Control system and control method for suppressing transient overvoltage of grid-connected inverter

Abstract

The invention relates to a control system and a control method for suppressing transient overvoltage of a grid-connected inverter, the recovery rate of a current reference is designed according to parameters of a filter and a current loop of the inverter so as to suppress the problem of transient overvoltage in a low voltage ride through recovery period, and the method comprises the following specific steps: 1, establishing a mathematical model of the grid-connected inverter; solving a time constant of current response according to a filter of the inverter and a current loop PI control parameter; 2, the recovery time of the current reference is limited, the recovery time of the active current reference is larger than the time constant of the current response, and the recovery time of the reactive reference is smaller than the time constant of the current response; and 3, recovering the current reference according to the recovery time selected in the step 2 after the fault is cleared. The suppression method is simple in design and convenient to implement, the transient overvoltage of the grid-connected point after mode switching can be suppressed based on the method, the suppression effect is achieved under different drop depths, and the transient characteristic of the grid-connected inverter in the low-voltage ride-through recovery period is improved.

Description

Technical Field

[0001] The present invention relates to a photovoltaic grid-connected inverter system, and in particular to a control system and method for suppressing transient overvoltage of a grid-connected inverter. Background Art

[0002] After a grid voltage dip fault occurs, according to national standards, a photovoltaic grid-connected inverter needs to enter the low-voltage ride-through mode and inject reactive current related to the grid connection point voltage to support the grid connection point voltage. However, after the fault is cleared, the current reference of the photovoltaic inverter needs to be switched from the low-voltage ride-through mode back to unity power factor operation. If the current reference withdrawal method of the photovoltaic inverter is improper, it will cause a transient overvoltage problem at the grid connection point, bringing a greater safety risk to the stable operation of the inverter.

[0003] The generation mechanism of transient overvoltage is mainly divided into two types. The first is the transient overvoltage caused by the slow withdrawal of reactive power, resulting in reactive power surplus of the inverter. The other is that the rapid withdrawal of active power causes overmodulation of the current loop, resulting in a transient overvoltage with a higher amplitude at the grid connection point. For the first reason, existing research believes that there is a phase mutation at the grid connection point voltage at the moment of fault clearing, and the phase-locked loop needs time to track the phase of the grid connection point, resulting in a slow actual reactive power withdrawal. In addition, phase misalignment will also cause the actual reactive current amplitude to be greater than the current reference amplitude, which may worsen the transient overvoltage. Therefore, existing literature has proposed an overvoltage suppression method that provides phase compensation to the phase-locked loop to improve the system response speed. However, in practice, the transient overvoltage caused by the second reason has a higher amplitude and worse transient characteristics, and it is easier to cause the photovoltaic inverter to trip off the grid.

[0004] For the second reason, the overmodulation of the current loop needs to be analyzed in combination with each loop of the inverter and the transient process of the line, and the influence of overmodulation on the transient overvoltage needs to be analyzed. Existing literature has established a grid-connected inverter model considering the current limiting link in the low-voltage ride-through mode, and at the same time pointed out the influence of the current reference recovery rate during fault recovery on the transient overvoltage at the grid connection point, but the selection range and reasons for the recovery rate have not been discussed.

[0005] Therefore, it is necessary to further study a transient overvoltage suppression method considering the active current reference recovery rate, so as to effectively suppress the transient overvoltage within a wide voltage dip range, improve the transient characteristics of the inverter during recovery, and ensure the safe and stable operation of the inverter. Summary of the Invention

[0006] Aiming at the problems in the above-mentioned prior art, the purpose of the present invention is to provide a method for suppressing transient overvoltage of a grid-connected inverter. This method designs the recovery rate of the current reference according to the system parameters, suppresses the transient overvoltage at the grid connection point during fault recovery, and improves the stability and reliability of the grid-connected inverter under weak grids.

[0007] To achieve the above object, the technical solution of the present invention is as follows:

[0008] On the one hand, the present invention provides a control system for suppressing transient overvoltage of a grid-connected inverter. The control system includes a phase-locked loop, a coordinate transformation module, a current regulator, a current reference setting module, and a voltage detection and comparison module. Among them, the coordinate transformation module performs dq transformation on the sampled Point of Common Coupling (PCC) voltage. The q-axis voltage is used as the input signal of the phase-locked loop. The phase-locked loop outputs a phase in the same direction as the d-axis and serves as the transformation angle of the coordinate transformation module. After calculating the amplitude of the PCC voltage, the grid-connected inverter uses it as the input of the voltage detection and comparison module. After comparing with the preset value in the module, a mode switching signal is given.

