CN107437821B - Island microgrid virtual machine differentiation fault ride-through system and implementation method thereof - Google Patents

Island microgrid virtual machine differentiation fault ride-through system and implementation method thereof Download PDF

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CN107437821B
CN107437821B CN201710820837.4A CN201710820837A CN107437821B CN 107437821 B CN107437821 B CN 107437821B CN 201710820837 A CN201710820837 A CN 201710820837A CN 107437821 B CN107437821 B CN 107437821B
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converter
current
fault
control module
voltage
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CN107437821A (en
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帅智康
沈超
涂春鸣
沈征
彭也伦
黄文�
沈霞
尹新
赵峰
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Hunan University
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Hunan University
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    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381Dispersed generators
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/46Controlling of the sharing of output between the generators, converters, or transformers
    • HELECTRICITY
    • H02GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02JCIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/388Islanding, i.e. disconnection of local power supply from the network

Abstract

The invention discloses a differentiated fault ride-through system of an island microgrid virtual machine and an implementation method thereof, wherein the system consists of a converter output voltage and current acquisition module, a fault detection and judgment module, a virtual synchronous control module and a differentiated fault ride-through control module; the converter output voltage and current acquisition module detects the voltage at two ends of a filter capacitor and the filter inductor current output by the converter and respectively feeds the voltage and the filter inductor current back to the virtual synchronous control module, the fault detection and judgment module and the differential fault ride-through control module; the fault detection and judgment module judges whether the converter fault occurs and detects the fault degree; and realizing the fault ride-through control of the virtual machine of the island microgrid by adopting a differential fault ride-through control module. The impact current can be limited to protect the converter during the fault period, the frequency stability of the microgrid is maintained, the isolated island microgrid is guaranteed to operate safely during the short-circuit fault, and the contradiction between two requirements of protecting the converter device and maintaining the stable operation of the microgrid in the existing fault process is solved.

Description

Island microgrid virtual machine differentiation fault ride-through system and implementation method thereof
Technical Field
The invention belongs to the technical field of microgrid fault control, and relates to an islanding microgrid virtual machine differentiation fault ride-through system and an implementation method thereof.
Background
A large number of distributed power supplies mainly based on wind energy and solar energy are connected into the power distribution network, so that the electric energy quality and the power supply reliability of the power distribution network are improved, and the trend complexity of the power distribution network side is increased. The microgrid has been widely noticed and popularized in recent years as a system capable of effectively integrating distributed energy, loads, energy storage devices and electric vehicles. In a microgrid, distributed energy is often connected through a converter device to realize flexible conversion of electric energy and improve controllability. Among these, the distributed energy source with the voltage-controlled converter arrangement as an interface plays an irreplaceable role in voltage and frequency support, especially under islanding conditions.
Common voltage control methods include constant voltage and constant frequency control and droop control. The constant voltage and constant frequency control can stabilize the voltage and frequency of the system under an island condition, and ensure better electric energy quality, but has higher requirement on the capacity of the converter. The droop control converter with the automatic load distribution characteristic can maintain the stability of the voltage and the frequency of the microgrid and can realize load sharing. However, droop control has a strict requirement on the line resistance characteristic, and extra control is often needed to adapt to a low-voltage network in which the line is resistive. In recent years, an inverter-virtual machine that simulates electromagnetic characteristics and rotation characteristics of a synchronous motor in terms of control has become a research hotspot due to strong adaptability and power grid friendliness. However, compared with a synchronous generator, the converter device has poor thermal overcurrent capability, and an impact current generated in the short-circuit fault process is likely to damage a power device, so that a safety problem is caused. Therefore, protection of the converter by switching control during a fault becomes an important means.
For common voltage control, some restraining methods exist for fault impact current, and a current limiting mode of increasing virtual impedance according to the magnitude of a voltage drop amplitude is provided, so that a droop controller can limit fault current in a transient process. However, when a near end fails (the voltage drop amplitude is large), the increased virtual impedance reaches an upper limit value, and the surge current is difficult to limit effectively. The output current of the converter can be effectively controlled by directly controlling the voltage of the modulation wave when the fault occurs. However, the method depends on communication seriously, and hidden danger is brought to system reliability. In addition, the current limiter of the current inner ring in the common voltage control can play a certain role in limiting the impact current caused by the short-circuit fault.
