CN114362198A - High-impedance design method for extra-high voltage limited power frequency overvoltage and secondary arc current - Google Patents
High-impedance design method for extra-high voltage limited power frequency overvoltage and secondary arc current Download PDFInfo
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Abstract
The invention belongs to the technical field of power transmission and transformation engineering, and particularly relates to a high-impedance design method for limiting power frequency overvoltage and secondary arc current by an extra-high voltage. The invention comprises the following steps: step 1, establishing an electromagnetic transient simulation-based model of a system; step 2, determining the power frequency overvoltage at the tail end of the no-load long line of the ultra-high voltage transmission line; step 3, solving the power frequency voltage rise caused by load shedding of the line; step 4, determining power frequency overvoltage caused by single-phase short circuit of the extra-high voltage line; step 5, determining a high impedance value of the extra-high voltage line; and 6, determining the neutral point reactance of the high-voltage shunt reactor. The method of the invention utilizes the high-voltage shunt reactor to be additionally provided with the neutral point reactance, reduces the secondary arc current and the recovery voltage, solves the problems of temporary power frequency overvoltage and secondary arc current exceeding of a remote extra-high voltage alternating current power grid, and improves the success rate of single-phase reclosing of an extra-high voltage line. The calculation method is simple and practical, and provides technical support for the power frequency overvoltage in the extra-high voltage power grid and the influence of the power frequency overvoltage on the power grid equipment.
Description
Technical Field
The invention belongs to the technical field of power transmission and transformation engineering, and particularly relates to a high-impedance design method for limiting power frequency overvoltage and secondary arc current of an extra-high voltage power transmission line, in particular to a method for limiting power frequency overvoltage and secondary arc current of an alternating-current extra-high voltage power transmission line by using an extra-high voltage shunt reactor and a small reactance of a neutral point of the extra-high voltage shunt reactor.
Background
The frequency of the power frequency overvoltage is power frequency or close to the power frequency. The reasons for the power frequency overvoltage include capacitance effect of a no-load long line, normal phase voltage rise caused by asymmetric earth fault, load sudden change and the like, and are related to system structure, capacity, parameters and operation mode.
The extra-high voltage power grid has the power transmission capacity with longer distance, larger capacity and lower loss. But the charging reactive power of the extra-high voltage transmission line can exceed 5.3MVA per 1km line length, and is about 4-6 times of that of a 500kV line under the same line length. In an extra-high voltage power system, the magnitude of power frequency overvoltage directly influences the magnitude of operation overvoltage and may endanger the safe operation of an equipment system. Meanwhile, the power frequency overvoltage is also an important basis for determining the rated voltage of the lightning arrester, so that the overvoltage level of the system is influenced.
Due to the requirement that the power grids in North, China and east China are interconnected by extra-high voltage power grids and supplied by the extra-high voltage power grids in the south and north directions and the like for long-distance power transmission, a considerable part of extra-high voltage lines are relatively long. The charging power of a single-section line is very high, and a high-voltage shunt reactor (high reactance for short) must be used for compensation. After the extra-high voltage line is connected with the parallel reactor, the inductive reactive power of the reactor partially compensates the capacitive reactive power of the line, which is equivalent to reducing the length of the line and reducing the power frequency voltage rise value.
According to the regulation of national standard GB/Z24842-2009 overvoltage and insulation coordination of 1000kV extra-high voltage alternating current transmission and transformation engineering, the power frequency overvoltage of a 1000kV system is generally limited below 1.3pu, and the line side can be allowed to be below 1.4pu for a short time (the duration time is not more than 0.5s) under the conditions of single-phase grounding and three-phase load shedding.
The degree of compensation for high impedance cannot be too high to make reactive compensation and voltage control difficult for high load operation. According to the experience of China in running extra-high voltage projects, the high-impedance compensation degree is controlled to be 80% -90% in the initial stage of extra-high voltage power grid construction. In areas with stronger power grids or when the ultra-high voltage transmission line is shorter, the compensation degree can be properly reduced.
The secondary arc current does not belong to overvoltage, but is an electromagnetic transient phenomenon which needs to be taken into account and is generated in the single-phase reclosing process. The secondary arc of the extra-high voltage line is large in secondary current, high in recovery voltage and difficult to extinguish, and the current-free intermittent time and success rate of single-phase reclosing can be influenced, so that the measures of limiting the secondary current and accelerating the extinguishing of the secondary arc need to be researched.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a high-impedance design method for limiting the power frequency overvoltage and the secondary arc current by the extra-high voltage. The invention aims to solve the problems of temporary power frequency overvoltage and excessive secondary current of a remote extra-high voltage alternating current power grid.
The technical scheme adopted by the invention for realizing the purpose is as follows:
a high-impedance design method for limiting power frequency overvoltage and secondary arc current by extra-high voltage comprises the following steps:
step 3, solving the power frequency voltage rise caused by load shedding of the line;
step 4, determining power frequency overvoltage caused by single-phase short circuit of the extra-high voltage line;
step 5, determining a high impedance value of the extra-high voltage line;
and 6, determining the neutral point reactance of the high-voltage shunt reactor.
Furthermore, the electromagnetic transient simulation-based model of the system established in the step 1 is based on the collected system parameters and an electromagnetic transient modeling method, and is used for establishing a distributed model of the extra-high voltage transmission and transformation line, a detailed model of a main transformer and extra-high voltage networking engineering and an equivalent model of a regional power grid power supply and load.
Furthermore, the collected system parameters comprise main parameters of a local synchronous motor and main parameters of a power transmission line; the main parameters of the local synchronous motor comprise: synchronous motor, main transformer, series compensation, low-voltage reactor compensation, low-voltage capacitor compensation, and section load system parameters; the main parameters of the power transmission line comprise: the parameters of the transmission line such as the ground capacitance, the interphase capacitance, the inductance per unit length and the susceptance (capacitance) line.
