Reactive compensation control method for network-forming type energy storage converter under three-phase imbalance working condition

Abstract

The invention relates to a reactive compensation control method for a network-forming type energy storage converter under a three-phase unbalanced working condition, and belongs to the technical field of power electronics. The method comprises a battery energy storage module, an NPC type three-level converter module and an energy storage converter control module. The DC-side voltage control module maintains the DC-side voltage constant, the acquisition module acquires the network-side voltage and current in real time and converts the network-side voltage and current into two-phase static coordinate system components, and active power P and reactive power Q are obtained through the positive and negative sequence separation module and the power calculation module. The network construction type control module generates a three-phase voltage vector through a VSG active-frequency control loop and a VSG reactive-voltage control loop, and the negative sequence current control module aims at eliminating negative sequence current and finally generates a control signal of a converter switching device. According to the invention, the reactive compensation function is integrated into the grid-forming type energy storage converter, the problems of current imbalance and low power factor under the three-phase imbalance working condition are effectively solved, and the stability of a power grid and the adaptability to renewable energy sources are improved. A simulation result shows that the method can realize three-phase current balance, the power factor is close to 1, and the robustness of a power grid and the comprehensive performance of an energy storage system are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a control method related to an energy storage converter, specifically a reactive power compensation control method for a grid-forming energy storage converter under three-phase unbalanced conditions. Technical Background

[0002] With the increasing proportion of renewable energy, the stability and quality of the power grid face increasing challenges. Especially in the case of high-penetration wind and solar power generation access, the volatility of the power grid frequency and voltage increases. Therefore, how to ensure the stability and quality of the power grid without relying on traditional rotating mechanical equipment has become an important direction in power system research.

[0003] Under this background, the virtual synchronous generator (VSG) control strategy emerged. By mimicking the characteristics of synchronous generators, the energy storage system can participate in the frequency regulation and stability of the power grid like traditional generators. The VSG-controlled energy storage converter can not only provide frequency response but also improve the adaptability of the power grid to renewable energy fluctuations through rapid power regulation. However, with the application and popularization of VSG technology, only having frequency regulation ability is not enough. In actual power grid operation, the power system also needs reactive power support to maintain the stability of the power grid voltage. Therefore, integrating the reactive power compensation function into the VSG-controlled energy storage converter can more comprehensively improve its support ability for the power grid. Summary of the Invention

[0004] Aiming at the deficiencies existing in the above-mentioned prior art, the present invention proposes a reactive power compensation control method for a grid-forming energy storage converter under three-phase unbalanced conditions, which can realize multiple regulations of frequency, voltage, and power quality in the power grid, and significantly improve the robustness of the power grid and its adaptability to renewable energy access.

[0005] The present invention is realized through the following technical solutions:

[0006] A reactive power compensation control method for a grid-forming energy storage converter under three-phase unbalanced conditions, characterized by comprising:

[0007] A battery energy storage module, an NPC-type three-level converter module, and an energy storage converter control module.

[0008] The battery energy storage module is used to support the converter to realize energy exchange with the power grid, including functions such as active power control, reactive power control, voltage regulation, and frequency support.

[0009] The NPC-type three-level converter module plays a role as a bridge to realize energy interaction between the battery energy storage module and the grid side, and has the functions of inversion and rectification.

[0010] The energy storage converter control module, which includes DC-side voltage control, grid-forming active and reactive power control, and negative-sequence current control, can solve the problems of unbalanced output current and low power factor of traditional grid-forming energy storage converters under three-phase unbalanced conditions, and significantly improve the active support ability of the energy storage converter.

[0011] A reactive power compensation control method for a grid-forming energy storage converter under three-phase unbalanced conditions, characterized by including:

[0012] 1) A DC-side voltage control module that constructs a control loop with the goal of controlling the output DC-side voltage of the battery energy storage module to be constant.

