Grid forming inverter and method for estimating grid connection impedance and voltage based on data

Abstract

One embodiment provides a grid forming inverter comprising: a power stage that converts power according to the on / off control of switching elements and outputs the converted power; and a control circuit that obtains measurements of active power, reactive power, output voltage, and output current of power output from the power stage to the grid, applies the measurements to an active power conservation formula, a reactive power conservation formula, and a Kirchhoff voltage formula between a grid node and the output node to estimate the grid voltage, calculates estimates of the grid voltage formed at the grid node and the grid connection impedance formed between the grid node and the output node, and controls the switching elements using Pulse Width Modulation (PWM) according to the estimates.

Description

Technology Field

[0001] This embodiment relates to a grid forming inverter. Background Technology

[0002] Conventional grid-following inverters are designed to operate in sync with the frequency and phase of the power grid, but this leads to several major problems. First, grid-following inverters cannot independently supply stable power when a problem occurs in the power grid. This can cause serious issues, particularly in situations where power grid stability is critical. Second, grid-following inverters have limited frequency and phase control capabilities, which can make it difficult to maintain power quality.

[0003] To address these issues, the development of grid-forming inverters is necessary. Grid-forming inverters can improve power grid stability by independently regulating frequency and phase, and possess the capability to supply power independently even in the event of a grid failure. This represents a significant technological advancement that can greatly enhance the reliability of power grid operations.

[0004] Accurately determining grid connection impedance and grid voltage is crucial for controlling grid-forming inverters. Grid connection impedance is closely related to various line components, such as transformers, transmission lines, and distribution lines; due to the complex interplay of these elements, accurately measuring the impedance is extremely difficult. In particular, as the measurement distance increases, the resistance and reactance of the lines also increase, making it even harder to determine the precise grid connection impedance. Therefore, advanced technology is required to accurately determine grid connection impedance, which is essential for enhancing the reliability and stability of grid-forming inverters. Furthermore, the accurate measurement of grid voltage is also critical, as it is necessary to maintain the stability of the power grid and ensure optimal power quality. The problem to be solved

[0005] Against this backdrop, the objective of the present embodiment is, in one aspect, to provide a more improved control technology for a grid-forming inverter. In another aspect, to provide a technology for more accurately estimating grid connection impedance and grid voltage. In yet another aspect, to provide a technology for estimating grid connection impedance and grid voltage using only data from the grid-forming inverter. means of solving the problem

[0006] To achieve the aforementioned objective, one embodiment provides a grid forming inverter comprising: a power stage that converts power according to the on / off control of switching elements and outputs the converted power; and a control circuit that obtains measurements of the active power, reactive power, output voltage, and output current of the power output from the power stage to the grid, applies the measurements to an active power conservation formula, a reactive power conservation formula, and a Kirchhoff voltage formula between a grid node and the output node to estimate the grid voltage, calculates the estimated values ​​of the grid voltage formed at the grid node and the grid connection impedance formed between the grid node and the output node, and controls the switching elements using Pulse Width Modulation (PWM) according to the estimated values.

[0007] The above active power conservation formula, the above reactive power conservation formula, and the above Kirchhoff voltage formula may have nonlinearity.

[0008] The above control circuit can calculate the estimates by finding a solution to an objective function such that the value calculated according to the estimates and the estimated residual of the measurements are minimized.

[0009] The above control circuit can find the above solution and calculate the above estimates using the above measurements obtained from a plurality of operating points.

[0010] The above active power conservation formula, the above reactive power conservation formula, and the above Kirchhoff voltage formula include the phase angle of the system voltage, and the control circuit can calculate the phase angle of the system voltage according to the steady-state power-phase angle relationship.

[0011] The above objective function can be composed of the following objective function formulas.

[0012] [Objective function formulas]

[0013]

[0014]

[0015] Here, the meaning of each variable is as follows.

