A control method and system for improving virtual inertia of grid-connected converter

Abstract

The present invention discloses a control method and system for improving the virtual inertia of a grid-type converter, and belongs to the field of control technology in renewable energy power generation. The frequency stability of the entire system is maintained from two aspects: slowing down the rate of change of the output frequency of the grid-type converter itself and slowing down the rate of change of the system frequency. By introducing a first-order inertia link and simulating the rotor motion equation of a synchronous generator, the change in the output frequency of the converter itself is linked to the deviation of the change in the input and output active power, so that the grid-type converter has an inertia similar to that of a synchronous generator. When the input and output active power are unbalanced, the rate of change of the output frequency of the grid-type converter itself is slowed down. By detecting the rate of change of the system frequency, the input active power of the grid-type converter is quickly adjusted to support the system power shortage / surplus, slow down the rate of change of the system frequency, and buy time for the primary frequency modulation of the system. The virtual inertia of the grid-type converter can be improved.

Description

Technical Field

[0001] The present invention belongs to the field of control technology in renewable energy power generation, and more specifically, relates to a control method and system for improving the virtual inertia of a grid-connected converter. Background Art

[0002] In order to cope with energy crisis and environmental pollution, countries around the world are vigorously developing new energy power generation. As the capacity of new energy such as wind power and photovoltaic power continues to increase, power electronic converters are gradually replacing synchronous generators in power systems. Since power electronic converters do not have mechanical equipment or rotating parts like synchronous generators, the inertia of the power system is reduced. When the power grid is disturbed, it will seriously affect the frequency stability.

[0003] According to the different synchronization methods with the power grid, new energy power electronic converters can be divided into two categories: one is the phase-locked synchronous converter represented by phase-locked loop synchronization; the other is the grid-type converter represented by active-frequency droop and virtual synchronous machine control. The grid-type converter can actively build voltage and frequency, provide voltage and frequency support for the system, and has the ability to operate with load without the need for an external power grid. In order to improve the inertia of the proportional electronic power system, it is necessary to start from the control method of the grid-type converter, so that the grid-type converter has an inertia similar to that of a synchronous generator, slow down the frequency change rate when a large power shortage occurs in the system, provide frequency support for the system, and improve the power system's ability to resist sudden load changes.

[0004] At present, domestic and foreign researchers have proposed many control methods to improve the virtual inertia of grid-type converters in order to improve the inertia of the system and reduce the rate of change of the frequency. (1) In order to improve the dynamic stability of the frequency, the inertia time constant T is adaptively changed according to the rate of change of the output frequency of the converter itself, that is, when the frequency change rate is small, the inertia time constant T takes a smaller value; when the frequency change rate is large, the inertia time constant T takes a larger value. This method improves the dynamic response of the output frequency of the grid-type converter itself and can effectively slow down the rate of change of the output frequency of the grid-type converter itself. However, the adaptive virtual inertia control strategy needs to detect the change of its own output frequency to change the inertia time constant, which is more complicated to apply, and cannot slow down the rate of change of the system frequency, and cannot provide inertia support power for the system. (2) Starting from slowing down the rate of change of the system frequency, a differential compensation link is introduced on the basis of the inherent frequency modulation strategy of the grid-type converter. By detecting the rate of change of the system frequency deviation, an auxiliary frequency modulation power related to the rate of change of the system frequency deviation is introduced to slow down the rate of change of the system frequency. However, this virtual inertia control strategy does not take into account the change in the converter's own output frequency, and cannot provide a good frequency support effect for the system when there is an active power shortage in the system. Summary of the invention

[0005] In view of the current research status of existing virtual inertia technology and actual engineering needs, the present invention provides a control method and system for improving the virtual inertia of a grid-type converter. The purpose is that when a large active power shortage suddenly occurs at a node in the system, the grid-type converter can provide frequency support for the system, and the virtual inertia of the system is enhanced by slowing down the change rate of the grid-type converter's own output frequency and slowing down the change rate of the system frequency.

[0006] To achieve the above object, according to one aspect of the present invention, a control method for improving the virtual inertia of a grid-connected converter is provided, comprising:

[0007] On the basis of active power-frequency droop control, a differential link between the given frequency and the system frequency deviation is introduced to obtain the input active power of the grid-connected converter.

