Mesh-forming converter and overload current amplitude limiting method thereof

Abstract

The invention discloses a network construction type converter and an overload current amplitude limiting method thereof, and the overload current amplitude limiting method comprises the steps: limiting the internal potential reference voltage under the premise of not changing the virtual impedance when the amplitude limiting is carried out on the output current of the network construction type converter. According to the invention, on the premise of not changing the virtual impedance, the current is limited by limiting the reference voltage of the internal potential, the adverse effect on the operation stability of the converter is avoided, the condition that the port voltage difference is under the triple overload condition can be ensured in real time, and the amplitude limiting strategy is simple, convenient, rapid and accurate.

Description

Technical Field

[0001] The present application relates to the technical field of grid-type converters, and in particular to a grid-type converter and an overload current limiting method thereof. Background Art

[0002] With the widespread application of grid-connected converters, the safety and stability of grid-connected converters are also issues of great concern.

[0003] As a controlled voltage source, the grid-type converter controls a constant voltage, and the demand for current may be extremely high. For example, in the latest grid-type test, the China Electric Power Research Institute simulated a short-circuit fault in the system to test the grid support capability and fault protection function of the grid-type converter. When a short-circuit fault occurs in the system, the grid-type converter instantly outputs a short-circuit current to support the grid. The current may exceed the allowable threshold of the device and even burn the device, so current limitation is a prerequisite for the safe and stable operation of the converter.

[0004] The current limiting strategy of the grid-type converter is usually achieved by dynamically adjusting the virtual impedance. However, the purpose of grid control is to maintain the port voltage stability. The increase in impedance will lead to an increase in the internal potential of the virtual synchronous machine, which may eventually lead to a limiting failure. Summary of the invention

[0005] In view of this, the purpose of the present application is to provide a grid-type converter and an overload current limiting method thereof, so as to solve the problem that the existing current limiting method by adjusting virtual impedance leads to unstable operation of the converter.

[0006] The technical solutions adopted by this application to solve the above technical problems are as follows:

[0007] On one hand, the present application provides an overload current limiting method for a grid-type converter, the overload current limiting method comprising:

[0008] When limiting the output current of the grid-type converter, the internal potential reference voltage is limited without changing the virtual impedance.

[0009] In one example, the limited internal potential reference voltage includes:

[0010] The internal potential limit is calculated based on the grid voltage, the maximum current limit value, the line impedance and the virtual impedance;

[0011] According to the internal potential limiting, the internal potential reference voltage is limited.

[0012] In one example, the calculating the internal potential amplitude limit according to the grid voltage, the maximum current amplitude limit value, the line impedance and the virtual impedance includes:

[0013] Calculating a maximum limiting voltage of the system impedance according to the maximum current limiting value, the line impedance and the virtual impedance;

[0014] The internal potential amplitude limit is calculated according to the grid voltage and the maximum limiting voltage of the system impedance.

[0015] In one example, calculating the maximum limiting voltage of the system impedance according to the grid voltage and the maximum limiting voltage of the system impedance includes:

[0016] Calculating a modulus value of system impedance according to the line impedance and the virtual impedance;

[0017] Based on the module value of the system impedance and the maximum current limit value, the maximum limit voltage of the system impedance is calculated.

[0018] In one example, limiting the internal potential reference voltage according to the internal potential amplitude limiting includes:

[0019] In a dq-axis coordinate system, a first internal potential amplitude limit and a second internal potential amplitude limit are calculated based on the dq-axis components of the grid voltage and the maximum limiting voltage of the system impedance;

[0020] Based on the first internal potential clipping and the second internal potential clipping, an internal potential reference voltage is limited.

[0021] In one example, the overload current limiting method further includes:

[0022] When the internal potential that meets the maximum current limit value condition cannot be constructed, the internal potential limit that meets the overload current limit requirement is constructed by adjusting the virtual impedance amplitude.

[0023] In one example, the method of adjusting the virtual impedance amplitude to construct an internal potential amplitude limiter that meets the overload current amplitude limiter requirement includes:

[0024] Determine the voltage drop that needs to be compensated and calculate the corresponding internal potential limit;

[0025] Based on the calculated internal potential limit, the impedance to be compensated is calculated and the proportionality coefficient of the impedance compensation is determined;

[0026] Based on the proportionality factor, a virtual impedance to be compensated is determined.

