CN113447803B - Short-circuit current calculation voltage coefficient value method for checking breaking capacity of circuit breaker - Google Patents
Short-circuit current calculation voltage coefficient value method for checking breaking capacity of circuit breaker Download PDFInfo
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Abstract
The invention relates to a method for calculating voltage coefficient value of short-circuit current for checking breaking capacity of a circuit breaker, which comprises the following steps: 1) Acquiring a node admittance matrix Y according to the network topology parameters of the power system; 2) Inverting the node admittance matrix Y to obtain a node impedance matrix Z; 3) Collecting generator output and terminal voltage data in a tide result in a maximum operation mode, and calculating sub-transient potential of each generator; 4) Calculating the injection current of the generator to the network according to the secondary transient potential; 5) Calculating node open circuit voltage U i according to a network equation; 6) The voltage coefficient c is calculated and the corresponding node short circuit current I i is calculated therefrom. Compared with the prior art, the invention has the advantages of meeting the actual power grid condition, having larger breaking margin, balancing the safety and the economy of the power grid, and the like.
Description
Technical Field
The invention relates to the field of breaker breaking capacity checking in a power system, in particular to a practical calculation voltage coefficient value method for short-circuit current used for breaker breaking capacity checking.
Background
In the calculation of the short-circuit current based on the equivalent voltage source method, the voltage coefficient c of the voltage class above 35kV is generally 1.1 in national standard, and the impedance correction is considered, so that the calculated short-circuit current is theoretically the maximum short-circuit current flowing through the short-circuit point after considering all possible operation states. However, in some actual power grids, the method for calculating the short-circuit current may cause problems of smaller or even out-of-limit open-circuit margin of the circuit breaker, poor economical efficiency, and the like, so that a practical voltage coefficient value method suitable for a specific power grid needs to be explored, so that the open-circuit margin of the circuit breaker (for example, annual) is more definite and is more suitable for the change condition of the actual power grid short-circuit current, and the balance between safety and economy is achieved.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provide a short-circuit current calculation voltage coefficient value method for checking the breaking capacity of a circuit breaker.
The aim of the invention can be achieved by the following technical scheme:
A short-circuit current calculation voltage coefficient value method for checking the breaking capacity of a circuit breaker comprises the following steps:
1) Acquiring a node admittance matrix Y according to the network topology parameters of the power system;
2) Inverting the node admittance matrix Y to obtain a node impedance matrix Z;
3) Collecting generator output and terminal voltage data in a tide result in a maximum operation mode, and calculating sub-transient potential of each generator;
4) Calculating the injection current of the generator to the network according to the secondary transient potential;
5) Calculating node open circuit voltage U i according to a network equation;
6) The voltage coefficient c is calculated and the corresponding node short circuit current I i is calculated therefrom.
In the step 1), the conditions for obtaining the node admittance matrix Y include:
a: adopting the average rated voltage of each voltage level as a voltage reference value;
B: neglecting all parallel branches in the network, namely neglecting the capacitance to ground of the transmission line, the excitation branch of the transformer and the static load;
c: the transformer adopts a non-reference transformation ratio, namely the per unit transformation ratio of the transformer is equal to the ratio of the actual transformation ratio of the transformer to the average rated voltage.
In the step 3), the calculation formula of the sub-transient potential E Gi of the node i where the generator is located is as follows:
wherein P Gi、QGi、UGi is the active and reactive output and the terminal voltage of the node i where the generator is located, X' di and R si are the sub-transient reactance and the armature resistance of the node i where the generator is located, the superscript and the matrix form are shown, and j represents the imaginary unit.
In the step 4), the current injected by the generator to the networkThe calculation formula of (2) is as follows:
in the step 5), the expression of the network equation is:
wherein the diagonal elements For the self-impedance of node i, n is the dimension of the node impedance matrix, and the off-diagonal element Z ij is the transimpedance between node i and node j,/>For injection of current column vectors, the elements in the injection of current column vectors are all 0,/>, except for the generator nodesIs the node open circuit voltage column vector.
In the step 6), the calculation formula of the voltage coefficient c i of the network where the node i is located is as follows:
ci＝Ui·UB/Un
Wherein, U B and U n are the reference voltage and the nominal voltage of the network where the node i is located, respectively.
In the step 6), the calculation formula of the node short-circuit current is as follows:
Wherein, S B system reference capacity, Z B reference impedance of corresponding voltage class.
Compared with the prior art, the invention has the following advantages:
The invention provides a method for calculating a voltage coefficient by short-circuit current, which is used for checking the breaking capacity of a circuit breaker.
In addition, the short-circuit current calculated by the voltage coefficient of the existing method is too large, so that the problem that the open-circuit margin remained by the circuit breaker is too low or even exceeds the standard is caused when the short-circuit current is used for checking the open-circuit capability of the circuit breaker in operation, and the result of the invention ensures that the circuit breaker is kept with larger open-circuit margin, thereby realizing the balance of the safety and the economy of a power grid; in addition, the invention is more in line with the actual power grid state, and can also provide reference for the design of local extension or reconstruction planning.
