Control method of power grid simulator with wide-frequency domain simulation function of transmission line impedance

Abstract

The present invention discloses a power grid simulator control method with a wide-frequency domain simulation function for transmission line impedance, and belongs to the field of power grid simulation control technology. First, the transmission line between the output end of the filter circuit in the power grid simulator and the high-voltage side of the transformer is regarded as a two-port network, and the relationship between the terminal voltage of the transmission line and the terminal current and the starting voltage is calculated; then the relationship is subjected to multi-order linearization processing and expressed in the form of a transfer function in the frequency domain; then the PCC point currents of the power grid simulator and the measured grid-connected converter are collected, and the PCC point reference voltage is calculated based on the transfer function and the power grid simulator reference voltage; then a voltage controller is used to control the difference between the PCC point reference voltage and the collected PCC point voltage, and the capacitor current is collected and fed back to the output of the voltage controller, thereby obtaining a modulation command signal. The present invention realizes wide-frequency domain, high-precision, dynamic, and real-time simulation of line impedance.

Description

Technical Field

[0001] The present invention relates to the technical field of power grid simulation control, and in particular to a power grid simulator control method with a transmission line impedance wide-frequency domain simulation function. Background Art

[0002] In new power systems dominated by renewable energy, grid voltage drops, asymmetric faults, and frequency fluctuations can cause grid-connected converters to disconnect from the grid at irregular intervals, seriously impacting the safe and stable operation of the grid. Therefore, grid adaptability testing is required before new energy grid-connected converters are put into operation. This requires specialized equipment—a grid simulator—to accurately simulate various typical grid operating and fault conditions.

[0003] In recent years, with the increase in the penetration rate of new energy, power grid simulators have attracted the attention of some scholars and have made certain progress in topology and control. For example, "A Feedback Power Grid Simulator and Its Working Method" (Publication No. CN117110771A) proposes a topological structure of a feedback power grid simulator, which is simple in structure and easy to control. It not only has a sinusoidal wave output mode and a variety of harmonic superposition output modes, but also has single-phase, two-phase or three-phase low (zero) voltage ride-through simulation and other functions, but does not consider the line impedance simulation function. Since new energy power generation systems such as photovoltaic and wind power are mostly distributed in remote areas such as deserts and islands, they need to be connected to the grid through long-distance transmission and distribution lines. At this time, the system line impedance will not only change the impedance characteristics of the power grid, making the power grid present a weak grid characteristic, but also affect the working performance of the grid-connected converter, causing system stability problems. Therefore, in order to more realistically simulate the actual power grid situation and detect the weak grid adaptability of the grid-connected converter, it is very necessary for the power grid simulator to have a wide-frequency domain simulation function of the long transmission line impedance.

[0004] Currently, existing research on line impedance simulation has consistently treated power grid transmission lines as equivalent to resistive inductance. For example, "A Control Method, Control Device, and Power Grid Simulator" (Publication No. CN116819210A) uses an improved voltage-current dual closed-loop control strategy to control the converter in the power grid simulator. This strategy reduces the discrepancy between the simulated power grid mirror and the actual power grid by simulating line impedance and the frequency fluctuation characteristics of the synchronous power supply. However, the proposed method can only simulate line impedance at the fundamental frequency, and the line impedance is only considered as resistive inductance. This not only differs from the current actual power grid but also fails to meet the development needs of new power systems. On the one hand, in actual power systems, renewable energy generation systems often require longer transmission lines for grid connection. At this point, not only the resistance-inductance characteristics of the lines must be considered, but also the distributed capacitance between the cables and the ground, and between the cables themselves. Therefore, the transmission lines exhibit more than just resistance-inductance. On the other hand, with the increasing penetration of renewable energy, a large number of power electronic converters are being incorporated into the grid, which in turn leads to the injection of a rich variety of harmonics. At this time, transmission line simulations that only cover low-frequency bands can no longer reflect the actual grid conditions, and a wider frequency domain must be considered. Therefore, to more realistically and accurately simulate the impact of transmission lines on grid characteristics, it is necessary to study line impedance simulation control methods that are wide-frequency, high-precision, real-time, and highly applicable. Summary of the Invention

