CN116031850B - Short-circuit current limiting method and system using instantaneous current to activate virtual impedance - Google Patents
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Abstract
The invention discloses a short-circuit current limiting method and a short-circuit current limiting system for activating virtual impedance by utilizing instantaneous current. The system consists of a traditional voltage type converter and a virtual current-limiting impedance controller. The method utilizes voltage negative feedback generated by the virtual current limiting impedance and the voltage command of the traditional voltage type converter to synthesize a new voltage command, thereby realizing the purpose of limiting the output current of the converter during short circuit fault. The control method provided by the invention has the characteristics of simplicity, high efficiency and the like, instantaneous current is adopted as a judging basis for fault occurrence, vector voltage amplitude change is adopted as a judging basis for fault clearing, quick activation and automatic exit of virtual current limiting impedance are realized, and after the fault is cleared, the converter automatically returns to a normal operation mode, so that the fault resistance of the converter is improved.
Description
Technical Field
The invention relates to the field of converter control, in particular to a short-circuit current limiting method and a short-circuit current limiting system for activating virtual impedance by utilizing instantaneous current.
Background
The energy storage converter is an interface circuit commonly used in energy storage power generation technology, and voltage control of the energy storage converter is beneficial to fundamentally changing low inertia, under damping and other weak network operation characteristics of the converter, and can be theoretically free of the constraint of a phase-locked loop.
The operation stability of the voltage type control converter in a weak current network environment is improved, dynamic voltage and frequency support can be provided for a power network, and the voltage type control converter is widely focused. However, the output current is not directly controlled, when the grid voltage has a short-circuit fault, the output current is limited by the overcurrent characteristic of the power electronic device, and the risk of overcurrent fault exists. In order to avoid the damage of the converter caused by faults, the most direct processing mode is tripping through a relay protection circuit and off-grid planning, but the method not only depends on the reliability of the protection circuit, but also can not provide voltage and frequency support for a power grid during faults. In this regard, a large number of scholars propose short-circuit fault ride-through control of a voltage-controlled converter.
The general topology and control method of the voltage-controlled converter is shown in fig. 1, wherein the converter adopts classical droop control, and the generated voltage command angular frequency omega i And amplitude E i Can be expressed as:
ω i ＝ω 0 -mP i
E i ＝E 0 -nQ i
ω 0 、E 0 the angular frequency and the amplitude of the no-load output voltage of the converter are respectively; m and n are sagging control coefficients; p (P) i And Q i The output active power and reactive power of the converter i are respectively. The deviation of frequency and voltage should be limited to a reasonable range, and can be set:
|ω i -ω 0 |<Δω max
|E i -E 0 |<ΔE max
△ω max 、△E max the allowed frequency and voltage maximum deviations, respectively. The current transformer generally adopts double-loop control by combining a capacitor voltage outer loop and an inductor current inner loop, wherein the voltage outer loop adopts proportional resonance control, and the inductor current inner loop adopts proportional control. In a static coordinate system, the virtual impedance is represented by Z V 、R FCL Two parts, wherein the virtual impedance Z V For decoupling active power and reactive power distribution control, as inductive-based resistive-inductive impedance R V +jX V 。R FCL For a resistive virtual current limiting impedance, it is activated only during a fault, and automatically exits after normal operation and fault clearance.
