JP3996119B2 - Leakage current measuring device - Google Patents

Description

  The present invention relates to a leakage current measuring device, and more particularly to a technique for measuring a leakage current of a distribution system.

In the distribution system, the insulation state is regularly measured to prevent leakage fires, but in the past, cables and facilities were mega-ringed (insulation resistance measurement) after a power failure. Further, when the power failure was not caused, the leakage current was measured with a leak tester or the like. These problems are that the mega ring method cannot be performed frequently because the circuit under test must be interrupted, and the measurement method using a leak tester is generally a leakage current measurement. There is a problem that it is not possible to measure the resistance leakage current flowing through the. The following Patent Documents 1 and 2 are proposed as means for solving these problems.
JP 2001-215247 A Japanese Patent Laid-Open No. 10-78462

  In the above-described known example, the effective component in the leakage current, that is, the resistance current can be measured. However, it is necessary to measure the magnitude of the voltage of the circuit to be measured. There is also a problem that it becomes expensive. In addition, the zero-phase current transformer has various phase characteristics depending on the product, and there is a problem that the accuracy is deteriorated and cannot be used in a general purpose combination. The present invention is to solve these problems.

  The present invention is an amplifier that amplifies an output signal from a zero-phase current transformer (ZCT) that is installed in a circuit under test to detect a leakage current, and converts an analog signal that is an output signal of the amplifier into a digital signal. An A / D conversion unit, a zero-cross detection circuit that detects a voltage zero-cross point of the circuit under test and outputs a sampling start signal, a storage unit that stores sine wave data having an effective value of 1, and a calculation as a main component A CPU that samples the leakage current by the sampling start signal, synchronizes the sampled output signal value with the stored digital data of the sine wave waveform of effective value 1, and calculates the average of the multiplication value This is a leakage current measuring device that obtains an effective current component.

  Further, according to the present invention, the storage unit subdivides and stores digital data having a sine wave waveform whose effective value is 1, which is the same as the phase advance or delay of the output signal of the zero-phase current transformer. This is a leakage current measuring device which performs multiplication using digital data whose amount is delayed or advanced.

  In the present invention, the sine wave waveform data having an effective value of 1 is stored as 30-degree digital data in the storage unit, multiplied by the sampled output signal value of the zero-phase current transformer, This is a leakage current measuring device for calculating an effective amount of leakage current of a phase delta circuit.

  According to the present invention, it is not necessary to measure the magnitude of the voltage of the circuit under test, and the components for this are unnecessary and simplified, so that the cost is reduced. Further, even a general-purpose zero-phase current transformer that can be easily procured can obtain a more accurate effective divided current.

  That is, by measuring the effective component current of the leakage current of the distribution system, it is possible to grasp the quality of the insulation state, prevent the leakage fire due to the leakage current, and the effect is great.

The best mode for carrying out the present invention will be described.
Embodiments of the leakage current measuring apparatus according to the present invention will be described below with reference to FIGS. FIG. 1 is a diagram for explaining an algorithm of the leakage current measuring apparatus according to the first embodiment. FIG. 2 is a configuration diagram of the leakage current measuring apparatus according to the first embodiment. FIG. 3 is a diagram illustrating a specific example of calculating the effective current of the leakage current in the first embodiment. FIG. 4 is a waveform diagram illustrating an algorithm of the leakage current measuring apparatus according to the first embodiment. FIG. 5 is a diagram illustrating an example of an effective current measuring method in consideration of the phase characteristics of the zero-phase current transformer in the first embodiment. FIG. 6 is a diagram illustrating an example of phase characteristics of the zero-phase current transformer. FIG. 7 is a vector diagram showing the leakage current phase of the three-phase delta circuit.

  FIG. 1 is a diagram illustrating an algorithm of the first embodiment, where 1 is a circuit to be measured, 2 is a zero-phase current transformer (ZCT), and 3 is a leakage current measuring device. Reference numeral 5 denotes a zero-cross detection circuit that detects a zero-cross point of the voltage waveform of the circuit under test and generates a sampling start signal, which will be described later. Reference numeral 6 denotes an output signal of the zero-cross detection circuit, and a sampling start signal. An amplifying circuit for amplifying the output signal of the detected leakage current, 12 digitally samples the amplified leakage current signal waveform at 121 by the CPU and stores the digital waveform in the storage unit 13 and also in the storage unit 14 The power calculation with sine wave waveform data having an effective value of 1 is mainly performed.

  That is, the leakage current power P is a multiplication (power factor) of the voltage V, the current I, and the phase angle Φ, and the calculation formula is represented by P = VI cos Φ. On the other hand, it can be easily understood from the above calculation formula that the effective current is obtained by dividing the leakage current power P by the voltage V.

In addition, the leakage current power p in the sampling method is an average of products (p = vi ) of instantaneous values of the voltage v and the current (leakage current) i (122 in the figure). When divided by v, an effective current is obtained (123 in the figure).

