled converter located on the inverter side in an embodiment of the present invention.

[0026] Figure 7 This is a simulation waveform diagram of the submodule capacitor voltage and DC bus voltage of the fully controlled converter located on the inverter side in an embodiment of the present invention. Detailed Implementation

[0027] The present invention will be further described and illustrated below with reference to specific embodiments. The embodiments described are merely examples of the content of this disclosure and do not limit the scope of the invention. The technical features of each embodiment in the present invention can be combined accordingly, provided that there is no mutual conflict.

[0028] This invention relates to a topology of a fully controlled converter with active commutation capability, as shown in the figure. Figure 1 As shown, the fully controlled converter includes three phase units: phase a, phase b, and phase c. These three phase units are connected in parallel on the DC side of the fully controlled converter. Each phase unit includes an upper bridge arm and a lower bridge arm connected in series. The DC side ends of the upper bridge arms of all phase units are interconnected, and the DC side ends of the lower bridge arms of all phase units are interconnected. The AC side ends of the upper and lower bridge arms in each phase unit are connected to the AC side of the corresponding phase unit.

[0029] Each bridge arm consists of N SM The full-bridge submodules SM and N DE All fully controlled components and bridge arm inductor L arm composition. Figure 1In this diagram, VT1, VT3, and VT5 correspond to the upper arms of phases a, b, and c, respectively, while VT4, VT6, and VT2 correspond to the lower arms of phases a, b, and c, respectively. Taking phase a as an example, the connection point A between the upper arm containing VT1 and the lower arm containing VT4 constitutes the AC side connection point for phase a; correspondingly, the connection point between VT3 and VT6 constitutes the AC side connection point for phase b, and the connection point between VT5 and VT2 constitutes the AC side connection point for phase c. The three AC side connection points output the AC current for phase a and the AC current i for phase b, respectively. b and c-phase alternating current i c The corresponding AC side voltages of phase a, phase b, and phase c are u, respectively. a u b and u c .in, Figure 1 i in dc This represents the DC current of a fully controlled converter, further expressed in u. dc Indicates the reference DC voltage of the bridge arm, u j and i j (j=a, b, c) represent the AC side voltage and AC side current of phase j, respectively, i jp and i jn U represents the upper arm current and lower arm current in phase j, respectively. jpSM and u jnSM These represent the output voltages of the full-bridge submodules in the upper and lower arms of phase j, respectively. Figure 1 The current reference direction is shown, and the DC current i of the fully controlled converter is... dc The current flows into the common DC side of the upper bridge arm and out through the common DC side of the lower bridge arm; the current i in the upper bridge arm of phase a. ap Phase b upper arm current i bp and the current i of the upper bridge arm of phase c cp The current i in the lower arm of phase a flows from the corresponding upper bridge arm to VT1, VT3, and VT5 respectively. an Phase b lower bridge arm current i bn and the lower bridge arm current i of phase c cn The AC current i flows to the corresponding lower bridge arm via VT4, VT6, and VT2 respectively. a i b and i c The reference direction is pointed from the fully controlled converter to the AC side.

[0030] Based on the above topology, when the bridge arm is turned on, the reference level of the full-bridge submodule in the upper and lower bridge arms is determined according to the bridge arm reference DC voltage and the AC side voltage of the corresponding phase unit. For phase j, the reference level of the full-bridge submodule in the upper bridge arm is... and the reference level of the full-bridge submodule in the lower bridge arm They are represented as follows:

[0031]

[0032] like Figure 4 As shown, each full-bridge submodule includes four power devices and one energy storage capacitor. The four power devices form two series branches, which are connected in parallel across the energy storage capacitor. The midpoints of the two series branches serve as the two AC output terminals of the full-bridge submodule. By controlling the switching states of the four power devices, the full-bridge submodule can achieve positive on, negative on, and bypass states to generate different output levels required by the bridge arms.