[0016] : Final value of line resistance estimate

[0017] : Estimated line resistance (value used to find the solution to the objective function)

[0018] : Final estimated value of line reactance

[0019] : Estimated line reactance (value used to find the solution to the objective function)

[0020] : Final value of system voltage estimate

[0021] : Estimated system voltage (value used to find the solution to the objective function)

[0022] : Active power estimation residual

[0023] : Reactive power estimation residual

[0024] : Output voltage estimation residual

[0025] : Measure of active power obtained at the K-th (K is a natural number)-th operating point

[0026] : Measurement of reactive power obtained at the Kth driving point

[0027] : Measured output voltage obtained at the Kth driving point

[0028] : Measured output current obtained at the Kth operating point

[0029] : Phase angle of the system voltage calculated for the Kth operating point

[0030] : Phase angle of the output current obtained at the Kth operating point

[0031] : Intermediate variable 1 used in the calculation for the Kth driving point

[0032] : Intermediate variable 2 used in the calculation for the Kth driving point

[0033] The phase angle of the above system voltage can be calculated using the following phase angle formula.

[0034] [Phase Angle Formula]

[0035]

[0036] The above control circuit can find a solution such that the sum of the estimated residuals described in (2), (3) and (4) among the above objective function equations is minimized.

[0037] The above control circuit can find solutions to the above objective function equations using a Grid Search Algorithm (GSA).

[0038] The control circuit above can set the minimum and maximum values ​​of the grid connection impedance estimates in the grid search algorithm to reference impedances of 0.05 and 0.4 times based on per-unit (pu).

[0039] The control circuit above can set the minimum and maximum values ​​of the grid voltage estimates in the grid search algorithm to reference phase-ground voltages of 0.95 and 1.05 times based on the per unit.

[0040] The above control circuit can set the estimated values ​​of the grid connection impedance and the grid voltage to a minimum value during the initialization step of the grid search algorithm.

[0041] The above control circuit can divide the minimum and maximum values ​​in the grid search algorithm into N intervals (N is a natural number greater than or equal to 2), calculate the sum of the estimated residuals in each interval, and determine the estimates as the smallest among the sums of the estimated residuals.

[0042] An LCL output filter is placed at the output terminal of the above power stage, and the output node can be formed on the LCL output filter.

[0043] The above LCL output filter includes an inverter-side inductor, a grid-side inductor, and an output capacitor, and the output node may be formed at the common junction of the inverter-side inductor, the grid-side inductor, and the output capacitor. Effects of the invention

[0044] As described above, according to the present embodiment, a more improved control technology for a grid forming inverter can be provided. Furthermore, according to the present embodiment, the grid connection impedance and grid voltage can be estimated more accurately. Additionally, according to the present embodiment, the grid connection impedance and grid voltage can be estimated using only the data of the grid forming inverter. Brief explanation of the drawing

[0045] Figure 1 is a diagram of a typical power system configuration. FIG. 2 is a configuration diagram of a power system according to one embodiment. FIG. 3 is a diagram showing a grid forming inverter according to one embodiment and a grid connection impedance formed around it. FIG. 4 is a configuration diagram of a control circuit according to one embodiment. FIG. 5 is an example configuration diagram of a phase control circuit according to one embodiment. Figure 6 is a simplified model of the grid connection impedance. FIG. 7 is a diagram showing an algorithm for finding a solution for a grid forming inverter according to one embodiment. Specific details for implementing the invention

[0046] Hereinafter, some embodiments of the present invention will be described in detail with reference to the exemplary drawings. It should be noted that in assigning reference numerals to the components of each drawing, the same components are given the same reference numeral whenever possible, even if they are shown in different drawings. Furthermore, in describing the present invention, if it is determined that a detailed description of related known components or functions could obscure the essence of the invention, such detailed description is omitted.

[0047] In addition, terms such as first, second, A, B, (a), (b), etc., may be used when describing the components of the present invention. These terms are intended only to distinguish the components from other components, and the essence, order, or sequence of the components is not limited by the terms. Where it is stated that a component is "connected," "combined," or "connected" to another component, it should be understood that the component may be directly connected or connected to the other component, but that another component may also be "connected," "combined," or "connected" between each component.

[0048] Figure 1 is a diagram of a typical power system configuration.

[0049] Referring to FIG. 1, a grid (GD) may be formed in the power system (10).

[0050] The grid (GD) is a large-scale power network that may consist of multiple synchronous generators (SGs). Examples of synchronous generators include hydroelectric generators, thermal power generators, and nuclear power generators. Hydroelectric generators utilize the flow of water from dams or rivers to rotate large turbines, and this rotation of the large turbines generates inertia. Thermal power generators generate electricity on the principle of burning fossil fuels such as coal, natural gas, and oil to boil water, and using the resulting steam to rotate turbines. Like hydroelectric generators, thermal power generators also acquire inertia as they rotate turbines. Similarly, nuclear power generators use nuclear energy to rotate turbines, and this process generates inertia.

