CN106771647A - A Method for Measuring Capacitive Current of Small Current Grounded Grid - Google Patents

Abstract

The invention relates to a method for measuring the capacitance current of a small current grounding power grid, which comprises an HC-1 type capacitance current tester and a measuring device, wherein the HC-1 type capacitance current tester injects a 5Hz constant-current square wave signal into a zero sequence loop of a power distribution network, and charges and discharges a line to ground capacitance, so as to measure the capacitance current to ground of the power distribution network.

Description

A Method for Measuring Capacitive Current of Small Current Grounded Grid

technical field

The invention belongs to the field of power system measurement, in particular to a method for measuring the capacitive current of a small-current grounded power grid.

Background technique

In my country, most of the line faults in the medium and low voltage distribution network are single-phase ground faults. Since the neutral point is not grounded, the line voltage vector triangle does not change when the single-phase ground fault occurs, and the neutral point voltage of the three phases remains unchanged. Under the condition that the capacitance current of the power grid is not large and the grounding arc can be self-extinguished, the power grid can continue to supply power for 1 to 2 hours with a fault. Therefore, the early medium and low voltage distribution networks in my country mostly adopted ungrounded methods to improve the reliability and continuity of power supply. With the rapid development of urban construction, more and more cable lines are used in urban power grids, which greatly increases the capacitive current of the power grid. According to statistics, many short-circuit faults in the distribution network are caused by the fact that the single-phase ground current of the line cannot be extinguished by itself. An unstable, intermittent ground arc that causes arcing ground overvoltages of high magnitude. The arc overvoltage lasts for a long time and affects a large area. It may cause insulation breakdown and expand the fault to the equipment with poor insulation in the power grid and the insulation weakness on the line. In addition, due to the instability of the neutral point, the single-phase grounding of the power grid is likely to cause electromagnetic resonance, which leads to accidents such as voltage transformer burnout and high-voltage fuse blown, which brings great harm to the safe and reliable operation of the power grid.

Traditional capacitive current measurement methods can be divided into direct method and indirect method.

Direct measurement methods such as the single-phase metal grounding method, in the earlier period, the single-phase metal grounding method was generally used to measure the capacitive current. The capacitive current into the ground is measured directly by means of a current transformer. The operation and wiring required by this method are very complicated, and it is possible to endanger the insulation at the weak point of the non-grounded phase insulation and cause two phases to be short-circuited to different grounds, which is very unsafe. The whole test work poses a threat to the safety of the test personnel and the power grid, and the test is very dangerous.

At present, there are many methods of indirectly measuring the capacitance current of the power grid, which can be divided into two categories according to whether they directly contact the primary side: the first type is the method of adding capacitance at the neutral point, which must directly deal with the primary side during the measurement process; The second type, such as the injection signal injection method, does not need to be in direct contact with the primary side because it injects the triangle through the PT opening and measures the feedback signal in real time during the measurement process. The indirect measurement methods that have to deal directly with the primary side, such as the neutral point external capacitance method, are currently used. Although they are simpler than the direct method and can measure the capacitance current value more accurately, they still have the following disadvantages: ①The measurement is still related to When dealing with one side, the safety of personnel and equipment cannot be guaranteed. ②Because primary equipment is involved, the operation is cumbersome and there is also the risk of misoperation. The preparation work is time-consuming and the measurement work efficiency is low. Usually most of the time is spent waiting for dispatching orders, issuing work tickets, switching operations, and taking safety measures. Only two to three stations can be measured a day, and the work efficiency is very low. Due to the above two reasons, it is hoped that there is a device that can directly measure the capacitive current of the system on the secondary side without involving the dangerous primary side; does not require cumbersome switching operations and waiting, and makes the measurement of the capacitive current simple and efficient , easy. In the capacitive current measurement method that does not need to be in direct contact with the primary side, it is mainly divided into two categories according to the source of the required calculation signal: the basic idea of the first category is to use the neutral point displacement voltage and neutral point displacement voltage during normal operation of the power grid. The total capacitive reactance of the power grid to ground is obtained by calculating parameters such as the point current and the inductance value of the arc suppressing coil; then, the capacitive current in the current operation mode is calculated from the zero-sequence circuit when a single-phase fault occurs. The second type is the injection signal method, which uses the feedback signal of the different frequency current injected from the outside of the system to calculate the capacitance current of the grid to ground through the corresponding equivalent circuit.

