Grid Unbalance Control Method of Load Virtual Synchronous Machine Using Complex Power Definition

Abstract

The present invention discloses a method for controlling the grid imbalance of a load virtual synchronous machine using complex power definition. The steps include: Step 1, collect the measured values of the three-phase unbalanced grid voltage, perform a balancing process, and input the obtained balanced voltage vector to the LVSM module; Step 2, solve the DC component command value P ref 0 of the active power and the DC component command value Q ref 0 of the reactive power, and the ideal current command value i b* abc output by the LVSM module; Step 3, utilize the voltage complex vector u αβ in the two-phase stationary αβ coordinate system, the voltage delayed complex vector u' αβ obtained in Step 1, and P ref 0, Q ref 0 obtained in Step 2 to solve the actual value of the current inner loop command. The method of the present invention has a shorter actual execution time of the program and does not lose control performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronic conversion device control, and relates to a method for controlling the imbalance of a load virtual synchronous machine in a power grid using complex power definition. Background Art

[0002] With the rapid development of power semiconductor technology, a large number of power electronic converters (non-linear loads) are connected to the power grid, which will cause the power grid to lack inertia and damping and affect the safety and stability of the power grid. To solve this problem, some scholars have proposed to adopt the technology of load virtual synchronous machine (LVSM) in the distribution network to simulate the inertia and damping of synchronous motors, improve the external characteristics of power electronic converter loads, contribute to the active frequency modulation and reactive voltage regulation of the power grid, and play a role in stabilizing the power grid.

[0003] Traditional LVSM control methods assume that the three-phase grid voltage is in an ideal situation, that is, the three phases are completely balanced. However, in actual power grid voltages, there will always be a certain degree of imbalance, resulting in double-frequency fluctuations of active power and reactive power in three-phase voltage source rectifiers using traditional LVSM control methods. Later, some scholars proposed LVSM control methods under unbalanced grid voltage conditions. However, most of these methods are based on instantaneous power theory. To deal with the double-frequency fluctuation problem, it is necessary to separately decompose the three-phase grid voltage and three-phase incoming line currents into positive and negative sequences. Subsequently, subsequent control units such as coordinate transformation, LVSM, and current inner loop also require separate positive sequence channels and negative sequence channels, making the structures of these LVSM control schemes complex, with low working efficiency and high control costs, and it is difficult to be successfully popularized and applied. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for controlling the imbalance of a load virtual synchronous machine in a power grid. Because it uses complex power definition, the mathematical expression of power double-frequency fluctuation is more concise, thus solving the problems existing in the prior art that each control unit requires an independent positive sequence channel and an independent negative sequence channel, resulting in a complex control scheme structure, low working efficiency, and high operation cost.

[0005] The technical solution adopted by the present invention is a method for controlling the imbalance of a load virtual synchronous machine using complex power definition, which is implemented according to the following steps:

[0006] Step 1: Collect the measured values of three-phase unbalanced grid voltage, perform balancing processing, and input the obtained balanced voltage vector to the LVSM module;

[0007] Step 2: Solve the direct current component command value of active power P ref 0, and the direct current component command value of reactive powerQ ref 0, the ideal current command value output by the LVSM module i b* abc ;

[0008] Step 3: Use the voltage complex vector in the two-phase stationary αβ coordinate system u αβ and the voltage delay complex vector obtained in Step 1 u' αβ and the P ref 0, Q ref 0 obtained in Step 2 to solve the actual value of the current inner loop command.

[0009] The beneficial effect of the present invention is that compared with the LVSM control method that requires positive and negative sequence separation, since there is no need for separate positive and negative sequence control channels for voltage and current and the associated coordinate transformation link is omitted, the actual execution time of the program of the method of the present invention is shorter and the control performance is not lost. Description of the Drawings

[0010] Figure 1 is the system control block diagram that requires positive and negative sequence separation when the existing LVSM control method is applied to a three-phase voltage source rectifier under unbalanced grid voltage conditions;

[0011] Figure 2 is the control block diagram of the load virtual synchronous machine grid voltage unbalance control method defined by complex power adopted by the present invention;

[0012] Figure 3 is the control block diagram of the LVSM;

