CN111830364B - Calculation method for calculating transient interference signals at two ends of secondary cable of transformer substation - Google Patents

Calculation method for calculating transient interference signals at two ends of secondary cable of transformer substation Download PDF

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CN111830364B
CN111830364B CN202010501271.0A CN202010501271A CN111830364B CN 111830364 B CN111830364 B CN 111830364B CN 202010501271 A CN202010501271 A CN 202010501271A CN 111830364 B CN111830364 B CN 111830364B
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point
ground
afk
secondary cable
equivalent impedance
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CN111830364A (en
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沈卫东
刘娟
聂文海
袁敬中
刘丽
张楠
朱全友
李顺昕
李海滨
周洁
李莉
张嵩
朱正甲
梁大鹏
宋堃
史智萍
张晋梅
赵芃
刘志雄
申惠琪
赵一男
宋斌
何金良
张波
段炼
王森
李志中
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Economic and Technological Research Institute of State Grid Hebei Electric Power Co Ltd
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/088—Aspects of digital computing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/083—Locating faults in cables, transmission lines, or networks according to type of conductors in cables, e.g. underground

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  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Mathematical Physics (AREA)
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  • Measurement Of Resistance Or Impedance (AREA)

Abstract

A calculation method for calculating transient interference signals at two ends of a secondary cable connected with equipment in a transformer substation comprises the following calculation steps: recording two ends of the secondary cable at the two ends of the ground point A, B to obtain equivalent impedance Z A of the point A and the ground point at infinity, as shown in figure 1 of the abstract drawing; and obtaining Z B by the same method; establishing a coaxial pi-type equivalent circuit of the secondary cable, and respectively connecting an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b at two ends of A, B; and calculating interference signals between the core wires at the two ends of the cable and the grounding point. The beneficial effects are as follows: the calculation method can accurately calculate the interference time domain signal in the cable when the secondary cable is interfered by external transient state, and the calculation method of the transmission line theory is adopted, so that the speed is high.