[0027] In one example, determining the voltage drop that needs to be compensated includes:

[0028] In the dq axis coordinate system, the voltage drop that needs to be compensated is determined based on the q axis component of the grid voltage and the maximum limiting voltage of the system impedance.

[0029] In one example, the overload current limiting method further includes:

[0030] When the grid-type converter outputs power, the compensated virtual impedance is attenuated.

[0031] On the other hand, the present application provides a grid-type converter, which is connected to a power grid. The grid-type converter also includes a control unit, which is configured to execute the overload current limiting method of the grid-type converter.

[0032] The grid-type converter and overload current limiting method thereof provided in the embodiment of the present application limit the current by limiting the internal potential reference voltage without changing the virtual impedance, which will not have an adverse effect on the stability of the converter operation, and can ensure in real time that the port voltage difference is under a three-fold overload condition, and the limiting strategy is simple, convenient, fast and accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 A schematic diagram of a grid-type converter provided in an embodiment of the present application;

[0034] Figure 2 A schematic diagram of an overload current limiting method for a grid-type converter provided in an embodiment of the present application;

[0035] Figure 3 A schematic diagram of the test results provided in the embodiment of the present application.

[0036] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with embodiments and with reference to the accompanying drawings. DETAILED DESCRIPTION

[0037] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application clearer and more understandable, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0038] Figure 1 A schematic diagram of a grid-connected converter provided in an embodiment of the present application.

[0039] like Figure 1 As shown, the DC side of the grid-type converter Q1 is connected to a DC source, and the AC side of the grid-type converter Q1 is connected to a grid, for example, through a filter inductor L pfc , transformers, etc. connected to the power grid.

[0040] Furthermore, the grid-type converter Q1 also includes a control unit, which is configured to execute an overload current limiting method for the grid-type converter, including voltage limiting, voltage control, current control and PWM wave generation links.

[0041] An embodiment of the present application provides an overload current limiting method for a grid-type converter. When limiting the output current of the grid-type converter, the internal potential limiting that meets the current limiting requirement is solved in the voltage limiting link, and the internal potential limiting is used to limit the internal potential reference voltage, thereby limiting the voltage difference between the internal potential reference voltage and the grid voltage. After the voltage control link, the current control link, and the PWM wave generation link, a drive signal for controlling Q1 is generated to achieve output current limiting.

[0042] like Figure 2 As shown, the overload current limiting method of the grid-type converter provided in the embodiment of the present application, the overload current limiting method S100 includes:

[0043] Step S101: When limiting the output current of the grid-type converter, the internal potential reference voltage is limited without changing the virtual impedance.

[0044] The existing technology generally limits the current by adjusting the virtual impedance, and adjusting the virtual impedance will cause the converter to run unstably. Since the internal potential can be regarded as the vector of another voltage source, the vector of the impedance (including the virtual impedance) and the grid voltage must satisfy the vector closure. By limiting the vector of the impedance and the internal potential voltage, the output current can be limited. When the angle between the vector of the system impedance and the vector of the grid voltage is less than 90°, within the range of the maximum current limit closed vector, a maximum current limit value condition can be constructed. Therefore, at this time, the output current can be limited by limiting the internal potential reference voltage without changing the virtual impedance, which will not have an adverse effect on the stability of the converter operation. The port voltage difference can be guaranteed to be under the condition of three times overload in real time, and the limiting strategy is simple, convenient, fast and accurate.

[0045] In one example, limiting the internal potential reference voltage includes:

[0046] The internal potential limit is calculated based on the grid voltage, the maximum current limit value, the line impedance and the virtual impedance;

[0047] According to the internal potential limit, the internal potential reference voltage is limited.

[0048] Among them, according to the grid voltage, the maximum current limit value, the line impedance and the virtual impedance, the internal potential limit is calculated to include:

[0049] According to the maximum current limit value, line impedance and virtual impedance, the maximum limit voltage of the system impedance is calculated;

[0050] The internal potential amplitude limit is calculated based on the grid voltage and the maximum limiting voltage of the system impedance.