Furthermore, the determination of the power frequency overvoltage at the tail end of the no-load long line of the ultra-high voltage transmission line in the step 2 is carried out by comparing the capacitance-rise effect of the line with the capacitance-rise effect of the no-load line after the power reactance is taken into consideration, so that the power reactance is equivalent to the increase of the line length, the power frequency overvoltage is further increased, and the resonance point is advanced;
when the no-load lossless line length is set as l, the current is measured at the end
Calculating the head voltage
Terminal voltage
And the voltage at a certain point in the line
Is represented by formula (1):
in the formula:
is the wave impedance;
is a phase constant; x is the distance from the receiving end, L0Reactance per unit length; c0Capacitance per unit length, l is line length;
defining the voltage transfer coefficient of the tail end to the head end of the line as formula (2):
in the formula: head end voltage
Terminal voltage
The voltage transfer coefficient of the end to the head of the line is K12；
From the equation (2), the voltage on the line is from the head end
Gradually rises and is distributed along the line according to a cosine curve, and the voltage at the tail end of the line
Reaches a maximum, e.g. when β l is 90 °, which corresponds to a series resonance, K, as seen from the head end of the line12→∞，
At the moment, the line length is 0.25 times of the wavelength of the voltage distribution on the power frequency no-load long line; meanwhile, the capacitance current of the no-load circuit can also form voltage rise on the reactance of the power supply, so that the voltage at the head end of the circuit is higher than the electromotive force of the power supply, and the power frequency overvoltage is further increased;
considering the power supply reactance, the relationship between the line end voltage and the power supply electromotive force can be obtained as follows:
in the above formula, the first and second carbon atoms are,
is the generator electromotive force, j is an imaginary number sign, ZCIs the wave impedance, XSIs a reactance of a power supply,
is the current at the head end of the line;
order:
by modifying the formula (3), the electrodynamic transfer function K of the line end voltage and the power supply can be obtained02The following formula:
power supply reactance XsInfluence of (2) through angle
Is shown as
Time K02→∞，
At this time
The length of the circuit is increased by the power supply reactance, the resonance point is advanced, and the power frequency voltage multiple is increased by the power supply reactance;
the transfer function of the line end voltage to the power supply motor type is expressed as follows:
in formula (5):
is the wave impedance;
is a phase constant.
Furthermore, the step 3 of solving the power frequency voltage rise caused by load shedding of the line is that when the system is in normal operation, the voltage at the head end of the line is
Head end current of line
Power factor of
The transmitted active power
Reactive power
If power supply reactance XsThen, the electric power generation mode E is:
before load shedding, if considerable active and inductive reactive power is transmitted on the line, the electric E of power supply generation is inevitably higher than the voltage value U of the head end of the line1，E＞U1；
After load shedding, the power supply transient electromotive force E 'is considered to be unchanged according to the principle of unchanged magnetic linkage'dE is approximately distributed, and E is the electromotive force of the generator; the circuit breaker at the tail end of the line is opened, so that the running mode of the power supply with no-load long line is formed; after the tail end of the line is subjected to load shedding, the voltage at the head end of the line is higher than the electromotive force of a power supply, and the overvoltage at the tail end of the long line is more serious.
Furthermore, the step 4 of determining the power frequency overvoltage caused by the single-phase short circuit of the extra-high voltage line includes:
setting the A phase in the system to generate single-phase earth fault, its boundary condition
Then there are:
in the formula
The positive sequence, negative sequence and zero sequence components of the voltage at the fault point;
the positive sequence, negative sequence and zero sequence components of the current at the fault are obtained;
according to the set boundary conditions, a composite sequence network is formed when the single phase is grounded, and the sequence network obtains a sequence current and a healthy phase voltage as follows:
in the above formula, a ═ ej120°；Z1、Z2、Z0The positive sequence impedance, the negative sequence impedance and the zero sequence impedance of the network are seen from a fault point,
is the phase voltage of the B phase of the power grid,
is the C-phase voltage of the power grid,
is the phase A electromotive force of a power supply; with K(1)The healthy phase voltage is increased after the single-phase earth fault is shown, and the formula (5) can be simplified as follows:
in the above formula, the first and second carbon atoms are,
the voltage is the healthy and complete phase of the power grid;
wherein:
in the above formula, Z1、Z2、Z0For the positive sequence, negative sequence and zero sequence impedance of the network seen from the fault point, the Z is generally available for the ultra-high voltage transmission system with larger system transmission capacity1≈Z2Neglecting the impedance resistance components in each sequence, simplifying as:
in the above formula：X0Is a zero sequence reactance, X1Is a positive sequence reactance;
according to the formula, the power frequency overvoltage and the X seen from the fault point0/X1(the ratio of zero sequence to positive sequence reactance) has a large relationship; x0/X1The increase of the voltage will lead the load shedding overvoltage of the single-phase earth fault to have an increasing trend;
x taking of typical ultra-high voltage transmission line0/X12.6; then calculate K(1)And U:
U＝1.21E
in the above formula: e is the power supply potential and U is the voltage.
