A hybrid vsc and current source acc medium voltage grid connected inverter and control method

Abstract

The application provides a kind of mixed VSC and current source ACC medium voltage grid-connected inverter and control method, belongs to medium voltage grid-connected inverter field, the medium voltage grid-connected inverter includes medium voltage ACC, three-phase low voltage VSC, power frequency transformer, AC reactor and DC reactor etc..During operation, ACC works in fundamental frequency switching mode, and outputs three-phase square wave current.VSC undertakes part of power flow, and simultaneously governs high-order harmonic in square wave current through active filtering function, and simultaneously improves grid current power factor through reactive power compensation function.Compared with prior art, the application combines the advantages of voltage source fast flexibility and current source high efficiency and economy, improves system power density and power transmission efficiency.

Description

Technical Field

[0001] This invention belongs to the field of medium-voltage grid-connected inverters, specifically relating to a hybrid VSC and current source ACC medium-voltage grid-connected inverter and its control method. Background Technology

[0002] The energy structure is undergoing profound changes, with renewable distributed energy, primarily solar and wind power, becoming increasingly prevalent in power distribution networks. Renewable distributed energy mainly uses DC power supply; to efficiently and safely connect to medium-voltage AC distribution networks, medium-voltage grid-connected inverters are needed for voltage conversion and power transmission. Common medium-voltage grid-connected inverters include power frequency transformer + voltage source converter (VSC), modular multilevel converter (MMC), and power electronic transformer (PET) solutions. In large-capacity applications, the power frequency transformer + voltage source converter (VSC) integrates multiple power frequency transformers, resulting in a large and heavy system. While the modular multilevel converter (MMC) can achieve high-power transmission, its integrated multiple power modules lead to low power density and high system cost. The power electronic transformer (PET) solution uses high-frequency transformer isolation, resulting in a relatively small size, but it has multiple power conversion stages, low power transmission efficiency, and often requires customized designs, further increasing system cost. Besides VSC medium-voltage grid-connected inverters, current source grid-commutated converters (LCCs) have attracted widespread attention from researchers due to their low cost, high power transmission efficiency, and high power density of valve group units. However, this circuit topology uses semi-controlled thyristor power devices and relies on the grid for commutation. Under weak AC grid conditions, there is a risk of commutation failure, resulting in low system reliability. Furthermore, LCCs have high harmonic and reactive power content in the output current, leading to poor power quality. When connected to the grid, a large number of filters and reactive power compensation devices are required, resulting in low system power density. Summary of the Invention

[0003] To overcome the shortcomings of existing technologies, this invention proposes a hybrid VSC and current source ACC medium-voltage grid-connected inverter and its control method. The medium-voltage grid-connected inverter includes a medium-voltage ACC, a three-phase low-voltage VSC, a power frequency transformer, an AC reactor, and a DC reactor. During operation, the ACC operates in baseband switching mode, outputting a three-level square wave current. The VSC, while handling a portion of the power flow, uses active filtering to mitigate high-order harmonics in the square wave current, and simultaneously utilizes reactive power compensation to improve the power factor of the grid-connected current.

[0004] To achieve the above objectives, the present invention adopts the following technical solution:

[0005] A medium-voltage grid-connected inverter with hybrid VSC and current source ACC includes a converter main circuit, which consists of a medium-voltage current source ACC, a low-voltage three-phase VSC, a power frequency transformer, and a DC reactor L. GIt consists of AC reactors La1, Lb1, Lc1, La2, Lb2, and Lc2; ACC represents an active phase-commutation converter, and VSC represents a voltage source converter; the DC side of the medium-voltage current source ACC is connected in series with the DC side of the low-voltage three-phase VSC and the DC reactors, and then connected to the medium-voltage DC grid; the AC side of the low-voltage three-phase VSC is connected in series with the AC reactors La2, Lb2, Lc2, and the power frequency transformer, and then connected in parallel with the AC side of the medium-voltage current source ACC, and then connected to the three-phase medium-voltage AC grid through the AC reactors La1, Lb1, and Lc1;

[0006] On the DC side, the medium-voltage current source ACC is composed of bridge arm units S1, S2, S3, S4, S5, and S6. The positive terminals of bridge arm units S1, S3, and S5 are connected in parallel to form terminal Np+, which is connected to the DC reactor L. G Connected on one side, DC reactor L G The other side is connected to the positive terminal P of the medium-voltage DC; the negative terminals of bridge arm units S1, S3, and S5 and the positive terminals of bridge arm units S4, S6, and S2 are connected to form AC terminals u1, v1, and w1; the negative terminals of bridge arm units S4, S6, and S2 are connected in parallel to form terminal Np-.

