Direct-current capacitance determination method for flexible interconnection device of three-port intensive low-voltage power distribution network

Abstract

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

Description

Technical Field

[0001] The present 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 Art

[0002] With the increasing proportion of renewable energy and the growing trend toward more diversified loads, low-voltage distribution networks are facing challenges such as voltage overshoot, line equipment overload, and insufficient capacity to absorb new energy. Traditional solutions involve expanding and renovating low-voltage distribution lines, but this approach carries significant drawbacks, including high costs, 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 regions has been introduced into distribution networks, giving rise to the concept of flexible interconnection.

[0003] Low-voltage flexible interconnection refers to the use of power electronic flexible interconnected devices (Flexible Interconnected Devices, FIDs) to upgrade or build distribution network connection nodes. By utilizing the FID's excellent dynamic flow control capabilities and the complementary spatiotemporal characteristics of loads, multiple power supply areas can be interconnected and supported. This changes the closed-loop design and open-loop operation limitations of traditional low-voltage AC distribution systems, enhances the interaction between sources, networks, and loads, and further improves the power supply reliability of low-voltage distribution networks.

[0004] However, the design of low-voltage distribution network FIDs in related technologies relies on back-to-back, two-level voltage source converters (VSCs) as the hardware foundation. These devices suffer from high cost, bulk, and low power density and efficiency, making them unable to meet the low-cost and scalable 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 (FID) with an ANPC (Active Neutral Point Clamped) topology is proposed, comprising three AC ports and one DC port. The DC capacitor is a crucial component of the DC port, ensuring DC port voltage stability. The AC port voltage is an inverted version of the DC port voltage, thus impacting the AC port voltage quality and, in turn, the power flow regulation performance. Related technologies often neglect the design of DC capacitors for multiple AC port scenarios, resulting in poor power flow regulation performance. 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 consider multi-AC port scenarios and improve the flow regulation effect in response to the above technical problems.

[0006] In a first aspect, an embodiment of the present application provides a method for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device, the device comprising three AC ports and one DC port, each of the AC ports being connected to a power grid via an AC feeder, and the DC port comprising a plurality of DC capacitors;

[0007] The method comprises:

[0008] Determining the active power and reactive power of the three AC feeders respectively, and determining the current and modulation voltage of the three AC ports respectively according to the active power and reactive power of the three AC feeders;

[0009] Determine the instantaneous value of the current of the DC capacitor excluding the DC component of the current within the fundamental wave period according to the current and the modulation voltage of the three AC ports;

[0010] The capacitance value of the DC capacitor is determined according to the capacitor voltage ripple requirement and the instantaneous current value of the DC capacitor.

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

[0012] In the case of transmitting active power on the AC feeder, determining the amplitude of the modulation voltage of the AC port according to 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 of the AC port is determined according to 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 of the AC port includes:

[0015] determining a first intermediate voltage value according to 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;

[0016] determining a second intermediate voltage value according to the DC voltage value;

[0017] determining the amplitude of the modulation voltage of the AC port according to the ratio of the first intermediate voltage value to the second intermediate voltage value; and

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

[0019] determining a third intermediate voltage value according to 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 of the AC port is determined according to an inverse tangent function of a 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] In the case where active power and reactive power are transmitted on the AC feeder, the amplitude of the modulation voltage of the AC port is determined according to 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 of the AC port includes:

[0024] determining a first phase according to the active power of the AC feeder and the reactive power of the AC feeder;

[0025] determining a fourth intermediate voltage value according to 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;

[0026] determining a fifth intermediate voltage value according to the DC voltage value;

[0027] The amplitude of the modulation voltage of the AC port is determined according to 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 excluding the DC component of the current within the fundamental wave period includes:

[0029] determining an average value of the DC capacitor current within a switching cycle based on the currents and modulation voltages of the three AC ports;

[0030] Determining an average value of the DC capacitor current within the fundamental wave period according to the average value of the DC capacitor current within the switching period;

[0031] The instantaneous value of the DC capacitor current is determined according to the average value of the DC capacitor current in the switching cycle and the average value of the DC capacitor current in the fundamental wave cycle.

