CN113009275A - Double-end fault location method for flexible direct-current access alternating-current hybrid line - Google Patents
Double-end fault location method for flexible direct-current access alternating-current hybrid line Download PDFInfo
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Abstract
The invention provides a flexible direct current access alternating current hybrid line double-end fault location method, which comprises the following steps: (1) after the fault of the alternating current overhead line-cable hybrid line is detected, voltage and current information of the alternating current power supply side of the line is collected, voltage information of a converter grid-connected point is collected, and negative sequence components of the voltage and the current are obtained. (2) And based on the distribution parameter model, calculating the negative sequence components of the voltage and the current collected at the alternating current power supply side to obtain the negative sequence voltage distribution along the line. (3) And the amplitude distribution of the negative sequence voltage along the line obtained by calculating the electric quantity information at the side of the alternating current power supply is compared with the amplitude of the negative sequence voltage collected at the grid-connected point of the converter, and the fault distance is determined through an amplitude difference function according to the characteristic that the amplitude difference between the two at the fault point is minimum.
Description
Technical Field
The invention belongs to the field of relay protection of power systems, and particularly relates to a flexible direct-current access alternating-current hybrid line double-end fault distance measurement method.
Background
Due to the outstanding advantages of flexible control, no commutation failure, low power transmission loss and the like, the flexible direct current has a great deal of practical engineering application in the aspects of large-area asynchronous interconnection of an alternating current power grid, reconstruction and reinforcement of a regional power transmission grid and the like in recent years, and the pattern of a flexible alternating current-direct current hybrid power transmission system is gradually formed. For an alternating current transmission line with flexible direct current access, in the process of transmitting electric energy to an urban load center substation, considering that investment cost and high failure rate of a middle joint of a high-voltage cable are high, a large-section long-distance cable line cannot be used. However, in order to match with the modern development of cities, the cable lines have the advantages of small floor space, no influence of bad weather, beautiful appearance of cities and the like, and the cable lines need to be changed into the cable lines on specific paths. The alternating current overhead line-cable hybrid line has the advantages of economy and environmental protection, and has wide application prospect. The line parameters of the alternating current overhead line-cable hybrid line are not uniform, the fault reasons and the properties of the overhead line and the cable are greatly different, and after the fault occurs, the fault position is accurately and effectively calculated and the fault is timely cleared, so that the method has important significance for improving the power supply reliability of a power grid. Therefore, it is necessary to research an effective fault ranging method.
At present, the existing alternating current hybrid line fault location method can be divided into three categories, namely a traveling wave method, an artificial intelligence algorithm and a fault analysis method. The traveling wave method mainly detects the traveling wave head of the transient current and is assisted by a signal processing means. But the required sampling frequency is extremely high, the refraction and reflection at the connecting point of the mixed line are complex, and the wave head identification is difficult. The artificial intelligence algorithm has a complex processing process, depends on a large amount of simulation data for training, and has performance to be tested by actual engineering. The fault analysis method can be divided into a single-end fault location method and a double-end fault location method, the principle of the single-end fault location method is simple, and errors caused by the influence of opposite-side system information are difficult to avoid. The double-end distance measurement method comprises the steps of respectively calculating voltage distribution along the line from two ends of the line, determining fault distance measurement according to the fact that voltage amplitudes at fault points are equal, and establishing a fault area identification function at a connecting point of a mixed line, determining a fault section and then calculating the fault distance. The double-ended ranging method is more accurate and reliable, but the process is very complicated compared with the single-ended ranging method, and the requirements on data synchronization and communication technology are higher.
The flexible direct current access brings new fault characteristics to the alternating current hybrid line, and the control strategy configured by the converter can influence the alternating current output characteristics, so that the performance of the existing double-end fault distance measuring method is influenced. According to the low-voltage ride-through control strategy of the converter, the positive sequence voltage at the grid-connected point and the flowing positive sequence current are subjected to nonlinear time variation in the fault transient regulation process, and the precision of the fault distance measurement method based on the double-end positive sequence electrical quantity is influenced due to the fact that the fundamental frequency electrical quantity cannot be accurately extracted. The negative sequence current suppression strategy of the converter enables the negative sequence current output by the converter to be suppressed to be zero, so that the fault distance measurement method based on the double-end negative sequence electric quantity is invalid. Therefore, it is necessary to propose a fault location method that is suitable in consideration of the influence of the converter control strategy.
Disclosure of Invention
The invention designs a double-end fault location method aiming at an alternating current overhead line-cable mixed line of flexible direct current access. Compared with the traditional double-end fault distance measurement method, the method starts from a control strategy configured by a flexible direct current converter, utilizes the fault characteristic that a negative sequence network under the control strategy only comprises an alternating current power supply side, and the negative sequence current flowing on a power transmission line between a grid-connected point and a fault point of the converter is zero, compares the distribution of negative sequence voltage amplitude values along the line obtained by calculating the electric quantity information of the alternating current power supply side with the negative sequence voltage amplitude value collected at the grid-connected point of the converter, finds the minimum point of amplitude difference to determine the fault distance, and effectively improves the distance measurement precision. The technical scheme of the invention is as follows:
a double-end fault location method for an alternating current hybrid line with flexible direct current access comprises the following steps:
(1) after the fault of the alternating current overhead line-cable hybrid line is detected, voltage and current information of the alternating current power supply side of the line is collected, voltage information of a converter grid-connected point is collected, and negative sequence components of the voltage and the current are obtained.