[0007] The low-voltage three-phase VSC consists of switching devices SW1, SW2, SW3, SW4, SW5, SW6, SW7, and SW8, as well as diodes D1 and D2. The collectors of switching devices SW1, SW3, and SW5 are connected in parallel and then connected to a DC capacitor C. L The positive terminal of the switch is formed by connecting the emitters of switching devices SW1, SW3, and SW5 in parallel with the collectors of switching devices SW2, SW4, and SW6 to form AC terminals u2, v2, and w2. The emitters of switching devices SW2, SW4, and SW6 are then connected in parallel to a DC capacitor C. L The negative terminal of diode D1; the cathode of diode D1 is connected in parallel with the collector of switching device SW8 and then connected to DC capacitor C. L The positive terminal of diode D1 is connected to the collector of switching device SW7 to form AC terminal u3. The emitter of switching device SW8 is connected to the cathode of diode D2 to form AC terminal v3. The emitter of switching device SW7 and the anode of diode D2 are connected in parallel to DC terminal C. L The negative electrode;

[0008] The medium-voltage current source ACC terminal Np- is connected to the low-voltage three-phase VSC AC terminal u3. The low-voltage three-phase VSC AC terminal v3 is connected to the medium-voltage DC negative terminal N. The medium-voltage current source ACC AC terminals u1, v1, and w1 are connected to the corresponding AC terminals A1, B1, and C1 on the high-voltage side of the power frequency transformer, as well as one end of the AC reactors La1, Lb1, and Lc1. The other end of the AC reactors La1, Lb1, and Lc1 is directly connected to the A, B, and C phase terminals of the three-phase medium-voltage AC power grid. The low-voltage three-phase VSC AC terminals u2, v2, and w2 are connected to the corresponding AC terminals a1, b1, and c1 on the low-voltage side of the power frequency transformer.

[0009] This invention also provides a control method for a medium-voltage grid-connected inverter that combines a hybrid VSC (Variable Voltage Controller) and a current source ACC (Adaptive Current Controller). For the low-voltage three-phase VSC, each phase adopts a capacitor voltage outer loop + current inner loop control strategy. To improve the power quality on the AC grid side, harmonic mitigation and reactive power compensation terms are introduced in the current inner loop. The switching devices SW1, SW2, SW3, SW4, SW5, and SW6 are turned on and off using a sinusoidal pulse width modulation strategy. For the medium-voltage current source ACC, a constant DC voltage control strategy is adopted. To improve power transmission efficiency, the switching on and off of each bridge arm unit is achieved using a baseband switching modulation strategy.

[0010] Compared with existing solutions, the advantages of the hybrid medium-voltage grid-connected inverter proposed in this invention are as follows:

[0011] 1) The main power transmission adopts a current source circuit topology with no internal energy storage capacitor, resulting in high power density. In addition, the bridge arm unit is composed of fully controlled current source devices that can actively turn on and off the current without relying on grid commutation, thus ensuring high operational reliability.

[0012] 2) Voltage source converters can provide reactive power compensation and harmonic mitigation functions, which can effectively improve the power quality on the grid side.

[0013] 3) Compared with voltage source converters, it can effectively isolate DC side faults and has fault ride-through capability. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the circuit topology of the medium-voltage grid-connected inverter with hybrid VSC and current source ACC of the present invention.

[0015] Figure 2a , Figure 2b , Figure 2cThis is a schematic diagram of the low-voltage three-phase VSC phase control strategy of the present invention; Figure 2a This is a schematic diagram of the low-voltage three-phase VSCA phase control strategy of the present invention; Figure 2b This is a schematic diagram of the low-voltage three-phase VSC B-phase control strategy of the present invention; Figure 2c This is a schematic diagram of the low-voltage three-phase VSC C-phase control strategy of the present invention. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0017] like Figure 1 As shown, a medium-voltage grid-connected inverter of the present invention, which combines a hybrid VSC and a current source ACC, includes a converter main circuit. The converter main circuit consists of a medium-voltage current source active commutation converter (ACC), a low-voltage three-phase voltage source converter (VSC), a power frequency transformer, and a DC reactor L. G It consists of AC reactors La1, Lb1, Lc1, La2, Lb2, and Lc2.

