System and method for power-voltage drop control of a DC microgrid

Abstract

The invention relates to a system and method for power-voltage droop control of a DC microgrid. 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 a measurement of a droop current amount that is not a DC bus current of the DC microgrid. The droop current amount is used to determine a droop power amount selected as a power amount in the PV relationship. The PV droop control loop is further configured to derive a current reference based on droop in the amount of droop power. The internal current control loop is configured to control operation of the power converter by deriving a measurement current corresponding to the control quantity based on a measurement of the amount of droop current, directly comparing the current reference with the measurement current, and generating a switching signal for a switch in the power converter.

Description

Technical Field

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

[0002] A microgrid is a local power grid and can operate in grid-connected mode by connecting to a macrogrid, and operate in island mode by disconnecting from the macrogrid and operate 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 adjusts the output DC-side power according to the DC bus voltage of the microgrid. Known systems and methods of PV droop control are disadvantageous in some aspects and need improvement. Summary of the Invention

[0003] In one aspect, 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 a measurement of a droop current quantity, which is a current quantity different from the DC bus current of the DC microgrid. In the PV droop control of the DC microgrid, the droop current quantity is used to determine a droop power quantity selected as the power quantity in the PV relationship between the power quantity and the DC bus voltage of the DC microgrid. The PV droop control loop is further configured to derive a current reference based on a voltage drop in the voltage-drop 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 the control quantity based on the measurement of the droop current quantity; directly comparing the current reference with the measured current; and generating a switching signal for a switch in the power converter based on the comparison.

[0004] On the other hand, a method for PV droop control in a DC microgrid is provided. The method includes selecting a droop power amount as the power amount in a PV relationship between the power amount and the DC bus voltage 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 different from the DC bus current of the DC microgrid. The method further includes deriving a droop-derived current reference based on the droop power amount, the current reference corresponding to a control amount of an internal current control loop of a power converter. The method further includes deriving a measured current corresponding to the control amount based on a measurement of the droop current amount. In addition, the method includes directly comparing the current reference with the measured current by controlling the operation of the power converter in the internal current control loop, and generating a switching signal for the switch in the power converter based on the comparison. Description of the Drawings

[0005] These and other features, aspects, 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 like characters represent like parts.

[0006] Figure 1A A known power-voltage (PV) droop control mechanism is shown.

[0007] Figure 1B A known PV droop control for an AC-to-DC (AC / DC) converter is shown.

[0008] Figure 1C Shows Figure 1B The step response of the known PV droop control of the AC / DC converter shown.

[0009] Figure 1D A known PV droop control for a DC / DC boost converter is shown.

[0010] Figure 2A Is a schematic diagram of an example microgrid.

[0011] Figure 2B Is a flowchart of the PV droop control method.

[0012] Figure 3A Is an example PV droop control for an AC / DC converter.

[0013] Figure 3B Is Figure 3A The step response of the control of the AC / DC converter shown.

[0014] Figure 3C Is an example PV droop control for a DC / DC boost converter.

[0015] Figure 3DIs an example of PV droop control for a DC / DC buck converter.

[0016] Figure 3E Is another example of PV droop control for an AC / DC converter.

[0017] Figure 4A Shows an example microgrid including multiple converters.

[0018] Figure 4B Shows another example microgrid including multiple converters.

[0019] Figure 5 Is Figure 3A The comparison of the impedance curve graphs of the PV droop control shown in Figure 1B And the known PV droop control.

[0020] Figure 6A Is Figure 3C The step response of the PV droop control shown in

[0021] Figure 6B Shows Figure 3C The impedance graph of the PV droop control shown in

[0022] Figure 7A Shows Figure 3E The step response of the PV droop control shown in

[0023] Figure 7B Shows Figure 3E The impedance graph of the PV droop control shown in Detailed Description

[0024] This disclosure includes power-voltage (PV) droop control for a direct current (DC) microgrid. Method aspects will be made partially apparent and partially explicitly discussed in the following description.

