CN111224409A - Virtual impedance-based over-current suppression method for direct current access device - Google Patents
Virtual impedance-based over-current suppression method for direct current access device Download PDFInfo
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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
- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
- H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
- H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
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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
- H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02M—APPARATUS FOR CONVERSION BETWEEN AC AND AC, BETWEEN AC AND DC, OR BETWEEN DC AND DC, AND FOR USE WITH MAINS OR SIMILAR POWER SUPPLY SYSTEMS; CONVERSION OF DC OR AC INPUT POWER INTO SURGE OUTPUT POWER; CONTROL OR REGULATION THEREOF
- H02M1/00—Details of apparatus for conversion
- H02M1/32—Means for protecting converters other than automatic disconnection
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Abstract
The invention provides a virtual impedance-based over-current suppression method for a direct current access device, which comprises the following steps of: adding a virtual impedance L on the output side of the direct-current micro-grid-connected converter; detecting the output current of the direct-current microgrid grid-connected converter in real time, and adjusting the voltage output of the direct-current microgrid grid-connected converter by using virtual impedance when the output current changes rapidly; and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable the voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of the voltage measurement value and the voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid. The invention is more flexible than actual equipment in the parameter adjusting process, can save the engineering cost, has better economy, and does not have the problems of difficult type selection and value setting of the current-limiting reactor and the like.
Description
Technical Field
The invention relates to the technical field of power grids, in particular to a virtual impedance-based over-current suppression method for a direct current access device.
Background
The direct-current microgrid is a microgrid formed by direct currents. The basic structure of the direct-current microgrid system adopting the direct-current microgrid grid-connected converter is shown in fig. 5, and electric energy generated by an alternating-current distribution network is rectified and regulated by the direct-current microgrid grid-connected converter and then is connected with a direct-current microgrid. When an interelectrode short circuit occurs, a port capacitor of the direct-current microgrid grid-connected converter is rapidly discharged, so that a large discharge current is generated, and the protection of the whole system is seriously influenced. The current common fault overcurrent suppression method mainly adopts a series current limiting reactor to slow down the discharge speed of a capacitor and reduce the fault current rise rate and peak value, thereby realizing overcurrent suppression. However, the adoption of the series current-limiting reactor has the defects of large reactor volume, high engineering cost and the like.
After searching, some typical prior arts are found, for example, patent application No. 201510046240.X discloses a virtual impedance-based method for suppressing fault current on the dc side of an MMC, which can effectively suppress the instantaneous rising speed of fault current when a fault occurs on the dc side of the MMC by mapping a virtual impedance parallel circuit in a primary system into a controller through a feedback function. For another example, the patent with application number 20201510046269.8 discloses a method for suppressing a fault current on an ac side of an MMC, which is based on virtual impedance, and the method can effectively suppress the instantaneous rising speed of the fault current when the ac side of the MMC fails by mapping a virtual impedance parallel circuit in a primary system to a controller through a feedback function.
Therefore, how to suppress the instantaneous fault overcurrent of the dc microgrid, many practical problems to be dealt with in practical application (for example, adopting the virtual impedance to save the engineering cost) have not been proposed.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a virtual impedance-based overcurrent suppression method for a direct current access device, which has the following specific technical scheme:
a virtual impedance-based over-current suppression method for a direct current access device comprises the following steps:
virtual impedance L is added on the output side of the direct-current micro-grid-connected converterGE；
Output current I of direct current micro-grid-connected converterGEPerforming real-time detection when the output current IGEWhen the voltage changes rapidly, the virtual impedance is utilized to adjust the voltage output of the direct-current micro-grid-connected converter;
and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable a voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of a voltage measurement value and a voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid.
Optionally, in the method for suppressing the overcurrent of the dc access device based on the virtual impedance, the dc microgrid grid-connected converter adopts a voltage-current double closed-loop control mode.