The virtual machine has more serious impact current in the short circuit process, and the virtual synchronous control lacks a current limiter, and the impact current cannot be limited by the original control. According to the fault ride-through mode of the traditional voltage control type converter, a mode smooth switching-based low voltage ride-through control method in a grid-connected mode is provided. The method limits fault current by adopting a traditional method of switching low voltage into current control, and can smoothly switch to virtual synchronous control after fault is removed. However, the method does not analyze the fault current component of the voltage control type converter, and the current limiting method lacks pertinence. The converter switched to current control cannot provide voltage and frequency support, so that the microgrid in the transient process becomes more fragile, and even the system is unstable.
The limiter in the controller of the converter has a certain limiting effect on the output current of the converter during the fault, however, the limitation cannot actively control the output power of the converter, and the impact is easily generated in the fault recovery stage. The existing active low voltage ride through control mainly comprises two methods of directly switching to direct current control or directly controlling the inner potential of the converter. Switching to direct current control can quickly control the current, but cannot support the network frequency; the direct control of the potential in the converter has the defects of communication delay, poor capability of inhibiting impact current and the like.
Disclosure of Invention
In order to achieve the purpose, the invention provides a differentiated fault ride-through system for virtual machines of an island microgrid, which can limit an impact current to protect a converter during a fault period, maintain the frequency stability of the microgrid at the same time, ensure the safe operation of the island microgrid during a short-circuit fault and solve the contradiction between two requirements of protecting a converter device and maintaining the stable operation of the microgrid in the conventional fault process.
The invention further aims to provide a realization method of the island microgrid virtual machine differentiation fault ride-through system.
The invention adopts the technical scheme that an island microgrid virtual machine differentiated fault ride-through system consists of a converter output voltage and current acquisition module, a fault detection and judgment module, a virtual synchronous control module and a differentiated fault ride-through control module; wherein the content of the first and second substances,
the converter output voltage and current acquisition module is used for detecting the voltage at two ends of a filter capacitor and the filter inductor current output by the converter and respectively feeding the detected voltage and current back to the virtual synchronous control module, the fault detection and judgment module and the differential fault ride-through control module;
the fault detection and judgment module is used for judging whether the converter fault occurs and detecting the fault degree;
the virtual synchronous control module is used for realizing output control when the converter operates normally;
the differential fault ride-through control module is used for realizing fault ride-through control of an island microgrid virtual machine and consists of a rapid current limiting control module and a virtual impedance current limiting control module, wherein the rapid current limiting control module is used for realizing control of limiting fault current of a current transformer when the fault degree is serious; and the virtual impedance current limiting control module is used for realizing the control of limiting the fault current of the converter when the fault degree is shallow.
The invention adopts another technical scheme that an implementation method of a virtual machine differentiation fault ride-through system of an island microgrid is specifically carried out according to the following steps:
step 1, a converter output voltage and current acquisition module detects voltage and filter inductance current at two ends of a filter capacitor output by a converter and respectively feeds the detected voltage and current back to a virtual synchronous control module, a fault detection and judgment module and a differential fault ride-through control module;
step 2, a fault detection and judgment module judges whether a converter fault occurs and detects the fault degree; when no fault occurs, the output control of the converter in normal operation is realized by adopting the virtual synchronous control module;
step 3, when a fault occurs and the fault degree is serious, the control of limiting the fault current of the converter is realized by adopting a rapid current-limiting control module; when a fault occurs and the fault degree is shallow, the control of limiting the fault current of the converter is realized by adopting the virtual impedance current-limiting control module.
Further, the fault detection and judgment module takes the voltage at two ends of a filter capacitor and a filter inductance current signal output by the converter as input, compares the instantaneous value of the current signal with a set current protection value, and when the instantaneous value of the current signal is less than or equal to the current protection value, the algorithm change-over switch is connected with the virtual synchronous control module; when the instantaneous value of the current signal is greater than the current protection value, a control switching signal is output, the converter is switched to enter a differential fault ride-through control module, meanwhile, a fault detection and judgment module judges the fault falling depth of the converter according to the magnitude of a voltage signal of a port of the converter, and when the voltage falling depth of the converter is greater than or equal to a set threshold value, an algorithm switching switch is connected with a rapid current limiting control module; when the voltage drop depth of the converter is smaller than the set threshold value, the algorithm change-over switch is connected with the virtual impedance current limiting control module.
