Distributed inertia enhancement control method for direct-current bus voltage of low-inertia direct-current micro-grid

Abstract

The invention discloses a direct-current micro-grid which can increase the voltage inertia of a direct-current bus without adding a large-scale physical capacitor. A virtual inertia enhancement controller (VIEC) of the DC-DC converter simulates mechanical inertia characteristics of a DC machine through a virtual capacitor, thereby compensating a power imbalance condition causing voltage fluctuation of a DC bus. When the battery is charged and the generated photovoltaic (PV) power is greater than the load power, the VIEC then controls a path from a DC-DC converter of the PV to a DC bus. When the battery discharges and the generated photovoltaic power is less than the load power, the VIEC controls the path from the DC-DC converter of the battery to the DC bus. When the generated photovoltaic power is greater than the load power and the battery is fully charged, the VIEC simultaneously controls the path from the DC-DC converter of the PV and the DC-DC converter of the battery to the DC bus. The VIEC can simulate an additional virtual capacitor connected in parallel with the DC link capacitor. By simulating the VIEC of the virtual capacitor, the voltage fluctuation of the direct-current bus is obviously reduced.

Description

Technical Field

[0001] The present invention relates to a low-inertia DC microgrid of renewable energy, and more particularly to a distributed inertia control method using a DC-DC converter on the microgrid. Background Art

[0002] Traditionally, electricity is delivered by large utilities through utility grids to thousands of connected customers. The large scale of the grid helps provide a stable voltage to customers. When loads are turned on or off, voltage fluctuations are reduced due to the inertia of the grid.

[0003] Recently, renewable energy generation has been added to the grid. Photovoltaic (PV) solar panels have a low inertia because the energy they produce fluctuates with sunlight, especially when clouds pass through. Battery storage systems are also added to store renewable energy for use when sunlight weakens.

[0004] Climate change exacerbates utility outages from events like hurricanes and wildfires. The grid can be offline for days after a hurricane. Utilities may shut down power lines preemptively during wind events to prevent them from sparking new fires. Some customers are also building their own microgrids that can disconnect from the utility grid so they can still provide power even during an outage.

[0005] Factories, residential complexes or some communities may have their own microgrids, which can be powered by their own renewable energy sources (such as solar arrays) and have batteries installed on their microgrids to store electricity for later use. An island microgrid is a microgrid that can be isolated from the utility grid. An island microgrid is a local grid with its own power generation and backup units, and can operate independently of the utility grid.

[0006] While the utility grid generates and delivers alternating current (AC), some microgrids also generate and deliver direct current (DC). For example, a DC microgrid might have an array of solar panels that convert solar energy into 750 volts of DC power, which is then carried by power lines within the microgrid. A town with its own microgrid might have 750 volt DC power lines on utility poles that carry power to customers' homes and either convert the DC voltage directly on-site to the DC loads or convert it to AC for use with standard AC appliances.

[0007] FIG1 shows a schematic diagram of a low-inertia DC microgrid of the prior art. The DC bus 10 is isolated from the AC grid and is prone to undesirable voltage fluctuations, which is particularly prone to occur on the common DC bus. The solar panels in the photovoltaic (PV) array 20 generate DC power through sunlight and provide current IPV and voltage VPV to the DC-DC converter 22. The DC-DC converter 22 is a switched-mode power supply (SMPS) for providing generated power PG to the DC bus 10. The output capacitor 24 near the DC-DC converter 22 is responsible for adjusting the output voltage and minimizing the ripple.

[0008] The switching signal SPV for the power semiconductors in the DC-DC converter 22 is provided by the PV controller 41. The Maximum Power Point Tracking (MPPT) 49 checks the output current IPV and voltage VPV of the PV array 20 and tracks the power (IPV x VPV) generated by the PV array 20. The MPPT 49 can adjust the power drawn from the PV array 20 by the DC-DC converter 22 by adjusting the control signal DPV and sending it to the pulse width modulator (PWM) 48, wherein the PWM 48 can be used to generate pulses on the switching signal SPV and control the power semiconductors. For example, the pulse of SPV can be adjusted by the PWM generator 48, which allows more power to flow from the PV array 20 to the DC bus 10, thereby increasing the power PG connected to the DC bus. The MPPT can also be used to periodically scan a series of DPV values ​​so that the DC-DC converter 22 draws a varying current IPV and a measured voltage VPV, tracks the power IPV*VPV, and then selects the DPV value that produces the maximum power.

[0009] The battery 30 can be charged by the DC bus 10, or can be discharged to the DC bus with a bidirectional current IB so that power PB flows into / out of the DC bus 10. The bidirectional DC-DC converter 32 connects the battery voltage VB to the DC bus voltage 10, which is called VDC. The battery controller 150 can generate a switching signal SB for the power semiconductor in the DC-DC converter 32. The DC bus voltage controller 54 compares the reference voltage point VDCREF with the DC bus voltage VDC 10 to generate a current reference point IBREF. The current controller 56 compares IBREF with the battery current IB to generate a control DB for the PWM 58, which generates a switching signal SB and can adjust its pulse width in response to DB.

[0010] Figures 2A and 2B show schematic diagrams of simulation results of fluctuations in the DC bus voltage of a low inertia DC microgrid due to changes in solar irradiation. In Figure 2A, the solar radiation falling on the photovoltaic solar panels varies with the time of day and also varies due to clouds blocking the sunlight. In Figure 2B, when the solar radiation changes, the DC bus voltage of the microgrid powered by the photovoltaic panels will overshoot or undershoot by up to 7%.

[0011] Figures 3A-3B show schematic diagrams of simulation results of fluctuations in the DC bus voltage of a low inertia DC microgrid due to load changes. In Figure 3A, the load current drawn from the microgrid varies over time. When the load suddenly switches to a higher load current, the DC bus voltage has a larger undershoot. The extent of these voltage drops can be as high as 6% to 14% of the nominal DC bus voltage, as shown in Figure 3B.

[0012] Figures 4A and 4B show schematic diagrams of simulation results of fluctuations in the DC bus voltage of a low inertia DC microgrid due to changes in solar radiation and load. In Figure 4A, the solar radiation falling on the photovoltaic solar panels varies with the time of day and also varies due to clouds blocking the sunlight. Figure 4B shows the combined effect of solar radiation and load changes on the DC bus voltage. The fluctuation of the DC bus voltage can be as high as 6% to 14%.

[0013] In practice, fluctuations in the DC bus voltage are undesirable as they may lead to power semiconductor failure in the DC-DC converter, rapid battery life degradation, and load shedding.

[0014] 5A to 5B show schematic diagrams of virtual inertia enhancement control principles. FIG. 5A shows an equivalent circuit of a DC motor. The DC motor rotates at an angular velocity (ω) and is driven by a prime mover having a moment of inertia (J). Due to electromagnetic induction, the induced voltage generated across the armature is EA, and IA is the current flowing through the armature winding (shown as an inductor and a resistor). VT is the terminal voltage of the DC motor. The speed of the DC motor can be controlled by its inertia characteristics.

