An ac voltage control method for a high-voltage meshed converter

Abstract

The application discloses an AC voltage control method of a high-voltage grid-forming type converter, and introduces a new type of outer loop voltage control of the converter based on a voltage-current proportional relationship, and forms a high-voltage grid-forming type converter control architecture together with an inner loop current control. The voltage-current proportional outer loop control is not dependent on a parallel capacitive filter on the AC side of the converter, is suitable for a high-voltage large-capacity multi-level converter with high output waveform quality, can cancel the parallel capacitive filter, and effectively reduces the system cost; when a power grid fault occurs, the grid-forming type converter can automatically and quickly enter a positive sequence current limiting and negative sequence current suppression state under the action of the voltage-current proportional control, so that overcurrent blocking and damage of the converter are avoided, and the converter can automatically return to normal operation after the power grid fault is removed.

Description

Technical Field

[0001] This invention relates to the field of converter control technology, and specifically to an AC voltage control method for a high-voltage grid-type converter. Background Technology

[0002] With the rapid growth in demand for DC transmission, flexible AC transmission, distributed energy generation, and energy storage, power electronic converters are becoming increasingly common in power grids. Grid-following converters synchronize their output AC voltage with the AC grid via a phase-locked loop (PLL) and exchange active and reactive power with the grid as instructed. Based on an inner-loop current controller, grid-following converters can rapidly regulate and limit the output AC current. Furthermore, when the grid is operating asymmetrically, a negative-sequence current controller can suppress negative-sequence current and maintain three-phase current balance. Since converters have limited overcurrent tolerance, the rapid current control capability of the inner-loop positive and negative-sequence current controllers can prevent overcurrent blocking and damage during grid disturbances, which is crucial for ensuring the safe operation of the converter. However, grid-following converters rely on AC power supplies and may not operate normally in passive or weak grid conditions.

[0003] In recent years, to compensate for the shortcomings of grid-connected converters and in the trend of developing 100% renewable energy grids, grid-forming converters, which can operate autonomously without relying on the AC grid, have gradually gained attention. The main difference between grid-connected and grid-forming converters lies in their control systems and corresponding operating modes; their hardware structures are the same or similar. A cascaded control architecture combining an inner-loop current controller and an outer-loop voltage controller has become the mainstream control architecture for grid-forming converters. This architecture not only enables autonomous construction of the grid voltage but also retains the inner-loop current controller to prevent overcurrent blocking and damage to the converter. In low-voltage grid-forming converter control, traditional dq outer-loop voltage control is based on the charging and discharging characteristics of the parallel capacitive filter on the AC side of the converter to construct the AC voltage. Currently, the practical application of low-voltage grid-forming converters is gradually increasing. Based on this, negative-sequence component control is introduced, enabling grid-forming converters to have the ability to ride through grid asymmetric faults.

[0004] On the other hand, with the rapid development of modular cascaded multilevel converters in high-voltage, high-capacity converter applications, the output levels of these converters can reach tens or even hundreds, such as flexible DC transmission converters which can reach hundreds of levels. The output AC voltage and current harmonic content are relatively low, meeting grid connection requirements and eliminating the need for AC-side high-voltage parallel filters, thus reducing system costs. Therefore, traditional dq outer-loop voltage control cannot be directly applied to high-voltage grid-type converters without parallel capacitive filters, necessitating the research of novel outer-loop voltage control methods.

[0005] For example, Chinese patent CN113962181A, published on January 21, 2022, discloses a dual-loop control parameter optimization design method for a grid-type voltage source converter. This method includes an inner-loop proportional parameter kc and an outer-loop resonant parameter kr, comprising the following steps: plotting the pole plot of the closed-loop transfer function as a function of the parameters, finding the parameter value corresponding to the dominant pole closest to the origin; plotting a Bode plot, determining whether the parameter value meets a predefined stability margin; if so, the parameter is the optimized control parameter for the control loop; otherwise, the parameter value should be increased or decreased to meet the requirements. This method optimizes the control parameters of the inner and outer loops independently, achieving both desired stability margins and the fastest dynamic performance; it directly utilizes pole plots and Bode plots, eliminating the need for complex formulas; compared to traditional proportional resonant controllers, applying a resonant controller only in the outer loop significantly improves system dynamic performance while reducing the complexity of the control system structure and design. This scheme also uses a parallel capacitive filter, but high-voltage parallel filters are not suitable for the control system of high-voltage grid-type converters due to their high cost. Summary of the Invention

[0006] The technical problem this invention aims to solve is that current high-voltage grid-connected converter control systems suffer from the high cost of high-voltage parallel filters, while traditional dq outer-loop voltage control cannot be directly applied to high-voltage grid-connected converters without parallel capacitive filters. This invention proposes a novel outer-loop voltage controller for high-voltage grid-connected converters, using voltage-current proportional control as the core. This method effectively reduces system costs by eliminating reliance on high-voltage parallel filters on the AC side of the converter.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: an AC voltage control method for a high-voltage grid-type converter, comprising the following steps: using a novel outer-loop voltage controller for outer-loop voltage control: controlling the outer-loop voltage of the high-voltage grid-type converter through voltage-current proportional control.