[0008] The first-order inertia link is introduced to simulate the rotor motion equation of the synchronous generator, and the deviation of the input and output active power changes of the grid-forming converter is mapped to the change of the output frequency of the grid-forming converter itself.

[0009] Based on the change in the output frequency of the grid-type converter itself and the dq-axis reference value of the output voltage of the grid-type converter, a PWM control signal is generated through voltage and current dual-loop control, and the switch tube of the grid-type converter is controlled according to the PWM control signal.

[0010] Furthermore, the input active power expression of the grid-connected converter is:

[0011] P in =P ref +K fp (f ref -f 系统 )+Ms(f ref -f 系统 )

[0012] Among them, P ref is the reference value of the DC side input active power of the grid-connected converter, K fp is the frequency active power droop coefficient, f ref For a given frequency, f 系统 is the system frequency, and M is the differential coefficient.

[0013] Furthermore, the mapping relationship between the change in the output frequency of the grid-type converter itself Δf(s) and the deviation ΔP(s) of the change in the input and output active power of the grid-type converter is:

[0014]

[0015] Wherein, T is the inertia time constant of the simulated synchronous generator, and D is the damping coefficient of the simulated synchronous generator.

[0016] Furthermore, the PWM control signal is generated through voltage and current dual-loop control based on the change in the output frequency of the grid-type converter itself and the dq-axis reference value of the output voltage of the grid-type converter, including:

[0017] The change in the output frequency of the grid-type converter itself, Δf, plus the given frequency f ref The output frequency f of the grid-type converter itself is obtained, and then the reference angle θ is obtained through the integration link; according to the reference angle θ and the dq axis reference value of the grid-type converter output voltage, a PWM control signal is generated through voltage and current dual-loop control.

[0018] Furthermore, the dq-axis reference value of the grid-connected converter output voltage is determined by:

[0019] Calculate the reactive power output of the grid-type converter, and obtain the output voltage amplitude reference value U generated by the reactive power output of the grid-type converter through the reactive-voltage droop link cref , and U cref The virtual excitation voltage E generated by the virtual excitation link is deducted from the voltage drop on the virtual impedance to generate the dq-axis reference value of the output voltage.

[0020] Furthermore, a first-order inertia link with a time constant τ is added after the differential link to achieve differentiation of the system frequency.

[0021] In another aspect, the present invention further provides a control system for improving the virtual inertia of a grid-type converter, characterized in that it comprises: a computer-readable storage medium and a processor;

[0022] The computer-readable storage medium is used to store executable instructions;

[0023] The processor is used to read the executable instructions stored in the computer-readable storage medium to execute the above-mentioned control method for improving the virtual inertia of the grid-connected converter.

[0024] In general, compared with the existing technical solutions, the above technical solutions conceived by the present invention have the following advantages:

[0025] (1) The control method for improving the virtual inertia of the grid-type converter provided by the present invention provides system frequency support from two aspects: slowing down the rate of change of the output frequency of the grid-type converter itself and slowing down the rate of change of the system frequency. By introducing the first-order inertia link and simulating the rotor motion equation of the synchronous generator, the change of the output frequency of the converter itself is linked to the deviation of the input and output active power changes, so that the grid-type converter has an inertia similar to that of the synchronous generator. When the input and output active power are unbalanced, the rate of change of the output frequency of the grid-type converter itself is slowed down; by detecting the rate of change of the system frequency, the input active power of the grid-type converter is quickly adjusted to support the system power shortage / surplus, slow down the rate of change of the system frequency, and buy time for the primary frequency modulation of the system.