[0015] Figure 5B shows a half-bridge based DC-DC converter circuit. The input supply voltage VIN will be converted to the output supply voltage VOUT. A common ground GND is used for both input and output, but in some systems, separate grounds can still be used.

[0016] The power inductor 314 is connected in series with the resistor 312 and is located between VIN and the source of the high-side power semiconductor 302. The source of the upper power semiconductor 302, the drain of the lower power semiconductor 304 and one side of the resistor 312 are connected together at the switch node.

[0017] The gate G1 of the upper power semiconductor 302 is driven to a high level to turn on the power semiconductor 302 and maintain it for a period of time (which is expressed as (1-D)*Ts, D is the duty cycle, and Ts is the switching period), so that VIN is connected in series with the power inductor 314 to provide a boosted DC output voltage VOUT, and the output capacitor 320 is used to adjust VOUT and minimize VOUT ripple. Once G1 is driven to a low level, the gate of the lower power semiconductor 304 is driven to a high level to charge the power inductor. G1 and G2 are usually signals of complementary clocks in the kHz frequency range, and by adjusting the duty cycle, the output voltage VOUT required for a specific input voltage VIN can be obtained. For example, by increasing the duty cycle of G2 relative to G1, a higher VOUT can be obtained.

[0018] The output capacitor 320 with capacitance CDC can filter the output to provide a more constant output voltage VOUT (or VDC). Node P is the connection point of the output capacitor 320, the current IDC enters the node P, and the output current IOUT flows out of the node P. The difference between IOUT and IDC is the current flowing into the output capacitor 320.

[0019] The power semiconductors 302 , 304 may be n-channel silicon-based Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), but recently power semiconductors based on Silicon Carbide (SiC) or Gallium Nitride (GaN) have also been used because they can provide higher currents for a given physical transistor size and have improved electrical and thermal performance.

[0020] Therefore, the current demand is to enhance the inertia of DC microgrids that are not connected to the public grid. It is also necessary to obtain a low-inertia DC microgrid with a more stable DC bus voltage without simply adding large capacitors. There is also a need for a controller for a DC-DC converter that can simulate the mechanical inertia characteristics of a DC motor through a virtual capacitor, and absorb or support an unbalanced amount of power when the DC bus voltage fluctuates. It is also necessary to be able to equip the DC-DC converter with a virtual inertia enhancement controller (Virtual Inertia Enhancement Controller; VIEC) by adjusting the pulse width (i.e., duty cycle) of the switching signal to virtually enhance the inertia of an island or low-inertia DC microgrid. In addition, there is a need to add VIEC to DC-DC converters for battery energy storage and photovoltaic power generation, thereby providing distributed inertia control for low-inertia DC bus microgrids. Summary of the invention

[0021] According to a first aspect of the present invention, a low-inertia DC microgrid with virtual inertia enhancement is provided, characterized in that it includes: a DC bus, a load, a second DC-DC converter, a second output capacitor, a second controller and a second virtual inertia enhancement controller (Virtual Inertia Enhancement Controller; VIEC). The DC bus has a DC bus voltage. The load draws load power from the DC bus. The second DC-DC converter is connected to a photovoltaic (PV) array, which receives a photovoltaic current with a photovoltaic voltage from the photovoltaic array, and the second DC-DC converter has a power semiconductor controlled by a second switching signal, which converts the photovoltaic voltage into the DC bus voltage on the second output. The second output capacitor is connected to the second output, wherein the second DC current generated by the second DC-DC converter flows into the second output capacitor, and the second load current flows out of the second output capacitor and flows to the DC bus. The second controller is used to generate the second switching signal to the second DC-DC converter to adjust the second DC current to maintain the DC bus voltage. A second virtual inertia enhancement controller is disposed in the second controller, and the second virtual inertia enhancement controller enables the second controller to adjust the second switch signal in response to a difference between the second DC current and a second load current to enhance the effective capacitance of the second output capacitor.

[0022] According to a second aspect of the present invention, a high-inertia DC microgrid is provided, characterized in that it includes: a DC bus, a load, a battery storage unit, a first output capacitor, a first DC-DC converter, a first controller and a first virtual inertia enhancement controller (Virtual Inertia Enhancement Controller; VIEC). The DC bus has a DC voltage and is connected to a plurality of nodes. The load is placed on the DC bus and receives load power. The battery storage unit is used to store energy. The first output capacitor is connected between the DC bus and a first output. The first DC-DC converter is connected between the battery storage unit and the first output. The first controller generates a first control signal to the first DC-DC converter. The first virtual inertia enhancement controller (VIEC) is used to generate a first difference between a first output current sent from the first DC-DC converter to the first output and a first load current sent from the first output capacitor to the DC bus, and the first virtual inertia enhancement controller (VIEC) enables the first controller to adjust the first control signal to increase the first output current by a function of the first difference and the inverse of a first virtual capacitance value. The first virtual inertia enhancement controller (VIEC) enables the first DC-DC converter to simulate a first virtual capacitor having the first virtual capacitance value.

[0023] According to a third aspect of the present invention, a virtual inertia enhancement controller (VIEC) is provided, characterized in that it includes: a first adder, a second adder, a first multiplier, a second multiplier, a third adder, and a third multiplier. The first adder receives an input current and is placed in an output capacitor between the output of a DC-DC converter and a DC bus on a microgrid. The first adder also receives an output current sent from the output capacitor to the DC bus, and the first adder generates a first difference between the input current and the output current. The second adder receives a feedback signal and the first difference and generates a second difference. The first multiplier divides the second difference by a virtual capacitance value to generate a first result. The second multiplier divides the first result by a proportional factor to generate a reference voltage. The third adder receives a nominal reference voltage and the reference voltage and generates a third difference. The third multiplier multiplies the third difference by a conductance value to generate a feedback signal. Furthermore, the reference voltage is compared with the DC bus voltage to generate an adjustment signal, so as to adjust the output current of the DC-DC converter by simulating the virtual capacitance value, thereby compensating for the voltage fluctuation on the DC bus. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] FIG1 shows a low inertia DC microgrid in the prior art.

[0025] Figures 2A and 2B show schematic diagrams of simulation results showing fluctuations in the DC bus voltage of a low inertia DC microgrid due to changes in solar irradiation.

[0026] 3A and 3B are schematic diagrams showing simulation results of fluctuations in the DC bus voltage of a low inertia DC microgrid due to load changes.

[0027] 4A and 4B are schematic diagrams showing simulation results of fluctuations in the DC bus voltage of a low-inertia DC microgrid due to changes in solar radiation and load.

