Control method for grid-connected direct-current component of converter

Abstract

The control method for the grid-connected direct-current component of the converter is suitable for converters of different topological structures adopting three-phase three-wire system grid connection. The method comprises the following steps: acquiring alternating current A, B and C three-phase currents of a three-phase three-wire system grid-connected converter, converting the alternating current A, B and C three-phase currents from a three-phase static coordinate system to a two-phase rotating coordinate system according to Clarke conversion and Park conversion, and representing symmetrical sine components in the alternating current A, B and C three-phase currents as direct current values on the two rotating coordinate systems; direct current components in alternating current A, B and C three-phase currents are represented as 50Hz fluctuation quantity on two rotating coordinate systems, a resonance control link aiming at the 50Hz fluctuation quantity is added on the basis of a traditional proportional-integral current closed-loop control strategy aiming at direct current quantity control, and control over grid-connected direct current components is achieved.

Description

Technical Field

[0001] The invention relates to the field of grid-connected converters, and in particular to the field of converters that require strict limitation of the DC component of the grid-connected current. Background Art

[0002] As global environmental pollution and fossil energy crises continue to intensify, clean renewable energy has received more and more attention. In recent years, non-hydropower renewable energy such as wind power and photovoltaics has developed rapidly, and a large number of large-scale wind power and photovoltaic power stations have been built in China. These new energy power generation systems need to be connected to the grid through converters. All grids need to meet the performance requirements of new energy power generation systems through corresponding grid-connected converters.

[0003] The grid-connected converter is an important interface for the new energy power generation system to access the power grid. With the continuous improvement of the penetration rate of the new energy power generation system, the grid-connected converter has become more and more prominent in the power grid. An excellent grid-connected converter can efficiently complete the rapid grid connection of the new energy power generation system and effectively reduce the impact of the grid-connected converter on the power grid. The grid-connected converter can virtualize the synchronous generator to complete autonomous operation, power quality management, system impedance detection, network impedance control and other auxiliary control functions.

[0004] The DC component of the grid-connected current is a very important performance indicator of the grid-connected converter. The DC component will cause the converter output voltage to deviate, which may lead to adverse consequences such as grid-connected voltage fluctuation, reduced power factor, and shortened equipment life. At the same time, if the DC component exceeds a certain level, it will also affect the safety and stability of the power grid. It is necessary to develop corresponding control technology to suppress the DC component of the converter current, which is also the background of the present invention. Summary of the invention

[0005] In order to meet the suppression of the DC component of the grid-connected current during the operation of the grid-connected converter, the present invention provides a control method for the DC component of the grid-connected converter. The AC A, B, and C three-phase currents of the three-phase three-wire grid-connected converter are collected, and the AC A, B, and C three-phase currents are transformed from the three-phase stationary coordinate system to the two-term rotating coordinate system according to the Clarke transformation and the Park transformation. The symmetrical sinusoidal components in the AC A, B, and C three-phase currents are expressed as DC quantities in the two-term rotating coordinate system; the DC components in the AC A, B, and C three-phase currents are expressed as 50Hz fluctuation quantities in the two-term rotating coordinate system. On the basis of the traditional proportional-integral current closed-loop control strategy for DC quantity control, a resonant control link for the 50Hz fluctuation quantity is added to realize the control of the grid-connected DC component.

[0006] The control method of the grid-connected DC component of the converter provided by the present invention is an extension of the traditional proportional-integral current closed-loop control strategy, does not change the original control method, and is simple to implement.

[0007] Technical Solution

[0008] A control method for a grid-connected DC component of a converter is used in a converter hardware that uses a three-phase three-wire AC / DC converter, the AC side is connected to the A, B, C AC grids, and the DC side is connected to the DC bus. The AC / DC can use a dual-level topology or a three-level topology, such as Figure 1 shown.

[0009] A control method for a grid-connected DC component of a current transformer collects the AC / DC converter AC side current I a ,I b ,I c The three-phase AC current is transformed from a three-phase stationary coordinate system to a two-phase rotating coordinate system through Clarke transformation and Park transformation, and is controlled in the two-phase rotating coordinate system.

