Online calculation method and system for active impedance of low-voltage light-load line

Abstract

The invention provides a low-voltage light-load line active impedance online calculation method and system.The low-voltage light-load line active impedance online calculation method comprises the steps that the frequency and voltage of a power grid are detected in real time at a first test point and a second test point at the two ends of a to-be-detected target section line respectively; obtaining a harmonic and inter-harmonic content data set; selecting a frequency band with the minimum voltage amplitude as a test frequency band in the harmonic and inter-harmonic content data set; injecting different high-frequency currents in a test frequency band into the power grid at the first test point and the second test point in sequence, and respectively calculating harmonic contents at the first test point and the second test point to obtain a first harmonic content data set and a second harmonic content data set; according to the first harmonic content data set and the second harmonic content data set, the impedance of the to-be-measured target section line is estimated, and therefore an impedance estimation scheme capable of being applied to the light-load line is provided.

Description

Technical Field

[0001] The present invention relates to the technical field of power system power management, and in particular to a method and system for online calculation of active impedance of a low-voltage light-load line. Background Art

[0002] Due to the complexity of the lines, the small impact of faults and cost issues, the flow calculation and line impedance analysis of low-voltage distribution networks have always been in a blind spot. However, with the increasing requirements for lean management, real-time analysis of the impedance changes of distribution network lines can play a positive role in early detection of line aging, short-circuit and open-circuit fault prediction, and line loss management, which can effectively improve power supply reliability.

[0003] At present, there are two main technical routes for impedance estimation of low-voltage lines: the first technical route is to install a micro PMU device, namely a synchronous phasor measurement unit (PMU), to collect the voltage and current vectors at the same time, calculate the voltage phase angle difference at both ends of the line, and then calculate the line impedance. This method requires high-precision synchronization and has a large investment cost. In addition, since the voltage drop of the distribution network is often small, the accuracy error caused by the transformer is even larger than the actual value, which brings about the error drowning problem. The second technical route is to calculate the line impedance using the voltage and current amplitude collected by the smart meter. This method is simple to calculate, but is greatly affected by the current phase angle, and can only calculate the modulus of the line impedance, and cannot accurately calculate the resistance and reactance of the line, which has certain limitations. The two existing technical routes are based on loaded or even heavily loaded lines. For a large number of low-voltage lines, there are a large number of spare intervals or lightly loaded lines. During daily operation, the power frequency current is very small or even zero. At this time, the line impedance can no longer be calculated online through the above two methods.

[0004] Therefore, it is necessary to provide a technical solution for impedance estimation of low-voltage light-load lines to solve the problem that line impedance cannot be estimated through current under long-term light-load conditions. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a method for online calculation of active impedance of a low-voltage light-load line, comprising the following specific steps:

[0006] The frequency and voltage of the power grid are respectively detected in real time at a first test point and a second test point at both ends of the target section line to be tested, so as to obtain a harmonic and interharmonic content data set;

[0007] In the harmonic and interharmonic content data set, a frequency band with the smallest voltage amplitude is selected as a test frequency band;

[0008] Injecting different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current;

[0009] Injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a second harmonic content data set and stop injecting the high-frequency current;

[0010] The impedance of the target section line to be measured is estimated according to the first harmonic content data set and the second harmonic content data set.

[0011] In a possible implementation manner, the frequency band of the high-frequency current is not within the range of the power frequency band during operation of the power grid.

[0012] In a possible implementation, calculating the harmonic content includes calculating harmonic power, harmonic voltage, and harmonic current.

[0013] In a possible implementation manner, the high-frequency current is generated by modulating a power switch and a high-frequency load together.

[0014] The present invention also provides a low-voltage light-load line active impedance online calculation system, comprising:

[0015] A measuring terminal is used to detect the frequency and voltage of the power grid in real time at a first test point and a second test point at both ends of the target section line to be measured, respectively, to obtain a harmonic and interharmonic content data set;

[0016] A fusion terminal, used for selecting a frequency band with a minimum voltage amplitude in the harmonic and interharmonic content data set as a test frequency band;

[0017] The measuring terminal is also used to inject different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculate the harmonic content at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current;

[0018] The measuring terminal is also used for injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point, obtaining a second harmonic content data set and stopping the injection of high-frequency current;

[0019] The fusion terminal is further used to estimate the impedance of the target section line to be measured according to the first harmonic content data set and the second harmonic content data set.

