Systems and methods of power-voltage droop control for a direct current (DC) microgrid

Abstract

A direct current (DC) microgrid is provided. The microgrid includes a power converter, a power-voltage (PV) droop control loop, and an internal current control loop of the power converter. The PV droop control loop is configured to receive measurements of droop current amounts, rather than the DC bus current of the DC microgrid. The droop current amounts are used to determine the droop power amount selected as the power amount in the PV relationship. The PV droop control loop is further configured to derive a current reference based on the droop in the droop power amount. The internal current control loop is configured to control the operation of the power converter by deriving a measured current corresponding to the control amount based on the measurements of the droop current amounts, directly comparing the current reference with the measured current, and generating switching signals for the switches of the power converter.

Description

Background Technology

[0001] The field of the present disclosure generally relates to microgrids, and more specifically, to systems and methods for controlling direct current (DC) microgrids.

[0002] A microgrid is a local power grid and can operate in grid-connected mode by being connected to a macrogrid, and in island mode by being disconnected from the macrogrid and functioning autonomously. In a DC microgrid, the bus current of the microgrid is DC. One method of controlling the output of a DC microgrid is power-voltage (PV) droop control, which regulates the output DC-side power as a function of the microgrid's DC bus voltage. Known systems and methods of PV droop control are disadvantageous in some aspects, and improvements are desired.

[0003] In one embodiment, a direct current (DC) microgrid is provided. The microgrid includes a power converter, a power-voltage (PV) droop control loop electrically coupled to the power converter, and an internal current control loop of the power converter. The PV droop control loop is configured to receive measurements of droop currents, which are current quantities other than the DC bus current of the DC microgrid. These droop currents are used to determine a droop power quantity selected as a power quantity in the PV relationship between the DC bus voltage and the power quantity of the DC microgrid in the PV droop control of the DC microgrid. The PV droop control loop is further configured to derive a current reference based on the droop in the droop power quantity, the current reference corresponding to a control quantity of the internal current control loop of the power converter. The internal current control loop of the power converter is electrically coupled to the power converter and the PV droop control loop. The internal current control loop is configured to control the operation of the power converter by deriving a measured current corresponding to a control amount based on measurements of droop current amounts, directly comparing a current reference with the measured current, and generating switching signals for the switches of the power converter based on the comparison.

[0004] In another embodiment, a method for PV droop control of a DC microgrid is provided. The method comprises selecting a droop power amount as a power amount in the PV relationship between the DC bus voltage and the power amount of the DC microgrid in the PV droop control of the DC microgrid. The droop power amount is based on a droop current amount, which is a current amount other than the DC bus current of the DC microgrid. The method also comprises deriving a current reference based on the droop in the droop power amount, wherein the current reference corresponds to a control amount of the internal current control loop of the power converter. The method further comprises deriving a measured current corresponding to the control amount based on the measurements of the droop current amount. Additionally, the method comprises controlling the operation of the power converter by directly comparing the current reference with the measured current in the internal current control loop, and generating switching signals of the switches of the power converter based on the comparison. Brief explanation of the drawing

[0005] These and other features, embodiments, and advantages of the present disclosure will be better understood when the following detailed description is read with reference to the accompanying drawings, in which the same letters indicate the same parts throughout the drawings. FIG. 1a illustrates a known power-voltage (PV) droop control mechanism. FIG. 1b illustrates known PV droop control for an alternating current to direct current (AC / DC) converter. FIG. 1c illustrates the step response of a known PV droop control for the AC / DC converter shown in FIG. 1b. FIG. 1d illustrates known PV droop control for a DC / DC boost converter. Figure 2a is a schematic diagram of an exemplary microgrid. Figure 2b is a flowchart of the method for PV droop control. Figure 3a is an exemplary PV droop control for an AC / DC converter. Figure 3b is the step response of the control for the AC / DC converter shown in Figure 3a. Fig. 3c is an exemplary PV droop control for a DC / DC boost converter. Figure 3d is an exemplary PV droop control of a DC / DC buck converter. Figure 3e is another exemplary PV droop control for an AC / DC converter. FIG. 4a illustrates an exemplary microgrid including a plurality of converters. FIG. 4b illustrates another exemplary microgrid including multiple converters. Figure 5 is a comparison of the impedance plots of the PV droop control shown in Figure 3a with the known PV droop control shown in Figure 1b. Figure 6a is the step response of the PV droop control shown in Figure 3c. Figure 6b shows the impedance plots of the PV droop control illustrated in Figure 3c. Figure 7a illustrates the step response of the PV droop control shown in Figure 3e. Figure 7b shows the impedance plots of the PV droop control illustrated in Figure 3e. Specific details for implementing the invention

[0006] The present disclosure includes power-voltage (PV) droop control for a direct current (DC) microgrid. Method embodiments will be partially obvious and partially explicitly discussed in the following description.

