Multiphase voltage source converter topology and four-phase voltage source converter control method

Abstract

The present invention discloses a multi-phase voltage source converter topology, including n mutually parallel bridge arms, the n mutually parallel bridge arms are connected in parallel with a DC voltage stabilizing capacitor, the two ends of the DC voltage stabilizing capacitor are respectively connected to a port to form a DC side port, the midpoint of each bridge arm is connected to one end of a filter inductor, and the other end of each filter inductor is used as an AC side port. The converter can be used for a three-phase to four-phase back-to-back converter, providing a bridge for energy transmission between a four-phase power system and a DC system, realizing the interconnection and mutual assistance between the four-phase power system and the DC system as well as the three-phase power system, and realizing the stable transmission of low-frequency four-phase power transmission.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic equipment topology and control, and in particular relates to a multi-phase voltage source converter topology, and also relates to a voltage source converter control method for a four-phase AC power system. Background Art

[0002] Driven by the "dual carbon" goals, it has become an inevitable trend to accelerate the construction of a new power system with new energy as the main body. With the substantial increase in the installed capacity of new energy, the long-distance transmission of new energy power generation faces key problems such as low transmission capacity and large transmission losses that need to be solved urgently. Compared with the traditional three-phase AC power system, the four-phase AC power system has significant advantages such as high power density, saving line corridors, and compact line layout, and is suitable for application in a variety of power transmission and distribution scenarios. The low-frequency four-phase transmission system not only has the above advantages but also has the function of reducing the pressure loss and power loss of the transmission line. It will have broad development prospects as a new transmission method. Therefore, it is urgent to conduct in-depth research on the key converter equipment for building a four-phase AC power system.

[0003] The four-phase AC power system can improve power density, save line corridors, and have compact line layout. It is suitable for power transmission and distribution systems in various scenarios, such as multi-terminal interconnected low-frequency four-phase transmission and distribution systems in urban power grids, low-frequency four-phase transmission and distribution systems interconnected by offshore island power grids, new offshore wind power low-frequency four-phase AC transmission and distribution systems, and submarine low-frequency four-phase transmission and distribution systems.

[0004] Based on the traditional three-phase voltage source converter, research on the four-phase voltage source converter has been carried out, which has broad application prospects in the four-phase transmission and distribution system. At present, there are relatively few studies on the four-phase voltage source converter, so it is urgent to study the multi-mode control strategy of the four-phase voltage source converter to support the stable operation of the four-phase AC power system. Summary of the invention

[0005] The purpose of the present invention is to provide a multi-phase voltage source converter topology, which can provide a bridge for energy transmission between a four-phase power system and a DC system and a three-phase power system, and realize the interconnection and mutual assistance between the four-phase power system and the DC system and the three-phase power system.

[0006] A second object of the present invention is to provide an application of a four-phase voltage source converter topology in a four-phase AC power system.

[0007] The third object of the present invention is to provide a voltage source converter control method for a four-phase AC power system to support the stable operation of the four-phase AC power system.

[0008] The first technical solution adopted by the present invention is a multi-phase voltage source converter topology, including n bridge arms connected in parallel, and n bridge arms connected in parallel are connected in parallel with a DC stabilizing capacitor, and the two ends of the DC stabilizing capacitor are respectively connected to a port to form a DC side port, and the midpoint of each bridge arm is connected to one end of a filter inductor, and the other end of each filter inductor serves as an AC side port.

[0009] The present invention is also characterized in that:

[0010] The bridge arm is one of the half-bridge, full-bridge, neutral point clamped three-level, and modular multi-level composed of switching tubes.

[0011] The value of n is 4.

[0012] The second technical solution adopted by the present invention is the application of a multi-phase voltage source converter topology in a four-phase AC power system.