[0013] 2) A collection module that real-time collects the grid-side voltage u gabc of the converter and the current i abc at the outlet of the converter, and after performing Clark transformation on them, obtains the components on the α-axis and β-axis in the two-phase stationary coordinate system, which are respectively represented as u ɑ , u β and i ɑ , i β ;

[0014] 3) A positive and negative sequence separation module and a power calculation module that respectively perform positive and negative sequence separation on u ɑ , u β and i ɑ , i β to obtain the positive sequence voltage components u ɑp , u βp the negative sequence voltage components u ɑn , u βn and the positive sequence current components i ɑp , i βp the negative sequence current components i ɑn , i βn , and then send them to the power calculation link to obtain the active output power P and reactive power Q of the converter;

[0015] 4) A grid-forming control module that constructs control loops for the grid-forming active control loop and the reactive control loop;

[0016] 5) A negative-sequence current control module that constructs a control loop with the negative-sequence current as the control target under a power grid with unbalanced voltage amplitude with reactive load, adjusts the output voltage signal and the voltage signal generated in step 3 to generate the final voltage signal, and then generates the control signal of the converter switching device through the modulation link.

[0017] The described battery energy storage module is composed of an energy storage battery and a bidirectional DC / DC circuit. After that, it outputs the DC side voltage as the input of the converter. By adjusting the flow direction of the given active power, the converter can switch between the inverter and rectifier working stages, enabling the energy storage battery to discharge and charge. Throughout the process, the DC side voltage is always maintained constant through the control loop.

[0018] In the described DC side voltage control module, the difference between the expected value U dcref of the DC side voltage and the actual sampled value U dc is used as the input of the voltage outer loop. After passing through the PI controller, it outputs the current signal i ref . Then, after taking the difference with the actual sampled current i output by the energy storage battery, it is used as the input of the current inner loop. After passing through the PI controller, it outputs the voltage signal. Finally, through the PWM modulator, the control signals s1 and s2 of the DC / DC switching device are generated to maintain the constancy of the DC side voltage.

[0019] In the described positive and negative sequence separation module and power calculation module, the u ɑ , u β and i ɑ , i β obtained from the acquisition module are subjected to the Hilbert transform, that is, u ɑ , u β and i ɑ , i β are separated into positive and negative sequences to obtain the positive sequence components u ɑβp , i ɑβp of the voltage and current and the negative sequence components u ɑβn , i ɑβn of the voltage and current. Then, they are sent to the power calculation link to obtain the active output power P and reactive power Q of the converter.

[0020] In the described grid-forming control module, the real-time value P of the active power obtained is sent to the VSG active-frequency control loop to obtain the angular frequency ω, and then the power angle δ is further obtained. The real-time value Q of the active power obtained in step 3 is sent to the VSG reactive-voltage control loop to obtain the voltage amplitude E m . Further, after combining the two, the three-phase voltage vector e abc output by the converter is obtained. After passing through the Clark transform, it outputs e αp , e βp .

[0021] In the described negative sequence current control module, for a power grid with an unbalanced voltage amplitude with reactive load, aiming to control the grid output current to be three-phase balanced and the energy storage converter to be able to supply reactive power to the grid, it is necessary to control the negative sequence current output by the grid to be 0 and make the grid voltage and current in the same phase; the negative sequence current component i ɑn, i βn After subtracting from the expected values respectively, they are used as the input terminals of the PR regulator, and the voltage components U αn and U βn of the α and β axes are output respectively. Then, they are added to e αp and e βp obtained in step 4. After passing through the Clark inverse transformation, the final voltage signal e ref is generated. Then, through the SVPWM modulation link, the control signals e ma , e mb , and e mc of the converter switching devices are generated.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] Compared with the prior art, this method of the present invention has higher accuracy and flexibility. Integrating the reactive power compensation function into the grid-forming energy storage converter can compensate reactive power in real time, reduce the grid voltage fluctuation, and improve the grid stability and the comprehensive performance of the energy storage system. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the following description of the embodiments in conjunction with the accompanying drawings, where:

[0025] Figure 1 is the topology diagram of the energy storage converter system of the present invention;

[0026] Figure 2 is the overall control block diagram of the present invention;

[0027] Figure 3 is the control block diagram for maintaining the DC side voltage constant of the present invention;

[0028] Figure 4 is the control block diagram for separating positive and negative sequence components of the present invention;