[0007] FIGS. 1a-1d illustrate known methods of droop control in a DC microgrid. FIG. 1a is a schematic diagram of a known power-voltage (PV) droop control mechanism. Droop control is used to control the flow of power between the converters, loads, and power sources of a microgrid. Droop control regulates the power supplied and consumed by the converters, loads, and power sources of the microgrid as a function of the DC bus voltage of the microgrid. As used herein, the DC bus voltage of the DC microgrid refers to the output voltage of the microgrid measured across the DC output bus of the microgrid. The DC bus current of the DC microgrid refers to the output current of the DC microgrid measured at the output bus of the microgrid. In PV droop control, the droop in the DC power output of the power converters is proportional to the droop in the voltage measured across the DC capacitance, and the output DC-side power is regulated as a function of the DC bus voltage. The ratio in the PV relationship between the DC bus voltage and the DC-side power is the droop gain (k). In known PV droop control mechanisms (see FIG. 1b and FIG. 1d), DC side quantities are used as inputs and a DC current reference is generated. A known PV droop equation is given as equation (1).

[0008]

[0009] V0 is the DC voltage reference and is the DC bus voltage when no load is connected to the DC bus. k is the reciprocal of the slope of the PV line in the PV droop characteristic plot (102) or the droop gain. v is the DC bus voltage of the microgrid.

[0010] DC side current reference ( ) can be derived based on the following mathematical formula (2).

[0011]

[0012] The DC bus voltage (v) is measured using a voltage sensor, and the DC side current reference ( It is used to generate ). The PV droop control loop (103) is a DC side current reference ( It is used to generate ).

[0013] In a known PV droop control (see FIG. 1b) for an AC to DC (AC / DC) converter or rectifier (104), the DC side current reference ( ) is input to an additional (extra) control loop (106), such as an external current control loop or a PI (proportional-integral) controller, as follows: d-axis current reference ( Creates )

[0014]

[0015] Three-phase AC electric quantities (i) of current (i) and voltage (e) a,b,c and e a,b,c ) is converted into a representation in the rotation reference frame of the dq axes, and the d-axis current (i d ) or d-axis voltage (v d ) can be derived. The dq reference frame may be a synchronous reference frame in which the d-axis of the frame is aligned with the AC side voltage measured at the point of common coupling (PCC) of the AC / DC converter (104) with the AC grid (not shown) where the AC / DC converter (104) receives AC power. The generated AC current reference in Equation (3) is an indication of power flow from the AC side to the DC microgrid. The AC current reference is provided to the AC side current controller or internal current control loop (108) to generate switching signals of the AC / DC converter.

[0016] As illustrated in FIG. 1b, a known PV droop controller (105) includes a droop control loop (103) and an additional control loop (106). The additional control loop (106) is a DC current reference ( AC d-axis current reference from ) ) is required to obtain. Furthermore, an additional PI control loop is required for the DC side current (i) which is typically not needed in the control of an AC / DC converter. dc It requires additional measurement of ).

[0017] The additional control loop (106) also creates stability issues in droop control, and the additional pole generated by the control loop (106) leads to an underdamped step response. FIG. 1c is a plot of the step response illustrating the effect of the load step response on the DC bus voltage at various values ​​of droop gain (k). The step response has an overshoot. For small loads, the overshoot can be mitigated through the PI controller design. For medium to large loads, the transient voltage drop is excessive and can destabilize the DC microgrid.

[0018] FIG. 1d illustrates known PV droop control for a DC-to-DC (DC / DC) boost converter (104-bs). In the boost converter, the output voltage is greater than the input voltage. In the known PV droop control for the DC / DC boost converter (104-bs), the DC current reference ( ) is provided to an additional control loop (106), such as a PI controller, as an input inductor current reference ( Create ) as follows:

[0019]

[0020] DC current reference ( ) and measured DC current (i dcThe error between ) is input to the PI controller (106) and the input inductor current reference ( Generates ). Generated current reference( ) is an indication of power flow from the input DC bus to the DC microgrid. Input inductor current reference ( ) is given as an internal current control loop (108) of the DC / DC converter (104-bs) to generate switching signals of the switches of the converter (104-bs).

[0021] As illustrated in FIG. 1d, known PV droop control is based on a DC current reference ( Input inductor current reference from ) An additional control loop (106) is required to obtain ). As with the AC / DC converter (104), the additional control loop (106) can create stability issues in droop control, and the additional poles generated by the PI controller can lead to an under-damped step response. Furthermore, the additional control loop (106) is a DC side current (i) that is not typically required in the control of the DC / DC converter. dc It requires additional measurement of ).

