Active damping control system for suppressing high-frequency resonance and MMC alternating current grid-connected system

Abstract

The utility model discloses an active damping control system for suppressing high-frequency resonance and an MMC alternating-current grid-connected system. The active damping control system comprises a voltage and current sampling module, a voltage and current double-closed-loop control module, a feed-forward voltage long-period sampling module, a voltage feed-forward control module, an adder and an NLC modulation and capacitance voltage-sharing module, the input end of the voltage and current sampling module is connected with voltage and current of a PCC point of a power grid, and the output end of the voltage and current sampling module is connected with the input end of the voltage and current double-closed-loop control module; the input end of the feed-forward voltage long-period sampling module is connected with a PCC point voltage of a power grid, and the output end of the feed-forward voltage long-period sampling module is connected with the input end of the voltage feed-forward control module; the output end of the voltage feed-forward control module and the output end of the voltage and current double-closed-loop control module are connected with the input end of the adder, the output end of the adder is connected with the input end of the NLC modulation and capacitance voltage-sharing module, and the output end of the NLC modulation and capacitance voltage-sharing module is connected with a switching device of the converter.

Description

TECHNICAL FIELD

[0001] The utility model belongs to the field of power electronics, specifically related to the active damping control system and MMC AC parallel operation system of high frequency resonance suppression. BACKGROUND

[0002] In recent years, with the gradual increase of new energy penetration rate, the increase of power electronic equipment, its potential stability problem is increasingly prominent. The new energy collection and through the converter sending out system shows low inertia, weak damping characteristics, the converter control link exists delay, usually 300~600us, makes the converter AC side impedance shows inductive negative damping characteristics in high frequency band, is easy to occur resonance with the capacitive frequency band of AC line, the high frequency resonance is extremely great, excessive resonance current can cause circuit breaker misoperation, has caused many engineering fault tripping, this unstable resonance problem will possibly cause the whole system instability even collapse.

[0003] For the high frequency oscillation problem of converter, the related research has proposed many feasible suppression strategies, which are roughly divided into active suppression strategy based on main controller and passive suppression strategy based on main circuit. On the one hand, the high frequency band negative damping characteristics of the converter can be weakened by adjusting the controller parameters, but it can only be used as an optimization method and cannot fundamentally solve the high frequency oscillation problem; Or is to add low pass, band pass, band stop filter to filter out high harmonic, increase high frequency band damping, but the effect of this method is still not good, will move the negative damping frequency band down, only transfer the high frequency resonance frequency band without really solving the high frequency resonance problem, and the passive suppression strategy based on main circuit needs to parallel RLC passive device at PCC point or increase RC passive branch in bridge arm, which increases the cost and needs to change the original main circuit structure, and is not worth the loss. Utility model content

[0004] The utility model aims at overcoming the problem of high frequency resonance caused by the inductive negative resistance characteristics of new energy converter in high frequency band and the interaction with capacitive line, and proposes an active damping control system and MMC AC parallel operation system for suppressing high frequency resonance.

[0005] In order to achieve the above purpose, the utility model adopts the following technical scheme:

[0006] An active damping control system for suppressing high frequency resonance comprises:

[0007] A voltage and current sampling module, a voltage and current double closed loop control module, a feedforward voltage long period sampling module, a voltage feedforward control module, an adder and an NLC modulation and capacitor voltage sharing module.

[0008] The input end of the voltage and current sampling module is connected with the voltage and current of the grid PCC point, and the output end of the voltage and current sampling module is connected with the input end of the voltage and current double closed-loop control module; the input end of the long-period sampling module of the feed-forward voltage is connected with the voltage of the grid PCC point, and the output end of the long-period sampling module of the feed-forward voltage is connected with the input end of the voltage feed-forward control module; the output end of the voltage feed-forward control module is connected with the input end of the adder of the output end of the voltage and current double closed-loop control module, the output end of the adder is connected with the input end of the NLC modulation and capacitor voltage sharing module, and the output end of the NLC modulation and capacitor voltage sharing module is connected with the switching device of the converter.

[0009] Further, the d-axis voltage of the grid PCC point is connected with the d-axis reference voltage in negative feedback, and then input into the outer loop of the voltage and current double closed-loop control module; the d-axis reference current is output from the outer loop of the voltage and current double closed-loop control module; the d-axis current of the grid PCC point is connected with the d-axis reference current in negative feedback, and then input into the inner loop of the voltage and current double closed-loop control module.

