A method of parallelizing a virtual synchronous machine and a droop-controlled converter-type voltage source

Abstract

The application discloses a kind of virtual synchronous machine and droop control converter type voltage source parallel operation method, comprising the following steps: step 1, the first converter type voltage source is started under no-load in droop control mode and runs;Step 2, according to parameter setting scheme, the droop coefficient kω of second converter, rotational inertia J, damping coefficient D are set;Step 3, the second converter type voltage source is started in virtual synchronous machine control mode, and a smaller active power instruction value than the first converter type voltage source is set to start running;Step 4, close parallel operation switch, while adjusting active power instruction value from set value in the same time interval ladder type increases, until reaching the rated power of second converter.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of power electronic device control, and particularly relates to a method for connecting a virtual synchronous machine and a droop control voltage source converter. BACKGROUND

[0002] As a terminal link for delivering electric energy, the micro-grid can be operated in grid-connected mode and self-sufficient mode, and has the ability to connect different types and different forms of power sources. Therefore, when the micro-grid operates in different mode conditions, whether it will affect the stable output of the distributed power source, it is necessary to consider not only the characteristics of the micro-grid itself, but also the differences between different types and different characteristics of power sources. In the micro-grid, there is no power source providing voltage and phase angle reference, and all power sources have consistent control mode, which is called peer-to-peer control of micro-grid system. In the peer-to-peer control system, each power source is equal and has no master-slave relationship. For each power source, there is no voltage and frequency reference, and the only available electrical quantity is the acquisition quantity at the port or grid-connected point. The following control is taken as an example. The droop control is the most representative control method in peer-to-peer control. Assuming that all power sources in the micro-grid use droop control, the output of each power source can be changed according to the different acquisition quantities due to the active-frequency and reactive-voltage droop characteristics. When the system operating state changes, the electrical quantities collected at the grid-connected point also change. The converter adjusts its output according to the reference value output by the droop control module. The whole process does not need dispatching signals or measurement values of other power sources, and realizes local control and independent autonomy.

[0003] In the peer-to-peer control micro-grid system, by changing the droop coefficient, the sensitivity of each power source to load changes can be different. When the load changes or the system operating mode changes, according to the specific conditions of each power source, such as considering distance, cost and other factors, the output of each power source is comprehensively matched to make the system operating mode more optimized.

[0004] The droop control method makes the inverter consistent with the traditional power generation equipment in steady-state response. The inverter static operating point is calculated by the droop equation, and the inverter output is controlled. However, the inverter with droop control has the possibility of subsynchronous oscillation when multiple inverters are connected in parallel, like traditional synchronous generators. In addition, the droop control only simulates the steady-state characteristics of the synchronous generator, and basically does not involve the simulation of the transient characteristics of the synchronous generator. Therefore, the transient response time constant of the inverter with droop control is small, and the inertia and damping are lower than those of the traditional synchronous generator. When a large number of inverters are connected to the grid, they will have a great impact on the inertia and stability of the overall grid.

[0005] The virtual synchronous machine control further simulates the external characteristic of the traditional synchronous generator on the basis of the droop control, considers the transient characteristic of the synchronous generator, and is closer to the demand of the smart grid on the new energy grid-connected power generation equipment "plug and play". In a stably operated micro-grid system, if a converter with other control modes is newly connected, there are some problems affecting the stable operation, and the expandability is poor. If a converter with the droop control is newly connected to the micro-grid system based on the virtual synchronous machine control, there are problems such as impact current.

[0006] Invention purposes

[0007] The purpose of the present application is to solve the problems in the prior art that when the virtual synchronous machine control converter and the droop control converter are connected in parallel, the impact current occurs after closing, and the impact current cannot be eliminated by PI regulation and parameter setting, and that during the current sharing process after parallel operation, unreasonable parameter configuration affects the current sharing effect. Invention contents

[0008] The present application provides a virtual synchronous machine and droop control converter type voltage source parallel operation method, the voltage source includes two converter type voltage sources, the method comprises the following steps:

[0009] Step 1, start the first converter type voltage source in no-load operation in the droop control mode; the droop control mode is to react to the operation state of the power grid through the droop relationship of active frequency and reactive voltage; the droop control has no inertia coefficient J and damping coefficient D of the virtual synchronous machine, and reacts more quickly to the change of the power grid state;

[0010] Step 2, set the droop coefficient k ω , the moment of inertia J and the damping coefficient D of the second converter according to the parameter setting scheme.

[0011] Step 3, start the second converter type voltage source in the virtual synchronous machine control mode, and set a smaller active power instruction value than the first converter type voltage source to start operation;

[0012] Step 4, close the parallel operation switch, and at the same time, adjust the active power instruction value to increase in steps from the set value in the same time interval until the rated power of the second converter is reached.

