CN114325107A - Method for judging accuracy of harmonic impedance calculation result - Google Patents
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Abstract
The invention discloses a method for judging the accuracy of a harmonic impedance calculation result, which divides nonlinear users into two types according to the condition that the nonlinear users stop operating: only cutting off the harmonic source and keeping the user side impedance connected with the power grid (such as a photovoltaic station and the like), and cutting off the harmonic source and the user side impedance (such as an electric arc furnace and the like); determining the difference between the IEC method and the superposition principle method and the respective application range thereof in the process of verifying and solving the accuracy of the harmonic impedance; based on the characteristics of the two methods and the category of the nonlinear user, the method capable of accurately verifying and obtaining the correctness of the harmonic impedance is provided.
Description
Technical Field
The invention relates to the technical field of power quality sharing, in particular to a method for judging the accuracy of a harmonic impedance calculation result.
Background
With the development of power electronic equipment and the increase of nonlinear loads, harmonic pollution in power systems is becoming more and more serious. Accurate calculation of harmonic impedance is one of the keys to the study of harmonic problems. Based on harmonic measurement data at the Point of Common Coupling PCC, several non-intrusive harmonic impedance estimation methods have been proposed by the prior scholars. After calculating the harmonic impedance, the correctness of the calculation result needs to be verified. Accurate estimation of harmonic impedance is of great significance to quantification of harmonic responsibility, filter design, harmonic control, system resonance prediction and the like. However, there is still no unified method for verifying the correctness of obtaining the harmonic impedance.
The main idea of the existing method is to compare the calculated background harmonic voltage with the actual measurement voltage of the PCC after the user quits the operation, and if the difference between the calculated background harmonic voltage and the actual measurement voltage of the PCC is not large, the harmonic impedance is calculated accurately; on the contrary, it shows that there is a large error in obtaining the harmonic impedance. The background harmonics can be calculated according to IEC61000-3-6 (referred to as the IEC method) or the superposition rationale. However, in the existing research, when the accuracy of the harmonic impedance is verified and obtained, a little is ignored, that is, the IEC method and the superposition principle method need to be selected according to the actual working condition and cannot be selected at will because of different basic principles, otherwise, the risk of misjudgment of the accuracy of the harmonic impedance may occur.
Disclosure of Invention
Aiming at the problems, the invention aims to divide nonlinear users into two types of models by an IEC method and a superposition principle, and analyze the difference and the application range of the two types of models in detail, thereby providing a method for judging the accuracy of a harmonic impedance calculation result. The technical scheme is as follows:
a method for judging the accuracy of a harmonic impedance calculation result comprises the following steps:
step 1: dividing the nonlinear users into A-type nonlinear users and B-type nonlinear users, and respectively calculating harmonic voltage measured at the PCC (resonance control) position when each type of nonlinear user exits the operation, namely reference voltage of background harmonic;
1) the class-A nonlinear user is a user with a harmonic source and harmonic impedance disconnected when the class-A nonlinear user stops running, and the class-A nonlinear user calculates background harmonic voltage according to a circuit model of an IEC method; a class reference voltage defining the background harmonic is
The following equation is obtained:
wherein Z isuAnd
respectively the harmonic impedance and the harmonic current at the system side;
after the system side harmonic impedance Zu is accurately obtained, the background harmonic voltage calculated according to the IEC method
Will be connected to a class A reference voltage
Equal;
2) the B-type nonlinear user is a user with the harmonic source cut off and the harmonic impedance still connected to the power grid when the B-type nonlinear user stops running, and the class-B nonlinear user calculates the background harmonic voltage according to a superposition principle model; class B reference voltage defining background harmonics of
The following equation is obtained:
wherein Z iscIs the user side harmonic impedance;
after the harmonic impedance Zc at the user side and the harmonic impedance Zu at the system side are accurately obtained, the background harmonic voltage is calculated according to the superposition principle
And a class B reference voltage
Equal;
step 2: the accuracy of the harmonic impedance is verified and obtained according to the magnitude relation between the harmonic impedance Zc at the user side and the harmonic impedance Zu at the system side, and the detailed process is as follows:
A) the condition of | Zc | is not much larger than | Zu |
Calculating the A-type nonlinear user by adopting an IEC method
Computing by adopting superposition principle for B-type nonlinear users
Thereby verifying and obtaining system side harmonicThe accuracy of the impedance Zu and the user side harmonic impedance Zc;
B) working condition that | ZC | is far greater than | Zu |
Due to | Zc|>>|ZuL, thus having Zu/ZcAnd 0, converting the formula of calculating the background harmonic voltage according to the superposition principle into the following formula:
then
And
the method is approximately equal to each other, and whether the users are nonlinear users of the A type or the B type, the accuracy of the system side harmonic impedance Zu can be verified and obtained based on the IEC method or the superposition principle.