[0013] The third technical solution adopted by the present invention is a control method for a voltage source converter of a four-phase AC power system, wherein the voltage source converter is a four-phase voltage source converter topology, wherein the DC side port is connected to a DC system, and the AC side port is connected to a four-phase AC power system, and the inner loop of the four-phase voltage source converter topology is always grid-side current control, and is specifically implemented according to the following steps:

[0014] Step 1. Set reference data: DC voltage reference value U dc_ref , DC current reference value I ref , AC voltage reference value u a_ref 、u b_ref ; Collect circuit data: DC side voltage U of the voltage across the DC voltage stabilizing capacitor dc 、AC side voltage u output from AC port a and u b , AC side current i a and i b ;

[0015] Step 2: Select reference data and circuit data to calculate and control each bridge arm drive signal according to actual needs to achieve control of the voltage source converter.

[0016] There are three actual requirements in step 2, specifically:

[0017] ① When the DC side voltage is controlled by a four-phase voltage source converter, the DC voltage reference value U dc_ref The DC voltage U collected dc The difference is then passed through a voltage controller to generate the AC side current reference amplitude I ref , and the AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave i of phase a and phase ba_ref and i b_ref ;

[0018] ② When the AC side voltage is controlled by a four-phase voltage source converter, the AC voltage reference wave u a_ref 、u b_ref The AC voltage u collected a 、u b The voltage controller generates the AC side current reference wave i a_ref and i b_ref ;

[0019] ③. By giving the current reference amplitude I ref With the AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave i of phase a and phase b a_ref and i b_ref .

[0020] The specific process of step 2 is: select one of the requirements through the multi-way switch selector, and convert the AC side current reference wave i obtained according to the requirement into a_ref and i b_ref The AC side current i a 、i b The difference is made, and the pulse width modulation wave is obtained through the current controller. The modulation wave is passed through the PWM generator to generate a PWM drive signal to drive the switch tube on the bridge arm.

[0021] The beneficial effects of the present invention are:

[0022] 1) The multiphase voltage source converter topology proposed in the present invention can be used for a three-phase to four-phase back-to-back converter, providing a bridge for energy transmission between the four-phase power system and the DC system as well as between the three-phase power system, thereby realizing the interconnection and mutual assistance between the four-phase power system and the DC system as well as the three-phase power system;

[0023] 2) The four-phase voltage source converter designed by the present invention can provide strong support for the safe and stable operation of the four-phase power system by selecting control methods of different working modes in the four-phase power system;

[0024] 3) The four-phase to three-phase conversion mode proposed in the present invention improves the ability of the power system to operate safely and stably. When a single-phase fault occurs in the four-phase system or phase-by-phase maintenance is performed, the four-phase system can be switched to a three-phase system, effectively improving power supply reliability and maintenance safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is the multi-phase voltage source converter topology of the present invention;

[0026] Figure 2It is the type of bridge arm that can be selected by the converter in the present invention;

[0027] Figure 3 Schematic diagram of the topology of the m-phase-n-phase back-to-back converter in the present invention;

[0028] Figure 4 It is a schematic diagram of a permanent magnet wind turbine transmission system using a multi-phase voltage source converter topology;

[0029] Figure 5 It is a schematic diagram of a doubly-fed wind turbine transmission system using a multi-phase voltage source converter topology;

[0030] Figure 6 This is a topological structure diagram of a multi-phase voltage source converter when n=4 according to the present invention;

[0031] Figure 7 is a control block diagram of a control method for a four-phase voltage source converter according to the present invention;

[0032] Figure 8 It is a waveform diagram of different control modes in the control method of the present invention.