[0010] The q-axis voltage of the grid PCC point is connected with the q-axis reference voltage in negative feedback, and then input into the outer loop of the voltage and current double closed-loop control module; the q-axis reference current is output from the outer loop of the voltage and current double closed-loop control module; the q-axis current of the grid PCC point is connected with the q-axis reference current in negative feedback, and then input into the inner loop of the voltage and current double closed-loop control module.

[0011] The q-axis current of the grid PCC point is connected with the output end of the inner loop of the voltage and current double closed-loop control module in positive feedback after decoupling, and then connected with the d-axis voltage after passing through the voltage feed-forward control module in positive feedback.

[0012] The d-axis current of the grid PCC point is connected with the output end of the inner loop of the voltage and current double closed-loop control module in negative feedback after decoupling, and then connected with the q-axis voltage after passing through the voltage feed-forward control module in positive feedback.

[0013] Further, the feed-forward voltage sampling period of the voltage feed-forward control module is configured as:

[0014]

[0015] Wherein, T The feed-forward voltage sampling period is represented by TFF, T s The voltage and current double closed-loop sampling period is represented by TDC, f 1, f 2) represents the target frequency band in which the new energy converter station is estimated to occur high-frequency resonance.

[0016] Further, the long-period sampling module of the feed-forward voltage is equivalent to a zero-order holder, and the transfer function of the zero-order holder is:

[0017]

[0018] wherein, G ZOH s ) is a zero-order hold transfer function, e -sT T represents a feed-forward voltage sampling period.

[0019] Further, the feed-forward voltage sampling period is twice the high-frequency resonance frequency, and the high-frequency resonance frequency is the frequency of the second notch position in the bode plot of the zero-order hold.

[0020] Further, the outer loop of the voltage-current double closed-loop control module adopts constant AC voltage control, and the inner loop of the voltage-current double closed-loop control module adopts current control.

[0021] Further, the outer loop voltage PI control of the voltage-current double closed-loop control module is:

[0022]

[0023] wherein, H v s ) is a voltage controller transfer function, k vp is a proportional coefficient of the voltage outer loop controller, k vi is an integral coefficient of the voltage outer loop controller.

[0024] Further, the inner loop current PI control of the voltage-current double closed-loop control module is:

[0025]

[0026] wherein, H i s ) is a current controller transfer function, k ip is a proportional coefficient of the current inner loop controller, k ii is an integral coefficient of the current inner loop controller.

[0027] Further, the impedance transfer function of the converter is:

[0028]

[0029] wherein, Z is the impedance of the converter, ​​​​Impedance transfer function of the converter when the feed-forward voltage long-period sampling module is not added to the damping control system, Transfer function of the feed-forward voltage long-period sampling module;

[0030] R is a bridge arm parasitic resistance, L is a bridge arm inductance, k ip is a proportional coefficient of the current inner loop controller, k ii is an integral coefficient of the current inner loop controller, k vp is a proportional coefficient of the voltage outer loop controller, k vi is an integral coefficient of the voltage outer loop controller, Signal transmission time delay transfer function of the damping control system, e Natural constant, T Feed-forward voltage sampling period.

[0031] An MMC AC grid-connected system uses the active damping control system for suppressing high-frequency resonance.

[0032] Compared with the prior art, the active damping control system for suppressing high-frequency resonance has the following beneficial technical effects:

[0033] The active damping control system for suppressing high-frequency resonance provided by the utility model discloses a feed-forward voltage long-period sampling module, which adopts a sampling period much larger than that of a voltage and current double closed-loop control module, and can significantly improve the high-frequency impedance characteristics of the converter by increasing the damping of the converter at the high-frequency band, greatly reduces the probability of high-frequency resonance fault, and guarantees the safe and stable operation of the converter station, without the need to increase any hardware equipment, thereby reducing the scheme cost. BRIEF DESCRIPTION OF DRAWINGS

[0034] The drawings described herein are only for the purpose of explanation, and are not intended to limit the scope of the utility model disclosure in any way. In addition, the shapes and scale sizes of the components in the drawings are only illustrative, and are used to help understand the utility model, and are not specific limitations on the shapes and scale sizes of the components of the utility model. In the drawings:

[0035] Figure 1 MMC converter grid-connected system power circuit and control loop connection schematic diagram;

[0036] Figure 2 Definite AC voltage double closed-loop control system structure diagram of the active damping control system for suppressing high-frequency resonance of the utility model;

[0037] Figure 3 Bode plot of the zero-order hold for a sampling rate of f;

[0038] Figure 4 Impedance characteristic curve of the MMC converter at the AC side in the high-frequency range for the feed-forward voltage sampling at different sampling periods;

[0039] Figure 5 Resonant frequency in the example f z = 1900 Hz, T Voltage waveform at the AC port of the converter under damping control with = 1.32 ms;

[0040] Figure 6 Resonant frequency in the example f z = 1250 Hz, T Voltage waveform at the AC port of the converter under damping control with = 1.32 ms;

[0041] Figure 7 Resonant frequency in the example f z Fast Fourier decomposition spectrum of the AC voltage at a resonant frequency of = 1900 Hz.