[0051] In a specific implementation, the maximum limiting voltage of the system impedance is calculated according to the grid voltage and the maximum limiting voltage of the system impedance, and includes:

[0052] According to the line impedance and the virtual impedance, the module value of the system impedance is calculated;

[0053] Based on the modulus of the system impedance and the maximum current limit value, the maximum limiting voltage of the system impedance is calculated.

[0054] For example, system resistance R = R load +R vir , R load is the line resistance, R vir is a virtual resistor; system reactance X = X load +X vir , X load is the line reactance, X vir is the virtual reactance. Based on the system resistance R and the system reactance X, the system impedance modulus is calculated Assume that the maximum current limit value is I m , then the maximum limiting voltage E of the calculated system impedance sys For E sys =ZI m .

[0055] In a specific implementation, limiting the internal potential reference voltage according to the internal potential limiting includes:

[0056] In the dq axis coordinate system, based on the dq axis components of the grid voltage and the maximum limiting voltage of the system impedance, a first internal potential amplitude limit and a second internal potential amplitude limit are calculated;

[0057] Based on the first internal potential clipping and the second internal potential clipping, the internal potential reference voltage is limited.

[0058] For example, in the dq axis coordinate system, the dq axis components of the grid voltage are u d 、u q , the maximum limiting voltage of the system impedance is E sys , the calculated first internal potential limit The calculated second internal potential limit When the angle between the vector of the system impedance and the vector of the grid voltage is less than 90°, there is no need to change the virtual impedance. ref Limit to And 0≤E ref .

[0059] Specifically, when When E ref =E ref ; when E ref Less than When When E ref Greater than When

[0060] It should be noted that in other examples, it is also feasible to replace the grid voltage in the above examples with the grid connection point voltage.

[0061] In one example, the overload current limiting method further includes:

[0062] When the internal potential that meets the maximum current limit value condition cannot be constructed, the internal potential limit that meets the overload current limit requirement is constructed by adjusting the virtual impedance amplitude.

[0063] Under the condition of maximum current limit, there exists a closed set of vectors. If it is not satisfied, it means that the current is either too large, then the sum of the grid voltage and the virtual impedance vector voltage cannot intersect with the vector of the existing internal potential and there is no solution. That is, when the angle between the system impedance vector and the grid voltage vector is greater than or equal to 90°, the virtual impedance vector can only be increased (the grid voltage vector remains fixed).

[0064] The internal potential limiter that meets the overload current limiter requirement is constructed by adjusting the virtual impedance amplitude, including:

[0065] Determine the voltage drop that needs to be compensated and calculate the corresponding internal potential limit;

[0066] Based on the calculated internal potential limit, the impedance to be compensated is calculated and the proportionality coefficient of the impedance compensation is determined;

[0067] Based on the proportionality factor, a virtual impedance to be compensated is determined.

[0068] In a specific implementation, determining the voltage drop that needs to be compensated includes:

[0069] In the dq axis coordinate system, the voltage drop that needs to be compensated is determined based on the q axis component of the grid voltage and the maximum limiting voltage of the system impedance.

[0070] For example, when the angle between the vector of the system impedance and the vector of the grid voltage is greater than or equal to 90°, it is impossible to construct an internal potential that meets the maximum current limit value condition, and it is necessary to compensate for the voltage drop by changing the virtual impedance, ΔU 2 =(u q ) 2 -|E sys | 2 , ΔU is the voltage drop that needs to be compensated, and the corresponding internal potential limit E′ is calculated. lmt , E ref=E′ lmt .

[0071] By limiting the internal potential E' lmt , calculate the impedance required for compensation:

[0072] 1. Calculate the compensation impedance Z' = E' lmt / I m , I m is the maximum current limit value;

[0073] 2. Calculate the proportional coefficient k of impedance compensation = Z' / Z. Based on the proportional coefficient k, determine the virtual resistance to be compensated as R' vir =k*RR load , the virtual reactance to be compensated is X′ vir =k*XX load .

[0074] In one example, the overload current limiting method further includes:

[0075] When the grid-type converter outputs power, the compensated virtual impedance is attenuated.