[0033] When a fully controlled converter needs to adapt to bidirectional power transmission, all four power devices in the full-bridge submodule are fully controlled, enabling the submodule to switch the corresponding conduction state according to the power transmission direction. Fully controlled devices include IGBTs, MOSFETs, or IGCTs. When the power transmission direction of the fully controlled converter is fixed, some of the fully controlled devices in the full-bridge submodule can be replaced with uncontrolled diodes according to the fixed power transmission direction, so that each full-bridge submodule consists of two fully controlled devices and two uncontrolled diodes. This reduces the number of fully controlled devices while maintaining the required submodule output state under the corresponding power transmission direction, thus lowering the converter's device cost and drive control complexity. Taking inverter operation as an example, T1 and T4 in the full-bridge submodule can be replaced with uncontrolled diodes.

[0034] In the above topology, the cascaded full-bridge submodules in each bridge arm are used to modulate and output multi-level stepped waves according to the reference level of the corresponding bridge arm. This ensures that the bridge arm output voltage simultaneously includes voltage components corresponding to the bridge arm reference DC voltage and the AC side voltage of the phase unit, achieving decoupling between the AC and DC side voltages. The reference level of the full-bridge submodules in the upper and lower bridge arms of the phase unit is obtained based on the bridge arm reference DC voltage and the AC side voltage of the phase unit, according to Kirchhoff's laws. During bridge arm commutation, the output voltage of the full-bridge submodules in the participating bridge arms is adjusted to reduce the commutation voltage based on the corresponding reference level. This commutation voltage acts on the bridge arm inductor to control the transfer of bridge arm current between different bridge arms, achieving active commutation.

[0035] Fully controlled devices connected in series in each bridge arm control the switching on and off of the corresponding bridge arm, and the bridge arm conduction sequence is formed by the complementary half-wave conduction of the upper and lower bridge arms of each phase. Since the current transfer between bridge arms is controlled by the commutation voltage actively generated by the full-bridge submodule, the commutation process does not rely on the AC grid to provide a natural commutation voltage, thereby reducing the risk of commutation failure when the AC grid is abnormal. The bridge arm inductors are used to establish the bridge arm current change process under the commutation voltage, assisting in current transfer and energy transfer between bridge arms.

[0036] In one embodiment of the present invention, the present invention also provides a modulation method for a fully controlled converter with active commutation capability, the method comprising the following steps:

[0037] The upper and lower bridge arms of each phase unit are controlled to conduct in a complementary half-wave manner. Based on the reference DC voltage of the bridge arm and the AC side voltage of each phase unit, the reference level of the full-bridge submodule in the upper and lower bridge arms of each phase unit is obtained according to Kirchhoff's laws. When the bridge arm is in the conducting state, the reference level is used as the modulation target. The switching state of the full-bridge submodule is determined by the nearest level approximation modulation, so that the output voltage of each full-bridge submodule is superimposed to form a multi-level stepped wave, thereby enabling the bridge arm output voltage to follow the corresponding reference level. When the bridge arm is in the off state, the full-bridge submodule is controlled to continuously output a preset level to reduce the voltage stress on the fully controlled devices connected in series in the bridge arm.

[0038] For a fully controlled converter located on the inverter side, within a 60° electrical angle after the arm to be turned on, the commutation voltage required for commutation is obtained based on the arm inductance and the preset rate of change of the arm current. Within this commutation interval, the full-bridge submodule in the arm to be turned on is modulated, causing its output voltage to decrease relative to the corresponding reference level by the commutation voltage. This creates a voltage across the arm inductor to drive the change in arm current, causing the current in the originally turned arm to transfer to the arm to be turned on according to a predetermined change process. By actively adjusting the output level of the full-bridge submodule, the commutation conditions required for current transfer between arms can be actively established, so that the commutation process no longer relies solely on the natural commutation conditions of the AC side voltage.

[0039] For a fully controlled converter located on the rectifier side, the arm conduction sequence corresponds to that on the inverter side, and the commutation modulation process employs a control method that is dual to that on the inverter side. Within a 60° electrical angle before the arm to be turned off, the commutation voltage is obtained based on the arm inductance and the preset rate of change of arm current of the arm to be turned off. The output voltage of the full-bridge submodule in the arm to be turned off is then controlled to decrease the commutation voltage relative to the corresponding reference level. The resulting commutation voltage acts on the arm inductor, causing the current in the arm to be turned off to gradually transfer to the next conducting arm until current commutation between the arms is completed. Through the dual modulation method on both the inverter and rectifier sides, the voltage required for commutation can be actively generated by the full-bridge submodule, thereby achieving active commutation.

