A current limiting method for grid-connected inverter based on virtual impedance switching

Abstract

The present invention provides a current limiting method for a grid-type inverter based on virtual impedance switching, and relates to the field of power electronic control technology. The method includes performing voltage and current dual closed-loop control on the output voltage and inductor current of the grid-type inverter, and then performing SPWM modulation on the control output to obtain a pulse signal for driving each switch tube in the grid-type inverter. Among them, when the instantaneous value of the three-phase current in the output current exceeds the preset current threshold, a virtual impedance is input in the voltage and current dual closed-loop control through the first state machine. After the power grid is restored, the second state machine controls the first state machine to switch the state and cut off the virtual impedance. The method can limit the output current in time during the fault period, avoid the inverter device from being subjected to excessive current shock, and protect its safety. At the same time, the active power output can be quickly restored after the fault is cleared, thereby improving the reliability and stability of the system.

Description

Technical Field

[0001] The invention relates to the technical field of power electronic control, and in particular to a current limiting method for a grid-connected inverter based on virtual impedance switching. Background Art

[0002] At present, most grid-connected inverters adopt a control mode that decouples the input power of the prime mover from the electromagnetic power on the grid side. They generally lack rotating reserve capacity and rotational inertia, and cannot provide inertial response similar to that of traditional synchronous generators. Therefore, the new power system characterized by a high proportion of new energy and a high proportion of power electronic equipment is prone to lack of necessary inertial support and damping response in the power grid, thereby affecting the stability of voltage and frequency. In addition, traditional grid-connected inverters also have problems such as easy loss of step when connected to weak power grids. Therefore, in order to suppress the frequency and voltage fluctuations of the power system and improve the system stability, and to provide reliable voltage and frequency inertial support and necessary damping response for the new power system with new energy as the main body, a grid-connected control strategy is proposed.

[0003] In order to solve the overcurrent problem of grid-type inverters when the grid voltage drops, power electronic devices with higher power levels can be used to withstand greater current shocks, but this method will significantly increase the cost of the inverter. Another method is to limit the current by limiting the reference value of the inner loop of the inverter current and protect the inverter device. This method easily leads to saturation of the outer loop of the voltage, causing the outer loop of the voltage to gradually lose control ability, which is not conducive to the stable operation of the inverter. Furthermore, this method limits the output current by limiting the output power of the system, but does not consider the response speed of the current limit.

[0004] Therefore, a method based on virtual impedance current limiting is proposed: by reducing the reference value of the voltage outer loop, the voltage controller is prevented from outputting an excessively large current reference value. Combining the concepts of virtual impedance and power limitation, a dynamic current limiting control strategy is proposed to ensure that there is always an intersection between the power reference value and the power angle curve of the system to achieve dynamic current limiting control. However, the method of investing in virtual impedance does not consider the problem of repeated switching of virtual impedance due to changes in the output current amplitude. In addition, during a fault, converting the grid-forming inverter to a grid-following inverter to limit the current through constant current control will cause the grid-forming inverter to lose its natural advantages, and it will be difficult to achieve control switching during fault recovery. Summary of the invention

[0005] In order to solve the technical problems existing in the above-mentioned prior art, the present invention proposes a current limiting method for a grid-type inverter based on virtual impedance switching. The method designs virtual impedance switching logic and makes full use of the output current amplitude over-limit signal and the grid voltage recovery signal to implement a current limiting strategy for the grid-type inverter during the fault overcurrent period and quickly restore active power after the fault is cleared.

[0006] The method includes:

[0007] Collect the output current I of the grid-connected inverter oabc And detect in real time through the current detection device;

[0008] If i oabc ≤I0, then output a Boolean value of 0 to the first state machine to keep it in state A; perform voltage and current dual closed-loop control, and perform SPWM modulation on the output of the voltage and current dual closed-loop control to obtain a pulse signal for driving each switch tube in the grid-type inverter;

[0009] If i oabc >I0, then output a Boolean value 1 to the first state machine to switch it to state B, input virtual impedance in the voltage and current dual closed-loop control, and perform SPWM modulation on the output of the voltage and current dual closed-loop control to obtain a pulse signal for driving each switch tube in the grid-type inverter; perform real-time grid recovery detection, and if the grid is restored, control the first state machine to switch to state A through the second state machine to remove the virtual impedance;

[0010] Among them, i oabc is the output current I oabc The instantaneous value of the three-phase current in I0 is the preset current threshold; the object of the voltage and current double closed-loop control is the output voltage V of the grid-connected point of the grid-connected inverter. abc and the inductor current of the grid-connected inverter.

