Small current grounding system bus capacitance measurement method

Abstract

The invention discloses a novel efficient small-current grounding system bus capacitance measurement method, and relates to the technical field of power system measurement. According to the method, under the condition that an arc suppression coil is normally connected to a system, accurate measurement of the bus grounding capacitance is realized by additionally adding a group of measurement capacitance and short-circuit current measurement links and combining an improved mathematical model. Compared with a traditional detection method, the new method does not need to pull open an arc suppression coil disconnecting link or operate other primary high-voltage equipment during measurement, an operation order application process which must be executed due to operation of the primary equipment is avoided, the measurement efficiency is remarkably improved, and the method is suitable for rapid general survey and daily operation and maintenance of a transformer substation and contributes to guaranteeing safe and stable operation of a power grid.

Description

Technical Field

[0001] This invention relates to the field of power system measurement technology, specifically to a method for measuring bus-to-ground capacitance in low-current grounding systems. Compared with the existing neutral point external capacitance method, its biggest advantage is that the bus capacitance can be accurately measured without opening the arc suppression coil switch. Background Technology

[0002] In power systems, systems with an ungrounded neutral point or grounded through a high impedance are called low-current grounding systems. Primary equipment in the system, such as transformers, busbars, and lines, have distributed capacitance to ground, known as busbar capacitance. When a single-phase ground fault occurs, this capacitance generates a ground fault current. If the fault current is too large (typically >5A), an arc suppression coil needs to be installed for compensation. As the grid load increases, the busbar capacitance value changes; therefore, it needs to be measured periodically to check and adjust the compensation parameters of the arc suppression coil. Currently, methods for measuring the busbar-to-ground capacitance in low-current grounding systems in power systems include the heterogeneous frequency signal injection method and the neutral point external capacitance method.

[0003] The working principle of the heterofrequency signal injection method is as follows: a constant current signal with a special frequency (such as 5Hz, 20Hz, 75Hz, etc., often referred to as "heterogeneous frequency") that is different from the power frequency (50Hz) is injected into the system. Since the impedance characteristics of the system to heterofrequency signals are mainly determined by its capacitance to ground, the total capacitance of the three phases to ground of the system can be calculated by accurately measuring the heterofrequency voltage response generated at the injection point.

[0004] The working principle of the neutral point external capacitor method is as follows: In a low-current grounding system, due to the incomplete balance of the three-phase-to-ground capacitance, the neutral point itself has a small (usually very small) asymmetrical voltage. When a temporary additional capacitor of known capacitance is connected between the system's neutral point and ground, the system's neutral point-to-ground potential (zero-sequence voltage) will change significantly. By accurately measuring the zero-sequence voltage before and after connecting the additional capacitor, the total three-phase-to-ground capacitance of the system can be calculated.

[0005] Both methods share a common feature: both require initial equipment operation on the high-voltage equipment. The heterogeneous frequency signal injection method necessitates disconnecting the bus PT, temporarily short-circuiting the high-voltage side harmonic suppression resistor, and then reactivating the bus PT. Simultaneously, the arc suppression coil disconnect switch must be opened. The neutral point external capacitor method requires opening the arc suppression coil disconnect switch before operation. All of these operations require a power outage of the high-voltage equipment.

[0006] In power systems, any operation of primary equipment or power outage requires a strict power outage application and work permit approval process, which is time-consuming and unpredictable. When encountering scenarios requiring rapid surveys and tests of a large number of substations, the aforementioned measurement methods are significantly limited in efficiency due to external factors such as power outage application and work permit approval processes. Therefore, there is an urgent need for a method that can accurately measure bus capacitance under normal operating conditions of primary equipment (bus PT, arc suppression coil, and harmonic suppression resistors are all connected) to simplify the testing process, improve efficiency, and ensure the safe and economical operation of the power grid. Summary of the Invention

[0007] The invention aims to overcome the shortcomings of existing technologies that require primary equipment operation (opening the arc suppression coil switch) to measure bus capacitance, and provides a highly efficient and accurate method for measuring external capacitance. This method, through improvements to the measurement circuit and calculation model, enables uninterrupted measurement of bus capacitance under normal system operation with the arc suppression coil connected.