[0026] (2) Simulations have shown that compared with the traditional virtual inertia control method, the virtual inertia control method of the present invention has smaller change rates of the output frequency of the grid-type converter and the system frequency when the system load suddenly increases with the same power, which is consistent with the theoretical analysis results and meets the frequency inertia requirements of the grid-type converter. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow chart of a control method for improving the virtual inertia of a grid-type converter provided by the present invention;

[0028] Figure 2 It is a schematic diagram of the topological structure of an independent power supply system of two grid-type converters provided in a specific embodiment of the present invention;

[0029] Figure 3 It is a control block diagram of the grid-connected converter proposed in the present invention;

[0030] Figure 4 It is the load active power waveform of the entire system before and after the load of the independent power supply system of the grid-type converter increases suddenly;

[0031] Figure 5 It is a waveform diagram of the output frequency of the grid-type converter 1 itself before and after the load of the independent power supply system of the grid-type converter increases suddenly;

[0032] Figure 6 It is the system frequency waveform diagram before and after the load of the independent power supply system of the grid-type converter increases suddenly;

[0033] Figure 7 It is the input active power waveform of the grid-type converter 1 before and after the load of the independent power supply system of the grid-type converter increases suddenly. DETAILED DESCRIPTION

[0034] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, so that those skilled in the art can better understand the present invention and implement it. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to be limiting of the present invention. The technical features involved in each embodiment of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0035] The following is combined with Figure 1-7 The working principle and specific implementation methods of the present invention are described.

[0036] When a node in the system experiences a sudden increase or decrease in load, resulting in a shortage or surplus of active power in the system, the grid-connected converter needs to provide frequency support capability to maintain the frequency stability of the entire system. Improving the frequency support capability can be achieved by slowing down the rate of change of the grid-connected converter's own output frequency and slowing down the rate of change of the system frequency. Figure 1 , combined with Figure 2 and Figure 3 The present invention provides a control method for improving the virtual inertia of a grid-connected converter, comprising the following steps:

[0037] S1, on the basis of active power-frequency droop control, introduces the differential link of the given frequency and the system frequency deviation value to obtain the input active power of the grid-connected converter.

[0038] Specifically, when the node location of the sudden increase / decrease in load is far away from the grid-type converter, the grid-type converter shares a small load increment, and the active power shortage / surplus causes the system frequency to change faster. On the basis of active-frequency droop control, a differential link between the given frequency and the system frequency deviation value is introduced. By detecting the rate of change of the system frequency, the input active power of the grid-type converter is quickly adjusted to support the system power shortage / surplus, slow down the rate of change of the system frequency, and buy time for the primary frequency regulation of the system.

[0039] More specifically, on the basis of active power-frequency droop control (equivalent to the primary frequency modulation of the system), a differential link between the given frequency and the system frequency deviation is introduced. When the system frequency changes, the DC input side of the grid-type converter generates an inertial support power P proportional to the inverse of the system frequency change. 1 , whose expression is:

[0040] P 1 =Ms(f ref -f 系统 )

[0041] Among them, f ref For a given frequency, f 系统is the system frequency, and M is the differential coefficient. After introducing the differential link of the given frequency and the system frequency deviation value, the input active power expression of the grid-connected converter is:

[0042] P in =P ref +K fp (f ref -f 系统 )+Ms(f ref -f 系统 )

[0043] Where P ref is the reference value of the DC side input active power of the grid-connected converter, K fp It is the frequency active power droop coefficient. When the node location of sudden increase / decrease in load is far away from the grid-type converter, the grid-type converter shares a small load increment, and the active power shortage / surplus causes the system frequency to change faster. Since the primary frequency regulation has not yet taken effect, by detecting the change rate of the system frequency, the input active power of the grid-type converter is quickly adjusted to support the system power shortage / surplus, slow down the change rate of the system frequency, and buy time for the primary frequency regulation of the system.

[0044] S2 introduces a first-order inertia link to simulate the rotor motion equation of the synchronous generator, and maps the deviation of the input and output active power change of the grid-forming converter to the change of the output frequency of the grid-forming converter itself.

[0045] Specifically, when the node position of the sudden increase / decrease in load is close to the grid-type converter, the grid-type converter needs to share most of the load increment, which leads to a faster rate of change of the grid-type converter's own output frequency, while the rate of change of the system frequency is not large at this time. By introducing the first-order inertia link and simulating the rotor motion equation of the synchronous generator, the change of the grid-type converter's own output frequency is linked to the deviation of the input and output active power changes, so that the grid-type converter has an inertia similar to that of the synchronous generator, and when the input and output active power are unbalanced, the rate of change of the grid-type converter's own output frequency is slowed down.

