Compensation method and system for eliminating the influence of time delay on grid-side current active damping

Abstract

The application discloses a compensation method and system for eliminating the influence of time delay on grid-side current active damping, and belongs to the technical field of power technology application, and the method comprises the following steps: sampling a grid-side current, and obtaining a feedback current by subjecting the grid-side current to a negative high-pass filter; acquiring a modulation signal output by a main control loop of a grid-connected inverter, and obtaining a feedback modulation signal by subjecting the modulation signal to a compensator; and superimposing the feedback current and the feedback modulation signal by means of an adder and then loading the superimposed current and modulation signal to an output of the main control loop of the grid-connected inverter; wherein the compensator eliminates the influence of time delay on grid-side current active damping, fully eliminates the adverse effects of time delay, ensures that the active damping is equivalent to a positive constant resistance, and makes the grid-connected inverter system not be affected by the impedance change of a power grid in a weak power grid condition, thereby improving the stability of the system.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of power technology application, and in particular to a compensation method and system for eliminating the influence of time delay on active damping of a grid-side current. BACKGROUND

[0002] Compared with an L-type single-inductor filter, an LCL-type filter has stronger attenuation capability for high-frequency components, and is thus applied to the field of distributed power generation. However, the LCL-type filter has a three-order structure, and is prone to resonance, thereby causing system instability. The resonance suppression method for the LCL-type inverter is generally divided into passive damping and active damping. The passive damping increases the system damping by connecting or paralleling a resistor to the filter capacitor branch, and the introduction of the resistor increases the system loss and affects the system efficiency. The active damping increases the damping by adding an additional feedback control, and can obtain the same resonance suppression effect as the passive damping. The active damping has flexible and adjustable implementation, and does not bring additional power consumption, and thus has become a research and application hotspot in recent years. However, the capacitor current and capacitor voltage feedback active damping needs to add an additional sensor to the system, which not only increases the hardware cost of the system, but also is not conducive to the reliability of the system. The grid-side current active damping can be equivalent to connecting a resistor in parallel to the filter capacitor by feeding back the grid-side current. The method is flexible and controllable, does not introduce an additional sensor, and has certain engineering application value.

[0003] However, in digital control, due to the inevitable system time delay, the virtual impedance expression equivalent to the grid-side current active damping is a frequency-dependent sinusoidal function. With the wide-range change of the grid impedance, the resonance frequency of the LCL filter moves to the low-frequency band, which causes the resistance part of the equivalent virtual impedance to change from negative to positive in a certain frequency range. If the resonance frequency is close to the turning frequency of the resistance equivalent to the virtual impedance from negative to positive, the system is extremely unstable.

[0004] In order to reduce the influence of the system time delay, in recent years, scholars have proposed many methods, which can be generally divided into two categories: one effective way is to reduce the time delay in the PWM implementation process, such as increasing the sampling frequency or changing the PWM reference update time. However, this method is difficult to avoid the influence of frequency aliasing and switching noise, and the method has very high requirements for the controller, and needs to calculate the PWM wave value in a very short time. Another way is to add delay compensation in the control loop, such as a first-order filter compensator and a second-order generalized integrator. However, these compensators can only compensate for a maximum delay of half a sampling period, and will amplify the high-frequency noise to a certain extent.

[0005] In the related art, a Chinese patent application with the publication number CN105591400A discloses a fast robust single current feedback control method for an LCL grid-connected inverter. The fast robust PWM control method is adopted to completely eliminate the one-beat lag calculation delay of the grid-connected current outer loop and the active damping inner loop without introducing the duty cycle limitation. However, as mentioned above, by reducing the delay in the PWM implementation process, it is difficult to avoid the influence of frequency aliasing and switching noise, and the requirement for the controller is extremely high. SUMMARY

[0006] The technical problem to be solved by the present application is how to make the grid-connected inverter system not be affected by the change of grid impedance in the case of weak grid.

[0007] The present application solves the above technical problems by the following technical means:

[0008] The present application proposes a compensation method for eliminating the influence of delay on the active damping of grid-side current, which comprises the following steps:

[0009] Sampling the grid-side current, and obtaining the feedback current by subjecting the grid-side current to a negative high-pass filter;

[0010] Obtaining the modulation signal output by the main control loop of the grid-connected inverter, and obtaining the feedback modulation signal by subjecting the modulation signal to a compensator, wherein the expression of the compensator G(s) is:

[0011] G(s)=G HPF (s)G LCL (s)-G HPF (s)G LCL (s)G d (s)

[0012]

[0013] In the formula, G HPF (s) is the expression of the negative high-pass filter; G LCL (s) is the expression of the LCL filter; G d (s) is the inherent delay of the system; L1 is the bridge-side inductance of the LCL filter; L2 is the grid-side inductance of the LCL filter; C is the capacitance of the LCL filter; L g is the grid impedance value corresponding to the resonance frequency with the turning frequency; and s is a differential operator.

