Voltage imbalance treatment method and system for microgrid black start process under unbalanced load

Abstract

The application provides a voltage imbalance treatment method and system for a micro-grid black start process under an unbalanced load, comprising the following steps: S1, determining that the focus point for treating voltage imbalance of a three-phase three-wire inverter is to eliminate negative sequence voltage; S2, introducing a mathematical model describing a two times power frequency oscillation disturbance signal characteristic generated by the negative sequence voltage into a control loop; and S3, using a proportional integral control method combined with a quasi-resonant controller to suppress the two times power frequency oscillation component in the control loop, so as to realize negative sequence voltage suppression and voltage imbalance treatment. The application analyzes the mechanism of voltage imbalance problems of the three-phase three-wire inverter under various unbalanced loads, and reveals that the focus point for treating voltage imbalance problems is to eliminate negative sequence voltage.

Description

Technical Field

[0001] This invention relates to the field of microgrid technology, which is composed of distributed renewable energy generation systems. Specifically, it relates to a method for managing voltage imbalance during the black start process of a microgrid under unbalanced loads. More specifically, it relates to a method for configuring an energy storage system to assist in achieving reliable black start of a microgrid with unbalanced loads. Background Technology

[0002] In recent years, driven by climate goals such as "dual carbon" (carbon diversification and carbon sequestration), the penetration rate of distributed generation devices for renewable energy, represented by wind and solar power, in the global power system has increased significantly. However, renewable energy output is highly volatile and susceptible to extreme weather and the weak load-bearing capacity of power electronic devices. To achieve rapid system recovery after power outages caused by the instability of microgrids composed of distributed renewable energy generation systems under extreme weather conditions, or to reduce the scale of power outages by constructing islanded microgrids based on distributed power sources in fault-free areas after large-scale grid blackouts, reliable black start of microgrids is of great significance. However, the reliable black start process needs to consider the problem of establishing unbalanced voltage in the three-phase three-wire grid-connected inverter of the energy storage system under unbalanced loads. This may cause instantaneous active and reactive power fluctuations in the converter, increasing the difficulty of system black start and the risk of microgrid recovery failure, thereby increasing system outage losses.

[0003] To address the issue of unbalanced voltage during black start-up in microgrids under unbalanced loads, existing mitigation methods include virtual impedance control and sequence control. Sequence control is more widely used, based on the symmetrical component method. It utilizes a reduced-order resonant regulator to separate the negative-sequence voltage component generated by the unbalanced load, constructing dual closed-loop control loops for positive and negative sequences to control the voltages separately, ultimately eliminating the negative-sequence component and mitigating the voltage imbalance. However, this method suffers from bandwidth limitations in the positive-sequence and negative-sequence voltage separation technique based on the reduced-order resonant regulator, which also increases control complexity to some extent. Furthermore, the dual closed-loop control strategy for sequence voltage and current in the synchronous coordinate system requires eight proportional-integral controllers. Therefore, these methods have limited control performance, high complexity in control architecture and parameter design, and are not conducive to achieving reliable black start-up of microgrids under complex load characteristics in practical engineering.

[0004] In summary, while existing sequential control methods can address the black-start voltage imbalance problem under unbalanced loads, they require complex control architectures and have limited control performance, making them unsuitable for widespread application in practical engineering.

