Grid-forming Inverter Harmonic Suppression Ability Test System and Its Testing and Evaluation Methods

Abstract

The present invention discloses a test system for the harmonic suppression ability of a grid-forming converter and its test and evaluation methods, including: a non-linear load and its control switch S load , the non-linear load is used to provide harmonics with different frequency characteristics, and its control switch S load is used to control the grid connection or disconnection of the non-linear load; an analog power grid and its control switch S Grid , the analog power grid is used to provide a stable voltage for the non-linear load and simulate faults, and its control switch S Grid is used to control the state of the analog power grid; a data acquisition device, which is used to collect the instantaneous values of the three-phase currents on the grid-forming converter side, the instantaneous values of the three-phase voltages on the grid side and the instantaneous values of the three-phase currents, and the instantaneous values of the three-phase currents on the non-linear load side. The structure of the present invention is simple and reliable, the operation is convenient, the test time and cost can be saved greatly, and the harmonic suppression performance index of the grid-forming converter for the power grid can be clarified.

Description

Technical Field

[0001] The present invention relates to the technical field of harmonic suppression of grid-forming converters in the power grid, and in particular to a harmonic suppression ability test system for grid-forming converters and its test and evaluation methods. Background Art

[0002] With the development of renewable energy power generation, the proportion of power electronic devices in the power grid has gradually increased, and the overall power grid shows trends such as reduced inertia and weakened strength. To ensure the stable and safe operation of the power grid, power electronic converters need to achieve and even replace the functions and roles of traditional synchronous motors, and grid-forming converters have emerged and gradually become the mainstream development trend. However, regarding the dynamic characteristics of grid-forming converters, especially the accurate definition and standardization of the harmonic suppression characteristics of the power grid, no consensus has been reached. Therefore, a set of standardized test methods are needed to define its control performance indicators, such as evaluating the dynamic stability of its harmonic suppression characteristics for the power grid. Summary of the Invention

[0003] The first object of the present invention is to overcome the deficiencies of the prior art and provide a harmonic suppression ability test system for grid-forming converters, which has a simple and reliable structure, is easy to operate, and can save a large amount of test time and cost.

[0004] The second object of the present invention is to provide a test method for a harmonic suppression ability test system of a grid-forming converter.

[0005] The third object of the present invention is to provide an evaluation method for a harmonic suppression ability test system of a grid-forming converter.

[0006] The first object of the present invention is achieved through the following technical solutions: A harmonic suppression ability test system for grid-forming converters includes:

[0007] Non-Linear Load and its control switch S load , the Non-Linear Load is used to provide harmonics with different frequency characteristics, and its control switch S load is used to control the grid connection or disconnection of the Non-Linear Load;

[0008] Simulated Grid and its control switch S Grid , the Simulated Grid is used to provide a stable voltage for the Non-Linear Load and simulate faults, and its control switch S Grid is used to control the state of the Simulated Grid;

[0009] A data acquisition device, which is used to collect the instantaneous values of the three-phase currents on the grid-forming converter side (i GFC,a 、iGFC,b , i GFC,c ), the instantaneous values of the three-phase voltages on the grid side (v Grid,a , v Grid,b , v Grid,c ) and the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ), and the instantaneous values of the three-phase currents on the non-linear load side (i Load,a , i Load,b , i Load,c );

[0010] Among them, the grid-forming converter GFC is configured with a grid-connected switch S for controlling its grid-connected / off-grid state GFC . The grid-forming converter GFC and the non-linear load Non-linear Load are connected in parallel through a cable and then connected in series with the simulated grid Grid through a cable. Due to its voltage source characteristics, the grid-forming converter GFC can provide voltage support for the non-linear load Non-Linear Load during a grid fault. The data acquisition device is connected to the signal acquisition points on the grid-forming converter GFC side, the simulated grid side, and the non-linear load side through signal lines.

[0011] Preferably, the non-linear load Non-linear Load is composed of 3 identical single-phase rectifiers. Each rectifier is composed of an H4 diode bridge, a resistor R on the DC side p and a capacitor C connected in parallel through a wire, and then connected in series with a resistor R on the AC side s through a wire.

[0012] Preferably, the sampling frequency of the data acquisition device is not less than 10 kHz.