[0042] Figure 8 Resonant frequency in the example f z Fast Fourier decomposition spectrum of the AC voltage at a resonant frequency of = 1250 Hz.

[0043] Figure 9 Modular multilevel converter MMC in the example using an active damping control system for suppressing high-frequency resonances.

[0044] Figure 2 In the example, AC voltage d-axis reference value, u d AC voltage d-axis actual value, AC voltage q-axis reference value, u q AC voltage q-axis actual value, H v s Transfer function of the voltage controller, Three-phase AC current reference d-axis component, i d Three-phase AC current actual d-axis component, Three-phase AC current reference q-axis component, i q Three-phase AC current actual q-axis component, H ​i s is a current controller transfer function, K id is a decoupling coefficient, m k is a modulation signal, H d s is a time delay transfer function.

[0045] Figure 9 R is a bridge leg parasitic resistance, L is a bridge leg inductance, i au 、i bu 、i bu are three-phase upper bridge leg currents, respectively, i al 、 i bl 、i bul are three-phase lower bridge leg currents, respectively, v a v b v c are three-phase AC side voltages, respectively. DETAILED DESCRIPTION

[0046] In order to make the personnel in the technical field better understand the practical new type scheme, the technical scheme in the practical new type embodiment will be described clearly and completely in the following by combining with the drawings in the practical new type embodiment. Obviously, the described embodiment is only a part of the embodiment of the practical new type, rather than all the embodiments. Based on the embodiment in the practical new type, all the other embodiments obtained by the person skilled in the art without making the creative labor should belong to the protection scope of the practical new type.

[0047] It should be noted that when an element is referred to as being "on" another element, it can be directly on the other element or intervening elements can also be present. When an element is referred to as being "connected" to another element, it can be directly connected to the other element or intervening elements can also be present. The terms "vertical", "horizontal", "left", "right" and similar expressions as used herein are for illustration purposes only and are not meant to be limiting.

[0048] ​​​​​Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0049] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this utility model are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the utility model described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0050] Example 1

[0051] See Figure 1 An active damping control system for suppressing high-frequency resonance includes:

[0052] Voltage and current sampling module 1, voltage and current dual closed-loop control module 2, feedforward voltage long-cycle sampling module 3, voltage feedforward control module 4, adder 5, and NLC modulation and capacitor voltage equalization module 6;

[0053] The input terminal of voltage and current sampling module 1 is connected to the voltage and current at the PCC point of the power grid, and the output terminal of voltage and current sampling module 1 is connected to the input terminal of voltage and current dual closed-loop control module 2. The input terminal of feedforward voltage long-cycle sampling module 3 is connected to the voltage at the PCC point of the power grid, and the output terminal of feedforward voltage long-cycle sampling module 3 is connected to the input terminal of voltage feedforward control module 4. The output terminal of voltage feedforward control module 4 and the output terminal of voltage and current dual closed-loop control module 2 are connected to the input terminal of adder 5. The output terminal of adder 5 is connected to the input terminal of NLC modulation and capacitor voltage equalization module 6. The output terminal of NLC modulation and capacitor voltage equalization module 6 is connected to the switching device of the converter.

[0054] PCC point (Point of Common Coupling) is the connection of more than one user load in the power system. The double closed loop voltage and current sampling module 1 of the embodiment can collect the voltage and current of the PCC point, input into the voltage and current double closed loop controller 2, and be used for voltage and current double closed loop control; the long period sampling module 3 of the feedforward voltage collects the voltage signal at a higher sampling period, and after the voltage feedforward control module 4, the output signal of the voltage and current double closed loop control module 2 is added to become the output modulation signal, and finally the NLC modulation and the capacitor voltage sharing control module 6 output the final switch device driving signal. The combination of the voltage and current double closed loop control module 2 and the voltage feedforward control module 4 can realize the accurate control of the power electronic system. The combination provides stability and accuracy through the double closed loop control, and the voltage feedforward control module 4 is used to improve the dynamic response speed of the system. The output of the feedforward control module 4 is directly added to the output of the voltage and current double closed loop control module 2. The response speed and stability of the system can be further improved. Because the voltage feedforward control module 4 can predict and compensate the change of the input voltage, the voltage and current double closed loop control module 2 can adjust its output faster to maintain the stability of the output voltage.

