Unified power flow controller and control method for power coupling of cascaded H-bridge partial modules

Abstract

The present invention discloses a unified power flow controller and control method for power coupling of cascaded H-bridge partial modules. The single-phase converter of the unified power flow controller includes: a sending-end power grid connection end, which is used to connect the sending-end power grid; a parallel unit, which is used for stable control of the voltage of the capacitor on the DC side of the H-bridge module; a series unit, which is used for line power flow regulation; a receiving-end power grid connection end, which is used to connect the receiving-end power grid; the parallel unit and the series unit are power coupled through a primary H-bridge module. The control method thereof includes the following steps: regulating the line power flow of the line through the series unit; and stably controlling the voltage of the capacitor on the DC side of the H-bridge module through the parallel unit. The present invention discloses a unified power flow controller. The series unit and the parallel unit of the unified power flow controller are power coupled only through a primary H-bridge module, and the parallel unit does not need a power frequency transformer to boost the voltage, which helps to reduce the total number of required H-bridge modules and the total cost of the unified power flow controller.

Description

Technical Field

[0001] The present invention belongs to the technical field of unified power flow controllers and control methods thereof, and in particular relates to a unified power flow controller and a control method thereof for power coupling of cascaded H-bridge partial modules. Background Art

[0002] When long-distance power transmission is carried out through the AC power grid, the impedance voltage drop generated on the long-distance line will seriously reduce the transmission capacity and synchronization stability margin. The unified power flow controller uses advanced power electronics technology to achieve flexible AC power transmission. It has rich functions such as AC voltage regulation, line impedance compensation, and power oscillation damping suppression, which can effectively improve the long-distance transmission capacity and synchronization stability of the AC power grid.

[0003] A unified power flow controller is generally composed of two groups of voltage source converters, in which the AC ports of one group of converters are connected in series to the AC line for power flow regulation, and the AC ports of the other group of converters are connected in parallel to the AC grid bus for reactive power compensation; the two groups of converters are interconnected through DC ports, and power coupling and energy balance are achieved through coordinated control.

[0004] Due to the high voltage level and large transmission power in the transmission network, the unified power flow controller needs to adopt a large-capacity converter circuit topology. Chinese patent CN106099909B proposes a unified power flow controller based on a modular multilevel converter (MMC). The unified power flow controller has excellent performance, but it needs to install power frequency transformers on both the AC series side and the AC parallel side, and contains a large number of power electronic semiconductor devices and capacitors, resulting in a large size and extremely high cost of the unified power flow controller.

[0005] In order to reduce the volume and cost, Chinese patent CN202550584U proposes a unified power flow controller based on H-bridge modules. Although the unified power flow controller omits the transformer on the AC series side, it uses a multi-winding transformer with a large volume and a relatively complex manufacturing process on the AC parallel side. In addition, the unified power flow controller includes multiple cascaded double H-bridge modules, and each double H-bridge module is interconnected on the DC side for power coupling, which requires a large number of power electronic semiconductor devices, resulting in high costs. Summary of the invention

[0006] The present invention is proposed to solve the problems existing in the prior art, and its purpose is to provide a unified power flow controller and a control method for power coupling of cascaded H-bridge partial modules.

[0007] The technical solution of the present invention is: a unified power flow controller with power coupling of cascaded H-bridge partial modules, wherein the single-phase converter of the unified power flow controller comprises:

[0008] A sending-end power grid connection terminal, used for connecting to a sending-end power grid;

[0009] Parallel unit, used for voltage stabilization control of capacitors on the DC side of the H-bridge module;

[0010] Series units are used for line flow regulation;

[0011] A receiving-end power grid connection terminal, used for connecting to a receiving-end power grid;

[0012] The parallel units and the series units are power coupled through a primary H-bridge module.

[0013] Furthermore, there are multiple single-phase converters in the unified power flow controller, and the structures of the single-phase converters are the same.

[0014] Furthermore, the parallel unit includes an AC filter inductor L sh , AC filter inductor L sh One end of is the sending-end grid connection end, which is connected to the sending-end grid bus BUS.