[0013] The second object of the present invention is achieved by the following technical solution: A test method for a grid-forming converter harmonic suppression ability test system, including the following steps:

[0014] 1) S Grid is in the normally closed state, and S load is in the normally open state;

[0015] 2) Set the characteristics and capacity of the non-linear load Non-Linear Load;

[0016] 3) Close S GFC , connect the grid-forming converter GFC to the simulated grid Grid, and set the power setting value of the grid-forming converter GFC to 0 to ensure synchronization and stable operation with the simulated grid Grid;

[0017] 4) Close S load, connect the Non-Linear Load to the simulated Grid;

[0018] 5) Turn on the data acquisition device and start the waveform recording function;

[0019] 6) Gradually increase the power setpoint P of the Grid-Forming Converter (GFC) in specific steps GFC,set , and maintain each step for a preset time until the power setpoint P GFC,set reaches the rated power of the Grid-Forming Converter (GFC), and the Grid-Forming Converter (GFC) operates stably at the rated power setpoint P n for a preset time;

[0020] 7) Disconnect S GFC , and cut out the Grid-Forming Converter (GFC);

[0021] 8) Stop the data acquisition device, analyze the collected data, and complete the test.

[0022] Preferably, in step 6), the specific step is 25% S N , where S N is the rated apparent power of the Grid-Forming Converter (GFC).

[0023] The third object of the present invention is achieved by the following technical solution: an evaluation method for a Grid-Forming Converter harmonic suppression ability test system, which is based on the data collected by the data acquisition device: the instantaneous values of the three-phase currents on the Grid-Forming Converter side (i GFC,a , i GFC,b , i GFC,c ), the instantaneous values of the three-phase voltages on the grid side (v Grid,a , v Grid,b , v Grid,c ), and the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ) and the instantaneous values of the three-phase currents on the non-linear load side (i Load,a , i Load,b , i Load,c ). By performing a fast Fourier analysis on the collected data, the grid harmonic voltage amplitude of each order of each phase grid harmonic current amplitude converter grid side harmonic current amplitude non-linear load side harmonic current amplitude h represents the order number, h = 2, 3, 4,..., 50;

[0024] To determine the impact of the grid-forming converter (GFC) on grid harmonics and evaluate which harmonics the GFC can compensate and to what extent, it is necessary to comparatively analyze the total harmonic distortion (THD) of the grid voltage, including 2nd to 50th order harmonics, i.e., harmonic frequencies from 100 Hz to 2.5 kHz, when the GFC operates at different powers and when the GFC is disconnected v and the total harmonic distortion (THD) of the grid current i as well as the content of each order of grid voltage harmonics V’ Grid,h ;

[0025]

[0026]

[0027]

[0028] In the formula, represents the amplitude of the grid harmonic voltage of the first order, represents the amplitude of the grid harmonic current of the first order;

[0029] It is defined that if the GFC reduces the THD v and THD i of the grid, that is, the THD v and THD i of the grid when the GFC is disconnected minus the THD v and THD i of the grid when the GFC is operating, and the result is positive, then it has a positive response to the existing harmonics. In this case, once the GFC is connected to the grid, the harmonic content at the connection point will be reduced;

[0030] On the other hand, it is also necessary to evaluate the harmonic current provided by the GFC to the non-linear load and injected into the grid. It is necessary to comparatively analyze the contribution of harmonic current from the grid and the contribution of harmonic current from the GFC

[0031]

[0032]

[0033] The total contribution of harmonic current from the GFC and the grid for each order Greater than or equal to 1; when greater than 1, it indicates that there is power exchange between the grid-forming converter GFC and the power grid, which is an unfavorable behavior; when equal to 1, it indicates that the grid-forming converter GFC does not inject additional harmonic current into the power grid, which is a satisfactory result.

[0034] Preferably, the grid-forming converter GFC operates at different powers of 0, 25% S N , 50% S N , 75% S N and 100% S N , where S N is the rated apparent power of the grid-forming converter GFC.

[0035] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0036] 1. The present invention can effectively target the characteristics of the grid-forming converter, verify its control performance and the ability to ensure the stable operation of the system, and clarify the harmonic suppression performance index of the grid-forming converter for the power grid.