[0005] Zhou Ziyang. Research on Control Strategy of Virtual Synchronous Generator under Unbalanced Load [D]. Yichang: Three Gorges University, 2022. This paper discloses that in order to achieve the goal of "carbon peaking and carbon neutrality," my country is vigorously developing distributed generation technology based on renewable energy. The power system is evolving from a high-inertia, high-damping system dominated by synchronous generators to a low-inertia, underdamped system dominated by power electronic equipment. The power grid will face difficulties in frequency and voltage regulation. Considering the excellent characteristics of traditional synchronous generators, the Virtual Synchronous Generator (VSG) technology is used to simulate the operation of a synchronous generator in terms of external characteristics, effectively improving the stability of the distributed system. This paper proposes an improved VSG control strategy to eliminate the negative sequence voltage component of the VSG output caused by three-phase load imbalance in actual operating conditions and to smooth the second harmonic fluctuation of the output power. Simulations verify the effectiveness of the proposed control strategy. First, the mathematical model of VSG is established and studied. Starting from a synchronous generator model, the inverter's structure is equivalent to that of a synchronous generator to obtain a VSG model. The active power frequency regulation and reactive power voltage regulation characteristics of a synchronous generator are simulated, and the active power-frequency regulation and reactive power-voltage control loops of the VSG are designed, completing the overall control design of the VSG. Next, the operating characteristics of the VSG under unbalanced loads are analyzed. When the three-phase load is unbalanced, the load voltage neutral point shifts, and the presence of negative sequence components causes imbalance in the VSG output voltage and current, resulting in second harmonic fluctuations in output power. Eliminating negative sequence voltage through positive and negative sequence separation technology can effectively suppress the imbalance phenomenon. Three commonly used positive and negative sequence separation methods are analyzed, and comparative simulations verify that the positive and negative sequence separation method based on a reduced order resonant regulator (ROR) can achieve accurate and rapid separation of positive and negative sequence components. Finally, a sequence control strategy for a VSG with an unbalanced load based on an ROR regulator is proposed. This paper utilizes a Resonant ROR regulator to separate the positive and negative sequence of voltage and current. The negative sequence voltage amplitude is used as the feedback quantity for the VSG excitation loop. The VSG voltage and current loops are divided into positive and negative sequence loops. Finally, key parameters of the VSG under load imbalance are designed to balance the grid voltage and smooth power second harmonic fluctuations. The effectiveness of the proposed control strategy is verified by Matlab / Simulink simulation. This paper uses a positive and negative sequence separation method based on a reduced-order resonant regulator to separate the positive and negative sequence components, which suffers from bandwidth limitations and increases the complexity of the overall control architecture. Subsequently, a positive and negative sequence control structure requiring eight proportional-integral controllers is constructed to control the positive and negative sequence voltages separately, thereby eliminating the negative sequence voltage and mitigating voltage imbalance. In practical applications, this method suffers from low bandwidth and complex control architecture and parameter design, lacking widespread applicability.

[0006] To address the black-start voltage imbalance problem in microgrids under unbalanced loads, this invention first analyzes the generation mechanism of unbalanced voltage, identifying the elimination of negative-sequence voltage as the key to mitigation. Subsequently, based on the internal model principle, a mathematical model describing the characteristics of the double-frequency oscillation disturbance signal generated by the negative-sequence voltage is introduced into the control loop. A combination of traditional proportional-integral control and a quasi-resonant controller is used to suppress the double-frequency oscillation component, thereby suppressing the negative-sequence voltage and mitigating the voltage imbalance problem. This invention eliminates the need for consuming control resources to separate the positive and negative sequence voltages of the grid, possessing effectiveness, reliability, ease of use, and significant potential for widespread application. This invention provides a new reference approach for mitigating voltage imbalance during black-start operations in microgrids under unbalanced loads. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method and system for mitigating voltage imbalance during the black start process of a microgrid under unbalanced load.

[0008] A method for mitigating voltage imbalance during the black start process of a microgrid under unbalanced load, provided by the present invention, includes:

[0009] Step S1: The key to addressing voltage imbalance in a three-phase three-wire inverter is eliminating negative sequence voltage;

[0010] Step S2: Introduce a mathematical model into the control loop to describe the characteristics of the double power frequency oscillation disturbance signal generated by the negative sequence voltage;

[0011] Step S3: In the control loop, the proportional-integral control method combined with the quasi-resonant controller method is used to suppress the double power frequency oscillation component, thereby suppressing the negative sequence voltage and addressing the voltage imbalance problem.

[0012] Preferably, step S1 includes: for two types of unbalanced loads, including: unbalanced single-phase loads only and unbalanced phase-to-phase loads containing both symmetrical single-phase loads and unbalanced phase-to-phase loads, the unbalanced voltage at the point of common coupling is divided into positive-sequence, negative-sequence, and zero-sequence components based on the symmetrical component method. The inability to eliminate the zero-sequence component of the phase voltage at the point of common coupling established in a three-phase three-wire inverter is analyzed. Finally, it is clarified that the focus of voltage imbalance management in a three-phase three-wire inverter is to eliminate the negative-sequence voltage.

[0013] Preferably, the mathematical model for the characteristics of the double power frequency oscillation disturbance signal in step S2 includes:

[0014]

[0015] Where, L{cos(2ω N t)} represents cos(2ω) N Laplace transform of the term t); 2ωN represents the oscillating component disturbance signal; s represents the complex variable in the Laplace transform.

