Direct-current bus start control method suitable for medium-voltage direct-current collection system

Abstract

The invention provides a direct current bus start control method suitable for a medium-voltage direct current collection system, which aims at a circuit structure that a low-voltage direct current bus exists between a front-stage converter and a rear-stage converter, a front-stage branch runs in a current source mode, and after a rear-stage low-voltage direct current bus is detected to be established through a control strategy, the front-stage branch runs in a current source mode; the problem of transient impact generated in the starting process is effectively restrained, smooth establishment of the low-voltage direct-current bus voltage and undifferentiated starting of the multiple confluence branches are achieved, and the method is particularly suitable for a structure with the front end of the low-voltage direct-current bus and the front end of the low-voltage direct-current bus and the front end of the low-voltage direct-current bus and the front end of the low-voltage direct-current bus are connected in parallel. And the converged medium-voltage direct-current converter is a direct-current boosting and converging system in which the power flows unidirectionally. According to the invention, smooth starting and stable control of the low-voltage direct-current bus in the direct-current boost collection system can be realized under the condition of one-way power flow, and self-starting without differences and interconnection lines can be realized among multiple collection branches on the low-voltage side.

Description

Technical Field

[0001] The present invention relates to the field of electromechanics, and in particular to a DC busbar startup control method suitable for a medium-voltage DC collection system. Background Art

[0002] Large-scale renewable energy power generation bases such as wind power / photovoltaic power generation have long collection distances. The use of AC collection, harmonic resonance, and reactive power transmission will cause many problems. The AC collection-DC transmission method has many power conversion links and high system costs. The organic combination of DC boost conversion technology and wind power / photovoltaic power generation to form a renewable energy DC power generation system is a new technical route that provides efficient and low-cost grid-connected solutions for large-scale wind and solar bases.

[0003] In traditional AC collection systems, for structures with multiple branches on the low-voltage side, the low-voltage DC bus voltage can be first established by reversely drawing power from the rear-stage AC grid through the bidirectional conversion function of the centralized inverter, and then the multiple branches of the front-stage convergence are restarted and directly enter the maximum power tracking operation. However, in the scenario where the low-voltage side of the wind power / photovoltaic DC boost collection system contains multiple branches, and the rear-stage DC boost converter has a unidirectional power flow characteristic, it is impossible to reversely draw power from the grid to establish the low-voltage DC bus voltage. Summary of the invention

[0004] In order to solve the problems of difficult DC bus startup control and large transient impact under unidirectional power flow of DC converter in renewable energy DC boost collection system, the present invention proposes a DC bus startup control method suitable for medium voltage DC collection system for DC boost collection system with unidirectional power flow, which can realize smooth startup and stable control of DC bus voltage under multi-branch collection structure, and is particularly suitable for DC boost collection system with unidirectional power flow.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] A DC busbar startup control method applicable to a medium-voltage DC collection system, wherein the medium-voltage collection system comprises n collection branches on the low-voltage DC side, and the n branches are converted into medium-voltage DC through a DC boost converter after being collected on the low-voltage DC side, so as to realize the collection of medium-voltage DC from renewable energy sources; the DC busbar startup control method comprises:

[0007] The converters in the n bus branches on the low-voltage DC side all have a unidirectional power flow characteristic, and the DC boost converter connected after the n branches are merged also has a unidirectional power flow characteristic, that is, the power reverse flow characteristic is not satisfied at any time;

[0008] Before starting, the capacitor voltage of the low-voltage DC bus is zero. At the starting moment, the n bus branches all output in current source mode to establish the subsequent low-voltage DC bus. Each branch uses the input current disturbance strategy in real time to determine whether the subsequent low-voltage DC bus voltage has been established during the operation of the current source mode. If the low-voltage DC bus voltage no longer changes with the change in the output current amplitude of the branch, it is determined that the subsequent low-voltage DC bus has been established. When it is determined that the DC bus voltage has been established, the working mode of the branch is switched from the current source mode to the maximum power tracking mode, achieving smooth mode switching and stable establishment of the low-voltage DC bus.

