ISOLATED NEUTRAL SYSTEM

TECHNICAL FIELD

The present invention relates to the field of power systems, and in particular, to a determining method and apparatus for a working voltage of an isolated neutral system.

BACKGROUND ART

In an isolated neutral system, a neutral point in a three-phase alternating current grid is not electrically connected to earth, that is, isolated neutral means that the neutral point is not manually connected to earth. FIG. 1 is a schematic structural diagram of an isolated neutral system. The system includes three phase wires and three capacitor banks, where the three phase wires are respectively a phase-A wire, a phase-B wire, and a phase-C wire, and the three capacitor banks are re- spectively a capacitor bank 14 on the phase-A wire, a capacitor bank 15 on the phase-B wire, and a capacitor bank 16 on the phase-C wire. One terminal of each capacitor bank is connected to one phase wire, and the respective other terminals of the capacitor banks are connected to one another to form a neutral point 101. Each capacitor bank includes a plurality of capacitors connected in series and/or in parallel. When a capacitance value of each capacitor bank is equal, that is, when a sampling value of a phase voltage of each phase wire is equal, where denotes a capacitance value of the phase-A wire denotes a capacitance value of the phase-B wire, denotes a capacitance value of the phase-C wire, denotes a current of the phase-A wire, denotes a current of the phase-B wire, and denotes a current of the phase-C wire. In this case, a working voltage of the isolated neutral system is 0.

However, in actual application, due to process errors or breakdowns of small in- ternal capacitors or other factors, capacitance values of the capacitors may not be consistent with a preset value, and it is not easy to measure a capacitance value of each capacitor. This may cause a case where the working voltage of the isolated neutral system exceeds a preset threshold when no fault occurs, further causing a relay protection apparatus corresponding to the capacitor bank to perform a false action.

SUMMARY OF THE INVENTION

In view of this, the present invention proposes a determining method for a work- ing voltage of an isolated neutral system, where a power system includes three phase wires and three capacitor banks, one terminal of each capacitor bank is connected to one phase wire, the respective other terminals of the capacitor banks are connected to one another to form a neutral point, and each capacitor bank in- cludes a plurality of capacitors; and the determining method includes : periodically sampling a phase voltage of each phase wire, to obtain a plu- rality of groups of sampling values of real-time phase voltages, where each group of sampling values of real-time phase voltages includes a sampling value of a phase voltage of each phase wire that is obtained by simultaneously measuring each phase wire; determining unbalance rates between the capacitor banks based on the plurality of groups of sampling values of real-time phase voltages, where the un- balance rate is a ratio between capacitance values of every two phase wires! and determining a working voltage of the isolated neutral system based on the unbalance rates.

According to the determining method as described above, optionally, the step of determining unbalance rates between the capacitor banks based on the plurality of groups of samphng values of real-time phase voltages includes: determining unbalance rates and between the capacitor banks based on the following formula: where denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the measurement, denotes a sampling value of a zero sequence voltage of the neutral point, denotes a sampling value of a phase voltage of a phase-A wire, denotes a sampling value of a phase voltage of a phase-B wire, denotes a sampling value of a phase voltage of a phase-C wire, denotes a capacitance value of the phase-A wire, denotes a capacitance value of the phase-B wire, and de- notes a capacitance value of the phase-C wire.

According to the determining method as described above, optionally, the working

. voltage is , and

According to the determining method as described above, optionally, the number of groups of sampling values of real-time phase voltages ranges from 2 to 10. According to the determining method as described above, optionally, after the step of determining a working voltage of the isolated neutral system based on the un- balance rates, the determining method further includes: if it is identified that a fault occurs in the isolated neutral system, deter- mining, based on the working voltage, whether the fault is an internal fault, where the internal fault indicates that a fault occurs in a capacitor in the capaci- tor bank; and the step of determining, based on the working voltage, whether the fault is an internal fault includes : if a value of the working voltage is greater than or equal to a preset threshold, determining that the fault is an internal fault.