[0051] In a general power system (10), the proportion of power generation occupied by the synchronous generator (SG) is overwhelmingly high, so the grid (GD) can operate stably by utilizing the inertia of the synchronous generator (SG).

[0052] A general power system (10) may include renewable energy generators (RG), such as solar power generators and / or wind power generators. The power produced by the renewable energy generators (RG) may not be in a form suitable for the grid (GD). For example, the power produced by a solar power generator is in the form of direct current voltage, so it cannot be directly input into the grid (GD), which has an alternating current voltage. In the case of a wind power generator, the frequency or voltage level of the power produced is different from the frequency or voltage level of the power used by the grid (GD), so it cannot be directly input into the grid (GD).

[0053] Accordingly, the power produced by the renewable energy generator (RG) can be converted into power through the inverter (11) and then supplied to the grid (GD).

[0054] A renewable energy generator (RG) connected to a general power system (10) primarily converts power through a grid-following inverter (11) and then supplies power to the grid (GD).

[0055] The grid-following inverter (11) is automatically synchronized with the voltage, frequency, and phase of the grid (GD), thereby allowing for easy connection to the grid (GD). It plays a role in stably supplying power produced from energy sources with high variability, such as solar or wind power, to the power grid. The design of the grid-following inverter (11) is relatively simple, making installation and operation easy. It has the advantages of low maintenance costs and high reliability. During the connection process with the grid (GD), the grid-following inverter (11) continuously detects the status of the grid (GD) and adjusts the output as needed.

[0056] However, if the grid (GD) is unstable or unavailable, the grid following inverter (11) may stop operating. This means that the grid following inverter (11) cannot supply power in situations where the grid (GD) is interrupted or malfunctioning. Additionally, the grid following inverter (11) may have difficulty adapting to the variability of the grid (GD), and combined with the high variability of the renewable energy generator (RG), this may affect the stability of the grid (GD).

[0057] To address climate change and prepare for the depletion of fossil fuels, synchronous generators (SGs) are being rapidly replaced by renewable energy generators (RGs). As SGs decrease and RGs increase, the sources capable of supplying inertia to the grid (GD) also diminish. If the grid's inertia decreases, voltage and frequency variability increase, and in severe cases, even small fluctuations can lead to grid collapse.

[0058] In order to cope with the rapid transition to renewable energy generators (RG), studies are emerging that use grid-forming inverters instead of grid-following inverters (11).

[0059] FIG. 2 is a configuration diagram of a power system according to one embodiment.

[0060] Referring to FIG. 2, a grid (GD) may be formed in the power system (100), and a plurality of renewable energy generators (RG) may be connected. And, these plurality of renewable energy generators (RG) can supply power generated through a grid forming inverter (110) to the grid (GD).

[0061] The grid forming inverter (110) can operate as a voltage source. The grid forming inverter (110) can have the ability to set and maintain voltage in the grid (GD). The grid forming inverter (110) can generate and regulate voltage on its own, thereby maintaining voltage and supplying power to the load even when disconnected from the grid (GD).

[0062] A grid forming inverter (110) does not provide physical inertia, but can provide virtual inertia or composite inertia to the grid (GD). This is a technique that controls the inverter's response through a control algorithm to make it behave like a synchronous generator with traditional inertia.

[0063] The virtual inertia function is designed so that the grid forming inverter (110) responds quickly to frequency fluctuations of the grid (GD). For example, if the load of the grid (GD) suddenly increases and the frequency drops, the grid forming inverter (110) can quickly supply additional power to mitigate the drop in frequency. Conversely, if the load of the grid (GD) decreases, the grid forming inverter (110) can reduce the power supply to suppress the rise in frequency.

[0064] The virtual inertia of such grid forming inverters (110) can play an important role in situations where the proportion of traditional synchronous generators in the grid (GD) decreases and the proportion of new and renewable energy generators (RG) increases. The virtual inertia can help maintain the stability of the grid (GD) and manage frequency fluctuations.

[0065] FIG. 3 is a diagram showing a grid forming inverter according to one embodiment and a grid connection impedance formed around it.

[0066] Referring to FIG. 3, the grid forming inverter (110) may include a power stage (310), a control circuit (320), a sensing circuit (330), and a communication circuit (340), etc.

