A Modeling Method for Decoupling Line Impedance of Active DC Distribution Network under Frequency Deviation

Abstract

The present invention provides a method for decoupling the impedance of a faulty line under frequency deviation in a flexible DC distribution network with distributed power sources connected. The method includes: analyzing the fault characteristics with frequency deviation problems after different types of faults occur in the DC distribution line, and giving the line impedance expression after decoupling the impedance of the faulty line under frequency deviation through the electrical quantity constraint conditions before and after the equivalent transformation of the faulty line impedance. The method of the present invention solves the problem that the fault responses of various distributed power sources affect each other due to the coupling impedance, facilitates subsequent fault calculations, and can provide certain reference for the coordination and setting of protection in actual engineering.

Description

[0001] The present invention relates to the field of flexible DC distribution fault protection, and particularly to a fault line impedance decoupling modeling method in a multi-voltage-level two-terminal flexible DC distribution network with distributed power sources. Background Art

[0002] With the increasing risk of energy crisis worldwide, countries have successively placed the development of new energy technologies in an important strategic position. At the same time, China has put forward a development strategy to build a new power system with new energy as the main body. Compared with traditional AC distribution networks, AC-DC hybrid distribution networks can effectively improve power quality, reduce power losses, and reduce energy conversion steps, which is conducive to the flexible access of a high proportion of new energy. Therefore, DC distribution networks with distributed power sources have received extensive attention from domestic and foreign scholars.

[0003] When a pole-to-pole short-circuit fault occurs in a DC distribution system, the fault resistance almost entirely comes from the line equivalent impedance. However, the line impedance exhibits different impedance characteristics for different types of distributed power sources. Existing fault loop modeling rarely considers the influence of coupling impedance parameters on equivalent fault modeling, resulting in the inability to conduct a combined analysis of the effects of different types of distributed power sources on the fault point and the inability to accurately solve the fault characteristics, which affects the reliability of subsequent protection schemes. Existing anti-decoupling modeling methods have large errors when applied to DC distribution systems and do not consider the influence brought by frequency changes during the fault transient process. However, after a fault occurs in a DC distribution system, the DC line impedance exhibits a dynamic impedance characteristic that changes with frequency according to the excitation of different distributed power sources, resulting in the influence of fault characteristics by frequency deviation, thus affecting the accuracy of fault characteristic analysis and the reliability of protection schemes. Summary of the Invention

[0004] In view of the deficiencies of the prior art, an embodiment of the present invention provides a fault line impedance decoupling modeling method for an active DC distribution network under frequency deviation. The method includes:

[0005] Based on the physical relationship, establish an equivalent circuit model of the distribution network after a line short-circuit fault from the structure of the distribution network with distributed power sources in actual engineering.

[0006] Based on Kirchhoff's law, ensure that the key parameters of the circuit remain unchanged before and after impedance decoupling, and judge the constraint conditions that need to be satisfied for impedance parameter decoupling.

[0007] According to the discharge frequency of each module at the fault point, make a correction of the equivalent impedance parameters of the fault line under frequency deviation during the impedance decoupling process of the line.

[0008] Preferably, starting from the actual application scenario, a fault line impedance decoupling modeling scheme considering frequency changes is proposed, and accurate fault initial conditions are given and an equivalent fault circuit is established.

[0009] Preferably, circuit analysis is performed on the fault equivalent model to ensure that the key circuit parameters remain unchanged before and after the decoupling of the line impedance, including:

[0010] After the decoupling of the line impedance, the output current and discharge frequency of the photovoltaic module and the energy storage module remain unchanged;

[0011] Ensure that the output voltage of each discharge module after the equivalent transformation is approximately equal to the DC bus voltage;

[0012] Calculate the line voltage drop of each module using the output current of the distributed module, and set the voltages at both ends of the short line in the active distribution network to be equal to obtain the decoupling modeling conditions for the fault impedance parameters;

[0013] Considering the main function of the DC link capacitor, the cable grounding capacitance is ignored.

[0014] Furthermore, the calculation formula for the precondition of short line impedance decoupling in the DC distribution network with distributed power sources is as follows:

[0015]

[0016] where, I p , I pp are the output currents of the photovoltaic module before and after the decoupling of the line impedance parameters, I s , I ss are the output currents of the energy storage module, s p , s s , s pp , s ss are the discharge frequencies of the photovoltaic and energy storage modules to the fault point before and after the decoupling of the line impedance parameters, respectively.

