CN109617109B - Method for analyzing direct-current disconnection fault of multi-terminal flexible direct-current power distribution system - Google Patents

Method for analyzing direct-current disconnection fault of multi-terminal flexible direct-current power distribution system Download PDF

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CN109617109B
CN109617109B CN201811518996.XA CN201811518996A CN109617109B CN 109617109 B CN109617109 B CN 109617109B CN 201811518996 A CN201811518996 A CN 201811518996A CN 109617109 B CN109617109 B CN 109617109B
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贾科
冯涛
赵其娟
毕天姝
王聪博
陈金锋
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North China Electric Power University
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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Abstract

The invention discloses a method for analyzing a direct-current disconnection fault of a multi-terminal flexible direct-current power distribution system, which comprises the following steps of: step 1: analyzing the difference of power flow transfer in the open-loop and closed-loop operation modes after a disconnection fault occurs aiming at the open-loop and closed-loop operation modes of the multi-end flexible direct-current power distribution system; step 2: analyzing the change of the energy state of the converters at two ends according to the difference of power flow transfer in an open-loop and closed-loop operation mode after the occurrence of the disconnection fault and the control method of the converters at the rectification side and the inversion side; and step 3: and establishing a model for analyzing a disconnection fault mechanism of the multi-end flexible direct-current power distribution system.

Description

Method for analyzing direct-current disconnection fault of multi-terminal flexible direct-current power distribution system
Technical Field
The invention relates to the technical field of control and protection of a flexible direct current power distribution system, in particular to a method for analyzing a direct current disconnection fault of a multi-terminal flexible direct current power distribution system.
Background
With the rise of new energy power generation and the development of direct current loads of electric vehicles and the like, the research of flexible direct current power distribution systems is drawing extensive attention at home and abroad. Compared with an alternating current distribution system, the direct current distribution system has outstanding advantages in the aspect of adapting to a distributed power supply and a direct current load, has flexible control, large transmission capacity, excellent electric energy quality and low current conversion loss, and is one of the development trends in the future distribution field. The types of dc system faults are mainly three: bipolar short circuit faults, unipolar grounded short circuit faults and unipolar broken line faults. Because short-circuit faults have strong short-circuit impact current, the research is relatively deep, and single-pole disconnection faults are difficult to research on disconnection protection of a direct-current system because of lack of obvious fault characteristics.
Therefore, it is desirable to have a method for analyzing a dc disconnection fault of a multi-terminal flexible dc power distribution system to solve the problems in the prior art.
Disclosure of Invention
The invention discloses a method for analyzing a direct-current disconnection fault of a multi-terminal flexible direct-current power distribution system, which comprises the following steps of:
step 1: analyzing the difference of power flow transfer in the open-loop and closed-loop operation modes after a disconnection fault occurs aiming at the open-loop and closed-loop operation modes of the multi-end flexible direct-current power distribution system;
step 2: analyzing the change of the energy state of the converters at two ends according to the difference of power flow transfer in an open-loop and closed-loop operation mode after the occurrence of the disconnection fault and the control method of the converters at the rectification side and the inversion side;
and step 3: and establishing a model for analyzing a disconnection fault mechanism of the multi-end flexible direct-current power distribution system.
Preferably, when the multi-end flexible direct-current power distribution system operates in an open loop mode in the step 1, the power flow direction is single, that is, power flows from the rectifying end to the inverting end through the direct-current bus, after a single-pole disconnection fault occurs, power transmission at two ends is terminated, the power flow direction keeps the output power of the rectifying end and transmits the output power to a direct-current load, the inverting side absorbs power, and the distributed photovoltaic power supply outputs power to the inverting side.
Preferably, when the multi-end flexible direct-current power distribution system operates in a closed loop in the step 1, a path for power flow transfer has diversity, and after a single-pole disconnection fault occurs, transmission power at two ends of the rectifying end and the inverting end is not terminated.
Preferably, in step 2, the control method of the rectifier side converter comprises: the rectifier side converter is controlled by constant direct current voltage; the inverter side converter control method comprises the following steps: the inverter side converter is controlled by constant active power.
