A virtual synchronous control method with current limiting function

Abstract

The present invention relates to a virtual synchronous control method with a current limiting function, comprising the following steps: (1) calculating a dq axis current reference value; (2) generating a voltage reference value in an αβ coordinate system; (3) generating a modulation voltage by a voltage inner loop, performing voltage inner loop calculation, and obtaining a modulation voltage v in the αβ coordinate system according to an inner loop output α_pwm and v β_pwm , where the voltage inner loop adopts quasi-proportional resonant control; the calculated v α_pwm and v β_pwm After the corresponding transformation, the modulation voltage value v in the abc coordinate system can be obtained. a_pwm 、v b_pwm 、v c_pwm Generate a modulation wave, control the IGBT in the converter, and obtain the output voltage.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a novel virtual synchronous control architecture with a current limiting function. Background Art

[0002] Microgrids are a key component of smart grids. They are clusters that integrate local loads and distributed power generation, such as wind turbines, solar photovoltaics and energy storage systems. With the continuous development of related technologies, microgrids and their technologies are receiving more and more attention from enterprises and scientific researchers.

[0003] At the same time, with the increasingly serious world energy crisis and the continuous development of my country's power grid, the power grid presents the characteristics of "high proportion of new energy" and "high proportion of power electronic equipment", that is, "double high". The power grid itself is undergoing a profound change, which makes many problems gradually prominent. For example, renewable energy itself has a large volatility, which leads to its unstable output power and large power fluctuations. At the same time, power electronic equipment itself has the characteristics of low damping and low inertia, which will have an adverse effect on the stability of its microgrid system. Therefore, in order to smooth out the fluctuation, when microgrids invest in renewable energy, they are generally equipped with a certain amount of energy storage systems. When the energy storage system is connected to the power grid, it needs to be connected to the corresponding converter. At the specific control level of the converter, virtual synchronous control technology is generally used to simulate the static power electronic device as a virtual synchronous generator (Virtual Synchronous Generator), and by simulating the primary frequency modulation of the synchronous generator, the inertia of the system is increased, thereby solving the problem of insufficient inertia in the system itself, and ultimately improving the stability of the AC microgrid.

[0004] The current basic virtual synchronization control architecture is as follows Figure 1 , that is, input active power reference and reactive power reference, and collect output port voltage and current to calculate real-time output active power and reactive power. The output phase angle is obtained by active power calculation, and the output voltage reference value is obtained by reactive power calculation.

[0005] However, the traditional virtual synchronous control architecture has the following problems: First, if the grid-side impedance of the system is too small, that is, when the grid is strong, the traditional virtual synchronous control will be difficult to integrate into the grid, and instability is very likely to occur; second, when the inner loop control in the control architecture does not include the current loop, but only includes the voltage loop, the traditional virtual synchronous control will not be able to limit the fault current when the system experiences a transient voltage drop, which will generate a large impact current. This may cause the system to become unstable and affect the transient stability of the system. Therefore, it is very important to propose a new control architecture that can suppress transient current when the system voltage drops.

[0006] In summary, in order to suppress transient current during voltage drop and improve transient stability of virtual synchronous control, it is of great significance and practical value to study a new virtual synchronous control architecture with current limiting function. Summary of the invention

[0007] In view of the above problems, the purpose of the present invention is to overcome the deficiency of the existing virtual synchronous control architecture with only a single voltage loop in the inner loop that cannot suppress the impact current when the grid voltage drops, and propose a new virtual synchronous control architecture, which comprehensively considers the addition of virtual reactance, etc., to achieve current limiting control of the output current when the grid voltage drops. The technical solution is as follows:

[0008] A virtual synchronous control method with a current limiting function comprises the following steps:

[0009] (1) Calculate the dq axis current reference value

[0010] The grid-side current is sampled and phase-locked to generate the grid-side phase angle θ PLL , and collect the AC side output current i of the converter abc , generate i through Clark transformation α_VSG and i β_VSG , and then the current i in the dq coordinate system is generated by Park transformation d_dulp and i q_dulp ; Then collect the output voltage v of the converter AC side abc , generate v through Clark transformation α_PLL and v β_PLL , and then the voltage v in the dq coordinate system is generated by Park transformation d_PLL and v q_PLL ;

[0011] Current reference value i in dq coordinate system dref and i qref It is expressed as the following formula:

[0012]

[0013]

[0014]

[0015] In the above formula, P set is the set active power reference value, v mag is the grid voltage amplitude, k p is the voltage amplitude ratio parameter, Q set is the set reactive power reference value;