[0037] 2. The system structure is simple and reliable, easy to operate, and can save a large amount of test time and cost.

[0038] 3. The present invention provides a systematic idea for the test and evaluation of the grid-forming converter, which is helpful for the rapid development of the grid-forming converter and even the grid-forming new energy power generation technology. At the same time, it can enhance the grid friendliness of new energy power generation mainly based on power electronic devices and enhance the stability of the power grid system. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 is the schematic diagram of the system of the present invention.

[0040] Figure 2 is the circuit diagram of the single-phase rectifier. DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention will be further described in detail below with reference to the embodiments and the drawings, but the embodiments of the present invention are not limited thereto.

[0042] As Figure 1 shown, this embodiment provides a test system for the harmonic suppression ability of a grid-forming converter, including:

[0043] Non-linear load Non-Linear Load and its control switch S load , the non-linear load Non-Linear Load is used to provide harmonics with different frequency characteristics, and its control switch S load is used to control the grid connection or disconnection of the non-linear load Non-Linear Load;

[0044] Simulated power grid Grid and its control switch S Grid , the simulated power grid Grid is used to provide a stable voltage for the non-linear load Non-LinearLoad and simulate faults, and its control switch S Grid is used to control the state of the simulated power grid Grid;

[0045] A data acquisition device for collecting the instantaneous values of the three-phase currents (i GFC,a , i GFC,b , i GFC,c ) on the side of the grid-forming converter, the instantaneous values of the three-phase grid voltages (v Grid,a , v Grid,b , v Grid,c ) and the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ) and the instantaneous values of the three-phase currents on the non-linear load side (i Load,a , i Load,b , i Load,c );

[0046] Among them, the grid-forming converter GFC is configured with a grid-connected switch S for controlling its own grid-connected / off-grid state GFC . The grid-forming converter GFC and the non-linear load Non-linear Load are connected in parallel by a cable and then connected in series with the simulated power grid Grid by a cable. Due to its voltage source characteristics, the grid-forming converter GFC can provide voltage support for the non-linear load Non-Linear Load during grid faults. The data acquisition device is connected to the signal acquisition points on the side of the grid-forming converter GFC, the simulated power grid side, and the non-linear load side through signal lines.

[0047] Preferably, the non-linear load Non-linear Load is composed of 3 identical single-phase rectifiers, as Figure 2 shown. Each rectifier is composed of an H4 diode bridge, a resistor R p and a capacitor C connected in parallel by wires, and then connected in series with a resistor R s on the AC side by wires.

[0048] Preferably, the sampling frequency of the data acquisition device is not less than 10 kHz.

[0049] This embodiment provides a test method for the above grid-forming converter harmonic suppression ability test system, including the following steps:

[0050] 1) S Grid is in the normally closed state, and S load is in the normally open state;

[0051] 2) Set the characteristics and capacity of the Non-Linear Load;

[0052] 3) Close S GFC , connect the grid-forming converter GFC to the simulated grid Grid, set the power setpoint of the grid-forming converter GFC to 0, and ensure synchronous and stable operation with the simulated grid Grid;

[0053] 4) Close S load , connect the Non-Linear Load to the simulated grid Grid;

[0054] 5) Turn on the data acquisition device and start the oscillograph function;

[0055] 6) Gradually increase the power setpoint P N (S N is the rated apparent power of the grid-forming converter GFC) of the grid-forming converter GFC in steps of 25%, and maintain a preset time for each step until the power setpoint P GFC,set reaches the rated power of the grid-forming converter GFC, and the grid-forming converter GFC operates stably at the rated power setpoint P GFC,set and maintain the preset time; n

[0056] 7) Open S GFC , disconnect the grid-forming converter GFC;

[0057] 8) Stop the data acquisition device, analyze the collected data, and complete the test.

