WO2019037333A1

WO2019037333A1 - Rapid fault recognition method based on curvature change of direct-current current waveform

Info

Publication number: WO2019037333A1
Application number: PCT/CN2017/115155
Authority: WO
Inventors: 贾科; 王聪博; 毕天姝; 李猛; 李晨曦; 宣振文; 冯涛; 赵琪娟
Original assignee: 华北电力大学
Priority date: 2017-08-22
Filing date: 2017-12-08
Publication date: 2019-02-28
Also published as: CN107677931A; CN107677931B

Abstract

A rapid fault recognition method based on a curvature change of a direct-current current waveform, comprising: for a multi-end flexible direct-current power distribution system containing a power electronic converter, according to an electrical quantity characteristic in each phase after the occurrence of a direct-current bipolar short circuit fault, obtaining a mathematical expression of a fault current after the direct-current line fault (1); according to the obtained mathematical expression of the fault current, recognising a fault line utilising the difference in the change of the curvature of a current waveform after the fault by means of a curvature algorithm (2); and then according to the change conditions of a direct-current change rate di/dt, selecting a line switching on/off action, to realise fault protection of the line (3). In the method, based on a sudden change characteristic of a line current after a fault, the sudden change of the curvature of a current waveform is characterised by a curvature algorithm, achieving the aim of recognising an external fault, and improving the selectivity of protection for a sudden current change quantity.

Description

A Fast Fault Identification Method Based on Difference of Curvature Change of DC Current Waveform

This application claims the priority of the Chinese Patent Application filed on August 22, 2017, the Chinese Patent Office, Application No. 201710724344.0, entitled "A Fast Fault Identification Method Based on the Difference of Curvature Variation of DC Current Waveforms", the entire contents of which is hereby incorporated by reference. This is incorporated herein by reference.

Technical field

The invention relates to the technical field of a DC distribution network, and in particular relates to a fast fault identification method based on a difference in curvature variation of a DC current waveform.

Background of the invention

With the rapid development of distributed generation devices and power electronics technology, the future development direction of smart grids is mainly concentrated in distribution networks, especially DC distribution networks to effectively accept distributed power supplies, efficient and stable voltage conversion and control, and system optimization. The characteristics of high power quality, strong power supply reliability and good economy have been widely concerned by scholars at home and abroad. As one of the key technologies of the system, the protection technology in the DC distribution network, including the fast and reliable identification technology of DC fault, is in the stage of theoretical research and experimental exploration.

Compared with the AC distribution network, DC distribution network protection is difficult to apply the protection method of the AC distribution network, mainly due to its fault type, fault development process, fault voltage and current characteristics, and fault consequences. Among them, the biggest problem for the realization of DC distribution network protection is the fastness and selectivity of protection. When any DC line in the system fails, overcurrent occurs in all DC lines. Because the overcurrent capability of power electronic devices is relatively weak, it is necessary to quickly identify DC faults within 2-5ms to prevent equipment damage. The use of overcurrent protection to achieve fault identification requires DC breakers to operate quickly within 2-5ms, which is difficult. Distance protection puts higher demands on precise distance measurement. The research on the flexible DC distribution network in the prior art mainly focuses on the system control design, etc., which is limited to simple qualitative simulation, and the research is less, and the protection principles proposed in the existing literature mostly pass the harmonic of the component frequency band in the DC system. The wave current is used for fault identification in the area and outside, the action time is 20ms (50Hz), and the electric current at both ends is required to calculate the line current separately and construct the action criterion, resulting in large protection calculation, complicated setting, no rapidity, and practical engineering. Poor sex.

Summary of the invention

The object of the present invention is to provide a fast fault identification method based on the difference of curvature variation of a direct current waveform, which is based on the abrupt characteristic of the line current after the fault, and the curvature algorithm is used to characterize the sudden change of the curvature of the current waveform to achieve the fault outside the identification area. The purpose is to increase the selectivity of current mutation protection.

A fast fault identification method based on a difference in curvature variation of a DC current waveform, the method comprising:

Step 1. For the multi-terminal flexible DC power distribution system with power electronic converter, obtain the mathematical expression of the fault current after the DC line fault according to the electrical quantity characteristics of each stage after the DC bipolar short-circuit fault occurs;

Step 2: According to the obtained mathematical expression of the fault current, the fault line is identified by the curvature algorithm by using the difference in curvature of the current waveform after the fault;

Step 3: According to the change of the DC current change rate di/dt, the line switch action is selected to realize the line fault protection.