[0014] The feedback current and the feedback modulation signal are superimposed by a summer and then loaded to the output of the main control loop of the grid-connected inverter.

[0015] The present application adds a feedback compensator in the main control loop of the grid-connected inverter, changes the impedance equivalent to the active damping of the grid-side current into a constant positive virtual impedance independent of frequency, avoids the situation that the resistance part of the virtual impedance is negative in a certain frequency range, and makes the grid-connected inverter system not affected by the change of grid impedance in the case of weak grid, thereby improving the stability of the system.

[0016] Further, the calculation process of the turning frequency equal to the grid impedance value corresponding to the resonance frequency comprises:

[0017] determining the negative high-pass filter parameter and the sampling period, wherein the negative high-pass filter parameter comprises a gain and a cutoff angular frequency;

[0018] based on the negative high-pass filter parameter and the sampling period, calculating the zero point of g R (ω) in the denominator of the resistance expression in the virtual impedance as the turning frequency ω R , wherein the expression of g R (ω) is:

[0019] g R (ω)=ωcos(1.5ωT s )+ω d sin(1.5ωT s )

[0020] In the formula, T s is the sampling period of the high-pass filter; ω is a frequency variable; ω d is the cutoff angular frequency of the high-pass filter.

[0021] When the turning frequency ω R is equal to the resonance frequency ω r , the corresponding grid impedance value L g is calculated.

[0022] Further, the calculation formula of the grid impedance value L g is:

[0023]

[0024] Further, the expression of the negative high-pass filter is:

[0025]

[0026] In the formula, K d is the gain of the high-pass filter; ω d is the cutoff angular frequency of the high-pass filter.

[0027] Further, the expression of the system inherent delay is:

[0028]

[0029] In the formula: T s is the sampling period of the high-pass filter.

[0030] In addition, the application further provides a compensation system for eliminating the influence of the delay on the active damping of the grid-side current, the system comprising:

[0031] an active damping circuit and a delay elimination circuit, the active damping circuit comprising a negative high-pass filter and a first adder, and the delay elimination circuit comprising a compensator and a second adder, wherein:

[0032] the negative high-pass filter is configured to process the grid-side current to obtain a feedback current;

[0033] the compensator is configured to process a modulation signal output by a main control circuit of the grid-connected inverter to obtain a feedback modulation signal;

[0034] the first adder is configured to superimpose the feedback current and the feedback modulation signal to obtain a total feedback quantity;

[0035] the second adder is configured to load the total feedback quantity to an output of the main control circuit of the grid-connected inverter;

[0036] wherein the expression of the compensator G(s) is:

[0037] G(s) = G HPF (s)G LCL (s)-G HPF (s)G LCL (s)G d (s)

[0038]

[0039] In the formula: G HPF (s) is the expression of the negative high-pass filter; G LCL (s) is the expression of the LCL filter; G d (s) is the inherent delay of the system; L1 is the bridge-side inductance of the LCL filter; L2 is the grid-side inductance of the LCL filter; C is the capacitance of the LCL filter; L g is the grid impedance value corresponding to the turning frequency equal to the resonance frequency; and s is a differential operator.

[0040] Further, the calculation process of the grid impedance value corresponding to the turning frequency equal to the resonance frequency comprises:

[0041] determining the negative high-pass filter parameters and the sampling period, wherein the negative high-pass filter parameters comprise a gain and a cutoff angular frequency;

[0042] Based on the negative high-pass filter parameter and sampling period, calculate g in the denominator of resistance expression in the virtual impedance R The zero point of (ω) as the turning frequency ω R Wherein, g R The expression of (ω) is:

[0043] g R (ω) = ωcos(1.5ωT s ) + ω d sin(1.5ωT s )

[0044] In the formula, T s is the sampling period of the high-pass filter; ω is the frequency variable; ω d is the cutoff angular frequency of the high-pass filter;

[0045] When the turning frequency ω R is equal to the resonant frequency ω r , the corresponding grid impedance value L g is calculated.