[0016] Preferably, step S3 includes: in the control loop, using a PI controller as the main controller to achieve accurate tracking of low-frequency changing reference signals;

[0017] The QR controller is used as an auxiliary controller to suppress the disturbance signal of twice the power frequency oscillation component under voltage imbalance.

[0018] Preferably, the QR controller in step S3 includes:

[0019]

[0020] Among them, K r ω is the gain coefficient of the quasi-resonant controller. c ω is the cutoff frequency of the quasi-resonant controller. r is the resonant frequency; s represents the complex frequency variable.

[0021] A voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load, provided by the present invention, includes:

[0022] Module M1: The key to addressing voltage imbalance in three-phase three-wire inverters is eliminating negative sequence voltage;

[0023] Module M2: Introduces a mathematical model into the control loop to describe the characteristics of the double power frequency oscillation disturbance signal generated by the negative sequence voltage;

[0024] Module M3: In the control loop, the proportional-integral control method combined with the quasi-resonant controller method is used to suppress the double power frequency oscillation component, thereby suppressing negative sequence voltage and addressing voltage imbalance.

[0025] Preferably, module M1 includes: for two types of unbalanced loads, including: unbalanced single-phase loads only and unbalanced phase-to-phase loads containing both symmetrical single-phase loads and unbalanced phase-to-phase loads, the unbalanced voltage at the point of common coupling is divided into positive-sequence, negative-sequence, and zero-sequence components based on the symmetrical component method. The non-elimination of the zero-sequence component of the phase voltage at the point of common coupling established in a three-phase three-wire inverter is analyzed, and finally it is clarified that the focus of voltage imbalance management in a three-phase three-wire inverter is to eliminate the negative-sequence voltage.

[0026] Preferably, the mathematical model for the characteristics of the double power frequency oscillation disturbance signal in module M2 includes:

[0027]

[0028] Where, L{cos(2ω N t)} represents cos(2ω)N Laplace transform of the term t); 2ω N represents the oscillating component disturbance signal; s represents the complex variable in the Laplace transform.

[0029] Preferably, the module M3 includes: in the control loop, a PI controller is used as the main controller to achieve accurate tracking of low-frequency changing reference signals;

[0030] The QR controller is used as an auxiliary controller to suppress the disturbance signal of twice the power frequency oscillation component under voltage imbalance.

[0031] Preferably, the QR controller in module M3 includes:

[0032]

[0033] Among them, K r ω is the gain coefficient of the quasi-resonant controller. c ω is the cutoff frequency of the quasi-resonant controller. r is the resonant frequency; s represents the complex frequency variable.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. This invention analyzes the mechanism of voltage imbalance problem in three-phase three-wire inverters under various unbalanced loads, and reveals that the key to solving the voltage imbalance problem is to eliminate negative sequence voltage;

[0036] 2. Based on the internal model principle, this invention can eliminate negative sequence voltage and manage voltage imbalance by introducing a mathematical model describing the characteristics of disturbance signals into the control loop, thereby eliminating the need to perform positive and negative sequence decomposition. It has simple control architecture and parameter design, as well as sufficient ease of use and promotion value.

[0037] 3. This invention does not require the consumption of control resources to separate the positive and negative sequence of grid voltage, and has the advantages of effectiveness, reliability, ease of use and good promotion value; this invention provides a new reference approach for the treatment of voltage imbalance problem in microgrids during black start under unbalanced load. Attached Figure Description

[0038] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0039] Figures 1a to 1b This is a schematic diagram of the equivalent circuit for two types of unbalanced loads in a microgrid.

[0040] Figure 2 This is a schematic diagram of the control architecture for the self-starting phase of an energy storage system.

[0041] Figures 3a to 3b This is a schematic diagram of the frequency response of a resonant (RC) controller and a quasi-resonant (QRC) controller.

[0042] Figure 4 This is a schematic diagram of a voltage and current dual closed-loop control strategy based on the proportional-integral (PI) principle combined with quasi-resonance.

[0043] Figures 5a to 5d A schematic diagram comparing the simulation waveforms of proportional-integral control and the proposed proportional-integral combined quasi-resonant control in response to unbalanced loads.