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

[0033] determining a charge function of the DC capacitor according to an instantaneous current value of the DC capacitor;

[0034] determining a peak-to-peak value of a charge change of the DC capacitor according to the charge function;

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

[0036] In a second aspect, an embodiment of the present application provides a device for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device, the flexible interconnection device comprising three AC ports and one DC port, each of the AC ports being connected to a power grid via an AC feeder, and the DC port comprising a plurality of DC capacitors;

[0037] The DC capacitance determining device comprises:

[0038] an AC port parameter module, configured to respectively determine the active power and reactive power of the three AC feeders, and to respectively determine the current and modulation voltage of the three AC ports based on the active power and reactive power of the three AC feeders;

[0039] A current instantaneous value module, configured to determine the instantaneous current value of the DC capacitor excluding the DC component of the current within a fundamental wave 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 according to the capacitor voltage ripple requirement and the instantaneous current value of the DC capacitor.

[0041] In a third aspect, an embodiment of the present application provides a computer device comprising a memory and a processor, wherein a computer program is stored in the memory, and when the processor executes the computer program, the method described in the first aspect is implemented.

[0042] In a fourth aspect, an embodiment of the present application provides a computer-readable storage medium having a computer program stored thereon, and when the computer program is executed by a processor, the method described in the first aspect is implemented.

[0043] In a fifth aspect, an embodiment of the present application provides a computer program product, including a computer program, which implements the method described in the first aspect when executed by a processor.

[0044] The above-described method, apparatus, computer device, 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 considers the impact of the current at the three AC ports of the intensive low-voltage distribution network flexible interconnection device on the DC capacitor voltage and determines the DC capacitance based on the capacitor voltage ripple requirement. The DC capacitance determination method proposed in the embodiments of the present application fully considers the maximum voltage fluctuation under power flow regulation conditions, thereby ensuring that the voltage ripple requirement is met under all power flow regulation conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] In order to more clearly illustrate the technical solutions in the embodiments of the present application or related technologies, the following briefly introduces the drawings required for use in the descriptions of the embodiments of the present application or related technologies. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying any creative labor.

[0046] Figure 1 This is a schematic diagram of the topological structure of a three-port intensive low-voltage distribution network flexible interconnection device;

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

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

[0049] Figure 4 The diagram below shows the comparison between the modulation wave and the carrier wave and the corresponding DC capacitor current.

[0050] Figure 5 This is the simulation waveform of the DC capacitor voltage under the maximum DC voltage fluctuation condition;

[0051] Figure 6 This is a structural block diagram of a DC capacitance determination device for a three-port intensive low-voltage distribution network flexible interconnection device. DETAILED DESCRIPTION

[0052] In order to make the above-mentioned objectives, technical solutions and advantages of this application more clearly understood, this application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0053] like Figure 1As shown in (a) of FIG, 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 capacitance of the embodiment of the present application is applicable to both DC capacitors C1 and C2.

[0054] In an exemplary embodiment, referring to Figure 2 , provides a method for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device, the method being executed by, for example but not limited to, a computer device, and the method may include the following steps S202~S206.

[0055] S202 : determining the active power and reactive power of the three AC feeders respectively, and determining the current and modulation voltage of the three AC ports respectively according to the active power and reactive power of the three AC feeders.

[0056] S204 , determining the instantaneous value of the current of the DC capacitor excluding the DC component of the current within the fundamental wave period according to the currents and the modulation voltages of the three AC ports.

[0057] S206 , determining the capacitance value of the DC capacitor according to the capacitor voltage ripple limit value and the instantaneous current value of the DC capacitor.

[0058] The DC capacitor of the embodiment of the present application is determined by considering the impact of the currents of the three AC ports on the voltage of the DC capacitor. The voltage of the DC capacitor is related to the current flowing into the DC port, and the currents of the three AC ports determine the current ultimately flowing into the DC port.

[0059] The current of the AC port is determined by the power flow regulation requirement, that is, it is related to the power flow regulation working condition. In the embodiment of the present application, in a three-port flexible interconnection scenario, the power flows of the three AC feeders when the current of the DC port is the maximum are determined by traversal.

[0060] The active power and reactive power of each AC feeder are traversed as variables. Since the intensive low-voltage distribution network flexible interconnection device does not have an internal energy storage part and cannot actively provide active power to the outside, there is an active power balance constraint between the three AC feeders, namely:

[0061]

[0062] Where P1, P2 and P3 are the active powers of the three AC feeders respectively. Assume that the active power flows from the flexible interconnection device to the AC feeder, and ignore the inherent losses of the line and the device P loss .

[0063] It should be noted that the functions of the flexible interconnection device include regulating the active power and reactive power of each AC feeder. Figure 1 As shown in (b) of Figure 1, an LCL filter is connected between the AC port and its corresponding AC feeder. If the LCL filter loss is ignored, the active power of the AC port and its corresponding AC feeder are 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 maximum.

