JPH0316069Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a grounding resistance meter that measures the grounding resistance of a protective grounding circuit of a power distribution line.

3 in the power distribution system, such as in European countries.
In places where a phase 4-wire system is used, the method shown in Figure 1 is used to measure the grounding resistance of the protective grounding circuit. In the figure, P is the power supply, L
is the line, L _O is the neutral line, V is the line voltage between line L and neutral line L _O , SW is the switch, R is the known resistance, r and r' are the voltage dividing resistors, E is the voltage across the resistor R, A is an amplifier (peak detector), D is a diode, C is a capacitor, and M is a meter. That is, conventionally, the line L was momentarily grounded by a switch SW through a known resistor R, and the scale of the meter M was read by peak-holding the value of the current I flowing at that time or the value E obtained by converting it into a voltage. (The scale of meter M has been converted to an ohm value.)

The current I flowing at this time is expressed as I=V/R+Z, where Z is the grounding resistance. That is, when Z=∞, I=
0, when Z=0, I=V/R, and the scale of the meter M becomes an unequal scale that gradually becomes smaller as it increases from 0. Therefore, with such conventional measuring methods, there was a dilemma in that when trying to measure low resistance, high resistance could not be measured, and when trying to measure high resistance, low resistance could not be measured with high accuracy. For this reason, two ranges, high and low, were installed, but in any case, highly accurate measurements could not be expected. Also, a bigger disadvantage of this method is that if the line voltage moves at the moment of measurement, or if the line-to-ground conduction period ends before the peak value of the sine wave is reached, the measured value will be affected. It was subject to change.

The present invention eliminates the above-mentioned drawbacks, and although the operation of instantaneously shorting the power line L to ground through the known resistance R is the same, the method of extracting the measured value is different. Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 2 is a schematic circuit diagram showing an embodiment of the present invention. In the figure, parts corresponding to those in FIG. 1 are designated by the same or similar symbols. D ₃ is a rectifier diode connected to both ends of resistor R. D ₄ is a rectifier diode connected between one end of the ground side of resistor R and the neutral line L _O. A/D is a double integration type analog/digital converter. Converter shown. First, the voltage E generated across the resistor R when the switch SW is momentarily closed is rectified and divided, this divided voltage is held at its peak, and the output is used as the reference signal.
Insert as E'ref into the reference voltage (reference) terminal of the analog-to-digital converter A/D. On the other hand, due to the ground short-circuit current I, a voltage E'(E'=IZ) equal to the voltage across the ground resistor Z is generated between one end on the ground side of the resistor R and the neutral line L _O. Here, the sum of voltages E and E' is line voltage V
There is an equal relationship with This voltage E' is rectified and divided at the same voltage division ratio as voltage E, the divided voltage is held at its peak, and the output is used as the input signal Ein.
input to the input terminal of the analog-to-digital converter A/D. Note that dividing the voltages E and E' is necessary because the voltages E and E' have very large values.
This is to lower the voltage to a level that can be processed by the IC.

Here, a well-known double integral type analog-to-digital converter will be briefly described. Figure 3 is its block diagram, Figures 4 and 5.
The figure is an explanatory diagram of the operation. First, the switch _S1 is turned on for a certain period of time t to integrate the input voltage Ein. This time t is set to the time required for the counter to count 1000, for example. At the end of the fixed time t, switch S ₁ is turned off and switch S ₂ is turned on, and the reference voltage Eref with the opposite polarity to the input voltage Ein is set.
Integrate. The moment the previously integrated input voltage becomes 0, the comparator operates and stops the operation of the integrating circuit. The counter counts the time t' from when the switch _S2 is turned on until the comparator operates, and enters the counted number on the display. In this case, Ein×
t=Eref×t′, therefore Ein=(Eref×t′)/
The relationship t holds true. Here, since t and Eref are constant, the input voltage Ein is proportional to t', that is, the count number. FIG. 4 shows how two voltages Ein1 and Ein2 with a magnitude ratio of 2:3 are represented by 1000 counts and 1500 counts. Then,
A double-integrating analog-to-digital converter can be used as a digital volt meter.