[0008] To solve the above-mentioned technical problems, the specific technical solution of the present invention is as follows:

[0009] A method for measuring bus capacitance in a low-current grounding system, the system including bus capacitance C0 and three-phase unbalanced potential E. p An arc-suppression coil Z, consisting of a resistor R and an inductor L, and an arc-suppression coil switch K, are connected to an external short-circuit measuring ammeter T, external measuring capacitors C1 and C2, and corresponding switching switches K0, K1, and K2. Only the switching switches of the external circuit are controlled to measure and calculate the bus capacitance C0. The external measuring capacitors C1 and C2, the short-circuit measuring ammeter T, and the arc-suppression coil Z are connected in parallel. The measurement steps are as follows:

[0010] The neutral point-to-ground voltage U0 is measured when the arc suppression coil switch K is always closed and K0, K1, and K2 are all open; the short-circuit current I is measured when K0 is closed and K1 and K2 are open. d When K1 is closed and K0 and K2 are open, measure the first voltage to ground U1; when K0 is open and K1 and K2 are closed, measure the second voltage to ground U2; then establish a mathematical model to solve for the bus capacitance C0.

[0011] When establishing a mathematical model to solve for the bus capacitance C0, the impedance of the arc suppression coil under normal operation is simplified to a pure resistance R, and its inductive reactance component is ignored. The specific solution for the bus capacitance C0 is as follows:

[0012] Using the capacitive reactance X0 of the bus capacitor as the unknown, the relationship between the equivalent resistance R of the arc suppression coil and the capacitive reactance X0 of the bus capacitor C0 is shown in the following formula:

[0013] ;

[0014] The expressions for U1 and U2 are as follows:

[0015] ;

[0016] ;

[0017] in, Let R be the angular frequency of the power grid frequency. Substituting R into the expressions for U1 and U2, we obtain two nonlinear equations containing only X0, and simplify the right-hand side of the expression using f(X0):

[0018] U1=f1(X0);

[0019] U2=f2(X0);

[0020] Where f1() is the equation for calculating U1, and f2() is the equation for calculating U2;

[0021] Define two residual functions:

[0022] F1(X0) = U1 - f1(X0);

[0023] F1(X0) = U2 - f2(X0);

[0024] The solution to the residual function is X0, which makes the residual equation F1=F2=0. Once X0 is obtained, the value of C0 can be calculated.

[0025] Traditional methods require operation of primary equipment to ensure measurement accuracy, and due to time constraints, it is impossible to complete all substation measurements within the specified time. The method proposed in this invention, however, uses external measuring switches, ammeters, and capacitors for measurement, eliminating the need for primary equipment operation and significantly reducing overall working time.

[0026] The method proposed in this invention has a significantly higher overall work efficiency than traditional methods when it is necessary to urgently complete the bus capacitance detection of low current grounding systems in multiple substations while maintaining the same measurement accuracy. Attached Figure Description

[0027] Figure 1 The equivalent working principle diagram of the existing neutral point external capacitor method;

[0028] Figure 2 This is the equivalent working principle diagram used in this invention;

[0029] Figure 3 The simplified working equivalent principle diagram of this invention is shown below, based on the impedance of the arc suppression coil. Detailed Implementation

[0030] To better understand the purpose, structure, and function of this invention, the invention will be described in further detail below with reference to the accompanying drawings.

[0031] To aid in understanding the innovativeness of this invention, the working process of the existing neutral point external capacitor method is analyzed below. The equivalent working principle of the existing neutral point external capacitor method is as follows: Figure 1 As shown, this includes the bus capacitance C0 and the system three-phase unbalanced electromotive force E. p Arc suppression coil Z (composed of R and L), arc suppression coil switch K, external measuring capacitor C1, switch K1 for external capacitor C1, N is the neutral point of the grounding transformer, and U is the voltage between the center point and ground.

[0032] When the system is working normally, switch K is in the closed state. The inductive reactance generated by the arc suppression coil Z compensates for the capacitive reactance generated by the bus capacitor C0, thereby compensating for the capacitive current generated by C0.