[0022] In contrast, the systems and methods disclosed herein solve the problems described above in known droop control mechanisms. Additional control loops are eliminated, thereby increasing the speed and stability of microgrid control. DC side current (i dc Additional measurement of ) is also eliminated, and thereby, the complexity of the system and the DC side bus current (i dc Reduces hardware and software costs for additional sensors.

[0023] FIG. 2a is a schematic diagram of an exemplary DC microgrid (200) implementing the PV droop control mechanisms disclosed herein. In an exemplary embodiment, the microgrid (200) includes a power converter (202). The power converter (202) is a DC converter that converts AC power or DC power into DC power. The power converter (202) may be an AC / DC converter (see FIG. 3a and FIG. 3e described later), a DC / DC boost converter (see FIG. 3c described later), and / or a DC / DC buck converter (see FIG. 3d described later). In a buck converter, the output voltage is smaller than the input voltage. The microgrid (200) also includes an internal current control loop (204) that is electrically coupled to the power converter (202) and configured to control the operation of the power converter (202). As used herein, ‘electrically coupled’ refers to two components being electrically connected directly or through electrical components between the two components, or receiving inputs from one of the two components to the other of the two components directly or through components between the two components, such as receiving digital inputs of firmware. The internal current control loop (204) is configured to generate switch signals of the switches of the power converter (202) based on a control amount so that the output power from the power converter (202) satisfies a desired specification, e.g., a desired output voltage.

[0024] In an exemplary embodiment, the DC microgrid (200) further includes a PV droop control loop (201) electrically coupled to a power converter (202) and an internal current control loop (204). The PV droop control loop (201) may also be referred to as a PV droop controller. The PV droop control loop (201) is configured to receive measurements of droop current amounts, which are current amounts other than the DC bus current of the DC microgrid (200). These droop current amounts are used to determine the droop power amount selected as the power amount in the PV relationship between the DC bus voltage and the power amount of the DC microgrid (200) in the PV droop control of the DC microgrid (200). The PV droop control loop (201) is also configured to derive a current reference based on the droop in the droop power amount, where the current reference corresponds to the control amount of the internal current control loop (204). Control in the internal control loop (204) is generated by a direct comparison between the current reference and the measured current corresponding to the control amount. The measured current corresponding to the controlled amount can be derived based on the measurements of the droop current amount.

[0025] FIG. 2b is a flowchart of an exemplary method for PV droop control. In an exemplary embodiment, the method (250) includes selecting a droop power amount as a power amount in the PV relationship between the DC bus voltage and the power amount in PV droop control of a DC microgrid (252). The droop power amount is based on a droop current amount, which is a current amount other than the DC bus current of the DC microgrid. The droop power amount may be a fictitious power resulting from the product of the droop current amount and the DC bus voltage. In some embodiments, the droop power amount is active power, which is the input power to the power converter. Since the droop current amount for the droop power amount is not the DC bus current, a sensor for the DC bus current is not required in PV droop control, thereby simplifying the configuration and reducing the costs associated with hardware and software for the sensor. Instead, the droop current amount is a controlled amount in the internal current control loop of the power converter or a quantity of the same type as the controlled amount. For example, in the case of an AC / DC converter, the droop current amount can be the input current of the converter, and the control amount of the internal current control loop can be the d-axis input current of the power converter. In the case of a DC / DC boost converter, both the droop current and the control amount can be the input current of the power converter. In the case of a DC / DC buck converter, both the droop current amount and the control amount can be the output current of the converter. Therefore, a sensor for the droop current amount is already included for the control of the power converter.

[0026] In an exemplary embodiment, the method (250) also includes deriving a current reference (254) based on the droop in the droop power amount, wherein the current reference corresponds to the control amount of the internal current control loop of the power converter. Thus, the current reference can be directly provided to the internal current control loop (204) for the control of the power converter (202), thereby eliminating the additional control loop (106) in the known droop control.

[0027] In an exemplary embodiment, the method (250) further comprises deriving a measured current corresponding to a control quantity based on measurements of droop current quantities (256). Since the droop current quantity is the same quantity or of the same type as the control quantity, the measured current corresponding to the control quantity is the measurements of the droop current quantity or a different expression of the measurements of the droop current quantity, and can be derived directly from the measurements of the droop current quantity. For example, in the case of a DC / DC boost converter or a DC / DC buck converter, the control quantity and the droop current quantity are the same quantity, and the measured current corresponding to the control quantity is the measurements of the droop current quantity. In the case of an AC / DC converter, the control quantity is the d-axis input current of the power converter, while the droop current quantity is the input current of the three phases. The d-axis input current is the dq-frame expression of the input current. The d-axis input current can be derived by converting the measured three-phase AC input current into a rotating dq-frame expression.

