JPH09229981A - Monitoring device used for monitoring harmonics and harmonic monitoring system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a monitoring device which is suitable for preventing a trouble due to a harmonic current and to provide a monitoring system which inspects its data in a concentrated manner and which can specify the generation source of harmonics. SOLUTION: The monitoring system is composed of local monitoring devices 4 (4a to 4d) which obtain current signals and voltage signals by current transformers 2 (2a to 2d) and VT's (transformers) installed at electric circuits 1 (1a to 1d) and of a central monitoring device 25 which transmits and receives signals from a plurality of local monitoring devices 4 via a transmission line 5. In every local monitoring device 4, a harmonic current signal is sampled, digitized and frequency-analyzed, and a monitoring value and a warning time limit are set so as to be output to a warning contact when they are exceeded. The central monitoring device 25 judges the direction of a harmonic power flow and specifies the generation source of harmonics on the basis of received data.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a monitoring device and a monitoring system for measuring and monitoring a harmonic current generated by an induction from a load such as an inverter connected to an electric line and a load near the electric line, and more particularly to a harmonic device. The present invention relates to a configuration of a monitoring device and a monitoring system which are effective for detecting the generation direction of a wave current and identifying the generation source.

[0002]

2. Description of the Related Art In recent years, the measurement of harmonic currents on electric circuits and the identification of their sources have been attracting attention due to the necessity of preventing damage such as damage to electrical equipment due to harmonic currents and the occurrence of fire accidents. . As a device for this purpose, for example, a handy-type device for measuring and displaying a harmonic current is known, which has a built-in current transformer configured so as to sandwich an electric path. In addition, a current transformer that matches the rated current of the electric circuit is installed, and the current and harmonic currents are measured and displayed from the secondary output of the current transformer, and a measurement and display device that transmits these values to the host device is available. Are known.

[0003]

In the former handy type device, it is possible to easily perform measurement without the need of installing a current transformer. However, it is not possible to centrally monitor harmonic currents over a wide range of individual circuits.
Further, since the harmonic current can be confirmed only at the measurement site and the recordability is insufficient, it is unsuitable for continuously monitoring the harmonic current in a wide range of electric circuits.

On the other hand, according to the technique of installing a current transformer and measuring and displaying the current and the harmonic current, the harmonic current can be intensively monitored and continuously over a wide range of electric circuits. Harmonic currents can be monitored.

However, it is necessary to investigate the generation source in order to prevent the disturbance due to the harmonic current.
In the above-mentioned conventional technology, it was difficult to determine the generation direction and generation source of the harmonic current.

The present invention provides a monitoring device suitable for preventing a failure due to a harmonic current, and a monitoring system capable of centrally reading the data and identifying the source of the harmonic. The purpose is to

[0007]

In order to achieve the above object, according to a first aspect of the present invention, in a monitoring device for monitoring harmonics flowing on a circuit, means for detecting a current on the circuit. And means for obtaining the magnitude of the current for the harmonic to be monitored from the detected current, means for monitoring whether or not the harmonics to be monitored exceed preset monitoring values, and means for monitoring In the above, when it is detected that any one of the harmonics exceeds the reference value, a monitoring is provided for each of the harmonics to be monitored after a predetermined time has elapsed, and means for outputting an alarm. A device is provided.

According to the second aspect of the present invention, a plurality of local monitoring devices provided in the power distribution system for measuring and monitoring harmonics, and data of the local monitoring devices should be fetched and monitored. Regarding harmonics, the central monitoring device that derives the harmonic flow and identifies the harmonic source is connected to each local monitoring device and the central monitoring device, and data is transmitted from each local monitoring device to the central monitoring device. The central monitoring device is provided with means for obtaining a phase difference of current for each harmonic to be monitored for harmonics of the circuit, and current for each harmonic taken from each local monitoring device. And a means for deriving a harmonic flow based on the magnitude of the value and the phase to identify the harmonic source.

In the above harmonic monitoring system, the monitoring device of the first aspect described above can be used as the local monitoring device.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The outline and details of the embodiments of the present invention will be sequentially described below.

An outline of the installation state of the monitoring device according to the present invention is shown in FIG. The monitoring device 4 shown in the figure is connected to the current transformer (CT) 2 and the transformer (VT) 3 installed in the electric circuit 1. The current transformer (CT) 2 and the transformer (VT) 3 transform the energizing current of the electric path and the circuit voltage. The signal obtained by the transformation is input to the monitoring device 4. This monitoring device 4
Outputs measurement data via the transmission path 5.

