CN110261713A - A method of diagnosis flexible direct current line commutation device exchange flanks earth fault - Google Patents

Abstract

The invention discloses a method for diagnosing the grounding fault of the AC side of a flexible DC power grid converter. A relay that can monitor the common-mode current and the common-mode voltage is respectively installed on the DC side of each AC/DC terminal of the pseudo-bipolar flexible DC network. For electrical protection devices, when the common-mode current is abnormally large, or there is a common-mode voltage, but the common-mode current is small, or there is a common-mode voltage, but the active power of the common-mode current voltage is negative, it is determined that the AC/DC terminal is AC A ground fault has occurred on the side. The present invention is applicable to a pseudo-bipolar flexible DC network including several AC/DC converters. When a ground fault occurs on the AC side of a certain AC/DC terminal, it can quickly determine the fault occurrence point and send a signal to inform the control system to cut off the terminal. The inverter ensures the safe and reliable operation of the flexible DC power grid.

Description

A method for diagnosing ground faults on the AC side of flexible DC grid converters

technical field

The invention relates to a relay protection method for a DC power grid, in particular to a method for diagnosing a grounding fault on the AC side of a flexible DC power grid converter.

Background technique

When a single-phase ground fault occurs on the AC side of any voltage source converter (AC/DC) in a multi-terminal flexible DC system, a common-mode voltage current will be generated on the DC side, and a zero-sequence voltage will be generated on the non-fault side, so it is necessary to quickly identify Fault end and isolate the fault end so as not to affect the safe operation of the entire DC grid.

In the three-phase AC ungrounded power grid, the zero-sequence voltage is usually used to detect whether a ground fault occurs, and the zero-sequence current is used to select the line. Existing literature has stated that zero-sequence current and voltage on the AC side will lead to common-mode current and voltage on the DC side.

However, since a single-phase ground fault occurs on the AC side of the voltage source converter at either end, the DC common-mode voltage current will pass through and generate a zero-sequence voltage at the non-fault end, so the zero-sequence voltage protection cannot determine which end of the converter has a ground fault . Since common-mode voltages will appear in the entire DC network, it is difficult to determine which end of the converter has a ground fault with a single common-mode voltage.

Contents of the invention

The purpose of the present invention is to provide a method for diagnosing the ground fault on the AC side of the flexible DC grid converter for the pseudo-bipolar flexible DC system. This solution only needs to collect the current and voltage of the DC side, and does not need to calculate the impedance angle. After the fault end is grounded, a signal is sent to inform the control system to cut off the converter at this end, so that the DC grid continues to operate safely and reliably.

In order to achieve the above purpose, the technical solution adopted in the present invention is: a method for diagnosing the ground fault of the AC side of the flexible direct current grid converter, the flexible direct current grid is a pseudo-bipolar flexible grid containing several AC/DC converters DC network, where each AC/DC converter is one end of the DC network, respectively monitor the common-mode current and common-mode voltage of the DC side of each AC/DC converter; if the i-th terminal AC/DC commutation The common-mode capacitance of the device is C _icom , and the vector of the fundamental common-mode current vector of the DC side at the i-th end is obtained through measurement and calculation: The fundamental common mode voltage vector is Among them, the positive direction of the current is from the DC bus to the AC/DC converter, then the following three criteria are used to diagnose whether a ground fault occurs on the AC/DC side of the i-th terminal,

Criterion 1: The common mode current is abnormally large, namely: and

Criterion 2: There is a common-mode voltage, but the common-mode current is small, that is: and

Criterion 3: There is a common-mode voltage, but the active power of the common-mode current and voltage is negative, that is and

Among them: k ₁ is the reliability coefficient of excess power, j represents the imaginary number ω is the fundamental angular frequency of the AC side, ε is the maximum error that may occur in common mode current measurement, u _zd is the common mode voltage amplitude that the DC power grid can withstand for a long time, k ₂ is the under-power reliability coefficient, Indicates the fundamental common mode active power;

If any of the above three criteria is established, it is diagnosed as a ground fault on the AC/DC side of the i-th end.