[0040] Furthermore, in this embodiment, the lead of the initial conduction phase of the upper bridge arm relative to the phase of the AC side voltage of the corresponding phase unit is defined as the phase shift angle. Since the phase of the AC side current is related to the conduction timing of the bridge arm, changing the phase shift angle can alter the phase relationship between the AC side current and the AC side voltage, thereby adjusting the active and reactive power output ratios of the fully controlled converter. Thus, without altering the basic active commutation control process described above, the converter's operating power state can be adjusted by regulating the conduction phase of the bridge arm.

[0041] Simultaneously, the phase shift angle, the DC current of the fully controlled converter, the reference DC voltage of the bridge arm, the bridge arm inductance, and the AC side voltage amplitude of the corresponding phase unit collectively satisfy the bridge arm energy balance constraint. This constraint ensures that the energy accumulation of the energy storage capacitor in the full-bridge submodule is zero within the conduction interval of the corresponding bridge arm over one power frequency cycle. This maintains energy balance during cyclic operation, preventing voltage shifts caused by continuous energy accumulation or release, and providing stable energy conditions for the full-bridge submodule to continuously modulate according to the reference level and commutation voltage.

[0042] Figure 2 Taking the upper bridge arm of phase a as the example of current transferring from the upper bridge arm of phase c to the upper bridge arm of phase a, a schematic diagram of the bridge arm conduction in the commutation state of the inverter side of a fully controlled converter is given. In this commutation state, the DC voltage and current are constant, the upper bridge arm of phase a, the upper bridge arm of phase c, and the lower bridge arm of phase b are simultaneously turned on, and the remaining bridge arms are turned off. The commutation state lasts for 60° electrical angle. Figure 2 The bridge arm indicated in red is the off bridge arm. Under the above commutation state, the electrical quantities satisfy:

[0043]

[0044] in, The output voltage of the full-bridge submodule in the upper arm of phase a; For bridge arm inductance The current of the upper arm of phase a Rate of change over time; The AC side voltage of phase a; This is the output voltage of the full-bridge submodule in the upper arm of phase c; for rate of change over time This refers to the current in the upper arm of phase c. For the DC current of a fully controlled converter; This is the AC side voltage of phase c.

[0045] During this commutation process, the output voltage of the full-bridge submodule in the upper arm of phase a is adjusted, while the full-bridge submodules in the other arms maintain their corresponding reference levels. The output voltage of the full-bridge submodule in the upper arm of phase a satisfies:

[0046]

[0047] in, The full-bridge submodule generates its own commutation voltage through modulation.

[0048] Therefore, given the bridge arm inductance and the required bridge arm current change rate during commutation, the corresponding commutation voltage can be determined. By reducing the commutation voltage of the full-bridge submodule in the bridge arm to be turned on from the corresponding reference level, the commutation voltage is applied to the bridge arm inductor, and the bridge arm current can be actively controlled to shift from the original conducting bridge arm to the bridge arm to be turned on according to the required change process.

[0049] Since the commutation voltage is not coupled with the AC grid voltage, the fully controlled converter breaks through the limitation of LCC (Line-Commutated Converter) based on the natural commutation point and has the ability to actively commutate at any time.

[0050] The commutation state analysis of the remaining bridge arms is the same as that of the upper bridge arm of phase a. The conduction sequence of the six bridge arms is as follows: VT1, VT2, VT3 conduct; VT2, VT3, VT4 conduct; VT3, VT4, VT5 conduct; VT4, VT5, VT6 conduct; VT5, VT6, VT1 conduct; VT6, VT1, VT2 conduct, and so on, cycling in the above order. Each state lasts for 60° electrical angle, and at any given time, there is always a bridge arm in the commutation process.

[0051] When the fully controlled converter is located on the rectifier side, its arm conduction sequence is consistent with that of the fully controlled converter located on the inverter side. The commutation modulation process is coupled with that of the fully controlled converter located on the inverter side. Specifically, within the 60° electrical angle before the arm to be turned off, the commutation voltage is determined based on the arm inductance and the preset arm current change rate of the arm to be turned off. The output voltage of the full-bridge submodule in the arm to be turned off is controlled to be the corresponding reference level minus the commutation voltage. The full-bridge submodules in the other arms can maintain the corresponding reference level.