[0009] Furthermore, the n branches on the low-voltage DC side start the DC bus indiscriminately and without sequence, that is, any branch first operates in the current source mode at any time, and then switches to the maximum power tracking mode after the control strategy determines that the DC bus has been established.

[0010] Furthermore, the DC type wind turbine power generation unit outside the bus branch on the low voltage DC side tempers the photovoltaic power generation unit, and the DC busbar startup control method is applicable to both the wind power DC boost collection system and the photovoltaic DC life collection system.

[0011] The main principle of the DC bus startup control method is: each branch is divided into two working processes in the startup stage. In the first working process, each branch works in the output current source mode and performs output current disturbance control in real time; in the second working process, after each branch detects that the voltage of the rear-stage low-voltage DC bus has been established, it switches from the output current source mode to the maximum power tracking mode, and each branch realizes a smooth transition from the startup process to the steady-state operation.

[0012] The beneficial effects of the present invention are:

[0013] In view of the circuit structure in which a DC bus exists between the front and rear converters, the present invention adopts a control method combining a current source and a power source to smoothly establish the DC bus voltage during the startup phase, effectively suppressing the transient impact problem generated during the startup process, and realizing the indiscriminate smooth startup of the DC bus voltage and multiple converters. It is particularly suitable for the front stage of the low-voltage DC bus including a multi-branch output parallel structure, which solves the problem that the low-voltage DC bus startup in the power unidirectional conversion system requires additional power or needs to add communication interconnection devices between different branches, and realizes the indiscriminate smooth startup of multiple branches and stable control of the DC bus voltage. Moreover, the medium-voltage DC converter is a DC boost collection system with unidirectional power flow. Compared with the conventional AC collection system solutions in renewable energy sources such as wind power / photovoltaic power, under the control method described in the present invention, the converter at the front stage in the DC boost collection system in the startup phase establishes the rear-stage DC bus in the form of a controllable current source. When the DC bus voltage is established to a predetermined value, the rear-stage DC converter will take over the control of the DC bus voltage. Thereafter, the converter of the front-stage branch switches to the maximum power tracking mode, that is, switches from the current source mode to the power source mode, so as to realize the smooth startup and stable control of the low-voltage DC bus in the DC boost collection system under unidirectional power flow, and multiple branches on the low-voltage side can be self-started without distinction and without interconnection. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a schematic diagram of the principle of a DC busbar startup control method applicable to a medium voltage DC collection system of the present invention;

[0015] Figure 2 It is a schematic diagram of the principle of a wind power / photovoltaic DC boost collection system including n low-voltage collection branches in a specific implementation manner;

[0016] Figure 3 It is a schematic diagram of the principle of a DC boost collection system including three low-voltage collection branches preferably in a specific implementation manner. DETAILED DESCRIPTION

[0017] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the protection scope of the present invention.

[0018] like Figure 2 As shown in the figure, the medium voltage DC collection system includes n bus branches on the low voltage DC side: 1# branch - n# branch (i.e. Figure 2In the figure, n branches (branch 1# to branch n#) converge on the low-voltage DC side and then use a DC boost converter to convert the low-voltage DC into a medium-voltage DC, thus realizing the collection of medium-voltage DC from renewable energy.

[0019] The front end of the medium voltage DC collection system includes multiple DC wind turbines or photovoltaic power generation units. For the wind power DC boost collection system, the front end DC wind turbine includes a wind turbine and an AC / DC converter. For the photovoltaic DC boost collection system, the front end photovoltaic power generation unit includes a photovoltaic cell array and a maximum power point tracking (MPPT) converter.

[0020] Each branch output on the low-voltage side of the medium-voltage DC collection system is a low-voltage DC voltage. The low-voltage DC voltages output by multiple branches are connected in parallel and then connected to the input end of the subsequent DC boost converter. After the DC boost conversion, the low-voltage DC voltage is converted into a medium-voltage DC voltage output, thereby realizing the collection and delivery of medium-voltage DC power from a wind power plant or a photovoltaic power station. The DC boost converter has a unidirectional power flow characteristic, that is, energy can only flow from the low-voltage side of the DC converter to the medium-voltage side.