[0005] The present invention provides a power grid simulator control method with a wide-frequency domain simulation function for transmission line impedance. Aiming at the problem of complex transmission line parameters and non-resistance-inductance characteristics in new power systems, a distributed parameter circuit model is used to process the transmission line. Through a multi-order linearization approximation method combined with a voltage and current closed-loop feedback control loop, a wide-frequency domain, high-precision, dynamic, and real-time simulation of line impedance is achieved.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0007] A method for controlling a power grid simulator with a wide-frequency domain simulation function for transmission line impedance, the power grid simulator comprising a DC power supply, an inverter unit, and a filter circuit, wherein the DC power supply is connected to an input of the inverter unit, an output of the inverter unit is connected to an input of the filter circuit, an output of the filter circuit is connected to a high-voltage side of a transformer, and a low-voltage side of the transformer is connected to a grid-connected converter under test;

[0008] The power grid simulator control method comprises the following steps:

[0009] (1) The transmission line between the output end of the filter circuit in the power grid simulator and the high-voltage side of the transformer is regarded as a two-port network, and the propagation coefficient and characteristic impedance of the transmission line are calculated;

[0010] (2) Combining the two-port network, the propagation coefficient and the characteristic impedance of the transmission line, the relationship between the terminal voltage, the terminal current and the starting voltage of the transmission line is obtained;

[0011] (3) Substitute the transmission circuit start-end voltage and terminal voltage in the relationship derived in step (2) with the grid simulator reference voltage and the PCC point reference voltage, respectively, to obtain the calculation relationship for the PCC point reference voltage when simulating line impedance;

[0012] (4) Perform multi-order linearization on the relationship obtained in step (3) to obtain a linearized PCC point reference voltage calculation formula, and express it in the form of a transfer function in the frequency domain;

[0013] (5) Collect the current at the point of common coupling (PCC) between the grid simulator and the grid-connected converter under test, and calculate the PCC point reference voltage based on the transfer function and the grid simulator reference voltage;

[0014] (6) The PCC point voltage is collected and subtracted from the calculated PCC point reference voltage. The voltage controller is used to control the difference between the two. The output of the voltage controller is subtracted from the capacitor current of the filter circuit in the power grid simulator (via the proportional coefficient) to obtain a modulation command signal. The drive signal is generated through PWM modulation to control the switch tube of the inverter unit in the power grid simulator.

[0015] Furthermore, the propagation coefficient and characteristic impedance of the transmission line are as follows:

[0016]

[0017] Where γ represents the propagation coefficient, Z C represents characteristic impedance, j represents imaginary unit, ω represents angular frequency, R0 represents resistance per unit length of the transmission line, L0 represents inductance per unit length of the transmission line, and C0 represents capacitance per unit length of the transmission line.

[0018] Furthermore, in step (2), the relationship between the terminal voltage of the transmission line, the terminal current, and the starting voltage is as follows:

[0019]

[0020] in, represents the transmission line terminal voltage, represents the voltage at the beginning of the transmission line, represents the transmission line terminal current, γ represents the propagation coefficient, Z C represents the characteristic impedance, and l represents the length of the transmission line.

[0021] Furthermore, in step (3), the transmission line starting voltage and terminal voltage are replaced by the grid simulator reference voltage and PCC point reference voltage respectively, and the transmission line terminal current is replaced by the PCC point current, and the PCC point reference voltage calculation expression when simulating line impedance is obtained:

[0022]

[0023] Furthermore, in step (4), the transfer function is expressed as:

[0024]

[0025] in, Indicates the PCC point reference voltage, represents the grid simulator reference voltage, represents the PCC point current, s represents a complex variable, G(s) represents the input channel transfer function, and H(s) represents the output channel transfer function.

[0026] Furthermore, the multi-order linearization process adopts the Padé approximation fitting method.

[0027] Furthermore, the input channel transfer function and the output channel transfer function in the transfer function are respectively expressed as:

[0028]

[0029] Wherein, R0 represents the resistance per unit length of the transmission line, L0 represents the inductance per unit length of the transmission line, C0 represents the capacitance per unit length of the transmission line, and l represents the length of the transmission line.

[0030] Furthermore, in step (5), the grid simulator reference voltage is multiplied by the input channel transfer function, the PCC point current is multiplied by the output channel transfer function, and the difference between the two is taken to obtain the PCC point reference voltage.