[0065] Where pu stands for per-unit power, 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 rated current at 80kVA). If the reference value for active power is 80kW, then the actual value is 80kW when the per-unit power value is 1 p.u.

[0066] like Figure 1 As shown in (b) of Figure 1, if AC feeder 1 corresponding to AC port 1 is used as the active balancing feeder, to simplify the analysis, the active power of AC feeder 1 is set to 1 p.u. and the reactive power of AC feeder 1 is set to 0 p.u. 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, at least the active power and reactive power of AC feeders 2 and 3 must be traversed to obtain the operating condition with maximum DC capacitor voltage ripple.

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

[0068] 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 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 set to 1 p.u., and the reactive power Q1 of AC feeder 1 is set to 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, when the low-voltage power grid is operating stably, 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. A voltage transformer can be set on the AC feeder to measure the three-phase voltage, which is generally considered to be a stable value. Based on this, under the condition that the voltage of the AC feeder 1 is known, the current of the AC port 1 is It can be calculated as:

[0073]

[0074] The current calculated here is The d-axis component of and q-axis component , when the three-phase voltage u of AC feeder 1 g1 When phase locking is performed, u g1d The rated voltage amplitude is 311V, u g1q is 0V, the above formula can be further changed to:

[0075]

[0076] For the current of AC port 2 and the current at AC port 3 The calculation is similar, and the three-phase voltage u of AC feeder 1 is also used. g1 The synchronous rotating coordinate system obtained by phase locking undergoes dq transformation:

[0077]

[0078]

[0079] To simplify the analysis, assuming that the voltages of the three AC feeders are the same, the above formula is further changed to:

[0080]

[0081]

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

[0083]

[0084]

[0085]

[0086] Considering the voltage drop across the two inductors in the LCL filter, according to Figure 3 The phasor relationship diagram shown calculates the modulation voltage of AC port 1 to AC port 3.

[0087] about Figure 3 Explanation of the phasor relationship diagram in [1]: According to Kirchhoff's voltage law for circuits, the sum of the closed-loop voltages is zero. This means that the sum of the voltage at the AC port, the voltage drop across the LCL filter, and the voltage on the AC feeder is zero. 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 vectors). Depending on the direction of active and reactive power transmission, the current direction also varies, and the relationship between the three phasors also varies.

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

[0089] Optionally, when active power is transmitted on the 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 according to 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 according to the DC voltage value; and the amplitude of the modulation voltage of the AC port is determined according to the ratio of the first intermediate voltage value to the second intermediate voltage value.

[0091] The third intermediate voltage value is determined according to the amplitude of the AC feeder current, the fundamental angular frequency and the equivalent inductance of the LCL filter; and the phase of the modulation voltage of the AC port is determined according to the inverse tangent 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 active power is AC port → grid, Figure 3 As shown in (a), the AC feeder voltage and the AC feeder current are in phase, that is, the phase difference is 0°, 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 AC feeder voltage is ahead of the phase of the AC feeder voltage. For the convenience of calculation, the LCL filter is simplified to an L filter, so the modulation voltage U of AC port 1 to AC port 3 is m for:

[0093]

[0094] Where U LCL is the equivalent L filter voltage, V dc is the DC voltage value, U g and I g where ω is the amplitude of the AC feeder voltage and current, respectively. ω is the fundamental angular frequency, and L is the equivalent inductance of the LCL filter. The LCL filter is equivalent to an L filter, ignoring the filter capacitor C. The inductance of the L filter is equal to the sum of the two inductances of the LCL filter.

[0095] If the LCL filter is equivalent to an 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 the filter capacitor branch is not ignored, 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 is zero. However, the current in the filter capacitor is generally very small, so the amplitude of the AC port current and the AC feeder current are approximately equal.

[0096] The AC port modulation voltage is a control variable of the control system. After modulation, it is finally reflected on the AC port, that is, the fundamental component of the AC port voltage (because the AC port voltage is a step wave composed of multiple levels). The sum of the AC port voltage, the LCL filter voltage drop, and the AC feeder voltage is 0. The LCL filter voltage drop 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 active power is grid → AC port, Figure 3 As shown in (b), the AC feeder voltage and the AC feeder current are in opposite phase, that is, 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 AC feeder voltage lags behind the phase of the AC feeder voltage, so the modulation voltage U of 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 active power and reactive power are transmitted on the AC feeder, the amplitude of the modulation voltage at 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 its corresponding AC feeder.