[0067] The power stage (310) can convert power according to the on / off control of the switching elements and output the converted power to the grid.

[0068] The input node (Ni) of the power stage (310) can be connected to a renewable energy generator. For example, the output of an energy storage device, a photovoltaic generator, etc., can be connected to the input of the power stage (310).

[0069] The output node (No) of the power stage (310) can be connected to the grid.

[0070] The power stage (310) may include a plurality of switching elements. And, the on / off state of these switching elements can be determined by a control signal (Gs) supplied from the control circuit (320).

[0071] The control signal (Gs) may be a gate control signal for switching elements. The gate control signal may be a Pulse Width Modulation (PWM) signal. The switching elements may be turned ON during a time interval in which the PWM signal indicates a high level, and turned OFF during a time interval in which it indicates a low level.

[0072] The control circuit (320) receives a sensing signal (Sv) from the sensing circuit (330) and can generate a control signal (Gs) using the sensing signal (Sv). The sensing signal (Sv) may be, for example, a sensing signal for an input voltage (Vi), an output voltage, an input current, an output current, etc.

[0073] The communication circuit (340) can exchange information with other devices via analog communication and / or digital communication. For example, the communication circuit (340) can receive command values ​​from a higher-level controller. The communication circuit (340) can receive active power command values ​​and / or reactive power command values ​​from the higher-level controller and transmit the corresponding command values ​​to the control circuit (320).

[0074] The control circuit (320) can estimate the grid connection impedance (Rg+jXg) and the grid voltage (Vg∠θg) using the sensing signal (Sv).

[0075] An output filter (350) may be placed at the output terminal of the grid forming inverter (110). The output filter (350) may have the form of an LCL and may include an inverter-side inductor (Lfl), a grid-side inductor (Lfg), and an output capacitor (Cf). An inverter-side inductor (Lfl) may be placed between the power stage (310) and the output node (No), an output capacitor (Cf) may be placed between the ground and the output node (No), and a grid-side inductor (Lfg) may be placed from the output node (No) toward the grid node (Ng).

[0076] The inverter-side inductor (Lfl) and the output capacitor (Cf) may be placed inside the case of the grid-forming inverter (110), and the grid-side inductor (Lfg) may be placed outside the case of the grid-forming inverter (110). Thus, the grid-forming inverter (110) can measure the voltage formed at the output node (No) as the output voltage (E∠θe). Of course, depending on the embodiment, the voltage formed at the output terminal of the grid-side inductor (Lfg) may also be measured, but for convenience of understanding, this specification describes measuring the voltage formed at the output node (No) as the output voltage (E∠θe).

[0077] The grid forming inverter (110) can calculate the grid connection impedance (Rg+jXg) and the grid voltage (Vg∠θg) using the output voltage (E∠θe) and the values ​​of the active power (P) and reactive power (Q) output through the output node (No).

[0078] Here, the grid connection impedance (Rg+jXg) may be a value including the grid-side inductor (Lfg) and the impedance of the power line after the grid connection point (PCC), and the grid voltage (Vg∠θg) may be a voltage formed in a part that can be viewed as a grid node (Ng).

[0079] FIG. 4 is a configuration diagram of a control circuit according to one embodiment.

[0080] Referring to FIG. 4, the control circuit (320) may include a power measurement circuit (410), a phase control circuit (420), a voltage control circuit (430), a gate control circuit (440), and a system estimation circuit (450), etc.

[0081] The power measurement circuit (410) can receive the output voltage or system voltage (Vg) and the output current (Io) to measure the reactive output power (Qo) and the active output power (Po). The power measurement circuit (410) is also called a power meter.

[0082] The voltage control circuit (430) can receive reactive output power (Qo) and reactive power command value (Qc) and generate a voltage control value (Vpwm).

[0083] The voltage control circuit (430) can calculate a reactive power reference by reflecting a reactive power droop control equation. The voltage control circuit (430) can form a reactive power droop gain equation using a reactive power droop gain and a value corresponding to the X-intercept or a value corresponding to the Y-intercept, and calculate a reactive power reference by substituting an output voltage or a grid voltage (Vg) into the reactive power droop gain equation. Alternatively, the voltage control circuit (430) can calculate a reactive power reference by combining the value calculated from the reactive power droop gain equation with a reactive power command value (Qc).