[0046] Further, the calculation formula of the grid impedance value L g is:

[0047]

[0048] Further, the expression of the negative high-pass filter is:

[0049]

[0050] In the formula, K d is the gain of the high-pass filter; ω d is the cutoff angular frequency of the high-pass filter.

[0051] Further, the expression of the system inherent delay is:

[0052]

[0053] In the formula, T s is the sampling period of the high-pass filter.

[0054] The advantages of the present application are:

[0055] (1) The present application adds a feedback compensator in the main control loop of the grid-connected inverter, changes the impedance equivalent to the active damping of the grid-side current into a positive virtual impedance that is constant regardless of frequency, avoids the case that the resistance part of the virtual impedance is negative in a certain frequency range, and makes the grid-connected inverter system not affected by the change of grid impedance in the case of weak grid, thereby improving the stability of the system.

[0056] Additional aspects and advantages of the present application will be better understood from the following descriptions, become apparent by the described embodiments, or learned from the practice of the application. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 is a flowchart of a compensation method for eliminating the influence of time delay on active damping of grid-side current in an embodiment of the present application;

[0058] Figure 2 is a flowchart of the construction of a compensator in an embodiment of the present application;

[0059] Figure 3 is a structural diagram of a compensation system for eliminating the influence of time delay on active damping of grid-side current in an embodiment of the present application;

[0060] Figure 4 is a grid-side current waveform diagram when only active damping strategy is used in the system in the presence of time delay in an embodiment of the present application;

[0061] Figure 5 is a grid-side current waveform diagram when a compensator is added in the system in the presence of time delay in an embodiment of the present application. DETAILED DESCRIPTION

[0062] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described below in a clear and complete manner in combination with the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, but not all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the protection scope of the present application.

[0063] As shown in Figure 1 an embodiment of the present application proposes a compensation method for eliminating the influence of time delay on active damping of grid-side current, comprising the following steps:

[0064] S10, sampling the grid-side current, and obtaining the feedback current by passing the grid-side current through a negative high-pass filter;

[0065] S20, obtaining the modulation signal output by the main control loop of the grid-connected inverter, and obtaining the feedback modulation signal by passing the modulation signal through a compensator, wherein the expression of the compensator G(s) is:

[0066] G(s)=G HPF (s)G LCL (s)-G HPF (s)G LCL (s)G d (s)

[0067]

[0068] G HPF (s) is a negative high-pass filter expression; G LCL (s) is an LCL filter expression; G d (s) is a system inherent delay; L1 is a bridge-side inductance of the LCL filter; L2 is a grid-side inductance of the LCL filter; C is a capacitance of the LCL filter; L g is a grid impedance value corresponding to a turning frequency equal to a resonance frequency; s is a differential operator;

[0069] S30, superimpose the feedback current and the feedback modulation signal through an adder and load to an output of a main control loop of the grid-connected inverter.

[0070] It should be noted that the current delay compensation strategy can only reduce the influence of the delay, but cannot eliminate the problem of the delay. In the embodiment, the compensator is constructed by calculating the grid impedance value corresponding to the turning frequency close to the resonance frequency, which can equivalently move the delay out of the active damping feedback loop, change the equivalent impedance of the grid-side current active damping into a constant positive virtual impedance independent of the frequency, avoid the case that the resistance part of the virtual impedance is negative in a certain frequency range, make the grid-connected inverter system not be affected by the grid impedance change in the case of weak grid, and make the system be able to stably operate under a wider range of grid impedance change, thereby improving the stability of the system.

[0071] In an embodiment, as shown in Figure 2 , the construction process of the compensator includes:

[0072] (1) determining inherent parameters

[0073] The grid-connected inverter needs to determine the following parameters: bridge-side inductance L1, grid-side inductance L2, capacitance C, high-pass filter gain K d , cutoff angular frequency ω d , and sampling period T s .

[0074] (2) calculating a turning frequency

[0075] The resistance expression R eq in the virtual impedance is calculated as follows:

[0076]

[0077] g R (ω)=ωcos(1.5ωT s )+ω a sin(1.5ωT s )

[0078] wherein: K PWM is the inverter gain; and ω is the frequency variable.

[0079] The positive and negative of the resistance in the virtual impedance is determined by the expression R eq The positive and negative of g R (ω) in the denominator results in that, after the negative high-pass filter parameters and the sampling frequency have been determined, the zero of g R (ω) can be calculated, i.e. when the frequency variable ω takes a value such that g R (ω) is zero, the frequency at this time is taken as the turning frequency ω R .