The more commonly used methods in the signal entry method include the three-frequency method and the frequency division method, but the three-frequency method can only be applied to the case where the line-to-ground capacitance is small. When the capacitance is large, the three-frequency method cannot be used anyway. Meet measurement needs. Compared with the three-frequency method, the stability of the frequency division method has been greatly improved, but when measuring a small capacitance, when the frequency of the injected signal is low, the impedance of the entire RLC series circuit will be very large, which is not much different from the excitation impedance of the PT. , In this case, a large measurement error will be caused, so when measuring a small capacitance, the frequency division method will have a large error.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide a method for measuring the capacitive current of a small current grounded power grid based on a square wave signal injection method.

The present invention solves its technical problem and realizes by taking the following technical solutions:

A method for measuring the capacitive current of a small-current grounded power grid, including a HC-1 capacitive current tester and a measuring device. The ground capacitance is charged and discharged, so as to measure the ground capacitance current of the distribution network. The measuring device is composed of AC and DC power supply systems, digital signal microprocessors, signal acquisition units and power electronic inverter circuits. The digital signal microprocessor controls The power electronic inverter circuit generates a constant current 5Hz square wave signal, which is injected into the zero-sequence circuit of the system through the open triangle of the system bus bar, the neutral point of the ground transformer or the neutral point of the capacitor bank. The signal acquisition system adopts a high-performance A/D sampling circuit and digital signal processing.

Moreover, the HC-1 capacitive current tester is connected to the secondary opening triangle of the busbar PT of the system under test through a special cable.

Moreover, the HC-1 capacitive current tester is connected to the secondary side of the supporting PT of the instrument through a special cable, and then the primary side of the matching PT of the instrument is connected to the neutral point of the grounding transformer through an insulating rod.

Moreover, the HC-1 capacitive current tester is connected to the secondary side of the supporting PT of the instrument through a special cable, and then the primary side of the supporting PT of the instrument is connected to a compensating capacitor bank through an insulating rod.

Moreover, if a constant current i ₀ is injected into the low-voltage side of the PT, there will be currents i ₁ , i ₂ , i ₃ on the high-voltage side of the PT, and PT excitation currents i _a , i _b , i _c , and the number of turns of the high-voltage and low-voltage windings of the PT is n respectively. ₁ and n ₂ have the following relationship:

n ₁ (i ₁ -i _a )=n ₂ i ₀

n ₁ (i ₂ -i _b )=n ₂ i ₀

n ₁ (i ₃ −i _c )=n ₂ i ₀ .

Moreover, after the injected signal is a constant current square wave signal of 5 Hz, Fourier transform and signal analysis are performed on the injected signal. For the injected square wave signal, the Fourier series can be expanded into the sum of sine waves with different angular frequencies, namely:

Sampling and data processing in the secondary opening triangle of the voltage transformer can obtain U ₅ , U ₁₅ , U ₂₅ and I ₅ , I ₁₅ , and I ₂₅ respectively:

The injected signal is calculated and analyzed to obtain the magnitude of the system capacitive current.

Moreover, the HC-1 type capacitive current tester is connected to the circuit of the secondary opening triangle of the busbar PT of the system under test through a special cable, including: HC-1 type capacitive current tester, insulation rod with fuse, grounding transformer , arc suppressing coil and bus transformer, in which the low voltage side of the bus transformer is delta connected, the arc suppressing coil exits, and the HC-1 capacitive current tester injects a 5Hz constant current square wave signal into the dedicated PT low voltage side of the instrument, and the red clip Connect the black clip to terminal a and connect to terminal x to form a loop on the low-voltage side of the dedicated PT, and on the high-voltage side of the PT to connect to the neutral point of the grounding transformer through an insulating rod with a fuse to form a loop with the line-to-ground capacitance, and to form a loop through the input square wave The signal is decomposed by Fourier series and the voltage and current of the PT low-voltage side of the instrument are measured to calculate the capacitance of the line to ground, and then the capacitance current.