An example of the structure of the monitoring device 4 is shown in FIG. The monitoring device 4 has, as its input / output section, an input terminal 6 for inputting a current signal from CT2 and an input terminal 7 for a voltage signal from VT3 for inputting a signal. The input terminal 6 is connected to the internal current transformer (CT) 10C, and the input terminal 7 is connected to the internal transformer (VT) 10T. Further, the monitoring device 4 has a power input terminal 8 for operating the monitoring device 4.
A transmission signal terminal 9 for transmission, an alarm contact 19 for performing a relay operation, a display element 13 for displaying, a keyboard 15 for inputting an instruction and an operation, and a monitoring device 4. And a rotary SW 20 for setting an address on the network. Here, the transmission signal terminal 9 is connected to the transmission driver / receiver 22.

Further, the monitoring device 4 has therein an internal current transformer (CT) 10C and an internal transformer (VT) 10T.
A / D converter 11 for converting each of the current and the voltage input via the digital signal into a digital signal, a CPU 12 for performing calculation and the like for measurement / control, and a memory 17 for storing set values, data and the like. The driver 14 of the display element 13, the decoder 16 for decoding the input from the keyboard 15, the relay driver 18 for driving the alarm contact 19, the decoder 21 for decoding the setting of the rotary SW 20, and these are connected. It has a bus 12a for transmitting and transmitting information and a power supply circuit 23. The A / D converter 11 includes an element that simultaneously samples and holds each analog input value.

The internal current transformer (CT) 10C, the internal transformer (VT) 10T and the A / D converter 11 function as means for detecting current and voltage on the electric path. Also, CP
U12 and the memory 17 exceed the preset monitoring value for each of the means for obtaining the magnitude of the current for the harmonic to be monitored from the current detected by the detecting means and the harmonic to be monitored. It acts as a means of monitoring. Further, when the CPU 12, the memory 17, and the relay driver 18 detect that one of the harmonics exceeds the reference value by the monitoring means,
For each harmonic to be monitored, it functions as a means for outputting an alarm after a predetermined time has elapsed. The correspondence relationship between the hardware resources and the functional configuration is an example, and the present invention is not limited to this.

The monitoring device 4 operates as follows. The internal current transformer (CT) 1 converts the current signal input to the input terminal 6
The data is transformed and synthesized by 0C, sampled and held by the A / D converter 11, and digitized. Similarly, the voltage signal input to the input terminal 7 is transferred to the internal transformer (V
T) It is transformed by 10T and sampled and held by the A / D converter 11 in synchronization with the current signal and digitized. The sampling is performed at a sampling frequency that is at least twice the frequency f _max of the highest-order harmonic to be measured, and the frequency band of the signal is limited to f _max or less by a low-pass filter before sampling.

The keyboard 15 is used to select display items and
It accepts operations such as metamorphosis ratio and monitoring values. The signal generated by the operation is coded by the decoder 16 and input to the CPU 12 via the bus 12a.

The CPU 12 sends data and set values such as the transformation ratio input from the keyboard 15 to the memory 17 for storage. Further, the CPU 12 takes in the digitized signal via the bus 12a and performs necessary processing such as calculation. The measured value and the like are displayed on the display element 13 via the driver 14. The CPU 12 monitors the measured value,
When the measured value exceeds the monitored value and a certain time has passed, the alarm contact 19 is driven via the relay driver 18 and a signal is output to the outside.

The power supply circuit 23 converts the electric power input from the power supply input terminal 8 into a required voltage and supplies it to each part of the monitoring device 4.

The monitoring device 4 of the present invention can be used not only as a single device but also as a local monitoring device together with other monitoring devices. That is, the plurality of local monitoring devices 4a to 4d and the central monitoring device 25 are connected and used via the transmission line 5 (see FIG. 3). That is, it constitutes a network together with other devices. Therefore, the local monitoring device 4
The data measured by is transmitted to the transmission path 5 by the CPU 12 via the transmission driver / receiver 22. Also,
A command sent from the central monitoring device 25 via the transmission path 5 is sent to the CPU 12 via the transmission driver / receiver 22.
Is received by

The rotary SW 20 sets the address of the local monitoring device 4 on the network. The local monitoring device 4 exchanges information with the set address.

Next, an embodiment of a monitoring system according to another invention of the present application, which incorporates a local monitoring device, will be described with reference to FIG.

