Inverter VSG control strategy introducing negative droop link and parameter design method thereof

Abstract

The invention provides an inverter VSG control strategy introducing a negative droop link and a parameter design method thereof, and belongs to the field of inverter control, and the method comprises the following steps: 1, constructing a VSG active control ring, introducing the negative droop link in the VSG active control ring, and changing a VSG active power reference value; 2, determining an inertia coefficient in a VSG active control ring according to the inertia supporting capability requirement; step 3, determining a damping coefficient in the VSG active control loop under the inertia coefficient in the step 2 according to the stable operation requirement of the inverter; step 4, determining an equivalent droop coefficient to be embodied by the inverter according to a primary frequency modulation requirement; step 5, configuring a gain coefficient of a negative droop link to enable an equivalent droop coefficient which should be reflected by the inverter to meet requirements; by introducing a negative droop link, parameter design difficulty caused by coupling of a damping characteristic and a droop characteristic in VSG control is overcome, and the droop coefficient of the inverter is within a reasonable range while the stability of the inverter under VSG control is ensured.

Description

Technical Field

[0001] The present invention relates to the technical field of inverter control, and in particular to an inverter VSG control strategy introducing a negative droop link and a parameter design method thereof. Background Art

[0002] The stable operation of traditional power systems depends largely on the support of synchronous machines. In recent years, with the continuous increase in the penetration rate of renewable energy power generation, the safe and stable operation of power systems has been challenged. Virtual synchronous generators (VSG) control has received widespread attention because it can make the inverter reflect the inertia support characteristics similar to synchronous machines.

[0003] Existing studies have shown that in a strong power grid environment, the inverter under VSG control may have a more serious instability problem. By increasing the damping coefficient D in the VSG control parameters, the stability of VSG control can be improved to a certain extent. The inertia support capacity of the inverter under VSG control is positively correlated with the inertia coefficient J. Generally speaking, the larger the inertia coefficient J is set, the larger the damping coefficient D needs to be set. That is, in order to achieve the required inertia support capacity while maintaining stability, there is a lower limit to the setting of the damping coefficient D.

[0004] On the other hand, the VSG control principle determines that its damping characteristics will be coupled with the droop characteristics. The increase in the damping coefficient D will naturally lead to an increase in the droop coefficient. The droop coefficient reflects the ability of the unit to provide power increment after the grid frequency disturbance. The larger the droop coefficient means the larger the power increment. In the current power system, the droop coefficient of the synchronous machine is usually set between 8 and 20. In order to coordinate with the synchronous machine, the droop coefficient of the grid-connected inverter should not be too large, which in turn sets an upper limit for the VSG damping coefficient D to a certain extent.

[0005] The upper and lower limits of the damping coefficient caused by the above reasons greatly reduce the flexibility of VSG control parameter design. The existing technology does not take into account the coupling between the droop coefficient and the damping coefficient when designing control parameters, and thus does not take into account the design constraints brought by this coupling. When the requirements are more stringent, the lower limit of the damping coefficient D may even be higher than the upper limit, making it impossible to complete the design. There is currently no effective solution to this problem. Summary of the invention

[0006] The technical problem to be solved by the present invention is how to ensure the stability of the inverter under VSG control while keeping its droop coefficient within a reasonable range when designing VSG control parameters.

[0007] The present invention solves the above technical problems through the following technical solutions: introducing a negative droop link inverter VSG control strategy and parameter design method thereof, comprising the following steps:

[0008] Step 1: construct a VSG active power control loop, introduce a negative droop link into the VSG active power control loop, and change the VSG active power reference value;

[0009] Step 2: Determine the inertia coefficient in the VSG active control loop according to the inertia support capacity requirements;

[0010] Step 3: According to the stable operation requirement of the inverter, determine the damping coefficient in the VSG active control loop under the inertia coefficient of step 2;

[0011] Step 4: Determine the equivalent droop coefficient that the inverter should reflect according to the primary frequency regulation requirements;

[0012] Step 5: Configure the negative droop link gain coefficient so that the equivalent droop coefficient of the inverter meets the requirements.