Optionally, in the method for suppressing an overcurrent of a dc access device based on a virtual impedance, when the output current I is greater than the threshold valueGEAt a fast change, the virtual impedance LGEThe increment of the voltage drop induced by the upper inductor is expressed by the formula delta UGE＝LGE·dIGE/dt。
Optionally, in the method for suppressing overcurrent of the dc access device based on the virtual impedance, a voltage output feedback value of the dc microgrid grid-connected converter is calculated according to a formula UGE＝UGE2+ΔUGE＝ UGE2+LGEdIGE(dt); wherein, UGE2Is a voltage measurement value, delta U, of a direct current micro-grid-connected converterGEIs the voltage drop increment.
In the virtual impedance-based over-current suppression method for the direct current access device, a current sensor is mounted at the output end of the direct current microgrid grid-connected converter so as to output current I to the direct current microgrid grid-connected converterGEAnd carrying out real-time detection.
The beneficial effects obtained by the invention comprise:
1. the photovoltaic system instantaneous fault overcurrent suppression method based on the virtual impedance controls the size of the virtual impedance through a mathematical model and an algorithm, so that the method is more flexible than actual equipment in the parameter adjustment process;
2. the effect of the current-limiting reactor is generated by the algorithm, and the current-limiting reactor of primary equipment cannot be added, so that the engineering cost can be saved, the economy is better, and the problems of difficult type selection and value setting of the current-limiting reactor and the like do not exist;
3. the virtual impedance-based DC access device overcurrent suppression method is based on the impedance effect generated under the condition that the current has large sudden change, so that the virtual impedance cannot be generated when the system normally operates, and the normal operation of the system cannot be influenced.
Drawings
The present invention will be further understood from the following description taken in conjunction with the accompanying drawings, the emphasis instead being placed upon illustrating the principles of the embodiments.
Fig. 1 is a schematic overall flowchart of a virtual impedance-based method for suppressing an overcurrent in a dc access device according to an embodiment of the present invention;
fig. 2 is a schematic structural diagram of adding a virtual impedance at the output side of the dc microgrid grid-connected converter according to one embodiment of the present invention;
fig. 3 is a schematic diagram illustrating voltage control for adding virtual impedance to a dc microgrid system to suppress overcurrent according to an embodiment of the present invention;
fig. 4 is a waveform of an output current of the dc microgrid grid-connected inverter when a fault occurs after virtual impedance control is added in one embodiment of the present invention;
fig. 5 is a schematic structural diagram of a dc microgrid grid-connection in the prior art;
fig. 6 is a schematic diagram of a topology of a dc microgrid grid-connected converter in the prior art;
fig. 7 is a schematic diagram of a fault location and a fault type of a dc microgrid system in the prior art;
fig. 8 is a waveform of an output current of a dc microgrid grid-connected inverter when a dc microgrid system fails in the prior art;
fig. 9 is a schematic diagram of the principle that the output end of the dc microgrid grid-connected inverter is connected with a current-limiting reactor in series in the prior art;
fig. 10 shows waveforms of output currents of a dc microgrid grid-connected inverter when a dc microgrid system fails in the prior art.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more clearly apparent, the present invention is further described in detail below with reference to embodiments thereof; it should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Other systems, methods, and/or features of the present embodiments will become apparent to those skilled in the art upon review of the following detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. Additional features of the disclosed embodiments are described in, and will be apparent from, the detailed description below.
The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there is an orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", etc. based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not intended to indicate or imply that the device or component referred to must have a specific orientation, be constructed in a specific orientation, and be operated, and therefore, the terms describing the positional relationship in the drawings are only for illustrative purposes and are not to be construed as limitations of the present patent, and specific meanings of the terms described above will be understood by those skilled in the art according to specific situations.
The invention relates to a virtual impedance-based over-current suppression method for a direct current access device, which comprises the following embodiments according to the description shown in the attached drawings:
with the continuous improvement of clean energy power generation technology and the continuous development of a direct-current microgrid, a distributed power supply with volatility and intermittence is efficiently and safely incorporated into a main network to become a research hotspot, and a direct-current microgrid grid-connected converter is also widely concerned, adopts a controllable turn-off power electronic device IGBT, can independently control active power and reactive power, and can supply power to a passive network; when the tide is reversed, the direction of the direct current is reversed, and the polarity of the direct current voltage is unchanged, so that the method has wide application prospects in the fields of construction of a distributed power generation system (such as wind power generation, solar power generation and the like), power supply to remote loads, construction of city direct current distribution networks and the like. The invention discloses a virtual impedance-based over-current suppression method for a direct current access device, which mainly aims at a direct current micro-grid-connected converter.