[0016] (2) Generate voltage reference value in αβ coordinate system

[0017] Based on the current reference value i in the dq coordinate system generated in the previous step dref and i qref , calculate the control phase angle θ of virtual synchronous control vsg , as shown below:

[0018] i d_err =(i dref -i d_dulp ) (4)

[0019]

[0020] Among them, i d_err is the deviation between the d-axis reference current and the actual current, ω0 is the reference angular frequency, and the grid-side angular frequency is taken; H is the virtual moment of inertia parameter of virtual synchronous control, D is the damping coefficient of virtual synchronous control, and s is the Laplace operator;

[0021] Based on the i in the dq coordinate system generated in the previous step qref , calculate the output voltage amplitude reference value v ref :

[0022] i q_err =(i qref -i q_dulp ) (6)

[0023]

[0024] Among them, i q_err is the deviation between the q-axis reference current and the actual current, k p1 is the proportional control parameter, k i1 is the integral control parameter, v0 is the reference voltage amplitude;

[0025] The control phase angle θ of the virtual synchronous control calculated by formula (5) is vsg , and add a virtual resistor R v and virtual impedance L v , calculate the reference voltage v in the αβ coordinate system α_ref and v β_ref , as shown below:

[0026] v α_ref =v ref cos(θ vsg )+i α_vsg R v +i β_vsg ω0L v (8)

[0027] v β_ref =v ref sin(θ vsg )+iβ_vsg R v -i α_vsg ω0L v (9)

[0028] (3) The modulation voltage is generated by the voltage inner loop

[0029] According to equations (8) and (9), v is calculated α_ref and v β_ref , perform voltage inner loop calculation, and output out according to the inner loop α and out β With v α_ref and v β_ref Get the modulation voltage v in the αβ coordinate system α_pwm and v β_pwm , where the voltage inner loop adopts quasi-proportional resonant control, as shown in the following equation:

[0030]

[0031]

[0032] v d_pwm = ( out α +vα_ref)cos ( θ vsg) + ( out β +vβ_ref)sin ( θ vsg) (12)

[0033] v q_pwm = ( out α +vα_ref)sin ( θ vsg) -(out β +vβ_ref)cos ( θ vsg) (13)

[0034] Among them, k pr is the proportionality coefficient, k rr is the integral coefficient, ω c is the cutoff frequency of the low-pass filter, ω 0r is the resonant frequency;

[0035] The calculated v α_pwm and v β_pwm After the corresponding transformation, the modulation voltage value v in the abc coordinate system can be obtained. a_pwm 、v b_pwm 、v c_pwmThus, the input modulation is generated to generate a modulation wave, control the IGBT in the converter, and obtain the output voltage.

[0036] Compared with the prior art, the present invention has the following advantages:

[0037] (1) The proposed new virtual synchronous control architecture can suppress transient impact current through virtual impedance when the grid voltage drops, and can control the output current of the converter after the voltage drops. At the same time, compared with the traditional control architecture that uses dual-loop control of voltage and current loops in the inner loop and limits the current in the current loop, the adoption of this new virtual synchronous control architecture can avoid the saturation of the overall control architecture when the voltage drops, that is, it can avoid the saturation of the control architecture before the current loop after the current control loop is limited in the traditional control architecture, and the loss of control function.

[0038] (2) Compared with the traditional virtual synchronous control architecture whose inner loop adopts voltage and current loop, the new virtual synchronous control architecture proposed in the present invention only includes voltage loop, which can respond to grid changes more quickly. That is, compared with the traditional virtual synchronous control, the virtual synchronous generator using this new control framework can be connected to a stronger grid and has better adaptability to the grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is a traditional virtual synchronous control architecture.

[0040] Figure 2 It is an extension method of the coordination controller in the present invention.

[0041] Figure 3 It is the specific control architecture in the simulation example of the present invention.

[0042] Figure 4 This is the simulation result of Case 1 of the simulation example of the present invention.

[0043] Figure 5 This is the simulation result of Case 2 of the simulation example of the present invention. DETAILED DESCRIPTION

[0044] The virtual synchronization control architecture proposed by the present invention will be described in detail below with reference to the accompanying drawings and specific implementations.