[0052] Figure 3 Taking phase a as an example, the turn-on strategy of the series-connected fully controlled devices and the AC side voltage (i.e. Figure 3 AC voltage in the middle), upper arm voltage, and conduction signal S of the series fully controlled device. ap and AC side current (i.e. Figure 3The phase relationship between the AC current in the phase is defined as follows: Specifically, the series-connected fully controlled devices control the complementary half-wave conduction of the upper and lower bridge arms of phase a. The initial conduction phase of the upper bridge arm is defined to lead the AC side voltage phase α of its phase unit, i.e., the phase shift angle is defined as α. Under the action of the self-generated commutation voltage of the full-bridge submodule, the bridge arm can achieve zero-current turn-on and zero-current turn-off. Among them, the AC side voltage of phase b lags behind the AC side voltage of phase a by 120°, and the AC side voltage of phase c leads the AC side voltage of phase a by 120°.

[0053] Figure 3 The upper bridge arm voltage waveform in the figure represents the voltage change of the upper bridge arm of phase a during the corresponding conduction and commutation processes, where 1 / 2u dc This represents the DC voltage component determined by the reference DC voltage of the bridge arm. When the bridge arm is normally conducting, the full-bridge submodule in the upper bridge arm is modulated according to the corresponding reference level. When the bridge arm enters the commutation process, the output voltage of the full-bridge submodule in the participating bridge arm is adjusted to reduce the commutation voltage based on the corresponding reference level. This commutation voltage acts on the bridge arm inductor to control the transfer of bridge arm current between different bridge arms. In this way, during the commutation process at the start and end of conduction of the bridge arm, the bridge arm current can be gradually built up from zero and gradually reduced to zero, thereby achieving zero-current turn-on and zero-current turn-off of the bridge arm.

[0054] The AC current of a corresponding phase unit is formed by the superposition of the upper and lower bridge arm currents of that phase. Figure 3 Taking the AC current of phase a as an example, its current gradually changes during the commutation of the bridge arm, maintains a corresponding current level in the non-commutation conduction range, and forms an alternating positive and negative AC current waveform as the conduction state of the upper and lower bridge arms switches. Since the starting phase of the AC current is consistent with the starting conduction phase of the upper bridge arm, the phase of the AC current leads the phase of the AC voltage of the corresponding phase unit by an angle α. By controlling the phase shift angle α, the phase relationship between the AC current and the AC voltage can be changed, thereby adjusting the ratio of active power to reactive power output.

[0055] Simultaneously, the phase shift angle, the DC current of the fully controlled converter, the reference DC voltage of the bridge arm, the bridge arm inductance, and the AC side voltage amplitude of the corresponding phase unit collectively satisfy the bridge arm energy balance constraint relationship. This bridge arm energy balance constraint relationship is used to limit the matching relationship between the above operating parameters, ensuring that the energy accumulation of the energy storage capacitor of the full-bridge submodule is zero within the conduction interval of the corresponding bridge arm in one power frequency cycle. This allows the energy storage capacitor to maintain energy balance during cyclic operation, avoiding periodic voltage shifts caused by continuous net energy accumulation or release, and providing stable energy conditions for the full-bridge submodule to continuously modulate according to the reference level and commutation voltage.

[0056] Specifically, under the above modulation strategy, combined with Figure 1Given the current reference direction, the upper arm current i of phase j can be derived. jp Represented as:

[0057]

[0058] in, It is the power frequency angular frequency. Let be the initial phase of the AC side voltage of the j-th phase unit.

[0059] Based on the complementary half-wave conduction relationship between the upper and lower bridge arms of each phase unit, the current i of the lower bridge arm of phase j can be obtained accordingly. jn .

[0060] As can be seen from the above relationship of the bridge arm current, after the bridge arm enters the commutation stage, the bridge arm current gradually increases according to the corresponding change process; within the stable conduction range, the bridge arm current maintains the corresponding current level; in the commutation stage of exiting conduction, the bridge arm current gradually decreases to zero, thus corresponding to the aforementioned active commutation process within 60° electrical angle and the zero-current turn-on and zero-current turn-off of the bridge arm.

