A transient adaptive virtual inertia control method for batteries in DC microgrids

Abstract

The present invention discloses a transient adaptive virtual inertia control method for a battery in a DC microgrid. The method comprises determining a system operating condition based on the DC bus voltage of the microgrid and its rate of change, determining an inertia compensation requirement and designing a virtual capacitor input amount based on the system operating condition, determining a droop coefficient increment based on the accumulated power on the source side and the virtual capacitor requirement, and influencing the transient duration after load fluctuation by adjusting the battery power output in real time. When the absolute value of the DC voltage deviation gradually increases, the virtual capacitor adaptive term provides stronger inertia support for the system. When the absolute value of the DC voltage deviation gradually decreases, the virtual capacitor adaptive term reduces the influence of the constant virtual capacitor and accelerates the voltage recovery speed, thereby providing the DC microgrid with flexible virtual inertia support capability and effectively improving the voltage quality and dynamic characteristics of the system.

Description

Technical Field

[0001] The present invention relates to the technical field of improving the dynamic performance of a DC microgrid, and in particular to a transient adaptive virtual inertia control method for a battery in a DC microgrid. Background Art

[0002] DC microgrids, as a valuable complement to larger power grids, offer flexible and controllable power regulation capabilities while minimizing frequency and power angle stability. They are currently an effective technological solution for integrating renewable energy networks and integrating distributed power sources and loads. However, the high degree of power electronics hides the inherent inertia of microgrid ports, while high renewable energy penetration can easily induce frequent DC voltage fluctuations. These "double highs" pose significant challenges to the stable operation of microgrids, making voltage quality a hot topic in microgrid research. To address this issue, experts and researchers have improved the DC voltage waveform by adding inertia. However, constant virtual capacitance struggles to cope with variable system operating conditions. While introducing virtual inertia provides voltage support, it also prolongs the voltage recovery process. Therefore, microgrids urgently need flexible inertia regulation capabilities to further improve the system's dynamic performance. Summary of the Invention

[0003] In order to solve the above problems, the purpose of the present invention is to provide a transient adaptive virtual inertia control method for batteries in a DC microgrid, which is used to improve the dynamic performance of the DC microgrid system.

[0004] To achieve the above technical objectives, the present application provides a method for transient adaptive virtual inertia control of batteries in a DC microgrid, wherein the DC microgrid is composed of a wind farm, a photovoltaic battery group, a battery, a DC load, an AC load, and a DC bus, including the following steps:

[0005] According to the DC bus voltage and its rate of change, the inertia compensation demand and virtual capacitor input amount of the DC microgrid are obtained;

[0006] Based on the accumulated power on the source side and the demand for virtual capacitance, the droop coefficient increment of the DC microgrid is obtained. By adjusting the battery power output in real time, the transient duration of the DC microgrid after load fluctuations is controlled, and the system inertia of the DC microgrid is dynamically supplemented.

[0007] Preferably, in the process of obtaining the inertia compensation demand and the amount of virtual capacitor input, the operating conditions are determined based on the DC bus voltage and its rate of change, and the inertia compensation demand is determined and the amount of virtual capacitor input is set based on the operating conditions, wherein the DC voltage offset is obtained through the DC bus voltage based on the DC voltage change rate to obtain the system operating conditions.

[0008] Preferably, in the process of obtaining the system operating condition, the system operating condition is expressed as:

[0009]

[0010] Among them, u dc Indicates the DC bus voltage, Indicates the rate of change, Δu dc Indicates the DC voltage offset, Δu dc =u dcref -u dc ,u dcref Indicates the DC voltage reference value.

[0011] Preferably, in the process of obtaining the inertia compensation demand, the inertia compensation demand of the DC microgrid under the operating condition is obtained according to the change trend of the absolute value of the DC voltage offset corresponding to the operating condition.

[0012] Preferably, in the process of setting the virtual capacitor input amount, the expression of the virtual capacitor input amount is:

[0013]

[0014] Wherein, the inertia coefficients k1 and k2 are both positive numbers.