[0028] FIG5A shows an equivalent circuit diagram of a DC machine.

[0029] FIG. 5B shows a circuit diagram of a DC-DC converter of a half-bridge based switch mode power supply (SMPS).

[0030] Figure 6 A schematic diagram of a virtual inertia enhancement controller (VIEC) is presented.

[0031] Figure 7 A schematic diagram of a low-inertia DC microgrid with a virtual inertia enhancement controller (VIEC) is presented.

[0032] FIG. 8A to FIG. 8B A flow chart of VIEC for realizing three control modes by selecting a DC-DC converter is shown.

[0033] Fig. 9 A schematic diagram of a low-inertia DC microgrid with a virtual inertia enhancement controller (VIEC) implemented for a photovoltaic DC-DC converter is presented.

[0034] Fig.10 A schematic diagram of a low-inertia DC microgrid with a virtual inertia enhancement controller (VIEC) for photovoltaic and battery DC-DC converters is presented.

[0035] FIG. 11A to FIG. 11C A schematic diagram of the simulation results of the DC bus voltage fluctuation of a low-inertia DC microgrid due to solar irradiation changes with and without VIEC is shown.

[0036] Figure 12A to Fig. 12C A schematic diagram of the simulation results of the DC bus voltage fluctuation of the low inertia DC microgrid due to load changes with and without VIEC is presented.

[0037] FIG. 13A to FIG. 13C A schematic diagram of the simulation results of the DC bus voltage fluctuation of a low-inertia DC microgrid due to solar irradiation and load changes with and without VIEC is shown.

[0038] Fig.14 A schematic diagram of a multi-PV low-inertia DC microgrid with a virtual inertia enhancement controller (VIEC) implemented for PV and battery DC-DC converters is presented.

[0039] Fig.15 A schematic diagram of the technical solution of using feedforward control in a battery controller to improve VIEC is presented. DETAILED DESCRIPTION

[0040] The technical solution to which the present invention relates includes improving the control of a DC-DC converter to provide enhanced virtual inertia to a low inertia DC microgrid. The following description is intended to enable a person of ordinary skill in the art to make and use the present invention in the context of a specific application and its requirements. It will be clear to those skilled in the art that various modifications to the preferred embodiments and the general principles defined herein can be applied to other embodiments. Therefore, the present invention is not limited to the specific embodiments shown and described, but should be given the widest scope consistent with the principles and novel features disclosed herein.

[0041] The inventors want to increase the inertia of a low-inertia DC microgrid. Increasing the DC bus capacitance by placing large capacitors on the DC bus can increase the inertia, but this is not ideal in practice. Instead, the inventors simulate the inertia characteristics of the DC machine by controlling the DC-DC converter, which will help to virtually increase the capacitance of the DC link.

[0042] The DC machine of Figure 5A has the mechanical DC machine equation:

[0043]

[0044] Applying Kirchhoff's current law to the node P of the DC-DC converter in FIG. 5B yields the following equation:

[0045]

[0046] The inventors realized that these equations are similar. Specifically:

[0047] The moment of inertia (J) in the DC motor of FIG. 5A is similar to the DC bus capacitor (C dc ).

[0048] The damping coefficient (D) (Figure 5A) corresponds to the parasitic conductance (G dc ), (Figure 5B).

[0049] Angular velocity (ω) corresponds to DC bus voltage (V dc ).

[0050] Mechanical torque (T m ) corresponds to the DC bus output current (i dc ).

[0051] The electric torque (T e ) corresponds to the DC bus input current (i out ).

[0052] The inventors have realized that the DC bus capacitance (C dc ) to increase the inertia of the DC bus voltage of the island or low inertia DC microgrid, which is similar to the inertia moment (J) in the DC machine of FIG. 5A due to the rotating turbine or rotor, and provides the inertia of the DC machine. The inventors have realized that increasing the DC bus capacitance (C dc ) can increase the inertia of the DC bus.

[0053] However, the inventors did not use a larger output capacitor 320 (FIG. 5B) in this regard, but virtually increased the DC bus capacitance (C dc ). Therefore, it can be determined that the virtual capacitor C vir is added to the physical output capacitor (C dc ), the total capacitance is C vir +C dc .

[0054] For example, when the load changes suddenly, the energy stored in the DC link capacitor can support the change in load power during the proposed transient according to the following equation:

[0055]

[0056] By increasing the virtual capacitor C vir value, reducing the rate of change of the DC bus voltage.

[0057] Applying Kirchhoff’s current law to the node to which the DC link capacitor of the DC-DC converter is connected yields the Virtual Inertia Enhancement (VIE) equation:

[0058]

[0059] A Virtual Inertia Enhancement Controller (VIEC) can be implemented using this VIE equation.

[0060] Figure 6 A Virtual Inertia Enhancement Controller (VIEC) is shown. The VIEC 52 can implement the VIE equations (equations as described above).

[0061] Adder 140 subtracts the DC load current IDC1 from the load current ILOAD1. Adder 144 subtracts the nominal DC voltage VDCNOM1 from the reference DC voltage VDCREF1, multiplier 154 multiplies the difference by the DC conductivity GDC, and adder 142 subtracts the product from the difference of adder 140. The result of adder 142 is:

[0062] i load1 -i dc1 -G dc (v dcref1 -V dcnom1 )

[0063] The above is the left side of the VIE equation.

[0064] The right side of the VIE equation is:

[0065]

[0066] Next, the inverse of the desired virtual capacitance CVIR is multiplied by both sides of the VIE equation. Multiplier 150 multiplies the left side of the VIE equation by 1 / CVIR. Then, a Laplace transform is performed to Integrate to obtain the DC reference voltage VDCREF1.

[0067] VIEC 52 can output a DC reference voltage VDCREF1, and this voltage will be used to control the DC-DC converter.

[0068] As shown in FIG5A , the rotation inertia J of the DC motor and the DC bus capacitor C of the DC-DC converter are dcIt can act as the inertia constant of DC bus voltage fluctuation. DC bus capacitance C dc The virtual inertia enhancement control (VIEC) can be used to virtually increase the inertia. VIEC controls the fluctuation of the DC bus voltage by controlling the DC-DC converter.

[0069] Figure 7 A low-inertia DC microgrid is shown, in which a virtual inertia enhancement controller (VIEC) is implemented on the battery 30 and interfaces with the DC-DC converter 32 to control the DC bus voltage 10 .

[0070] The battery controller 50 uses VIEC 52 as a power semiconductor in the DC-DC converter 32 to generate a switching signal SB. VIEC 52 compares the current IDC1 output from the DC-DC converter 32 with the current ILOAD1 flowing into the DC bus. In a steady state, these currents should be equal, but when a mismatch occurs, voltage fluctuations occur on the DC bus 10. For example, when the load suddenly increases, ILOAD1 increases, causing the DC bus voltage to fluctuate. Through VIEC52, the energy stored in the DC link capacitor is immediately used to support additional load power requirements without disturbing the DC bus voltage. The virtual capacitor 36 (Cvir1) is simulated by VIEC 52 to provide virtual inertia to the DC bus voltage 10. VIEC 52 generates a reference voltage VDCREF1 for the DC bus voltage controller 54. In a steady state, the battery 30 will provide more current IB to support additional load requirements.