[0028] In an exemplary embodiment, the method (250) further comprises controlling the operation of the power converter (258) by directly comparing a current reference with a measured current in an internal current control loop of the power converter and generating switching signals of the switches of the power converter based on the comparison.

[0029] By eliminating an additional control loop for generating a current reference for a controlled amount in an internal current control loop and an additional sensor for the DC bus current, the microgrids and methods disclosed herein are advantageous compared to known PV droop control in simplifying hardware and software and increasing the speed in controlling power converters and the stability of microgrids.

[0030] Figures 3a-3d show the virtual power (P) in the PV relationship. ficExemplary PV droop control mechanisms using ) as the droop power are illustrated. In exemplary embodiments, P fic is the product of the DC bus voltage and the converter inductor current as in the following mathematical formula (5).

[0031]

[0032] Converter Internal Current Reference ( ) is generated directly from the DC side bus voltage as shown in the mathematical equation (6) below.

[0033]

[0034] Current reference ( ) differs for different types of converters. In an AC / DC converter (202-ac), the current reference ( ) is the AC side d-axis current ( ) is (see Fig. 3a). In a DC / DC boost converter, the current reference ( ) is the DC side input current ( ) is. In exemplary embodiments, the output of the PV droop control loop (201) serves as a reference input for d-axis current control of the AC / DC converter (202-ac) (Fig. 3a) or input inductor control of the DC / DC boost converter (202-bs) (Fig. 3c).

[0035] In exemplary embodiments, for the AC / DC converter (202-ac), the PV droop is an amount equal to the virtual or power (v×i) with respect to the droop gain (k') and the droop in the DC bus voltage. d Adjusts ). The droop gain (k') is the droop gain (k) in conventional PV droop control and the d-axis grid voltage (E) in the no-load condition. d It is related to ) as follows:

[0036]

[0037] The following mathematical formulas (8) and (9) are derived by bringing mathematical formula (7) back to mathematical formula (6).

[0038]

[0039]

[0040] v d is the d-axis voltage at the input side of the converter. Since converter losses are negligible under normal operating conditions, the ratio of the difference between voltage droops in PV droop equations (1) and (9) is. Under normal operating conditions, the voltage droops in equations (1) and (9) are approximately the same or have a difference ratio ( ) is close to 1, the reason being the ratio between the no-load voltages of the AC and DC sides ( ) is the ratio between the nominal voltages of the DC and AC sides ( This is because it is approximately the opposite of ). When the load level is higher, the difference between the voltage droops in equations (1) and (9) becomes larger, or the difference ratio becomes lower, because, considering that the converters are often placed close to the grid bus, the converter losses are greater than the line losses. The changes in the difference ratio indicate that, under heavy load conditions, the PV droop control disclosed herein provides better control than known droop control, which is confirmed by the higher voltage level in FIG. 3b compared to FIG. 1c.

[0041] In exemplary embodiments, for a DC / DC boost converter (see FIG. 3c), PV droop control is an amount equal to the virtual or power (v×i) with respect to the droop gain (k') and the droop in the DC bus voltage. L Adjusts ). The droop gain (k') is related to the droop gain (k) of conventional PV droop control and the input DC source voltage (E) at no load as follows:

[0042]

[0043] The following mathematical formulas (11) and (12) are obtained by taking mathematical formula (10) back to mathematical formula (6).

[0044]

[0045]

[0046] v in is the voltage at the input side of the converter (202). Since converter losses are negligible under normal operating conditions, the ratio of the difference between voltage droops in PV droop equations (1) and (12) is. Under normal operating conditions, the voltage droops in equations (1) and (12) are approximately the same as each other, or the difference ratio is close to 1. At higher load levels, the difference between the voltage droops in equations (1) and (12) is higher, or the difference ratio ( ) becomes lower. The increase in the difference at higher load levels indicates that voltage regulation using the systems and methods disclosed herein is better than known PV droop control, particularly under heavy load conditions.

[0047] In some embodiments, the droop gain (k') is a variable that varies as a function of time. For example, k' = f(t). In other embodiments, the droop gain (k') is a variable that varies as a function of another variable. For example, k' = f(x), where x is another variable in the converter or DC microgrid.

[0048] FIG. 3d illustrates another exemplary PV droop control mechanism using virtual power for other types of converters, such as DC / DC buck converters (202-bk). In an exemplary embodiment, compared to the PV droop control illustrated in FIG. 3a and FIG. 3c, where the droop current is the input current of the converter (202-ac, 202-bs), the droop current is the output current of the converter (202-bk). Instead of inductors being at the input of the AC / DC converter (202-ac) or the DC / DC boost converter (202-bs), the inductor (304) is placed at the output of the DC / DC buck converter (202-bk). The inductor current reference is directly controlled by the output of the PV droop control loop (201).