[0020] In a possible implementation, in the low-voltage light-load line active impedance online calculation system, the frequency band of the high-frequency current is not within the range of the power frequency band when the power grid is operating.

[0021] In a possible implementation, in a low-voltage light-load line active impedance online calculation system, the calculation of harmonic content includes the calculation of harmonic power, harmonic voltage, and harmonic current.

[0022] In a possible implementation, in a low-voltage light-load line active impedance online calculation system, the high-frequency current is generated by modulating a power switch and a high-frequency load.

[0023] The technical solution provided by the present invention has at least the following beneficial effects:

[0024] By real-time detecting the frequency and voltage of the power grid at the first test point and the second test point at both ends of the target section line to be tested, a harmonic and interharmonic content data set is obtained, and the frequency band with the smallest voltage amplitude is selected as the test frequency band accordingly. High-frequency current is injected into the power grid under this test frequency band, which can minimize the impact on the power quality of the power grid. At the same time, the impedance of the target section line to be tested is estimated by the obtained first harmonic content data set and second harmonic content data set, thereby realizing the impedance estimation of the low-voltage light-load line. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A flow chart of a method for online calculation of active impedance of a low-voltage light-load line provided by an embodiment of the present invention;

[0026] Figure 2 A circuit schematic diagram of a measurement terminal provided by an embodiment of the present invention;

[0027] Figure 3 A schematic diagram of a high-frequency component network provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to deepen the understanding of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The embodiments are only used to explain the present invention and do not limit the protection scope of the present invention.

[0029] Please refer to Figures 1 to 3 The present invention provides a low-voltage light-load line active impedance online calculation method, comprising the following specific steps:

[0030] S100: Real-time detection of the frequency and voltage of the power grid at a first test point and a second test point at both ends of the target section line to be tested, respectively, to obtain a harmonic and interharmonic content data set;

[0031] S200: Selecting a frequency band with the smallest voltage amplitude in the harmonic and interharmonic content data set as a test frequency band;

[0032] S300: injecting different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current;

[0033] S400: injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a second harmonic content data set and stop injecting the high-frequency current;

[0034] S500: Estimate the impedance of the target line segment to be measured according to the first harmonic content data set and the second harmonic content data set.

[0035] In this embodiment, the power grid is constructed by connecting transformers, branch boxes, meter boxes, etc. Figure 3 The transformer is powered by the main power supply and outputs the transformed electric energy to the outside. The impedance of the external power grid at the transformer input can be expressed as Z s The line between the transformer outlet and the branch box can be regarded as the first segment line, and its impedance can be expressed by Z l1 The line between the branch box and the meter box can be regarded as the second segment line, and its impedance can be expressed by Z l2 The impedance from the meter box to the home can be expressed as Z L If the target segment line to be tested is the first segment line, the first test point can be set at the branch box, and the second test point can be set at the transformer outlet. If the target segment line to be tested is the second segment line, the first test point can be set at the meter box, and the second test point can be set at the branch box.

[0036] In a specific implementation, it is assumed that the target segment line to be tested is the first segment line, such as Figure 3 , install measurement terminals at the first test point and the second test point (i.e., two equipment installation points). In step S100, when the power grid is operating normally, the measurement terminals at each equipment installation point detect the frequency and voltage of the power grid in real time, calculate the harmonic and interharmonic content in real time, obtain the harmonic and interharmonic content data set, and transmit the results to the fusion terminal by carrier. In the specific implementation, the carrier mainly uses HPLC carrier. The power grid frequency at the installation point is basically within 50Hz±0.1Hz, the harmonics are 1-20 harmonics, and the interharmonics are generally 1.1-19.9 harmonics.

[0037] After executing step S100, in step S200, the fusion terminal counts the harmonic content of each order at each device installation point according to the harmonic and interharmonic content data set, and finds a frequency point, i.e., frequency band f, according to the counted harmonic content, which satisfies:

[0038] V f =min(V1,V 1.1 ,V 1.2 ,...,V 25 )

[0039] Where V f Indicates the harmonic voltage content corresponding to the frequency point f, V1 indicates the 1st harmonic, i.e., the 50Hz voltage content, V 1.1 Indicates the 1.1th harmonic, i.e. the 55Hz interharmonic voltage content, V 1.2 Indicates the 1.2th harmonic, i.e. the 60Hz interharmonic voltage content, V 25 Indicates the 25th harmonic voltage content. This formula indicates that all harmonic voltages and interharmonic voltages of the power grid from 1 to 25, i.e., 50 to 1250 Hz (in steps of 5 Hz), are calculated in real time, and the frequency band f with the smallest harmonic voltage amplitude is selected as the subsequent test frequency band.