The basic structure of the direct-current microgrid system adopting the direct-current microgrid grid-connected converter is shown in fig. 5, and electric energy generated by an alternating-current distribution network is rectified and regulated by the direct-current microgrid grid-connected converter and then is connected with a direct-current microgrid. Fig. 6 shows a topology structure of a common dc microgrid grid-connected converter, in which electric energy of an ac distribution network is connected in parallel with a dc microgrid through an ac side and a dc side. When an inter-electrode short circuit occurs, for example, f1 or f2, f3 shown in fig. 7 is short-circuited, the voltage across the port capacitor of the dc microgrid grid-connected inverter instantaneously decreases to be close to 0, the capacitor rapidly discharges, a large discharge current is generated, and the protection, setting and the like of the whole system are seriously affected.
Fig. 8 shows the output current waveform of the dc microgrid grid-connected inverter when a fault occurs at time f2 of 0.3s, and the system rated voltage is UGE750v, the output current of the alternating current power supply under normal operation is 0.374kA, the short-circuit current is 2.55kA, and the capacitance C at the outlet of the direct current micro-grid-connected converterGE1950 uF. As can be seen from fig. 8, at the moment of a fault, the capacitor at the port of the dc microgrid grid-connected converter is rapidly discharged, the output current of the dc microgrid grid-connected converter rapidly rises, reaches a peak value of 5.726kA after about 8.6ms, the fault current drops to an extremely low value after about 21.6ms, then the current briefly oscillates and begins to drop, the current drops to the lowest point after about 0.4071s, and then the capacitor at the port of the dc microgrid grid-connected converter, the current-limiting reactor, the line reactance and the like form an oscillation loop, which is stabilized at about 2.55 kA.
The current common fault overcurrent suppression method mainly adopts a series current limiting reactor as shown in fig. 9, namely, the current limiting reactor is connected in series at the outlet of the direct-current microgrid grid-connected converter, so that the discharging speed of a capacitor is reduced, the rising rate and the peak value of fault current are reduced, and overcurrent suppression is realized. Fig. 10 shows that after a positive electrode and a negative electrode are respectively connected in series with a 3mH current-limiting reactor, the output current waveform of the direct-current microgrid grid-connected inverter appears after 68.5ms of the peak value of short-circuit current when f2 fails, and the peak current is reduced to 4.4143kA, so that the peak current is reduced by about 22.9%; after about 0.783s, the fault current is reduced to an extremely low value, then a port capacitor of the direct-current micro-grid-connected converter, a current-limiting reactor, a line reactor and the like form an oscillation loop, and the short-circuit current continuously oscillates near 2.55 kA. Comparing fig. 10 and fig. 8, it can be seen that the peak value of the fault current, the rising rate of the short-circuit current, and the discharging speed of the port capacitor are suppressed after the series current limiting reactor is connected.
Although the principle of the series current impeder is simple, easy to implement, and has a significant over-current suppression effect, the series current impeder has several disadvantages:
1. the type selection of the current-limiting reactor is difficult. The iron core reactor is easy to saturate under large current and loses the current limiting function; the air-core reactor has large magnetic leakage, is easy to influence peripheral equipment, has large volume and needs larger floor area.
2. The current-limiting reactor is used for increasing equipment parts, so that project cost, occupied area and the like are correspondingly increased, and particularly, when rated current is large, the reactor is large in size for facilitating heat dissipation of the reactor.
3. The inductance value of the current limiting reactor is not well determined. The inductance value is too small, the current limiting effect is not obvious, the inductance value is too large, the size of the reactor is large, and system instability is easily caused. In addition, the reactor may also resonate with a capacitor or the like in the system.
In order to solve the defects existing in the series current limiting reactor, the invention provides a virtual impedance-based over-current suppression method for a direct current access device.