[0018] The medium-voltage current source ACC DC side is connected in series with the low-voltage three-phase VSC DC side and the DC reactor, and then connected to the medium-voltage DC grid. The low-voltage three-phase VSC AC side is connected in series with AC reactors La2, Lb2, Lc2, and the power frequency transformer, and then connected in parallel with the medium-voltage current source ACC AC side, and then connected to the three-phase medium-voltage AC grid through AC reactors La1, Lb1, and Lc1. On the DC side, the medium-voltage current source ACC is composed of bridge arm units S1, S2, S3, S4, S5, and S6. Each bridge arm unit contains multiple autonomous switching power semiconductor devices connected in series, with the number of devices in series ranging from [4 40].

[0019] The positive terminals of bridge arm units S1, S3, and S5 are connected in parallel to form terminal Np+, which is connected to DC reactor L. G Connected on one side, DC reactor L G The other side is connected to the medium-voltage DC positive terminal P. The negative terminals of bridge arm units S1, S3, and S5, and the positive terminals of bridge arm units S4, S6, and S2 are connected to form AC terminals u1, v1, and w1. The negative terminals of bridge arm units S4, S6, and S2 are connected in parallel to form terminal Np-.

[0020] The low-voltage three-phase VSC consists of switching devices SW1, SW2, SW3, SW4, SW5, SW6, SW7, SW8, and diodes D1 and D2.

[0021] Preferably, the switching devices SW1, SW2, SW3, SW4, SW5, SW6, SW7, and SW8 can be insulated gate bipolar transistors (IGBTs) or silicon carbide field-effect transistors (SiC-MOSFETs).

[0022] The collectors (drains) of switching devices SW1, SW3, and SW5 are connected in parallel and then connected to a DC capacitor C. L The positive terminal, the emitter (source) of switching devices SW1, SW3, and SW5, and the collector (drain) of switching devices SW2, SW4, and SW6 are connected in parallel to form AC terminals u2, v2, and w2. The emitter (source) of switching devices SW2, SW4, and SW6 are then connected in parallel to a DC capacitor C. L The negative electrode.

[0023] The cathode of diode D1 is connected in parallel with the collector (drain) of switching device SW8, and then connected to a DC capacitor C. L The positive terminal of diode D1 is connected to the collector (drain) of switching device SW7 to form AC terminal u3. The emitter (source) of switching device SW8 is connected to the cathode of diode D2 to form AC terminal v3. The emitter (source) of switching device SW7 and the anode of diode D2 are connected in parallel to DC terminal C. L The negative electrode.

[0024] The Np- terminal of the medium-voltage current source ACC is connected to the AC terminal u3 of the low-voltage three-phase VSC. The AC terminal v3 of the low-voltage three-phase VSC is connected to the negative terminal N of the medium-voltage DC circuit. The AC terminals u1, v1, and w1 of the medium-voltage current source ACC are connected to one end of the corresponding AC terminals A1, B1, and C1 on the high-voltage side of the power frequency transformer, as well as one end of the AC reactors La1, Lb1, and Lc1. The other end of the AC reactors La1, Lb1, and Lc1 is directly connected to the terminals of phases A, B, and C of the three-phase medium-voltage AC power grid. The AC terminals u2, v2, and w2 of the low-voltage three-phase VSC are connected to one end of the AC reactors La2, Lb2, and Lc2, and the other end is connected to the corresponding AC terminals a1, b1, and c1 on the low-voltage side of the power frequency transformer.