[0011] Furthermore, the method further comprises:

[0012] Collect output voltage V abc , and calculate the reactive power reference value Q ref and active power reference value P ref ;

[0013] Q ref =1.5u pd i q ;

[0014]

[0015]

[0016] In the formula, u pd is the output voltage V abc The d-axis component, i q is the reactive current required to be output when the grid voltage drops, i q (t_) is the value of reactive current output before the grid voltage drops, ΔU is the voltage change at the grid connection point, U is the rated voltage at the grid connection point, I n is the rated current value of the grid connection point, id is the active current;

[0017] When the grid voltage drops more than 0.1pu, the original power reference value is switched to the recalculated reactive power reference value Q ref and active power reference value P ref ; When the power grid is restored, switch back to the original power reference value.

[0018] Preferably, the preset current threshold is set to be less than 1.3 pu.

[0019] Preferably, virtual impedance is added in the dq rotating coordinate system.

[0020] Preferably, the method for determining whether the power grid has been restored includes:

[0021] Real-time calculation of output voltage V abc The difference d between the instantaneous value of the three-phase voltage and the preset rated value;

[0022] When |d|≤D, the power grid is restored, and a Boolean value 1 is output to the second state machine; when |d|>D, the power grid is not restored, and a Boolean value 0 is output to the second state machine; wherein D is a preset threshold.

[0023] Preferably, the first state machine and the second state machine are SR latches.

[0024] It can be seen that this method can limit the output current in time during the fault period to prevent the inverter device from being subjected to excessive current shock to protect its safety. At the same time, the active power output can be quickly restored after the fault is cleared, so that the reliability and stability of the power system can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The figure is a schematic diagram of a grid-type inverter topology and its control system in one embodiment of the present invention.

[0026] Figure 2 It is a schematic block diagram of virtual impedance switching logic control in one embodiment of the present invention.

[0027] Figure 3 The figure is a schematic block diagram of voltage and current dual closed-loop control of virtual impedance switching in one embodiment of the present invention.

[0028] Figure 4 The figure is a schematic block diagram of power limitation control of a grid-connected inverter in one embodiment of the present invention.

[0029] Figure 5 The waveform diagram of the output current of the grid-connected inverter without dynamic current limiting after the grid voltage drops by 40% in one embodiment of the present invention;

[0030] Figure 6The waveform diagram of the output current of the grid-connected inverter with dynamic current limiting after the grid voltage drops by 40% in one embodiment of the present invention.

[0031] Figure 7 The waveform diagram of the output current of the grid-connected inverter without dynamic current limiting after the grid voltage drops by 60% in one embodiment of the present invention.

[0032] Figure 8 The waveform diagram of the output current of the grid-connected inverter with dynamic current limiting after the grid voltage drops by 60% in one embodiment of the present invention.

[0033] Fig. 9 FIG. 4 is a schematic diagram of virtual impedance switching logic in an embodiment of the present invention.

[0034] Fig.10 The figure is a waveform diagram of active power outputted by a grid-connected inverter with dynamic current limiting in one embodiment of the present invention. DETAILED DESCRIPTION

[0035] The inventor will further explain the present invention in detail below with reference to the accompanying drawings and specific embodiments.

[0036] Example 1

[0037] like Figure 1 As shown in the figure, in the grid-type inverter topology and its control system, L f is the AC side filter inductor, C f is the AC side filter capacitor, L g is the AC line inductance; V abc is the output voltage of the inverter, I oab c is the output current of the inverter, e is the grid voltage, E is the voltage reference value of the droop control output, and θ is the phase information of the droop control output. Its control system can collect the voltage and current of the grid connection point for the power calculation link to calculate the actual power, and obtain the voltage vector reference value through the power control link, and then use the voltage and current double closed-loop control to control the output voltage and the inductor current of the grid-type inverter, and obtain the pulse signal to drive each switch tube by sinusoidal pulse width modulation (SPWM) of the control output.

[0038] like Figure 2As shown, firstly, by detecting whether the instantaneous value of the three-phase current exceeds the set current threshold, when overcurrent is detected, a high level is output. Since once an overcurrent is detected during the entire fault process, the virtual impedance needs to be continuously invested to limit the overcurrent, and at this time the overcurrent detection device cannot detect the overcurrent signal, that is, the overcurrent detection output is low level, but the virtual impedance needs to be kept invested during the fault, so a state machine is required to store the overcurrent signal. That is, when there is no overcurrent signal, the state machine is kept in state A, and the virtual impedance is not invested; when an overcurrent signal appears, the state machine is switched to state B, and the virtual impedance is invested.

