JPH1151993A - Zero-phase current measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make accurate detection by providing signal input sections with different measuring ranges for individual phases, and obtaining the zero-phase current with the outputs of the other input sections when the overflow of the output of an input section with a small range is detected. SOLUTION: Current transformers CT1-CT3 connected in series to the primary side windings are connected to three phases respectively, and individual secondary winding outputs are inputted to operation amplifiers op1-op6 with the same characteristic such as gain. The number of the secondary windings is set so that the measuring ranges of the signal input sections constituted of the CT1-CT3 and op1-op6 are made the 5 A-range of the stationary current measuring range and the 100 A-range of the accident current measuring range for individual phases. An overflow judgment section 4 invariably monitors the signals Ias , Ibs , Ics of the 5 A-range, judges the input signals as the overflow state when one or more signals exceed a fixed level, and switches a switch 5 from the normal contact S side to the O side. An I0 calculation section 6 calculates the zero-phase current I0 based on the signals Iao , Ibo , Ico of the 100 A-range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zero-phase current measuring device for measuring a zero-phase current generated as an excessive fault current at the time of a ground fault in a system.

[0002]

2. Description of the Related Art As a conventional apparatus for measuring a zero-sequence current at the time of a ground fault, for example, as shown in FIG.
(Current transformer) ZCT and its zero-phase current I _{0 as} its output current
And those composed of the I ₀ measurement unit 20 for measuring a respective phase currents I _a, I _b, a I _b detected by CT1 to CT3,
An apparatus configured with an I ₀ calculating unit 21 that calculates I ₀ by inputting a signal in a predetermined measurement range and vector-calculating these phase currents has been put to practical use.

[0003]

However, ZCT
However, the use of an apparatus using the method has a problem in that the cost increases because an expensive ZCT must be newly provided. In contrast, each of the phase currents I _a ,
A device that detects I _b and I _b and calculates I ₀ by vector calculation has an advantage that it can be realized at low cost. However, the zero-phase current is small in relation to the magnitude of the ground fault current and the measurement range of the I ₀ calculation unit 21. It is not always possible to accurately detect I ₀ .

That is, the neutral point of the power system is usually grounded in order to suppress overvoltage in the event of an accident and to reliably operate the protection device. A direct grounding method is used, and a resistance grounding method is used for a power distribution system. For this reason, the magnitude of I ₀ at the time of a ground fault is completely different depending on the grounding method, and in a conventional device having only one measurement range, I ₀
Has exceeded the measurement range, and on the contrary, I ₀ is extremely small with respect to the measurement range, and a certain accuracy cannot be obtained.

An object of the present invention is to provide a zero-phase current measuring device capable of accurately detecting a zero-phase current irrespective of the magnitude of the zero-phase current generated when a ground fault occurs or the like. .

[0006]

The invention according to claim 1 is
A plurality of signal input units having different current measurement ranges are provided for each phase, and among the outputs of the respective signal input units, it is detected whether or not the output of the first signal input unit having a small measurement range has overflowed. If not, a zero-sequence current is calculated based on the output of the first signal input unit. If an overflow occurs, a zero-sequence current measurement unit that calculates a zero-phase current based on the output of another signal input unit is provided. It is characterized by having been provided.

[0007] The plurality of signal inputs provided for each phase typically include a first signal input having a steady-state current measurement range and a second signal input having a fault current measurement range. It consists of. In this case, the ranges for measuring the fault current may be divided into two types, and a signal input unit corresponding to each range may be provided. Whether the output of the first signal input unit having a small measurement range (for example, a signal input unit having a steady-state current measurement range) among the plurality of signal input units provided as described above overflows the measurement range. When no overflow occurs, the zero-phase current I ₀ is obtained by vector calculation based on the output of the first signal input section. When the overflow occurs, another signal input section (eg, abnormal The zero-phase current I ₀ is obtained by vector calculation based on the output of the signal input unit). The detection of overflow is
I _a, I _b, all I _c becomes the target, either when one phase current is overflowed the zero-phase current I ₀ based on the output of the other signal input unit is determined, all the phase currents Only when the overflow does not occur, the zero-phase current I ₀ is obtained based on the output of the first signal input unit. Further, when the zero-phase current I ₀ as determined at the zero-phase current measuring unit is caused by a ground fault or the like, is often to record the waveform of the zero-phase current I ₀ and the other signals, the In this case, the zero-phase current measurement unit may be activated based on the contact output of the protection device that detected the accident, or the zero-phase current measurement unit may always be operated, and only when an accident is detected, the zero-phase current obtained at that time. Waveforms and other signal waveforms may be recorded.

According to a second aspect of the present invention, the first signal input section having a small current measurement range is a signal input section for a steady current measurement range, and the other signal input section is a signal input section for a fault current measurement range. Part.

In general, a plurality of signal input sections are constituted by a signal input section having a steady-state current measurement range and a signal input section having a fault current measurement range, so that an ultra-high voltage system in which a neutral point is a direct grounding system is used. However, the present invention can also be applied to a power distribution system in which the neutral point is a resistance grounding system. As an example, the steady current measurement range and the fault current measurement range are 5 A and 1 respectively.
00A can be set.