Description

Calculation method for calculating transient interference signals at two ends of secondary cable of transformer substation
Technical Field
The invention relates to calculation and evaluation of cable transient interference, which can be used for interference signals on secondary cables after transient interference such as direct lightning strike or lightning invasion waves of a transformer substation.
Background
Because the secondary system of the transformer substation is subject to outstanding electromagnetic disturbance, electric workers at home and abroad develop a great deal of researches. The special research work aiming at the electromagnetic transient of the transformer substation, which is carried out by the electric science institute in 1978, accumulates a great deal of data and experience. Studies have shown that switching operations, system short-circuit faults and lightning strokes of substations may lead to high-frequency electromagnetic interferences with great intensity.
The secondary cable is often used as a main carrier of communication and control signals in the transformer substation, and is often used for carrying the task of transmitting the control signals of weak current equipment to strong current equipment or transmitting data signals collected on the strong current equipment to the weak current equipment. The normal operation of the secondary cable can be said, the stable transmission of important signals in the station is ensured, and the safe and stable operation of the transformer substation is ensured.
Communication cables in substations often have shielding and armouring layers in addition to the core wires, which play the roles of shielding interfering signals and supporting protection core wires, etc., whereas in order to ensure reliable operation of the secondary cable, the shielding and armouring layers need to be grounded at one or both ends. The difference of the grounding modes of the shielding layer and the armor layer influences the stable operation of the secondary cable under faults and lightning strokes. A simple schematic of the secondary cable setup within a substation is shown in fig. 2.
The secondary cable core is twisted pair, the potential difference between the two cores is the transmitted signal, the shielding layer and the armor layer are grounded at two ends (not necessarily grounded at two ends), zs is the output impedance of the signal output device, zl is the input impedance of the signal receiving device, the device shell is grounded through a grounding wire, and in order to achieve the purpose of inhibiting electromagnetic interference and the like, the ground on the secondary device is generally connected with the device shell.
When short circuit fault or lightning strike occurs in the station, the disturbance signals generate disturbance signals in the core wires through capacitive and inductive coupling and other ways, and as the paired core wires generally adopt twisted pair wires, the differential mode disturbance signals among the core wires are smaller, and are mainly common mode disturbance signals. Therefore, the potential difference that threatens the safety within the station is mainly represented by the potential difference between the core and the shielding layer, which would break down the cable, and the potential difference between the end of the core and the "ground" of the device, which would break down the device. In addition, when the two ends of the shielding layer or the armor layer are grounded, a disturbance current flows in the shielding layer or the armor layer, and when the current is too large, the cable may be burnt.
The grounding network and the secondary cable are mainly connected through grounding points at two ends of the cable, and lightning current is injected through a certain grounding point on the grounding network, as shown in fig. 3.
The whole system can be divided into three parts, namely a lightning current source, a grounding grid with three external ports and a secondary cable, wherein the three external ports are respectively a lightning current injection port and two end interfaces of the secondary cable. When lightning current is injected into the ground network, a potential difference is generated between the grounding points at two ends of the cable, and at this time, if the shielding layer or the armor layer of the cable is grounded at the same time, a loop is formed, and current is generated. This current, through coupling between the layers of the cable, creates a voltage across the cable core, sheath, and ground.
The current secondary cable transient interference simulation calculation still has certain defects: a part of the method does not consider the coupling of the ground network, and directly adds an interference signal to the secondary cable model; a portion of which takes into account the coupling of the counterpoise but does not take into account the transient impedance of the counterpoise; the other method utilizes the transmission line theory to model the whole transformer substation, but the input signal is transient current injected into the ground network, and the input signal is an interference signal of the secondary cable, and the modeling is complete, but the calculation model is excessively complex, so that the calculation time is longer, and the error is larger.
The invention considers the grounding connection between the secondary system and the primary grounding network, the proper amount of matching method considers the high-frequency transient impedance of the grounding network, the whole model is simple, the input signal is the ground potential time domain waveform of the two end connection points of the cable, the output is the time domain waveform of the interference signal, and the calculation speed is high.
Disclosure of Invention
The invention aims to solve the problems, and designs a calculation method for calculating transient interference signals at two ends of a secondary cable connected with equipment in a transformer substation. The specific design scheme is as follows:
a calculation method for calculating transient interference signals at two ends of a secondary cable connected with equipment in a transformer substation comprises the following calculation steps:
Step one, recording time domain waveforms u a and u b of ground potential rise at two ends of a secondary cable at a point A, B;
Performing FFT (fast Fourier transform) on the u a and u b waveforms to find out a main frequency point f 1,f2,…,fn;
Step three, calculating the injection amplitude value of the ground net A point as I Afk and the frequency as f k, (k=1, 2, …, n), and marking the ground potential rise of the ground net at the A point as U Afk, (k=1, 2, …, n);
Step four, calculating the equivalent impedance Z Afk of the ground network A point and the ground point at the frequency f k, (k=1, 2, …, n);
Step five, finding out equivalent impedance frequency response F A(s) of the point A and the ground point at infinity by adopting a vector matching method through Z Afk, and further obtaining equivalent impedance Z A of the point A and the ground point at infinity;
Step six, obtaining Z B by the same method;
Establishing a coaxial pi-type equivalent circuit of the secondary cable, and respectively connecting an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b at two ends of A, B;
and step eight, calculating interference signals between core wires at two ends of the cable and a grounding point by a solving circuit.
In the fourth step, the equivalent impedance Z Afk, (k=1, 2, …, n) is calculated as:
In the fifth step, the step of the vector matching method is as follows:
Step five-1 linearly interpolates n data sets (f k,ZAfk) over the interval 0.001, max (f k), where max (f k) is the maximum value in f k, (k=1, 2, …, n), resulting in a new set of data sets (fcal m,ZcalAfm) to be matched in the right amount (m=1, 2, …, g),
