Smooth switching control method for network following type converter and network constructing type converter

Abstract

The invention belongs to the technical field of grid-connected control, and particularly relates to a grid-following type and grid-constructing type smooth switching control method for a three-level converter based on LC filtering, and the method comprises the steps: collecting the grid-side voltage uabc of the converter and the current iabc at the outlet of the converter in real time, and obtaining the active output power P and reactive power Q of the converter; constructing a control loop for smooth switching of phase angles of active control loops and phase-locked loops of the following network type and the construction network type according to the topological structures and the mathematical models of the following network type and the construction network type converters; and constructing a following network type and constructing network type d and q axis current reference value smooth switching control loop. Compared with the prior art, the method has higher accuracy and flexibility, power and frequency fluctuation in the converter switching process can be better restrained, the grid-connected stability is improved, and safe and stable operation of the converter under the condition that the power grid strength is greatly changed is achieved.

Description

Technical Field

[0001] The present application relates to the technical field of grid-connected control, and in particular to a method for smooth switching control of grid-following type and grid-building type converters. Background Art

[0002] With the acceleration of global energy transformation, the penetration rate of new energy sources, mainly wind power and photovoltaic power, has gradually increased. Unlike traditional power generation methods, new energy power generation needs to be connected to the grid through power electronic equipment. A high proportion of power electronic equipment will change the characteristics of traditional power grids, further reduce the strength of the power grid, and the power system will gradually show a weak network trend. The power generation units in existing new energy sites are all connected to the grid using the grid-following (GFL) control strategy, and use the phase-locked loop (PLL) to follow the grid voltage phase to achieve synchronization.

[0003] However, studies have shown that GFL control is equivalent to a current source, which has a greater risk of instability under weak grid conditions and is difficult to adapt to the trend of future weak grids. Therefore, in recent years, more and more research has been conducted on grid-forming (GFM) control, which uses a self-synchronous control method to overcome the subsynchronous oscillation problem caused by the PLL link in GFL control under weak grid conditions.

[0004] Studies have shown that the GFL-controlled converter has stronger stability under strong power grids and weaker stability under weak power grids; while the GFM-controlled converter has stronger stability under weak power grids and weaker stability under strong power grids. Therefore, the existing converter working mode is difficult to adapt to safe and stable operation under large-scale changes in grid impedance. To address this problem, the main solution is to use grid strength as the switching indicator and study the smooth switching strategy of GFL / GFM, that is, to use GFL control under strong power grids and GFM control under weak power grids. The current GFL / GFM switching scheme is not mature in design, and the power and frequency fluctuate greatly during the switching process, which results in poor grid-connected stability. Summary of the invention

[0005] In view of the shortcomings existing in the above-mentioned prior art, the present invention proposes a smooth switching control method for a converter with grid-following / grid-forming type control. The method designs a GFL / GFM smooth switching control strategy based on a mathematical model of the converter in a synchronous coordinate system. The purpose is to better suppress power and frequency fluctuations during converter switching, improve grid-connected stability, and achieve safe and stable operation of the converter under large changes in grid strength.

[0006] The present invention is achieved through the following technical solutions:

[0007] A method for smooth switching control of a grid-following type and a grid-building type converter is provided, comprising the following steps:

[0008] Step 1: Real-time acquisition of the grid-side voltage u of the converter abc and the current i at the converter outlet abc , after Parker transformation, we can get the components of d-axis and q-axis in the two-phase rotating coordinate system, which are expressed as u d 、u q and i d 、i q , will u d 、u q and i d 、i q After being sent to the power calculation link, the active output power P and reactive power Q of the converter are obtained;

[0009] Step 2: Construct the control loop for smooth switching of the active power control loop and the phase angle of the phase-locked loop for the following and building networks: send the real-time value of the active power P in step 1 to the building network active power-frequency control loop to obtain the output angular frequency ω GFM , will u abc Sent to the grid-following PLL link to obtain the output angular frequency ω GFL , a first switch and a second switch are set between the phase-locked loop of the grid-following control and the active-frequency control loop of the grid-building control, and the first switch is set at the output angular frequency ω of the PLL part GFL And the active-frequency control loop output angular frequency ω GFM The second switch is arranged before the Parker transformation, its S1 end is connected to the phase angle θ output by the system, S2 is connected to the integrator output of the phase-locked loop, and the first switch and the second switch are synchronously controlled, that is, the S1 end is connected at the same time or the S2 end is connected at the same time;