[0040] After executing step S200, in steps S300 and S400, the fusion terminal controls each measurement terminal in turn, and injects high-frequency pulse disturbance currents, i.e., high-frequency currents, with different duty cycles and current sizes in the frequency band f in turn. Each measurement terminal detects the harmonic content of the specified frequency band f, including harmonic power, harmonic voltage, and harmonic current. When the harmonic current exceeds a certain threshold for a period of time, for example, if the harmonic current in this frequency band exceeds 1A for 0.5s, it is considered that a device in the network has injected high-frequency harmonics, i.e., high-frequency currents, and the first harmonic content data set and the second harmonic content data set are obtained respectively, and the results are transmitted to the fusion terminal via a carrier mode. The carrier mode here is mainly the HPLC modulation mode.

[0041] After executing steps S300 and S400, in step S500, the fusion terminal obtains the measurement information of the upper and lower ends of the line, namely the first harmonic content data set and the second harmonic content data set, and then calculates the line impedance:

[0042]

[0043] In the formula, represents the estimated value of the line impedance, Represents the high-frequency harmonic amplitude matrix of multiple measurements, I n Indicates the harmonic current amplitude measured by the nth injection on the line, express The transpose of It represents the harmonic power difference matrix measured at two points above and below the line, that is, the loss of harmonic current on the line, ΔP n Indicates the active power difference measured for the nth time between two points on the line, ΔQ n It represents the reactive power difference measured for the nth time between two points above and below the line, and j represents the imaginary part.

[0044] It should be noted that if Figure 2 When high-frequency current is injected into the grid connection point, it can be understood as injecting harmonics, and the injection source can be equivalent to a high-frequency disturbance. Figure 3 , due to the high-frequency current I of the disturbance device H It is known that, therefore, in the high-frequency component network, it can be equivalent to a high-frequency current source. By measuring the harmonic voltage and harmonic current at each device installation point, the impedance of each segmented line can be estimated. In specific implementation, the high-frequency current I H It can be calculated based on the voltage and modulation resistance of the access point, i.e., the grid connection point. The low-voltage light-load line active impedance online calculation method provided by the present invention is sensitive and reliable, and the modulation adopts a micro-current mode. The modulation circuit is simple, the power consumption is small, and the impact on the power quality of the power grid is small. The injection signal can avoid the power frequency band and is not interfered by the operation of the power grid. It belongs to active measurement and can also be used for light-load lines.

[0045] In a possible implementation manner, the frequency band of the high-frequency current is not within the range of the power frequency band during operation of the power grid.

[0046] In this embodiment, the injection frequency band of high-frequency current, i.e. harmonics (i.e., the carrier signal frequency is at least several hundred kHz) and the duty cycle are controllable, and the line impedance of the full frequency band can be measured, and the power grid power frequency (50Hz) and the harmonics of the power frequency integer multiples, i.e., the power frequency band range, can be avoided, and the interference from the power grid operation is small. The low-voltage light-load line active impedance online calculation method in the present invention belongs to an active measurement method, and can also be applied to light-load lines.

[0047] In a possible implementation, calculating the harmonic content includes calculating harmonic power, harmonic voltage, and harmonic current.

[0048] In a possible implementation manner, the high-frequency current is generated by modulating a power switch and a high-frequency load together.

[0049] In this embodiment, Figure 2 , the power switch can be a transistor, the high-frequency load can be a modulation resistor, the base of the transistor is triggered by the PWM generator, so as to modulate and generate the corresponding high-frequency current at the grid connection point. The high-frequency current can be regarded as a high-frequency pulse disturbance current, the frequency and duty cycle of which can be adjusted by the PWM generator, and the current size can be adjusted by the modulation resistor.

[0050] The present invention also provides a low-voltage light-load line active impedance online calculation system, comprising:

[0051] A measuring terminal is used to detect the frequency and voltage of the power grid in real time at a first test point and a second test point at both ends of the target section line to be measured, respectively, to obtain a harmonic and interharmonic content data set;

[0052] A fusion terminal, used for selecting a frequency band with a minimum voltage amplitude in the harmonic and interharmonic content data set as a test frequency band;

[0053] The measuring terminal is also used to inject different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculate the harmonic content at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current;

[0054] The measuring terminal is also used for injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point, obtaining a second harmonic content data set and stopping the injection of high-frequency current;

[0055] The fusion terminal is further used to estimate the impedance of the target section line to be measured according to the first harmonic content data set and the second harmonic content data set.

