阅读说明：本技术 一种直流滤波器高压电容器接地故障保护方法 (Ground fault protection method for high-voltage capacitor of direct-current filter ) 是由 李永丽 张云柯 宋金钊 于 2020-05-08 设计创作，主要内容包括：本发明涉及一种直流滤波器高压电容器接地故障保护方法,包括步骤如下：采集得到直流滤波器首端电压u和不平衡电流i<Sub>T2</Sub>,获得离散的首端电压和不平衡电流序列；计算虚拟电容C<Sub>zd</Sub>；根据高压电容器的桥臂电容确定保护整定值C<Sub>set</Sub>,当虚拟电容C<Sub>zd</Sub>大于保护整定值C<Sub>set</Sub>时,则保护判定为区内故障；反之,则保护判定为区外故障。(The invention relates to a ground fault protection method for a high-voltage capacitor of a direct-current filter, which comprises the following steps: acquiring a head end voltage u and an unbalanced current i of the direct current filter T2 Obtaining a discrete head end voltage and unbalanced current sequence; calculating a virtual capacitance C zd (ii) a Determining a protection setting value C according to the bridge arm capacitance of the high-voltage capacitor set When the virtual capacitor C zd Greater than the protection setting value C set If so, the protection is judged as an intra-area fault; otherwise, the protection is determined to be an out-of-area fault.)

1. A method for protecting a high-voltage capacitor ground fault of a direct current filter comprises the following steps:

(1) acquiring a head end voltage u and an unbalanced current i of the direct current filter_T2Obtaining discrete head end voltage and unbalanced current sequences

(2) Calculating a virtual capacitance C_zdThe calculation formula is

In the formula, N_TThe total number of sampling points in the time window length;

(3) determining a protection setting value C according to the bridge arm capacitance of the high-voltage capacitor_setWhen the virtual capacitor C_zdGreater than the protection setting value C_setIf so, the protection is judged as an intra-area fault; otherwise, the protection is determined to be an out-of-area fault.

2. The method of claim 1, wherein the setting value C is protected_setIs calculated by the formula

C_set＝k_setC

In the formula, k_setTo protect the setting of the action, k_setTaking 0.001-0.05.

Technical Field

The invention relates to the field of relay protection of power systems, in particular to a method for protecting a ground fault of a high-voltage capacitor of a direct-current filter.

Background

The HVDC technology has the advantages of long transmission distance, large transmission capacity, low loss, flexible control and the like, and is widely applied to high-voltage and extra-high-voltage power grids in China. In the LCC-HVDC system, the direct current filters are configured at two ends of the direct current transmission line so as to filter characteristic harmonics injected to the direct current side by the converter, and therefore hazards of communication interference, heating of direct current side power equipment, reduction of electric energy quality and the like caused by the characteristic harmonics are effectively avoided.

The high-voltage capacitor is a core element of the dc filter and bears most of the dc-side voltage. After the high-voltage capacitor has a ground fault, the tuning frequency is shifted due to the change of the resonant circuit of the filter, which not only causes the filtering effect to be poor, but also causes the filter element to be damaged due to overcurrent or overvoltage. In actual engineering, differential protection is used for judging various ground faults of a direct current filter, but fault positions are difficult to determine, and meanwhile, the phenomenon that false operation occurs due to the fact that protection is affected by the fact that transient characteristics of a current transformer are inconsistent occurs. Therefore, the direct current filter high-voltage capacitor ground fault protection is researched, the reliability and the sensitivity of the protection action are improved, and the method has great significance for improving the safety and the stability of a direct current system.

Disclosure of Invention

Aiming at the problems, the invention provides a method for protecting the ground fault of a high-voltage capacitor of a direct-current filter. The method constructs the ground fault protection of the high-voltage capacitor of the direct-current filter based on the virtual capacitance characteristics of the high-voltage capacitor when the inside and the outside of the high-voltage capacitor are in fault, overcomes the defect that differential protection is difficult to determine the fault position, and only utilizes single-end electric quantity. The technical scheme of the invention is as follows:

a method for protecting a high-voltage capacitor ground fault of a direct current filter comprises the following steps:

(1) acquiring a head end voltage u and an unbalanced current i of the direct current filter_T2Obtaining discrete head end voltage and unbalanced current sequences

j and k are positive integers, 1, 2, … and N are taken, and N is the total sequence point number.