[0015] Preferably, in the process of obtaining the droop coefficient increment, the accumulated power on the source side is expressed as:

[0016]

[0017] Among them, u dc_L Indicates the DC voltage on the load side, Δu dc_L Indicates the DC voltage deviation on the load side.

[0018] Preferably, in the process of obtaining the droop coefficient increment, the droop coefficient increment is expressed as:

[0019]

[0020] Where △k is the increment of droop coefficient, and are the first-order and second-order derivatives of the DC voltage on the load side of the system, respectively.

[0021] Preferably, in the process of dynamically supplementing the system inertia of the DC microgrid, based on the droop coefficient increment, the battery is controlled to adjust the output to provide additional virtual capacitance for the DC microgrid, thereby dynamically supplementing the system inertia of the DC microgrid.

[0022] Preferably, a transient adaptive virtual inertia control system for implementing the transient adaptive virtual inertia control method includes:

[0023] The compensation demand module is used to obtain the inertia compensation demand and virtual capacitor input amount of the DC microgrid based on the DC bus voltage and its change rate;

[0024] The dynamic replenishment module is used to obtain the droop coefficient increment of the DC microgrid based on the accumulated power on the source side and the virtual capacitor demand. By adjusting the battery power output in real time, it controls the transient duration of the DC microgrid after load fluctuations and dynamically replenishes the system inertia of the DC microgrid.

[0025] The present invention discloses the following technical effects:

[0026] When the DC voltage fluctuates due to a sudden change in the equivalent load, the present invention allows the battery to provide additional virtual capacitance for the DC microgrid by adjusting its output. Compared with constant virtual capacitance control, transient adaptive virtual inertia control can provide stronger inertial support when the voltage deviates, delaying the deviation of the DC voltage. At the same time, when the voltage recovers, the impact of the additional virtual capacitance is reduced, accelerating the recovery of the DC voltage, thereby achieving flexible inertial support and comprehensively improving the transient performance of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 This is a five-terminal island DC microgrid used to verify the effectiveness of the present invention.

[0029] Figure 2 It is a flow chart of the method shown in the present invention;

[0030] Figure 3 It is a block diagram of the transient adaptive virtual inertia control strategy of the energy storage unit shown in the present invention;

[0031] Figure 4 1 is a comparison chart of simulation results of the present invention's method, transient adaptive inertia control method, and fixed droop control in simulation example 1 shown in the present invention;

[0032] Figure 5 2 is a comparison chart of simulation results of the method of the present invention, the transient adaptive inertia control method, and the fixed droop control method shown in the simulation example 2 of the present invention;

[0033] Figure 6 3 is a comparison chart of simulation results of the present invention's method, transient adaptive inertia control method, and fixed droop control;

[0034] Figure 7 3 is a comparison chart of simulation results of simulation example 4 shown in the present invention using the method of the present invention, the transient adaptive inertia control method, and the fixed droop control method. DETAILED DESCRIPTION

[0035] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the application for protection, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of this application.

[0036] like Figure 1-7 As shown, the present invention provides a transient adaptive virtual inertia control method for batteries in a DC microgrid, which is beneficial to improving the power quality of the DC microgrid.

[0037] Figure 2 The flowchart of the transient adaptive virtual inertia control method of the battery implemented in the present invention includes the following steps:

[0038] Step 1: Determine the system operating conditions based on the microgrid DC bus voltage and its change rate;

[0039] Step 2: Determine the inertia compensation requirements based on the system's operating conditions and design the amount of virtual capacitor input;

[0040] Step 3: Determine the droop coefficient increment based on the accumulated power on the source side and the virtual capacitor demand, and adjust the battery power output in real time to influence the transient duration after the load fluctuation.

[0041] For multi-terminal DC microgrids, DC voltage is a key indicator of power quality, and the voltage level is directly and negatively correlated with load conditions. When the load increases, DC capacitors discharge to maintain system power balance, causing the DC voltage to drop. When the load decreases, the power supply output current exceeds the load current, and the DC capacitors absorb the charge, causing the voltage to rise. Considering the varying signs of DC voltage deviations and the random fluctuations of renewable energy sources, the system's operating conditions become even more complex and variable.