[0046] More specifically, in the active frequency Pf control loop of the grid-connected converter, the following formula is used to simulate the rotor motion of the synchronous generator to realize the virtual inertia of the output frequency of the grid-connected converter itself:

[0047]

[0048] Where T is the inertia time constant of the simulated synchronous generator; P in P is the active power input to the DC side of the grid-connected converter; outis the output active power of the grid-type converter; D is the damping coefficient of the simulated synchronous generator; f is the output frequency of the grid-type converter itself; f ref is the reference frequency.

[0049] Linearize the above equation and we get:

[0050]

[0051] Let ΔP = ΔP in -ΔP out , ΔP is the difference between the input active power change and the output active power change of the grid-type converter. By performing Laplace transform on the above equation, the relationship between the change of the grid-type converter's own frequency and the change of the input and output active power can be obtained:

[0052]

[0053] As can be seen from the above formula, the active frequency Pf control of the grid-type converter introduces a first-order inertia link, simulates the rotor motion equation of the synchronous generator, and links the change in the converter's own output frequency with the deviation of the input and output active power, so that the grid-type converter has an inertia similar to that of a synchronous generator. When the input and output active power are unbalanced, the rate of change of the grid-type converter's own output frequency is slowed down. This allows the grid-type converter's own output frequency change to have a certain transition time, thereby suppressing the sudden change of its own output frequency, enhancing its own frequency's ability to resist load changes, and is conducive to improving the stability of the grid-type converter's own output frequency.

[0054] S3, based on the change in the output frequency of the grid-type converter and the dq-axis reference value of the output voltage of the grid-type converter, a PWM control signal is generated through voltage and current dual-loop control, and the switch tube of the grid-type converter is controlled according to the PWM control signal.

[0055] Specifically, 1. Detect the input active power of the grid-type converter and detect the output voltage of the grid-type converter (such as the attached Figure 2 U c1 ) and output current (such as Figure 2 I o1 ), calculate the output active power (such as the attached Figure 2 P out1 ), simulate the rotor motion equation of the synchronous generator, map the power difference ΔP to the difference Δf of the converter's own output frequency, and add the difference of its own output frequency to the given frequency f ref The output frequency f of the converter itself is obtained, and the reference angle θ is obtained through the integration link of the output frequency itself.

[0056] 2. Detect the output voltage of the grid-type converter (such as the attached Figure 2 U c1 ) and output current (such as Figure 2 I o1 ), calculate the output reactive power (such as the attached Figure 2 Q out1 ). The reactive power fed back by calculation is used to generate the output voltage amplitude reference value U through the reactive voltage droop link. cref , a virtual excitation voltage E is generated through a virtual excitation link. In order to achieve power decoupling, a virtual impedance link is added to the control, and the virtual excitation voltage minus the voltage drop on the virtual impedance generates the dq axis reference value of the output voltage.

[0057] 3. The reference angle θ and the dq axis reference value of the output voltage are converted into a PWM signal through a voltage and current double loop and rotation transformation. The PWM signal controls the output voltage and current of the grid-type converter through the drive device and reaches the AC bus through the AC network. The frequency measured at the AC bus point can be considered as the system frequency f 系统 According to the characteristics of system frequency and AC lines, when there is a sudden increase in load (power shortage) in the system, the system frequency will drop. In order to adjust the deviation and change rate of the system frequency, it is necessary to compensate for the system power shortage in time. Therefore, it is necessary to control the grid-connected converter and adjust its output voltage and current in time, that is, to adjust its output power.

[0058] A specific embodiment is to build a simulation model of an independent power supply system composed of two grid-type converters running in parallel and under load in Matlab / Simulink for simulation verification. The two grid-type converters have the same capacity and the same parameters. The parameters of the independent power supply system composed of the grid-type converters are shown in Table 1. In order to further intuitively demonstrate the superiority of the control method for improving the virtual inertia of the grid-type converter proposed in the present invention, the grid-type converter is compared with the virtual inertia control method of the present invention and the traditional virtual inertia control method.