[0076] Specifically, when the angle between the vector of the system impedance and the vector of the grid voltage rotates from greater than or equal to 90° to less than 90°, the compensated virtual impedance needs to be linearly attenuated. The attenuation method can be shown in the following formula: vir =Z vir (t0-t), Z′ vir is the virtual impedance after attenuation, t0 is the attenuation time reference, t is the attenuation time constant, Z vir is the virtual impedance for compensation.

[0077] Based on the virtual impedance and voltage limit range obtained above, the safe operating range of the internal potential is finally obtained. Figure 3 This is the test effect diagram, u abc Represents the three-phase voltage, i abc It represents the three-phase current of the power grid. Current overload occurs from the 2nd to the 10th second. The overload current is finally limited to three times the current, and the loop control runs stably without sudden changes.

[0078] The preferred embodiments of the present application are described above with reference to the accompanying drawings, but the scope of the present application is not limited thereby. Any modification, equivalent substitution and improvement made by a person skilled in the art without departing from the scope and essence of the present application shall be within the scope of the present application.

Claims

1. A method for limiting overload current of a grid-connected converter, characterized in that: The overload current limiting method comprises: When limiting the output current of the grid-type converter, the internal potential reference voltage is limited without changing the virtual impedance.

2. The overload current limiting method according to claim 1, characterized in that: The limiting internal potential reference voltage comprises: The internal potential limit is calculated based on the grid voltage, the maximum current limit value, the line impedance and the virtual impedance; According to the internal potential limit, the internal potential reference voltage is limited.

3. The overload current limiting method according to claim 2, characterized in that: The calculation of the internal potential limit according to the grid voltage, the maximum current limit value, the line impedance and the virtual impedance includes: Calculating a maximum limiting voltage of the system impedance according to the maximum current limiting value, the line impedance and the virtual impedance; The internal potential amplitude limit is calculated according to the grid voltage and the maximum limiting voltage of the system impedance.

4. The overload current limiting method according to claim 3, characterized in that: The calculating the maximum limiting voltage of the system impedance according to the grid voltage and the maximum limiting voltage of the system impedance comprises: Calculating a modulus value of system impedance according to the line impedance and the virtual impedance; Based on the module value of the system impedance and the maximum current limit value, the maximum limit voltage of the system impedance is calculated.

5. The overload current limiting method according to claim 3, characterized in that: The limiting the internal potential reference voltage according to the internal potential amplitude limiting comprises: In a dq-axis coordinate system, a first internal potential amplitude limit and a second internal potential amplitude limit are calculated based on the dq-axis components of the grid voltage and the maximum limiting voltage of the system impedance; Based on the first internal potential clipping and the second internal potential clipping, an internal potential reference voltage is limited.

6. The overload current limiting method according to claim 1, characterized in that: The overload current limiting method further includes: When the internal potential that meets the maximum current limit value condition cannot be constructed, the internal potential limit that meets the overload current limit requirement is constructed by adjusting the virtual impedance amplitude.

7. The overload current limiting method according to claim 6, characterized in that: The method of adjusting the virtual impedance amplitude to construct an internal potential amplitude limiter that meets the overload current amplitude limiter requirement includes: Determine the voltage drop that needs to be compensated and calculate the corresponding internal potential limit; Based on the calculated internal potential limit, the impedance to be compensated is calculated and the proportionality coefficient of the impedance compensation is determined; Based on the proportionality factor, a virtual impedance to be compensated is determined.

8. The overload current limiting method according to claim 7, characterized in that: Determining the pressure drop that needs to be compensated includes: In the dq axis coordinate system, the voltage drop that needs to be compensated is determined based on the q axis component of the grid voltage and the maximum limiting voltage of the system impedance.

9. The overload current limiting method according to claim 7, characterized in that: The overload current limiting method further includes: When the grid-type converter outputs power, the compensated virtual impedance is attenuated.

10. A grid-type converter, characterized in that: The grid-type converter is connected to a power grid, and the grid-type converter further comprises a control unit, wherein the control unit is configured to execute the overload current limiting method for the grid-type converter according to any one of claims 1 to 9.

Images