A method and system for selecting the inductance value of a flexible direct-current distribution network smoothing reactor

Abstract

The application provides a flexible direct current power distribution network smoothing reactor inductance value selection method and system, including the following steps: an analysis formula between the fault current in the flexible direct current power distribution network and the inductance value of the smoothing reactor is proposed; the parameters of the MMC converter in the flexible direct current power distribution network are input into the analysis formula; the parameters of the circuit breaker in the flexible direct current power distribution network are input into the analysis formula; according to the parameters input into the analysis formula, the lower limit of the inductance value of the smoothing reactor is obtained, and the inductance value of the smoothing reactor should be equal to or greater than the value. The application performs circuit analysis on the fault current of the flexible direct current power distribution network, proposes an expression between the fault current and the smoothing reactor, and finally determines the lower limit of the inductance value of the smoothing reactor in combination with the performance of the circuit breaker configured in the system. The application provides a reference for the selection of the inductance value of the smoothing reactor of the flexible direct current power distribution network, takes into account the safety and economy, avoids excessive investment in the smoothing reactor, and has important significance for the construction of the flexible direct current power distribution network.

Description

Technical Field

[0001] This invention relates to the field of power system fault analysis, and in particular to a method and system for selecting the inductance value of a smoothing reactor in a flexible DC distribution network. Background Technology

[0002] With the development of power electronics technology, flexible DC distribution networks are gradually becoming an important choice for future distribution network architectures. They offer advantages such as good power quality, low line losses, and ease of control, and can be readily integrated with distributed power sources such as photovoltaics, wind turbines, and energy storage. However, compared to AC grids, DC grids experience faster and larger fault currents after a fault, making them more difficult to interrupt. Interrupting DC fault currents typically relies on DC circuit breakers, which are expensive, with their price directly proportional to their breaking capacity. Considering the investment required for flexible DC distribution networks, only DC circuit breakers with limited breaking capacity can be installed. Therefore, measures must be taken to suppress fault currents to ensure that any fault current in the system can be interrupted. Smoothing reactors at the converter output have the ability to suppress fault currents; the larger the inductance value of the smoothing reactor, the stronger its ability to suppress fault currents. Theoretically, the inductance value of the smoothing reactor should be as large as possible, but again, considering investment factors, the inductance value of the smoothing reactor cannot be increased indefinitely. How to select the inductance value of the smoothing reactor is a current challenge. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the prior art.

[0004] The technical solution of this invention is: a method for selecting the inductance value of a smoothing reactor in a flexible DC distribution network, comprising the following steps:

[0005] 1) Determine the parameters of the MMC converter;

[0006] 2) Determine the parameters at the MMC converter outlet during steady-state operation;

[0007] 3) Determine the time t from the occurrence of the fault to the start of circuit breaker operation. op ;

[0008] 4) Substitute the parameters from steps 1)-3) into the fault current calculation formula to obtain the result based on the inductance value L of the smoothing inductor. s The fault current i is the independent variable. f (L s ,t op );

[0009] 5) Based on the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy i f (L smin ,top )|=I max Therefore, the value of the smoothing reactor should satisfy L s ≥L smin .

[0010] The parameters in step 1) include: the number of MMC converter arm submodules N, the MMC converter submodule capacitance C0, the arm inductance L0, and the on-resistance of the switching devices R. c .

[0011] The parameters in step 2) include: DC voltage value U d and DC current value I d .

[0012] In step 4), the formula for calculating the fault current is:

[0013]

[0014] In the formula, C e L is the equivalent capacitance value. e R is the equivalent inductance value. e R0 is the equivalent resistance value, σ is the on-resistance value, ω is the real part of the characteristic roots of the equation, A is the amplitude of the fault current, θ-β is the phase angle constant of the fault current, and t is time.

[0015] A system for selecting the inductance value of a smoothing reactor in a flexible DC distribution network includes:

[0016] The parameter module is used to determine the parameters of the MMC converter;

[0017] The output module is used to determine the parameters at the output of the MMC converter during steady-state operation.

[0018] The time module is used to determine the time t from the occurrence of a fault to the start of circuit breaker operation. op ;

[0019] The calculation module is used to substitute the parameters from the parameter module, output module, and time module into the fault current calculation formula to obtain the result based on the inductance value L of the smoothing inductor. s The fault current i is the independent variable. f (L s ,t op );

[0020] The selection module is used to select the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy | i f (L smin ,t op )|=I maxTherefore, the value of the smoothing reactor should satisfy L s ≥L smin .

