CN103529347A - Cascade inverter H-bridge unit fault detecting method based on harmonic analysis - Google Patents

Abstract

The invention discloses a cascade inverter H-bridge unit short circuit fault detecting method based on harmonic analysis of switching frequency, which is suitable for a carrier phase-shifting PWM (pulse width modulation) manner based on the harmonic analysis to output phase voltage and comprises three parts, namely output alternating voltage sampling, extracting and processing of harmonic wave close to the switching frequency, and fault judging and fault locating. Three-phase output voltage of a cascade inverter is respectively subjected to low-pass filtering and A/D (analog-to-digital conversion) sampling, and is subjected to discrete Fourier transformation to obtain characteristic harmonic; a comparison result of the characteristic harmonic amplitude and a fault threshold is utilized as the evidence of fault judging, wherein the fault threshold is obtained by carrying out a series of calculation on command voltage. The fault locating is determined through a phase angle range of the harmonic of the switching frequency. According to the cascade inverter H-bridge unit short circuit fault detecting method based on the harmonic analysis of the switching frequency, which is disclosed by the invention, the three-phase output voltage of the inverter is collected, and no other hardware circuit is additionally arranged, so that the cost is reduced, and the efficiency is improved; the application capability is strong, and the application value in the study on the real time fault detection of the cascade inverter exists.

Description

A Fault Detection Method for Cascaded Inverter H-Bridge Units Based on Harmonic Analysis

technical field

The invention belongs to the technical field of application of high-voltage and high-power power electronics technology in power systems, and relates to the short-circuit fault detection of H-bridge units of cascaded inverters under the carrier phase-shifting PWM (pulse width modulation) mode based on switching frequency harmonic analysis method.

Background technique

In recent years, with the development of industry and power technology, the H-bridge cascaded structure has become a research hotspot in the fields of grid-connected inverter, energy storage, and power electronic transformers. The H-bridge cascaded inverter has the advantages of large power capacity, small switching device stress, and low harmonic content, so it is widely used in various electrical devices.

Cascaded inverters are structurally easy to realize modularly, and have lower switching losses and harmonic output. However, because it adopts a structure in which multiple H-bridge units are connected in series, its structural complexity and control complexity increase the possibility of failure. Cascaded inverters are often used in important occasions such as wind turbines and power electronic transformers. If a fault occurs but is not dealt with in a timely and effective manner, the consequences will be very serious. This puts higher requirements on the redundancy technology of cascaded inverters, and the premise of redundant technology is fault detection, including fault judgment and fault location, that is, it can judge whether the cascaded inverter is A fault occurs, and its location is determined.

The fault detection of the H-bridge cascaded inverter can be realized by adding additional hardware circuits, such as installing a voltage sensor on the output side of each H-bridge unit, but this method not only greatly increases the cost, but also increases the system cost. Complexity reduces the reliability of the device. In terms of software methods, there are very few studies on the fault detection of cascaded inverters. Considering that each H-bridge unit in the single-phase link of the cascaded inverter has a certain angle of phase shift due to the triangular carrier, then each The harmonics at the switching frequency in the output voltage waveform of the H-bridge unit will also be phase-shifted by a certain angle. According to the analysis of the single-phase output voltage of the cascaded inverter, the harmonic amplitudes and phases near the switching frequency are obtained. The angle can be used to judge and locate the short-circuit fault of the H-bridge unit of the cascaded inverter.

Contents of the invention

Purpose of the invention: for the above-mentioned cascaded inverter fault detection problem, the purpose of the present invention is to propose a cascaded inverter H-bridge unit fault detection method based on harmonic analysis, the method is based on switching frequency harmonic analysis A short-circuit fault detection method for cascaded inverter H-bridge units under the carrier phase-shift PWM modulation mode, which can perform fault judgment and location without adding hardware circuits, and improve the redundancy of the device.

