Inertia support control method and device for cascaded multilevel converter

Abstract

The invention discloses a cascade multi-level converter inertia support control method and device, and the method comprises the steps: building a mathematical model of a cascade multi-level converter, and calculating the capacitor voltage fluctuation components of sub-modules under different working conditions; the method comprises the following steps: detecting a frequency change rate in a power system frequency event, determining a cascaded multilevel converter sub-module capacitor voltage safety regulation domain according to sub-module capacitor voltage fluctuation components under different working conditions, and determining a sub-module capacitor voltage maximum regulation quantity; constructing a droop control link, and multiplying the frequency deviation of the power system by a droop coefficient to serve as a sub-module capacitor voltage change reference value; predicting the load variation of the power system by combining the inertia identification result of the power system, and solving the optimal droop coefficient by taking the condition that the capacitance voltage variation reference value of the sub-module is equal to the maximum capacitance voltage regulating variable of the sub-module as a constraint condition; through droop control, voltage control, current control and modulation links, the capacitor voltage of the cascaded multilevel converter sub-module is adjusted, and inertia support is provided for a power system. The device comprises a frequency change rate calculation module, a load prediction module, a droop control module, a voltage control module, a current control module and a modulation module. According to the cascade multi-level converter inertia support control method and device disclosed by the invention, capacitor voltage out-of-limit caused by capacitor voltage double frequency fluctuation during inertia support can be avoided, and the inertia support capability of the cascade multi-level converter is improved through optimal droop coefficient selection.

Description

Technical Field

[0001] This invention relates to the field of control technology for multilevel converters, and in particular to a method and apparatus for inertia support control of cascaded multilevel converters. Background Technology

[0002] Cascaded multilevel converters, through the cascading of multiple sub-modules, can provide reactive power compensation for power systems and have broad application prospects in power electronic power systems.

[0003] The cascaded multilevel converter has a large number of sub-modules, and the energy stored in the capacitors of each sub-module is considerable. By adjusting the capacitor voltage of the sub-modules, short-term inertia support can be provided during power system frequency events.

[0004] Existing inertia support control mainly employs two methods: virtual synchronous machines and droop control. Virtual synchronous machine control constructs rotor motion equations to simulate the rotor characteristics of a synchronous machine, providing indirect inertia support for the power system. Droop control provides direct inertia support by multiplying the frequency deviation by a droop coefficient as a reference value for capacitor voltage changes; the provided inertia level can be quantified. However, traditional droop control considers the second harmonic fluctuation component of the submodule capacitor voltage during multi-condition operation of the cascaded multilevel converter, which can easily lead to capacitor voltage exceeding limits when providing inertia support. Furthermore, existing droop control methods lack exploration of the maximizing inertia support capability of cascaded multilevel converters, making it difficult to fully utilize their inertia support capacity.

[0005] Therefore, based on the actual operating characteristics of cascaded multilevel converters, it is necessary to develop an inertia support control method suitable for cascaded multilevel converters to ensure the safe operation of cascaded multilevel converters and provide maximum inertia support. Summary of the Invention

[0006] This invention provides a method and apparatus for controlling the inertia support of a cascaded multilevel converter. It solves the problem of capacitor voltage exceeding limits caused by the second harmonic fluctuation of the capacitor voltage in the factor module of the cascaded multilevel converter during the inertia support process, ensuring the safety of the inertia support of the cascaded multilevel converter and maximizing the utilization of the inertia support potential of the cascaded multilevel converter. See the description below for details: In a first aspect, a method for inertia support control of a cascaded multilevel converter, the method comprising: A mathematical model of a cascaded multilevel converter is established, and the voltage fluctuation components of its submodule capacitors under different operating conditions are calculated. The frequency change rate in power system frequency events is detected, and the safe adjustment range of the submodule capacitor voltage is determined based on the voltage fluctuation component of the submodule capacitor under different operating conditions, and the maximum adjustment amount of the submodule capacitor voltage is determined. A droop control loop is constructed, and the power system frequency deviation multiplied by the droop coefficient is used as the reference value for the change in the capacitor voltage of the submodule. Based on the power system inertia identification results, the load change of the power system is predicted. The optimal droop coefficient is solved with the constraint that the reference value of the submodule capacitor voltage change is equal to the maximum adjustment of the submodule capacitor voltage. By adjusting the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control, and modulation, inertia support is provided for the power system.