[0031] Furthermore, the high-voltage side of the transformer is a common coupling point between the power grid simulator and the grid-connected converter under test, namely, a PCC point.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] (1) The present invention can simulate the impedance of a transmission line of any length with high precision, and can change the tested power grid environment by setting the transmission line length online, thus meeting the high real-time requirements for power grid simulation in actual tests.

[0034] (2) A multi-order linear expression is used to simulate the line impedance. The order used is related to the length of the simulated transmission line and the power grid environment. This not only conforms to the characteristics of the actual transmission line, but also has the characteristics of strong versatility and wide frequency domain.

[0035] (3) The present invention uses the grid simulator reference voltage and the PCC point current sampling value to calculate the PCC point reference voltage, and then controls the PCC point voltage through the voltage loop controller, and realizes active damping through capacitor current acquisition and feedback, ultimately meeting the high-precision and high-dynamic simulation requirements of the grid simulator line impedance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is the circuit diagram of the power grid simulator of the present invention;

[0037] Figure 2 This is the overall control block diagram of the wide-frequency domain simulation function of the superimposed line impedance of the present invention

[0038] Figure 3 The voltage and current waveforms at the PCC point when the power grid simulator is connected to the actual transmission line;

[0039] Figure 4 The voltage and current waveforms at the PCC point when the power grid simulator simulates the transmission line; DETAILED DESCRIPTION

[0040] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific implementation methods.

[0041] Figure 1 The figure shows a topological diagram of a power grid simulation system, including a power grid simulator, a transformer and a grid-connected converter under test. The power grid simulator consists of a DC power supply, an inverter unit and a filter circuit. The DC power supply is connected to the input end of the inverter unit, the output end of the inverter unit is connected to the input end of the filter circuit, the output end of the filter circuit is connected to the high-voltage side of the transformer, and the low-voltage side of the transformer is connected to the grid-connected converter under test.

[0042] The high-voltage side of the transformer is the common coupling point between the power grid simulator and the converter under test, namely the PCC point.

[0043] Figure 2The control block diagram of the present invention is shown. It utilizes a control strategy that includes an outer voltage loop and an inner capacitor current loop. The outer voltage loop is controlled by a voltage controller. The PCC reference voltage is calculated based on the grid simulator reference voltage and the PCC current, combined with a transmission line model. The PCC voltage is then measured and subtracted from the reference voltage. This difference is then controlled by a voltage controller. The voltage controller output is then subtracted from the capacitor current (via a proportional coefficient) to generate a modulation command signal. The control method of the present invention is not limited to a specific grid simulator inverter unit circuit; it can be applied to any DC / AC conversion circuit suitable for a grid simulator.

[0044] Specifically, a control method for a power grid simulator with a wide-frequency domain simulation function for transmission line impedance includes the following steps:

[0045] (1) If the transmission line is regarded as a two-port network, the output voltage of the filter circuit is the starting voltage of the two-port network, the output current of the filter circuit is the starting current of the two-port network, the voltage on the high-voltage side of the transformer is the terminal voltage of the two-port network, and the current on the high-voltage side of the transformer is the terminal current of the two-port network.

[0046] In a specific implementation of the present invention, the physical quantities are defined as follows:

[0047] The voltage at the start of the two-port network is The starting current of the two-port network is The high voltage side of the transformer is the common coupling point between the grid simulator and the converter under test, namely the PCC point. The voltage at the PCC point is The current at the PCC point is The length of the transmission line is l, in km, the resistance per unit length is R0, the inductance per unit length is L0, the capacitance per unit length is C0, and the conductance per unit length is ignored.

[0048] Propagation coefficient γ and characteristic impedance Z of the transmission line C Expressed as:

[0049]

[0050] Here, j represents the imaginary unit and ω represents the angular frequency.

[0051] (2) The voltage-current relationship between the start and end of the transmission line is obtained by combining the two-port network. The relationship is sorted out to obtain the relationship between the terminal voltage of the transmission line and the terminal current and the start voltage. The relationship contains transcendental terms.