[0025] Figures 1A - 1D Shows a known method of droop control in a DC microgrid. Figure 1AIt is a schematic diagram of a known power-voltage (PV) droop control mechanism. The droop control is used to control the power flow among converters, loads, and power sources in a microgrid. The droop control regulates the power supplied and consumed by the converters, loads, and power sources in the microgrid according to 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 at 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 of the DC power output of the power converter is proportional to the voltage droop measured across the DC capacitor, and the output DC-side power is regulated as a function of the DC bus voltage. The proportionality in the PV relationship between the DC bus voltage and the DC-side power is the droop gain k. In the known PV droop control mechanism (see Figure 1B and Figure 1D ), the DC-side quantities are used as inputs, and a DC current reference is generated. The known PV droop equation is given as Eqn(1).

[0026]

[0027] V0 is the DC voltage reference and the DC bus voltage when no load is connected to the DC bus. k is the reciprocal of the droop gain or slope of the P-V line in the P-V droop characteristic curve 102. v is the DC bus voltage of the microgrid.

[0028] DC-side current reference can be derived based on the following Eqn(2).

[0029]

[0030] The DC bus voltage v is measured using a voltage sensor, and the DC bus voltage v is used to generate the DC-side current reference The PV droop control loop 103 is used to generate the DC-side current reference

[0031] In the known PV droop control for an alternating current (AC) to direct current (DC) (AC / DC) converter or rectifier 104 (see Figure 1B ), the DC-side current reference is input into an external current control loop or an additional control loop 106 (such as a PI (proportional integral) controller) to generate the d-axis line current reference as follows

[0032]

[0033] The three-phase AC electrical quantities i of current i and voltage e a,b,c and e a,b,ccan be transformed into a representation in a rotating reference frame on the d-q axes to derive the d-axis current i d or the d-axis voltage v d . The d-q reference coordinate system can be a synchronous reference coordinate system, where the d-axis of the coordinate system is aligned with the AC-side voltage measured at the point of common coupling (PCC) between the AC / DC converter 104 and the AC power grid (not shown), and the AC / DC converter 104 receives AC power from this AC power grid. The AC current reference generated by Equation (3) is an indication of the power flow from the AC side to the DC microgrid. The AC current reference is provided to the AC-side current controller or the internal current control loop 108 to generate the switching signal of the AC / DC converter.

[0034] As Figure 1B shown, the known PV droop controller 105 includes a droop control loop 103 and an additional control loop 106. The additional control loop 106 is required to obtain the d-axis current reference of AC from the DC current reference In addition, the additional PI control loop requires an additional measurement of the DC-side current i , which is not usually required in the control of the AC / DC converter. dc The additional control loop 106 also causes stability problems in the droop control, and the additional poles generated by the control loop 106 result in an underdamped step response.

[0035] is a graph of the step response, showing the effect of the load step response on the DC bus voltage at various droop gain k values. The step response has an overshoot. The overshoot can be mitigated by the PI controller design for small loads. For medium to large loads, the transient voltage drop may be too large, making the DC microgrid unstable. Figure 1C shows the known PV droop control for the 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

[0036] Figure 1D is provided to an additional control loop 106 such as a PI controller to generate the input inductor current reference as follows The error between the DC current reference

[0037]

[0038] and the measured DC current i is input to the PI controller 106 to generate the input inductor current reference dc The generated current reference The generated current reference It is an indication of the power flow from the input DC bus to the DC microgrid. The input inductor current reference is provided to the internal current control loop 108 of the DC / DC converter 104-bs to generate a switching signal for the switches in the converter 104-bs.

[0039] As Figure 1D shown, known PV droop control requires an additional control loop 106 to obtain the input inductor current reference from the DC current reference Similar to that in the AC / DC converter 104, the additional control loop 106 may cause stability problems in the droop control, and the additional poles generated by the PI controller may result in an underdamped step response. In addition, the additional control loop 106 requires an additional measurement of the DC-side current i dc which is not usually required in the control of the DC / DC converter.

[0040] In contrast, the systems and methods disclosed herein solve the above problems in the known droop control mechanisms. The additional control loop is eliminated, thereby improving the control speed and stability of the microgrid. The additional measurement of the DC-side current is also eliminated, thereby reducing the system complexity and the hardware and software costs of the additional sensors for the DC-side bus current.