The present invention further provides a determining apparatus for a working voltage of an isolated neutral system, where a power system includes three phase wires and three capacitor banks, one terminal of each capacitor bank is connected to one phase wire, the respective other terminals of the capacitor banks are con- nected to one another to form a neutral point, and each capacitor bank includes a plurality of capacitors; and the determining apparatus includes: a sampling unit configured to periodically sample a phase voltage of each phase wire, to obtain a plurality of groups of sampling values of real-time phase voltages, where each group of sampling values of real-time phase voltages in- cludes a sampling value of a phase voltage of each phase wire that is obtained by simultaneously measuring each phase wire; a first determining unit configured to determine unbalance rates between the capacitor banks based on the plurahty of groups of sampling values of re- al-time phase voltages, where the unbalance rate is a ratio between capacitance values of every two phase wires; and a second determining unit configured to determine a working voltage of the isolated neutral system based on the unbalance rates.

According to the determining apparatus as described above, optionally, the first determining unit is specifically configured to: determine unbalance rates and between the capacitor banks based on the following formula: where denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a sampltng value of a zero sequence voltage of the neutral point, denotes a sampling value of a phase voltage of a phase-A wire, denotes a sampling value of a phase voltage of a phase-B wire, denotes a sampling value of a phase voltage of a phase-C wire, denotes a capacitance value of the phase-A wire, denotes a capacitance value of the phase-B wire, and de- notes a capacitance value of the phase-C wire.

According to the determining apparatus as described above, optionally, the working voltage is

_,

According to the determining apparatus as described above, optionally, the num- ber of groups of sampling values of real-time phase voltages ranges from 2 to 10.

According to the determining apparatus as described above, optionally, the de- termining apparatus further includes : a third determining unit configured to ^: if it is identified that a fault occurs in the isolated neutral system, determine, based on the working voltage, whether the fault is an internal fault, where the internal fault indicates that a fault occurs in a capacitor in the capacitor bank; and the step of determining, based on the working voltage, whether the fault is an internal fault includes : if a value of the working voltage is greater than or equal to a preset threshold, determining that the fault is an internal fault.

The present invention further provides a determining apparatus for a working voltage of an isolated neutral system, where a power system includes three phase wires and three capacitor banks, one terminal of each capacitor bank is connected to one phase wire, the respective other terminals of the capacitor banks are con- nected to one another to form a neutral point, and each capacitor bank includes a plurality of capacitors; and the determining apparatus includes: at least one memory configured to store instructions; and at least one processor configured to perform, according to the instructions stored in the memory, the determining method for a working voltage of an isolated neu- tral system according to any one of the foregoing embodiments.

The present invention further provides a readable storage medium, where the readable storage medium stores machine-readable instructions that, when exe- cuted by a machine, cause the machine to perform the determining method for a working voltage of an isolated neutral system according to any one of the forego- ing embodiments.

The unbalance rates between the capacitor banks are obtained based on the sampling values of the phase voltages measured in real time, and the working voltage is accurately determined based on the unbalance rates, so that subsequent operations can be accurately performed.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features and advantages of the present invention will be more apparent to those of ordinary skill in the art from the detailed description of preferred embodiments of the present invention with reference to the accompa- nying drawings, in which:

FIG. 1 is a schematic structural diagram of an isolated neutral system according to an embodiment of the present invention;

FIG. 2 is a schematic flowchart of a determining method for a working voltage of an isolated neutral system according to another embodiment of the present in- vention;

FIG. 3 is a schematic flowchart of a determining method for a working voltage of an isolated neutral system according to still another embodiment of the present invention;

FIG. 4Ais a schematic structural diagram of a determining apparatus for a working voltage of an isolated neutral system according to an embodiment of the present invention; and

FIG. 4B is a schematic structural diagram of a determining apparatus for a working voltage of an isolated neutral system according to another embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In order to make the objectives, technical solutions, and advantages of the present invention more apparent, the present invention will be described in further detail by way of embodiments hereinafter.