Furthermore, the step 5 of determining the high impedance value of the extra-high voltage line is to comprehensively determine the high impedance value of the extra-high voltage line according to the power frequency overvoltage under three conditions and by combining reactive compensation of typical engineering;
the lossless long line equation of the line voltage and the current of the extra-high voltage transmission line is as follows:
in the formula:
is the wave impedance;
is a phase constant; x is the distance from the receiving end, x0Reactance per unit length; b0For susceptance per unit length, the voltage at a point in the line is
Current is
A receiving terminal voltage of
Current is
The imaginary number symbol is j, sin beta x is a sine function, and cos beta x is a cosine function;
at the receiving end voltage
For reference, if the receiving end transmission power is:
Sr＝Pr+jQr (13)
in the above formula: srFor receiving end transmission power, PrIs active power, QrIs reactive power, j is an imaginary number symbol;
the expression of equation (12) can be expressed as:
reactive charge generated by a line of length l, using susceptance b per unit length0Represents the charging power integral form of:
in the above formula: qCFor the purpose of charging the power for a long line,
is the conjugate of the voltage at point x, b0Is unit susceptance, ZCIs the wave impedance, x is the line point length,
is the voltage at a certain point in the line;
long line charging power Q in combination with the equation of voltage in equation (13)CComprises the following steps:
in the above formula, PrIs active power, QrIs reactive power;
as seen from equation (16), the line charging reactive power is related to the line transmission power, the line length and the unit reactance and susceptance; charging power Q of high impedance design according to long line in power transmission processc，QcAs the active power increases slightly, but the capacitance of the line plays a major role; meanwhile, the high-impedance design meets the highest voltage under the condition of restraining the light load, so that the light load line parameters are used as the control conditions of the high-impedance value;
equation (16) reduces to:
in the above formula: qcCharging reactive power for long lines, UeRating the phase voltage for the system;
the high impedance design is determined by the following formula:
in the above formula, QLFor high impedance compensation capacity, QcAnd charging reactive power for the long line, wherein B is a compensation coefficient.
Further, step 6, determining the neutral point reactance of the high-voltage shunt reactor, wherein when a single-phase earth fault occurs to the line, after the circuit breakers at two ends of a fault phase trip, other two phases still operate and the working voltage is kept; due to interphase capacitance C12And certain current still flows through the fault point under the action of the interphase mutual inductance M
Namely a secondary current, and the electric arc is called as a secondary arc;
the secondary current comprises: a capacitive component and an inductive component;
the capacitance component refers to the voltage of the normal phase passing through the interphase capacitance C12A current provided to a fault point; meanwhile, the load current on the normal phase induces electromotive force on the fault phase through the interphase mutual inductance, the electromotive force provides current for a fault point through a loop formed by the phase-to-ground capacitor and the high reactance, and the capacitor component plays a main role under the condition of no compensation;
the inductance component is that the load current on the normal phase induces electromotive force on the fault phase through the interphase mutual inductance, and the electromotive force provides current for the fault point through a loop formed by the phase-to-ground capacitance and the high reactance;
the primary current and recovery voltage should be limited to small values, when the primary current is large and the recovery voltage is high, in order to limit the primary current and its recovery voltage, the method of adding the neutral point reactance of high-voltage shunt reactor is used to reduce the primary current and recovery voltage, including: two dimensions of accelerating the extinction of the latent supply arc and restraining the resonance overvoltage are required, and the value of the reactance of the neutral point of the high-voltage shunt reactor is calculated through the determined line capacitance reactance and high reactance value.
Furthermore, a small reactance is selected according to the requirement of the accelerated latent arc extinguishing;
the secondary arc current is less than (15-20) A;
from the perspective of compensating the inter-phase capacitance, the small reactance value can be approximated by equation (19):
in the formula: x0Reactance value of neutral point small reactance, XLPositive sequence reactance value, X, of a shunt reactor12The interphase capacitive reactance value of the line;
selecting a small reactance according to the requirement of inhibiting the resonance overvoltage:
in order to inhibit the power frequency transmission resonance overvoltage, the neutral point small reactance is calculated according to the formula (20):
in the formula: xL0Zero sequence reactance value of a parallel reactor, for a single-phase reactor, XL0＝XL(ii) a For three-phase three-column reactor, XL0＝XL/2；
The high-impedance compensation of the extra-high voltage line adopts a single-phase reactor, and the formulas (19) and (20) are the same;
calculating a parallel high-impedance value by analyzing factors such as reactive compensation of the line and the like under the conditions of long-line capacity-rise effect, line load shedding and single-phase earth fault of the line; and determining a final neutral point small reactance value according to the determined high reactance value and the line parameters.
A computer storage medium having a computer program stored thereon, the computer program when executed by a processor implementing the steps of the method for designing a high impedance for very high voltage limited power frequency overvoltage and secondary current.
The invention has the following beneficial effects and advantages:
the invention utilizes the method that the high-voltage shunt reactor is additionally provided with the neutral point reactance (also called small reactance), reduces the secondary arc current and recovers the voltage, thereby solving the problems of temporary power frequency overvoltage and secondary arc current exceeding of a remote extra-high voltage alternating current power grid in the prior art and improving the success rate of single-phase reclosing of an extra-high voltage line.
The calculation method is simple and practical, can be more easily used in engineering practice, and provides basic technical support for the power frequency overvoltage in the extra-high voltage power grid and the influence of the power frequency overvoltage on the power grid equipment.
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The above and/or additional aspects and advantages of the present invention will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:
FIG. 1 is a graph of the capacitive effect of an extra-high voltage unloaded line of the present invention;
FIG. 2 is a composite grid of the present invention with a single phase grounded;
FIG. 3 is a schematic diagram of the secondary current of the present invention;
fig. 4 is an equivalent circuit diagram of the extra-high voltage long line of the invention.
In the figure:
Detailed Description
In order that the above objects, features and advantages of the present invention can be more clearly understood, a more particular description of the invention will be rendered by reference to the appended drawings. It should be noted that the embodiments and features of the embodiments of the present application may be combined with each other without conflict.
In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention, however, the present invention may be practiced in other ways than those specifically described herein, and therefore the scope of the present invention is not limited by the specific embodiments disclosed below.
The solution of some embodiments of the invention is described below with reference to fig. 1-4.
Example 1
The invention also provides an embodiment, which is a high-impedance design method for limiting the power frequency overvoltage and the secondary arc current in the extra-high voltage, as shown in fig. 1, fig. 1 is a capacitance effect diagram of the extra-high voltage no-load circuit.
The invention relates to a high-impedance design method for limiting power frequency overvoltage and secondary arc current by extra-high voltage, which specifically comprises the following steps:
And establishing a distributed model of the extra-high voltage transmission and transformation line, a detailed model of a main transformer and extra-high voltage networking project and an equivalent model of a regional power grid power supply and load based on an electromagnetic transient modeling method according to the collected system parameters.