Firstly, the transient process of three-phase short circuit of the power grid is analyzed without considering the virtual current limiting impedance effect. The circuit model of the voltage type converter with load is shown in fig. 2, f in the figure (3) Representing a three-phase short circuit, the pre-fault circuit is in a steady state, wherein the equivalent impedance from the equivalent internal potential of the converter to the fault point is zeq=req+jXeq, and comprises a virtual impedance Z V And the impedance Z of the connection line between the outlet voltage of the converter and the fault point Line . Point of failure to load equivalentImpedance is Z L ＝R L +jX L . The voltage loop of the typical voltage type converter has slower response speed, and the potential vector U in the voltage loop g The amplitude and phase of the converter can be approximately assumed to be unchanged before and during the fault, and alpha is the voltage phase angle of the converter at the moment of the fault, and can be expressed as:
U g (0 - )＝U g (0 + )＝E i sin(ω i t+α)
because the three-phase circuit is symmetrical, taking the phase A as an example, the current differential equation of the converter before the symmetrical fault of the power grid is obtained is as follows:
the pre-fault current transformer can be solved as follows:
considering the most severe short-circuit fault, i.e. a three-phase symmetrical ground short-circuit, the whole circuit is divided into two independent circuits at the time of the fault, wherein the right circuit becomes a short-circuit without power supply, and the current will decay from the initial value at the moment of the short-circuit to zero. The left hand circuit is still connected to the current transformer and the current transformer will gradually change from the pre-short value to the new steady state value. Taking phase a as an example, assuming that a short circuit occurs at time t=0, the current differential circuit equation of the converter when the grid symmetrical fault occurs is obtained as follows:
according to the open-close law of the circuit, the current in the inductor can not be suddenly changed at the moment before and after the short circuit, I S 、I T The steady-state working current amplitude and the transient current amplitude of the converter during the fault period can be obtained by solving:
from the above equation, the steady-state operating current of the current transformer during a fault is proportional to the internal potential vector of the current transformer and inversely proportional to the equivalent impedance of the current transformer to the point of the fault. Transient current of converter during fault time constant τ=l eq /R eq Attenuation. In particular when the converter is previously in an idle state, i.e. I m =0, and the fault time satisfies α - θ=3pi/2, substituting the above formula to obtain:
i(t)＝I S (e -t/τ -cosω i t)
the maximum instantaneous value of the short-circuit current of the converter appears in half cycle after the fault occurs, T is the power grid cycle, and the maximum impulse current expression is:
I im ＝I S +I S e -T/2τ
the analysis shows that the voltage-type control converter cannot directly inhibit fault current, and the safety and the stability of the voltage-type control converter are threatened. In the conventional converter control method, some scholars adopt mode switching control, such as the literature entitled virtual synchronous converter fault impact current rapid suppression method based on quasi-PR control (China Motor engineering report, 2018 (16): pages 4768-4776) proposes that when faults occur, voltage type control is switched into current type control, and output current is reasonably limited by directly controlling network side current.
However, the mode switching process is complicated, a voltage type and current type one-main-one-standby-two control method is required to be prepared, when the power grid fault is cleared, the normal running state cannot be automatically restored, the voltage type control can be restored through the reverse switching control, and a current suppression strategy based on virtual impedance is provided for some students.
The virtual current-limiting impedance can increase the output impedance of the converter, change the output impedance characteristic, and ideally, the virtual current-limiting impedance R FCL Will be introduced at the moment of fault occurrence, and the equivalent impedance from the equivalent internal potential of the converter to the fault point is changed to be req+R FCL +jXeq, likewiseTaking phase a as an example, considering the virtual current limiting impedance, t=0 the current differential circuit equation of the current transformer after the grid fault is expressed as:
after introducing the virtual impedance, let I S0 、I T0 The steady-state operating current amplitude and the transient current amplitude of the converter during the fault are respectively taken into account the virtual current limiting impedance. The current of the converter can be obtained by:
from the above, a virtual current limiting resistor R is introduced FCL The current limiting resistor can effectively limit the steady-state working current of the current transformer during faults, simultaneously accelerate transient current attenuation of the current transformer, and effectively inhibit transient impact current and steady-state overload current.
The fault current is suppressed by changing virtual impedance in the literature titled A fault clearing method in converter-dominated microgrids with conventional protection means (IEEE Transactions on Power Electronics,2016 (6): 4628-4640), but the method adopts the amplitude of the output current at the net side to generate virtual impedance, the time delay of amplitude detection influences the instantaneous performance of the virtual impedance, the instantaneous impact current cannot be effectively suppressed, and overload damage of power electronic devices is easily caused.
In order to reduce the adverse effect of detection delay as much as possible, the present patent uses instantaneous current as the judgment basis for virtual current limiting impedance activation.