Faults occurring in an isolated neutral system may be classified as an internal fault and an external fault. The internal fault indicates that a fault such as an earth fault or a capacitor breakdown occurs in a capacitor bank of the isolated neutral system. The external fault indicates that a fault does not occur in a ca- pacitor, but occurs in a device beyond the protection of a relay protection appa- ratus corresponding to the capacitor bank. When the external fault occurs, the relay protection apparatus corresponding to the capacitor bank should not per- form an action. Whether the relay protection apparatus should perform an action may be determined by determining whether a working voltage exceeds a preset threshold. If the working voltage is greater than or equal to the preset threshold, protection should be activated. If the working voltage is less than the preset threshold, no action should be performed. Therefore, accuracy of the working voltage plays a key role. The inventor found that the working voltage is related to a ratio between capacitor banks, and based on this, the inventor provides a method for determining a ratio between capacitor banks, to obtain an accurate working voltage.

Embodiment 1

This embodiment provides a determining method for a working voltage of an iso- lated neutral system, and the method is performed by a determining apparatus for a working voltage of an isolated neutral system. The apparatus may be inte- grated into a relay protection apparatus, or may be independently disposed.

FIG. 2 is a schematic flowchart of the determining method for a working voltage of an isolated neutral system according to this embodiment. The determining method for a working voltage of an isolated neutral system includes the following steps.

In step 201, a phase voltage of each phase wire is periodically sampled, to obtain a plurality of groups of sampling values of real-time phase voltages, where each group of sampling values of real-time phase voltages includes a samphng value of a phase voltage of each phase wire that is obtained by simultaneously measuring each phase wire.

A phase voltage of each phase wire is continuously sampled periodically and a period may be set according to actual requirements, for example, may be set to one milhsecond. In addition, each phase wire is simultaneously sampled. Specifi- cally, a sampling value of a phase voltage may be obtained by using a voltage sensor.

The number of groups of sampling values of real-time phase voltages may range from 2 to 10. In this way, enough data can be provided to accurately determine the working voltage subsequently and the working voltage can be determined as quickly as possible.

In step 202, unbalance rates between the capacitor banks are determined based on the plurality of groups of sampling values of real-time phase voltages, where the unbalance rate is a ratio between capacitance values of every two phase wires.

Due to process errors or other factors, a capacitance value of each capacitor bank may not be equal, and it is very difficult to measure a capacitance value of each capacitor in each capacitor bank. The inventor uses a manner of determining a ratio between capacitance values of the capacitor banks based on a sampling value of a real-time phase voltage to determine the working voltage with higher accuracy.

For example, unbalance rates and between the capacitor banks are determined based on the following formula: where denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a sampling val· ue of a zero sequence voltage of the neutral point, denotes a samphng value of a phase voltage of a phase-A wire, denotes a samphng value of a phase volt- age of a phase-B wire, denotes a sampling value of a phase voltage of a phase-C wire, denotes a capacitance value of the phase-A wire, denotes a capacitance value of the phase-B wire, and de- notes a capacitance value of the phase-C wire.

In step 203, the working voltage of the isolated neutral system is determined based on the unbalance rate.

The working voltage may be defined according to requirements, for example, may be configured based on a corresponding relay protection apparatus. In the present invention, the working voltage is, for example, where denotes a capacitance value of the phase-A wire, denotes a capacitance value of the phase-B wire, and denotes a capacitance value of the phase-C wire.

Optionally, after step 203, the determining method further includes: returning to perform step 201, and periodically repeating the foregoing steps, to continuously determine a sampling value of a phase voltage. In this embodiment, working personnel may trigger step 201 according to actual requirements, or steps 201 to 203 may be periodically performed by the apparatus to automatically determine the working voltage.