[0008] This invention introduces voltage-current proportional control as a novel outer-loop voltage controller for high-voltage grid-connected converters. Its voltage control principle does not rely on the high-voltage parallel filter on the AC side of the converter and can form a cascaded control architecture with the inner-loop current controller. Furthermore, the inner-loop positive-sequence current controller can be used to limit the positive-sequence current, and the inner-loop negative-sequence current controller can be used to suppress the negative-sequence current, preventing overcurrent blocking and damage to the converter under grid fault conditions.

[0009] Preferably, the command value of the dq-axis component of the positive-sequence current in the positive-sequence inner-loop current control is determined by the positive-sequence outer-loop voltage control. The calculation formula for the positive-sequence outer-loop voltage control of the converter is:

[0010]

[0011]

[0012] in, and These are the command values ​​for the dq-axis components of the positive-sequence AC voltage at PCC, where PCC is the common coupling point. d + and u q + These are the dq-axis components of the positive-sequence AC voltage at PCC. and These are the command values ​​for the dq-axis components of the positive-sequence AC current output by the converter, i. Ld + and i Lq + These are the dq-axis components of the positive-sequence AC current output by the converter, and z is the proportionality constant.

[0013] Combining positive sequence inner loop current control, the positive sequence outer loop voltage control of the converter based on voltage-current proportional control provides the command value for positive sequence inner loop current control.

[0014] As a preferred method, the converter is protected under grid fault conditions by using an inner-loop positive-sequence current controller to limit the positive-sequence current and an inner-loop negative-sequence current controller to suppress the negative-sequence current.

[0015] Preferably, the process of suppressing negative-sequence current includes: setting all command values ​​of the negative-sequence current dq-axis component in the negative-sequence inner-loop current control to 0, and not adding negative-sequence outer-loop voltage control. For the positive-sequence component, voltage-current proportional control is used to control the positive-sequence dq-axis voltage, and inner-loop current control is used to control the positive-sequence dq-axis current. For the negative-sequence component, inner-loop current control is used to suppress the negative-sequence dq-axis current, and all command values ​​of the negative-sequence dq-axis current are set to 0.

[0016] Preferably, the command value of the dq-axis component of the negative-sequence current in the negative-sequence inner-loop current control is determined by the negative-sequence outer-loop voltage control. The calculation formula for the negative-sequence outer-loop voltage control of the converter is as follows:

[0017]

[0018]

[0019] in, These are the command values ​​for the dq-axis components of the negative sequence AC voltage at PCC, u d - and u q - These are the dq-axis components of the negative sequence AC voltage at PCC. and These are the command values ​​for the dq-axis components of the negative-sequence AC current output by the converter, i. Ld - and i Lq - These are the command values ​​for the dq-axis components of the negative-sequence AC current output by the converter. When the command value of the dq-axis component of the negative-sequence current is not set to 0, a negative-sequence outer-loop voltage control needs to be added. As can be seen from the above formula, the construction of the new outer-loop voltage control does not depend on the parallel capacitive filter and is suitable for high-voltage grid-type converters without parallel capacitive filters.

[0020] Preferably, in a per-unit system, the proportional constant z in the formula for the positive-sequence outer loop voltage control of the converter ranges from 0.2 to 5.

[0021] Preferably, in a per-unit system, the proportional constant z in the negative-sequence outer-loop voltage control calculation formula of the converter ranges from 0.2 to 5. Note that the proportional constant z is the only control parameter for voltage-current proportional control. In a per-unit system, z can generally be selected within the range of 0.5-2 to ensure that the voltage and current proportional relationship are on the same order of magnitude, thus fully utilizing the proportional control effect of voltage and current.

[0022] The substantial effects of the present invention are: (1) The present invention introduces a new type of outer loop voltage control for converters by introducing a voltage-current ratio relationship, which together with the inner loop current control forms a high-voltage grid-forming converter control architecture. The voltage-current ratio outer loop control does not depend on the parallel capacitive filter on the AC side of the converter, and is suitable for high-voltage, high-capacity, multi-level converters with high output waveform quality. The parallel capacitive filter can be eliminated.

[0023] (2) During normal operation and disturbance of the power grid, under the voltage-current proportional control, the AC voltage at the PCC and the AC current of the converter can track the corresponding command value, thus achieving the control target of the grid-type converter.