[0025] like Figure 2a , Figure 2b , Figure 2c As shown, the medium-voltage grid-connected inverter employs a capacitor voltage outer loop + current inner loop control strategy for each phase of the low-voltage three-phase VSC. To improve the power quality on the AC grid side, harmonic mitigation and reactive power compensation terms are introduced in the current inner loop. The switching of devices SW1, SW2, SW3, SW4, SW5, and SW6 is achieved through a sinusoidal pulse width modulation (SPWM) strategy. For the medium-voltage current source ACC, a constant DC voltage control strategy is adopted. To improve power transmission efficiency, the switching of each bridge arm unit is achieved through a baseband switching modulation strategy, specifically including:

[0026] 1) By collecting the three-phase AC grid side line voltage u AB u BC u CA and the three-phase line voltage u on the low-voltage side of the power frequency transformer a1b1 u b1c1 u c1a1 The phase θ1 of the three-phase AC grid voltage and the phase θ2 of the power frequency three-phase AC grid voltage are obtained using a phase-locked loop (PLL). Figure 2a As shown, for a low-voltage three-phase VSC, the drive signal generation method for the A-phase switch device on the three-phase AC grid side is as follows: by comparing the DC voltage reference value with the acquired actual value u CL The inner loop current reference value is obtained by multiplying the difference by sinθ2 after PI control; to achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value; for reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gc The reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cosθ2 via PI control; for harmonic mitigation, the output current i of phase A on the AC side of the ACC is collected. a1 Through a band-stop filter and -k TF Multiplying them yields the harmonic mitigation current feedforward term, k. TF The voltage turns ratio of the power frequency transformer; the reference value of the inner loop current introduced into the feedforward term and the phase current i of the low-voltage three-phase VSCA are used. a2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the voltage u of phase A on the three-phase AC grid side. a1After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSC A-phase voltage reference value. The drive signals for switching devices SW1 and SW2 are obtained using a sinusoidal pulse width modulation strategy.

[0027] I q_feb =2*[i ga sinθ1+i gb sin(θ1-2π / 3)+i ga sin(θ1+2π / 3)] / 3 (1)

[0028] like Figure 2b As shown, for a low-voltage three-phase VSC, the drive signal generation method for the B-phase switch device on the three-phase AC grid side is as follows: by comparing the DC voltage reference value with the acquired actual value u CL The difference is then multiplied by sin(θ²-2π / 3) after PI control to obtain the inner loop current reference value; to achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value; for reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gc The reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cos(θ²-2π / 3) via PI control; for harmonic mitigation, the ACC AC side current i is collected. b1 Through a band-stop filter, with -k TF Multiplying these yields the harmonic mitigation current feedforward term; the inner loop current reference value introduced into the feedforward term is then multiplied by the low-voltage three-phase VSCB phase current i. b2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the voltage u of phase B on the three-phase AC grid side. b1 After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSC B-phase voltage reference value. The drive signals for switching devices SW3 and SW4 are obtained using a sinusoidal pulse width modulation strategy.

[0029] like Figure 2c As shown, for a low-voltage three-phase VSC, the driving signal generation method for the C-phase switch device on the three-phase AC grid side is as follows: by comparing the DC voltage reference value with the acquired actual value u CL The difference is then multiplied by sin(θ² + 2π / 3) after PI control to obtain the inner loop current reference value. To achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value. For reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gcThe reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cos(θ² + 2π / 3) via PI control; for harmonic mitigation, the ACC AC side current i is collected. c1 Through a band-stop filter, with -k TF Multiplying these yields the harmonic mitigation current feedforward term; the inner loop current reference value introduced into the feedforward term is then multiplied by the low-voltage three-phase VSC C-phase current i. c2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the C-phase voltage u of the three-phase AC grid side. c1 After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSC C-phase voltage reference value. The drive signals for switching devices SW5 and SW6 are obtained using a sinusoidal pulse width modulation strategy.