The invention according to claim 3 is characterized in that each of the plurality of signal input sections is constituted by a plurality of operational amplifier circuits having different gains connected to the secondary side output of the current transformer. I do.

In the present invention, since the measurement ranges of the plurality of signal input units need only be different, the secondary-side output of one current transformer having a different gain can be used without providing a plurality of expensive current transformers. A configuration in which amplification is performed by a plurality of operational amplifier circuits may be employed. In this way, downsizing and cost reduction can be achieved.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic configuration diagram of a main part of a zero-phase current measuring device according to an embodiment of the present invention. CT1 to CT3 connected in series to the primary side winding are respectively three phases a to
Connected to the c-phase, the respective secondary winding outputs of the CTs 1 to 3 are input to operational amplifiers OP1 to OP6 as shown in the figure. The operational amplifiers OP1 to OP6 have the same gain and other characteristics. However, the number of secondary windings of each of the CTs 1 to 3 is set to n1 and n2 (n1> n2).
Therefore, in the signal input section composed of the CT and the operational amplifier, the measurement range of each signal input section is CT1 to CT3.
In this example, the measurement range for each phase is 5 amps (5 amps), which is the steady-state current measurement range.
The number of secondary windings of n1 and n2 is determined so that A) range and the fault current measurement range of 100 amperes (100A) range. In the example shown in FIG. 1, in the a phase,
I _aS, which is the output of OP1, becomes a signal in the 5A range, and I _ao, which is the output of the operational amplifier OP2, becomes a signal in the 10A range. Similarly, in the b phase, _Ibs is a signal in the 5A range, and _Ibo is a signal in the 100A range. Also,
In the c phase, I _cs is a signal in the 5A range, and I _co is 10
The signal is in the 0A range.

The overflow judging section 4 constantly monitors the signals I _aS , I _bs , and I _cs in the 5A range, and any one or more of these signals is in an overflow state exceeding a certain level. When it is determined that the operation has been completed, the switch 5 is switched. Normally, the switch 5 has a contact connected to the s side, and the I _aS , I _bs , and I _cs signals are transmitted to the I ₀ calculating unit 6.
To be entered. However, if it is determined that any one of these signals has overflowed beyond a certain level, the contact of the switch 5 is switched to the o side. As a result, the I ₀ calculator 6 calculates the zero-phase current I ₀ by vector calculation based on the signals I _ao , I _bo , and I _co that are signals in the 100 A range.

FIG. 2 shows a specific configuration diagram of the zero-phase current measuring device.

The zero-phase current measuring device has a line voltage measuring terminal as well as a current measuring terminal for each phase. The line voltages V _ab , V _bc , and V _ca are respectively the instrument transformer PT
The signal is input to the low-pass filter LPF via the operational amplifiers OP 10 to OP 12 and OP amplifiers OP 12. The LPF prevents a folding error at the time of quantization, and its output is input to the CPU 20 as digital data via the sample / hold circuit S / H, the multiplexer MPX21 and the A / D converter. The phase currents I _a , I _b , and I _c are respectively connected to CT12, CT3-4, and CT56 whose primary sides are connected in series.
1, CT2, 4, and 6 have n2 windings (n1> n2). In this embodiment, the measurement range by the combination of the subsequent OPs 13 to 18 and each CT is CT1,
The combination of 3, 5, and OP results in 5A, CT2,
The gain of each OP and the number of windings n1 and n2 are set so as to be 100 A in the combination of 4, 6, and the OP.

The CPU 20 determines the line voltages V _ab , V _bc ,
Sequentially switched by MPX21 for V _ca input,
The phase currents I _a , I _b , and I _c are sequentially switched by the MPX 22 and taken in.

[0017] ROM23 stores a program for calculating the _{I 0} based on the data inputted through the A / D converter. The RAM 24 sequentially stores data input from the A / D converter and stores other data during operation by background processing of the CPU 20. When the storage area is full, the data input from the A / D converter is sequentially stored by repeating the FIFO processing in which the data stored most recently is discarded and stored at the head. The I / O 27 includes a print control unit 25
And the display control unit 26 are connected, and the print control unit 25 prints the output result on paper, and the display control unit 26
Displays the output result on the display. Here, the result recorded as output is a waveform.

Next, the operation of the zero-phase current measuring device will be described with reference to FIG. FIG. 3 is a flowchart showing an operation of calculating and outputting an _I0 waveform or the like when an accident is detected by the CPU 20.

In step ST1, line voltages V _ab , V _bc , V _ca from the A / D converter and phase currents I _a , I _b , I a
_c. The sampling input data of _c
4 in a predetermined area. This operation is actually a CPU
It is always performed in 20 backgrounds. ST2
Determines whether the input signal waveform of the rated current measurement range, that is, the 5A range overflows a certain level.

This determination is YES if any one of the three input signal waveforms in the 5A range overflows. If none of the three input signal waveforms has overflowed, the process proceeds to ST4, where I ₀ is calculated with an input signal in the 5A range (rated current measurement range), and the value is stored in RAM2.
4 in a predetermined area. What is calculated is a continuous value of the instantaneous value of I ₀ , and as a result, an I ₀ waveform is obtained and stored. In ST2, if any one of the three input signals overflows, the process proceeds to ST3, where I ₀ is calculated with the input signal in the 100A range (fault current measurement range), and the value is stored in a predetermined area of the RAM 24. .