[0024] (3) In the event of a grid fault, the grid-type converter can automatically and quickly enter the positive sequence current limiting and negative sequence current suppression state under the voltage and current proportional control, so as to avoid overcurrent blocking and damage to the converter, and automatically return to normal operation after the grid fault is cleared.

[0025] (4) The voltage and current proportional outer loop controller can control the AC voltage, realize the positive sequence current limiting and negative sequence current suppression of the converter under the asymmetrical fault of the power grid, and the voltage and current proportional outer loop controller combined with the synchronous machine simulation operation can enable the grid-type converter to simulate the inertial characteristics of the synchronous motor and the primary frequency regulation and secondary frequency regulation functions. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a low-voltage grid-type converter system.

[0027] Figure 2 This is a control block diagram of the grid-type converter in this embodiment;

[0028] Figure 3 This is a schematic diagram of the high-voltage grid-type converter system structure in this embodiment;

[0029] Figure 4 One of the simulation waveform diagrams for the synchronous operation of the converter under a load step.

[0030] Figure 5 The second schematic diagram of the simulation waveforms for the synchronous operation of the converter under a step load.

[0031] Figure 6 One of the simulation waveform diagrams for constant voltage operation of a converter under a single-phase ground fault.

[0032] Figure 7 The second schematic diagram of the simulation waveform of the converter under constant voltage operation under single-phase ground fault;

[0033] Figure 8 One of the simulation waveform diagrams for the synchronous operation of a converter under a single-phase ground fault.

[0034] Figure 9 The second schematic diagram of the simulation waveforms for the synchronous operation of the converter under a single-phase ground fault.

[0035] The components include: 1. Modular multilevel converter, 2. Reactor, 3. Transformer, and 4. Load. Detailed Implementation

[0036] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0037] This embodiment introduces voltage-current proportional control as a novel outer-loop voltage controller for high-voltage grid-connected converters. Its voltage control principle does not rely on the high-voltage parallel filter on the AC side of the converter and can form a cascaded control architecture with the inner-loop current controller. The inner-loop positive-sequence current controller is used to limit the positive-sequence current, and the inner-loop negative-sequence current controller is used to suppress the negative-sequence current, so as to avoid overcurrent blocking and damage to the converter under grid fault conditions.

[0038] 1. Traditional dq vector control

[0039] 1.1 The system structure diagram of a low-voltage grid-type converter that supplies power to a passive network is shown in Figure 1.1. Figure 1 As shown. Where, v abc and i abcThese are the three-phase AC voltage and three-phase AC current output from the converter, u abc This refers to the three-phase AC voltage at the point of common coupling (PCC) of the converter. Low-voltage grid-type converters can use low-cost two-level or three-level converters. Due to the limited number of output levels, high-frequency pulse-width modulation (PWM) is required. To filter out high-order harmonics at the low-voltage converter output, it is generally necessary to use... Figure 1 The LC-type low-pass filter system shown has L and R being the equivalent reactance and equivalent resistance of the commutator reactor, respectively, and C being the equivalent capacitance of the parallel filter. d and I d These are the DC voltage and DC current of the converter, respectively.

[0040] The voltage-current relationship of the converter system can be expressed as:

[0041] L(di abc / dt)=-Ri abc +v abc -u abc (1)

[0042] Through dq coordinate transformation, the positive sequence voltage and current of the converter can be represented by the positive sequence dq axis components of equations (2) and (3), and the negative sequence current and voltage of the converter can be represented by the negative sequence dq axis components of equations (4) and (5).

[0043]

[0044]

[0045]

[0046]

[0047] Where s is the Laplace operator, and v is the rated angular frequency of the power grid. d + v q + and i d + i q + These are the dq-axis components of the positive-sequence AC voltage and positive-sequence AC current output from the converter, respectively. d + u q + These are the dq-axis components of the positive-sequence AC voltage at PCC. d - v q- and i d - i q - These are the dq-axis components of the negative-sequence AC voltage and negative-sequence AC current output from the converter, respectively. d - u q - These are the dq-axis components of the negative sequence AC voltage at PCC.

[0048] A key characteristic of grid-type converter control is the need to control the AC voltage. For example... Figure 2 As shown, traditional dq vector control is a cascaded control structure, where current control serves as the inner loop to regulate the converter output current, and voltage control serves as the outer loop to regulate the converter grid-side voltage. The output of the outer loop voltage control serves as the input current command for the inner loop current control.