[0033] In existing operations, when measuring the bus capacitance C0, to avoid the influence of the arc suppression coil Z on the measured C0 value, K needs to be disconnected before measurement. Then, the measurement is performed according to the following steps:

[0034] Step 1: Measure the voltage U with K and K1 disconnected. At this time, the value of U is equal to E. p The value of this voltage, which measures the three-phase unbalanced electromotive force of the system, is U. p .

[0035] Step 2: With K open, close K1 and measure the value of U. The value of U at this point is the value after capacitor C1 is connected. c1 .

[0036] Step 3: Based on the voltage division relationship of capacitors in series, the value of bus capacitor C0 can be calculated.

[0037]

[0038] In this process, switch K is a primary device, and its activation and deactivation require an application and work permit process. K1, however, is an auxiliary measuring switch, and its activation and deactivation do not require any application process.

[0039] This invention adds a measuring capacitor C2 and a short-circuit measuring ammeter T, along with a corresponding measuring switch, to the existing neutral point external capacitor method. During the measurement process, the arc suppression coil switch K remains in a conducting state. The equivalent circuit is as follows: Figure 2 As shown in the figure, C0 is the bus capacitance, E pThe system's three-phase unbalanced electromotive force is represented by Z, the arc suppression coil (composed of resistor R and inductor L), K, the arc suppression coil disconnect switch, C1 and C2 are external measuring capacitors, K1 and K2 are the switching switches for C1 and C2, N is the neutral point of the grounding transformer, U is the neutral point-to-ground voltage, and T is an ammeter used to measure the neutral point-to-ground short-circuit current I. d K0 serves as a short-circuit switch. One end of the resistor R, inductor L, external measuring capacitors C1 and C2, arc suppression coil switch K, and ammeter T is grounded. The other end of the resistor R and inductor L is connected to the other end of the arc suppression coil switch K. The other end of the ammeter T and external measuring capacitors C1 and C2 is connected to one end of the short-circuit switch K0 and the switching switches K1 and K2, respectively. The other end of the short-circuit switch K0 and the switching switches K1 and K2 is grounded.

[0040] The present invention first measures the bus capacitance C0 according to the following steps, wherein the arc suppression coil switch K is always in a closed state:

[0041] Step 1: With K0, K1, and K2 all disconnected, measure the voltage U. The voltage of the neutral point of the grounding transformer to ground when no measuring switch is engaged is U0.

[0042] Step 2: Close K0, open K1 and K2, and measure the short-circuit current I. d The measurement results are used to increase the constraints on the mathematical model and improve the stability of the numerical solution.

[0043] Step 3: Disconnect K0, close K1, keep K2 disconnected, measure the voltage U, and the voltage to ground of the neutral point of the grounding transformer after the measuring switch K1 is turned on is the first voltage to ground U1;

[0044] Step 4: Keep K0 open, keep K1 closed, close K2, and measure the voltage U. The voltage to ground of the neutral point of the grounding transformer after switching K1 and K2 is turned on is the second voltage to ground, U2.

[0045] Where U0, U1, U2, I d The measurement order can be different, but the state of switches K0~K2 must be the same as the corresponding switch states in steps one to four when measuring each value.

[0046] Then, a mathematical model is established to solve for the bus capacitance C0. Since the inductive reactance of the arc suppression coil is much greater than its equivalent parallel resistance under normal operating conditions (low excitation state), the impedance of the arc suppression coil under normal operating conditions can be simplified to a pure resistance R, ignoring its inductive reactance component. This simplification significantly reduces the complexity of the mathematical model. Based on the above simplification, the equivalent working principle diagram of this invention becomes... Figure 3 As shown.