[0021] In its operation, this invention performs circuit analysis on the fault current of a flexible DC distribution network, proposes an expression for the relationship between the DC fault current and the smoothing reactor, and, in conjunction with the performance of the circuit breakers configured in the system, finally determines the lower limit of the inductance value of the smoothing reactor.

[0022] This invention provides a reference for the selection of smoothing reactors in flexible DC distribution networks, taking into account both safety and economy, avoiding excessive investment in smoothing reactors, and is of great significance for the construction of flexible DC distribution networks. Attached Figure Description

[0023] Figure 1 This is a flowchart of the present invention.

[0024] Figure 2 This is a diagram of the internal structure of the submodule.

[0025] Figure 3 This is a diagram of the internal structure of an MMC converter.

[0026] Figure 4 This is the equivalent circuit diagram of an MMC converter.

[0027] Figure 5 It is the equivalent circuit diagram of the most severe fault.

[0028] Figure 6 This is a schematic diagram of a smoothing reactor.

[0029] Figure 7 This is the fault equivalent circuit diagram containing a smoothing reactor.

[0030] Figure 8 It is a different L s Fault current waveform under the given values,

[0031] Figure 9 It is t op fault current i at any time f (L s ,t op The value of the smoothing reactor L s Relationship diagram. Detailed Implementation

[0032] The present invention is as follows Figure 1 As shown, a method for selecting the inductance value of a smoothing reactor in a flexible DC distribution network includes the following steps:

[0033] 1) Determine the parameters of the MMC converter;

[0034] 2) Determine the parameters at the MMC converter outlet during steady-state operation;

[0035] 3) Determine the time t from the occurrence of the fault to the start of circuit breaker operation. op ;

[0036] 4) Substitute the parameters from steps 1)-3) into the fault current calculation formula to obtain the result based on the inductance value L of the smoothing inductor. s The fault current i is the independent variable. f (L s ,t op );

[0037] 5) Based on the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy | i f (L smin ,t op )|=I max Therefore, the value of the smoothing reactor should satisfy L s ≥L smin .

[0038] The parameters in step 1) include: the number of MMC converter arm submodules N, the MMC converter submodule capacitance C0, the arm inductance L0, and the on-resistance of the switching devices R. c .

[0039] The parameters in step 2) include: DC voltage value U d and DC current value I d .

[0040] This invention takes into account both safety and economy. Under the premise of ensuring that the circuit breaker can interrupt the most severe fault current, it selects an appropriate inductance value for the smoothing reactor, maximizes the utilization of the smoothing reactor's capacity, reduces excessive investment, and improves the economic efficiency of flexible DC distribution network construction.

[0041] The principle of this invention is analyzed as follows:

[0042] 1. Introduction to the working principle of MMC

[0043] The MMC converter consists of a series of sub-modules. The internal components of each sub-module include IGBT transistors VT1 and VT2 that can be controlled to turn on and off, corresponding anti-parallel diodes VD1 and VD2, and sub-module capacitor C0.

[0044] like Figure 2As shown, to control the input of submodule capacitor C0, an on-signal is applied to VT1 and an off-signal is applied to VT2, as shown in Figures (a) and (b). In Figure (a), although VT1 has an on-signal, VT1 is subjected to reverse voltage and cannot be turned on; current flows through the anti-parallel diode VD1. In Figures (a) and (b), submodule capacitor C0 is input, u SM It equals the voltage of the submodule capacitor.

[0045] When control submodule C0 is disconnected, a turn-off signal is applied to VT1 and an turn-on signal is applied to VT2, as shown in Figures (c) and (d). In Figure (d), although VT2 has an turn-on signal, VT2 is subjected to reverse voltage and cannot be turned on; current flows through the anti-parallel diode VD2. In Figures (c) and (d), module capacitor C0 is disconnected, u SM It equals zero.

[0046] In reality, every switching device has a very low on-resistance, but when a certain number of sub-modules are connected in series, the overall on-resistance cannot be ignored. Assume that the on-resistance of a single IGBT and the anti-parallel diode is R. c When calculating the fault current of an MMC converter, multiple R values ​​need to be considered. c The effects caused when they are connected in series.