The document titled Virtual Impedance Current Limiting for Inverters in Microgrids With Synchronous Generators (IEEE Transactions on Industry applications,2015 (2): pages 1630-1638) proposes that the virtual current limiting impedance automatically exits when the absolute value of the current transformer instantaneous current is less than the threshold current, i.e., |i| < Ith. However, the judgment logic is too simple, and the current of the converter is automatically limited below the threshold current after the virtual current limiting impedance is input, and the virtual current limiting impedance exiting mechanism is triggered, so that the judgment logic can lead the virtual current limiting impedance to enter and exit the cyclic process.
In order to avoid current oscillation caused by frequent switching of the virtual current limiting resistor during fault as much as possible, the method adopts the change of vector voltage amplitude as the judgment basis for clearing three-phase short circuit faults
Disclosure of Invention
The invention aims to provide a virtual impedance-based current limiting strategy which utilizes instantaneous current to quickly activate virtual current limiting impedance to inhibit instantaneous surge current and steady-state overload current, and can be directly integrated into the existing voltage-type control strategy.
In order to solve the problems of the prior art that the detection delay affects the instantaneous performance of virtual impedance and the current oscillation caused by frequent switching of the virtual current limiting resistor, the invention provides a short-circuit current limiting method for activating the virtual impedance by utilizing instantaneous current.
As one of the improvements of the above technical solutions, the method specifically includes the following steps:
s1, sampling three-phase output instantaneous current i, and judging whether the absolute value of the three-phase output instantaneous current i is larger than threshold current Ith, wherein the threshold current Ith is the safety current of the converter;
s2, when the absolute value of the sampling instantaneous current i is larger than the threshold current Ith, activating the virtual current limiting impedance R FCL For virtual current limiting impedance R FCL The generated voltage negative feedback is summed with a voltage command of the traditional voltage type converter, and finally a control value of the converter is obtained through pulse width modulation so as to reduce the amplitude of the output current of the converter during short circuit; if the absolute value of the output current is smaller than the threshold current, returning to the step S1;
s3, after waiting for a period of time and stabilizing the circuit, sampling and calculating the minimum value U of the three-phase outlet voltage amplitude of the converter 0 ；
S4, the steps are carried outS3, obtaining a minimum value U of the amplitude of the three-phase outlet vector voltage 0 Reference voltage amplitude E generated by sag control of converter i Comparing, when the condition is satisfied: u (U) 0 Greater than E i 2, automatically exiting the virtual current limiting impedance; if not, returning to the step S3.
As one of the improvements of the above technical solution, in the step S2, the instantaneous current of the converter is adopted as the criterion for activating the virtual impedance, specifically: when the output current of any phase in the sampled three-phase outlet current is larger than the threshold current, the virtual current limiting impedance is activated; and when the output currents of three phases in the three-phase outlet currents are smaller than the threshold current, continuing to sample.
The invention provides a short-circuit current limiting system for activating virtual impedance by utilizing instantaneous current, which is respectively connected with a three-phase transmission line, wherein the system comprises a traditional voltage type converter and a virtual current limiting impedance controller;
the virtual current-limiting impedance controller is provided with a current sampling unit, a voltage sampling unit, a data processing unit, an activation processing unit and an execution unit;
the current sampling unit is used for sampling the instantaneous current of the three-phase outlet respectively and transmitting the sampled current instantaneous value to the data processing unit;
the voltage sampling unit is used for respectively sampling the voltage instantaneous values of the three-phase outlets after the virtual current limiting impedance is activated and transmitting the sampled voltage instantaneous values to the data processing unit;
the data processing unit is provided with threshold current and reference voltage amplitude values;
the data processing unit is used for receiving the sampled current instantaneous value transmitted by the current sampling unit and the sampled voltage instantaneous value transmitted by the voltage sampling unit;
after receiving the sampling current instantaneous value, the data processing unit compares the absolute value of the sampling current instantaneous value with the threshold current and gives a virtual current limiting impedance activation instruction to the execution unit according to the comparison result;
after the data processing unit receives the sampled voltage instantaneous value, calculating a voltage amplitude value according to the received sampled voltage instantaneous value (the voltage amplitude value is obtained by carrying out Fourier transform calculation on the voltage instantaneous value), comparing the minimum value of the calculated voltage amplitude value with a reference voltage amplitude value, and then issuing a virtual current limiting impedance exit instruction to the execution unit according to a comparison result;
the execution unit is used for executing the activation and the exit of the virtual current limiting impedance according to the received instruction;
the activation processing unit is used for carrying out summation processing on voltage negative feedback generated after the virtual current limiting impedance is activated and a voltage command of the traditional voltage type converter;
the summed voltage command is regulated by a voltage-current control loop, and finally a control signal of the converter is obtained through Pulse Width Modulation (PWM) so as to reduce short-circuit current.