  Here, the present embodiment is characterized in that sinusoidal waveform data having an effective value of 1 is used without measuring the voltage. That is, the zero-cross point of the voltage waveform is set as the sampling start point, and is replaced with data of a sine wave waveform having an effective value of 1 synchronized with the voltage waveform. Since the leakage current power described above multiplies the leakage current by a voltage and further divides the power by the voltage, if the data waveform has an effective value of 1, the leakage current power becomes an effective current. In addition, since there are other elements such as the amplification unit 7 described later, the leakage coefficient is obtained by multiplying the result by the current coefficient.

  By the way, the voltage imbalance rate of the circuit under test is generally ± 2% or less, and the influence on the leakage current power due to the voltage imbalance is very small. It can be done.

  The effective amount of the leakage current is measured by the above algorithm. FIG. 2 is a block diagram of the leakage current measuring apparatus 3 that embodies the above contents. In addition, description is abbreviate | omitted about the component demonstrated above. 4 is a power supply circuit that supplies a stable voltage to the leakage current measuring device, and 7 is an amplifier circuit that converts the detection signal (current signal) of the zero-phase current transformer (ZCT) into a voltage signal and has an appropriate size. It amplifies. 8 is a sample-and-hold circuit (S / H circuit) for temporarily holding via the signal line 9 in accordance with an instruction from the CPU 12 because the converted voltage signal changes every moment. This is an A / D conversion circuit that performs analog / digital conversion of the output signal via the signal line 11 in accordance with an instruction from the CPU 12, and the converted data becomes sampling data of leakage current.

  The CPU 12 includes a non-volatile storage unit 14 that stores data of a sine wave waveform having an effective value 1 and a volatile storage unit 13 that stores the sampled data, and the calculation result is stored in the volatile storage unit 13. Is done.

  Reference numeral 15 is a display circuit for displaying the above-mentioned effective current and leakage current values, 17 is a setting circuit for selecting the display contents and selecting and setting the type of the zero-phase current transformer, and 16 is the display contents. This is a communication circuit for transmitting the same content to a host device (not shown). The host device can remotely measure the effective current and leakage current via the communication circuit 16 and can record them as a record, so that a trend graph of the effective current can be displayed. Accordingly, since the effective current, that is, the insulation state can be always displayed, the deterioration state can be grasped.

  FIG. 3 and FIG. 4 are diagrams showing more concretely the calculation operation in the effective current measurement by the above-described algorithm, and the sampling timing starts from the sampling start signal as described above and is one AC period as shown in FIG. Is divided into 32, the leakage current waveform is sampled, the obtained digital value is sequentially stored in the storage unit 13, and the digital value is multiplied by the sine wave waveform data of the effective value 1 stored in the storage unit 14 to obtain a result. Is stored in the storage unit 13. When 32 sampling is completed, an average of the multiplied values is taken, and an effective current can be obtained by multiplying the value divided by 32 for 32 sampling by a coefficient not shown. The coefficient is uniquely determined by the current transformation ratio of the zero-phase current transformer, the burden resistance during current / voltage conversion, the amplification factor of the amplification circuit, and the resolution of the A / D conversion circuit.

  Here, the contents of the storage unit 14 in FIG. 3 will be described. FIG. 3 shows a state in which sinusoidal waveform data having an effective value 1 is stored corresponding to each angle of 32 samplings per AC cycle, so that it can be easily understood. The angle is shown in parentheses, and the actual data is shown on the right. That is, the angle at the sampling timing 1 is 0 degree, the data of the effective value 1 is 0, the angle at the sampling timing 2 is 11.25 degrees (an angle obtained by dividing 360 degrees by 32), and the effective The data of value 1 is 0.276 (rounded off to the fourth decimal place). These values are sine functions with respect to the angle of each sampling timing of a sine waveform having a peak value of √2.

  As described above, the leakage current power can be calculated and the effective current can be measured without measuring the voltage value of the circuit under test. In addition, the electric power of FIG. 4 shows leakage current electric power.

  Next, FIGS. 5 and 6 will be described. A zero-phase current transformer (ZCT) has good phase characteristics as long as it is highly accurate, but a general-purpose type has different phase characteristics depending on the frequency, and the phase changes depending on the magnitude of leakage current to be measured. FIG. 6 shows an example of the frequency / current phase characteristics. Further, the phase characteristics differ depending on the type of the zero-phase current transformer, and the phases of the output signals of the circuit under test and the zero-phase current transformer are shifted, making it impossible to obtain a correct effective current. A method for solving this is shown in FIG. 5 in which values of sin functions corresponding to angles before and after each point of each sampling timing in FIG. 3 are arranged. FIG. 5 shows an example in which the value of the sin function of 1.125 degrees obtained by dividing 10 times during one sampling timing (11.25 degrees) is arranged, and if the value of the sin function is arranged at all during this one sampling timing. Obviously, various phase shifts can be accommodated. Moreover, although the said example is an example of 10 division | segmentation, if the division | segmentation number is increased, more accurate effective component current can be measured and calculated. FIG. 5 shows a case where the phase of the leakage current is about one degree ahead of the phase of the zero phase current transformer in comparison with the phase in the circuit under test. In this case, the sine wave waveform data having an effective value of 1 is delayed by one degree. The data value is multiplied. That is, the leakage current power is calculated by delaying the output phase of the zero-phase current transformer (equivalent to making it the same as the phase of the circuit under test). Therefore, the effective component current is correctly measured and the accuracy is improved. In the case of the phase characteristics as shown in FIG. 6, it is necessary to calculate by delaying the phase by about 23 degrees at 50 Hz, but the frequency can be detected by measuring the period of the sampling start signal output by the zero cross detection circuit. Information can be obtained, and the magnitude of the current can be known by calculating the effective value of the sampled leakage current, so it can be determined how many times the phase should be delayed.