[0021] Each branch on the low-voltage DC side of the medium-voltage DC collection system has two steady-state working modes: current source mode and maximum power tracking mode.

[0022] like Figure 1 As shown, a DC bus startup control method applicable to a medium voltage DC collection system according to an embodiment of the present invention includes:

[0023] The converters in the n bus branches on the low voltage DC side all have unidirectional power flow characteristics, and the DC boost converter connected after the n branches are merged also has unidirectional power flow characteristics, that is, the power reverse flow characteristic is not satisfied at any time.

[0024] Before starting, the low-voltage DC bus capacitor voltage is zero. At the start-up time, the n bus branches are all output in the current source mode to establish the low-voltage DC bus at the next stage, and each branch uses the output current disturbance strategy in real time during the operation of the current source mode to determine whether the voltage of the low-voltage DC bus at the next stage has been established. Among them, the principle of the output current disturbance strategy is: while the branch converter is running in the output current source mode, the output current reference value is changed in real time at intervals, and the voltage value of the low-voltage DC bus at the next stage changes accordingly with the change of the branch output current reference value. If the voltage of the low-voltage DC bus at the next stage increases with the increase of the branch output current reference value, it indicates that the voltage of the low-voltage DC bus at the next stage has not been established. At this time, the branch converter should continue to operate in the current source mode; if the voltage value of the low-voltage DC bus at the next stage no longer changes accordingly with the change of the branch output current reference value, it can be determined that the low-voltage DC bus at the next stage has been established. When it is determined that the low-voltage DC bus voltage has been established, the operating mode of the branch is switched from the current source mode to the maximum power tracking mode, thereby achieving smooth mode switching and stable establishment of the low-voltage DC bus.

[0025] Preferably, the n branches on the low-voltage DC side can perform the above startup process indiscriminately and without sequence, that is, any branch can operate in the current source mode at any time, and then switch to the maximum power tracking mode after determining that the DC bus has been established through the above-mentioned output current disturbance strategy.

[0026] Preferably, the bus branch on the low-voltage DC side can be either a DC wind turbine power generation unit or a photovoltaic power generation unit, and the control method is applicable to both a wind power DC boost collection system and a photovoltaic DC collection system.

[0027] like Figure 3 As shown, a DC boost collection system including three photovoltaic bus branches on the low-voltage side is taken as an example to illustrate the control method of the present invention. Figure 3 The low-voltage side includes three branches, each of which is composed of a photovoltaic array and a corresponding maximum power point tracking (MPPT) converter. Each branch has its own controller, and each branch controller can determine the working mode of the branch. The working modes of each branch mainly include two types: output current source mode and maximum power point tracking mode.

[0028] Before starting, the low-voltage DC (LVDC) bus voltage of the DC boost collection system is zero. At the same time, since the grid-connected converter on the grid side is essentially a DC / AC inverter, it has the function of bidirectional power flow. Therefore, before the low-voltage side is started, the medium-voltage DC (MVDC) bus voltage can be established by drawing power from the grid through the grid-connected converter.

[0029] After the start-up, the three branches can be started and operated without distinction and in no order. Taking time 0 as the starting point, assuming that branch 1#, branch 2# and branch 3# (i.e. Figure 3 The actual starting times of branch 1#, branch 2#, and branch 3# are t1, t2, and t3 respectively.

[0030] At time t1, branch 1# starts to start, and branch 1# first operates in output current source mode. At this time, the voltage of the low-voltage DC bus capacitor of the subsequent stage begins to gradually build up. Since the DC boost converter of the subsequent stage is a unidirectional power conversion, the DC boost converter has not yet established a normal current path and is in an actual cut-off state. As branch 1# outputs to the low-voltage capacitor of the subsequent stage in current source mode, the low-voltage DC bus voltage gradually increases. Thereafter, during the output current source mode, branch 1# monitors the establishment state of the DC bus voltage of the subsequent stage through the output current disturbance strategy. When the output current reference value increases, if the DC bus voltage of the subsequent stage increases accordingly, it indicates that the DC bus voltage has not yet been established. When the branch controller detects that the voltage of the low-voltage DC bus of the subsequent stage has stopped changing, it indicates that the DC boost converter of the subsequent stage has been working normally, and the DC boost converter operates in the stable low-voltage DC bus mode, and the control of the low-voltage DC bus voltage is realized. At this time, the low-voltage DC bus voltage has been established.