[0013] Figure 4 is the system control block diagram when the method of the present invention is applied to a three-phase voltage source rectifier;

[0014] Figure 5a is the simulation waveform of the system current when the system eliminates active power fluctuations in Embodiment 1; Figure 5b is the simulation waveform of the system power corresponding to Embodiment 1;

[0015] Figure 6a is the simulation waveform of the system current when the system eliminates reactive power fluctuations in Embodiment 1; Figure 6b is the simulation waveform of the system power corresponding to Embodiment 1;

[0016] Figure 7a is the simulation waveform of the system power when the three-phase voltage source rectifier adopting the method of the present invention participates in grid regulation under the condition of grid frequency fluctuation in Embodiment 2;Figure 7b is the simulation waveform of the DC-side voltage corresponding to Embodiment 2;

[0017] Figure 8a is the simulation waveform of the system power when the three-phase voltage source rectifier using the method of the present invention participates in grid regulation under the condition that the amplitude of the grid voltage fluctuates in Embodiment 3; Figure 8b is the simulation waveform of the DC-side voltage corresponding to Embodiment 3. Detailed implementation manners

[0018] Referring to Figure 1 , it is the system control block diagram of the three-phase voltage source rectifier that requires the LVSM control method for positive and negative sequence separation of voltage and current in the prior art. It can be seen that this control method requires separate positive and negative sequence decomposition of the three-phase grid voltage and the three-phase incoming line current. Subsequently, subsequent control units such as coordinate transformation, LVSM, and current inner loop also require separate positive sequence channels and negative sequence channels, making the structure of such LVSM control schemes complex.

[0019] Referring to Figure 2 , it is the grid unbalance control method of the load virtual synchronous machine using the complex power definition proposed by the present invention. Figure 2 The LVSM structure diagram used in the method of the present invention is as shown in Figure 3 shown.

[0020] Figure 3 The meanings of the variables recorded in J refer to the moment of inertia, with the unit of kg·m 2 ; T m refers to the virtual mechanical torque, with the unit of N·m; T e refers to the virtual electromagnetic torque, with the unit of N·m; P m refers to the mechanical power, with the unit of W; P e refers to the electromagnetic power, with the unit of W; ω refers to the angular velocity of the motor, with the unit of rad / s; ω n refers to the rated angular velocity of the motor, with the unit of rad / s; E refers to the amplitude of the LVSM induced electromotive force, with the unit of V; e a , e b , e c refer to the three-phase induced electromotive forces of the LVSM, all with the unit of V; i * a 、 i* b , i * c refer to the reference values of the three-phase incoming line currents, and the unit of all is A; θ is the phase angle of the LVSM induced electromotive force, and the unit is rad; U * dc refer to the reference value of the DC side voltage, and the unit is V; U dc refer to the DC side voltage, and the unit is V; Q * refer to the reference value of the reactive power, and the unit is Var; Q refer to the reactive power, and the unit is Var; D p refer to the damping coefficient, D q refer to the reactive power droop coefficient, K p refer to the active power droop coefficient; L v refer to the virtual inductor of the LVSM, and the unit is H; R v refer to the virtual resistance of the LVSM, and the unit is Ω.

[0021] The method of the present invention uses the complex power definition to express the grid voltage imbalance information, and the overall function expression is as follows in Equation (1):

[0022] (1)

[0023] To simplify the expression form, in Equation (1), the coefficients of the sine and cosine terms are respectively denoted as k 1, k 2, k 3, k 4, and the transformed expression is as follows in Equation (2):

[0024] (2)

[0025] In Equation (1) and Equation (2), u αβ , i αβ respectively represent the voltage and current complex vectors in the two-phase stationary αβ coordinate system, and the units are V and A respectively; u' αβ , i' αβ respectively represent the voltage and current 1 / 4 cycle delayed complex vectors in the two-phase stationary αβ coordinate system, and the units are V and A; P0 and Q 0 represent the DC components of the instantaneous active power and the instantaneous reactive power respectively, with the units of W and Var; P s2 and P c2 represent the sinusoidal double-frequency fluctuation component and the cosine double-frequency fluctuation component of the instantaneous active power respectively, with the unit of W; Q s2 and Q c2 represent the sinusoidal double-frequency fluctuation component and the cosine double-frequency fluctuation component of the instantaneous reactive power respectively, with the unit of Var.