[0071] The DC bus voltage controller 54 compares the reference voltage VDCREF1 from VIEC 52 with the bus voltage VDC of the DC bus 10 to generate a current reference IBREF. The current controller 56 compares IBREF with the battery current IB to generate a control signal DB to the PWM 58, which can generate a switching signal SB and adjust its pulse width to control the power semiconductor.

[0072] The DC bus 10 is isolated from the AC grid and is susceptible to undesirable voltage fluctuations. The solar panels in the photovoltaic (PV) array 20 can generate DC power from sunlight and provide a current IPV and a voltage VPV to the DC-DC converter 22. The DC-DC converter 22 is a SMPS that has power semiconductors that are turned on and off at the order of tens of kHz, thereby providing the generated power PG to the DC bus 10. The output capacitor 24 close to the DC-DC converter 22 also plays the role of smoothing the ripple caused by the switch and regulating the output voltage.

[0073] The PV controller 41 provides a switching signal SPV to the power semiconductors in the DC-DC converter 22. The MPPT 49 checks the current IPV and the voltage VPV of the PV array 20 and tracks the power (IPV x VPV) produced by the PV array 20. The MPPT 49 can adjust the power drawn from the PV array 20 by the DC-DC converter 22 by adjusting the control signal DPV sent to the PWM generator 48, wherein the PWM generator 48 can generate the PWM signal SPV for the power semiconductors. For example, the PWM 48 can increase the duty cycle of the power semiconductors to increase the output voltage to the DC bus 10. The MPPT 49 can also continuously scan a series of DPV values ​​to cause the DC-DC converter 22 to draw higher and lower current IPV values, then measure the voltage VPV, track the power IPV*VPV, and then select the DPV value that produces the maximum power.

[0074] The load power PL can be provided by the DC bus 10 to the DC load 12, and the DC load 12 can be a DC power supply device. The DC / AC converter 13 converts the DC into AC for use by the AC load 15, which can include common household appliances. The capacitor 14 is connected across the load, which smoothes the ripple in the DC bus voltage and adjusts the output voltage. The load power PL includes the power of the DC load 12 and the power of the AC load 15.

[0075] Figure 7 Mode 1 is shown, where the DC-DC converter 32 of the battery 30 can implement Virtual Inertia Enhancement (VIE). However, VIE may be implemented on other DC-DC converters on the DC microgrid. In Mode 2, the DC-DC converter 22 of the PV array 20 is used to implement VIE. In Mode 3, VIE can be implemented on both the PV and battery DC-DC converters.

[0076] These control modes are determined by the power level (P) generated (PG) and the power level (P g ,P l) and the battery's state of charge (SOC).

[0077] FIG. 8A to FIG. 8B The flow chart of VIEC for realizing three control modes by selecting DC-DC converter is shown. Fig. 8A In step 202, the generated power PG, the load power PL and the SOC of the battery 30 are input. In step 204, when the battery SOC is less than 40%, the process proceeds to step 206, where the battery 30 is replaced with a newly charged battery, and the process is repeated from step 202.

[0078] In step 208, the reference power PREF is calculated by subtracting the load power PL from the generated power PG, or using PREF=PG-PL.

[0079] exist Figure 8B In step 210, when PREF is negative, that is, when the load power exceeds the generated power, the process proceeds to step 220 to activate control mode 1. VIEC can be implemented on a DC-DC converter of a battery.

[0080] In step 210, when PREF is positive, that is, when the generated power exceeds the load power, the process proceeds to step 212, and when the battery SOC is greater than 95%, the process proceeds to step 230 to activate control mode 3. VIEC can be implemented on either the battery DC-DC converter or the PV DC-DC converter.

[0081] In step 210, when PREF is positive, that is, when the generated power exceeds the load power, the process proceeds to step 212, and when the battery SOC is not higher than 95%, the process proceeds to step 240 to activate control mode 2. VIEC can be implemented on a PV DC-DC converter.

[0082] Fig. 9 The implementation of a virtual inertia enhanced controller (VIEC) for a photovoltaic DC-DC converter is presented. Mode 2 is described here.

[0083] The PV controller 40 uses VIEC 42 to generate a switching signal SPV for the power semiconductors in the DC-DC converter 22. VIEC 42 compares the current IDC2 output from the DC-DC converter 22 with the current ILOAD2 flowing into the DC bus 10. In a steady state, these currents should be equal, but when a mismatch occurs, voltage fluctuations occur on the DC bus 10. For example, when the PV power suddenly increases, IDC2 will increase, causing voltage fluctuations on the DC bus. Through VIEC 42, the additional power generated can be immediately stored in the DC link capacitor 24 without disturbing the DC bus voltage. The virtual capacitor 26 (Cvir2) can be simulated by VIEC 42 to provide virtual inertia to the DC bus voltage 10. VIEC 42 can generate a reference voltage VDCREF2 for the DC bus voltage controller 44.

[0084] The DC bus voltage controller 44 compares the reference voltage VDCREF2 from VIEC 42 with the bus voltage VDC of the DC bus 10 to generate a current reference IPVREF. The current controller 46 compares IPVREF with the PV current IPV to generate a control DPV and sends it to the PWM 48, where the PWM 48 generates a switching signal SPV and responds to the DPV to adjust its pulse width. In this mode, the MPPT 49 is disabled.

[0085] The battery 30 may be charged from the DC bus 10 by the charging current IB. When the load suddenly increases, the battery 30 may be discharged to the DC bus 10 through the DC-DC converter 32. The battery controller 50 disables the VIEC 52 in Mode 2.

[0086] A stable or constant reference voltage VDCREF1 may be applied to the DC bus voltage controller 54. The DC bus voltage controller 54 compares the reference voltage VDCREF1 with the bus voltage VDC of the DC bus 10 to generate a current reference IBREF. The current controller 56 compares IBREF with the battery current IB to generate a control DB and sends it to the PWM 58, which generates a switching signal SB and adjusts its pulse width in response to DB.

[0087] The DC bus 10 is isolated from the AC grid and is susceptible to undesirable voltage fluctuations. Solar panels in a photovoltaic (PV) array 20 generate DC power from sunlight and provide a current IPV and a voltage VPV to a DC-DC converter 22. Both DC-DC converters 22 and 32 are SMPSs that have power semiconductors that can be operated to switch on and off to provide power to the DC bus 10. An output capacitor 24 near the DC-DC converter 22 can smooth the ripple caused by the switch and regulate the output voltage.