[0049] 1.2 Inner Loop Current Control:

[0050] Since converter transformers typically use a Y / Δ connection, when an asymmetrical fault occurs in the AC power grid, the converter transformer can isolate the zero-sequence component on the grid side. Therefore, only the positive-sequence and negative-sequence components can be transmitted to the AC side of the converter. When the power grid is in a normal symmetrical operating state, the three-phase AC voltage and current only have positive-sequence components; when the power grid is in an asymmetrical operating state, the three-phase AC voltage and current will contain both positive-sequence and negative-sequence components. Figure 2 The positive sequence inner loop current control of the converter is represented by equations (6) and (7), and the negative sequence inner loop current control is represented by equations (8) and (9).

[0051]

[0052]

[0053]

[0054]

[0055] in These are the command values ​​for the dq-axis components of the positive-sequence AC current output by the converter, and their values ​​are given by the positive-sequence voltage controller of the converter's outer loop. These are the command values ​​for the dq-axis components of the negative-sequence AC current output by the converter; such as Figure 2 As shown, when a strategy to suppress negative sequence current is adopted, its value is 0; when a negative sequence voltage control strategy is adopted, its value is given by the negative sequence voltage control of the converter's outer loop. p1 and k i1 These are the proportional coefficient and integral coefficient of the inner loop current controller, respectively.

[0056] 1.3 Outer Loop Voltage Control

[0057] Figure 2 The positive sequence outer loop voltage control of the converter is represented by equations (10) and (11); the negative sequence outer loop voltage control is represented by equations (12) and (13).

[0058]

[0059]

[0060]

[0061]

[0062] in These are the command values ​​for the dq-axis components of the positive-sequence AC voltage at PCC. These are the command values ​​for the dq-axis components of the negative sequence AC voltage at PCC. Ld + i Lq + These are the dq-axis components of the positive-sequence AC current output from the grid side of the parallel capacitor bank, i Ld - i Lq - These are the dq-axis components of the negative-sequence AC current output from the parallel capacitor grid side, respectively. p2 and k i2 These are the proportional coefficient and integral coefficient of the inner loop current controller, respectively. Because the traditional outer loop voltage control of a low-voltage grid converter is based on the electrical relationship of the equivalent capacitance of the parallel filter, equations (10) to (13) include the equivalent capacitance C of the parallel capacitive filter.

[0063] 2. Voltage and current proportional control

[0064] 2.1 High-voltage converter system

[0065] Figure 3 This is a system structure diagram of a high-voltage grid-type converter that supplies power to a passive network, including a modular multilevel converter 1, a reactor 2, a transformer 3, and a load 4.

[0066] To achieve higher output voltage levels, high-voltage grid-type converters typically use modular multilevel converters. Increasing the number of output levels reduces the converter's harmonic content and eliminates the need for parallel capacitive filters. On the other hand, the cost of parallel capacitive filters in high-voltage systems is significantly higher than in low-voltage systems; therefore, to improve economic efficiency, parallel capacitive filters are usually not installed in high-voltage systems. L and R represent the total equivalent reactance and total equivalent resistance of the converter and transformer, respectively.

[0067] According to equations (10) to (13), the traditional outer-loop voltage control of low-voltage grid-connected converters requires construction based on the equivalent capacitance of parallel filters, which cannot be used for the outer-loop voltage control of high-voltage grid-connected converters. Therefore, while retaining the inner-loop current control of low-voltage grid-connected converters, the control of high-voltage grid-connected converters needs to construct a new type of outer-loop voltage control that does not rely on parallel capacitive filters, and adjust the current command value of the inner-loop current control to achieve control of the AC voltage on the grid side of the converter.

[0068] 2.2 Novel outer-loop voltage control combined with positive-sequence inner-loop current control. Here, voltage-current proportional control based on the proportional coupling relationship between converter voltage and current is proposed as the outer-loop voltage control of the grid-type converter. Equations (14) and (15) are the positive-sequence outer-loop voltage control of the converter based on voltage-current proportional control, and the command values ​​of the positive-sequence inner-loop current control are given.

[0069]

[0070]

[0071] Similarly, converter negative sequence outer loop voltage control can be constructed as shown in equations (16) and (17).

[0072]

[0073]

[0074] Where z is a proportionality constant. Under voltage-current proportional control, the deviations of the dq-axis components of the converter output positive and negative sequence AC current relative to their command values ​​are proportional to the deviations of the dq-axis components of the converter output positive and negative sequence AC voltage relative to their command values.

[0075] Combining equations (6), (7), (14), and (15), we can obtain

[0076]

[0077]

[0078] Combining equations (2), (3), (18), and (19), we can obtain

[0079]

[0080]

[0081] u d + ,u q +These are the command values ​​for the dq-axis components of the positive-sequence AC voltage at PCC.