Moreover, the HC-1 type capacitive current tester is connected to the secondary side of the supporting PT of the instrument through a special cable, and then the primary side of the supporting PT of the instrument is connected to the neutral point circuit of the grounding transformer through an insulating rod, including: HC- Type 1 capacitive current tester, insulating rod with fuse, compensating capacitor bank, arc suppressing coil and bus transformer, among them, the low voltage side of the bus transformer is delta connected, and the arc suppressing coil exits. Connect the red clip of the HC-1 capacitance current tester to terminal a and the black clip to terminal x to form a loop on the low-voltage side of the dedicated PT, and connect the neutral point of the capacitor bank to the line pair on the high-voltage side of the PT through an insulating rod with a fuse. The ground capacitance forms a loop. Through Fourier series decomposition of the input square wave signal and the voltage and current measurement of the PT low-voltage side of the instrument, the ground capacitance of the line is calculated, and then the compensation capacitance is subtracted according to the conversion formula to obtain Get the real capacitance, and then find the capacitance current.

Moreover, the HC-1 type capacitive current tester is connected to the secondary side of the supporting PT of the instrument through a special cable, and then the primary side of the supporting PT of the instrument is connected to a circuit of a compensating capacitor bank through an insulating rod, including: HC- Type 1 capacitive current tester, insulating rod with fuse, compensating capacitor bank, arc suppressing coil and bus transformer, in which the low voltage side of the bus transformer is connected in delta, the secondary harmonic elimination device is disconnected, and the high-impedance resonance elimination device is shorted. Connect the red clip of the HC-1 type capacitance current tester to the L terminal and the black clip to the N terminal to form a loop on the low-voltage side of the dedicated PT. Through Fourier series decomposition of the input square wave signal and the voltage and Current measurement, calculate the ground capacitance of the line, and then calculate the capacitance current.

Advantage and positive effect of the present invention are:

The advantage of the present invention is that for distribution networks with different voltage levels, special single-phase voltage transformers are customized to reduce the impact of excitation current on the measurement accuracy of capacitive current; the injected signal is a 5Hz constant current square wave signal, and the injected signal is Fourier transform and signal analysis, the zero-sequence loop of the system is injected into the system’s voltage transformer twice, and the signal acquisition system uses high-performance A/D sampling loop and digital signal processing to calculate and analyze the injected signal, so as to obtain the system The size of the capacitor current; the square wave signal injection method is more conducive to reducing the impact of the three-phase voltage unbalance on the measurement accuracy; this method can use a variety of signal injection methods, namely grounding transformer neutral point injection, compensation capacitor neutral point injection and Triangular injection of busbar PT opening; this method only needs an external voltage transformer and an insulating rod with a fuse, does not need to add additional equipment and instruments, and can be measured when the power grid is running normally without changing the power grid wiring, which can be safe and Quickly and accurately measure the system capacitance current value.

Description of drawings

Fig. 1 is a block diagram of circuit principle of the present invention;

Fig. 2 is to adopt busbar opening triangular PT injection method to measure wiring diagram among the present invention;

Fig. 3 is to adopt grounding transformer neutral point injection method to measure wiring diagram among the present invention;

Fig. 4 is a measurement wiring diagram using the compensation capacitor bank neutral point injection method in the present invention.

detailed description

The present invention will be further described in detail below in conjunction with the accompanying drawings and through specific embodiments. The following embodiments are only descriptive, not restrictive, and cannot limit the protection scope of the present invention.