Drawings
Fig. 1 is a network with a short circuit at point k1 in an embodiment of the invention.
Fig. 2 is a comparison of errors from different methods.
Fig. 3 is a flow chart of the method of the present invention.
Detailed Description
The invention will now be described in detail with reference to the drawings and specific examples.
As shown in FIG. 3, the invention provides a method for calculating the voltage coefficient of a short-circuit current for checking the breaking capacity of a circuit breaker, an equivalent voltage source method is widely used for short-circuit current engineering calculation, and an equivalent voltage source is used for a short-circuit point when the equivalent voltage source method is used for calculating the short-circuit currentInstead, the voltage source is the only voltage source of the network, and the other power sources (such as the potential of synchronous generators, synchronous motors, asynchronous motors and feed networks) are all considered to be zero and replaced with their own internal impedance.
According to GB/T15544.1-2013 three-phase alternating current system short-circuit current calculation, the voltage coefficient c is essentially the ratio of the highest allowable operating voltage at the initial end of the element to the nominal voltage. However, the maximum operating value of the terminal voltage of some components is often more easily obtained, so that the impedance correction is performed in this case. However, in the practice of calculating the short-circuit current, impedance correction is often not performed, which results in that the voltage coefficient recommended in the standard cannot be used for the value.
The invention relates to a method for taking a node voltage coefficient in an electric power network as a value when impedance correction is not carried out, and takes a k1 point short circuit in fig. 1 as an example to explain the basic idea of the invention.
In order to calculate the voltage coefficient used by the k1 point short circuit, the invention obtains the open circuit voltage (starting end voltage) and input impedance of the k1 point through a network equation (node voltage equation) on the premise of neglecting all parallel branches in the power network, further obtains the voltage coefficient c, and then uses the equivalent voltage source voltageThe short-circuit current at the k1 point can be obtained by dividing the input impedance, and the impedance in the equivalent network is not required to be corrected when the voltage coefficient c calculated by the invention is used for calculating the short-circuit current.
The practical voltage coefficient value calculation method based on the improved classical assumption comprises the following steps:
When the existing classical assumption is adopted to calculate short-circuit current, all parallel branches in a network are ignored, a transformer adopts a reference transformation ratio, a voltage coefficient c is calculated by taking the ratio of the reference voltage of the network to a nominal voltage, namely c is 1.05, and the detailed steps of the invention for improving the practical value of the voltage coefficient of the classical assumption are introduced.
The node admittance matrix Y is formed by utilizing the calculated network topology data, the node admittance matrix Y is inverted to obtain the node impedance matrix Z, then the injection current of the generator node is calculated by the tide result data, and then the open circuit voltage of each node is calculated by the network equation U=ZI, so that the node voltage coefficient c can be calculated, and the method can be summarized as follows:
1) Reading network related topology parameters, and forming a node admittance matrix Y under the following conditions:
a: voltage reference value: adopting the average rated voltage of each voltage level as a voltage reference value;
b: neglecting all parallel branches in the network, namely neglecting the capacitance to ground of a transmission line, the excitation branch of a transformer, static load and the like;
C: the transformer adopts a non-reference transformation ratio, namely the per unit transformation ratio of the transformer is equal to the ratio of the actual transformation ratio of the transformer to the average rated voltage;
2) Inverting the node admittance matrix Y to obtain a node impedance matrix Z;
3) Reading data such as generator output, terminal voltage and the like in a power flow result in a maximum operation mode, and calculating sub-transient potential E Gi of each generator according to a formula (1):
Wherein P Gi、QGi、Ugi is the active and reactive output and the terminal voltage of the generator i, and X' di and R si are the secondary transient reactance and armature resistance of the generator i;
4) The injection current of the generator into the network is determined by equation (2):
5) Based on the results of step 2) and step 4), a node open circuit voltage U i is found using the network equation of equation (3), in which a current column vector is injected The elements in the two-phase power generator are 0 except the generator node;
Namely:
Wherein the diagonal element Z ii is the self-impedance of the node i, the off-diagonal element Z ij is the transimpedance between the node i and the node j, n is the dimension of the node impedance matrix, and the current column vector is injected The elements in (a) are 0 except the generator node.
6) The value of the voltage coefficient c is obtained by the formula (5), and the network equation is calculated by using the per unit value, so that the value is calculated to be a named value, and then the voltage coefficient is calculated according to definition.
ci＝Ui·UB/Un (5)
Wherein, U B and U n are the reference voltage and the nominal voltage of the network where the node i is located, respectively.
The corresponding node short circuit current can be further calculated from equation (6):
Wherein, S B system reference capacity, reference impedance for the corresponding voltage class of Z B.