[0039] Furthermore, when the virtual impedance needs to be removed, a second state machine is introduced. The state of the second state machine is determined according to the recovery of the power grid. If the power grid is not restored, it is kept in state A without changing the state of the other state machine. When the power grid is restored, it is switched to state B, and the other state machine is controlled to switch the state to state A, and the virtual impedance is removed.

[0040] This embodiment proposes an SR latch to implement the above logic, and its truth table is shown in Table 1. When overcurrent is detected, S=1, R=0, and the output is high level. When virtual impedance is input to limit the current, S=0, R=0, and the previous output is maintained, which is also high level.

[0041] In addition, since the grid recovery detection will output a high level only when the grid is restored, that is, the SR latch of the grid recovery detection part outputs a high level, before that, the SR latch of the grid recovery detection part outputs a low level. Therefore, when the grid is restored, since the R of the SR latch that determines the input and removal of the virtual impedance is set to 1, it outputs a low level, thereby removing the virtual impedance.

[0042] Table 1 SR latch truth table

[0043] S R state 1 0 1 0 1 0 0 0 Keep

[0044] Optionally, the logic control function of the SR latch may also be replaced by a JK latch or other components to achieve this.

[0045] In addition, the voltage and current double closed-loop control block diagram after adding the virtual impedance link is as follows Figure 3 As shown, U cvd with U cvq It is the voltage drop on the d-axis and d-axis after dq decomposition on the virtual impedance.

[0046]

[0047] In the formula, R v is the virtual resistance, ω is the actual angular frequency output by the grid-type inverter, L v is the virtual inductance, id is the active current, i q It is the reactive current required to be output when the grid voltage drops.

[0048] Adding a virtual impedance link in the dq rotating coordinate system can not only utilize the output current of the grid-type inverter after decoupling in the dq rotating coordinate system, but also weaken the influence of the high-order harmonics of the output current on the virtual impedance link, which is easy to implement in engineering.

[0049] However, if the system is not power-limited, the P-δ curve (power angle curve) may drop due to a severe drop in grid voltage, resulting in no intersection between the active power reference value and the P-δ curve, causing system instability.

[0050] Therefore, the power of the system can be further limited to ensure stable operation of the system. According to the grid connection standard of the German Energy and Water Association (Bundesverband Der Energie-und Wasserwirtschaft, BDEW), the power current that should be injected into the grid is:

[0051]

[0052] In the formula, i q (t_) is the reactive current output before the grid voltage drops, ΔU is the voltage change at the grid connection point, U is the rated voltage at the grid connection point, I n It is the current rating of the grid connection point.

[0053] Since the inner loop control of the grid-type inverter is not a constant current control, its reactive current cannot be controlled. However, the key to the grid-type control strategy is to adjust the output voltage by adjusting the active power and reactive power. Therefore, the injected reactive current can be modified to the reactive power reference value, thereby providing corresponding reactive support for the system. The modified reactive power reference value Q ref as follows:

[0054]

[0055] In the formula, u pd is the d-axis component of the grid-connected point voltage, ΔU is the voltage change at the grid-connected point, and U is the rated value of the grid-connected point voltage.

[0056] Generally, when the fault current is less than the rated current, the active current can be set according to the reactive current, as shown in the following formula:

[0057]

[0058] In the formula, i qI is the reactive current required to be output when the grid voltage drops. n is the rated current value of the grid connection point, i d is the active current.

[0059] However, when the reactive current setting value is too large, the active current reference value needs to be reduced to ensure that the output current does not over-flow. When the reactive current setting value exceeds the rated current, the active current, i.e., the active power reference value, can be directly set to 0 to inject reactive current to the maximum extent.

[0060] The power limiting strategy of dynamic current limiting of grid-connected inverter based on droop control of pre-low-pass filter is as follows Figure 4 As shown, V g is the grid voltage, V n is the rated grid voltage, P and Q are the actual output power, D f With D q To control the active loop and reactive loop proportional coefficients respectively, ω ref is the rated angular frequency, It is an integral link. After detecting that the grid voltage drops by more than 0.1pu, the reference power of the grid-building inverter is re-adjusted, and the active power and reactive power reference values ​​are switched to ensure that the system will not become unstable and effectively limit the output current. After the fault is cleared, switch to the original active power and reactive power setting values. Since fault clearing only requires cutting off the virtual impedance link and switching to the original power reference value, the fault recovery process is relatively simple. In addition, since the virtual impedance link and power limit both dynamically limit the current during the fault, the low voltage ride-through of the grid-building inverter can be finally achieved, ensuring no overcurrent during the fault, fast fault recovery, and maximum reactive current injection to ensure stable system operation.