[0041] Figure 2A is a schematic diagram of an exemplary DC microgrid 200 implementing the PV droop control mechanism disclosed herein. In the 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 can be an AC / DC converter (see Figure 3A and Figure 3E described later), a DC / DC boost converter (see Figure 3C described later), and / or a DC / DC buck converter (see Figure 3D described later). In the buck converter, the output voltage is less 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 is configured to control the operation of the power converter 202. As used herein, "electrically coupled" means that two components are electrically connected directly or via electrical components between the two components, or directly receive the input of the other from one of the two components, or receive the input of the other of the two components from one of the two components via the components between the two components, such as receiving a digital input in firmware. The internal current control loop 204 is configured to generate a switching signal for the switches in the power converter 202 based on a control quantity such that the output power from the power converter 202 meets the desired specifications, such as the desired output voltage.

[0042] In an exemplary embodiment, the DC microgrid 200 further includes a PV droop control loop 201 electrically coupled to the power converter 202 and the 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 a measurement of a droop current amount, which is an amount of current different from the DC bus current of the DC microgrid 200. In the PV droop control of the DC microgrid 200, the droop current amount is used to determine a droop power amount that is selected as the power amount in the PV relationship between the power amount and the DC bus voltage of the DC microgrid 200. The PV droop control loop 201 is further configured to derive a current reference based on a voltage drop in the voltage drop power amount, where the current reference corresponds to a control amount of the internal current control loop 204. The control in the internal control loop 204 is generated by a direct comparison between the current reference and a measured current corresponding to the control amount. The measured current corresponding to the control amount may be derived based on the measurement of the droop current amount.

[0043] Figure 2B is a flowchart of an example method of PV droop control. In an example embodiment, method 250 includes selecting 252 a droop power amount as the power amount in the PV relationship between the power amount and the DC bus voltage in the PV droop control of the DC microgrid. The droop power amount is based on a droop current amount, which is an amount of current different from the DC bus current of the DC microgrid. The droop power amount may be a virtual power that is the product of the droop current amount and the DC bus voltage. In some embodiments, the droop power amount is an actual power that is the input power of the power converter. Since the droop current amount used for the droop power amount is not the DC bus current, a sensor for the DC bus current is not required in the PV droop control, thus simplifying the configuration and reducing the cost associated with the hardware and software for the sensor. Instead, the droop current amount is a control amount or an amount of the same type as the control amount in the internal current control loop of the power converter. For example, for an AC / DC converter, the droop current amount may be the input current of the converter, and the control amount of the internal current control loop may be the d-axis input current of the power converter. For a DC / DC boost converter, both the droop current and the control amount may be the input current of the power converter. For a DC / DC buck converter, both the droop current amount and the control amount may be the output current of the converter. Thus, a sensor for the droop current amount is already included for the control of the power converter.

[0044] In an example embodiment, method 250 further includes deriving 254 a current reference based on the droop in the droop power amount, where the current reference corresponds to a control amount of an internal current control loop of the power converter. Thus, the current reference can be directly provided to the internal current control loop 204 for control of the power converter 202, thereby eliminating the additional control loop 106 in known droop control.

[0045] In an exemplary embodiment, method 250 further includes deriving 256 a measured current corresponding to the control amount based on a measurement of the droop current amount. Since the droop current amount is the same amount or the same type of amount as the control amount, the measured current corresponding to the control amount is a measurement value of the droop current amount or a different representation of the measurement value of the droop current amount and can be directly derived from the measurement value of the droop current amount. For example, for a DC / DC boost converter or a DC / DC buck converter, the control amount and the droop current amount are the same amount, and the measured current corresponding to the control amount is the measurement value of the droop current amount. For an AC / DC converter, the control amount is the d-axis input current of the power converter, and the droop current amount is the input current in three phases. The d-axis input current is the d-q reference frame representation of the input current. The d-axis input current can be derived by converting the measured three-phase AC input current into a representation in a rotating d-q reference frame.

[0046] Further, in an exemplary embodiment, method 250 includes controlling the operation 258 of the power converter by directly comparing the current reference with the measured current in an internal current control loop of the power converter and generating a switching signal for a switch in the power converter based on the comparison.