[0015] Furthermore, the parallel unit includes an H-bridge module, which is n in number and cascaded into a group, and the upper end of the uppermost H-bridge module is connected to the AC filter inductor L sh The bottom end of the lowest H-bridge module is grounded.

[0016] Furthermore, the series unit includes an H-bridge module H se , H-bridge module H se The AC side passes through the AC filter inductor L f , AC filter capacitor C f Connected to the secondary winding of the series-compensated transformer T1.

[0017] Furthermore, the primary winding of the series-compensated transformer T1 is connected in series to the corresponding AC line.

[0018] Furthermore, the H bridge module H in the series unit se The DC side of the parallel unit is connected to the nth H-bridge module H sh_n The DC side passes through the DC filter capacitor C dc connected.

[0019] A control method for a unified power flow controller with power coupling of cascaded H-bridge partial modules comprises the following steps:

[0020] A. Through the series connection unit, the line flow is regulated;

[0021] B. Through the parallel unit, the DC side capacitor voltage of the H-bridge module is stably controlled.

[0022] Furthermore, step A adjusts the line flow of the line through the series connection unit. The specific process is as follows:

[0023] The series connection of the units is achieved by adjusting the H-bridge module H se The modulation ratio controls the AC filter capacitor C f The voltage v cf And the primary winding voltage v of the series-compensated transformer T1 se , to achieve line flow regulation.

[0024] Furthermore, step B performs stable control on the capacitor voltage on the DC side of the H-bridge module through the parallel unit, and the specific process is as follows:

[0025] The parallel units control the output current i sh Realize reactive power compensation for the AC grid point BUS, and use the cascaded H-bridge module voltage balancing algorithm to achieve the DC side capacitor voltage V of all H-bridge modules dc_k (k=1…n) stable and balanced.

[0026] The beneficial effects of the present invention are as follows:

[0027] The present invention discloses a unified power flow controller, in which the series units and parallel units of the unified power flow controller are power coupled only through a primary H-bridge module, and the parallel units do not need an industrial frequency transformer to boost the voltage, which helps to reduce the total number of required H-bridge modules and the total cost of the unified power flow controller. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a circuit principle diagram of the unified power flow controller of the present invention applied to a three-phase power system;

[0029] Figure 2 It is an internal circuit schematic diagram of a single-phase converter in a unified power flow controller of the present invention;

[0030] Figure 3 It is a circuit principle diagram of the unified power flow controller of the present invention applied to a single-phase power system;

[0031] Figure 4 It is a circuit topology diagram of the H-bridge module in the present invention. DETAILED DESCRIPTION

[0032] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings and embodiments:

[0033] like Figures 1 to 4 As shown, a unified power flow controller with power coupling of cascaded H-bridge partial modules, a single-phase converter of the unified power flow controller includes:

[0034] A sending-end power grid connection terminal, used for connecting to a sending-end power grid;

[0035] Parallel unit, used for voltage stabilization control of capacitors on the DC side of the H-bridge module;

[0036] Series units are used for line flow regulation;

[0037] A receiving-end power grid connection terminal, used for connecting to a receiving-end power grid;

[0038] The parallel units and the series units are power coupled through a primary H-bridge module.

[0039] Furthermore, there are multiple single-phase converters in the unified power flow controller, and the structures of the single-phase converters are the same.

[0040] Furthermore, the parallel unit includes an AC filter inductor L sh , AC filter inductor L sh One end of is the sending-end grid connection end, which is connected to the sending-end grid bus BUS.

[0041] Furthermore, the parallel unit includes an H-bridge module, which is n in number and cascaded into a group, and the upper end of the uppermost H-bridge module is connected to the AC filter inductor L sh The bottom end of the lowest H-bridge module is grounded.

[0042] Furthermore, the series unit includes an H-bridge module H se , H-bridge module H se The AC side passes through the AC filter inductor L f , AC filter capacitor C f Connected to the secondary winding of the series-compensated transformer T1.

[0043] Furthermore, the primary winding of the series-compensated transformer T1 is connected in series to the corresponding AC line.

[0044] Furthermore, the H bridge module H in the series unit se The DC side of the parallel unit is connected to the nth H-bridge module H sh_n The DC side passes through the DC filter capacitor C dc connected.