[0010] Step 3: Construct the reactive power control loop, power control loop d and q-axis current reference value smooth switching control loop of the grid-following and grid-building types: convert the reactive power Q and voltage u in step 1 into abc The effective value is sent as the input signal to the reactive power-voltage control loop of the network to obtain the output voltage signal E m , further m Perform Park transformation to obtain the voltage reference signal u dref_GFM and u qref_GFM , the voltage reference value u dref_GFM 、u qref_GFM and voltage u d 、u q As the input signal, it is sent to the voltage inner loop in the network inner loop controller to generate the current reference value i dref_GFM and i qref_GFM ; The active power P and reactive power Q in step 1 are sent as input signals to the grid control outer loop to obtain the current reference value i generated by the grid loop dref_GFL and iqref_GFL For the control of the d-axis, three switches are set respectively: the first switch is set before the PI link PI_GFL of the grid active power control, S1 is connected to the active power reference value under the grid control, S2 is connected to P ref_k1 , represents the active power reference value of the grid-type outer loop in the grid-type control mode; the second switch is set before the PI link PI_GFM of the voltage control, S1 connects u dref_k2 , represents the d-axis voltage reference value of the inner loop controller of the network-following control mode, S2 connects the u of the network control dref_GFM The third switch is set at the system output i dref Previously, S1 and S2 were connected to the d-axis current reference value i in the grid-following control mode. dref_GFL and the d-axis current reference value i in the grid control mode dref_GFM , realize i dref Switching between the two control modes. Similarly, three switches are set for the control of the q axis: the first switch is set before the PI link PI-GFL of reactive power control, S1 is connected to the Q under grid control ref , S2 connects to Q ref_k3 , represents the reactive power reference value of the outer loop of the grid-type control in the grid-type control mode; the second switch is set before the PI link PI-GFM of the q-axis voltage control, and S1 is connected to u qref_k4 , represents the q-axis voltage reference value of the inner loop controller of the grid-following control mode, S2 connects the u qref_GFM ; The third switch is located at the system output i qref Previously, S1 and S2 were connected to the q-axis current reference value i in the grid-following control mode. qref_GFL and the q-axis current reference value i in the grid control mode qref_GFM , realize i qref Switching between two control modes. PI_k1, PI_k2, PI_k3, and PI_k4 are all PI links of switching control, and all switching switches are controlled synchronously, that is, S1 is connected at the same time or S2 is connected at the same time;

[0011] Step 4: Set the d-axis and q-axis current reference values ​​i generated in step 3 dref 、i qref The input signal is sent to the current inner loop, and the final voltage signal e is generated after conditioning by the current inner loop. dref 、e qref , and then generate the control signal e of the converter switch device through the PWM modulation link ma 、e mb 、e mc ;

[0012] As a further improvement of the present invention, in step 2,

[0013] The grid-type active power control loop adopts virtual synchronous machine control.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] Compared with the prior art, the present invention has higher accuracy and flexibility, and organically combines the grid-following and grid-building control strategies, which helps to better suppress power and frequency fluctuations during converter switching, improve grid-connected stability, and achieve safe and stable operation of the converter under large changes in grid strength. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and / or additional aspects and advantages of the present invention will become apparent and easily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0017] Figure 1 It is a topological diagram of the grid-connected system of the grid-following type and grid-building type converters of the present invention;

[0018] Figure 2 It is a schematic diagram of the grid-type power control loop and the current inner loop control of the present invention;

[0019] Figure 3 It is a schematic diagram of the grid-type active-frequency control loop and reactive-voltage control loop of the present invention;

[0020] Figure 4 It is the mathematical model of the meshed inner loop controller of the present invention;

[0021] Figure 5 The present invention provides a control loop for smooth switching of phase angles;

[0022] Figure 6 It is a structural block diagram of the smooth switching control of d-axis and q-axis current reference values ​​provided by the present invention. DETAILED DESCRIPTION

[0023] In order to more clearly illustrate the specific implementation of the present invention or the technical solution in the prior art, the present invention is further described in detail below in conjunction with the drawings and specific implementation. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0024] Example 1

[0025] like Figure 1 As shown, the topology diagram of the grid-connected system of the grid-following type and grid-building type converter of the present invention specifically includes:

[0026] Distributed power sources are connected in parallel on the DC side, and the DC voltage U dcAfter being inverted by a full-bridge inverter, it is connected to the grid at the common coupling point (PCC) through the LC filtering link; the grid-following control link is mainly composed of power calculation, phase-locked loop, power control, current control, PWM modulation and other links; the grid-building control link is mainly composed of power calculation module, power control, dual closed-loop control, PWM modulation and other links.