(2) Calculating a virtual capacitance C_zdThe calculation formula is

In the formula, N_TThe total number of sampling points in the time window length;

(3) determining a protection setting value C according to the bridge arm capacitance of the high-voltage capacitor_setWhen the virtual capacitor C_zdGreater than the protection setting value C_setIf so, the protection is judged as an intra-area fault; otherwise, the protection is determined to be an out-of-area fault.

Further, protecting the setting value C_setIs calculated by the formula

C_set＝k_setC

In the formula, k_setTo protect the setting of the action, k_setTaking 0.001-0.05.

The invention provides a method for protecting a high-voltage capacitor ground fault of a direct-current filter, aiming at the defects of the traditional direct-current filter high-voltage capacitor ground fault protection. Compared with the prior art, the method has the following advantages:

(1) the invention provides the ground fault protection of the high-voltage capacitor of the direct-current filter based on the virtual capacitance characteristics of the high-voltage capacitor in the internal and external faults, and has perfect protection theory and good selectivity.

(2) The method only utilizes single-end electric quantity and is not influenced by the inconsistency of the transformation ratio characteristics of the sensor.

Drawings

FIG. 1 is a schematic diagram of a model HP12/24 DC filter.

Fig. 2 is an equivalent circuit of the upper bridge arm ground fault of the high-voltage capacitor.

FIG. 3 is an equivalent circuit of the ground fault of the lower bridge arm of the high-voltage capacitor.

The reference numbers in the figures illustrate:

the DC filter of FIG. 1 is composed of a high-voltage capacitor C₁Resistance R₁Inductor L₁Capacitor C₂And an inductance L₂Composition is carried out; high-voltage capacitor C₁The capacitance of each bridge arm is C; c_T1、C_T3The current transformers are respectively a head end current transformer and a tail end current transformer; c_T2An unbalanced current transformer; f. of₁、f₂Are respectively a high-voltage capacitor C₁Grounding fault points of the upper bridge arm and the lower bridge arm on the right side; f. of₃Is a ground fault point in the area below the high-voltage capacitor; AC is an alternating current system power supply; z_AFThe equivalent impedance of the filter at the AC bus; l is_srIs a smoothing reactor.

FIG. 2(a) shows the bridge leg connection of the high-voltage capacitorA ground fault complex frequency domain equivalent circuit; (b) the figure is a time domain equivalent circuit of the grounding fault of the upper bridge arm of the high-voltage capacitor; u(s) and u (t) are frequency domain and time domain equivalent voltage sources respectively; i is_T1(s) and i_T1(t) frequency domain and time domain head end currents respectively; i is_T3(s) and i_T3(t) are frequency domain and time domain tail end currents respectively; i is_T2(s) is the frequency domain unbalanced current; i is_f1(s) and I_f2(s) respectively passing through the top of the upper bridge arm of the high-voltage capacitor and a fault point f₁Fault current, fault point f of capacitor between₁Fault current with the capacitance between the unbalanced bridge; i.e. i_f(t) is time domain equivalent fault current; z(s) is the frequency domain equivalent impedance of the elements in the area below the high-voltage capacitor; lambda is from the top of the upper bridge arm of the high-voltage capacitor to the fault point f₁Percentage of the middle capacitance to the upper bridge arm capacitance C on the right side (0)<λ<1)；Z_eqIs the equivalent impedance; c_eqIs an equivalent capacitance.

Fig. 3 (a) is a complex frequency domain equivalent circuit of the ground fault of the lower bridge arm of the high-voltage capacitor; (b) the figure is a time domain equivalent circuit of the grounding fault of a lower bridge arm of the high-voltage capacitor; u(s) and u (t) are frequency domain and time domain equivalent voltage sources respectively; i is_T1(s) and i_T1(t) frequency domain and time domain head end currents respectively; i is_T3(s) and i_T3(t) are frequency domain and time domain tail end currents respectively; i is_T2(s) is the frequency domain unbalanced current; i is_f1(s) and I_f2(s) respectively passing through the tail part of the lower bridge arm of the high-voltage capacitor and a fault point f₂Fault current, unbalanced bridge and fault point f of capacitor between₂Fault current of the capacitor in between; i.e. i_f(t) is time domain equivalent fault current; z(s) is the frequency domain equivalent impedance of the elements in the area below the high-voltage capacitor;for high-voltage capacitor unbalance bridging to fault point f₂The percentage of the middle capacitance to the right lower bridge arm capacitance CZ′_eqIs the equivalent impedance; c'_eqIs an equivalent capacitance.

