A short-circuit fault characteristic calculation method for a flexible HVDC power distribution network containing distributed power sources

Abstract

The application provides a short-circuit fault characteristic calculation method for a flexible DC power distribution network containing distributed power sources. The method comprises the following steps: dividing each distributed discharge module after an inter-pole short-circuit fault of a power distribution line into fault stages, correcting an equivalent impedance model of a fault line through discharge frequency changes in each fault stage, making the fault characteristic analysis have frequency variation characteristics, and deducing a fault current expression of each discharge module after a short-circuit fault of the line. The method of the application does not need an additional iterative algorithm when calculating the fault current, has small calculation error, and can provide certain reference for cooperation and setting of protection in actual engineering.

Description

[0001] This invention relates to the field of fault protection for flexible DC power transmission and distribution, and in particular to a method for calculating the transient characteristics of short-circuit faults in flexible DC distribution networks containing distributed power sources. Background Technology

[0002] With the integration of various types and capacity levels of distributed power sources at different locations, DC distribution networks exhibit complex fault characteristics when different types of faults occur. Furthermore, the inherent low damping and low inertia of DC distribution networks lead to a high fault current rise rate, posing new challenges to subsequent fault type identification and effective fault detection, and significantly impacting the reliability of protection schemes. In addition, during the integration of distributed power sources into DC distribution networks through various types of power electronic converters, several issues remain to be addressed regarding steady-state operation, fault detection, and fault isolation. Therefore, as the foundation of distribution network protection, it is necessary to conduct precise analysis of the fault characteristics of DC distribution networks under different fault types.

[0003] Currently, existing analysis processes focus on scenarios without distributed power source access. Considering the large number of power electronic converters in DC distribution networks, it is necessary to analyze the fault characteristics after a short-circuit fault occurs on the DC side in order to ensure the reliable operation of the converters and systems. It is also necessary to conduct further research on issues such as unclear fault transient characteristics and insufficient theoretical demonstration of fault characteristics after a ground fault occurs in DC distribution networks with multiple distributed power sources, so as to provide theoretical guidance for DC protection. Summary of the Invention

[0004] This invention provides a method for calculating the transient characteristics of short-circuit faults in flexible DC distribution networks containing distributed power sources, overcoming the shortcomings of existing technologies. A general formula for calculating fault currents in multi-voltage DC distribution network systems with various distributed power sources is derived, providing a reference for component selection and power grid planning.

[0005] To achieve the above objectives, the present invention adopts the following technical solution, including:

[0006] A method for calculating transient fault current during the AC feed-in stage after a two-level VSC is blocked includes the following steps:

[0007] Establish an equivalent circuit model for short-circuit faults at both poles of the low-voltage side DC distribution network;

[0008] Fault stage classification after a short circuit fault occurs between two poles of the line in the DAB low-voltage side module, distributed photovoltaic module, and energy storage module;

[0009] Based on circuit theorems and the defined fault stages, calculate the short-circuit current and output voltage of each module at the fault point.

[0010] Preferably, the DC distribution network model is a 10kV high-voltage AC voltage level, which is connected to a ±10kV high-voltage DC distribution line through a two-level voltage source converter (VSC). This allows for bidirectional power flow while dealing with grid disturbances. The low-voltage DC distribution network has a voltage level of ±375V and is interconnected through a dual-active bridge (DAB) DC transformer. The photovoltaic power generation system and the energy storage power generation system are connected to the low-voltage DC bus through Buck / Boost converters and Boost converters, respectively, forming a hybrid AC / DC distribution system.

[0011] Furthermore, the initial stage of each module after a bipolar short-circuit fault occurs in the DC distribution network is solved, and the fault stages of each module after a bipolar short-circuit fault occurs in the DC distribution network are divided. Finally, the fault current and voltage values ​​of each distributed power source are solved.

[0012] Preferably, to solve for the initial voltage value of the capacitor and the initial current value of the inductor of the DAB module after a two-pole short-circuit fault occurs in the DC distribution network, according to Kirchhoff's voltage law, we can obtain:

[0013]

[0014] Where i1-i4 are the branch currents in the DAB low-voltage side equivalent circuit, and u1-u3 are the steady-state operating voltages of each capacitor, approximately u1=u2=u3=Udc, we can obtain:

[0015]

[0016] Furthermore, we solve for the initial voltage values ​​of the capacitors and the initial current values ​​of the line inductors in the Boost converter after a short-circuit fault occurs in the DC distribution network. The initial fault conditions of the photovoltaic module at time t0 when the fault begins are:

[0017]

[0018] Among them, U oc It is the open-circuit output voltage of the photovoltaic unit, I opv It is the output current of the photovoltaic module.