Optionally, after step 203, the determining method further includes: if it is iden- tified that a fault occurs in the isolated neutral system, determining, based on the working voltage, whether the fault is an internal fault, where the internal fault indicates that a fault occurs in a capacitor in the capacitor bank. If it is deter- mined that the fault is an internal fault, a relay protection apparatus corre- sponding to the capacitor bank should perform an action to clear the fault. If it is determined that the fault is a non-internal fault, that is, it is determined that the fault is an external fault, the relay protection apparatus corresponding to the capacitor bank should not perform an action. As an exemplary description, if a value of the working voltage is less than a preset threshold, it is determined that the fault is an external fault. If the value of the working voltage is greater than or equal to the preset threshold, it is determined that the fault is an internal fault. The preset threshold herein may be determined according to actual requirements, and details are not repeated herein.

In this embodiment, the unbalance rates between the capacitor banks are ob- tained based on the sampling values of the phase voltages measured in real time, and the working voltage is accurately determined based on the unbalance rates, so that subsequent operations can be accurately performed.

Embodiment 2

This embodiment further supplements the descriptions of the determining method for a working voltage of an isolated neutral system in Embodiment 1. FIG. 3 is a schematic flowchart of the determining method for a working voltage of an isolated neutral system according to this embodiment. The determining method for a working voltage of an isolated neutral system includes the following steps. In step 301, when it is determined that a fault occurs in a capacitor bank, a phase voltage of each phase wire is periodically sampled, to obtain a plurahty of groups of sampling values of real-time phase voltages, where each group of sampling values of real-time phase voltages includes a samphng value of a phase voltage of each phase wire that is obtained by simultaneously measuring each phase wire.

Specific operations of step 301 are consistent with those of step 201, and details are not repeated herein.

Herein, the number of groups of sampling values of real-time phase voltages may range from 2 to 10, and the number of 5 groups may be specifically selected.

In step 302, unbalance rates and between the capacitor banks are de- termined based on the following formula: w here denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a sampling value of a zero sequence voltage of the neutral point of the isolated neutral system, denotes a samphng value of a phase voltage of a phase-A wire, denotes a sampling value of a phase voltage of a phase-B wire, denotes a sampling value of a phase voltage of a phase-C wire, denotes a capacitance value of the phase- A wire, denotes a capacitance value of the phase-B wire, and denotes a capacitance value of the phase-C wire.

This is because, for the isolated neutral system whose capacitor bank is not faulty, that is, for the isolated neutral system where sampling values of phase voltages of the three phase wires are equal, a sum of currents of the three phase wires is 0, that is: where denotes a current of the phase-A wire, denotes a current of the phase-B wire, denotes a current of the phase-C wire, denotes a sampling value of a phase voltage of the phase-A wire, denotes a samphng value of a phase voltage of the phase-B wire, denotes a sampling value of a phase volt- age of the phase-C wire, denotes a samphng value of a zero sequence voltage of the neutral point of the isolated neutral system, denotes a capacitance value of the phase-A wire, denotes a capacitance value of the phase-B wire, denotes a capacitance value of the phase-C wire, j denotes the imaginary number symbol, and ω denotes an angular frequency of the isolated neutral system.

The inventor found, through creative efforts, that after the foregoing formula is divided by the foregoing formula (l. l) is rewritten into the following for- mula (1.2):

Assuming that the fore- going formula (1.2) is rewritten into the following formula (1.3):

It is assumed that where denotes the working voltage. To calculate , that is, . In this way, the fore- going formula (1.3) is rewritten into the following formula (1.4): Assuming that and the foregoing formula (1.4) is rewritten into According to this formula, assum- ing that a value of obtained in the first measurement is and a value of obtained in the second measurement is and , and by anal· ogy, after a total of five measurements, the following formulas are obtained:

The foregoing formulas are rewritten into the following matrix formula (1.5): where In other words, the matrix formula (1.5) is XR = Y. According to the least square method, the following formula (1.6) is obtained ac- cording to the matrix formula (1.5): where denotes a transposed matrix of the matrix denotes an in- verse of and a calculation method thereof belongs to the prior art, and will not be repeated herein. Since a corresponding error may occur in each measure- ment, a value of R may be finally obtained according to the foregoing matrix for- mula, and and are further obtained. After the unbalance rates and are put into the foregoing matrix formula (1.5), an accurate value of can be obtained.