The control method for limiting short-circuit current by utilizing instantaneous current to activate virtual impedance provided by the invention uses the instantaneous current of the converter as an activation criterion of the virtual current-limiting impedance, and uses the change of the vector amplitude of the voltage vector at the outlet of the converter as a judgment basis for clearing three-phase short-circuit faults. After the fault is cleared, the converter can automatically recover to a normal operation mode, so that the fault processing flow is simplified, and the fault resistance of the converter is improved.
Drawings
Fig. 1 is a schematic diagram of an overall frame of a voltage-type control converter structure and a limiting method and system according to the present invention;
fig. 2 is a schematic diagram of an equivalent circuit when the voltage-type control converter is short-circuited;
fig. 3 is an equivalent circuit of the converter after the virtual current limiting impedance is input;
FIG. 4 is a flow chart of virtual current limiting impedance input and output according to the present invention;
FIG. 5 is a current waveform diagram of a three-phase ground fault without current limiting strategy;
FIG. 6 is a current waveform diagram of a phase short fault without the current limiting strategy;
FIG. 7 is a current waveform diagram of a three-phase ground fault when a virtual current limiting impedance strategy is adopted, wherein FIG. 7 (a) is a current waveform diagram of the whole process of the occurrence and clearing of the three-phase ground fault, and FIG. 7 (b) is a current waveform diagram of suppressing the moment of the fault when the three-phase ground fault is adopted;
fig. 8 is a current waveform diagram of a phase-to-phase short-circuit fault using a virtual current limiting impedance strategy, wherein fig. 8 (a) is a current waveform diagram of the whole process of phase-to-phase short-circuit fault occurrence and clearing, and fig. 8 (b) is a current waveform diagram of suppressing a fault instant at the phase-to-phase short-circuit fault.
Detailed Description
The technical scheme provided by the invention is further described below with reference to the accompanying drawings and the embodiments.
The invention provides a virtual impedance-based current limiting control method, which comprises the following steps:
s1, sampling three-phase output instantaneous current i, and judging whether the absolute value of the three-phase output instantaneous current i is larger than threshold current Ith, wherein the threshold current Ith is the safety current of the converter;
s2, when the absolute value of the sampling instantaneous current i is larger than the threshold current Ith, activating the virtual current limiting impedance R FCL For virtual current limiting impedance R FCL The generated voltage negative feedback is summed with a voltage command of the traditional voltage type converter, and finally a control value of the converter is obtained through pulse width modulation so as to reduce the amplitude of the output current of the converter during short circuit; if the absolute value of the output current is smaller than the threshold current, returning to the step S1;
s3, after waiting for a period of time and stabilizing the circuit, sampling and calculating the minimum value U of the three-phase outlet voltage amplitude of the converter 0 ；
S4, enabling the amplitude value of the three-phase outlet vector voltage obtained in the step S3 to be the minimum value U 0 Reference voltage amplitude E generated by sag control of converter i Comparing, when the condition is satisfied: u (U) 0 Greater than E i 2, automatically exiting the virtual current limiting impedance; if not, returning to the step S3.
In step S2, in order to reduce the intermediate delay link from the fault occurrence to the virtual current limiting impedance input, the invention adopts the instantaneous current of the converter as an activation criterion, and when the instantaneous output current of any phase is larger than the threshold current, the virtual current limiting impedance is activated. In the whole fault process, firstly, the period from the occurrence of the fault to the input of the virtual current limiting impedance should be ensured, the possible maximum impact current of the current transformer should be smaller than the allowable maximum impact current of the current transformer, and meanwhile, after the short-circuit fault circuit enters a new steady state, the steady state fault current amplitude of the current transformer should be smaller than the threshold current.