[0056] In this embodiment, in order to solve the problem that the line impedance cannot be estimated by current under the above-mentioned long-term light-load condition, the present invention provides a low-voltage light-load line active impedance online calculation system to implement the above-mentioned low-voltage light-load line active impedance online calculation method, which is mainly based on a fusion terminal and a measuring device, namely a measuring terminal, and the power grid is in normal operation. In the specific implementation, the fusion terminal sends a command to the measuring terminal, and the measuring terminal injects a high-frequency current with adjustable frequency and duty cycle into the power grid through a power switch and a high-frequency load, and calculates the line impedance through the high-frequency current and high-frequency power collected from the upstream and downstream of the line. The network can be used for data transmission between the measuring terminal and the fusion terminal.

[0057] In a possible implementation, in the low-voltage light-load line active impedance online calculation system, the frequency band of the high-frequency current is not within the range of the power frequency band when the power grid is operating.

[0058] In a possible implementation, in a low-voltage light-load line active impedance online calculation system, the calculation of harmonic content includes the calculation of harmonic power, harmonic voltage, and harmonic current.

[0059] In a possible implementation, in a low-voltage light-load line active impedance online calculation system, the high-frequency current is generated by modulating a power switch and a high-frequency load.

[0060] The above embodiments should not limit the present invention in any way, and any technical solutions obtained by equivalent replacement or equivalent conversion shall fall within the protection scope of the present invention.

Claims

1. A method for online calculation of active impedance of low-voltage light-load lines, characterized in that: The specific steps include: The frequency and voltage of the power grid are respectively detected at a first test point and a second test point at both ends of the target section line to be tested in real time to obtain a harmonic and interharmonic content data set; In the harmonic and interharmonic content data set, a frequency band with the smallest voltage amplitude is selected as a test frequency band; Injecting different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current; Injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point to obtain a second harmonic content data set and stop injecting the high-frequency current; The impedance of the target section line to be measured is estimated according to the first harmonic content data set and the second harmonic content data set.

2. The method for online calculation of active impedance of low-voltage light-load lines according to claim 1 is characterized in that: The frequency band of the high-frequency current is not within the range of the power frequency band during operation of the power grid.

3. The method for online calculation of active impedance of low-voltage light-load lines according to claim 1 is characterized in that: Calculation of harmonic content includes calculation of harmonic power, harmonic voltage, and harmonic current.

4. The method for online calculation of active impedance of low-voltage light-load lines according to claim 1 is characterized in that: The high-frequency current is generated by modulating the power switch and the high-frequency load.

5. A low-voltage light-load line active impedance online calculation system, characterized in that: include: A measuring terminal is used to detect the frequency and voltage of the power grid in real time at a first test point and a second test point at both ends of the target section line to be measured, respectively, to obtain a harmonic and interharmonic content data set; A fusion terminal, used for selecting a frequency band with a minimum voltage amplitude in the harmonic and interharmonic content data set as a test frequency band; The measuring terminal is also used to inject different high-frequency currents in the test frequency band into the power grid at the first test point, and respectively calculate the harmonic content at the first test point and the second test point to obtain a first harmonic content data set and stop injecting the high-frequency current; The measuring terminal is also used for injecting different high-frequency currents in the test frequency band into the power grid at the second test point, and respectively calculating the harmonic contents at the first test point and the second test point, obtaining a second harmonic content data set and stopping the injection of high-frequency current; The fusion terminal is further used to estimate the impedance of the target section line to be measured according to the first harmonic content data set and the second harmonic content data set.

6. The low-voltage light-load line active impedance online calculation system according to claim 5 is characterized in that: The frequency band of the high-frequency current is not within the range of the power frequency band during operation of the power grid.

7. The low-voltage light-load line active impedance online calculation system according to claim 5 is characterized in that: Calculation of harmonic content includes calculation of harmonic power, harmonic voltage, and harmonic current.

8. The low-voltage light-load line active impedance online calculation system according to claim 5 is characterized in that: The high-frequency current is generated by modulating the power switch and the high-frequency load.

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