FIG. 3 shows an example of the configuration of the monitoring system. As shown in FIG. 3, the monitoring system of the present invention includes a plurality of local monitoring devices 4a to 4d provided in a power distribution system for measuring and monitoring harmonics, and the local monitoring device 4a.
The central monitoring device 25 that takes in the data of 4d, derives the flow of harmonics, and specifies the harmonic source for the harmonics to be monitored. These are connected via a transmission line 5 to form a network. That is, the transmission line 5
Are the local monitoring devices 4a-4d and the central monitoring device 25.
And data is transmitted from each of the local monitoring devices 4a to 4d to the central monitoring device 25. Also, in FIG. 3, the secondary side electric circuit 1a of the transformer 24 of the power source is shown.
1 shows an example of a system for performing monitoring in an electric line configured to branch the electric lines 1b to 1d. In FIG. 3, as an example of the branch circuit, 1b, 1c, 1d
However, in an actual electric circuit, the circuit voltage and the number of branch circuits may be various.

In the electric circuit 1a, a current transformer (C
T) 2a is installed, and CT2 is installed in the electric circuits 1b to 1d.
b-2d are installed, respectively. A local monitoring device 4a is connected to the current transformer (CT) 2a, and local monitoring devices 4b to 4d are correspondingly connected to the CTs 2b to 2d. Although not shown in FIG. 3, the VT for voltage measurement is similarly connected to the local monitoring devices 4a to 4d. The VT input is common in the same circuit voltage range. The power input is also connected in the same manner, but not shown.

Here, a capacitor 32 is connected to the electric line 1b via a reactor 31. Loads 34 and 36 are connected to the electric paths 1c and 1d via inverters 33 and 35, respectively.

Although not shown, the central monitoring unit 25 includes a central processing unit (CPU), a memory, an interface, an input unit, an output unit, a communication unit and the like.
The input device includes a keyboard and the like, and the output device includes a display device. Further, a printing device is connected as needed. In addition to this, it has an external storage device. In this central monitoring device, the communication device receives the data sent from the local monitoring devices 4a to 4d via the transmission path 5,
Store in memory. Then, each CPU executes the program stored in the memory, whereby the central monitoring device 2
5 is a means for obtaining the phase difference of the current for each harmonic to be monitored for the harmonics of the circuit, and the magnitude of the current value for each harmonic fetched from each local monitoring device and the phase based on the phase. And a means for identifying a harmonic source by deriving a wave flow.

Next, the monitoring operation of the monitoring device of the present invention will be described.

With the widespread use of power electronic equipment such as inverters, the current waveform has become a distorted waveform including harmonics. FIG. 4 shows an example of the current waveform of the electric path. In the monitoring device of the present invention, such distorted current waveform is subjected to frequency analysis to obtain a value for each harmonic order.

The following four harmonics are generally considered (IEC 1000-4-7, item 4.1). a) quasi-stationary (gradual fluctuation) harmonics b) fluctuating harmonics c) large fluctuation harmonics (sudden harmonics) d) intermediate harmonics Here, in the present invention, quasi-stationary harmonics of a). The main target is waves.

In the monitoring of the present invention, the current waveform is accumulated in a constant time width (Tw), sampled, digitized, and analyzed by discrete Fourier transform (hereinafter referred to as DFT). The current waveform input to the input terminal is, for example, that shown in FIG.

In monitoring, first, a current waveform and a voltage waveform are sampled. The data of the current waveform sampled by the A / D converter 11 is as shown in FIG. 5, for example. The voltage waveform is generally a sine wave. After that, the A / D converter 11 converts each sampled value into digital data. In addition, monitoring is
Perform for each phase of R, S, T.

Sampling is performed as follows. That is, as shown in FIG. 6, when the power of the local monitoring device 4 is turned on, the CPU 12 first performs an initialization process (step S001). Checking the memory 17, reading the address on the network of the local monitoring device 4 set by the rotary SW 20 via the decoder 21, reading the rated current setting value input by the keyboard 15 via the decoder 16, etc. Is processed. When the initialization process is completed, the CPU 12 performs a timer process (step S002). That is, a counter with a count value n = N is set, a timer (not shown) is activated, and an interrupt cancellation instruction is issued. As shown in FIG. 9, the timer repeats until an interrupt request is generated and the CPU 12 gives an instruction to cancel the interrupt at Δt intervals.

