CN104113052A - Method for protecting active power filter (APF) - Google Patents
Method for protecting active power filter (APF) Download PDFInfo
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Abstract
The invention discloses a method for protecting an active power filter (APF). In the method for protecting the APF provided by the invention, by current limiting of instruction current, compensating current actually output by the APF is limited within a reasonable range, thereby preventing the instruction current from being too large, causing APF overcurrent and damaging the APF. In the method in the invention, proportional current limiting and switch-off current limiting are performed on the instruction current in sequence, and dual protection is realized through two times of current limiting, thereby improving reliability of a system, better protecting the APF, and prolonging service life.
Description
Technical Field
The invention belongs to the technical field of power electronics, and particularly relates to a protection method of an active power filter.
Background
The increasing development and maturity of power electronic technology and the wide application of power automation equipment also cause serious power grid harmonic pollution problems while promoting the development of industrial productivity and the improvement of electrical automation level, and Active Power Filters (APFs) are produced at the same time.
An Active Power Filter (APF) is comprised of a main circuit (converter), a drive circuit and a control system. The compensation process comprises the steps of firstly obtaining current signals of system voltage and a nonlinear load through a voltage transformer (PT) and a Current Transformer (CT), processing the obtained voltage signals and current signals by a control system, calculating instruction current corresponding to each subharmonic needing to be compensated, then generating driving pulse for driving a converter according to the instruction current, rapidly controlling the on-off of a switching device (power device) in a main circuit in real time, outputting actual compensation current needing to be compensated, offsetting the compensation current and each subharmonic needing to be compensated in the load current, and finally obtaining expected sinusoidal power grid current, so that the electric energy quality of a power grid is improved.
As a power grid harmonic and reactive dynamic compensation device, the APF not only needs to have good steady-state compensation performance and dynamic response speed, but also ensures stable and reliable operation. The precondition of stable operation of the APF is that the compensation capacity of the APF cannot exceed the rated capacity of the APF, and because the power unit in the converter is very sensitive to instantaneous overcurrent and overvoltage, which may cause damage to the switching tube, the excessive operation of the APF is avoided as much as possible.
Normally, there are two situations that may cause the APF to run excessively: (1) a non-linear load surge connected to the utility grid; (2) the load connected into the public power grid has faults such as short circuit or open circuit, and the like, so that the asymmetric component of the system is increased suddenly. Both of these conditions, if they cause the APF to operate over-capacity, are primarily due to sudden increases in the harmonic components of the load current resulting in sudden increases in the command current. Therefore, to limit the over-capacity operation of the APF, it is critical to limit the command current within the self-compensation capacity.
Yangyu et al propose a software current-limiting control strategy in the article entitled parallel active power filter current-limiting compensation strategy research (power automation equipment 2006,26(3):21-25), which only processes reference current, is simple to control and achieves better current-limiting effect. However, it has one of the most critical problems that have not been solved: maximum absolute value i of command currentrefmaxAnd the system maximum allowable compensation current value imaxThe definition of (1). Conventional cutoff and proportional limiting strategies for command current maximum absolute value irefmaxIs defined as the absolute value of the peak value of the command current, and the maximum allowable compensation current value i of the systemmaxIs defined as a fixed value according to the system capacity.
In fact, for an APF system capable of compensating any subharmonic within 0-2.5 kHz, the maximum absolute value i of the command currentrefmaxIt cannot be simply defined as the absolute value of the command current peak. In addition to the peak value at which the effective value of the command current reaches the rated capacity of the system when compensating for the full-band harmonics, the peak value at which the effective value of the command current reaches the rated capacity of the system when compensating for the low order or single harmonic is considered. The rated capacity of an APF is usually defined in terms of the effective value of the rated harmonic current that can be compensated for per phase. KnotIn practical terms, when the APF needs to compensate full-band harmonics, if the effective value of the compensation current of the system reaches the rated value, the peak value of the command current is usually much higher than the peak value of the command current when the APF system only compensates low-order or single-order harmonics and the effective value of the compensation current reaches the rated value. Therefore, if the maximum value i of the command current is simply setrefmaxDefined as a peak, then the following two cases may occur:
(1) if the effective value reaches the rated value when the full-band harmonic wave is compensated according to the APF, the peak value of the command current is defined as the maximum allowable compensation current peak value i of the systempeakWhen the APF compensates the low order or single harmonic, even if the effective value of the compensation current exceeds the rated value, the peak value of the command current does not reach the maximum allowable compensation current peak value i set by the systempeakThe command current will continue to increase until the peak reaches the set ipeakAnd the effective value of the command current far exceeds the rated effective value, so that the APF is caused to overcurrent.
(2) The peak value of the command current when the effective value reaches the rated value when compensating low order or single harmonic according to APF is defined as the maximum allowable compensation current peak value i of the systempeak(i.e., maximum allowable current), then when the APF compensates for the full band harmonics, the peak value of the command current reaches the set ipeakIn time, the effective value of the command current is far from the rated value of the system, so that the utilization rate of the APF is low.