[0061] Under the aforementioned modulation strategy, to maintain the energy conservation of the submodule capacitor, the energy accumulation of the capacitor must be zero within the conduction half-cycle. Taking the upper bridge arm of phase j as an example, the energy accumulation of the energy storage capacitor of its full-bridge submodule within the conduction interval of one power frequency cycle of the corresponding bridge arm is denoted as E. jp ,but:

[0062]

[0063] Substituting the above relationships of bridge arm currents and full-bridge submodule output voltages, we get:

[0064]

[0065] in, The energy accumulation of the energy storage capacitor of the full-bridge submodule in the upper arm of phase j within the conduction range of that arm; This represents the AC side voltage amplitude of the corresponding phase unit.

[0066] exist Under operating conditions, the above energy balance relationship can be further written as:

[0067]

[0068] The above relationship is the energy balance constraint relationship of the bridge arm. From this relationship, it can be seen that the phase shift angle, the DC current of the fully controlled converter, the reference DC voltage of the bridge arm, the amplitude of the bridge arm inductance and the AC side voltage of the corresponding phase unit together constitute the combination of operating parameters that meet the energy balance requirements of the bridge arm.

[0069] The energy balance constraint relationship of the bridge arm does not require that the phase shift angle must be uniquely determined by other operating parameters, but rather it restricts the matching relationship that the phase shift angle must satisfy with other operating parameters. Therefore, when changing the phase shift angle to adjust the phase relationship between the AC side current and the AC side voltage, the corresponding operating parameters still need to satisfy the above-mentioned energy balance constraint relationship of the bridge arm, so as to take into account both the adjustment of the active power and reactive power output ratio and the periodic energy balance of the energy storage capacitor of the full bridge submodule.

[0070] Furthermore, under the condition that the energy conservation of the submodule capacitors is satisfied during operation, the capacitor voltages of each full-bridge submodule are balanced and regulated. Figure 1 The reference direction of the bridge arm current is shown. When the fully controlled converter on the inverter side operates according to the above modulation strategy, its bridge arm current is always positive. In each control cycle, the capacitor voltage of each full-bridge submodule is sampled, and each full-bridge submodule is sorted from high to low according to the capacitor voltage. At the same time, the number of full-bridge submodules put into operation in the current control cycle is determined according to the nearest level approximation modulation.

[0071] When a full-bridge submodule is positively engaged, it is selected in ascending order of capacitor voltage, corresponding to the number of modules to be engaged, while the remaining full-bridge submodules are bypassed. When a full-bridge submodule is negatively engaged, it is selected in descending order of capacitor voltage, corresponding to the number of modules to be engaged, while the remaining full-bridge submodules are bypassed. By selecting the appropriate full-bridge submodules for positive or negative engagement based on the capacitor voltage ranking, the voltage difference between different full-bridge submodules can be reduced, achieving balanced control of the capacitor voltage of each full-bridge submodule. The full-bridge submodule switching states under the energy storage capacitor voltage balanced control method are as follows: Figure 4 As shown. Figure 4 In the diagram, the black power devices represent those that are in the on state under the corresponding switching state, and the red power devices represent those that are in the off state under the corresponding switching state.

[0072] The simulation verification results provided in the embodiments of this application are presented below. The simulation references the electrical parameters of a DC back-to-back interconnection project in a certain region: rated power of 2500MW, reference DC voltage of the bridge arm of 840kV, rated current of 2976A, and AC phase voltage of 525kV. When the converter of the DC back-to-back interconnection project in a certain region adopts the MMC scheme, the rated voltage of the submodule is 1.56kV, the submodule capacitance is 12mF, and 540 submodules are required for a single bridge arm to support the DC bus voltage. Using the fully controlled converter with active commutation capability provided by this invention, the number of full-bridge submodules required for a single bridge arm under the same operating conditions can be reduced by half, that is, only 270 full-bridge submodules and 270 series-connected fully controlled devices are required. Based on the above parameters, the inverter side simulation is performed. Substituting the energy balance constraint of this invention, and operating at the common maximum active power (α=0), the bridge arm inductance L is obtained.arm It is 183mH.