[0030] 2) For the medium-voltage current source ACC, the constant DC voltage control and baseband switching modulation strategy proposed in this invention is as follows: the medium-voltage DC voltage u is controlled by adjusting the conduction time of bridge arm units S1, S2, S3, S4, S5, and S6. dcnp Control is implemented as follows: Within each grid base frequency cycle 1 / F, the A-phase voltage Usa and C-phase voltage Usc of the medium-voltage three-phase AC grid are defined as positive, and the time when Usa = Usc is defined as T0; F represents the switching frequency of each bridge arm device in the ACC; within each grid base frequency cycle 1 / F, the conduction angle is... All shut-off angles are The adjustment time is T. R ,and The turn-on time of the bridge arm unit S1 within the medium-voltage current source ACC is T0+TR, and the turn-off time is... The turn-on time of bridge arm unit S2 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S3 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S4 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S5 within the medium-voltage current source ACC is The turn-off time is T0+TR+1 / F; the turn-on time of bridge arm unit S6 in the medium-voltage current source ACC is... The shutdown time is If any of the above-mentioned on / off times is greater than 1 / F, then 1 / F is subtracted from this on / off time to obtain the actual on / off time; if any of the above-mentioned on / off times is less than 0, then 1 / F is added to this on / off time to obtain the actual on / off time. The control of the medium-voltage current source ACC also includes changing the adjustment time T. R For medium-voltage DC voltage u dcnp Control is performed using medium-voltage DC voltage u dcnp The value is always positive, if the medium-voltage DC voltage u dcnp When the voltage is less than its reference value, the adjustment time TR is reduced by a preset step size until the medium-voltage DC voltage u is reached. dcnp If the value is equal to its reference value, otherwise increase the adjustment time T. R .

[0031] During normal operation, the medium-voltage current source ACC and the low-voltage three-phase VSC operate according to the processes described in points 1) and 2). The low-voltage three-phase VSC can be bypassed on the DC side by turning on switching device SW8 or SW7. By turning on switching devices SW7 and SW8, the low-voltage three-phase VSC can output a negative voltage, thereby reducing the DC bus voltage. In the event of a fault on the DC side, the medium-voltage current source ACC achieves fault isolation through direct connection to bridge arm units S1 and S4, or S3 and S6, or S5 and S2. The low-voltage three-phase VSC is also isolated through the turning on switching device SW8 or SW7.

[0032] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A medium-voltage grid-connected inverter combining VSC and current source ACC, characterized in that, The converter main circuit includes a medium-voltage current source ACC, a low-voltage three-phase VSC, a power frequency transformer, and a DC reactor L. G It consists of AC reactors La1, Lb1, Lc1, La2, Lb2, and Lc2; ACC represents an active phase-commutation converter, and VSC represents a voltage source converter; the DC side of the medium-voltage current source ACC is connected in series with the DC side of the low-voltage three-phase VSC and the DC reactors, and then connected to the medium-voltage DC grid; the AC side of the low-voltage three-phase VSC is connected in series with the AC reactors La2, Lb2, Lc2, and the power frequency transformer, and then connected in parallel with the AC side of the medium-voltage current source ACC, and then connected to the three-phase medium-voltage AC grid through the AC reactors La1, Lb1, and Lc1; On the DC side, the medium-voltage current source ACC is composed of bridge arm units S1, S2, S3, S4, S5, and S6. The positive terminals of bridge arm units S1, S3, and S5 are connected in parallel to form terminal Np+, which is connected to the DC reactor L. G Connected on one side, DC reactor L G The other side is connected to the positive terminal P of the medium-voltage DC; the negative terminals of bridge arm units S1, S3, and S5 and the positive terminals of bridge arm units S4, S6, and S2 are connected to form AC terminals u1, v1, and w1; the negative terminals of bridge arm units S4, S6, and S2 are connected in parallel to form terminal Np-. The low-voltage three-phase VSC consists of switching devices SW1, SW2, SW3, SW4, SW5, SW6, SW7, and SW8, as well as diodes D1 and D2. The collectors of switching devices SW1, SW3, and SW5 are connected in parallel and then connected to a DC capacitor C. L The positive terminal of the switch is formed by connecting the emitters of switching devices SW1, SW3, and SW5 in parallel with the collectors of switching devices SW2, SW4, and SW6 to form AC terminals u2, v2, and w2. The emitters of switching devices SW2, SW4, and SW6 are then connected in parallel to a DC capacitor C. L The negative terminal of diode D1; the cathode of diode D1 is connected in parallel with the collector of switching device SW8 and then connected to DC capacitor C. L The positive terminal of diode D1 is connected to the collector of switching device SW7 to form AC terminal u3. The emitter of switching device SW8 is connected to the cathode of diode D2 to form AC terminal v3. The emitter of switching device SW7 and the anode of diode D2 are connected in parallel to DC terminal C. L The negative electrode; The medium-voltage current source ACC terminal Np- is connected to the low-voltage three-phase VSC AC terminal u3. The low-voltage three-phase VSC AC terminal v3 is connected to the medium-voltage DC negative terminal N. The medium-voltage current source ACC AC terminals u1, v1, and w1 are connected to the corresponding phase AC terminals A1, B1, and C1 on the high-voltage side of the power frequency transformer, as well as one end of the AC reactors La1, Lb1, and Lc1. The other end of the AC reactors La1, Lb1, and Lc1 is directly connected to the A, B, and C phase terminals of the three-phase medium-voltage AC power grid. The low-voltage three-phase VSC AC terminals u2, v2, and w2 are connected to the corresponding phase AC terminals a1, b1, and c1 on the low-voltage side of the power frequency transformer.