Meanwhile, an effective harmonic detection method is needed to obtain an effective value of the command current for the proportional current limiting link and realize selective harmonic compensation of the APF. Zhang Tree et al (Chinese Motor engineering reports 2010(03):55-62) propose a selective harmonic current detection algorithm based on synchronous rotation coordinate transformation under the problem of multiple synchronous rotation coordinate systems, but the method needs to be subjected to multiple times of rotation coordinate transformation, has large calculation amount, needs to design a low-pass or band-pass filter, has detection precision of harmonic components depending on the design of the filter, and has direct influence on the detection precision due to improper parameter design of the filter.
In addition, voltage fluctuation on the direct current side of the APF is caused when a large-capacity nonlinear load suddenly changes, and if overvoltage occurs on the direct current side, the switching devices connected in parallel to two ends of the direct current bus capacitor may be damaged due to transient high-voltage breakdown.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a protection method of an active power filter.
The protection method of the active power filter comprises the following steps:
(1) extracting each subharmonic component needing to be compensated in the load current, and superposing the extracted each subharmonic component to obtain an instruction current;
(2) setting a first threshold value and a second threshold value in sequence according to a rated compensation current effective value of the active power filter and the highest harmonic frequency needing to be compensated;
(3) proportionally limiting the instruction current by taking the first threshold as the maximum allowable current to obtain a first instruction current;
(4) taking the second threshold value as an interruption peak current to carry out interruption current limiting on the first instruction current to obtain a second instruction current;
(5) and generating a driving pulse for driving the converter in the active power filter according to the second instruction current so as to enable the converter to output corresponding compensation current.
The protection method of the active power filter provided by the invention limits the compensation current actually output by the active power filter within a reasonable range by limiting the instruction current, prevents the active power filter from being damaged due to APF overcurrent (the compensation current actually output exceeds the maximum allowable value born by a switching device) caused by overlarge instruction current, sequentially performs proportional current limiting and cutoff current limiting on the instruction current, realizes double protection by twice current limiting, improves the reliability of the system, can better protect the active power filter, and prolongs the service life.
Before proportional current limiting and cutoff current limiting processing, related parameters of proportional current limiting and cutoff current limiting are set according to compensation frequency set by a user. Therefore, relevant parameters are set according to actual conditions, and compared with the condition that a single parameter is fixedly used under any working condition in the prior art, the method is more flexible, the output capacity of the APF can be further improved under the condition that the APF only compensates low-order harmonics, the utilization efficiency of the APF is improved, and even under the condition of full-band compensation, the compensation current can be effectively limited within the range of rated output capacity. The proportion current limiting and cutoff current limiting method with adjustable parameters is beneficial to realizing the maximization of APF output capacity and the unification of high reliability under different working conditions.
Preferably, the step (1) extracts each subharmonic component to be compensated based on a sliding window iterative DFT algorithm.
The method overcomes the defects of complex coordinate transformation and difficult parameter design in a harmonic current detection algorithm based on a synchronous rotating coordinate system based on a sliding window iteration DFT algorithm, improves the traditional sliding window DFT algorithm by canceling a periodic averaging filter, greatly reduces the occupation of program space resources, realizes single or superposed extraction of any subharmonic, and has high precision.
The specific process of extracting n-order harmonic classification based on the sliding window iteration DFT algorithm is as follows:
a, initializing a sliding window, wherein the length of the sliding window is the total number of sampling points in one fundamental wave period, and is marked as N, assuming that data sampled by the sliding window is stored in an array X [ N ] with the size of N, and the values of all elements in the sliding window are 0 under an initial condition, namely X [0] -X [ N-1] ═ 0;
b, collecting new data x (k tau) (x (k tau) is sampling data at a certain moment, wherein k represents the number of a current sampling point, the value range of k is 0-N-1, and tau is a sampling time interval), and updating the sampling data corresponding to the sampling point before a fundamental wave period of a sliding window;
c. calculating the phase of the n-th harmonic wave to be extracted at the current sampling point, specifically:
one period is 2 pi, and one period samples N sampling points, so that the phase difference between each sampling point isIn the invention, the phase at the k-th sampling point is assumed to be 0Since it is an nth harmonic, the phase of the nth harmonic at the kth sampling point is
d. Judging whether the number k of the current sampling point meets the condition that k is equal to N-1, wherein N is the total number of sampling points in one fundamental wave period:
if yes, the order is:
real part A of nth harmonic component for instantaneous value calculation of harmonic component at current sampling pointnAnd an imaginary part BnRespectively as follows: a. then＝an，Bn＝bn；
Instantaneous value x of n-th harmonic component at current sampling pointn(k) Comprises the following steps:
xn(k)＝ancos(2πnk/N)+bnsin(2πnk/N)，
wherein,
an＝an+x(kτ)*cos(2πnk/N)，
bn＝bn+x(kτ)*sin(2πnk/N)。
otherwise, let:
real part A of instantaneous value of nth harmonic component for calculation at current sampling pointnAnd an imaginary part BnRespectively as follows: a. then＝A'n，Bn＝B'nWherein A'nAnd B'nRespectively the real part A of the nth harmonic component of the previous fundamental periodnAnd an imaginary part BnCalculating the real part and the imaginary part of the harmonic component at the current sampling point, not updating, and continuing to use the value of the previous period;
further, according to the real part AnAnd an imaginary part BnObtaining the instantaneous value x of the n-th harmonic component at the current sampling pointn(k) Comprises the following steps:
xn(k)＝Ancos(2πnk/N)+Bnsin(2πnk/N)，
e. and after the sampling point number k is added with 1, returning to continue executing the step b until the k is equal to N, and ending.