The collected system parameters comprise main parameters of a local synchronous motor and main parameters of a power transmission line.
Wherein, local synchronous machine main parameters include: the system comprises system parameters such as a synchronous motor, a main transformer, series compensation, low-voltage reactor compensation, low-voltage capacitor compensation, section load and the like.
The main parameters of the transmission line include: the main line parameters of the transmission line such as ground capacitance, interphase capacitance, unit length inductance, susceptance (capacitance) and the like.
the capacity rising effect of the circuit is simply considered and compared with the capacity rising effect of the no-load circuit after the power supply reactance is considered, the power supply reactance is obtained, the length of the circuit is increased, the power frequency overvoltage is further increased, and the resonance point is advanced.
For long power transmission lines, the entrance impedance of the line is capacitive when the tail is unloaded. When the influence of impedance (inductance) in a power supply is taken into consideration, the capacitance effect not only enables the voltage at the tail end of the circuit to be higher than that at the head end, but also enables the voltage at the head end and the tail end of the circuit to be higher than the electromotive force of the power supply, which is one of the reasons for generating power frequency overvoltage of a no-load long circuit.
The method is characterized in that the end power frequency overvoltage of the extra-high voltage no-load long line is determined, the no-load lossless line with the length of l is shown in figure 1, and figure 1 is a capacitance effect diagram of the extra-high voltage no-load line. When the terminal current flows
The head voltage can be calculated
Terminal voltage
And the voltage at a certain point in the line
Is represented by formula (1):
in the formula:
is wave impedance；
Is a phase constant; x is the distance from the receiving end, L0Reactance per unit length; c0Capacitance per unit length, l is line length.
Here, the voltage transfer coefficient from the end of the line to the head end is defined as formula (2):
in the formula: voltage at head end of line
Terminal voltage
The voltage transfer coefficient of the end to the head of the line is K12。
As can be seen from equation (2), the voltage on the line is from the head end
Gradually rises and is distributed along the line according to a cosine curve, and the voltage at the tail end of the line
Reaches a maximum, e.g. when β l is 90 °, which corresponds to a series resonance, K, as seen from the head end of the line12→∞，
At the moment, the line length is 0.25 times of the wavelength of the voltage distribution on the power frequency no-load long line. Meanwhile, the capacitance current of the no-load circuit can also form voltage rise on the reactance of the power supply, so that the voltage at the head end of the circuit is higher than the electromotive force of the power supply, and the power frequency overvoltage is further increased.
Considering the power supply reactance, the relationship between the line termination voltage and the power supply electromotive force can be obtained as follows:
in the above formula, the first and second carbon atoms are,
is the generator potential, j is an imaginary symbol, ZCIs the wave impedance, XSFor the reactance of the power supply, the head-end current of the line is
Order:
by modifying the formula (3), the electrodynamic transfer function K of the line end voltage and the power supply can be obtained02The following formula:
power supply reactance XsInfluence of (2) through angle
Is shown as
Time K02→∞，
At this time
The power supply reactance is equivalent to the increase of the line length, the resonance point is advanced, and the power supply reactance can also increase the power frequency voltage multiple.
The transfer function of the line end voltage to the power supply dynamics can be expressed as:
in formula (5):
is the wave impedance;
is a phase constant.
For analysis, the calculation is performed by a case, and a certain power plant machine takes 2 1000MW units and is connected to a system bus through 2 27/1000kV step-up transformers with 1120MVA capacity. Here, a 2-loop extra-high voltage line access system is used.
Taking 100MVA as a base value and X 'of each unit as main calculation parameters of several devices'dThe value is 0.024, and each 27/1000kV booster transformer is XT1Is 0.014, and the wave impedance Z of the single-loop extra-high voltage linec0.025 and beta 0.001. The calculation results are shown in table 1.
TABLE 1 calculation of overvoltage over long no-load line
| Distance (km) | 100 | 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 |
| Multiple of overvoltage | 1.06 | 1.10 | 1.15 | 1.20 | 1.27 | 1.34 | 1.42 | 1.52 | 1.64 |
And 3, solving the power frequency voltage rise caused by load shedding of the circuit.
When the power transmission line runs under heavy load, a circuit breaker at the tail end of the line suddenly trips to throw off the load due to some reason, so that the power frequency voltage is increased, and the load throwing effect is generally called.
When the system is in normal operation, the voltage at the head end of the line is set to be
Head end current of line
Power factor of
The transmitted active power
Reactive power
If power supply reactance XsThen, the electric power generation mode E is:
in the above formula, the first and second carbon atoms are,
is the line head end voltage.
Before load shedding, if considerable active and inductive reactive power is transmitted on the line, the generating electromotive force E of the power supply is inevitably higher than the voltage value U of the head end of the line1，E＞U1. The overvoltage multiple of the line can be obtained by parameter calculation in the case, and the calculation result is shown in table 2.
TABLE 2 overvoltage multiple after load shedding of the line
| Distance (km) | 100 | 150 | 200 | 300 | 350 | 400 | 450 | 500 |
| Multiple of overvoltage | 1.01 | 1.01 | 1.02 | 1.05 | 1.06 | 1.09 | 1.11 | 1.14 |
After load shedding, the power supply transient electromotive force E 'can be simply considered as unchanged according to the principle of unchanged magnetic linkage'dE is electric for generating electricity. And the circuit breaker at the tail end of the line is opened, so that the running mode of the power supply with no-load long line is formed. After the tail end is unloaded, the influence of the capacitance effect of the long line on the power frequency voltage rise is calculated, as shown in the formula (6), the voltage at the head end of the line is higher than the electromotive force of the power supply, and the overvoltage at the tail end of the long line is more serious. The overvoltage multiple of the line can be calculated according to the parameters, and the calculation result is shown in table 2.
And 4, determining power frequency overvoltage caused by single-phase short circuit of the extra-high voltage line.