[0009] According to different mode switching signals, different current references are selected and used as the input of the current regulator. The current regulator outputs a modulation wave signal to the grid-connected inverter to control the driving of the switching tubes.

[0010] Furthermore, the current reference setting module includes Mode 1: Low Voltage Ride Through (LVRT) mode, and Mode 2: LVRT recovery mode. The transient overvoltage at the PCC is suppressed by setting the recovery time of the current reference in Mode 2.

[0011] On the other hand, the present application provides a control method for suppressing transient overvoltage of a grid-connected inverter. The control method includes the following steps:

[0012] 1) Establish a mathematical model of the grid-connected inverter, and obtain the time constant of the current response according to the filter of the inverter and the PI control parameters of the current loop:

[0013] 2) Set the recovery time of the current reference. Among them, the recovery time of the active current reference is greater than the time constant of the current response, and the recovery time of the reactive reference is less than the time constant of the current response.

[0014] 3) When it is detected that the PCC voltage recovers above the preset value, the current reference setting module switches from Mode 1 to Mode 2, and the current reference recovers according to the recovery time selected in step 2).

[0015] Furthermore, in the mathematical model of step 1), the differential and analytical expressions of the current response are:

[0016]

[0017] where L f is the filter inductor of the inverter, i d , i q are the dq-axis components of the grid-connected current, i dref , iqref is the dq-axis reference for the grid-connected current, k p,acc , k i,acc and k 1 are the proportional gain and integral gain of the current-loop PI controller, τ 2 and τ 1 are the time constants of the current response, K 4 -K

[0018] Furthermore, the expression for the current response time constant in step 1) is:

[0019]

[0020] where τ 1 is the larger value of the two current response time constants, dominating the current response process.

[0021] Furthermore, the defined range of the current reference recovery time in step 2) is:

[0022] T req < τ 1 < T red

[0023] where T req is the recovery time of the reactive current reference, and T red is the recovery time of the active current reference.

[0024] The transient overvoltage suppression method of the present invention has the following characteristics:

[0025] 1) Compared with other overvoltage suppression schemes that improve the reactive power withdrawal rate under a wide time scale, this method is remarkable in suppressing the transient overvoltage caused by overmodulation of the current loop, improving the transient characteristics during the inverter recovery period;

[0026] 2) The active power recovery rate is simply designed and easy to program, providing feasibility for engineering practice;

[0027] 3) It can achieve the suppression of the transient overvoltage at the PCC point under different voltage dip depths. Description of the Drawings

[0028] Figure 1 is the main circuit structure of the grid-connected inverter and the structure diagram of the transient overvoltage suppression method at the PCC point based on the current-loop time constant in the present invention;

[0029] Figure 2 is the mode switching logic block diagram during LVRT recovery;

[0030] Figure 3 is the internal structure schematic diagram of the current regulator;

[0031] Figure 4(a) shows the simulation waveforms of the PCC point voltage with and without the overvoltage suppression strategy after the grid voltage recovers from 0.4 p.u.

[0032] Figure 4(b) shows the simulation waveforms of the active and reactive current references and the actual currents without the overvoltage suppression strategy after the grid voltage recovers from 0.4 p.u.

[0033] Figure 4(c) shows the simulation waveforms of the active and reactive current references and the actual currents with the overvoltage suppression strategy after the grid voltage recovers from 0.4 p.u.

[0034] Figure 5(a) shows the simulation waveforms of the grid - connected point voltage with and without the overvoltage suppression strategy after the grid voltage recovers from 0.1 p.u.

[0035] Figure 5(b) shows the simulation waveforms of the grid - connected point voltage when the recovery rate of the active current reference is within and outside the constraint range after the grid voltage recovers from 0.1 p.u.

[0036] Figure 5(c) shows the simulation waveforms of the active and reactive current references and the actual currents without the overvoltage suppression strategy after the grid voltage recovers from 0.1 p.u.