[0013] The present invention eliminates the coupling of the droop coefficient and the damping coefficient by introducing a negative droop link, thereby overcoming the parameter design difficulty caused by the coupling of the damping characteristic and the droop characteristic in VSG control. While ensuring the stability of the inverter under VSG control, the droop coefficient is kept within a reasonable range, thereby coordinating with the synchronous machine during grid-connected operation to avoid taking on excessive disturbance power.

[0014] Preferably, in the step 1, a VSG active control loop is constructed by simulating the synchronous machine rotor motion equation, and the active transfer function of the VSG active control loop under the small signal model is expressed as formula (1):

[0015]

[0016] In formula (1), is the per unit value of the grid-connected active power increment, s represents the differential operator, is the per unit value of the basic active power reference value increment, is the per-unit value of the grid angular frequency increment, J is the inertia coefficient in the VSG control loop, D is the damping coefficient in the VSG control loop, G(s) is the active response second-order system transfer function, ξ is the damping ratio of the active response second-order system transfer function, ω n is the natural response frequency of the active response second-order system transfer function, ω gN is the grid angular frequency rating, K p is the maximum transmission active power, are the per unit values ​​of the inverter AC side voltage and the grid voltage respectively, It is the per unit value of the reactance between the inverter output and the grid.

[0017] Preferably, in step 1, a negative droop link is introduced into the VSG active power control loop, and the per-unit value of the power reference value increment of the negative droop link is calculated using formula (2), which is added to the per-unit value of the basic active power reference value increment. superior:

[0018]

[0019] In formula (2), K is the per unit value of the increment of the reference value of the negative droop link active power, neg is the negative droop link gain coefficient, K neg ≤0.

[0020] Preferably, in step 1, a negative droop link is introduced into the VSG active control loop, and formula (3) is used to calculate the relationship between the per-unit value of the inverter grid-connected active power increment and the per-unit value of the grid angular frequency increment under the VSG control strategy with the negative droop link introduced, and then the theoretical equivalent droop coefficient K is calculated using formula (4): equ :

[0021]

[0022] In formula (3), is the per-unit value of the inverter grid-connected active power increment when only the frequency is disturbed;

[0023]

[0024] In formula (4), K equ is the equivalent droop coefficient.

[0025] Preferably, the process of determining the inertia coefficient in the VSG active control loop in step 2 includes:

[0026] Step 2.1: Refer to the grid connection standard to determine the equivalent inertia constant H equ ;

[0027] Step 2.2: Determine the value of the inertia coefficient J in the VSG control loop using formula (5):

[0028] J=2H equ (5)

[0029] Preferably, the process of determining the damping coefficient in the VSG active control loop under the inertia coefficient in step 2 in step 3 includes:

[0030] Step 3.1: Calculate the maximum transmission active power K according to the impedance of the system to which the inverter is connected p , calculate the grid angular frequency rating ω according to the rated frequency of the system to which the inverter is connectedgN , determine an initial value for the parameter design of the damping coefficient D under the current inertia coefficient J;

[0031] Step 3.2: Build a single inverter grid-connected simulation model according to the actual parameters of the system to which the inverter is connected. The inverter adopts VSG control and is set to the parameters initially determined in step 3.1. The negative droop link gain coefficient K neg Set to 0;

[0032] Step 3.3: Verify the stability of the inverter under the current parameter settings through simulation experiments. If it cannot run stably, increase the damping coefficient D.

[0033] Step 3.4: Appropriately reduce the impedance between the inverter output and the system point in the simulation model to verify the stability of the inverter after the grid strength increases. If it cannot operate stably, increase the damping coefficient D. Under the premise of ensuring the stability margin, determine the value of the damping coefficient D.

[0034] Preferably, in step 3.1, an initial value is determined for the parameter design of the damping coefficient D under the current inertia coefficient J by formula (6):

[0035]

[0036] Under this initial value, the damping ratio of the active response second-order system transfer function is ξ=0.4.

