CN102033177A - Method and system for measuring power angle of electric power circuit of distribution network - Google Patents

Abstract

The invention discloses a method and a system for measuring a power angle of an electric power circuit of a distribution network, which belong to the technical field of power electronics. The system for measuring the power angle is positioned at a power substation end; a main transformer is connected with main station equipment through a used transformer; and a user transformer at a low-voltage user end is connected with a power angle measurement end. The method comprises the following steps of: controlling the conduction of a silicon controlled rectifier by a signlechip on a low voltage side of the used transformer inside a power substation, and generating a voltage distorted signal when short circuit happens; after a voltage waveform with the distorted signal is received by the user end, extracting a single distorted signal from a network voltage waveform by time domain differential technology, and accumulating a plurality of distorted signals so as to form a synthesized signal; simultaneously, performing time-frequency analysis processing on the synthesized signal by combining Wigner-Ville distribution with short-time Fourier transform, determining the time difference between the distorted signal and a local voltage zero crossing point according to the time distribution condition of power parameters of the synthesized signal, and combining a connection group relation of the transformer so to obtain the power angle of the circuit. The method is low in cost, is easy to operate, is not influenced by the electric power network structure, and is widely used.

Description

A kind of power distribution network power circuit power angle measuring method and measuring system

Technical field

The invention belongs to electric and electronic technical field, particularly a kind of power distribution network power circuit power angle measuring method and measuring system.

Background technology

In power distribution network, the power circuit power angle is an important parameter of electric system, is the main state variable that can the measurement system stable operation.Expansion of power distribution network scale in recent years and intelligentized development have proposed more strict requirement to the stable operation of power distribution network, and power distribution network merit angle is as the important parameter of operation of power networks state, and its effective measurement detects Operation of Electric Systems and is extremely important.

Measurement to the power circuit power angle at present mainly contains following two kinds of methods:

1) GPS synchronous clock mensuration, this method is as topmost measuring power angle means, and complicated operation, application cost height are mainly used in the large-scale high voltage power transmission electrical network, and in the huge low and medium voltage distribution network of quantity, still do not have widespread use, can not be as desirable power distribution network method for measuring power angle.

2) electric power system tide computing method, this method under the prerequisite of known line impedance, user load according to circuit in power flow direction utilize mathematical formulae to calculate the line power angle.But line construction and user load and unstable in actual electric network, the two often changes, and therefore can't utilize the formula Calculation Method to obtain the power angle of circuit then by they are tested in advance.

Summary of the invention

The measuring method and the measuring system that the purpose of this invention is to provide a kind of power distribution network power circuit power angle.

A kind of power distribution network power circuit measuring power angle system is characterized in that, this measuring power angle system is made up of the power angle measuring terminals that is used to produce the main website equipment of voltage distortion signal and be positioned at the low-voltage customer end that is positioned at electric substation's end.At electric substation's end, main-transformer connects main website equipment by house transformer; At the low-voltage customer end, user transformers connects the power angle measuring terminals; When needs carried out the power angle measurement, main website equipment produced distorted signal by the instantaneous short-circuit of control house transformer low-pressure side near voltage over zero, the basic voltage over zero moment of just holding in electric substation of its energy peak; This signal spans transformer platform district transfers to user side; The power angle measuring terminals receives, the processes voltage distorted signal.

The secondary side of described house transformer connects a, b, the c three-phase power line, be provided with on any therein two phase lines, be connected a, the voltage distortion modulating apparatus of b phase line, the voltage distortion modulating apparatus is mainly by single-chip microcomputer, signal modulation circuit, signal filter circuit and holding circuit constitute, at a, voltage transformer pt on the b phase line connects signal filter circuit, signal filter circuit is connected to single-chip microcomputer, single-chip microcomputer is connected to PC by RS485 again, and single-chip microcomputer also is connected to the silicon controlled control utmost point by pulsed triggering circuit, is connected the holding circuit process resistance R on a phase line, be connected behind the controllable silicon on the b phase line; Conducting by machine controllable silicon, a-b two-phase circuit is carried out short circuit, short trouble is eliminated the controllable silicon shutoff when treating the a-b line voltage zero-cross, this process produces a voltage distortion signal on electrical network a-b line voltage, and this signal spans transformer platform district transfers to user side.