The step (2) calculates the first threshold value i according to the following formulamax：
imax＝Ki*ip，
Wherein ipIs the rated compensation current effective value;
the first proportionality coefficient is Ki：
Wherein h is the highest harmonic number to be compensated.
The step (2) calculates the second threshold value i according to the following formulapeak：
ipeak＝Kp*imax，
Wherein imaxIs a first threshold value;
Kpis the second proportionality coefficient:
wherein h is the highest harmonic number to be compensated.
The principle that the APF operation efficiency is maximized by setting the first proportional coefficient and the second proportional coefficient according to the highest harmonic frequency to be compensated ensures that the APF can safely, reliably and stably operate under various different working conditions and the utilization efficiency of a switching device (power unit) of the converter is highest. The specific selection process is as follows: firstly, establishing a simulation related APF model through a Simulink simulation function of Matlab, initializing and setting two proportionality coefficients (namely a first proportionality coefficient and a second proportionality coefficient and also including how frequency bands are divided and initial value selection of proportionality coefficient values in each frequency band) according to set harmonic times needing compensation and harmonic current waveform characteristics (mainly the relation between a harmonic current effective value and a harmonic current peak value) under the corresponding compensation harmonic times, and carrying out effect verification after properly increasing the proportionality coefficients if the actual peak current exceeds a maximum allowable compensation current peak value and the actual compensation current effective value exceeds the maximum allowable compensation current effective value under the action of the initialized proportionality coefficient values of the simulated APF model; on the contrary, if the ratio exceeds the above range, the ratio is appropriately decreased to verify the effect. After multiple feedback adjustment and verification, two optimal proportionality coefficients can be obtained, and the gradient type set value provided by the invention can be used as optimization.
The step (3) comprises the following steps:
(3-1) calculating an effective value of the command current, and using a quotient obtained by dividing the first threshold value by the larger of the effective value of the command current and the first threshold value as a proportional current limiting factor;
(3-2) according to the formula:
i'ref＝k*iref
calculating a first instruction current i'ref，irefK is the proportional current limiting factor for the command current.
Taking the effective value i of the instruction currentrmsAnd a first threshold value imaxThe larger of them is marked as iMaxReuse the first threshold imaxDivided by the larger iMaxThe quotient obtained is used as the proportional limiting factor k, i.e. k ═ imax/iMAX. When the effective value of the command current is larger than a first threshold value, iMAX＝irmsAt the moment, the proportional current limiting factor k is less than 1, and the obtained first command current i'refLess than the command current iref(original command current), namely, the function of limiting current. When the effective value of the command current is less than the first threshold value, iMAX＝imaxAt this time, the proportional efficiency factor k is 1, and the obtained first command current is equal to the command current, which is equivalent to outputting the originally calculated command current as it is.
The invention is based on:
calculating the effective value i of the command currentrmsN is the total number of sampling points in a fundamental wave period, h is the highest compensation harmonic frequency, and N represents the compensation harmonic frequency.
In the step (4), according to a formula:
the first command current is cut off and limited.
The output of the proportional current-limiting link is used as the input of the cut-off current-limiting link, so that the double current-limiting of cutting off the current-limiting after proportional current-limiting is formed, and the second command current can be obtained after the cut-off current-limitingStream i ″)refThe second command current is used for generating a driving pulse for driving the converter, and the amplitude of the first command current is limited to [ -i ] by cutting off the current limitpeak,ipeak]Further preventing the compensating current from being too large.
The protection method also comprises the steps of detecting the direct current side voltage (namely the voltage at two ends of the direct current bus capacitor) of the active power filter in real time, and carrying out hardware blocking and current limiting on the active power filter according to the direct current side voltage, wherein the specific process comprises the following steps:
(S1) detecting a dc side voltage of the active power filter in real time;
(S2) conditioning the detected direct current side voltage to a set voltage range to obtain a corresponding equivalent direct current side voltage;
(S3) comparing the conditioned equivalent dc side voltage with a third threshold, and according to the comparison result, performing the following operations:
if the voltage of the equivalent direct current side is greater than or equal to the third threshold value, blocking the driving pulse of the converter in the active power filter until the voltage of the equivalent direct current side is reduced to the fourth threshold value, and removing the blocking to enable the corresponding compensation current output by the converter;
otherwise, no operation is performed.