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

[0102] determining a first phase according to the active power of the AC feeder and the reactive power of the AC feeder;

[0103] The fourth intermediate voltage value is determined according to 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 according to the DC voltage value; and the amplitude of the modulation voltage of the AC port is determined according to the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

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

[0107]

[0108] For example, as in case 5: 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 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 of the AC feeder voltage is smaller than that of the AC port modulation voltage U m The phase of the AC feeder voltage is ahead of the phase of the AC feeder voltage, so the modulation voltage U of AC port 1 to AC port 3 is m for:

[0109]

[0110] For example, as in case 6: if active power and reactive power are transmitted on the AC feeder, and the direction of active power is grid → AC port, 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 AC feeder voltage lags behind the phase of the AC feeder voltage, so the modulation voltage U of AC port 1 to AC port 3 m for:

[0111]

[0112] Where P and Q represent the active power and reactive power on the AC feeder, respectively. Using these formulas, we can calculate the three-phase modulation waveforms at the input and output AC ports under sinusoidal pulse width modulation (SPWM).

[0113] The average value of the current flowing through the DC capacitor in one switching cycle and one fundamental wave cycle is calculated, and the instantaneous value of the current in the DC capacitor is further obtained.

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

[0115] The value of the modulation wave corresponds to the duty cycle. When its value is greater than 0, the port level switches between OL and P at a high frequency; when its value is less than 0, the port level switches between OU and N at a high frequency. Since the three-phase modulation waves are 120° apart, there are only two situations: two phases are positive and one phase is negative, and one phase is positive and two phases are negative. According to the above preprocessing method, it becomes two phases positive and one phase is 0, or one phase is positive and two phases are 0, such as Figure 4 As shown, two special cases are given where phases a and b are positive and phase a is positive.

[0116] When the level of a phase is P, the DC capacitor current includes the current of this phase. Figure 4 Explain (a) in the figure. The modulation waves of phases a and b are both greater than 0, and the levels switch between OL and P. Therefore, when the levels of phases a and b are both OL, the current flowing through the DC capacitor is 0. When the level of phase a is P and the level of phase b is OL, the current flowing through the DC capacitor is i a (It can be understood that the three phases a, b and c are equivalent. When the level of phase b is P and the level of phase a is OL, the current flowing through phase a and phase b is i. b ). When the levels of phases a and b are both P, the current flowing through the DC capacitor is i a +i b ; Figure 4 The same applies to (b) in the figure. Therefore, the average current of the DC capacitor in one switching cycle is:

[0117]

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

[0119] After obtaining the average current value of the DC capacitor within a switching cycle, the average current value of the DC capacitor within a fundamental wave cycle is calculated, that is, the DC component of the current flowing through the DC capacitor.

[0120] By integrating and averaging, the average current value of the DC capacitor within a fundamental wave cycle is calculated as:

[0121]

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

[0123]

[0124] Where, f sis the switching frequency, f is the fundamental frequency, and Δθ is the angle. This formula can be used to calculate the average current flowing through the DC capacitor during one fundamental frequency cycle, which is the DC component of the DC capacitor current.

[0125] Subtract the average value of the DC capacitor current in one switching cycle from the average value of the DC capacitor current in one fundamental wave cycle to obtain the instantaneous value of the DC capacitor current excluding the DC component of the current in one fundamental wave cycle:

[0126]

[0127] According to the physical theory of capacitance, the amount of charge during capacitor charging and discharging 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] Where ω is the fundamental angular frequency.

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

[0131]

[0132] Where Q Cmax and Q Cmin Q C The maximum and minimum values ​​of (θ).

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

[0134]

[0135] In summary, the embodiment of the present application proposes a method for determining the DC capacitor taking into account the three-port current. By calculating the instantaneous value of the capacitor current and considering the capacitor voltage ripple requirements, the lower limit of the DC capacitor value is calculated, that is, the DC capacitor parameters are determined based on a quantitative analysis of the impact of the three-port current on the DC side voltage.