[0055] Referring to Figure 2 , the voltage and current of the grid PCC point are feedback and decoupling controlled in the voltage and current double closed loop control system after d-q transformation;

[0056] The d-axis voltage of the grid PCC point u d and the d-axis reference voltage After negative feedback connection, the outer ring of the voltage and current double closed loop control module 2 is input, and the d-axis reference current output by the outer ring of the voltage and current double closed loop control module 2 The d-axis current of the grid PCC point i d and the d-axis reference current After negative feedback connection, the inner ring of the voltage and current double closed loop control module 2 is input;

[0057] The q-axis voltage of the grid PCC point u q and the q-axis reference voltage After negative feedback connection, the outer ring of the voltage and current double closed loop control module 2 is input, and the q-axis reference current output by the outer ring of the voltage and current double closed loop control module 2 The q-axis current of the grid PCC point i q and the q-axis reference current After negative feedback connection, the inner ring of the voltage and current double closed loop control module 2 is input;

[0058] The q-axis current of the grid PCC pointi q decoupling coefficient K id ) is connected to the output of the inner loop of the voltage and current double closed loop control module 2 in positive feedback, and the d-axis voltage u d feedback connection after passing through the voltage feedforward control module 4.

[0059] The d-axis current of the grid PCC point i d decoupling coefficient K id ) is connected to the output of the inner loop of the voltage and current double closed loop control module 2 in negative feedback, and the q-axis voltage u q feedback connection.

[0060] The voltage and current double closed loop control loop of the converter and the voltage feedforward control loop both have time delay, which is caused by signal sampling, signal transmission of the control link, and running delay of the NLC and the voltage sharing algorithm of the sub-module. The time delay of the voltage feedforward channel is also the direct reason for the converter system to show inductive-negative resistance characteristics at high frequency, that is, the fundamental reason for the occurrence of high frequency resonance problem. The feedforward voltage sampling module adopts a longer sampling period.

[0061] The configuration of the feedforward voltage sampling period T of the voltage feedforward control module 4 is:

[0062]

[0063] Among them, T indicates the feedforward voltage sampling period, T s indicates the voltage and current double closed loop sampling period, f 1, f 2) indicates the target frequency band of the new energy converter station estimated to occur high frequency resonance.

[0064] The feedforward voltage long period sampling module 3 is equivalent to a zero-order holder, and the transfer function of the zero-order holder is:

[0065]

[0066] Among them, G ZOH ( s ) indicates the transfer function of the zero-order holder, e -sT indicates the signal transmission time delay transfer function, T indicates the feedforward voltage sampling period.

[0067] Transfer function is a mathematical tool used to describe the relationship between input and output of a linear time-invariant system, usually represented by H ( s ) or G ( s ). In transfer function, S represents a complex variable, which is the variable in Laplace transform. The real part of the complex variable usually represents the decay or growth characteristics in the time domain, while the imaginary part is related to frequency and represents the frequency response of the system.

[0068] In time delay transfer function, the symbol e represents the natural constant, whose value is approximately 2.71828. In time delay transfer function, e usually appears together with the exponential function, forming e -τs such an expression, where τ represents the time delay time, and s is the complex variable of Laplace transform. This expression describes the decay effect of the signal due to time delay during transmission. Specifically, when a signal passes through a time delay element, its output signal can be represented in the form of input signal multiplied by e -τs . Here, e -τs is a time delay transfer function, which represents the exponential decay characteristics of the signal within the time delay time τ .

[0069] Zero-Order Holder (ZOH) is a commonly used signal processing element, which plays an important role in digital control systems and sampling systems. Its characteristic is to maintain the value of the input signal constant within each sampling period, thus forming a step-shaped output signal. Zero-Order Holder has low-pass characteristics, i.e., it allows low-frequency signals to pass through while attenuating high-frequency signals. However, unlike ideal low-pass filters, the amplitude-frequency characteristic curve of Zero-Order Holder is not monotonically decreasing, but may have peaks or notches at certain frequency points.