Because the load is suddenly changed, the direct-current side overvoltage condition easily occurs, in order to adjust the direct-current side voltage to restore the direct-current side voltage to a normal level, the controller superposes a fundamental wave active component in the original command current obtained according to the load harmonic current detection, and the two components are superposed, so that the maximum compensation current allowable value of the APF can be possibly exceeded, and the APF overcurrent is caused. However, when the load fluctuates, because there is usually a certain delay in the change and tracking of the command current, the control of the voltage on the dc side is completed by superimposing a fundamental wave active component in the command current, a certain delay is required, and it is likely that the APF is damaged due to overvoltage on the dc side when the command current is not yet adjusted. According to the invention, the voltage of the direct current side is detected in real time, when overvoltage of the direct current side is detected, the driving pulse of the power module in the APF is directly blocked, the damage of a switch device caused by the overvoltage is prevented, and simultaneously, the follow current effect of the diode on the direct current side is gradually reduced, so that the adjustment of the voltage of the direct current side is completed, the time delay can be effectively eliminated, the real-time performance is strong, the impact current is well inhibited, the control is flexible, and the realization is simple.
The principle of hardware blocking current limiting is as follows: when the equivalent voltage value of the instantaneous value of the direct current side voltage exceeds the maximum set value (third threshold), the driving pulse is forcibly pulled down (namely, the driving pulse of the converter in the active power filter is blocked), the switching device (power unit) which is conducted by the converter is blocked, a circuit enters a follow current state, the direct current side voltage is gradually reduced through the follow current effect of diodes at two ends of the switching device, when the equivalent voltage value of the instantaneous value of the direct current side voltage is reduced to the minimum set value (fourth threshold), the driving pulse is reopened, the power unit restores to normal operation, and the effect of protecting the active power filter under the condition of overvoltage at the direct current side is achieved.
Because the collected direct-current side voltage under the normal operation condition of the APF is usually higher voltage (about 700 +/-50V), direct comparison protection cannot be carried out, and for convenience, the collected direct-current side voltage of the active power filter is regulated to be within a set voltage range to obtain corresponding equivalent direct-current side voltage; a comparison is then done with the equivalent dc side voltage to determine if hardware lockout current limiting is required. In the invention, a proper equivalent conditioning transformation ratio K is set according to the actual application requirementTIn the normal case KTThe value range of (a) is 0.004-0.006. Using a set equivalent conditioning ratio KTThe detected DC side voltage V is calculated according to the following formuladcConditioning to a set voltage range to obtain an equivalent direct current side voltage V'dc：
V′dc＝KT*Vdc。
In fact, if the technical conditions allow, the subsequent comparison may be performed directly with the collected dc side voltage without conditioning.
Said third threshold value V1According to the formula:
V1＝KT*(Vdcp+ΔVh)
calculation of where VdcpIndicating the DC-side voltage rating, Δ VhIndicating the overvoltage margin of the overvoltage protection on the DC side, KTRepresenting the equivalent conditioning ratio.
The fourth threshold value V2According to the formula:
V2＝KT*(Vdcp-ΔVl)
calculation of VdcpIndicating the DC-side voltage rating, Δ VlUndervoltage margin, K, representing the release of the overvoltage protection on the DC sideTRepresenting the equivalent conditioning ratio.
The selection of the overvoltage allowance of the direct current side overvoltage protection and the undervoltage allowance of the direct current side overvoltage protection release can directly influence the sensitivity of the protection, and the selection principle needs to consider the reliability of the protection on one hand, so the value cannot be too large, otherwise the sensitivity is too low; on the other hand, the size of the APF is too small, otherwise, misoperation can occur, and the working efficiency of the APF is affected. Preferably, the overvoltage allowance DeltaV of the direct-current side overvoltage protectionh50-100V can be taken, and undervoltage allowance delta V of overvoltage protection removal at direct current sidelThe voltage can be 100-150V.
Compared with the prior art, the invention has the following advantages:
(1) the advantages of proportional current limiting and cut-off current limiting are integrated, and the reliability of PWM driving pulse is ensured by adopting double protection of proportional current limiting first and cut-off current limiting later.
(2) The current waveform characteristics of APF when compensating full-band harmonic waves and only compensating low-order or single harmonic waves are comprehensively considered, the threshold values of proportional current limiting and cut-off current limiting are flexibly set according to the practical application condition, the magnitude of the instruction current can be effectively limited, the current limiting effect on the compensation current output by the APF is further achieved, and the utilization efficiency of the APF under various working conditions can be guaranteed.