[0019] Furthermore, the initial voltage values ​​of the capacitors and the initial current values ​​of the line inductors in the Buck / Boost converter are calculated after a bipolar short-circuit fault occurs in the energy storage module in the DC distribution network. The initial fault conditions of the energy storage module at time t0 when the fault begins are:

[0020]

[0021] Among them, E bat Ibat I represents the output voltage and current of the energy storage battery's equivalent capacitance. osg Output current to the energy storage module.

[0022] Preferably, after a two-pole short-circuit fault occurs in each module of the DC distribution network, a fault stage division is made, including:

[0023] The fault response process of the DAB module is divided into the low-voltage side capacitor discharge stage and the bridge arm freewheeling stage.

[0024] The photovoltaic module fault response process is divided into three stages: capacitor discharge stage, multi-branch power supply stage, and fault power flow stabilization stage.

[0025] The fault response process of the energy storage module is divided into a multi-branch power supply stage and a fault power flow stabilization stage.

[0026] Furthermore, the fault current and voltage during the low-voltage side capacitor discharge stage of the DAB module are calculated and solved. The formulas for fault current and voltage are as follows:

[0027]

[0028]

[0029]

[0030]

[0031] Where u CL It is the voltage of the DAB low-voltage side support capacitor, R bd1 L bd1 These are the equivalent resistance and inductance of the low-voltage side distribution lines, respectively.

[0032] Furthermore, the feature is that the fault current during the freewheeling phase of the DAB module is calculated, and the fault current formula is:

[0033]

[0034] Where Itd1 is the initial current value of the freewheeling current of the line inductor Lbd1.

[0035] Furthermore, the feature is that the fault current during the capacitor discharge stage of the photovoltaic module is calculated using the following formula:

[0036] i fp (t)=i cb (t)+i pv

[0037]

[0038]

[0039]

[0040]

[0041] Among them, i fp It is the fault current output by the photovoltaic module, i cb It is the discharge current of the DC line voltage regulator capacitor, u cb It is the voltage of the DC line voltage regulator capacitor, i pv It is the output current of the photovoltaic power source, R bdp L bdp These are the equivalent resistance and inductance of the power distribution lines near the photovoltaic module, respectively.

[0042] Furthermore, the fault current in the multi-branch feeding stage of the photovoltaic module is calculated using the following formula:

[0043] i fp (t)=i cp (t)+i cb (t)+i pv

[0044]

[0045] Among them, i cp It is the discharge current of the capacitor in the Boost converter of the photovoltaic module.

[0046] Furthermore, the fault current during the multi-branch power supply stage of the energy storage module is calculated using the following formula:

[0047]

[0048]

[0049]

[0050]

[0051] Among them, i bs It is the discharge current of the capacitor in the Buck / Boost converter of the energy storage module, u cbs It is the voltage of the DC line voltage regulator capacitor, R. bds L bds These are the equivalent resistance and inductance of the power distribution lines near the photovoltaic module, respectively. Attached Figure Description

[0052] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0053] Figure 1 This is a schematic diagram of the fault equivalent circuit of a two-level voltage source converter.

[0054] Figure 2 This is the equivalent circuit diagram for the initial stage of a DAB module failure.

[0055] Figure 3 This is the equivalent circuit diagram of each fault stage on the low-voltage side of the DAB after a line short-circuit fault.

[0056] Figure 4 This is the equivalent circuit diagram of the photovoltaic module after a short circuit fault.

[0057] Figure 5 This is the equivalent circuit diagram of the energy storage module after a line short-circuit fault. Detailed implementation method:

[0058] The present invention will now be described in detail with reference to the accompanying drawings, but this is not intended to limit the scope of protection of this application.

[0059] Figure 1 This invention proposes an equivalent circuit for the AC feed-in stage of a two-level voltage source converter. The DC distribution network model is a 10kV high-voltage AC voltage level. A ±10kV high-voltage DC distribution line is formed through a two-level voltage source converter (VSC), which enables bidirectional power flow while coping with grid disturbances. The low-voltage DC distribution network voltage level is ±375V, which is interconnected through a dual active bridge (DAB) DC transformer. The photovoltaic power generation system and the energy storage power generation system are connected to the low-voltage DC bus through Buck / Boost converters and Boost converters, respectively, forming an AC / DC hybrid power distribution system.