[0058] This embodiment also provides an evaluation method for the above-mentioned grid-forming converter harmonic suppression ability test system, which is characterized in that the method is based on the data collected by the data acquisition device: the instantaneous values of the three-phase currents on the grid-forming converter side (i GFC,a , i GFC,b , i GFC,c ), the instantaneous values of the three-phase voltages on the grid side (v Grid,a , v Grid,b , v Grid,c ), and the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ) and the instantaneous values of the three-phase currents on the non-linear load side (i Load,a , i Load,b , i Load,c ). By performing a fast Fourier analysis on the collected data, the amplitude of the grid harmonic voltage the amplitude of the grid harmonic current the amplitude of the harmonic current on the grid side of the converter the amplitude of the harmonic current on the non-linear load side h represents the order number, h = 2, 3, 4,..., 50;

[0059] To determine the impact of the grid-forming converter GFC on the grid harmonics and evaluate which harmonics the grid-forming converter GFC can compensate and to what extent, it is necessary to comparatively analyze the total harmonic distortion THD N of the grid voltage and the total harmonic distortion THD N of the grid current, as well as the content V’ N of each order of the grid voltage harmonics, when the grid-forming converter GFC operates at different powers (i.e., 0, 25% S N 、50% S v 、75% S i 、100% S Grid,h ), and when the grid-forming converter GFC is disconnected. The harmonics include the 2nd to 50th order harmonics, i.e., the harmonic frequencies are 100 Hz - 2.5 kHz;

[0060]

[0061]

[0062]

[0063] In the formula, represents the amplitude of the grid harmonic voltage of the first order, represents the amplitude of the grid harmonic current of the first order;

[0064] It is defined that if the grid-forming converter GFC reduces the THD v and THD i of the grid, that is, the THD v and THD i of the grid when the grid-forming converter GFC is disconnected minus the THD v and THD i of the grid when the grid-forming converter GFC is operating results in a positive value, then it makes a positive response to the existing harmonics. In this case, once the grid-forming converter GFC is connected to the grid, the harmonic content at the connection point will be reduced;

[0065] On the other hand, it is also necessary to evaluate the harmonic current provided by the grid-forming converter GFC to the non-linear load and injected into the grid. It is necessary to comparatively analyze the contribution of the harmonic current from the grid of each order when the grid-forming converter GFC operates at different powers (i.e., 0, 25% S N 、50% S N 、75% S N 、100% S N ); Harmonic current contributions from the grid-forming converter GFC

[0066]

[0067]

[0068] Total harmonic current contributions from the grid-forming converter GFC and the power grid at each order Greater than or equal to 1; when greater than 1, it indicates that there is power exchange between the grid-forming converter GFC and the power grid, which is an adverse behavior; when equal to 1, it indicates that the grid-forming converter GFC does not inject additional harmonic current into the power grid, which is a satisfactory result.

[0069] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A test system for the harmonic suppression ability of a network-forming converter, characterized in that, Including: Non-Linear Load and its control switch S load , the Non-Linear Load is used to provide harmonics with different frequency characteristics, and its control switch S load is used to control the grid connection or disconnection of the Non-Linear Load; Simulated power grid Grid and its control switch S Grid , the simulated power grid Grid is used to provide a stable voltage for the non-linear load Non-Linear Load and simulate faults, and its control switch S Grid is used to control the state of the simulated power grid Grid; A data acquisition device for collecting the instantaneous values of the three-phase currents (i GFC,a , i GFC,b , i GFC,c ) on the grid-forming converter side, the instantaneous values of the three-phase voltages (v Grid,a , v Grid,b , v Grid,c ) on the grid side, the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ), and the instantaneous values of the three-phase currents (i Load,a , i Load,b , i Load,c ) on the non-linear load side; Among them, the grid-forming converter GFC is configured with a grid connection switch S for controlling its own grid-connected / off-grid state GFC , the grid-forming converter GFC and the non-linear load Non-linear Load are connected in parallel through a cable and then connected in series with the simulated grid Grid through a cable. Due to its voltage source characteristics, the grid-forming converter GFC can provide voltage support for the non-linear load Non-Linear Load during grid faults. The data acquisition device is connected to the signal acquisition points on the side of the grid-forming converter GFC, the simulated grid side, and the non-linear load side through signal lines; The test method of the test system for harmonic suppression ability of the network-forming converter includes the following steps: 1) S Grid is in the normally closed state, and S load is in the normally open state; 2) Set the characteristics and capacity of the Non-Linear Load; 3) Close S GFC , connect the grid-forming converter GFC to the simulated grid Grid. The power setpoint of the grid-forming converter GFC is 0 to ensure synchronization and stable operation with the simulated grid Grid; 4) Close S load , connect the Non-Linear Load to the Grid; 5) Turn on the data acquisition device and start the waveform recording function; 6) Gradually increase the power setpoint P of the grid-forming converter GFC in specific steps GFC,set , and maintain each step for a preset time until the power setpoint P GFC,set reaches the rated power of the grid-forming converter GFC, and the grid-forming converter GFC operates stably at the rated power setpoint P n for a preset time; 7) Disconnect S GFC , and cut out the grid-forming converter GFC; 8) Stop the data acquisition device, analyze the collected data, and complete the test.