[0088] The load power PL is provided by the DC bus 10 to a DC load 12, wherein the DC load 12 may be a DC power supply device. A capacitor 14 is connected across the DC load 12 to smooth the ripple in the DC bus voltage and regulate the output voltage.

[0089] Fig.10 A low inertia DC microgrid with a Virtual Inertia Enhancement Controller (VIEC) for PV and battery DC-DC converters is presented. Mode 3 is described here.

[0090] The PV controller 40 uses VIEC 42 to generate a switching signal SPV for the power semiconductors in the DC-DC converter 22. VIEC 42 compares the current IDC2 output from the DC-DC converter 22 with the current ILOAD2 flowing into the DC bus 10. In a steady state, these currents should be equal, but when a mismatch occurs, voltage fluctuations occur on the DC bus 10. For example, when the power generated by PV suddenly increases, IDC2 will increase, causing the DC bus voltage to fluctuate. Through VIEC 42, the additional power generated is immediately stored in the DC link capacitor without disturbing the DC bus voltage. The virtual capacitor 26 (Cvir2) is simulated by VIEC 42, thereby providing virtual inertia to the DC bus voltage 10. VIEC 42 can generate a reference voltage VDCREF2 and send it to the DC bus voltage controller 44.

[0091] The DC bus voltage controller 44 compares the reference voltage VDCREF2 from VIEC 42 with the bus voltage VDC of the DC bus 10 to generate a current reference IPVREF. The current controller 46 compares IPVREF with the PV current IPV to generate a control DPV and sends it to the PWM 48, where the PWM 48 generates a switching signal SPV and adjusts its pulse width in response to the DPV. In this mode, the MPPT 49 is disabled.

[0092] The battery 30 can be charged from the DC bus 10, or can be discharged using the battery current IB to supply power to the DC bus 10. The DC-DC converter 32 converts the battery voltage VB to the voltage VDC of the DC bus 10. When the battery state is charged from VDC to VB, the process is reversed.

[0093] The battery controller 50 uses VIEC 52 to generate a switching signal SB to send to the power semiconductors in the DC-DC converter 32. VIEC 52 compares the current IDC1 flowing into the capacitor 34 with the current ILOAD1 flowing out of the capacitor 34. In a steady state, these currents should be equal, but when a mismatch occurs, voltage fluctuations occur on the DC bus 10. For example, when the load suddenly increases, ILOAD1 increases, causing the DC bus voltage to fluctuate. Through VIEC 52, the energy stored in the DC link capacitor can be immediately used to support additional load power requirements without disturbing the DC bus voltage. The virtual capacitor 36 (Cvir1) is simulated by VIEC 52 to provide virtual inertia to the DC bus voltage 10. VIEC 52 generates a reference voltage VDCREF1 and sends it to the DC bus voltage controller 54.

[0094] The DC bus voltage controller 54 compares the reference voltage VDCREF1 from VIEC 52 with the bus voltage VDC of the DC bus 10 to generate a current reference IBREF. The current controller 56 compares IBREF with the battery current IB to generate a control signal DB and sends it to the PWM generator 58, which generates a switching signal SB with a desired duty cycle.

[0095] The DC bus 10 is isolated from the AC grid and is susceptible to undesirable voltage fluctuations. The solar panels in the photovoltaic (PV) array 20 generate DC power from sunlight and generate a current IPV and a voltage VPV. The DC-DC converter 22 is a SMPS that has power semiconductors that can be operated to turn on and off to provide a generated power PG to the DC bus 10. The output capacitor 24 near the DC-DC converter 22 can smooth the ripple caused by the switch and adjust the output voltage.

[0096] The load power PL is provided by the DC bus 10 to the DC load 12, wherein the DC load 12 may be a DC power supply device. The capacitor 14 is connected across the load to smooth the ripple of the DC bus voltage and adjust the output voltage.

[0097] FIG. 11A to FIG. 11C A schematic diagram of the simulation results of the DC bus voltage fluctuation of a low-inertia DC microgrid due to solar irradiation changes with and without VIEC is presented. Fig.11A The amount of solar radiation falling on photovoltaic solar panels varies with the time of day and also due to clouds blocking the sunlight. Fig. 11B In the proposed method, without adopting Virtual Inertia Enhancement (VIE), the DC bus voltage of a microgrid powered by photovoltaic panels may fluctuate by up to 7% when the solar irradiance changes.

[0098] Fig. 11C A PV controller 40 having a VIEC 42 is shown to enable the DC-DC converter 22 to simulate a virtual capacitor 26 (eg, Fig. 9 ), which can significantly reduce the DC bus voltage fluctuation caused by changes in solar radiation.

[0099] Figure 12A to Fig. 12C A schematic diagram of the simulation results of the DC bus voltage fluctuation of a low inertia DC microgrid due to load changes with and without VIEC is presented. In FIG12A , the load current drawn from the microgrid varies with time. When the load suddenly switches to a higher load current, the DC bus voltage may drop. As shown in FIG12B , the voltage drop phenomenon that occurs may be as high as 6% to 14%.

[0100] Fig. 12C A battery controller 50 having a VIEC 52 is shown, which is used to make the DC-DC converter 32 simulate a virtual capacitor 36 (eg, Figure 7 ), thus significantly reducing the DC bus voltage fluctuations caused by load changes.

[0101] FIG. 13A to FIG. 13C A schematic diagram of the simulation results of the DC bus voltage fluctuation of a low-inertia DC microgrid due to solar irradiation and load changes with and without VIEC is presented. Fig.13A The solar radiation falling on photovoltaic solar panels varies with the time of day and also due to clouds blocking the sunlight, which is also Fig.11A In addition, the load also changes, as shown in Figure 12A. Fig. 13B The combined effect of solar radiation and load variation on the DC bus voltage is shown. Overall, the DC bus voltage can have voltage fluctuations as high as 6% to 14%.

[0102] Fig. 13C A photovoltaic controller 40 having a VIEC 42 is shown to enable the DC-DC converter 22 to simulate a virtual capacitor 26 (eg, Fig.10), which can significantly reduce the DC bus voltage fluctuation caused by solar radiation changes. In addition, the battery controller 50 has a VIEC 52 to enable the DC-DC converter 32 to simulate a virtual capacitor 36 (such as Fig.10 ), which significantly reduces the DC bus voltage fluctuations due to load changes. These DC voltage fluctuations are undesirable in practice because they can cause switching failures in the DC-DC converter, rapid degradation of battery life, and load shedding.