Step five-2, the frequency response obtained by proper matching is as follows:
Step five-3, R ch,Cch,Rl,Ll is calculated by the following formula (h=1, 2, …, N)
Rl＝d,Ll＝e，
In the seventh step, the grounding point of the whole circuit is an infinity zero potential point, and the grounding point is the same as the infinity grounding point in the fourth step and the fifth step.
The method for calculating the transient interference signals at the two ends of the secondary cable connected with the equipment in the transformer substation by the technical scheme has the beneficial effects that:
the calculation method can accurately calculate the interference time domain signal in the cable when the secondary cable is interfered by external transient state, and the calculation method of the transmission line theory is adopted, so that the speed is high.
Drawings
FIG. 1 is an equivalent network diagram obtained by a vector matching algorithm;
FIG. 2 is a schematic diagram of a secondary cable connection arrangement within a substation;
FIG. 3 is a schematic diagram of a ground network connected to a secondary cable;
FIG. 4 is a modeling diagram of a counterpoise in an example of an embodiment;
FIG. 5 is a diagram of cable parameters used in an example of an embodiment;
FIG. 6 is a time domain waveform of ground potential rise across a cable;
fig. 7 is a circuit simulation diagram in embodiment 1;
FIG. 8 is a graph showing comparison of the results in example 1;
FIG. 9 is a graph showing comparison of the results in example 2;
FIG. 10 is a graph of quality matching parameter results in example 3;
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
A calculation method for calculating transient interference signals at two ends of a secondary cable connected with equipment in a transformer substation comprises the following calculation steps:
Step one, recording time domain waveforms u a and u b of ground potential rise at two ends of a secondary cable at a point A, B;
Performing FFT (fast Fourier transform) on the u a and u b waveforms to find out a main frequency point f 1,f2,…,fn;
Step three, calculating the injection amplitude value of the ground net A point as I Afk and the frequency as f k, (k=1, 2, …, n), and marking the ground potential rise of the ground net at the A point as U Afk, (k=1, 2, …, n);
Step four, calculating the equivalent impedance Z Afk of the ground network A point and the ground point at the frequency f k, (k=1, 2, …, n);
Step five, finding out equivalent impedance frequency response F A(s) of the point A and the ground point at infinity by adopting a vector matching method through Z Afk, and further obtaining equivalent impedance Z A of the point A and the ground point at infinity;
Step six, obtaining Z B by the same method;
Establishing a coaxial pi-type equivalent circuit of the secondary cable, and respectively connecting an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b at two ends of A, B;
and step eight, calculating interference signals between core wires at two ends of the cable and a grounding point by a solving circuit.
In the fourth step, the equivalent impedance Z Afk, (k=1, 2, …, n) is calculated as:
In the fifth step, the step of the vector matching method is as follows:
Step five-1 linearly interpolates n data sets (f k,ZAfk) over the interval 0.001, max (f k), where max (f k) is the maximum value in f k, (k=1, 2, …, n), resulting in a new set of data sets (fcal m,ZcalAfm) to be matched in the right amount (m=1, 2, …, g),
Step five-2, the frequency response obtained by proper matching is as follows:
Step five-3, R ch,Cch,Rl,Ll is calculated by the following formula (h=1, 2, …, N)
Rl＝d,Ll＝e，
In the seventh step, the grounding point of the whole circuit is an infinity zero potential point, and the grounding point is the same as the infinity grounding point in the fourth step and the fifth step.
The method for calculating the transient interference signals at the two ends of the secondary cable connected with the equipment in the transformer substation by the technical scheme has the beneficial effects that:
the calculation method can accurately calculate the interference time domain signal in the cable when the secondary cable is interfered by external transient state, and the calculation method of the transmission line theory is adopted, so that the speed is high.
Modeling is performed on a 110kV transformer substation grounding grid, as shown in fig. 4, the size of a grounding grid test field is 70.5mX52.5 m, the black part on the right side of the drawing is a master control room grounding grid, the rest of the drawing is a grid of 50 mX52.5 m, and all sizes are marked in the drawing. The point A is the end point position of the cable duct in the area A, the point B is the end point position of the cable duct in the area B, namely, two points A, B are grounding points at two ends of the cable. Points A1 and B1 are lightning current injection points. The parameters of the cable are shown in fig. 5.
Example 1
Step one, when 8/20us transient lightning current with peak value of 2.88kA is injected into the center of the local network (point B1), recording time domain waveforms u a and u b of ground potential rise at two ends of two termination points A, B of the secondary cable, see FIG. 6
Performing FFT (fast Fourier transform) on the u a and u b waveforms to find out a main frequency point f 1,f2,…,f23;
Step three, calculating the injection amplitude value of the ground net A point as I Afk and the frequency as f k, (k=1, 2, …, 23), and marking the ground potential rise of the ground net at the A point as U Afk, (k=1, 2, …, 23);
Step four, calculating the equivalent impedance Z Afk of the ground network a point and the ground point at the frequency f k, (k=1, 2, …, 23);
Step five, finding out equivalent impedance frequency response F A(s) of the point A and the ground point at infinity by adopting a vector matching method through Z Afk, and further obtaining equivalent impedance Z A of the point A and the ground point at infinity;
Step six, obtaining Z B by the same method;
and seventhly, establishing a coaxial pi-type equivalent circuit of the secondary cable, wherein the grounding mode of the 1# cable is that the single end of the shielding layer is grounded, the two ends of the armoring layer are grounded, the grounding mode of the 2# cable is that the two ends of the shielding layer are grounded, the single end of the armoring layer is grounded, the two ends of A, B are respectively connected with an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b, and the circuit diagram is shown in fig. 7.
And step eight, calculating interference signals between core wires at two ends of the cable and a grounding point by a solving circuit. FIG. 8 is a graph comparing calculated values with measured values.
Example 2
The injection current peaks were varied to 1.24kA, 2.08kA,2.88Ka, respectively. Recording a point voltage u a1-ua3 and a point voltage u b1-ub3;
Only a coaxial pi-type equivalent circuit of the secondary cable is established, the grounding mode is that a single end of a shielding layer is grounded, two ends of an armor layer are grounded, two ends of A, B are respectively connected with an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b,
The interference signal between the core wires at the two ends of the cable and the grounding point is calculated, as shown in fig. 9. In the figure, the solid line represents the actual measurement value, and the broken line represents the calculation value.
Example 3
Step five-1 linearly interpolates 23 data sets (f k,ZAfk) over the interval 0.001, max (f k), where max (f k) is the maximum 170666.7Hz in f k, (k=1, 2, …, 23), resulting in a new set of data sets (fcal m,ZcalAfm) to be matched in the right amount (m=1, 2, …, 340),
Step five-2, the frequency response obtained by proper matching is as follows:
step five-3, R ch,Cch,Rl,Ll, (h=1, 2, …, N) was calculated by the following formula, see fig. 10 for specific parameters.
Rl＝d,Ll＝e，