Further, the method for determining the magnitude relationship between | Zc | and | Zu | is as follows:
when | Zc |>>When | Zu | is Zu/ZcIs approximately equal to 0, thus having
Wherein,
for the harmonic current measured on the common line,
harmonic current at the user side;
by quantization
And
similarity between them, indirectly evaluating | Zc|>>|ZuIf | is true.
Further, the evaluation | Zc|>>|ZuIn the process of judging whether the I is established or not, reconstructing harmonic current of the user side of the source signal by an independent component method
According to the Nonton equivalent circuit, when a nonlinear user is connected with the system, there are
Wherein, the matrix X is the harmonic voltage measured by the PCC point of the observation signal
And harmonic current measured on the common line
Forming; harmonic current of matrix I from harmonic source signal system side
With harmonic current at the subscriber side
Forming; the matrix Z is composed of harmonic impedances on two sides of the PCC points;
firstly, extracting the fast-changing component of the signal by a median filtering technique
And
reconstructing the harmonic source signal by ICA algorithm
While the signal
And
the similarity between the two is quantified through a correlation coefficient;
if the correlation coefficient is greater than the set value, | Zc | > > | Zu |, otherwise | Zc | is not much greater than | Zu |.
The invention has the beneficial effects that: the invention divides the nonlinear users into two types according to the stop operation condition of the nonlinear users: 1) cutting off only the harmonic source while leaving its customer side impedance connected to the grid (e.g., photovoltaic stations, etc.), 2) cutting off the harmonic source and the customer side impedance (e.g., electric arc furnace, etc.). Then, the difference between the IEC method and the superposition principle method and the respective application range of the IEC method and the superposition principle method are researched in the process of verifying and solving the accuracy of the harmonic impedance. Finally, based on the characteristics of the two methods and the category of the nonlinear user, the method capable of accurately verifying and obtaining the correctness of the harmonic impedance is provided.
Drawings
FIG. 1 shows an equivalent circuit corresponding to the IEC method.
Fig. 2 shows equivalent circuits corresponding to the superposition principle method.
Fig. 3 is a flowchart of a method for determining the accuracy of the harmonic impedance calculation result.
FIG. 4 is photovoltaic field 9 harmonic data; (a) harmonic current (b) harmonic voltage.
FIG. 5 shows measured data of the electric arc furnace at the harmonic 5 th order; (a) PCC points actually measure harmonic voltage (b) public line actually measures harmonic current.
FIG. 6 is arc furnace 5 harmonic data; (a) harmonic current analysis (b) harmonic voltage analysis.
Detailed Description
The invention is described in further detail below with reference to the figures and specific embodiments. In actual engineering, the accuracy of obtaining the harmonic impedance can be indirectly verified by comparing the obtained background harmonic voltage with the voltage measured by the PCC point when the user does not work. In general, there are two methods for background harmonic calculation: the IEC method and the stacking principle method. The two methods are specifically based as follows.
(1) Calculating background harmonic voltage according to IEC method
In the Norton equivalent circuit corresponding to the IEC method shown in FIG. 1, Zu, Zc, Zc,
The harmonic impedance and the harmonic current source are respectively at the system side and the user side.
And
respectively, the harmonic voltage measured at the PCC point and the harmonic current measured on the common line.
When the switch is closed, the customer is connected to the grid, and the harmonic voltage at PCC is
When the switch is turned off, the user side stops operating, and the harmonic voltage at the PCC is
According to the definition of harmonic emission level by the IEC61000-3-6 standard, the harmonic voltage emission level of a nonlinear customer is the difference in the harmonic voltage at the PCC before the customer side is connected to the network and after the customer exits from service. Referring to the standard, the harmonic emission levels at the user side and the system side may be expressed as
(2) Calculating background harmonic voltage according to superposition principle
Fig. 2 shows a norton circuit corresponding to the stacking principle method. When the system side harmonic source and the nonlinear user harmonic source work independently, the harmonic voltage of the PCC points is the respective harmonic voltage emission level of the system side and the user side, which are respectively expressed as
(3) Method for verifying and solving accuracy of harmonic impedance
When a nonlinear user accesses a power grid, if the Zu and the Zc can be accurately estimated, the obtained background harmonic voltage is also accurate. The accuracy of obtaining the harmonic impedance can be indirectly verified by using the principle. When the nonlinear user exits operation, the harmonic voltage measured at the PCC may be considered a reference voltage for the background harmonic. Therefore, by comparing the obtained background harmonic voltage with the reference voltage, the correctness of obtaining Zu and Zc can be verified.
In practice, nonlinear users are diverse and complex. The present invention classifies non-linear users into the following two categories based on how they exit their operations.