Further, the measurement calculation obtains that the fundamental common-mode current of the DC side at the i-th end is The fundamental common-mode voltage is Specifically: voltage sensors and current sensors are installed on the positive and negative poles of the DC output end of the AC/DC converter, and the polarity of the current sensor is positive from the DC bus to the AC/DC converter; the positive current i _p is obtained by sampling , _{negative current i n} , _positive voltage _{u p} , and _negative voltage _un , the positive and negative electrodes of the converter have a capacitance value of C to the ground, and the common-mode capacitance of this terminal is C _com =2C. The fundamental common mode voltage vector of this terminal can be obtained by Fourier algorithm and the fundamental common-mode current vector

Further, the value range of k ₁ is k ₁ >1; the value range of k ₂ is 0<k ₂ <1.

Further, u _zd takes 10% of the rated phase-to-phase voltage of the AC side of the AC/DC converter.

Further, ε takes 5% of the rated DC current of the AC/DC converter.

Compared with the prior art, the method for diagnosing the ground fault on the AC side of the flexible DC power grid converter described in the present invention has the beneficial effect that when searching for the ground fault point, it is only necessary to collect the current and voltage of the DC side without calculating the impedance If there is a ground fault at the local end, it can immediately send a signal to inform the local control system to remove the faulty converter, so that the DC power grid continues Safe and reliable operation.

Description of drawings

Fig. 1 is a schematic diagram of the installation of the protection device in the method for diagnosing the ground fault on the AC side of the flexible DC grid converter according to the present invention;

Fig. 2 is a logical block diagram of fault protection in the method for diagnosing the ground fault on the AC side of the flexible DC grid converter according to the present invention.

Detailed ways

The patent of the present invention will be further described below in conjunction with the accompanying drawings.

Refer to Figure 1 and Figure 2 for coordination. Firstly, as shown in Figure 1, the AC side of the AC/DC terminal in a flexible DC grid is connected to the AC grid through a transformer, and the DC side of the AC/DC terminal is connected to the DC bus. Several other DC lines are also connected to the DC bus. Connect the primary side of the voltage sensor to the positive and negative DC busbars and ground respectively, and connect the secondary side of the voltage sensor to the analog quantity acquisition module of the relay protection device; install the current sensor on the positive and negative poles of the AC/DC DC output terminal, and the secondary The side is connected to the analog quantity acquisition module of the relay protection device, and the polarity of the current sensor is positive from the DC bus to the AC/DC converter; the protection device can sample the positive current _{i p} , the negative current in , the positive voltage _up , the negative Voltage u _n . Through the calculation of the central processing unit, the common-mode voltage u _com =(u _p +u _n )/2 and the common-mode current i _com =( _{ip +i n )} /2 of the terminal can be obtained. The capacitance value is C, and the common mode capacitance of this terminal is C _com =2C. The fundamental common mode voltage vector of this terminal can be obtained by Fourier algorithm and the fundamental common-mode current vector

The block diagram of the protection logic for diagnosing the ground fault on the AC side of the flexible DC grid converter is shown in Figure 2. When there is no ground fault on the AC side of all AC/DC terminals of the flexible DC grid, there will be no common-mode voltage and common-mode current, and the common-mode voltage amplitude calculated by the relay protection and common-mode current magnitude Both will not be greater than the maximum measurement error, and will not be misjudged; when a ground fault occurs on the AC/DC side of the other end of the flexible DC grid, The measured fundamental active power is the loss on the capacitor, the direction is positive, and the three criteria will not be misjudged. When a ground fault occurs on the AC/DC side of the local end, it is equivalent to connecting a common-mode fundamental power supply at the local end, and the fundamental common-mode current measured at the local end is the DC current flowing to the entire DC grid. If the entire DC grid The equivalent common-mode impedance is small, and the amplitude of the common-mode current is large, that is far greater than The first criterion will be able to accurately determine that a ground fault has occurred on this side; if the equivalent common-mode impedance of the entire DC power grid is large, a large common-mode voltage will appear, namely: But the amplitude of the common mode current is small, that is much smaller than The second criterion can accurately determine that a ground fault has occurred on the local side; if the equivalent common-mode impedance of the entire DC grid is close to the capacitive impedance of the local side, there will be both a large common-mode voltage and a large common-mode voltage. Mode current, but the measured fundamental common mode active power is the loss of the DC grid, and the power direction is negative, that is: The third criterion will be able to accurately determine that a ground fault has occurred on this side.