[0070] Notch refers to the phenomenon of sharp decline in the frequency characteristic curve near a certain frequency point, which is usually the result of the combined action of phase lag and gain change of the system. In the frequency characteristic of Zero-Order Holder, the first notch usually appears near half of the sampling frequency (i.e., Nyquist frequency). Due to the influence of various factors such as sampling period, system parameters, etc., the exact description of the second notch position is not fixed, and the second notch position of Zero-Order Holder can be determined through frequency characteristic analysis, which can be realized by drawing the Bode plot (amplitude-frequency characteristic and phase-frequency characteristic graph) of Zero-Order Holder. In the Bode plot, the change of amplitude-frequency characteristic curve with frequency can be observed, and the position and depth of the notch can be determined.

[0071] Referring to Figure 3 , the feed-forward voltage sampling period is twice the high-frequency resonance frequency, which adopts the frequency of the second notch position in the bode diagram of the zero-order holder.

[0072] The outer ring of the voltage-current double closed-loop control module adopts constant AC voltage control, and the inner ring of the voltage-current double closed-loop control module adopts current control. On the one hand, it can stabilize the grid point voltage, and on the other hand, it can stabilize the grid frequency.

[0073] The outer ring voltage PI control of the voltage-current double closed-loop control module is:

[0074]

[0075] Wherein, H v ( s ) is the transfer function of the voltage controller, k vp Kp is the proportional coefficient of the voltage outer ring controller, k vi Ki is the integral coefficient of the voltage outer ring controller.

[0076] The inner ring current PI control of the voltage-current double closed-loop control module is:

[0077]

[0078] Wherein, H i ( s ) is the transfer function of the current controller, k ip Kp is the proportional coefficient of the current inner ring controller, k ii Ki is the integral coefficient of the current inner ring controller.

[0079] The impedance transfer function of the converter is:

[0080]

[0081] Wherein, Z is the impedance of the converter, Z0 represents the impedance transfer function of the converter when the feed-forward voltage long-period sampling module is not added to the damping control system, Z1 represents the transfer function of the feed-forward voltage long-period sampling module;

[0082] R is the bridge arm parasitic resistance, and L is the bridge arm inductance, k ip Kp is the proportional coefficient of the current inner ring controller, k iiis an integral coefficient of the current inner loop controller, k vp is a proportional coefficient of the voltage outer loop controller, k vi is an integral coefficient of the voltage outer loop controller, represents a signal transmission time delay transfer function of the damping control system, e represents a natural constant, T represents a feed-forward voltage sampling period.

[0083] Since the AC voltage outer loop bandwidth in the voltage-current double closed loop module is much smaller than the current inner loop, the voltage outer loop has little effect on the high frequency band (>700Hz) impedance characteristics of the MMC converter. Here, the effect of the voltage outer loop on the converter impedance model is ignored, and only the current inner loop, control link time delay and voltage feed-forward branch are considered.

[0084] Embodiment two

[0085] Referring to Figure 1 and Figure 9 , an MMC AC grid-connected system uses an active damping control system for suppressing high-frequency resonance as described in embodiment one.

[0086] The MMC converter is composed of a plurality of sub-modules with the same structure connected in series, each of which has its own voltage source and switching device. These sub-modules are connected through a specific topology, which can generate multiple levels of voltage waveform, thereby reducing pulsation and harmonics, and improving the efficiency and stability of power transmission. The working principle of the MMC converter is based on the switching control of power electronic devices, which adjusts the output voltage and current by adjusting the state of the switching device, realizing flexible control and optimal scheduling of the power grid. The MMC converter AC grid-connected system usually includes the MMC converter itself, the control system, the protection system, and the connection part with the AC power grid. Among them, the MMC converter is the core part of the system, responsible for converting DC power into AC power; the control system is responsible for monitoring and controlling the operating state of the MMC converter, ensuring its stable operation and meeting the requirements of the power grid; the protection system is used to protect the safety of the MMC converter and the power grid in the case of failure; the connection part with the AC power grid includes transformers, filters and other devices, which are used to match the voltage and current levels of the power grid and reduce harmonic pollution. Using an active damping control system for suppressing high-frequency resonance can improve the stability of the MMC AC grid-connected system, suppress high-frequency resonance, optimize the sampling period, enhance the adaptability of the system and improve the energy utilization efficiency.