When the CPU 12 receives an interrupt request from the timer, the CPU 12 performs the process shown in FIG. That is, it is checked whether the count value is n = 0 (step S003), 0
If so, the counter with the count value n = N is set (step S004). After setting the count value or n
If is not 0, a signal instructing sampling and holding of measurement data is output to the A / D converter 11 (step S005). The A / D converter 11 receives this and takes in the data. Next, the CPU 12
A signal instructing to start A / D conversion is output to the A / D converter 11 (step S006). In response,
The A / D converter 11 converts the held measurement data into a digital value. When the A / D conversion is completed, the A / D converter 11 turns on the A / D conversion end signal. CP
The U12 waits for the A / D converter 11 to turn on the A / D conversion end signal (step S007). When the A / D conversion end signal is turned on, the CPU 12 takes in the converted digital data and stores it in a predetermined area of the memory 17 via the bus 2a (step S008). In the case of FIG. 6, the number of stored data is N in two cycles. N is determined by the current cycle and the sampling frequency, but can be 128, for example. And C
The PU 12 increments the count value by n-1, and ends the data acquisition process.

Next, the CPU 12 performs DFT processing on the digitized sampling value. The DFT determines the intensity of the current for each harmonic of each order. Based on this, the ratio (content ratio) contained in the whole of each harmonic is obtained. FIG. 10 shows an example of the content rate for each harmonic order,
It shows an example of setting the monitoring value for the harmonic order (frequency). In the figure, the ratio of the harmonics contained in the current waveform is set as a monitoring value for each order. When the order is high, the monitored value is set to the monitored value (lower right characteristic) because the allowable value in the electric path decreases. Note that FIG. 11 shows an example of a state in which the content rate of the third harmonic exceeds the set value.

The CPU 12 uses the data accumulated in the memory 17 until the timer issues an interrupt request.
Perform various calculations. For example, by a fast Fourier transform (hereinafter referred to as FFT) that saves the number of DFT operations,
The calculation for obtaining the current content rate for each harmonic is executed.
Further, using the obtained calculation result, comparison with the monitoring value is performed for each harmonic. The CPU 12 does not drive the alarm contact 19 when the harmonic content does not exceed the monitoring value. When the harmonic generation is remarkable and the harmonic content obtained by the FFT exceeds the monitored value in any order, as shown in FIG. 7, for example, a different time (alarm time After the time limit), the relay driver 18
The alarm contact 19 is driven via.

The monitoring value and alarm time limit for the harmonic current differ depending on the harmonic order and have the characteristics shown in FIG. For example, in the case of the 7th harmonic, the content rate is 20% and the alarm time limit is about 2.7 seconds, and in the case of the 5th harmonic, the content rate is 40%.
The alarm time limit is about 2 seconds in%.

The local monitoring device 4 has a function of transmitting measured amounts of current, voltage, power, power factor, etc. in addition to the above-mentioned harmonic alarm function. The transmission destination may be, for example, the central monitoring device 25 shown in FIG.

FIG. 8 shows the processing when a transmission request is interrupted by the central monitoring device 25. When the CPU 12 receives an interrupt request which is input to the transmission signal terminal 9 via the transmission path 5 and sent via the transmission driver / receiver 22, the CPU 12 checks whether the request is addressed to its own device (step S01).
0). That is, address verification is performed. The address verification is performed on the device address read during the initialization processing shown in FIG. 6 and the transmitted destination address. If it is not addressed to its own device, the interrupt request is ignored.
On the other hand, when it is addressed to the own device, the command sent is checked (step S011). That is, it checks whether it is a data transmission request. If it is not a data transmission request, the process moves to the corresponding process not shown in this flow.

On the other hand, if the request is for data transmission, the requested data is set (step S01).
2). Then, those data are transmitted (step S
013). As the requested data, for example, the sampled current value and voltage value, the electric power obtained from them, the power factor, and the like are sent. If the power and power factor are not calculated by the local monitoring device 4 but by the central monitoring device 25, the power and power factor are not transmitted.

By the above processing, the local monitoring device 4
Transmits the data obtained by the measurement to the central monitoring device 25. The transmission is sent from the transmission signal terminal 9 via the transmission line 5 via the transmission driver / receiver 22.

Next, the operation of the harmonic monitoring system shown in FIG. 3 will be described.

In FIG. 3, the local monitoring devices 4a to 4d provided on each electric line individually perform the operation described in the above embodiment. Each monitoring set value is set according to the withstand capability of the electric line and the connected load against harmonics. In addition, each of the local monitoring devices 4a to 4d is connected to the central monitoring device 25 via the transmission path 5, and transmits measured amounts of harmonic current, voltage, power, power factor, etc. of the electric path to the central monitoring device 25. To do.

