Method for determining dc capacitor of three-port intensive low-voltage distribution network flexible interconnection device

Abstract

The application provides a DC capacitor determination method of a three-port intensive low-voltage power distribution network flexible interconnection device, the flexible interconnection device comprises three AC ports and one DC port, each AC port is connected to a power grid through an AC feeder, and the DC port comprises a plurality of DC capacitors; the method comprises the following steps: determining the active power and the reactive power of the three AC feeders respectively, and determining the current and the modulation voltage of the three AC ports according to the active power and the reactive power of the three AC feeders; determining the current instantaneous value of the DC capacitor without the current DC component in the fundamental period according to the current and the modulation voltage of the three AC ports; and determining the capacitance value of the DC capacitor according to the capacitor voltage ripple limit value and the current instantaneous value of the DC capacitor; the scene of multiple AC ports is considered, and the regulation effect of power flow is improved.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, and in particular to a method for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device. Background Technology

[0002] With the increasing proportion of renewable energy and the growing trend of load diversification, low-voltage distribution networks will face problems such as voltage exceeding limits, line equipment overload, and insufficient capacity to absorb new energy. The traditional solution is to expand and upgrade low-voltage distribution lines, but this has significant drawbacks such as high cost, wasted capacity, and frequent planned power outages. Therefore, to meet the higher performance requirements of distribution networks in the new energy era, the concept of "interconnection and mutual assistance" between power supply areas has been introduced into the distribution network, thus forming the concept of flexible interconnection.

[0003] Low-voltage flexible interconnection refers to upgrading or constructing distribution network interconnection nodes using flexible interconnected devices (FIDs). By leveraging the FID's excellent dynamic power flow control capabilities and the complementary spatiotemporal characteristics of loads, multiple power supply areas are interconnected and mutually supportive. This changes the limitations of closed-loop design and open-loop operation in traditional low-voltage AC distribution systems, enhances the interaction capabilities between sources, grids, and loads, and further improves the power supply reliability of low-voltage distribution networks.

[0004] However, in related technologies, the design of low-voltage distribution network FID (Flexible Interconnection) is based on back-to-back two-level voltage source converters (VSCs), which suffers from drawbacks such as high cost, large size, and low power density and efficiency, failing to meet the low-cost and large-scale application requirements of low-voltage distribution networks for flexible interconnection devices. To address this, a three-port intensive low-voltage distribution network flexible interconnection device is proposed, with an ANPC (Active Neutral Point Clamped) topology, including three AC ports and one DC port. The DC capacitor is a crucial component of the DC port, affecting its voltage stability. The AC port voltage is an inverter of the DC port voltage, thus affecting its quality and, consequently, its power flow regulation. In related technologies, the design of the DC capacitor largely neglects consideration of multi-AC port scenarios, resulting in suboptimal power flow regulation. Summary of the Invention

[0005] Based on this, it is necessary to provide a method, device, computer equipment, computer-readable storage medium, and computer program product for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device that can take into account multi-AC port scenarios and improve power flow regulation effect, in order to address the above-mentioned technical problems.

[0006] In a first aspect, embodiments of this application provide a method for determining the DC capacitance of a three-port integrated low-voltage distribution network flexible interconnection device. The device includes three AC ports and one DC port. Each AC port is connected to the power grid through an AC feeder, and the DC port includes multiple DC capacitors.

[0007] The method includes:

[0008] The active power and reactive power of the three AC feeders are determined respectively, and the current and modulation voltage of the three AC ports are determined respectively based on the active power and reactive power of the three AC feeders.

[0009] Based on the current and modulation voltage of the three AC ports, determine the instantaneous current value of the DC capacitor without a DC current component within the fundamental period;

[0010] The capacitance value of the DC capacitor is determined based on the capacitor voltage ripple requirements and the instantaneous current value of the DC capacitor.

[0011] In one embodiment, determining the modulation voltages of the three AC ports includes:

[0012] When active power is transmitted on the AC feeder, the amplitude of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and the AC feeder; and,

[0013] The phase of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter.

[0014] In one embodiment, when active power is transmitted on the AC feeder, determining the amplitude of the modulation voltage at the AC port includes:

[0015] The first intermediate voltage value is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter.

[0016] Based on the DC voltage value, determine the second intermediate voltage value;

[0017] The amplitude of the modulation voltage at the AC port is determined based on the ratio of the first intermediate voltage value to the second intermediate voltage value; and,

[0018] Determining the phase of the modulation voltage at the AC port includes:

[0019] The third intermediate voltage value is determined based on the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter.

[0020] The phase of the modulation voltage at the AC port is determined based on the arctangent function of the ratio of the third intermediate voltage value to the amplitude of the AC feeder voltage.