[0033] Among them, 1-wind turbine, 2-3(4) phase-to-4 phase back-to-back converter, 3-first low-frequency four-phase step-up transformer, 4-second low-frequency four-phase step-up transformer, 5-submarine cable, 6-onshore voltage and frequency conversion station. DETAILED DESCRIPTION

[0034] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0035] The multi-phase voltage source converter topology of the present invention is as follows: Figure 1 As shown, it includes n mutually parallel bridge arms, and n mutually parallel bridge arms are connected in parallel with a DC voltage stabilizing capacitor. The two ends of the DC voltage stabilizing capacitor are respectively connected to a port to form a DC side port, and the midpoint of each bridge arm is connected to one end of a filter inductor, and the other end of each filter inductor is used as an AC side port. On the basis of a three-phase voltage source converter, the present invention changes the number of bridge arms to achieve conversion from DC to n-phase AC, and then constructs an m-phase-n-phase back-to-back converter topology to achieve conversion from three-phase AC to four-phase AC, and there is a DC decoupling port, and frequency conversion is achieved at the same time. It is applied in the field of low-frequency four-phase power transmission, which can improve the transmission capacity and reduce line pressure loss and power loss; it is also suitable for establishing a bridge for energy transmission between a three-phase system and a four-phase system to achieve energy mutual assistance.

[0036] The multi-phase voltage source converter topology submodule of the present invention is as follows: Figure 2 As shown, ①-half-bridge structure, ②-full-bridge structure, ③-neutral point clamped three-level structure, and ④-modular multi-level structure can be adopted.

[0037] like Figure 3 As shown, an m-phase-n-phase back-to-back converter topology constructed by a multi-phase voltage source converter topology of the present invention has an m-phase AC port, a DC port and an n-phase AC port, u 3a 、u 3b to u 3m is the m-phase AC voltage, i 3a 、i 3b ~i 3m is the m-phase AC current, u 4a 、u 4b 、u 4c ~u 4n is the n-phase AC voltage, i 4a 、i 4b 、i 4c ~i 4n is the n-phase alternating current, U dc The DC port voltage is used to establish a bridge for energy transmission between the m-phase AC power system and the DC system and the n-phase AC power system.

[0038] For wind turbines, a low-frequency four-phase transmission system can be realized by combining a 3-phase to 4-phase back-to-back converter topology without modifying the existing three-phase generators. Figure 4 As shown, it includes 1-wind turbine, 2-3 (4) phase-4 phase back-to-back converter, 3-first low-frequency four-phase step-up transformer, 4-low-frequency four-phase step-up transformer, 5-submarine cable, 6-onshore voltage-conversion and frequency-conversion station, which improves the transmission capacity and reduces the line voltage loss and power loss. The wind turbine 1 includes a permanent magnet generator or a double-fed wind turbine. The permanent magnet generator converts the power frequency three-phase electricity generated by the wind turbine into low-frequency four-phase electricity by equipping a 3-phase-to-4-phase back-to-back converter. The outlet voltage of the wind turbine is increased to the voltage level of the collection bus through the first low-frequency four-phase step-up transformer 3. The collection bus voltage is increased to a higher voltage level through the second low-frequency four-phase step-up transformer 4 for external transmission. Finally, it is connected to the onshore power grid through the onshore voltage-conversion and frequency-conversion station 6, so as to realize the continuous transmission of offshore wind power to the onshore power consumption area.

[0039] The permanent magnet wind turbine can also be replaced with a doubly fed wind turbine 1, by changing the number of phases of its rotor and stator, and combining it with a 4-phase-4-phase back-to-back converter topology to realize a low-frequency four-phase transmission system, such as Figure 5 As shown, it includes 1-doubly-fed wind turbine, 2-4-phase conversion-4-phase back-to-back converter, 3-low-frequency four-phase step-up transformer ①, 4-low-frequency four-phase step-up transformer ②, 5-submarine cable, 6-onshore voltage and frequency conversion station, which can improve the transmission capacity and reduce the line pressure loss and power loss.

[0040] The doubly-fed wind turbine unit is equipped with a 4-phase-4-phase back-to-back converter to directly output low-frequency four-phase electricity. The outlet voltage of the wind turbine is increased to the voltage level of the collection bus through a low-frequency four-phase step-up transformer①. The collection bus voltage is then increased to a higher voltage level through a low-frequency four-phase step-up transformer② for external transmission. Finally, it is connected to the onshore power grid through an onshore voltage and frequency conversion station, thereby realizing the continuous transmission of offshore wind power to onshore power consumption areas.

