JP2696513B2 - Electrical capacitance measurement method for ground - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for measuring a ground capacitance of an electric circuit or the like in a live state. (Prior Art) Conventionally, in order to find a trouble in an electric circuit such as an electric leakage, it has been general to measure an insulation resistance and monitor an electric circuit state. However, when the load Z is connected to the electric circuit, the measurement is made as if the insulation resistance was lowered despite the normal electric circuit and the load due to the leakage current leaking to the ground from the capacitance between the load and the ground. In order to monitor the state of the electric circuit, it is necessary to measure the electrostatic capacitance to the ground, that is, the capacitance between the electric circuit itself and the ground or the capacitance of the connected load, and to monitor the capacitance together with the insulation resistance. FIG. 2 shows a method of measuring the electrostatic capacitance generally used in the related art. Low frequency signal oscillator which the ground line L _E of the power receiving transformer T having a load Z, becomes different frequency ₁ from the commercial power source frequency
Or allowed to penetrate to the connected transformer OT to OSC, or by applying a low-frequency voltage measured by, for example connecting insert the oscillator in series with the path 1 and path 2 to the ground line L _E, the ground line L _E The leakage current generated by the low-frequency voltage that is fed back to the ground line via the insulation resistance Ro existing between the electric circuit and the ground and the ground stray capacitance Co is detected by the current transformer ZCT penetrating through the current transformer ZCT and amplified by the amplifier AMP. After that, filter FIL
Only the component of frequency ₁ is selected, and this is synchronously detected by the multiplier MULT using the voltage obtained by applying the output signal of the oscillator OSC to the 90 ° phase shifter PSS, for example. The electrostatic capacitance of the electric circuit is measured by detecting the component (that is, the component shifted by 90 ° from the applied low frequency voltage). The measurement theory will be further described to assist in understanding the present invention. If Vsinomega ₁ t and (omega _{1 =} 2 [pi ₁₎ a low-frequency signal voltage applied to the ground line L _E, for example, as a sine wave, the frequency ₁ which returns to the ground line L _E via the ground point E leakage current I is If a component that is 90 ° out of phase with the applied AC voltage, that is, a value proportional to the component of the second term on the right side of the above equation (1), is detected by means such as synchronous detection, this value is determined by the capacitance. Since it is proportional to Co, the capacitance of the electric circuit can be obtained from this. However, in the conventional method in which the leakage current returning to the ground line is detected by the current transformer ZCT and the leakage current component of frequency ₁ included in the current transformer output is selected and output by the filter FIL, the current is usually changed. Since the phase of the leakage current at frequency ₁ is shifted in the flower → amplifier → filter system,
In order to obtain a value proportional to Co from these synchronous detection outputs, it is necessary to compensate for this phase shift. For this purpose, a fixed phase shifter PS is inserted into the first input terminal or the second input terminal of the synchronous detector MULT as shown in the figure, thereby correcting the phase shift and synchronizing with each other. . However, in the conventional method as described above, the phase characteristics of the current transformer ZCT, filter FIL, phase shifter PS, etc. fluctuate due to temperature changes or aging of the characteristics of the parts used, and as a result, the phase error from the initial set value This has the disadvantage that correct measurement results cannot be provided. In order to cope with these, the influence of the phase error has been minimized by adopting an extremely high-quality current transformer or filter with little characteristic fluctuation, but it is still difficult to completely eliminate the influence. Met. (Object of the Invention) The present invention has been made in order to eliminate the disadvantages of the conventional method for measuring the capacitance to ground as described above.
It is an object of the present invention to provide a method for measuring the capacitance to ground that can always correct the phase shift of a measurement signal at low cost without requiring expensive components and can always provide accurate measurement results. SUMMARY OF THE INVENTION In order to achieve this object, the invention of claims 1, wherein in the earth capacitance measuring method of path according to the present invention, low for measurement of different frequencies f ₁ and path of the commercial frequency applying a frequency signal voltage, at the earth capacitance measuring method path for detecting a leakage current of a frequency f ₁ to return to the path of the ground line, generated in a system for detecting a leakage current of the frequency f ₁ leakage The phase transition of the current is θ, the phase transition of the phase shifter PS is θ ₁ ,
The phase transition when PS and phase shifter PSSS are connected in series is θ ₂
(Where θ ₁ ≠ θ ₂ ), the relationship between the phase transition θ and the phase transition amount θ ₁ in the phase shifter PS is | θ−θ ₁ | ≪1, the phase transition θ and the phase transition θ ₂ and of the relationship | θ-θ ₂ | set the phase shift amount of the phase shifter PSSS with adjusting the phase shift amount of << 1 become as the phase shifter PS, the leakage current the frequency f ₁ a component to obtain a first output by synchronous detection by using the voltage obtained phase remained only theta ₁ by applying the low-frequency signal voltage for measurement to said phase shifter PS, the frequency f The leak current component of _{1 and} the low frequency signal voltage for measurement are
Via the PS obtain a second output by synchronous detection by using the voltage obtained phase remained only theta ₂ by applying to the phase shifter PSSS, the first output and the second output And the capacitance of the electric circuit to ground was measured using the difference between (Embodiments of the Invention) Hereinafter, the present invention will be described in detail based on the illustrated embodiments. FIG. 1 is a block diagram showing one embodiment of the present invention, and the same symbols as those in FIG. 2 have the same meanings. Ie, T At a drawing transformers, 1 and 2 this transformer the electric path 2 a secondary low path ground line L _E which has been subjected to second type grounding is connected. The grounding line L _E are combined with trans OT and current transformer ZCT, the output end of the displacement current transformer ZCT an amplifier
Connected to AMP, the output of which is connected to filter FIL. The output of the filter FIL is connected to one input terminal of the synchronous detector MULT1, the other input terminal is connected to the output terminal of the first phase shifter PS, and the input terminal of the PS is connected to the output terminal of the oscillator OSC. Connecting.