[0021] In one embodiment, determining the modulation voltages of the three AC ports includes:

[0022] When active power and reactive power are transmitted on the AC feeder, the amplitude of the modulation voltage of the AC port is determined based on the active power of the AC feeder, the reactive power of the AC feeder, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and the AC feeder.

[0023] In one embodiment, when active power and reactive power are transmitted on the AC feeder, determining the amplitude of the modulation voltage at the AC port includes:

[0024] The first phase is determined based on the active power and reactive power of the AC feeder.

[0025] The fourth intermediate voltage value is determined based on the first phase, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter.

[0026] Based on the DC voltage value, determine the fifth intermediate voltage value;

[0027] The amplitude of the modulation voltage at the AC port is determined based on the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

[0028] In one embodiment, determining the instantaneous current value of the DC capacitor, which does not contain a DC current component, within the fundamental period includes:

[0029] The average value of the DC capacitor current during the switching cycle is determined based on the current and modulation voltage of the three AC ports.

[0030] The average value of the DC capacitor current during the fundamental period is determined based on the average value of the DC capacitor current during the switching cycle.

[0031] The instantaneous current value of the DC capacitor is determined based on the average DC capacitor current during the switching cycle and the average DC capacitor current during the fundamental cycle.

[0032] In one embodiment, determining the capacitance value of the DC capacitor includes:

[0033] Determine the charge function of the DC capacitor based on the instantaneous current value of the DC capacitor;

[0034] The peak-to-peak value of the charge change of the DC capacitor is determined based on the charge quantity function;

[0035] The capacitance value of the DC capacitor is determined based on the peak-to-peak value of the charge change and the capacitor voltage ripple limit value.

[0036] Secondly, this application provides a DC capacitor determination device for a three-port integrated low-voltage distribution network flexible interconnection device. The flexible interconnection device includes three AC ports and one DC port. Each AC port is connected to the power grid through an AC feeder. The DC port includes multiple DC capacitors.

[0037] The DC capacitance determination device includes:

[0038] The AC port parameter module is used to determine the active power and reactive power of the three AC feeders respectively, and to determine the current and modulation voltage of the three AC ports respectively based on the active power and reactive power of the three AC feeders.

[0039] The instantaneous current module is used to determine the instantaneous current value of the DC capacitor without a DC current component within the fundamental period based on the current and modulation voltage of the three AC ports.

[0040] The capacitance value module is used to determine the capacitance value of the DC capacitor based on the capacitor voltage ripple requirements and the instantaneous current value of the DC capacitor.

[0041] Thirdly, embodiments of this application provide a computer device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the method described in the first aspect.

[0042] Fourthly, embodiments of this application provide a computer-readable storage medium having a computer program stored thereon, wherein the computer program, when executed by a processor, implements the method described in the first aspect.

[0043] Fifthly, embodiments of this application provide a computer program product, including a computer program that, when executed by a processor, implements the method described in the first aspect.

[0044] The aforementioned method, apparatus, computer equipment, computer-readable storage medium, and computer program product for determining the DC capacitor of a three-port intensive low-voltage distribution network flexible interconnection device consider the influence of the current at the three AC ports of the intensive low-voltage distribution network flexible interconnection device on the voltage of the DC capacitor, and determine the DC capacitor based on the capacitor voltage ripple requirements. The DC capacitor determination method proposed in this application fully considers the maximum voltage fluctuation under power flow regulation conditions, thereby ensuring that the voltage ripple requirements are met under all power flow regulation conditions. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the description of the embodiments of this application or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 A schematic diagram of the topology of a three-port intensive low-voltage distribution network flexible interconnection device;

[0047] Figure 2 A schematic diagram of the method for determining the DC capacitance of a three-port integrated low-voltage distribution network flexible interconnection device;

[0048] Figure 3 A schematic diagram of the voltage and current phasor relationship for different feeder power;

[0049] Figure 4 A schematic diagram showing the comparison between the modulated wave and the carrier wave, and the corresponding DC capacitor current;

[0050] Figure 5 The simulated waveform of the DC capacitor voltage under the condition of maximum DC voltage fluctuation is shown.

[0051] Figure 6 A structural block diagram of the DC capacitor determination device for a three-port intensive low-voltage distribution network flexible interconnection device. Detailed Implementation

[0052] To make the above-mentioned objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit this application.

[0053] like Figure 1As shown in (a), the topology of the three-port intensive low-voltage distribution network flexible interconnection device includes one DC port and three AC ports. The DC port is supported by two DC capacitors C1 and C2 connected in series. The DC capacitors C1 and C2 have the same capacitance value and the same withstand voltage. The method for determining the DC capacitor in the embodiments of this application is applicable to both DC capacitor C1 and DC capacitor C2.

[0054] In one exemplary embodiment, reference is made to Figure 2 A method for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device is provided. The method is performed by, for example but not limited to, a computer device, and may include the following steps S202 to S206.