After calculating _I0 and obtaining its waveform as described above, an accident such as a ground fault is detected in ST5.
The accident detection is performed by, for example, the line voltages V _ab , V
Either _bc or _Vca is below a predetermined voltage (<
UV), or one of the phase currents I _a , I _b , I _c exceeds a predetermined current (> OC), or the zero-phase voltage V ₀ exceeds a predetermined voltage (> OV), or zero-phase Current I ₀
Is greater than a predetermined current (> OC). If the accident detection is yes, ST6, ST
For a 3 or ST4, printed or displayed outputs the stored waveform of I _0. In this embodiment, the object to be printed or displayed and output is a waveform of _I0 and a line voltage V _abc (V
_ab , _Vbc , _Vca ), phase current _Iabc ( _Ia , _Ib ,
I _c ) and the zero-phase voltage V ₀ . In addition, since accident detection requires at least one cycle of waveform data to improve its accuracy, it is required to obtain the _I0 waveform from data slightly before the accident detection timing. In the example shown in FIG. 3, since the I ₀ waveform is always obtained and stored before executing the accident detection step, when an accident is detected in ST5, the data of the I ₀ waveform after the timing slightly before the detection timing is obtained. Is printed or displayed. In the example shown in FIG.
3, ST4 are to be calculated and stored at all times I ₀ in, but may be output to calculate the I ₀ only when fault detection. In the case of such control, an accident is detected in ST11 as shown in FIG. Since the fault detection in this case cannot be performed at I ₀ or V ₀ , for example, one of the line voltages V _ab , V _bc , V _ca is lower than a predetermined voltage (<UV), or Phase currents I _a , I _b , I
_This is performed when any one of _c exceeds a predetermined current (> OC). When an accident is detected, data from a timing before that timing is read out from the RAM 24 and I ₀ calculation is performed.

FIG. 5 shows the accident detection timing and the _I0 waveform printed or displayed and output. As shown in the figure,
The waveform from a point shortly before the accident detection timing is displayed. The same applies to the waveforms of the line voltage V _abc (V _ab , V _bc , V _ca ), the phase current I _abc (I _a , I _b , I _c ), and the zero-phase voltage V ₀ .

In the above embodiment, as shown in FIG. 2, by connecting the primary sides of two CTs having different numbers of secondary windings in series, the signal input sections for the steady-state current measurement range and the fault current measurement range are connected. However, as shown in FIG.
And the secondary-side output of the two OPs with different gains
(OP-A and OP-B), it is also possible to provide a signal input section for a steady-state current measurement range and a fault current measurement range. Normally, the linearity of the CT is quite good, so even if configured in this way, the accuracy will not be degraded, and the cost and size of the OP can be reduced because the OP is inexpensive and small compared to the CT. is there.

The number of current measurement ranges is not limited to two, but may be more.

[0025]

According to the first aspect of the present invention, even if the magnitude of the zero-phase current generated at the time of the occurrence of a ground fault or the like is small enough to be measurable in the steady-state current measurement range, the range of the steady-state current is reduced. , The measurement of the zero-sequence current can be performed without automatically changing the measurement range and deteriorating the accuracy.

According to the second aspect of the present invention, by setting one of the signal input sections to the steady-state current measurement range, normal steady-state current measurement can be shared.

According to the third aspect of the present invention, the cost and size of the OP can be reduced because the OP is inexpensive and small in size as compared with the CT.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a main part of a zero-phase current measuring device according to an embodiment of the present invention.

FIG. 2 is a detailed block diagram of the zero-phase current measuring device.

FIG. 3 is a flowchart showing the operation of the zero-phase current measuring device.

FIG. 4 is a flowchart showing the operation of another example of the zero-phase current measuring device.

FIG. 5 is a diagram showing an accident detection timing and a recorded waveform example.

FIG. 6 is a diagram showing another configuration example of the signal input unit of the zero-phase current measuring device.

FIG. 7 is a configuration diagram of a conventional zero-sequence current measuring device.

Claims (3)

[Claims] A plurality of signal input units having different current measurement ranges are provided for each phase, and it is determined whether an output of a first signal input unit having a smaller measurement range among outputs of each signal input unit overflows. And when no overflow occurs, a zero-phase current is obtained based on the output of the first signal input unit,
A zero-phase current measuring device, comprising: a zero-phase current measuring unit that obtains a zero-phase current based on an output of another signal input unit when an overflow occurs. 2. The method according to claim 1, wherein the first signal input portion having a small current measurement range is a signal input portion for a steady current measurement range, and the other signal input portion is a signal input portion for a fault current measurement range. The zero-sequence current measuring device according to claim 1, wherein 3. The apparatus according to claim 1, wherein each of the plurality of signal input units is constituted by a plurality of operational amplifier circuits connected to a secondary side output of a current transformer and having different gains. 2. The zero-phase current measuring device according to 2.