[0013] Preferably, the active frequency control method of the virtual synchronous machine comprises: subtracting the output port angular frequency of the converter from the rated angular frequency, multiplying the difference by the droop coefficient to obtain the corresponding active power part on the droop curve, and then adding the rated active power to obtain the virtual mechanical power P M , and then subtracting the electromagnetic power P eThe active power variation ΔP is obtained. Then, referring to the rotor motion equation, the virtual torque variation ΔT is obtained by dividing ΔP by the rated angular frequency, and then passing through the inertia and damping links, the angular frequency reference value ω of the converter output is obtained ref .

[0014] Preferably, the virtual synchronous machine reactive power voltage control method comprises: comparing the sample measurement of the reactive power with its rated value, taking the difference, and multiplying the difference by the corresponding droop coefficient to obtain the voltage difference corresponding to the reactive power variation, adding the voltage difference to the given value of the system voltage to obtain the voltage output by the converter, comparing the feedback voltage with the obtained reference value, taking the difference, and adjusting through the PI regulator to obtain the virtual field current.

[0015] Further preferably, the control process of the virtual synchronous machine comprises: obtaining the power P e , Q e output by the inverter bridge arm port by measuring the voltage and current values output by the inverter bridge arm port; obtaining the amplitude E ref and the frequency instruction value ω ref of the port voltage instruction value through the active frequency droop characteristic and the reactive voltage droop characteristic of the virtual synchronous generator; and obtaining the required modulation wave signal through the voltage-current double closed loop control, and generating the required PWM signal through PWM modulation.

[0016] Preferably, when the inertia coefficient is 0 and the damping coefficient is 1, the active power of the second-order virtual synchronous machine VSG control is converted to the first-order droop control. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 is a virtual synchronous machine active frequency control block diagram.

[0018] Figure 2 is a virtual synchronous machine reactive voltage control block diagram.

[0019] Figure 3 is a virtual synchronous machine control block diagram.

[0020] Figure 4 is a droop control block diagram.

[0021] Figure 5 is an active frequency droop characteristic curve.

[0022] Figure 6 is a reactive voltage droop characteristic curve.

[0023] Figure 7 is the equivalent conversion of virtual synchronous machine control and droop control.

[0024] Figure 8 is a converter-type voltage source parallel system schematic diagram of the two control modes.

[0025] Figure 9 is the active power instruction of the voltage source converter with virtual synchronous machine control over time.

[0026] Figure 10 is the flow chart of the voltage source converter with virtual synchronous machine and droop control. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, but not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0028] Those skilled in the art should understand that the step numbers used in the present application are only for the convenience of description, and are not limited to the execution sequence of the steps. The terms used in the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the present application specification and the appended claims, unless otherwise clearly indicated by the context, the singular forms "a", "an" and "the" are intended to include the plural forms. The terms "comprise" and "include" indicate the presence of described features, integers, steps, operations, elements, and / or components, but do not exclude one or more other features, integers, steps, operations, elements, components, and / or sets thereof. The term "and / or" refers to any combination of the associated listed items and all possible combinations, and includes these combinations.

[0029] Figure 1 is the active power frequency control block diagram of the virtual synchronous machine. In order to simulate the active power and frequency droop characteristics of the synchronous generator, the difference between the output port angular frequency of the converter and the rated angular frequency is multiplied by the droop coefficient to obtain the corresponding active power part on the droop curve, and then the rated active power is added to obtain the virtual mechanical power PM, and then the electromagnetic power P e of the converter output is subtracted to obtain the active power change ΔP. Then, referring to the rotor motion equation, ΔP is divided by the rated angular frequency to obtain the virtual torque change ΔT, and then passes through the inertia and damping link to obtain the angular frequency reference value ω ref .

[0030] Figure 2is the virtual synchronous machine reactive voltage control block diagram, the reactive power sampling measurement and its rated value comparison difference, and the difference value multiplied by the corresponding droop coefficient, the reactive power change corresponding to the voltage difference, the voltage difference value and the given value of the system voltage, the converter output voltage, the feedback voltage and the reference value comparison difference, through the PI regulator adjustment to get the virtual excitation current.

[0031] Figure 3 is the virtual synchronous machine control block diagram, we get the inverter output power P e , Q e by measuring the voltage and current values of the inverter bridge arm port output, through the active frequency droop characteristic and the reactive voltage droop characteristic of the virtual synchronous generator to get the amplitude E ref and frequency command value ω ref of the port voltage instruction value. And through the voltage current double closed loop control to get the required modulation wave signal, through the PWM modulation to generate the required PWM signal.

[0032] Figure 4 is the droop control block diagram, the overall idea of droop control is also through the droop relationship of active frequency and reactive voltage to respond to the operation state of the power grid. The difference is that the droop control has no inertia coefficient J and damping coefficient D of the virtual synchronous machine, and the response to the change of the power grid state is more rapid.

[0033] Figure 5 is the active frequency droop characteristic curve, the output active power and frequency of the synchronous generator in the primary frequency modulation process of the synchronous generator are embodied as droop characteristics.

[0034] Figure 6 is the reactive voltage droop characteristic curve, the reactive voltage droop characteristic is embodied on the external characteristic in the reactive voltage control process of the synchronous generator.