[0047] By eliminating the additional sensor for the DC bus current and the additional control loop for generating the current reference for the control amount in the internal current control loop, the microgrid and method disclosed herein are superior to known PV droop control in terms of simplifying the hardware and software and increasing the stability of the microgrid and the speed in the control of the power converter.

[0048] Figures 3A - 3D An example PV droop control mechanism using the virtual power P fic as the droop power amount in the PV relationship is shown. In an exemplary embodiment, P fic is the product of the DC bus voltage and the inductor current of the converter, as in the following equation (5).

[0049]

[0050] Converter internal current reference is directly generated from the DC side bus voltage, as shown in the following equation (6).

[0051]

[0052] For different types of converters, the current reference is different. In the AC / DC converter 202-ac, the current reference is the d-axis line current on the AC side (see Figure 3A ). In the DC / DC boost converter, the current reference is the DC-side input current In an exemplary embodiment, the output of the PV droop control loop 201 is used as the reference input for the d-axis line current control of the AC / DC converter 202-ac( Figure 3A ) or the input inductor control of the DC / DC boost converter 202-bs( Figure 3C ).

[0053] In an example embodiment, for the AC / DC converter 202-ac, the PV voltage drop adjusts a virtual or power-like quantity v×i with respect to the voltage drop and droop gain k' in the DC bus voltage d . The droop gain k' is related to the droop gain k in the conventional PV droop control and the d-axis grid voltage under no-load conditions as follows:

[0054]

[0055] The following equations (8) and (9) are derived by substituting equation (7) back into equation (6).

[0056]

[0057] v d is the d-axis voltage on the input side of the converter. Since converter losses are negligible under normal operating conditions, the difference ratio between the voltage droops in the PV droop equations (1) and (9) is Under normal operating conditions, due to the ratio between the no-load voltages on the AC and DC sides being approximately the inverse of the ratio between the nominal voltages on the DC and AC sides , the voltage droops in equations (1) and (9) are approximately the same or the difference ratio is close to 1. If the load level is high, the difference between the voltage droops in equations (1) and (9) is greater, or the difference ratio becomes lower because converter losses are higher than line losses, considering that converters are often placed close to the grid bus. The variation in the difference ratio indicates that under heavy load conditions, the PV droop control disclosed herein provides better regulation than known droop control, as confirmed by Figure 3B the higher voltage level in Figure 1C compared to

[0058] In an exemplary embodiment, for a DC / DC boost converter (see Figure 3C ), the PV droop control regulates the amount of virtual or virtual - like power v×i for droop in the DC bus voltage and droop gain k'. L The droop gain k' is related to the droop gain k of the conventional PV droop control and the input DC source voltage E at no - load:

[0059]

[0060] The following equations (11) and (12) are obtained by taking equation (10) back to equation (6).

[0061]

[0062] v in is the voltage at the input side of the converter 202. Since converter losses are negligible under normal operating conditions, the difference ratio between the voltage droops in the PV droop equations (1) and (12) is Under normal operating conditions, the voltage droops in equations (1) and (12) are approximately equal to each other, or the difference ratio is close to 1. At higher load levels, the difference between the voltage drops in equations (1) and (12) is higher, or the difference ratio becomes lower. The increase in the difference at higher load levels indicates that the voltage regulation using the systems and methods disclosed herein is superior to known PV droop control, especially under heavy - load conditions.

[0063] In some embodiments, the droop gain k' is a time - varying variable. 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.

[0064] Figure 3D Another example of PV droop control mechanism using virtual power for other types of converters such as the DC / DC buck converter 202 - bk is shown. In an exemplary embodiment, compared with the PV droop control shown in Figure 3A and Figure 3C , where the droop current amount is the input current of the converters 202 - ac, 202 - bs, the droop current amount is the output current of the converter 202 - bk. The inductor 304 is located at the output of the DC / DC buck converter 202 - bk, rather than at the output of the AC / DC converter 202 - ac or the DC / DC boost converter 202 - bk. The inductor current reference is directly controlled by the output of the PV droop control loop 201.

[0065] The PV droop control loop 201 using virtual power is superior to the known PV droop controller 105. The PV droop control loop 201 is less complex than the known PV droop controller 105 because the additional control loop 106 and the additional DC current measurement are eliminated in the PV droop control loop 201. The elimination of the additional control loop also increases the control speed and stability by eliminating the control delay from the additional control loop 106.