The central monitoring device 25 accumulates and analyzes the received data from each of the local monitoring devices 4a-4d. In particular, the value of each harmonic of the electric circuit for each harmonic and the phase difference with reference to the voltage phase are obtained, and the harmonic flow is derived from this phase difference. Further, the directions of the harmonic sources are detected and the harmonic sources are specified by mutually comparing the flows of the harmonics.

As a premise, steady-state values of harmonics are accumulated, and values separated for each harmonic order are used.

Although the reactors and stray capacitances of the electric lines are distributed in the electric lines, they can be simplified as shown in FIG. That is, as a distributed constant or a lumped constant, a stray capacitance, a capacitive load such as a line filter, or the like, and an inductive load such as a line impedance or a reactor are connected in series. In the figure, the values of C and L have various values depending on the wiring material and method, but they function as a low-pass filter for harmonics. Therefore,
The phase is delayed with respect to the bank reference phase (voltage phase) as the distance from the generation source increases. The higher the order, the more difficult it is to grasp this phase difference, but by installing multiple local monitoring devices in the electric circuit and performing sampling time intervals in close proximity,
It can be processed by interpolation and extrapolation.

The concept for deriving the harmonic flow from the phase difference will be described. First, the first method is
Effective power (power factor) for each harmonic from active power and apparent power
Ask for. The power factor is obtained by calculating (active power / apparent power). The power factor tells whether the electrical angle (phase) is advanced or delayed. In the second method, for each phase of R, S, and T, the voltage waveform and the current waveform are band-limited to a frequency equal to or lower than the highest-order harmonic to be analyzed, and then sampling and digitization are performed. Then, DFT is performed on the data, and the amplitude and phase angle on the complex plane are calculated.

The cosine of the phase difference of the current phase of each harmonic with respect to the voltage phase shows the power factor at that harmonic, and the component in phase with the voltage phase shows the effective component of the current at that harmonic, and the voltage phase The component of the phase deviated from the quarter period from is the reactive component of the current in the harmonic.

Based on the phase difference between the amplitude and the voltage phase for each harmonic, which is obtained by sampling the current waveform and performing DFT, the intensity of each harmonic and the lead or lag of the phase are measured at the waveform measurement positions. By comparing,
Locate the bank that is causing the current distortion.

That is, FIG. 5 shows the R, S, and T phases of the current waveform including distortion as shown in FIG.
The time-division sampling as shown in (3) is performed, the sampling value at each time is digitized, and the DFT is performed. To specifically execute the calculation, the number of calculations can be saved by the FFT method. For example, as shown in FIG. 9, with respect to a waveform including a distortion having a basic period T, a constant time interval Δ
At T, sampling is performed for 2T, which is twice the cycle of the fundamental wave, and N (= 2T / ΔT) sampling values i
_{Let 1} , i ₂ , i ₃ , ... i _N be obtained.

Here, j is an imaginary unit, and the complex representation of the current is #I _k for the _kth harmonic component.

[0050]

[Equation 1]

For each harmonic, obtained by the DFT represented by

[0052]

[Equation 2]

The amplitude I _k and the phase θ _k are obtained.

Also for the voltage waveform, the complex display #V for the fundamental wave is obtained from the data sampled at the same time.

[0055]

(Equation 3)

Calculated by

[0057]

(Equation 4)

Then, the amplitude V and the phase θ _V are obtained.

Here, assuming that the voltage waveform is hardly distorted, it is assumed that all harmonics of the voltage have the phase θ _V. From this, the kth harmonic of
The phase difference between voltage and current is (2) and (4),

[0060]

(Equation 5)

Therefore, the power factor Pf _{K of} the kth harmonic is
Is

[0062]

(Equation 6)

Then, the reactive component Ir _{k of the kth} harmonic current
Is from (2) and (6)

[0064]

(Equation 7)

To obtain

The above operation is performed for each local monitoring device provided on the transmission line, and the position of the harmonic source is specified based on the measured current phase and reactive component for each harmonic.

High speed digital signal processing capability is required for each of the local monitoring device and the central monitoring device, but by performing distributed processing individually, it is possible to grasp the harmonic power flow as a system.

[0068]

According to the local monitoring apparatus of the present invention, since the monitoring value and the alarm time limit are different for each harmonic, the setting can be made according to the connection load. Therefore,
It is possible to detect the open-phase operation of the motor by the harmonic current and avoid damage to the cable or bus duct due to the increase of the harmonic current in the neutral wire of the three-phase four-wire type electric circuit. In addition, even-order harmonics increase due to damage to the electronic equipment, and damage due to biased magnetism of the transformer can be avoided.