[0049] A PV droop control loop (201) using virtual power is advantageous compared to a known PV droop controller (105). The PV droop control loop (201) is less complex than a known PV droop controller (105) because additional control loops (106) and additional DC current measurements are eliminated from the PV droop control loop (201). The elimination of additional control loops also increases control speed and stability by eliminating control delays from additional control loops (106).

[0050] Referring again to FIG. 3b, the step response (306) of an AC / DC converter (202-ac) having a droop control loop (201) is shown. The plots in FIG. 3b can be directly compared with the plots in FIG. 1c showing the step response (107) of an AC / DC converter (104) having a known droop controller (105), because the test conditions are made as identical as possible when generating the plots in both figures. As shown in FIG. 1c and FIG. 3b, both the known droop controller (105) and the droop control loop (201) stabilize at a steady-state DC voltage of 350 V after an initial startup transient from zero voltage. At 0.5 seconds, a load step of 5 ohms is applied on the DC bus by closing the switch. As discussed above, when comparing FIG. 3b with FIG. 1c, the PV droop control loop (201) is better than the known PV droop controller (105) in controlling the DC bus voltage. Furthermore, when comparing performance in equivalent droop gains according to Equation (4), the voltage drop is lower when using the PV droop control disclosed herein than when using the known PV droop controller (105). Through the PV droop control loop (201), the step response (306) of the PV droop control loop (201) is over-attenuated (see FIG. 3b) compared to the under-attenuated step response (see FIG. 1c) of the known PV droop controller.

[0051] FIG. 3e illustrates another exemplary PV droop control mechanism. Compared to the PV droop control loop (201) illustrated in FIG. 3a, 3c, and 3d, which uses virtual power in the PV relationship, the PV droop control loop (201-r) illustrated in FIG. 3e uses active power in the PV relationship. In an exemplary embodiment, neglecting converter losses, the DC-side power can be expressed by the AC-side power as follows.

[0052]

[0053] AC side d-axis current reference from DC side bus voltage ( PV droop control that directly generates ) is provided as follows.

[0054]

[0055] v d is the d-axis voltage on the AC side. Using the measured AC side voltage, v d Instead of inducing, v d can be obtained from the measured grid-side voltage and AC-side current. PV droop control is based on the AC-side active power (3 / 2 v) relative to the droop and droop gain (k) at the DC bus voltage. d ×i d ) controls. The droop gain (k) is functionally identical to the droop gain in known PV droop control. In some embodiments, the droop gain (k) is a variable that varies as a function of time. For example, k = f(t). In other embodiments, the droop gain (k) is a variable that varies as a function of another variable. For example, k = f(x), where x is another variable in the converter or DC microgrid.

[0056] In an exemplary embodiment, if losses between the AC grid and the converter, such as line losses and converter filter losses, are ignored, the AC side power can be expressed as follows.

[0057]

[0058] PV droop control is based on the AC side d-axis current ( It can be used to derive ) as follows.

[0059]

[0060] E, which is the d-axis grid voltage dThe AC grid voltage is generally stable and the changing power flow mainly affects the voltage angle, so it is mostly constant.

[0061] In an exemplary embodiment, the output of the PV droop control serves as a reference input for d-axis current control of the AC / DC converter (202-ac). FIG. 3e illustrates a block diagram of the PV droop control. Internal current control is maintained as in a conventional AD / DC rectifier, and the droop control is a current reference ( The grid-side voltage (E) being introduced to generate ) d It changes according to ).

[0062] Compared to the known PV droop control illustrated in FIG. 1a, 1b, and 1d, which regulates the DC bus voltage using DC-side power, the PV droop control illustrated in FIG. 3e regulates the DC bus voltage using AC-side input power. The active power on the AC side is given as follows:

[0063]

[0064] A synchronously rotating frame v d In the case of alignment with, v q = 0. The AC side active power equation of equation (17) is as follows:

[0065]

[0066] The relationship between the AC side active power and the DC output power can be written as follows:

[0067]

[0068] Typically, AC / DC converters operate at high efficiencies, such as over 99%. Losses in AC filters can be minimized through design. Therefore, This is applied in an AC / DC converter system. However, PV droop control using AC side power is more advantageous than known PV droop control by simplifying hardware and software and increasing the speed of dynamic response and stability of the AC / DC converter by eliminating additional measurement of DC bus current and additional control loop (106).