1) Model A:
for non-linear users such as arc furnaces, the harmonic source and harmonic impedance are disconnected when the user is not working. For such users, the background harmonic voltage should be calculated according to the circuit model of the IEC method. The reference voltage is defined as
By mixing
Is replaced by
Can calculate
Therefore, after Zu is accurately obtained,
will be mixed with
Are equal. It is to be noted that this method cannot verify the correctness of obtaining Zc because Zc is not contained in formula (4).
2) Model B:
for non-linear consumers such as wind farms, photovoltaic farms, etc., when the consumer stops operating, the harmonic source is cut off while the harmonic impedance is still connected to the grid. For such users, the background harmonic voltage should be calculated according to the superposition principle model. The reference voltage is defined as
By mixing
Is replaced by
Can calculate
When Zc and Zu are accurately obtained, there are
And
are equal. It should be noted that, since formula (5) contains both Zc and Zu, the method can verify the correctness of Zc and Zu at the same time.
After the user category is determined, the accuracy of the harmonic impedance can be verified and obtained according to the magnitude relation between the Zc and the Zu. The detailed process is as follows.
A) The condition of | Zc | is not much larger than | Zu |
When | Zc | is not much larger than | Zu |, the background harmonic voltage obtained from the equations (4) and (5)
And
are not equal to each other. Therefore, the temperature of the molten metal is controlled,
will not be equal to
And is
Is not equal to
Theoretically, for model a, the background harmonic voltage calculated by the superposition principle does not match the harmonic voltage measured after the user has been cut. Also, for model B, the IEC method calculated background harmonic voltage does not match the measured voltage. Therefore, if the method of calculating the background harmonic voltage (i.e., the IEC method or the superposition principle) is selected incorrectly, even if the calculation result is accurate, it is mistaken that the harmonic impedance is found incorrectly.
In order to avoid the misjudgment, it is necessary to match the user model using a correct method (IEC method or superposition principle method).
B) Working condition that | ZC | is far greater than | Zu |
Under this condition, the error of obtaining Zc is usually large, so only the accuracy of obtaining Zu needs to be verified. Due to | Zc|>>|ZuL, thus having Zu/ZcAnd ≈ 0, the formula (5) can be converted into the formula (7). Thus, the background harmonic voltage can be calculated by the superposition principle without knowledge of Zc.
It is to be noted that, in this case, the formula (7) is equivalent to the formula (5), and obtained
And
approximately equal to each other. Therefore, the accuracy of Zu can be verified and obtained based on the IEC method or the superposition principle no matter the type A or the type B nonlinear users.
C) Method for judging size relationship between | ZC | and | Zu |
When | Zc |>>When | Zu | is Zu/ZcIs approximately equal to 0, thus having
Thus, by quantization
And
similarity between them, can be indirectly evaluatedc|>>|ZuIf | is true. In this evaluation process, the source signal can be reconstructed by Independent Component Analysis (ICA)
ICA is a blind source separation technique that extracts latent variables and the original signal from the observed signal.
According to the Nonton equivalent circuit (FIG. 1 or FIG. 2), when a nonlinear user is connected to the system, there is
In the formula: matrix X is formed by observation signals
And
formed by a harmonic source signal
And
the matrix Z is formed by harmonic impedances on both sides of the PCC point. Before the ICA algorithm is used, the fast-varying component of the signal needs to be extracted by a median filtering technique to ensure the independence between source signals. Reconfigurable harmonic source signal through ICA algorithm
. While the signal
And
the similarity between them is quantified by a correlation coefficient. The larger the correlation coefficient, the more similar the two signals are.
Fig. 3 shows a flowchart of the method for determining the accuracy of the harmonic impedance.
Example verification:
when the photovoltaic farm is shut down at night, the relevant electrical components (such as transformers, lines and filters, etc.) remain connected to the grid, so that only the harmonic sources are disconnected from the circuit, while the harmonic impedances of the photovoltaic stations remain connected to the system side. The method provided by the invention is proved to be correct by taking a certain photovoltaic power generation field in China as a typical nonlinear user of a model B.
For the 9 th harmonic, satisfy | Zc uti>>L Zu | as shown in FIG. 4(a)Due to harmonic currents determined by ICA
And measured
Almost coincide, and
and
the correlation coefficient between the two is large and is 0.998, so that the absolute value of Z isc|>>|ZuL. At this time, it is theoretically possible to compare
And
(or
) The accuracy of Zu is judged according to the correlation between the Zu and the Zu. While figure 4(b) shows that,
and
all can be combined with
And matching, thereby showing that the Zu is obtained with high precision. Further demonstrating the correctness of the proposed method.
The best embodiment of the specific application is as follows:
the arc furnace is a typical non-linear user in model a, which can be used to verify the correctness of the proposed method. Harmonic data was measured from a 150kV busbar in a 100MW DC arc furnace at a sampling frequency of 6400 Hz. And analyzing the sample data of voltage and current per minute by using fast Fourier transform to obtain the harmonic wave of each harmonic wave.