[0029] Figure 5 is the control block diagram of the VSG active control loop and reactive power control loop of the present invention;

[0030] Figure 6 is the simulation waveform diagram of the DC side voltage;

[0031] Figure 7 is the simulation waveform diagram of the single-phase grid voltage and current;

[0032] Figure 8 is the simulation waveform diagram of the three-phase grid current. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0034] Embodiment 1

[0035] As Figure 1 shown, the topology diagram of the grid-connected system of the energy storage converter of the present invention specifically includes:

[0036] The energy storage battery outputs the DC-side voltage U dc after passing through the DC / DC converter. The DC voltage U dc is then connected to the three-phase voltage amplitude unbalanced grid with reactive load after passing through the NPC three-level converter and then through the LC filtering link. The set working condition is: the voltage amplitude of phase A of the grid drops by 150V and the load absorbs inductive reactive power.

[0037] DC-side voltage control module: Construct a control loop with the goal of controlling the output DC-side voltage of the battery energy storage module to be constant.

[0038] Specifically, after subtracting the DC-side voltage expected value U dcref = 750V from the actual sampled value U dc , the result is used as the input of the voltage outer loop. After passing through the PI controller, the current signal i ref is output. Then, after subtracting it from the actual sampled current i output by the energy storage battery, the result is used as the input of the current inner loop. After passing through the PI controller, the voltage signal is output. Finally, the control signals s1 and s2 of the DC / DC switching device are generated through the PWM modulator to maintain the DC-side voltage constant. The specific control block diagram for maintaining the DC-side voltage constant and the simulation waveform diagram of the DC-side voltage are respectively as shown in Figure 3 and Figure 6 .

[0039] Acquisition module: Real-time collect the grid-side voltage u gabc of the converter and the current i abc at the outlet of the converter. After performing the Clark transformation on them, the components on the α-axis and β-axis in the two-phase stationary coordinate system are obtained, which are respectively represented as u ɑ , u β and i ɑ , iβ;

[0040] Positive and negative sequence separation module and power calculation module: Respectively separate the positive and negative sequences of u ɑ , u β and i ɑ , i β to obtain the positive sequence component u ɑp , u βp of the voltage, the negative sequence component uɑn , u βn and the positive sequence component i of the current ɑp , i βp negative sequence component i of the current ɑn , i βn , and then send them to the power calculation link to obtain the active output power P and reactive power Q of the converter;

[0041] Specifically, for u ɑ , u β and i ɑ , i β after Hilbert transform, that is, u ɑ , u β and i ɑ , i β are separated into positive and negative sequences to obtain the positive sequence components u ɑβp , i ɑβp of voltage and current and the negative sequence components u ɑβn , i ɑβn of voltage and current, and then send them to the power calculation link to obtain the active output power P and reactive power Q of the converter;

[0042] Furthermore, Figure 4 a control block diagram for separating the positive and negative sequence components of the grid current is given, where the method of separating the positive and negative sequence components is the Hilbert transform method. Specifically, the Hilbert transformer converts the three-phase signal into an analytic signal and uses the symmetrical component method to realize the separation of the positive and negative sequence components;

[0043] Furthermore, the expressions of the positive and negative sequence components of the grid current after Hilbert transform are given:

[0044]

[0045] Among them, is the positive sequence component, is the negative sequence component, is the analytic signal on the α, β axes;

[0046] Furthermore, according to the instantaneous power theory, the instantaneous power injected by the VSG into the grid can be expressed as:

[0047]

[0048] Among them, is the positive sequence component, is the negative sequence component, and p, q are the average component and the fluctuating component of the power respectively;

[0049] Grid-forming control module: Construct the control loops of the grid-forming active control loop and the reactive control loop;

[0050] Specifically, the real-time active power value P obtained by the power calculation module is sent to the VSG active power-frequency control loop to obtain the angular frequency ω, and then the power angle δ is further obtained. The real-time reactive power value Q obtained by the power calculation module is sent to the VSG reactive power-voltage control loop to obtain the voltage amplitude E m , and further, after combining the two, the three-phase voltage vector e output by the converter is obtained abc , and then after Clark transformation, e is output αp 、e βp .