[0082] Traditional outer-loop voltage control is constructed based on parallel capacitive filters. For example... Figure 2 As shown, voltage-current proportional control is used as a new type of outer-loop voltage control to replace the traditional outer-loop voltage control. From equations (14) to (17), it can be seen that the construction of the new outer-loop voltage control does not depend on the parallel capacitive filter and is suitable for high-voltage grid-type converters without parallel capacitive filters. In equations (20) and (21), there is a strong coupling and nonlinear relationship between AC voltage and current, and AC current becomes the interference term in the outer-loop voltage control.

[0083] To reveal the working mechanism of voltage-current proportional control, equation (20) is processed and the final value theorem is applied.

[0084]

[0085] in This is used to represent the magnitude of the change in the positive-sequence d-axis current component before and after a system disturbance. In steady state, according to equation (22), the d-axis component of the positive-sequence AC voltage will track its command value under voltage-current proportional control. Similarly, it can be seen that the q-axis component of the positive-sequence AC voltage will also track its command value.

[0086] Note that the proportional constant z is the only control parameter for voltage-current proportional control. In a per-unit system, z can generally be selected within the range of (0.5-2) to ensure that the voltage and current proportional relationships are on the same order of magnitude, thus fully utilizing the proportional control effect of voltage and current. To eliminate coupling, a feedforward term can be specifically introduced in the outer-loop voltage control to cancel out interference terms. For the sake of simplifying the analysis, this invention does not consider feedforward decoupling.

[0087] 2.3 Overall Control Architecture

[0088] The overall control block diagram of the high-voltage grid-type converter of this invention is as follows: Figure 2 As shown. The synchronization phase of the AC component dq-axis transformation is:

[0089] θ=∫ω * dt+θ0 (23)

[0090] Where θ0 is the initial phase; ω * This is the commanded value of the AC voltage angular frequency, which is the rated angular frequency of the power grid. Common methods for determining its value include:

[0091] (1) When operating under constant voltage, the AC voltage angular frequency command value is equal to the grid angular frequency rated value.

[0092] ω * =ω (24)

[0093] (2) When using a synchronous machine for simulation operation, the AC voltage angular frequency command value is determined by the following formula.

[0094]

[0095] Where P* and P are the commanded and actual values ​​of the active power output of the converter, respectively. D is the proportionality coefficient between the active power deviation and the angular frequency deviation, and T1 and T2 are two time constants. Equation (25) simulates the inertial characteristics of the synchronous motor and the proportional relationship between the frequency deviation and the active power deviation in primary frequency regulation.

[0096] By changing the initial phase θ0, the d-axis can be aligned with the phase angle of the A-phase voltage output from the converter station. The command value for the dq-axis component of the positive-sequence voltage output from the converter is:

[0097]

[0098]

[0099] Among them U * This is the commanded value for the AC voltage amplitude at the converter's PCC. For example... Figure 2 As shown, by performing inverse dq coordinate transformation on equations (6) to (9), the positive and negative sequence three-phase AC voltage modulation waves of the converter output can be obtained. By superimposing the two, the three-phase AC voltage modulation waves of the converter output can be obtained.

[0100] Under normal steady-state conditions, the converter output AC current will track its command value, and the converter's AC voltage and current will remain within their limits. Positive-sequence inner-loop current control provides rapid current regulation, through methods such as... Figure 2 The positive sequence current command value input is limited, which can achieve the purpose of automatic fault current limiting and avoid overcurrent blocking and damage to the converter during grid faults.

[0101] The negative-sequence outer-loop voltage control of the converter can also suppress the negative-sequence AC voltage output by the converter; considering the prevention of overcurrent blockage and damage to the converter, the negative-sequence component control in this invention mainly uses a strategy to suppress negative-sequence current, such as... Figure 2 As shown, the command values ​​of the negative sequence current dq axis components in the negative sequence inner loop current control are all set to 0, and the negative sequence outer loop voltage control is no longer added.

[0102] 3. Simulation Test

[0103] Establish as Figure 3 The simulation system of a high-voltage grid-type converter supplying power to a passive network is shown in Table 1. The system parameters are listed in Table 1. The grid-type converter uses a modular multilevel converter and has no high-voltage parallel filter on the AC side. The control block diagram of the grid-type converter is shown below. Figure 2As shown in Table 2, the control parameters are as follows. For the positive sequence component, voltage-current proportional control is used to control the positive sequence dq-axis voltage, and inner-loop current control is used to control the positive sequence dq-axis current. For the negative sequence component, inner-loop current control is used to suppress the negative sequence dq-axis current, and the negative sequence dq-axis current command value is set to 0. The converter on the opposite side uses constant DC voltage control to provide a stable DC-side voltage for the grid-type converter on this side. The modular multilevel converter uses nearest-level approximation modulation and sorted capacitor voltage balance control, and no longer configures circulating current suppression control.