[0017] Furthermore, the calculation formulas for the output voltages of each discharge module before and after decoupling are as follows:

[0018] I p (s p L bp +R bp )+I f (s f L bd2 +R bd2 )=I pp (s pp L bdp +R bdp )

[0019] I s (s s L bs +R bs )+I f (s f L bd2 +R bd2) = I ss (s ss L bds +R bds )

[0020] where I f and s f are the short - circuit current and frequency at both poles of the DC side. The left side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before decoupling of the line impedance, and the right side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before decoupling after decoupling of the line impedance.

[0021] Furthermore, the calculation formulas for the output current of each discharge module before and after decoupling are as follows:

[0022] I p +I s = I pp +I ss

[0023] where the left side of the equal sign is the short - circuit fault current value before decoupling of the line impedance, and the right side of the equal sign is the short - circuit fault current value after decoupling of the line impedance.

[0024] Furthermore, the calculation formula for the output line voltage of each distributed module is as follows:

[0025] I p (s p L bp +R bp ) = I s (s s L bs +R bs )

[0026] where the left side of the equal sign is the line voltage drop of the photovoltaic module output, and the right side of the equal sign is the line voltage drop of the energy storage module output.

[0027] Preferably, according to the decoupling modeling conditions of the fault impedance parameters, the calculation formula for line impedance decoupling is obtained. The decoupling modeling parameters of the fault line impedance under frequency deviation are as follows: <[

[0028]

[0029]

[0030]

[0031]

[0032] where R bdp is the equivalent inductance of the photovoltaic module line, L bdp is the equivalent resistance of the photovoltaic module line, R bds is the equivalent inductance of the energy storage module line, Lbds Equivalent resistance of the energy storage module circuit Description of the Drawings

[0033] Figure 1 is the equivalent circuit of a dual - terminal multi - voltage - level DC distribution network with distributed power sources provided by the present invention;

[0034] Figure 2 is the schematic diagram of decoupling modeling of the impedance of the short - circuit fault line in the distribution network provided by the present invention;

[0035] Figure 3 is the equivalent circuit diagram of the two - pole short - circuit fault of the low - voltage side DC distribution network provided by the present invention;

[0036] Figure 4 is the comparison diagram of the short - circuit fault circuit calculation results before and after the decoupling modeling of the line impedance under frequency deviation provided by the present invention. Detailed Description of the Invention

[0037] The following further elaborates on the specific implementation manners of the present invention with reference to the drawings.

[0038] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0039] The present invention provides an equivalent circuit of a dual - terminal multi - voltage - level DC distribution network with distributed power sources, as Figure 1 shown.

[0040] All DC cables and DC buses are modeled using lumped - parameter models, which are suitable for mathematical analysis of fault response characteristics and are widely used in DC transient fault analysis methods.

[0041] Among them, R c1 , R c2 , L c1 , L c2 are the equivalent resistance and inductance of the high - voltage distribution line respectively, C H , C L are the DC bus capacitors on the high - and low - voltage sides of the DAB converter respectively, R bd , R bs , R bp , L bd , L bs , L bp are the equivalent resistance and inductance of the low - voltage side distribution line respectively, L b , Lbb , C pv , C b , C bs are the inductor and capacitor in the power electronic converter respectively, and R load is the equivalent DC load.

[0042] A decoupling modeling scheme for the impedance of the distribution line after a fault is proposed for the coupling problem in the DC distribution network with distributed power sources due to the change of the line impedance parameters with the discharge frequency after the fault as Figure 2 shown. Based on Kirchhoff's law, it is ensured that the key parameters of the circuit remain unchanged before and after impedance decoupling, and the constraint conditions for judging the decoupling of the impedance parameters are included:

[0043] Step 1: After the line impedance is decoupled, the output current and discharge frequency of the photovoltaic module and the energy storage module remain unchanged;

[0044] Step 2: Ensure that the output voltage of each discharge module after the equivalent transformation is approximately equal to the DC bus voltage;

