CN112993963A - Grounding circuit of flexible direct current distribution network and protection method thereof - Google Patents

Grounding circuit of flexible direct current distribution network and protection method thereof Download PDF

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Publication number
CN112993963A
CN112993963A CN201911293298.9A CN201911293298A CN112993963A CN 112993963 A CN112993963 A CN 112993963A CN 201911293298 A CN201911293298 A CN 201911293298A CN 112993963 A CN112993963 A CN 112993963A
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current
direct current
voltage
line
transformer
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赵健龙
石勇
侯炜
陈俊
王文龙
刘永生
卢宇
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NR Electric Co Ltd
NR Engineering Co Ltd
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NR Engineering Co Ltd
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/08—Limitation or suppression of earth fault currents, e.g. Petersen coil
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R31/00—Arrangements for testing electric properties; Arrangements for locating electric faults; Arrangements for electrical testing characterised by what is being tested not provided for elsewhere
    • G01R31/08—Locating faults in cables, transmission lines, or networks
    • G01R31/081—Locating faults in cables, transmission lines, or networks according to type of conductors
    • G01R31/086—Locating faults in cables, transmission lines, or networks according to type of conductors in power transmission or distribution networks, i.e. with interconnected conductors
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/04—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for transformers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/26—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured
    • H02H7/268—Sectionalised protection of cable or line systems, e.g. for disconnecting a section on which a short-circuit, earth fault, or arc discharge has occured for dc systems
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/04—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess voltage
    • H02H9/041—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess voltage using a short-circuiting device
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/60—Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Emergency Protection Circuit Devices (AREA)

Abstract

The invention discloses a grounding circuit of a flexible direct current distribution network.A lightning arrester is connected in parallel with a neutral point of an alternating current side transformer of a flexible direct current distribution network system or a neutral point led out by a grounding transformer. The invention also discloses a protection method, which comprises the following steps: collecting the current of the head ends and the tail ends of the positive electrode line and the negative electrode line, and collecting the voltage of the positive electrode line and the voltage of the negative electrode line on the direct current side; when the differential current of the positive line is out of limit or the differential current of the negative line is out of limit, the direct current line is judged to have an internal fault, and then if the unbalanced voltage of the positive line and the voltage of the negative line is out of limit, the single-pole ground fault is judged. According to the technical scheme, the neutral point of the alternating current side connection transformer is grounded through the parallel connection lightning arrester, the neutral point is led out through the grounding transformer when the transformer has no neutral point, and the lightning arrester is in a low-resistance state and has larger fault current to pass through when a single-pole grounding fault occurs on the direct current side by utilizing the variable resistance characteristic of the lightning arrester, so that the effect of protecting the transformer is achieved.