[0103] Fig.14 A schematic diagram of a multi-photovoltaic low-inertia DC microgrid with a virtual inertia enhancement controller (VIEC) implemented for photovoltaic and battery DC-DC converters is shown. In this embodiment, there is more than one PV array 20. The additional PV array 520 provides PV power PG1 to the DC bus 10 through a DC-DC converter 522. The capacitor 524 can filter the ripple from the DC-DC converter 522. The PV controller 540 has an MPPT controller 549, which multiplies the voltage VPV1 from the PV array 520 by the current IPV1 to obtain the PV power from the PV array 520, and then adjusts the DPV and sends it to the PWM generator 548 to generate a switching signal SPV of the power device and send it to the DC-DC converter 522.

[0104] However, VIEC is not implemented on the battery controller 50. Instead, VIEC 42 subtracts IDC2 from ILOAD2 and performs Fig.14 The other operations shown in , to achieve the adjustment of VDCREF2. The operation of the DC bus voltage controller 44, the current controller 46 and the PWM generator 48 can be as described above. Fig. 9 When VIEC 42 is disabled, MPPT 49 is activated and used in Mode 1. MPPT 49 obtains the PV power of PV array 20 by multiplying VPV2 by IPV2.

[0105] Therefore, in this embodiment, VIEC is implemented on only one of the PV arrays (PV array 20) instead of on all PV arrays 520. There may also be multiple PV arrays 520, each with its own DC-DC converter 522.

[0106] Fig.15Another DC microgrid with feedforward control is shown. In the battery controller 50', the feedforward controller 55 reads the load current ILOAD1 input to VIEC52 and adds a signal to the reference current IBREF based on the value of ILOAD1, and this signal comes from the DC bus voltage controller 54. The combined value obtained by the adder 57 can be used as IBREF and applied to the current controller 56.

[0107] The feedforward controller 55 bypasses the delays from VIEC 52 and the DC bus voltage controller 54 by directly passing the change in load current ILOAD1 to the current controller 56, which improves the response speed of the battery controller 50'. The amount of IBREF change caused by ILOAD1 can be adjusted, for example, by a scaling factor, and thus the sensitivity of the feedforward can be adjusted.

[0108] Although VIEC 52 and the DC bus voltage controller 54 can still adjust IBREF as before, the adder 57 allows feed-forward control of the load current ILOAD1 to be achieved. Alternative Implementation

[0109] The inventors have also contemplated several other embodiments. For example, multiple combinations and variations of low-inertia DC microgrids and DC-DC converters are possible. Using a DC-DC converter to simulate a virtual capacitor can save the cost, space requirements, and power loss of a physical capacitor while also achieving improved inertia, thereby achieving better DC voltage stability on a low-inertia DC microgrid.

[0110] VIEC 52 can be implemented in various ways. Figure 6 The blocks in the figure can be hardware blocks or programmable logic arrays. Figure 6 Some or all of the operations in can be implemented by a processor, and the term is a digital signal processor (DSP).

[0111] Although the configuration of a single battery 30 and its DC-DC converter 32 has been described, it can be applied to a configuration with multiple batteries and multiple DC-DC converters. VIEC 52 can implement the above-mentioned technical solution for only one of these batteries, or in a subgroup of batteries, or the technical features of VIEC 52 can be replicated for each battery controller 50 of each battery. The load solution may be more complex, and multiple DC-DC converters can be configured for each subgroup of loads. Other devices and subsystems can also be added to the low inertia DC microgrid and connected to the DC bus 10. Although, Figure 7 The DC / AC converter 13 and the AC load 15 are shown in FIG. Figure 7, Fig. 9 , Fig.10 , Fig.14 and Fig.15 It can appear or be unused.

[0112] Although in Figure 8, SOC targets such as 40% and 95% are described, other SOC limits can be used instead. In addition to switching modes at PREF = 0, other values ​​of PREF can also be mode inflection points. Other modes can also be added, especially for more complex low inertia DC microgrid implementations.

[0113] Although the battery controller 50 and PV controller 40 have been described with a DC bus voltage controller driving a current controller which then controls the PWM, other control devices and variations may be substituted. The PI controller used in the voltage and current control may be replaced with a PID, model predictive, adaptive or fuzzy logic controller. Although a battery storage device has been described, other storage devices may be substituted, such as supercapacitors, ultracapacitors or different types of batteries, such as lithium-ion or lead-acid batteries. Although a DC voltage of 750 volts has been described, DC voltages of other strengths may be substituted. Multiple low inertia DC microgrids may be connected together to the DC bus. When a DC-AC converter is provided on the DC bus 10, the load devices may also include devices for AC loads.

[0114] The DC bus 10 may occasionally be connected to the AC grid. While an island DC microgrid may never be connected to the utility grid, other low-inertia DC microgrids may be connected to the utility grid. The DC bus of a grid-connected low-inertia DC microgrid may be connected to the utility grid using a voltage source converter (VSC). The connection to the grid may be temporary or permanent. Although the voltage of the DC bus 10 is shown as 750 volts DC in the figure, it may be replaced by other voltages. This may be part of a subgrid or DC bus 10 connected to other parts, or there may be a DC-DC converter between DC buses with different DC voltages, or it may be part of a photovoltaic generator for different loads or generators.

[0115] The relative size levels of virtual capacitor 26 relative to capacitor 24, or virtual capacitor 36 relative to capacitor 34 may vary. The target ratio may be 1:1 or 1:4, or the ratio may be 1:1 to 1:5.

[0116] The DC-DC converter can be a buck converter, a boost converter, an LLC resonant converter, and use advanced transistors such as silicon carbide (SiC), gallium nitride (GaN), or other advanced technologies.

[0117] At the Maximum Power Point (MPP), the difference between power and voltage is zero. A PV system should operate at or close to the MPP of the PV panel under different conditions (such as varying solar radiation, temperature, and load).

[0118] Maximum power point tracking (MPPT) can be an algorithm implemented in a photovoltaic (PV) array interface DC-DC converter that continuously adjusts the impedance seen by the PV array (or its input impedance) so that the PV operates at the MPP. This is achieved by changing the duty cycle (DPV) of the PWM block, which generates the switching signal (SPV) for the DC-DC converter. The MPPT can continuously measure the voltage VPV, current IPV, and power P = IPV*VPV, and track the change in P (dP) and the change in V (dV) in each cycle to achieve the MPP. If dP>0 and dV>0, then the DPV should be increased, if dP>0 and dV<0, then the DPV should be decreased, if dP<0 and dV>0, then the DPV should be decreased, if dP>0 and dV<0, then the DPV should be increased. The MPPT can use hill climbing to achieve the MMP.

[0119] More complex buffers, level shifters, or other components can be substituted or added. Inverters can be added at various locations. Other delays, hysteresis, and output waveform shaping can be added.

[0120] Different transistor, capacitor, resistor, and other device sizes can be used, and various layout arrangements can be used, such as multi-leg, ring, toroidal, or irregularly shaped transistors. Current can be positive or negative, and can flow in either direction. Many second- and third-order circuit effects may be present and may be significant, especially for smaller device sizes. Circuit simulation can be used to account for these secondary factors during design.