[0007] Specifically, the mathematical model for establishing the cascaded multilevel converter and calculating the voltage fluctuation components of the sub-module capacitors under different operating conditions is as follows: The cascaded multilevel converter includes three bridge arms, each consisting of... N It is composed of cascaded sub-modules and connected to the power grid via a filter inductor, the inductance of which is [value missing]. L During normal grid-connected operation, the phase voltage of the three-phase power grid v sj symmetry, j Indicates the phase sequence, which can be taken respectively. a , b , c Three-phase power grid phase voltage v sj The expression is as follows: In the formula, V m This represents the amplitude of the phase voltage of the power grid. ω Angular frequency; Under normal operation, the three phases are symmetrical. The expression for the three-phase output phase voltage of the cascaded multilevel converter is as follows: In the formula, v dc This represents the average voltage of the submodule capacitors. m The modulation ratio is the sum of the amplitude of the output phase voltage and the modulation ratio. Nv dc The ratio; Cascaded multilevel converter three-phase output current i j The expression is as follows: In the formula, I m The magnitude of the output current. The power factor angle; The three-phase instantaneous power expression of the cascaded multilevel converter is as follows: Cascaded multilevel converter three-phase instantaneous power p j The expression for the relationship between the voltage of the submodule capacitor is as follows: In the formula, E C The energy stored in the capacitors of each phase submodule. v dc0 This refers to the rated voltage of the submodule capacitor. C dc The capacitance value of the submodule; The instantaneous value expression of the submodule capacitor voltage is obtained as follows: In the formula, Δ v dc2 This represents the second harmonic frequency fluctuation component of the submodule capacitor voltage during normal operation.

[0008] Specifically, the frequency change rate in the detected power system frequency events is determined based on the voltage fluctuation components of the submodule capacitors under different operating conditions. The safe adjustment range of the cascaded multilevel converter submodule capacitor voltage is then determined, and the maximum adjustment amount of the submodule capacitor voltage is determined as follows: Detecting the rate of frequency change in power system frequency events RoCoF The expression is as follows: In the formula, f For power system frequency; Based on the voltage fluctuation components of the submodule capacitors under different operating conditions, the safe adjustment range of the submodule capacitor voltage in the cascaded multilevel converter is determined, and the expression for the maximum adjustment amount of the submodule capacitor voltage is determined as follows: In the formula, v dcmax This is the upper limit of the capacitor voltage in the submodule. v dcmin The lower limit of the capacitor voltage of the submodule, Δ v dcmax Δ is the maximum adjustable value of the capacitor voltage in the submodule. v dc2m This represents the amplitude of the second harmonic fluctuation component of the submodule capacitor voltage during normal operation.

[0009] Specifically, the droop control mechanism, which uses the power system frequency deviation multiplied by a droop coefficient as a reference value for the submodule capacitor voltage change, works as follows: A droop control loop is constructed, which uses the power system frequency deviation multiplied by a droop coefficient as a reference value for the submodule capacitor voltage change. The expression is: In the formula, k The droop coefficient is... v dcmin The lower limit of the capacitor voltage of the submodule, Δ ω pu The per-unit value of the power system frequency deviation is expressed as: In the formula, ω 0 is the rated angular frequency. f 0 represents the rated frequency.