[0044] Figures 6a to 6e This is a schematic diagram of key simulation waveforms during the zero-voltage start-up phase of an energy storage system. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0046] Example 1

[0047] This invention provides a method and system for mitigating voltage imbalance during the black start process of a microgrid under unbalanced load. It eliminates the need for control resources to separate the positive and negative sequence voltages of the grid. Starting with an analysis of the unbalanced voltage generation mechanism, it identifies eliminating the negative sequence voltage as the key focus of mitigation. Based on the internal model principle, a mathematical model describing the characteristics of the double-frequency oscillation disturbance signal generated by the negative sequence voltage is introduced into the control loop. A combination of traditional proportional-integral control and a quasi-resonant controller is used to suppress the double-frequency oscillation component, thereby suppressing the negative sequence voltage and mitigating the voltage imbalance problem. This invention eliminates the need for control resources to separate the positive and negative sequence voltages of the grid, possessing effectiveness, reliability, ease of use, and significant potential for wider application. It provides a new reference approach for mitigating voltage imbalance during the black start process of microgrids under unbalanced load.

[0048] The voltage imbalance mitigation method for the microgrid black start process under unbalanced load includes:

[0049] Step 1: The key to addressing voltage imbalance in a three-phase three-wire inverter is eliminating negative sequence voltage;

[0050] This step addresses two types of unbalanced loads, using the symmetrical component method to divide the unbalanced voltage at the point of common coupling into positive-sequence, negative-sequence, and zero-sequence components. It analyzes the inescapability of the zero-sequence component of the phase voltage at the point of common coupling established in a three-phase three-wire inverter, ultimately clarifying that the key to addressing voltage imbalance in a three-phase three-wire inverter is eliminating the negative-sequence voltage.

[0051] Microgrid loads are complex and uncertain. Under unbalanced load conditions, energy storage systems (ESS) will generate unbalanced voltage at the point of common coupling (PCC) during the black start phase, specifically during the zero-load voltage ramp-up process. However, unbalanced voltage conditions can easily generate active and reactive power oscillations at twice the power frequency under traditional control architectures, posing challenges to the reliable operation of grid-connected converters during black start. For example, it can exacerbate DC bus fluctuations in energy storage, photovoltaic, and wind power systems, leading to over / under voltage risks; or it can weaken voltage stability at the PCC, increasing the control burden on grid-connected converters during black start, thus increasing the risk of black start failure and microgrid outage losses. Therefore, it is necessary to first analyze the mechanism of unbalanced voltage generation, explore key points for unbalanced voltage mitigation, and avoid the negative impacts of unbalanced voltage that could lead to black start failure.

[0052] This invention primarily considers the output voltage imbalance characteristics of a three-phase three-wire GSC under the following two types of unbalanced loads. The first type is a load containing only an unbalanced single phase, such as... Figure 1a As shown, Z La Z Lc Z Lc Unbalanced; the second category includes symmetrical single-phase loads and asymmetrical phase-to-phase loads, such as... Figure 1b As shown, Z La Z Lc Z Lc Balanced, but there is a load Z between phases AC. Lac It is worth noting that when Figure 1b China Z La Z Lc Z Lc When unbalanced, its voltage imbalance characteristics are similar to those of the first type of unbalanced load, so it will not be analyzed separately here.

[0053] Unbalanced load type 1: The voltage at the point of common coupling is:

[0054] In the formula, These are the phase voltages of phases A, B, and C at the point of common coupling. The current flowing from the point of common coupling to the stage two microgrid load and the single-phase load current of the microgrid are respectively:

[0055] In the formula, These are the phase currents flowing out from the common junction of phases A, B, and C, respectively. These represent the currents flowing to the load from phases A, B, and C, respectively. The equivalent line impedance of the three phases flowing to the load at the point of common coupling is Z. Line The single-phase load is:

[0056] Z L =[Z La Z Lb Z Lc In the formula, Z La Z Lb Z Lc Let A, B, and C be the single-phase load impedances, respectively. Therefore, the voltage at the point of common coupling can be calculated as follows:

[0057] U P =I P Z Line +I L Z L When a microgrid has only a single-phase load, there is no phase-to-phase path, therefore I P with I L Consistent. Based on the symmetrical component method, the voltage at the point of common coupling can be decomposed into...

[0058] U P =U P(1) +U P(2) +U P(0) In the formula U P(1) U P(2) U P(0) These are the positive-sequence, negative-sequence, and zero-sequence components, respectively. The zero-sequence voltage component is...