Technical solution: In order to solve the above-mentioned technical problems, the present invention provides a method for detecting faults in H-bridge units of cascaded inverters based on harmonic analysis, the method comprising the following steps:

Step 1: Output AC voltage sampling:

Step 1.1: Use the voltage sensor to collect the three-phase voltage of the cascaded inverter as v _xo , where x represents phase x, that is, x=a, b, c, a, b, c are three-phase symbols; the simulated The signal v _xo is filtered by a low-pass filter, and the filtered signal is v′ _xo , the cut-off frequency of the low-pass filter is f _lp = 3f _sw , and f _sw is the switching frequency of the output voltage of the H-bridge unit of the cascaded inverter ;

Step 1.2, A/D sampling is carried out to the analog signal v′ _xo after the filtering, the sampling frequency f _s is an integer multiple of the switching frequency f _sw of the output voltage of the H-bridge unit, and the discrete signal obtained by sampling is denoted as v _x (k) , where 0≤k≤N-1, N is the number of sampling points in a power frequency cycle;

Step 2: Harmonic extraction and processing near the switching frequency:

Step 2.1: Calculate the discrete Fourier transform result of the discrete signal v _x (k) near the H-bridge unit output voltage switching frequency f _sw , which is denoted as:

{V _x (n _sw -n ₀ ),V _x (n _sw -n ₀ +1),…,V _x (n _sw ),…,V _x (n _sw +n ₀ )}

Among them, n _sw =f _sw /f ₀ , n _sw is the number of switching frequency harmonics, and the number of harmonics near f _sw taken by each phase is 2n ₀ +1; H-bridge unit output voltage switching frequency f _sw , f ₀ = 50Hz.

其中x代表x相，x＝a,b,c，a,b,c为三相符号，m为谐波次数，即m＝n_sw-n₀,n_sw-n₀+1,…,n_sw+n₀，

为m次谐波幅值，

为m次谐波相角。Step 2.2: Calculate the harmonic amplitude and phase angle; multiply the results of the discrete Fourier transform of the x-phase voltage by 1/N and transform it into an exponential form to obtain multiple harmonics near the switching frequency f _sw of each phase wave, recorded as

Where x represents phase x, x=a,b,c, a,b,c are three-phase symbols, m is the harmonic order, that is, m=n _sw -n ₀ ,n _sw -n ₀ +1,…,n _sw +n ₀ ,

is the mth harmonic amplitude,

is the phase angle of the mth harmonic.

中，取幅值最大的一项，记为

即作为级联型逆变器输出相电压开关频率附近特征谐波；其中x代表x相，x＝a,b,c，a,b,c为三相符号，|C_xsw|为特征谐波幅值，即

为m次谐波幅值，

为特征谐波相角。Step 2.3: Calculate the harmonics around the switching frequency; in the complex sequence

Among them, take the item with the largest amplitude and record it as

That is, as a cascaded inverter output phase voltage switching frequency near the characteristic harmonics; where x represents the x phase, x = a, b, c, a, b, c are three-phase symbols, |C _xsw | is the characteristic harmonic amplitude, ie

is the mth harmonic amplitude,

is the characteristic harmonic phase angle.

Step 3: Fault judgment and fault location;

将

移相角度得到n个电压信号量

n为每相H桥单元的个数，i＝1,2,…,n，f₀为工频50Hz，f_sw为H桥单元输出电压开关频率；x相参考开关频率谐波为 $C_{\mathrm{xsw}}^{*}=\frac{{2v}_{\mathrm{dc}}}{\mathrm{\π}}\underset{i=1}{\overset{n}{\mathrm{\Σ}}}\sin \left(\mathrm{\πM}\frac{v_{\mathrm{xi}}^{*}}{v_{\mathrm{xm}}^{*}}\right)e^{j(i-1)\mathrm{\Δ\Φ}},$ 其中v_dc为单个H桥直流侧电压，为指令电压幅值，i＝1,2,…,n，M为调制比，ΔΦ＝2π/n，x相的故障阈值 $\left|C_{\mathrm{xth}}\right|=\left|C_{\mathrm{xsw}}^{*}\right|+0.05v_{\mathrm{dc}};$ Step 3.1: Calculate the fault threshold; the cascaded inverter x-phase command voltage is