Further, the virtual synchronous control module outputs a current signal i by the convertersFor input, the reference value of the three-phase voltage at this time is used as the voltage, the voltage and the current isMultiplying as power input to the virtual synchronization control module with the active power reference value P*From the rated angular velocity omega*Quotient T ofmComparing, wherein an active inertia droop ring in the virtual synchronous control module receives active power input and then performs inertia calculation and droop calculation to obtain a reference wave angular velocity omega, and meanwhile, the reference wave angular velocity omega is integrated to obtain a phase angle theta of a reference wave; after receiving reactive power input, the reactive inertia droop ring in the virtual synchronous control module and the rated reactive power Q*Comparing, and calculating inertia and droop to obtain virtual excitation MfifVirtual excitation MfifAnd multiplying the amplitude by the reference wave angular velocity omega to obtain the amplitude of the output signal, and multiplying the amplitude by the sine value of the phase angle theta of the reference wave to serve as the output of the virtual synchronous control module to finish the output control when the converter normally operates.
Furthermore, the quick flow limiting control module detects the publicVoltage phase theta plus phase shift angle of common connection pointWith IsetGenerating a three-phase current reference value i as a reference current amplitudeabcrefAnd the converter outputs three-phase current iabcAnd (4) performing difference, and generating a driving signal through the hysteresis control module to control the on-off of the IGBT switching device, so that the limitation on the fault current of the converter is realized.
Further, the virtual impedance current limiting control module controls the modulation voltage V output by the module in a virtual synchronous mannerref abcFor input, modulating the voltage Vref abcAnd the converter outputs three-phase current iabcAt a virtual impedance ZvAnd (3) performing SPWM modulation after high-frequency harmonic waves are eliminated by the voltage drop difference generated above and the low-pass filter LPF, and driving the on-off of an IGBT switching device, thereby realizing the fault current-limiting control of the converter.
The method has the advantages that aiming at the characteristics that the fault process caused by poor thermal overcurrent capacity of the converter is easy to burn out, the frequency of the island microgrid is supported by the voltage control type converter and the like, the low-voltage ride-through control mode required by the converter is judged by detecting the voltage drop depth of each converter port in the microgrid and the output current, namely, the proper fault ride-through control mode is selected according to the fault state of the converter, so that the safe and stable operation of the island microgrid under the fault condition is realized. When a micro-grid fails, a fault near-end converter with a deep port voltage drop is switched to fast current-limiting control, output current is fed back to a fast current-limiting control module, a hysteresis control module is adopted to realize fast control of impact current, the output current is reasonably distributed, the micro-grid voltage is supported, the impact current is inhibited, and the converter is protected; for a fault far-end converter with shallow port voltage drop, switching to virtual impedance current limiting control, feeding back an output current value to a virtual impedance current limiting control module, and modulating a voltage Vref abcAnd the converter outputs three-phase current iabcAt a virtual impedance ZvThe voltage drop generated in the step A is differentiated, SPWM modulation is carried out, and driving is carried outAnd the IGBT switching device is switched on and off, so that short-circuit current is limited, and frequency support is provided for the micro-grid. The invention ensures the stable and safe operation of the whole microgrid during the fault process, and realizes the actual requirements of not only preventing the device damage possibly caused by the impact current, but also keeping the voltage and the frequency of the microgrid stable.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a topological structure diagram of an island microgrid according to an embodiment of the invention.
Fig. 2 is a diagram of implementing virtual synchronization control according to an embodiment of the present invention.
Fig. 3 is a diagram of an implementation of fast current limit control according to an embodiment of the present invention.
Fig. 4 is a diagram of an implementation of virtual impedance current limiting control according to an embodiment of the present invention.
Fig. 5 is a differentiated fault-ride-through control implementation diagram according to an embodiment of the present invention.
Fig. 6 is a SPWM modulation schematic diagram of an embodiment of the present invention.
Fig. 7a is a graph of the output current variation of the converter DG1 after the differential fault ride-through system control according to the present invention.
Fig. 7b is a graph of the output current variation of the converter DG2 after the differential fault ride-through system control according to the present invention.
Fig. 7c is a graph of the output current variation of the converter DG3 after the differential fault ride-through system control according to the present invention.
Fig. 7d is a frequency waveform diagram of the common bus controlled using the differential fault ride-through system of the present invention.
Fig. 7e is a graph of the voltage change on the common bus after differential fault ride-through system control using the present invention.