Described power angle measuring terminals is installed in the low-pressure side of user transformers, voltage transformer pt on a, b phase line connects signal filter circuit, signal filter circuit is connected to 16 A/D sample circuits and positive and negative comparator circuit respectively, 16 A/D sample circuits are connected the DSP signal processing module respectively with positive and negative comparator circuit, and the DSP signal processing module also connects keyboard input, LED display respectively and is connected to PC by RS485; The voltage distortion signal that DSP collects the AD sample circuit carries out time frequency analysis to determine its time-domain position on local voltage waveform, obtains the power angle of transmission line of electricity indirectly.

A kind of measuring method of power distribution network power circuit power angle is characterized in that, comprises the following steps:

1) generation of voltage distortion signal and extraction, the conducting by machine controllable silicon in electric substation produces the voltage distortion signal with the short circuit of house transformer low-pressure side; When the line voltage between any two-phase circuit of house transformer low-pressure side is about to zero passage, this two-phase circuit is carried out short circuit, stop short circuit when treating voltage zero-cross, the short-circuit current that this process produces can be superimposed upon on this line voltage inducing an instantaneous voltage drop on its leakage inductance when flowing through main-transformer, produce the voltage distortion signal, this voltage distortion signal spans transformer platform district transfers to user side; Before user side carries out time frequency analysis to the voltage distortion signal, at first it to be extracted from local voltage waveform, user side is to the detection and the analysis of distorted signal for convenience, when carrying out the power angle measurement, carry out the transmission of a plurality of distorted signals continuously, and each distorted signal carries by two adjacent voltage waveforms at system's head end;

2) measurement of the time frequency analysis of user side voltage distortion signal and power circuit power angle, after receiving the voltage waveform that contains distorted signal at user side, use the time-domain difference technology that the individual distortion signal is extracted from grid voltage waveform, and with a plurality of distorted signals formation composite signal that adds up, use Wei Gela to distribute simultaneously and composite signal is carried out the time frequency analysis processing in conjunction with Short Time Fourier Transform, determine the time difference of distorted signal and local voltage over zero according to the time distribution situation of its energy parameter, and obtain the power angle of circuit in conjunction with the connection group relation of transformer.

Described short-circuit current can be superimposed upon on this line voltage inducing an instantaneous voltage drop on its leakage inductance when flowing through main-transformer, produces the voltage distortion signal, and the distorted signal s (t) that obtains by difference method can be expressed as:

s(t)＝u(t)-u(t-T)＝γ(t)+n(t)

U in the formula (t) is a mains voltage signal; T is the power frequency period of line voltage, T=0.02s; γ (t) is the virtual voltage distorted signal; N (t) disturbs for electric network noise.

Described electric network noise disturbs when interference ratio is serious, be difficult to that the two differentiates with distorted signal and noise interferences, carry out the transmission of N distorted signal continuously in electric substation's side for this reason, N the distorted signal that front and back voltage cycle signal subtraction is obtained at user side adds up and forms synthetic distorted signal s (t) then, that is:

$s\left(t\right)=\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}(u_k\left(t\right)-u_k(t-T))=\mathrm{N\γ}\left(t\right)+\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}{n}_k\left(t\right)$

The composition of distorted signal obtains the N multiplication by force among the s (t) like this, and the front and back voltage signal is made the undesired signal n that difference obtains _k(t) owing to have random character, during adding up the amplitude enhancing very limited.When N is enough big, in the composite signal resolution of distortion composition be improved significantly, wherein get N=7～10.

The Wigner distribution of described synthetic distorted signal s (t) is:

$W_s(t,\mathrm{\ω})=\underset{-\∞}{\overset{+\∞}{\∫}}s(t+\mathrm{\τ}/2)s^{*}(t-\mathrm{\τ}/2)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}$

The Short Time Fourier Transform of synthetic distorted signal s (t) is expressed as:

$F(t,\mathrm{\ω})={\left|\underset{-\∞}{\overset{+\∞}{\∫}}s\left(\mathrm{\τ}\right)h(\mathrm{\τ}-t)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}\right|}^2$

Wherein, s ^*(t) be the conjugate function of s (t); The window function of h (t) for being adopted in the short time discrete Fourier transform, selecting amplitude here is 1 rectangular window, i.e. h (t)=1; W is the angular frequency of synthetic distorted signal s (t), w=2 π (200～600) rad/s; τ represents time variable; J is an imaginary unit, and j ²=-1.