[0059] In the formula Let A, B, and C be the zero-sequence components of the phase voltages. Based on the rules for solving zero-sequence components, the zero-sequence components can be derived as follows:

[0060] Assuming a three-phase three-wire GC system can achieve balanced phase voltages at the point of common coupling under unbalanced load, then there is no zero-sequence voltage, meaning the above equation is 0. Since Z... La Z Lc Z Lc Imbalance leads to

[0061] In the formula, The equation above indicates that a zero-sequence current exists under this assumption. However, for a three-phase three-wire inverter, there is no zero-sequence current path, meaning the zero-sequence current cannot exist, thus rendering the above assumption invalid. In other words, under unbalanced loads in a microgrid, the phase voltage at the point of common coupling established by a three-phase three-wire inverter cannot eliminate the zero-sequence voltage to achieve three-phase phase voltage balance. It is worth noting that although the three-phase phase voltages cannot be balanced, the zero-sequence voltage is eliminated when the phase voltages differ. Therefore, if the negative-sequence voltage is eliminated, three-phase line voltage balance under unbalanced loads can still be achieved. The above analysis defines the limitations of the three-phase three-wire inverter's ability to manage voltage imbalances.

[0062] For unbalanced load type two, the following current relationship exists.

[0063] In the formula, Let be the current of the AC phase-to-phase load. Although a new phase-to-phase current path is added to the network, the zero-sequence voltage at the point of common coupling can still be calculated as follows:

[0064] The above equation shows that, unlike the first type of load, under the second type of unbalanced load, the phase voltage at the point of common coupling established by the three-phase three-wire inverter does not contain a zero-sequence component, and its negative-sequence component can be eliminated to balance it.

[0065] In summary, for the point of common coupling voltage established by a three-phase three-wire inverter, under the first type of unbalanced load, eliminating the negative sequence voltage component can alleviate phase voltage imbalance and achieve line voltage balance; while under the second type of unbalanced load, eliminating the negative sequence voltage component can simultaneously achieve phase voltage and line voltage balance. This reveals the capability of three-phase three-wire inverters to manage voltage imbalance and establishes that the key to managing black-start voltage imbalance under unbalanced loads is the elimination of negative sequence voltage.

[0066] Step 2: Quasi-resonant control method based on internal mode principle;

[0067] The equivalent architecture of the self-starting process of the energy storage system of a three-phase three-wire inverter is as follows: Figure 2 As shown. The energy storage system uses an LC filter to filter out switching frequency interference before being connected to the point of common coupling. It is worth noting that the step-up transformer primarily affects the zero-voltage start-up time setting, so its connection is omitted here. For the control architecture, a dual-loop control strategy of voltage and current is adopted, where the reference value of the outer voltage loop is obtained through an improved V / f control based on zero-voltage start-up.

[0068] Considering the existence of negative sequence voltage in a three-phase three-wire inverter under unbalanced load and its elimination method, it can be seen that the voltage at the point of common coupling, after Clarke and Park transformations, is the voltage v in the dq coordinate system.pd v pq There will be twice the power frequency 2ω N Oscillating component. Therefore, the goal of addressing voltage imbalance can be converted into eliminating the double-frequency oscillation component of the voltage at the point of common coupling in the dq coordinate system. According to the internal model principle, for a closed-loop control system, a mathematical model describing the dynamic characteristics of the command signal, input signal, and disturbance signal, i.e., the internal model, needs to be included in the feedback control loop. Under symmetrical load operating conditions, v pd v pq Since there is no double-frequency oscillation component, the control system only needs to achieve zero steady-state error tracking of the step signal. The transfer function of the proportional-integral (PI) controller includes the Laplace transform (1 / s) of the step signal, thus achieving zero steady-state error tracking. Similarly, under asymmetrical load conditions, to achieve zero steady-state error tracking of 2ω... N Suppressing oscillating disturbance signals requires the controller to include a mathematical model for a double-frequency oscillation component, i.e., cos(2ω). N Laplace transform of term t)

[0069] Therefore, a resonant controller (RC) can be considered, with the following transfer function:

[0070] Where K r ω is the gain coefficient of the resonant controller. r The resonant frequency is K. The frequency response of the resonant controller varies with K. r The changing curve is as follows Figure 3a As shown, where ω r Set to 2ω N That is, (2π×100) rad / s, it can be seen that the resonant controller has a high gain at a frequency of 100Hz, and when K r As the frequency increases, the amplitude-frequency response curve will shift upwards while the phase-frequency response curve remains unchanged. However, the grid frequency f... N There is a fluctuation range; applying a resonant controller may cause deviations from f. N The gain of the resonant controller drops sharply, causing it to lose its regulating capability. Therefore, consider using... Figure 3b The quasi-resonant controller (QRC) with the frequency characteristics shown has the following transfer function:

[0071] In the formula, ω c This is the cutoff frequency of the quasi-resonant controller. A comparison of the frequency response curves shows that the gain decay of the quasi-resonant controller is relatively slower, allowing it to adapt to a certain frequency offset range. When ω... cAs the frequency increases, the gain decays more slowly, meaning the bandwidth is larger and the gain is also greater. Considering a standard frequency fluctuation range of ±0.5%, and taking a certain margin into account by selecting a range of 49.7-50.3Hz, then ω c It is (0.6×π) rad / s.

[0072] In summary, to eliminate negative sequence components during black start, a controller is selected based on the internal model principle, combining a PI controller that tracks the step signal and suppresses 2ω. N A QR controller for oscillating component disturbance signals can obtain... Figure 4 The diagram illustrates a voltage-current dual-loop control strategy combining proportional-integral (PI) and quasi-resonant control in the dq coordinate system. Specifically, the PI controller acts as the main controller, exhibiting high gain in the low-frequency range to accurately track low-frequency changing reference signals; while the QR controller acts as the auxiliary controller, possessing a resonant frequency ω. r The high gain characteristic in the vicinity is used to suppress disturbance signals of oscillation components at twice the power frequency (i.e., the set resonant frequency of the QR controller) under voltage imbalance. Additionally, v refdq Zero-start boost voltage command, v Pdq Let i be the voltage at the point of common coupling. Pdq For the load-side current, i Ldq For the inverter filter inductor current, L f C f These are the filter inductor and capacitor, respectively. The output of the control loop is v. mdq Subsequently, Clarke transform and inverse Park transform are used for SPWM modulation to generate the inverter's switching drive signal. The voltage loop and current loop are respectively controlled by ω... N C f Item and ω N L f The operation of the term implements the dq axis coupling term in the LC filter type inverter, and realizes the dq axis decoupling control.

[0073] The present invention also provides a voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load. The voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load can be implemented by executing the process steps of the voltage imbalance mitigation method for the black start process of a microgrid under unbalanced load. That is, those skilled in the art can understand the voltage imbalance mitigation method for the black start process of a microgrid under unbalanced load as a preferred embodiment of the voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load.

[0074] Example 2

[0075] Example 2 is a preferred example of Example 1.

[0076] Existing sequence control methods require consuming control resources to separate the positive and negative sequence voltages of the power grid to achieve separate control of positive and negative sequence voltages. This invention provides a voltage imbalance mitigation method and system for the black start process of a microgrid under unbalanced load. Based on the internal model principle, it introduces a mathematical model describing the characteristics of the double-frequency oscillation disturbance signal generated by the negative sequence voltage into the control loop. It utilizes a combination of traditional proportional-integral control and a quasi-resonant controller to suppress the double-frequency oscillation component, thereby suppressing the negative sequence voltage and mitigating the voltage imbalance problem.

[0077] To verify the effectiveness of this invention, the voltage imbalance mitigation method proposed in this invention was used to evaluate the operating characteristics of a microgrid energy storage system during the zero-voltage start-up phase. Both the voltage imbalance mitigation method proposed in this invention and the traditional PI control strategy were applied to the simulation model, and the simulation waveforms under unbalanced load are shown in Figure 5. Figure 5a Given the amplitude of the negative sequence voltage, it can be seen that under traditional PI control, the negative sequence voltage is approximately 3V. The proposed PI combined with QR control strategy can eliminate the negative sequence voltage. Voltage imbalance is measured using the IEEE PES (Power Quality Engineering Society) standard document, "IEEE Standard Test Procedures for Multiphase Induction Motors and Generators," which defines voltage imbalance as the percentage of the maximum voltage difference between the effective voltage value and the average three-phase voltage, relative to the average three-phase voltage. The standard strictly defines that the voltage imbalance cannot exceed 0.5%. Figure 5b In the voltage unevenness measurement shown, under proportional-integral control, K u It remains at approximately 0.84%, while under the proposed proportional-integral combined quasi-resonant control strategy, K u K remains at a low level close to zero; after amplification, it can be determined that... u It has consistently remained below 0.01%. Figure 5c , Figure 5d The paper demonstrates the voltage deviation between the actual and reference voltage values ​​in the dq-axis control loop under two control strategies. Under proportional-integral (PI) control, the voltage deviation waveform clearly exhibits an oscillating component at twice the power frequency, confirming the presence of negative-sequence voltage. However, under the PI combined with quasi-resonant control strategy, the twice-power-frequency oscillation component is completely eliminated, with only errors caused by the limited bandwidth of the control loop remaining. In summary, the proposed PI combined with quasi-resonant control strategy effectively suppresses the twice-power-frequency oscillation component in the dq-axis voltage error compared to the traditional PI control strategy, thereby effectively eliminating negative-sequence voltage under unbalanced loads and effectively mitigating voltage imbalance.