As shown in fig. 1, the voltage-type control converter structure, the limiting method and the system overall frame method provided by the invention are schematically shown, and the invention adds virtual current-limiting impedance control on the basis of typical virtual impedance-based droop control, so that the output current of the converter can be effectively reduced when faults occur. The invention adds a virtual current-limiting impedance controller on the basis of the existing typical traditional voltage type converter to form a limiting system of the invention; the virtual current limiting impedance adopts the instantaneous current as an activation criterion, so that the delay time from the occurrence of faults to the investment of the virtual current limiting impedance is effectively reduced. Meanwhile, the change of vector voltage amplitude at the outlet side of the converter is used as a criterion of virtual current limiting impedance withdrawal, so that current oscillation caused by frequent switching cycle is avoided;
according to the daving equivalent transformation, the voltage-controlled converter can be equivalently a series model of an ideal voltage source and equivalent impedance, and according to fig. 1, the output impedance of the converter can be expressed as after introducing a virtual impedance in consideration of the influence of a transfer function
Z out ＝G(s)(Z V +R FCL )+Z eq (1)
In the formula (1), G(s) is the transfer function of the voltage of the converter, Z eq The equivalent impedance of the converter is expressed as respectively
In the formula (2), L and C are filter inductance and filter electricity of the converterCapacity, k e Is an inductance current inner loop proportional controller, k PWM For converter gain, T u (s) is a voltage outer loop proportional resonant controller, the transfer function of which can be expressed as
As can be seen from the above analysis and the workflow chart of fig. 4, the virtual impedance current limiting control method of the present invention comprises the following steps:
s1, sampling a three-phase output current instantaneous value i, and judging whether the absolute value of the three-phase output current instantaneous value i is larger than threshold current Ith. In practical application, considering the influence of current detection and control response delay, short circuit fault occurs to virtual current limiting impedance input effect, and a short transition delay exists. During this time delay, the maximum current surge that the converter may exhibit should be less than the maximum current surge I that the converter can tolerate max 。
S2, when the instantaneous value of the output current of any one phase is larger than the threshold current, activating the virtual current limiting impedance R FCL If not, the three-phase circuit line is in the normal working mode, and the step S1 is returned. And synthesizing voltage negative feedback generated by the virtual current limiting impedance with a voltage reference value generated by the droop control module based on the virtual impedance to obtain a new voltage instruction. The command passes through a voltage and current control loop, and finally a control signal of the converter is obtained through PWM modulation. The input of the negative feedback voltage effectively reduces the output current of the converter during faults and plays a role in current limiting.
S3, after the circuit is stabilized, sampling and calculating the minimum value U of the vector voltage amplitude of the three-phase outlet of the converter 0 . Different from the traditional virtual impedance current limiting control, the invention adopts the change of the three-phase outlet vector voltage amplitude of the converter as the basis for exiting the virtual current limiting impedance.
S4, minimizing the amplitude value U of the three-phase outlet voltage obtained in the step S3 0 Reference voltage amplitude E generated by sag control of converter i Comparing when U 0 Greater than E i At/2, the virtual current limiting impedance exitsCompleting the limitation of short-circuit current and restoring the circuit to normal operation; if not, returning to the step S3.
As shown in FIG. 3, after the virtual current limiting impedance is input, the equivalent impedance from the equivalent internal potential of the converter to the outlet voltage side is Z 1 Mainly comprises virtual impedance Z V And a virtual current limiting impedance R FCL . The equivalent impedance from the outlet voltage side of the converter to the load end is Z 2 . Converter outlet side voltage amplitude U 0 Can be expressed as
Z when the symmetrical grounding short-circuit fault occurs and enters a steady state 2 ＝Z Line Since the virtual impedance and virtual current limiting impedance are typically much greater than the link impedance, we approximate |Z here 1 |>3|Z 2 I, can get
Z after the fault is cleared and normal operation is restored 2 ＝Z Line +Z L Due to load re-input into the circuit, |Z 2 |>|Z 1 I, can get
The formulas (5) and (6) can be used as logic judgment basis for judging whether the short circuit fault is eliminated or not, and whether the short circuit fault is eliminated or not can be reasonably judged by analyzing the change of the vector voltage amplitude of the outlet of the converter, and the logic judgment basis is used as judgment basis for the withdrawal of the virtual current limiting impedance.