The first embodiment is as follows:
as shown in fig. 1 and 2, a method for suppressing an overcurrent in a dc access device based on a virtual impedance includes the following steps:
virtual impedance L is added on the output side of the direct-current micro-grid-connected converterGE；
Output current I of direct current micro-grid-connected converterGEPerforming real-time detection when the output current IGEWhen the voltage changes rapidly, the virtual impedance is utilized to adjust the voltage output of the direct-current micro-grid-connected converter;
and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable a voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of a voltage measurement value and a voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid.
In this embodiment, through a scheme of mathematical modeling, characteristics of actual circuit elements (particularly, current-limiting reactors) are mapped into a controller of the dc microgrid system, and a method for suppressing instantaneous fault and overcurrent of a direct access device based on virtual impedance is established, which can play a role of impedance, and achieve the purpose of suppressing instantaneous fault and overcurrent of the dc microgrid system without adding additional electrical elements (such as current-limiting reactors), so as to solve the disadvantages of the current-limiting reactors in the process of suppressing instantaneous fault and overcurrent.
According to the photovoltaic system instantaneous fault overcurrent suppression method based on the virtual impedance, the virtual impedance is controlled through a mathematical model and an algorithm, so that the method is more flexible than actual equipment in the parameter adjustment process.
The effect of the current-limiting reactor is generated by the algorithm, and the current-limiting reactor of the primary equipment cannot be added, so that the engineering cost can be saved, the economy is better, and the problems of difficult type selection and value setting of the current-limiting reactor and the like do not exist.
Example two:
as shown in fig. 1 and 2, a method for suppressing an overcurrent in a dc access device based on a virtual impedance includes the following steps:
virtual impedance L is added on the output side of the direct-current micro-grid-connected converterGE；
Output current I of direct current micro-grid-connected converterGEPerforming real-time detection when the output current IGEWhen the voltage changes rapidly, the virtual impedance is utilized to adjust the voltage output of the direct-current micro-grid-connected converter;
and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable a voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of a voltage measurement value and a voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid.
In this embodiment, through a scheme of mathematical modeling, characteristics of actual circuit elements (particularly, current-limiting reactors) are mapped into a controller of the dc microgrid system, and a method for suppressing instantaneous fault and overcurrent of a direct access device based on virtual impedance is established, which can play a role of impedance, and achieve the purpose of suppressing instantaneous fault and overcurrent of the dc microgrid system without adding additional electrical elements (such as current-limiting reactors), so as to solve the disadvantages of the current-limiting reactors in the process of suppressing instantaneous fault and overcurrent.
According to the photovoltaic system instantaneous fault overcurrent suppression method based on the virtual impedance, the virtual impedance is controlled through a mathematical model and an algorithm, so that the method is more flexible than actual equipment in the parameter adjustment process.
The effect of the current-limiting reactor is generated by the algorithm, and the current-limiting reactor of the primary equipment cannot be added, so that the engineering cost can be saved, the economy is better, and the problems of difficult type selection and value setting of the current-limiting reactor and the like do not exist.
In order to improve the response speed of the direct-current microgrid grid-connected converter, a double-ring control system based on the thought of industrial cascade control is adopted, namely, the current magnitude which changes rapidly is added in a target voltage ring to serve as a feedback variable so as to improve the working frequency of the control system. Therefore, in the method for suppressing the overcurrent of the direct current access device based on the virtual impedance, the direct current microgrid grid-connected converter adopts a voltage and current double closed-loop control mode.
As shown in fig. 2 and 3, in the method for suppressing the overcurrent of the dc access device based on the virtual impedance, when the output current I is larger than the predetermined valueGEAt a fast change, the virtual impedance LGEThe increment of the voltage drop induced by the upper induction is expressed by a formula delta UGE＝LGE·dIGE/dt。
In the virtual impedance-based over-current suppression method for the direct current access device, the voltage output feedback value of the direct current microgrid grid-connected converter passes through a formula UGE＝UGE2+ΔUGE＝UGE2+LGEdIGE(dt); wherein, UGE2Is a voltage measurement value, delta U, of a direct current micro-grid-connected converterGEIs the voltage drop increment. In the virtual impedance-based over-current suppression method for the direct current access device, a current sensor is mounted at the output end of the direct current microgrid grid-connected converter so as to output current I to the direct current microgrid grid-connected converterGEAnd carrying out real-time detection.