Asymmetric short circuit is the most common fault mode of a power transmission line, and the zero sequence component of short circuit current can increase power frequency voltage of a healthy phase, which is often called as asymmetric effect. Asymmetric short circuit faults in the system are most common single-phase earth faults, and when one end of a line trips to dump load, power frequency overvoltage can be further increased due to the fact that the faults still exist. And (4) analyzing and extracting the ratio of the zero sequence reactance to the positive sequence reactance of the typical extra-high voltage project through calculation. And obtaining the single-phase short-circuit power frequency overvoltage of the typical ultra-high voltage transmission line.
Suppose that the A phase of the system has single-phase earth fault, and the boundary condition is
Then there are:
in the formula
The positive sequence, negative sequence and zero sequence components of the voltage at the fault point;
the positive sequence, negative sequence and zero sequence components of the current at the fault are shown.
Based on the assumed boundary conditions, a composite sequence network is formed for single-phase grounding, from which a sequence current and a robust phase voltage can be derived, as shown in fig. 2.
Wherein a ═ ej120°；Z1、Z2、Z0The positive sequence impedance, the negative sequence impedance and the zero sequence impedance of the network are seen from a fault point,
is the phase voltage of the B phase of the power grid,
is the C-phase voltage of the power grid,
is the phase A electromotive force of the power supply.
With K(1)The healthy phase voltage is increased after the single-phase earth fault is shown, and the formula (5) can be simplified as follows:
in the above formula, the first and second carbon atoms are,
and the voltage is the healthy and complete phase voltage of the power grid.
Wherein:
in the above formula, Z1、Z2、Z0For the positive sequence, negative sequence and zero sequence impedance of the network seen from the fault point, the Z is generally available for the ultra-high voltage transmission system with larger system transmission capacity1≈Z2Neglecting the resistance components of each sequence, the method can be simplified as follows:
in the above formula: x0Is a zero sequence reactance, X1Is a positive sequence reactance.
According to the formula, the power frequency overvoltage and X seen from the fault point0/X1(ratio of zero sequence to positive sequence reactance) has a large relationship. X0/X1The increase will tend to increase the single-phase earth fault load shedding overvoltage.
X taking of typical ultra-high voltage transmission line0/X12.6. Then K can be found(1)And U:
U＝1.21E
in the above formula: e is the power supply potential and U is the voltage.
The line overvoltage multiple of 1.21 in a typical ultrahigh voltage single-phase fault can be obtained through calculation.
And 5, determining a high impedance value of the extra-high voltage line.
Specifically, according to power frequency overvoltage under three conditions, reactive compensation of typical engineering is combined, and a high-impedance value of the extra-high voltage line is comprehensively determined.
As shown in fig. 4, fig. 4 is an equivalent circuit diagram of the extra-high voltage long line of the present invention. The lossless long line equation of the line voltage and the current of the extra-high voltage transmission line is as follows:
in the formula:
is the wave impedance;
is a phase constant; x is the distance from the receiving end, x0Reactance per unit length; b0For susceptance per unit length, the voltage at a point in the line is
Current is
A receiving terminal voltage of
Current is as
The ordinal number symbol is j, sin β x is a sine function, and cos β x is a cosine function.
At the receiving end voltage
For reference, if the receiving end transmission power is:
Sr＝Pr+jQr (13)
in the above formula: srFor receiving end transmission power, PrIs active power, QrIs the reactive power, j is the imaginary symbol.
The expression of equation (12) can be expressed as:
as shown in FIG. 4, the reactive power generated by the charging of the line of length l is represented by the susceptance b per unit length0Represents the charging power integral form of:
in the above formula: qCFor the purpose of charging the power for a long line,
is the conjugate of the voltage at point x, b0Is unit susceptance, ZCIs the wave impedance, d is the sign of the differential operation, x is the length of a certain point of the line,
is the voltage at a certain point in the line.
Long line charging power Q in combination with the equation of voltage in equation (13)CComprises the following steps:
in the above formula, PrIs active power, QrIs reactive power.
As can be seen from equation (16), the line charging reactive power is related to the line transmission power, the line length and the unit reactance and susceptance. The design of high impedance mainly depends on the charging power Q of long line in the process of power transmissionc，QcWill increase slightly with active power, but the capacitance of the line plays a major role. Meanwhile, the high impedance design meets the highest voltage under the condition of restraining the light load, so the light load line parameters are used as the control conditions of the high impedance value. Equation (16) can be simplified as:
in the above formula: qcCharging reactive power for long lines, UeRating the phase voltage for the system;
get UeIs 1000kV, and the line capacitance C of unit length is taken0It was 0.01378 uF/km. The charging reactive power of the line is calculated as shown in table 3 below.
TABLE 3 result of settlement of reactive power generated by extra-high voltage line
| Line distance (km) | 100 | 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 |
| Charging power (Mvar) | 433 | 649 | 865 | 1082 | 1298 | 1514 | 1731 | 1947 | 2163 |
The high impedance design is determined by the following formula:
in the above formula, QLFor high impedance compensation capacity, QcAnd charging reactive power for the long line, wherein B is a compensation coefficient.
The compensation degree of the high impedance is controlled to be 80% -90% at the initial stage of the construction of the extra-high voltage power grid, wherein the value of B is 90%, and the calculated high impedance value is shown in the following table 4.
TABLE 4 calculation results of Compensation for high reactance
| Line distance (km) | 100 | 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 |
| Charging power (Mvar) | 195 | 292 | 389 | 487 | 584 | 681 | 779 | 876 | 974 |
And 6, determining the design of the neutral point reactance of the high-voltage shunt reactor.
Single-phase reclosure is generally adopted by ultrahigh voltage transmission lines in China so as to improve the stability of system operation, and the overvoltage of the single-phase reclosure is much lower than that of a three-phase reclosure. The ultra-high voltage line is also supposed to adopt a single-phase reclosing. In order to improve the success rate of single-phase reclosing, the problems of secondary arc current and recovery voltage in the line reclosing process must be considered.