The following description is provided by using three measurements as an exam- ple. The foregoing formulas are rewritten into the following formula: XR = Y.

Since values in the matrices X and Y may be obtained through measurement, a value of R may be determined according to the following formula, that is, Assuming that where

Since the foregoing variables can be directly obtained through measurement, corresponding values of unbalance rates and can be calculated. In step 303, the working voltage of the isolated neutral system is determined based on the unbalance rates.

Values in both steps 301 and 302 are determined when no fault occurs in the iso- lated neutral system, and the working voltage of the isolated neutral system may be subsequently determined in real time based on the unbalance rates that are obtained when no fault occurs, for example, the working voltage may be deter- mined based on the following formula:

In step 304, whether the working voltage is greater than or equal to a preset threshold is determined, and if it is determined that the working voltage is greater than or equal to the preset threshold, it is determined that an internal fault occurs in the isolated neutral system.

If the working voltage is still 0 or less than the preset threshold, this indicates that no internal fault occurs. If the working voltage is greater than or equal to the preset threshold, this indicates that the internal fault occurs, and a corresponding relay protection apparatus is required to perform an action to ehminate the fault. The preset threshold is, for example, 0.5 V. If the working voltage is less than the preset threshold, this indicates that an external fault occurs.

In this way, whether the internal fault or the external fault occurs may be deter- mined based on the accurate working voltage, to avoid a false action of the relay protection apparatus. When a fault, for example, a breakdown, occurs in a capac- itor in a capacitor bank, a capacitance value of a corresponding phase wire is not equal to capacitance values of the other two phase wires, causing a voltage of the corresponding phase wire to be unequal to voltages of the other two phase wires.

According to this embodiment, an actual working voltage can be determined by determining the unbalance rates between the capacitor banks. In this way, the working voltage may be continuously monitored to determine whether a fault occurs in a capacitor in a capacitor bank, and further determine whether a cor- responding relay protection apparatus is required to perform an action.

Embodiment 3

This embodiment provides a determining apparatus for a working voltage of an isolated neutral system. A power system includes three phase wires and three capacitor banks. One terminal of each capacitor bank is connected to one phase wire, and the respective other terminals of the capacitor banks are connected to one another to form a neutral point. Each capacitor bank includes a plurahty of capacitors, that is, each capacitor bank is composed of a plurality of capacitors connected in series and in parallel.

FIG. 4Ais a schematic structural diagram of a determining apparatus for a working voltage of an isolated neutral system according to this embodiment. The determining apparatus for a working voltage of an isolated neutral system in- cludes a sampling unit 401, a first determining unit 402, and a second determin- ing unit 403.

The sampling unit 401 is configured to periodically sample a phase voltage of each phase wire, to obtain a plurality of groups of samphng values of real-time phase voltages, where each group of samphng values of real-time phase voltages in- cludes a sampling value of a phase voltage of each phase wire that is obtained by simultaneously measuring each phase wire. The first determining unit 402 is configured to determine unbalance rates between the capacitor banks based on the plurality of groups of sampling values of real-time phase voltages, where the unbalance rate is a ratio between capacitance values of every two phase wires. The second determining unit 403 is configured to determine a working voltage of the isolated neutral system based on the unbalance rates.

Optionally, the first determining unit 402 is specifically configured to ^: determine unbalance rates and between the capacitor banks based on the following formula : where denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a value of obtained in the n ^th measurement, denotes a sampling val- ue of a zero sequence voltage of the neutral point, denotes a samphng value of a phase voltage of a phase-A wire, denotes a samphng value of a phase volt- age of a phase-B wire, denotes a sampling value of a phase voltage of a phase-C wire, denotes a capacitance value of the phase-Awire, denotes a capacitance value of the phase-B wire, and de- notes a capacitance value of the phase-C wire.

Optionally, the working voltage is and

Optionally, the number of groups of sampling values of real-time phase voltages ranges from 2 to 10.