2. The harmonic suppression ability test system of a network-forming converter according to claim 1, characterized in that: The non-linear load is composed of three identical single-phase rectifiers, and each rectifier consists of an H4 diode bridge, a resistor R on the DC side p and a capacitor C which are connected in parallel by wires and then connected in series with a resistor R on the AC side s through wires.

3. The harmonic suppression ability test system of a network-forming converter according to claim 1, characterized in that: The sampling frequency of the data acquisition device is not less than 10 kHz.

4. A test system for the harmonic suppression ability of a network-forming converter according to claim 1, characterized in that, In step 6), the specific step size is 25%S N , where S N is the rated apparent power of the grid-forming converter GFC.

5. The evaluation method of the harmonic suppression ability test system of the network-forming converter according to any one of claims 1-4, characterized in that, This method is based on the data collected by the data acquisition device: the instantaneous values of the three-phase currents on the grid-forming converter side (i GFC,a , i GFC,b , i GFC,c ), the instantaneous values of the three-phase voltages on the grid side (v Grid,a , v Grid,b , v Grid,c ), and the instantaneous values of the three-phase currents (i Grid,a , i Grid,b , i Grid,c ) and the instantaneous values of the three-phase currents on the non-linear load side (i Load,a , i Load,b , i Load,c ). By performing a fast Fourier analysis on the collected data, the amplitude of the grid harmonic voltage the amplitude of the grid harmonic current the amplitude of the harmonic current on the converter grid side the amplitude of the harmonic current on the non-linear load side where h represents the order number, h = 2, 3, 4,..., 50; To determine the impact of the grid-forming converter (GFC) on grid harmonics, and to evaluate which harmonics the GFC can compensate and to what extent, it is necessary to comparatively analyze the total harmonic distortion (THD) of the grid voltage, including the 2nd to 50th order harmonics (i.e., harmonic frequencies from 100 Hz to 2.5 kHz), when the GFC operates at different powers and when the GFC is disconnected v and the THD of the grid current i as well as the content V' of each order of grid voltage harmonics Grid,h ; Wherein, represents the amplitude of the grid harmonic voltage of the first order, represents the amplitude of the grid harmonic current of the first order; Define that if the grid-forming converter GFC reduces the THD of the power grid v and THD i , that is, the THD of the power grid when the grid-forming converter GFC is disconnected v and THD i minus the THD of the power grid when the grid-forming converter GFC is operating v and THD i results in a positive value, then it makes a positive response to the existing harmonics. In this case, once the grid-forming converter GFC is connected to the power grid, the harmonic content at the connection will be reduced; On the other hand, it is also necessary to evaluate the harmonic current situation provided by the grid-forming converter GFC to the non-linear load and injected into the power grid, and it is necessary to compare and analyze the harmonic current contributions from the power grid at each order when the grid-forming converter GFC operates at different powers. and the harmonic current contribution from the grid-forming converter GFC The total harmonic current contribution from the grid-forming converter GFC and the power grid at each order is greater than or equal to 1; when it is greater than 1, it indicates that there is power exchange between the grid-forming converter GFC and the power grid, which is an adverse behavior; when it is equal to 1, it indicates that the grid-forming converter GFC does not inject additional harmonic current into the power grid, which is a satisfactory result.

6. The evaluation method of the harmonic suppression ability test system of the network-forming converter according to claim 5, characterized in that The grid-forming converter GFC operates at different powers of 0, 25% S N , 50% S N , 75% S N and 100% S N , where S N is the rated apparent power of the grid-forming converter GFC.

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