[0055] S202, determine the active power and reactive power of the three AC feeders respectively, and determine the current and modulation voltage of the three AC ports respectively based on the active power and reactive power of the three AC feeders.

[0056] S204 determines the instantaneous current value of the DC capacitor without a DC current component within the fundamental period based on the current and modulation voltage of the three AC ports.

[0057] S206. Determine the capacitance value of the DC capacitor based on the capacitor voltage ripple limit value and the instantaneous current value of the DC capacitor.

[0058] The determination of the DC capacitor in this embodiment takes into account the influence of the current at the three AC ports on the voltage of the DC capacitor. Specifically, the voltage of the DC capacitor is related to the current flowing into the DC port, while the current at the three AC ports determines the final current flowing into the DC port.

[0059] The current at the AC port is determined by the power flow regulation requirements, i.e., it is related to the power flow regulation operating conditions. In this embodiment of the application, in a three-port flexible interconnection scenario, the power flow of the three AC feeders when the DC port current is at its maximum is determined by a traversal method.

[0060] By iterating through the active and reactive power of each AC feeder as variables, and considering that the intensive low-voltage distribution network flexible interconnection device lacks internal energy storage and cannot actively provide active power, there is an active power balance constraint among the three AC feeders:

[0061]

[0062] Wherein, P1, P2, and P3 represent the active power of the three AC feeders. It is assumed that the active power flows from the flexible interconnect device to the AC feeders, and the losses of the lines and the device themselves are ignored. loss .

[0063] It should be noted that the flexible interconnection device functions to regulate the active and reactive power of each AC feeder. For example... Figure 1 As shown in (b), an LCL filter is connected between the AC port and its corresponding AC feeder. If the loss of the LCL filter is ignored, the active power of the AC port and its corresponding AC feeder is equal. However, the inductance and current of the LCL filter can provide or absorb reactive power, so the reactive power of the AC port and its corresponding AC feeder is usually not equal.

[0064] When the current at the AC port reaches the rated value of 1 p.u., its impact on the voltage of the DC capacitor is the greatest. Therefore, when selecting variables, the current at the AC port should be as large as possible, even if the apparent power is large, so as to obtain the operating condition when the DC capacitor voltage ripple is the largest.

[0065] Where pu represents per-unit value, which is the actual value divided by the reference value. For example, the reference value for DC voltage is 750V, and the reference value for AC current is 171A (calculated based on the current rating under an 80kVA capacity); if the reference value for active power is 80kW, then when the per-unit value of active power is 1p.u., its actual value is 80kW.

[0066] like Figure 1 As shown in (b), if AC feeder 1 corresponding to AC port 1 is taken as the active power balancing feeder, for the sake of simplifying the analysis, the active power of AC feeder 1 is configured as 1 p.u. and the reactive power of AC feeder 1 is 0 p.u., while the active power of AC feeders 2 and 3 is allocated, and the reactive power of AC feeders 2 and 3 is calculated using the rated apparent power and the allocated active power. Therefore, it is necessary to traverse the active power and reactive power of AC feeders 2 and 3 at least to obtain the operating condition when the DC capacitor voltage ripple is at its maximum.

[0067] In an exemplary embodiment, determining the modulation voltages of the three AC ports in step S102 may include the following:

[0068] When transmitting active power on an AC feeder, the amplitude of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and its corresponding AC feeder; and the phase of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter.

[0069] Specifically, the active power P1 of AC feeder 1 is configured to be 1 p.u., and the reactive power Q1 of AC feeder 1 is 0 p.u.; the active power P2 and reactive power Q2 of AC feeder 2, and the active power P3 and reactive power Q3 of AC feeder 3 can be calculated as follows:

[0070]

[0071]

[0072] Furthermore, under normal low-voltage power grid operation, the effective value of the line voltage of the AC feeder is stable at 380V, and the effective value of the phase voltage is stable at 220V. Voltage transformers can be installed on the AC feeder to measure the three-phase voltage, which is generally considered to be a stable value. Therefore, given the voltage of AC feeder 1, the current at AC port 1... It can be calculated as follows:

[0073]

[0074] This calculates the current. d-axis components and q-axis components When the three-phase voltage u of AC feeder 1 g1 When performing phase-locked loop, u g1d The rated voltage amplitude is 311V, u g1q If the value is 0V, the above formula can be further modified as follows:

[0075]

[0076] For the current at AC port 2 and the current at port 3 The calculation is similar, also using the three-phase voltage u of AC feeder 1. g1 The synchronous rotating coordinate system obtained by phase-locked loop is transformed by dq as follows:

[0077]

[0078]

[0079] To simplify the analysis, assuming that the voltages of the three AC feeders are the same, the above formula can be further modified as follows:

[0080]

[0081]

[0082] In the dq coordinate system, the current amplitude and phase of AC ports 1 to 3 can be obtained from the dq components:

[0083]

[0084]

[0085]

[0086] Considering the voltage drop across the two inductors in an LCL filter, according to... Figure 3 The modulation voltages of AC ports 1 to 3 can be calculated from the phasor diagram shown.