[0035] Figure 7 is the equivalent conversion of virtual synchronous machine control and droop control, when the inertia coefficient is 0 and the damping coefficient is 1, the second-order VSG control of active power is converted into first-order droop control.

[0036] Figure 8 is the schematic diagram of the converter type voltage source parallel system of the two control methods, in the figure, I1, I2 are the currents output by the two inverters respectively, U1, U2 are the voltages output by the two inverters respectively, are the phase angles of the voltages output by the two inverters relative to the common load point voltage, P1, P2, Q1, Q2 are the active power and reactive power output by the two inverters respectively. Z1, Z2 are the line impedances of the two inverters to the common load, and θ is the impedance angle of the line.

[0037] Figure 9 is the change relationship of active power instruction with time when the voltage source converter controlled by virtual synchronous machine is connected in parallel, the output power of the converter is increased step by step in the same time period, so that the transient process of parallel connection is smooth.

[0038] Figure 10 is the flow chart of the converter type voltage source connected in parallel and operated by droop control and virtual synchronous machine control.

[0039] Table 1 is the parameter setting scheme when the converter type voltage source controlled by virtual synchronous machine and droop control is connected in parallel:

[0040] Table 1 is the parameter setting scheme when the converter type voltage source controlled by virtual synchronous machine and droop control is connected in parallel

[0041]

[0042] As shown in Table 1, the setting is made according to the influence of the three parameters, and the setting results are as follows:

[0043] If the power output by the converter can be evenly divided in proportion to the rated capacity, it is necessary to ensure that the ratio of the droop coefficients is equal to the ratio of the rated capacities; in order to make the transition time Δt of each converter type voltage source equal, it is necessary to make the inertia constant J proportional to the rated capacity of the converter; the larger damping coefficient D can improve the transient performance of active power. The increase of D reduces the influence of the mismatch of inertia coefficient on the transient distribution of active power, and the system becomes non-overshoot, but the damping coefficient D cannot be too large, so as to avoid slow response speed caused by over-damping.

Claims

1. A method for parallel operation of a converter-type voltage source with a virtual synchronous machine and droop control, wherein the voltage source comprises two converter-type voltage sources, characterized in that, The method includes the following steps: Step 1: Start the first converter-type voltage source under no-load conditions using droop control mode; the droop control mode responds to the operating status of the power grid through the droop relationship between active frequency and reactive voltage; the droop control does not have the inertia coefficient J and damping coefficient D of a virtual synchronous machine, and responds more quickly to changes in the power grid status. Step 2: Adjust the droop coefficient k of the second converter according to the parameter tuning scheme. ω The moment of inertia J and damping coefficient D are set. Step 3: Start the second converter-type voltage source using virtual synchronous machine control mode, and set an active power command value smaller than that of the first converter-type voltage source to start operation; Step 4: Close the parallel operation switch and simultaneously adjust the active power command value to increase stepwise from the set value within the same time interval until the rated power of the second converter is reached.

2. The method for parallel operation of a virtual synchronous machine and a converter-type voltage source with droop control according to claim 1, characterized in that, The active frequency control method of the virtual synchronous machine includes: taking the difference between the angular frequency at the converter output port and the rated angular frequency, multiplying the difference by a droop coefficient to obtain the active power portion corresponding to the droop curve, and adding the rated active power to obtain the virtual mechanical power P. M Subtract the converter output electromagnetic power P e The change in active power ΔP is obtained. Then, referring to the rotor motion equation, ΔP is divided by the rated angular frequency to obtain the virtual torque change ΔT. After passing through inertia and damping elements, the reference value of the converter output angular frequency ω is obtained. ref .

3. The method for parallel operation of a virtual synchronous machine and a converter-type voltage source with droop control according to claim 1, characterized in that, The reactive voltage control method of the virtual synchronous machine includes: comparing the sampled measurement of reactive power with its rated value and taking the difference, multiplying the difference by the corresponding droop coefficient to obtain the voltage difference corresponding to the change in reactive power, adding the voltage difference to the given value of the system voltage to obtain the voltage output of the converter, comparing the feedback voltage with the obtained reference value and taking the difference, and obtaining the virtual excitation current through the adjustment of the PI regulator.

4. The method for parallel operation of a virtual synchronous machine and a converter-type voltage source with droop control according to claim 3, characterized in that, The control process of the virtual synchronous machine includes: obtaining the inverter output power P by measuring the voltage and current values ​​output from the inverter bridge arm ports. e Q e The amplitude E of the port voltage command value is obtained by using the active frequency droop characteristic and reactive voltage droop characteristic of the virtual synchronous generator. ref and frequency command value ω ref The required modulation wave signal is obtained through voltage and current dual closed-loop control, and the required PWM signal is generated through PWM modulation.

5. The method for parallel operation of a virtual synchronous machine and a converter-type voltage source with droop control according to claim 1, characterized in that, When the inertia coefficient is 0 and the damping coefficient is 1, the second-order virtual synchronous machine VSG control with active power is converted into a first-order droop control.

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