In the step 1, the process of obtaining the mathematical expression of the fault current after the DC line fault is:

Let the initial value of the capacitor voltage be V ₀ , the initial value of the inductor current be I ₀ , the DC resistance of the reactor, the series impedance of the capacitor, and the contact resistance of the metal components of the discharge circuit are uniformly represented by R _Stray , then the capacitor voltage after the fault is solved by the second order. The differential equation is obtained:

In the formula,

The time constant for the decay of the discharge current;

Is the natural angular frequency of the discharge circuit;

The angular frequency of the oscillating discharge current;

The initial phase angle of the discharge current caused by the initial current;

The mathematical expression of the fault current after further DC line fault is:

Where U _dc is the sum of the capacitor voltages of the bridge arm input submodules, C ₀ is the initial value of the submodule capacitor, n is the number of submodules, σ is the time constant of the discharge current decay; ω is the angular frequency of the oscillating discharge current ; α is the initial phase angle of the discharge current caused by the initial current.

In the step 2,

First, the obtained mathematical expression of the fault current is differentiated, and the rate of change of the fault component current is expressed as:

Further, the mathematical expression of the fault current is brought into the curvature formula, and the degree of bending of the direct current at the initial time of the fault is calculated as:

The degree of bending is related to the equivalent resistance and reactance of the DC power distribution system, and can simultaneously reflect the physical characteristics of the resistor R and the inductor L, and then identify the faulty line by the curvature algorithm.

The specific process of step 3 is as follows:

If the system is running stably, the DC current rate of change di/dt remains at zero;

In the event of a fault, if the calculated di/dt value is greater than the local line fault setting value Imax, then both converters are blocked and the local line switch is activated;

If the di/dt is started and the Imax value is not reached, the fault circuit is identified by the curvature algorithm of the step 2, and then the protection device selectively blocks the converter station according to the calculation result, and finally the switching operation.

It can be seen from the technical solution provided by the above invention that the above method is based on the abrupt characteristic of the line current after the fault, and the curvature algorithm is used to characterize the sudden change of the curvature of the current waveform, thereby achieving the purpose of identifying the fault outside the region and improving the protection of the current sudden change. Selectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly described below. It is obvious that the drawings in the following description are only some embodiments of the present invention, Those of ordinary skill in the art will be able to obtain other figures from these drawings without the inventive effort.

1 is a schematic flow chart of a method for quickly identifying faults based on a difference in curvature variation of a DC current waveform according to an embodiment of the present invention;

2 is a schematic structural diagram of a multi-terminal DC power distribution system according to an embodiment of the present invention;

3 is a simplified equivalent circuit diagram of a fault state according to an embodiment of the present invention;

4 is a schematic diagram of geometric characterization of fault current according to an embodiment of the present invention;

5 is a schematic diagram of simulation of a DC current measured after a DC bipolar short circuit occurs at each fault point according to an embodiment of the present invention;

FIG. 6 is a schematic diagram of corresponding sampling points according to an embodiment of the present invention; FIG.

7 is a schematic diagram of simulation results of current sudden amount protection according to an embodiment of the present invention;

FIG. 8 is a schematic diagram of simulation results of current waveform curvature protection according to an embodiment of the present invention; FIG.

FIG. 9 is a schematic diagram showing simulation results of current waveform curvature protection according to an embodiment of the present invention; FIG.

FIG. 10 is a schematic diagram showing simulation results of current waveform curvature (including noise) according to an embodiment of the present invention.

Mode for carrying out the invention

The technical solutions in the embodiments of the present invention are clearly and completely described in the following with reference to the drawings in the embodiments of the present invention. It is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present invention without creative efforts are within the scope of the present invention.

The embodiments of the present invention are further described in detail below with reference to the accompanying drawings. FIG. 1 is a schematic flowchart of a fast fault identification method based on a difference in curvature variation of a DC current waveform according to an embodiment of the present invention, where the method includes:

In this step, FIG. 2 is a schematic structural diagram of a multi-terminal DC power distribution system according to an embodiment of the present invention, and FIG. 2 is marked with a fault point position. When a DC line has a bipolar short-circuit fault, the CDSM-MMC The series capacitor firstly discharges to the fault point through the DC line, causing the module capacitor voltage to drop, the DC current to rise, the valve level device will be locked after 2-5ms after the fault, and the fault current contains the submodule capacitor discharge current and before the inverter is blocked. The AC system feeds current. The fault sub-module is still running normally, and the bridge arm sub-module capacitors are alternately discharged in sequence. Under the premise of neglecting the inverter control strategy and the pre-fault load current, the fault state can be equivalent to the normal running state and fault attachment according to the superposition principle. The superposition of states, so that the fault-added state simplified equivalent circuit can be obtained. FIG. 3 is a simplified equivalent circuit diagram of the fault state according to the embodiment of the present invention. The resistance, reactance, and capacitance in FIG. 3 are all associated with the reference direction. .