(3) The protection method based on hardware blocking is carried out according to the direct current side voltage of the APF while the command current is subjected to proportional current limiting and cutoff current limiting in sequence, the defect that certain delay exists when the direct current side voltage is adjusted by superposing one fundamental wave active component in the command current is effectively overcome, the switching device of the converter can be protected rapidly to prevent the direct current side from over-voltage damage to the switching device, the impact load sudden change is well inhibited, and the control is flexible and simple to realize.
(4) The improved sliding window iteration DFT algorithm overcomes the defects of complex coordinate transformation and difficult parameter design of a harmonic current detection algorithm based on a synchronous rotating coordinate system, and the traditional sliding window DFT algorithm is improved by canceling a periodic averaging filter, so that the occupation of program space resources is greatly reduced, the single or overlapping extraction of any subharmonic is realized, and the precision is high.
Drawings
Fig. 1 is a flowchart of a protection method for an active power filter according to the present embodiment;
FIG. 2 is a reference voltage circuit of the present embodiment;
FIG. 3 is a hysteresis comparator circuit of the present embodiment;
fig. 4 is a schematic structural diagram of the SAPF system according to the present embodiment;
FIG. 5 is a full-band steady-state compensation waveform of the SAPF under a rated condition according to the present embodiment;
FIG. 6 is a steady state compensation waveform of the SAPF performing current limit compensation according to the present embodiment;
fig. 7 is a dynamic compensation waveform of current limit compensation performed by the SAPF according to the present embodiment.
Detailed Description
The invention is further described with reference to the following figures and specific examples.
The protection method of the active power filter of the present embodiment, as shown in fig. 1, includes the following steps:
(1) extracting each subharmonic component needing to be compensated in the load current, and superposing the extracted each subharmonic component to obtain an instruction current;
in this embodiment, each subharmonic component to be compensated is extracted based on the sliding window iterative DFT algorithm, and a specific process of extracting n-th harmonic classification based on the sliding window iterative DFT algorithm is as follows:
a, initializing a sliding window, wherein the length of the sliding window is the total number of sampling points in one fundamental wave period, and is denoted as N (in the embodiment, N is 200), assuming that data sampled by the sliding window is stored in an array X [ N ] with the size of N, and the values of all elements in the sliding window are 0 under an initial condition, that is, X [0] -X [ N-1] ═ 0;
b, collecting a new data x (k tau), wherein k represents a number corresponding to a current sampling point in a fundamental wave period, tau is a sampling time interval (1/N times of the fundamental wave period), and updating the sampling data corresponding to the sampling point before one fundamental wave period of a sliding window;
c. calculating the phase of the n-th harmonic wave to be extracted at the current sampling point, specifically:
one period is 2 pi, and one period samples N points, so that the phase difference between each sampling point isIn the invention, the phase at the k-th sampling point is assumed to be 0Since it is an nth harmonic, the phase of the nth harmonic at the kth sampling point is
d. Judging whether the number k of the current sampling point meets the condition that k is equal to N-1, wherein N is the total number of sampling points in one fundamental wave period:
if yes, the order is:
real part A of nth harmonic component for instantaneous value calculation of harmonic component at current sampling pointnAnd an imaginary part BnRespectively as follows: a. then＝an，Bn＝bn；
The instantaneous value of the n-th harmonic component at the current sampling point is as follows:
xn(k)＝ancos(2πnk/N)+bnsin(2πnk/N)，
wherein,
an＝an+x(kτ)*cos(2πnk/N)，
bn＝bn+x(kτ)*sin(2πnk/N)，
otherwise, let:
real part A of nth harmonic component for instantaneous value calculation of harmonic component at current sampling pointnAnd an imaginary part BnRespectively as follows: a. then＝A'n，Bn＝B'nWherein A'nAnd B'nRespectively the real part A of the nth harmonic component of the previous fundamental periodnAnd an imaginary part BnCalculating the real part and the imaginary part of the harmonic component at the current sampling point, not updating, and continuing to use the value of the previous period;
the instantaneous value of the nth harmonic component at the current sampling point is:
xn(k)＝Ancos(2πnk/N)+Bnsin(2πnk/N)，
e. and after the sampling point number k is added with 1, returning to continue executing the step b until the k is equal to N, and ending.
At the end of each cycle, the nth harmonic component (i.e. the instantaneous value of the nth harmonic component) at the current sampling point and the corresponding A are obtainednAnd Bn。
(2) Setting a first threshold value and a second threshold value in sequence according to the rated compensation current effective value of the active power filter and the highest harmonic frequency needing to be compensated;
calculating the first threshold i according to the following formulamax：
imax＝Ki*ip，
Wherein ipIs the rated compensation current effective value;
Kifirst scale factor:
wherein h is the highest harmonic number to be compensated.
Calculating the second threshold value i according to the following formulapeak：
ipeak＝Kp*imax，
Wherein imaxIs a first threshold value;
Kpis the second proportionality coefficient:
wherein h is the highest harmonic order to be compensated.