Preferably, the step of analyzing the change of the energy state of the two-terminal converter in the step 2 comprises:
when a single-pole disconnection fault occurs in an open-loop operation mode of a multi-terminal flexible direct-current power distribution system, the internal energy of a rectification side converter Modular Multilevel Converter (MMC) controlled by constant direct-current voltage is not changed (delta P)MMC10), the submodule dc capacitor voltage does not change (Δ U)c10), the voltage at two ends of the direct current load is stable, and the internal energy change (delta P) of the inverter side converter Modular Multilevel Converter (MMC) controlled by constant active power is adoptedMMC2Not equal to 0), the direct current capacitor of the submodule continues to charge, and the voltage rises (delta U)c2>0) Interpolar electricityIncreasing the pressure;
when a single-pole disconnection fault occurs in a closed-loop operation mode of the multi-end flexible direct-current power distribution system, because the direction of the power flow is transferred after the disconnection fault occurs, the internal energy states of the rectification side converter controlled by adopting the constant direct-current voltage and the inversion side converter controlled by adopting the constant active power are not changed.
Preferably, the step 3 is a specific step of establishing a model for analyzing a disconnection fault mechanism of the multi-terminal flexible direct-current power distribution system:
when the multi-end flexible direct current power distribution system operates in an open loop mode, the broken line fault current is as follows:
Figure BDA0001902742500000021
wherein, U2、U1Respectively a virtual electromotive force direct current component at the rectifier valve side and a virtual electromotive force direct current component at the inverter valve side, RgIs a ground resistor, Uc1And Uc2Respectively obtaining a rectification side converter submodule capacitor voltage and an inversion side converter submodule capacitor voltage, wherein N is the number of the submodules thrown into each phase of upper and lower bridge arms of the converter at any moment;
obtaining DC bias voltage delta U from broken line fault current, i.e. fault loopa1For the relationship between the direct-current side voltage of the rectifier side valve and the alternating-current feed-in and the internal energy change of the converter, the relationship between the positive and negative electrode voltages and the bridge arm voltage of the converter is as follows:
Figure BDA0001902742500000031
wherein, Delta Ua1For rectifying side A-phase fault DC voltage component, ua1For rectifying side A phase voltage, ua1(0) For A-phase voltage transients, u, before a fault occurspa1、una1The upper bridge arm voltage and the lower bridge arm voltage i of the A phase at the rectification side respectivelypa1、ina1Respectively an upper bridge arm current and a lower bridge arm current of a phase at a rectification side, L is a bridge arm inductance, p represents a positive electrode or an upper bridge arm, n represents a negative electrode orA lower bridge arm;
For the inverter side, the calculation formula of the bridge arm voltage and the capacitor voltage is as follows:
Figure BDA0001902742500000032
wherein u ispa2、una2The voltage of an upper bridge arm and the voltage of a lower bridge arm on an inversion side A phase are respectively Udc2For inverting the inter-electrode voltage u of the side DC busa2For inverting side phase A voltage, ua2(0) For instantaneous value of phase voltage of phase A before occurrence of fault, Uc2(0) For inverting side before fault to change initial value of DC capacitor voltage, delta Uc2Is the capacitor voltage increment after the fault;
according to the energy relation, the following steps are carried out:
Figure BDA0001902742500000033
wherein, Δ WεAdditional energy, Δ P, for charging the DC capacitor after a faultMMC2Is the extra power of the inverter side converter, CeqThe direct current bus voltage is finally up to a stable voltage which is 2 times of the amplitude of the phase voltage on the valve alternating current side;
when a multi-end flexible direct current power distribution system operates in a closed loop mode, energy transfer of alternating current systems at two ends of a rectifying end and an inverting end is not stopped, the energy state of a Modular Multilevel Converter (MMC) of the MMC at the two ends is not changed, the change trend of fault current of a fault electrode and a non-fault electrode is reversed, two processes before and after the fault are analyzed by using a superposition theorem, a current conversion end controlled by fixed direct current voltage is equivalent to a voltage source, an active power current conversion end and a photovoltaic current conversion end are equivalent to a current source, and the current of positive and negative circuits before the fault is:
Figure BDA0001902742500000041
Wherein, Uip-n(0)、Ujp-n(0) Respectively represent the positive and negative electrode line voltage before i end fault, the positive and negative electrode line voltage before j end fault, ZkIs the impedance corresponding to the ij line;
after the fault, according to the voltage division principle, the DC load impedance is greater than the line impedance, all non-fault polar line currents are unchanged, and a fault current component is superposed on the fault polar line current. The current at the fault point is:
If＝I12p(0)
wherein, I12p(0) The current flowing to the inversion side from the rectification side of the positive line before the fault is generated.