[0045] Step 3: Calculate the line voltage drop of each module by using the output current of the distributed module, and set the voltages at both ends of the short line in the active distribution network to be equal to obtain the decoupling modeling conditions for the fault impedance parameters;

[0046] The calculation formula for the precondition of the short line impedance decoupling in the DC distribution network with distributed power sources in Step 1 is as follows:

[0047]

[0048] Among them, I p , I pp are the output currents of the photovoltaic module before and after the decoupling of the line impedance parameters, I s , I ss are the output currents of the energy storage module, and s p , s s , s pp , s ss are the discharge frequencies of the photovoltaic and energy storage modules to the fault point before and after the decoupling of the line impedance parameters respectively.

[0049] In Step 1, the calculation formulas for the output currents of each discharge module before and after decoupling are as follows:

[0050] I p + I s = I pp + I ss

[0051] Among them, the left side of the equal sign is the short-circuit fault current value before the line impedance is decoupled, and the right side of the equal sign is the short-circuit fault current value after the line impedance is decoupled.

[0052] In Step 2, the calculation formulas for the output voltages of each discharge module before and after decoupling are as follows:

[0053] I p (s p L bp +R bp )+I f (s f L bd2 +R bd2 )=I pp (s pp L bdp +R bdp )

[0054] I s (s s L bs +R bs )+I f (s f L bd2 +R bd2 )=I ss (s ss L bds +R bds )

[0055] Among them, I f and s f are the short-circuit currents and frequencies at both poles of the DC side. The left side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before line impedance decoupling, and the right side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before line impedance decoupling after decoupling.

[0056] In Step 3, the calculation formula for the output line voltage of each distributed module is as follows:

[0057] I p (s p L bp +R bp )=I s (s s L bs +R bs )

[0058] Among them, the left side of the equal sign is the output line voltage drop of the photovoltaic module, and the right side of the equal sign is the output line voltage drop of the energy storage module.

[0059] Starting from the actual application scenario, a modeling scheme for decoupling the line impedance of a faulty distribution network considering frequency changes is proposed, and the precise fault initial conditions are given and an equivalent fault circuit is established, as Figure 3 shown.

[0060] According to the decoupling modeling conditions of the fault impedance parameters, the decoupled calculation formula of the line impedance is obtained. The decoupled modeling parameters of the fault line impedance under frequency deviation are as follows:

[0061]

[0062]

[0063]

[0064]

[0065] According to the proposed method for decoupling the fault line impedance modeling of a flexible DC distribution network with distributed power sources under frequency deviation, based on the equivalent circuit of a two-pole short-circuit fault in the low-voltage side DC distribution network, an equivalent simulation model of a flexible DC distribution network with distributed power source access is built using the PSCAD / EMTDC simulation software. Through the short-circuit fault circuit calculation results before and after the line impedance decoupling modeling, the proposed fault line impedance decoupling modeling scheme under frequency deviation is verified, as Figure 4 shown. The method of the present invention does not require an additional iterative algorithm when calculating the DC side fault current, has a small calculation error, and can provide a certain reference for the coordination and setting of protection in practical engineering.

[0066] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: the specific implementation manners of the present invention can still be modified or equivalently replaced, and any modification or equivalent replacement that does not depart from the spirit and scope of the present invention shall be covered by the protection scope of the claims of the present invention.

Claims

1. A method for decoupling and modeling the line impedance of an active DC distribution network under frequency deviation, characterized in that, Including: Step 1: Establish an equivalent fault circuit model of the distribution network after a line short-circuit fault for the physical relationship from the structure of the actual engineering distribution network with distributed power sources. Step 2: According to Kirchhoff's law, ensure that the key parameters of the circuit remain unchanged before and after impedance decoupling, and judge the constraint conditions that need to be satisfied for impedance parameter decoupling. Including: After the line impedance is decoupled, the output current and discharge frequency of the photovoltaic module and the energy storage module remain unchanged. Ensure that the output voltage of each discharge module after the equivalent transformation is approximately equal to the DC bus voltage. Calculate the line voltage drop of each module using the output current of the photovoltaic module and the energy storage module, and set the voltages at both ends of the short line in the active distribution network to be equal. Considering the main function of the DC link capacitor, the cable grounding capacitance is ignored. Step 3: Based on the discharge frequency of each module at the fault point, make a correction of the equivalent impedance parameters of the fault line under frequency deviation for the impedance decoupling process of the line.