[0087] about Figure 3 Explanation of the phasor relationship diagram: According to Kirchhoff's voltage law, the sum of voltages in a closed loop is 0, meaning the sum of the voltage at the AC port, the voltage drop across the LCL filter, and the voltage of the AC feeder is 0. In the complex plane, a sinusoidal quantity can be represented as a phasor with a specific direction, amplitude, and phase angle. The three phasors form a triangle (similar to a vector). Since the directions of active and reactive power transmission differ, the current direction also differs, and the relationship between the three phasors also differs.

[0088] Assuming the positive direction of the current on the AC feeder is AC port → power grid, the following cases can be distinguished based on the direction of active power and reactive power transmitted on the AC feeder (the modulation voltage phase calculated below is phase a; the modulation voltage phases of phases b and c can be obtained by lag or lead by 120°).

[0089] Optionally, when transmitting active power on an AC feeder, determining the amplitude and phase of the modulation voltage at the AC port may include the following:

[0090] The first intermediate voltage value is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter; the second intermediate voltage value is determined based on the DC voltage value; and the amplitude of the modulation voltage at the AC port is determined based on the ratio of the first intermediate voltage value to the second intermediate voltage value.

[0091] The third intermediate voltage value is determined based on the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter; the phase of the modulation voltage at the AC port is determined based on the arctangent function of the ratio of the third intermediate voltage value to the amplitude of the AC feeder voltage.

[0092] For example, in case 1: if only active power is transmitted on the AC feeder, and the direction of the active power is AC port → power grid, such as... Figure 3 As shown in (a), the AC feeder voltage and AC feeder current are in phase, i.e., 0° out of phase, and the AC port modulation voltage U m The amplitude is greater than the amplitude of the AC feeder voltage, and the AC port modulation voltage Um The phase of the voltage leads the phase of the AC feeder voltage. For ease of calculation, the LCL filter is simplified to an L filter; therefore, the modulation voltage U at AC port 1 to AC port 3... m for:

[0093]

[0094] In the formula, U LCL It is the voltage of the equivalent L filter, V dc It is the DC voltage value, U g and I g Let ω represent the amplitudes of the AC feeder voltage and AC feeder current, respectively, ω be the fundamental angular frequency, and L be the equivalent inductance of the LCL filter. The LCL filter is equivalent to an L-filter, neglecting the filter capacitor C. The inductance of the L-filter is equal to the sum of the two inductance values ​​of the LCL filter.

[0095] If we treat an LCL filter as an equivalent L filter, the filter capacitor branch will not shunt current, and the current at the AC port will be the same as the current in the AC feeder. If we don't neglect the filter capacitor branch, then according to Kirchhoff's current law, the sum of the current at the AC port, the current in the filter capacitor, and the current in the AC feeder should be zero. However, the current in the filter capacitor is generally very small, so the amplitudes of the current at the AC port and the current in the AC feeder are approximately equal.

[0096] The AC port modulation voltage is a control quantity of the control system. After modulation, it is finally reflected on the AC port, which is the fundamental component of the AC port voltage (because the AC port voltage is a stepped wave composed of multiple levels). The sum of the AC port voltage, the voltage drop of the LCL filter, and the AC feeder voltage is 0. The voltage drop of the LCL filter is generally small, so the AC port modulation voltage and the AC feeder voltage are basically equal.

[0097] For example, in case 2: if only active power is transmitted on the AC feeder, and the direction of the active power is from the power grid to the AC port, such as... Figure 3 As shown in (b), the AC feeder voltage and AC feeder current are out of phase, i.e., 180° out of phase, and the AC port modulation voltage U m The amplitude is greater than the amplitude of the AC feeder voltage, and the AC port modulation voltage U m The phase of the voltage lags behind the phase of the AC feeder voltage, therefore the modulation voltage U at AC port 1 to AC port 3... m for:

[0098]

[0099] In an exemplary embodiment, determining the modulation voltages of the three AC ports in step S102 may further include the following:

[0100] When transmitting active and reactive power on an AC feeder, the amplitude of the modulation voltage at the AC port is determined based on the active power, reactive power, amplitude of the AC feeder voltage, amplitude of the AC feeder current, fundamental angular frequency, DC voltage value, and the equivalent inductance of the LCL filter connected between the AC port and its corresponding AC feeder.

[0101] Optionally, when transmitting active and reactive power on an AC feeder, determining the amplitude of the modulation voltage at the AC port may include the following:

[0102] The first phase is determined based on the active power and reactive power of the AC feeder.