[0045] A control method for a unified power flow controller with power coupling of cascaded H-bridge partial modules comprises the following steps:

[0046] A. Through the series connection unit, the line flow is regulated;

[0047] B. Through the parallel unit, the DC side capacitor voltage of the H-bridge module is stably controlled.

[0048] Furthermore, step A adjusts the line flow of the line through the series connection unit. The specific process is as follows:

[0049] The series connection of the units is achieved by adjusting the H-bridge module H seThe modulation ratio controls the AC filter capacitor C f The voltage v cf And the primary winding voltage v of the series-compensated transformer T1 se , to achieve line flow regulation.

[0050] Furthermore, step B performs stable control on the capacitor voltage on the DC side of the H-bridge module through the parallel unit, and the specific process is as follows:

[0051] The parallel units control the output current i sh Realize reactive power compensation for the AC grid point BUS, and use the cascaded H-bridge module voltage balancing algorithm to achieve the DC side capacitor voltage V of all H-bridge modules dc_k (k=1…n) stable and balanced.

[0052] Specifically, step B includes establishing an output current expression of the parallel unit in stabilizing the voltage of the capacitor on the DC side of the H-bridge module through the parallel unit. The specific process is as follows:

[0053] Output current reference value i of parallel unit sh_ref This is achieved by the following expression:

[0054]

[0055] Among them, θ vs1 is the grid voltage v at the sending end extracted by the phase-locked loop s1 The phase of

[0056] I shd_ref is i sh_ref The active component of

[0057] I shq_ref is i sh_ref The reactive component.

[0058] More specifically, I shd_ref This is achieved by the following expression:

[0059]

[0060] Among them, K p_vdc , K i_vdc They are the proportional and integral coefficients of the PI regulator respectively;

[0061] v dc_ref is the DC capacitor voltage reference value of the phase converter,

[0062] v dc_ave is the average voltage of the DC capacitors of all H-bridge modules in the phase converter. More specifically, v dc_ave This is achieved by the following expression:

[0063]

[0064] Among them, k is the subscript of the parallel unit H bridge module number,

[0065] n is the total number of parallel unit H bridge modules.

[0066] More specifically, I shq_ref This is achieved by the following expression:

[0067]

[0068] Among them, Q sh_ref is the reactive power compensation reference value,

[0069] V s1 is the sending-end grid voltage v s1 Valid values.

[0070] As an application illustration, the unified power flow controller can be used in three-phase power systems and single-phase power systems.

[0071] The following is a detailed description with reference to the accompanying drawings.

[0072] Figure 1 The schematic diagram of the circuit of the unified power flow controller with power coupling of the cascaded H-bridge partial modules of the present invention applied in a three-phase power system.

[0073] The unified power flow controller is composed of three independent single-phase converters with completely the same structure, and each single-phase converter is composed of a parallel unit and a series unit.

[0074] The parallel unit of the single-phase converter consists of an AC filter inductor L sh , n H-bridge modules (n≥2) are cascaded, and the n H-bridge modules are numbered as follows: H sh_1 , H sh_2 ,…H sh_k ,…H sh_n .

[0075] The series unit of the single-phase converter consists of an H-bridge module, an AC filter inductor L f , AC filter capacitor C f , series-compensated transformer T1, the H-bridge module is numbered H se .

[0076] Figure 2 The internal circuit schematic diagram of a single-phase converter in a unified power flow controller with power coupling of cascaded H-bridge partial modules of the present invention.

[0077] The parallel cells and series cells of each single-phase converter are connected as follows: The first H-bridge module H sh_1 The upper end of the AC port is connected through the AC filter inductor Lsh Connected to the sending end power grid bus BUS, the first H bridge module H sh_1 The AC port of the lower end is connected to the second H-bridge module H sh_2 The kth H-bridge module H sh_k The upper end of the AC port is connected to the k-1th H-bridge module H sh_k-1 The kth H-bridge module H sh_k The lower end of the AC port is connected to the k+1th H bridge module H sh_k+1 The upper end of the AC port is connected to (k=2…n-1); the nth H bridge module H sh_n The upper end of the AC port is connected to the n-1th H bridge module H sh_n-1 The lower end of the AC port is connected to the nth H bridge module H sh_n The lower end of the AC port is grounded.