[0066] Return reference Figure 3B , showing the step response 306 of the AC / DC converter 202-ac with the droop control loop 201. Figure 3B The curves in Figure 1C can be directly compared with the curves in Figure 1C , showing the step response 107 of the AC / DC converter 104 with the known droop controller 105, because the test conditions are as identical as possible when generating the curves in the two figures. As shown in Figure 1C and Figure 3B , after the initial start-up transient from zero voltage, both the known droop controller 105 and the droop control loop 201 stabilize at a steady-state DC voltage of 350V. At 0.5 seconds, a 5-ohm load step is applied to the DC bus by closing the switch. Comparing Figure 3B and Figure 1C , as described above, the PV droop control loop 201 is better than the known PV droop controller 105 in regulating the DC bus voltage. In addition, when comparing the performance at equivalent droop gains according to Equation (4), the voltage drop using the PV droop control disclosed herein is less than the voltage drop using the known PV droop controller 105. With the PV droop control loop 201, compared with the underdamped step response of the known PV droop controller (see Figure 1C ), the step response 306 of the PV droop control loop 201 is overdamped (see Figure 3B ).

[0067] Figure 3E shows another exemplary PV droop control mechanism. Compared with the PV droop control loop 201 shown in Figure 3A , Figure 3C and 3D , the PV droop control loop 201-r shown in Figure 3E uses the actual power in the PV relationship. In the exemplary embodiment, ignoring the converter losses, the DC-side power can be represented by the AC-side power as follows.

[0068]

[0069] The PV droop control that directly generates the AC-side d-axis current reference from the DC-side bus voltage is provided as follows. ​

[0070]

[0071] v d is the d-axis line voltage on the AC side. Instead of using the measured AC-side voltage to derive v d v d can be obtained from the measured grid-side voltage and AC-side current. The PV droop control adjusts the actual AC-side power 3 / 2v d ×i d with respect to the droop of the DC bus voltage and the droop gain k. The droop gain k is functionally the same as the droop gain in the known PV droop control. In some embodiments, the droop gain k is a time-varying variable k = f(t). For example. 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.

[0072] In an exemplary embodiment, ignoring the losses between the AC grid and the converter, such as line losses and converter filter losses, the AC-side power can be expressed as follows.

[0073]

[0074] The PV droop control can be used to derive the reference of the d-axis line current on the AC side as follows

[0075]

[0076] Since the amplitude of the AC grid voltage is usually stable and the varying power flow mainly affects the voltage angle, the d-axis grid voltage E d is usually constant.

[0077] In an exemplary embodiment, the output of the PV droop control is used as the reference input for the d-axis line current control of the AC / DC converter 202-ac. Figure 3E A block diagram of the PV droop control is shown. The internal current control remains the same as that of a traditional AD / DC rectifier, and the droop control changes with the introduction of the grid-side voltage E d to generate the current reference

[0078] Compared with Figure 1A , Figure 1B and Figure 1D the known PV droop control shown, where the known PV droop control uses the DC-side power to regulate the DC bus voltage, Figure 3E the PV droop control shown in

[0079]

[0080] When the synchronous rotating coordinate system is aligned with v d v = 0. The actual power equation (17) on the AC side becomes as follows: q When the synchronous rotating coordinate system is aligned with v, v = 0. The actual power equation (17) on the AC side becomes as follows:

[0081]

[0082] The relationship between the actual power on the AC side and the DC output power can be written as follows:

[0083] P ac = P dc + Converter losses + AC filter losses (19)

[0084] Typically, the AC / DC converter operates with a high efficiency of, for example, 99% or higher. The losses in the AC filter can be minimized by design. Therefore, P ac ≈ P dc is applicable to the AC / DC converter system. However, by eliminating the additional measurement of the DC bus current and the additional control loop 106, the PV droop control using the AC side power is superior to the known PV droop control, thus simplifying the hardware and software and increasing the dynamic response speed and stability of the AC / DC converter.