Will

phase shift Angle to get n voltage semaphores

n is the number of H-bridge units in each phase, i=1,2,...,n, f ₀ is the power frequency 50Hz, f _sw is the switching frequency of the output voltage of the H-bridge unit; the harmonic of the x-phase reference switching frequency is $C_{\mathrm{wxya}}^{*}=\frac{{2v}_{\mathrm{dc}}}{\mathrm{\π}}\underset{i=1}{\overset{\mathrm{no}}{\mathrm{\Σ}}}\sin \left(\mathrm{\πM}\frac{v_{\mathrm{xi}}^{*}}{v_{\mathrm{xm}}^{*}}\right)e^{j(i-1)\mathrm{\Δ\Φ}},$ Where v _dc is the DC side voltage of a single H-bridge, is the command voltage amplitude, i=1,2,...,n, M is the modulation ratio, ΔΦ=2π/n, the fault threshold of phase x $\left|C_{\mathrm{xth}}\right|=\left|C_{\mathrm{wxya}}^{*}\right|+0.05v_{\mathrm{dc}};$

Step 3.2: Fault judgment; the characteristic harmonics near the switching frequency of the x-phase output voltage of the cascaded inverter The amplitude of |C _xsw | is compared with the fault threshold |C _xth |, if |C _xsw |>|C _xth |, it is judged that phase x is faulty, otherwise it is normal operation;

故障定位判据为：若第i个H桥单元发生故障，则

落在

范围内，其中t₀为t时刻之前x相第一个H桥单元三角载波v_cx1为最大值或最小值的时刻，t_sw＝1/f_sw，

为所述步骤1.1中低通滤波器在f_sw处的相移。Step 3.3: Fault location; if it is judged at time t that the H-bridge unit short-circuit fault occurs in phase x of the inverter, take the harmonic at f _sw calculated in step 2.2 phase angle of

The fault location criterion is: if the i-th H-bridge unit fails, then

fall on

In the range, where t ₀ is the moment when the triangular carrier v _cx1 of the first H-bridge unit of phase x is the maximum value or the minimum value before time t, t _sw =1/f _sw ,

is the phase shift of the low-pass filter at f _sw in the step 1.1.

Beneficial effect:

(1) This method mainly relies on software implementation, which can be realized conveniently without configuring additional complicated hardware detection circuits;

(2) This method can not only quickly and accurately detect faults, but also quickly perform fault location. As a prerequisite for device redundancy technology, it can provide guarantee for subsequent fault processing solutions;

(3) Compared with the fault detection function of the IGBT drive protection circuit, it has a wider detection range.

Description of drawings

Figure 1 is a structural diagram of the H-bridge unit cascaded grid-connected inverter;

Figure 2 is a phasor diagram of switching frequency harmonics of each unit of a single-phase chain link;

Figure 3 is a block diagram of the fault detection method.

Detailed ways

The present invention will be further explained below in conjunction with the accompanying drawings.

A kind of cascaded inverter H-bridge unit fault detection method based on harmonic analysis provided by the invention, the method comprises the following steps:

Step 1: Output AC voltage sampling:

Step 1.1: Use the voltage sensor to collect the three-phase voltage of the cascaded inverter as v _xo , where x represents phase x, that is, x=a, b, c, a, b, c are three-phase symbols; the simulated The signal v _xo is filtered by a low-pass filter, and the filtered signal is v′ _xo , the cut-off frequency of the low-pass filter is f _lp = 3f _sw , and f _sw is the switching frequency of the output voltage of the H-bridge unit of the cascaded inverter ;

Step 1.2, A/D sampling is carried out to the analog signal v′ _xo after the filtering, the sampling frequency f _s is an integer multiple of the switching frequency f _sw of the output voltage of the H-bridge unit, and the discrete signal obtained by sampling is denoted as v _x (k) , where 0≤k≤N-1, N is the number of sampling points in a power frequency cycle;

Step 2: Harmonic extraction and processing near the switching frequency:

Step 2.1: Calculate the discrete Fourier transform result of the discrete signal v _x (k) near the H-bridge unit output voltage switching frequency f _sw , which is denoted as:

{V _x (n _sw -n ₀ ),V _x (n _sw -n ₀ +1),…,V _x (n _sw ),…,V _x (n _sw +n ₀ )}

Among them, n _sw =f _sw /f ₀ , n _sw is the number of switching frequency harmonics, and the number of harmonics near f _sw taken by each phase is 2n ₀ +1; H-bridge unit output voltage switching frequency f _sw , f ₀ = 50Hz;