[0010] Specifically, the prediction of power system load changes based on power system inertia identification results, with the constraint that the reference value of submodule capacitor voltage change equals the maximum adjustment of submodule capacitor voltage, is used to solve for the optimal droop coefficient. Based on the power system inertia identification results, combined with the frequency change rate RoCoF It can predict changes in power system load, and its per-unit value is denoted as ; Considering the role of the droop control element, the frequency extreme value of the power system in a frequency event. f ext and and k Related, can be represented as Using the constraint that the reference value of the submodule capacitor voltage change equals the maximum adjustment amount of the submodule capacitor voltage, the per-unit value Δ of the maximum adjustment amount of the submodule capacitor voltage can be obtained. v dcmaxpu The expression is as follows: Based on the above formula, the optimal droop coefficient can be solved, which can be expressed as: In the formula, Δ f maxpu The per-unit value of the maximum frequency deviation can be expressed as: By selecting the optimal droop coefficient, the inertia support capability of the cascaded multilevel converter is maximized within the allowable range of submodule capacitor voltage transformation.

[0011] Specifically, the method of adjusting the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control, and modulation to provide inertia support for the power system involves: The reference value of the capacitor voltage change of the cascaded multilevel converter submodule is obtained through the droop control circuit. ; Through a voltage control circuit, the capacitor voltage of the cascaded multilevel converter submodule is controlled to track its reference value, thereby obtaining the current. d Axis component reference values; Current q The shaft component reference value is given by the control system; Using current control circuit to control current dq The axis components are controlled to track their respective reference values; The switching signals of each switching device in the cascaded multilevel converter are obtained through the modulation stage, and the conduction of the switching devices is controlled to realize the inertia support control of the cascaded multilevel converter.

[0012] Secondly, a cascaded multilevel converter inertia support control device, the device comprising: The frequency change rate calculation module is used to calculate state variables such as power system frequency deviation and frequency change rate. The load prediction module is used to predict load changes during frequency events based on system inertia identification results and frequency change rate. The droop control module is used to select the optimal droop coefficient and multiply the power system frequency deviation by the droop coefficient as the reference value for the change in the capacitor voltage of the submodule. The voltage control module is used to control the capacitor voltage of the cascaded multilevel converter submodule to track its reference value, thereby obtaining the current. d Axis component reference values; The current control module is used to control the current of the cascaded multilevel converter. dq Axial components; The modulation module converts the control signal obtained from the current control module into a modulation signal, and uses a modulation algorithm to obtain the switching signals of each switching device in the cascaded multilevel converter, thereby controlling the conduction of the switching devices.

[0013] The beneficial effects of the technical solution provided by this invention are: 1. The inertia support control method and device for cascaded multilevel converters proposed in this invention can solve the problem of capacitor voltage exceeding the limit caused by the second harmonic frequency fluctuation of the capacitor voltage of the factor module of the cascaded multilevel converter during the inertia support process, and ensure the safety of the inertia support of the cascaded multilevel converter. 2. The inertia support control method and device for cascaded multilevel converters proposed in this invention can select the optimal droop coefficient, maximize the utilization of the inertia support capability of the cascaded multilevel converter, and improve the utilization rate of the cascaded multilevel converter; 3. The cascaded multilevel converter inertia support control method and device proposed in this invention do not require the addition of additional hardware circuits, and can provide short-term inertia support for the power system, thereby improving the frequency stability of the power system. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the process of the present invention; Figure 2 This is a topology diagram of a cascaded multilevel converter; Figure 3 This is a schematic diagram of the droop control mechanism; Figure 4 This is a schematic diagram of the overall control strategy; Figure 5 This is a prototype diagram of a three-phase cascaded multilevel converter; Figure 6 Waveform diagram for the inertia support control experiment of a cascaded multilevel converter; Among them, Figure (a) shows the capacitor voltage waveform of the cascaded multilevel converter submodule, Figure (b) shows the power system frequency waveform, and Figure (c) shows the output active power waveform of the cascaded multilevel converter. Figure 7 Waveform diagram of traditional control experiment for cascaded multilevel converter; Figure (a) shows the capacitor voltage waveform of the cascaded multilevel converter submodule, Figure (b) shows the power system frequency waveform, and Figure (c) shows the output active power waveform of the cascaded multilevel converter. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0016] To address the problems existing in the background technology and avoid the capacitor voltage exceeding the limit caused by the second harmonic fluctuation of the capacitor voltage in the factor module of the cascaded multilevel converter during the inertia support process, thereby improving the inertia support capability of the cascaded multilevel converter, this invention provides an inertia support control method for a cascaded multilevel converter, taking a three-phase cascaded multilevel converter as the research object.