[0010] When there is no DC quantity in the three-phase AC current, the value of the three-phase AC current transformed to the two-phase rotating coordinate system is the DC quantity. When there is a DC quantity in the three-phase AC current, the value of the three-phase AC current transformed to the two-phase rotating coordinate system contains a 50Hz AC fluctuation quantity in addition to the DC quantity.

[0011] A control method for the grid-connected DC component of a converter of the present invention adds a resonant control link for 50Hz fluctuation quantity on the basis of the conventional proportional-integral current closed-loop control strategy for DC quantity control, adds a resonant regulator to both the d-axis and q-axis proportional-integral closed-loop regulation in the two-phase rotating coordinate system control link, and sets the resonant frequency to 50Hz. Figure 2 shown.

[0012] Due to the existence of the resonant regulator, the DC amount in the AC / DC converter grid-connected current can be controlled according to the given instruction. When the given instruction is 0, the DC component in the AC / DC converter grid-connected current can be controlled to be 0.

[0013] Beneficial Effects

[0014] A control method for the grid-connected DC component of a converter collects the three-phase currents of AC A, B, and C of a three-phase three-wire grid-connected converter, and expresses the DC component in the three-phase currents of AC A, B, and C as a 50Hz fluctuation in a two-term rotating coordinate system. A resonant control link for the 50Hz fluctuation is added on the basis of the traditional proportional-integral current closed-loop control strategy for DC quantity control to achieve control of the grid-connected DC component. A control method for the grid-connected DC component of a converter is an extension of the traditional proportional-integral current closed-loop control strategy, which does not change the original control method and is simple to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1It is a device diagram for applying the grid-connected DC component control method.

[0016] Figure 2 It is a specific implementation block diagram of the grid-connected DC component control method.

[0017] Figure 1 middle

[0018] A, B, and C are the AC side connection terminals of the AC / DC converter;

[0019] DC+ and DC- are the DC side connection terminals of the AC / DC converter;

[0020] Ia, Ib, and Ic are the three-phase currents of the AC / DC converter connected to the grid;

[0021] Figure 2 middle

[0022] I dref ,I qref It is a given instruction for the grid-connected current of the AC / DC converter;

[0023] I a ,I b ,I c is the sampling value of the three-phase current of the AC / DC converter connected to the grid;

[0024] I α ,I β is the value of the AC / DC converter grid-connected three-phase current after Clarke transformation;

[0025] I d ,I q is the value of the three-phase current of the AC / DC converter after Park transformation;

[0026] U dref , U qref It is the voltage command output after the closed-loop control of the control loop is adjusted; DETAILED DESCRIPTION

[0027] like Figure 1 As shown, the converter hardware used in the control method of the grid-connected DC component of the converter of the present invention is a three-phase three-wire AC / DC converter, the AC side is connected to the A, B, C AC power grids, and the DC side is connected to the DC bus. The AC / DC can adopt a dual-level topology or a three-level topology.

[0028] The invention discloses a control method for a grid-connected DC component of a converter, collects three-phase AC currents Ia, Ib and Ic of a three-phase three-wire AC / DC converter, and reflects the DC component in the three-phase AC current in a two-phase rotating coordinate system through Clarke transformation and Park transformation.

[0029] Due to the three-phase three-wire system, the sum of Ia+Ib+Ic must be 0, but the three-phase AC currents Ia, Ib, and Ic may all have certain DC components.