[0026] The method of the present invention uses the above complex power definition to express the grid voltage imbalance information, and adopts the complex vectors u αβ and i αβ and their 1 / 4 cycle delayed complex vectors u' αβ and i' αβ , to replace the original positive and negative sequence components in the instantaneous power theory, and obtain a new instantaneous power expression, so that the LVSM control process of the grid voltage imbalance no longer needs to process the positive sequence component and the negative sequence component separately. After these improvements, compared with the Figure 1 shown in the existing LVSM control method applied to the three-phase voltage source rectifier system, the control process of the method of the present invention is more concise.

[0027] Referring to Figure 2 , Figure 3 , the method of the present invention is implemented according to the following steps:

[0028] Step 1: Collect the measured values of the three-phase unbalanced grid voltage, perform a balancing process, and input the obtained balanced voltage vector to the LVSM module,

[0029] The specific process is:

[0030] 1.1) Transform the three-phase grid voltage vector u abc from the three-phase stationary coordinate system to the two-phase stationary coordinate system, and transform it into the two-phase grid voltage vector u αβ , where u α is the u αβ axis component of α , u β is the u αβ axis component of β , and the units are all V;

[0031] 1.2) Use the Second-Order General Integrator (SOGI) to perform a 1 / 4 cycle delay processing on the two-phase grid voltage vector u αβ to obtain the delayed grid voltage vector u' αβ , where u' α is u' αβ 's α axis component, u' β is u' αβ 's β axis component, and the units are both V;

[0032] 1.3) Solve through the functional u b α = (1 / 2)∙( u α - u' β ) and u b β = (1 / 2)∙( u' α + u' β ) to obtain the voltage vector balance component u b αβ 's αβ axis component u b α and u b β , and the units are both V.

[0033] Step 2: Solve the DC component command value of the active power P ref 0, the DC component command value of the reactive power Q ref 0, the ideal current command value output by the LVSM module i b* abc ,

[0034] The specific process is:

[0035] 2.1) According to the voltage vector balance component u b αβand the ideal current command value output by the LVSM module i b* abc , calculate the DC component command value of the active power P ref 0, the DC component command value of the reactive power Q ref 0, and the functional formulas are respectively:

[0036] P ref 0 = u b a ∙ i b* a + u b b ∙ i b* b + u b c ∙ i b* c ,

[0037] Q ref 0 = (( u b b -u b c ) ∙ i b* a + ( u b c -u b a ) ∙ i b* b + ( u b a -u b b ) ∙ i b* c ) / ;

[0038] Among them, P ref 0 is the DC component command value of the active power, with the unit of W; Q ref0 is the DC component command value of reactive power, with the unit of Var; u b a and u b b and u b c are respectively a phase, b phase, c phase grid voltage balance components, with the unit of V; i b* a and i b* b and i b* c are respectively the a phase, b phase, c phase current command values output by LVSM, with the unit of A;

[0039] 2.2) On the premise of assuming that the current inner loop control effect is completely ideal, that is, the current output i b abc completely tracks the current command i b* abc , in order to save the calculation amount and not introduce interference, when calculating the LVSM reactive power feedback Q 0, the balanced current command i b* abc is used to replace the balanced component i b abc of the current feedback, and the functional formula is:

[0040] Q 0 = (( u b b -u b c ) • i b a + ) ( u b c -u b a ) • i b b + ) ( ub a -u b b )∙ i b c ) / ;

[0041] wherein, Q 0 is the LVSM reactive power feedback, with the unit of Var; i b a , i b b , i b c are respectively the balanced components of the three-phase current feedback, with the unit of A;

[0042] Calculated Q After 0 is input into the LVSM module, the ideal value of the current command i b* abc .