[0087] Embodiment three

[0088] The embodiment provides an active damping control system for inhibiting high-frequency resonance, relates to a control system composed of an MMC converter main control loop and a feedforward voltage loop, and mainly comprises the following modules: a voltage and current sampling module 1, a voltage and current double closed-loop control module 2, a feedforward voltage long-period sampling module 3, a voltage feedforward control module 4, an adder 5 and an NLC modulation and capacitor voltage equalization module 6. Wherein, the measured PCC point voltage and current are input signals of the voltage and current sampling module 1 and the feedforward voltage long-period sampling module 3, the output signal of the voltage feedforward control module 4 is added with the output signal of the voltage and current double closed-loop control module 2 in the adder 5, and finally the trigger signal is generated to control the switching device to be turned on or off through the NLC modulation & capacitor voltage equalization module 6.

[0089] The double closed-loop voltage and current sampling module (the sampling period is T s ) can collect the PCC point voltage and current and input into the voltage and current double closed-loop controller for voltage and current double closed-loop control; the voltage long-period sampling module collects the voltage signal at a higher sampling period , and the voltage feedforward module is added with the voltage and current double closed-loop control output signal to become the output modulation signal, and finally the switching device driving signal is output through the nearest level approximation modulation (NLC) and the capacitor voltage equalization control module.

[0090] The converter voltage and current double closed-loop control loop and the voltage feedforward control loop both have time delay, which is caused by signal sampling, signal transmission of the control link and NLC and sub-module voltage equalization algorithm running delay; the time delay of the voltage feedforward channel is also the direct reason for the inductive-negative resistance characteristic of the converter system at the high frequency band, that is, the fundamental reason for the high-frequency resonance problem.

[0091] The feedforward voltage long-period sampling module has a parameter, which is the sampling period T , and directly affects the damping effect, system dynamic characteristics and bandwidth. If the sampling period is too small, the damping effect cannot be achieved and the system cannot be inhibited from high-frequency resonance; if the sampling period is too large, the voltage feedforward branch bandwidth is reduced and the system dynamic characteristics are deteriorated.

[0092] The feedforward voltage sampling module is equivalent to the effect of a zero-order holder, as Figure 3 shown, has the effects of low pass and band stop, and has a series of notch points in the bode diagram of the zero-order holder, which are all integer multiples of the sampling frequency.

[0093] Considering the system dynamic characteristics, resonance damping effect and guaranteeing the integrity of the feedforward voltage signal and the double closed-loop main control signal without conflict, the notch points of the zero-order holder are selected for resonance damping.

[0094] The voltage and current double closed loop controller adopts a fixed alternating voltage outer loop controller and a current inner loop controller, which can stabilize the grid point voltage and stabilize the grid frequency.

[0095] In some specific examples of the utility model, taking the MMC converter grid-connected system shown in the figure as an example, the voltage and current sampling module is periodical Figure 1 The voltage and current sampling module is periodical T s The voltage and current signal is collected, and a first output signal is generated through a voltage and current double closed loop control module; the voltage long period sampling module is periodical T The voltage signal is collected, and a second output signal is generated through a voltage feedforward module; the first output signal and the second output signal are added through an adder to obtain a modulation wave, and the modulation wave is input into an NLC modulation and capacitor voltage sharing module to generate a driving signal to drive the converter power device.

[0096] In combination with Figure 1 , a design example of the voltage feedforward long period sampling module is given. When the feedforward voltage long period sampling module is not added to the system, the impedance transfer function of the MMC is:

[0097]

[0098] Among them is the current inner loop PI controller transfer function, is the control system delay transfer function.

[0099] The feedforward voltage long period sampling module transfer function is represented as a zero-order holder transfer function:

[0100]

[0101] Among them is the sampling period, and the impedance transfer function of the MMC after adding the sampling module is:

[0102]

[0103] Because the alternating voltage outer loop bandwidth in the voltage and current double closed loop module is much smaller than the current inner loop, the voltage outer loop has little effect on the high frequency band (>700Hz) impedance characteristics of the MMC converter, and the effect of the voltage outer loop on the converter impedance model is ignored here, and only the current inner loop, the control link time delay and the voltage feedforward branch proposed in the embodiment are considered.

[0104] In the utility model, in order to realize the active damping of the long period sampling module and guarantee the best damping effect, the second notch position of the zero-order holder is set as the resonance frequency f z Under the design, the utility model can maximize the suppression of the resonance frequencyf 1, f 2) target interval possible high frequency resonance, wherein f 1, f 2) / 2= f z As Figure 3 shown, that is, the trap frequency of the point position is selected as the resonance frequency, that is, the relationship between the feedforward voltage sampling period and the high frequency resonance frequency satisfies: f z =2 / T .