[0080] (3) Calculation of the grid impedance value at the turning frequency

[0081] When the turning frequency ω R is equal to the resonance frequency ω r , the corresponding grid impedance value L g at this time can be calculated by the following formula:

[0082]

[0083] wherein ω r is the resonance angular frequency. If the frequency is known, the grid impedance value L g can be solved.

[0084] (4) Construction of the compensator

[0085] The expression of the negative high-pass filter is determined as:

[0086]

[0087] The expression of the system inherent delay is determined as:

[0088]

[0089] The expression of the LCL type filter is determined as:

[0090]

[0091] Thus, the compensator is constructed as:

[0092] G(s) = G HPF (s)G LCL (s)-G HPF (s)G LCL (s)G d (s).

[0093] In addition, as Figure 3As shown, another embodiment of the present application also proposes a compensation system for eliminating the influence of time delay on the active damping of grid-side current, which comprises: an active damping loop and a time delay elimination loop, the active damping loop comprises a negative high-pass filter 10 and a first adder 20, the time delay elimination loop comprises a compensator 30 and a second adder 40, wherein:

[0094] The negative high-pass filter 10 is used for processing the grid-side current to obtain a feedback current;

[0095] The compensator 30 is used for processing a modulation signal output by a main control loop of the grid-connected inverter to obtain a feedback modulation signal;

[0096] The first adder 20 is used for superimposing the feedback current and the feedback modulation signal to obtain a total feedback quantity;

[0097] The second adder 40 is used for loading the total feedback quantity to an output of the main control loop of the grid-connected inverter;

[0098] The compensator G(s) has an expression as follows:

[0099] G(s)=G HPF (S)G LCL (S)-G HPF (S)G LCL (S)G d (S)

[0100]

[0101] In the formula, G HPF (s) is an expression of the negative high-pass filter; G LCL (s) is an expression of the LCL filter; G d (s) is a system inherent time delay; L1 is a bridge-side inductance of the LCL filter; L2 is a grid-side inductance of the LCL filter; C is a capacitance of the LCL filter; L g is an impedance value of the power grid corresponding to a turning frequency equal to a resonance frequency; and s is a differential operator.

[0102] In an embodiment, the calculation process of the impedance value of the power grid corresponding to the turning frequency equal to the resonance frequency comprises:

[0103] Determining the negative high-pass filter parameters and the sampling period, wherein the negative high-pass filter parameters comprise a gain and a cutoff angular frequency;

[0104] Based on the negative high-pass filter parameters and the sampling period, calculating a zero point of g R (ω) in the denominator of the resistance expression in the virtual impedance as the turning frequency ω R , wherein g RThe expression of (ω) is:

[0105] g R (ω) = ω cos (1.5 ωT s )+ ω d sin (1.5 ωT s )

[0106] In the formula, T s is the sampling period of the high-pass filter; ω is the frequency variable; ω d is the cut-off angular frequency of the high-pass filter.

[0107] When the turning frequency ω R is equal to the resonance frequency ω r , the corresponding grid impedance value L g is calculated.

[0108] In an embodiment, the calculation formula of the grid impedance value L g is:

[0109]

[0110] In an embodiment, the expression of the negative high-pass filter is:

[0111]

[0112] In the formula, K d is the gain of the high-pass filter; ω d is the cut-off angular frequency of the high-pass filter.

[0113] In an embodiment, the expression of the system inherent delay is:

[0114]

[0115] In the formula, T s is the sampling period of the high-pass filter.

[0116] Figure 4 is the grid-side current waveform of the system when only the active damping strategy is used in the presence of the delay, and it can be seen that the current has high-frequency harmonics and the system is unstable. Figure 5 is the grid-side current waveform of the system when the compensator is added in the presence of the delay, and it can be seen that the sinusoidal degree of the current is very good, the system is stably operated, and the given current is reduced from 50 A to 30 A at the moment of 0.2 s, and the system can also be well tracked.

[0117] In the description of the specification, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the description of the specification, the illustrative description of the above terms does not necessarily mean the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0118] In addition, the terms "first", "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, for example, two, three, etc., unless otherwise specifically limited.

[0119] Although the embodiments of the present application have been shown and described above, it is understood that the above-described embodiments are exemplary and cannot be construed as limiting the present application, and those skilled in the art can make changes, modifications, replacements and variations to the above-described embodiments within the scope of the present application.