A small current grounding grid capacitance current measurement method, the test principle shown in Figure 1, mainly includes the following parts: HC-1 capacitance current tester, instrument supporting PT, insulation rod with fuse, grounding transformer, capacitor bank, Busbar PT, where the busbar PT open delta connection method, the system arc suppressing coil exits, the HC-1 capacitance current tester is connected to the secondary open triangle of the busbar PT of the system under test through a special cable, or the special cable of the tester is connected to the instrument The secondary side of the matching PT, and then connect the primary side of the supporting PT of the instrument to the neutral point of the compensation capacitor bank (or grounding transformer) through an insulating rod. The zero-sequence circuit injects a 5Hz constant current square wave signal to charge and discharge the line-to-ground capacitance, thereby measuring the ground-to-ground capacitance current of the distribution network.

Using a dedicated PT for capacitive current measurement

Inject a constant current i ₀ at the low-voltage side of the PT, then there will be currents i ₁ , i ₂ , i ₃ at the high-voltage side of the PT, and PT excitation currents i _a , i _b , i _c , and the number of turns of the PT high-voltage and low-voltage windings is respectively n ₁ , n ₂ has the following relationship:

n ₁ (i ₁ -i _a )=n ₂ i ₀

n ₁ (i ₂ -i _b )=n ₂ i ₀

n ₁ (i ₃ -i _c )=n ₂ i ₀

Usually we think that since excitation impedance>>winding resistance R, excitation impedance>>leakage reactance X _L , excitation impedance>> capacitive reactance X _C , so i _a =i _b = _ic =0, namely i ₁ =i ₂ = i ₃ . But in the actual situation i _a ≠ i _b ≠ i _c ≠ 0, the excitation current of PT will have a great influence on the error of our measurement, especially the number of turns of the winding will also affect the accuracy of our measurement. In this case, we properly adjusted the number of turns of the PT winding and the winding method, effectively reducing the influence of the excitation current on the measurement accuracy of the capacitive current.

The injection signal is a constant current square wave signal of 5Hz

The injected signal is a 5Hz constant current square wave signal, and Fourier transform and signal analysis are performed on the injected signal. For the injected square wave signal, the Fourier series can be expanded into the sum of sine waves with different angular frequencies, namely:

Sampling and data processing in the secondary opening triangle of the voltage transformer can obtain U ₅ , U ₁₅ , U ₂₅ and I ₅ , I ₁₅ , and I ₂₅ respectively:

The microprocessor can accurately calculate the capacitance C by solving the equations, so as to calculate the value of the capacitance current. The principle block diagram of the measurement device is shown in Figure 1. The device is mainly composed of AC and DC power supply systems, digital signal microprocessors, signal acquisition units and power electronic inverter circuits.

The digital signal microprocessor controls the power electronic inverter circuit to generate a constant current 5Hz square wave signal, which is injected into the zero-sequence circuit of the system through the open triangle of the system bus bar, the neutral point of the ground transformer or the neutral point of the capacitor bank. The signal acquisition system adopts high Performance A/D sampling circuit and digital signal processing, calculate and analyze the injected signal, so as to obtain the size of the system capacitor current.

Different injection methods will be described below through three embodiments.

Example 1

As shown in Figure 2, a method for measuring the capacitive current of a small-current grounded power grid is measured by the busbar opening triangle PT injection method, including: HC-1 capacitive current tester, insulating rod with fuse, grounding transformer, arc suppression coil and busbar transformer , where the low voltage side of the bus transformer is connected in a triangle, the arc suppression coil exits, and the HC-1 capacitive current tester injects a 5Hz constant current square wave signal into the dedicated PT low voltage side of the instrument, and connects the red clip to terminal a and the black clip to x The terminal forms a loop on the low-voltage side of the dedicated PT, and the high-voltage side of the PT is lapped to the neutral point of the grounding transformer through an insulating rod with a fuse to form a loop with the capacitance of the line to ground, and the Fourier series is performed on the input square wave signal Decompose and measure the voltage and current of the PT low-voltage side of the instrument, calculate the ground capacitance of the line, and then calculate the capacitance current.