2. A medium-voltage grid-connected inverter with hybrid VSC and current source ACC as described in claim 1, characterized in that, Each bridge arm unit contains multiple autonomous switching power semiconductor devices connected in series, with the number of devices connected in series ranging from [4 to 40].

3. A medium-voltage grid-connected inverter with hybrid VSC and current source ACC as described in claim 1, characterized in that, Switching devices SW1, SW2, SW3, SW4, SW5, SW6, SW7, and SW8 are selected from insulated gate bipolar transistors or silicon carbide field-effect transistors.

4. A control method for a medium-voltage grid-connected inverter with hybrid VSC and current source ACC according to any one of claims 1-3, characterized in that, For low-voltage three-phase VSC, each phase adopts a capacitor voltage outer loop + current inner loop control strategy. To improve the power quality on the AC grid side, harmonic mitigation and reactive power compensation terms are introduced in the current inner loop. The switching devices SW1, SW2, SW3, SW4, SW5, and SW6 are turned on and off through a sinusoidal pulse width modulation strategy. For medium-voltage current source ACC, a constant DC voltage control strategy is adopted. To improve power transmission efficiency, the switching on and off of each bridge arm unit is achieved through a base frequency switching modulation strategy.

5. The control method according to claim 4, characterized in that, By collecting the three-phase AC grid side line voltage u AB u BC ,u CA and the low-voltage side line voltage u of the power frequency transformer a1b1 ,u b1c1 ,u c1a1 The phase phase θ1 of the three-phase AC grid side and the phase phase θ2 of the three-phase AC grid side at power frequency are obtained based on the phase-locked loop.

6. The control method according to claim 5, characterized in that, For low-voltage three-phase VSC, the drive signal for the A-phase switching device on the three-phase AC grid side is generated by: comparing the DC voltage reference value with the acquired actual value u. CL The inner loop current reference value is obtained by multiplying the difference by sinθ2 after PI control; to achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value; for reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gc The reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cosθ2 via PI control; for harmonic mitigation, the output current i of phase A on the AC side of the ACC is collected. a1 Through a band-stop filter and -k TF Multiplying them yields the harmonic mitigation current feedforward term, k. TF The voltage turns ratio of the power frequency transformer; the reference value of the inner loop current introduced into the feedforward term and the phase current i of the low-voltage three-phase VSCA are used. a2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the voltage u of phase A on the three-phase AC grid side. a1 After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSCA phase voltage reference value. The drive signals for switching devices SW1 and SW2 are obtained using a sinusoidal pulse width modulation strategy. I q_feb =2*[i ga sinθ1+i gb sin(θ1-2π / 3)+i gc sin(θ1+2π / 3)] / 3 (1) For low-voltage three-phase VSC, the drive signal for the B-phase switch device on the three-phase AC grid side is generated by: comparing the DC voltage reference value with the acquired actual value u. CL The difference is then multiplied by sin(θ²-2π / 3) after PI control to obtain the inner loop current reference value; to achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value; for reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gc The reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cos(θ²-2π / 3) via PI control; for harmonic mitigation, the ACC AC side current i is collected. b1 Through a band-stop filter, with -k TF Multiplying these yields the harmonic mitigation current feedforward term; the inner loop current reference value introduced into the feedforward term is then multiplied by the low-voltage three-phase VSCB phase current i. b2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the voltage u of phase B on the three-phase AC grid side. b1 After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSC B-phase voltage reference value. The drive signals for switching devices SW3 and SW4 are obtained using a sinusoidal pulse width modulation strategy. For low-voltage three-phase VSC, the drive signal for the C-phase switching device on the three-phase AC grid side is generated by: comparing the DC voltage reference value with the acquired actual value u. CL The difference is then multiplied by sin(θ² + 2π / 3) after PI control to obtain the inner loop current reference value; to achieve reactive power compensation and harmonic mitigation, a feedforward term is introduced into the inner loop current reference value; for reactive power compensation, the three-phase AC grid side current i is collected. ga i gb i gc The reactive current I is calculated using equation (1). q_feb The reference value 0 is compared with the reactive current I. q_feb After subtraction, the reactive power compensation current feedforward term is obtained by multiplying it by cos(θ² + 2π / 3) via PI control; for harmonic mitigation, the ACC AC side current i is collected. c1 Through a band-stop filter, with -k TF Multiplying these yields the harmonic mitigation current feedforward term; the inner loop current reference value introduced into the feedforward term is then multiplied by the low-voltage three-phase VSC C-phase current i. c2 The actual acquired values ​​are subtracted and then passed through a repetitive controller to obtain a dual-closed-loop output. To improve the dynamic response speed, a voltage feedforward term is introduced into the dual-closed-loop output, which is obtained by feeding forward the C-phase voltage u of the three-phase AC grid side. c1 After passing through a bandpass filter, the voltage feedforward term is subtracted from the dual closed-loop output to obtain the low-voltage three-phase VSC C-phase voltage reference value. The drive signals for switching devices SW5 and SW6 are obtained using a sinusoidal pulse width modulation strategy.