(3) Carrying out proportional current limiting on the instruction current by taking the first threshold as the maximum allowable current to obtain a first instruction current;
(3-1) according to:
calculating the effective value i of the command currentrmsN is the total number of sampling points in a fundamental wave period, h is the highest harmonic frequency needing to be compensated, and N represents the harmonic frequency needing to be compensated (only odd number of compensation is carried out, namely N is an odd number);
dividing the first threshold value by the larger of the effective value of the command current and the first threshold value to obtain a quotient which is used as a proportional current limiting factor;
(3-2) according to the formula:
i'ref＝k*iref
calculating a first instruction current i'ref，irefK is the proportional current limiting factor for the command current.
(4) Cutting off and limiting the first instruction current by taking the second threshold as cut-off peak current to obtain second instruction current; in particular according to the formula:
the first instruction current is cut off and limited, namely the output of the proportional current limiting link is used as the input of the cut-off current limiting link, so that double current limiting of cutting off and limiting after proportional current limiting is formed, and the second instruction current i' can be obtained after cutting off and limitingrefI ″ "ofrefI.e. the final reference command current, the magnitude of the first command current being limited to-i by interrupting the current limitingpeak,ipeak]From beginning to endOne step prevents the compensating current from being too large.
(5) And generating a driving pulse for driving the converter of the active power filter according to the second command current, so that the converter outputs corresponding compensation current.
The protection method of the embodiment further includes detecting a dc side voltage (voltage across a dc bus capacitor) of the active power filter in real time, and performing hardware lockout and current limitation on the active power filter according to the dc side voltage, and the specific process is as follows:
(S1) acquiring the direct current side voltage of the active power filter in real time;
(S2) conditioning the detected DC side voltage to a set voltage range to obtain a corresponding equivalent DC side voltage, which is specifically as follows:
in this embodiment, the detected dc-side voltage V is regulated to 5V or less, the equivalent regulation transformation ratio KT is set to 0.00517, and the detected dc-side voltage V is regulated according to the following formuladcConditioning to a set voltage range to obtain an equivalent direct current side voltage V'dc：
V′dc＝KT*Vdc。
(S3) comparing the regulated dc-side equivalent voltage with a third threshold, and according to the comparison result, performing the following operations:
if the voltage of the equivalent direct current side is greater than or equal to the third threshold value, blocking the driving pulse of the converter in the active power filter until the voltage of the equivalent direct current side is reduced to the fourth threshold value, and then removing the blocking to further finish the adjustment of the compensation current output by the converter;
otherwise, no operation is performed.
In this embodiment, the rated value of the dc side voltage is 750V, and the overvoltage margin Δ V of the dc side overvoltage protectiondcTaking an undervoltage allowance delta' V of 100V for removing overvoltage protection at a direct current sidedcThe actual DC side overvoltage protection value is 850V and the overvoltage protection release value is 600V because 150V is taken.
According to the formula:
V1＝KT*(Vdcp+ΔVh)
calculating a third threshold value V1Wherein V isdcpIndicating the DC-side voltage rating, Δ VhIndicating the overvoltage margin of the overvoltage protection on the DC side, KTExpressing the equivalent conditioning transformation ratio to obtain a third threshold value V1＝4.4V。
According to the formula:
V2＝KT*(Vdcp-ΔVl)
calculating a fourth threshold value V2，VdcpIndicating the rated value of the voltage at the DC side, Deltavl indicating the undervoltage margin for releasing the overvoltage protection at the DC side, KTExpressing the equivalent conditioning transformation ratio to obtain a fourth threshold value V1＝3.1V。
The hardware blocking and current limiting are completed based on the following system, wherein the system comprises a voltage conditioning circuit, a reference voltage circuit, a hysteresis comparison circuit and a drive protection circuit, and the hardware blocking and current limiting system comprises:
the voltage conditioning circuit is used for conditioning the collected direct current side voltage to a set voltage range and outputting equivalent direct current side voltage;
the reference voltage circuit is used for generating corresponding reference voltage according to a set voltage range;
the hysteresis comparison circuit takes a reference voltage and an equivalent direct current side voltage as input, outputs a corresponding level signal, and the corresponding threshold values of the hysteresis comparison circuit are respectively a third threshold value and a fourth threshold value, namely 4.4V and 3.1V;
and the driving protection circuit receives the output of the hysteresis comparison circuit, and outputs a driving protection signal when the output level of the hysteresis comparison circuit is high level, wherein the driving protection signal is used for blocking the driving pulse of the converter in the APF.
The reference voltage circuit is shown in fig. 2 and includes a zener diode U1, a first pin 1 of the zener diode U1 is connected to a power source DVCC through a current limiting resistor R1, a second pin 2 of the zener diode U1 is connected to GND, a bypass capacitor C3 is connected between the first pin 1 and the second pin 2, a third pin 3 of the zener diode U1 is connected to the first pin through a first voltage dividing resistor R2, and is connected to the second pin through a second voltage dividing resistor R3, in order to achieve stable output of a reference voltage, the power source DVCC is connected to DGND through a first filter capacitor C1 and a second filter capacitor C2 which are connected in parallel.