Description

Grounding circuit of flexible direct current distribution network and protection method thereof
Technical Field
The invention belongs to the field of relay protection of power systems, and particularly relates to a grounding circuit of a flexible direct-current distribution network and a protection method thereof.
Background
The selection of the grounding mode has important influence on the steady-state characteristic and the transient characteristic of the flexible direct-current power distribution system, on one hand, grounding provides a reference potential for the whole system, and on the other hand, different grounding modes can cause the system to generate different fault responses, so that fault detection and protection of the system are influenced.
The flexible direct current distribution network is in a starting stage, and the overvoltage and insulation matching is one of key problems which need to be solved in the direct current distribution network engineering application based on flexibility and straightness. According to the literature 'design of grounding mode of multi-end flexible direct-current distribution network' (reported by Chinese Motor engineering), aiming at a multi-end VSC flexible direct-current distribution network system, the optimal transient performance of a direct-current side non-grounding mode under system faults is obtained through simulation comparison. The study of the grounding scheme of the +/-10 kV flexible direct-current power distribution system (Guangdong power) only qualitatively analyzes the overvoltage mechanism and level of the double-end alternating-current and direct-current hybrid system under various fault conditions through simulation; the paper "analysis of influence of a flexible direct-current distribution network grounding mode on fault characteristics" (power grid technology) analyzes the influence of a direct-current side grounding mode through a capacitor on the alternating-current and direct-current side fault characteristics. A study on overvoltage mechanism and protective measures of a flexible direct-current power distribution network based on an overhead line under lightning surge is carried out in a paper 'lightning intrusion wave overvoltage simulation of a flexible direct-current-based 10kV power distribution network' (high-voltage technology), but at present, the flexible direct-current power distribution network is mostly transmitted by cables in cities.
Fig. 1 is directed to a flexible dc power distribution system including a Modular Multilevel Converter (MMC). The factors such as cost, efficiency and occupied area are considered comprehensively, at present, the mode that the neutral point of the transformer is connected on the alternating current side and grounded through a high resistor (without a parallel arrester in the figure 1) and the direct current side is not grounded is adopted, so that active and reactive power loss and harmonic zero sequence loops in steady operation can be avoided, the transient influence of different types of faults on the system can be reduced to the maximum extent, and the method is suitable for the requirement of a flexible direct current distribution network.
In cities, direct current distribution lines usually adopt a cable structure, and when the insulation of the cable structure is damaged, a cable core is directly connected with a metal protective shell to be grounded, so that the single-pole grounding is metallic grounding. When the neutral point of the coupling transformer is grounded through a high resistance, the voltage of the neutral point is deviated by about 10kV, but the fault current of the grounding point is only a few amperes due to the high resistance grounding of nearly a few kiloohms, and the fault current is difficult to be detected by the protection device. The paper of +/-10 kV direct current distribution system overvoltage and insulation coordination (southern power grid technology) proposes a mode of switching a small parallel resistor by two converter stations to increase fault current aiming at a single-pole ground fault, but the small parallel resistor needs to be put into the converter stations after the fault voltage is detected, the system needs to be in a fault state for a period of time, the stable operation of the system is not facilitated, and the influence on the system and the communication are required due to the fact that the small parallel resistors are arranged on the other two converter stations. In the article, "research on operating overvoltage of flexible direct current distribution network system" (power grid technology), MMC is used as an alternating current-direct current interconnection interface of the system, an overvoltage generation mechanism of the system under various faults is researched, corresponding protection measures are provided, and a method for distinguishing various fault types is not mentioned.
Disclosure of Invention
The invention aims to provide a grounding circuit of a flexible direct current distribution network and a protection method thereof.
In order to achieve the above purpose, the solution of the invention is:
the grounding circuit of the flexible direct current distribution network comprises an arrester, wherein the arrester is connected in parallel with a neutral point of an alternating current side transformer of the flexible direct current distribution network system or connected in parallel with a neutral point led out by a grounding transformer of the flexible direct current distribution network system. When the system has ground fault, the lightning arrester connected in parallel and grounded can provide fault signals for the protection device at the direct current side, so that rapid fault location and protection measures can be carried out.
One end of the lightning arrester is connected with a neutral point of a transformer or a neutral point led out through a grounding transformer at the alternating current side of the flexible direct current distribution network system, and the other end of the lightning arrester is grounded.
The lightning arrester is in a high-resistance state when a system normally operates, and is converted into a low-resistance state and flows fault current after a ground fault occurs. After the lightning arrester has a ground fault, the voltage at two ends is kept at a residual voltage level, and the fault voltage of a neutral point is weakened.
The protection method for the grounding circuit of the flexible direct current distribution network comprises the following steps:
step 1, collecting currents at the head end and the tail end of a positive line and currents at the head end and the tail end of a negative line, and collecting voltage of the positive line and voltage of the negative line on a direct current side;
step 2, when the differential current of the positive circuit is out of limit or the differential current of the negative circuit is out of limit, judging that the direct current circuit has an internal fault, and then entering step 3;
and 3, when the unbalanced voltage of the positive line voltage and the negative line voltage is out of limit, determining that the single-pole ground fault occurs.
In the step 2, the positive line differential current is an absolute value of a vector sum of the head end current and the tail end current of the positive line, and the negative line differential current is an absolute value of a vector sum of the head end current and the tail end current of the negative line.
In step 3, the unbalanced voltage is an absolute value obtained by subtracting the absolute values of the positive line voltage and the negative line voltage.
After the scheme is adopted, the invention has the beneficial effects that:
(1) according to the invention, the neutral point on the alternating current side is connected with the arrester in parallel for grounding, and when a single-pole grounding fault occurs, the arrester presents a low-resistance state and passes a large fault current after bearing overvoltage, so that a fault signal which is easy to detect is provided for a protection device, and therefore, rapid fault positioning and protection measures are carried out;
(2) according to the invention, the neutral point on the alternating current side is connected with the arrester in parallel for grounding, the arrester has quick response, and can instantly act after a single-pole grounding fault occurs, so that the protection device can rapidly remove the fault, and the fault state running time of the flexible direct current distribution network is reduced;
(3) according to the invention, the neutral point on the alternating current side is grounded in parallel with the lightning arrester, and the voltage amplitude of the neutral point is limited to be close to the residual voltage after a single-pole grounding fault occurs, so that the effect of protecting the transformer is achieved;
(4) according to the invention, the neutral point on the alternating current side is connected with the arrester in parallel for grounding, and the arrester cannot be conducted due to the constant neutral point potential when a bipolar short circuit fault occurs, so that the system and the protection are not influenced.
Drawings
FIG. 1 is a schematic diagram of the grounding of a neutral point parallel arrester of a flexible DC distribution network adopted by the invention;
FIG. 2 is a schematic view of a single pole ground fault of a DC line;
FIG. 3 is a DC equivalent circuit of a DC line in case of single-pole ground fault;
fig. 4 is a block diagram of a single-pole ground fault determination and protection strategy for a dc line.
Detailed Description
The technical solution and the advantages of the present invention will be described in detail with reference to the accompanying drawings.
The invention provides a grounding circuit of a flexible direct current distribution network and a protection method thereof.A novel grounding mode that a lightning arrester is connected in parallel with a neutral point of a transformer or a neutral point led out by a grounding transformer to be grounded is adopted in a flexible direct current distribution network system, when the system normally operates, the lightning arrester is in a high-resistance state, and when the system has a ground fault, the voltage of two ends of the lightning arrester rises to be in a low-resistance state and a larger fault current passes through, so that a fault signal is provided for a protection device, thereby carrying out rapid fault location and protection measures.
As shown in fig. 1, the ac side is a 10kV ac grid, the dc side is a ± 10kV voltage class, the two are connected to the MMC converter station through a 10/10kV junction transformer, the ac side is connected to the neutral point of the transformer or the neutral point led out by the grounding transformer when the transformer has no neutral point, and is grounded through the high-resistance parallel arrester. The neutral point of the transformer is connected with the alternating current side or the neutral point led out by the grounding transformer when the transformer has no neutral point is grounded through the high-resistance parallel arrester. The high resistance is connected with the arrester in parallel, the upper end of the high resistance is connected with the neutral point on the AC side, and the lower end of the high resistance is grounded.
Taking the positive dc line single pole grounding as an example, let G point generate single pole metallic grounding fault, as shown in fig. 2. RhighIs a high grounding resistance, A is a parallel lightning arrester, usa,usb,uscIs the equivalent alternating voltage u on the valve side of the transformerap,uanIs the voltage between the valves of the upper and lower bridge arms of phase a, up,unIs the positive and negative voltages of the DC line, L1，L2Is respectively a bridge arm inductor and a current-limiting reactor uo1,uo2Respectively, the neutral point voltage of the junction transformer and the equivalent neutral point voltage of the DC side, iap,ianIs a-phase upper and lower bridge arm current, idcIs a dc line current. Respectively arranging current transformers 1 and 2 at the head end and the tail end of the positive direct current line for measuring the current at the head end and the tail end of the positive direct current line, and arranging a voltage transformer 1 for measuring the voltage u of the positive direct current linepCurrent transformers 3 and 4 are respectively arranged at the head end and the tail end of the positive direct current line and used for measuring the negative direct current lineThe head end and the tail end of the current are provided with a voltage transformer 2 for measuring the voltage u of the negative direct current linen。
During normal operation, the neutral point voltage u of the transformer can be obtainedo1And the equivalent neutral point voltage u of the DC sideo2The expression of (a) is:
Figure BDA0002319832160000041
and voltage balance between the positive electrode and the negative electrode of the direct current line:
|up+un|＝0 (2)
metallic ground fault occurs at point G and the system reference potential point shifts to point G to 0. Since the interpolar voltage of the dc line is stable, it can be seen from equation (3) that the equivalent neutral point voltage on the dc side is shifted by-10 kV, and the voltages to the positive and negative electrodes of the dc line are shifted by-10 kV:
uo2＝uG-Udc/2＝-Udc/2 (3)
the voltages of the positive electrode and the negative electrode of the direct current line are unbalanced:
|up.G+un.G|≠0 (4)
the dc equivalent loop is shown in fig. 3, where Ig is the ground current at the fault point G and R is the equivalent impedance of the ground system. So that the neutral point voltage u of the transformer can be obtainedo1And fault point earth current IgThe expression of (a) is:
Figure BDA0002319832160000051
from the equation (5), the neutral point voltage u of the transformero1A voltage shift of-10 kV also occurs. Similarly, when the negative DC line is grounded in a single pole, the voltage u at the neutral point of the transformer iso1A voltage shift of +10kV may also occur. When the neutral point of the traditional transformer is grounded through a high resistance, the equivalent resistance R of the grounding system is the grounding high resistance RhighThen the earth current I at the fault pointg＝-Udc/2RhighA fault current of only a few amperes is difficult to detect by the protection device compared to a rated current of hundreds of thousands of amperes.
The grounding circuit is added with a parallel lightning arrester, so that the problem can be solved. The resistance of the arrester has a highly non-linear characteristic, i.e. a sudden voltage-current characteristic, which is characterized by a very high resistance and only a very small leakage current through the arrester at a certain voltage. In the event of an overvoltage, a very low resistance is present, a large current can be passed and the overvoltage is reduced to the residual voltage of the arrester. And after the overvoltage disappears, the lightning arrester restores to the original state.
Under the steady-state operation and non-monopole grounding fault of the flexible direct current distribution network system, the voltage at two ends of the lightning arrester is close to 0, the lightning arrester presents very high resistance which is actually equivalent to an insulator, and the grounding high resistance RhighThe equivalent resistance value is basically unchanged after the lightning arresters are connected in parallel, so that the flexible direct-current distribution network operates under the normal working condition that the alternating-current neutral point is grounded through a high resistance. When the positive pole or the negative pole direct current line is in single-pole grounding, the two ends of the lightning arrester bear the voltage of about 10kV, the voltage exceeds the action of the lightning arrester, the lightning arrester presents low resistance and flows large fault current Ig, and if the positive pole line is in single-pole grounding at the G point, the fault current Ig is measured by the current transformer 1 and is transmitted to the protection device. The protection device can be combined with the fault voltage signal and the fault current signal to judge the single-pole grounding fault and quickly cut off. In addition, the arrester can limit the voltage amplitude of the neutral point to be close to the residual voltage when in action, and the effect of protecting the transformer is achieved.
In one embodiment, the protection device samples the positive and negative voltages U of the DC line in real timep,UnPositive terminal current I1，I2Current I at both ends of the negative electrode3，I4。Iset,UsetSetting values of current unbalance and voltage unbalance are respectively set. Program logic diagram as shown in fig. 4, the starting condition of the first step protection device is equation (6) in order to distinguish between a fault state and a normal operation state.
|I1+I2|≥IsetOr | I3+I4|≥Iset (6)
In FIG. 4, for the positive DC line, Im1Is I1，Im2Is I2(ii) a For negative DC lines, Im1Is I3，Im2Is I4. Thus, equation (6) can be rewritten as:
|Im1+Im2|≥Iset (7)
||Up|-|Un||≥Uset (8)
during a double short-circuit fault, the voltage of the dc line is balanced, so that the second step can distinguish between a unipolar earth fault and a non-unipolar earth fault by means of a voltage imbalance as in equation (8). If the formula (8) is satisfied, the result is a unipolar ground fault, and the current characteristics are further used to distinguish between an in-region unipolar ground fault and an out-of-region unipolar ground fault. In the event of an out-of-region monopole ground fault, current Im1,Im2Same direction and same magnitude, and single-pole earth fault in region, current Im1,Im2The direction and the size are changed, and the criterion is shown as an equation (9). The single-pole grounding fault characteristic can be amplified by the formula (9), and the misoperation, K, of the device caused by measurement error can be avoidedBL1To protect the ratio coefficient of the device, Kset1And the motion setting value is used as the motion setting value. And if the single-pole grounding fault in the area is judged to occur, the protection device cuts off the fault line and carries out fault clearing and recovery actions. If the single-pole earth fault outside the area is judged, the protection device does not act.
Figure BDA0002319832160000061
Considering the fluctuations of the direct current and voltage and the measurement error of about 5% in total in normal operation, I in the formulae (7) and (8)set,UsetNeed to satisfy IsetRated current, U, of not less than 0.1 times of DC sidesetNot less than 0.1 times the rated voltage of the DC side. When the error is considered to be 5% in the case of an out-of-region fault, K isBL1Should be no more than 0.05, and the numerator is large and the denominator is small when the fault is in the region, KBL1Is of great valueThus, K can be amplified appropriatelyset1Taking the value of 0.5-0.8. Taking a connection transformer 10/10kV and a rated current of a direct current line as 500A as an example, I can be takenset＝50A，Uset＝2000V，Kset1＝0.5。
The above embodiments are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modifications made on the basis of the technical scheme according to the technical idea of the present invention fall within the protection scope of the present invention.