[0045] (1) Calculate the dq axis current reference value

[0046] The specific control architecture of the present invention is as follows Figure 2 As shown. First, the grid-side current is sampled and phase-locked. Thus, the grid-side phase angle θ is generated. PLL , and collect the AC side output current i of the converter abc , generate i through Clark transformation α_VSG and iβ_VSG , and then the current i in the dq coordinate system is generated by Park transformation d_dulp and i q_dulp . Then collect the output voltage v of the converter AC side abc , generate v through Clark transformation α_PLL and v β_PLL , and then the voltage v in the dq coordinate system is generated by Park transformation d_PLL and v q_PLL .

[0047] The current reference value i in the dq coordinate system dref and i qref It can be expressed as the following formula:

[0048]

[0049]

[0050]

[0051] In the above formula, P set is the set active power reference value, v mag is the grid voltage amplitude, k p is the voltage amplitude ratio parameter, Q set It is the set reactive power reference value.

[0052] (2) Generate voltage reference value in αβ coordinate system

[0053] Based on the current reference value i in the dq coordinate system generated in the previous step dref and i qref , will be based on i dref Calculate the control phase angle θ of virtual synchronous control vsg , as shown below.

[0054] i d_err =(i dref -i d_dulp ) (4)

[0055]

[0056] Among them, i d_err is the deviation between the d-axis reference current and the actual current, ω0 is the reference angular frequency, which is generally the grid-side angular frequency, that is, 2π in radians. H is the virtual moment of inertia parameter of virtual synchronous control, D is the damping coefficient of virtual synchronous control, and s is the Laplace operator.

[0057] Based on the dq coordinate system generated in the previous step, i qref Calculate the output voltage amplitude reference value vref .

[0058] i q_err =(i qref -i q_dulp ) (6)

[0059]

[0060] Among them, i q_err is the deviation between the q-axis reference current and the actual current, k p1 is the proportional control parameter, k i1 is the integral control parameter.

[0061] Then the virtual synchronization phase angle θ can be calculated using formula (5): vsg , and add a virtual resistor R v and virtual impedance L v , calculate the reference voltage v in the αβ coordinate system α_ref and v β_ref , as shown below

[0062] v α_ref =v ref cos(θ vsg )+i α_vsg R v +i β_vsg ω0L v (8)

[0063] v β_ref =v ref sin(θ vsg )+i β_vsg R v -i α_vsg ω0L v (9)

[0064] (3) The modulation voltage is generated by the voltage inner loop

[0065] According to formula (8) and (9), v is calculated α_ref and v β_ref Then enter the voltage inner loop calculation and output the inner loop out α and out β With v α_ref and v β_ref Get the modulation voltage v in the αβ coordinate system α_pwm and v β_pwm Here, the voltage inner loop adopts quasi-proportional resonant control, as shown in the following formula:

[0066]

[0067]

[0068] v d_pwm = ( out α +vα_ref)cos ( θ vsg) + ( out β +vβ_ref)sin ( θ vsg) (12)

[0069] v q_pwm = ( out α +vα_ref)sin ( θ vsg) -(out β +vβ_ref)cos ( θ vsg) (13)

[0070] Among them, k pr is the proportionality coefficient, k rr is the integral coefficient, ω c is the cutoff frequency of the low-pass filter, ω 0r is the resonant frequency.

[0071] Calculate v α_pwm and v β_pwm After the corresponding transformation, the modulation voltage value v in the abc coordinate system can be obtained. a_pwm 、v b_pwm 、v c_pwm Thus, the input modulation is generated to generate a modulation wave, and the actual IGBT in the converter is controlled to obtain the output voltage.

[0072] 3. Simulation example

[0073] The system used in this simulation example is Figure 3 , which consists of a single converter connected to an infinite power grid. The converter is connected to energy storage on the DC side, and the new control architecture is used for control. Two sets of simulation cases are specifically designed to perform current limiting control under frequency fluctuation and voltage drop, respectively, to verify the effectiveness of the proposed control architecture. The simulation model of the master-slave control system in the present invention is built in Pscad software. The specific simulation system structure is as follows Figure 3 The system parameters are shown in Table 1. The system simulation step size is set to 2×10 -6 s, the switching frequency is 3.2kHz.

[0074] Table 1

[0075]

[0076] Case 1: Press Figure 2 The structure is set at the initial power P ref =2.5MW, the grid-side frequency is dropped from 50Hz to 49.5Hz and increased from 50Hz to 50.5Hz, respectively, to achieve the power support function of virtual synchronous control. Figure 4 (a)(b) and Figure 4 It can be seen from the results of (c) and (d) that the new virtual synchronization control framework proposed in the present invention can realize the frequency support function.