In this case, two single-tuned filters are installed on the user side to mitigate the 5 th and 7 th harmonics. The installation of the filter will result in | Zc | not being much larger than | Zu |. The 5 th harmonic voltage and current are shown in figure 5. After 1000 minutes, the furnace was shut down for a short period of time (corresponding to the down time).
Calculating harmonic impedance at two sides of PCC point by ICA algorithm to obtain background harmonic current
And separating the resulting user side harmonic currents
As shown in fig. 6 (a). It can be seen that
And obtaining
There is a large difference between them. In addition to this, the present invention is,
and
the correlation coefficient ratio between them is small, only 0.33. Thus illustrating | ZcI is not much greater than ZuL. Based on the process obtained in accordance with the present invention,
should be in accordance with
Match with
And not matched. FIG. 6(b) shows that
And obtained
The amplitudes are substantially equal, demonstrating the correctness of the sought Zu. In contrast to this, the present invention is,
and
the difference between the two is large, which means that if the correctness of the background harmonic is analyzed and obtained by using the superposition principle method, the obtained Zu is mistakenly considered to have a large calculation error.
Claims (3)
1. A method for judging the accuracy of a harmonic impedance calculation result is characterized by comprising the following steps:
step 1: dividing the nonlinear users into A-type nonlinear users and B-type nonlinear users, and respectively calculating harmonic voltage measured at the PCC (resonance control) position when each type of nonlinear user exits the operation, namely reference voltage of background harmonic;
1) the class-A nonlinear user is a user with a harmonic source and harmonic impedance disconnected when the class-A nonlinear user stops running, and the class-A nonlinear user calculates background harmonic voltage according to a circuit model of an IEC method; a class reference voltage defining the background harmonic is
The following equation is obtained:
wherein Z isuAnd
respectively the harmonic impedance and the harmonic current at the system side;
after accurately obtaining the system side harmonic impedance Zu, according to the IEC squareBackground harmonic voltage of method calculation
Will be connected to a class A reference voltage
Equal;
2) the B-type nonlinear user is a user with the harmonic source cut off and the harmonic impedance still connected to the power grid when the B-type nonlinear user stops running, and the class-B nonlinear user calculates the background harmonic voltage according to a superposition principle model; class B reference voltage defining background harmonics of
The following equation is obtained:
wherein Z iscIs the user side harmonic impedance;
after the harmonic impedance Zc at the user side and the harmonic impedance Zu at the system side are accurately obtained, the background harmonic voltage is calculated according to the superposition principle
And a class B reference voltage
Is equal to
Step 2: the accuracy of the harmonic impedance is verified and obtained according to the magnitude relation between the harmonic impedance Zc at the user side and the harmonic impedance Zu at the system side, and the detailed process is as follows:
A) the condition of | Zc | is not much larger than | Zu |
Calculating the A-type nonlinear user by adopting an IEC method
Computing by adopting superposition principle for B-type nonlinear users
Verifying and obtaining the accuracy of the system side harmonic impedance Zu and the user side harmonic impedance Zc;
B) working condition that | ZC | is far greater than | Zu |
Due to | Zc|>>|ZuL, thus having Zu/ZcAnd 0, converting the formula of calculating the background harmonic voltage according to the superposition principle into the following formula:
then
And
the method is approximately equal to each other, and whether the users are nonlinear users of the A type or the B type, the accuracy of the system side harmonic impedance Zu can be verified and obtained based on the IEC method or the superposition principle.
2. The method for determining the accuracy of the harmonic impedance calculation result according to claim 1, wherein the magnitude relationship between | Zc | and | Zu | is determined by:
when | Zc |>>When | Zu | is Zu/ZcIs approximately equal to 0, thus having
Wherein,
for the harmonic current measured on the common line,
harmonic current at the user side;
by quantization
And
similarity between them, indirectly evaluating | Zc|>>|ZuIf | is true.
3. The method of claim 2, wherein the evaluation | Z is evaluatedc|>>|ZuIn the process of judging whether the I is established or not, reconstructing harmonic current of the user side of the source signal by an independent component method
According to the Nonton equivalent circuit, when a nonlinear user is connected with the system, there are
Wherein, the matrix X is the harmonic voltage measured by the PCC point of the observation signal
And harmonic current measured on the common line
Forming; harmonic current of matrix I from harmonic source signal system side
With harmonic current at the subscriber side
Forming; the matrix Z is composed of harmonic impedances on two sides of the PCC points;
firstly, extracting the fast-changing component of the signal by a median filtering technique
And
reconstructing the harmonic source signal by ICA algorithm
While the signal
And
the similarity between the two is quantified through a correlation coefficient;
if the correlation coefficient is greater than the set value, | Zc | > > | Zu |, otherwise | Zc | is not much greater than | Zu |.
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