[0047] Multiple sub-modules are connected together to form the main part of the MMC converter. The internal structure of the MMC converter is as follows: Figure 3 As shown;

[0048] Figure 3 in, u s The power supply is a three-phase AC power source, connected to the MMC converter via a converter transformer. The MMC converter has three parallel phase units, each consisting of 2N sub-modules connected in series, with N sub-modules in each of the upper and lower arms. Each arm also contains an arm reactor with an inductance of L0, primarily used to suppress interphase circulating current. This reactor is connected between the AC system and the converter and is therefore also called an AC reactor. The converter can be controlled by adjusting the capacitors in each sub-module.

[0049] 2. Calculation of DC fault current

[0050] After a fault occurs on the DC side, the fault current rises rapidly. The fault current mainly comes from the discharge of the submodule capacitors through the DC circuit, while the current feed in from the AC side is almost negligible. Therefore, in the following analysis, the influence of the AC power supply on the fault current is ignored, and the DC circuit is analyzed separately.

[0051] Based on the working principle and internal structure of the MMC converter, during the steady-state operation phase before a fault occurs, the DC-side voltage and current remain at their rated values, and the sum of the number of submodules engaged in each phase's upper and lower arms is also equal to the rated value. Let the DC voltage value before the fault be denoted as U. d The DC current value before the fault is denoted as I. d Let N be the sum of the number of submodules engaged in each phase's upper and lower arms. To simplify the calculation of fault current, the concept of an equivalent circuit for a phase unit is introduced. A phase unit of an MMC converter consists of 2N submodules connected in series with arm inductors. N submodules are engaged, and the other N submodules are bypassed (disconnected). However, the engaged and bypassed submodules are constantly changing. On average, the 2N submodules in a phase unit can be considered to have the same capacitor voltage U. c If, based on the principles of constant energy storage and constant total voltage across capacitors, 2N submodules are equivalently represented by a single capacitor, the following relationship holds:

[0052]

[0053] The equivalent capacitance C can be obtained. ph :

[0054]

[0055] In the formula, U dc It is a DC voltage.

[0056] Therefore, the equivalent circuit of the MMC converter in the instant before a DC-side fault occurs is as follows: Figure 4 As shown;

[0057] Figure 4 In this context, R0 equals the sum of the on-resistances of all switching devices (including IGBTs and diodes), R0 = NR c L0 is the bridge arm inductance. L represents the equivalent capacitance of each phase unit. line R is the equivalent inductance of the line. line R is the equivalent resistance of the circuit. f This is the transition resistance at the fault point. Figure 4 (a) in the diagram is a schematic diagram of the three-phase equivalent circuit of the converter. Figure 4 (b) in the diagram represents the single-phase equivalent circuit for calculating the faulty circuit, u C i is the voltage across the equivalent capacitance. f This is the fault current.

[0058] When the most severe bipolar metallic short-circuit fault occurs at the converter outlet:

[0059] R f =0,R line =0,Lline =0

[0060] In the formula, R f R is the transition resistance at the fault point. line L is the equivalent resistance of the line. line This is the equivalent inductance of the line.

[0061] The corresponding equivalent circuit diagram is as follows Figure 5 As shown, therefore, according to Figure 5 The circuit equations are as follows:

[0062]

[0063] In the formula, C e L is the equivalent capacitance value. e R is the equivalent inductance value. e d is the equivalent resistance value, t is the differential, and i is time. f The expression for the fault current (a quantity to be determined);

[0064] The second-order circuit is analyzed, in which... Therefore, the eigenvalues ​​are in the form of conjugate complex numbers:

[0065]

[0066] In the formula, λ is the characteristic root, σ is the real part of the characteristic root, j is the imaginary unit, and ω is the imaginary part of the characteristic root.

[0067] The DC voltage value before the fault is denoted as U. d The DC current value before the fault is denoted as I. d The initial value u of the capacitor voltage of the cascaded submodule can be approximated. C (0 - )=U d Therefore, the capacitor voltage u can be obtained. C and fault current i f The transient solutions are respectively

[0068]

[0069] in,

[0070] 3. Method for determining the value of smoothing reactor

[0071] When a DC-side fault occurs in the system, the fault current is interrupted by the DC circuit breaker. If the fault current amplitude is too large or rises too rapidly, the circuit breaker may fail to interrupt it. Therefore, measures to limit the short-circuit current are needed based on the capacity of the circuit breakers in the system. A smoothing reactor is connected between the converter and the DC line to suppress the rise of the fault current. The location of the smoothing reactor is as follows: Figure 6 As shown.