[0037] Preferably, in step 5, the negative droop link gain coefficient is configured using formula (7):

[0038] K neg =K equ -D (7)

[0039] In formula (7), K neg is the negative droop link gain coefficient, K neg ≤0, K equ is the equivalent droop coefficient, and D is the damping coefficient.

[0040] Preferably, the equivalent inertia constant H in the grid connection standard in step 2.1 equ 4s-6s.

[0041] Preferably, the equivalent droop coefficient K that the inverter should reflect in step 4 is equ Between 5-20.

[0042] The advantages provided by the present invention are:

[0043] (1) The present invention eliminates the coupling between the droop coefficient and the damping coefficient by introducing a negative droop link, thereby overcoming the parameter design difficulties caused by the coupling of the damping characteristic and the droop characteristic in VSG control. While ensuring the stability of the inverter under VSG control, the droop coefficient is kept within a reasonable range, thereby coordinating with the synchronous machine during grid-connected operation to avoid taking on excessive disturbance power.

[0044] (2) The present invention provides a new idea for the parameter design of the VSG control strategy. Under the conventional VSG control strategy, the damping characteristic is coupled with the droop characteristic, and the equivalent droop coefficient is determined by the damping coefficient. The design of the damping coefficient is limited by the droop characteristic requirements, and the flexibility is low. The introduction of the negative droop link in the present invention adds an extra degree of freedom to the equivalent droop coefficient of the inverter, so that the damping coefficient does not need to consider the constraints of the droop characteristic when designing, and can focus on pursuing good dynamic characteristics.

[0045] (3) The negative droop link introduced in the present invention has a clear principle and is easy to implement. Based on the existing VSG control strategy, it only needs to introduce a frequency measurement signal and simply modify the controller to complete it, which is convenient for promotion in practical applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A flow chart of a control strategy for an inverter VSG introducing a negative droop link and a parameter design method thereof provided by an embodiment of the present invention;

[0047] Figure 2 A schematic diagram of an inverter VSG control strategy and parameter design method for introducing a negative droop link provided in an embodiment of the present invention;

[0048] Figure 3 A single direct-drive wind turbine grid-connected system model built in simulation verification for the inverter VSG control strategy and parameter design method for introducing a negative droop link provided in an embodiment of the present invention;

[0049] Figure 4 The inverter VSG control strategy and parameter design method for introducing a negative droop link provided in an embodiment of the present invention are shown in the inverter active output waveform in the simulation verification. DETAILED DESCRIPTION

[0050] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the technical solution of the present invention is clearly and completely described below in combination with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0051] The present invention aims at the problem that the prior art fails to consider the coupling of the droop coefficient and the damping coefficient when designing control parameters, and thus fails to consider the design constraints brought by such coupling, which greatly reduces the flexibility of VSG control parameter design. By making improvements based on the current commonly used VSG control strategy and introducing a negative droop link, the coupling of the droop coefficient and the damping coefficient is eliminated, so that there is no need to consider the constraints of the coupling when designing control parameters, thereby improving the flexibility of VSG control parameter design. The relevant technical terms of the present invention are explained below:

[0052] Negative droop refers to a characteristic in which active power decreases as frequency decreases and increases as frequency increases, which is the opposite of the effect of the conventional droop control link in the power system.

[0053] The negative droop link refers to a control link that is introduced into the VSG active power control loop to make the inverter reflect the negative droop characteristics.

[0054] like Figure 1 As shown, this embodiment provides an inverter VSG control strategy and parameter design method introducing a negative droop link, including the following steps:

[0055] Step 1: Based on the basic principle of VSG control, an active control loop is constructed, and a negative droop link is introduced into the active control loop. The VSG active control reference value is changed to obtain the inverter VSG control strategy and its parameters with the negative droop link introduced. The schematic diagram is shown in Figure 2 As shown in the figure, is the per unit value of the grid angular frequency rating, ω gN is the grid angular frequency rating, is the per unit value of the grid angular frequency, K neg is the negative droop link gain coefficient, is the per unit value of the basic active power reference value, is the per-unit value of the active power reference value after the negative droop link is introduced, J is the inertia coefficient in the VSG control loop, D is the damping coefficient in the VSG control loop, s is the differential operator, is the per-unit value of the VSG output angular frequency increment, is the per-unit value of the VSG output angular frequency, θ vsg is the voltage phase angle at the VSG output.