As shown in FIG. 3, FIG. 3 is a schematic diagram of the secondary current of the present invention.
When a single-phase (A-phase) ground fault occurs in a line, after circuit breakers at two ends of a fault phase trip, other two phases (B-phase and C-phase) still operate and maintain working voltage. Due to interphase capacitance C12And certain current still flows through the fault point under the action of the interphase mutual inductance M
I.e. a secondary current, the arc of which is called a secondary arc.
The secondary current consists of two parts, a capacitive component and an inductive component (also called the transverse component and the longitudinal component), respectively.
Wherein, the capacitance component refers to the voltage of the normal phase passing through the interphase capacitance C12The current provided to the fault point. Meanwhile, the load current on the normal phase induces electromotive force on the fault phase through the interphase mutual inductance, the electromotive force provides current for a fault point through a loop formed by the phase-to-ground capacitor and the high reactance, and the capacitor component plays a main role under most uncompensated conditions.
The inductance component is that the load current on the normal phase induces electromotive force on the fault phase through the mutual inductance between phases, and the electromotive force provides current for the fault point through a loop formed by the relative ground capacitance and the high impedance.
In order to improve the success rate of the single-phase automatic reclosing, the secondary current and the recovery voltage are limited to small values. The secondary arc current and the recovery voltage of the line are related to the parameters of the power transmission line, the compensation condition of the line, the operating voltage and the transmission power flow at two ends of the line, and the influence of the network structures at two sides of the line on the secondary arc current and the recovery voltage is small. When the secondary arc current is larger and the recovery voltage is higher, measures are taken to accelerate the extinguishing of the secondary arc.
In order to limit the secondary current and the recovery voltage thereof, the secondary current and the recovery voltage are reduced by adding a neutral point reactance of a high-voltage shunt reactor. The neutral point reactance of the high-voltage parallel reactor is also called small reactance,
and calculating the value of the reactance of the neutral point of the high-voltage shunt reactor according to the determined line-to-line capacitance reactance and high reactance value according to two dimensions required by accelerated latent arc extinguishing and resonance overvoltage suppression.
(1) The small reactance is selected according to the requirement of accelerating the latent arc extinction.
The extra-high voltage line usually adopts single-phase reclosing as a measure for improving dynamic stability. However, when a single-phase earth fault occurs, because of capacitive coupling and mutual inductance coupling of the line, the secondary arc current of the earth point is difficult to self-extinguish, and the success rate of single-phase reclosing is reduced. After the neutral point of the shunt reactor is connected with a small reactance, the interphase capacitance can be compensated, the mutual inductance component can be partially compensated, and the amplitude of the secondary arc current is reduced. When a small resistance is added to the small reactance, the phase can also be changed, thereby accelerating the extinction of the latent arc. The optimum compensation of the small reactance is related to the system parameters, the degree of compensation of the shunt reactor, the installation location and the failure mode. In the engineering design, the system specializes to calculate the latent power supply current and the recovery voltage of various schemes and select the optimal reactance value. The secondary arc current should be less than (15-20) A.
The small reactance value can be approximated by equation (19) from the point of view of compensating the inter-phase capacitance alone:
in the formula: x0Reactance value of neutral point small reactance, XLPositive sequence reactance value, X, of a shunt reactor12And the interphase capacitive reactance value of the line.
(2) The small reactance is selected as required to suppress resonant overvoltage.
In order to inhibit the power frequency transmission resonance overvoltage, the small neutral point reactance can be calculated according to the formula (20):
in the formula: xL0Zero sequence reactance value of a parallel reactor, for a single-phase reactor, XL0＝XL(ii) a For three-phase three-column reactor, XL0＝XL/2。
The high impedance compensation of the extra-high voltage line generally adopts a single-phase reactor, then the formulas (19) and (20) are the same, C0Taking the zero sequence capacitance of the extra-high voltage circuit as 0.00852uF/km and the positive sequence capacitance C of the extra-high voltage circuit1At 0.01378uF/km, the interphase capacitive reactance C12This can be obtained by the following formula (21).
The neutral point small reactor values obtained from table 4 and the line capacitance parameters are shown in the following table.
TABLE 5 calculation results to compensate for high reactance neutral point and small reactance
| Line distance (km) | 100 | 150 | 200 | 250 | 300 | 350 | 400 | 450 | 500 |
| Corresponding small reactance value (omega) | 1261 | 841 | 630 | 504 | 420 | 360 | 315 | 280 | 252 |
According to the calculation results, the high reactance compensation value at one end of the 300km line is 584Mvar, and the value of the single-side high reactance small reactor is appropriate to be 420 omega.
In the steps, the parallel high-impedance value is calculated by analyzing the long-line capacity-rise effect, the load shedding of the line and the single-phase earth fault condition of the line and considering the factors of reactive compensation and the like of the line. And determining a final neutral point small reactance value according to the determined high reactance value and the line parameters. A series of measures provide basic technical support for power frequency overvoltage in an extra-high voltage power grid and influence of the power frequency overvoltage on power grid equipment.
The extra-high voltage shunt reactor and the method for evaluating the high-impedance neutral point small reactance provided by the embodiment of the invention strictly follow the circuit and magnetic circuit law, and the three-phase excitation impedance is considered to be the same in calculation, so all deductions are based on the premise that the open-phase overvoltage is close to the rated voltage amplitude, and the condition is satisfied by calculating and selecting the reactor and the neutral point small reactance value and vice versa. The calculation method is simple and practical, and can be easily used in engineering practice.
Example 2
Based on the same inventive concept, an embodiment of the present invention further provides a computer storage medium, where a computer program is stored on the computer storage medium, and when the computer program is executed by a processor, the steps of the method for designing a high impedance for an extra-high voltage limited power frequency overvoltage and a secondary current according to embodiment 1 or 2 are implemented.