[0121] Terms such as up, down, above, below, horizontal, vertical, inside, outside, etc. are relative and depend on the viewpoint and are not meant to limit the invention to a particular viewing angle. The device can be rotated so that vertical is horizontal and horizontal is vertical, so these terms depend on the observer.

[0122] The background section of the present invention may contain background information about the problem or environment of the present invention rather than describing the prior art of others. Therefore, the inclusion of material in the background section does not constitute an admission by the applicant of prior art.

[0123] Any method or process described herein is machine-implemented or computer-implemented, intended to be performed by a machine, computer or other device, and is not intended to be performed solely by a human being without the assistance of a machine. Tangible results generated may include reports or displays on other machine-generated display devices (such as computer monitors, projection devices, audio generating devices, and related media devices), and may include hard copy printouts also generated by a machine. Computer control of other machines is another tangible result.

[0124] Any advantages and benefits described may not apply to all embodiments of the invention. The word "means" is often preceded by a label consisting of one or more words. The word or words preceding the word "means" are labels intended to facilitate reference to claim elements and are not intended to convey structural limitations. Such means-plus-function claims are intended to cover not only structures described herein for performing the functions and their structural equivalents, but also equivalent structures. For example, although a nail and a screw have different structures, they are equivalent structures because they both perform the function of fastening. The signal is typically an electronic signal, but can be an optical signal, such as an optical signal that can be transmitted over a fiber optic line.

[0125] The foregoing description of the embodiments of the present invention is presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. In view of the above teachings, many modifications and variations may be made. The scope of the present invention is not limited by this detailed description, but by the appended claims.

Claims

1. A low-inertia DC microgrid with virtual inertia enhancement, characterized in that: include: a DC bus having a DC bus voltage; a load, which draws load power from the DC bus; a second DC-DC converter connected to a photovoltaic (PV) array, receiving a photovoltaic current having a photovoltaic voltage from the PV array, the second DC-DC converter having a power semiconductor controlled by a second switching signal, converting the photovoltaic voltage into the DC bus voltage on a second output; a second output capacitor connected to the second output, wherein a second DC current generated by the second DC-DC converter flows into the second output capacitor, and a second load current flows out of the second output capacitor and onto the DC bus; a second controller, configured to generate the second switching signal to the second DC-DC converter to adjust the second DC current to maintain the DC bus voltage; as well as Second Virtual Inertia Enhancement Controller (Virtual Inertia Enhancement Controller; VIEC), disposed in the second controller, the second virtual inertia enhancement controller enables the second controller to adjust the second switch signal in response to a difference between the second DC current and a second load current to enhance the effective capacitance of the second output capacitor.

2. The low-inertia DC microgrid with virtual inertia enhancement according to claim 1, characterized in that: Also includes: a first DC-DC converter connected to a battery and receiving a battery current having a battery voltage from the battery, the first DC-DC converter having a power semiconductor controlled by a first switching signal, which converts the battery voltage into the DC bus voltage on a first output; a first output capacitor connected to the first output, wherein a first DC current generated by the first DC-DC converter flows into the first output capacitor, and a first load current flows out of the first output capacitor and onto the DC bus; a first controller, configured to generate the first switching signal to the first DC-DC converter to adjust the first DC current to maintain the DC bus voltage; as well as First Virtual Inertia Enhancement Controller (Virtual Inertia Enhancement Controller; VIEC), disposed in the first controller, the first virtual inertia enhancement controller enables the first controller to adjust the first switching signal in response to a difference between the first DC current and a first load current to enhance the effective capacitance of the first output capacitor.

3. The low-inertia DC microgrid with virtual inertia enhancement according to claim 2, characterized in that: The second controller also includes: a second virtual inertia enhancement controller (VIEC), disposed in the second controller, the second virtual inertia enhancement controller adjusting a second reference voltage in response to a difference between the second DC current and the second load current; a second DC bus voltage controller, which compares the DC bus voltage with the second reference voltage to generate a second reference current; a second current controller, which compares the photovoltaic current with the second reference current to generate a second pulse width control signal; The second pulse width modulation (PWM) generator generates the second switch signal, and the pulse width of the second switch signal is controlled by the second pulse width control signal.

4. The low-inertia DC microgrid with virtual inertia enhancement according to claim 3, characterized in that: The second controller also includes: A maximum power point tracking (MPPT) controller multiplies the photovoltaic current and the photovoltaic voltage to obtain photovoltaic power, and the maximum power point tracking controller adjusts the second pulse width control signal to enable the photovoltaic array to operate at a maximum power point.

5. The low-inertia DC microgrid with virtual inertia enhancement according to claim 4, characterized in that: The first controller also includes: A first VIEC is disposed in the first controller, and the first VIEC adjusts a first reference voltage in response to a difference between the first DC current and the first load current; A first DC bus voltage controller, which compares the DC bus voltage with a first reference voltage to generate a first reference current; a first current controller, which compares the battery current with the first reference current to generate a first pulse width control signal; The first PWM generator generates the first switching signal, and the pulse width of the first switching signal is controlled by the first pulse width control signal.

6. The low-inertia DC microgrid with virtual inertia enhancement according to claim 5, characterized in that: The photovoltaic power is the photovoltaic voltage multiplied by the photovoltaic current; wherein, when the photovoltaic (PV) power exceeds the load power, the second virtual inertia enhancement controller (VIEC) is enabled, and, when the load power exceeds the PV power, the second virtual inertia enhancement controller (VIEC) is disabled; When the load power exceeds the photovoltaic power and the state of charge (SOC) value of the battery exceeds a first target SOC value, the first virtual inertia enhancement controller (VIEC) is enabled.

7. The low-inertia DC microgrid with virtual inertia enhancement according to claim 6, characterized in that: The first virtual inertia enhancement controller (VIEC) is disabled when the PV power exceeds the load power and the SOC value of the battery does not exceed a second target SOC value greater than the first target SOC value.

8. The low-inertia DC microgrid with virtual inertia enhancement according to claim 7, characterized in that: When the PV power exceeds the load power and the SOC value of the battery exceeds the second target SOC value, a first virtual inertia enhancement controller (VIEC) and a second virtual inertia enhancement controller (VIEC) are enabled.

9. The low-inertia DC microgrid with virtual inertia enhancement according to claim 8, characterized in that: When the second virtual inertia enhancement controller (VIEC) is enabled, the MPPT controller is disabled.