[0042] In step 1, the system operating conditions are determined based on the microgrid DC bus voltage and its change rate, specifically:

[0043] Real-time monitoring system DC voltage udc and its rate of change Determine the system DC voltage offset Δu based on the DC bus voltage dc =u dcref -u dc , according to the DC voltage offset Δu dc and DC voltage change rate The actual operating conditions of the system are divided into symbols, specifically

[0044]

[0045] In step 2, the inertia compensation requirement is determined based on the system's operating conditions and the amount of virtual capacitor input is designed, specifically:

[0046] The four operating conditions in step 1 can be divided into two categories: operating conditions 1 and 3 correspond to the process of DC voltage recovery, and the absolute value of the DC voltage deviation in this transient process |Δu dc This process helps reduce DC voltage deviation and improve power quality. To shorten voltage recovery time, the amount of virtual capacitors must be reduced.

[0047] Operating conditions 2 and 4 correspond to the transient process of DC voltage deviation. The absolute value of the system DC voltage deviation in this process |Δu dc | gradually increases. In order to prevent voltage deviation, it is necessary to increase the amount of virtual capacitance.

[0048] In summary, in order to meet the inertia compensation requirements of the DC microgrid under four operating conditions, the virtual capacitor input is designed to be

[0049]

[0050] In the above formula, both inertia coefficients k1 and k2 are positive numbers, and k1 is a constant virtual capacitance term. The second term It is a virtual capacitance adaptive term, and its sign depends on the signs of the voltage deviation and the voltage change rate. It is negative for operating conditions one and three and positive for operating conditions two and four, which meets the inertia compensation requirements.

[0051] In step 3, the present invention improves the droop coefficient by introducing the DC voltage and its rate of change into the calculation of the droop coefficient. By establishing an equation between the transient additional power on the battery side and the power external characteristic of the variable capacitor, the transient effect of the variable virtual capacitor is achieved. Specifically,

[0052] For DC microgrids, due to the existence of line impedance, there is a slight difference between the DC voltages of each port, but when the system is operating stably, the change trend of the port voltages is the same.

[0053] The battery has the ability of bidirectional power throughput and flexible power regulation. It usually adopts droop control to achieve DC voltage balance regulation. Taking UI droop control as an example, the battery port external characteristic can be expressed as

[0054] u dc_B =u dc_Bref -k0i dc

[0055] In the above formula, u dc_B is the system DC voltage, u dc_Bref is the DC voltage reference value, k0 is the fixed droop coefficient, i dc This is the DC side output current of the battery port.

[0056] In order to put the virtual adaptive capacitor designed in step 2 into the DC microgrid, it is necessary to improve the droop coefficient of the droop control. The charge accumulated by the battery terminal capacitor in the time period [t0, t] using the improved droop control is

[0057]

[0058] In the above formula, u dc_L is the DC voltage on the load side, R1 and L1 are the equivalent resistance and reactance of the line, C1 is the sum of the DC capacitance of the ports other than the battery, k B is the improved droop coefficient of the battery, which can be expressed as a fixed droop coefficient and a droop coefficient increment, i.e. k B =k0+Δk.

[0059] It can be seen from the above formula that after adopting the improved droop control, the additional capacity of the battery DC side capacitor is

[0060]

[0061] If the virtual adaptive capacitor designed in step 2 is used, the charge increment accumulated by the DC capacitor at the battery port during the time [t0, t] is

[0062]

[0063] The above two equations are the external characteristics of the additional power on the source side and the power of the variable virtual capacitor after the droop coefficient is improved. By establishing an equation between the two, the droop coefficient increment of the improved droop control can be calculated as

[0064]

[0065] Where Δk is the increment of droop coefficient, and are the first-order and second-order derivatives of the DC voltage on the load side of the system, respectively.

[0066] As can be seen from the above formula, by improving the calculation formula of the droop coefficient using DC voltage and its derivative, a virtual adaptive capacitor can be provided for the DC microgrid. The amount of virtual capacitor input can be changed in response to the system operating conditions when the load fluctuates, thereby dynamically supplementing the system inertia.