As shown in Figure 3, a method for measuring the capacitive current of a small-current grounded power grid uses the neutral point injection method of the grounding transformer for measurement, injects a signal at the neutral point of the compensation capacitor bank, and selects the neutral point injection mode of the capacitor bank for measurement, which mainly includes : HC-1 capacitance current tester, insulating rod with fuse, compensating capacitor bank, arc suppressing coil and bus transformer, among them, the low voltage side of the bus transformer is delta connected, and the arc suppressing coil exits. Connect the red clip of the HC-1 capacitance current tester to terminal a and the black clip to terminal x to form a loop on the low-voltage side of the dedicated PT, and connect the neutral point of the capacitor bank to the line pair on the high-voltage side of the PT through an insulating rod with a fuse. The ground capacitance forms a loop. Through Fourier series decomposition of the input square wave signal and the voltage and current measurement of the PT low-voltage side of the instrument, the ground capacitance of the line is calculated, and then the compensation capacitance is subtracted according to the conversion formula to obtain Get the real capacitance, and then find the capacitance current.

See Figure 4. A method for measuring the capacitive current of a small-current grounded power grid. The grounding transformer neutral point injection method is used to measure the wiring, and the signal is injected into the triangle of the bus transformer opening. It includes: HC-1 type capacitive current tester, insulating rod with fuse , Compensation capacitor bank, arc suppressing coil and bus transformer, in which the low voltage side of the bus transformer is connected in delta, the secondary harmonic elimination device is disconnected, and the high-impedance harmonic elimination device is shorted. Connect the red clip of the HC-1 type capacitance current tester to the L terminal and the black clip to the N terminal to form a loop on the low-voltage side of the dedicated PT. Through Fourier series decomposition of the input square wave signal and the voltage and Current measurement, calculate the ground capacitance of the line, and then calculate the capacitance current.

Although the embodiments and drawings of the present invention are disclosed for the purpose of illustration, those skilled in the art can understand that various replacements, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims Therefore, the scope of the present invention is not limited to what is disclosed in the embodiments and drawings.

Claims (8)

1. a kind of low current neutral grounding electric network capacitance current measurement method, including HC-1 types Capacitive current tester and measurement apparatus, It is characterized in that：HC-1 types Capacitive current tester injects 5Hz constant current square-wave signals to power distribution network zero sequence loop, to line-to-ground Electric capacity carries out discharge and recharge, and so as to be measured to power distribution network ground capacity electric current, measurement apparatus are by AC and DC power-supply system, number Word signal microprocessor, signal gathering unit and power electronic circuit.capacitor composition, digital signal microprocessor control electric power electricity Sub- inverter circuit produces a constant current 5Hz square-wave signals, and neutral point or capacitor are become by system busbar open delta, ground connection The zero sequence loop of group neutral point injected system, signal acquiring system uses high-performance A/D sampling circuits and Digital Signal Processing.

2. low current neutral grounding electric network capacitance current measurement method according to claim 1, it is characterised in that：The HC-1 types Capacitive current tester is connected to bis- open deltas of system under test (SUT) bus PT by Special cable wire.

3. low current neutral grounding electric network capacitance current measurement method according to claim 1, it is characterised in that：The HC-1 types Capacitive current tester is connected to the secondary side of the supporting PT of instrument by private cable, then by insulating bar by the supporting PT of instrument once Side is connected to grounding transformer neutral point.

4. low current neutral grounding electric network capacitance current measurement method according to claim 1, it is characterised in that：It is described HC-1 types Capacitive current tester is connected to the secondary side of the supporting PT of instrument by private cable, then by insulating bar that instrument is supporting PT primary sides connect a compensation capacitor group.

5. low current neutral grounding electric network capacitance current measurement method according to claim 1, it is characterised in that：In PT low-pressure sides Injection constant current i₀, then have electric current i in PT high-pressure sides₁, i₂, i₃, PT exciting currents i_a, i_b, i_cIf, PT high-low pressure umber of turns Respectively n₁, n₂Then there is following relation:

n₁(i₁-i_a)=n₂i₀

n₁(i₂-i_b)=n₂i₀

n₁(i₃-i_c)=n₂i₀,

After Injection Signal is for the constant current square-wave signal of 5Hz, the signal to injecting carries out Fourier transformation and signal analysis.For The square-wave signal of injection by the deployable sine wave sum different into angular frequency of fourier series, i.e.,：