The reference voltage circuit takes a first pin of a voltage stabilizing diode U1 as an output end to output a reference voltage V of an equivalent direct current side voltagerefOutput reference voltage VrefIs determined by the R2, the R3 and the reference voltage of the Zener diode, and satisfies the following relations:
in the formula, VTIs the reference voltage of the used stabilivolt.
Different reference voltages can be obtained by adjusting related parameters according to the set voltage range.
In this embodiment, the zener diode U1 in the reference voltage circuit is of TL431 type, the reference voltage is 2.5V, in this embodiment, R2 is 1.2k Ω, R3 is 3k Ω, and the rest of the parameters are set according to conventional selection, respectively, C1 is 0.1uF, C2 is 0.01uF, C3 is 1nF, R1 is 120 Ω, and the corresponding output reference voltage is:
FIG. 3 shows a hysteresis comparator circuit, which includes a comparator U2, an equivalent DC side voltage VdcA reference voltage V connected to the positive input terminal 5(V +) of the comparator U2 via a resistor R4refIs connected to the inverting input 6(V-) of the comparator U2 through a resistor R5. The positive input end 5 is grounded DGND through a capacitor C4 to play a role of filtering, the output end 7 of a comparator U2 is connected to a power supply DVCC (5V) through a resistor R7 to play a role of pulling up, a feedback resistor R6 is connected between the positive input end and the output end, and the output end of the comparator is connected to a next-stage drive protection circuit.
The voltage range set in this embodiment is 5V or less, and the corresponding third threshold V1And a fourth threshold value V2The voltage is 4.4V and 3.1V respectively, parameters in the hysteresis comparison circuit need to be reasonably set, and the setting principle is as follows:
wherein, VpHAnd VpLThe output voltage levels of the hysteresis circuit are respectively high and low.
In the hysteresis comparison circuit of this embodiment, VpH＝0V，VpLCorresponding to 5V, R4 ═ 6.2k Ω and R6 ═ 24k Ω are set. The other parameters are set according to conventional selection, and are respectively as follows:
C4＝1nF，R5＝5.1kΩ，R7＝39kΩ。
under normal conditions, the input equivalent DC side voltage is between 3.1V and 4.4V, and the output level signal V is at the momentpIs 0V. When the input equivalent DC side voltage is greater than or equal to 4.4V (fourth threshold V)2) Time, output level signal VpHigh level until the equivalent DC side voltage is less than or equal to 3.1V (third threshold value V)1) Time, output level signal VpAnd then low again.
In order to further verify the beneficial effects of the invention, corresponding experimental verification is carried out on a built 14kVA three-phase four-wire system SAPF prototype, and the structure of the prototype system is shown in FIG. 4. In the figure, uSa、uSb、uScFor three-phase voltages of the grid, iSa、iSb、iSc、iSNThree-phase current and neutral current of the power grid, Lga、Lgb、LgcFor each phase leakage inductance of the grid, iLa、iLb、iLc、iLNRespectively the load three-phase current and the neutral current, iCa、iCb、iCc、iCNTo compensate for the three-phase current and the neutral current, respectively. The nonlinear load adopts a three-phase uncontrolled rectifier bridge with inductance load Lc、Ls、RdAnd CfAn AC side output filter is formed to filter out the switching ripple and unnecessary high frequency components in the output current. The main parameters of the three-phase four-wire system SAPF prototype system are shown in table 1.
TABLE 1
| Symbol | Description of the parameters | Numerical value |
| us | Network phase voltage | 220V |
| fs | Frequency of the grid | 50Hz |
| Vdc | Voltage on the direct current side | 750V |
| Cdc | DC side total capacitance | 11.75mF |
| fsm | Sampling frequency | 10kHz |
| fsw | Switching frequency | 10kHz |
FIG. 5 is a full-band steady-state compensation waveform under the rated full-load condition of the 14kVA three-phase four-wire system SAPF prototype, wherein IL、IC、ISRespectively load current, compensation current and net side current. Rated compensation current effective value ip20A. It can be seen that, under the rated working condition, the composite current-limiting method based on the instruction current limitation does not work, the instruction current value is the detected load Harmonic current value, the compensated current waveform on the power grid side is a sine wave, and the total Harmonic distortion thd (total Harmonic distortion) is 3.6%, which indicates that the sample machine has good Harmonic compensation capability.