Taking a certain power grid as an example of the invention, selecting part 220kV and 500kV nodes for calculation, and in order to conveniently compare the difference of short-circuit current results calculated by different methods, defining the following calculation methods:
method 1: setting a voltage coefficient c=1.05, wherein the transformer adopts a reference transformation ratio, namely the per unit transformation ratio of the transformer is equal to 1, and other steps are the same as those of the invention;
method 2: calculating according to a national standard method;
Method 3: let the voltage coefficient c=1.1, the other steps are the same as in the present invention.
The method of the invention is used for calculating the voltage coefficient of each node, further calculating the short-circuit current, and taking the calculation result of the short-circuit current based on the tide as a standard comparison. In addition, in order to further verify the accuracy and the practicability of the method, the calculation results of the methods 1 to 3 are compared with the standard, and the calculation results are shown in the table 1:
TABLE 1 comparison of the short-circuit Current results of the Voltage coefficient values and different methods of the invention
Note that:
As can be seen from Table 1, most of the absolute values of errors of the short-circuit current obtained by the voltage coefficient obtained by the method are distributed within 5%, and the superiority of the method, namely the actual condition of the system is relatively close to that of the system, can be intuitively seen by comparing the errors of the different methods of FIG. 2.
In general, the open current of a 220kV voltage class breaker is 50kA, and the open current of a 500kV voltage class breaker is 63kA, and in order to further analyze the superiority of the invention in checking the open capability of the breaker, the open margin left by the short circuit current calculated by different methods is compared, as shown in Table 2. As is apparent from table 2, for the same node, the breaker open margin left by the invention is the largest; for the non-computational methods, in particular methods 2 and 3, the partial node breaker opening margin is already close to the critical value, even if the maximum short-circuit current that can be opened by the breaker is higher, but in practice such severe situations will not occur in most cases. Therefore, the invention has the advantages that the invention is more close to the actual power grid condition, is more suitable for checking the breaking capacity of the breaker in operation, and achieves the balance of safety and economy.
Table 2 comparison of open margin left by circuit breakers under different methods
Note that:
From the analysis results, the voltage coefficient calculated by the method is relatively close to the real system condition. The voltage coefficient has different values according to different standards and methods, but the short-circuit current calculated by the value result of the existing method is larger than that of an actual power grid, so that the problems of excessively low breaking margin, excessive standard and low economy and the like of the circuit breaker are caused, the calculated result of the invention is similar to the actual result, compared with the different methods, the calculated short-circuit current better accords with the actual power grid condition, other results are generally larger, and the short-circuit current which can be broken by the circuit breaker is limited, so that the result of the invention ensures that the circuit breaker has larger breaking margin, and the balance of the safety and the economy of the power grid is realized; in addition, the invention is more in line with the actual power grid state, and can also provide reference for the design of local extension or reconstruction planning.
Claims (2)
1. The method for calculating the voltage coefficient of the short-circuit current for checking the breaking capacity of the circuit breaker is characterized by comprising the following steps of:
1) Acquiring a node admittance matrix Y according to the network topology parameters of the power system;
Specifically, the conditions for acquiring the node admittance matrix Y include:
a: adopting the average rated voltage of each voltage level as a voltage reference value;
B: neglecting all parallel branches in the network, namely neglecting the capacitance to ground of the transmission line, the excitation branch of the transformer and the static load;
C: the transformer adopts a non-reference transformation ratio, namely the per unit transformation ratio of the transformer is equal to the ratio of the actual transformation ratio of the transformer to the average rated voltage;
2) Inverting the node admittance matrix Y to obtain a node impedance matrix Z;
3) Collecting generator output and terminal voltage data in a tide result in a maximum operation mode, and calculating sub-transient potential of each generator;
4) Calculating the injection current of the generator to the network according to the secondary transient potential;
5) Calculating node open circuit voltage U i according to a network equation;
6) Calculating a voltage coefficient c and calculating a corresponding node short-circuit current I i according to the voltage coefficient c;
in step 3), the calculation formula of the sub-transient potential E Gi of the node i where the generator is located is:
Wherein P Gi、QGi、UGi is the active power, the reactive power and the terminal voltage of the node i where the generator is located, X' di and R si are the secondary transient reactance and the armature resistance of the node i where the generator is located, j represents an imaginary unit;
In step 4), the injection current of the generator to the network The calculation formula of (2) is as follows:
in step 5), the expression of the network equation is:
wherein the diagonal elements For the self-impedance of node i, n is the dimension of the node impedance matrix, off-diagonal element Z ij is the transimpedance between node i and node j,
For injection of current column vectors, the elements in the injection of current column vectors are all 0,/>, except for the generator nodesAn open-circuit voltage column vector for a node;
in step 6), the calculation formula of the voltage coefficient c i of the network where the node i is located is:
ci＝Ui·UB/Un
Wherein, U B and U n are the reference voltage and the nominal voltage of the network where the node i is located, respectively.
2. The method for calculating the voltage coefficient of the short-circuit current for checking the breaking capacity of the circuit breaker according to claim 1, wherein in the step 6), the calculation formula of the node short-circuit current is as follows:
Wherein, S B system reference capacity, Z B reference impedance of corresponding voltage class.
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