A four-quadrant power ac tie-line protection method based on current probability distribution

Abstract

The four-quadrant power supply represented by the battery energy storage station and the flexible HVDC system accesses the power grid with power electronic converters as the energy conversion interface, so the fault current characteristics are quite different from those of the traditional synchronous machine. Compared with new energy power supply, four-quadrant power supply has a wider operating range, and the fault current characteristics are more complex than those of new energy power supply. Therefore, the line protection method designed for the fault characteristics of synchronous power supply and new energy power supply has the risk of sensitivity decline or even refusal after the four-quadrant power supply is connected. In order to solve the above problems, a four-quadrant power supply AC tie line protection method based on current probability distribution is disclosed. The protection method converts the currents on both sides into discrete probability distribution by a new probabilistic method, and identifies the internal and external faults based on the difference between the current probability distributions on both sides. The method disclosed in the application can quickly and accurately identify internal and external faults of the line and adapt to the fault characteristics of the four-quadrant power supply, ensuring the safe and stable operation of the power system, and has a broad application prospect in the field of relay protection.

Description

Technical Field

[0001] This invention relates to a four-quadrant AC tie-line protection method based on current probability distribution, belonging to the field of power system relay protection technology. Background Technology

[0002] The rapid increase in installed capacity of renewable energy has profoundly changed the operating characteristics of the power system. The randomness, intermittency, and volatility of renewable energy seriously affect the safe and stable operation of the power system. Energy storage is an effective means to solve these problems, not only participating in the regulation of grid voltage and frequency but also effectively improving power supply reliability. Therefore, in recent years, new energy storage technologies, represented by battery energy storage stations, have received widespread attention from the engineering and academic communities, with installed capacity increasing rapidly and developing towards centralization and large-scale deployment. Meanwhile, development models such as large-scale development in desert and Gobi areas, offshore wind power cluster development, and integrated hydro-wind-solar development in southeastern Tibet have created an urgent demand for new transmission methods such as flexible direct current transmission (VSC-HVDC) technology, and more and more VSC-HVDC projects will be put into use in the future.

[0003] Battery storage stations and VSC-HVDCs have multiple operating modes, capable of absorbing / generating active and reactive power, and possess four-quadrant power control and operation capabilities, hence the name four-quadrant power sources. Four-quadrant power sources are connected to the grid via converter interfaces, with limited fault current amplitude and controlled phase angle, significantly different from traditional synchronous generators. Furthermore, unlike renewable energy power plants connected via converter interfaces, four-quadrant power sources have a wider operating range and more complex fault current characteristics. Currently, there is little research on the protection of AC tie lines for four-quadrant power sources. Existing protection methods are mostly designed based on the fault characteristics of traditional synchronous generators or renewable energy power plants, which pose a risk of low sensitivity or even failure to operate when four-quadrant power sources are connected, seriously threatening the safe and stable operation of the power system. Summary of the Invention

[0004] To address the problem that existing protection methods suffer from decreased sensitivity or even failure to operate when applied to AC lines connected to four-quadrant power supplies, this invention provides a protection method for AC tie lines of four-quadrant power supplies based on current probability distribution.

[0005] A protection method for AC tie lines of a four-quadrant power supply based on current probability distribution includes the following steps:

[0006] Step 1: Detect the current i at the protection installation points on both sides of the transmission line. g and i b The subscripts b and g represent the power supply side and grid side in the four quadrants, respectively.

[0007] Step 2: Detect the maximum and minimum values ​​of the current on both sides within one power frequency cycle, and calculate the peak-to-peak value of the current on both sides; considering a certain margin, multiply the smaller peak-to-peak value by 1.5 and then divide it into 4 equal sub-intervals, with ε... base The sub-intervals are used as the width of the current partition; the baseline is calculated, and four widths of ε are divided outwards from the baseline in both positive and negative directions. base A subinterval of equal width and an infinite subinterval;