[0078] To verify the application of the proposed control strategy in the zero-voltage start-up phase of the energy storage system to address voltage imbalance under unbalanced load, dynamic simulation was performed for this phase. The zero-voltage start-up time was set to 1.5s to avoid overcurrent problems caused by inrush current at the step-up transformer. Figure 6a The positive sequence voltage amplitude waveform shown indicates that the startup process starts at t = 20ms. Under zero-voltage startup V / f control, after a continuous linear rise of 1.5s, the positive sequence voltage amplitude reaches around 311V. Figure 6b The negative sequence voltage amplitude waveform shown indicates that during zero-voltage start-up, the negative sequence voltage under the proportional-integral (PI) combined quasi-resonant control strategy remains at a low level (approximately 0.34V), while the negative sequence voltage under PI control cannot be effectively suppressed. For Figure 6c The unbalance measurement waveforms shown indicate that the unbalance under the proportional-integral (PI) combined quasi-resonant control strategy meets the 0.5% unbalance standard set by the IEEE PES power quality standard for most of the zero-voltage start-up process, while the unbalance under PI control struggles to meet this standard. For Figure 6d , Figure 6e The dynamic waveform of the dq-axis voltage error shown demonstrates that the voltage error under the proportional-integral (PI) combined quasi-resonant control strategy is consistently kept below 1V, with no double-frequency oscillation component. In contrast, the voltage error under PI control exhibits a double-frequency oscillation component during the zero-voltage start-up phase, and the amplitude of this oscillation component continuously increases. In summary, the proposed PI combined quasi-resonant control strategy effectively addresses voltage imbalance caused by load imbalance during the zero-voltage start-up phase of the energy storage system, compared to the traditional PI control strategy. This helps avoid active and reactive power oscillations caused by voltage imbalance, enhances the stability of the energy storage system's self-starting process and its ability to assist other distributed power systems in starting up, and ultimately improves the reliability of the microgrid during black-start processes, reducing losses caused by grid outages.

[0079] Existing methods can also address voltage imbalance, but they require consuming control resources to separate the negative-sequence voltage component generated by the unbalanced load, suffer from bandwidth limitations, and increase the complexity of control architecture and parameter design, thus lacking widespread applicability. The examples above demonstrate that this invention utilizes a combination of traditional proportional-integral control and a quasi-resonant controller to suppress the double-frequency oscillation component, thereby suppressing negative-sequence voltage and addressing voltage imbalance. This invention eliminates the need for consuming control resources to separate the positive and negative sequence of grid voltage, possessing effectiveness, reliability, ease of use, and significant potential for widespread application. It provides a new reference approach for addressing voltage imbalance during black start-up of microgrids under unbalanced loads.

[0080] Those skilled in the art will understand that, besides implementing the system and its various devices, modules, and units provided by this invention in the form of purely computer-readable program code, the same functions can be achieved entirely through logical programming of the method steps, making the system and its various devices, modules, and units of this invention function as logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers. Therefore, the system and its various devices, modules, and units provided by this invention can be considered as a hardware component, and the devices, modules, and units included therein for implementing various functions can also be considered as structures within the hardware component; alternatively, the devices, modules, and units for implementing various functions can be considered as both software modules implementing the method and structures within the hardware component.