[0104] Table 1 Simulation System Parameters

[0105] parameter numerical values Rated frequency of power grid / Hz 50 Rated AC voltage of power grid / kV 35 Converter rated capacity / MVA 20 Maximum overcurrent ratio of converter 1.5 Converter rated DC voltage / kV + / -30 Converter rated AC voltage / kV 31 Number of bridge arm sub-modules / each 49 Bridge arm reactance / H 0.053 Submodule capacitor / uF 4650 Converter transformer rated capacity / MVA 30 Converter transformer turns ratio 36 / 31 Converter transformer leakage reactance / pu 0.08 Active load / MW 25 Reactive load / Mvar 2.5

[0106] Table 2 Control System Parameters

[0107]

[0108] 3.1 Simulation Operation Example of Synchronous Machine under Load Step

[0109] Under normal grid operation, the grid-connected converter is in normal AC voltage control mode. The outer loop voltage control, based on the AC voltage command at the PCC, provides the current command for the inner loop current control; then, the inner loop current control determines the converter output AC voltage command, ultimately achieving the goal of controlling the AC voltage at the PCC.

[0110] This example tests the system response of a grid-type converter operating under synchronous machine simulation during a normal step load change in the AC grid. At 3.5 seconds, 5.5 MW of load is disconnected from the AC grid to test the system response under disturbance. To eliminate steady-state frequency deviation, at 4 seconds, the load is switched on...

[15] The secondary frequency regulation function of the synchronous machine simulates operation, and adjusts the active power command value of the converter through the integral link of frequency negative feedback, so that the grid frequency after the disturbance gradually returns to the rated value. Figure 4 and Figure 5 These are the simulation waveforms of various electrical quantities during the operation of the converter synchronous machine under a load step. Figure 4 (a) in u a and Figure 4 (b) in i a These are the AC voltage and current of phase a at PCC, respectively. Figure 4 (c) and These are the positive-sequence d-axis voltage output from the converter and its command value. Figure 4 (d) and These are the positive-sequence q-axis voltage output by the converter and its command value. Figure 5 (a) and These are the positive-sequence d-axis current output by the converter and its command value. Figure 5 (b) and These are the positive-sequence q-axis current output by the converter and its command value. Figure 5 (c) and These are the negative sequence d-axis and q-axis currents output by the converter, respectively. Figure 5 In (d), P and Q are the active and reactive power outputs of the converter, respectively. All variables in the above figure are per-unit values. Figure 5 In (e), f is the converter frequency command value.

[0111] After 22% of the load on the AC power grid is cut off, such as Figure 5 As shown, the active power and positive-sequence d-axis current output by the converter decrease proportionally. Under the inertial characteristics and primary frequency regulation ratio of equation (25), the converter frequency gradually increases to 50.03Hz; Figure 4 As shown in (c) and (d), the positive and negative sequence dq-axis voltages and currents also exhibit short-term slight disturbances, which are essentially eliminated within 0.1 seconds. At 4 seconds, after the secondary frequency regulation function of the synchronous machine simulation operation is activated, the converter frequency is as follows... Figure 5 As shown in (e), the frequency will slowly recover to the rated frequency. Due to the slow secondary frequency regulation response, the frequency recovery process takes a long time, a characteristic similar to that of a traditional generator's secondary frequency regulation. In summary, under normal grid operation and disturbances, the AC voltage at the PCC and the AC current of the grid-connected converter can track their corresponding command values ​​under voltage-current proportional control, thus achieving the control objective of the grid-connected converter.

[0112] 3.2 Case Study of Constant Voltage Operation under Single-Phase Ground Fault

[0113] When a sudden grid fault occurs, the target of normal AC voltage control for the converter cannot be achieved. At this time, the positive sequence current command value given by the positive sequence outer loop voltage control will be limited, the outer loop voltage control fails, and the positive sequence inner loop current control automatically enters the fault current limiting state to prevent overcurrent blocking and damage to the converter. The strategy of suppressing negative sequence current can suppress negative sequence current under asymmetrical grid faults, but the active and reactive power of the converter will exhibit oscillation components at twice the power frequency.

[0114] This example tests the system response of a grid-type converter operating under constant voltage in the event of a single-phase ground fault in the AC power grid. The commanded value of the AC voltage angular frequency of the converter is taken as the rated angular frequency of the power grid. At 1.1 seconds, a ground fault occurs in phase A of the AC power grid, and the fault is cleared after 0.2 seconds. Figure 4 (a) and (b) represent the three-phase AC voltage and current at PCC, respectively. Figure 4 (c) is the positive-sequence d-axis voltage output by the converter and its command value. Figure 4 (d) is the positive-sequence q-axis voltage output by the converter and its command value. Figure 5 (a) shows the positive-sequence d-axis current output by the converter and its command value. Figure 5 (b) is the positive-sequence q-axis current output by the converter and its command value. Figure 5 (c) represents the negative-sequence d-axis and q-axis currents at the converter output. Figure 5 (d) represents the active and reactive power output of the converter.