[0027] Step 1: Real-time acquisition of the grid-side voltage u of the converter abc and the current i at the converter outlet abc , after Parker transformation, we can get the components of d-axis and q-axis in the two-phase rotating coordinate system, which are expressed as u d 、u q and i d 、i q , will u d 、u q and i d 、i q After being sent to the power calculation link, the active output power P and reactive power Q of the converter are obtained;

[0028] Specifically, the power calculation process in step 1 is:

[0029]

[0030] Among them, P is the active output power of the converter, Q is the reactive output power of the converter, and u d 、u q is the grid-side voltage u of the converter abc After Parker transformation, the components of the d-axis and q-axis in the two-phase rotating coordinate system are obtained, i d 、i q is the current i at the converter outlet abc After Parker transformation, the components of the d-axis and q-axis in the two-phase rotating coordinate system are obtained;

[0031] Step 2: Construct the control loop for smooth switching of the active power control loop and the phase angle of the phase-locked loop: replace u in step 1 abc Sent to the grid-following PLL link to obtain the output angular frequency ω GFL , the real-time value of active power P is sent to the active power-frequency control loop of the network to obtain the output angular frequency ω GFM A first switch and a second switch are set between the phase-locked loop of the grid-following control and the active-frequency control loop of the grid-building control. The first switch is set at the output angular frequency ω of the PLL part. GFL And the active-frequency control loop output angular frequency ω GFMThe second switch is arranged before the Parker transformation, its S1 end is connected to the phase angle θ output by the system, S2 is connected to the integrator output of the phase-locked loop, and the first switch and the second switch are synchronously controlled, that is, the S1 end is connected at the same time or the S2 end is connected at the same time;

[0032] Specifically, Figure 2 The schematic diagram of the grid-following power control loop and the current inner loop control is given. In the figure, P and Q are the active power and reactive power output by the converter respectively; P ref and Q ref are the reference values ​​of the active power and reactive power output by the converter respectively; i od and i oq is the grid-side current i oabc After Parker transformation, the components of the d-axis and q-axis in the two-phase rotating coordinate system are obtained;

[0033] Furthermore, the expressions of the grid-following power control loop and the current inner loop are given:

[0034]

[0035] Among them, K p and K i is the proportional and integral coefficient of the grid-type power control loop PI controller, K ip and K ii are the proportional and integral coefficients of the current inner loop PI controller; i dref and i qref is the current inner loop d-axis current reference signal and q-axis current reference signal; L f is the filter inductance, ω is the system angular frequency; e dref and e qref It is the voltage control signal obtained by the current inner loop.

[0036] Specifically, Figure 3 A schematic diagram of a grid-type active-frequency control loop and a reactive-voltage control loop is given. The grid-type control is a virtual synchronous machine control. Furthermore, expressions of the grid-type active-frequency control loop and the reactive-voltage control loop are given.

[0037]

[0038] Among them, P m is the mechanical power of the virtual synchronous machine, ω 0 is the rated angular frequency of the converter output, U N and U are the system rated voltage and system output voltage; J and D are the virtual inertia and damping coefficient in the virtual synchronous machine control, respectively; k p and k u is the frequency modulation coefficient and voltage modulation coefficient, k i is the first-order inertia coefficient, θGFM is the power angle;

[0039] Specifically, Figure 5 The control loop of phase angle smooth switching is given. The detailed control logic of phase angle smooth switching is as follows: If the converter operates in the grid-following control mode, the switches are all in S1 and θ = θ GFL Since P=P under steady-state conditions ref ,but Figure 3 In the shown GFM =ω 0 When the converter switches to grid-type control, the switch is switched from S1 to S2. Due to the presence of the integrator after the switch, its output θ will not change suddenly, so the system power angle remains stable during the switching.

[0040] If the converter operates in the grid-type control mode, the switches are all in S2, and θ = θ GFM Since the phase-locked loop PLL can be synchronized with the power grid, there is always θ GFL =θ GFM When the converter switches to grid-following control, the switch switches from S2 to S1, and its output θ does not change suddenly, so the system power angle remains stable during switching.