[0047] Based on the measurement results of the first and second steps in the parameter measurement, using the capacitive reactance X0 of the bus capacitor as the unknown, the relationship between the equivalent resistance R of the arc suppression coil and the capacitive reactance X0 of the bus capacitor C0 can be obtained as shown in the following formula:

[0048]

[0049] Based on the measurement results from steps three and four, the expressions for U1 and U2 can be obtained as follows:

[0050]

[0051]

[0052] in, The angular frequency of the power grid ( Substituting R into the expressions for U1 and U2, we obtain two nonlinear equations containing only X0, and the right-hand side of the expression is simplified to f(X0):

[0053] U1=f1(X0)

[0054] U2=f2(X0)

[0055] Where f1() is the equation for calculating U1, and f2() is the equation for calculating U2;

[0056] Define two residual functions:

[0057] F1(X0) = U1 - f1(X0)

[0058] F1(X0) = U2 - f2(X0)

[0059] The solution to the residual function is X0, which makes the residual equation F1=F2=0. Once X0 is obtained, the value of C0 can be calculated.

[0060] Using the calculation model proposed in this invention, a bus capacitance test was performed on a 10kV II section busbar of an actual substation without an arc suppression coil. The capacitors used for testing, C1 and C2, have values ​​of 47uF and 49uF respectively. The measured data are: U0 = 42.2V, I... d =0.473A, U1=18.9V, U2=11.8V, the calculated result C0=30uF, which is consistent with the conventional test results and basically consistent with the bus capacitance value C0=30.4uF calculated by the traditional test method.

[0061] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

Claims

1. A method for measuring bus capacitance in a low-current grounding system, the system comprising bus capacitance C0 and system three-phase unbalanced potential E. p An arc-suppression coil Z, consisting of a resistor R and an inductor L, and an arc-suppression coil switch K, are characterized in that... The bus capacitance C0 is measured and calculated by using an external short-circuit measuring ammeter T, external measuring capacitors C1 and C2, and corresponding switching switches K0, K1, and K2. Only the switching switches of the external circuit are controlled. The external measuring capacitors C1 and C2, the short-circuit measuring ammeter T, and the arc suppression coil Z are connected in parallel. The measurement steps are as follows: The neutral point-to-ground voltage U0 is measured when the arc suppression coil switch K is always closed and K0, K1, and K2 are all open; the short-circuit current I is measured when K0 is closed and K1 and K2 are open. d When K1 is closed and K0 and K2 are open, measure the first voltage to ground U1; when K0 is open and K1 and K2 are closed, measure the second voltage to ground U2; then establish a mathematical model to solve for the bus capacitance C0.

2. The method for measuring bus capacitance in a low-current grounding system according to claim 1, characterized in that, When establishing the mathematical model to solve for the bus capacitance C0, the impedance of the arc suppression coil under normal operation is simplified to a pure resistance R, and its inductive reactance component is ignored.

3. The method for measuring bus capacitance in a low-current grounding system according to claim 1, characterized in that, The specific steps for establishing a mathematical model to solve for the bus capacitance C0 are as follows: Using the capacitive reactance X0 of the bus capacitor as the unknown, the relationship between the equivalent resistance R of the arc suppression coil and the capacitive reactance X0 of the bus capacitor C0 is shown in the following formula: ； The expressions for U1 and U2 are as follows: ； ； in, Let R be the angular frequency of the power grid frequency. Substituting R into the expressions for U1 and U2, we obtain two nonlinear equations containing only X0, and simplify the right-hand side of the expression using f(X0): U1=f1(X0); U2=f2(X0); Where f1() is the equation for calculating U1, and f2() is the equation for calculating U2; Define two residual functions: F1(X0) = U1 - f1(X0); F1(X0) = U2 - f2(X0); The solution to the residual function is X0, which makes the residual equation F1=F2=0. Once X0 is obtained, the value of C0 can be calculated.

4. The method for measuring bus capacitance in a low-current grounding system according to claim 1, characterized in that, The equivalent circuit of the measurement method is as follows: one end of resistor R, inductor L, external measuring capacitors C1 and C2, arc suppression coil switch K, and ammeter T is grounded; the other end of resistor R and inductor L is connected to the other end of arc suppression coil switch K; the other end of ammeter T and external measuring capacitors C1 and C2 is connected to one end of switching switches K0, K1, and K2 respectively; and the other end of switching switches K0, K1, and K2 is grounded.

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