DC distribution network fault line selection method based on AC-DC converter VSC signal injection

Abstract

The invention provides a DC distribution network fault line selection method based on AC-DC converter VSC signal injection, and the method comprises the steps: superposing a group of symmetrical harmonic sinusoidal signals uj in a three-phase modulation wave generated by a converter VSC on the basis of the original double-closed-loop control of the converter VSC, and carrying out the superposition of a group of symmetrical harmonic sinusoidal signals uj on the basis of guaranteeing that the modulation ratio is smaller than 1. SPWM modulation is carried out on a composite signal formed by the three-phase modulation wave and the harmonic sinusoidal signal uj, so that required harmonic signals, namely signals required to be acquired for active fault positioning, are contained in the output voltage and current of the converter VSC; fault line selection is carried out by identifying that the phase of the fault feeder zero-mode current is opposite to the phase of the sound feeder zero-mode current; by applying the technical scheme, the sensitivity of fault detection can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of DC distribution network fault detection, and in particular to a DC distribution network fault line selection method based on VSC signal injection. Background Art

[0002] The fault location principle of the traditional fault analysis method is affected by factors such as transition resistance, long-line distributed capacitance, differential term error, frequency-varying parameters, etc., and the accuracy is not high. The fault location principle of the fault analysis method combined with the exploration of the high controllability of the converter is currently being explored in the industry. The present invention targets the limitations of the fault analysis method and combines the high controllability of the converter to study the principle of DC line fault selection. Summary of the invention

[0003] In view of this, an object of the present invention is to provide a DC distribution network fault line selection method based on VSC signal injection to improve the sensitivity of fault detection.

[0004] To achieve the above object, the present invention adopts the following technical solution: a DC distribution network fault line selection method based on AC / DC converter VSC signal injection, on the basis of the original double closed-loop control of the converter VSC, a group of symmetrical harmonic sinusoidal signals u are superimposed on the three-phase modulation wave generated by the converter VSC j , on the basis of ensuring that the modulation ratio is less than 1, the three-phase modulation wave and the harmonic sinusoidal signal u j The formed composite signal is subjected to SPWM modulation to ensure that the output voltage and current of the converter VSC contain the required harmonic signals, that is, the signals required to be collected for active fault location; the fault line is selected by identifying that the zero-mode current of the faulty feeder is in opposite phase to the zero-mode current of the sound feeder.

[0005] In a preferred embodiment, a global harmonic sinusoidal signal u is injected. j Active detection is performed in a manner similar to that of

[0006] In a preferred embodiment, the composite modulated wave is

[0007]

[0008] Among them, A and A' are the amplitudes of the original modulation wave and the additional modulation wave respectively; ω and ω' are the angular frequencies of the original modulation wave and the additional modulation wave respectively.

[0009] In a preferred embodiment, the harmonic sinusoidal signal u is actively injected j The frequency is lower than the resonant frequency of the entire low voltage DC distribution system.

[0010] In a preferred embodiment, the voltage and current at the protection installation are decoupled;

[0011]

[0012] In the formula, the subscripts 1 and 0 represent the line mode and zero mode components of voltage and current respectively; the subscripts p and n represent the positive and negative poles respectively.

[0013] Compared with the prior art, the present invention has the following beneficial effects: when a ground fault occurs on the low-voltage side of the converter, since the system uses a grounding resistor to limit the current, the system can continue to operate for a period of time in the event of a fault. Therefore, a signal of a specific frequency can be injected into the system by changing the control strategy of the controller, and the characteristic signal will detect the fault through the fault time period. A harmonic sinusoidal signal is injected into the sinusoidal modulation signal to improve the output capacity of the converter and perform circulating current suppression. Because the converter output is linear, that is, when a fixed-frequency harmonic is injected, a fixed-frequency harmonic will definitely be output, which provides a basis for fault determination using harmonic injection. The frequency of the harmonic sinusoidal signal is lower than the resonant frequency of the entire low-voltage DC power distribution system to prevent the characteristic signal from causing new resonance in the system, destroying the stability of the system, and further expanding the fault. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 A schematic diagram of an active injection additional control strategy according to a preferred embodiment of the present invention;

[0015] Figure 2 This is a harmonic analysis diagram of the AC side grid-connected current of a preferred embodiment of the present invention;

[0016] Figure 3 This is a schematic diagram of the DC bus voltage after harmonic injection according to a preferred embodiment of the present invention;

[0017] Figure 4 This is a schematic diagram of a fault equivalent circuit of a DC power distribution network under an injection frequency according to a preferred embodiment of the present invention;

[0018] Figure 5 A schematic diagram of a fault zero-mode equivalent circuit of a preferred embodiment of the present invention;

[0019] Figure 6 A schematic diagram of a voltage modulated wave signal after superimposing an injection signal according to a preferred embodiment of the present invention;

[0020] Figure 7 Schematic diagram of original waveform of zero-mode current of each feeder line according to a preferred embodiment of the present invention. DETAILED DESCRIPTION

[0021] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0022] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0023] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application; as used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprise" and / or "include" are used in this specification, they indicate the presence of features, steps, operations, devices, components and / or their combinations.