As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.
The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.
These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.
Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.
Claims (10)
1. A high-impedance design method for limiting power frequency overvoltage and secondary arc current by extra-high voltage is characterized by comprising the following steps: the method comprises the following steps:
step 1, establishing an electromagnetic transient simulation-based model of a system;
step 2, determining the power frequency overvoltage at the tail end of the no-load long line of the ultra-high voltage transmission line;
step 3, solving the power frequency voltage rise caused by load shedding of the line;
step 4, determining power frequency overvoltage caused by single-phase short circuit of the extra-high voltage line;
step 5, determining a high impedance value of the extra-high voltage line;
and 6, determining the neutral point reactance of the high-voltage shunt reactor.
2. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: the establishment of the electromagnetic transient simulation-based model of the system in the step 1 is based on the collected system parameters and an electromagnetic transient modeling method, and is used for establishing a distributed model of an extra-high voltage transmission and transformation line, a detailed model of a main transformer and an extra-high voltage networking project and an equivalent model of a regional power grid power supply and load.
3. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 2, is characterized in that: the collected system parameters comprise main parameters of a local synchronous motor and main parameters of a power transmission line; the main parameters of the local synchronous motor comprise: synchronous motor, main transformer, series compensation, low-voltage reactor compensation, low-voltage capacitor compensation, and section load system parameters; the main parameters of the power transmission line comprise: the parameters of the transmission line such as the ground capacitance, the interphase capacitance, the inductance per unit length and the susceptance (capacitance) line.
4. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: step 2, determining the power frequency overvoltage at the tail end of the no-load long line of the ultra-high voltage transmission line, comparing the capacitance-rise effect of the line with the capacitance-rise effect of the no-load line after the power reactance is considered, and obtaining that the power reactance is equivalent to the increase of the line length, so that the power frequency overvoltage is further increased, and the resonance point is advanced;
when the no-load lossless line length is set as l, the current is measured at the end
Calculating the head voltage
Terminal voltage
And the voltage at a certain point in the line
Is represented by formula (1):
in the formula:
is the wave impedance;
is a phase constant; x is the distance from the receiving end, L0Reactance per unit length; c0Capacitance per unit length, l is line length;
defining the voltage transfer coefficient of the tail end to the head end of the line as formula (2):
in the formula: head end voltage
Terminal voltage
The voltage transfer coefficient of the end to the head of the line is K12；
From the equation (2), the voltage on the line is from the head end
Gradually rises and is distributed along the line according to a cosine curve, and the voltage at the tail end of the line
Reaches a maximum, e.g. when β l is 90 °, which corresponds to a series resonance, K, as seen from the head end of the line12→∞，
At the moment, the line length is 0.25 times of the wavelength of the voltage distribution on the power frequency no-load long line; meanwhile, the capacitance current of the no-load circuit can also form voltage rise on the reactance of the power supply, so that the voltage at the head end of the circuit is higher than the electromotive force of the power supply, and the power frequency overvoltage is further increased;
considering the power supply reactance, the relationship between the line end voltage and the power supply electromotive force can be obtained as follows:
in the above formula, the first and second carbon atoms are,
is the generator electromotive force, j is an imaginary number sign, ZCIs the wave impedance, XSIs a reactance of a power supply,
is the current at the head end of the line;
order:
by modifying the formula (3), the electrodynamic transfer function K of the line end voltage and the power supply can be obtained02The following formula:
power supply reactance XsInfluence of (2) through angle
Is shown as
Time K02→∞，
At this time
The length of the circuit is increased by the power supply reactance, the resonance point is advanced, and the power frequency voltage multiple is increased by the power supply reactance;
the transfer function of the line end voltage to the power supply motor type is expressed as follows:
in formula (5):
is the wave impedance;
is a phase constant.
5. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: step 3, solving the power frequency voltage rise caused by load shedding of the circuit, namely, when the system is set to normally operate, the voltage at the head end of the circuit is
Head end current of line
Power factor of
The transmitted active power
Reactive power
If power supply reactance XsThen, the electric power generation mode E is:
before load shedding, if considerable active and inductive reactive power is transmitted on the line, the electric E of power supply generation is inevitably higher than the voltage value U of the head end of the line1，E＞U1；
After load shedding, the power supply transient electromotive force E 'is considered to be unchanged according to the principle of unchanged magnetic linkage'dE is approximately distributed, and E is the electromotive force of the generator; the circuit breaker at the tail end of the line is opened, so that the running mode of the power supply with no-load long line is formed; after the tail end of the line is subjected to load shedding, the voltage at the head end of the line is higher than the electromotive force of a power supply, and the overvoltage at the tail end of the long line is more serious.
6. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: step 4, determining the power frequency overvoltage caused by the single-phase short circuit of the extra-high voltage line comprises the following steps:
setting the A phase in the system to generate single-phase earth fault, its boundary condition
Then there are:
in the formula
The positive sequence, negative sequence and zero sequence components of the voltage at the fault point;
the positive sequence, negative sequence and zero sequence components of the current at the fault are obtained;
according to the set boundary conditions, a composite sequence network is formed when the single phase is grounded, and the sequence network obtains a sequence current and a healthy phase voltage as follows:
in the above formula, a ═ ej120°；Z1、Z2、Z0The positive sequence impedance, the negative sequence impedance and the zero sequence impedance of the network are seen from a fault point,
is the phase voltage of the B phase of the power grid,
is the C-phase voltage of the power grid,
is the phase A electromotive force of a power supply; with K(1)The healthy phase voltage is increased after the single-phase earth fault is shown, and the formula (5) can be simplified as follows:
in the above formula, the first and second carbon atoms are,
the voltage is the healthy and complete phase of the power grid;
wherein:
in the above formula, Z1、Z2、Z0For the positive sequence, negative sequence and zero sequence impedance of the network seen from the fault point, the Z is generally available for the ultra-high voltage transmission system with larger system transmission capacity1≈Z2Neglecting the impedance resistance components in each sequence, simplifying as:
in the above formula: x0Is a zero sequence reactance, X1Is a positive sequence reactance;
according to the formula, the power frequency overvoltage and the X seen from the fault point0/X1(the ratio of zero sequence to positive sequence reactance) has a large relationship; x0/X1The increase of the voltage will lead the load shedding overvoltage of the single-phase earth fault to have an increasing trend;
x taking of typical ultra-high voltage transmission line0/X12.6; then calculate K(1)And U:
U＝1.21E
in the above formula: e is the power supply potential and U is the voltage.
7. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: step 5, determining the high impedance value of the extra-high voltage line is comprehensively determined according to the power frequency overvoltage under three conditions and by combining reactive compensation of typical engineering;
the lossless long line equation of the line voltage and the current of the extra-high voltage transmission line is as follows:
in the formula:
is the wave impedance;
is a phase constant; x is the distance from the receiving end, x0Reactance per unit length; b0For susceptance per unit length, the voltage at a point in the line is
Current is
A receiving terminal voltage of
Current is
The imaginary number symbol is j, sin beta x is a sine function, and cos beta x is a cosine function;
at the receiving end voltage
For reference, if the receiving end transmission power is:
Sr＝Pr+jQr (13)
in the above formula: srFor receiving end transmission power, PrIs active power, QrIs reactive power, j is an imaginary number symbol;
the expression of equation (12) can be expressed as:
reactive charge generated by a line of length l, using susceptance b per unit length0Represents the charging power integral form of:
in the above formula: qCFor the purpose of charging the power for a long line,
is the conjugate of the voltage at point x, b0Is unit susceptance, ZCIs the wave impedance, x is the line point length,
is the voltage at a certain point in the line;
long line charging power Q in combination with the equation of voltage in equation (13)CComprises the following steps:
in the above formula, PrIs active power, QrIs reactive power;
as shown in equation (16), the line is chargedReactive power is related to line transmission power, line length and unit reactance and susceptance; charging power Q of high impedance design according to long line in power transmission processc，QcAs the active power increases slightly, but the capacitance of the line plays a major role; meanwhile, the high-impedance design meets the highest voltage under the condition of restraining the light load, so that the light load line parameters are used as the control conditions of the high-impedance value;
equation (16) reduces to:
in the above formula: qcCharging reactive power for long lines, UeRating the phase voltage for the system;
the high impedance design is determined by the following formula:
in the above formula, QLFor high impedance compensation capacity, QcAnd charging reactive power for the long line, wherein B is a compensation coefficient.
8. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 1, is characterized in that: step 6, determining a neutral point reactance of the high-voltage shunt reactor, and when a single-phase earth fault occurs to the line, after circuit breakers at two ends of a fault phase trip, operating other two phases and keeping working voltage; due to interphase capacitance C12And certain current still flows through the fault point under the action of the interphase mutual inductance M
Namely a secondary current, and the electric arc is called as a secondary arc;
the secondary current comprises: a capacitive component and an inductive component;
the capacitance component refers to the voltage of the normal phase passing through the interphase capacitance C12A current provided to a fault point; meanwhile, the load current on the normal phase induces electromotive force on the fault phase through the interphase mutual inductance, the electromotive force provides current for a fault point through a loop formed by the phase-to-ground capacitor and the high reactance, and the capacitor component plays a main role under the condition of no compensation;
the inductance component is that the load current on the normal phase induces electromotive force on the fault phase through the interphase mutual inductance, and the electromotive force provides current for the fault point through a loop formed by the phase-to-ground capacitance and the high reactance;
the primary current and recovery voltage should be limited to small values, when the primary current is large and the recovery voltage is high, in order to limit the primary current and its recovery voltage, the method of adding the neutral point reactance of high-voltage shunt reactor is used to reduce the primary current and recovery voltage, including: two dimensions of accelerating the extinction of the latent supply arc and restraining the resonance overvoltage are required, and the value of the reactance of the neutral point of the high-voltage shunt reactor is calculated through the determined line capacitance reactance and high reactance value.
9. The method for designing the high impedance of the extra-high voltage limited power frequency overvoltage and the secondary current according to claim 8, is characterized in that: selecting a small reactance according to the requirement of the accelerated latent arc extinguishing;
the secondary arc current is less than (15-20) A;
from the perspective of compensating the inter-phase capacitance, the small reactance value can be approximated by equation (19):
in the formula: x0Reactance value of neutral point small reactance, XLPositive sequence reactance value, X, of a shunt reactor12The interphase capacitive reactance value of the line;
selecting a small reactance according to the requirement of inhibiting the resonance overvoltage:
in order to inhibit the power frequency transmission resonance overvoltage, the neutral point small reactance is calculated according to the formula (20):
in the formula: xL0Zero sequence reactance value of a parallel reactor, for a single-phase reactor, XL0＝XL(ii) a For three-phase three-column reactor, XL0＝XL/2；
The high-impedance compensation of the extra-high voltage line adopts a single-phase reactor, and the formulas (19) and (20) are the same;
calculating a parallel high-impedance value by analyzing factors such as reactive compensation of the line and the like under the conditions of long-line capacity-rise effect, line load shedding and single-phase earth fault of the line; and determining a final neutral point small reactance value according to the determined high reactance value and the line parameters.
10. A computer storage medium, characterized by: the computer storage medium has a computer program stored thereon, and the computer program when executed by a processor implements the steps of the method for designing a high impedance for ultra-high voltage limited power frequency overvoltage and secondary current according to claims 1 to 9.
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| CN114784805A (en) * | 2022-06-20 | 2022-07-22 | 中国科学院合肥物质科学研究院 | Load shedding overvoltage suppression method for power supply system |
| CN118472951A (en) * | 2024-07-12 | 2024-08-09 | 安徽沃华电力设备有限公司 | Low-voltage self-healing type high-capacity voltage limiting system |
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