Since the fault instantaneous DC voltage U _dc is the sum of the capacitor voltages of the bridge arm input sub-modules, and in the actual system, the capacitor discharge process is a second-order oscillating circuit. In this embodiment, the initial value of the capacitor voltage is V ₀ , and the initial value of the inductor current is obtained. For I ₀ , the DC impedance of the reactor, the series impedance of the capacitor, and the contact resistance of the metal components of the discharge circuit are uniformly represented by R _Stray . The capacitor voltage after the fault can be obtained by solving the second-order differential equation:

In the formula:

The time constant for the decay of the discharge current;

Is the natural angular frequency of the discharge circuit;

The angular frequency of the oscillating discharge current;

The initial phase angle of the discharge current caused by the initial current. In general,

Far less than

It can be considered that ω = ω ₀ .

Where: U _dc is the sum of the capacitor voltages of the bridge arm input submodules, C ₀ is the initial value of the submodule capacitor, n is the number of submodules, σ is the time constant of the discharge current decay; ω is the angular frequency of the oscillating discharge current ; α is the initial phase angle of the discharge current caused by the initial current.

Step 2: According to the obtained mathematical expression of the fault current, using the curvature difference of the curvature of the current waveform after the fault, the fault circuit is identified by the curvature algorithm;

In this step, the DC current rapidly changes (rises) when a DC bipolar short-circuit fault occurs on the DC line. Amplifying the fault point, as shown in FIG. 4 is a schematic diagram of the geometric representation of the fault current according to the embodiment of the present invention. Referring to FIG. 4: the change pattern of the DC current after the fault (10 Hz sampling rate) is the same as the geometric property defined by the curvature in the mathematics. That is: from the perspective of geometric properties, when the DC system is in normal operation, the DC current remains in a straight line. When a fault occurs, the DC current changes and the straight line bends. From the perspective of physical properties, the current sudden change is protected. The current differential (di/dt) is defined to express the amount of change in the direct current. From the principle of curvature, the curvature is also defined by differential, that is, the amount of change in the direct current can also be expressed.

Through the above analysis, the obtained mathematical expression of the fault current is first differentiated, and the rate of change of the fault component current (the rate of change of the fault current is the same as the rate of change of the fault component current) is expressed as:

Among them, the inductor current is much smaller than the capacitor discharge current, so the inductor current can be neglected. Therefore, it can be considered that the fault current is mainly composed of the capacitor discharge current, and the rising rate of the fault current is related to the equivalent reactance of the system, that is, the physics of the inductor L is reflected. characteristic.

Further, the fault current expression is brought into the curvature formula to calculate the degree of bending of the direct current at the initial fault:

In this step, the specific process is:

Theoretically, if the system is operating stably, the DC current rate of change di/dt remains zero (di/dt = 0) without considering the power transfer;

If the calculation of di/dt is started and the Imax value is not reached, the fault circuit is identified by the curvature algorithm of step 2, and then according to the calculation result, the protection device selectively blocks the converter station and finally switches.

The following method is used to verify the above method with specific simulation examples. Considering the protection action speed and the actual device hardware level, the sampling rate is set to 10 kHz. This simulation example is based on the protection of the fault point 11 in Figure 2, and selects the fault before, After 1ms as a data window, the sampling points are directly intercepted for protection criteria.

FIG. 5 is a schematic diagram of simulation of DC current measured after DC bipolar short circuit of each fault point in the embodiment of the present invention, the fault time is 0.8s, and the position includes the near end (F1) and the far end of the area respectively. F2), the outer end of the area (F3), as shown in Figure 6, is the corresponding sampling point diagram.

FIG. 7 is a schematic diagram showing simulation results of current sudden-change protection according to an embodiment of the present invention. It can be seen from FIG. 7 that the current variation amount and F1 (near-end) when a F1 (near-end) DC double-pole short-circuit fault occurs. The amount of current change during the DC bipolar high-impedance ground fault is basically the same, and the theoretical analysis of step 2 is verified, that is, the di/dt protection is not affected by the resistance. When a DC bipolar short-circuit fault occurs at F2 (remote), the current change amount is smaller than the F1 (near-end) DC bipolar short-circuit fault regardless of whether the fault point is high-resistance grounding. The reason is that the fault points F1 and F2 The presence of inductance in the DC line further verifies the authenticity and accuracy of the analysis in step 2. When an out-of-zone (F3) fault occurs, when a DC bipolar short-circuit fault occurs in F3 (outside-area), the current waveform curvature can be used to clearly distinguish the outer fault. As shown in FIG. 8, the current is the current according to the embodiment of the present invention. A schematic diagram of the simulation results of the waveform curvature protection, from FIG. 8 can verify the accuracy of the protection method proposed by the present invention and the reliability of the current waveform curvature algorithm.