[0069] Compared to the PV droop controls illustrated in FIG. 3a, 3c, and 3d, which use virtual power in the PV relationship, the PV droop control illustrated in FIG. 3e uses active power in the PV relationship. When active power is used, the input current and input voltage of the AC / DC converter are measured. The AC voltage is noisier than the DC voltage as measured in the methods illustrated in FIG. 3a, 3c, and 3d, because the power converter (202) can inject switching harmonics into the AC side voltage and the capacitance on the AC side is relatively small. When virtual power is used, the DC voltage on the output side is measured instead, which has a much larger capacitance than the capacitance on the AC side, for example, 1000 times larger, and therefore, is much less noisier than the AC voltage on the input side. Due to noise, when active power is used, software and hardware are required for signal processing and filtering of the AC side voltage, increasing the complexity of the circuit design.

[0070] FIGS. 4a and 4b illustrate that the PV droop control mechanisms disclosed herein can be used for multiple converters of a DC microgrid. The converters (202) can be connected in a ring configuration (402) (Fig. 4a). Alternatively or additionally, the converters (202) can be connected in a distributed configuration (404) (Fig. 4b). Each converter (202) is electrically connected to a source or a load.

[0071] The performance of the droop control loop (201) for the known droop controller (105) for AC / DC converters is further analyzed by comparing impedance Bode plots. FIG. 5 shows the impedance (Z) of the AC / DC converter (202-ac). s Plots of (s) are plotted, wherein the only difference between plots (502 and 504) is the type of PV droop control being used. Plots (502) are plots of impedance magnitudes (502-m) and phases (502-p) using a known droop controller (105). Plots (504) are plots of impedance magnitudes (504-m) and phases (504-p) using a droop control loop (201). The impedance plots (502, 504) are aligned so that the x and y axes of plots (502, 504) are identical, facilitating a direct visual comparison between the two plots (502, 504). From the magnitude plots, the PV droop in the PV droop control loop (201) has a lower quality factor than the PV droop in the known PV droop controller (105). Horizontal lines (Z l1 and Z l2 ) represents two different loads applied to the DC terminals of the AC / DC converter (202-ac). Z l1 > Z l2 However, here, the load (l2) has a higher power consumption than the load (l1).

[0072] The power flow stability of the system can be determined by the Middlebrook standard, where

[0073] am.

[0074] Magnitude of system impedance (|Z s (s)|) is plotted in the plots (502-m and 504-m). The magnitude of the load impedance (|Z l(s)|) is represented by two lines (506) in the plots (502-m, 504-m). For system stability, if a PV droop control loop (201) is used, |Z for all frequencies. s (s)| << |Z l (s)| is. In contrast, when a known PV droop controller (105) is used, Z l2 The load line for intersects the magnitude impedance plot (502). The differences indicate that the PV droop control loop (201) is more stable than the known PV droop controller (105), especially for relatively large loads on the DC bus such as the load (l2).

[0075] Step response (306-dc) (Fig. 6a) and impedance plots (Fig. 6b) of a PV droop control loop (201) used in a DC / DC boost converter (202-bs) having virtual power in a PV relationship are provided. The step response (306-dc / dc) and impedance plot (504-dc / dc) are similar to the step response (306) shown in Fig. 3b and the impedance plots (504) shown in Fig. 5. The step response of the PV droop control loop (201) is over-attenuated, and the PV droop control loop (201) is more stable than a known PV droop controller (105).

[0076] Step response (306-r) (Fig. 7a) and impedance plots (Fig. 7b) of a PV droop loop (201) used in an AC / DC converter (202-ac) having active power in a PV relationship are provided. As shown in Fig. 1c and Fig. 7a, both the known droop controller (105) and the droop control loop (201) stabilize at a steady-state DC voltage of 350 V after an initial startup transient from zero voltage. At 0.5 seconds, a load step of 5 ohms is applied on the DC bus by closing the switch. Comparing Fig. 7a with Fig. 1c, the PV droop control loop (201) is better than the known PV droop controller (105) in regulating the DC bus voltage.

[0077] The performance of the droop control loop (201) using active power in the PV relationship for the announced droop controller (105) is further analyzed by comparing impedance Bode plots. FIG. 7b shows the impedance (Z) of the AC / DC converter (202-ac). s Plots of (s) are plotted, wherein the only difference between plots (502 and 504-r) is the type of PV droop controller being used. Plots (502) are plots of the magnitudes (502-m) and phases (502-p) of impedance using a known droop controller (105). Plots (504-r) are plots of the magnitudes (504-rm) and phases (504-rp) of impedance using a droop control loop (201). The impedance plots (502, 504-r) are aligned so that the x and y axes of the plots (502, 504-r) are identical, facilitating a direct visual comparison between the two plots (502, 504-r). From the magnitude plots, the PV droop in the PV droop control loop (201) has a lower quality factor than the PV droop in the known PV droop controller (105). Horizontal lines (Z l1 and Z l2) represents two different loads applied to the DC terminals of the AC / DC converter (202-ac). Z l1 > Z l2 However, here, the load (l2) has a higher power consumption than the load (l1).