[0051] Specifically, Figure 3 a schematic diagram of the grid-forming active power-frequency control loop and reactive power-voltage control loop is given. The grid-forming control is virtual synchronous generator control. Further, expressions of the grid-forming active power-frequency control loop and reactive power-voltage control loop are given

[0052]

[0053] where P m is the mechanical power of the virtual synchronous generator, ω0 is the rated angular frequency output by the converter, U N and U are the system rated voltage and the system output voltage; J and D are the virtual inertia and damping coefficient in the virtual synchronous generator control respectively; k p and k u are the frequency modulation coefficient and voltage regulation coefficient respectively, k i is the first-order inertia coefficient, and θ is the power angle;

[0054] Negative sequence current control module: Under a power grid with unbalanced voltage amplitude with reactive load, a control loop with the negative sequence current as the control target is constructed. The output voltage signal and the voltage signal generated by the positive and negative sequence separation module are adjusted to generate the final voltage signal, and then the control signal of the converter switching device is generated through the modulation link

[0055] Specifically, Figure 2 a topology diagram of the energy storage converter system is given. The set working condition is: the voltage amplitude of phase A of the power grid drops by 150V and it is loaded with an inductive reactive power absorption load. Taking the control of the grid output current to be three-phase balanced and the energy storage converter to be able to supply reactive power to the grid as the goal, it is necessary to control the negative sequence current output by the grid to be 0, and make the grid voltage and current in the same phase; specifically, the negative sequence current components i ɑn 、i βn obtained by the positive and negative sequence separation module are respectively subtracted from the expected value and used as the input of the PR regulator, and the voltage components U αn 、U βn on the α and β axes are respectively output, and then combined with e αp 、e βpAfter addition, the final voltage signal e is generated through the Clark inverse transformation. ref , and then the control signals e ma , e mb , and e mc of the converter switching devices are generated through the SVPWM modulation link.

[0056] Specifically, based on MATLAB / Simulink, Figure 7 and Figure 8 respectively give the simulation waveform diagrams of the single-phase grid voltage and current and the three-phase grid current. It can be seen that the three-phase grid current is basically balanced, and the grid voltage and current are basically in the same phase, with a power factor of 0.9951. Thus, it can be proved that a grid-forming energy storage converter control method with reactive power compensation function proposed by the present invention can integrate the reactive power compensation function into the grid-forming energy storage converter, improving the grid stability and the comprehensive performance of the energy storage system.

[0057] The content not described in detail in this specification belongs to the prior art well-known to those skilled in the art. Finally, it should be noted that the above are only preferred examples of the present invention and are not used to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions recorded in the foregoing embodiments or perform equivalent replacements for some of the technical features. 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 present invention.

Claims

1. A reactive power compensation control method for a network-forming energy storage converter under three-phase unbalanced conditions, characterized in that, It includes: a battery energy storage module, an NPC-type three-level converter module, and an energy storage converter control module. The battery energy storage module is used to support the converter to achieve energy exchange with the power grid, including functions such as active power control, reactive power control, voltage regulation, and frequency support. The NPC-type three-level converter module plays a role as a bridge for realizing energy interaction between the battery energy storage module and the grid side, and has the functions of inversion and rectification. The energy storage converter control module includes DC-side voltage control, grid-forming active and reactive power control, and negative-sequence current control, which can solve the problems of unbalanced output current and low power factor of traditional grid-forming energy storage converters under three-phase unbalanced conditions, and significantly improve the active support ability of the energy storage converter.