[0103] The fourth intermediate voltage value is determined based on the first phase, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter; the fifth intermediate voltage value is determined based on the DC voltage value; and the amplitude of the modulation voltage at the AC port is determined based on the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

[0104] For example, in case 3: if active power and reactive power are transmitted on the AC feeder, and the direction of active power is AC port → grid, the value of reactive power is positive. Specifically, reactive power is positive when the phase of the AC feeder current lags behind the phase of the AC feeder voltage; and negative when the phase of the AC feeder current leads the phase of the AC feeder voltage. Figure 3 As shown in (c), the phase of the AC feeder voltage leads the phase of the AC feeder current, and the AC port modulation voltage U m The amplitude is greater than the amplitude of the AC feeder voltage, and the AC port modulation voltage U m The phase of the voltage leads the phase of the AC feeder voltage, therefore the modulation voltage U at AC port 1 to AC port 3... m for:

[0105]

[0106] For example, in case 4: if active power and reactive power are transmitted on the AC feeder, and the direction of active power is from the power grid to the AC port, and the value of reactive power is positive, such as... Figure 3 As shown in (d), the phase of the AC feeder voltage leads the phase of the AC feeder current, and the AC port modulation voltage U m The amplitude is less than the amplitude of the AC feeder voltage, and the AC port modulation voltage U m The phase of the voltage lags behind the phase of the AC feeder voltage, therefore the modulation voltage U at AC port 1 to AC port 3... m for:

[0107]

[0108] For example, in case 5: if active power and reactive power are transmitted on the AC feeder, and the direction of active power is AC port → power grid, and the value of reactive power is negative, such as... Figure 3 As shown in (e), the phase of the AC feeder voltage lags behind the phase of the AC feeder current, and the AC port modulation voltage U m The amplitude is less than the amplitude of the AC feeder voltage, and the AC port modulation voltage U m The phase of the voltage leads the phase of the AC feeder voltage, therefore the modulation voltage U at AC port 1 to AC port 3... m for:

[0109]

[0110] For example, in case 6: if active power and reactive power are transmitted on the AC feeder, and the direction of active power is from the grid to the AC port, and the value of reactive power is negative, such as... Figure 3 As shown in (f), the phase of the AC feeder voltage lags behind the phase of the AC feeder current, and the AC port modulation voltage U m The amplitude is greater than the amplitude of the AC feeder voltage, and the AC port modulation voltage U m The phase of the voltage lags behind the phase of the AC feeder voltage, therefore the modulation voltage U at AC port 1 to AC port 3... m for:

[0111]

[0112] In the formula, P and Q represent the active power and reactive power on the AC feeder, respectively. Using the above formula, the three-phase modulated waveforms at the input AC port and the output AC port can be calculated under Sinusoidal Pulse Width Modulation (SPWM) mode.

[0113] Calculate the average current flowing through the DC capacitor within one switching cycle and one fundamental cycle, and further obtain the instantaneous current value of the DC capacitor.

[0114] The AC port output levels of the integrated low-voltage flexible interconnection device include levels P, OU, OL, and N. First, the three-phase modulation waves of AC ports 1 to 3 are preprocessed: when the modulation wave value is greater than 0, no changes are made; when the modulation wave value is less than 0, it is changed to 0. Current will flow through the DC capacitor whenever the output level is P.

[0115] The value of the modulation wave corresponds to the duty cycle. When its value is greater than 0, the port level switches frequently between OL and P; when its value is less than 0, the port level switches frequently between OU and N. Since the three-phase modulation waves are 120° apart, only two cases will occur: two phases are positive and one phase is negative, or one phase is positive and two phases are negative. Following the preprocessing method described above, this becomes two phases positive and one phase is zero, or one phase is positive and two phases are zero, as shown below. Figure 4 As shown, two special cases are given: phases a and b are positive, and phase a is positive.

[0116] When the voltage level of a phase is P, the DC capacitor current includes the current of that phase. Figure 4 Explaining (a) in the diagram, both phases a and b have modulation waves greater than 0, and their levels switch between OL and P. Therefore, when both phases a and b have OL, the current flowing through the DC capacitor is 0. When phase a has P and phase b has OL, the current flowing through the DC capacitor is i. a (Understandably, phases a, b, and c are equivalent. When phase b's voltage level is P and phase a's voltage level is OL, it's simply a matter of swapping phases a and b, and the current flowing through them is i.) b When both phases a and b are at voltage level P, the current flowing through the DC capacitor is i. a +i b ; Figure 4 Similarly, in (b), the average current of the DC capacitor during one switching cycle is:

[0117]

[0118] In the formula, θ is the angle corresponding to the current switching cycle, and sign() is defined as... .

[0119] After obtaining the average current of the DC capacitor within one switching cycle, calculate the average current of the DC capacitor within one fundamental cycle, which is the DC component of the current flowing through the DC capacitor.