Claims (6)

1. The utility model provides a flexible direct current joins in marriage ground circuit of net which characterized in that: the lightning arrester is connected in parallel with a neutral point of an alternating current side transformer of the flexible direct current distribution network system or connected in parallel with a neutral point led out by a grounding transformer of the flexible direct current distribution network system.
2. The grounding circuit of the flexible direct current distribution network according to claim 1, wherein: one end of the lightning arrester is connected with a neutral point of a transformer or a neutral point led out through a grounding transformer at the alternating current side of the flexible direct current distribution network system, and the other end of the lightning arrester is grounded.
3. The grounding circuit of the flexible direct current distribution network according to claim 1, wherein: the lightning arrester is in a high-resistance state when a system normally operates, and is converted into a low-resistance state and flows fault current after a ground fault occurs.
4. The method for protecting the ground circuit of the flexible direct current distribution network according to claim 1, comprising the following steps:
step 1, collecting currents at the head end and the tail end of a positive line and currents at the head end and the tail end of a negative line, and collecting voltage of the positive line and voltage of the negative line on a direct current side;
step 2, when the differential current of the positive circuit is out of limit or the differential current of the negative circuit is out of limit, judging that the direct current circuit has an internal fault, and then entering step 3;
and 3, when the unbalanced voltage of the positive line voltage and the negative line voltage is out of limit, determining that the single-pole ground fault occurs.
5. The protection method according to claim 4, characterized in that: in the step 2, the differential current of the positive line is the absolute value of the vector sum of the head end current and the tail end current of the positive line, and the differential current of the negative line is the absolute value of the vector sum of the head end current and the tail end current of the negative line.
6. The protection method according to claim 4, characterized in that: in the step 3, the unbalanced voltage is an absolute value obtained by subtracting the absolute values of the positive line voltage and the negative line voltage.
CN201911293298.9A 2019-12-16 2019-12-16 Grounding circuit of flexible direct current distribution network and protection method thereof Pending CN112993963A (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114744596A (en) * 2022-02-22 2022-07-12 华北电力大学 Flexible direct current system pilot protection method and system based on voltage matching factor

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114744596A (en) * 2022-02-22 2022-07-12 华北电力大学 Flexible direct current system pilot protection method and system based on voltage matching factor

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