[0084] Also, the voltage control circuit (430) can calculate a voltage control value (Vpwm) based on the reactive power reference. The voltage control circuit (430) can calculate a voltage control value (Vpwm) by applying a PI (Proportional Integral) control circuit to the difference between the reactive power reference and the reactive output power (Qo).

[0085] The phase control circuit (420) can calculate a phase control value (θpwm) using the difference between the active power reference (Pref) and the active output power (Po). Additionally, the gate control circuit (440) can generate a control signal (Gs) for PWM controlling switching elements according to the voltage control value (Vpwm) and the phase control value (θpwm).

[0086] Here, the active power reference (Pref) may be the same value as the active power command value. The following description focuses on an example where the active power command value and the active power reference are the same value.

[0087] The system estimation circuit (450) can estimate the system connection impedance and system voltage (Vg) based on the sensing signal. The system estimation circuit (450) can also transmit the estimated values ​​to other components of the control circuit (320).

[0088] FIG. 5 is an example configuration diagram of a phase control circuit according to one embodiment.

[0089] Referring to FIG. 5, the phase control circuit (420) can generate an angular velocity control value (ωm) using an inertia model (521) and calculate a phase control value (θpwm) by integrating the angular velocity control value (ωm).

[0090] The phase control circuit (420) can calculate a damping value (D(ωm - ωg)) by multiplying the value obtained by subtracting the angular velocity value (ωg) of the grid voltage from the angular velocity control value (ωm) by a damping coefficient (D) to apply further damping, and input the damping value to the inertia model (521) through a feedback loop. Here, the angular velocity value (ωg) of the grid voltage can be measured by a PLL (Phase Lock Loop) circuit.

[0091] The phase control circuit (420) may include a virtual inertia model (520). The virtual inertia model (520) may include an inertia model (521) and a damping model (522).

[0092] The inertia model (521) can integrate the difference between the active power command value (Pref) and the active output power (Po) of the power stage. Additionally, the inertia model (521) can integrate the value obtained by subtracting the output (D(ωm - ωg)) of the damping model (522) from the difference between the active power command value (Pref) and the active output power (Po).

[0093] And, the phase control circuit (420) can set the output of the inertia model (521) as the angular velocity control value (ωm), and can calculate the phase control value (θpwm) by multiplying the angular velocity control value (ωm) by a constant coefficient (ωb) and integrating.

[0094] The damping model (522) can be negatively fed back to the inertia model (521) after multiplying the value obtained by subtracting the angular velocity value (ωg) of the system voltage from the angular velocity control value (ωm) by the damping coefficient (D).

[0095] According to this feedback loop, the phase control circuit (420) can supply the value obtained by subtracting D(ωm - ωg) from the difference (Pref - Po) between the active power command value (Pref) and the active output power (Po) as the input to the inertia model (521).

[0096] With this structure, the phase control circuit (420a) can apply both inertia (M) and damping (D).

[0097] The inertia model (521) can calculate an angular velocity control value (ωm) or a phase control value (θpwm) by integrating the difference between the effective power command value (Pref) and the effective output power (Po) of the power stage.

[0098] The damping model (522) can negatively feed back the difference between the angular velocity control value (ωm) and the angular velocity value (ωg) of the grid power by multiplying it by a damping coefficient (D) as input to the inertia model (521).

[0099] The inertia model (521) can calculate the output value of the angular velocity control value (ωm) by subtracting the value obtained by multiplying the difference between the angular velocity control value (ωm) and the angular velocity value (ωg) of the grid power by the damping coefficient (D) from the difference between the effective power command value (Pref) and the effective output power (Po) of the power stage, and dividing by the preset inertia value (M) and integrating.

[0100] As such, calculations in control circuits require values ​​for various variables, which may include the grid connection impedance and grid voltage. However, as mentioned above, these values ​​are difficult to measure in real-world conditions. To overcome this difficulty, one embodiment estimates the grid connection impedance and grid voltage based on data.

[0101] Figure 6 is a simplified model of the grid connection impedance.

[0102] Referring to FIG. 6, the grid connection impedance can be modeled by placing a line resistance (Rg) and a line reactance (Xg) in series between the output node (No) and the grid node (Ng) of the grid forming inverter.