[0120] The average current of the DC capacitor over one fundamental cycle is calculated by integration and averaging:

[0121]

[0122] Discretizing the above integral formula, it can be transformed into the form of integrating and summing the average current values ​​of each switching cycle within a fundamental period:

[0123]

[0124] In the formula, f sLet f be the switching frequency, f be the fundamental frequency, and Δθ be the angular element. From this formula, the average current flowing through the DC capacitor within one fundamental cycle can be calculated, that is, the DC component of the DC capacitor current.

[0125] Subtracting the average DC capacitor current over one switching cycle from the average DC capacitor current over one fundamental cycle yields the instantaneous DC capacitor current value without a DC current component over one fundamental cycle.

[0126]

[0127] According to the relevant physical theories of capacitance, the amount of charge generated during the charging and discharging of a capacitor is equal to the time integral of the instantaneous value of the capacitor current. The charge function of a DC capacitor can be expressed as:

[0128]

[0129] In the formula, ω is the fundamental angular frequency.

[0130] The peak-to-peak value of the charge change is:

[0131]

[0132] In the formula, Q Cmax and Q Cmin Q C The maximum and minimum values ​​of (θ).

[0133] Assuming the capacitor voltage ripple requirement is that the capacitor voltage fluctuation is less than U... C,pp According to the definition of capacitance The capacitance value of the DC capacitor should meet the following requirements:

[0134]

[0135] In summary, this application proposes a method for determining DC capacitor based on three-port current. By calculating the instantaneous value of the capacitor current and considering the capacitor voltage ripple requirement, the lower limit of the DC capacitor value is calculated. That is, the DC capacitor parameter is determined based on the quantitative analysis of the influence of three-port current on DC side voltage.

[0136] The following is a detailed explanation using specific simulation examples:

[0137] Simulation Example 1: Method for Determining DC Capacitor for Three-Port Current. Taking an 80kVA three-port intensive low-voltage flexible interconnection device as an example, the main circuit parameters are shown in Table 1, and the parasitic resistance is calculated as 5% of the impedance.

[0138] Table 1

[0139]

[0140] The active power of AC feeder 1 is determined to be 1 p.u., and the reactive power to be 0 p.u. First, the active power of AC feeder 2 is determined. Then, based on the apparent power of 1 p.u., the reactive power of AC feeder 2 is calculated. Since reactive power can be positive or negative, one active power corresponds to two possible reactive power values. The active power of AC feeder 3 is determined based on the balance of active power across the three feeders, while its reactive power is calculated based on the apparent power of 1 p.u., also resulting in two possible values. Therefore, after determining the active power of AC feeder 2, there are four possible combinations of reactive power for AC feeders 2 and 3, which need to be iterated sequentially. The results are shown below:

[0141] (1) When the reactive power of AC feeders 2 and 3 is positive, the required DC capacitor is at least 22.8mF.

[0142] (2) When the reactive power of AC feeder 2 is positive and the reactive power of AC feeder 3 is negative, the required DC capacitor is at least 19.3mF.

[0143] (3) When the reactive power of AC feeder 2 is negative and the reactive power of AC feeder 3 is positive, the required DC capacitor is at least 19.3mF.

[0144] (4) When the reactive power of AC feeders 2 and 3 is negative, the required DC capacitor is at least 31.3mF.

[0145] The results above show that the required DC capacitor is maximized when the reactive power of AC feeders 2 and 3 is negative. The relevant quantities involved in the calculation are as follows:

[0146] The three-phase current at AC port 1 is:

[0147]

[0148] The three-phase current at AC port 2 is:

[0149]

[0150] The three-phase current at AC port 3 is:

[0151]

[0152] The three-phase SPWM modulation waveform of AC port 1 is:

[0153]

[0154] The three-phase SPWM modulation waveform of AC port 2 is:

[0155]

[0156] The three-phase SPWM modulation waveform at AC port 3 is:

[0157]

[0158] according to Figure 2 The steps shown involve calculating the average capacitor current during each switching cycle, integrating to obtain the average capacitor current during one fundamental cycle, subtracting the average capacitor current from the average capacitor current to obtain the instantaneous capacitor current without DC component, and finally calculating the peak-to-peak value of the capacitor charge change:

[0159]

[0160] The capacitor voltage ripple is generally required to not exceed 2% of the capacitor voltage, i.e., 0.01 pu DC-side voltage. Therefore, the lower limit for the DC-side capacitor is:

[0161]

[0162] The application of the methods described in the above embodiments will be further illustrated below with specific simulation examples. The system will be simulated and verified using MATLAB / Simulink 2021a software.