其中x代表x相，x＝a,b,c，a,b,c为三相符号，m为谐波次数，即m＝n_sw-n₀,n_sw-n₀+1,…,n_sw+n₀，

为m次谐波幅值，

为m次谐波相角。Step 2.2: Calculate the harmonic amplitude and phase angle; multiply the results of the discrete Fourier transform of the x-phase voltage by 1/N and transform it into an exponential form to obtain multiple harmonics near the switching frequency f _sw of each phase wave, recorded as

Where x represents phase x, x=a,b,c, a,b,c are three-phase symbols, m is the harmonic order, that is, m=n _sw -n ₀ ,n _sw -n ₀ +1,…,n _sw +n ₀ ,

is the mth harmonic amplitude,

is the phase angle of the mth harmonic.

中，取幅值最大的一项，记为即作为级联型逆变器输出相电压开关频率附近特征谐波；其中x代表x相，x＝a,b,c，a,b,c为三相符号，|C_xsw|为特征谐波幅值，即

为m次谐波幅值，为特征谐波相角。Step 2.3: Calculate the harmonics around the switching frequency; in the complex sequence

Among them, take the item with the largest amplitude and record it as That is, as a cascaded inverter output phase voltage switching frequency near the characteristic harmonics; where x represents the x phase, x = a, b, c, a, b, c are three-phase symbols, |C _xsw | is the characteristic harmonic amplitude, ie

is the mth harmonic amplitude, is the characteristic harmonic phase angle.

Step 3: Fault judgment and fault location;

将

移相角度得到n个电压信号量

n为每相H桥单元的个数，i＝1,2,…,n，f₀为工频50Hz，f_sw为H桥单元输出电压开关频率；x相参考开关频率谐波为 $C_{\mathrm{xsw}}^{*}=\frac{{2v}_{\mathrm{dc}}}{\mathrm{\π}}\underset{i=1}{\overset{n}{\mathrm{\Σ}}}\sin \left(\mathrm{\πM}\frac{v_{\mathrm{xi}}^{*}}{v_{\mathrm{xm}}^{*}}\right)e^{j(i-1)\mathrm{\Δ\Φ}},$ 其中v_dc为单个H桥直流侧电压，

为指令电压幅值，i＝1,2,…,n，M为调制比，ΔΦ＝2π/n，x相的故障阈值 $\left|C_{\mathrm{xth}}\right|=\left|C_{\mathrm{xsw}}^{*}\right|+0.05v_{\mathrm{dc}};$ Step 3.1: Calculate the fault threshold; the cascaded inverter x-phase command voltage is

Will

phase shift Angle to get n voltage semaphores

n is the number of H-bridge units in each phase, i=1,2,...,n, f ₀ is the power frequency 50Hz, f _sw is the switching frequency of the output voltage of the H-bridge unit; the harmonic of the x-phase reference switching frequency is $C_{\mathrm{wxya}}^{*}=\frac{{2v}_{\mathrm{dc}}}{\mathrm{\π}}\underset{i=1}{\overset{\mathrm{no}}{\mathrm{\Σ}}}\sin \left(\mathrm{\πM}\frac{v_{\mathrm{xi}}^{*}}{v_{\mathrm{xm}}^{*}}\right)e^{j(i-1)\mathrm{\Δ\Φ}},$ Where v _dc is the DC side voltage of a single H-bridge,

is the command voltage amplitude, i=1,2,...,n, M is the modulation ratio, ΔΦ=2π/n, the fault threshold of phase x $\left|C_{\mathrm{xth}}\right|=\left|C_{\mathrm{wxya}}^{*}\right|+0.05v_{\mathrm{dc}};$

的幅值|C_xsw|与故障阈值|C_xth|比较，若|C_xsw|>|C_xth|，则判定为x相发生故障，否则为正常运行；Step 3.2: Fault judgment; the characteristic harmonics near the switching frequency of the x-phase output voltage of the cascaded inverter

The amplitude of |C _xsw | is compared with the fault threshold |C _xth |, if |C _xsw |>|C _xth |, it is judged that phase x is faulty, otherwise it is normal operation;