[0103] In addition, the grid forming inverter can verify the values ​​of the output voltage (E∠0) formed at the output node (No) and the output current (I∠Φ) flowing from the output node (No) to the grid node (Ng) through actual measurement or other measuring instruments, and can verify the active power (P) and reactive power (Q) output through the output node (No) through actual measurement or other measuring instruments.

[0104] Here, the values ​​of active power (P), reactive power (Q), output voltage (E∠0), and output current (I∠Φ) are referred to as the measurements.

[0105] At this time, when determining the phase of other components by setting the phase of the output voltage (E∠0) to a reference angle of 0 degrees, the output current can be expressed as I∠Φ and the grid voltage can be expressed as Vg∠-δ. The phase angle ∠Φ of the output current is a measured value and can be included in the aforementioned measurements. In addition, the phase angle ∠-δ of the grid voltage can be calculated by the method described below.

[0106] The contents described below as being calculated by the grid forming inverter can be calculated by a specific component of the grid forming inverter, specifically by the control circuit. For the sake of convenience of explanation, it is described below that the calculations are performed by the grid forming inverter.

[0107] A grid forming inverter—for example, a control circuit—can formulate the estimation problem to calculate grid connection impedance and grid voltage based on data, as shown in the following objective function equations.

[0109] [Objective function formulas]

[0110]

[0111]

[0112] Here, the meaning of each variable is as follows.

[0113] : Final value of line resistance estimate

[0114] : Estimated line resistance (value used to find the solution to the objective function)

[0115] : Final estimated value of line reactance

[0116] : Estimated line reactance (value used to find the solution to the objective function)

[0117] : Final value of system voltage estimate

[0118] : Estimated system voltage (value used to find the solution to the objective function)

[0119] : Active power estimation residual

[0120] : Reactive power estimation residual

[0121] : Output voltage estimation residual

[0122] : Measure of active power obtained at the K-th (K is a natural number)-th operating point

[0123] : Measurement of reactive power obtained at the Kth driving point

[0124] : Measured output voltage obtained at the Kth driving point

[0125] : Measured output current obtained at the Kth operating point

[0126] : Phase angle of the system voltage calculated for the Kth operating point

[0127] : Phase angle of the output current obtained at the Kth operating point

[0128] : Intermediate variable 1 used in the calculation for the Kth driving point

[0129] : Intermediate variable 2 used in the calculation for the Kth driving point

[0131] A grid forming inverter can formulate objective function equations as shown above to minimize the estimated residuals between the values ​​calculated based on the estimates and the measured values. Furthermore, the grid forming inverter can calculate the estimates by finding a solution to this objective function.

[0132] The estimated residual is, for example, the active power estimated residual ( It can be, and the reactive power estimation residual( It can be, and the output voltage estimation residual( It can be. For the Kth operating point, the grid forming inverter measures ( , , ) can be obtained. And, these measurements ( , , After calculating values ​​that are substantially identical to ) using estimates, those values ​​are the measured values ​​( , , Estimated residuals can be calculated by subtracting from ).

[0133] Estimated line resistance in objective function formulas ( ), line reactance estimate( ), System voltage estimate( ) and phase angle of system voltage( ) can be an unknown. In this case, to find a solution for the estimates using three estimated residual equations, the phase angle ( ) can be calculated using the steady-state power-phase angle relationship.

[0135] [Phase Angle Formula]

[0136]

[0138] The values ​​used in the phase angle formula are steady-state values, so calculation can be easy.

[0139] If we use the steady-state power-phase angle relationship in this way, the unknown is the line resistance estimate ( ), line reactance estimate( ), System voltage estimate( With these three, calculations can be made easier.

[0140] These objective function formulas can be derived from the active power conservation formula, the reactive power conservation formula, and the Kirchhoff voltage formula.

[0142] [Effective Power Conservation Formula and Reactive Power Conservation Formula]

[0143]

[0145] [Kirchhoff's Voltage Formula]

[0146]

[0148] By organizing these active power conservation formulas, reactive power conservation formulas, and Kirchhoff's voltage formulas, the formulas used in the objective function equations can be derived as follows.

[0149]

[0151] These active power conservation formulas, reactive power conservation formulas, and Kirchhoff's voltage formulas are equations that hold between the grid node where the grid voltage is to be estimated and the output node of the grid forming inverter.