[0073] Specifically, such as Figure 5 As shown, it gives the output level u of the full-bridge submodule in the upper arm of phase a. jpSM and bridge arm inductor voltage u jpL The simulated waveform. Among them, Figure 5 The upper half is the bridge arm inductor voltage waveform of the upper bridge arm of phase a. Figure 5 The lower half of the waveform represents the output voltage waveform of the full-bridge submodule in the upper arm of phase a. From... Figure 5 As can be seen from the diagram, during the commutation of the bridge arm, by adjusting the output voltage of the full-bridge submodule in the upper bridge arm a, the output voltage is reduced from the corresponding reference level to decrease the commutation voltage. This generates a voltage on the bridge arm inductor that corresponds to the change in bridge arm current, thereby controlling the bridge arm current to transfer according to a predetermined change process and achieving active commutation.

[0074] Figure 6 Simulated waveforms of the upper arm current of phase a and the alternating current of phase a are given. Figure 6 As can be seen, under the action of the commutation voltage actively generated by the full-bridge submodule, the current of the upper arm of phase a can gradually increase from zero with a controllable slope until its peak value reaches the corresponding rated DC current. At the same time, zero-current turn-on and zero-current turn-off can be achieved when the bridge arm enters and exits the conduction state. The AC side current of phase a is formed by the superposition of the current of the upper and lower arms of phase a, and the simulation results correspond to the above-mentioned bridge arm conduction and commutation mechanism.

[0075] Figure 7 The upper half is the simulated waveform of the capacitor voltage of the full-bridge submodule in the upper arm of phase a unit. Figure 7 The lower half represents the DC output voltage u of the fully controlled converter. dc The simulated waveform. (From...) Figure 7 As can be seen from the upper part, under the above balancing strategy, the capacitor voltage of each full-bridge submodule fluctuates periodically within the corresponding voltage range, and the capacitor voltage between different full-bridge submodules can remain relatively consistent. The capacitor voltage fluctuation rate of the submodule is about 6.5%, indicating that the full-bridge submodule switching method based on capacitor voltage sorting can achieve capacitor voltage balancing between each full-bridge submodule.

[0076] Furthermore, considering the commutation effect and the impact of the bridge arm commutation process on the DC-side voltage, the DC output voltage should be 300L lower than the bridge arm reference DC voltage. arm i dc According to the simulation parameters of this embodiment, the DC output voltage is approximately 680kV. Figure 7The lower part shows that the DC output voltage remains stable around 680kV, accompanied by periodic fluctuations with small amplitude, and the DC output voltage fluctuation rate is less than 1%. This indicates that a relatively stable DC output voltage can still be obtained without additional DC filter capacitors. The simulation results are consistent with the theoretical analysis above.

[0077] As can be seen from the above embodiments, the proposed fully controlled converter with active commutation capability and its modulation method, while possessing the advantage of high power density, can effectively realize AC / DC decoupling, active and reactive power allocation, and resist commutation failure, and has low operating losses, showing good application prospects in the field of high voltage DC transmission.

[0078] The above-described embodiments are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. Those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A fully controlled converter with active commutation capability, characterized in that, It includes three phase units, each phase unit comprising an upper bridge arm and a lower bridge arm connected in series. The DC-side terminals of the upper bridge arms of all phase units are interconnected, and the DC-side terminals of the lower bridge arms of all phase units are interconnected. The AC-side terminals of the upper and lower bridge arms in each phase unit are connected to the AC-side of the corresponding phase unit. Each bridge arm includes multiple full-bridge sub-modules, multiple fully controllable devices, and a bridge arm inductor connected in series. The fully controllable devices are used to control the complementary half-wave conduction of the upper and lower bridge arms of each phase unit. Specifically, the reference level of the full-bridge submodule in the upper and lower bridge arms of the phase unit is obtained based on the reference DC voltage of the bridge arm and the AC side voltage of the phase unit. When the bridge arm is turned on, the full-bridge submodule outputs a multi-level stepped wave with the reference level as the modulation target. When the bridge arm is commutated, the commutation voltage is obtained based on the bridge arm inductance and the bridge arm current change rate of the bridge arm participating in the commutation. The output voltage of the full-bridge submodule in the bridge arm participating in the commutation is the corresponding reference level minus the commutation voltage, so that the commutation voltage acts on the bridge arm inductance, controls the transfer of bridge arm current between different bridge arms, and completes active commutation.