The invention has the beneficial effects as follows and proposed a kind of new method of measuring power distribution network power circuit power angle, artificial instantaneous short-circuit fault by the house transformer secondary side in electric substation produces a distorted signal at the line voltage near zero-crossing point, utilize the characteristics of this distorted signal no time delay in transmission course, at user side to its time frequency analysis, distribute according to time of its energy parameter and to estimate voltage phase difference between house transformer low-pressure side and the user transformers low-pressure side, obtain the power angle of circuit in conjunction with the connection group relation of two class transformers.This method is with low cost, simple to operate, can ignore by the influence of electric network composition, is widely used.

Description of drawings

Fig. 1 is a power angle measurement mechanism structural drawing.

Fig. 2 is a measuring power angle system architecture synoptic diagram

Fig. 3 is a voltage distortion signal modulation waveform

Fig. 4 is a measuring power angle system first and last terminal voltage signal waveform

Fig. 5 is the main website equipment structure chart

Fig. 6 is a power angle measuring terminals structural drawing

Embodiment

The invention provides a kind of measuring method and measuring system of power distribution network power circuit power angle.Illustrated below in conjunction with accompanying drawing.

1. general structure

In the measuring power angle system architecture synoptic diagram as shown in Figure 1, 2, the measuring power angle system is made up of the power angle measuring terminals that is used to produce the main website equipment of voltage distortion signal and be positioned at the low-voltage customer end that is positioned at electric substation's end.At electric substation's end, main-transformer connects main website equipment by house transformer; At the low-voltage customer end, user transformers connects the power angle measuring terminals; When needs carried out the power angle measurement, main website equipment produced distorted signal by the instantaneous short-circuit of control house transformer low-pressure side near voltage over zero, the basic voltage over zero moment of just holding in electric substation of its energy peak; This signal spans transformer platform district transfers to user side; The power angle measuring terminals receives, the processes voltage distorted signal.The A point is the head end of power circuit among the figure; The B point is the end of power circuit; The C point is the low pressure end of user transformers; The D point is the low pressure end of applied voltage device.The voltage phase difference that A, B are 2 is the power angle of power circuit.

Figure 5 shows that the main website equipment structure chart.Secondary side at house transformer connects a, b, the c three-phase power line, be provided with on any therein two phase lines, be connected a, the voltage distortion modulating apparatus of b phase line, the voltage distortion modulating apparatus is mainly by single-chip microcomputer, signal modulation circuit, signal filter circuit and holding circuit constitute, at a, voltage transformer pt on the b phase line connects signal filter circuit, signal filter circuit is connected to single-chip microcomputer, single-chip microcomputer is connected to PC by RS485 again, and single-chip microcomputer also is connected to the silicon controlled control utmost point by pulsed triggering circuit, is connected the holding circuit process resistance R on a phase line, be connected behind the controllable silicon on the b phase line; Conducting by machine controllable silicon, a-b two-phase circuit is carried out short circuit, short trouble is eliminated the controllable silicon shutoff when treating the a-b line voltage zero-cross, this process produces a voltage distortion signal on electrical network a-b line voltage, and this signal spans transformer platform district transfers to user side.

Behind the a-b line voltage process signal filter circuit that voltage sensor PT obtains from the house transformer low-pressure side, positive and negative comparator circuit, convert the square wave that frequency is 50Hz to by sine wave, as the down trigger source of single-chip microcomputer, make single-chip microcomputer can determine the zero crossing moment of a-b line voltage.Single-chip microcomputer and common PC are carried out exchanges data by the RS485 interface, when needs carry out the power angle measurement, issue the continuous number that distorted signal sends the distorted signal of instruction and required transmission by PC to single-chip microcomputer, after receiving the instruction of PC, near single-chip microcomputer gating pulse trigger circuit voltage over zero make the controllable silicon conducting, and the a-b two-phase circuit of house transformer secondary side is carried out short circuit.PC can also be used to observing the position and the shape of distorted signal on the line voltage, so that the process of transmitting of distorted signal is monitored.Select resistance, inductance and controllable silicon as the transformer short-circuit branch road, adjust the size and the ratio of resistance and inductance, just can change the waveform of the voltage distortion signal of generation.In order to guarantee device security; in the transformer short-circuit branch road, holding circuit is housed; holding circuit is made up of air switch and fastp-acting fuse; holding circuit can be cut off modulation branch rapidly when short-circuit current is very big; prevent from house transformer is caused damage, after fault is got rid of, can conveniently recover.