As shown in fig. 2 and 3, when a bipolar short-circuit fault occurs in the line, the output current I of the dc microgrid grid-connected converterGERapidly change at the virtual impedance LGEUp-sensing a voltage drop increment Δ UGEAnd thus the voltage measurement on the output side. During normal operation of the system, the current fluctuation of the distributed ac power supply will also generate a voltage drop Δ U across the virtual impedanceGEHowever, because the current change rate is small under normal operation, the voltage drop value is small, the normal operation is hardly affected, and the effect of inhibiting the output voltage fluctuation to a small extent can be achieved. When a bipolar short-circuit fault occurs in the system, the output current of the direct-current micro-grid-connected converter changes rapidly, a large voltage drop is generated, and the virtual impedance control shows a relatively obvious current suppression effect.
Fig. 4 shows an output current waveform when a fault occurs at time f2 of 0.3s after a virtual impedance control module is added to a control strategy of the dc microgrid grid-connected inverter. The peak of the short circuit current appeared after about 8.7ms and the peak current was reduced to 4.5765kA, which was about 20.07%; after about 18.2ms, the fault current is reduced to an extremely low value, then the current starts to oscillate and rise, the current reaches a maximum value in about 0.4s, then the fault current starts to fall, the current falls to an extremely low value in about 0.53s, then an oscillation loop is formed by a port capacitor of the direct-current microgrid grid-connected converter, a current-limiting reactor, a line reactance and the like, and the short-circuit current continuously oscillates near 2.55 kA.
As can be known from comparison between fig. 4 and fig. 10, after the virtual impedance is added to the dc microgrid system controller, the instantaneous fault current waveforms have strong similarity to those of the dc microgrid system controller and the dc microgrid system controller, compared with the case where the current-limiting reactor is added to the primary circuit. That is to say, by adding the virtual impedance to the dc microgrid system controller, it can also effectively suppress transient fault overcurrent. The over-current suppression method of the direct current access device based on the virtual impedance is based on the impedance effect generated under the condition that the current has large sudden change, so that the virtual impedance cannot be generated when the system normally operates, and the normal operation of the system cannot be influenced.
Example three:
as shown in fig. 1 and 2, a method for suppressing an overcurrent in a dc access device based on a virtual impedance includes the following steps:
virtual impedance L is added on the output side of the direct-current micro-grid-connected converterGE；
Output current I of direct current micro-grid-connected converterGEPerforming real-time detection when the output current IGEWhen the voltage changes rapidly, the virtual impedance is utilized to adjust the voltage output of the direct-current micro-grid-connected converter;
and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable a voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of a voltage measurement value and a voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid.
In this embodiment, through a scheme of mathematical modeling, characteristics of actual circuit elements (particularly, current-limiting reactors) are mapped into a controller of the dc microgrid system, and a method for suppressing instantaneous fault and overcurrent of a direct access device based on virtual impedance is established, which can play a role of impedance, and achieve the purpose of suppressing instantaneous fault and overcurrent of the dc microgrid system without adding additional electrical elements (such as current-limiting reactors), so as to solve the disadvantages of the current-limiting reactors in the process of suppressing instantaneous fault and overcurrent.
According to the photovoltaic system instantaneous fault overcurrent suppression method based on the virtual impedance, the virtual impedance is controlled through a mathematical model and an algorithm, so that the method is more flexible than actual equipment in the parameter adjustment process.
The effect of the current-limiting reactor is generated by the algorithm, and the current-limiting reactor of the primary equipment cannot be added, so that the engineering cost can be saved, the economy is better, and the problems of difficult type selection and value setting of the current-limiting reactor and the like do not exist.
In order to improve the response speed of the direct-current microgrid grid-connected converter, a double-ring control system based on the thought of industrial cascade control is adopted, namely, the current magnitude which changes rapidly is added in a target voltage ring to serve as a feedback variable so as to improve the working frequency of the control system. Therefore, in the method for suppressing the overcurrent of the direct current access device based on the virtual impedance, the direct current microgrid grid-connected converter adopts a voltage and current double closed-loop control mode.