[0037] Figure 5(d) shows the simulation waveforms of the active and reactive current references and the actual currents when the recovery rate of the active current reference is outside the constraint range after the grid voltage recovers from 0.1 p.u.

[0038] Figure 5(e) shows the simulation waveforms of the active and reactive current references and the actual currents when the recovery rate of the active current reference is within the constraint range after the grid voltage recovers from 0.1 p.u. Detailed implementation manners

[0039] The present invention will be further described in detail below in conjunction with the accompanying drawings of the specification and the detailed implementation manners.

[0040] Embodiment 1:

[0041] Figure 1 This is the main circuit structure diagram of the grid - connected inverter applied to the transient overvoltage suppression method of the grid - connected inverter in the present invention. Among them, the main power circuit of the grid - connected inverter consists of an IGBT inverter bridge and an LC filter, and is connected to the AC grid through the grid - side impedance. The AC grid adopts the Thevenin equivalent form of the voltage source u s in series with the grid impedance. The control part of the grid - connected inverter mainly includes a current loop, a phase - locked loop, a current reference setting, and a mode switching, etc. The grid - connected inverter samples the grid - connected point voltage u PCC for coordinate transformation to be used as the input of the phase - locked loop. u d 、uq are the d - axis and q - axis components of the PCC point voltage. The phase - locked loop synchronizes the in - grid current and the grid - connected point voltage by locking u q to zero.

[0042] As Figure 1 shown, on the one hand, the present invention provides a control system for a grid - connected inverter to suppress transient over - voltage. The control system includes a phase - locked loop, a coordinate transformation module, a current regulator, a current reference setting module, and a voltage detection and comparison module. Among them, the coordinate transformation module performs dq transformation on the sampled PCC voltage. The q - axis voltage is used as the input signal of the phase - locked loop. The phase - locked loop outputs a phase in the same direction as the d - axis and serves as the transformation angle of the coordinate transformation module. After calculating the amplitude of the PCC voltage, the grid - connected inverter uses it as the input of the voltage detection and comparison module. After comparing with the preset value in the module, a mode - switching signal is given. Different current references are selected according to different mode - switching signals and used as the input of the current regulator. The current regulator outputs a modulation wave signal to the grid - connected inverter to control the drive of the switching tubes.

[0043] Under the grid voltage dip fault, the inverter operates in the LVRT mode, and the expressions of the current references \(i\) dref 、\(i\) qref are respectively:

[0044]

[0045] where \(K\) is the reactive power injection coefficient, \(U\) pcc is the voltage amplitude of the actual PCC point, \(U\) pcc0 is the voltage amplitude of the PCC point under unity power factor operation, and \(I\) max is the current limit value of the grid - connected inverter.

[0046] When the voltage dip fault is cleared, the inverter needs to switch back to the original unity power factor operation mode. However, directly switching from the LVRT mode to the unity power factor operation mode will cause transient over - voltage problems at the PCC point. Therefore, the present invention adds a mode 2, that is, the LVRT recovery mode, to suppress transient over - voltage by reasonably selecting the recovery time of the current reference.

[0047] Figure 2 is the mode - switching logic block diagram during LVRT recovery. When it is detected that the PCC point voltage recovers above 0.9 p.u., the inverter enters the LVRT recovery mode, and the current reference is recovered according to the designed recovery time.

[0048] Figure 3 is the internal structure schematic diagram of the current regulator. Among them, GVF is the grid - connected point voltage feed - forward coefficient, \(G\) d (s) is the digital control delay link, \(u\)o and u m are the output of the current loop and the modulated output voltage respectively. Based on this, the dq-axis expressions of the output u o of the grid-connected inverter current loop can be written as follows:

[0049]

[0050] where k p,acc and k i,acc are the proportional gain and integral gain of the current loop PI controller respectively. ω 0 = 100π rad / s is the fundamental grid angular frequency.

[0051] It can be seen from Equation (2) that the instantaneous change of the current loop output is mainly determined by the proportional gain of each dq-axis multiplied by the change of the current reference. If the current reference changes stepwise according to the traditional method, the proportional term will become a sudden change, resulting in a sudden increase in the current loop output. Usually, the inverter will limit the amplitude of the modulation wave, and the sudden increase in the current loop output will cause the modulation wave to saturate, thus causing the output of the inverter bridge arm to distort the high voltage. Therefore, to suppress the transient overvoltage, it is necessary to weaken the sudden increase in the current loop output. The current reference recovery time design method proposed by the present invention can make the current reference increase slowly, while making the actual current quickly track the current reference, thereby suppressing the magnitude and duration of this sudden increase.