Figure 6 shows that power angle measuring terminals structural drawing.The power angle measuring terminals is installed in the low-pressure side of user transformers, voltage transformer pt on a, b phase line connects signal filter circuit, signal filter circuit is connected to 16 A/D sample circuits and positive and negative comparator circuit respectively, 16 A/D sample circuits are connected the DSP signal processing module respectively with positive and negative comparator circuit, and the DSP signal processing module also connects keyboard input, LED display respectively and is connected to PC by RS485; The voltage distortion signal that DSP collects the AD sample circuit carries out time frequency analysis to determine its time-domain position on local voltage waveform, obtains the power angle of transmission line of electricity indirectly.

Measuring terminals utilizes a-b line voltage signal that voltage transformer pt obtains from the user transformers secondary side after filtration behind the wave circuit, its higher hamonic wave composition is suppressed significantly, by being converted into the square-wave signal of 50Hz behind the positive and negative comparator circuit, make DSP can determine the zero crossing moment of this ground voltage.Beginning DSP constantly from voltage over zero utilizes 16 A/D collection voltage signals and therefrom voltage distortion signal extraction is come out to form synthetic distorted signal, by it is carried out the zero crossing time difference that time frequency analysis is determined system's first and last terminal voltage, calculate the power angle of power circuit in conjunction with the house transformer and the different connection group relations of user transformers.The keyboard input can on-the-spot be provided with the connected mode code of user transformers and house transformer among the figure, and LCD display can show the power angle measurement result, and DSP is connected the test transmission result by the RS485 interface with upper PC.

In system for distribution network of power, because factors such as supply line, load characteristic, make and have tangible phase differential between the transmission line of electricity first and last terminal voltage, this phase differential power angle that is otherwise known as, be called for short the merit angle, be to weigh the main state variable that can power distribution network stable operation, its measurement monitors the stability of power distribution network and has great importance.Traditional power angle metering system application cost height, operation easier are big, are difficult to be applied in the huge low and medium voltage distribution network of number of nodes.This patent produces a voltage distortion signal at the line voltage near zero-crossing point, utilize distorted signal in transmission course, not have the characteristics of time delay substantially, proposed a kind of by this distorted signal being carried out the method and apparatus that time frequency analysis realizes that power distribution network power circuit power angle is measured.

Above-mentioned DSP selects TMS320LF2407 for use, and AD selects ADS8364 for use, and the RS485 interface circuit is selected MAXIM1485 for use, and single-chip microcomputer is selected ATmega128 for use.

2. the generation of voltage distortion signal and extraction

This patent produces the voltage distortion signal on electrical network a-b line voltage.In house transformer secondary side a-b line voltage negative near preceding 30 ° of zero crossing, conducting by machine controllable silicon is carried out short circuit with a-b two-phase circuit, short trouble is eliminated the controllable silicon shutoff when treating the a-b line voltage zero-cross, this process can produce a voltage distortion signal on electrical network a-b line voltage, this signal can transfer to user side in the extensions road.

Before user side carries out time frequency analysis to the voltage distortion signal, at first it to be extracted from local voltage waveform.The extraction of user side distorted signal for convenience, carry a distorted signal by two adjacent voltage waveforms, and distorted signal is superimposed upon the near zero-crossing point of previous voltage waveform, by the time-domain difference method front and back voltage signal is made difference at user side like this and obtain distorted signal.When containing noise in the electrical network, the distorted signal s (t) that obtains by difference method can be expressed as:

s(t)＝u(t)-u(t-T)＝γ(t)+n(t)

U in the formula (t) is a mains voltage signal; T is the power frequency period of line voltage, T=0.02s; γ (t) is the virtual voltage distorted signal; N (t) disturbs for electric network noise.