As shown in fig. 2 and 3, in the method for suppressing the overcurrent of the dc access device based on the virtual impedance, when the output current I is larger than the predetermined valueGEAt a fast change, the virtual impedance LGEThe increment of the voltage drop induced by the upper induction is expressed by a formula delta UGE＝LGE·dIGE/dt。
In the virtual impedance-based over-current suppression method for the direct current access device, the voltage output feedback value of the direct current microgrid grid-connected converter passes through a formula UGE＝UGE2+ΔUGE＝UGE2+LGEdIGE(dt); wherein, UGE2Is a voltage measurement value, delta U, of a direct current micro-grid-connected converterGEIs the voltage drop increment. In the virtual impedance-based over-current suppression method for the direct current access device, a current sensor is mounted at the output end of the direct current microgrid grid-connected converter so as to output current I to the direct current microgrid grid-connected converterGEAnd carrying out real-time detection.
As shown in fig. 2 and 3, when a bipolar short-circuit fault occurs in the line, the output current I of the dc microgrid grid-connected converterGERapidly change at the virtual impedance LGEUp-sensing a voltage drop increment Δ UGEAnd thus the voltage measurement on the output side. During normal operation of the system, the current fluctuation of the distributed ac power supply will also generate a voltage drop Δ U across the virtual impedanceGEHowever, because the current change rate is small under normal operation, the voltage drop value is small, the normal operation is hardly affected, and the effect of inhibiting the output voltage fluctuation to a small extent can be achieved. When a bipolar short-circuit fault occurs in the system, the output current of the direct-current micro-grid-connected converter changes rapidly, a large voltage drop is generated, and the virtual impedance control shows a relatively obvious current suppression effect.
Fig. 4 shows an output current waveform when a fault occurs at time f2 of 0.3s after a virtual impedance control module is added to a control strategy of the dc microgrid grid-connected inverter. The peak of the short circuit current appeared after about 8.7ms and the peak current was reduced to 4.5765kA, which was about 20.07%; after about 18.2ms, the fault current is reduced to an extremely low value, then the current starts to oscillate and rise, the current reaches a maximum value in about 0.4s, then the fault current starts to fall, the current falls to an extremely low value in about 0.53s, then an oscillation loop is formed by a port capacitor of the direct-current microgrid grid-connected converter, a current-limiting reactor, a line reactance and the like, and the short-circuit current continuously oscillates near 2.55 kA.
As can be known from comparison between fig. 4 and fig. 10, after the virtual impedance is added to the dc microgrid system controller, the instantaneous fault current waveforms have strong similarity to those of the dc microgrid system controller and the dc microgrid system controller, compared with the case where the current-limiting reactor is added to the primary circuit. That is to say, by adding the virtual impedance to the dc microgrid system controller, it can also effectively suppress transient fault overcurrent. The over-current suppression method of the direct current access device based on the virtual impedance is based on the impedance effect generated under the condition that the current has large sudden change, so that the virtual impedance cannot be generated when the system normally operates, and the normal operation of the system cannot be influenced.
In this embodiment, the output end of the dc microgrid grid-connected inverter is provided with a current sensor to output a current I to the dc microgrid grid-connected inverterGEAnd carrying out real-time detection. The DC micro-grid system controller receives the real-time output current of the current converter and calculates the output current IGEThe rate of change of (c).
In addition, a preset threshold is also arranged in a controller of the direct-current microgrid system, and when the output current I isGEAnd if the change rate is greater than a preset threshold value, judging that the instantaneous fault overcurrent signal occurs in the direct-current microgrid system, then adjusting the voltage output of the direct-current microgrid grid-connected converter by using the virtual impedance by using the controller, and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable the voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of the voltage measurement value and the voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of the instantaneous fault overcurrent of the direct-current microgrid.
As a preferable technical solution, a wireless communication module is further provided in the controller of the dc microgrid system. The controller connects the output current I of the direct-current microgrid grid-connected converter through the wireless communication module and the current sensorGEThe change rate is fed back to a control center or a data server in real time, and a technician performs wireless communication on a preset threshold value and an output current I of a current sensor to the direct current microgrid grid-connected converterGEThe sampling frequency of the sampling device is adjusted to meet different working requirements.