Further, according to the monitoring system of the present invention, since it is possible to identify the harmonic source in the power distribution system, it is possible to determine whether the harmonic is generated on the power supply side or the power demand side. Further, it is possible to determine the electric circuit and the load for taking effective measures. Moreover, since the order and value of the harmonics are obtained, the reactor and the active filter of the capacitor as the countermeasure filter can be efficiently selected.

Furthermore, according to the present invention, since the harmonic current can be continuously grasped over the entire power supply system, a predictive warning and preventive maintenance can be performed, and a sudden power supply or equipment failure can be prevented. It can be prevented and the availability of power supply equipment can be improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing an installed state of one device of a local monitoring device of the present invention.

FIG. 2 is a block diagram showing a hardware system configuration of a local monitoring device of the present invention.

FIG. 3 is a block diagram showing an outline of a configuration of a monitoring system of the present invention.

FIG. 4 is a waveform diagram showing an example of a harmonic current waveform for each of R, S, and T phases.

FIG. 5 is an explanatory diagram showing an example of sampling of a current waveform.

FIG. 6 is a flowchart showing a main flow of a processing procedure of the monitoring device of the present invention.

FIG. 7 is a flowchart showing a procedure for controlling data sampling and A / D conversion processing of the monitoring device of the present invention.

FIG. 8 is a flowchart showing the procedure of data transmission processing in the monitoring device of the present invention.

FIG. 9 is an explanatory diagram showing details of a sampling state of a harmonic current.

FIG. 10 is an explanatory diagram showing a content rate of each harmonic order of the monitoring device of the present invention and a setting example of monitoring values for them.

FIG. 11 is an explanatory diagram showing an example in which the content rate in a partial order exceeds the monitoring value in FIG. 6;

FIG. 12 is an explanatory diagram showing an example of a relationship between a harmonic content rate and an alarm time period characteristic of the monitoring device of the present invention.

FIG. 13 is an explanatory diagram for explaining the concept of determining the direction of the harmonic current in the system of the present invention.

[Explanation of symbols]

1 ... Electric line, 2 ... CT, 3 ... VT, 4 ... Local monitoring device, 5 ... Transmission line, 11 ... A / D converter, 12 ... CPU,
13 ... Display element, 19 ... Alarm contact, 24 ... Transformer, 25
… Central monitoring device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Internal reference number FI Technical indication location H02J 13/00 301 H02J 13/00 301A 301J (72) Inventor Satoko Goto Nakajo Town, Kitakanbara-gun, Niigata Prefecture 46 Tomioka 1 Industrial Equipment Division, Hitachi, Ltd.

Claims (3)

[Claims] 1. A monitoring device for monitoring harmonics flowing on an electric path, wherein a means for detecting at least a current on the electric path, and a current for a harmonic to be monitored are detected from the current detected by the detecting means. A means for obtaining the magnitude, a means for monitoring whether or not each harmonic to be monitored exceeds a preset monitoring value, and a means for monitoring detects that one of the harmonics exceeds a reference value. In such a case, the monitoring device is provided with a unit that outputs an alarm after a lapse of a predetermined time for each of the harmonics to be monitored. 2. A harmonic monitoring system for monitoring harmonics flowing on an electric line, comprising: a plurality of local monitoring devices provided in a distribution system for measuring and monitoring harmonics; and data of the local monitoring device. For the harmonics to be monitored and derived, derive the harmonic flow and connect the central monitoring device that identifies the harmonic source to each local monitoring device and the central monitoring device. The central monitoring device includes means for obtaining the phase difference of the current for each harmonic to be monitored for the harmonics of the power line, and the acquisition from each local monitoring device. A harmonics monitoring system, comprising: means for deriving a harmonic flow based on the magnitude and phase of the current value for each harmonic and identifying the harmonic source. 3. The local monitoring device according to claim 2, wherein each of the local monitoring devices detects a current and a voltage on the electric circuit, and a means for obtaining the magnitude of the current for the harmonic to be monitored from the detected current. For each harmonic to be monitored, a means to monitor whether it exceeds a preset monitoring value, and if the monitoring means detects that one of the harmonics exceeds the reference value, monitor it. A harmonic monitoring system, comprising: means for outputting an alarm after a lapse of a predetermined time for each harmonic.