[0115] During the fault, such as Figure 4 As shown, the phase A voltage drops to near 0, resulting in a severe imbalance in the three-phase AC voltage. Negative-sequence inner-loop current control can suppress the negative-sequence current, and the three-phase AC current remains balanced. Due to the severe voltage drop in the grid, such as... Figure 5 (a) The command value of the d-axis current control in the positive sequence inner loop current control is limited by the limiting circuit to prevent the converter from entering overcurrent lockout, which also leads to... Figure 4 (c) The d-axis voltage in the positive-sequence outer-loop voltage control shown cannot be maintained at its commanded value. Due to the presence of the negative-sequence voltage, the active and reactive power of the converter exhibits the following characteristics during faults: Figure 5 (d) shows the fluctuation characteristics at twice the power frequency. After the fault is cleared, the converter voltage and current gradually return to normal operating conditions. In summary, during a grid fault, the grid-type converter operating at constant voltage can automatically and quickly enter the positive sequence current limiting and negative sequence current suppression states under voltage-current proportional control, avoiding overcurrent lockout and damage to the converter, and automatically returning to normal operation after the grid fault is cleared.

[0116] 3.3 Simulation Example of Synchronous Machine Operation under Single-Phase Ground Fault

[0117] This example is used to test the system response of a grid-type converter operating under synchronous machine simulation when a single-phase ground fault occurs in the AC power grid. The command value of the AC voltage angular frequency of the converter is determined by equation (25). At 1.1 seconds, a ground fault occurs in phase a of the AC power grid, and the fault is cleared after 0.2 seconds. Figure 5 (a) and (b) represent the three-phase AC voltage and current at PCC, respectively. Figure 5 (c) is the positive-sequence d-axis voltage output by the converter and its command value. Figure 5 (d) is the positive-sequence q-axis voltage output by the converter and its command value. Figure 6 (a) shows the positive-sequence d-axis current output by the converter and its command value. Figure 6 (b) is the positive-sequence q-axis current output by the converter and its command value. Figure 6 (c) represents the negative-sequence d-axis and q-axis currents at the converter output. Figure 6 (d) represents the active and reactive power output of the converter.

[0118] Figure 8 and Figure 9 These are the simulation waveforms of various electrical quantities during the operation of the converter synchronous machine under a single-phase ground fault. Figure 8 (a) in u abc and Figure 8 (b) in i abc These are the three-phase AC voltage and current at the PCC. Figure 8 (c) and These are the positive-sequence d-axis voltage output from the converter and its command value. Figure 8 (d) and These are the positive-sequence q-axis voltage output by the converter and its command value. Figure 9 (a) and These are the positive-sequence d-axis current output by the converter and its command value. Figure 9 (b) and These are the positive-sequence q-axis current output by the converter and its command value. Figure 9 (c) and These are the negative sequence d-axis and q-axis currents output by the converter, respectively. Figure 9 In (d), P and Q are the active and reactive power outputs of the converter, respectively. All variables in the above figure are per-unit values.

[0119] During the fault, such as Figure 8 As shown, the phase a voltage drops to near 0, resulting in a severe imbalance in the three-phase AC voltage. The negative-sequence inner-loop current control also suppresses the negative-sequence current, and the three-phase AC current remains balanced. Due to the severe voltage drop in the grid, such as... Figure 9 (a) The command value of the d-axis current controller in the positive sequence inner loop current control is limited by the limiting circuit to prevent the converter from entering overcurrent lockout, which also leads to... Figure 8 (c) shows that the d-axis voltage in the outer loop positive sequence voltage control cannot be maintained at its commanded value. Due to the presence of negative sequence voltage, the active and reactive power of the converter exhibits the following characteristics during faults: Figure 9 (d) shows the fluctuation characteristics at twice the power frequency. After the fault is cleared, the converter voltage and current gradually return to normal operating conditions. In summary, during a grid fault, the grid-type converter operating under synchronous machine simulation can automatically and quickly enter the positive sequence current limiting and negative sequence current suppression states under voltage and current proportional control, avoiding overcurrent blocking and damage to the converter, and automatically returning to normal operation after the grid fault is cleared.

[0120] Similarly, when a three-phase symmetrical ground fault occurs in the power grid, the grid-type converter can automatically and quickly enter the positive sequence current limiting mode under the voltage and current proportional control to avoid overcurrent blocking and damage to the converter, and automatically return to normal operation after the power grid fault is cleared.

