CN113949048A - Direct-current microgrid fault current controller and control method thereof - Google Patents

Direct-current microgrid fault current controller and control method thereof Download PDF

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Publication number
CN113949048A
CN113949048A CN202111210770.5A CN202111210770A CN113949048A CN 113949048 A CN113949048 A CN 113949048A CN 202111210770 A CN202111210770 A CN 202111210770A CN 113949048 A CN113949048 A CN 113949048A
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China
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voltage
fault current
current controller
duty ratio
switching tube
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CN202111210770.5A
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高滢
曾杰
陈锐
张弛
黄辉
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China Southern Power Grid Power Technology Co Ltd
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China Southern Power Grid Power Technology Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H3/00—Emergency protective circuit arrangements for automatic disconnection directly responsive to an undesired change from normal electric working condition with or without subsequent reconnection ; integrated protection
    • H02H3/02—Details
    • H02H3/06—Details with automatic reconnection
    • H02H3/066—Reconnection being a consequence of eliminating the fault which caused disconnection

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Abstract

The application discloses direct current microgrid fault current controller and control method thereof, the controller includes: the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor; the collector of the first switching tube is connected to the anode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the cathode of the energy storage element; the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube. The interpolar voltage is rated voltage during normal operation, the current change is small, the control duty ratio is a first duty ratio, and the constant voltage control is performed at the moment. And when the interelectrode voltage is smaller than a preset threshold value, judging that a fault occurs, switching the duty ratio to a second duty ratio, and changing the duty ratio to constant current control. After the fault is cleared, the inter-pole voltage rises, the duty ratio is switched to the first duty ratio, and the constant-voltage control mode is recovered. The technical problems of poor current limiting effect and poor controllability in the prior art are solved.