[0056] Step 1.1, simulate the synchronous machine rotor motion equation and construct the VSG active control loop. The active transfer function under the small signal model is shown in formula (1):

[0057]

[0058] In formula (1), is the per unit value of the grid-connected active power increment, s represents the differential operator, is the per unit value of the basic active power reference value increment, is the per-unit value of the grid angular frequency increment, J is the inertia coefficient in the VSG control loop, D is the damping coefficient in the VSG control loop, G(s) is the active response second-order system transfer function, ξ is the damping ratio of the active response second-order system transfer function, ω n is the natural response frequency of the active response second-order system transfer function, ω gN is the grid angular frequency rating, K p is the maximum transmission active power, are the per unit values ​​of the inverter AC side voltage and the grid voltage respectively, It is the per unit value of the reactance between the inverter output and the grid.

[0059] Step 1.2: Introduce a negative droop link into the active power control loop, use formula (2) to calculate the per-unit value of the power reference value increment of the negative droop link, and add it to the per-unit value of the basic active power reference value increment. superior:

[0060]

[0061] In formula (2), K is the per unit value of the increment of the reference value of the negative droop link active power, neg is the negative droop link gain coefficient, K neg ≤0.

[0062] Step 1.3: Use formula (3) to calculate the per-unit value of the basic active power reference value increment under the VSG control strategy with the negative droop link introduced, when only the frequency disturbance is considered: When the inverter is 0, the relationship between the per-unit value of the grid-connected active power increment and the per-unit value of the grid angular frequency increment is calculated using formula (4) to obtain the theoretical equivalent droop coefficient K equ :

[0063]

[0064] In formula (3), is the per-unit value of the inverter grid-connected active power increment when only the frequency is disturbed;

[0065]

[0066] In formula (4), K equ is the equivalent droop coefficient.

[0067] Step 2: Determine the value of the inertia coefficient J in the VSG control loop according to the inertia support capacity requirements.

[0068] Step 2.1: Refer to the grid connection standard and determine a suitable equivalent inertia constant H equ .

[0069] Step 2.2: Determine the value of the inertia coefficient J in the VSG control loop using formula (5):

[0070] J=2H equ (5)

[0071] Step 3: Combine theoretical calculation and simulation experiments to determine the value of the damping coefficient D in the VSG control loop under the current inertia coefficient J to ensure that the inverter can operate stably under this parameter configuration.

[0072] Step 3.1: Calculate K based on the impedance of the system to which the inverter is connected p , calculate ω based on the rated frequency of the system to which the inverter is connected gN , use formula (6) to determine an initial value for the parameter design of the damping coefficient D under the current inertia coefficient J. Under this initial value, the damping ratio ξ of the active response second-order system transfer function is 0.4:

[0073]

[0074] Step 3.2: Build a single inverter grid-connected simulation model according to the actual parameters of the system to which the inverter is connected. The inverter adopts VSG control and is set to the parameters initially determined in step 3.1. The negative droop link gain coefficient K neg Temporarily set to 0.

[0075] Step 3.3: Verify the stability of the inverter under the current parameter settings through simulation experiments. If it cannot run stably, increase the damping coefficient D.

[0076] Step 3.4: Appropriately reduce the impedance between the inverter output terminal and the system point in the simulation model to verify the stability of the inverter after the grid strength increases. If it cannot operate stably, increase the damping coefficient D and determine the value of the damping coefficient D while ensuring a certain stability margin.

[0077] Step 4: According to the primary frequency regulation requirements, determine the equivalent droop coefficient K that the inverter should reflect equ .