[0121] This embodiment introduces a novel outer-loop voltage control for the converter by constructing a voltage-current ratio relationship, which together with the inner-loop current control forms a high-voltage grid-forming converter control architecture. The voltage-current proportional outer loop control does not rely on the parallel capacitive filter on the AC side of the converter, making it suitable for high-voltage, high-capacity, multi-level converters with high output waveform quality. It eliminates the need for the parallel capacitive filter, effectively reducing system costs. During normal grid operation and disturbances, the AC voltage at the PCC and the AC current of the grid-type converter, under voltage-current proportional control, can track their corresponding command values, achieving the control objective of the grid-type converter. During grid faults, the grid-type converter, under voltage-current proportional control, can automatically and quickly enter positive-sequence current limiting and negative-sequence current suppression states, preventing overcurrent blocking and damage to the converter, and automatically recovering to normal operation after the grid fault is cleared. The voltage-current proportional outer loop controller can control the AC voltage, achieving positive-sequence current limiting and negative-sequence current suppression under grid asymmetric faults. Furthermore, combining the voltage-current proportional outer loop controller with synchronous machine simulation operation allows the grid-type converter to simulate the inertial characteristics of a synchronous motor and its primary and secondary frequency regulation functions.

[0122] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention.

Claims

1. A method for controlling the AC voltage of a high-voltage grid-connected converter, characterized in that, Includes the following steps: A novel outer-loop voltage controller is introduced as a voltage-current proportional control for high-voltage grid-connected converters. The outer-loop voltage is controlled by the novel outer-loop voltage controller. In the positive sequence inner loop current control, the command value of the dq-axis component of the positive sequence current is determined by the positive sequence outer loop voltage control. The calculation formula for the positive sequence outer loop voltage control of the converter is: Among them, u d +* and u q +* These are the command values ​​for the dq-axis components of the positive-sequence AC voltage at PCC, u d + and u q + These are the dq-axis components of the positive-sequence AC voltage at PCC, i d +* and i q +* These are the command values ​​for the dq-axis components of the positive-sequence AC current output by the converter, i. Ld + and i Lq + These are the dq-axis components of the positive-sequence AC current output by the converter, and z is the proportionality constant. In a per-unit system, the proportional constant z in the formula for calculating the positive-sequence outer-loop voltage control of the converter ranges from 0.2 to 5.

2. The AC voltage control method for a high-voltage grid-type converter according to claim 1, characterized in that, Protecting the converter under grid fault conditions: Use an inner-loop positive-sequence current controller to limit the positive-sequence current.

3. The AC voltage control method for a high-voltage grid-type converter according to claim 1, characterized in that, The grid-type converter adopts a modular multilevel converter, and there is no high-voltage parallel filter on the AC side.

4. The AC voltage control method for a high-voltage grid-type converter according to claim 1, characterized in that, The opposite-side converter uses constant DC voltage control to provide a stable DC-side voltage for the grid-type converter on this side.

5. The AC voltage control method for a high-voltage grid-type converter according to claim 1, characterized in that, The modular multilevel converter uses nearest-level approximation modulation and sorted capacitor voltage balance control, and no longer configures circulating current suppression control.

6. A negative-sequence AC voltage control method for a high-voltage grid-connected converter, applicable to the method described in any one of claims 1 to 5, characterized in that, A novel outer-loop voltage controller for high-voltage grid-connected converters is introduced using voltage-current proportional control; the outer-loop voltage of the high-voltage grid-connected converter is controlled through voltage-current proportional control. In the negative-sequence inner-loop current control, the command value of the dq-axis component of the negative-sequence current is determined by the negative-sequence outer-loop voltage control. The calculation formula for the negative-sequence outer-loop voltage control of the converter is: Among them, u d -* ,u q -* These are the command values ​​for the dq-axis components of the negative sequence AC voltage at PCC, u d - and u q - These are the dq-axis components of the negative sequence AC voltage at PCC, i d -* and i q -* These are the command values ​​for the dq-axis components of the negative-sequence AC current output by the converter, i. Ld - and i Lq - These are the command values ​​for the dq-axis components of the negative sequence AC current output by the converter; In a per-unit system, the proportional constant z in the formula for calculating the negative-sequence outer loop voltage control of the converter ranges from 0.2 to 5.

7. The AC voltage negative sequence control method for a high-voltage grid-type converter according to claim 6, characterized in that, Protecting the converter under grid fault conditions: using an inner-loop negative-sequence current controller to suppress negative-sequence current.

8. The AC voltage negative sequence control method for a high-voltage grid-type converter according to claim 6, characterized in that, The process of suppressing negative sequence current includes setting the command values ​​of the negative sequence current dq axis components in the negative sequence inner loop current control to 0, and not adding negative sequence outer loop voltage control.

Images