Description

Direct-current microgrid fault current controller and control method thereof
Technical Field
The application relates to the technical field of electric power, in particular to a fault current controller of a direct-current micro-grid and a control method of the fault current controller.
Background
Because the microgrid coverage area is small, the power supply line is short, the line impedance is small, and under the combined action of a plurality of distributed power supplies, the rising speed of the short-circuit current of the direct-current microgrid is high, the amplitude is large, and serious impact can be caused to the system. In order to ensure the safe operation of the system, a simple and cheap overcurrent suppression method should be considered as far as possible to ensure the safe operation of the equipment.
The fault current suppression method applied in the current engineering mainly comprises the steps of configuring a current-limiting reactor, and limiting the rising speed of fault current by using the characteristic that the inductive current cannot suddenly change. Further, some researchers have proposed an overcurrent suppression method based on active control, which utilizes a control technique to change the on-off strategy of a device during a period from the occurrence of a fault to the fault isolation or before the blocking of the power electronic device of the converter, thereby realizing overcurrent suppression. However, this method has a limited current suppressing effect and can be used only as an auxiliary means.
Disclosure of Invention
The application provides a direct-current microgrid fault current controller and a control method thereof, which are used for solving the technical problems of poor current limiting effect and poor controllability in the prior art.
In view of this, the first aspect of the present application provides a dc microgrid fault current controller, where the controller includes:
the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor;
the collector of the first switching tube is connected to the positive electrode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the negative electrode of the energy storage element;
the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube.
Optionally, the method further comprises: an inductance;
the first end of the inductor is connected to the first end of the capacitor, and the second end of the inductor is connected between the first switch tube and the second switch tube.
Optionally, the first end of the inductor and the first end of the capacitor are both connected to a voltage source converter, so that the fault current controller is connected to the dc microgrid through the voltage source converter.
Optionally, the relation between the average value of the output voltage of the fault current controller and the average value of the output voltage of the direct current side of the voltage source type converter is as follows:
U2＝Ud-U1；
in the formula of U2Is a DC bus voltage, UdIs the average value of the output voltage of the DC side of the voltage source type converter, U1Is the average value of the output voltage of the fault current controller.
Optionally, the output voltage of the fault current controller is less than the dc side output voltage of the voltage source converter.
Optionally, the first switching tube and the second switching tube are both IGBTs.
A second aspect of the present application provides a control method for a dc microgrid fault current controller, which is applied to the dc microgrid fault current controller of the first aspect, and the method includes:
DC bus voltage U2Monitoring the size of the sample;
when the DC bus voltage U2When the voltage is the rated voltage, controlling the duty ratio to be a first duty ratio so that the fault current controller is in a constant voltage control mode;
when the DC bus voltage U2When the current is smaller than the preset threshold value, the duty ratio is switched to a second duty ratio, so that the fault current controller is in a constant current control mode until the direct current bus voltage U is lower than the preset threshold value2Switching the duty cycle to the first duty cycle for a nominal voltage.
According to the technical scheme, the method has the following advantages:
the application provides a direct current microgrid fault current controller includes: the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor; the collector of the first switching tube is connected to the anode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the cathode of the energy storage element; the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube. When the fault current controller operates normally, the interelectrode voltage (direct current bus voltage) is rated voltage, the current change is small, and the control duty ratio is a first duty ratio which is constant voltage control. And when the interelectrode voltage is smaller than a preset threshold value, judging that a fault occurs, switching the duty ratio to a second duty ratio, and changing the duty ratio to constant current control. After the fault is cleared, the interelectrode voltage rises, the duty ratio is switched to the first duty ratio, and the constant-voltage control mode is resumed.
The fault current controller can realize accurate control of the magnitude of the fault current and has strong controllability; the magnitude of the fault current can be greatly reduced; when the power grid normally operates, the output voltage can be further stabilized; after the fault disappears, the normal operation state can be automatically recovered, so that the technical problems of poor current limiting effect and poor controllability in the prior art are solved.
Drawings
Fig. 1 is a schematic structural diagram of a dc microgrid fault current controller provided in an embodiment of the present application;
fig. 2 is a schematic structural diagram of a dc microgrid fault current controller connected to a dc microgrid through a voltage source converter according to an embodiment of the present application;
fig. 3 is a schematic flowchart of a control method of a dc microgrid fault current controller provided in an embodiment of the present application;
FIG. 4 is an equivalent circuit of the DC side of the AC distribution network system connected to the voltage source type converter;
FIG. 5 is a schematic diagram of a fault current controller;
fig. 6 is a control block diagram of a dc microgrid fault current controller according to the present application;
fig. 7 is a simplified direct-current microgrid structure diagram for simulation;
FIG. 8 is a schematic of DC voltage waveforms before and after a fault;
fig. 9 is a schematic diagram of dc current waveforms before and after a fault.
Detailed Description
In order to make the technical solutions of the present application better understood, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is obvious that the described embodiments are only a part of the embodiments of the present application, and not all of the embodiments. 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 application.
Referring to fig. 1, a dc microgrid fault current controller provided in an embodiment of the present application includes: the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor;
the collector of the first switching tube is connected to the anode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the cathode of the energy storage element; the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube.
It should be noted that, the first switching tube and the second switching tube of this embodiment are both IGBTs, and a person skilled in the art can select the switching tube according to actual situations except the IGBTs, which is not limited herein.