[0017] Example 1 A method for inertia support control of a cascaded multilevel converter, the flowchart of which is shown below. Figure 1 As shown, the method specifically includes the following steps: Step 101: Establish a mathematical model of the cascaded multilevel converter and calculate the voltage fluctuation components of the submodule capacitors under different operating conditions. Step 102: Detect the frequency change rate in the power system frequency event, determine the safe adjustment range of the submodule capacitor voltage based on the voltage fluctuation component of the submodule capacitor under different operating conditions, and determine the maximum adjustment amount of the submodule capacitor voltage. Step 103: Construct a droop control loop, multiplying the power system frequency deviation by the droop coefficient as the reference value for the change in the submodule capacitor voltage; Step 104: Based on the power system inertia identification results, predict the power system load change. With the submodule capacitor voltage change reference value equal to the submodule capacitor voltage maximum adjustment as the constraint, solve for the optimal droop coefficient. Step 105: Adjust the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control and modulation links to provide inertia support for the power system; Step 106: Experimental testing verifies the effectiveness of the proposed cascaded multilevel converter inertia support control method; Example 2 The scheme in Example 1 will be further described below with reference to specific calculation formulas, accompanying drawings, and examples. See the description below for details: Step 201: Establish a mathematical model of the cascaded multilevel converter and calculate the voltage fluctuation components of the submodule capacitors under different operating conditions; Cascaded multilevel converter topology as follows Figure 2 As shown, it includes three phase units, each phase unit including N A cascaded H-bridge submodule and a filter inductor L Each H-bridge submodule contains four power devices, four anti-parallel diodes, and one capacitor. The three-phase grid voltage in the diagram is... v sj , j For phase sequence, j ={a, b, c}, the equivalent inductance of the power grid is... L s The output current of the cascaded multilevel converter is i j The output phase voltage is v j The AC output voltage of each submodule is v jk , k This refers to the sequence number of each submodule within each phase. k ={1, 2, ..., N The capacitor voltage of each submodule is... v dcjk .

[0018] During normal grid-connected operation, the phase voltage of the three-phase power grid v sj Symmetrical, three-phase power grid phase voltage v sj The expression is as follows: (1) In the formula, V m This represents the amplitude of the phase voltage of the power grid. ω Angular frequency; Under normal operation, the three phases are symmetrical. The expression for the three-phase output phase voltage of the cascaded multilevel converter is as follows: (2) In the formula, v dc This represents the average voltage of the submodule capacitors. m The modulation ratio is the sum of the amplitude of the output phase voltage and the modulation ratio. Nv dc The ratio; Cascaded multilevel converter three-phase output current i j The expression is as follows: (3) In the formula, I m The magnitude of the output current. The power factor angle; The three-phase instantaneous power expression of the cascaded multilevel converter is as follows: (4) Cascaded multilevel converter three-phase instantaneous power p j The expression for the relationship between the voltage of the submodule capacitor is as follows: (5) In the formula, E C The energy stored in the capacitors of each phase submodule. v dc0 This refers to the rated voltage of the submodule capacitor. C dc The capacitance value of the submodule; The instantaneous value expression of the submodule capacitor voltage is obtained as follows: (6) In the formula, Δ v dc2 This represents the second harmonic frequency fluctuation component of the submodule capacitor voltage during normal operation.