[0060] All DC cables and DC buses employ a lumped parameter model, which is suitable for mathematical analysis of fault response characteristics and widely used in DC transient fault analysis methods. (Figure R) c1 R c2 L c1 L c2 These are the equivalent resistance and inductance of a high-voltage power distribution line, C. H C L These are the high- and low-voltage side DC bus capacitors of the DAB converter, R. bd Rbs R bp L bd L bs L bp These are the equivalent resistance and inductance of the low-voltage side distribution line, respectively. b L bb C pv C b C bs These are the inductor and capacitor in a power electronic converter, R. load It is an equivalent DC load.

[0061] The equivalent circuit of the DAB module during steady-state operation of a DC distribution network is as follows: Figure 2 As shown, ignoring the voltage drop caused by the DC line inductance during steady-state operation, the following relationship is satisfied at time t0 according to Kirchhoff's voltage law:

[0062]

[0063] In actual engineering scenarios, we have approximately u1 = u2 = u3 = U dc Then, the initial conditions for a fault on the low-voltage side of the DAB after a short-circuit fault occurs in the DC line are:

[0064] During the discharge phase of the DAB low-voltage side capacitor, the short-circuit current on the low-voltage DC side is mainly driven by the rapid discharge of the DAB low-voltage side capacitor. Figure 3 As shown.

[0065] Based on the initial state of the DAB module failure, in the case of C L R bd1 L bd1 In the circuit, such as Figure 3 As shown in Figure a, ignoring the voltage drop across the line inductance, applying KVL yields:

[0066]

[0067] In the DAB low-voltage side discharge module, C is determined by system parameters. L R c1 2 / 4L c1 Since the value is less than 1, the fault circuit operates in an underdamped state. Therefore, the time-domain solution for the discharge from the DAB low-voltage side to the fault point is:

[0068]

[0069] in

[0070]

[0071] When the capacitance C LWhen the voltage decays and oscillates across zero, the back electromotive force generated by the short-circuit reactance of the DC line causes all four diodes on the low-voltage side of the DAB to conduct simultaneously, forming a new energy discharge circuit on the DC side, namely a first-order discharge circuit, such as... Figure 3 As shown in b.

[0072] Ignoring the forward conduction voltage of the diode, the low-voltage side output voltage of the DAB is clamped to zero by the conducting diode. Therefore, there is no charging or discharging process of the DC-side capacitor during this stage, where C... L The time when the voltage drops to zero is t. d1 The time for the DAB low-side voltage to drop to zero is:

[0073]

[0074] Line inductance L bd1 The initial current value I of the freewheeling td1 for:

[0075]

[0076] At this time, the short-circuit current of the DAB module at the fault point is:

[0077]

[0078] The current flowing through the diode is:

[0079]

[0080] To describe the short-circuit fault characteristics of distributed photovoltaic modules in DC distribution networks, the fault response process of the module is divided into three stages: capacitor discharge stage, multi-branch power supply stage, and fault power flow stabilization stage. Figure 4 This is the equivalent circuit for the photovoltaic module after a short-circuit fault, where the photovoltaic unit is connected to the low-voltage DC bus via a Boost circuit. During steady-state operation of the power distribution system, C... pv C b The voltage across the terminals depends directly on the photovoltaic output voltage and the DC line voltage, with initial values ​​for the photovoltaic open-circuit voltage U, respectively. oc and U dc During steady-state operation of the system, the diode current i flowing through the photovoltaic-side Boost converter is... D (t) can be represented as:

[0081]

[0082] Where I pv T is the output current of the photovoltaic equivalent current source. s This represents the period of the control signal, where D represents the duty cycle of the high level in the control signal. In practical engineering, T... s The unit is in microseconds, ignoring U. dc(1-D)DT s / 2L b The initial conditions for a photovoltaic side fault after a short-circuit fault occurs on the DC line are as follows:

[0083]

[0084] Among them, U oc It is the open-circuit output voltage of the photovoltaic unit, I opv It is the output current of the photovoltaic module.

[0085] When a fault occurs, the capacitor C near the fault point b During rapid discharge, the IGBT in the Boost converter latches up within microseconds according to its protection strategy, at which point diode D... b It operates in forward conduction mode to maintain the energy discharge circuit after a photovoltaic power supply failure. During this stage, the photovoltaic side voltage regulator capacitor C... pv There was no obvious discharge process at the fault point.