故障定位判据为：若第i个H桥单元发生故障，则

落在

范围内，其中t₀为t时刻之前x相第一个H桥单元三角载波v_cx1为最大值或最小值的时刻，t_sw＝1/f_sw，

为所述步骤1.1中低通滤波器在f_sw处的相移。Step 3.3: Fault location; if it is judged at time t that the H-bridge unit short-circuit fault occurs in phase x of the inverter, take the harmonic at f _sw calculated in step 2.2 phase angle of

The fault location criterion is: if the i-th H-bridge unit fails, then

fall on

In the range, where t ₀ is the moment when the triangular carrier v _cx1 of the first H-bridge unit of phase x is the maximum value or the minimum value before time t, t _sw =1/f _sw ,

is the phase shift of the low-pass filter at f _sw in the step 1.1.

The present invention is used in cascaded inverter H-bridge unit short-circuit fault detection under carrier phase-shifting PWM modulation mode. The H-bridge units are connected in series, and the three phases are connected in star form. In addition, the present invention is also applicable to a delta connection.

As shown in Figure 2, if the cascaded inverter has n H-bridge units per phase, then the harmonic v _sw of the output voltage of each phase at the switching frequency f _sw is the switching frequency harmonic output by each H-bridge unit Waves v _sw1 , v _sw2 ,…, v _swn are superimposed:

${\stackrel{<span\; class="notranslate">\; \&Center\; Dot;</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}}_{\mathrm{<span\; class="notranslate">\; sw</span>}}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; x</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\overset{\mathrm{<span\; class="notranslate">\; no</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\stackrel{<span\; class="notranslate">\; \&CenterDot;</span>}{\mathrm{<span\; class="notranslate">\; v</span>}}}_{\mathrm{<span\; class="notranslate">\; swx</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

Due to the phase shift of the PWM triangular carrier of each unit, there is also a certain angle difference between v _sw1 , v _sw2 ,..., v _swn , and its phasor diagram is shown in Figure 2a. Theoretically, v _sw is zero when the inverter is running normally; when a certain H-bridge unit is short-circuited, the phasor diagram is shown in Figure 2b (unit 1 is short-circuited), and v _sw is not zero at this time, and its magnitude is equal to v The magnitude of _sw1 , the phase is opposite to v _sw1 . In actual situations, v _sw is not zero, so a threshold should be set to determine whether a fault occurs, and the threshold can prevent misjudgment when the command voltage changes. Moreover, when a fault occurs in an actual situation, the harmonic with a larger amplitude is not exactly at the switching frequency, but near the switching frequency.

在这个谐波序列中，选取幅值最大的谐波为

作为级联型逆变器x相输出电压开关频率附近的特征谐波。短路故障的阈值|C_xth|由指令电压

经运算得到，故障判断依据为特征谐波幅值大于故障阈值，故障定位则是根据输出电压的f_sw/f₀次谐波的相角将故障定位到具体某个H桥单元。As shown in Figure 3, the single-phase output voltage v _xo of the cascaded inverter is first filtered by a low-pass filter, and then sampled by A/D to obtain a discrete sampling value v _x (k); and then passed through a discrete Fourier transform Transform to obtain multiple harmonics around the switching frequency f _sw of a single H-bridge unit, denoted as

In this harmonic sequence, the harmonic with the largest amplitude is selected as

As a characteristic harmonic near the switching frequency of the x-phase output voltage of the cascaded inverter. The short-circuit fault threshold | _Cxth | is determined by the command voltage

After calculation, the basis for fault judgment is that the amplitude of the characteristic harmonic is greater than the fault threshold, and the fault location is based on the phase angle of the f _sw /f _0th harmonic of the output voltage to locate the fault to a specific H-bridge unit.

The above is only a preferred embodiment of the present invention, it should be pointed out that, for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications can also be made. It should be regarded as the protection scope of the present invention.