Although the above description shows a case where the circuit under test is a single-phase circuit or a single-phase three-wire circuit, the three-phase delta circuit will be described with reference to FIG. In the three-phase delta circuit of the low voltage circuit, the second phase (S phase) is grounded. Therefore, in the phase voltage phase, as shown in FIG. 7, the first phase (R phase) voltage and the third layer (T phase) voltage It is known that the third phase is advanced by 60 degrees. Here, when the leakage current (Izr) of the first phase (R phase) and the leakage current (Izt) of the third phase (T phase) are generated as shown in FIG. 7, the above-described zero-phase current transformer ( The leakage current (Iz) phase detected by ZCT is the combined leakage current (Iz) of Izr and Izt. The effective component of the first phase (R phase) is Izr · cos Φ1, and the effective component of the third phase (T phase) is Izr · cos Φ2, and the effective component (Ir) is the first component as shown in FIG. Appears at a position 30 degrees ahead of the phase (R phase). Here, cos Φ1 is a phase angle between the R phase voltage phase and Izr, and cos Φ2 is a phase angle between the T phase voltage phase and Izr. That is, it is the same as the effective amount of the leakage current (Iz). By the way, the effective component must be in phase with the voltage phase, and therefore, the sine wave waveform data having an effective value of 1 is calculated by being advanced by 30 degrees, whereby an effective component current can be obtained.
The correction method considering the phase characteristics of the zero-phase current transformer described with reference to FIGS. 5 and 6 can be applied in the same manner, and an accurate effective component current of the leakage current can be obtained.

  In addition, the leakage current often includes harmonics, and a filter circuit (low-pass filter) may be provided to remove these harmonics. In this case, a phase delay is caused by the filter circuit, so the phase must be advanced. In some cases, it is obvious that the characteristics of the filter can be grasped, so that it can be easily handled.

  Furthermore, although the phase characteristics differ depending on the type of the zero-phase current transformer, if the characteristics are known in advance, it can be dealt with by setting the angle for phase correction from the setting unit 17 described above.

The figure explaining the algorithm of the leakage current measuring apparatus of Example 1. FIG. The block diagram of the leakage current measuring apparatus of Example 1. FIG. FIG. 6 is a diagram illustrating a specific example of calculating an effective current component of a leakage current in the first embodiment. The wave form diagram explaining the algorithm of the leakage current measuring apparatus of Example 1. FIG. The figure which shows the example of the measuring method of the effective current which considered the phase characteristic of the zero phase current transformer in Example 1. FIG. The figure which shows an example of the phase characteristic of a zero phase current transformer. The vector diagram which shows the leakage current phase of a three-phase delta circuit.

Explanation of symbols

1 Circuit under test 2 Zero-phase current transformer (ZCT)
DESCRIPTION OF SYMBOLS 3 Leakage current measuring apparatus 4 Power supply circuit 5 Zero cross detection circuit 6 Sampling start signal line 7 Amplifier 8 S / H circuit 9 Signal line 10 A / D conversion circuit 11 Signal line 12 CPU
DESCRIPTION OF SYMBOLS 13 Volatile memory | storage part 14 Non-volatile memory | storage part 15 Display circuit 16 Communication circuit 17 Setting circuit 121 Digital sampling process 122 Power calculation process 123 Leakage current effective part calculation process

Claims (3)

  An amplifier for amplifying an output signal of a zero-phase current transformer that is installed in a circuit under test to detect leakage current; an A / D converter for converting an analog signal that is an output signal of the amplifier to a digital signal; A zero-cross detection circuit that detects a voltage zero-cross point of the circuit under test and outputs a sampling start signal, a storage unit that stores digital data of a sine wave waveform whose effective value is 1, and a CPU that mainly performs computations And sampling the output signal of the zero-phase current transformer by the sampling start signal, synchronizing the sampled output signal value with the stored digital data of the sine wave waveform of the effective value 1, and averaging the multiplication values A leakage current measuring device characterized by obtaining an effective divided current of the leakage current. In the leakage current measuring device according to claim 1,
The storage unit subdivides and stores digital data having a sine wave waveform whose effective value is 1, and is delayed or advanced by the same amount as the phase advance or delay of the output signal of the zero-phase current transformer. Leakage current measuring device characterized by performing multiplication using digital data. In the leakage current measuring device according to claim 1 or 2,
The digital product obtained by advancing the sinusoidal waveform data of effective value 1 by 30 degrees and the sampled output signal value of the zero-phase current transformer are averaged to obtain an effective current of the leakage current of the three-phase delta circuit Leakage current measuring device characterized by that.

Images