Claims (4)

1. A calculation method for calculating transient interference signals at two ends of a secondary cable connected with equipment in a transformer substation is characterized by comprising the following calculation steps:
Step one, recording time domain waveforms u a and u b of ground potential rise at two ends of a secondary cable at a point A, B;
Performing FFT (fast Fourier transform) on the u a and u b waveforms to find out a main frequency point f 1,f2,…,fn;
Step three, calculating the injection amplitude value of the ground net A point as I Afk and the frequency as f k, (k=1, 2, …, n), and marking the ground potential rise of the ground net at the A point as U Afk, (k=1, 2, …, n);
Step four, calculating the equivalent impedance Z Afk of the ground network A point and the ground point at the frequency f k, (k=1, 2, …, n);
Step five, finding out equivalent impedance frequency response F A(s) of the point A and the ground point at infinity by adopting a vector matching method through Z Afk, and further obtaining equivalent impedance Z A of the point A and the ground point at infinity;
Step six, obtaining Z B by the same method;
Establishing a coaxial pi-type equivalent circuit of the secondary cable, and respectively connecting an equivalent impedance Z A, an A ground potential rising waveform u a, an equivalent impedance Z B and a ground potential rising waveform u b at two ends of A, B;
step eight, the solving circuit calculates interference signals between core wires at two ends of the cable and the grounding point,
In the fifth step, the step of the vector matching method is as follows:
Step five-1 linearly interpolates n data sets (f k,ZAfk) over the interval 0.001, max (f k), where max (f k) is the maximum value in f k, (k=1, 2, …, n), resulting in a new set of data sets (fcal m,ZcalAfm) to be matched in the right amount (m=1, 2, …, g),
Step five-2, the frequency response obtained by proper matching is as follows:
Step five-3, R ch,Cch,Rl,Ll is calculated by the following formula (h=1, 2, …, N)
Step five-4, the impedance network is shown in figure 1.
2. The method for calculating transient interference signals at two ends of a secondary cable connected to equipment in a transformer substation according to claim 1, wherein in the fourth step, the equivalent impedance Z Afk, (k=1, 2, …, n) is calculated by the following formula:
ZAfk＝UAfk/IAfk。
3. The method of claim 1, wherein a h,ch, d, e are real numbers and N is a frequency response equation medium.
4. The method for calculating transient interference signals at two ends of a secondary cable connected to equipment in a transformer substation according to claim 1, wherein in the seventh step, the grounding point of the whole circuit is an infinity zero potential point, which is the same as the grounding point of infinity in the fourth and fifth steps.
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