CN110261713B - Method for diagnosing AC side ground fault of converter of flexible DC power grid - Google Patents

Method for diagnosing AC side ground fault of converter of flexible DC power grid Download PDF

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CN110261713B
CN110261713B CN201910367307.8A CN201910367307A CN110261713B CN 110261713 B CN110261713 B CN 110261713B CN 201910367307 A CN201910367307 A CN 201910367307A CN 110261713 B CN110261713 B CN 110261713B
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current
converter
common mode
common
direct current
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CN110261713A (en
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周永荣
秦文康
孙军
董荣森
葛佳盛
徐习东
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NARI Group Corp
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    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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Abstract

The invention discloses a method for diagnosing a grounding fault on an AC side of a converter of a flexible direct-current power grid, which is characterized in that a relay protection device capable of monitoring common-mode current and common-mode voltage is respectively arranged on each DC side of an AC/DC end of a pseudo-bipolar flexible direct-current network, and when the common-mode current is abnormally large, or the common-mode voltage exists, but the common-mode current is slightly small, or the common-mode voltage exists, but the active power of the common-mode current voltage is negative, the AC/DC end is judged to have the grounding fault. The invention is suitable for a pseudo bipolar flexible direct current network comprising a plurality of AC/DC converters, can quickly judge a fault occurrence point and send a signal to inform a control system to cut off the converter at one AC/DC end when a ground fault occurs at the AC side of the AC/DC end, and ensures the safe and reliable operation of the flexible direct current network.