[0052] Embodiment 2:

[0053] Based on the control system provided in the embodiment, the control system for suppressing transient overvoltage of the grid-connected inverter proposed by the present invention includes the following steps:

[0054] Step 1: Establish a mathematical model of the grid-connected inverter, and obtain the time constant of the current response according to the filter of the inverter and the PI control parameters of the current loop;

[0055] Step 2: Define the recovery time of the current reference. The recovery time of the active current reference is greater than the time constant of the current response, and the recovery time of the reactive reference is less than the time constant of the current response;

[0056] Step 3: When it is detected that the voltage at the PCC point recovers to above 0.9 p.u., switch the current reference setting to the LVRT recovery mode, and make the current reference recover according to the recovery time selected in Step 2.

[0057] Among them, the expressions of the filter of the grid-connected inverter and the grid-side dynamics are:

[0058]

[0059] In the formula, i L1d and i L1qare the dq components of the inductor current on the inverter side, C f and R d are the filter capacitor and the passive resistor respectively, R g and L g are the resistive and inductive components of the grid impedance respectively, U s is the amplitude of the grid voltage. Since the current in the capacitor branch is very small, the influence of the delay link and the voltage feedforward is ignored at the same time. Combining with Equation (2), the dq-axis decoupling of the current loop can be achieved, and its differential and analytical expressions are:

[0060]

[0061] In the formula, L f is the filter inductor of the inverter, k p,acc , k i,acc are the proportional gain and integral gain of the PI controller of the current loop respectively, τ 1 , τ 2 are the response times of the current loop, K 1 -K 4 is the amplitude coefficient, and the superscript “.” represents the differential of the physical quantity.

[0062] Among them, the expressions for the response times τ 1 , τ 2 of the current loop are:

[0063]

[0064] In the formula, it can be seen that τ 1 >τ 2 , that is, τ 1 is the time constant of the current response. It can be seen from the grid-side dynamics of the last two lines in Equation (3) that after the grid voltage U s is restored, the active current i d will decrease, and the reactive current i q will increase. Therefore, in order to reduce the sudden increase of the current loop output and make the actual current quickly track the current reference, the range of the recovery time of the active and reactive current references is selected according to τ 1 as:

[0065] T req <τ 1 <T red (6)

[0066] In the formula, T req is the recovery time of the reactive current reference, and T red is the recovery time of the active current reference.

[0067] Description of the effects of the present invention:

[0068] Figure 4(a) shows the simulated waveforms of the PCC point voltage with and without the overvoltage suppression strategy after the grid voltage recovers from 0.4 p.u. As can be seen from the figure, if the current reference is directly switched, a relatively serious transient overvoltage phenomenon occurs at the PCC point, with the peak reaching 1.6 p.u.; if the recovery time of the current reference is within the range constrained by the proposed suppression method, the transient overvoltage at the PCC point is well suppressed, and the peak is only 1.2 p.u. Combining Figure 4(b) and Figure 4(c), it can be seen that in Figure 4(b), the current reference is recovered in a step manner, and the sudden increase in the output of the current loop at the recovery moment causes the modulation wave to saturate, resulting in the distorted transient overvoltage in Figure 4(a); while in Figure 4(c), the recovery time of the current reference is within the range constrained by the proposed suppression method, and the actual current quickly tracks the current reference, weakening the influence of overmodulation, thus effectively suppressing the transient overvoltage. This demonstrates the effectiveness of the overvoltage suppression method proposed in the present invention.

[0069] Figure 5(a) shows the simulated waveforms of the PCC point voltage with and without the overvoltage suppression strategy after the grid voltage recovers from 0.1 p.u. Basically the same as Figure 4(a), the proposed suppression method in Figure 5(a) can effectively suppress the peak value of the transient overvoltage. In addition, due to the increase in the depth of voltage sag, if the current reference is directly recovered, the transient overshoot of the current is more serious, as shown in Figure 5(c), which may cause the PCC point voltage to drop below 0.9 p.u., thus triggering the low voltage ride through again (the anti-secondary-trigger judgment is added in the simulation, so the LVRT is not triggered again), affecting the safe operation of the inverter. While the recovery time of the current reference is within the constrained range, as shown in Figure 5(e), since the actual current quickly tracks the current reference, there will be no situation of current transient overshoot, nor will there be a situation of triggering the low voltage ride through again.