[0030] Assumptions:

[0031] Ia current is I a =I m cosωt+I dca

[0032] Ib current is I b =I m cos(ωt-120°)+I dcb

[0033] Ic current is I c =I m cos9ωt+120°)+I dcc

[0034] I dca +I dcb +I dcc =0

[0035] According to the commonly used Clarke transformation formula, the current in the two-phase stationary coordinate system is

[0036] I α =I a

[0037]

[0038] According to the commonly used Park transformation formula, the current in the two-phase rotating coordinate system is

[0039] I d =I α cosωt+I β sinωt

[0040] I q =-I α sinωt+I β cosωt

[0041] Substituting the DC components in Ia, Ib and Ic, the current in the two-phase rotating coordinate system is:

[0042]

[0043] From the above, it can be seen that due to the DC components in Ia, Ib and Ic, the three-phase currents Ia, Ib and Ic are not DC quantities in the two-phase rotating coordinate system, but a fluctuation of 50Hz (angular velocity wt) is added on the basis of the DC quantity. Therefore, on the basis of controlling the DC quantity, it is necessary to control the fluctuation of 50Hz and control the fluctuation of 50Hz to 0 to ensure that the DC components in Ia, Ib and Ic of the inverter grid-connected current are 0.

[0044] A control method for the grid-connected DC component of a converter of the present invention adds a resonance control link for 50Hz fluctuation quantity on the basis of the conventional proportional-integral current closed-loop control strategy for DC quantity control, adds an R regulator to both the d-axis and q-axis closed-loop regulation of the two-phase rotating coordinate system control link, and sets the resonance frequency to 50Hz. Figure 2 shown.

[0045] The existence of R regulator can realize the error-free regulation of 50Hz signal. dref ,I qref There is no instruction for 50Hz fluctuation, which is equivalent to giving a 50Hz fluctuation instruction of 0. Due to the existence of the 50Hz resonant R regulator, the 50Hz fluctuation in the feedback current Id and Iq is 0, and then the DC component in Ia, Ib and Ic is controlled to be 0.

Claims

1. A method for controlling a grid-connected DC component of a converter, characterized in that: The method adds a resonant control link for 50Hz fluctuation quantity on the basis of the traditional proportional-integral current closed-loop control strategy for DC quantity control. Resonant regulators are added to the d-axis and q-axis proportional-integral closed-loop regulation of the two-phase rotating coordinate system control link, and the resonant frequency is set to 50Hz.

2. A method for controlling a DC component of a converter connected to a grid according to claim 1, characterized in that The three-phase alternating currents Ia, Ib, and Ic of the three-phase three-wire AC / DC converter are collected, and the DC component in the three-phase alternating current is reflected in the two-phase rotating coordinate system through Clarke transformation and Park transformation.

3. A method for controlling a DC component of a converter connected to a grid according to claim 2, characterized in that Due to the three-phase three-wire system, the sum of Ia+Ib+Ic must be 0, but the three-phase AC currents Ia, Ib, and Ic may all have a certain DC component. Assumptions: Ia current is I a =I m cosωt+I dca Ib current is I b =I m cos(ωt-120°)+I dcb Ic current is I c =I m cos(ωt+120°)+I dcc I dca +I dcb +I dcc =0 According to the commonly used Clarke transformation formula, the current in the two-phase stationary coordinate system is I α =I a According to the commonly used Park transformation formula, the current in the two-phase rotating coordinate system is I d =I α cosωt+I β sinωt I q =-I α sinωt+I β cosωt Substituting the DC components in Ia, Ib and Ic, the current in the two-phase rotating coordinate system is: From the above, it can be seen that due to the DC components in Ia, Ib and Ic, the three-phase currents Ia, Ib and Ic are not DC quantities in the two-phase rotating coordinate system, but a fluctuation of 50Hz (angular velocity wt) is added on the basis of the DC quantity. Therefore, it is necessary to control the fluctuation of 50Hz on the basis of controlling the DC quantity.

4. A method for controlling the grid-connected DC component of a converter according to claims 1 and 3, characterized in that The existence of the resonant R regulator can achieve zero-difference regulation of the 50Hz signal. dref ,I qref There is no instruction for 50Hz fluctuation, which is equivalent to giving a 50Hz fluctuation instruction of 0. Due to the existence of the 50Hz resonant R regulator, the 50Hz fluctuation in the feedback current Id and Iq is 0, and then the DC component in Ia, Ib and Ic is controlled to be 0.

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