FIG. 6 is the steady state test results for current limit compensation on the prototype, where IL、IC、ISRespectively load current, compensation current and net side current. The effective value of the load harmonic current is set to be 60A, and the selective harmonic compensation frequency is set to be 3, 5, 7 and 11. The highest harmonic number h to be compensated is 11, and the corresponding command current limit defines the maximum effective compensation current value (i.e. the first threshold) i allowed by the systemmax30A, peak (i.e., second threshold) ipeak69A. As can be seen from the figure, although the effective value of the harmonic current to be compensated in the load current is 3 times of the rated compensation capacity of the prototype and far exceeds the compensation range of the prototype, the effective value of the system compensation current is always limited to imaxAnd the condition of serious overcurrent operation can not occur. In addition, the compensation current is subjected to spectrum analysis by wavestar, and the effective value is 29.4A and is very close to the first threshold value imaxThe error is within 2 percent, and the control precision is high. Meanwhile, compared with the traditional method for defining the maximum compensation current effective value of the system to always obtain the rated value, the method has the advantages that the harmonic current close to 10A is compensated through multiple outputs on the premise of ensuring safe and reliable operation of the SAPF, and the utilization efficiency of the device is improved by 50% under the condition of compensating low-order harmonics.
FIG. 7 is the result of a dynamic experiment for current limiting compensation on the prototype, in which IL、IC、ISRespectively load current, compensation current and net side current. Setting the idle-40A harmonic load dynamic switching, the compensation frequency of SAPF is still 3, 5, 7, 11. It can be seen that when the load is suddenly applied, although it is compensated forThe load harmonic current far exceeds the compensation capability of the SAPF, but the effective value of the actual compensation current output by a prototype is well limited within 30A, and the compensation current exceeds the maximum allowable value of the system at the moment of no load sudden change so that the SAPF is in overcurrent operation, which shows that the current limiting strategy can also have a good control effect when the load is dynamically switched.
The above description is only for the specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention.
Claims (8)
1. A protection method of an active power filter is characterized by comprising the following steps:
(1) extracting each subharmonic component needing to be compensated in the load current, and superposing the extracted each subharmonic component to obtain an instruction current;
(2) setting a first threshold value and a second threshold value in sequence according to a rated compensation current effective value of the active power filter and the highest harmonic frequency needing to be compensated;
(3) proportionally limiting the instruction current by taking the first threshold as the maximum allowable current to obtain a first instruction current;
(4) taking the second threshold value as an interruption peak current to carry out interruption current limiting on the first instruction current to obtain a second instruction current;
(5) and generating a driving pulse for driving the converter in the active power filter according to the second instruction current so as to enable the converter to output corresponding compensation current.
2. The active power filter protection method of claim 1, wherein the step (1) extracts each subharmonic component to be compensated based on a sliding window iterative DFT algorithm.
3. The protection method of an active power filter according to claim 1, wherein the step (2) calculates the first threshold i according to the following formulamax：
imax＝Ki*ip，
Wherein ipIs the rated compensation current effective value;
Kifirst scale factor:
wherein h is the highest harmonic number to be compensated.
4. The protection method of an active power filter according to claim 1, wherein the step (2) calculates the second threshold i according to the following formulapeak：
ipeak＝Kp*imax，
Wherein imaxIs a first threshold value;
Kpis the second proportionality coefficient:
wherein h is the highest harmonic number to be compensated.
5. The protection method of an active power filter according to claim 1, wherein the step (3) is performed as follows:
(3-1) calculating an effective value of the command current, and using a quotient obtained by dividing the first threshold value by the larger of the effective value of the command current and the first threshold value as a proportional current limiting factor;
(3-2) according to the formula:
i'ref＝k*iref
calculating a first instruction current i'ref，irefK is the proportional current limiting factor for the command current.
6. The method according to claim 1, wherein the method further comprises detecting a dc side voltage of the active power filter in real time, and performing hardware blocking and current limiting on the active power filter according to the dc side voltage, and the following steps:
(S1) detecting a dc side voltage of the active power filter in real time;
(S2) conditioning the detected direct current side voltage to a set voltage range to obtain a corresponding equivalent direct current side voltage;
(S3) comparing the conditioned equivalent dc side voltage with a third threshold, and according to the comparison result, performing the following operations:
if the voltage of the equivalent direct current side is greater than or equal to the third threshold value, blocking the driving pulse of the converter in the active power filter until the voltage of the equivalent direct current side is reduced to the fourth threshold value, and removing the blocking to enable the converter to output corresponding compensation current;
otherwise, no operation is performed.
7. The method for protecting an active power filter according to claim 6, wherein said third threshold V is set to1According to the formula:
V1＝KT*(Vdcp+ΔVh)
calculation of VdcpIndicating the DC-side voltage rating, Δ VhIndicating the overvoltage margin of the overvoltage protection on the DC side, KTRepresenting the equivalent conditioning ratio.
8. The method for protecting an active power filter according to claim 7, wherein said fourth threshold V is set to2According to the formula:
V2＝KT*(Vdcp-ΔVl)
calculation of VdcpIndicating the DC-side voltage rating, Δ VlUndervoltage margin, K, representing the release of the overvoltage protection on the DC sideTRepresenting the equivalent conditioning ratio.
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