[0019] Step 202: Detect the frequency change rate in the power system frequency event, determine the safe adjustment range of the submodule capacitor voltage based on the voltage fluctuation component of the submodule capacitor under different operating conditions, and determine the maximum adjustment amount of the submodule capacitor voltage. Detecting the rate of frequency change in power system frequency events RoCoF The expression is as follows: (7) In the formula, f For power system frequency; Based on the voltage fluctuation components of the submodule capacitors under different operating conditions, the safe adjustment range of the submodule capacitor voltage in the cascaded multilevel converter is determined, and the expression for the maximum adjustment amount of the submodule capacitor voltage is determined as follows: (8) In the formula, v dcmax This is the upper limit of the capacitor voltage in the submodule. v dcmin The lower limit of the capacitor voltage of the submodule, Δ v dcmax Δ is the maximum adjustable value of the capacitor voltage in the submodule. v dc2m This represents the amplitude of the second harmonic fluctuation component of the submodule capacitor voltage during normal operation.

[0020] Step 203: Construct a droop control loop, multiplying the power system frequency deviation by a droop coefficient as a reference value for the change in the submodule capacitor voltage; A schematic diagram of the droop control mechanism is shown below. Figure 3 As shown.

[0021] A droop control loop is constructed, which uses the power system frequency deviation multiplied by a droop coefficient as a reference value for the submodule capacitor voltage change. The expression is: (9) In the formula, k The droop coefficient is... v dcmin The lower limit of the capacitor voltage of the submodule, Δ ω pu The per-unit value of the power system frequency deviation is expressed as: (10) In the formula, ω 0 is the rated angular frequency. f 0 represents the rated frequency.

[0022] Step 204: Based on the power system inertia identification results, predict the power system load change. With the submodule capacitor voltage change reference value equal to the submodule capacitor voltage maximum adjustment as the constraint, solve for the optimal droop coefficient. Based on the power system inertia identification results, combined with the frequency change rate RoCoF It can predict changes in power system load, and its per-unit value is denoted as ; Considering the role of the droop control element, the frequency extreme value of the power system in a frequency event. f ext and and k Related, can be represented as (11) Using the constraint that the reference value of the submodule capacitor voltage change equals the maximum adjustment amount of the submodule capacitor voltage, the per-unit value Δ of the maximum adjustment amount of the submodule capacitor voltage can be obtained. v dcmaxpu The expression is as follows: (12) Based on the above formula, the optimal droop coefficient can be solved, which can be expressed as: (13) In the formula, Δ f maxpu The per-unit value of the maximum frequency deviation can be expressed as: (14) By selecting the optimal droop coefficient, the inertia support capability of the cascaded multilevel converter is maximized within the allowable range of submodule capacitor voltage transformation.

[0023] Step 205: Adjust the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control and modulation links to provide inertia support for the power system; The reference value of the capacitor voltage change of the cascaded multilevel converter submodule is obtained through the droop control circuit. ; Through a voltage control circuit, the capacitor voltage of the cascaded multilevel converter submodule is controlled to track its reference value, thereby obtaining the current. d Axis component reference values; Current q The shaft component reference value is given by the control system; Using current control circuit to control current dq The axis components are controlled to track their respective reference values; The switching signals of each switching device in the cascaded multilevel converter are obtained through the modulation stage, and the conduction of the switching devices is controlled to realize the inertia support control of the cascaded multilevel converter. The above-mentioned components constitute the overall control strategy of the cascaded multilevel converter, as shown in the schematic diagram below. Figure 4 As shown.

[0024] Step 206: Experimentally test and verify the effectiveness of the proposed cascaded multilevel converter inertia support control method and device; To further verify the effectiveness of the proposed inertia support control method, the following was conducted: Figure 5 The experimental platform for the three-phase cascaded multilevel converter shown was used for experimental verification, and its main circuit parameters are listed in Table 1. In the experiment, the cascaded multilevel converter was connected to a simulated power grid with inertia characteristics, and the power grid was simulated equivalently through a two-level converter and a DC source.