As can be seen from the above equation, disturb by front and back voltage cycle signal being carried out contain the part electric network noise in the distorted signal that difference obtains, and when the electric network noise interference ratio is serious, be difficult to the two is differentiated.This patent carries out the transmission of N distorted signal continuously in electric substation's side for this reason, and as shown in Figure 3, N the distorted signal that front and back voltage cycle signal subtraction is obtained at user side adds up and form synthetic distorted signal s (t) then, that is:

$s\left(t\right)=\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}(u_k\left(t\right)-u_k(t-T))=\mathrm{N\γ}\left(t\right)+\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}{n}_k\left(t\right)$

The composition of distorted signal obtains the N multiplication by force among the s (t) like this, and the front and back voltage signal is made the undesired signal n that difference obtains _k(t) owing to have random character, amplitude strengthens very limited in adding up.When N is enough big, in the composite signal resolution of distortion composition be improved significantly, this patent is got N=7～10.

3. the measurement of power circuit power angle

Because the voltage distortion signal does not almost have time delay when transmitting in line of electric force, under the effect of the factors such as connection group of power circuit power angle, substation transformer, when being modulated near the house transformer low-pressure side voltage over zero distorted signal and transferring to user terminal, its energy peak constantly the time difference can occur with user's voltage over zero, as shown in Figure 4.Therefore, when trying to achieve the time-domain position of voltage distortion signal on the user side voltage waveform, voltage over zero time difference Δ T as can be known between house transformer low-pressure side and the user side.

Because the voltage distortion signal mainly is modulated in 30 ° of zones, voltage over zero front and back, its frequency range is between 150Hz to 600Hz, therefore the residing time domain of distorted signal on local voltage waveform, energy in 150Hz to the 600Hz frequency range has remarkable advantages than other time domains, therefore, calculate in the voltage distortion signal time distribution situation of the energy parameter of frequency content between the 150Hz to 600Hz by Time-Frequency Analysis Method, just can access the time-domain position of voltage distortion signal on the user side voltage waveform.

Wigner the distribute energy distribution have higher time frequency resolution, comparatively to concentrate and instantaneous frequency tracking characteristics preferably are well suited for distorted signal is carried out time frequency analysis.But Wigner distribution belongs to nonlinear transformation, when synthetic distorted signal is carried out time frequency analysis calculating, can cause the cross interference between the different frequency composition.Compare with Wigner distribution,, belong to linear transformation, when carrying out Fourier transform, do not have cross influence between each frequency content though the short time discrete Fourier transform time frequency resolution is low.Therefore this patent combines the cross influence that reduces Wigner distribution with short time discrete Fourier transform and Wigner distribution.

The Wigner distribution of synthetic distorted signal s (t) is:

$W_s(t,\mathrm{\ω})=\underset{-\∞}{\overset{+\∞}{\∫}}s(t+\mathrm{\τ}/2)s^{*}(t-\mathrm{\τ}/2)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}$

The Short Time Fourier Transform of synthetic distorted signal s (t) is expressed as:

$F(t,\mathrm{\ω})={\left|\underset{-\∞}{\overset{+\∞}{\∫}}s\left(\mathrm{\τ}\right)h(\mathrm{\τ}-t)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}\right|}^2$

Wherein, h (t) is the window function of short time discrete Fourier transform, and h in this patent (t) is chosen as rectangular window.

Having the Wigner distribution that suppresses cross influence is: W _s^ (t, ω)=W _s(t, ω) F (t, w)

In the application of this patent, because synthetic distorted signal s (t) is a sampled value of utilizing the finite length that 16 bit AD sample circuit obtain, when it was carried out time frequency analysis, actual what carry out was that the Wigner distribution of discretize is calculated.In calculating, the Short Time Fourier Transform of discretize is expressed as:

$F(n,w)=\underset{i=-p/2}{\overset{p/2}{\mathrm{\Σ}}}s(n+i)\exp (-\mathrm{jiw}{T}_0)$

The Wigner distribution table of discretize is shown:

$W_s(n,w)=2\underset{k=0}{\overset{2l-1}{\mathrm{\Σ}}}g\left(k\right)s(n+k)s^{*}(n-k)\exp (-j2\mathrm{kw}{T}_o)$

In two formulas, s (n) is the sampled value of synthetic distorted signal, n=1,2,3...M in the above; M is the sampling number in each power frequency period; T _oBe the sampled point interval; P is the length of window of selected short time discrete Fourier transform; L is the computational length of discretize Wigner distribution; The window function that g (k) distributes for Wigner, its effect is to exceed the signal zero padding of computational length; W is the signal angular frequency.