[0081] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A method for mitigating voltage imbalance during the black start process of a microgrid under unbalanced load, characterized in that, include: Step S1: The key to addressing voltage imbalance in a three-phase three-wire inverter is eliminating negative sequence voltage; Step S2: Introduce a mathematical model into the control loop to describe the characteristics of the double power frequency oscillation disturbance signal generated by the negative sequence voltage; Step S3: In the control loop, the proportional-integral control method combined with the quasi-resonant controller method is used to suppress the double power frequency oscillation component, thereby suppressing the negative sequence voltage and addressing the voltage imbalance problem.

2. The voltage imbalance mitigation method for the black start process of a microgrid under unbalanced load as described in claim 1, characterized in that, Step S1 includes: for two types of unbalanced loads, including: unbalanced single-phase loads only and unbalanced phase-to-phase loads containing both symmetrical single-phase loads and unbalanced phase-to-phase loads, the unbalanced voltage at the point of common coupling is divided into positive-sequence, negative-sequence, and zero-sequence components based on the symmetrical component method. The inability to eliminate the zero-sequence component of the phase voltage at the point of common coupling established in a three-phase three-wire inverter is analyzed. Finally, it is clarified that the focus of voltage imbalance management in a three-phase three-wire inverter is to eliminate the negative-sequence voltage.

3. The voltage imbalance mitigation method for the black start process of a microgrid under unbalanced load as described in claim 1, characterized in that, The mathematical model for the characteristics of the double power frequency oscillation disturbance signal in step S2 includes: Where, L{cos(2ω N t)} represents cos(2ω) N Laplace transform of the term t); 2ω N represents the oscillating component disturbance signal; s represents the complex variable in the Laplace transform.

4. The method for mitigating voltage imbalance during the black start process of a microgrid under unbalanced load as described in claim 1, characterized in that, Step S3 includes: in the control loop, using the PI controller as the main controller to achieve accurate tracking of the low-frequency changing reference signal; The QR controller is used as an auxiliary controller to suppress the disturbance signal of twice the power frequency oscillation component under voltage imbalance.

5. The voltage imbalance mitigation method for the black start process of a microgrid under unbalanced load as described in claim 4, characterized in that, The QR controller in step S3 includes: Among them, K r ω is the gain coefficient of the quasi-resonant controller. c ω is the cutoff frequency of the quasi-resonant controller. r is the resonant frequency; s represents the complex frequency variable.

6. A voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load, characterized in that, include: Module M1: The key to addressing voltage imbalance in three-phase three-wire inverters is eliminating negative sequence voltage; Module M2: Introduces a mathematical model into the control loop to describe the characteristics of the double power frequency oscillation disturbance signal generated by the negative sequence voltage; Module M3: In the control loop, the proportional-integral control method combined with the quasi-resonant controller method is used to suppress the double power frequency oscillation component, thereby suppressing negative sequence voltage and addressing voltage imbalance.

7. The voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load as described in claim 6, characterized in that, The module M1 includes: for two types of unbalanced loads, including: unbalanced single-phase loads only and unbalanced phase-to-phase loads containing both symmetrical single-phase loads and unbalanced phase-to-phase loads, the unbalanced voltage at the point of common coupling is divided into positive-sequence, negative-sequence, and zero-sequence components based on the symmetrical component method. The non-elimination of the zero-sequence component of the phase voltage at the point of common coupling established in a three-phase three-wire inverter is analyzed, and finally it is clarified that the focus of voltage imbalance management in a three-phase three-wire inverter is to eliminate the negative-sequence voltage.

8. The voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load as described in claim 6, characterized in that, The mathematical model for the characteristics of the double power frequency oscillation disturbance signal in module M2 includes: Where, L{cos(2ω N t)} represents cos(2ω) N Laplace transform of the term t); 2ω N represents the oscillating component disturbance signal; s represents the complex variable in the Laplace transform.

9. The voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load as described in claim 6, characterized in that, The module M3 includes: in the control loop, a PI controller is used as the main controller to achieve accurate tracking of low-frequency changing reference signals; The QR controller is used as an auxiliary controller to suppress the disturbance signal of twice the power frequency oscillation component under voltage imbalance.

10. The voltage imbalance mitigation system for the black start process of a microgrid under unbalanced load as described in claim 9, characterized in that, The QR controller in module M3 includes: Among them, K r ω is the gain coefficient of the quasi-resonant controller. c ω is the cutoff frequency of the quasi-resonant controller. r is the resonant frequency; s represents the complex frequency variable.

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