Claims (1)

1. the cascaded inverter H bridge cell failure detection method based on frequency analysis, is characterized in that, the method comprises the steps:

Step 1: output AC voltage sampling:

Step 1.1: the three-phase voltage that utilizes voltage sensor to gather cascaded inverter is v _xo, wherein x represents x phase, i.e. x=a, and b, c, a, b, c is three-phase symbol; By described simulating signal v _xoby low-pass filter, carry out filtering, filtered signal is v ' _xo, the cutoff frequency f of low-pass filter _lp=3f _sw, f _swswitching frequency for cascaded inverter H bridge unit output voltage;

Step 1.2, to described filtered simulating signal v ' _xocarry out A/D sampling, sample frequency f _sfor H bridge unit output voltage switching frequency f _swintegral multiple, sampling gained discrete signal be designated as v _x(k), 0≤k≤N-1 wherein, N is the number of sampled point in a power frequency period;

Near step 2: switching frequency, harmonic wave extracts and processes:

Step 2.1: calculate described discrete signal v _x(k) at H bridge unit output voltage switching frequency f _swnear discrete Fourier transformation result, is designated as:

{V _x(n _sw-n ₀),V _x(n _sw-n ₀+1),…,V _x(n _sw),…,V _x(n _sw+n ₀)}

N wherein _sw=f _sw/ f ₀, n _swfor the number of times of switching frequency harmonic wave, f that every phase is got _swnear harmonic wave number is 2n ₀+ 1; H bridge unit output voltage switching frequency f _sw, f ₀=50Hz;

Step 2.2: calculate harmonic amplitude and phase angle; The result of described x phase voltage discrete Fourier transformation is multiplied each other with 1/N respectively and turns to exponential form, obtain each phase switching frequency f _swnear multiple harmonic, is designated as

wherein x represents x phase, x=a, and b, c, a, b, c is three-phase symbol, m is overtone order, i.e. m=n _sw-n ₀, n _sw-n ₀+ 1 ..., n _sw+ n ₀,

for m subharmonic amplitude, for m subharmonic phase angle;

Near step 2.3: harmonic wave compute switch frequency; At described sequence of complex numbers

in, get amplitude maximum one, be designated as

as near characteristic harmonics cascaded inverter output phase voltage switching frequency; Wherein x represents x phase, x=a, and b, c, a, b, c is three-phase symbol, | C _xsw| be characteristic harmonics amplitude,

for m subharmonic amplitude,

for characteristic harmonics phase angle;

Step 3: fault judgement and localization of fault;

Step 3.1: calculate fault threshold; Cascaded inverter x phase command voltage is

will

phase shift angle obtains n voltage signal amount

n is the number of every phase H bridge unit, i=1, and 2 ..., n, f ₀for power frequency 50Hz, f _swfor H bridge unit output voltage switching frequency; X with reference to switching frequency harmonic wave is $C_{\mathrm{xsw}}^{*}=\frac{{2v}_{\mathrm{dc}}}{\mathrm{\&pi;}}\underset{i=1}{\overset{n}{\mathrm{\&Sigma;}}}\sin \left(\mathrm{\&pi;M}\frac{v_{\mathrm{xi}}^{*}}{v_{\mathrm{xm}}^{*}}\right)e^{j(i-1)\mathrm{\&Delta;\&Phi;}},$ V wherein _dcfor single H bridge DC side voltage,

for command voltage amplitude, i=1,2 ..., n, M is modulation ratio, ΔΦ=2 π/n, the fault threshold of x phase $\left|C_{\mathrm{xth}}\right|=\left|C_{\mathrm{xsw}}^{*}\right|+0.05v_{\mathrm{dc}};$

Step 3.2: fault judgement; By near characteristic harmonics described cascaded inverter x phase output voltage switching frequency

amplitude | C _xsw| with fault threshold | C _xth| relatively, if | C _xsw|>| C _xth|, be judged to be x and break down mutually, otherwise be normal operation;

Step 3.3: localization of fault; If be constantly judged as inverter x at t, there is mutually H bridge unit short trouble, get the f calculating in described step 2.2 _swplace's harmonic wave

phase angle

localization of fault criterion is: if i H bridge unit breaks down,

drop on in scope, t wherein ₀for t constantly before x first H bridge unit triangular carrier v mutually _cx1for the moment of maximal value or minimum value, t _sw=1/f _sw,

for low-pass filter in described step 1.1 is at f _swthe phase shift at place.

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