Description

Method for diagnosing AC side ground fault of converter of flexible DC power grid
Technical Field
The invention relates to a relay protection method for a direct current power grid, in particular to a method for diagnosing a grounding fault on an alternating current side of a converter of a flexible direct current power grid.
Background
When a single-phase earth fault occurs on an alternating-current side of a voltage source converter (AC/DC) at any end in a multi-end flexible direct-current system, common-mode voltage and current can be generated on the direct-current side, and zero-sequence voltage is generated on a non-fault side, so that the fault side needs to be quickly identified and isolated so as not to influence the safe operation of the whole direct-current power grid.
In a three-phase alternating-current ungrounded grid, whether a ground fault occurs is usually detected through zero-sequence voltage, and line selection is performed through zero-sequence current. The existing literature describes that the occurrence of zero-sequence current and voltage on the ac side can cause the occurrence of common-mode current and voltage on the dc side.
However, since a single-phase ground fault occurs on the ac side of any one of the voltage source converters, a zero-sequence voltage is generated on the non-fault end through the dc common-mode voltage current, and therefore, the zero-sequence voltage protection cannot determine which one of the voltage source converters has a ground fault. Since common mode voltage appears in the full dc network, it is difficult to determine which inverter has a ground fault.
Disclosure of Invention
The invention aims to provide a method for diagnosing the grounding fault of the AC side of a flexible DC power grid converter for a pseudo-bipolar flexible DC system.
In order to achieve the purpose, the invention adopts the technical scheme that: a method for diagnosing a ground fault on an alternating current side of a flexible direct current power grid converter is provided, wherein the flexible direct current power grid is a pseudo bipolar flexible direct current network comprising a plurality of AC/DC converters, each AC/DC converter is one end of the direct current network, and common mode current and common mode voltage of the direct current side of each AC/DC converter are respectively monitored; if the common-mode capacitance of the ith terminal AC/DC converter is CicomThe common-mode current vector of the side group wave of the direct current at the ith end is obtained by measurement and calculation
Figure BDA0002048633020000011
Fundamental common mode voltage vector of
Figure BDA0002048633020000012
Wherein, the positive direction of the current is from the direct current bus to the AC/DC converter, the I-th AC/DC AC side is diagnosed whether the earth fault occurs or not through the following three criteria,
criterion one is as follows: the common mode current is abnormally large, namely:
Figure BDA0002048633020000013
and is
Figure BDA0002048633020000014
Criterion two: there is a common mode voltage, but the common mode current is small, i.e.:
Figure BDA0002048633020000015
and is
Figure BDA0002048633020000016
Criterion three: there is a common mode voltage, but the active power of the common mode current voltage is negative, i.e.
Figure BDA0002048633020000017
And is
Figure BDA0002048633020000018
Wherein: k is a radical of1J represents an imaginary number omega as the angular frequency of the AC side group wave, epsilon is the maximum error possibly occurring in the measurement of the common-mode current, and u is an over-current reliability coefficientzdCommon-mode voltage amplitude, k, that can be sustained for a long time by a DC network2The reliability factor of the power shortage is obtained,
Figure BDA0002048633020000021
representing fundamental common mode active power;
if any one of the three criteria is true, the grounding fault is diagnosed to occur on the ith terminal AC/DC alternating current side.
Further, the common mode current of the side direct current fundamental wave at the ith end is obtained by measurement and calculation
Figure BDA0002048633020000022
Fundamental common mode voltage of
Figure BDA0002048633020000023
The method specifically comprises the following steps: a voltage sensor and a current sensor are arranged on the positive pole and the negative pole of the direct current output end of the AC/DC converter, and the polarity of the current sensor is positive when the direct current bus points to the AC/DC converter; sampling to obtain a positive electrode current ipNegative electrode current inPositive electrode voltage upNegative electrode voltage unThen the common mode voltage u of the terminalcom＝(up+un) /2, common mode current icom＝(ip+in) And/2, the capacitance value of the positive electrode and the negative electrode of the converter to the ground is C, and the common-mode capacitance of the end is Ccom2C. The fundamental wave common mode voltage vector of the end can be obtained through Fourier algorithm
Figure BDA0002048633020000024
And fundamental common mode current vector
Figure BDA0002048633020000025
Further, k is1Value range of k1>1；k2Value range of 0<k2<1。
Further, uzdThe AC side of the AC/DC converter is rated for 10% of the inter-phase voltage.
Furthermore, epsilon is 5% of rated direct current of the AC/DC converter.