The invention discloses a method for analyzing a direct-current disconnection fault of a multi-terminal flexible direct-current power distribution system, which has the following beneficial effects:
(1) the direct current disconnection fault characteristics of the flexible direct current power distribution system in open-loop and closed-loop operation modes are qualitatively analyzed, and the change process of a converter and line current after the fault is clear;
(2) a model for analyzing the disconnection fault is established, and the research on the direct-current disconnection protection of the flexible direct-current power distribution system is facilitated through the analysis of the fault characteristics.
Drawings
Fig. 1 is a flowchart of an analysis method for a dc disconnection fault of a multi-terminal flexible dc power distribution system according to the present invention.
FIG. 2 is a schematic diagram of a four-terminal flexible DC power distribution system topology;
FIG. 3 is a schematic diagram of a fault loop in an open loop mode of operation;
FIG. 4 is a schematic diagram of a fault-front-and-back equivalent circuit in a closed-loop operating mode;
FIG. 5 is a schematic diagram comparing rectifier side and inverter side converter sub-module capacitor voltage waveforms in open and closed loop modes of operation;
fig. 6 is a line current schematic in a closed loop mode of operation.
Detailed Description
In order to make the implementation objects, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be described in more detail below with reference to the accompanying drawings in the embodiments of the present invention. In the drawings, the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The described embodiments are only some, but not all embodiments of the invention. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
As shown in fig. 1, the present invention provides a method for analyzing a dc disconnection fault of a multi-terminal flexible dc power distribution system, which includes the following steps:
Step 1, analyzing differences of power flow transfer in an open-loop operation mode and a closed-loop operation mode of a multi-end flexible direct-current power distribution system after a disconnection fault occurs;
step 2, based on the difference in the step 1, analyzing the change of the energy states of the converters at two ends aiming at the difference of the control strategies of the converter at the rectifying side and the converter at the inverting side;
and 3, establishing a model for analyzing the disconnection fault mechanism of the multi-terminal flexible direct-current power distribution system based on the steps 1 and 2.
Figure 2 is a schematic diagram of a four-terminal flexible dc power distribution system topology. The rated voltage grade of a direct current line is +/-10 kV, alternating current systems on two sides are connected with the direct current line through a clamping double-submodule-based modular multi-level converter (CDSM-MMC), a centralized photovoltaic system is merged into a medium-voltage direct current distribution system through a boost type DC/DC converter, a rectification side converter of the system is controlled by constant direct current voltage, and an inversion side converter is controlled by constant active power. If the open-loop operation mode is adopted, the disconnecting switches SW43 and SW34 at the head end and the tail end of the Line4 (in a dotted Line in the figure) are disconnected, and the fault is that the point of the positive Line of the Line1 is disconnected.
Fig. 3 is a schematic diagram of a fault loop in an open loop mode of operation. Because the sub-module capacitor voltage of the inverter side converter CDSM-MMC2 is changed and the sub-module capacitor voltage of the rectifier side converter CDSM-MMC1 is unchanged, the potential difference of the neutral point of the alternating current system to the ground voltage is no longer 0, and a fault loop is formed between the potential difference and the ground. The fault current is calculated as follows:
Figure BDA0001902742500000061
Wherein, U2、U1Respectively a virtual electromotive force direct current component at the rectifier valve side and a virtual electromotive force direct current component at the inverter valve side, RgTo ground resistance, Uc1And Uc2Respectively obtaining a rectification side converter submodule capacitor voltage and an inversion side converter submodule capacitor voltage, wherein N is the number of the submodules thrown into each phase of upper and lower bridge arms of the converter at any moment;
line1 fault pole Line current is 0 and non-fault pole Line current is IfThe sum of the positive and negative electrode currents of the out-of-range line is If。
Fig. 4 is a schematic diagram of the equivalent current of the system before and after a fault in a closed-loop operation mode. At least one voltage node is needed to be used as a support in the multi-terminal direct current power distribution system, therefore, a rectification side converter controlled by constant direct current voltage is equivalent to a direct current voltage source, and because the inversion side converter is controlled by constant active power, the current of the inversion side is constant according to the relation between power and voltage current, so that the inversion side converter is equivalent to a direct current source, and a photovoltaic terminal is equivalent to a direct current source due to constant power. According to the superposition principle, after a fault, a voltage source is short-circuited, a current source is open-circuited, the direct current load impedance is far larger than the line impedance, and the direct current load current shunt is far smaller than the line shunt connected with the direct current load in parallel, so that the structure in a dashed frame can be ignored, and the fault current only forms a loop between the anode lines. The line current before and after the fault can be calculated as:
Figure BDA0001902742500000062
Wherein, I12p(0) The current flowing to the inversion side from the rectification side of the positive Line of Line1 before the fault is rectified.