The current waveform curvature protection simulation comparison result, when the DC bipolar short circuit fault occurs in F5 and F3 in FIG. 2, FIG. 9 is a schematic diagram showing the simulation result comparison of the current waveform curvature protection according to the embodiment of the present invention, from FIG. 9 It can be seen that the algorithm can distinguish the out-of-zone faulty lines in the multi-terminal DC distribution network by the magnitude of k. At the same time, in order to distinguish between F4 (outside the end) and F2 (distal in the area), the protection 11 can correctly identify the fault in the zone or outside the zone, and an appropriate delay can be used.

In addition, white noise with a signal-to-noise ratio of 40dB can be added to the simulation to test the noise interference for the proposed protection. The effect, as shown in FIG. 10 is a schematic diagram of the simulation result of the current waveform curvature (including noise) exemplified in the embodiment of the present invention, and the current waveform curvature algorithm provided by the present invention can still be correctly calculated.

It can be seen from the above simulation results that the fast fault identification method based on the difference of the curvature variation of the DC current waveform provided by the embodiment of the present invention is based on the abrupt characteristic of the line current after the fault, and the curvature algorithm is used to characterize the sudden change of the curvature of the current waveform to reach the identification area. The purpose of the external fault is to increase the selectivity of the current sudden change protection. The advantage of this method is that the action speed is fast, it can move quickly about 1ms after the fault, and has better selectivity, which can effectively identify the fault outside the zone, and has no special requirements on the sampling rate of the system, strong anti-noise ability and calculation amount. Small and does not require communication.

The above is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily think of changes or within the technical scope of the present disclosure. Alternatives are intended to be covered by the scope of the present invention. Therefore, the scope of the invention should be determined by the scope of the claims.

Claims

A fast fault identification method based on a difference in curvature of a DC current waveform, wherein the method comprises:

Step 1. For the multi-terminal flexible DC power distribution system with power electronic converter, obtain the mathematical expression of the fault current after the DC line fault according to the electrical quantity characteristics of each stage after the DC bipolar short-circuit fault occurs;

Step 2: According to the obtained mathematical expression of the fault current, the fault line is identified by the curvature algorithm by using the difference in curvature of the current waveform after the fault;

Step 3: According to the change of the DC current change rate di/dt, the line switch action is selected to realize the line fault protection.
The method of claim 1 wherein in said step 1, the process of obtaining a mathematical expression of the fault current after a DC line fault is:

Let the initial value of the capacitor voltage be V 0 , the initial value of the inductor current be I 0 , the DC resistance of the reactor, the series impedance of the capacitor, and the contact resistance of the metal components of the discharge circuit are uniformly represented by R Stray , then the capacitor voltage after the fault is solved by the second order. The differential equation is obtained:

In the formula,
The time constant for the decay of the discharge current;
Is the natural angular frequency of the discharge circuit;
The angular frequency of the oscillating discharge current;
The initial phase angle of the discharge current caused by the initial current;

The mathematical expression of the fault current after further DC line fault is:

Where U dc is the sum of the capacitor voltages of the bridge arm input submodules, C 0 is the initial value of the submodule capacitor, n is the number of submodules, σ is the time constant of the discharge current decay; ω is the angular frequency of the oscillating discharge current ; α is the initial phase angle of the discharge current caused by the initial current.
The method of claim 1 wherein in said step 2

First, the obtained mathematical expression of the fault current is differentiated, and the rate of change of the fault component current is expressed as:

Further, the mathematical expression of the fault current is brought into the curvature formula, and the degree of bending of the direct current at the initial time of the fault is calculated as:

The degree of bending is related to the equivalent resistance and reactance of the DC power distribution system, and can simultaneously reflect the physical characteristics of the resistor R and the inductor L, and then identify the faulty line by the curvature algorithm.
The method according to claim 1, wherein the specific process of step 3 is:

If the system is running stably, the DC current rate of change di/dt remains at zero;

In the event of a fault, if the calculated di/dt value is greater than the local line fault setting value Imax, then both converters are blocked and the local line switch is activated;

If the di/dt is started and the Imax value is not reached, the fault circuit is identified by the curvature algorithm of the step 2, and then the protection device selectively blocks the converter station according to the calculation result, and finally the switching operation.