[0043] Step 3: Use the voltage complex vector αβ in the two-phase stationary u αβ coordinate system and the voltage delay complex vector u' αβ obtained in Step 1 and P ref 0, Q ref 0 obtained in Step 2 to solve the actual value of the current inner loop command,

[0044] The specific process is as follows:

[0045] Situation 1: When selecting to eliminate the active power fluctuation as the control target, it is necessary to suppress the double-frequency fluctuation of the active power, that is, the following formula (3) needs to be satisfied:

[0046] (3)

[0047] Solve formula (3) to obtain the actual value of the current inner loop command when eliminating the active power fluctuation, and the expression is as follows formula (4):

[0048] (4)

[0049] wherein, i α ref_P is the actual value of the α axis current command when eliminating the active power fluctuation, with the unit of A; iβ ref_P To eliminate the β actual value of the shaft current command during active power fluctuations, with the unit of A.

[0050] Situation 2: When selecting to eliminate reactive power fluctuations as the control target, the double-frequency fluctuation component of reactive power is zero, that is, the following formula (5) needs to be satisfied:

[0051] (5)

[0052] Solve formula (5) to obtain the actual value of the current inner-loop command when eliminating reactive power fluctuations, and the expression is as follows formula (6):

[0053] (6)

[0054] where i α ref_Q is the actual value of the shaft current command when eliminating reactive power fluctuations, with the unit of A; α is the actual value of the shaft current command when eliminating reactive power fluctuations, with the unit of A. i β ref_Q is the actual value of the shaft current command when eliminating reactive power fluctuations, with the unit of A. β is the actual value of the shaft current command when eliminating reactive power fluctuations, with the unit of A.

[0055] Situation 3: To meet more application scenarios and implement other compromise operation schemes, this step sets the weight coefficient λ , and by adjusting the weight coefficient λ the system can operate in different modes, as shown in the following formula (7):

[0056] (7)

[0057] where the value range of the weight coefficient λ is 0 to 1; λ = 1 means eliminating active power fluctuations, λ = 0 means eliminating reactive power fluctuations; when λ takes a value between 0 and 1, it means implementing a compromise operation, trading a certain degree of fluctuations in active and reactive power for the balance of three-phase currents, that is, successful.

[0058] To make the objectives, technical solutions, and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the present invention in conjunction with the embodiments and drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Usually, the components of the embodiments of the present invention described in the drawings here can be arranged and designed in various different configurations.

[0059] Embodiment 1

[0060] Controlled according to the steps of the method of the present invention as Figure 4 shown, applying the method of the present invention to a three-phase voltage source rectifier system, without participating in grid regulation, by adjusting the weight coefficient λ to make the system operate in different modes.

[0061] Build a simulation model in the MATLAB / Simulink environment to verify the method of the present invention through simulation. The simulation shows the power fluctuation conditions of the method of the present invention under the operating modes of eliminating active power fluctuation and eliminating reactive power fluctuation. All the simulation conditions under the grid voltage unbalance are set as a the phase voltage amplitude drops to 0.8 pu, b and c the amplitudes of two-phase voltages remain 1 pu.

[0062] Figure 5a And Figure 5b are respectively the simulation waveforms of current and power when the system eliminates active power fluctuation in Embodiment 1. Among them, Figure 5a is the three-phase incoming line current waveform, Figure 5b is the active and reactive power waveforms. It can be seen from the simulation waveforms that the method of the present invention can better achieve the elimination of active power fluctuation.

[0063] Figure 6a And Figure 6b are respectively the simulation waveforms of current and power when the system eliminates reactive power fluctuation in Embodiment 1. Among them, Figure 6a is the three-phase incoming line current waveform, Figure 6b is the active and reactive power waveforms. It can be seen from the simulation waveforms that the method of the present invention can better achieve the elimination of reactive power fluctuation.

[0064] Embodiment 2

[0065] The same as Embodiment 1 is that it is controlled according to the steps of the method of the present invention as Figure 4 shown, applying the method of the present invention to a three-phase voltage source rectifier system. The difference is that this Embodiment 2 shows the process of the system participating in grid frequency regulation.

[0066] Adopt simulation to simulate the process of the system participating in grid regulation when the grid has frequency fluctuations. The set grid voltage unbalance conditions are a the phase voltage amplitude drops to 0.8 pu, b and c the amplitudes of two-phase voltages remain 1 pu, the DC side voltage command is 650 V, the reactive power command is 0 Var, and the frequency fluctuation is ±0.2 Hz.