[0105] At the same time, the sampling period of the feedforward voltage sampling module and the sampling period of the voltage and current double closed loop sampling module must be ensured to be an integer multiple;

[0106] Specifically, the integer multiple relationship between the sampling period of the damping control loop and the sampling period of the voltage and current loop is ensured, and the signal is complete and not lost. The damping controller parameter T needs to satisfy:

[0107]

[0108] Wherein, n is a positive integer;

[0109] The damping controller parameter T needs to consider the damping effect of the high frequency resonance of the new energy converter station, that is, to ensure that the new energy converter station does not occur high frequency resonance phenomenon in the frequency band , and the parameter T is designed as:

[0110]

[0111] Figure 4 The MMC converter AC side high frequency band impedance characteristic curve under different sampling periods of the feedforward voltage sampling module of the utility model; according to the method given by the utility model, the amplitude-frequency characteristic of the MMC converter after designing the feedforward voltage sampling module can be seen. The designed MMC control system hardly has inductive-negative resistance frequency band, and the risk of high frequency resonance with capacitive line is greatly reduced.

[0112] The utility model verifies through a specific example, taking the MMC converter grid-connected system shown in Figure 1 and Figure 9 as an example, the effectiveness of the active damping control system for suppressing high frequency resonance and the rationality of parameter selection proposed in the embodiment are verified. In the example, time delay links are added in the voltage feedforward module and the voltage and current double closed loop control module to simulate signal time delay in the system, and different resonance frequency high frequency oscillation is simulated by changing the AC side line parameters.

[0113] According to the high frequency resonance occurring in the actual HVDC project at present, the resonance frequency is almost distributed in the frequency band of 1000-2000 Hz. The example aims to improve the impedance characteristics in this frequency band and suppress the occurrence of resonance in this frequency band. According to the parameter design in the embodiment, the voltage feedforward sampling period T in this example is 1.32 ms.

[0114] The following two high frequency resonance working conditions are analyzed: (1) f z = 1900 Hz; (2) f z = 1250 Hz.

[0115] In the first case, as shown in FIG. 6, the system runs stably before 0.5 s. At the moment of 0.5 s, the delay module is put into operation, and the system starts to produce high frequency resonance near 1250 Hz. At 0.7 s, the virtual damping feedforward control is added, and the system recovers to be stable, which verifies the effectiveness of the damping control method. Figure 5

[0116] In the second case, as shown in FIG. 7, the alternating voltage oscillates at a frequency of Figure 6 z = 1900 Hz. At 0.7 s, the damping control is added, and the system recovers to be stable, which verifies the effectiveness of the virtual damping method proposed in the embodiment. f

[0117] Figure 7 , Figure 8 The fast Fourier decomposition harmonic spectrum of the alternating voltage under the two high frequency resonances is given.

[0118] Table 1 Main parameters of the implementation cases

[0119]

[0120] Table 2 RLC parameters table of the two cases

[0121]

[0122] Many embodiments and many applications besides those provided in the foregoing description will occur to those skilled in the art upon reading the previous description. Accordingly, the present teachings should not be understood as being limited to the embodiments described herein and illustrated in the drawings, but rather can be embodied in a variety of ways as apparent to those skilled in the art. The scope of the claims should not be determined by the preferred implementation but should be given a full scope consistent with the full scope and spirit of the disclosure as defined by the claims and their equivalents. For purposes of completeness, all articles and references including patents and patent documents are incorporated herein by reference in their entirety. The omission of any aspect of the subject matter disclosed herein from any claim does not preclude that aspect from being claimed in any other claim that broadens the claims beyond the prior art.

[0123] ​​The above is a further detailed description of the present application, which cannot be deemed as limiting the specific embodiments of the present application to the above. For those skilled in the art to which the present application belongs, without departing from the concept of the present application, some simple deductions or substitutions can be made, which shall be deemed as falling within the protection scope of the present application as defined by the submitted claims.

Claims

1. An active damping control system for suppressing high frequency resonance, characterized by, include: Voltage and current sampling module, voltage and current dual closed-loop control module, feedforward voltage long-cycle sampling module, voltage feedforward control module, adder and NLC modulation and capacitor voltage equalization module; The input terminal of the voltage and current sampling module is connected to the voltage and current at the PCC point of the power grid, and the output terminal of the voltage and current sampling module is connected to the input terminal of the voltage and current dual closed-loop control module. The input terminal of the feedforward voltage long-cycle sampling module is connected to the voltage at the PCC point of the power grid, and the output terminal of the feedforward voltage long-cycle sampling module is connected to the input terminal of the voltage feedforward control module. The output terminal of the voltage feedforward control module and the output terminal of the voltage and current dual closed-loop control module are connected to the input terminal of the adder. The output terminal of the adder is connected to the input terminal of the NLC modulation and capacitor voltage equalization module, and the output terminal of the NLC modulation and capacitor voltage equalization module is connected to the switching device of the converter.