In the figure, 1, a virtual synchronous control module, 2, a virtual impedance current limiting control module, 3, a rapid current limiting control module, 4, a hysteresis control module, 5, a low pass filter LPF, 6, a fault detection and judgment module, 7, a differential fault ride-through control module and 8, a PLL phase-locked loop module.
Detailed Description
The invention is further explained in detail below with reference to the figures and the specific examples. It should be understood that the examples are only for illustrating the present invention and are not intended to limit the scope of the present invention. Furthermore, it should be understood that various changes or modifications can be made by those skilled in the art after reading the description of the present invention, and such equivalents also fall within the scope of the protection defined by the present application.
The island microgrid virtual machine differentiated fault ride-through system consists of a converter output voltage and current acquisition module, a fault detection and judgment module 6, a virtual synchronous control module 1 and a differentiated fault ride-through control module 7; wherein the content of the first and second substances,
the converter output voltage and current acquisition module is used for detecting the voltage at two ends of a filter capacitor and the filter inductor current output by the converter and respectively feeding the detected voltage and current back to the virtual synchronous control module 1, the fault detection and judgment module 6 and the differentiated fault ride-through control module 7;
the fault detection and judgment module 6 is used for judging whether the converter fault occurs and detecting the fault degree;
the virtual synchronous control module 1 is used for realizing output control when the converter operates normally;
the differential fault ride-through control module 7 is used for realizing fault ride-through control of an islanding microgrid virtual machine and comprises a rapid current-limiting control module 3 and a virtual impedance current-limiting control module 2, wherein the rapid current-limiting control module 3 is used for realizing control of limiting fault current of a current transformer when the fault degree is serious; and the virtual impedance current limiting control module 2 is used for realizing the control of limiting the fault current of the converter when the fault degree is shallow.
The method for realizing the island microgrid virtual machine differentiation fault ride-through system specifically comprises the following steps:
step 1, a converter output voltage and current acquisition module detects voltage and filter inductance current at two ends of a filter capacitor output by a converter, and respectively feeds the detected voltage and current back to a virtual synchronous control module 1, a fault detection and judgment module 6 and a differentiation fault ride-through control module 7;
step 2, a fault detection and judgment module 6 judges whether the converter fault occurs and detects the fault degree; when no fault occurs, the virtual synchronous control module 1 is adopted to realize the output control of the converter during normal operation;
step 3, when a fault occurs and the fault degree is serious, the control of limiting the fault current of the converter is realized by adopting a rapid current limiting control module 3; when a fault occurs and the fault degree is shallow, the virtual impedance current limiting control module 2 is adopted to realize the control of limiting the fault current of the converter.
The island microgrid topology structure of the embodiment of the invention, as shown in fig. 1, comprises a current transformer DG1, a current transformer DG2, a current transformer DG3 and a load, wherein the current transformer DG1 and an LC filter ZLC1Connected to the current transformer DG2 and the LC filter ZLC2Connected to the current transformer DG3 and the LC filter ZLC3And the converter DG1 and the converter DG3 simulate three-phase grounding short-circuit fault impedance Z to the power grid through the transmission lines line1 and line3 respectivelygSupplying power; the converter DG2 is connected with the load impedance Z through the transmission line2loadSupplying power; the transmission lines line1, line2 and line3 are all connected with the Bus.
Each converter comprises a main circuit, a converter output voltage and current acquisition module, a virtual synchronous control module 1 and a driving module; the converter output voltage and current acquisition module respectively acquires voltages at two ends of three converter output filter capacitors and filter inductor currents, and transmits the voltages and the filter inductor currents to the virtual synchronous control module 1, so that virtual synchronous control under normal operation conditions and differential fault ride-through control under fault conditions are realized, a driving signal is transmitted to the hardware driving circuit, switching-on and switching-off of a switch device are controlled, and electric energy conversion is realized.