Fig. 5-8 are experimental results of a semi-physical simulation platform of RT-LAB, in which the current transformer, the line impedance and the load are simulated in the simulation platform of RT-LAB, a real-time hardware controller is used to test the current limiting control method provided herein, and the maximum impact current I allowed by the current transformer is set max 55A and 40A. In order to verify the effectiveness of the short-circuit fault protection method on different types of short-circuit faults, simulation verification is carried out on two fault types, namely a three-phase symmetrical grounding short-circuit fault and an asymmetrical interphase short-circuit fault. Interphase short circuit faults are simulated by series-feeding 8mH reactance at the load side AB.
Fig. 5 and 6 are respectively the whole-process current waveforms of the three-phase ground short-circuit fault at the load end and the inter-phase short-circuit fault at the load end when the current limiting strategy is not adopted. When a three-phase grounding short circuit fault occurs, the maximum instantaneous value of the short circuit current of the converter is up to 255A, and in the fault stage, the current is required to bear the short circuit current, the steady-state working current amplitude of the converter is about 176A, and the maximum impact current allowed by the inverter is far exceeded, so that the safe and stable operation of power electronic devices is seriously threatened. When an interphase short-circuit fault occurs, the impact current at the moment of the short-circuit fault is up to 171A, and enters a fault steady-state stage, and the AB phase current far exceeds the maximum impact current allowed by the converter.
Fig. 7 is a current waveform diagram of a three-phase ground fault when a virtual current limiting impedance strategy is adopted, wherein fig. 7 (a) is a current waveform diagram of the whole process of the occurrence and the clearing of the three-phase ground fault, and fig. 7 (b) is a current waveform diagram of suppressing the moment of the fault when the three-phase ground fault is adopted. As can be seen from the figure, after the fault occurs, the maximum impact current is about 44A, no obvious oscillation exists in the whole process, and the transition is smooth. When a virtual current limiting strategy is adopted, at the moment of fault occurrence, although impact current exists under the influence of delay input of the virtual current limiting impedance, the impact current is smaller than the maximum impact current which can be born by the converter, and the instantaneous current is adopted as an activation criterion, when the fault occurs, the virtual current limiting impedance R FCL The method is fast in input, the transient impact current is restrained from rising, the steady-state working current amplitude is greatly reduced, and the output current of the converter in the short circuit period is limited within threshold current. After the fault is cleared, the virtual current limiting impedance automatically exits according to the voltage vector at the outlet end of the converter, and the converter is restored to the normal running state.
Fig. 8 is a current waveform diagram of a phase-to-phase short circuit fault by using a virtual current limiting impedance strategy, wherein fig. 8 (a) is a current waveform diagram of the whole process of phase-to-phase short circuit fault occurrence and clearing, and as can be seen from the figure, after using the virtual current limiting impedance strategy, output current is limited within the maximum allowable current at the moment of fault and at the stage of fault steady state, and fig. 8 (b) is a current waveform diagram for suppressing the moment of fault when the phase-to-phase short circuit fault, because the virtual current limiting impedance is a three-phase symmetrical resistor, the three-phase output current is still asymmetrical after the fault occurs, but the virtual current limiting impedance guarantees the operation safety of the converter during the asymmetrical short circuit fault, and the effectiveness of the virtual impedance current limiting strategy on the asymmetrical short circuit fault is verified.
The experimental result proves that the control method for limiting the short-circuit current by utilizing the instantaneous current to activate the virtual impedance has the characteristics of simplicity, high efficiency and the like, adopts the three-phase instantaneous current as the judging basis of fault occurrence, realizes the quick activation of the virtual current-limiting impedance, adopts the change of the vector voltage amplitude as the judging basis of fault clearing, realizes the automatic withdrawal of the virtual current-limiting impedance, and can automatically recover to a normal operation mode after the fault is cleared, thereby simplifying the fault processing flow and improving the fault resistance of the converter.