2. An active damping control system to suppress high frequency resonance as recited in claim 1, wherein, After the d-axis voltage of the power grid PCC point is negatively fed back to the d-axis reference voltage, it is input to the outer loop of the voltage and current dual closed-loop control module. The outer loop of the voltage and current dual closed-loop control module outputs the d-axis reference current. After the d-axis current of the power grid PCC point is negatively fed back to the d-axis reference current, it is input to the inner loop of the voltage and current dual closed-loop control module. After the q-axis voltage of the power grid PCC point is negatively fed back to the q-axis reference voltage, it is input to the outer loop of the voltage and current dual closed-loop control module. The output of the outer loop of the voltage and current dual closed-loop control module is the q-axis reference current. After the q-axis current of the power grid PCC point is negatively fed back to the q-axis reference current, it is input to the inner loop of the voltage and current dual closed-loop control module. After the q-axis current decoupling of the power grid PCC point is positively fed back to the output of the inner loop of the voltage and current dual closed-loop control module, it is connected to the d-axis voltage positive feedback through the voltage feedforward control module. After the d-axis current decoupling of the power grid PCC point is negatively fed back to the output of the inner loop of the voltage and current dual closed-loop control module, it is connected to the q-axis voltage positive feedback through the voltage feedforward control module.

3. The active damping control system of claim 1, wherein, The voltage feedforward control module is configured with the following feedforward voltage sampling period: wherein, T represents a feed-forward voltage sampling period, T s represents a voltage-current double closed-loop sampling period, f 1, f 2) represents a target frequency band in which high-frequency resonance is estimated to occur in the new energy converter station.

4. The active damping control system of claim 1, wherein, The feedforward voltage long-period sampling module is equivalent to a zero-order hold, and the transfer function of the zero-order hold is: wherein, G ZOH s represents a zero-order hold transfer function, e -sT represents a signal transmission time delay transfer function, T represents a feedforward voltage sampling period.​ 5. An active damping control system to suppress high frequency resonance as defined in claim 4, wherein, The sampling period of the feedforward voltage is twice the high-frequency resonant frequency, and the high-frequency resonant frequency is the frequency of the second notch position in the Bode plot of the zero-order hold.

6. The active damping control system to suppress high frequency resonance of claim 1, wherein, The outer loop of the voltage and current dual closed-loop control module adopts constant AC voltage control, and the inner loop of the voltage and current dual closed-loop control module adopts current control.

7. An active damping control system to suppress high frequency resonance as defined in claim 6, wherein, The outer loop voltage PI control of the voltage-current dual closed-loop control module is as follows: wherein H v ( s ) is a voltage controller transfer function, k vp Kv is a proportional coefficient of the voltage outer loop controller, k vi Ki is an integral coefficient of the voltage outer loop controller.

8. An active damping control system to suppress high frequency resonance as defined in claim 6, wherein, The inner loop current PI control of the voltage and current dual closed-loop control module is as follows: wherein, H i s is a current controller transfer function, k ip is a proportional coefficient of the current inner loop controller, k ii is an integral coefficient of the current inner loop controller.​ 9. The active damping control system to suppress high frequency resonance of claim 1, wherein, The impedance transfer function of the converter is: Wherein, Z is the impedance of the converter, Z represents the impedance transfer function of the converter when the damping control system does not add the long-period sampling module of the feed-forward voltage, Z represents the transfer function of the long-period sampling module of the feed-forward voltage. R is the bridge arm parasitic resistance, L is the bridge arm inductance, k ip is the proportional coefficient of the current inner loop controller, k ii is the integral coefficient of the current inner loop controller, k vp is the proportional coefficient of the voltage outer loop controller, k vi is the integral coefficient of the voltage outer loop controller, represents the signal transmission time delay transfer function of the damping control system, e represents the natural constant, T represents the feedforward voltage sampling period.

10. A MMC AC grid-connected system, characterized in that, Use an active damping control system for suppressing high-frequency resonance as described in any one of claims 1-9.

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