[0041] Step 3: Construct the reactive power control loop and power control loop of the grid-following and grid-building type, and the q-axis current reference value smooth switching control loop: convert the reactive power Q and voltage u in step 1 into abc The effective value is sent as the input signal to the reactive power-voltage control loop of the network to obtain the output voltage signal E m , further m Perform Park transformation to obtain the voltage reference signal u dref_GFM and u qref_GFM , the voltage reference value u dref_GFM 、u qref_GFM and voltage u d 、u q As the input signal, it is sent to the voltage inner loop in the network inner loop controller to generate the current reference value i dref_GFM and i qref_GFM ; The active power P and reactive power Q in step 1 are sent as input signals to the grid control outer loop to obtain the current reference value i generated by the grid loop dref_GFL and i qref_GFL For the control of the d-axis, three switches are set respectively: the first switch is set before the PI link PI_GFL of the grid active power control, S1 is connected to the active power reference value under the grid control, S2 is connected to P ref_k1, represents the active power reference value of the grid-type outer loop in the grid-type control mode; the second switch is set before the PI link PI_GFM of the voltage control, S1 connects u dref_k2 , represents the d-axis voltage reference value of the inner loop controller of the network-following control mode, S2 connects the u of the network control dref_GFM The third switch is set at the system output i dref Previously, S1 and S2 were connected to the d-axis current reference value i in the grid-following control mode. dref_GFL and the d-axis current reference value i in the grid control mode dref_GFM , realize i dref Switching between the two control modes. Similarly, three switches are set for the control of the q axis: the first switch is set before the PI link PI-GFL of reactive power control, S1 is connected to the Q under grid control ref , S2 connects to Q ref_k3 , represents the reactive power reference value of the outer loop of the grid-type control in the grid-type control mode; the second switch is set before the PI link PI-GFM of the q-axis voltage control, and S1 is connected to u qref_k4 , represents the q-axis voltage reference value of the inner loop controller of the grid-following control mode, S2 connects the u qref_GFM ; The third switch is located at the system output i qref Previously, S1 and S2 were connected to the q-axis current reference value i in the grid-following control mode. qref_GFL and the q-axis current reference value i in the grid control mode qref_GFM , realize i qref Switching between two control modes. PI_k1, PI_k2, PI_k3, and PI_k4 are all PI links of switching control, and all switching switches are controlled synchronously, that is, S1 is connected at the same time or S2 is connected at the same time;

[0042] Specifically, Figure 4 The mathematical model of the grid-type inner loop controller is given, and further, the expression of the grid-type voltage inner loop is given.

[0043]

[0044] Among them, K up and K ui are the proportional and integral coefficients of the voltage inner loop PI controller; u dref and u qref is the voltage inner loop d-axis current reference signal and q-axis current reference signal; C f is the filter capacitor;

[0045] The inner loop expression of current is consistent with the inner loop expression of grid-type current.

[0046] Specifically, Figure 6 The control structure block diagram of the smooth switching of the d-axis and q-axis current reference values ​​is given. The detailed control logic is as follows: If the converter operates in the grid-following control mode, all switches are set to S1, and there is i dref =i dref_GFL and i qref =i qref_GFL , the outer ring adopts Figure 2 The power control loop shown in the figure outputs i after the PI link dref_GFL , and at the same time make the reactive power Q track its reference value Q ref To obtain i qref_GFL The d-axis and q-axis of the grid-type control will also operate independently, making i dref_GFM and i qref_GFM Track i separately dref_GFL and i qref_GFL , to ensure that the d and q axis current reference values ​​are synchronized in the two control modes, to ensure that when switching from S1 to S2, that is, when switching from grid-following control to grid-forming control, i dref and i qref Similarly, if the converter operates in the grid-type control mode, all switches are set to S2, and there is i dref =i dref_GFM and i qref =i qref_GFM , the outer ring adopts Figure 4 The control method of the voltage inner loop is shown in the figure. The d-axis and q-axis of the grid-type control will also operate independently, making i dref_GFL and i qref_GFL Track i separately dref_GFM and i qref_GFM , to ensure that the d and q axis current reference values ​​are synchronized in the two control modes, to ensure that when switching from S2 to S1, that is, when the grid-forming control switches to the grid-following control, i dref and i qref No mutation occurs.

[0047] Step 4: Set the d-axis and q-axis current reference values ​​i generated in step 3 dref 、i qref The input signal is sent to the current inner loop, and the final voltage signal e is generated after conditioning by the current inner loop. dref 、e qref , and then generate the control signal e of the converter switch device through the PWM modulation link ma 、e mb 、e mc .