[0025] Table 1 Experimental parameters

[0026] Experimental results of the cascaded multilevel converter under the proposed cascaded multilevel converter inertia support control method are as follows: Figure 6 As shown. To verify the effectiveness of the proposed inertia support control method and device for cascaded multilevel converters, the experimental conditions were set as follows: reactive current amplitude 2A, load power per unit value jumping from 0.5 to 0.7. After detecting a power system frequency deviation greater than a certain value, the proposed inertia support control method for cascaded multilevel converters was used to control the cascaded multilevel converters. The voltage changes of the sub-module capacitors of the cascaded multilevel converter are shown in the figure. Figure 6 As shown in (a), during the experiment, the voltage drop of the submodule capacitor was 5.5V, which did not exceed the minimum limit of the submodule capacitor voltage, and the cascaded multilevel converter remained within the safe operating range; the power system frequency change was as follows. Figure 6 As shown in (b), the lowest frequency of the system is 49.834Hz, and the maximum frequency deviation is 0.166Hz, indicating that the frequency fluctuation of the power system is effectively suppressed; the active power output of the cascaded multilevel converter is as follows: Figure 6 As shown in (c), during power system frequency drops, the cascaded multilevel converter inertia support control strategy is operational, actively outputting power to provide virtual inertia support and rapidly responding to power system frequency regulation demands. In contrast, experimental results using the traditional control method without inertia support are as follows: Figure 7 As shown. Under the traditional control method, the voltage change of the sub-module capacitors of the cascaded multilevel converter is as follows: Figure 7 As shown in (a), during the frequency decrease process, the capacitor voltage of the cascaded multilevel converter submodule remains almost constant and does not participate in the inertia support process; the power system frequency change is as follows: Figure 7 As shown in (b), the lowest frequency is 49.823Hz, and the maximum frequency deviation is 0.177Hz. The frequency fluctuation amplitude is greater than that when the proposed inertia-supported control method is used. The active power output of the cascaded multilevel converter is as follows: Figure 7 As shown in (c), its active power is always zero, and it does not provide an active power response during the grid frequency drop, thus failing to achieve inertia support. By comparison, it can be seen that compared with the traditional control method, the proposed cascaded multilevel converter inertia support control method reduces the power system frequency deviation by approximately 6.2%. The proposed method can achieve inertia support by precisely adjusting the active power output of the cascaded multilevel converter, effectively suppressing the power system frequency deviation, while ensuring that the capacitor voltage of the cascaded multilevel converter submodules remains within a safe range, verifying the effectiveness of the proposed method and device.

[0027] In summary, the advantages of this cascaded multilevel converter inertia support control method are as follows: 1. The inertia support control method and device for cascaded multilevel converters proposed in this invention can solve the problem of capacitor voltage exceeding the limit caused by the second harmonic frequency fluctuation of the capacitor voltage of the factor module of the cascaded multilevel converter during the inertia support process, and ensure the safety of the inertia support of the cascaded multilevel converter. 2. The inertia support control method and device for cascaded multilevel converters proposed in this invention can select the optimal droop coefficient, maximize the utilization of the inertia support capability of the cascaded multilevel converter, and improve the utilization rate of the cascaded multilevel converter; 3. The cascaded multilevel converter inertia support control method and device proposed in this invention do not require the addition of additional hardware circuits, and can provide short-term inertia support for the power system, thereby improving the frequency stability of the power system.

[0028] A cascaded multilevel converter inertia support control device, the device comprising: The frequency change rate calculation module is used to calculate state variables such as power system frequency deviation and frequency change rate. The load prediction module is used to predict load changes during frequency events based on system inertia identification results and frequency change rate. The droop control module is used to select the optimal droop coefficient and multiply the power system frequency deviation by the droop coefficient as the reference value for the change in the capacitor voltage of the submodule. The voltage control module is used to control the capacitor voltage of the cascaded multilevel converter submodule to track its reference value, thereby obtaining the current. d Axis component reference values; The current control module is used to control the current of the cascaded multilevel converter. dq Axial components; The modulation module converts the control signal obtained from the current control module into a modulation signal, and uses a modulation algorithm to obtain the switching signals of each switching device in the cascaded multilevel converter, thereby controlling the conduction of the switching devices.