Finally, it should be noted that the above embodiments are only for illustrating the technical solution of the present invention and are not limiting. Although the present invention has been described in detail with reference to the embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the present invention, which is intended to be covered by the appended claims.
Claims (2)
1. A short circuit current limiting method using an instantaneous current to activate a virtual impedance, the method using the instantaneous current to quickly activate the virtual current limiting impedance to suppress an instantaneous rush current and a steady state overload current, and controlling exit of the virtual current limiting impedance according to an exit vector voltage amplitude of a current transformer, the method comprising the steps of:
s1, sampling three-phase output instantaneous current i, and judging whether the absolute value of the three-phase output instantaneous current i is larger than threshold current Ith, wherein the threshold current Ith is the safety current of the converter;
s2, when sampling is instantaneousWhen the absolute value of the current i is larger than the threshold current Ith, the virtual current limiting impedance R is activated FCL For virtual current limiting impedance R FCL The generated voltage negative feedback is summed with a voltage command of the traditional voltage type converter, and finally a control value of the converter is obtained through pulse width modulation so as to reduce the amplitude of the output current of the converter during short circuit; if the absolute value of the output current is smaller than the threshold current, returning to the step S1;
s3, after waiting for a period of time and stabilizing the circuit, sampling and calculating the minimum value U of the three-phase outlet voltage amplitude of the converter 0 ；
S4, enabling the amplitude value of the three-phase outlet vector voltage obtained in the step S3 to be the minimum value U 0 Reference voltage amplitude E generated by sag control of converter i Comparing, when the condition is satisfied: u (U) 0 Greater than E i 2, automatically exiting the virtual current limiting impedance; if not, returning to the step S3;
in the step S2, the instantaneous current of the converter is used as the basis for determining the activated virtual impedance, which specifically includes: when the output current of any phase in the sampled three-phase outlet current is larger than the threshold current, the virtual current limiting impedance is activated; and when the output currents of three phases in the three-phase outlet currents are smaller than the threshold current, continuing to sample.
2. A short-circuit current limiting system using instantaneous current to activate virtual impedance based on the method of claim 1, said system being separately connected to three-phase transmission lines, said system comprising a conventional voltage-type current transformer, characterized in that said system further comprises a virtual current limiting impedance controller;
the virtual current-limiting impedance controller is provided with a current sampling unit, a voltage sampling unit, a data processing unit, an activation processing unit and an execution unit;
the current sampling unit is used for sampling the instantaneous current of the three-phase outlet respectively and transmitting the sampled current instantaneous value to the data processing unit;
the voltage sampling unit is used for respectively sampling the voltage instantaneous values of the three-phase outlets after the virtual current limiting impedance is activated and transmitting the sampled voltage instantaneous values to the data processing unit;
the data processing unit is provided with threshold current and reference voltage amplitude values;
the data processing unit is used for receiving the sampled current instantaneous value transmitted by the current sampling unit and the sampled voltage instantaneous value transmitted by the voltage sampling unit;
after receiving the sampling current instantaneous value, the data processing unit compares the absolute value of the sampling current instantaneous value with the threshold current and gives a virtual current limiting impedance activation instruction to the execution unit according to the comparison result;
after receiving the sampled voltage instantaneous value, the data processing unit calculates a voltage amplitude according to the received sampled voltage instantaneous value, compares the minimum value of the calculated voltage amplitude with a reference voltage amplitude, and then gives a virtual current limiting impedance exit instruction to the execution unit according to the comparison result;
the execution unit is used for executing the activation and the exit of the virtual current limiting impedance according to the received instruction;
the activation processing unit is used for carrying out summation processing on voltage negative feedback generated after the virtual current limiting impedance is activated and a voltage command of the traditional voltage type converter;
the summed voltage command is regulated by a voltage current control loop, and finally a control signal of the converter is obtained through pulse width modulation, so that short-circuit current is reduced.
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