[0052] In a specific embodiment of the present invention, to address the problem that transmission line parameters in new power systems are complex and not just exhibit resistance-inductance characteristics, a distributed parameter circuit model is used to process the transmission line. Based on the mathematical model of the transmission line, the relationship between the voltage and current at the beginning and the voltage and current at the end of the transmission line is obtained:

[0053]

[0054] The relationship between the terminal voltage, terminal current and starting voltage is obtained from formula (2):

[0055]

[0056] In formula (3), and Replaced with the grid simulator reference voltage and PCC point reference voltage Will Replaced by PCC point current The following relationship is obtained:

[0057]

[0058] From formula (4), it can be seen that the PCC point reference voltage of the transmission line can be represented by the grid simulator reference voltage and the PCC point current of the transmission line. Therefore, the function of simulating the transmission line impedance characteristics can be realized by controlling the reference voltage of the PCC point.

[0059] (3) The PCC point reference voltage calculation expression containing the transcendental term is linearized to obtain the linearized PCC point reference voltage calculation expression.

[0060] In a specific implementation of the present invention, in order to linearize the obtained transcendental equation, the Padé approximation fitting method is selected to linearize the complex transfer function, but the linearization method is not limited to this method.

[0061] The hyperbolic function Substitute into formula (3) and further organize it to obtain:

[0062]

[0063] For the exponential function e x , its second-order linear approximation is:

[0064]

[0065] Select Equation (6) to linearize Equation (5) and express it as a transfer function in the frequency domain. Then, the voltage at the PCC point can be adjusted by the controller to simulate the impedance characteristics of the transmission line.

[0066] Here, we take the second-order linear approximation formula (6) as an example, and formula (5) can be transformed into:

[0067]

[0068] Where s represents a complex variable, G(s) represents the input channel transfer function, and H(s) represents the output channel transfer function.

[0069] In addition to the second-order linearization illustrated above, other multi-order linearization processing methods can also be used.

[0070] (4) Collect the current at the PCC point and use the transfer function obtained in step (3) to calculate the reference voltage at the PCC point during the transmission line impedance simulation.

[0071] In a specific implementation of the present invention, the grid simulator reference voltage is multiplied by the input channel transfer function G(s), and the PCC point current is multiplied by the output channel transfer function H(s). The difference between the two is obtained to obtain the PCC point reference voltage. Therefore, the PCC point current is collected as The grid simulator reference voltage is Based on formula (4), the PCC point reference voltage is calculated as

[0072] (5) Collecting PCC point voltage and its reference voltage The difference between the two is obtained by subtracting them, and the voltage controller is used to control the difference between the two. The capacitor current is collected and the proportional coefficient k is used to calculate the value of the capacitor current. c Feedback to the voltage controller output to obtain the final modulation command signal

[0073] In a specific implementation of the present invention, the reference voltage of the PCC point obtained according to step (4) is subtracted from the collected PCC point voltage feedback value, and the difference between the two is controlled by a voltage controller. The voltage controller output and the feedback filter circuit capacitor current (via a proportional coefficient) are subtracted to obtain a modulation command signal, which is then modulated by PWM to generate a drive signal to control the switch tube of the inverter unit.

[0074] In order to verify the control strategy proposed in this invention, a Figure 1 The simulation model shown here uses the following parameters: a DC link voltage of 5 kV, a grid simulator active power of 15 MW, a switching frequency of 1.5 kHz, a filter circuit inductance of 4.5 mH, and a filter circuit capacitance of 1.2 μF. The simulated transmission line parameters are: a unit resistance of 0.01273 Ω / km, a unit inductance of 0.9337 mH / km, a unit capacitance of 12.74 μF / km, a transformer high-voltage side rated voltage of 35 kV, and a transformer low-voltage side rated voltage of 690 V. The effectiveness of the proposed strategy is verified by comparing the simulated transmission line impedance results with the actual line impedance.

[0075] Figure 3The voltage and current waveforms at the PCC point when the power grid simulator is connected to the actual transmission line, with line lengths of l = 30 km and l = 50 km respectively; Figure 4 The voltage and current waveforms at the PCC point when the power grid simulator simulates the transmission line are shown, with line lengths of l = 30 km and l = 50 km respectively. It can be seen that the present invention can simulate the power grid operation conditions with high accuracy when considering the transmission line.