In the virtual synchronization control according to the embodiment of the present invention, as shown in fig. 2, formula (1) is as follows,
in the above formula, TeFor virtual electromagnetic torque of the virtual machine, Q represents output reactive power of the virtual machine, e is the amplitude of modulation wave voltage generated by a virtual synchronous control algorithm, MfifFor the virtual flux linkage, ω is the virtual machine angular velocity, i represents the virtual machine output current, and θ is the virtual machine phase angle. Virtual synchronous control module 1 outputs current signal i by current transformersFor input, the reference value of the three-phase voltage at this time is used as the voltage, the voltage and the current isMultiplying as power input to the virtual synchronous control module 1 and with the active power reference value P*From the rated angular velocity omega*Quotient T ofmComparing, and performing inertia calculation and droop calculation after an active inertia droop ring in the virtual synchronous control module receives active power input to obtain a reference wave angular velocity omega, and meanwhile, integrating the reference wave angular velocity omega to obtain a phase angle theta of a reference wave; after receiving the reactive power input, the reactive inertia droop ring in the virtual synchronous control module 1 and the rated reactive power Q*Comparing, and calculating inertia and droop to obtain virtual excitation MfifVirtual excitation MfifAnd finally, multiplying the amplitude by the sine value of the phase angle theta of the reference wave to serve as the output of the virtual synchronous control module, and finishing the output control when the converter normally operates. K in FIG. 2s、JsIntegral constants of the reactive loop and the active loop, DpAnd DqThe sag coefficients of the active and reactive rings, respectively, E*And E are the voltage reference amplitude and the virtual machine output voltage amplitude, respectively, and △ T is the virtual torque compensation value.
The converter DG1, the converter DG2 and the converter DG3 in the microgrid are connected with an alternating current bus to jointly supply power to a load. The three converters are used for supplying power to the load in a common mode, generality of the micro-grid structure in the embodiment can be guaranteed, and various transient response processes of the island micro-grid in the embodiment when short-circuit faults occur are fully reflected. The structures of the converters DG1, DG2 and DG3 comprise a direct current micro source and a DC/AC converter for converting electric energy. The direct current micro source includes, but is not limited to, a direct current type power source such as an energy storage battery and a photovoltaic panel. Direct current input is converted into alternating current through a DC/AC converter and is connected to a public bus, so that high electric energy quality and high electric energy controllability can be guaranteed.
The output end of the DC/AC converter is connected with an LC filter inductance capacitor, so that the aim of filtering higher harmonics of voltage and current is fulfilled. And the load at the alternating current side of the DC/AC converter is connected with the bus, and the voltage and current information of the filtering port at the alternating current side are collected and transmitted to the virtual synchronous control module. The voltage and current transmission modes include, but are not limited to, constant impedance wires, digital signal transmission, and the like. The virtual synchronous control module obtains a modulation wave signal through algorithm calculation and carries out SPWM modulation, thereby driving an IGBT switch device in the converter.
In the fast current limiting control of the embodiment of the present invention, as shown in fig. 3, the fast current limiting control module 3 detects the voltage phase θ plus the phase shift angle of the common connection pointWith IsetGenerating a three-phase current reference value i as a reference current amplitudeabcrefAnd the converter outputs three-phase current iabcAnd (4) generating a driving signal through the hysteresis control module 4 to control the on and off of the IGBT switching device, thereby realizing the limitation of the fault current of the converter. The voltage phase theta is obtained by collecting the voltage e at the output end of the virtual machineabcObtained via the PLL module 8.
As shown in fig. 4, in the virtual impedance current limiting control module according to the embodiment of the present invention, the virtual impedance current limiting control module 2 uses the modulation voltage V output by the virtual synchronous control module 1ref abcAnd as input, carrying out SPWM modulation after virtual impedance current limiting processing, thereby driving the IGBT switching device. Virtual synchronous control module 1 to refer to active power P*Reactive power Q*Reference voltage E*Reference angular velocity omega*Output voltage amplitude E of virtual machinemAnd an output current iabcAs input, a modulation voltage V is generatedref abc(ii) a The virtual impedance current limiting control module 2 modulates the voltage Vref abcAnd the converter outputs three-phase current iabcAt a virtual impedance ZvAnd the generated voltage drop is subjected to difference, high-frequency harmonic waves are eliminated through a low-pass filter LPF 5, SPWM modulation is carried out, and the IGBT switching device is driven to be switched on and switched off, so that fault current-limiting control of the converter is realized.