[0152] The grid forming inverter can calculate estimates of the grid voltage formed at the grid node and the grid connection impedance formed between the grid node and the output node by applying measurements taken at the output node to these equations. Then, the grid forming inverter can PWM control the switching elements using the methods described with reference to FIG. 4 and / or FIG. 5 according to the calculated estimates.

[0153] Meanwhile, the active power conservation formula, the reactive power conservation formula, and the Kirchhoff voltage formula have nonlinearity and also include an unknown variable called the phase angle of the grid voltage, so if the solution is obtained by solving independent equations, an inaccurate value may be obtained. Accordingly, a grid forming inverter according to one embodiment first reduces the number of unknown variables to three by calculating the phase angle of the grid voltage according to the steady-state power-phase angle relationship. Then, the grid forming inverter can improve accuracy by calculating estimates by finding the optimal solution for the objective function rather than independent equations. The objective function is a function that minimizes the sum of the estimated residuals described in (2), (3), and (4) among the aforementioned [objective function formulas], and the grid forming inverter can perform the process of finding the optimal solution of such a function.

[0154] Grid forming inverters can find solutions to objective function equations using the Grid Search Algorithm (GSA).

[0155] FIG. 7 is a diagram showing an algorithm for finding a solution for a grid forming inverter according to one embodiment.

[0156] Referring to Fig. 7, the grid forming inverter can find a solution using a grid search algorithm.

[0157] The grid search algorithm does not fall into a local optimum and minimizes the track resistance estimate ( ), line reactance estimate( ), System voltage estimate( A global optimal solution can be derived to find ).

[0158] The grid search algorithm can set minimum and maximum values. Then, the minimum and maximum values ​​can be divided into N intervals (N is a natural number greater than or equal to 2), and the sum of the estimated residuals in each interval can be calculated. Then, the grid search algorithm can determine the estimates by selecting the smallest sum among those estimated residuals.

[0159] Grid forming inverters can set the minimum and maximum values ​​of each estimate in the grid search algorithm, and since the per-unit (pu) value of the power grid impedance is generally between 0.05 and 0.4, grid forming inverters can set the minimum and maximum values ​​of the grid connection impedance estimates to 0.05 and 0.4 times the reference impedance (Zbase) based on the per-unit.

[0160] Rg,min = 0.05·Zbase, Rg,max = 0.4·Zbase

[0161] (Rg,min : Minimum estimated line resistance, Rg,max : Maximum estimated line resistance)

[0162] Xg,min = 0.05·Zbase, Xg,max = 0.4·Zbase

[0163] (Xg,min : Minimum value of estimated line reactance, Xb,max : Maximum value of estimated line reactance)

[0164] In addition, in normal operation, the pu value of the system voltage can be set to 0.95 and 1.05 times the reference phase-to-ground voltage (Vbase).

[0165] Vg,min = 0.95·Vbase, Vg,max = 1.05·Vbase

[0166] (Vg,min : Minimum estimated system voltage, Vg,max : Maximum estimated system voltage)

[0167] Here, Zbase can satisfy the following equation.

[0168]

[0169] (Sbase: Standard rated capacity)

[0171] The grid forming inverter can set the grid connection impedance and grid voltage estimates to minimum values ​​during the initialization phase of the grid search algorithm. Accordingly, the grid forming inverter can calculate the sum of the estimated residuals.

[0172] In addition, the grid forming inverter divides the range between the minimum and maximum values ​​into N intervals and calculates the sum of estimated residuals in each interval; if the calculated value is smaller than the previous calculated value, it can find the minimum value by calculating the sum of estimated residuals.

[0173] As described above, according to the present embodiment, a more improved control technology for a grid forming inverter can be provided. Furthermore, according to the present embodiment, the grid connection impedance and grid voltage can be estimated more accurately. Additionally, according to the present embodiment, the grid connection impedance and grid voltage can be estimated using only the data of the grid forming inverter.

[0174] Terms such as "include," "compose," or "have" as described above, unless specifically stated otherwise, mean that the relevant component may be inherent; therefore, they should be interpreted as allowing for the inclusion of additional components rather than excluding them. All terms, including technical or scientific terms, have the same meaning as generally understood by those skilled in the art to which the present invention pertains, unless otherwise defined. Commonly used terms, such as those defined in advance, should be interpreted in accordance with their meaning in the context of the relevant technology and should not be interpreted in an ideal or overly formal sense unless explicitly defined in the present invention.