[0029] The execution entities of the above modules and units can be devices with computing functions such as computers, microcontrollers, and single-chip microcomputers. In specific implementation, the embodiments of the present invention do not limit the execution entities and can select them according to the needs of actual applications.

[0030] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not limited, and any device that can perform the above functions is acceptable.

[0031] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0032] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for inertia support control of a cascaded multilevel converter, characterized in that, The method includes: A mathematical model of a cascaded multilevel converter is established, and the voltage fluctuation components of its submodule capacitors under different operating conditions are calculated. The frequency change rate in power system frequency events is detected, and the safe adjustment range of the submodule capacitor voltage is determined based on the voltage fluctuation component of the submodule capacitor under different operating conditions, and the maximum adjustment amount of the submodule capacitor voltage is determined. A droop control loop is constructed, and the power system frequency deviation multiplied by the droop coefficient is used as the reference value for the change in the capacitor voltage of the submodule. Based on the power system inertia identification results, the load change of the power system is predicted. The optimal droop coefficient is solved with the constraint that the reference value of the submodule capacitor voltage change is equal to the maximum adjustment of the submodule capacitor voltage. By adjusting the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control, and modulation, inertia support is provided for the power system.

2. The inertia support control method for a cascaded multilevel converter according to claim 1, characterized in that, The mathematical model for establishing the cascaded multilevel converter is specifically used to calculate the voltage fluctuation components of the submodule capacitors under different operating conditions. The cascaded multilevel converter includes three bridge arms, each consisting of... N It is composed of cascaded sub-modules and connected to the power grid via a filter inductor, the inductance of which is [value missing]. L During normal grid-connected operation, the phase voltage of the three-phase power grid v sj symmetry, j Indicates the phase sequence, which can be taken respectively. a , b , c Three-phase power grid phase voltage v sj The expression is as follows: In the formula, V m This represents the amplitude of the phase voltage of the power grid. ω Angular frequency; Under normal operation, the three phases are symmetrical. The expression for the three-phase output phase voltage of the cascaded multilevel converter is as follows: In the formula, v dc This represents the average voltage of the submodule capacitors. m The modulation ratio is the sum of the amplitude of the output phase voltage and the modulation ratio. Nv dc The ratio; Cascaded multilevel converter three-phase output current i j The expression is as follows: In the formula, I m The magnitude of the output current. The power factor angle; The three-phase instantaneous power expression of the cascaded multilevel converter is as follows: Cascaded multilevel converter three-phase instantaneous power p j The expression for the relationship between the voltage of the submodule capacitor is as follows: In the formula, E C The energy stored in the capacitors of each phase submodule. v dc0 This refers to the rated voltage of the submodule capacitor. C dc The capacitance value of the submodule; The instantaneous value expression of the submodule capacitor voltage is obtained as follows: In the formula, Δ v dc2 This represents the second harmonic frequency fluctuation component of the submodule capacitor voltage during normal operation.

3. The cascaded multilevel converter inertia support control method according to claim 1, characterized in that, The frequency change rate in the detected power system frequency events is used to determine the safe adjustment range of the cascaded multilevel converter submodule capacitor voltage based on the submodule capacitor voltage fluctuation components under different operating conditions. The maximum adjustment amount of the submodule capacitor voltage is specifically determined as follows: Detecting the rate of frequency change in power system frequency events RoCoF The expression is as follows: In the formula, f For power system frequency; Based on the voltage fluctuation components of the submodule capacitors under different operating conditions, the safe adjustment range of the submodule capacitor voltage in the cascaded multilevel converter is determined, and the expression for the maximum adjustment amount of the submodule capacitor voltage is determined as follows: In the formula, v dcmax This is the upper limit of the capacitor voltage in the submodule. v dcmin The lower limit of the capacitor voltage of the submodule, Δ v dcmax Δ is the maximum adjustable value of the capacitor voltage in the submodule. v dc2m This represents the amplitude of the second harmonic fluctuation component of the submodule capacitor voltage during normal operation.