2. The method according to claim 1, wherein After a two-pole short-circuit fault occurs on the DC side line, within milliseconds, the impedance of the distribution line is coupled by the discharges of different types of distributed power sources. The coupled line impedance on the distributed power source side shows different response characteristics with frequency changes, making it difficult to accurately analyze the fault process of the distribution network with multiple types of distributed power sources. Starting from the actual application scenario, a modeling scheme for impedance decoupling of the fault line in the distribution network considering frequency changes is proposed, and accurate fault initial conditions are given and an equivalent fault circuit model is established.

3. The method according to claim 1, wherein The calculation formula for the premise of impedance decoupling of the short line in the DC distribution network with distributed power sources is as follows: Among them, I p , I pp are the output currents of the photovoltaic module before and after decoupling of the line impedance parameters, I s , I ss are the output currents of the energy storage module, s p , s s , s pp , s ss are the discharge frequencies of the photovoltaic and energy storage modules to the fault point before and after decoupling of the line impedance parameters respectively.

4. The method according to claim 1, wherein The calculation formula for the output voltage of each discharge module before and after decoupling is as follows: I p (s p L bp +R bp )+I f (s f L bd2 +R bd2 )=I pp (s pp L bdp +R bdp ) I s (s s L bs +R bs )+I f (s f L bd2 +R bd2 )=I ss (s ss L bds +R bds ) Among them, I f and s f are the short-circuit current and frequency at both poles on the DC side. The left side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before decoupling of the line impedance, and the right side of the equal sign is the line voltage drop of each module in the DC distribution network with distributed power sources before decoupling after decoupling of the line impedance; I p and I pp are the output currents of the photovoltaic module before and after decoupling of the line impedance parameters, I s and I ss are the output currents of the energy storage module, s p and s s and s pp and s ss are the discharge frequencies of the photovoltaic and energy storage modules to the fault point before and after decoupling of the line impedance parameters respectively; R bs and R bp and L bs and L bp are the equivalent resistance and inductance of the low-voltage side distribution line respectively; R bdp is the equivalent resistance of the photovoltaic module line after decoupling, L bdp is the equivalent inductance of the photovoltaic module line after decoupling, R bds is the equivalent resistance of the energy storage module line after decoupling, L bds is the equivalent inductance of the energy storage module line after decoupling.

5. The method according to claim 1, wherein The calculation formula for the output current of each discharge module before and after decoupling is as follows: I p +I s =I pp +I ss Among them, the left side of the equal sign is the short-circuit fault current value before line impedance decoupling, and the right side of the equal sign is the short-circuit fault current value after line impedance decoupling; I p 、I pp are the output currents of the photovoltaic module before and after line impedance parameter decoupling, and I s 、I ss are the output currents of the energy storage module.

6. The method according to claim 1, wherein Calculate the line voltage drop of each module using the output current of the photovoltaic module and the energy storage module, and set the voltages at both ends of the short line in the active distribution network to be equal. The calculation formula is: I p (s p L bp +R bp ) = I s (s s L bs +R bs ) Among them, the left side of the equal sign is the voltage drop of the output line of the photovoltaic module, and the right side of the equal sign is the voltage drop of the output line of the energy storage module; I p is the output current of the photovoltaic module before decoupling of the line impedance parameter, I s is the output current of the energy storage module; s p 、s s are the discharge frequencies of the photovoltaic and energy storage modules to the fault point before decoupling of the line impedance parameter, respectively; R bs 、R bp 、L bs 、L bp are the equivalent resistance and inductance of the low-voltage side distribution line, respectively.

7. The method according to claim 4, characterized in that According to the decoupling modeling conditions of the fault impedance parameters, the decoupled calculation formula of the line impedance is obtained. The decoupled modeling parameters of the fault line impedance under frequency deviation are as follows: where R bdp is the equivalent resistance of the decoupled photovoltaic module circuit, L bdp is the equivalent inductance of the decoupled photovoltaic module circuit, R bds is the equivalent resistance of the decoupled energy storage module circuit, L bds is the equivalent inductance of the decoupled energy storage module circuit.

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