(2) And based on the distribution parameter model, calculating the negative sequence components of the voltage and the current collected at the alternating current power supply side to obtain the negative sequence voltage distribution along the line.
(3) And the amplitude distribution of the negative sequence voltage along the line obtained by calculating the electric quantity information at the side of the alternating current power supply is compared with the amplitude of the negative sequence voltage collected at the grid-connected point of the converter, and the fault distance is determined through an amplitude difference function according to the characteristic that the amplitude difference between the two at the fault point is minimum.
Further, in the step (1), triangular window filtering processing is carried out on the sampling data window information, corresponding fundamental frequency quantity is extracted through Fourier transform, and then negative sequence components of voltage and current are obtained through transformation by a symmetric component method.
Further, in step (3), the amplitude difference function g (x) is as follows:
in the formula (I), the compound is shown in the specification,
representing the amplitude of a fault voltage negative sequence component at the position x away from the n end of the AC power supply side on the mixed line;
representing the m-terminal negative sequence voltage amplitude at the grid-connected point of the converter; n represents the number of calculations.
Compared with the prior art, the invention provides the flexible direct current access alternating current hybrid line double-end fault distance measurement method by considering the alternating current fault characteristic under the control strategy of the flexible direct current converter, effectively improves the distance measurement precision and has the following advantages:
(1) the influence of a converter control strategy is considered in the distance measurement principle, and based on the characteristic that negative sequence current flowing on a transmission line between a converter grid-connected point and a fault point is zero, single negative sequence voltage amplitude distribution along the line is calculated by only needing alternating current power supply side electric quantity information, so that a large amount of calculated quantity is saved, and calculation errors are reduced.
(2) The negative sequence voltage amplitude at the converter grid-connected point is transmitted to the distance measuring device at the alternating current power supply side through the communication device only once, so that the data transmission quantity is small, and the data synchronization problem does not exist.
(3) The applicability is wide. The method is suitable for single-phase earth faults, interphase short-circuit faults and three-phase short-circuit faults, and is suitable for non-mixed lines and multi-section mixed lines. In addition, the method is suitable for the alternating current transmission line accessed by the inverter type distributed power supply.
Drawings
Fig. 1 is a wiring diagram of an ac hybrid line for flexible dc access.
Fig. 2 is a single-phase earth fault composite sequence network diagram.
Fig. 3 is a flow chart of a double-ended fault location method.
Detailed Description
The invention is described in further detail below with reference to the figures and examples. Fig. 1 shows a typical wiring diagram of a flexible dc-switched ac hybrid line, in which a fault occurs on a three-section ac hybrid line mn composed of an overhead line, a cable and an overhead line. In the figure, xFRepresenting the distance from the fault point F to the n end of the alternating current power supply side of the mixed line;
and
the phase voltage and the current phasor of j collected by the end m of the converter grid-connected point and the end n of the alternating current power supply are represented respectively, and j is A, B and C. The invention provides a flexible direct current access alternating current hybrid line double-end fault location method which mainly comprises two parts of line negative sequence voltage amplitude calculation and location criterion.
1. Negative sequence voltage amplitude calculation along the line
The protection of the mixed line configuration carries out fault type judgment and fault phase selection after the fault is detected to occur. The voltage and current information of the n end of the alternating current power supply side shown in the figure 1 is collected, and the voltage information of the m end of the converter grid-connected point is collected for fault location. And (4) acquiring the electrical quantity information of a 40ms data window starting from the 2 nd period after the fault by considering that the transient state after the fault contains various harmonics and attenuated direct current components. And carrying out triangular window filtering processing on the information of the sampling data window, extracting corresponding 50Hz fundamental frequency quantity through Fourier transform, moving each data window for 20ms to obtain three groups of fundamental frequency quantity through the data window, carrying out multiple calculation for averaging in the subsequent fault location, and reducing calculation and sampling errors.
Electromagnetic induction exists among three phases of the power transmission line, accuracy in calculation of voltage distribution along the line is affected, and negative sequence components of voltage and current are obtained through conversion by a symmetric component method. The decoupling matrix T is shown below (1).
In the formula, a represents a rotational component,
and based on the distribution parameter model, calculating the negative sequence components of the voltage and the current collected at the alternating current power supply side to obtain the negative sequence voltage distribution along the line. For a single line, either overhead or cable, the negative sequence component of the voltage at x from the line start Y-end can be calculated from equation (2):
in the formula (I), the compound is shown in the specification,
representing a fault voltage negative sequence component at a position x away from a line starting end Y on the mixed line;
representing a voltage negative sequence component obtained after decoupling of the electric quantity at the Y end of the line starting end;
representing a current negative sequence component obtained after decoupling of the electric quantity at the Y end of the line starting end; zc2Representing the characteristic impedance of the line under the negative sequence component; gamma ray2Representing the propagation constant under the negative sequence component of the line.