4. The inertia support control method for a cascaded multilevel converter according to claim 1, characterized in that, The construction of the droop control loop, specifically using the power system frequency deviation multiplied by the droop coefficient as the reference value for the submodule capacitor voltage change, is as follows: A droop control loop is constructed, which uses the power system frequency deviation multiplied by a droop coefficient as a reference value for the submodule capacitor voltage change. The expression is: In the formula, k The droop coefficient is... v dcmin The lower limit of the capacitor voltage of the submodule, Δ ω pu The per-unit value of the power system frequency deviation is expressed as: In the formula, ω 0 is the rated angular frequency. f 0 represents the rated frequency.

5. The inertia support control method for a cascaded multilevel converter according to claim 1, characterized in that, The method for predicting power system load changes by combining power system inertia identification results, and using the constraint that the reference value of submodule capacitor voltage change equals the maximum adjustment of submodule capacitor voltage, is specifically as follows: Based on the power system inertia identification results, combined with the frequency change rate RoCoF It can predict changes in power system load, and its per-unit value is denoted as ; Considering the role of the droop control element, the frequency extreme value of the power system in a frequency event. f ext and and k Related, can be represented as Using the constraint that the reference value of the submodule capacitor voltage change equals the maximum adjustment amount of the submodule capacitor voltage, the per-unit value Δ of the maximum adjustment amount of the submodule capacitor voltage can be obtained. v dcmaxpu The expression is as follows: Based on the above formula, the optimal droop coefficient can be solved, which can be expressed as: In the formula, Δ f maxpu The per-unit value of the maximum frequency deviation can be expressed as: By selecting the optimal droop coefficient, the inertia support capability of the cascaded multilevel converter is maximized within the allowable range of submodule capacitor voltage transformation.

6. The inertia support control method for a cascaded multilevel converter according to claim 1, characterized in that, The method of adjusting the capacitor voltage of the cascaded multilevel converter submodule through droop control, voltage control, current control, and modulation to provide inertia support for the power system specifically involves: The reference value of the capacitor voltage change of the cascaded multilevel converter submodule is obtained through the droop control circuit. ; Through a voltage control circuit, the capacitor voltage of the cascaded multilevel converter submodule is controlled to track its reference value, thereby obtaining the current. d Axis component reference values; Current q The shaft component reference value is given by the control system; Using current control circuit to control current dq The axis components are controlled to track their respective reference values; The switching signals of each switching device in the cascaded multilevel converter are obtained through the modulation stage, and the conduction of the switching devices is controlled to realize the inertia support control of the cascaded multilevel converter.

7. A cascaded multilevel converter inertia support control device, characterized in that, The device includes: The frequency change rate calculation module is used to calculate state variables such as power system frequency deviation and frequency change rate. The load prediction module is used to predict load changes during frequency events based on system inertia identification results and frequency change rate. The droop control module is used to select the optimal droop coefficient and multiply the power system frequency deviation by the droop coefficient as the reference value for the change in the capacitor voltage of the submodule. The voltage control module is used to control the capacitor voltage of the cascaded multilevel converter submodule to track its reference value, thereby obtaining the current. d Axis component reference values; The current control module is used to control the current of the cascaded multilevel converter. dq Axial components; The modulation module converts the control signal obtained from the current control module into a modulation signal, and uses a modulation algorithm to obtain the switching signals of each switching device in the cascaded multilevel converter, thereby controlling the conduction of the switching devices.

Images