[0078] The power flow stability of the system can be determined by the Middlebrook criterion as expressed in Equation (20). The magnitude of the system impedance (|Z s (s)|) is plotted in the plots (502-m and 504-rm). The load impedance magnitude (|Z l (s)|) is represented by two lines (506) in the plots (502, 504-r). When the PV droop control loop (201) is used, |Z for all frequencies s (s)| << |Z l (s)| is. In contrast, when a known PV droop controller (105) is used, Z l2 The load line for intersects the magnitude impedance plot (502). The differences indicate that the PV droop controller (201) is more stable than the known PV droop controller (105), especially for relatively large loads on the DC bus such as the load (l2).

[0079] In some embodiments, the PV droop control loop (201) may be implemented on a controller board using analog circuitry and operational amplifiers. The input voltage is scaled down to a desired low voltage suitable for use on the controller boards using a resistor divider or a capacitive voltage divider. Equation (6) is implemented on the controller board using operational amplifiers by creating a subtractor circuit. Then, the output of the subtractor is given as a reference input for the current control algorithm of the DC microgrid converter.

[0080] In other embodiments, the PV droop control loop (201) may be implemented on a controller board within a microcontroller or FPGA firmware. The input voltage is scaled down to a desired low voltage suitable for use on the controller board using a resistor divider or a capacitive voltage divider. The scaled-down voltage is given to an analog-to-digital converter (ADC) connected to or included in the microcontroller for use as a variable in the firmware. Equation (6) is numerically implemented in the firmware to calculate the converter internal loop current reference, which is then given as a control algorithm for the DC microgrid converter.

[0081] As used herein, the terms “processor” and “computer,” and related terms, e.g., “processing device,” “computing device,” and “controller,” are not limited merely to integrated circuits referred to as computers in the technical field, but broadly refer to microcontrollers, microcomputers, analog computers, programmable logic controllers (PLCs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and other programmable circuits, and these terms are used interchangeably herein. In the embodiments described herein, “memory” may include, but is not limited to, computer-readable media, such as random-access memory (RAM) and computer-readable non-volatile media, such as flash memory. Alternatively, floppy disks, compact disc-read only memory (CD-ROM), magneto-optical disks (MODs), and / or digital versatile discs (DVDs), and / or other optical media storage devices may also be used. Additionally, in the embodiments described herein, additional input channels may be computer peripherals associated with an operator interface, such as touchscreens, mice, and keyboards, but are not limited thereto. Alternatively, other computer peripherals may also be used, such as scanners, but are not limited thereto.Furthermore, in exemplary embodiments, additional output channels may include, but are not limited to, an operator interface monitor or a heads-up display. Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor, a processing device, or a controller, such as a general-purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an ASIC, a programmable logic controller (PLC), a field programmable gate array (FPGA), a digital signal processing (DSP) device, and / or any other circuit or processing device capable of performing the functions described herein. The methods disclosed herein may be encoded as executable instructions embodied in a computer-readable medium, including, without limitation, a storage device and / or a memory device. When such instructions are executed by a processing device, they cause the processing device to perform at least some of the methods disclosed herein. The above examples are not intended to limit the definition and / or meaning of the terms processor and processing device in any way.

[0082] At least one technical effect of the systems and methods disclosed herein includes (a) PV droop control without additional measurement of the DC bus current of the microgrid; and (b) PV droop control without an additional controller for generating a current reference in the internal current control loop of the converter.

[0083] Exemplary embodiments of systems and methods for PV droop control have been described in detail above. Systems and methods are not limited to the specific embodiments described herein; rather, components of systems and / or operations of methods may be utilized independently and separately from other components and / or operations described herein. Furthermore, described components and / or operations may also be defined within other systems, methods, and / or devices, or used in combination with other systems, methods, and / or devices, and are not limited to implementations using only the systems described herein.

[0084] As used herein, an element or step referred to in the singular and preceded by the word "a" or "an" should be understood as not excluding plural elements or steps unless the exclusion of plural elements or steps is explicitly stated. Furthermore, references to "examples" or "one example" in this disclosure are not intended to be interpreted as excluding the existence of additional examples that also incorporate the mentioned features. Moreover, where the terms "includes," "including," "has," "contains," and their variations are used herein, such terms are intended to include in a manner similar to the term "comprises" as an open transition, without excluding any additional or other elements.