In the embodiment of the invention, differential fault current-limiting control is performed, as shown in fig. 5, a fault detection and judgment module 6 takes voltage at two ends of a filter capacitor and a filter inductor current signal output by a converter as input, compares an instantaneous value of the current signal with a set current protection value, when the instantaneous value of the current signal is smaller than or equal to the current protection value, and a microgrid normally runs, an algorithm change-over switch stays at a position 1' and is connected with a virtual synchronous control module 1, and the converter adopts virtual synchronous control to enhance virtual inertia and damping of a system. Under the condition of a fault, on the basis of rapid current-limiting control and virtual impedance current-limiting control, judging a fault ride-through control mode required by the converter by detecting the voltage drop depth of a port of the converter; when the instantaneous value of the current signal is greater than the current protection value, a control switching signal is output, the converter is switched to enter a differential fault ride-through control module, meanwhile, a fault detection and judgment module 6 judges the fault drop depth of the converter according to the magnitude of the voltage signal of the converter port, when the voltage drop depth of the converter is greater than or equal to a set threshold value, the voltage drop of the converter port is deeper, an algorithm switching switch is switched to a position 3', the algorithm switching switch is connected with a rapid current limiting control module 3, the fault current is directly controlled by adopting rapid current limiting control, and a three-phase current reference value i is generatedabcrefAnd the converter outputs three-phase current iabcMaking a difference, and generating a driving signal through the hysteresis control module 4 so as to drive an IGBT switching device, limit impact current and protect the safety of the converter under the fault condition; when the voltage drop depth of the converter is smaller than the set threshold value, the voltage drop of the converter port is shallow, the algorithm change-over switch is switched to the 2' position, the virtual impedance current limiting control module 2 is connected, the virtual impedance current limiting control is adopted, and the modulation voltage V is generatedref abcAnd SPWM modulation is carried out, so that an IGBT switching device is driven, and the realizationLimiting short circuit current while providing frequency support for the microgrid.
In the embodiment of the invention, based on the SPWM modulation principle, as shown in FIG. 6, the SPWM driving circuit receives the modulation voltage V output by the virtual impedance current limiting control module 2ref abcThen, modulating the voltage Vref abcThe signal is compared with sawtooth wave with 5kHz frequency in the SPWM driver, when the signal value of the modulation voltage is less than or equal to the signal value of the sawtooth wave, the SPWM driver transmits a high-level turn-on driving signal to an IGBT switching device in the DC converter, and when the modulation voltage V is less than or equal to the signal value of the sawtooth wave, the signal is transmitted to the IGBT switching deviceref abcWhen the signal value is larger than the sawtooth wave signal value, the SPWM driver transmits a low-level turn-off signal to an IGBT switching device in the direct current converter; the present embodiment is only exemplified by the frequency of the sawtooth wave signal of the conventional model being 5kHz, and the frequency is not limited to this value in practical application.
In the invention, the far-end virtual machine with a shallow fault degree adopts virtual impedance current-limiting control to limit short-circuit current and provide stable frequency for the micro-grid; the near-end virtual machine with serious fault degree adopts rapid current limiting control to inhibit impact current and reasonably distribute power, thereby ensuring the stable and safe operation of the whole microgrid in the fault process.
When the micro-grid has three-phase symmetrical voltage drop in 2s and the fault is cleared in 2.5s, the differential fault ride-through system in the embodiment of the invention is adopted to control in the fault process, and the output currents of the converter DG1, the converter DG2 and the converter DG3 change, as shown in FIGS. 7a-7 c; common bus frequency waveforms are as in fig. 7 d; common bus voltage variation as in fig. 7 e; as can be seen from the figure, the differential fault ride-through system in the embodiment of the present invention can well limit the short-circuit inrush current, and meanwhile, the frequency and the voltage of the common bus can be maintained within the allowable range.
The above description is only for the preferred embodiment of the present invention, and is not intended to limit the scope of the present invention. Any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention shall fall within the protection scope of the present invention.