[0048] The contents not described in detail in this specification belong to the prior art known to professional and technical personnel in this field. Finally, it should be noted that the above description is only a preferred example of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, it is still possible for those skilled in the art to modify the technical solutions described in the aforementioned embodiments or to replace some of the technical features therein by equivalents. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims

1. A smooth switching control method for grid-following and grid-building converters, characterized in that: The following steps are involved: Step 1: Real-time acquisition of the grid-side voltage u of the converter abc and the current i at the converter outlet abc , after Parker transformation, we can get the components of d-axis and q-axis in the two-phase rotating coordinate system, which are expressed as u d 、u q and i d 、i q , will u d 、u q and i d 、i q After being sent to the power calculation link, the active output power P and reactive power Q of the converter are obtained; Step 2: Construct a control loop for smooth switching of the active power control loop and the phase angle of the phase-locked loop of the grid-following type and the grid-building type; Step 3: Construct the grid-following and grid-building reactive power control loops, power control loops, and d- and q-axis current reference value smoothing switching control loops. Step 4: The current signal generated in step 3 is conditioned by the current inner loop to generate a final voltage signal, and then a control signal of the converter switch device is generated through a modulation link.

2. A method for controlling smooth switching of grid-following and grid-forming converters according to claim 1, characterized in that: In step 2, The real-time value of active power P in step 1 is sent to the active power-frequency control loop of the network to obtain the output angular frequency ω GFM , will u abc Sent to the grid-following PLL link to obtain the output angular frequency ω GFL , a first switch and a second switch are set between the phase-locked loop of the grid-following control and the active-frequency control loop of the grid-building control, and the first switch is set at the output angular frequency ω of the PLL part GFL And the active-frequency control loop output angular frequency ω GFM The second switch is arranged before the Park transformation, its S1 end is connected to the phase angle θ of the system output, S2 is connected to the integrator output of the phase-locked loop, and the first switch and the second switch are controlled synchronously, that is, the S1 end is connected at the same time or the S2 end is connected at the same time.

3. A method for controlling smooth switching of grid-following and grid-forming converters according to claim 1, characterized in that: In the step 3, specifically including: The reactive power Q and voltage u in step 1 are abc The effective value is sent as the input signal to the reactive power-voltage control loop of the network to obtain the output voltage signal E m , further m Perform Park transformation to obtain the voltage reference signal u dref_GFM and u qref_GFM , the generated voltage reference value u dref_GFM 、u qref_GFM and voltage u d 、u q As the input signal, it is sent to the voltage inner loop in the network inner loop controller to generate the current reference value i dref_GFM and i qref_GFM ; The active power P and reactive power Q in step 1 are sent as input signals to the grid control outer loop to obtain the current reference value i generated by the grid loop dref_GFL and i qref_GFL For the control of the d-axis, three switches are set respectively: the first switch is set before the PI link PI_GFL of the grid active power control, S1 is connected to the active power reference value under the grid control, S2 is connected to P ref_k1 , represents the active power reference value of the grid-type outer loop in the grid-type control mode; the second switch is set before the PI link PI_GFM of the voltage control, S1 connects u dref_k2 , represents the d-axis voltage reference value of the inner loop controller of the network-following control mode, S2 connects the u of the network control dref_GFM The third switch is set at the system output i dref Previously, S1 and S2 were connected to the d-axis current reference value i in the grid-following control mode. dref_GFL and the d-axis current reference value i in the network control mode dref_GFM , realize i dref Switching between the two control modes. Similarly, three switches are set for the control of the q axis: the first switch is set before the PI link PI-GFL of reactive power control, S1 is connected to the Q under grid control ref , S2 connects to Q ref_k3 , represents the reactive power reference value of the outer loop of the grid-type control in the grid-type control mode; the second switch is set before the PI link PI-GFM of the q-axis voltage control, and S1 is connected to u qref_k4 , represents the q-axis voltage reference value of the inner loop controller of the grid-following control mode, S2 connects the u qref_GFM ; The third switch is located at the system output i qref Previously, S1 and S2 were connected to the q-axis current reference value i in the grid-following control mode. qref_GFL and the q-axis current reference value i in the grid control mode qref_GFM , realize i qref Switching between two control modes. PI_k1, PI_k2, PI_k3, and PI_k4 are all PI links of switching control, and all switching switches are controlled synchronously, that is, S1 is connected at the same time or S2 is connected at the same time.

4. A method for controlling smooth switching of grid-following and grid-forming converters according to claim 1, characterized in that: In said step 4, specifically including: The d-axis and q-axis current reference values ​​i generated in step 3 are dref 、i qref The input signal is sent to the current inner loop, and the final voltage signal e is generated after conditioning by the current inner loop. dref 、e qref , and then generate the control signal e of the converter switch device through the PWM modulation link ma 、e mb 、e mc。 5. A method for controlling smooth switching of grid-following and grid-forming converters according to claim 1, characterized in that: In step 2, the network-forming active power control loop adopts virtual synchronous machine control.

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