Having the discretize Wigner distribution that suppresses cross influence like this is expressed as:

W _s^(n，ω)＝W _s(n，ω)F(n，w)

Because the operand of discretize Wigner distribution is very big, and does not still have various fast algorithms, be difficult to all radio-frequency components of synthetic distorted signal are analyzed, so this patent only calculates several principal ingredients in the distorted signal frequency range.Like this, the energy distribution function W (n) that meets voltage distortion signal frequency domain scope is:

$W\left(n\right)=\underset{i=1}{\overset{4}{\mathrm{\Σ}}}\left|W_s^{^}(n,w_i)\right|$

W wherein _iRepresent 200Hz in the distorted signal, 300Hz, 4 kinds of pairing angular frequencies of main frequency composition of 400HZ, 500Hz.

The peak value that the voltage distortion signal energy that obtains by W (n) parameter at user side distributes is n constantly _pBe the time-domain position of voltage distortion signal on the user side voltage waveform, the time difference Δ T of it and local voltage over zero can be expressed as Δ T=(M/2-n _p) T _o, can calculate the voltage phase difference θ of house transformer low-pressure side and user transformers low-pressure side in view of the above, promptly

When considering being connected group and concerning of house transformer and user transformers, the power angle of power circuit can be expressed as:

Wherein β is offset for the voltage-phase that the connection group by substation transformer causes.Mainly contain Yyn0 at China's substation transformer commonly used and be connected group for two kinds, differ 30 ° for the former its high and low pressure side voltage-phase, for its high and low pressure side voltage homophase of the latter with Dyn11.Under the different connection groups of house transformer and user transformers, the value of β is as shown in table 1.When α＞0, represent that line end voltage lags behind head end voltage on phase place; When α＜0, represent that line end voltage is ahead of head end voltage on phase place.

The different transformers of table 1 connect the value condition of β under the group condition

House transformer connects group	User transformers connects group	β
			Yyn0	Yyn0	0

Yyn0	Dyn11	-30°
			Dyn11	Yyn0	30°
Dyn11	Dyn11	0

Claims (6)

1. a power distribution network power circuit measuring power angle system is characterized in that, this measuring power angle system is made up of the power angle measuring terminals that is used to produce the main website equipment of voltage distortion signal and be positioned at the low-voltage customer end that is positioned at electric substation's end.At electric substation's end, main-transformer connects main website equipment by house transformer; At the low-voltage customer end, user transformers connects the power angle measuring terminals; When needs carried out the power angle measurement, main website equipment produced distorted signal by the instantaneous short-circuit of control house transformer low-pressure side near voltage over zero, the basic voltage over zero moment of just holding in electric substation of its energy peak; This signal spans transformer platform district transfers to user side; The power angle measuring terminals receives, the processes voltage distorted signal.

2. according to the described power distribution network power circuit of claim 1 measuring power angle system, it is characterized in that, the secondary side of described house transformer connects a, b, the c three-phase power line, be provided with on any therein two phase lines, be connected a, the voltage distortion modulating apparatus of b phase line, the voltage distortion modulating apparatus is mainly by single-chip microcomputer, signal modulation circuit, signal filter circuit and holding circuit constitute, be installed in a, voltage transformer pt on the b phase line connects signal filter circuit, signal filter circuit is connected to single-chip microcomputer, single-chip microcomputer is connected to PC by RS485 again, and single-chip microcomputer also is connected to the silicon controlled control utmost point by pulsed triggering circuit, is connected the holding circuit process resistance R on a phase line, inductance L, be connected behind the controllable silicon on the b phase line; Conducting by machine controllable silicon, a-b two-phase circuit is carried out short circuit, short trouble is eliminated the controllable silicon shutoff when treating the a-b line voltage zero-cross, this process produces a voltage distortion signal on electrical network a-b line voltage, and this signal spans transformer platform district transfers to user side.

3. according to the described power distribution network power circuit of claim 1 measuring power angle system, it is characterized in that, described power angle measuring terminals is installed in the low-pressure side of user transformers, the voltage transformer pt that is installed on a, the b phase line connects signal filter circuit, signal filter circuit is connected to 16 A/D sample circuits and positive and negative comparator circuit respectively, 16 A/D sample circuits are connected the DSP signal processing module respectively with positive and negative comparator circuit, and the DSP signal processing module also connects keyboard input, LED display respectively and is connected to PC by RS485; The voltage distortion signal that DSP collects the AD sample circuit carries out time frequency analysis to determine its time-domain position on local voltage waveform, obtains the power angle of transmission line of electricity indirectly.