[0024] A DC distribution network fault line selection method based on VSC signal injection, reference Figure 1-7 On the basis of the original double closed-loop control of the converter VSC, a set of symmetrical harmonic sinusoidal signals u is superimposed on the three-phase modulation wave generated by the converter VSC. j , on the basis of ensuring that the modulation ratio is less than 1, the three-phase modulation wave and the harmonic sinusoidal signal u j The composite signal is modulated by SPWM to achieve the output voltage and current of the converter VSC containing the required harmonic signal, that is, the signal required for active fault location. When the converter VSC is locked, the actively injected harmonic sinusoidal signal u j It cannot be output through the converter VSC, and at the moment of locking, the harmonic sinusoidal signal u j It will remain in the output voltage and current of the converter VSC and last for a period of time, and is used for fault line selection signal acquisition; the fault line selection is performed by identifying that the zero-mode current of the faulty feeder is in opposite phase to the zero-mode current of the healthy feeder.

[0025] Depend on Figure 1 It can be seen that the modulated wave after compounding is

[0026]

[0027] Among them, A and A' are the amplitudes of the original modulation wave and the additional modulation wave respectively; ω and ω' are the angular frequencies of the original modulation wave and the additional modulation wave respectively.

[0028] The basic principle of active harmonic signal frequency selection is to make the fault response characteristics as obvious as possible under the premise of equipment performance constraints, thereby achieving more sensitive detection. The frequency should be lower than the resonant frequency of the entire low-voltage DC distribution system to prevent the characteristic signal from causing new resonance in the system, destroying the stability of the system and further expanding the fault.

[0029] The present invention uses Z iP represents the equivalent impedance of the positive pole of the i-th feeder to the ground, ZiN represents the equivalent impedance of the negative pole of the ith feeder to the ground, Z sP , Z sN Then it is the equivalent impedance to ground after the other branch feeders are combined, U ctrl Converter and AC system equivalent controlled source, and It is the voltage of positive and negative busbar to ground.

[0030] Due to the coupling effect between the positive and negative DC lines, the voltage and current at the protection installation must be decoupled first.

[0031]

[0032]

[0033] In the formula, the subscripts 1 and 0 represent the line mode and zero mode components of voltage and current respectively; the subscripts p and n represent the positive and negative poles respectively.

[0034] Since the zero-mode current characteristics at each monitoring point distributed along the line are uniform within the characteristic frequency band and are easy to analyze, the zero-mode current is selected for analysis.

[0035] In order to ensure the rapidity of fault line selection, the present invention adopts the method of global harmonic injection for active detection, and abandons the method of starting the injection strategy after the fault occurs. In order to verify the rationality of the proposed fault location method, it is necessary to detect whether the power quality is qualified under the condition of harmonic injection. Since the grid-connected system is connected to the AC power grid, the AC side voltage is limited to the grid voltage, so it is only necessary to verify whether the AC current and DC voltage meet the power quality.

[0036] Figure 2-3 It can be seen that under the control strategy and harmonic injection range designed by the present invention, after filtering on the AC side, the harmonic THD of the grid-connected point current is less than 3%, which meets the AC power quality. The allowable deviation range of the voltage of the low-voltage DC distribution system below 1.5kV is -20% to +5%. The figure shows that although the DC bus voltage fluctuates compared to when no harmonics are injected, it is still below 5%, and the DC side power quality is also satisfied. Therefore, the global strategy of injecting additional harmonics designed by the present invention is feasible.

[0037] The equivalent circuit of a single-pole grounding fault on a feeder is as follows: Figure 4 As shown, Z sLk is the line series impedance of the kth feeder, Z sLk =R sLk +jX sLk , R sLk , X sLk Z sLk Corresponding resistance and reactance, k = 1, 2, ..., n; ZpLk is the line-to-ground impedance of the kth feeder, Z pLk =R pLk +jX pLk , R pLk , X pLk Z pLk The corresponding resistance and reactance; Z loadk is the load connected to the kth feeder. Assume that the nth feeder fails, R f is the transition resistance of the fault branch.