[0136] The following is a detailed description based on a specific simulation example:

[0137] Simulation Example 1 considers the method for determining the DC capacitance of a three-port current. Taking an 80kVA three-port integrated low-voltage flexible interconnect 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 is 0 p.u. First, determine the active power of AC feeder 2, and then calculate its reactive power based on the apparent power of 1 p.u. Since reactive power can be positive or negative, one active power corresponds to two reactive powers, positive and negative. The active power of AC feeder 3 is determined based on the active power balance of the three feeders, while the reactive power is calculated based on the apparent power of 1 p.u., which also has positive and negative cases. 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 traversed in sequence. The results are shown below:

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

[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 capacitance is at least 19.3 mF.

[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 capacitance is at least 19.3 mF.

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

[0145] From the above results, we can see that when the reactive power of AC feeders 2 and 3 is negative, the required DC capacitance is the largest. The relevant quantities involved in the calculation process are as follows:

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

[0147]

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

[0149]

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

[0151]

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

[0153]

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

[0155]

[0156] The three-phase SPWM modulation wave of AC port 3 is:

[0157]

[0158] according to Figure 2 The steps shown are to calculate the average value of the capacitor current in each switching cycle, then integrate to obtain the average value of the capacitor current in one fundamental cycle, and subtract the instantaneous value of the capacitor current without the DC component. Finally, the peak-to-peak value of the capacitor charge change is:

[0159]

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

[0161]

[0162] The application of the method in the above embodiment is further illustrated below with reference to a specific simulation example. MATLAB / Simulink 2021a software is used to perform simulation verification on the system.

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

[0164] Table 2

[0165]

[0166] As attached Figure 5 As shown, it is the voltage V of the upper DC capacitor of the DC port of the three-port low-voltage flexible interconnection device. dc1 The horizontal axis is time t, in seconds (s), and the vertical axis is capacitor voltage, in pu (1 pu represents 750 V). The simulated waveform shows 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 0.01 pu range.

[0167] The above describes specific embodiments of the present application. It should be understood that the present application is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the substantive content of the present application. In the absence of conflict, the embodiments of the present application and the features in the embodiments may be combined with each other in any manner.

[0168] It should be understood that, although the various steps in the flowcharts involved in the various embodiments described above are displayed in sequence according to the instructions of the arrows, these steps are not necessarily executed in sequence in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order restriction on the execution of these steps, and these steps can be executed in other orders. Moreover, at least a portion of the steps in the flowcharts involved in the various embodiments described above can include multiple steps or multiple stages, and these steps or stages are not necessarily executed and completed at the same time, but can be executed at different times, and the execution order of these steps or stages is not necessarily to be carried out in sequence, but can be executed in turn or alternately with other steps or at least a portion of steps or stages in other steps.

[0169] Based on the same inventive concept, an embodiment of the present application also provides a device for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device for implementing the method for determining the DC capacitance of the three-port intensive low-voltage distribution network flexible interconnection device involved above. The implementation solution provided by the device for determining the DC capacitance of the three-port intensive low-voltage distribution network flexible interconnection device is similar to the implementation solution described in the method for determining the DC capacitance of the three-port intensive low-voltage distribution network flexible interconnection device mentioned above. Therefore, the specific limitations in the embodiments of the device for determining the DC capacitance of one or more three-port intensive low-voltage distribution network flexible interconnection devices provided below can be found in the limitations of the method for determining the DC capacitance of the three-port intensive low-voltage distribution network flexible interconnection device above, and will not be repeated here.

[0170] In an exemplary embodiment, referring to Figure 6 , provides a DC capacitance determination device for a three-port intensive low-voltage distribution network flexible interconnection device, comprising:

[0171] AC port parameter module 610, configured to determine the active power and reactive power of the three AC feeders, and determine the current and modulation voltage of the three AC ports according to the active power and reactive power of the three AC feeders;

[0172] The current instantaneous value module 620 is used to determine the instantaneous value of the current of the DC capacitor excluding the DC component of the current within the fundamental wave period based on the current and modulation voltage of the three AC ports;

[0173] The capacitance value module 630 is configured to determine the capacitance value of the DC capacitor according to 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 determine, when active power is transmitted on the AC feeder, 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 of the LCL filter connected between the AC port and its corresponding AC feeder; and 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 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 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 of the LCL filter; and determine the phase of the modulation voltage of the AC port based on an inverse tangent 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 active power and reactive power are transmitted on the AC feeder.

[0177] In an exemplary embodiment, the AC port parameter module 610 is further configured to determine a first phase based on the active power and reactive power of the AC feeder when active power and reactive power are transmitted on 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 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 a ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

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

[0179] In an exemplary embodiment, a computer device is provided, including a memory and a processor, wherein a computer program is stored in the memory, and when the processor executes the computer program, the steps in the above-mentioned embodiments of the method for determining the DC capacitance of each three-port intensive low-voltage distribution network flexible interconnection device are implemented.