[0078] H-bridge module H for series connection of units se The DC port of the parallel unit is connected to the nth H-bridge module H sh_n The DC port passes through the DC filter capacitor C dc connected.

[0079] H-bridge module H for series connection of units se The AC port is connected through the AC filter inductor L f , AC filter capacitor C f It is connected to the secondary winding of the series-compensated transformer T1; the primary winding of the series-compensated transformer T1 is connected in series to the AC line of the corresponding phase.

[0080] Figure 3 The schematic diagram of the circuit for the application of the unified power flow controller with power coupling of the cascaded H-bridge partial modules in a single-phase power system of the present invention.

[0081] The parallel unit of the single-phase converter in the unified power flow controller is composed of an AC filter inductor L sh , n H-bridge modules (n≥2) are cascaded, and the n H-bridge modules are numbered as follows: H sh_1 , H sh_2 ,…H sh_k ,…H sh_n The series unit of the single-phase converter consists of an H-bridge module, an AC filter inductor L f , AC filter capacitor C f , series-compensated transformer T1, the H-bridge module is numbered H se .

[0082] Figure 4 The circuit topology diagram of the H-bridge module in the present invention is shown. The H-bridge module includes four IGBT modules, and the ports on both sides are the upper end and the lower end of the AC port and the P end and the N end of the DC port.

[0083] The present invention overcomes the disadvantage of high cost caused by the existing unified power flow controller based on H-bridge modules including a large number of dual H-bridge modules, and proposes a unified power flow controller with power coupling of cascaded H-bridge partial modules and a control method thereof.

[0084] The present invention discloses a unified power flow controller, in which the series units and parallel units of the unified power flow controller are power coupled only through a primary H-bridge module, and the parallel units do not need an industrial frequency transformer to boost the voltage, which helps to reduce the total number of required H-bridge modules and the total cost of the unified power flow controller.

Claims

1. A unified power flow controller with power coupling of cascaded H-bridge modules, characterized by: The single-phase converter of the unified power flow controller comprises: A sending-end power grid connection terminal, used for connecting to a sending-end power grid; Parallel unit, used for voltage stabilization control of capacitors on the DC side of the H-bridge module; Series units are used for line flow regulation; A receiving-end power grid connection terminal, used for connecting to a receiving-end power grid; The parallel units and series units are power coupled through a primary H-bridge module; There are multiple single-phase converters in the unified power flow controller, and the structures of the single-phase converters are the same; The parallel unit consists of an AC filter inductor L sh The AC filter inductor L sh The other end not connected to the cascade H bridge is directly connected to the sending-end power grid bus BUS; The series unit consists of a primary H-bridge module H se The AC end is connected in series with the AC filter inductor L f After the parallel AC filter capacitor C f and the secondary winding of the series-compensated transformer T1, and the first-level H-bridge module H of the series unit se The DC end of the parallel unit is connected to the last H-bridge module H sh_n The DC side connection realizes power coupling.

2. The control method of the unified power flow controller of the cascaded H-bridge partial module power coupling according to claim 1 is characterized in that: The following steps are involved: A. Through the series connection unit, the line flow is regulated; B. Through the parallel unit, the DC side capacitor voltage of the H-bridge module is stably controlled; Step A adjusts the line flow through the series connection unit. The specific process is as follows: The series connection of the units is achieved by adjusting the H-bridge module H se The modulation ratio controls the AC filter capacitor C f The voltage V Cf And the primary winding voltage V of the series-compensated transformer T1 se , to achieve line flow regulation.

3. The control method of the unified power flow controller with power coupling of cascaded H-bridge partial modules according to claim 2 is characterized in that: Step B uses the parallel unit to stably control the capacitor voltage on the DC side of the H-bridge module. The specific process is as follows: The parallel unit achieves i by controlling the output current sh The reactive power compensation of the AC grid-connected point BUS is realized by adopting the cascaded H-bridge module voltage balancing algorithm to realize the DC side capacitor voltage V of all H-bridge modules dc_k Stable and balanced, where k=1…n.

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