[0067] Figure 7a And Figure 7bThey are the simulation waveforms of the power and DC-side voltage of the three-phase voltage source rectifier adopting the method of the present invention when the grid frequency fluctuates in Embodiment 2. At this time, the weight coefficient λ is set to 1, that is, it operates in the mode of eliminating the active power fluctuation. It can be seen that when the grid frequency drops, the method of the present invention can automatically participate in the grid regulation, so that the system active power and output voltage decrease accordingly; when the grid frequency rises, the system active power and output voltage increase accordingly.

[0068] Embodiment 3

[0069] The same as Embodiment 1 is that it is controlled according to the above steps of the method of the present invention. As Figure 4 shown, the method of the present invention is applied to the three-phase voltage source rectifier system. The difference is that Embodiment 3 shows the process of the system participating in the grid voltage regulation.

[0070] The process of the system participating in the grid regulation when the grid voltage fluctuates is simulated. The set grid voltage unbalance condition is that a the phase voltage amplitude drops to 0.8 pu, b , c the amplitudes of two-phase voltages remain 1 pu, the DC-side voltage command is 650 V, the reactive power command is 0 Var, and the overall three-phase grid voltage changes by ±20%.

[0071] Figure 8a And Figure 8b They are the simulation waveforms of the power and DC-side voltage of the three-phase voltage source rectifier adopting the method of the present invention when the grid voltage amplitude fluctuates in Embodiment 3. When the grid voltage amplitude changes, the method of the present invention participates in the grid regulation by changing the consumed reactive power. The weight coefficient λ is set to 0, that is, it operates in the mode of eliminating the reactive power fluctuation. When the grid voltage amplitude drops, it indicates that the grid reactive power is insufficient. At this time, the system can emit reactive power to assist the grid to restore the voltage; when the grid voltage amplitude rises, it indicates that the grid reactive power is excessive. At this time, the system can act as a reactive power load to assist the grid to restore the voltage.

Claims

1. A method for controlling grid imbalance of a load virtual synchronous machine using complex power definition, characterized in that, Implement according to the following steps: Step 1: Collect the measured values of the three-phase unbalanced grid voltage, perform balancing processing, and input the obtained balanced voltage vector to the LVSM module. The specific process is as follows: 1.1) Transform the three-phase grid voltage vector u abc through the coordinate transformation from the three-phase stationary coordinate system to the two-phase stationary coordinate system into the two-phase grid voltage vector u αβ , where u α is the u αβ axis component of α , and u β is the u αβ axis component of β , and the units are both V; 1.2) Use a second-order generalized integrator to perform a 1 / 4 cycle delay process on the two-phase grid voltage vector u αβ to obtain a delayed grid voltage vector u' αβ , where u' α is the u' αβ axis component of α , and u' β is the u' αβ axis component of β , and the units are both V; 1.3) Through the functional u b α = (1 / 2)∙( u α - u' β ) and u b β = (1 / 2)∙( u' α + u' β ) to solve, and obtain the voltage vector balance components u b αβ of αβ axis component u b α and u b β , with the unit of V for both; Step 2: Solve the DC component command value of the active power P ref 0, the DC component command value of the reactive power Q ref 0, the ideal current command value output by the LVSM module i b* abc , and the specific process is as follows: 2.1) According to the voltage vector balance component u b αβ and the ideal current command value output by the LVSM module i b* abc , calculate the DC component command value of the active power P ref 0, the DC component command value of the reactive power Q ref 0, and the functional expressions are respectively: P ref 0 = u b a ∙ i b* a + u b b ∙ i b* b + u b c ∙ i b* c ， Q ref 0 = (( u b b -u b c )∙ i b* a + ( u b c -u b a )∙ i b* b + ( u b a -u b b )∙ i b* c ) / ； Wherein, P ref 0 is the DC component command value of the active power, with the unit of W; Q ref 0 is the DC component command value of the reactive power, with the unit of Var; u b a 、 u b b 、 u b c are respectively a phase, b phase, c phase grid voltage balance components, with the unit of V for all; i b* a 、 i b* b 、 i b* c are respectively the a phase, b phase, c phase current command values output by LVSM, with the unit of A for all; 2.2) On the premise that the control effect of the current inner loop is completely ideal, that is, the current output i b abc completely tracks the current command i b* abc , in order to save computational effort and not introduce interference, when calculating the LVSM reactive power feedback Q at 0, the balanced current command i b* abc is used to replace the balanced component of the current feedback i b abc , and the functional form is: Q 0 = (( u b b -u b c )∙ i b a + ( u b c -u b a )∙ i b b + ( u b a -u b b )∙ i b c ) / ； Among them, Q 0 is the LVSM reactive power feedback, with the unit of Var; i b a , i b b , i b c are respectively the balanced components of the three-phase current feedback, and the unit of all is A; Calculated Q After 0 is input into the LVSM module, the ideal current command value is obtained i b* abc ; Step 3: Use the voltage complex vector in the two-phase stationary αβ coordinate system u αβ and the voltage-delayed complex vector obtained in Step 1 u' αβ and the P ref 0, Q ref 0 obtained in Step 2 to solve the actual value of the current inner-loop command.