On the other hand, one input terminal of the synchronous detector MULT2 is connected to the output terminal of the filter FIL, and the other input terminal is connected to the output terminal of a second phase shifter PSSS connected to the output terminal of the phase shifter PS. . The respective output terminals of the synchronous detectors MULT1 and MULT2 are connected to the input terminals of a subtraction circuit SUB. In the circuit configured in this way, the signal input from the oscillator OSC to the ground line via the transformer OT is fed back via the current transformer ZCT, and the phase shift that occurs when passing through the amplifier AMP and filter FIL system If θ is θ, the leakage current component at frequency _{1 at} the filter FIL output is And input to one input terminal of each of the synchronous detectors MULT1 and MULT2. On the other hand, the output of the oscillator OSC is applied to another input terminal of the first synchronous detector MULT1 via the phase shifter PS. This voltage is a _o sin
(Ω ₁ t + θ ₁ ), the output M _{1 of} the first synchronous detector
Is Becomes On the other hand, the output of the phase shifter PS is added to the phase shifter PSSS having a fixed fixed phase amount, and the output is a _o cos (ω ₁ t + θ ₂ )
Then, the output M ₂ of the second synchronous detector is Becomes If the phase shifter PS is further adjusted to | θ−θ ₁ | ≪1, | θ−θ ₂ | ≪1, cos (θ−θ ₁ ) 1, cos (θ−θ ₂ ) 1, sin
(Θ−θ ₁ ) θ−θ ₁ , sin (θ−θ ₂ ) θ−θ ₂ , M ₁ and M ₂ in the equations (3) and (4) are Is approximated. Note that, in order to satisfy | θ−θ ₁ | ≪ and | θ−θ ₂ | PS1, the specific method of adjusting the PS and setting the PSSS is given by the following equation (3) when | θ−θ ₁ | ≪1 The second term on the right side of is "0"
Since the output of MULT ₁ of the synchronous detector becomes the maximum,
Transfer device P so that the output of synchronous detector MULT ₁ is almost maximum.
With adjusting _{S, | θ-θ 2 |} « case (4) of the second term on the right-hand side becomes" 0 ", the output of the synchronous detector MULT ₂ is maximum, the synchronous detector MULT ₂ Output Should be set such that θ ₁ is close to the maximum and θ ₁ ≠ θ ₂ . Therefore, if the outputs of the synchronous detectors MULT1 and MULT2 are input to the subtractor SUB, the output of the SUB Is obtained. Equation (7) in theta ₁ - [theta] ₂ to is a fixed phase shift amount by the phase shifter PSSS, further angular frequency omega _1, the voltage V, a _o is the output of the subtraction circuit SUB if a constant value Is the ground capacitance Co
Can be obtained, and the ground capacitance Co can be calculated from the voltage value without being affected by the phase characteristics due to the aging and temperature change of each component in the circuit. In the above description, only the case of a single-phase two-wire circuit has been described. However, the present invention is not limited to this. Further, it is apparent that the commercial frequency voltage can be used as a substitute for the measurement low frequency signal voltage when measuring each phase of the single-phase two-wire circuit and the three-phase circuit. (Effects of the Invention) Since the present invention is constructed and operates as described above, it is possible to compensate for the influence of the fluctuation of the phase characteristic in the ground capacitance measuring circuit with a simple circuit configuration, and to obtain an accurate ground capacitance. This is very effective in measuring the capacity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a block diagram showing a conventional method for measuring a ground capacitance. T… Transformer, 1,2… Electrical path, L _E … Ground line, E… Ground point, MULT, MULT1, MULT2… Synchronous detector, ZCT… Current transformer, AMP… Amplifier, FIL… … Filter, OSC …… Oscillator, OT …… Injection transformer, PS, PSS, PSSS …… Phase shifter.

Claims (1)

(57) [Claims] Applying a low-frequency signal voltage for the measurement of different frequencies f ₁ and path of the commercial frequency, the frequency f ₁ is fed back to the path of the ground line
Leakage current at the earth capacitance measuring method path for detecting the frequency f ₁ of the phase transition of the leakage current generated in a system for detecting a leakage current theta, the phase shifter a phase transition of the PS theta ₁ ,
If the phase shift when the phase shifter PS and the phase shifter PSSS are connected in series is θ ₂ (where θ ₁ ≠ θ ₂ ), the phase shift θ and the phase shift amount θ ₁ in the phase shifter PS relationship with the | θ-θ ₁ | «1, the relationship between the phase shift theta and the phase shift theta ₂ is | θ-θ ₂ | «1 and the phase shifter so that
Set the phase shift amount of the phase shifter PSSS with adjusting the phase shift amount of PS, a leakage current component of the frequency f _1, by applying a low-frequency signal voltage the measurement to the phase shifter PS The phase obtained is θ
_{A first} output is obtained by performing synchronous detection using the voltage shifted by 1 and the phase shift of the leakage current component of the frequency f ₁ and the measurement low-frequency signal voltage via the phase shifter PS is performed. obtain a second output by synchronous detection by using the voltage obtained phase remained only theta ₂ by applying to the vessel PSSS, path using a difference between the first output and the second output A method for measuring the capacitance of an electric circuit to ground, characterized by measuring the capacitance to ground.