$u\left(t\right)=\frac{2U_m}{\mathrm{\π}}({\mathrm{sin\ω}}_{5}t+\frac{1}{3}{\mathrm{sin\ω}}_{15}t+\frac{1}{5}{\mathrm{sin\ω}}_{25}t+\mathrm{...})$

Sampling is carried out in voltage transformer secondary open delta and data processing respectively obtains U₅, U₁₅, U₂₅And I₅, I₁₅, I₂₅Then have：

$U_5=I_5\sqrt{R^2+{({\mathrm{\ω}}_{5}L-\frac{1}{{\mathrm{\ω}}_{5}C})}^2}$

$U_{15}=I_{15}\sqrt{R^2+{({\mathrm{\ω}}_{15}L-\frac{1}{{\mathrm{\ω}}_{15}C})}^2}$

$U_{25}=I_{25}\sqrt{R^2+{({\mathrm{\ω}}_{25}L-\frac{1}{{\mathrm{\ω}}_{25}C})}^2}$

Signal to injecting carries out calculating analysis, so as to obtain the size of system capacitive current.

6. low current neutral grounding electric network capacitance current measurement method according to claim 2, it is characterised in that：The HC-1 types Capacitive current tester is connected in bis- circuits of open delta of system under test (SUT) bus PT by Special cable wire, including：HC- 1 type Capacitive current tester, band fuse insulating bar, grounding transformer, arc suppression coil and busbar transformer, wherein busbar transformer Low-pressure side delta connection, arc suppression coil is exited, and HC-1 types Capacitive current tester is noted to the supporting special PT low-pressure sides of instrument Enter the constant current square-wave signal of 5Hz, red clip is connect into a ends black clip connects x ends in special PT low-pressure sides formation loop, and in PT High-pressure side snaps into grounding transformer neutral point and forms loop with line mutual-ground capacitor by the insulating bar with fuse, by defeated The square-wave signal for entering carries out Fourier space decomposition and PT low-pressure side voltages supporting to instrument and current measurement, calculates circuit Ground capacitance, and then obtain capacitance current.

7. low current neutral grounding electric network capacitance current measurement method according to claim 3, it is characterised in that：The HC-1 types Capacitive current tester is connected to the secondary side of the supporting PT of instrument by private cable, then by insulating bar by the supporting PT of instrument once Side is connected in the circuit of grounding transformer neutral point, including：HC-1 types Capacitive current tester, band fuse insulating bar, compensation Capacitor bank, arc suppression coil and busbar transformer, wherein busbar transformer low-pressure side delta connection, arc suppression coil are exited.Will HC-1 types Capacitive current tester red clip connects a ends black clip and connects x ends and forms loop in special PT low-pressure sides, and high in PT Pressure side snaps into capacitor bank neutral point and forms loop with line mutual-ground capacitor by the insulating bar with fuse, by input Square-wave signal carries out Fourier space decomposition and PT low-pressure side voltages supporting to instrument and current measurement, calculates circuit over the ground Capacitance, then subtracts compensating electric capacity amount according to reduction formula, obtains real capacitance, and then obtain capacitance current.

8. low current neutral grounding electric network capacitance current measurement method according to claim 4, it is characterised in that：It is described HC-1 types Capacitive current tester is connected to the secondary side of the supporting PT of instrument by private cable, then by insulating bar that instrument is supporting PT primary sides are connected in the circuit of a compensation capacitor group, including：HC-1 types Capacitive current tester, band fuse insulating bar, benefit Capacitor bank, arc suppression coil and busbar transformer, wherein busbar transformer low-pressure side delta connection are repaid, secondary resonance elimination device breaks Open, high resistant resonance eliminator short circuit.HC-1 types Capacitive current tester red clip is connect into L ends black clip, and to connect N-terminal low in special PT Pressure side forms loop, and Fourier space decomposition and PT low-pressure side voltages supporting to instrument are carried out by the square-wave signal being input into And current measurement, the ground capacitance of circuit is calculated, and then obtain capacitance current.