[0038] By decoupling with the help of Karrenbauer transformation, we can obtain Figure 4 The corresponding fault zero-mode equivalent circuit is as follows: Figure 5 As shown, Z s0Lk is the zero-mode series impedance of the kth feeder, Z p0Lk is the zero-mode impedance of the kth feeder to the ground branch, is the calculated value of zero-mode current at the head end of the kth feeder, is the calculated value of zero-mode current at the head end of fault feeder n, It is the calculated value of zero-mode current at the first end of the converter.

[0039] Depend on Figure 5 It can be seen that the calculated zero-mode current flows out from the faulty feeder and flows into the healthy feeder and the grounding branch of the converter. For the healthy feeder,

[0040]

[0041] For the faulty feeder,

[0042]

[0043] From formula (4), we can see that the bus zero mode voltage The phase of is the reference phase, and the zero-mode current phase calculated at the head end of the healthy feeder is the impedance angle of the sum of the zero-mode series impedance of the line and the impedance of the ground branch. Therefore, the zero-mode current phase calculated for each healthy feeder is the same. It is much smaller than the calculated zero-mode current of each feeder. Therefore, it can be seen from equation (5) that the calculated zero-mode current of the faulty feeder is opposite to that of the healthy feeder.

[0044] Based on this, the ground fault line selection can be completed according to the characteristics that the zero-mode current calculated at the head end of each healthy feeder is the same in phase, but opposite in phase to the zero-mode current calculated at the head end of the faulty feeder. The present invention processes the current of each feeder and collects the positive and negative current i at the head end of the kth feeder. Pk 、i Nk , which is used to identify the faulty feeder. The measured feeder current is filtered using a bandpass filter, and ω is set inis the specific frequency of the detection signal, then the positive and negative currents at the head end of the kth feeder after filtering are i Pk (ω in ),i Nk (ω in ). Then, the Karrenbauer transform is used to obtain the zero-mode component of each feeder line on the filtered current time domain signal.

[0045]

[0046] Build a 750V DC distribution network model in MATLAB / Simulink, with 3 feeder lines and an active injection signal frequency ω in For 150Hz, the transition resistance R f Take 0.1Ω.

[0047] Figure 6 It is the voltage modulated wave signal after the injection signal is superimposed.

[0048] Assume that a positive ground fault occurs at the head end of feeder 1. The simulation results are as follows: Figure 7 shown.

[0049] Depend on Figure 7 It can be seen that when no single-pole grounding fault occurs in the three feeders, the calculated values ​​of the zero-mode current are all zero; when a single-pole grounding fault occurs in feeder 1 at 0.5s, the phase of its feeder zero-mode current is opposite to the zero-mode current phase of the healthy feeders (feeder 2 and feeder 3), which verifies the fault line selection basis proposed in the present invention.

[0050] The present invention provides a line selection method for a single-pole grounding fault in a low-voltage DC distribution network. The method utilizes the following fault characteristics: the zero-mode current phase of the faulty feeder is opposite to that of the sound feeder. A line selection criterion is designed based on this fault characteristic, and the effectiveness of the proposed criterion is verified by simulation results. The method can identify the occurrence of a single-pole grounding fault in each feeder.

Claims

1. A DC distribution network fault line selection method based on VSC signal injection, characterized in that: On the basis of the original double closed-loop control of the converter VSC, a set of symmetrical harmonic sinusoidal signals u is superimposed on the three-phase modulation wave generated by the converter VSC. j , on the basis of ensuring that the modulation ratio is less than 1, the three-phase modulation wave and the harmonic sinusoidal signal u j The formed composite signal is subjected to SPWM modulation to ensure that the output voltage and current of the converter VSC contain the required harmonic signals, that is, the signals required to be collected for active fault location; the fault line is selected by identifying that the zero-mode current of the faulty feeder is in opposite phase to the zero-mode current of the sound feeder.

2. The DC distribution network fault line selection method based on VSC signal injection according to claim 1 is characterized in that: Using global injection of harmonic sinusoidal signal u j Active detection is performed in a manner similar to that of 3. The DC distribution network fault line selection method based on VSC signal injection according to claim 1 is characterized in that: The modulated wave after compounding is Among them, A and A' are the amplitudes of the original modulation wave and the additional modulation wave respectively; ω and ω' are the angular frequencies of the original modulation wave and the additional modulation wave respectively.

4. The DC distribution network fault line selection method based on VSC signal injection according to claim 1, characterized in that: Active injection of harmonic sinusoidal signal u j The frequency is lower than the resonant frequency of the entire low voltage DC distribution system.

5. The DC distribution network fault line selection method based on VSC signal injection according to claim 1, characterized in that: Decouple the voltage and current at the protection installation location; In the formula, the subscripts 1 and 0 represent the line mode and zero mode components of voltage and current respectively; the subscripts p and n represent the positive and negative poles respectively.

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