[0085] Compared with Figure 3A , Figure 3C and Figure 3D the PV droop control using virtual power in the PV relationship shown in Figure 3E the PV droop control shown in Figure 3A uses the actual power in the PV relationship. When using the actual power, the input current and input voltage of the AC / DC converter are measured. As measured in the schemes shown in

[0086] Figure 4A and Figure 4B the AC voltage is noisier than the DC voltage 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 using virtual power, instead, the DC voltage on the output side is measured, which has a much larger capacitance, for example, a thousand times larger, than the capacitance on the AC side and is thus much less noisy than the AC voltage on the input side. Due to the noise, if the actual power is used, software and hardware for signal processing and filtering of the AC side voltage are required, which increases the complexity of the circuit design. Figure 4A ) connected. Optionally or additionally, the converter 202 is in a distributed configuration 404(Figure 4B ) Connections. Each converter 202 is electrically connected to a source or a load.

[0087] By comparing the impedance Bode plots, the performance of the droop control loop 201 of the AC / DC converter was further analyzed with respect to the known droop controller 105. Figure 5 The impedance curves of the AC / DC converter 202-ac are shown, where the only difference between curves 502 and 504 is the type of PV droop control used. Curve 502 is the curve of the magnitude 502-m and phase 502-p of the impedance using the known droop controller 105. Curve 504 is the curve of the magnitude 504-m and phase 504-p of the impedance using the droop control loop 201. The impedance curves 502, 504 are aligned so that the x-axis and y-axis of curves 502, 504 are the same, facilitating a direct visual comparison between the two curves 502, 504. According to the magnitude plot, 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. The horizontal lines represent two different loads applied at the DC terminals of the AC / DC converter 202-ac. Wherein the load has a higher power consumption than the said load.

[0088] The power flow stability of the system can be determined by the Middlebrook criterion, where

[0089]

[0090] The system impedance magnitude |Z s (s)| is shown in curves 502-m and 504-m. The load impedance magnitude |Z l (s)| is represented by two lines 506 in curves 502-m, 504-m. For system stability, when using the PV droop control loop 201, for all frequencies |Z s (s)| << |Z l (s)|. Conversely, when using the known PV droop controller 105, the load line Z l2 intersects the magnitude impedance curve 502. The difference indicates 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 load 12.

[0091] The step response 306-dc( Figure 6A ) and impedance plot ( Figure 6B ) of the PV droop control loop 201 used in the DC / DC boost converter 202-bs are provided, where the virtual power is in a PV relationship. The step response 306-dc / dc and the impedance curve 504-dc / dc are similar to Figure 3B the step response 306 shown andFigure 5 The impedance curve 504 shown. The step response of the PV droop control loop 201 is overdamped, and the PV droop control loop 201 is more stable than the known PV droop controller 105.

[0092] The step response 306-r of the PV droop loop 201 used in the AC / DC converter 202-ac with real power in the PV relationship is provided Figure 7A ) and impedance diagram ( Figure 7B ). As Figure 1C and Figure 7A shown, after the initial start-up transient starting from zero voltage, both the known droop controller 105 and the droop control loop 201 stabilize at a steady-state DC voltage of 350V. At 0.5 seconds, a 5-ohm load step is applied to the DC bus by closing the switch. Comparing Figure 7A and Figure 1C , the PV droop control loop 201 is better than the known PV droop controller 105 in regulating the DC bus voltage.

[0093] By comparing the impedance Bode plots, the performance of the droop control loop 201 using real power in the PV relationship is further analyzed relative to the known droop controller 105. Figure 7B The impedance curves of the AC / DC converter 202-ac are shown, where the only difference between curves 502 and 504-r is the type of PV droop controller used. Curve 502 is the curve of the magnitude 502-m and phase 502-p of the impedance using the known droop controller 105. Curve 504-r is the curve of the magnitude 504-r-m and phase 504-r-p of the impedance using the droop control loop 201. Align the impedance curves 502, 504-r so that the x-axis and y-axis of curves 502, 504-r are the same, facilitating a direct visual comparison between the two curves 502, 504-r. According to the magnitude plot, 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. The horizontal lines Z l1 and Z l2 represent two different loads applied at the DC terminals of the AC / DC converter 202-ac. Z l1 >Z l2 , where the load l2 has a higher power consumption than the load l1.