[0180] In an exemplary embodiment, a computer-readable storage medium is provided, on which a computer program is stored. When the computer program is executed by a processor, the steps in the above-mentioned DC capacitance determination method embodiments of each three-port intensive low-voltage distribution network flexible interconnection device are implemented.

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

[0182] Those skilled in the art will understand that all or part of the processes in the above-mentioned embodiments can be implemented by configuring the relevant hardware through a computer program. The computer program can be stored in a non-volatile computer-readable storage medium. When the computer program is executed, it can include the processes of the above-mentioned embodiments. Among them, any reference to memory, database 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 various forms, such as static random access memory (SRAM) or dynamic random access memory (DRAM). The databases involved in the various embodiments provided herein may include at least one of a relational database and a non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the various embodiments provided herein may be, but are not limited to, general-purpose processors, central processing units (CPUs), graphics processing units (GPUs), digital signal processors (DSPs), programmable logic devices (PLDs), quantum computing-based data processing logic devices, artificial intelligence (AI) processors, and the like.

[0183] The technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, 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 above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present application shall 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 of the AC ports is connected to the power grid via an AC feeder, and the DC port includes a plurality of DC capacitors connected in series; The method comprises: Determining the active power and reactive power of the three AC feeders respectively, and determining the current and modulation voltage of the three AC ports respectively according to the active power and reactive power of the three AC feeders; Determining the instantaneous value of the current of the DC capacitor excluding the DC component of the current within the fundamental wave period according to the current and the modulation voltage of the three AC ports; The capacitance value of the DC capacitor is determined according to 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 The determining of the modulation voltages of the three AC ports includes: In the case of transmitting active power on the AC feeder, determining the amplitude of the modulation voltage of the AC port according to 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 of the AC port is determined according to 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 In a case where active power is transmitted on the AC feeder, determining the amplitude of the modulation voltage of the AC port includes: determining a first intermediate voltage value according to 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; determining a second intermediate voltage value according to the DC voltage value; determining the amplitude of the modulation voltage of the AC port according to the ratio of the first intermediate voltage value to the second intermediate voltage value; and Determining the phase of the modulation voltage of the AC port includes: determining a third intermediate voltage value according to 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 of the AC port is determined according to an inverse tangent function of a ratio of the third intermediate voltage value to the amplitude of the AC feeder voltage.

4. The method according to claim 1, wherein The determining of the modulation voltages of the three AC ports includes: In the case where active power and reactive power are transmitted on the AC feeder, the amplitude of the modulation voltage of the AC port is determined according to 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 In a case where active power and reactive power are transmitted on the AC feeder, determining the amplitude of the modulation voltage of the AC port includes: determining a first phase according to the active power of the AC feeder and the reactive power of the AC feeder; determining a fourth intermediate voltage value according to 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; determining a fifth intermediate voltage value according to the DC voltage value; The amplitude of the modulation voltage of the AC port is determined according to the ratio of the fourth intermediate voltage value to the fifth intermediate voltage value.

6. The method according to claim 1, wherein The determining of the instantaneous value of the current of the DC capacitor excluding the DC component of the current within the fundamental wave period includes: determining an average value of the DC capacitor current within a switching cycle based on the currents and modulation voltages of the three AC ports; Determining an average value of the DC capacitor current within the fundamental wave period according to the average value of the DC capacitor current within the switching period; The instantaneous value of the DC capacitor current is determined according to the average value of the DC capacitor current in the switching cycle and the average value of the DC capacitor current in the fundamental wave cycle.

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

8. A device for determining the DC capacitance of a three-port intensive low-voltage distribution network flexible interconnection device, characterized in that: The flexible interconnection device includes three AC ports and one DC port, each of the AC ports is connected to the power grid via an AC feeder, and the DC port includes a plurality of DC capacitors connected in series; The DC capacitance determining device comprises: an AC port parameter module, configured to respectively determine the active power and reactive power of the three AC feeders, and to respectively determine the current and modulation voltage of the three AC ports based on the active power and reactive power of the three AC feeders; A current instantaneous value module, configured to determine the instantaneous current value of the DC capacitor excluding the DC component of the current within a fundamental wave 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 according to 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: A computer program is stored in the memory, and when the processor executes the computer program, the method according to any one of claims 1 to 7 is implemented.

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

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