[0031] At time t2 and time t3, branch 2# and branch 3# start to start respectively, and branch 2# and branch 3# also operate in output current source mode first, and the subsequent operation mode switching and control strategy transformation process are similar to branch 1#.

[0032] At the start-up time of branch 2# and branch 3#, regardless of whether the voltage of the low-voltage DC bus at the subsequent stage has been established, branch 2# and branch 3# can proceed according to the set start-up strategy, that is, output in current source mode first, and realize smooth switching of working mode according to the real-time detection of DC bus voltage. For example, at the start-up time of branch 2#, if the voltage of the low-voltage DC bus at the subsequent stage has been established and the DC boost converter has been working in the stable low-voltage bus voltage mode, branch 2# can also output in current source mode and output power to the low-voltage DC bus at the subsequent stage. When the controller of branch 2# detects that the DC bus voltage has been established, the working mode of the branch is switched from current source mode to maximum power tracking mode.

[0033] Similarly, at the start-up time of branch 3#, if the voltage of the downstream low-voltage DC bus has not been established, branch 3# will also output in current source mode first, output power to the downstream low-voltage DC bus to continue to establish the low-voltage DC bus. When the branch 3# controller detects that the DC bus voltage has been established, it will switch from current source mode to maximum power tracking mode.

[0034] From the above analysis, it can be seen that when the low-voltage DC side contains multiple branches, each branch can be started in any order without distinction, and no communication interconnection device is required between the branches. Each branch first outputs in the current source mode during the startup phase, and uses the output current disturbance strategy during the current source working mode to determine whether the subsequent DC bus voltage has been established. When the subsequent low-voltage DC bus voltage has been established through real-time detection, the branch working mode is switched from the current source mode to the maximum power tracking mode.

[0035] The above description is only a preferred embodiment of the present invention, and does not limit the scope of the present invention. Any equivalent structure or process transformation made by using the contents of the present invention specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the protection scope of the present invention.

Claims

1. A DC bus startup control method applicable to a medium voltage DC collection system, characterized in that: The medium-voltage collection system comprises n collection branches on the low-voltage DC side. After the n branches are collected on the low-voltage DC side, the low-voltage DC is converted into medium-voltage DC through a DC boost converter, thereby realizing the collection of medium-voltage DC from renewable energy sources. The DC bus startup control method includes: The converters in the n bus branches on the low-voltage DC side all have a unidirectional power flow characteristic, and the DC boost converter connected after the n branches are merged also has a unidirectional power flow characteristic, that is, the power reverse flow characteristic is not satisfied at any time; Before starting, the capacitor voltage of the low-voltage DC bus is zero. At the starting moment, the n bus branches all output in current source mode to establish the subsequent low-voltage DC bus. Each branch uses the input current disturbance strategy in real time to determine whether the subsequent low-voltage DC bus voltage has been established during the operation of the current source mode. If the low-voltage DC bus voltage no longer changes with the change in the output current amplitude of the branch, it is determined that the subsequent low-voltage DC bus has been established. When it is determined that the DC bus voltage has been established, the working mode of the branch is switched from the current source mode to the maximum power tracking mode, achieving smooth mode switching and stable establishment of the low-voltage DC bus.

2. The DC busbar startup control method applicable to a medium voltage DC collection system according to claim 1 is characterized in that: The n branches on the low-voltage DC side start the DC bus indiscriminately and without sequence, that is, any branch operates in the current source mode at any time, and then switches to the maximum power tracking mode after the control strategy determines that the DC bus has been established.

3. The DC busbar startup control method applicable to a medium voltage DC collection system according to claim 1 is characterized in that: The DC type wind turbine power generation unit on the periphery of the low voltage DC side of the bus branch tempers the photovoltaic power generation unit, and the DC busbar startup control method is applicable to both the wind power DC boost collection system and the photovoltaic DC life collection system.

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