[0094] The power flow stability of the system can be determined by the Middlebrook criterion expressed in Equation (20). The system impedance magnitude |Z s (s)| is shown in curves 502-m and 504-r-m. The load impedance magnitude |Z l(s) | Represented by two lines 506 in the curves 502, 504 - r. When the PV droop control loop 201 is used, for all frequencies | Z s (s) | << | Z l (s) |. In contrast, when the known PV droop controller 105 is used, the load line | Z l2 intersects the magnitude impedance curve 502. These 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 load 12.

[0095] In some embodiments, the PV droop control loop 201 can be implemented on a controller board using analog circuits and operational amplifiers. The input voltage is scaled down to the required low voltage suitable for use on the controller board using a resistive voltage divider or a capacitive voltage divider. Equation (6) is implemented on the controller board using an operational amplifier by creating a subtractor circuit. Then the output of the subtractor is provided as a reference input to the current control algorithm of the DC microgrid converter.

[0096] In other embodiments, the PV droop control loop 201 can be implemented on a controller board within a microcontroller or FPGA firmware. The input voltage is scaled down to the required low voltage suitable for use on the controller board using a resistive voltage divider or a capacitive voltage divider. The scaled - down voltage is provided 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 implemented in the firmware to digitally calculate the converter inner - loop current reference, which is then provided to the control algorithm of the DC microgrid converter.

[0097] As used herein, the terms "processor" and "computer" and related terms (e.g., "processing device", "computing device", and "controller") are not limited to those integrated circuits referred to as computers in the art, but rather 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 may be 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), computer-readable non-volatile media such as flash memory. Alternatively, floppy disks, compact disk-read only memory (CD-ROM), magneto-optical disk (MOD), and / or digital versatile disk (DVD) and / or other optical media storage devices may also be used. Additionally, in the embodiments described herein, additional input channels may be, but are not limited to, computer peripherals associated with operator interfaces such as touchscreens, mice, and keyboards. Alternatively, other computer peripherals may be used, which may include, for example but not limited to, scanners. Additionally, in an exemplary embodiment, additional output channels may include, but are not limited to, an operator interface monitor or a heads-up display. Some embodiments relate to the use of one or more electronic or computing devices. Such devices typically include a processor, processing device, or controller such as a general-purpose central processing unit (CPU), graphics processing unit (GPU), microcontroller, reduced instruction set computer (RISC) processor, ASIC, programmable logic controller (PLC), field programmable gate array (FPGA), 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 contained in a computer-readable medium, which includes (but is not limited to) storage devices and / or memory devices. When executed by a processing device, such instructions cause the processing device to perform at least a portion of the methods disclosed herein. The above examples are not intended to limit in any way the definition and / or meaning of the terms processor and processing device.

[0098] At least one technical effect of the systems and methods disclosed herein includes: (a) PV droop control without the need for additional measurement of the DC bus current of the microgrid; and (b) PV droop control without an additional controller to generate a current reference in the internal current control loop of the converter.

[0099] The above has described in detail example embodiments of a system and method for PV droop control. The system and method are not limited to the specific embodiments described herein, but rather the components of the system and / or the operations of the method can be used independently and separately from other components and / or operations described herein. Additionally, the described components and / or operations can also be defined in other systems, methods, and / or devices, or used in combination with other systems, methods, and / or devices, and are not limited to being practiced solely using the system described herein.

[0100] As used herein, an element or step recited in the singular and proceeding with the word "a" or "an" should be understood as not excluding a plurality of elements or steps, unless such exclusion is explicitly recited. Additionally, a reference to "an example" or "one example" of the present disclosure is not to be construed as excluding the existence of additional examples that also incorporate the recited features. Further, with respect to the terms "comprising," "including," "having," "containing," and variations thereof used herein, these terms are intended to be inclusive in a manner similar to the term "including" as an open transitional word and do not exclude any additional or other elements.

[0101] Although specific features of various embodiments of the present invention may be shown in some figures and not in others, this is merely for convenience. In accordance with the principles of the present invention, any feature of a figure can be referenced and / or claimed in combination with any feature of any other figure.