[0086] The fault current output by the photovoltaic power supply is determined by the output current i of the photovoltaic power supply. pv DC line voltage regulator capacitor C b The discharge current i cb Composition. The short-circuit current i at the fault point of the photovoltaic module. fb It can be represented as:

[0087] i fp (t)=i cb (t)+i pv

[0088] According to KVL, in C b R bdp L bdp The differential equation for the circuit is as follows:

[0089]

[0090] The transient time-domain solution expression of the fault characteristics can be obtained from differential equation (23):

[0091]

[0092]

[0093] in:

[0094]

[0095] The time interval for the capacitor discharge phase is t. p0 -t p1 From the moment the fault occurred until C b With Cpv The voltage across both ends ends at this point.

[0096] When C b Discharge to C pv When the voltages are equal, the photovoltaic module enters the multi-branch feeding stage, at which point the fault current is generated by i. pv i cb And photovoltaic power supply voltage regulator capacitor C pv The discharge current i cp Composition, i fp It can be represented as:

[0097] i fp (t)=i cp (t)+i cb (t)+i pv

[0098] For each of the components i fp Solve for the three current components of (t), using u cpv u cb i db i fp As state variables, the state equations can be derived based on the equivalent circuit after the fault:

[0099]

[0100] According to the state equation, i can be derived cp Transient time-domain solution:

[0101]

[0102] During the stable phase of a photovoltaic (PV) module fault flow, the output voltage and current of the PV module will be relatively stable. Due to the IGBT's latch-up, the DC-DC converter operates without a control strategy. Considering that environmental factors such as temperature and illumination remain essentially constant, the PV output panel can be considered an equivalent stable current source. Even with continuous discharge from the inductive and capacitive energy storage components, the fault current will still decay in an oscillating manner, gradually bringing the fault flow to a stable stage.

[0103] To describe the DC distribution network short-circuit fault characteristics of the energy storage module, based on its initial state, the fault response process of the module is divided into a multi-branch power supply stage and a fault power flow stabilization stage.

[0104] Figure 5 This is the equivalent circuit of the photovoltaic module after a short-circuit fault, in which the energy storage unit is connected to the low-voltage DC bus via a Buck / Boost converter.

[0105] The initial state of a fault in an energy storage module is similar to that of a photovoltaic module. Therefore, the initial conditions for a fault in an energy storage module after a short circuit in the DC line are as follows:

[0106]

[0107] Among them, E bat I bat I represents the output voltage and current of the energy storage battery's equivalent capacitance. osg Output current to the energy storage module.

[0108] In the multi-branch power supply stage of the energy storage module, after a fault occurs, the fault current output by the energy storage module is generated by the energy storage equivalent capacitor C. s Discharge current i cs DC line voltage regulator capacitor C bs The discharge current i bs Composition. i fs (t) can be represented as:

[0109] i fs (t)=i cs (t)+i bs (t)

[0110] For each of the components i fp Solve for the two current components of (t), with u cs u cbs i cs i fs As state variables, the state equations can be derived based on the equivalent circuit after the fault:

[0111]

[0112] Solving the state equations yields the following:

[0113]

[0114] Solving the differential equation yields the transient time-domain solution expression for the fault characteristics:

[0115]

[0116]

[0117] in:

[0118]

[0119] According to the state equation, i can be derived cs Transient time-domain solution:

[0120]

[0121] The transient fault current of the energy storage module can be obtained. The time interval of the multi-branch feeding stage is t.s0 -t s1 The timeframe starts from the moment the fault occurs and ends when the power flow output by the photovoltaic module remains stable. s1 After a certain time, the inductive and capacitive energy storage elements in the energy storage module continue to discharge and enter the fault power flow stabilization stage. According to the capacitor discharge time constant τ=RC, the multi-branch power supply stage of the energy storage module lasts longer than that of the photovoltaic module.