[0077] Case 2: Press Figure 2 The structure of the initial setting power P ref =2.5MW, the system voltage drops from 10kV to 2kV. When the voltage drops, the virtual impedance is adaptively transformed, and R v , L v Switch to R v1 , L v1 . Combined Figure 5 It can be seen from the results that the present invention can better limit the current when the system voltage drops, thereby achieving low voltage ride through.

[0078] 4. Conclusion

[0079] The case results show that:

[0080] 1) The novel virtual synchronous control architecture proposed in the present invention is the same as the traditional virtual synchronous control framework, which can realize the frequency support of the system, improve the system inertia, and increase the system stability.

[0081] 2) Using the novel virtual synchronous control framework proposed in the present invention, when the grid strength is very strong, the transient current can be limited when the grid voltage drops in the case of only a voltage loop, thereby achieving low voltage ride through. It is only necessary to adaptively transform the virtual impedance according to the voltage drop amplitude without switching the mode.

[0082] 3) The method proposed in the present invention can be used for low-inertia systems containing a large number of distributed power sources to increase the inertia of the system, such as large-scale wind farms, etc. It is particularly suitable for independent microgrids in remote areas with inconvenient transportation and equipped with a large number of distributed power sources.

Claims

1. A virtual synchronous control method with current limiting function, comprising the following steps: (1) Calculate the dq axis current reference value The grid-side current is sampled and phase-locked to generate the grid-side phase angle θ PLL , and collect the AC side output current i of the converter abc , generate i through Clark transformation α_VSG and i β_VSG , and then the current i in the dq coordinate system is generated by Park transformation d_dulp and i q_dulp ; Then collect the output voltage v of the converter AC side abc , generate v through Clark transformation α_PLL and v β_PLL , and then the voltage v in the dq coordinate system is generated by Park transformation d_PLL and v q_PLL ; Current reference value i in dq coordinate system dref and i qref It is expressed as the following formula: In the above formula, P set is the set active power reference value, v mag is the grid voltage amplitude, k p is the voltage amplitude ratio parameter, Q set is the set reactive power reference value; (2) Generate voltage reference value in αβ coordinate system Based on the current reference value i in the dq coordinate system generated in the previous step dref and i qref , calculate the control phase angle θ of virtual synchronous control vsg , as shown below: i d_err =(i dref -i d_dulp ) (4) in, i d_err is the deviation between the d-axis reference current and the actual current, ω0 is the reference angular frequency, and the grid side angular frequency is taken; H is the virtual moment of inertia parameter of virtual synchronous control, D is the damping coefficient of virtual synchronous control, and s is the Laplace operator; Based on the i in the dq coordinate system generated in the previous step qref , calculate the output voltage amplitude reference value v ref : i q_err =(i qref -i q_dulp ) (6) Among them, i q_err is the deviation between the q-axis reference current and the actual current, k p1 is the proportional control parameter, k i1 is the integral control parameter, v0 is the reference voltage amplitude; The control phase angle θ of the virtual synchronous control calculated by formula (5) is vsg , and add a virtual resistor R v and virtual impedance L v , calculate the reference voltage v in the αβ coordinate system α_ref and v β_ref , as shown below: v α_ref =v ref cos(θ vsg )+i α_vsg R v +i β_vsg ω0L v (8) v β_ref =v ref sin(θ vsg )+i β_vsg R v -i α_vsg ω0L v (9) (3) The modulation voltage is generated by the voltage inner loop According to equations (8) and (9), v is calculated α_ref and v β_ref , perform voltage inner loop calculation, and output out according to the inner loop α and out β With v α_ref and v β_ref Get the modulation voltage v in the αβ coordinate system α_pwm and v β_pwm , where the voltage inner loop adopts quasi-proportional resonant control, as shown in the following equation: v d_pwm = ( out α +vα_ref)cos ( i vsg) + ( out β +vβ_ref)sin ( i vsg) (12) v q_pwm = ( out α +vα_ref)sin ( i vsg) -(out) β +vβ_ref)cos ( i vsg) (13) Among them, k pr is the proportionality coefficient, k rr is the integral coefficient, ω c is the cutoff frequency of the low-pass filter, ω 0r is the resonant frequency; The calculated v α_pwm and v β_pwm After the corresponding transformation, the modulation voltage value v in the abc coordinate system can be obtained. a_pwm 、v b_pwm 、v c_pwm , generate a modulation wave, control the IGBT in the converter, and obtain the output voltage.

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