Claims (4)

1. A virtual machine differentiated fault ride-through system of an island microgrid is characterized by comprising a converter output voltage and current acquisition module, a fault detection and judgment module (6), a virtual synchronous control module (1) and a differentiated fault ride-through control module (7); wherein the content of the first and second substances,
the converter output voltage and current acquisition module is used for detecting the voltage at two ends of a filter capacitor and the filter inductor current output by the converter and respectively feeding back the detected voltage and current to the virtual synchronous control module (1), the fault detection and judgment module (6) and the differentiated fault ride-through control module (7);
the fault detection and judgment module (6) is used for judging whether the converter has faults and detecting the fault degree;
the virtual synchronous control module (1) is used for realizing output control when the converter operates normally;
the differential fault ride-through control module (7) is used for realizing fault ride-through control of an island microgrid virtual machine and comprises a rapid current limit control module (3) and a virtual impedance current limit control module (2), wherein when the voltage drop depth of a converter is larger than or equal to a set threshold value, an algorithm change-over switch is connected with the rapid current limit control module (3); when the voltage drop depth of the converter is smaller than the set threshold value, the algorithm change-over switch is connected with the virtual impedance current limiting control module (2);
the rapid current limiting control module (3) detects the voltage phase of the common connection pointθPlus phase shift angleφTo do so byI set Generating three-phase current reference values as reference current amplitudesi abcref And the current transformer outputs three-phase currenti abc Making a difference, and generating a driving signal through the hysteresis control module (4) to control the on-off of an IGBT switching device, thereby realizing the limitation of the fault current of the converter;
the virtual impedance current limiting control module (2) controls the modulation voltage output by the virtual synchronous control module (1)V ref abc For input, modulating the voltageV ref abc And the converter outputs three-phase currenti abc At the virtual impedanceZ vAnd the generated voltage drop is subjected to difference, high-frequency harmonic waves are eliminated through a low-pass filter LPF (5), SPWM modulation is carried out, and the IGBT switching device is driven to be switched on and switched off, so that the fault current-limiting control of the converter is realized.
2. The method for implementing the virtual machine differentiated fault ride-through system of the islanded microgrid according to claim 1 is specifically performed according to the following steps:
step 1, a converter output voltage and current acquisition module detects voltages at two ends of a filter capacitor and filter inductor currents output by a converter, and respectively feeds the detected voltages and currents back to a virtual synchronous control module (1), a fault detection and judgment module (6) and a differentiation fault ride-through control module (7);
step 2, a fault detection and judgment module (6) judges whether the converter fault occurs and detects the fault degree; when no fault occurs, the output control of the converter in normal operation is realized by adopting the virtual synchronous control module (1);
step 3, when a fault occurs and the voltage drop depth of the converter is greater than or equal to a set threshold value, a fast current limiting control module (3) is adopted to realize the control of the converter for limiting the fault current; when a fault occurs and the voltage drop depth of the converter is smaller than a set threshold value, the control of limiting the fault current of the converter is realized by adopting a virtual impedance current limiting control module (2).
3. The method for realizing the virtual machine differentiation fault ride-through system of the islanded microgrid according to claim 2, characterized in that the fault detection and judgment module (6) takes a voltage across a filter capacitor and a filter inductor current signal output by a converter as input, compares an instantaneous value of the current signal with a set current protection value, and when the instantaneous value of the current signal is less than or equal to the current protection value, the algorithm change-over switch is connected with the virtual synchronous control module (1); when the instantaneous value of the current signal is greater than the current protection value, a control switching signal is output, the converter is switched to enter a differential fault ride-through control module, meanwhile, a fault detection and judgment module (6) judges the fault falling depth of the converter according to the magnitude of a voltage signal of a converter port, and when the voltage falling depth of the converter is greater than or equal to a set threshold value, an algorithm switch is connected with a rapid current limiting control module (3); when the voltage drop depth of the converter is smaller than the set threshold value, the algorithm change-over switch is connected with the virtual impedance current limiting control module (2).
4. An implementation method of an islanded microgrid virtual machine differentiated fault ride-through system according to claim 2, characterized in that the virtual synchronous control module (1) outputs a current signal by a converteri s For input, the reference value of the three-phase voltage is used as voltage, the voltage and currenti s Multiplying as power input of the virtual synchronization control module (1) with the active power reference valueP *From rated angular velocityω *Quotient of (1)T m Comparing, the active inertia droop ring in the virtual synchronous control module (1) receives the active power input and then carries out inertia calculation and droop calculation to obtain the angular velocity of the reference waveωWhile simultaneously aligning the angular velocity of the reference waveωIntegrating to obtain the voltage phase of the reference waveθ(ii) a The reactive inertia droop ring in the virtual synchronous control module (1) receives reactive power input and then shares the rated reactive power with the reactive power inputQ *Comparing, calculating inertia and droop to obtain virtual excitationM f i f Virtual excitationM f i f Multiplied by the angular velocity of the reference waveωObtaining the amplitude of the output signal, which is multiplied by the voltage phase of the reference waveθThe sine value of the control module is used as the output of the virtual synchronous control module (1) to complete the output control of the converter during normal operation.
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