Compared with the prior art, the method for diagnosing the grounding fault on the alternating current side of the converter of the flexible direct current power grid has the beneficial effects that: when a ground fault point is searched, only the current and voltage of the direct current side are collected, an impedance angle is not required to be calculated, whether the local end has the ground fault or not can be diagnosed through the common-mode current, the voltage and the active power direction, and if the local end has the ground fault, a signal can be immediately sent to inform a local end control system to remove a fault current converter, so that the direct current power grid can continuously, safely and reliably operate.
Drawings
Fig. 1 is a schematic installation diagram of a protection device in the method for diagnosing the ac-side ground fault of the converter of the flexible dc power grid according to the present invention;
fig. 2 is a logic block diagram of fault protection in the method for diagnosing the ac-side ground fault of the converter of the flexible dc power grid according to the present invention.
Detailed Description
The invention will be further explained with reference to the accompanying drawings.
Referring to fig. 1 and 2, firstly, as shown in fig. 1, an AC side of an AC/DC terminal in a flexible DC power grid is connected to an AC power grid through a transformer, a DC side of the AC/DC terminal is connected to a DC bus, and a plurality of other DC lines are connected to the DC bus. The primary side of a voltage sensor is respectively connected to a positive direct current bus and a negative direct current bus and grounded, and the secondary side of the voltage sensor is connected to a relay protection device analog quantity acquisition module; the positive pole and the negative pole of the AC/DC direct current output end are provided with current sensors, the secondary sides of the current sensors are connected to the analog quantity acquisition module of the relay protection device, and the polarity of the current sensors is positive when the direct current bus points to the AC/DC converter; the protection device can obtain the anode current i by samplingpNegative electrode current inPositive electrode voltage upNegative electrode voltage un. The common mode voltage u of the end can be obtained by the calculation of the central processing unitcom＝(up+un) /2, common mode current icom＝(ip+in) And/2, the capacitance value of the positive electrode and the negative electrode of the converter to the ground is C, and the common-mode capacitance of the end is Ccom2C. The fundamental wave common mode voltage vector of the end can be obtained through Fourier algorithm
Figure BDA0002048633020000031
And fundamental common mode current vector
Figure BDA0002048633020000032
A protection logic block diagram for diagnosing a fault on the ac side of the dc-dc converter is shown in fig. 2. When all AC/DC end AC sides of the flexible DC power grid do not have ground faults, common mode voltage and common mode current do not appear, and the common mode voltage amplitude value obtained through relay protection calculation
Figure BDA0002048633020000033
And common mode current amplitude
Figure BDA0002048633020000034
Both the error values are not larger than the maximum measurement error, and misjudgment is avoided; when the AC/DC alternating current side at the other end of the flexible direct current power grid has ground fault,
Figure BDA0002048633020000035
the measured fundamental wave active power is loss on the capacitor, the direction is positive, and the three criteria can not be misjudged.When the AC/DC AC side of the local end has a ground fault, the method is equivalent to the situation that the local end is connected with a common mode fundamental wave power supply, fundamental wave common mode current measured by the local end is direct current flowing to the whole direct current power grid, and if the equivalent common mode impedance of the whole direct current power grid is small, the common mode current amplitude is large, namely
Figure BDA0002048633020000036
Far greater than
Figure BDA0002048633020000037
The first criterion can accurately judge that the local side has the ground fault; if the equivalent common-mode impedance of the whole dc power grid is large, a large common-mode voltage will appear, that is:
Figure BDA0002048633020000038
but the common mode current is small in magnitude, i.e.
Figure BDA0002048633020000039
Much less than
Figure BDA00020486330200000310
The second criterion can accurately judge that the local side has the ground fault; if the equivalent common-mode impedance of the whole direct-current power grid is close to the impedance of the capacitor on the side, then the direct-current power grid has a larger common-mode voltage and a larger common-mode current, but the measured fundamental wave common-mode active power is the loss of the direct-current power grid, and the power direction is negative, namely:
Figure BDA00020486330200000311
the third criterion can accurately judge that the ground fault occurs at the side.
While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention. Various modifications and alterations to this invention will become apparent to those skilled in the art upon reading the foregoing description. Accordingly, the scope of the invention should be determined from the following claims.