[0061] In order to verify the proposed virtual impedance switching logic and dynamic current limiting scheme, the present invention uses the Simulink simulation platform to build relevant models. The simulation duration is set to 2s, and the three-phase voltage of the time-varying power supply drops by 40% and 60% at 0.5s, and returns to normal at 1.5s, to simulate the three-phase voltage drop of the power grid during 0.5s-1.5s. Among them, the main parameters of the simulation model are shown in Table 2.

[0062] Table 2 Main simulation parameters

[0063] parameter Numeric parameter Numeric <![CDATA[DC bus voltage U dc > 800V <![CDATA[Grid-side equivalent inductance L g > 50mH <![CDATA[Switching frequency f s > 10kHz <![CDATA[Grid-side equivalent resistance R g > 0.9Ω <![CDATA[Filter inductor L f > 10mH <![CDATA[Reactive droop coefficient D q > <![CDATA[3*10 -4 ]]> <![CDATA[Filter capacitor C f > 50μF <![CDATA[Active droop coefficient D p > <![CDATA[3.33*10 -4 ]]> <![CDATA[Current loop proportionality coefficient k ip > 50 <![CDATA[Active power reference value P ref > 20kW <![CDATA[Virtual resistor R v > 1Ω <![CDATA[Voltage outer loop proportionality coefficient k up > 0.5 <![CDATA[Virtual inductor L v > 5mH <![CDATA[Voltage outer loop integral coefficient k ui > 100 Voltage amplitude reference value E 311V <![CDATA[Frequency reference value ω ref > 50Hz

[0064] like Figure 5 As shown in the figure, after the three-phase grid voltage drops by 40%, the output current of the grid-connected inverter without dynamic current limiting is 1.84pu, and the steady-state current value is 1.52pu, which shows that both are overcurrent. Figure 6As shown in the figure, after adding dynamic current limiting, the output current of the grid-type inverter has an instantaneous maximum current of 1.18pu, and the steady-state current is obviously not over-current. Figure 6 , Figure 8 As shown in Figure 1, after 1.5s, the fault is cleared, that is, the grid voltage is restored, and the output current of the grid-connected inverter also returns to normal. Fig. 9 As shown, when system overcurrent is detected, the virtual impedance input logic is at a high level. When the grid voltage is restored and the fault is cleared, the virtual impedance input logic is at a low level, thereby realizing the input and output of the virtual impedance during the fault period.

[0065] like Figure 7 As shown in the figure, after the grid voltage drops by 60%, the grid-connected inverter output current without dynamic current limiting is unstable. This phenomenon occurs because when the grid voltage drops severely, the P curve drops, resulting in no intersection between the active power reference value and the P curve. Figure 8 As shown in the figure, after adding the dynamic current limiting strategy, the system did not become unstable, and its instantaneous maximum current was 1.22pu, and the steady-state current was obviously not over-current. Fig.10 As shown in the figure, in order to inject reactive current to the maximum extent and limit the output steady-state current, the reference value of active power can be set to 0. After the fault is cleared, the active power will recover to the original level relatively quickly.

[0066] Preferably, considering that in the case of severe grid voltage drop, when the grid-type inverter is fully loaded, the output current should always be within the tolerance range, that is, the output current amplitude should be less than 1.3pu, and the fault current amplitude caused by the grid voltage drop should be less than 0.3pu. Therefore, the overcurrent threshold can be set to 1.2pu or other values ​​less than 1.3pu to retain a certain margin.

[0067] Preferably, combined with the voltage and current double closed-loop control, the dq component of the voltage drop on the virtual impedance is used to reduce the reference value of the voltage outer loop, thereby preventing the voltage controller from outputting an excessively large current reference value. Adding a virtual impedance link in the dq rotating coordinate system can not only utilize the output current of the grid-type inverter after decoupling in the dq rotating coordinate system, but also weaken the impact of the output current high-order harmonics on the virtual impedance link, which is easy to implement in engineering.

[0068] Preferably, the method for judging whether the output current amplitude exceeds the limit is: compare the amplitude of the output current of the grid-type inverter monitored in real time with the set overcurrent threshold through the Compare comparator, and output a Boolean value as an over-limit signal. When the amplitude of the output current is greater than or equal to the overcurrent threshold, the over-limit signal is 1; when the amplitude of the output current is less than the overcurrent threshold, the over-limit signal is 0. This signal is used to determine whether an overcurrent occurs, thereby triggering the current limiting strategy.