[0059] Simulation conditions: At the initial moment, two grid-connected converters are connected to the common grid point U pcc When the load is connected in parallel, the load increases suddenly at node a at t=3s, and the load model adopts the constant impedance model. The simulation results are shown in Figure 4-Figure 7 shown.

[0060] Table 1 Simulation parameters of grid-connected converter independent power supply system

[0061]

[0062] The simulation results are as follows:

[0063] It should be noted that before t = 3s, the independent power supply system composed of grid-connected converters operated stably with load at the public grid connection point. When the system ran to t = 3s, the active load suddenly increased by 1.938MW at node a, causing the entire system to redistribute the load, and the output frequency of the grid-connected converter itself and the system frequency changed, deviating from the initial value. Figure 4 It is the load active power waveform of the whole system. The active load of node a suddenly increases by 1.938MW, which makes the total active power of the system increase from 8.153MW to 10.091MW. Figure 5 is the output frequency waveform of the grid-type converter 1 itself, Figure 5 It can be seen that the virtual inertia control method proposed in the present invention can effectively slow down the rate of change of the output frequency of the grid-type converter itself when the load suddenly increases. Figure 6 is the system frequency waveform, Figure 6 It can be seen that the virtual inertia control method proposed in the present invention can effectively slow down the rate of change of system frequency when the load suddenly increases. Figure 7 is the input active power waveform of the grid-connected converter 1, Figure 7 It can be seen that the virtual inertia control method proposed in the present invention can quickly adjust the input active power of the grid-connected converter when the load suddenly increases, compared with the traditional virtual inertia control method, to provide power support for the system.

[0064] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A control method for improving the virtual inertia of a grid-connected converter, characterized in that: include: On the basis of active power-frequency droop control, the differential link between the given frequency and the system frequency deviation is introduced to obtain the input active power of the grid-connected converter; the input active power expression of the grid-connected converter is: P in =P ref +K fp (f ref -f 系统 )+Ms(f ref -f 系统 ) Among them, P ref is the reference value of the DC side input active power of the grid-connected converter, K fp is the frequency active power droop coefficient, f ref For a given frequency, f 系统 is the system frequency, M is the differential coefficient; The first-order inertia link is introduced to simulate the rotor motion equation of the synchronous generator, and the deviation of the input and output active power changes of the grid-forming converter is mapped to the change of the output frequency of the grid-forming converter itself; the mapping relationship between the change of the output frequency of the grid-forming converter itself Δf(s) and the deviation of the input and output active power changes of the grid-forming converter ΔP(s) is: Where, T is the inertia time constant of the simulated synchronous generator, and D is the damping coefficient of the simulated synchronous generator; Based on the change in the output frequency of the grid-type converter itself and the dq axis reference value of the output voltage of the grid-type converter, a PWM control signal is generated through voltage and current dual loop control, including: The change in the output frequency of the grid-type converter itself, Δf, plus the given frequency f ref The output frequency f of the grid-type converter is obtained, and then the reference angle θ is obtained through the integration link; according to the reference angle θ and the dq axis reference value of the output voltage of the grid-type converter, a PWM control signal is generated through the voltage and current dual loop control; And the switch tube of the grid-connected converter is controlled according to the PWM control signal.

2. The control method for increasing the virtual inertia of a grid-connected converter according to claim 1, characterized in that: The dq axis reference value of the grid-connected converter output voltage is determined by: Calculate the reactive power output of the grid-type converter, and obtain the output voltage amplitude reference value U generated by the reactive power output of the grid-type converter through the reactive-voltage droop link cref , and U cref The virtual excitation voltage E generated by the virtual excitation link is deducted from the voltage drop on the virtual impedance to generate the dq-axis reference value of the output voltage.

3. The control method for increasing the virtual inertia of a grid-connected converter according to claim 1, characterized in that: A first-order inertia link with a time constant τ is added after the differential link to achieve differentiation of the system frequency.

4. A control system for improving the virtual inertia of a grid-connected converter, characterized in that: include: A computer readable storage medium and a processor; The computer-readable storage medium is used to store executable instructions; The processor is used to read the executable instructions stored in the computer-readable storage medium and execute the method according to any one of claims 1 to 3.

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