[0078] Step 5: Use formula (7) to configure the negative droop link gain coefficient so that the equivalent droop coefficient of the inverter meets the requirements:

[0079] K neg =K equ -D (7)

[0080] The present invention overcomes the parameter design difficulties caused by the coupling of damping characteristics and droop characteristics in VSG control by introducing a negative droop link. While ensuring the stability of the inverter under VSG control, the droop coefficient is within a reasonable range, thereby coordinating with the synchronous machine during grid-connected operation to avoid taking on excessive disturbance power.

[0081] Simulation Verification

[0082] The inverter VSG control strategy and parameter design method of the present invention introducing a negative droop link are explained and verified in combination with a set of specific data:

[0083] In the PSCAD simulation software, build Figure 3 The single direct-drive wind turbine grid-connected system model shown in FIG. 1 has circuit parameters as shown in Table 1, wherein the inverter adopts the VSG control strategy of introducing a negative droop link proposed in the present invention.

[0084] Table 1 Circuit parameters of single direct-drive wind turbine grid-connected system model

[0085] parameter Numeric Direct drive fan rated capacity / MVA 5 Direct drive fan output voltage / kV 3.1 Back-to-back converter DC link capacitance / mF 5 Filter inductance / mH 5 Step-up transformer rated capacity / MVA 6 Step-up transformer ratio 3.1 / 35 Step-up transformer equivalent reactance / pu 0.0576 Grid rated voltage / kV 35 Rated grid frequency / Hz 50

[0086] Refer to the equivalent inertia constant H in the wind farm grid connection standard equ Generally, the requirement is set to 4s-6s, and the expected equivalent inertia constant H equ Set to 4s, and then J=2H equ The inertia coefficient J=8 is calculated.

[0087] K is calculated from the circuit parameters in Table 1 p =1.43,ω gN =314, and then by The calculated value is D=47.95. The VSG control parameters in the simulation model are set to J=8, D=47.95. After a simulation experiment, it is finally determined that the damping coefficient D=50 can ensure the stable operation of the inverter.

[0088] Since the synchronous machine droop coefficient is usually set between 5 and 20, the expected equivalent droop coefficient K equ Set it to 20 to prevent the inverter from bearing too large a power increment in a frequency modulation, which can be controlled by K neg =K equ -D calculates the negative droop link gain coefficient K neg =-30.

[0089] In the simulation experiment, the grid frequency is set to step down by 0.5Hz every 10s. The inverter active output waveform is as follows: Figure 4As shown, it shows that the inverter can operate stably; the average active output value of 9-10s and the average active output value of 15-16s are measured, and the actual value of the equivalent droop coefficient is calculated. The results are shown in Table 2. The equivalent inertia constant is consistent with the design expectation.

[0090] Table 2 Calculation results

[0091] Measurand Numeric Frequency before disturbance / pu 50 Frequency after disturbance / pu 49.5 9-10s average active output value / pu 0.500 15-16s average active output value / pu 0.700 Calculated value of equivalent droop coefficient 20.0

[0092] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that the technical solutions described in the aforementioned embodiments may still be modified, or some of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. The inverter VSG control strategy and parameter design method with negative droop link are introduced, which are characterized by: The following steps are involved: Step 1: construct a VSG active power control loop, introduce a negative droop link into the VSG active power control loop, and change the VSG active power reference value; Step 2: Determine the inertia coefficient in the VSG active control loop according to the inertia support capacity requirements; Step 3: According to the stable operation requirements of the inverter, determine the damping coefficient in the VSG active control loop under the inertia coefficient of step 2; Step 4: Determine the equivalent droop coefficient that the inverter should reflect according to the primary frequency regulation requirements; Step 5: Configure the negative droop link gain coefficient so that the equivalent droop coefficient of the inverter meets the requirements.