[0085] Specific features of various embodiments of the present invention may be illustrated in some drawings and not in others, but this is merely for convenience. In accordance with the principles of the present invention, any feature of a drawing may be referenced and / or claimed in combination with any feature of any other drawing.

[0086] The written description herein uses examples to disclose the invention, including in the best mode, and also to enable any person skilled in the art to practice the invention, including manufacturing and using any devices or systems and performing any integrated methods. The patentable scope of the invention is defined by the claims and may include other examples conceived by those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that are not different from the literal language of the claims, or if they include equivalent structural elements that have insignificant differences from the literal language of the claims.

Claims

Claim 1 A direct current (DC) microgrid comprises: a power converter; and a power-voltage (PV) droop control loop electrically coupled to the power converter, wherein the PV droop control loop is configured to: receive measurements of droop current amounts, which are current amounts other than the DC bus current of the DC microgrid—wherein the droop current amounts are used to determine a droop power amount selected as said power amount in the PV relationship between the DC bus voltage and the power amount of the DC microgrid in PV droop control of the DC microgrid—; and derive a current reference—wherein the current reference corresponds to a control amount of the internal current control loop of the power converter—based on the droop in said droop power amount; and wherein the internal current control loop of the power converter is electrically coupled to the power converter and the PV droop control loop, and the internal current control loop derives a measured current corresponding to said control amount based on the measurements of said droop current amounts; and directly compares said current reference with said measured current. A DC microgrid configured to control the operation of the power converter by generating switching signals of the switches of the power converter based on the above comparison. Claim 2 A DC microgrid according to claim 1, wherein the droop power is a fictitious power including the result of multiplying the DC bus voltage and the droop current. Claim 3 In paragraph 2, the above-mentioned droop current amount is the input current of the power converter, a DC microgrid. Claim 4 In paragraph 3, the power converter is a DC microgrid that is an alternate current (AC) to DC converter and / or a DC to DC boost converter. Claim 5 In paragraph 2, the power converter is a DC-to-DC buck converter, and the droop current amount is the output current of the power converter, a DC microgrid. Claim 6 A DC microgrid according to any one of claims 2 to 5, wherein the droop gain of the PV droop control increases as the load level of the DC microgrid increases. Claim 7 A DC microgrid according to any one of claims 1 to 5, wherein the droop gain of the PV droop control is a function of time. Claim 8 A DC microgrid according to any one of claims 1 to 5, wherein the droop gain of the PV droop control is a function of a variable in the power converter and / or in the DC microgrid. Claim 9 In any one of claims 1 to 5, the DC microgrid comprises a plurality of power converters. Claim 10 A DC microgrid, wherein in any one of claims 1 to 5, the droop power amount is the input power of the power converter. Claim 11 A method for power-voltage (PV) droop control in a direct current (DC) microgrid, comprising: selecting a droop power amount as the power amount in the PV relationship between the DC bus voltage and the power amount of the DC microgrid in the PV droop control of the DC microgrid, wherein the droop power amount is based on a droop current amount which is a current amount other than the DC bus current of the DC microgrid; deriving a current reference based on the droop in the droop power amount, wherein the current reference corresponds to a control amount of an internal current control loop of a power converter; deriving a measured current corresponding to the control amount based on measurements of the droop current amount; and controlling the operation of the power converter by: in the internal current control loop, directly comparing the current reference with the measured current; and generating switching signals of the switches of the power converter based on the comparison. Claim 12 In claim 11, the step of selecting the droop power amount further comprises the step of selecting the droop power amount as a virtual power including the result of the product of the DC bus voltage and the droop current amount. Claim 13 In paragraph 12, the above droop current amount is the input current of the power converter, method. Claim 14 In paragraph 13, the power converter is an alternating current (AC) to DC converter and / or a DC to DC boost converter. Claim 15 In paragraph 12, the power converter is a DC-to-DC buck converter, and the droop current amount is the output current of the power converter. Claim 16 A method according to any one of claims 12 to 15, further comprising the step of selecting a droop gain of the PV droop control, wherein the droop gain increases as the load level of the DC microgrid increases. Claim 17 A method comprising, in any one of claims 11 to 15, further a step of selecting the droop gain of the PV droop control as a function of time. Claim 18 A method comprising, in any one of claims 11 to 15, further selecting the droop gain of the PV droop control as a function of a variable in the power converter and / or in the DC microgrid. Claim 19 A method according to any one of claims 11 to 15, wherein the DC microgrid comprises a plurality of power converters. Claim 20 A method according to any one of claims 11 to 15, wherein the step of selecting the droop power amount further comprises the step of selecting the droop power amount as the input power of the power converter.

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