[0102] This written description uses examples to disclose the present invention, including the best mode, and also enables any person skilled in the art to practice the present invention, including making and using any device or system and performing any incorporated method. The patentable scope of the present invention is defined by the claims and may include other examples that occur to those skilled in the art. If these other embodiments have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements that do not differ materially from the literal language of the claims, then these other embodiments are intended to be within the scope of the claims.

Claims

1. A direct current (DC) microgrid, comprising: Power converters; A power-voltage PV droop control loop is electrically coupled to the power converter, wherein the PV droop control loop is configured to: receiving a measurement of a droop current amount, the droop current amount being an amount of current different from a DC bus current of a DC microgrid, wherein the droop current amount is used to determine a droop power amount, the droop power amount being selected as an amount of power in a PV relationship between the power amount in PV droop control of the DC microgrid and a DC bus voltage of the DC microgrid; as well as deriving a current reference based on the droop in the droop power amount, the current reference corresponding to a control amount of an internal current control loop of the power converter; The inner current control loop of the power converter is electrically coupled to the power converter and the PV droop control loop, the inner current control loop being configured to control the operation of the power converter by: deriving a measured current corresponding to the control amount based on the measured value of the droop current amount; directly comparing the current reference to the measured current; as well as A switching signal for a switch in the power converter is generated based on the comparison. 2 . The DC microgrid according to claim 1 , wherein the droop power amount is a virtual power including a product of the DC bus voltage and the droop current amount. 3 . The DC microgrid of claim 2 , wherein the droop current amount is an input current of the power converter.

4. The DC microgrid according to claim 3, wherein the power converter is an alternating current (AC) to DC converter and / or a DC to DC boost converter.

5. The DC microgrid of claim 2, wherein the power converter is a DC to DC buck converter, and the droop current amount is an output current of the power converter.

6. The DC microgrid according to any one of claims 2 to 5, wherein a droop gain of the PV droop control increases as a load level of the DC microgrid increases.

7. The 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.

8. A DC microgrid according to any one of claims 1 to 5, wherein a droop gain of the PV droop control is a function of variables in the power converter and / or the DC microgrid.

9. The DC microgrid according to any one of claims 1 to 5, wherein the DC microgrid comprises a plurality of power converters.

10. The DC microgrid according to any one of claims 1 to 5, wherein the amount of droop power is an input power of the power converter.

11. A method for power-voltage PV droop control in a DC microgrid, the method comprising: selecting a droop power amount as a power amount in a PV relationship between the power amount and a DC bus voltage of the DC microgrid in a PV droop control of the DC microgrid, wherein the droop power amount is based on a droop current amount, the droop current amount being a current amount different from a DC bus current of the DC microgrid; deriving a current reference based on the droop in the droop power amount, the current reference corresponding to a control amount of an internal current control loop of the power converter; deriving a measured current corresponding to the control amount based on the measured value of the droop current amount; and The operation of the power converter is controlled by: In the inner current control loop, directly comparing the current reference to the measured current; and A switching signal for a switch in the power converter is generated based on the comparison. 12 . The method of claim 11 , wherein selecting the droop power amount further comprises selecting the droop power amount as a virtual power comprising a product of the DC bus voltage and the droop current amount. The method of claim 12 , wherein the droop current amount is an input current of the power converter.

14. The method of claim 13, wherein the power converter is an alternating current (AC) to DC converter and / or a DC to DC boost converter.

15. The method of claim 12, wherein the power converter is a DC to DC buck converter and the droop current amount is an output current of the power converter.

16. The method according to any one of claims 12 to 15, further comprising: A droop gain of the PV droop control is selected, wherein the droop gain increases as a load level of the DC microgrid increases.

17. The method according to any one of claims 11 to 15, further comprising: The droop gain of the PV droop control is selected as a function of time.

18. The method according to any one of claims 11 to 15, further comprising: A droop gain of the PV droop control is selected as a function of variables in the power converter and / or the DC microgrid.

19. The method of any one of claims 11 to 15, wherein the DC microgrid comprises a plurality of power converters.

20. The method of any one of claims 11 to 15, wherein selecting the droop power amount further comprises selecting the droop power amount as an input power for the power converter.

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