Claims

1. A compensation method for eliminating the influence of time delay on active damping of grid-side current, characterized in that, The method comprises: sampling a grid-side current to obtain a feedback current through a negative high-pass filter; The modulation signal outputted by the main control loop of the grid-connected inverter is acquired, the modulation signal is subjected to inherent delay of the system and LCL type filter to obtain grid-side current, and the modulation signal is subjected to compensator to obtain feedback modulation signal, wherein the compensator The expression is: wherein: is a negative high-pass filter expression; is an LCL filter expression; is a system inherent delay; is a bridge-side inductance of the LCL filter; is a grid-side inductance of the LCL filter; C is a capacitance of the LCL filter; is a grid impedance value corresponding to a break frequency equal to a resonance frequency; is a differential operator; Wherein, the grid impedance value The calculation formula is: for the resonance frequency, when the frequency variable is made zero, the frequency variable at this time is taken as the turning frequency ; superimposing the feedback current and a feedback modulation signal through an adder and loading the superimposed result to an output of a main control loop of the grid-connected inverter.

2. The compensation method for eliminating the influence of time delay on active damping of grid-side current according to claim 1, characterized in that, The calculation process of the turning frequency equal to the grid impedance value corresponding to the resonance frequency comprises: determining a negative high-pass filter parameter and a sampling period, wherein the negative high-pass filter parameter comprises a gain and a cutoff angular frequency; based on the negative high pass filter parameter and the sampling period, calculating a zero point in a denominator of a resistance expression in the virtual impedance as a turning frequency wherein wherein the expression is In the formula: is the sampling period of the high pass filter; is the frequency variable; is the cut-off angular frequency of the high pass filter; when the turning frequency is equal to the resonance frequency the corresponding grid impedance value is calculated.

3. The compensation method for eliminating the influence of time delay on active damping of grid-side current according to claim 1, characterized in that, an expression of the negative high-pass filter is: wherein: G is the gain of the high pass filter; ωc is the cut-off angular frequency of the high pass filter.

4. The compensation method for eliminating the effect of time delay on active damping of grid-side current as claimed in claim 1, wherein, an expression of the system inherent delay is: In the formula: is the sampling period of the high-pass filter.

5. A compensation system for eliminating the effect of time delay on active damping of grid-side current, characterized in that, The system comprises: an active damping loop and a delay elimination loop, the active damping loop comprising a negative high-pass filter and a first adder, and the delay elimination loop comprising a compensator and a second adder, wherein: the negative high-pass filter is configured to process the grid-side current to obtain the feedback current; the compensator is configured to process a modulation signal output by the main control loop of the grid-connected inverter to obtain the feedback modulation signal; the first adder is configured to superimpose the feedback current and the feedback modulation signal to obtain a total feedback quantity; the second adder is configured to load the total feedback quantity to the output of the main control loop of the grid-connected inverter; The compensator The expression is: wherein: is a negative high-pass filter expression; is an LCL filter expression; is the system inherent delay; is the bridge-side inductance of the LCL filter; is the grid-side inductance of the LCL filter; C is the capacitance of the LCL filter; is the grid impedance value corresponding to the resonance frequency for the turning frequency; is the differential operator; Wherein, the grid impedance value The calculation formula is: for the resonance frequency, when the frequency variable such that for zero, the frequency variable as the turning frequency ; wherein the modulation signal is processed through the system inherent delay and an LCL type filter to obtain the grid-side current.

6. The compensation system to eliminate the effect of time delay on active damping of grid-side current as claimed in claim 5 wherein, The calculation process of the turning frequency equal to the grid impedance value corresponding to the resonance frequency comprises: determining a negative high-pass filter parameter and a sampling period, wherein the negative high-pass filter parameter comprises a gain and a cutoff angular frequency; based on the negative high pass filter parameter and the sampling period, calculating a zero of a denominator of a resistance expression in the virtual impedance as a turning frequency wherein wherein is given by In the formula: is the sampling period of the high pass filter; is the frequency variable; is the cut-off angular frequency of the high pass filter; when the turning frequency is equal to the resonance frequency the corresponding grid impedance value is calculated.

7. The compensation system to eliminate the effect of time delay on active damping of grid-side current as claimed in claim 5 wherein, an expression of the negative high-pass filter is: wherein: G is the gain of the high pass filter; ωc is the cut-off angular frequency of the high pass filter.

8. The compensation system to eliminate the effect of time delay on active damping of grid-side current as claimed in claim 5 wherein, an expression of the system inherent delay is: In the formula: is the sampling period of the high-pass filter.

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