[0070] Taking the active current reference as an example, Figure 5(b) shows the simulated waveforms of the grid-connected point voltage when the recovery time of the active current reference is within and outside the constrained range after the grid voltage recovers from 0.1 p.u. It can be seen that although the transient overvoltage at the grid-connected point during the switching instant can also be suppressed when the active current reference is outside the constrained range, the duration of the overvoltage is significantly longer than that when the active current reference is within the constrained range. This is because the active current reference still recovers faster than the response of the current loop. As shown in Figure 5(d), the active current does not track the current reference during the recovery period. Although the current overshoot is smaller than that in Figure 5(c), the transient characteristics are worse than those in Figure 5(e). This demonstrates that when the recovery time of the active current reference is within the constrained range, there is a better transient overvoltage suppression effect and better transient characteristics during the recovery stage.

[0071] In summary, the transient overvoltage suppression method for grid-connected inverters proposed by the present invention can effectively suppress the transient overvoltage at the PCC point during the recovery period under different voltage dip depths, thereby improving the transient characteristics and ride-through performance of the inverter during the recovery period and ensuring the safe and stable operation of the inverter.

[0072] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, several improvements and refinements can be made without departing from the principle of the present invention, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A control system for suppressing transient overvoltage of a grid-connected inverter, characterized in that: The control system includes a phase-locked loop, a coordinate transformation module, a current regulator, a current reference setting module and a voltage detection and comparison module; wherein the coordinate transformation module performs a dq transformation on the sampled common coupling point PCC voltage, and the q-axis voltage is used as the input signal of the phase-locked loop, and the phase output of the phase-locked loop is in the same direction as the d-axis and serves as the transformation angle of the coordinate transformation module; the grid-connected inverter calculates the common coupling point PCC voltage amplitude as the input of the voltage detection and comparison module, and gives a mode switching signal after comparing it with the preset value in the module; Different current references are selected according to different mode switching signals and used as inputs of the current regulator. The current regulator outputs a modulated wave signal to the grid-connected inverter to control the drive of the switch tube.

2. A control system for suppressing transient overvoltage of a grid-connected inverter according to claim 1, characterized in that: The current reference setting module includes mode 1: low voltage ride-through LVRT mode, and mode 2: LVRT recovery mode. The transient overvoltage at the PCC is suppressed by the recovery time of the current reference given by mode 2.

3. A method for controlling a grid-connected inverter to suppress transient overvoltage, characterized in that: The control method is based on the control system according to claim 1, and comprises the following steps: 1) Establish a mathematical model of the grid-connected inverter and obtain the time constant of the current response based on the PI control parameters of the inverter filter and current loop: 2) Setting the recovery time of the current reference, wherein the recovery time of the active current reference is greater than the time constant of the current response, and the recovery time of the reactive current reference is less than the time constant of the current response; 3) When it is detected that the PCC voltage recovers to above the preset value, the current reference setting module switches from mode 1 to mode 2, so that the current reference is restored according to the recovery time selected in step 2).

4. A method for controlling a grid-connected inverter to suppress transient overvoltage according to claim 3, characterized in that: The differential and analytical expressions of the current response in the mathematical model of step 1) are: Among them, L f is the filter inductance of the inverter, i d 、i q is the dq axis component of the grid current, i dref 、i qref is the dq axis reference of the grid current, k p,acc , k i,acc are the proportional gain and integral gain of the current loop PI controller, τ1 and τ2 are the time constants of the current response, K1-K4 are the amplitude coefficients, and the superscript ". " represents the differentiation of the physical quantity.

5. A method for controlling transient overvoltage of a grid-connected inverter according to claim 3, characterized in that: The expression of the current response time constant in step 1) is: Among them, τ1 is the larger value of the two current response time constants and dominates the current response process.

6. A method for controlling grid-connected inverters to suppress transient overvoltage according to claim 3, characterized in that: The limit range of the current reference recovery time in step 2) is: T req <τ1<T red Where T req is the recovery time of the reactive current reference, T red is the recovery time of the active current reference.

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