2. Reactive power compensation control method for a network-forming energy storage converter under three-phase unbalanced conditions, characterized in that, It includes: 1) A DC-side voltage control module that constructs a control loop with the goal of controlling the DC-side voltage output by the battery energy storage module to be constant. 2) Acquisition module, which acquires the grid-side voltage u of the converter in real time gabc and the current i at the outlet of the converter abc , and after performing Clark transformation on them, the components on the α-axis and β-axis in the two-phase stationary coordinate system are obtained, which are respectively expressed as u ɑ , u β and i ɑ , i β ; 3) Positive and negative sequence separation module and power calculation module respectively separate u ɑ , u β and i ɑ , i β into positive and negative sequences to obtain the positive sequence components of voltage u ɑp , u βp , the negative sequence components of voltage u ɑn , u βn and the positive sequence components of current i ɑp , i βp , the negative sequence components of current i ɑn , i βn . Then, they are sent to the power calculation link to obtain the active output power P and reactive power Q of the converter; 4) A grid-forming control module that constructs control loops for the grid-forming active power control loop and the reactive power control loop. 5) A negative-sequence current control module that constructs a control loop with the negative-sequence current as the control target under a power grid with unbalanced voltage amplitude with reactive load, adjusts the output voltage signal and the voltage signal generated by the positive and negative sequence separation module to generate the final voltage signal, and then generates the control signal of the converter switching device through the modulation link.

3. The reactive power compensation control method of the network-forming energy storage converter under the three-phase unbalanced condition according to claim 1, wherein The battery energy storage module is composed of an energy storage battery and a bidirectional DC / DC circuit, and outputs the DC-side voltage as the input end of the converter. By adjusting the flow direction of the given active power, the converter can switch between the inversion and rectification working stages, realizing the discharge and charge of the energy storage battery. Throughout the process, the DC-side voltage is always maintained constant through the control loop.

4. The reactive power compensation control method for a network-forming energy storage converter under a three-phase unbalanced condition according to claim 2, wherein, In the DC-side voltage control module, The DC-side voltage reference value U dcref and the actual sampled value U dc are subtracted from each other and used as the input of the outer voltage loop. After passing through a PI controller, the current signal i ref is output. Then, after subtracting the actual sampled current i output by the energy storage battery, it is used as the input of the inner current loop. After passing through a PI controller, a voltage signal is output. Finally, control signals s1 and s2 for the DC / DC switching device are generated through a PWM modulator to maintain the constancy of the DC-side voltage.

5. The reactive power compensation control method of the network-forming energy storage converter under the three-phase unbalanced working condition according to claim 2, characterized in that, In the positive and negative sequence separation module and the power calculation module, u obtained from the acquisition module ɑ , u β and i ɑ , i β are subjected to Hilbert transform, that is, u ɑ , u β and i ɑ , i β are separated into positive and negative sequences to obtain the positive sequence components u ɑβp , i ɑβp of voltage and current and the negative sequence components u ɑβn , i ɑβn of voltage and current, and then they are sent to the power calculation link to obtain the active output power P and reactive power Q of the converter.

6. The reactive power compensation control method for a network-forming energy storage converter under three-phase unbalanced conditions according to claim 2, characterized in that, In the described network-forming control module, the real-time value P of the active power obtained by the power calculation module is sent to the VSG active-power - frequency control loop to obtain the angular frequency ω, and then the power angle δ is further obtained. The real-time value Q of the active power obtained by the power calculation module is sent to the VSG reactive-power - voltage control loop to obtain the voltage amplitude E m , and further, after combining the two, the three-phase voltage vector e output by the converter is obtained abc , and after passing through the Clark transformation, e αp , e βp .

7. The reactive power compensation control method of the network-forming energy storage converter under the three-phase unbalanced working condition according to claim 2, wherein, In the negative-sequence current control module, under a power grid with unbalanced voltage amplitude with reactive load, with the goal of controlling the grid output current to be three-phase balanced and the energy storage converter to be able to supply reactive power to the grid, it is necessary to control the negative-sequence current output by the grid to be 0 and make the grid voltage and current in the same phase. After subtracting the negative-sequence current components \(i\) ɑn and \(i\) βn obtained from the positive-negative sequence separation module from their respective expected values, they are used as the inputs of the PR regulator, and the voltage components \(U\) αn and \(U\) βn on the \(\alpha\) and \(\beta\) axes are respectively output. Then, after adding them to \(e\) αp and \(e\) βp obtained from the network-forming control module, the final voltage signal \(e\) ref is generated after the Clark inverse transformation. Then, the control signals \(e\) ma for the converter switching devices are generated through the SVPWM modulation link. and \(e\) mb and \(e\) mc .

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