Claims (5)

1. A method for diagnosing a ground fault on an alternating current side of a flexible direct current power grid converter is characterized in that the flexible direct current power grid is a pseudo bipolar flexible direct current network comprising a plurality of AC/DC converters, wherein each AC/DC converter is one end of the direct current network, and common mode current and common mode voltage of the direct current side of each AC/DC converter are respectively monitored; if the common-mode capacitance of the ith terminal AC/DC converter is CicomThe common-mode current vector of the side group wave of the direct current at the ith end is obtained by measurement and calculation
Figure FDA0002971877740000011
Fundamental common mode voltage vector of
Figure FDA0002971877740000012
Wherein, the positive direction of the current is from the direct current bus to the AC/DC converter, the I-th AC/DC AC side is diagnosed whether the earth fault occurs or not through the following three criteria,
criterion one is as follows: the fundamental wave common mode current vector magnitude is unusually large, namely:
Figure FDA0002971877740000013
and is
Figure FDA0002971877740000014
Criterion two: the fundamental wave common mode voltage vector amplitude is larger than the common mode voltage amplitude which can be endured by the direct current power grid for a long time, but the fundamental wave common mode current vector amplitude is smaller, namely:
Figure FDA0002971877740000015
and is
Figure FDA0002971877740000016
Criterion three: the fundamental wave common mode voltage vector amplitude is larger than the common mode voltage amplitude which can be endured by the direct current power grid for a long time, but the fundamental wave common mode active power is negative, namely
Figure FDA0002971877740000017
And is
Figure FDA0002971877740000018
Wherein: k is a radical of1J represents an imaginary number, omega is the angular frequency of an alternating side group wave, epsilon is the maximum error possibly occurring in the measurement of the common-mode current, and u is the reliable coefficient of the over-currentzdCommon-mode voltage amplitude, k, that can be sustained for a long time by a DC network2The reliability factor of the power shortage is obtained,
Figure FDA0002971877740000019
representing fundamental common mode active power;
if any one of the three criteria is true, the grounding fault is diagnosed to occur on the ith terminal AC/DC alternating current side.
2. The method for diagnosing the ac-side ground fault of the converter of the flexible dc power grid according to claim 1, wherein the measurement and calculation result is that the common mode current of the i-th side of the dc-side fundamental wave is
Figure FDA00029718777400000110
Fundamental common mode voltage of
Figure FDA00029718777400000111
The method specifically comprises the following steps: a voltage sensor and a current sensor are arranged on the positive pole and the negative pole of the direct current output end of the AC/DC converter, and the polarity of the current sensor is positive when the direct current bus points to the AC/DC converter; sampling to obtain a positive electrode current ipNegative electrode current inPositive electrode voltage upNegative electrode voltage unThen the common mode voltage u of the terminalcom＝(up+un) /2, common mode current icom＝(ip+in) And/2, the capacitance value of the positive electrode and the negative electrode of the converter to the ground is C, and the common-mode capacitance of the end is Ccom2C; obtaining the fundamental wave common mode voltage vector of the end through a Fourier algorithm
Figure FDA00029718777400000112
And fundamental common mode current vector
Figure FDA00029718777400000113
3. Method for diagnosing an ac-side earth fault of a converter of a flexible dc-grid according to claim 1, characterized in that k is1Value range of k1>1；k2Value range of 0<k2<1。
4. Method for diagnosing an ac-side earth fault of a converter of a flexible dc-grid according to claim 1, characterized in that uzdThe AC side of the AC/DC converter is rated for 10% of the inter-phase voltage.
5. A method of diagnosing an AC-side ground fault of a flexible DC grid converter according to claim 1, characterized in that e is 5% of the rated DC-current of the AC/DC converter.
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CN102645607A (en) * 2011-02-21 2012-08-22 洛克威尔自动控制技术股份有限公司 Modular line-to-ground fault identification
CN103887772A (en) * 2013-09-11 2014-06-25 南京南瑞继保电气有限公司 Flexible DC power transmission grounding measurement device and protection method
CN104950207A (en) * 2014-03-25 2015-09-30 艾默生网络能源有限公司 Short circuit detecting method and device for photovoltaic inverter
CN207705796U (en) * 2018-01-10 2018-08-07 重庆聚陆新能源有限公司 A kind of multifunctional ligand power grid flexible ground device
EP3435503A1 (en) * 2017-07-27 2019-01-30 Siemens Aktiengesellschaft Localisation of an earth fault in an it network

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US20170227590A1 (en) * 2016-02-05 2017-08-10 Hamilton Sundstrand Corporation High impedance arc fault detection

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN102645607A (en) * 2011-02-21 2012-08-22 洛克威尔自动控制技术股份有限公司 Modular line-to-ground fault identification
CN103887772A (en) * 2013-09-11 2014-06-25 南京南瑞继保电气有限公司 Flexible DC power transmission grounding measurement device and protection method
CN104950207A (en) * 2014-03-25 2015-09-30 艾默生网络能源有限公司 Short circuit detecting method and device for photovoltaic inverter
EP3435503A1 (en) * 2017-07-27 2019-01-30 Siemens Aktiengesellschaft Localisation of an earth fault in an it network
CN207705796U (en) * 2018-01-10 2018-08-07 重庆聚陆新能源有限公司 A kind of multifunctional ligand power grid flexible ground device

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