2. The inverter VSG control strategy and parameter design method for introducing a negative droop link according to claim 1, characterized in that: In the step 1, the VSG active control loop is constructed by simulating the synchronous machine rotor motion equation. The active transfer function of the VSG active control loop under the small signal model is expressed as formula (1): In formula (1), is the per unit value of the grid-connected active power increment, s represents the differential operator, is the per unit value of the basic active power reference value increment, is the per-unit value of the grid angular frequency increment, J is the inertia coefficient in the VSG control loop, D is the damping coefficient in the VSG control loop, G(s) is the active response second-order system transfer function, ξ is the damping ratio of the active response second-order system transfer function, ω n is the natural response frequency of the active response second-order system transfer function, ω gN is the grid angular frequency rating, K p is the maximum transmission active power, are the per unit values ​​of the inverter AC side voltage and the grid voltage respectively, It is the per unit value of the reactance between the inverter output and the grid.

3. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 1, characterized in that: In step 1, a negative droop link is introduced into the VSG active power control loop, and the per-unit value of the power reference value increment of the negative droop link is calculated using formula (2), which is added to the per-unit value of the basic active power reference value increment. superior: In formula (2), K is the per unit value of the increment of the reference value of the negative droop link active power, neg is the negative droop link gain coefficient, K neg ≤0.

4. The inverter VSG control strategy and parameter design method for introducing a negative droop link according to claim 3 is characterized in that: In step 1, a negative droop link is introduced into the VSG active control loop, and formula (3) is used to calculate the relationship between the per-unit value of the inverter grid-connected active power increment and the per-unit value of the grid angular frequency increment under the VSG control strategy with the negative droop link introduced, and then formula (4) is used to calculate the theoretical equivalent droop coefficient K equ : In formula (3), is the per-unit value of the inverter grid-connected active power increment when only the frequency is disturbed; In formula (4), K equ is the equivalent droop coefficient.

5. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 1, characterized in that: The process of determining the inertia coefficient in the VSG active control loop in step 2 includes: Step 2.1: Refer to the grid connection standard to determine the equivalent inertia constant H equ ; Step 2.2: Determine the value of the inertia coefficient J in the VSG control loop using formula (5): J=2H equ 。 (5)。 6. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 1, characterized in that: The process of determining the damping coefficient in the VSG active control loop under the inertia coefficient in step 2 in step 3 includes: Step 3.1: Calculate the maximum transmission active power K according to the impedance of the system to which the inverter is connected p , calculate the grid angular frequency rating ω according to the rated frequency of the system to which the inverter is connected gN , determine an initial value for the parameter design of the damping coefficient D under the current inertia coefficient J; Step 3.2: Build a single inverter grid-connected simulation model according to the actual parameters of the system to which the inverter is connected. The inverter adopts VSG control and is set to the parameters initially determined in step 3.

1. The negative droop link gain coefficient K neg Set to 0; Step 3.3: Verify the stability of the inverter under the current parameter settings through simulation experiments. If it cannot run stably, increase the damping coefficient D. Step 3.4: Appropriately reduce the impedance between the inverter output and the system point in the simulation model to verify the stability of the inverter after the grid strength increases. If it cannot operate stably, increase the damping coefficient D. Under the premise of ensuring the stability margin, determine the value of the damping coefficient D.

7. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 6, characterized in that: In step 3.1, an initial value is determined for the parameter design of the damping coefficient D under the current inertia coefficient J by formula (6): Under this initial value, the damping ratio of the active response second-order system transfer function is ξ=0.

4.

8. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 1, characterized in that: In step 5, the negative droop link gain coefficient is configured using formula (7): K neg =K equ -D (7) In formula (7), K neg is the negative droop link gain coefficient, K neg ≤0,K equ is the equivalent droop coefficient, and D is the damping coefficient.

9. The inverter VSG control strategy and parameter design method with negative droop link introduced in claim 5, characterized in that: The equivalent inertia constant H in the grid connection standard in step 2.1 equ 4s-6s.

10. The inverter VSG control strategy and parameter design method with negative droop link introduced according to claim 1, characterized in that: The equivalent droop coefficient K that the inverter should reflect in step 4 equ Between 5-20.

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