[0067] When the DC voltage fluctuates due to a sudden change in the equivalent load, the battery can provide additional virtual capacitance for the DC microgrid by adjusting its output. Compared with constant virtual capacitance control, transient adaptive virtual inertia control can provide stronger inertial support when the voltage deviates, delaying the deviation of the DC voltage. At the same time, when the voltage recovers, it reduces the impact of the additional virtual capacitance and accelerates the recovery of the DC voltage, thus achieving flexible inertial support and comprehensively improving the transient performance of the system.

[0068] like Figure 3 Shown is the block diagram of the battery transient adaptive virtual inertia control strategy.

[0069] The present invention builds the following on Matlab / Simulink platform Figure 1 The multi-port DC microgrid simulation model shown in Figure 1 has model parameters listed in Table 1. The permanent magnet wind turbine system and photovoltaic array achieve DC interconnection via a W-VSC and PV-DC, respectively. The energy storage unit connects to the DC microgrid via a B-DC. The AC load and DC load draw energy from the DC microgrid via a L-VSC and L-DC, respectively. In the simulation results, the DC load is considered positive as input. The basic parameters of the simulation model are listed in Table 1.

[0070] Table 1

[0071]

[0072]

[0073] Simulation Example 1 of the present invention is that at the initial moment, the wind speed is set to the rated wind speed of 12m / s and the irradiance is set to 1000W / m 2 At this point, the wind turbine and photovoltaic power generation unit are both operating at rated capacity, with outputs of 19 kW and 20 kW, respectively. The AC load is 4 kW, and the DC load is 40 kW. At this point, 5 kW of power is being exchanged between the battery and the DC microgrid, and the DC voltage of the microgrid is approximately 497.5 V. At 3 seconds, the DC load suddenly drops to 37.5 kW.

[0074] Before the load fluctuation, the DC voltage was below the rated voltage until a sudden drop in DC load power at 3 seconds disrupted the initial stable operating state. At this point, the DC microgrid had a transient excess power of approximately 2.5kW, which needed to be balanced by the battery to achieve a new steady state. As the DC capacitors gradually absorbed charge, the system's DC voltage gradually increased and eventually stabilized at around 500V. During this period, the signs of the system's DC voltage deviation and its derivative were both positive, indicating the first operating condition.

[0075] Combine Figure 4 As shown in (a) and (c), the battery using IVC control has the strongest inertia, so its transient duration is about 50% longer than that of the fixed droop control. However, the battery using AVC control can accelerate the recovery of the DC voltage by releasing power, and its transient recovery time is even 50% shorter than that of the fixed droop control.

[0076] In the simulation example 2 of the present invention, at the initial stage of the simulation, the wind turbine and photovoltaic power generation unit are in rated operation, with AC load and DC load of 4kW and 37.5kW respectively. At this stage, the DC voltage of the system is about 500V. Until the DC load suddenly decreases by 3kW at 2s, the changes in the system parameters during this transient process are as follows: Figure 5 shown.

[0077] Initially, the system DC voltage is at its rated value. At the moment the microgrid experiences a sudden load drop at 2 seconds, approximately 3 kW of excess power exists within the system. The DC capacitors absorb this power, causing the voltage to slowly increase. This transient process represents the second operating condition. During this period, the adaptive term in AVC control is positive, so the transient voltage deviation is approximately 0.1 seconds longer than in IVC control.

[0078] In simulation example 3 of the present invention, during the initial phase, the wind turbine and photovoltaic power generation units operated at rated conditions, with the AC and DC loads absorbing 4 kW and 34.5 kW of power, respectively. At this point, the microgrid operated at approximately 503 V. At 2 seconds, the DC load suddenly increased to 37.5 kW, disrupting the system's original stability.

[0079] Initially, the microgrid's DC voltage is approximately 503 V. After a sudden load increase at 2 seconds, a power shortage of approximately 3 kW occurs within the system. The DC capacitors release power, causing a DC voltage drop. This process corresponds to the third operating condition described above.

[0080] Figure 6The curves showing the DC voltage and virtual capacitance during this transient process show that the adaptive term of AVC control is negative during the transient period, and its transient recovery time is even about 60% shorter than that of fixed droop control. However, due to the additional inertia of IVC control, its transient time is the longest.