[0041] like Figure 6 As shown in FIG. 1 , when n is 4, the multiphase voltage source converter topology is a four-phase voltage source converter topology, that is, it includes four bridge arms: bridge arm a, bridge arm b, bridge arm c and bridge arm d. dc is the DC side voltage, u a 、u b 、u c and u d is the AC side voltage, i a 、i b 、i c and i d is the AC side current, S a , S b , S c and S d is the switch drive signal on the four bridge arms, S a '、S b '、S c ' and S d ' is the switch driving signal under the four bridge arms. When n=4, the phase voltage source converter topology is applied in the four-phase AC power system.

[0042] The voltage source converter control method of the four-phase AC power system of the present invention is a multi-phase voltage source converter topology. The DC side port is connected to the DC system, and the AC side port is connected to the four-phase AC power system. The control block diagram is as follows Figure 7 As shown, the control modes include ①-DC voltage control, ②-AC voltage control, ③-AC current command, and ④-AC current control. The inner loop of the multi-phase voltage source converter topology is always grid-side current control. Different external control loops are selected according to different control requirements. The specific implementation is as follows:

[0043] Step 1. Set reference data: DC voltage reference value U dc_ref , DC current reference value I ref , AC voltage reference value u a_ref 、u b_ref ; Collect circuit data: DC side voltage U of the voltage across the DC voltage stabilizing capacitor dc 、AC side voltage u output from AC port a and u b , AC side current ia and i b ;

[0044] Step 2: Select reference data and circuit data to calculate and control each bridge arm drive signal according to actual needs to achieve control of the voltage source converter.

[0045] There are three actual demand control modes in step 2, specifically:

[0046] ① When the DC side voltage is controlled by a four-phase voltage source converter (rectifier mode, grid-connected mode), the DC voltage reference value U dc_ref The DC voltage U collected dc The difference is then passed through a voltage controller to generate the AC side current reference amplitude I ref , and the AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave i of phase a and phase b a_ref and i b_ref ;

[0047] ② When the AC side voltage is controlled by a four-phase voltage source converter (island mode), the AC voltage reference wave u a_ref 、u b_ref The AC voltage u collected a 、u b The voltage controller generates the AC side current reference wave i a_ref and i b_ref ;

[0048] ③. When neither the DC side voltage nor the AC side voltage requires the control of the four-phase voltage source converter and only the power flow between the AC side and the DC side is controlled (grid-connected mode), the current reference amplitude I is given. ref With the AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave i of phase a and phase b a_ref and i b_ref .

[0049] The specific process of step 2 is: select one of the requirements through the multi-way switch selector, and convert the AC side current reference wave i obtained according to the requirement into a_ref and i b_ref The AC side current i a 、i b The difference is made, and the pulse width modulation wave is obtained through the current controller. The modulation wave is passed through the PWM generator to generate a PWM drive signal to drive the switch tube on the bridge arm.

[0050] Under the four-phase balanced working condition, the phase difference between phase a and phase c is 180 degrees, and the phase difference between phase b and phase d is 180 degrees, so the control can adopt a double H-bridge control strategy to simplify the control; the modulation strategy adopts a sinusoidal pulse width modulation strategy, and the modulation waves of the four phases a, b, c, and d differ by 90 degrees respectively.