[0008] Step 3: Calculate the probability that the current sampling point falls within each sub-interval within the 10ms data window, obtaining the discrete probability distribution of the current on both sides; the sub-intervals are labeled from negative to positive as sub-interval 1 to sub-interval 10, and the probabilities that the current sampling points on both sides fall within sub-interval 1 to sub-interval 10 are respectively denoted by η. g(b)1 -η g(b)10 express;

[0009] Step 4: Determine the maximum probability η of the sub-interval of the current with the smaller peak-to-peak value. max Is it less than η? THR If η max Greater than η THR D JS =0; if η max Less than or equal to η THR Calculate D JS The calculated maximum probability of a subinterval under normal system operation is approximately 0.54. Considering a certain margin, η THR Set to 0.6;

[0010] Step 5: Calculate D within the 10ms data window JS The average value of DA JS ;

[0011] Step 6: Set the protection action threshold to DA JSTHR Determine DA JS and DA JSTHR The size relationship, if DA JS >DA JSTHR If the fault occurs, it is determined to be an intra-zone fault, and the protection system will activate; if DA JS <DA JSTHR If the fault is outside the designated area, the protection system will not operate. Taking into account both the reliability and security of the protection system, DA... JSTHR Set it to 0.25.

[0012] This invention discloses a protection method for AC tie lines of four-quadrant power supplies based on current probability distribution. This method can adapt to the complex fault characteristics of four-quadrant power supplies and solves the problem of decreased sensitivity or even failure to operate in traditional protection methods after the connection of four-quadrant power supplies. When a fault occurs in the AC line connected to a four-quadrant power supply, the protection can act quickly and sensitively, ensuring the safe and stable operation of the power system. Attached Figure Description

[0013] Figure 1 A schematic diagram of a four-quadrant power source connected to the power grid;

[0014] Figure 2 A flowchart illustrating the fault identification method for AC tie-line protection of four-quadrant power supplies based on current probability distribution.

[0015] Figure 3 This is a schematic diagram of the probabilistic partitioning of the current on both sides. Detailed Implementation

[0016] The invention will now be further described with reference to the accompanying drawings.

[0017] A protection method for AC tie lines of a four-quadrant power supply based on current probability distribution includes the following steps:

[0018] Step 1: As Figure 1 and Figure 2 Detect the current i at the protection installation points on both sides of the transmission line. b and i g The subscripts b and g represent the power supply side and grid side in the four quadrants, respectively. The current sampling sequences on both sides are as follows:

[0019] i b =[i b (1),i b (2),···,i b (k),···,i b (n)] (1)

[0020] i g =[-i g (1),-i g (2),···,-i g (k),···,-i g (n)] (2)

[0021] Where n represents the number of sampling points within a 10ms data window.

[0022] Step Two: As Figure 2 and Figure 3The maximum and minimum values ​​of the current on both sides are detected within one power frequency cycle, and the peak-to-peak values ​​of the current on both sides are calculated. Considering a certain margin, the smaller peak-to-peak value is multiplied by 1.5 and then divided into 4 sub-intervals. These sub-intervals are used as the current partitioning reference. The baseline is calculated, and the current is divided into four sub-intervals with ε as the center. base Divide the width into four subintervals in both positive and negative directions, and one infinite subinterval. The subinterval width is ε. base The specific calculation formula for baseline is as follows:

[0023] ε base =1.5×min{I gmax -I gmin ,I bmax -I bmin} / 4 (3)

[0024]

[0025] Step 3: As Figure 2 Calculate the probability that the current sampling point falls within each sub-interval within a 10ms data window to obtain the discrete probability distribution of the current on both sides. The sub-intervals are labeled from negative to positive as sub-interval 1 to sub-interval 10, and the probabilities that the current sampling points on both sides fall within sub-interval 1 to sub-interval 10 are respectively denoted by η. g(b)1 -η g(b)10 express.