In an optional embodiment, the dc microgrid fault current controller of the present application further includes: an inductance; the first end of the inductor is connected to the first end of the capacitor, and the second end of the inductor is connected between the first switch tube and the second switch tube.
It should be noted that the inductor of the present embodiment can play a role in reducing dc current ripple.
In a specific embodiment, the first end of the inductor and the first end of the capacitor are both connected to the voltage source type converter, so that the fault current controller is connected to the dc microgrid through the voltage source type converter.
It should be noted that, a manner of accessing the fault current controller to the ac power distribution network is shown in fig. 2, specifically, the first end of the inductor and the first end of the capacitor are both connected to the voltage source type converter, so that the fault current controller is accessed to the dc microgrid through the voltage source type converter.
The embodiment provides a dc microgrid fault current controller, including: the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor; the collector of the first switching tube is connected to the anode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the cathode of the energy storage element; the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube. When the fault current controller operates normally, the interelectrode voltage (direct current bus voltage) is rated voltage, the current change is small, and the control duty ratio is a first duty ratio which is constant voltage control. And when the interelectrode voltage is smaller than a preset threshold value, judging that a fault occurs, switching the duty ratio to a second duty ratio, and changing the duty ratio to constant current control. After the fault is cleared, the interelectrode voltage rises, the duty ratio is switched to the first duty ratio, and the constant-voltage control mode is resumed.
The technical principle of the direct-current microgrid fault current controller is as follows:
the direct-current microgrid comprises various direct-current power supplies, some direct-current power supplies are characterized by voltage sources, such as alternating-current distribution network systems and energy storage systems which are connected based on a voltage source type converter, and some direct-current power supplies are characterized by current sources, such as photovoltaic systems. The fault current of the direct-current microgrid is mainly provided by a power supply end, wherein the fault current provided by the power supply with the characteristic of a voltage source is used as a main fault current. Taking an ac distribution network system accessed based on a voltage source type converter as an example, on the dc side, the ac distribution network can be equivalent to an adjustable dc voltage source, as shown in fig. 4 below. When an inter-pole fault occurs, the fault current provided by the ac distribution network can be represented by the following equation:
i＝ud/(Z+Rf)
in the formula udFor the equivalent voltage on the DC side of the AC distribution network, Z is the impedance between the fault point and the voltage source converter, RfIs a fault transition resistance. It can be seen that the fault current depends mainly on the equivalent voltage udThe size of (2).
According to the principle of the voltage source type converter, the average voltage u on the DC sidedAmplitude U of voltage of alternating-current side voltage lineacSatisfies the following relation:
Ud＝Uac/m
wherein m is a modulation ratio and 0< m < 1. Therefore, the voltage regulating range of the direct-current side of the voltage source type converter is up to the amplitude of the alternating-current distribution network line voltage. Therefore, the fault current cannot be effectively controlled by adjusting the output voltage of the dc side of the voltage source converter.
If a reverse controllable voltage source, i.e. a negative pressure source u, is connected in series at the outlet of the voltage source type inverter1As shown in fig. 5. When the microgrid runs normally, the output voltage of the controllable voltage source is controlled to be 0, so that the normal running of the microgrid can not be influenced; when the interelectrode fault occurs, the fault current supplied from the AC distribution network is changed as follows
i＝(ud-u1)/(Z+Rf)
Therefore, the output voltage of the controllable voltage source u1 at the time of the fault is controlled, namely the magnitude of the fault current can be controlled arbitrarily.
Therefore, the inventors propose a fault current controller of the present application:
during normal operation, the controllable voltage source u1The output voltage of the grid voltage regulator is close to 0 so as to reduce the influence of a series voltage source on the normal operation of the direct-current micro-grid; in case of failure, the controllable voltage source u1Should be increased appropriately. When u is1＝udWhen the fault current provided by the alternating current power grid to the fault point is reduced to 0. Thus, the controllable voltage source u1Should be between 0 and uContinuously adjustable (u)<ud). Controllable voltage source u taking into account the power bidirectional flow characteristics of the microgrid1But also has the capability of bi-directional current flow.
As shown in FIG. 1, wherein EbIs the terminal voltage of the energy storage element. When the direct-current microgrid operates normally, the switch tube S1 is normally open, the switch tube S2 is normally closed, and the controllable voltage source u1The output voltage of the inductor is about the conduction voltage drop of the switching tube, the influence on normal operation is small, and the inductor L can also play a role in reducing direct current ripples. When the inter-electrode fault of the direct-current microgrid occurs, the conduction duty ratio of the switch tube S1 and the switch tube S2 is controlled, and the output voltage u can be adjusted1And thus arbitrarily control the magnitude of the fault current.
After the voltage source type converter is connected to an alternating current distribution network system of a direct current microgrid and the fault current controller shown in fig. 1 is connected in series, the circuit structure is shown in fig. 2. The average value of the output voltage on the DC side of a VSC (Voltage Source converter) is udThe mean value of the output voltage of the fault current controller is U1The DC bus voltage is U2The three satisfy the following relational expression
U2＝Ud-U1；
In the formula of U2Is a DC bus voltage, UdIs the average value of the output voltage on the DC side of the voltage source type converter, U1Is the average value of the output voltage of the fault current controller.
The above is a dc microgrid fault current controller provided in the embodiment of the present application, and the following is a control method of a dc microgrid fault current controller provided in the embodiment of the present application.
Referring to fig. 3, in an embodiment of the present application, a method for controlling a dc microgrid fault current controller includes:
step 101, comparing the DC bus voltage U2Is monitored.
Step 102, when the direct current bus voltage U is obtained2And when the voltage is the rated voltage, controlling the duty ratio to be a first duty ratio so that the fault current controller is in a constant voltage control mode.
Step (ii) of103. When the DC bus voltage U2When the current is smaller than the preset threshold value, the duty ratio is switched to a second duty ratio, so that the fault current controller is in a constant current control mode until the direct current bus voltage U is reached2The duty cycle is switched to a first duty cycle for the nominal voltage.
As shown in fig. 2, when the inter-electrode fault occurs on the dc side, the inter-electrode voltage (dc bus voltage U) is described2) Decreases rapidly and the current I rises rapidly. Therefore, the interelectrode voltage U2The amplitude of the current and the current rise rate can be used as the basis of fault diagnosis.