2. The method for controlling grid imbalance of a load virtual synchronous machine using complex power definition according to claim 1, characterized in that, In Step 3, the specific process is as follows: In Case 1, when selecting to eliminate the active power fluctuation as the control target, it is necessary to suppress the second-harmonic fluctuation of the active power, that is, the following formula (3) needs to be satisfied: （3） Among them, u αβ , i αβ respectively represent the voltage and current complex vectors in the two-phase stationary αβ coordinate system, and the units are V and A respectively; u ' αβ , i' αβ respectively represent the voltage and current 1 / 4 cycle delayed complex vectors in the two-phase stationary αβ coordinate system, and the units are V and A. Solve formula (3) to obtain the actual value of the current inner-loop command when eliminating the active power fluctuation. The expression is as follows formula (4): （4） Among them, i α ref_P to eliminate the α actual value of the shaft current command when eliminating active power fluctuations, with the unit of A; i β ref_P to eliminate the β actual value of the shaft current command when eliminating active power fluctuations, with the unit of A.

3. The method for controlling grid imbalance of a load virtual synchronous machine using complex power definition according to claim 1, characterized in that, In Step 3, the specific process is as follows: In Case 2, when selecting to eliminate the reactive power fluctuation as the control target, the second-harmonic fluctuation component of the reactive power is zero, that is, the following formula (5) needs to be satisfied: （5） Among them, u αβ and i αβ respectively represent the complex voltage and current vectors in the two-phase stationary αβ coordinate system, with the units of V and A respectively; u ' αβ and i' αβ respectively represent the 1 / 4-period delayed complex voltage and current vectors in the two-phase stationary αβ coordinate system, with the units of V and A; Solve formula (5) to obtain the actual value of the current inner-loop command when eliminating the reactive power fluctuation. The expression is as follows formula (6): （6） Among them, i α ref_Q To eliminate the α Actual value of the shaft current command when eliminating reactive power fluctuations, unit: A; i β ref_Q To eliminate the β Actual value of the shaft current command when eliminating reactive power fluctuations, unit: A.

4. The method for controlling grid imbalance of a load virtual synchronous machine using complex power definition according to claim 1, characterized in that, In Step 3, the specific process is as follows: Situation 3: To meet more application scenarios and implement other compromise operation schemes, a weight coefficient is set in this step λ , and by adjusting the weight coefficient λ , the system can be operated in different modes, as shown in the following formula (7): （7） Among them, i α ref_P To eliminate the α Actual value of the shaft current command when eliminating active power fluctuations, unit: A; i β ref_P To eliminate the β Actual value of the shaft current command when eliminating active power fluctuations, unit: A; i α ref_Q To eliminate the α Actual value of the shaft current command when eliminating reactive power fluctuations, unit: A; i β ref_Q To eliminate the β Actual value of the shaft current command when eliminating reactive power fluctuations, unit: A; Weight coefficient λ The value range is 0 to 1; λ When = 1, it means eliminating active power fluctuations, λ When = 0, it means eliminating reactive power fluctuations; When λ The value is between 0 and 1, it means achieving a compromise operation, exchanging a certain degree of fluctuations in active and reactive powers for the balance of three-phase currents.

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