Therefore, after the fault, all negative electrode Line currents are kept unchanged, the Line1 fault electrode Line is 0, and the other Line positive electrode currents are superposed with a fault current Ibf. The positive and negative currents are no longer balanced.
Fig. 5 is a comparison of rectifier side and inverter side converter sub-module capacitor voltage waveforms for open and closed loop modes of operation. It can be seen from the change of the sub-module capacitor voltage of the open-loop rectification side converter and the inversion side converter that the rectification side direct current capacitor is not charged or discharged and is in a stable state, and the inversion side direct current capacitor continues to be charged under the action of the difference power. According to the change of the capacitor voltage of the sub-modules of the converters on the closed-loop rectification side and the inversion side, the internal energy state of the converters at two ends of the closed-loop system is unchanged, the direct current capacitors are in a stable state, and the voltages of the positive electrode and the negative electrode of all circuits are kept unchanged.
Fig. 6 is a schematic diagram of line current in a closed loop mode of operation. It can be clearly seen from the line current diagram in the closed-loop operation mode that, during closed-loop operation, all positive line currents (fault poles) are superimposed with one fault component, and all negative line currents (non-fault poles) are kept unchanged, so that the correctness of model establishment in fig. 4 is verified.
Finally, it should be pointed out that: the above examples are only intended to illustrate the technical solution of the present invention, and not to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims (5)

1. A method for analyzing a direct current disconnection fault of a multi-terminal flexible direct current power distribution system is characterized by comprising the following steps:
step 1: analyzing the difference of power flow transfer in the open-loop and closed-loop operation modes after a disconnection fault occurs aiming at the open-loop and closed-loop operation modes of the multi-end flexible direct-current power distribution system;
step 2: analyzing the change of the energy state of the converters at two ends according to the difference of power flow transfer in an open-loop and closed-loop operation mode after the occurrence of the disconnection fault and the control method of the converters at the rectification side and the inversion side;
and step 3: establishing a model for analyzing a disconnection fault mechanism of the multi-end flexible direct-current power distribution system;
The step 3 of establishing a model for analyzing the disconnection fault mechanism of the multi-terminal flexible direct-current power distribution system specifically comprises the following steps:
when the multi-end flexible direct current power distribution system operates in an open loop mode, the broken line fault current is as follows:
Figure FDA0003368400540000011
wherein, IfFor line break fault currents, U2、U1Respectively a virtual electromotive force direct current component at the rectifier valve side and a virtual electromotive force direct current component at the inverter valve side, RgTo ground resistance, Uc1And Uc2Respectively obtaining a rectification side converter submodule capacitor voltage and an inversion side converter submodule capacitor voltage, wherein N is the sum of the numbers of the submodules input by an upper bridge arm and a lower bridge arm at any time;
deriving DC-bias voltage DeltaU from line-break fault current, i.e. fault loopa1For the relationship between the direct-current side voltage of the rectifier side valve and the alternating-current feed-in and the internal energy change of the converter, the relationship between the positive and negative electrode voltages and the bridge arm voltage of the converter is as follows:
Figure FDA0003368400540000012
wherein, Delta Ua1For rectifying side A-phase fault DC voltage component, ua1For rectifying side A phase voltage, ua1(0) For instantaneous value of phase voltage of phase A before occurrence of fault, Udcp1、Udcn1Are respectively a straight-flow outletVoltage of the anode and cathode upa1、una1The upper bridge arm voltage and the lower bridge arm voltage i of the A phase at the rectification side respectivelypa1、ina1Respectively an upper bridge arm current and a lower bridge arm current of a phase A at a rectification side, wherein L is a bridge arm inductance, p represents a positive pole or an upper bridge arm, and n represents a negative pole or a lower bridge arm;
For the inverter side, the calculation formula of the bridge arm voltage and the capacitor voltage is as follows:
Figure FDA0003368400540000021
wherein u ispa2、una2Respectively an inversion side A phase upper bridge arm voltage and a lower bridge arm voltage, Udc2For inverting the inter-electrode voltage u of the side DC busa2For inverting side A phase voltage, ua2(0) For instantaneous value of phase voltage of phase A before occurrence of fault, Uc2(0) For inverting side before fault to change initial value of DC capacitor voltage, delta Uc2Is the capacitor voltage increment after the fault;
according to the energy relation, the following steps are carried out:
Figure FDA0003368400540000022
wherein, Δ WεAdditional energy, Δ P, for charging the DC capacitor after a faultMMC2Is the extra power of the inverter side converter, CeqThe direct current bus voltage is finally up to a stable voltage, namely 2 times of the amplitude of the phase voltage on the valve alternating current side;
when the multi-end flexible direct current power distribution system operates in a closed loop mode, energy transfer of alternating current systems at two ends of a rectifying end and an inverting end is not stopped, the energy state of a modular multilevel converter of the converter at the two ends is not changed, the change trend of fault pole and non-fault pole fault current is reversed, two processes before and after a fault are analyzed by using a superposition theorem, the converter end controlled by fixed direct current voltage is equivalent to a voltage source, the active power converter end and the photovoltaic converter end are equivalent to a current source, and the positive and negative circuit currents before the fault are:
Figure FDA0003368400540000031
Wherein, Uip(0)、Uin(0) Respectively representing the voltages of positive and negative electrode lines before the i-end fault; u shapejp(0)、Ujn(0) The positive and negative electrode line voltages before the j end fault are respectively; z is a linear or branched memberkIs the impedance corresponding to the ij line;
according to the partial pressure principle after the trouble, direct current load impedance is greater than line impedance, and all non-trouble polar line currents do not change, and trouble utmost point line current stack fault current component, the electric current of fault point is:
If＝I12p(0)
wherein, I12p(0) The current flowing to the inversion side from the rectification side of the positive line before the fault.
2. The method for analyzing the direct-current disconnection fault of the multi-terminal flexible direct-current power distribution system according to claim 1, wherein the method comprises the following steps: when the multi-end flexible direct-current power distribution system operates in an open loop mode in the step 1, the power flow direction is single, namely, power flows to the inversion end from the rectification end through the direct-current bus, after a single-pole disconnection fault occurs, power transmission at two ends is stopped, the power flow direction keeps the output power of the rectification end and transmits the output power to a direct-current load, the inversion side absorbs power, and the distributed photovoltaic power supply outputs power to the inversion side.
3. The method for analyzing the direct-current disconnection fault of the multi-terminal flexible direct-current power distribution system according to claim 1, wherein the method comprises the following steps: when the multi-end flexible direct-current power distribution system operates in a closed loop mode in the step 1, paths for power flow transfer have diversity, and after a single-pole disconnection fault occurs, transmission power at two ends of the rectifying end and the inverting end is not stopped.
4. The method for analyzing the direct-current disconnection fault of the multi-terminal flexible direct-current power distribution system according to claim 1, wherein the method comprises the following steps: the control method of the rectifier side converter in the step 2 comprises the following steps: the rectifier side converter is controlled by constant direct current voltage; the inverter side converter control method comprises the following steps: the inverter side converter adopts constant active power control.
5. The method for analyzing the direct current disconnection fault of the multi-terminal flexible direct current power distribution system according to claim 4, wherein the method comprises the following steps: the step of analyzing the change of the energy state of the converters at two ends in the step 2 comprises the following steps:
when a single-pole disconnection fault occurs in an open-loop operation mode of the multi-end flexible direct-current power distribution system, the internal energy of the rectification side converter modular multilevel converter controlled by constant direct-current voltage is not changed, the direct-current capacitor voltage of the submodule is not changed, and the voltages at two ends of a direct-current load are maintained stable; the internal energy of the inverter side converter modular multilevel converter controlled by constant active power is changed, the direct current capacitor of the submodule continues to charge, and the voltage is increased by delta Uc2>0, the interelectrode voltage increases;
when a single-pole disconnection fault occurs in a closed-loop operation mode of the multi-end flexible direct-current power distribution system, the internal energy state of the rectification side converter controlled by constant direct-current voltage and the internal energy state of the inversion side converter controlled by constant active power are not changed.
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CN104820158A (en) * 2015-04-30 2015-08-05 国家电网公司 Direct-current broken-line fault determination method of flexible direct-current power transmission system

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