Optionally, as shown in FIG. 4B, the determining apparatus further includes a third determining unit 404, which is configured to ^: if it is identified that a fault occurs in the isolated neutral system, determine, based on the working voltage, whether the fault is an internal fault, where the internal fault indicates that a fault occurs in a capacitor in the capacitor bank.

The step of determining, based on the working voltage, whether the fault is an internal fault includes : if a value of the working voltage is greater than or equal to a preset threshold, determining that the fault is an internal fault.

A working method of each unit in this embodiment is the same as that in the foregoing embodiment, and will not be repeated herein.

According to the determining apparatus for a working voltage of an isolated neu- tral system, the unbalance rates between the capacitor banks are obtained based on the sampling values of the phase voltages measured in real time, and the working voltage is accurately determined based on the unbalance rates, so that subsequent operations can be accurately performed.

The present invention further provides a determining apparatus for a working voltage of an isolated neutral system. A power system includes three phase wires and three capacitor banks. One terminal of each capacitor bank is connected to one phase wire, and the respective other terminals of the capacitor banks are connected to one another to form a neutral point. Each capacitor bank includes a plurality of capacitors connected in series and/or in parallel. The determining apparatus includes at least one memory and at least one processor. The memory is configured to store instructions. The processor is configured to perform, according to the instructions stored in the memory, the determining method for a working voltage of an isolated neutral system according to any one of the foregoing em- bodiments.

An embodiment of the present invention further provides a readable storage me- dium. The readable storage medium stores machine-readable instructions that, when executed by a machine, cause the machine to perform the determining method for a working voltage of an isolated neutral system according to any of the foregoing embodiments.

The readable storage medium stores the machine-readable instructions that, when executed by a processor, cause the processor to execute any of the foregoing methods. Specifically, a system or an apparatus with a readable storage medium may be provided, software program code for implementing the functions of any one of the foregoing embodiments is stored in the readable storage medium, and a computer or a processor of the system or apparatus is caused to read and execute the machine-readable instructions stored in the readable storage medium.

In this condition, the program code itself read from the readable storage medium may implement the functions of any one of the foregoing embodiments, and therefore machine-readable code and the readable storage medium storing the machine-readable code constitute a part of the present invention.

The embodiments of the readable storage medium include a floppy disk, a hard disk, a magnetic optical disc, an optical disc (such as CD-ROM, CD-R, CD-RW, DVD-ROM, DVD-RAM, DVD-RW and DVD+RW), a magnetic tape, a non-volatile memory card and a ROM. Optionally, the program code may be downloaded from a server computer or a cloud via a communication network.

It should be understood by those skilled in the art that various variations and modifications may be made to the embodiments disclosed above without departing from the essence of the present invention. Therefore, the protection scope of the present invention shall be limited by the appended claims.

It should be noted that not all the steps and units in the flows and structural di- agrams of the system described above are necessary, and some steps or units may be omitted according to practical requirements. The execution order of the various steps is not fixed and may be adjusted according to requirements. A structure of the apparatus described in the foregoing embodiments may be a physical struc- ture, or may be a logical structure. In other words, some units may be imple- mented by the same physical entity, or some units may be implemented sepa- rately by a plurahty of physical entities, or may be implemented together by some components in a plurahty of independent devices.

In the foregoing embodiments, a hardware unit may be implemented mechani- cally or electrically. For example, a hardware unit or a processor may include a permanent dedicated circuit or logic (such as a dedicated processor, FPGA or ASIC) to accomplish a corresponding operation. The hardware unit or processor may also include a programmable logic or circuit (such as a general-purpose processor or another programmable processor), and may be set temporarily by software to ac complish a corresponding operation. The specific implementation (mechanical manner, or a dedicated permanent circuit, or a temporarily set circuit) may be determined in consideration of cost and time. The above description is merely illustrative of the preferred embodiments of the present invention and is not intended to limit, the present invention. Any modifi- cations, equivalent substitutions, improvements, and the like made within the spirit and principles of the present invention shall fall within the protection scope of the present invention.