[0072] In theory, a larger inductance value for the smoothing reactor is better, but this increases the system's investment cost. Therefore, it is necessary to select an appropriate inductance value for the smoothing reactor to ensure that the circuit breaker can interrupt fault current while reducing system investment.

[0073] Let the inductance value of the smoothing reactor be denoted as L. s After adding the smoothing reactor, L e The value will increase, as shown in the following formula:

[0074]

[0075] Its equivalent circuit is as follows Figure 7 As shown.

[0076] By changing L s The value of L is changed. e This affects the characteristic roots λ, A, and β, ultimately limiting the fault current.

[0077] With all other parameters fixed, let time t and L be the starting points. s The fault current i is the independent variable. f Let i be the dependent variable. f (L s ,t), fault current i f (L s The formula for calculating t is:

[0078]

[0079] Let the time of the fault occurrence be recorded as time 0, and the time from the protection system identifying the fault to the circuit breaker starting to operate be recorded as t. op The maximum current that a circuit breaker can interrupt is denoted as I. max The inductance value L of the smoothing reactor s The choice should satisfy the condition of equation (5).

[0080] |i f (L s ,t op )|≤I max (5)

[0081] Under the condition of satisfying equation (5), L should be made as much as possible. s The smaller the better, to reduce system investment.

[0082] Therefore, a method for calculating the inductance value of a flexible DC smoothing reactor is as follows:

[0083] 1) Record the MMC converter parameters, including: the number of MMC converter arm submodules N, the MMC converter submodule capacitance C0, the arm inductance L0, and the on-resistance R of the switching devices. c ;

[0084] 2) Determine the DC voltage value U at the converter outlet during steady-state operation. d DC current value I d ;

[0085] 3) Determine the time t from the occurrence of the fault to the start of circuit breaker operation. op ;

[0086] 4) Substitute the parameters from 1), 2), and 3) into the fault current calculation formula (4) to obtain the fault current expression i with the inductance value of the smoothing inductor as the independent variable. f (L s ,t op );

[0087] 5) Based on the maximum current value I that the circuit breaker can interrupt. max , making |i f (L smin ,t op )|=I max ;

[0088] 6) The value of the smoothing reactor should satisfy L s ≥L smin .

[0089] A system for selecting the inductance value of a smoothing reactor in a flexible DC distribution network includes:

[0090] The parameter module is used to determine the parameters of the MMC converter;

[0091] The output module is used to determine the parameters at the output of the MMC converter during steady-state operation.

[0092] The time module is used to determine the time t from the occurrence of a fault to the start of circuit breaker operation. op ;

[0093] The calculation module is used to substitute the parameters from the parameter module, output module, and time module into the fault current calculation formula to obtain the result based on the inductance value L of the smoothing inductor. s The fault current i is the independent variable. f (L s ,t op );

[0094] The selection module is used to select the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy | i f (Lsmin ,t op )|=I max Therefore, the value of the smoothing reactor should satisfy L s ≥L smin .

[0095] Specific examples are as follows:

[0096] Referring to the Suzhou Tongli ±10kV flexible DC power distribution system, the invention is further explained in specific embodiments. 1)

[0098] Taking the ±10kV Suzhou Tongli Flexible DC Distribution System as an example, the MMC converter parameters are shown in the table below:

[0099] <![CDATA[Sub-module capacitor C0]]> 2.8mF Number of submodules N 24 <![CDATA[On-resistance R c > 5e-3ohm <![CDATA[Bridge arm inductor L0]]> 2mH

[0100] 2) Referring to the Suzhou Tongli project, during steady-state operation of the system, the DC inter-electrode voltage U at the MMC converter outlet is... d =20kV, steady-state operating DC current I d =0.5kV.

[0101] To demonstrate the limiting effect of the smoothing reactor's inductance value on the fault current in a specific embodiment, L is selected sequentially. s The values ​​are 10mH, 20mH, 30mH, and 50mH, respectively. These are then substituted into the fault current i along with parameters 1) and 2). f (L s The calculation formula for ,t) yields 5 different L values. s The fault current waveform under the given value is as follows: Figure 8 As shown.