Claims

1. A method for calculating the short-circuit fault characteristics of a flexible DC distribution network containing distributed power sources, characterized in that, include: Step 1: Establish an equivalent circuit model for a short-circuit fault between the two poles of the low-voltage side DC distribution network; Step 2: Determine the fault stages after a short circuit occurs between the two poles of the DAB low-voltage side module, distributed photovoltaic module, and energy storage module; Step 3: Based on circuit theorems and the divided fault stages, calculate the short-circuit current and output voltage of each module at the fault point; The initial stage of each module after a bipolar short-circuit fault occurs in the DC distribution network is determined. The fault stages of each module after a bipolar short-circuit fault occurs in the DC distribution network are divided. Finally, the fault current and voltage values ​​of each distributed power source are determined. The fault stages are divided according to the occurrence of a bipolar short-circuit fault in each module of the DC distribution network, including: The fault response process of the DAB module is divided into the low-voltage side capacitor discharge stage and the bridge arm freewheeling stage. The photovoltaic module fault response process is divided into three stages: capacitor discharge stage, multi-branch power supply stage, and fault power flow stabilization stage. The fault response process of the energy storage module is divided into a multi-branch power supply stage and a fault power flow stabilization stage. Fault current during the low-voltage side capacitor discharge phase of the DAB module ,Voltage The calculation and solution for the fault current and voltage are as follows: ， ， ， ， Where u CL It is the voltage of the DAB low-voltage side support capacitor, R bd1 L bd1 These are the equivalent resistance and inductance of the low-voltage side distribution lines, respectively; C L For the low-voltage side DC bus capacitor of the DAB converter; L c1 U is the equivalent inductance of a high-voltage power distribution line; t is time; U dc This refers to the DC line voltage. Fault current during the freewheeling phase of the DAB module Solving for the fault current formula, we get: ， Among them, I td1 For line inductance L bd1 The initial current value of the freewheeling current; The fault current formula for solving the fault current during the capacitor discharge phase of a photovoltaic module is as follows: ， ， ， ， Among them, i fp It is the fault current output by the photovoltaic module, i cb It is the discharge current of the DC line voltage regulator capacitor, u cb It is the voltage of the DC line voltage regulator capacitor, i pv It is the output current of the photovoltaic power source, R bdp L bdp These are the equivalent resistance and inductance of the power distribution lines near the photovoltaic module, respectively; C b For DC line voltage regulation capacitor; Fault current during multi-branch feeding stage of photovoltaic modules Solving for the fault current formula, we get: ， ， Among them, i cp It is the discharge current of the capacitor in the photovoltaic module's Boost converter; C pv For photovoltaic power supply voltage regulation capacitor; i pv L is the output current of the photovoltaic power source. b Inductance in a power electronic converter; Fault current during the multi-branch power supply stage of the energy storage module Solving for the fault current formula, we get: ， ， ， Among them, i bs It is the discharge current of the capacitor in the Buck / Boost converter of the energy storage module, u cbs It is the voltage of the DC line voltage regulator capacitor, R. bds L bds These are the equivalent resistance and inductance of the power distribution lines near the photovoltaic module, respectively. C bs This is a voltage stabilizing capacitor for DC lines.

2. The method according to claim 1, characterized in that, The DC distribution network model uses a 10kV high-voltage AC voltage level, which is converted into ±10kV high-voltage DC distribution lines through a two-level voltage source converter (VSC). This enables bidirectional power flow while dealing with grid disturbances. The low-voltage DC distribution network has a voltage level of ±375V and is interconnected through a dual active bridge (DAB) DC transformer. The photovoltaic power generation system and the energy storage power generation system are connected to the low-voltage DC bus through Buck / Boost converters and Boost converters, respectively, forming a hybrid AC / DC distribution system.

3. The method according to claim 1, characterized in that, To determine the initial voltage and current of the capacitor in the DAB module after a bipolar short-circuit fault occurs in the DC distribution network, according to Kirchhoff's voltage law, we obtain: ， Where i1-i4 are the branch currents in the DAB low-voltage side equivalent circuit, and u1-u3 are the steady-state operating voltages of each capacitor, approximately u1=u2=u3=U dc We can obtain: ； Among them, R bd R bs R bp R is the equivalent resistance of the low-voltage side distribution line; load It is an equivalent DC load.

4. The method according to claim 1, characterized in that, Solve for the initial voltage values ​​of the capacitor and the initial current values ​​of the line inductor in the Boost converter after a bipolar short-circuit fault occurs in the photovoltaic module in the DC distribution network. The initial fault conditions of the photovoltaic module at time t0 when the fault begins are: ， Among them, U oc It is the open-circuit output voltage of the photovoltaic unit, I opv It is the output current of the photovoltaic module; I pv The output current is the photovoltaic equivalent current source; i d This refers to the diode current flowing through the photovoltaic-side Boost converter during steady-state operation of the system.

5. The method according to claim 3, characterized in that, Solve for the initial voltage values ​​of the capacitors and the initial current values ​​of the line inductors in the Buck / Boost converter after a bipolar short-circuit fault occurs in the energy storage module in the DC distribution network. The initial fault conditions of the energy storage module at time t0 when the fault begins are: ， Among them, E bat I bat I represents the output voltage and current of the energy storage battery's equivalent capacitance. osg Output current to the energy storage module; i cs The energy storage equivalent capacitance C s Discharge current; i fs The fault current is output by the energy storage module after a fault occurs.

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