[0163] Reference Figure 2 and Figure 5 This is simulation example two of this application, with simulation parameters shown in Table 2. This example provides simulation verification of the effectiveness of the DC capacitance determination method in a three-port low-voltage flexible interconnect device, as detailed below:

[0164] Table 2

[0165]

[0166] As attached Figure 5 As shown, the voltage V of the upper DC capacitor at the DC port of the three-port low-voltage flexible interconnect device is... dc1 The horizontal axis represents time t in seconds (s), and the vertical axis represents capacitor voltage in pU (1 pU represents 750V). Simulation waveforms show that the maximum peak-to-peak fluctuation of the DC capacitor voltage is around 0.0085 pU, and the capacitor voltage ripple can be limited to the required range of 0.01 pU.

[0167] The specific embodiments of this application have been described above. It should be understood that this application 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 substantive content of this application. Unless otherwise specified, the embodiments and features described in the embodiments of this application can be arbitrarily combined with each other.

[0168] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages of other steps.

[0169] Based on the same inventive concept, this application also provides a DC capacitance determination device for a three-port integrated low-voltage distribution network flexible interconnection device, used to implement the DC capacitance determination method for the three-port integrated low-voltage distribution network flexible interconnection device described above. The solution provided by this DC capacitance determination device for a three-port integrated low-voltage distribution network flexible interconnection device is similar to the solution described in the above-described DC capacitance determination method for a three-port integrated low-voltage distribution network flexible interconnection device. Therefore, the specific limitations in one or more embodiments of the DC capacitance determination device for a three-port integrated low-voltage distribution network flexible interconnection device provided below can be found in the limitations of the DC capacitance determination method for a three-port integrated low-voltage distribution network flexible interconnection device described above, and will not be repeated here.

[0170] In one exemplary embodiment, reference is made to Figure 6 A DC capacitance determination device for a three-port intensive low-voltage distribution network flexible interconnection system is provided, comprising:

[0171] The AC port parameter module 610 is used to determine the active power and reactive power of the three AC feeders respectively, and to determine the current and modulation voltage of the three AC ports respectively based on the active power and reactive power of the three AC feeders.

[0172] The instantaneous current value module 620 is used to determine the instantaneous current value of the DC capacitor without DC current component within the fundamental period based on the current and modulation voltage of the three AC ports.

[0173] The capacitance value module 630 is used to determine the capacitance value of a DC capacitor based on the capacitor voltage ripple limit value and the instantaneous current value of the DC capacitor.

[0174] In an exemplary embodiment, the AC port parameter module 610 is further configured to, when transmitting active power on the AC feeder, determine the amplitude of the modulation voltage of the AC port based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and its corresponding AC feeder; and to determine the phase of the modulation voltage of the AC port based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter.

[0175] In an exemplary embodiment, the AC port parameter module 610 is further configured to, when transmitting active power on the AC feeder, determine a first intermediate voltage value based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter; determine a second intermediate voltage value based on the DC voltage value; determine the amplitude of the modulation voltage of the AC port based on the ratio of the first intermediate voltage value to the second intermediate voltage value; and determine a third intermediate voltage value based on the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter; and determine the phase of the modulation voltage of the AC port based on the arctangent function of the ratio of the third intermediate voltage value to the amplitude of the AC feeder voltage.

[0176] In an exemplary embodiment, the AC port parameter module 610 is further configured to determine the amplitude of the modulation voltage of the AC port based on the active power of the AC feeder, the reactive power of the AC feeder, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and its corresponding AC feeder when transmitting active power and reactive power on the AC feeder.

[0177] In an exemplary embodiment, the AC port parameter module 610 is further configured to, when transmitting active and reactive power on the AC feeder, determine a first phase based on the active and reactive power of the AC feeder; determine a fourth intermediate voltage value based on the first phase, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter; determine a fifth intermediate voltage value based on the DC voltage value; and determine the amplitude of the modulation voltage of the AC port based on the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

[0178] The various modules in the DC capacitance determination device of the aforementioned three-port intensive low-voltage distribution network flexible interconnection device can be implemented entirely or partially through software, hardware, or a combination thereof. These modules can be embedded in the processor of a computer device in hardware form or independent of it, or stored in the memory of a computer device in software form, so that the processor can call and execute the corresponding operations of each module.

[0179] In one exemplary embodiment, a computer device is provided, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the steps in the embodiments of the DC capacitance determination method for each of the three-port intensive low-voltage distribution network flexible interconnection devices described above.

[0180] In one exemplary embodiment, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the steps in the embodiments of the DC capacitance determination method for each of the three-port intensive low-voltage distribution network flexible interconnection devices described above.

[0181] In one exemplary embodiment, a computer program product is provided, including a computer program that, when executed by a processor, implements the steps in the embodiments of the DC capacitance determination method for each of the three-port intensive low-voltage distribution network flexible interconnection devices described above.