4. the measuring method of a power distribution network power circuit power angle is characterized in that, comprises the following steps:

1) generation of voltage distortion signal and extraction, conducting by machine controllable silicon in electric substation with the short circuit of house transformer low-pressure side so that produce the voltage distortion signal; When the line voltage between any two-phase circuit of house transformer low-pressure side is about to zero passage, this two-phase circuit is carried out short circuit, stop short circuit when treating voltage zero-cross, the short-circuit current that this process produces can be superimposed upon on this line voltage inducing an instantaneous voltage drop on its leakage inductance when flowing through main-transformer, produce the voltage distortion signal, this voltage distortion signal spans transformer platform district transfers to user side; Before user side carries out time frequency analysis to the voltage distortion signal, at first it to be extracted from local voltage waveform, user side is to the detection and the analysis of distorted signal for convenience, when carrying out the power angle measurement, carry out the transmission of a plurality of distorted signals continuously, and each distorted signal carries by two adjacent voltage waveforms at system's head end;

2) measurement of the time frequency analysis of user side voltage distortion signal and power circuit power angle, after receiving the voltage waveform that contains distorted signal at user side, use the time-domain difference technology that the individual distortion signal is extracted from grid voltage waveform, and with a plurality of distorted signals formation composite signal that adds up, use Wei Gela to distribute simultaneously and composite signal is carried out the time frequency analysis processing in conjunction with Short Time Fourier Transform, determine the time difference of distorted signal and local voltage over zero according to the time distribution situation of its energy parameter, and obtain the power angle of circuit in conjunction with the connection group relation of transformer.

5. according to the described power distribution network power circuit of claim 4 method for measuring power angle, it is characterized in that, described short-circuit current can be superimposed upon on this line voltage inducing an instantaneous voltage drop on its leakage inductance when flowing through main-transformer, produce the voltage distortion signal, the distorted signal s (t) that obtains by difference method can be expressed as:

s(t)＝u(t)-u(t-T)＝γ(t)+n(t)

U in the formula (t) is a mains voltage signal; T is the power frequency period of line voltage, T=0.02s; γ (t) is the virtual voltage distorted signal; N (t) disturbs for electric network noise.

6. according to the described power distribution network power circuit of claim 4 method for measuring power angle, it is characterized in that, described electric network noise disturbs when interference ratio is serious, be difficult to that the two differentiates with distorted signal and noise interferences, carry out the transmission of N distorted signal continuously in electric substation's side for this reason, N the distorted signal that front and back voltage cycle signal subtraction is obtained at user side adds up and forms synthetic distorted signal s (t) then, that is:

$s\left(t\right)=\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}(u_k\left(t\right)-u_k(t-T))=\mathrm{N\γ}\left(t\right)+\underset{k=1}{\overset{k=N}{\mathrm{\Σ}}}{n}_k\left(t\right)$

The composition of distorted signal obtains the N multiplication by force among the s (t) like this, and the front and back voltage signal is made the undesired signal n that difference obtains _k(t) owing to have random character, during adding up the amplitude enhancing very limited.When N is enough big, in the composite signal resolution of distortion composition be improved significantly, wherein get N=7～10.

The Wigner distribution of described synthetic distorted signal s (t) is:

$W_s(t,\mathrm{\ω})=\underset{-\∞}{\overset{+\∞}{\∫}}s(t+\mathrm{\τ}/2)s^{*}(t-\mathrm{\τ}/2)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}$

The Short Time Fourier Transform of synthetic distorted signal s (t) is expressed as:

$F(t,\mathrm{\ω})={\left|\underset{-\∞}{\overset{+\∞}{\∫}}s\left(\mathrm{\τ}\right)h(\mathrm{\τ}-t)\exp (-\mathrm{j\ω\τ})\mathrm{d\τ}\right|}^2$

Wherein, s ^*(t) be the conjugate function of s (t); The window function of h (t) for being adopted in the short time discrete Fourier transform, selecting amplitude here is 1 rectangular window, i.e. h (t)=1; W is the angular frequency of synthetic distorted signal s (t), w=2 π (200～600) rad/s; τ represents time variable; J is an imaginary unit, and j ²=-1.

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