In summary, the method for suppressing an overcurrent in a dc access device based on a virtual impedance according to the present invention has the following beneficial effects:
1. the photovoltaic system instantaneous fault overcurrent suppression method based on the virtual impedance controls the size of the virtual impedance through a mathematical model and an algorithm, so that the method is more flexible than actual equipment in the parameter adjustment process;
2. the effect of the current-limiting reactor is generated by the algorithm, and the current-limiting reactor of primary equipment cannot be added, so that the engineering cost can be saved, the economy is better, and the problems of difficult type selection and value setting of the current-limiting reactor and the like do not exist;
3. the virtual impedance-based DC access device overcurrent suppression method is based on the impedance effect generated under the condition that the current has large sudden change, so that the virtual impedance cannot be generated when the system normally operates, and the normal operation of the system cannot be influenced.
Although the invention has been described above with reference to various embodiments, it should be understood that many changes and modifications may be made without departing from the scope of the invention. That is, the methods, systems, and devices discussed above are examples, and various configurations may omit, replace, or add various processes or components as appropriate. For example, in alternative configurations, the methods may be performed in an order different than that described and/or various components may be added, omitted, and/or combined. Moreover, features described with respect to certain configurations may be combined in various other configurations, such as different aspects and elements of a configuration may be combined in a similar manner. Furthermore, elements may be updated as technology evolves, i.e., many of the elements are examples and do not limit the scope of the disclosure or claims.
Specific details are set forth in the description in order to provide a thorough understanding of the exemplary configurations including implementations. However, configurations may be practiced without these specific details, such as well-known circuits, processes, algorithms, structures, and techniques, which have been shown without unnecessary detail in order to avoid obscuring the configurations. This description provides example configurations only, and does not limit the scope, applicability, or configuration of the claims. Rather, the foregoing description of the configurations will provide those skilled in the art with an enabling description for implementing the described techniques. Various changes may be made in the function and arrangement of elements without departing from the spirit or scope of the disclosure.
It is intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of this invention. The above examples are to be construed as merely illustrative and not limitative of the remainder of the disclosure. After reading the description of the invention, the skilled person can make various changes or modifications to the invention, and these equivalent changes and modifications also fall into the scope of the invention defined by the claims.
Claims (5)
1. A virtual impedance-based over-current suppression method for a direct current access device is characterized by comprising the following steps:
virtual impedance L is added on the output side of the direct-current micro-grid-connected converterGE；
Output current I of direct current micro-grid-connected converterGEPerforming real-time detection when the output current IGEWhen the voltage changes rapidly, the virtual impedance is utilized to adjust the voltage output of the direct-current micro-grid-connected converter;
and correcting the voltage output of the direct-current microgrid grid-connected converter so as to enable the voltage output feedback value of the direct-current microgrid grid-connected converter to be the sum of the voltage measurement value and the voltage drop increment of the direct-current microgrid grid-connected converter, and finishing the suppression of instantaneous fault overcurrent of the direct-current microgrid.
2. The virtual impedance-based over-current suppression method for the direct current access device according to claim 1, wherein the direct current microgrid grid-connected converter adopts a voltage-current double closed-loop control mode.
3. The method as claimed in claim 2, wherein the virtual impedance-based over-current suppressing method is applied when the output current I is larger than the threshold voltageGEAt a fast change, the virtual impedance LGEThe increment of the voltage drop induced by the upper inductor is expressed by the formula delta UGE＝LGE·dIGE/dt。
4. The virtual impedance-based over-current suppression method for the direct current access device according to claim 3, wherein a voltage output feedback value of the direct current microgrid grid-connected inverter is obtained through a formula UGE＝UGE2+ΔUGE＝UGE2+LGEdIGE/dt；
Wherein, UGE2Is a voltage measurement value, delta U, of a direct current micro-grid-connected converterGEIs the voltage drop increment.
5. The virtual impedance-based over-current suppression method for the direct current access device according to claim 4, wherein a current sensor is installed at an output end of the direct current microgrid grid-connected inverter so as to output a current I to the direct current microgrid grid-connected inverterGEAnd carrying out real-time detection.
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