2. The fully controlled converter with active commutation capability according to claim 1, characterized in that, The fully controllable device is an IGBT, MOSFET, or IGCT.

3. The fully controlled converter with active commutation capability according to claim 1, characterized in that, When the fully controlled converter is used for unidirectional power transmission, the four power devices in each full-bridge submodule include two fully controlled devices and two uncontrolled diodes.

4. The fully controllable converter according to claim 1, characterized in that, Based on the reference DC voltage of the bridge arm and the AC side voltage of each phase unit, the reference level of the full-bridge submodule in the upper and lower bridge arms of each phase unit is obtained according to Kirchhoff's laws.

5. The fully controlled converter with active commutation capability according to claim 1, characterized in that, When the bridge arm is turned off, the full-bridge submodule continuously outputs a preset level to reduce the voltage stress on the fully controlled devices in the corresponding bridge arm.

6. A modulation method for a fully controlled converter according to any one of claims 1-5, characterized in that, Includes the following steps: The upper and lower bridge arms of each phase unit are controlled to conduct complementary half-waves. Based on the reference DC voltage of the bridge arm and the AC side voltage of each phase unit, the reference level of the full-bridge submodule in the upper and lower bridge arms of each phase unit is obtained according to Kirchhoff's laws. When the bridge arm is on, the reference level is used as the modulation target, and the full-bridge submodule is controlled to output a multi-level stepped wave by the nearest level approximation modulation. When the bridge arm is off, the full-bridge submodule is controlled to continuously output a preset level. When the fully controlled converter is located on the inverter side, within a 60° electrical angle after the bridge arm to be turned on, the commutation voltage is obtained based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned on. The output voltage of the full-bridge submodule in the bridge arm to be turned on is the reference level minus the commutation voltage, so that the bridge arm current is transferred from the original conducting bridge arm to the bridge arm to be turned on. When the fully controlled converter is located on the rectifier side, within a 60° electrical angle before the bridge arm to be turned off, the commutation voltage is obtained based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned off. The output voltage of the full-bridge submodule in the bridge arm to be turned off is the reference level minus the commutation voltage, so that the bridge arm current is transferred from the bridge arm to be turned off to the next conducting bridge arm, thereby completing active commutation on the inverter side or rectifier side.

7. The modulation method according to claim 6, characterized in that, The step of obtaining the commutation voltage based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be conducted includes: multiplying twice the bridge arm inductance by the preset bridge arm current change rate of the bridge arm to be conducted to obtain the commutation voltage. The step of obtaining the commutation voltage based on the bridge arm inductance and the preset bridge arm current change rate of the bridge arm to be turned off includes: multiplying twice the bridge arm inductance by the preset bridge arm current change rate of the bridge arm to be turned off to obtain the commutation voltage.

8. The modulation method according to claim 6, characterized in that, The lead of the initial conduction phase of the upper bridge arm relative to the AC side voltage phase of its phase unit is defined as the phase shift angle; by adjusting the phase shift angle, the active power and reactive power output ratio of the fully controlled converter can be adjusted. The phase shift angle, the DC current of the fully controlled converter, the reference DC voltage of the bridge arm, the bridge arm inductance, and the AC side voltage of the corresponding phase unit satisfy the bridge arm energy balance constraint relationship.

9. The modulation method according to claim 8, characterized in that, The energy balance constraint relationship of the bridge arm is to ensure that the energy accumulation of the energy storage capacitor of the full bridge submodule is zero during the conduction interval of the bridge arm in one power frequency cycle.

10. The modulation method according to claim 6, characterized in that, In each control cycle, the capacitor voltage of the energy storage capacitor of each full-bridge submodule is sampled, and the full-bridge submodules are sorted from high to low according to the capacitor voltage. The number of full-bridge submodules to be put into operation is determined according to the nearest level approximation modulation. When a full-bridge submodule is being put into operation, the full-bridge submodules corresponding to the number of submodules to be put into operation are selected in order of capacitor voltage from low to high, and the other full-bridge submodules are bypassed. When a full-bridge submodule is being put into operation, the full-bridge submodules corresponding to the number of submodules to be put into operation are selected in order of capacitor voltage from high to low, and the other full-bridge submodules are bypassed, so as to balance the capacitor voltage of the energy storage capacitor of each full-bridge submodule.

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