[0182] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by configuring related hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, artificial intelligence (AI) processors, etc., and are not limited to these.

[0183] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0184] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A method for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device, characterized in that, The device includes three AC ports and one DC port. Each AC port is connected to the power grid via an AC feeder, and the DC port includes multiple DC capacitors connected in series. The method includes: The active and reactive power of each of the three AC feeders are determined, and based on the active and reactive power of the three AC feeders, the current and modulation voltage of each of the three AC ports are determined; there is an active power balance constraint among the three AC feeders, which is: Wherein, P1, P2, and P3 represent the active power of the three AC feeders, respectively. P loss For the losses of the lines and equipment themselves; Based on the current and modulation voltage of the three AC ports, determine the instantaneous current value of the DC capacitor without a DC current component within the fundamental period; The capacitance value of the DC capacitor is determined based on the capacitor voltage ripple limit value and the instantaneous current value of the DC capacitor.

2. The method according to claim 1, characterized in that, Determining the modulation voltages of the three AC ports includes: When active power is transmitted on the AC feeder, the amplitude of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and the AC feeder; and, The phase of the modulation voltage at the AC port is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter.

3. The method according to claim 2, characterized in that, When active power is transmitted on the AC feeder, determining the amplitude of the modulation voltage at the AC port includes: The first intermediate voltage value is determined based on the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter. Based on the DC voltage value, determine the second intermediate voltage value; The amplitude of the modulation voltage at the AC port is determined based on the ratio of the first intermediate voltage value to the second intermediate voltage value; and, Determining the phase of the modulation voltage at the AC port includes: The third intermediate voltage value is determined based on the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance of the LCL filter. The phase of the modulation voltage at the AC port is determined based on the arctangent function of the ratio of the third intermediate voltage value to the amplitude of the AC feeder voltage.

4. The method according to claim 1, characterized in that, Determining the modulation voltages of the three AC ports includes: When active power and reactive power are transmitted on the AC feeder, the amplitude of the modulation voltage of the AC port is determined based on the active power of the AC feeder, the reactive power of the AC feeder, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, the DC voltage value, and the equivalent inductance value of the LCL filter connected between the AC port and the AC feeder.

5. The method according to claim 4, characterized in that, Determining the amplitude of the modulation voltage at the AC port when transmitting active and reactive power on the AC feeder includes: The first phase is determined based on the active power and reactive power of the AC feeder. The fourth intermediate voltage value is determined based on the first phase, the amplitude of the AC feeder voltage, the amplitude of the AC feeder current, the fundamental angular frequency, and the equivalent inductance value of the LCL filter. Based on the DC voltage value, determine the fifth intermediate voltage value; The amplitude of the modulation voltage at the AC port is determined based on the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

6. The method according to claim 1, characterized in that, The determination of the instantaneous current value of the DC capacitor within the fundamental period, excluding the DC current component, includes: The average value of the DC capacitor current during the switching cycle is determined based on the current and modulation voltage of the three AC ports. The average value of the DC capacitor current during the fundamental period is determined based on the average value of the DC capacitor current during the switching cycle. The instantaneous current value of the DC capacitor is determined based on the average DC capacitor current during the switching cycle and the average DC capacitor current during the fundamental cycle.

7. The method according to claim 1, characterized in that, Determining the capacitance value of the DC capacitor includes: Determine the charge function of the DC capacitor based on the instantaneous current value of the DC capacitor; The peak-to-peak value of the charge change of the DC capacitor is determined based on the charge quantity function; The capacitance value of the DC capacitor is determined based on the peak-to-peak value of the charge change and the capacitor voltage ripple limit value.

8. A DC capacitance determination device for a three-port intensive low-voltage distribution network flexible interconnection system, characterized in that, The flexible interconnection device includes three AC ports and one DC port. Each AC port is connected to the power grid via an AC feeder, and the DC port includes multiple DC capacitors connected in series. The DC capacitance determination device includes: The AC port parameter module is used to determine the active power and reactive power of the three AC feeders respectively, and to determine the current and modulation voltage of the three AC ports respectively based on the active power and reactive power of the three AC feeders; there is an active power balance constraint among the three AC feeders, which is: Wherein, P1, P2, and P3 represent the active power of the three AC feeders, respectively. P loss For the losses of the lines and equipment themselves; The instantaneous current module is used to determine the instantaneous current value of the DC capacitor without a DC current component within the fundamental period based on the current and modulation voltage of the three AC ports. The capacitance value module is used to determine the capacitance value of the DC capacitor based on the capacitor voltage ripple limit value and the instantaneous current value of the DC capacitor.

9. A computer device, comprising a memory and a processor, characterized in that, The memory stores a computer program, and when the processor executes the computer program, it implements the method of any one of claims 1 to 7.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the computer program is executed by a processor, it implements the method of any one of claims 1 to 7.

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