10. The low-inertia DC microgrid with virtual inertia enhancement according to claim 6, characterized in that: The first virtual inertia enhancement controller (VIEC) further comprises: a first subtractor, configured to subtract the first DC current from the first load current to generate a first difference; a first feedback subtractor, configured to subtract a first feedback signal from the first difference to generate a feedback difference; a first multiplier, configured to multiply the feedback difference by a reciprocal of a first virtual capacitance value to generate the first reference voltage; a first feedback path subtractor for subtracting a nominal reference voltage from the first reference voltage to generate a first path signal; a first conductance multiplier, configured to multiply the first path signal by a first conductance value to generate the first feedback signal; The second virtual quantitative enhancement controller (VIEC) further comprises: a second subtractor, configured to subtract the second DC current from the second load current to generate a second difference; a second feedback subtractor, configured to subtract a second feedback signal from the second difference to generate a feedback difference; a second multiplier, configured to multiply the feedback difference by a reciprocal of a second virtual capacitance value to generate the second reference voltage; a second feedback path subtractor for subtracting a nominal reference voltage from the second reference voltage to generate a second path signal; and The second conductance multiplier is used to multiply the second path signal by a second conductance value to generate the second feedback signal.

11. The low-inertia DC microgrid with virtual inertia enhancement according to claim 10, characterized in that: The inverse of the second virtual capacitance value is scaled.

12. The low-inertia DC microgrid with virtual inertia enhancement according to claim 5, characterized in that: The DC bus is isolated from the public power grid.

13. The low-inertia DC microgrid with virtual inertia enhancement according to claim 5, characterized in that: The first controller also includes: A feedforward controller forwards the first load current to be combined with an input of the first current controller.

14. A high inertia DC microgrid, characterized in that: include: A DC bus having a DC voltage and connected to a plurality of nodes; A load is placed on the DC bus and receives load power; battery storage units for storing energy; a first output capacitor connected between the DC bus and the first output; a first DC-DC converter connected between the battery storage unit and the first output; a first controller, generating a first control signal to the first DC-DC converter; a first virtual inertia enhancement controller (VIEC) for generating a first difference between a first output current delivered from the first DC-DC converter to the first output and a first load current delivered from the first output capacitor to the DC bus, the first virtual inertia enhancement controller (VIEC) causing the first controller to adjust the first control signal to increase the first output current by a reciprocal function of the first difference and a first virtual capacitance value; The first virtual inertia enhancement controller (VIEC) enables the first DC-DC converter to simulate a first virtual capacitor having the first virtual capacitance value.

15. The high inertia DC microgrid according to claim 14, characterized in that: Also includes: Photovoltaic PV) array for generating PV electricity; a second output capacitor connected between the DC bus and the second output; a second DC-DC converter connected between the photovoltaic array and the second output; a second controller, configured to generate a second control signal to the second DC-DC converter; as well as a second virtual inertia enhancement controller (VIEC) generating a second difference between a second output current delivered from the second DC-DC converter to the second output and a second load current delivered from the second output capacitor to the DC bus, the second virtual inertia enhancement controller (VIEC) causing the second controller to adjust the second control signal to increase the second output current by a reciprocal function of the second difference and a second virtual capacitance value; wherein the second virtual inertia enhancement controller (VIEC) enables the second DC-DC converter to simulate a second virtual capacitor having the second virtual capacitance value; wherein when the state of charge (SOC) of the battery storage unit is less than a target SOC, the first virtual inertia enhancement controller (VIEC) is disabled; Wherein, when the load power exceeds the photovoltaic power provided by the photovoltaic array, the second virtual inertia enhancement controller (VIEC) is disabled; Thereby, the DC bus inertia is increased by the virtual inertia enhancement controller, resulting in the DC-DC converter being able to simulate a virtual capacitor.

16. The high inertia DC microgrid according to claim 15, characterized in that: The first virtual inertia enhancement controller (VIEC) further comprises: a first feedback adder that subtracts a first feedback signal from the first difference to generate a first correction difference; a first multiplier, which multiplies the first correction difference by the inverse of the first virtual capacitance value to generate a first reference voltage; a first nominal feedback adder that subtracts a first nominal voltage from the first reference voltage to generate a first corrective feedback signal; A first feedback multiplier multiplies the first correction feedback signal by a first conductance to generate the first feedback signal.

17. The high inertia DC microgrid according to claim 16, characterized in that: The second virtual inertia enhancement controller (VIEC) further comprises: a second feedback adder that subtracts a second feedback signal from the second difference to generate a second corrected difference; a second multiplier, configured to multiply the second correction difference by a reciprocal of the second virtual capacitance value to generate a second reference voltage; a second nominal feedback adder that subtracts a second nominal voltage from the second reference voltage to generate a second corrective feedback signal; A second feedback multiplier multiplies the second correction feedback signal by a second conductance to generate the second feedback signal.

18. The high inertia DC microgrid according to claim 17, characterized in that: The first controller also includes: a first DC bus voltage controller, configured to generate a first reference current according to a difference between the first reference voltage of the first virtual inertia enhancement controller (VIEC) and the DC bus voltage; a first current controller generating a first pulse width control signal according to the first reference current and a first current flowing from the battery storage unit into the first DC-DC converter; a first pulse width modulator (PWM) generating a first switching signal to power a power transistor in the first DC-DC converter, and the first PWM adjusting a pulse width of the first switching signal in response to the first pulse width control signal; Wherein, the second controller further includes: a second DC bus voltage controller, which generates a second reference current according to a difference between the second reference voltage of the second virtual inertia enhancement controller (VIEC) and the DC bus voltage; a second current controller, which generates a second pulse width control signal according to the second reference current and a second current sent from the photovoltaic array to the second DC-DC converter; A second pulse width modulator (PWM) generates a second switching signal to power the power transistor in the second DC-DC converter, and the second PWM adjusts the pulse width of the second switching signal in response to the second pulse width control signal.

19. The high-inertia DC microgrid according to claim 18, characterized in that: The second controller also includes: A Maximum Power Point Tracking (MPPT) module, wherein when the second virtual inertia enhancement controller (VIEC) is disabled, the MPPT module generates the second pulse width control signal and sends it to the second PWM, and the MPPT adjusts the second pulse width control signal to maximize the PV power from the PV array.

20. A Virtual Inertia Enhancement Controller; VIEC), characterized in that include: A first adder receiving an input current and disposed in an output capacitor between an output of the DC-DC converter and a DC bus on the microgrid, the first adder also receiving an output current sent from the output capacitor to the DC bus, and the first adder generating a first difference between the input current and the output current; a second adder receiving the feedback signal and the first difference value and generating a second difference value; a first multiplier, which divides the second difference by the virtual capacitance value to generate a first result; a second multiplier that divides the first result by a scale factor to generate a reference voltage; a third adder receiving the nominal reference voltage and the reference voltage and generating a third difference value; as well as a third multiplier that multiplies the third difference by the conductance value to generate a feedback signal; The reference voltage is also compared with the DC bus voltage to generate an adjustment signal to adjust the output current of the DC-DC converter by simulating the virtual capacitor value, thereby compensating for voltage fluctuations on the DC bus.

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