The control block diagram is shown in fig. 6, and it can be understood that, in normal operation, the interpolar voltage is a rated voltage, the current change is small, and the duty ratio K is equal to K1(first duty ratio), and this time, constant voltage control. When the interelectrode voltage is less than a preset threshold UsetOr the current rise rate is higher than the set value IsetWhen a failure occurs, the duty ratio K is switched to K2(second duty ratio), change to constant current control. After the fault is cleared, the interelectrode voltage rises, and the duty ratio K is switched to K1(first duty cycle) and then returns to the constant voltage control mode.
The following is a description of simulation experiments provided in the examples of the present application:
in the embodiment of the application, a simplified microgrid structure shown in fig. 6 is used for establishing a simulation model. The detailed circuits of the VSC and the fault current controller are shown in fig. 2. In the simulation, the voltage between DC bus electrodes is set to 750V, R1、R2Are all 2.5 omega. The VSC pulse triggering is started at 0.1s, the interpolar fault f1 occurs at 0.3s, and the fault disappears after 50 ms. The VSC employs constant voltage control. The fault current controller is controlled by a bus voltage at a constant direct current side during normal operation, and is controlled by a constant current under fault, wherein the control target is 0.5 kA.
As shown in fig. 8 and 9, after the pulse trigger starts, the VSC output voltage is adjusted for a while and then stabilized at 750V. Comparing the VSC output voltage and the DC bus voltage, the fault current controller can shorten the process of adjusting the DC bus voltage, so that the DC bus voltage is stabilized near the rated voltage faster than the VSC output voltage. After a fault occurs, the fault current controller is switched to be under constant current control, the VSC output current can be adjusted to be stable to be close to the control target 0.5kA within about 10ms, the maximum value of the short-circuit current is only 1.45 times (0.87kA) of the rated operation current, the duration is extremely short, and the current drops below the rated operation current within 1 ms. After the fault disappears, the fault current controller is switched to be controlled by constant voltage, the bus voltage is adjusted for a period of time and then is recovered to 750V, the load current is recovered immediately, and the direct-current microgrid is recovered to normally operate. As can be seen from the voltage waveforms, during the fault current controller mode switching, there is a short rise of voltage across the VSC of small magnitude, but not exceeding the insulation withstand voltage of the device.
In the fault current controller of the control method of the embodiment, when the fault current controller operates normally, the interpolar voltage (dc bus voltage) is a rated voltage, the current change is small, and the control duty ratio is the first duty ratio, which is the constant voltage control. And when the interelectrode voltage is smaller than a preset threshold value, judging that a fault occurs, switching the duty ratio to a second duty ratio, and changing the duty ratio to constant current control. After the fault is cleared, the interelectrode voltage rises, the duty ratio is switched to the first duty ratio, and the constant-voltage control mode is resumed. According to the control method of the fault current controller of the direct-current microgrid, the fault current can be accurately controlled, and the controllability is strong; the magnitude of the fault current can be greatly reduced; when the power grid normally operates, the output voltage can be further stabilized; after the fault disappears, the normal operation state can be automatically recovered, so that the technical problems of poor current limiting effect and poor controllability in the prior art are solved.
The terms "first," "second," "third," "fourth," and the like in the description of the present application and in the above-described drawings are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the data so used is interchangeable under appropriate circumstances such that the embodiments of the application described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms "comprises," "comprising," and "having," and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, system, article, or apparatus that comprises a list of steps or elements is not necessarily limited to those steps or elements expressly listed, but may include other steps or elements not expressly listed or inherent to such process, method, article, or apparatus.
It should be understood that in the present application, "at least one" means one or more, "a plurality" means two or more. "and/or" for describing an association relationship of associated objects, indicating that there may be three relationships, e.g., "a and/or B" may indicate: only A, only B and both A and B are present, wherein A and B may be singular or plural. The character "/" generally indicates that the former and latter associated objects are in an "or" relationship. "at least one of the following" or similar expressions refer to any combination of these items, including any combination of single item(s) or plural items. For example, at least one (one) of a, b, or c, may represent: a, b, c, "a and b", "a and c", "b and c", or "a and b and c", wherein a, b, c may be single or plural.
In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus and method may be implemented in other manners. For example, the above-described apparatus embodiments are merely illustrative, and for example, the division of the units is only one logical division, and other divisions may be realized in practice, for example, a plurality of units or components may be combined or integrated into another system, or some features may be omitted, or not executed. In addition, the shown or discussed mutual coupling or direct coupling or communication connection may be an indirect coupling or communication connection through some interfaces, devices or units, and may be in an electrical, mechanical or other form.
The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one place, or may be distributed on a plurality of network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of the embodiment.
In addition, functional units in the embodiments of the present application may be integrated into one processing unit, or each unit may exist alone physically, or two or more units are integrated into one unit. The integrated unit can be realized in a form of hardware, and can also be realized in a form of a software functional unit.
The integrated unit, if implemented in the form of a software functional unit and sold or used as a stand-alone product, may be stored in a computer readable storage medium. Based on such understanding, the technical solution of the present application may be substantially implemented or contributed to by the prior art, or all or part of the technical solution may be embodied in a software product, which is stored in a storage medium and includes instructions for causing a computer device (which may be a personal computer, a server, or a network device) to execute all or part of the steps of the method according to the embodiments of the present application. And the aforementioned storage medium includes: a U disk, a removable hard disk, a Read-Only Memory (ROM), a Random Access Memory (RAM), a magnetic disk or an optical disk, and other various media capable of storing program codes.
The above embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should 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 in the embodiments of the present application.