Although the content of the present invention has been described in detail through the above preferred embodiments, it should be understood that the above description should not be considered as limiting the present invention. Various modifications and alterations to the present invention will become apparent to those skilled in the art upon reading the above disclosure. Therefore, the protection scope of the present invention should be defined by the appended claims.

Claims (5)

1. a kind of diagnosis flexible direct current line commutation device exchanges the method for flanking earth fault, which is characterized in that the flexible direct current Power grid is the pseudo- bipolar flexible DC network comprising several AC/DC inverters, wherein each AC/DC inverter is direct current net One end of network, monitors the common mode current and common-mode voltage of the DC side of each AC/DC inverter respectively；If the i-th end AC/DC The common mode capacitance of inverter is C_icom, the DC side fundamental wave common mode current vector for obtaining the i-th end through survey calculation isFundamental wave Common-mode voltage vector isWherein, electric current positive direction is to be directed toward AC/DC inverter by DC bus, then passes through following three Criterion diagnoses whether the i-th end AC/DC exchange side occurs ground fault,

Criterion one: common mode current is extremely big, it may be assumed thatAnd

Criterion two: there are common-mode voltages, but common mode current is less than normal, it may be assumed thatAnd

Criterion three: there are the active power of common-mode voltage, but common mode current voltage to be negative, i.e.,And

Wherein: k₁To cross electricity safety factor, j indicates that imaginary number ω is exchange side group angular frequency, and ε is that common mode current measurement may The worst error of appearance, u_zdFor the common-mode voltage amplitude that DC grid can be born for a long time, k₂To owe electricity safety factor,Indicate fundamental wave common mode active power；

If any one of three above criterion is set up, all it is diagnosed as the i-th end AC/DC exchange side and ground fault occurs.

2. diagnosis flexible direct current line commutation device exchanges the method for flanking earth fault as described in claim 1, which is characterized in that institute It states survey calculation and obtains the DC side fundamental wave common mode current at the i-th end and beFundamental wave common-mode voltage isSpecifically: in AC/ Voltage sensor, current sensor are installed on the positive and negative electrode of the DC output end of DC inverter, current sensor polarity is served as reasons DC bus is directed toward AC/DC converter and is positive；Sampling obtains positive electrode current i_p, cathodal current i_n, cathode voltage u_p, cathode voltage u_n, then the common-mode voltage u at the end_com=(u_p+u_n)/2, common mode current i_com=(i_p+i_n)/2, the inverter positive and negative anodes direct-to-ground capacitance Value is C, then the common mode capacitance at the end is C_com=2C.Pass through the fundamental wave common-mode voltage vector at the available end of Fourier algorithmAnd fundamental wave common mode current vector

3. diagnosis flexible direct current line commutation device exchanges the method for flanking earth fault as described in claim 1, which is characterized in that k₁ Value range is k₁>1；k₂Value range is 0 < k₂<1。

4. diagnosis flexible direct current line commutation device exchanges the method for flanking earth fault as described in claim 1, which is characterized in that u_zd Take the 10% of the exchange specified voltage between phases in side of AC/DC inverter.

5. diagnosis flexible direct current line commutation device exchanges the method for flanking earth fault as described in claim 1, which is characterized in that ε Take the 5% of AC/DC inverter rated direct current.