Detailed Description

The invention is described in detail below with reference to the figures and examples.

The invention relates to a ground fault protection method for a high-voltage capacitor of a direct current filter, which mainly utilizes virtual capacitance characteristics to realize the discrimination of internal and external faults of a high-voltage capacitor area and comprises the following specific steps:

(1) fig. 1 is a schematic diagram of a high-voltage dc filter specifically applied in this embodiment. Acquiring the head end voltage u and the unbalanced current i of the direct current filter by a voltage and current acquisition device_T2Obtaining discrete head end voltage and unbalanced current sequences

j and k are positive integers, 1, 2, … and N can be taken, and N is the total sequence point number;

(2) calculating a virtual capacitance C_zdThe calculation formula is

In the formula, N_TThe total number of sampling points within 5ms of the time window;

(3) when the virtual capacitor C_zdGreater than the protection setting value C_setIf so, the protection is judged as an intra-area fault; otherwise, the protection is judged as an out-of-area fault; protection setting value C_setIs calculated by the formula

C_set＝k_setC

In the formula, k_setComprehensively considering the measurement errors, k, of the voltage transformer and the current transformer for protecting the setting value of the action_set0.001-0.05 is taken, and C is the bridge arm capacitance of the high-voltage capacitor.

The identification of faults inside and outside the high-voltage capacitor area is realized based on the virtual capacitance characteristics, and the principle is as follows:

and (3) an equivalent circuit of the upper bridge arm ground fault of the high-voltage capacitor is shown in figure 2. From FIG. 2(a) and kirchhoff's law, it can be derived

In the formula I_f(s) is a current transformer C not passing through_T3The fault current of (2).

The formula (1) is simplified and subjected to inverse Laplace transform to obtain the product

Current i_f1And i_f2The fault current i can be considered to be a fault current when only the capacitance element flows_fFlows through the equivalent capacitance C_eq. In combination with the above analysis, the DC filter is simplified and equivalent to an impedance Z_eqAnd a capacitor C_eqAnd (3) obtaining a time domain equivalent circuit of the ground fault of the upper bridge arm of the high-voltage capacitor in a parallel connection mode, as shown in fig. 2 (b).

According to FIG. 2(b), the voltage u (t) and the current i can be obtained_f(t) has the following relationship

The virtual capacitor C can be obtained by combining the vertical type (2) and the formula (3)_zdThe calculation formula is as follows

From the equation (4), the virtual capacitance C_zdAnd equivalent capacitance C_eqProportional relation is formed, and the ratio of the two is lambda/2; when the upper bridge arm f of the high-voltage capacitor₁When a point generates a grounding fault, an unbalanced transformer C is utilized_T2Measured current i_T2The virtual capacitor C can be obtained by calculating the head end voltage u of the direct current filter in real time_zd(ii) a If the DC filter parameters are fixed and unchanged, the virtual capacitor C_zdThe value of (2) is only dependent on the position of the fault point, and is independent of the running mode of the direct current system.

And (3) a ground fault equivalent circuit of a lower bridge arm of the high-voltage capacitor, as shown in figure 3. The same virtual capacitor can be obtainedC_zdThe calculation formula is as follows

From the equation (5), the virtual capacitance C_zdAnd equivalent capacitance C'_eqIs in direct proportion and the ratio between the two isLower bridge arm f of high-voltage capacitor₂After the point generates the earth fault, the unbalanced transformer C is utilized_T2Measured current i_T2The virtual capacitor C can be obtained by calculating the head end voltage u of the direct current filter in real time_zd(ii) a If the DC filter parameters are fixed and unchanged, the virtual capacitor C_zdThe value of (2) is only dependent on the position of the fault point, and is independent of the running mode of the direct current system.

When the high-voltage capacitor is normally operated, the high-voltage capacitor C₁The voltages at the two ends of the unbalanced bridge are always the same, and the unbalanced current transformer CT₂The theoretical value of the measured current is 0 and is not influenced by the running mode of the direct current system. Therefore, the virtual capacitance C is used in normal operation of the high-voltage capacitor_zdIs 0.

Although the specific embodiments of the present invention have been described with reference to specific examples, it is not intended to limit the scope of the present invention, and it should be understood by those skilled in the art that various modifications and variations can be made without inventive faculty based on the technical solutions of the present invention.