Claims (7)

1. A direct current microgrid fault current controller is characterized by comprising: the energy storage device comprises a first switching tube, a second switching tube, an energy storage element and a capacitor;
the collector of the first switching tube is connected to the positive electrode of the energy storage element, the emitter of the first switching tube is connected to the collector of the second switching tube, and the emitter of the second switching tube is connected to the negative electrode of the energy storage element;
the first end of the capacitor is connected between the first switch tube and the second switch tube, and the second end of the capacitor is connected to the emitter of the second switch tube.
2. The dc microgrid fault current controller of claim 1, further comprising: an inductance;
the first end of the inductor is connected to the first end of the capacitor, and the second end of the inductor is connected between the first switch tube and the second switch tube.
3. The dc microgrid fault current controller of claim 2, wherein the first end of the inductor and the first end of the capacitor are both connected to a voltage source converter, such that the fault current controller is connected to the dc microgrid via the voltage source converter.
4. The dc microgrid fault current controller of claim 3, wherein the relation between the average value of the output voltage of the fault current controller and the average value of the output voltage at the dc side of the voltage source converter is as follows:
U2＝Ud-U1；
in the formula of U2Is a DC bus voltage, UdIs the average value of the output voltage of the DC side of the voltage source type converter, U1Is the average value of the output voltage of the fault current controller.
5. The DC microgrid fault current controller of claim 4, wherein an output voltage of the fault current controller is less than a DC side output voltage of the voltage source converter.
6. The dc microgrid fault current controller of claim 1, wherein the first switching tube and the second switching tube are both IGBTs.
7. A control method of a direct current microgrid fault current controller is applied to the direct current microgrid fault current controller of any one of claims 1 to 6, and the method comprises the following steps:
DC bus voltage U2Monitoring the size of the sample;
when the DC bus voltage U2When the voltage is the rated voltage, controlling the duty ratio to be a first duty ratio so that the fault current controller is in a constant voltage control mode;
when the DC bus voltage U2When the current is smaller than the preset threshold value, the duty ratio is switched to a second duty ratio, so that the fault current controller is in a constant current control mode until the direct current bus voltage U is lower than the preset threshold value2Switching the duty cycle to the first duty cycle for a nominal voltage.
CN202111210770.5A 2021-10-18 2021-10-18 Direct-current microgrid fault current controller and control method thereof Pending CN113949048A (en)

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