[0081] Simulation Example 4 of the present invention is that in the initial stage of the simulation, the wind speed is set to 12m / s and the irradiance is set to 1000W / m 2 , the wind turbine and photovoltaic power generation unit are operating at rated conditions, and the AC load and DC load absorb 4kW and 37.5kW of power respectively. At this time, the microgrid is operating at a rated voltage of 500V. At 1.3s, the DC load suddenly increases to 40kW. The parameter changes in the transient process are as follows Figure 7 shown.

[0082] After the microgrid experiences a sudden load increase at 2 seconds, there is a power shortage of about 2.5kW in the system. The DC capacitor releases its charge, causing the DC voltage to drop. This process corresponds to the fourth operating condition described above.

[0083] During this transient period, since the adaptive term of AVC control adds additional inertia to the microgrid, the transient process of its voltage deviation is prolonged compared with IVC control, and its transient duration is about 25% longer than that of IVC control.

[0084] The simulation results verify the correctness of the proposed transient adaptive virtual inertia control method for batteries in DC microgrids.

[0085] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, a feature specified as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, "plurality" means two or more, unless otherwise specifically defined.

[0086] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A transient adaptive virtual inertia control method for batteries in a DC microgrid, wherein the DC microgrid comprises a wind farm, a photovoltaic battery group, batteries, a DC load, an AC load, and a DC bus, characterized in that: The following steps are involved: Obtaining the inertia compensation requirement and virtual capacitor input amount of the DC microgrid according to the DC bus voltage and its change rate; According to the accumulated power on the source side and the virtual capacitor demand, the droop coefficient increment of the DC microgrid is obtained, and the transient duration of the DC microgrid after the load fluctuation is controlled by adjusting the battery power output in real time, thereby dynamically replenishing the system inertia of the DC microgrid; In the process of obtaining the inertia compensation requirement and the amount of virtual capacitor input, the operating condition is determined based on the DC bus voltage and its rate of change. Based on the operating condition, the inertia compensation requirement is determined and the amount of virtual capacitor input is set. The DC voltage offset is obtained through the DC bus voltage based on the DC voltage rate of change to obtain the system operating condition. In the process of obtaining the system operating condition, the system operating condition is expressed as: Among them, u dc Indicates the DC bus voltage, Indicates the rate of change, Δu dc Indicates the DC voltage offset, Δu dc =u dcref -u dc ,u dcref Indicates the DC voltage reference value; In the process of obtaining the inertia compensation demand, obtaining the inertia compensation demand of the DC microgrid under the operating condition according to a change trend of the absolute value of the DC voltage offset corresponding to the operating condition; In the process of setting the virtual capacitor input amount, the expression of the virtual capacitor input amount is: In the formula, the inertia coefficients k1 and k2 are both positive numbers; In the process of obtaining the droop coefficient increment, the source side accumulated electricity is expressed as: Among them, u dc_L Indicates the DC voltage on the load side, Δu dc_L Indicates the DC voltage deviation on the load side; In the process of obtaining the droop coefficient increment, the droop coefficient increment is expressed as: Where Δk is the increment of droop coefficient, and are the first and second derivatives of the DC voltage on the load side of the system, respectively; In the process of dynamically supplementing the system inertia of the DC microgrid, based on the droop coefficient increment, the battery is controlled to adjust the output to provide additional virtual capacitance for the DC microgrid, thereby dynamically supplementing the system inertia of the DC microgrid.

2. The method for transient adaptive virtual inertia control of batteries in a DC microgrid according to claim 1, characterized in that: A transient adaptive virtual inertia control system for implementing a transient adaptive virtual inertia control method includes: A compensation demand module is used to obtain the inertia compensation demand and virtual capacitor input amount of the DC microgrid according to the DC bus voltage and its change rate; The dynamic replenishment module is used to obtain the droop coefficient increment of the DC microgrid based on the accumulated power on the source side and the virtual capacitor demand, control the transient duration of the DC microgrid after load fluctuations by adjusting the battery power output in real time, and dynamically replenish the system inertia of the DC microgrid.

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