[0026] Step Four: As Figure 2 Determine the maximum probability η of the sub-interval of the current with the smaller peak-to-peak value. max Is it less than η? THR If η max Greater than η THR D JS Set to 0; if η max Less than or equal to η THR Calculate D JS The calculated maximum probability of a subinterval under normal system operation is approximately 0.54. Considering a certain margin, η THR Set to 0.6; D JS The calculation formula is as follows:

[0027]

[0028] Step 5: As Figure 2 Calculate D within a 10ms data window JS The average value of DA is obtained. JS The specific calculation formula is as follows:

[0029]

[0030] Step Six: As Figure 2The protection action threshold is set to DA. JSTHR Determine DA JS and DA JSTHR The size relationship, if DA JS >DA JSTHR If the fault occurs, it is determined to be an intra-zone fault, and the protection system will activate; if DA JS <DA JSTHR If the fault is outside the designated area, the protection system will not operate. Taking into account both the reliability and security of the protection system, DA... JSTHR Set it to 0.25.

[0031] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements, substitutions, modifications and refinements can be made without departing from the principles and spirit of the present invention, and these improvements, substitutions, modifications and refinements should also be considered within the scope of protection of the present invention.

Claims

1. A protection method for AC tie lines of a four-quadrant power supply based on current probability distribution, comprising the following steps: Step 1: Detect the current i at the protection installation points on both sides of the transmission line. b and i g The subscripts b and g represent the power supply side and grid side in the four quadrants, respectively. Step 2: Detect i within one power frequency cycle. g and i b Calculate the peak-to-peak value of the current on both sides by taking the maximum and minimum values ​​of the current. Considering a certain margin, multiply the smaller peak-to-peak value by 1.5 and then divide it into 4 equal sub-intervals, with the width denoted as ε. base The sub-interval is used as the current partitioning reference; the baseline is calculated, and four divisions with a width of ε are made outward from the baseline in both positive and negative directions. base A subinterval and an infinite subinterval; Step 3: Calculate the probability that the current sampling points on both sides fall within each sub-interval within the 10ms data window, obtaining the discrete probability distribution of the current on both sides; the sub-intervals are labeled from negative to positive as sub-interval 1 to sub-interval 10, and the probabilities that the current sampling points on both sides fall within sub-interval 1 to sub-interval 10 are respectively denoted by η. g(b)1 -η g(b)10 express; Step 4: Determine the maximum probability η of the sub-interval of the current with the smaller peak-to-peak value. max Is it greater than η? THR If η max Greater than η THR D JS =0; if η max Less than or equal to η THR Calculate D JS The calculated maximum probability of a subinterval under normal system operation is approximately 0.

54. Considering a certain margin, η THR Set to 0.6; Step 5: Calculate D within the 10ms data window JS The average value of DA JS ; Step 6: Set the protection action threshold to DA JSTHR Determine DA JS and DA JSTHR The size relationship, if DA JS >DA JSTHR If the fault occurs, it is determined to be an intra-zone fault, and the protection system will activate; if DA JS <DA JSTHR If the fault is outside the designated area, the protection will not operate; considering both the reliability and security of the protection, DA JSTHR Set it to 0.

25.

2. The method for protecting AC tie lines of a four-quadrant power supply based on current probability distribution according to claim 1, characterized in that: Step 2: Calculate the peak-to-peak values ​​of the currents on both sides within one power frequency cycle. Considering a certain margin, multiply the smaller peak-to-peak value by 1.5 and then divide it into four equal sub-intervals, with the width denoted as ε. base Calculate the baseline, with the baseline as the center and ε as the boundary. base Divide the width into four sections, each with a width of ε, in both positive and negative directions. base Given a subinterval of ε and an infinite subinterval, calculate ε. base The calculation formulas for baseline and baseline are shown in equation (1) and equation (2) respectively: ε base =1.5×min{I gmax -I gmin ,I bmax -I bmin } / 4 (1) In the formula, I g(b)max and I g(b)min These represent the maximum and minimum values ​​of the current on both sides within one power frequency cycle, respectively. The subscripts b and g represent the power supply side current and grid side current in the four quadrants, respectively.

3. The method for protecting AC tie lines of a four-quadrant power supply based on current probability distribution according to claim 1, characterized in that: Indicator D described in step four JS The calculation formula is:

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