7. The control method according to any one of claims 4-6, characterized in that, For the medium-voltage current source ACC, the constant DC voltage control and base frequency switching modulation strategy is as follows: the medium-voltage DC voltage u is controlled by adjusting the conduction time of bridge arm units S1, S2, S3, S4, S5, and S6. dcnp To take control.

8. The control method according to claim 7, characterized in that, Within each grid fundamental frequency cycle 1 / F, the time when both phase A voltage Usa and phase C voltage Usc of the medium-voltage three-phase AC grid are positive and Usa = Usc is defined as T0; F represents the switching frequency of each bridge arm device in the ACC; within each grid fundamental frequency cycle 1 / F, the conduction angle is... All shut-off angles are The adjustment time is T. R ,and The turn-on time of the bridge arm unit S1 within the medium-voltage current source ACC is T0+TR, and the turn-off time is... The turn-on time of bridge arm unit S2 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S3 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S4 within the medium-voltage current source ACC is The shutdown time is The turn-on time of bridge arm unit S5 within the medium-voltage current source ACC is The turn-off time is T0+TR+1 / F; the turn-on time of bridge arm unit S6 in the medium-voltage current source ACC is... The shutdown time is If any of the above-mentioned turn-on or turn-off times is greater than 1 / F, then 1 / F is subtracted from this turn-on or turn-off time to obtain the actual turn-on or turn-off time; if any of the above-mentioned turn-on or turn-off times is less than 0, then 1 / F is added to this turn-on or turn-off time to obtain the actual turn-on or turn-off time.

9. The control method according to claim 8, characterized in that, This includes changing the adjustment time T R For medium-voltage DC voltage u dcnp Control is performed using medium-voltage DC voltage u dcnp The value is always positive, if the medium-voltage DC voltage u dcnp When the value is less than the reference value, the adjustment time T is reduced by a preset step size. R Until the medium-voltage DC voltage u dcnp If the value is equal to its reference value, otherwise increase the adjustment time T. R .

10. The control method according to claim 4, characterized in that, During normal operation, the medium-voltage current source ACC and the low-voltage three-phase VSC bypass the low-voltage three-phase VSC on the DC side through the switching device SW8 or the switching device SW7. Through the switching devices SW7 and SW8, the low-voltage three-phase VSC outputs a negative voltage, thereby reducing the DC bus voltage. When a fault occurs on the DC side, the medium-voltage current source ACC achieves fault isolation through the direct-connection bridge arm unit S1, bridge arm unit S4, or bridge arm unit S3, bridge arm unit S6, or bridge arm unit S5, bridge arm unit S2. The low-voltage three-phase VSC is also isolated through the switching device SW8 or the switching device SW7.

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