[0051] For the verification of the control method of the present invention, a simulation model of a four-phase voltage source converter was built to verify the effectiveness of the control methods of the rectification mode, island model, grid-connected mode and four-phase-to-three-phase conversion mode. The DC voltage level was set to 750V, and the AC voltage level was set to 220V (phase voltage effective value). The waveforms of each working condition are shown in the figure below. Figure 8 As shown, a) Rectification mode waveform: AC side power supply (220V / 15Hz), DC side voltage is controlled to 750V, 0.3s load sudden increase, DC voltage drops, 0.35s returns to 750V for stable operation; b) Island mode waveform: DC side power supply (750V), AC side voltage is controlled to 220V / 15Hz, 0.3s load sudden increase, AC side voltage is re-controlled to 220V / 15Hz after a short transient process; c) Grid-connected mode waveform: DC side power supply (750V), AC side power supply (220V / 15Hz), grid-connected current amplitude is controlled at 50A, 0.3s grid-connected current amplitude command suddenly changes to 100A, the current loop responds quickly, and reaches a new steady state in 0.305s. d) Four-phase-to-three-phase conversion mode waveform: Taking the island mode as an example, it works in the four-phase mode 0.3s ago, one phase is disconnected at 0.3s, and it switches to the three-phase mode. After a half-cycle transient, the four-phase mode with a 90-degree difference is switched to the three-phase mode with a 120-degree difference.

[0052] When overhauling transmission lines, the switch from four-phase to three-phase working mode creates an environment for phase-by-phase off-line maintenance, making the maintenance work safer and more reliable; or in the case of a single-phase fault, the faulty phase can be quickly removed and switched to three-phase working mode to improve power supply reliability.

Claims

1. A voltage source converter control method for a four-phase AC power system, characterized in that: The voltage source converter comprises n bridge arms connected in parallel to each other, the n bridge arms connected in parallel to each other are connected in parallel to a DC voltage stabilizing capacitor, the two ends of the DC voltage stabilizing capacitor are respectively connected to a port to form a DC side port, the midpoint of each bridge arm is connected to one end of a filter inductor, and the other end of each filter inductor is used as an AC side port, the bridge arm is one of a half-bridge, a full-bridge, a neutral point clamped three-level, and a modular multi-level composed of switch tubes, and the value of n is 4; The DC side port is connected to the DC system, and the AC side port is connected to the four-phase AC power system. The inner loop of the four-phase voltage source converter topology is always grid-side current control, which is specifically implemented in the following steps: Step 1. Set reference data: DC voltage reference value U dc_ref , DC current reference value I ref , AC voltage reference value u a_ref , u b_ref ; Collect circuit data: DC side voltage of the voltage across the DC voltage stabilizing capacitor U dc , AC side voltage output from the AC port u a and u b , AC side current i a and i b ; Step 2: Select reference data and circuit data to calculate and control each bridge arm drive signal according to actual needs to achieve control of the voltage source converter; The actual requirements described in step 2 include three types, specifically: ① When the DC side voltage is controlled by a four-phase voltage source converter, the DC voltage reference value is U dc_ref The DC voltage collected U dc The difference is then passed through a voltage controller to generate the AC side current reference amplitude. I ref , and the AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave of phase a and phase b i a_ref and i b_ref ; ② When the AC side voltage is controlled by a four-phase voltage source converter, the AC voltage reference wave u a_ref , u b_ref The AC voltage collected u a , u b The voltage controller generates the AC side current reference wave i a_ref and i b_ref ; ③. Neither the DC side voltage nor the AC side voltage requires the four-phase voltage source converter to control. When only the power flow between the AC side and the DC side is controlled, the current reference amplitude is given. I ref With AC side voltage u a The phase angle information obtained after phase locking generates the AC side current reference wave of phase a and phase b i a_ref and i b_ref .

2. The voltage source converter control method for a four-phase AC power system according to claim 1, characterized in that: The specific process of step 2 is: select one of the requirements through the multi-way switch selector, and convert the AC side current reference wave obtained according to the requirement into i a_ref and i b_ref The AC side current collected i a , i b The difference is made, and the pulse width modulation wave is obtained through the current controller. The modulation wave is passed through the PWM generator to generate a PWM drive signal to drive the switch tube on the bridge arm.

Images