The mixed line parameters are not uniform, and different line parameters are substituted for segment calculation when the negative sequence voltage distribution along the line is calculated. And substituting the negative sequence components of the voltage and the current collected at the n end of the alternating current power supply side as the initial end boundary into corresponding characteristic impedance and propagation constant parameters, taking the negative sequence components of the voltage and the current at the calculated connecting point as a new initial end boundary, modifying the line parameters, and continuing to carry out derivation calculation, thereby completing the calculation of the whole-line negative sequence voltage distribution.
It is worth mentioning that the single-phase earth fault and the interphase short-circuit fault can be directly converted into the negative sequence component, and the method for calculating the negative sequence voltage amplitude along the line can be directly used. For a three-phase short circuit fault, manual adding delay processing needs to be carried out on one phase data window. Corresponding time delay is added to the A-phase filtering data window of the alternating current power supply side, B, C two phases are unchanged, the three-phase short-circuit fault is artificially converted into the BC phase-to-phase short-circuit fault, a negative sequence component is manufactured, and the BC phase-to-phase short-circuit fault is converted into the phase-to-phase short-circuit fault.
2. Distance measurement criterion
Taking the example of the occurrence of a phase-a ground fault in a hybrid line, the principle of ranging will be briefly explained, and a composite sequence diagram of a single-phase ground fault is shown in fig. 2. The low voltage ride through control strategy of the converter enables the positive sequence voltage at the grid-connected point and the flowing positive sequence current to present nonlinear time variation in the fault transient regulation process, and in the positive sequence network of the composite sequence network, the converter is equivalent to a controlled positive sequence current source
Therefore, fundamental frequency electrical quantity at the grid-connected point of the converter cannot be accurately extracted, and the precision of the fault distance measurement method based on the double-end positive sequence electrical quantity is influenced. In the negative sequence network, the negative sequence current suppression strategy of the converter suppresses the output negative sequence current to be zero, which is embodied as infinite resistance RmInfinity, corresponds to an open circuit,
and carrying out fault distance measurement by using the fault characteristics. In the figure, the position of the upper end of the main shaft,
and
respectively representing fault sequence component currents flowing into a fault point from the m end and the n end of the hybrid line; zm1，Zm2，Zm0And Zn1，Zn2，Zn0Respectively representing equivalent sequence component impedance between the fault point and m and n ends of the hybrid line bus; zseq1，Zseq2，Zseq0Respectively representing equivalent sequence component impedance of an alternating current system behind the n ends of the buses of the hybrid line;
representing the equivalent voltage of the ac system behind the n-terminal of the hybrid line bus.
And comparing the negative sequence voltage amplitude along the line obtained by calculating the electric quantity information at the side of the alternating current power supply with the negative sequence voltage amplitude collected at the grid-connected point of the converter, and determining the fault distance by using an amplitude difference function, wherein the amplitude difference function G (x) is shown as the following formula (3).
In the formula (I), the compound is shown in the specification,
representing the amplitude of a fault voltage negative sequence component at the position x away from the n end of the AC power supply side on the mixed line;
representing the m-terminal negative sequence voltage amplitude at the grid-connected point of the converter; and N represents the calculation times, and three groups of fundamental frequency quantity obtained by sampling are calculated for three times. And calculating and sampling errors are reduced by repeating the calculation for three times and solving the average value.
According to the characteristic that the difference function of the two amplitude values at the fault point is minimum, fault distance measurement is realized through the distance measurement criterion of the following formula (4):
G(xF)＝min[G(x)] (4)
in the formula, xFIndicating the distance to failure.
In summary, a flowchart of the double-ended fault location method is shown in fig. 3.
Claims (3)
1. A double-end fault location method for an alternating current hybrid line with flexible direct current access comprises the following steps:
(1) after the fault of the alternating current overhead line-cable hybrid line is detected, voltage and current information of the alternating current power supply side of the line is collected, voltage information of a converter grid-connected point is collected, and negative sequence components of the voltage and the current are obtained.
(2) And based on the distribution parameter model, calculating the negative sequence components of the voltage and the current collected at the alternating current power supply side to obtain the negative sequence voltage distribution along the line.
(3) And the amplitude distribution of the negative sequence voltage along the line obtained by calculating the electric quantity information at the side of the alternating current power supply is compared with the amplitude of the negative sequence voltage collected at the grid-connected point of the converter, and the fault distance is determined through an amplitude difference function according to the characteristic that the amplitude difference between the two at the fault point is minimum.
2. The fault location method according to claim 1, wherein in step (1), triangular window filtering processing is performed on the sampled data window information, corresponding fundamental frequency quantity is extracted through Fourier transform, and then negative sequence components of voltage and current are obtained through symmetrical component method transformation.
3. The fault location method of claim 1, wherein in step (3), the amplitude difference function G (x) is given by:
in the formula (I), the compound is shown in the specification,
representing the amplitude of a fault voltage negative sequence component at the position x away from the n end of the AC power supply side on the mixed line;
representing the m-terminal negative sequence voltage amplitude at the grid-connected point of the converter; n represents the number of calculations.
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