[0102] Figure 8 In the diagram, the vertical axis represents the instantaneous value of the fault current, and the horizontal axis represents time (the moment the fault occurs is marked as time 0). From... Figure 8 As can be seen, the larger the value of the smoothing reactor, the more obvious the suppression effect on the rise of fault current.

[0103] 3) Assume the time from the occurrence of the fault to the start of operation of the DC circuit breaker is t. op =5ms

[0104] 4) Substitute the above parameters into the fault current calculation formula (4) to obtain t op At time i, the fault current i f (L s ,t op The value of the smoothing reactor L s The expression is given by equation (6):

[0105]

[0106]

[0107]

[0108]

[0109]

[0110] According to expression (6), i can be drawn. f (L s ,t op ) and L s Relationship curves, such as Figure 9 As shown.

[0111] 5) Take the maximum current value I that the circuit breaker can interrupt. max =2.5kA, represented by the dashed line in the diagram above;

[0112] 6) Solid line i f (L s ,t op ) and dashed line L s The intersection point corresponds to the minimum inductance value L of the smoothing reactor on the x-axis. smin ;

[0113] As shown in the figure above, when L s =L smin When ≈0.036,

[0114] i f (L smin ,t op ) = i f (0.036, 0.005) = 2.49918

[0115] i f (L smin ,t op ) = i f (0.036, 0.005) ≤ I max =2.5

[0116] Therefore, the smoothing reactor inductance L s The value should satisfy

[0117] L s ≥L smin =0.036H

[0118] That is, in order to meet the circuit breaker operating time t op =5ms and maximum breaking short-circuit current I maxTo meet the requirement of 2.5kA, a smoothing reactor with an inductance of at least 0.036H must be installed between the two poles of the converter outlet, that is, a smoothing reactor with an inductance of at least 0.018H must be installed on each pole.

[0119] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for selecting the inductance value of a smoothing reactor in a flexible DC distribution network, characterized in that, Includes the following steps: 1) Determine the parameters of the MMC converter; 2) Determine the parameters at the MMC converter outlet during steady-state operation; 3) Determine the time t from the occurrence of the fault to the start of circuit breaker operation. op ; 4) Substitute the parameters from steps 1)-3) into the fault current calculation formula to obtain the result based on the smoothing reactor inductance value L. s The fault current i is the independent variable. f (L s ,t op ); 5) Based on the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy | i f (L smin ,t op )|=I max Therefore, the value of the smoothing reactor should satisfy L s ≥L smin ; The parameters in step 1) include: the number of MMC converter arm submodules N, the MMC converter submodule capacitance C0, the arm inductance L0, and the on-resistance of the switching devices R. c ; The parameters in step 2) include: DC voltage value U d and DC current value I d ; In step 4), the formula for calculating the fault current is: In the formula, C e L is the equivalent capacitance value. e R is the equivalent inductance value. e R0 is the equivalent resistance value, σ is the on-resistance value, ω is the real part of the characteristic roots of the equation, A is the amplitude of the fault current, θ-β is the phase angle constant of the fault current, and t is time.

2. A system for selecting the inductance value of a smoothing reactor in a flexible DC distribution network, characterized in that, include: The parameter module is used to determine the parameters of the MMC converter, including: the number of MMC converter arm submodules N, the MMC converter submodule capacitance C0, the arm inductance L0, and the on-resistance R of the switching devices. c ; The output module is used to determine the parameters at the MMC converter output during steady-state operation, including the DC voltage value U. d and DC current value I d ; The time module is used to determine the time t from the occurrence of a fault to the start of circuit breaker operation. op ; The calculation module is used to substitute the parameters from the parameter module, output module, and time module into the fault current calculation formula to obtain the result based on the smoothing reactor inductance value L. s The fault current i is the independent variable. f (L s ,t op ); The selection module is used to select the maximum current value I that the circuit breaker can interrupt. max The lower limit L of the smoothing reactor inductance value. smin Satisfy | i f (L smin ,t op )|=I max Therefore, the value of the smoothing reactor should satisfy L s ≥L smin ; The formula for calculating fault current is: In the formula, C e L is the equivalent capacitance value. e R is the equivalent inductance value. e R0 is the equivalent resistance value, σ is the on-resistance value, ω is the real part of the characteristic roots of the equation, A is the amplitude of the fault current, θ-β is the phase angle constant of the fault current, and t is time.

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