The above was a case where the reference voltage Eref was constant, but
Consider the case where the reference voltage Eref is a changing voltage E′ref. When E'ref is equal to the input voltage Ein, the slope of the integral is the same for both Ein/R'C' as shown in Figure 5, so the count by E'ref is equal to the integral count of 1000 for the input voltage Ein. It's the same. When E′ref is 1/2 of the input voltage Ein, the slope is Ein/
Since 2R'C', Ein×t=Ein/2×t', therefore, t'=2t and the count is 2000. Conversely, E′ref
When is 2Ein, the slope is 2Ein/R′C′ and t′=t/2
Therefore, the display count will be 500. That is,
Varying voltage instead of constant reference voltage Eref
E′ref, this analog-to-digital converter calculates the ratio between the input voltage Ein and the reference voltage E′ref.
Ein/E′ref will be displayed.

The present invention utilizes the properties of such a double integral type analog-to-digital converter. Returning to Figure 2 again, if the resistance R is 10Ω and the power supply voltage is 240V, the short circuit current I will be 240/10+10=12amp. The respective voltage drops E and E' are both 120V. If this voltage is divided to 1/100 and inputted to the converter A/D as E'ref and Ein, Ein/E'ref (=E'/E) = 1.2/1.2 = 1 as described above.
Display. Here, since E′=IZ, E=IR
E′/E=Z/R, and R is known, so display
Ein/E′ref is proportional to the grounding resistance Z. Therefore, if the digital point is set in advance to display a number multiplied by 10, then in the above case 10 will be displayed and the converter A/D will directly display the value of the ground resistance Z of 10Ω. In other words, when the grounding resistance Z is 15Ω, I = 240/10 + 15 = 9.6amp, and the voltage drops E and E' are 96V and 144V, respectively.If you divide this into 1/100 and input it to the converter A/D, , 1.44/0.96 × 10 = 15 (Ω) is displayed.

In addition, the power supply voltage is 24V, which is extremely 1/10 from 240V.
Even if it becomes , resistance R = 10Ω, grounding resistance Z =
When 10Ω, I = 24/10 + 10 = 1.2amp, the respective voltage drops will be 12V and 12V, and if you divide this into 1/100 and input it to the converter A/D, the display will be
0.12/0.12×10=10(Ω). Similarly, when Z=15Ω, 0.144/0.096×10=15(Ω) is displayed. In this way, the display of the converter A/D linearly represents the ground resistance itself with extremely high accuracy, regardless of the power supply voltage.

As explained above, according to the present invention, three-phase four-phase
The grounding resistance of the grounding circuit of a distribution line with a neutral line, such as a wire system, can be determined with high accuracy regardless of whether it is low or high, and is independent of line voltage fluctuations and the magnitude of the instantaneous voltage applied during measurement. can be measured.

Note that this invention is not limited to three-phase, four-wire systems.
If the distribution line uses a neutral line, it can also be applied to a single-phase three-wire type.

[Brief explanation of the drawing]

Fig. 1 is a schematic circuit diagram showing a conventional example, Fig. 2 is a schematic circuit diagram showing an embodiment of the present invention, Fig. 3 is a block diagram of a double integral type analog-to-digital converter in the present invention, and Fig. 4 and FIG. 5 is an explanatory diagram of its operation. L... Line, SW... Switch R... Known resistance, D ₃ , r, r'... First rectification/voltage dividing means, L _O ...
...neutral line, D ₄ , r, r' ... second rectification/pressure dividing means, (A ₁ , D ₁ , C ₁ ), (A ₂ , D ₂ , C ₂ )
...Peak hold circuit, A/D...Double integral type analog-to-digital converter.

Claims (1)

[Scope of utility model registration request] A switch and a known resistor are connected in series to the distribution line, one end of the known resistor is grounded, a first rectifier/voltage dividing means is connected to both ends of the known resistor, and one end of the ground side of the known resistor is connected to both ends of the known resistor. A second rectifier/voltage dividing means is connected between the power line and the neutral line of the distribution line, and when the switch is momentarily turned on, the output of the first rectifier/voltage dividing means is held at its peak. The reference voltage is applied to the reference voltage terminal of the double integral analog-to-digital converter, and the output of the second rectifying/dividing means is applied to the peak voltage.
A digital ground resistance meter that holds the voltage and applies it to an input terminal of the analog-to-digital converter to cause the analog-to-digital converter to display the ground resistance of the ground circuit of the distribution line.