[0069] Preferably, the method for judging the grid voltage recovery is: calculating the absolute value of the voltage change (actual grid voltage and rated grid voltage), comparing the absolute value of the difference between the amplitude of the real-time monitored grid voltage and the set rated value through the Compare comparator, judging whether it is within the error range, and outputting a Boolean value as a voltage recovery signal. When the absolute value of the difference is within the error range, the voltage recovery signal is 1; when the absolute value of the difference is outside the error range, the voltage recovery signal is 0. This signal is used to judge whether the grid voltage has returned to normal, so as to timely remove the virtual impedance and restore the active power output.

[0070] It can be seen that this method can limit the output current in time during the fault period to prevent the inverter device from being subjected to excessive current shock to protect its safety. At the same time, the active power output can be quickly restored after the fault is cleared, so that the reliability and stability of the power system can be improved.

[0071] While achieving the above-mentioned beneficial effects, each preferred scheme further achieves the following beneficial effects: switching the power reference value can enhance the stability of the power grid system; the preset current threshold value is less than 1.3pu, which can achieve the maximum output current and leave a margin when the grid-type inverter is fully loaded, thereby improving the fault tolerance rate; adding virtual impedance in the dq rotating coordinate system can weaken the influence of the output current high-order harmonics on the virtual impedance, which is easy to implement in engineering; setting the logical relationship between the input signal and the output current amplitude over-limit signal and the voltage recovery signal can realize the virtual impedance switching operation during fault and recovery, which is suitable for actual engineering applications; setting the output current amplitude over-limit signal and the grid voltage recovery signal and the control logic of the SR latch can avoid frequent switching of the virtual impedance when the output current amplitude changes; and removing the virtual impedance can make the output current quickly return to normal working state, improve the reliability and stability of the system, so as to restore the power supply as soon as possible and reduce the impact on users.

[0072] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the invention.

Claims

1. A current limiting method for a grid-connected inverter based on virtual impedance switching, characterized in that: include: Collect the output current I of the grid-connected inverter oabc And detect in real time through the current detection device; If i oabc ≤I0, the current detection device outputs a Boolean value 0 to the first state machine, so that it remains in state A; Perform voltage and current double closed-loop control, and perform SPWM modulation on the output of the voltage and current double closed-loop control to obtain a pulse signal to drive each switch tube in the grid-type inverter; If i oabc >I0, the current detection device outputs a Boolean value 1 to the first state machine, causing it to switch to state B, input a virtual impedance into the voltage and current dual closed-loop control, and perform SPWM modulation on the output of the voltage and current dual closed-loop control to obtain a pulse signal to drive each switch tube in the grid-type inverter; Perform real-time grid recovery detection. If the grid is restored, the second state machine controls the first state machine to switch to state A to remove the virtual impedance. Among them, i oabc is the output current I oabc The instantaneous value of the three-phase current in I0 is the preset current threshold; the object of the voltage and current double closed-loop control is the output voltage V of the grid-connected point of the grid-connected inverter. abc and the inductor current of the grid-connected inverter; Also includes: Collect output voltage V abc , and calculate the reactive power reference value Q ref and active power reference value P ref ; Q ref =1.5u pd i q ; In the formula, u pd is the output voltage V abc The d-axis component, i q is the reactive current required to be output when the grid voltage drops, i q (t_) is the value of reactive current output before the grid voltage drops, ΔU is the voltage change at the grid connection point, U is the rated voltage at the grid connection point, I n is the rated current value of the grid connection point, i d is the active current; When the grid voltage drops more than 0.1pu, the original power reference value is switched to the recalculated reactive power reference value Q ref and active power reference value P ref ; When the power grid is restored, switch back to the original power reference value.

2. A grid-connected inverter current limiting method based on virtual impedance switching as claimed in claim 1, characterized in that: The preset current threshold is less than 1.3 pu.

3. A current limiting method for a grid-connected inverter based on virtual impedance switching as claimed in claim 1, characterized in that: The input virtual impedance is: adding virtual impedance in the dq rotating coordinate system.

4. A current limiting method for a grid-connected inverter based on virtual impedance switching as claimed in claim 1, characterized in that: Methods for determining grid restoration include: Real-time calculation of output voltage V abc The difference d between the instantaneous value of the three-phase voltage and the preset rated value; When |d|≤D, the power grid is restored, and a Boolean value 1 is output to the second state machine; when |d|>D, the power grid is not restored, and a Boolean value 0 is output to the second state machine; wherein D is a preset threshold.

5. A current limiting method for a grid-connected inverter based on virtual impedance switching as claimed in claim 1, characterized in that: The first state machine and the second state machine are SR latches.

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