[0076] The embodiments described above provide a detailed description of the technical solutions and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements and equivalent substitutions made within the scope of the principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A control method for a power grid simulator capable of wide-frequency simulation of transmission line impedance, the power grid simulator comprising a DC power supply, an inverter unit, and a filter circuit, wherein the DC power supply is connected to the input of the inverter unit, the output of the inverter unit is connected to the input of the filter circuit, the output of the filter circuit is connected to the high-voltage side of a transformer, and the low-voltage side of the transformer is connected to a grid-connected converter under test; It is characterized by: The power grid simulator control method comprises the following steps: (1) The transmission line between the output end of the filter circuit in the power grid simulator and the high-voltage side of the transformer is regarded as a two-port network, and the propagation coefficient and characteristic impedance of the transmission line are calculated; (2) Combining the two-port network, the propagation coefficient and the characteristic impedance of the transmission line, the relationship between the terminal voltage, the terminal current and the starting voltage of the transmission line is obtained; (3) Replace the starting voltage and terminal voltage in the relationship derived in step (2) with the grid simulator reference voltage and PCC point reference voltage, respectively, and replace the terminal current with the PCC point current, thereby obtaining the calculation relationship of the reference voltage during line impedance simulation; (4) Perform multi-order linearization on the relationship obtained in step (3) to obtain a linearized PCC point reference voltage calculation expression, and express it in the form of a transfer function in the frequency domain; (5) Collect the PCC point current of the grid simulator and the grid-connected converter under test, and calculate the PCC point reference voltage based on the transfer function and the grid simulator reference voltage; (6) The PCC point voltage is collected and subtracted from the calculated PCC point reference voltage. The voltage controller is used to control the difference between the two. The output of the voltage controller is subtracted from the capacitor current of the filter circuit in the power grid simulator after scaling by the proportional coefficient to obtain a modulation command signal. The drive signal is generated through PWM modulation to control the switch tube of the inverter unit in the power grid simulator.

2. The power grid simulator control method with transmission line impedance wide-frequency domain simulation function according to claim 1, characterized in that: The propagation coefficient and characteristic impedance of the transmission line are as follows: Where γ represents the propagation coefficient, Z C represents characteristic impedance, j represents imaginary unit, ω represents angular frequency, R0 represents resistance per unit length of the transmission line, L0 represents inductance per unit length of the transmission line, and C0 represents capacitance per unit length of the transmission line.

3. The power grid simulator control method with transmission line impedance wide-frequency domain simulation function according to claim 1, characterized in that: In step (2), the relationship between the terminal voltage, the terminal current, and the starting voltage of the transmission line is as follows: in, represents the transmission line terminal voltage, represents the voltage at the beginning of the transmission line, represents the transmission line terminal current, γ represents the propagation coefficient, Z C represents the characteristic impedance, and l represents the length of the transmission line.

4. The power grid simulator control method with transmission line impedance wide-frequency domain simulation function according to claim 3, characterized in that: In step (3), the starting voltage and the terminal voltage are replaced by the grid simulator reference voltage With PCC point reference voltage Replace the terminal current with the PCC point current The calculation expression of the reference voltage at the PCC point during line impedance simulation is obtained:

5. The power grid simulator control method with transmission line impedance wide-frequency domain simulation function according to claim 1, characterized in that: In step (4), the transfer function is expressed as: in, Indicates the PCC point reference voltage, represents the grid simulator reference voltage, represents the PCC point current, s represents a complex variable, G(s) represents the input channel transfer function, and H(s) represents the output channel transfer function.

6. The control method of a power grid simulator with a transmission line impedance wide-frequency domain simulation function according to claim 5, characterized in that: The multi-order linearization process adopts the Padé approximation fitting method.

7. The power grid simulator control method with transmission line impedance wide-frequency domain simulation function according to claim 6, characterized in that: The input channel transfer function and the output channel transfer function in the transfer function are respectively expressed as: Wherein, R0 represents the resistance per unit length of the transmission line, L0 represents the inductance per unit length of the transmission line, C0 represents the capacitance per unit length of the transmission line, and l represents the length of the transmission line.

8. The control method of a power grid simulator with a transmission line impedance wide-frequency domain simulation function according to claim 4, characterized in that: In step (5), the grid simulator reference voltage is multiplied by the input channel transfer function, the PCC point current is multiplied by the output channel transfer function, and the difference between the two is taken to obtain the PCC point reference voltage.

9. The control method of a power grid simulator with a transmission line impedance wide-frequency domain simulation function according to claim 4, characterized in that: The high-voltage side of the transformer is the PCC point between the power grid simulator and the grid-connected converter under test.

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