[0175] The foregoing description is merely an illustrative explanation of the technical concept of the present invention, and those skilled in the art to which the present invention pertains will be able to make various modifications and variations within the scope of the essential characteristics of the present invention. Accordingly, the embodiments disclosed in the present invention are intended to explain, not limit, the technical concept of the present invention, and the scope of the technical concept of the present invention is not limited by these embodiments. The scope of protection of the present invention shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present invention.

Claims

Claim 1 A power stage that converts power according to the on / off control of switching elements and outputs the converted power; The control circuit includes a control circuit that obtains measurements of the active power, reactive power, output voltage, and output current of the power output from the power stage to the grid at the output node, applies the measurements to an active power conservation formula, a reactive power conservation formula, and a Kirchhoff voltage formula between the grid node and the output node to estimate the grid voltage, calculates the estimated values ​​of the grid voltage formed at the grid node and the grid connection impedance formed between the grid node and the output node, and controls the switching elements by PWM (Pulse Width Modulation) according to the estimates, wherein the active power conservation formula, the reactive power conservation formula, and the Kirchhoff voltage formula have nonlinearity, and the control circuit calculates the estimates by finding a solution to an objective function such that the value calculated according to the estimates and the estimated residual of the measurements are minimized, finds the solution using the measurements obtained from a plurality of operating points, and calculates the estimates. A grid forming inverter, wherein the phase angle of the grid voltage is included in the active power conservation formula, the reactive power conservation formula, and the Kirchhoff voltage formula, and the control circuit calculates the phase angle of the grid voltage according to the steady-state power-phase angle relationship, the control circuit finds a solution to the formulas constituting the objective function using a Grid Search Algorithm (GSA), the control circuit sets the grid connection impedance and the estimated grid voltage to a minimum value during the initialization phase of the Grid Search Algorithm, divides the minimum and maximum values ​​in the Grid Search Algorithm into N (N is a natural number greater than or equal to 2) intervals, calculates the sum of the estimated residuals in each interval, and determines the estimated values ​​as the smallest among the sums of the estimated residuals. Claim 2 delete Claim 3 delete Claim 4 delete Claim 5 delete Claim 6 In claim 1, the grid forming inverter, wherein the objective function is composed of the following objective function formulas. [Objective function formulas] Here, the meaning of each variable is as follows. : Final value of line resistance estimate : Estimated line resistance (value used to find the solution to the objective function) : Final estimated value of line reactance : Estimated line reactance (value used to find the solution to the objective function) : Final value of system voltage estimate : Estimated system voltage (value used to find the solution to the objective function) : Active power estimation residual : Reactive power estimation residual : Output voltage estimation residual : Measure of active power obtained at the K-th (K is a natural number)-th operating point : Measurement of reactive power obtained at the Kth driving point : Measured output voltage obtained at the Kth driving point : Measured output current obtained at the Kth operating point : Phase angle of the system voltage calculated for the Kth operating point : Phase angle of the output current obtained at the Kth operating point : Intermediate variable 1 used in the calculation for the Kth driving point : Intermediate variable 2 used in the calculation for the Kth driving point Claim 7 In Clause 6, a grid forming inverter in which the phase angle of the system voltage is calculated using the following phase angle formula. [Phase Angle Formula] Claim 8 In claim 7, the control circuit is a grid forming inverter that finds a solution such that the sum of the estimated residuals described in (2), (3) and (4) of the objective function equations is minimized. Claim 9 delete Claim 10 In claim 8, the control circuit is a grid forming inverter that sets the minimum and maximum values ​​of the grid connection impedance estimates in the grid search algorithm to reference impedances of 0.05 and 0.4 times based on per-unit (pu). Claim 11 In claim 10, the control circuit sets the minimum and maximum values ​​of the grid voltage estimates in the grid search algorithm to reference phase-ground voltages of 0.95 and 1.05 times based on per unit, respectively, in a grid forming inverter. Claim 12 delete Claim 13 delete Claim 14 A grid forming inverter according to claim 1, wherein an LCL output filter is disposed at the output terminal of the power stage, and the output node is formed in the LCL output filter. Claim 15 In claim 14, the LCL output filter comprises an inverter-side inductor, a grid-side inductor, and an output capacitor, and the output node is formed at the common junction of the inverter-side inductor, the grid-side inductor, and the output capacitor, a grid-forming inverter.

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